Multifunctional electrospun PVDF/CS-Al/Cu-LDH hybrid nanofiber membrane with superior Hg(II) adsorption efficiency and dual anticancer–antioxidant bioactivity

Abstract

The analysis at hand discusses the creation and multifunctional testing of an electrospun nanofiber membrane completed from polyvinylidene fluoride (PVDF), chitosan (CS), and Al/Cu-layered double hydroxide (Al/Cu-LDH), which is meant for the simultaneous adsorption of Hg(II) ions and the testing of its biological properties. Thorough physicochemical characterization techniques such as FT-IR, XRD, BET surface area analysis, XPS, and SEM-EDS confirmed the effective incorporation of Al/Cu-LDH and CS into the PVDF matrix to arrangement a highly porous membrane with enriched hydroxyl and amino practical groups that are important in binding interactions with metal ions. Batch adsorption studies revealed an appreciable influence of adsorbent quantity, solution pH, and interaction time, with a maximum Langmuir monolayer adsorption capacity of 422.68 mg.g⁻¹ at pH 6 after 100 min and a quantity of 0.02 g. Adsorption kinetics were best fitted to the pseudo-second-order model, whereas thermodynamic study reflected that the adsorption method was spontaneous and endothermic. Response Surface Methodology (RSM) using a Box–Behnken design (BBD) was applied for the statistical optimization of adsorption parameters with a desirability value of 0.99 and a respectable fit among experimental and projected results. The uptake of Hg(II) ions occurs through surface complexation and ion exchange pathways involving –OH and –NH₂ functional groups, as further validated by XPS investigations. Besides its adsorption abilities, the membrane showed significant biological efficiency, presenting dose-dependent cytotoxicity against MCF-7 (breast cancer) besides HePG-2 (liver cancer) cell lines, along with remarkable antioxidant activity in DPPH radical scavenging tests. These results highlight its potential for dual applications in environmental cleanup and biomedical fields. Altogether, the PVDF/CS-Al/Cu-LDH nanofiber membrane comes out as a high-capacity, regenerable, and biologically active material with future prospects for advanced water purification technologies plus multifunctional therapeutic applications.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1038/s41598-026-54200-0.

Introduction

Mercury (Hg) is recognized as one of the most hazardous heavy metals present in the environment, attributable to its formidable toxicity, persistence in ecosystems, and propensity for bioaccumulation and biomagnification within both aquatic and terrestrial food webs¹. Even at minimal concentrations, mercury represents a significant risk to human health, being associated with a range of severe conditions including neurological, renal, and developmental disorders. The primary anthropogenic sources responsible for mercury contamination in aquatic systems include industrial discharges from processes such as chlor-alkali production, coal burning, metal plating, mining operations, and the manufacturing of batteries and fluorescent lamps. Due to its high mobility, mercury can manifest in various oxidation states, namely elemental (Hg⁰), mercurous (Hg⁺), and mercuric (Hg²⁺), with the latter being the most stable and toxic form in aqueous environments². Because of this, the efficient removal and recovery of mercury ions (Hg(II)) from industrial wastewater has become a major environmental issue, underscoring the pressing need for the creation of cutting-edge, effective, and sustainable remediation solutions. Chemical precipitation, membrane filtration, ion exchange, electrochemical treatment, and adsorption are the conventional methods used to remove mercury and other heavy metals from wastewater. Among these techniques, adsorption has been found to be particularly successful, flexible, and economical due to its ease of use, potential for reuse, and ability to extract trace amounts of pollutants without causing secondary contamination³. However, the efficacy of traditional adsorbents, including activated carbon, clays, zeolites, and metal oxides, is frequently restricted by factors such as limited surface area, inadequate selectivity, and constrained regeneration capabilities. In order to overcome these constraints, investigators are increasingly concentrating on the development of advanced hybrid nanomaterials that exhibit elevated adsorption capacities, robust structural integrity, and modifiable surface chemistry, thus facilitating the selective capture of mercury ions through various physicochemical interactions⁴.

Layered double hydroxides (LDHs), commonly referred to as hydrotalcite-like resources, have garnered significant scholarly interest as prospective adsorbents for heavy metal ions. This interest stems from their distinctive two-dimensional lamellar architecture, elevated anion exchange capacity, and plentiful hydroxyl groups that purpose as active places for the coordination of metal ions⁵. The inherent structural versatility of LDHs facilitates meticulous modulation of their physicochemical characteristics, which encompass surface charge, interlayer spacing, and chemical reactivity. Notably, among the various LDH configurations, bimetallic Al/Cu-LDHs exhibit commendable adsorption properties attributed to the synergistic interaction between Al³⁺ and Cu²⁺ ions, which bolsters surface basicity, redox activity, and the affinity for soft metal cations like Hg(II). Additionally, the incorporation of Cu²⁺ yields supplementary sites that can establish stable surface complexes with mercury species via coordination and ion-exchange processes⁶. Notwithstanding their exceptional adsorption capabilities, layered double hydroxide (LDH) powders frequently encounter challenges such as particle aggregation, inadequate mechanical strength, and difficulties associated with separation and reuse post-adsorption^7,8. To mitigate these concerns, LDHs are commonly immobilized or combined with polymeric substrates to enhance their structural integrity and facilitate their effective utilization in aqueous environments. For creating nanofibrous membranes with high porosity, large surface area, and interconnected pore networks which offer a multitude of active sites for adsorption and filtration applications electrospinning has become a powerful and versatile method. Polyvinylidene fluoride (PVDF) is frequently employed as a matrix polymer owing to its remarkable mechanical properties, chemical stability, and resilience against degradation in severe conditions^9,10. However, PVDF, when used in isolation, lacks adequate hydrophilicity and active sites requisite for effective binding of metal ions, thus necessitating surface modification or the incorporation of functional additives. Chitosan (CS), a biopolymer derived from chitin, presents a sustainable and eco-friendly approach to modification owing to its abundant amino (–NH₂) and hydroxyl (–OH) functional groups, which can interact with metal ions via chelation and hydrogen bonding mechanisms. By integrating Al/Cu-LDHs and chitosan within an electrospun PVDF matrix, it is possible to develop a hybrid nanofibrous membrane that features multifunctional adsorption sites and enhanced surface accessibility, thereby uniting the mechanical fortitude of PVDF with the hydrophilicity and metal-binding traits of chitosan and LDH nanoparticles. This tri-phase nanocomposite architecture not only improves the dispersion and stability of the LDH within the polymeric matrix, but also generates numerous interfacial active sites adept at capturing mercury ions through complexation, ion exchange, and electrostatic forces. Furthermore, the nanofiber membrane’s interlinked porous structure promotes quick mass transfer and effective interaction between the Hg(II) ions and the functional adsorption sites, leading to increased removal efficacy and quick adsorption dynamics¹¹.

Numerous investigations have substantiated the efficacy of LDH-based polymeric composites in the removal of hazardous metal ions counting Cd(II), Hg(II), Pb(II), and Cr(VI)¹². However, many of these systems encounter challenges pertaining to limited selectivity, structural degradation, or inadequate reusability. The innovation of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane aims to mitigate these issues by harnessing the synergistic interactions between the inorganic and organic constituents. The Al/Cu-LDH component facilitates the presence of active hydroxyl groups and redox-active copper sites that are instrumental in the complexation of mercury¹³. Concurrently, chitosan introduces amino and hydroxyl functional groups that augment the chemical affinity and adsorption capabilities. The PVDF backbone is critical for providing mechanical strength, durability, and resistance to chemical degradation, enabling the membrane to sustain its performance across continuous-flow or multi-cycle adsorption settings¹⁴. Moreover, the process of electrospinning employed in the fabrication of these nanofibrous composites ensures the nanoscale dispersion of the active LDH particles, thereby significantly increasing the number of accessible reactive sites for effective mercury capture. The Hg(II) adsorption mechanism on the Hg@PVDF/CS-Al/Cu-LDH membrane is characterized by a synergistic integration of surface complexation, ion exchange, and electrostatic connections. The incorporation of nitrogen and oxygen donor atoms within the chitosan and LDH layers promotes the establishment of stable Hg–N and Hg–O coordination bonds, while the Cu(II) sites embedded in the LDH lattice enhance surface complexation through partial electron transfer. The collective interplay of these mechanisms produces high binding affinity, rapid adsorption kinetics, and strong chemisorption of mercury ions onto the composite’s surface. Furthermore, the hierarchical architecture of the electrospun membrane improves the accessibility to adsorption sites and inhibits particle agglomeration, thereby preserving a high level of efficiency throughout the entirety of the adsorption method¹⁵.

The novelty of this study investigated the synthesis, structural characterization, and adsorption performance of the PVDF/CS-Al/Cu-LDH nanofiber membrane towards Hg(II), which functions as an innovative multifunctional adsorbent for mercury extraction from aqueous systems. The integration of Al/Cu-LDH and chitosan into the PVDF matrix via electrospinning produces a resilient hybrid structure that demonstrates improved chemical reactivity, mechanical strength, and environmental durability. Comprehensive characterization utilizing methodologies like FT-IR, XRD, BET, SEM-EDX, and XPS confirms the effective synthesis of the composite and clarifies the adsorption mechanisms associated with Hg(II) ions. This research search for to donate to the expansion of advanced nanostructured materials specifically designed for water purification and environmental cleanup, thereby offering a feasible and sustainable strategy for mitigating mercury pollution in industrial discharges.

Experimental

Resources and apparatuses

Tables S1 and S2 provide a complete description of the resources and apparatuses utilized.

Preparation of adsorbent

Preparation of Al/Cu-LDH

The production of layered double hydroxide (LDH) materials via the coprecipitation technique adhered to the methodology depicted in Fig. 1. Firstly, solution A was constituted by liquifying 4.59 mmol (0.612 g) of aluminum chloride (AlCl₃) alongside 10.34 mmol (1.39 g) of copper chloride (CuCl₂) in 60 mL of distilled water. At the same time, 5 mmol of sodium carbonate (Na₂CO₃) and 27.57 mmol (1.102 g) of sodium hydroxide (NaOH), were dissolved with the help of an extra 60 mL of distilled water, creating solution B. Upon achieving a pH within the range of 9 to 10 for solution A, incremental additions of solution B were performed. This mixture was then maintained under a nitrogen atmosphere and allowed to homogenize at ambient temperature for a duration of 24 h. The resulting solids were subsequently processed into Al/Cu-LDH precipitate by subjecting them to centrifugation at 8000 rpm for 10-min intervals, utilizing ethanol and distilled water to eliminate excess ions. Lastly, the solids were dried for six hours at 80 °C in a lab oven¹⁶.

Fig. 1.

Diagrammatic representation of PVDF/CS-Al/Cu-LDH nanofiber membrane synthesis for Hg(II) adsorption.

Preparation of PVDF/CS-Al/Cu-LDH nanofiber membrane

Nanofiber membrane assemblies were synthesized through the electrospinning technique, using a polymer blend consisting of Polyvinylidene fluoride (PVDF) and chitosan (CS) in an 80:20 weight ratio. The solution of polymer was formulated at a concentration of 15% weight/volume by dissolving the constituents in a solvent comprising 90% acetic acid, augmented with 5% weight/weight of Al/Cu-LDH)¹⁷. A glass syringe with a blunt-tip 20-gauge needle held this final solution. A syringe pump device was used to precisely manage the polymer solution’s flowing rate at 1 mL/h. An electrical voltage between 27 and 28.5 kV was utilized during the electrospinning process, and the needle was placed 15 cm away from the grounded collection plate (Fig. 1).

Removal and batch studies of Hg(II) via PVDF/CS-Al/Cu-LDH nanofiber membrane

A comprehensive investigation was undertaken to calculate the adsorption efficacy of the PVDF/CS-Al/Cu-LDH nanofiber membrane. This study prioritized the examination of numerous factors, including pH levels, the quantity of the adsorbent employed, the original concentrations of Hg(II), the duration necessary to attain equilibrium, and the variations in temperature¹⁸. An analytical batch adsorption investigation was directed to evaluate the effectiveness of Hg(II) ion removal. A stock solution was diluted to generate a 25 mL aliquot for the study, which examined adsorption isotherms at different concentrations spanning from 20 to 590 mg/L. During the course of the experiments, the aliquot volume was consistently held at 25 mL, the pH was regulated at 6, an adsorbent mass of 0.02 g was employed, and the duration of contact was standardized at 100 min under a controlled temperature of 25 °C. In addition, to examine the impact of contact time on adsorption efficiency, supplementary experiments were performed utilizing a constant concentration of 400 mg/L, during which the interaction duration was systematically varied between 5 and 100 min, while other temperature and solution parameters were kept constant. At a concentration of 400 mg/L and a set contact period of 100 min, the impact of temperature was also thoroughly evaluated. A constant volume of 25 mL, a pH of 6, and an adsorbent mass of 0.02 g were maintained while the temperature variation was methodically adjusted from 20 to 45 °C. Both sodium hydroxide and hydrochloric acid solutions, each at 0.1 N, were used for pH modifications in order to maintain constant pH values throughout the methods of experimentation¹⁹. The collected data were then analyzed to look at adsorption isotherms and kinetic behaviors, as well as to assess how temperature affected the thermodynamic characteristics related to the adsorption procedures. Before and after the adsorption phase, the concentrations of Hg(II) ions in the solutions were measured applying a Perkin-Elmer flame atomic absorption spectrometer. Equations (1) and (2) served as a fundamental basis for calculating the adsorption capacity (q_e) and the quantity of metal ions effectively immobilized by the PVDF/CS-Al/Cu-LDH nanofiber membrane.

1

2

Biological experimental

The cell lines utilized in this investigation were procured from a recognized cell repository. In particular, the MCF-7 cell line, which is characteristic of human breast adenocarcinoma, and the HepG-2 cell line, indicative of human hepatocellular carcinoma, were given by the American Type Culture Collection (ATCC, Manassas, VA, USA). The supplier’s recommended growth conditions were followed for these cell lines.

Experimental design

Response Surface Methodology (RSM) is used to create statistical models for issues with several possible answers²⁰. Clarifying the relationship between experimental variables and their corresponding outcomes is the main goal. Additionally, RSM serves as a useful technique to improve process effectiveness²¹. In order to pinpoint the optimal solution amidst a diverse technical environment, RSM executes a sequence of systematically organized experiments that are guided by the essential appearances of the procedure in question. RSM is recognized as a widely employed method for optimizing process parameters in this setting. The amount of metal ions that can be adsorbed per gram of PVDF/CS-Al/Cu-LDH nanofiber membrane at different time intervals (t) is quantitatively evaluated in Eq. (1). On the other hand, Eq. (2) makes it possible to assess how well the metal ion removal procedure works^22,23. The weight of the adsorbent, the length of contact, and the pH of the solution have all been found to be crucial variables that serve as process parameters. These factors were selected because, as Table S3 shows, they had significant negative effects on adsorption capacity. Furthermore, the analysis provides a thorough assessment of the limitations associated with each parameter, outlining their respective maximum and minimum values recorded during the study. Table 1, constructed utilizing Design Expert Software, provides an extensive overview of multiple combinations of parameters and their associated outcomes. A comprehensive investigation of the correlations between the parameters and the resulting data is made possible by the inclusion of “2×m” axial runs, “2^m” factorial runs, and P center runs in this analysis. Equation (3) can be used to determine the required number of evaluations, which is determined by the number of input variables included in the study.

3

Table 1.

The response surface methodology framework was utilized to evaluate the adsorption capacity of Hg(II) ions, and the Box-Behnken Design (BBD) was employed for a systematic study.

Run	Real variables			qe (mg/g)
	pH	Time (min.)	Dose	Experimental	Expected	Residual error
1	5	100	0.5	265.125	228.11	37.01
2	2	52.5	0.5	195.082	213.08	-18.00
3	5	52.5	0.26	315.13	315.13	0.0000
4	5	52.5	0.26	315.13	315.13	0.0000
5	2	5	0.26	212.265	153.42	58.85
6	5	100	0.02	424.2	383.35	40.85
7	5	5	0.5	26.66	67.51	-40.85
8	8	5	0.26	38.62	19.61	19.01
9	8	52.5	0.02	327.74	309.74	18.00
10	5	52.5	0.26	315.13	315.13	0.0000
11	2	52.5	0.02	256.045	277.88	-21.84
12	2	100	0.26	219.793	238.80	-19.01
13	5	52.5	0.26	315.13	315.13	0.0000
14	8	100	0.26	326.31	385.16	-58.85
15	5	52.5	0.26	315.13	228.11	37.01
16	5	5	0.02	56.005	213.08	-18.00
17	8	52.5	0.5	215.617	315.13	0.0000

The approach defines “p” as the required number of the experimental trials and “N” as the number of process parameters influencing the results. The variable “m” in this study has a fixed value of 3²⁴. The central compound design encompasses three fundamental stages: first, the determination of the model coefficients; next, the formulation of the experimental structure; and finally, the valuation of the model’s effectiveness through outcome analysis. After this analysis is finished, a solid empirical model has been built that methodically investigates the function’s performance in various input variable configurations. As shown in Eq. (4), a quadratic regression model has been developed as a result:

4

In this analysis, we use the letter “i” to represent the resistance constant and the letter “j” to represent the velocity constant. The different constants associated with resistance, interaction, velocity, and an extra constant are represented by the parameters β₀, β_i, β_ii, and β_ij. There is a methodical framework to this classification. We look at the metrics R², R²_Adj, and R²_Pred to evaluate the efficiency of the polynomial model computations. A higher correlation between the computed findings and the experimental data set is indicated by a higher R² value, which enhances the model’s ability to accurately represent the observed events²⁵.

Results and discussion

Characterization of PVDF/CS-Al/Cu-LDH

X-ray diffraction (XRD) patterns

The X-ray diffraction (XRD) patterns of the PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membranes, as exposed in Fig. 2a, validate the effective combination and structural development of the LDH embedded within the polymeric matrix, both preceding to and subsequent the adsorption of mercury. The unaltered PVDF/CS-Al/Cu-LDH demonstrates characteristic LDH diffraction peaks at approximately 2θ = 11.5°, 23°, 34°, and 60°, which resemble to the (003), (006), (012), and (110) crystallographic planes associated with hydrotalcite-like structures. Additionally, a broad halo is observed around 2θ = 18–20°, indicative of the semi-crystalline β-phase of PVDF. The distinct peaks attributed to the LDH reveal a well-ordered lamellar architecture, confirming the effective integration of Al/Cu-LDH nanoparticles into the PVDF/CS nanofibrous matrix without any compromise to structural integrity. Following mercury adsorption, the Hg@PVDF/CS-Al/Cu-LDH pattern exhibits a significant decrease in diffraction intensity and a broadening of the LDH peaks, particularly observable in the (003) and (006) reflections, alongside a slight shift of the basal (003) peak to a lower 2θ value. These alterations are indicative of partial distortion and increased disorder of the LDH layers, which likely arise from the intercalation or surface complexation of Hg(II) ions through the hydroxyl group functionalities of the LDH and the amino groups present in chitosan, resulting in increased interlayer spacing and diminished crystallinity. Notwithstanding this structural change, the continuity of the amorphous halo of PVDF substantiates the stability of the polymeric framework following the incorporation of Hg(II) ions. In summary, the transformations observed through XRD corroborate that mercury ions were effectively immobilized within the composite via ion exchange and coordination mechanisms, culminating in a partially amorphized yet structurally stable Hg-loaded PVDF/CS-Al/Cu-LDH nanofiber membrane²⁶.

Fig. 2.

(a) XRD of PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane, (b) N₂ adsoprtion/desorption isotherm of PVDF/CS-Al/Cu-LDH, (c) Pore radius, (d) Pore size, (e) FT-IR (f) Asynchronous, (g) Synchronous of PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH, (h) EDX of PVDF/CS-Al/Cu-LDH, and (i) EDX of Hg@PVDF/CS-Al/Cu-LDH.

Isotherm of N₂ adsorption/desorption

The isotherm of nitrogen adsorption–desorption and the pore structure analysis of PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membranes are presented in Fig. 2b–d, elucidating the textural characteristics and the effects of Hg(II) adsorption on the surface traits of the composite materials. According to the International Union of Pure and Applied Chemistry (IUPAC), both samples exhibit a classic type IV isotherm, as shown in Fig. 2b, indicating that they are mesoporous materials²⁷. The isotherms exhibit a distinct H3-type hysteresis loop, revealing of slit-shaped pores designed due to the aggregation of plate-like LDH nanoparticles in conjunction with the interstitial connections within the PVDF/CS nanofibrous matrix. The PVDF/CS-Al/Cu-LDH composite reveals a relatively elevated nitrogen uptake at increased relative pressures (P/P₀ > 0.8), attesting to its well-developed mesoporous architecture and considerable surface area. In the case of mercury adsorption, the Hg@PVDF/CS-Al/Cu-LDH membrane shows a significant decrease in nitrogen adsorption capacity, which implies partial blockage of pores or surface coverage resulting from the Hg(II) adsorption. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, diminishes from 93.21 m²/g for the pristine PVDF/CS-Al/Cu-LDH nanofiber membrane to 84.36 m²/g following the Hg(II) adsorption, suggesting the occupation of active sites by mercury ions and a corresponding reduction in the accessible surface area. The analysis of pore radius distribution, illustrated in Fig. 2c, substantiates this observation: the average pore radius contracts from 2.44 nm in PVDF/CS-Al/Cu-LDH to 1.88 nm in Hg@PVDF/CS-Al/Cu-LDH, thereby indicating a reduction of pore channels due to the deposition and intercalation of mercury species within the galleries of the LDH and the polymer matrix. Moreover, the pore size distribution illustrated in Fig. 2d indicates a decline in total pore volume from 0.168 cm³/g to 0.136 cm³/g subsequent to Hg(II) loading, aligning with indications of partial filling or obstruction of mesopores during the adsorption process. Collectively, these textural alterations evidence that Hg(II) ions engage significantly with the composite surface via chemisorption and ion exchange mechanisms, leading to diminishing characteristics of surface area, pore size, and overall pore volume while preserving the mesoporous structure. Hence, the results derived from the N₂ adsorption–desorption experiments reinforce the structural integrity and enhanced adsorption efficacy of the PVDF/CS-Al/Cu-LDH nanofiber for the elimination of mercury, affirming its potential utility in the effective capture of heavy metals²⁷.

FT-IR

The Fourier-transform infrared (FT-IR) spectra and two-dimensional correlation infrared analyses of the PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membranes, as depicted in Figs. 2e–g, offer comprehensive insights into the practical groups that performance a role in mercury adsorption and the associated molecular interactions. In Fig. 2e, the FT-IR spectrum of the unmodified PVDF/CS-Al/Cu-LDH nanofiber membrane reveals distinct absorption bands that are characteristic of its composite materials. The broad band observed at approximately 3420 cm⁻¹ can be credited to the stretching vibrations associated with –OH groups originating from interlayer water molecules and hydroxyl groups present in the LDH framework. The peaks that appear in the range of 2920–2850 cm⁻¹ are linked to the asymmetric and symmetric stretching vibrations of –CH₂ groups from both PVDF and chitosan²⁷. The band situated at 1630 cm⁻¹ is related to the bending vibrations of adsorbed H₂O molecules as well as the N–H bending vibrations of the amine groups found in chitosan. Absorption peaks that are noted within the range of 1400–1470 cm⁻¹ correspond to C–H bending and O–H deformation vibrations, whereas the pronounced bands located between 1010 and 1100 cm⁻¹ are credited to C–O–C and C–F stretching vibrations, thereby confirming the existence of PVDF and chitosan within the nanofiber matrix. The distinctive peaks associated with the LDH layers are observed around 620 cm⁻¹ and are linked to the stretching vibrations of M–O and M–O–M (specifically Al–O and Cu–O), thus affirming the structural integrity of the Al/Cu-LDH phase embedded in the polymeric composite. Post-mercury adsorption, the FT-IR spectrum of the Hg@PVDF/CS-Al/Cu-LDH reveals several significant alterations²⁸. The –OH stretching band broadens and shows a slight shift toward lower wavenumbers, which indicates a robust interaction and hydrogen bonding among Hg(II) ions and the hydroxyl groups located on the surface of the LDH. Concurrently, the N–H bending vibration of chitosan diminishes in intensity and shifts, which signifies coordination among the Hg(II) ions and the nitrogen atoms from the amino groups. Further supporting the creation of Hg–O bonds is a decrease in the intensity of the M–O vibration bands around 620 cm⁻¹, which suggests that complexation and ion exchange mechanisms inside the LDH structure and at the polymer’s active regions immobilize mercury ions²⁹. A more thorough knowledge of these interactions is provided by the two-dimensional association infrared spectra. The asynchronous 2D correlation spectrum illustrated in Fig. 2f displays cross-peaks predominantly situated in regions associated with –OH (3600–3200 cm⁻¹), C–H (2900–2850 cm⁻¹), and C–O/C–F (1400–1000 cm⁻¹) vibrations. This observation suggests that there are sequential and site-specific alterations in these functional groups because of Hg(II) adsorption. The synchronous spectrum illustrated in Fig. 2g reveals pronounced auto peaks within comparable regions, thereby corroborating the simultaneous perturbations of hydroxyl, amino, and carbonyl functional groups. Collectively, these two-dimensional correlation findings substantiate that the adsorption of Hg(II) is characterized by multidentate coordination interactions involving –OH, –NH₂, and oxygen-containing groups, in addition to potential electrostatic interactions with the LDH structure. In summary, the integrated analyses of FT-IR spectroscopy and two-dimensional correlation techniques affirm that the Hg(II) adsorption onto PVDF/CS-Al/Cu-LDH through a synergistic mechanism encompasses surface complexation, and ion exchange, without any indicators of structural degradation within the composite framework²⁶.

EDX analysis

The energy-dispersive X-ray spectroscopy (EDX) spectrum obtained for the PVDF/CS-Al/Cu-LDH nanofiber membrane, as demonstrated in Fig. 2h, provides compelling evidence for the actual integration and uniform distribution of all anticipated elements in the compound matrix. The identified elements—carbon (C), nitrogen (N), oxygen (O), fluorine (F), aluminum (Al), and copper (Cu)—align closely with the chemical components of the composite: chitosan contributing C, N, and O; PVDF contributing C and F; and the Al/Cu-LDH phase contributing Al, Cu, and O. Quantitative elemental analysis indicates that fluorine constitutes the majority element, representing 72.01%, which underscores the substantial presence of the PVDF polymer in the composite. The concentrations of carbon (10.85%), oxygen (9.45%), and nitrogen (4.08%) predominantly derive from the chitosan and PVDF constituents, with nitrogen indicating the existence of amino functional groups in chitosan that facilitate active binding places for metal ions. The quantities of metallic elements aluminum (1.92%) and copper (1.7%) corroborate the incorporation of Al/Cu-LDH nanoparticles within the polymeric framework³⁰. The simultaneous observation of metallic peaks alongside organic elements signifies a well-integrated hybrid structure, suggesting the uniform dispersion of LDH nanoplates throughout the PVDF/CS matrix, with no indications of aggregation. The pronounced fluorine signal and the proportionate presence of aluminum and copper peaks further affirm that the nanofiber membrane retains its intended composition and structural integrity post-synthesis. In summary, the EDX analysis substantiates the formation of a chemically homogeneous and compositionally cohesive PVDF/CS-Al/Cu-LDH nanofiber membrane, with the observed elemental distribution validating the effective integration of LDH into the polymeric host, as well as the existence of active locations amenable for future heavy metal adsorption³¹.

The EDX analysis of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane, depicted in Fig. 2i, provides unequivocal indication of the actual adsorption and immobilization of mercury ions on the composite surface. The EDX spectrum identifies characteristic peaks corresponding to Hg, alongside the fundamental elemental constituents of the composite, namely C, N, O, F, Al, and Cu. This observation indicates that the structural veracity of the PVDF/CS-Al/Cu-LDH membrane maintains post-adsorption of Hg(II) ions. Quantitative elemental analysis reveals that mercury accounts for 23.49% of the total detected elements, underscoring a significant loading efficiency and a strong empathy of the composite for mercury ions. The substantial presence of Hg, along with discernible peaks at approximately 2.2, 10.0, and 11.5 keV, validates the chemisorption of Hg(II) species onto both the LDH and polymeric active sites. In comparison to the unmodified PVDF/CS-Al/Cu-LDH membrane, the relative proportions of C (decreased to 5.42%), N (3.07%), O (8.75%), and F (55.96%) exhibit slight reductions, indicative of partial surface coverage by adsorbed mercury ions, which diminishes the signal intensities of the lighter elements. The levels of Al (1.71%) and Cu (1.59%) remain nearly constant, suggesting that the LDH framework retains stability and is not leached during the adsorption method. The emergence of pronounced Hg peaks and the concurrent decrease in the ratios of the other surface elements provide compelling evidence that mercury ions are effectively sequestered through mechanisms like ion exchange, surface complexation, and electrostatic interactions with hydroxyl, amino, and oxygen-containing practical groups present on the PVDF/CS-Al/Cu-LDH architecture. In summary, the EDX results robustly validate the effective synthesis of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane and affirm its efficacy in mercury adsorption. The high mercury percentage combined with the conservation of the LDH framework integrity highlight the composite’s exceptional chemical stability and strong binding capacity, placing it as a promising applicant for requests in heavy metal removal³².

SEM analysis

The SEM images of the PVDF/CS-Al/Cu-LDH nanofiber membrane show a well-structured three-dimensional network of smooth, continuous, and bead-free nanofibers with uniform morphology, indicating effective synthesis through electrospinning. Fiber diameters range from 68.38 to 116.9 nm, with a mean of approximately 81.06 nm, reflecting controlled electrospinning parameters. The nanofibers are randomly oriented and highly interconnected, creating a porous structure that enhances surface accessibility and promotes effective adsorbate diffusion³³. At higher magnifications, the fibers’ surfaces appear slightly textured due to the uniform dispersion of PVDF/CS-Al/Cu-LDH. The absence of nanoparticle agglomeration and consistent nanoparticle distribution suggest robust interfacial interactions, likely from hydrogen bonding and electrostatic forces between LDH hydroxyl groups and the chitosan and PVDF amino and fluorinated groups. This results in a highly porous membrane with a large specific surface area, providing numerous active adsorption sites necessary for heavy metal removal. SEM analysis confirms that the PVDF/CS-Al/Cu-LDH nanofiber membrane has a homogeneous, flexible morphology, making it suitable for efficient adsorption of Hg(II) and other environmental remediation applications (Fig. 3a).

Fig. 3.

(a) SEM of PVDF/CS-Al/Cu-LDH, (b) SEM mapping of PVDF/CS-Al/Cu-LDH, (c) SEM of Hg(II)@PVDF/CS-Al/Cu-LDH, and (d) SEM mapping of Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane.

The elemental mapping of the PVDF/CS-Al/Cu-LDH nanofiber membrane using Scanning Electron Microscopy Energy Dispersive Spectroscopy (SEM-EDS) shows a uniform distribution of elements C, N, O, F, Al, and Cu within the fibrous structure, confirming the successful fabrication of a homogeneous hybrid membrane. The central SEM image depicts a well-interconnected nanofibrous network, while the mapping indicates that carbon and nitrogen from chitosan are evenly dispersed³⁴. The consistent distribution of oxygen atoms associated with functional groups in both chitosan and LDH suggests active moieties that enhance metal ion binding. The prominent fluorine signal makes up about 75% of the elemental composition, highlighting the PVDF phase’s predominance and the support of its fluoropolymer backbone’s structural integrity³⁵. The aluminum and copper distribution maps indicate uniformity without aggregation, confirming effective embedding of PVDF/CS-Al/Cu-LDH nanofibers and their even dispersion throughout the network. This dispersion signifies good interfacial compatibility among the inorganic LDH and organic polymer medium, enhancing the membrane’s structural stability and reactivity³⁶. The quantitative composition of 5% C, 4% N, 9% O, 75% F, 5% Al, and 3% Cu further validates the successful integration of components. In conclusion, SEM-EDS mapping confirms that the PVDF/CS-Al/Cu-LDH nanofiber membrane has a uniform elemental composition, establishing a robust hybrid architecture with many active sites for effective heavy metal adsorption (Fig. 3b).

The SEM images of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane, shown in Fig. 3c, demonstrate the morphological integrity and surface changes after Hg(II) adsorption. This confirms the structural stability and high adsorption capacity of the composite membrane³⁷. The membrane maintains its three-dimensional, randomly oriented, and bead-free fibrous network with smooth interconnections, indicating that the electrospun structure is intact during adsorption. The nanofiber diameters range from 67.49 to 169.3 nm, slightly larger than the pristine PVDF/CS-Al/Cu-LDH membrane due to mercury ions depositing on fiber surfaces and partially occluding pores in the matrix³⁸. The fibers exhibit a rougher texture and denser appearance, likely due to strong interactions among Hg(II) ions and active functional groups like hydroxyl (–OH) from LDH layers, amino (–NH₂) from chitosan, and other oxygen-containing groups from both CS and PVDF. This suggests surface complexation and ion exchange mechanisms aid in mercury immobilization. Despite adsorption-induced changes, the overall fibrous morphology remains well-defined, with no signs of fiber fusion, indicating mechanical resilience and chemical stability. The porous, interconnected structure ensures continuous accessibility of active sites, benefiting subsequent adsorption cycles and enhancing mass transfer efficiency. Consequently, the SEM analysis shows that the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane sustains a stable and uniform nanofibrous morphology, with increased surface roughness, confirming successful Hg(II) adsorption through chemisorption and complexation mechanisms while maintaining structural integrity and reusability for environmental remediation³⁹.

The elemental mapping via SEM-EDS of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane illustrates the elemental arrangement and spatial preparation. This analysis confirms uniform incorporation of constituents and effective adsorption of Hg(II) ions. The SEM image shows a consistent nanofibrous structure without beads, maintaining integrity post-mercury adsorption, indicating the durability of the electrospun composite. The mapping results show thorough dispersion of C, N, O, F, Al, Cu, and Hg, reflecting the hybrid nature of the material. Carbon and nitrogen signals arise from chitosan, ensuring even distribution within the PVDF matrix. The oxygen signal stems from hydroxyl and carbonyl groups in chitosan and LDH, which facilitate binding to mercury ions³⁹. The fluorine, making up 66% of elemental distribution, supports structural continuity and adds flexibility to the membrane. Aluminum and copper from LDH nanoplates are distributed evenly without aggregation, demonstrating compatibility between inorganic LDH and organic matrices. The mercury mapping shows a well-distributed signal across nanofibers, confirming effective immobilization of Hg(II) ions through interactions with hydroxyl, amino, and oxygen-rich groups, as well as ion exchange in the LDH interlayers. The elemental composition is 5% C, 4% N, 9% O, 66% F, 6% Al, 4% Cu, and 5% Hg, indicating uniform chemical integration and efficient mercury loading. Thus, the SEM-EDS mapping analysis validates the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane as having well-distributed composition, stability, and effective Hg(II) adsorption, making it a promising option for environmental remediation (Fig. 3d).

XPS

The X-ray photoelectron spectroscopy (XPS) analysis of C 1s spectra for PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH membranes provides insights into surface chemical composition and bonding related to Hg(II) adsorption. For the unmodified PVDF/CS-Al/Cu-LDH membrane, the C 1s spectrum shows peaks at 284.11, 285.53, and 292.24 eV, linked to C–C/C = C, C–O/C–N, and O–C = O/C–F bonds. The strongest peak at 285.53 eV (52.93%) indicates carbon bonded to oxygen or nitrogen, reflecting integration of organic and inorganic phases. The peak at 292.24 eV (33.83%) corresponds to C–F bonds in PVDF, confirming the fluoropolymer’s presence, while the peak at 284.11 eV (13.74%) relates to aliphatic carbon in the chains. After mercury adsorption, the Hg@PVDF/CS-Al/Cu-LDH membrane displays similar peaks at 284.42, 285.78, and 290.54 eV, with shifts to higher binding energies, indicating strong interactions with Hg(II) ions. The shift of the C–O/C–N peak to 285.78 eV, along with a decrease in C–F intensity (from 33.83% to 25.75%), suggests engagement of functional sites in binding the metal. Additionally, the rise of the C–C/C = C peak (to 19.44%) signifies a charge redistribution in the polymer matrix due to complexation. These changes confirm that Hg(II) ions mostly form coordination bonds with chitosan and LDH’s hydroxyl, amino, and carbonyl groups, along with interactions with PVDF’s fluorinated segments⁴⁰. Overall, the XPS analysis indicates that mercury adsorption on the PVDF/CS-Al/Cu-LDH membrane occurs through chemisorption mechanisms involving electron sharing with donor sites, while preserving the hybrid polymeric framework’s stability (Fig. 4).

Fig. 4.

XPS of PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane.

The nitrogen (N 1s) analysis via XPS for PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH membranes illustrates the chemical state of nitrogen and its interactions with mercury during adsorption⁴¹. The PVDF/CS-Al/Cu-LDH membrane displays a significant N 1s peak at 399.7 eV, indicating nitrogen in amine forms from chitosan, confirming chitosan’s integration in the PVDF matrix and interaction with Al/Cu-LDH layers through hydrogen bonding and electrostatic forces. After mercury adsorption, the N 1s spectrum shifts positively to 400.4 eV in the Hg@PVDF/CS-Al/Cu-LDH membrane, reflecting variations in the nitrogen electronic situation due to coordination with Hg(II) ions. This increase in binding energy suggests the creation of Hg–N coordination bonds, resulting in diminished electron density around nitrogen. Additionally, a lack of new peaks or broadening in the N 1s region indicates chemical stability of amine groups, supporting their role in mercury complex formation. The findings substantiate chemisorption, highlighting nitrogen as a key site for Hg(II) immobilization through coordination. The N 1s XPS results affirm the strong binding of Hg(II) ions to chitosan’s amino groups within the PVDF/CS-Al/Cu-LDH matrix, demonstrating nitrogen’s vital role in mercury capture and the composite’s stability post-adsorption, highlighting its potential in heavy metal removal requests (Fig. 4).

The XPS analyses of the O 1s region pertaining to the PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membranes, as illustrated in Fig. 4, provide intricate visions into the oxygen-containing practical groups and their roles in mercury adsorption. Within the pristine PVDF/CS-Al/Cu-LDH membrane, the O 1s spectrum reveals two distinct peaks at 531.54 eV and 532.99 eV, which are indicative of metal-oxygen interactions (M–O, where M denotes Al or Cu) and the incidence of hydroxyl or adsorbed oxygen species (–OH, C–O, or O–C = O), individually. The primary peak at 532.99 eV (70.71%) signifies a high prevalence of surface hydroxyl and oxygenated groups arising from the LDH layers as well as chitosan, thereby highlighting the material’s pronounced hydrophilic characteristics and its potential to function as active sites for metal ion adsorption⁴². Conversely, the second peak at 531.54 eV (29.29%) corroborates the existence of lattice oxygen within the framework of Al/Cu-LDH, thereby affirming the structural integrity of the LDH phase. Following the adsorption of Hg(II), the O 1s spectrum associated with Hg@PVDF/CS-Al/Cu-LDH exhibits two peaks that have been shifted to 531.95 and 533.63 eV, accompanied by significant alterations in their intensities. The predominant peak at 531.95 eV (68.39%) corresponds to oxygen coordinated to metal atoms, suggesting the establishment of Hg–O coordination bonds between mercury ions and the oxygen sites associated with hydroxyl and carboxyl groups. The secondary component at 533.63 eV (31.61%) pertains to surface hydroxyl and carbonyl oxygen atoms that have undergone a positive binding energy shift as a result of electron donation to Hg(II), thereby indicating the participation of these groups in the chemical complexation process. The observed shifts in binding energy, along with variations in peak proportions, substantiate a chemisorption mechanism, wherein oxygen-containing groups, particularly hydroxyl and carboxyl functionalities, serve as pivotal coordination sites for the immobilization of Hg(II). Collectively, the findings from the O 1s XPS analyses elucidate that robust Hg–O interactions transpire via electron sharing and surface complexation, thereby affirming the essential role played by oxygen-rich groups in the adsorption of mercury and the chemical stability of the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane throughout the adsorption procedure.

XPS results for the F 1s region of the PVDF/CS-Al/Cu-LDH and Hg(II)PVDF/CS-Al/Cu-LDH nanofiber membranes provide insights into fluorine bonding and PVDF structural integrity before and after mercury adsorption. In the unmodified PVDF/CS-Al/Cu-LDH membrane, a prominent peak in the F 1s spectrum at 688.21 eV corresponds to C–F bonds in the PVDF polymer, indicating strong covalent interactions between carbon and fluorine that impart PVDF’s chemical resistance, hydrophobic properties, and mechanical strength. The symmetrical peak profile suggests a homogeneous PVDF distribution and minimal chemical reactivity between fluorine atoms and Al/Cu-LDH or chitosan. After mercury adsorption, the Hg@PVDF/CS-Al/Cu-LDH membrane shows a slight peak shift to 688.35 eV and a slight reduction in peak intensity, implying minor changes in the fluorine atoms’ electronic environment due to weak electrostatic connections with Hg(II) ions, not chemical bonding or structural changes. The F 1s peak position and profile remain stable, confirming C–F bonds’ preservation and PVDF’s chemical and structural integrity post-adsorption. These findings emphasize PVDF’s inert and resilient nature, supporting the active Al/Cu-LDH and chitosan components for adsorption. Thus, the F 1s XPS analysis confirms that the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane maintains its fluorinated architecture following Hg(II) adsorption, ensuring stability, durability, and reusability for heavy metal removal applications⁴³.

The XPS analyses of the Al 2p region for the PVDF/CS-Al/Cu-LDH and the Hg(II)@PVDF/CS-Al/Cu-LDH nanofiber membranes, as illustrated in Fig. 4, provide comprehensive insights into the chemical state and coordination characteristics of aluminum within the LDH configuration as well as its behavior subsequent to mercury adsorption. In the unaltered PVDF/CS-Al/Cu-LDH membrane, the Al 2p peak is situated at a binding energy of 75.59 eV, which is indicative of Al³⁺ ions residing in an octahedral coordination environment (Al–OH) within the LDH framework. This binding energy corresponds with aluminum that is coordinated to hydroxyl groups typical of hydrotalcite-like LDH structures, thereby verifying the existence of aluminum in its trivalent oxidation state and its effective incorporation into the composite matrix. The narrow and symmetrical shape of the peak indicates a homogenous chemical environment, suggesting that aluminum atoms are uniformly distributed throughout the LDH layers and are robustly integrated within the PVDF/CS nanofibrous structure without any distortion of the lattice. Post-mercury adsorption, the Al 2p peak for the Hg@PVDF/CS-Al/Cu-LDH sample exhibits a slight shift to a lower binding energy of 74.73 eV, indicative of subtle electronic modifications arising from interactions among Hg(II) ions and the surface of the LDH. This negative shift implies a minor increase in electron density surrounding the aluminum centers, likely attributed to partial charge transfer or electronic redistribution during the complexation process of Hg(II) with adjacent hydroxyl groups linked to the Al–O layers. Notably, the consistency in peak shape reinforces that aluminum maintains its oxidation state and coordination geometry even after the adsorption process, and the integrity of the LDH structure remains unaffected. Consequently, this marginal shift substantiates that mercury adsorption transpires chiefly through surface complexation and electrostatic interactions involving oxygenated functional groups proximate to aluminum sites, as opposed to lattice substitution or structural degradation. In summary, the Al 2p XPS findings validate that the PVDF/CS-Al/Cu-LDH nanofiber membrane sustains its structural and chemical stability post-Hg(II) adsorption, with aluminum serving an essential role in anchoring active sites and promoting robust binding of Hg(II) via surface coordination mechanisms⁴³.

The XPS spectra of the Cu 2p area for PVDF/CS-Al/Cu-LDH and Hg@PVDF/CS-Al/Cu-LDH nanofiber membranes, as shown in Fig. 4, provide visions into the oxidation states and coordination of copper in the layered double hydroxide (LDH) lattice and its interaction with Hg(II) ions after adsorption. The PVDF/CS-Al/Cu-LDH membrane shows characteristic Cu 2p doublet peaks indicative of Cu 2p₃/₂ and Cu 2p₁/₂ within 933–955 eV, with notable satellite features confirming the existence of Cu²⁺ and Cu⁺ types. The prevalence of Cu²⁺ peaks highlight copper ion coordination with hydroxyl and oxygen atoms (Cu–O and Cu–OH bonds) within the LDH framework, indicating Cu²⁺ in octahedral positions. This mixed oxidation state enhances redox activity and charge transfer, improving the composite material’s adsorption capability. In contrast, after mercury adsorption, the Cu 2p spectrum for Hg@PVDF/CS-Al/Cu-LDH shows a slight shift to lower binding energies and reduced intensity of Cu²⁺ satellite peaks, indicating slight electronic interactions between Cu and the adsorbed Hg(II) ions. The decreased binding energy suggests electron density redistribution and potential weak charge transfer between Hg(II) ions and Cu–O groups, limiting transitions from Cu²⁺ to Cu⁺, while preserving the LDH structure’s integrity. The stability of Cu 2p peaks after adsorption confirms copper’s role as an active site for electron exchange without significant displacement or reduction. Thus, XPS results demonstrate that Hg(II) adsorption occurs mainly through surface complexation and electronic interactions involving hydroxyl and oxygen functional groups linked to Cu, rather than through structural substitution or degradation. Overall, the PVDF/CS-Al/Cu-LDH membrane maintains its mixed-valence copper configuration and structural stability post-mercury adsorption, with copper contributing to enhanced adsorption efficiency via redox mediation and electronic polarization effects⁴³.

The XPS examination of the Hg 4f region pertaining to the Hg@PVDF/CS-Al/Cu-LDH nanofiber membrane, as exemplified in Fig. 4, offers substantial evidence of mercury adsorption and elucidates the oxidation state and bonding configuration of Hg species on the composite surface. The spectrum reveals two prominent peaks at 100.67 and 104.67 eV, which correspond to the Hg 4f₇/₂ and Hg 4f₅/₂ spin–orbit components, respectively. The approximate energy separation of 4.0 eV is indicative of divalent mercury (Hg²⁺). Furthermore, the peak intensity ratio—57.69% for Hg 4f₇/₂ and 42.31% for Hg 4f₅/₂ reinforces the assertion that mercury is predominantly present in the oxidized Hg²⁺ state within the composite material. The binding energies observed in this analysis are marginally higher than those typically associated with pure HgO, suggesting that the Hg²⁺ ions are firmly adhered to the surface through chemical coordination mechanisms rather than mere physical adsorption. This increase in binding energy implies a transfer of electrons from electronegative atoms—mainly oxygen and nitrogen found in hydroxyl (–OH), amino (–NH₂), and carboxyl (–COOH) functional groups—to the mercury ions, thereby leading to the formation of stable Hg–O and Hg–N coordination bonds. These results corroborate that the mechanism of adsorption is primarily chemisorptive, featuring complexation and electron-sharing connections among Hg(II) ions and the active functional sites provided by the Al/Cu-LDH and chitosan components. The lack of any indicators for low-binding-energy states associated with metallic mercury (Hg⁰) suggests that no reduction occurs during the adsorption process and that mercury remains in its divalent ionic form. Consequently, the XPS findings affirm that Hg²⁺ ions are successfully captured and stabilized within the hybrid structure of the Hg@PVDF/CS-Al/Cu-LDH membrane through robust coordination interactions with donor atoms of oxygen and nitrogen. This highlights the composite’s significant affinity, chemical resilience, and efficacy in the sequestration of mercury, where chemisorption and surface complexation are predominant processes that guarantee the stable and irreversible immobilization of Hg(II)⁴³.

The XPS survey spectra of the PVDF/CS-Al/Cu-LDH and Hg(II)@PVDF/CS-Al/Cu-LDH nanofiber membranes, depicted in Fig. 4, provide compelling evidence on elemental composition and mercury adsorption on the composite surface. The unmodified PVDF/CS-Al/Cu-LDH membrane spectrum displays peaks for C 1s, O 1s, N 1s, F 1s, Al 2p, and Cu 2p, representative the presence of various components in the hybrid material. The prominent C 1s and F 1s peaks reveal the PVDF polymer backbone, while O 1s and N 1s peaks signify chitosan’s hydroxyl, carboxyl, and amino groups, highlighting the functional groups available for metal ion coordination. The Al 2p and Cu 2p peaks support the integration of the Al/Cu-LDH phase, underscoring the organic-inorganic nanocomposite architecture. After mercury adsorption, the Hg@PVDF/CS-Al/Cu-LDH membrane’s spectrum retains all original peaks and introduces a distinct Hg 4f peak, confirming mercury ions on the membrane surface. Changes in binding energies and intensities of O 1s, N 1s, and Cu 2p suggest strong interactions among Hg(II) ions and the composite’s practical groups, indicating Hg–O and Hg–N coordination. These interactions reveal that mercury adsorption primarily occurs through chemisorption and surface complexation rather than physical adsorption. Notably, all other elemental peaks remain intact, indicating that the membrane’s structural integrity and chemical composition persist post-mercury uptake, with no evidence of leaching or degradation. In summary, the XPS spectra demonstrate the successful integration of PVDF/CS-Al/Cu-LDH matrix, and the effective adsorption of Hg(II) ions on Hg@PVDF/CS-Al/Cu-LDH, approving its potential utility as an adsorbent for metals⁴³.

Zero point of charge

The zero point of charge (pH_zpc) for the PVDF/CS-Al/Cu-LDH nanofiber membrane, as depicted in Fig. 5a, was established from the intersection of the ΔpH versus pH curve, where ΔpH is equal to zero, resulting in a pH_zpc value of 5.3. This outcome proposes that at a pH of 5.3, the membrane’s surface exhibits an overall neutral charge, with an equimolar distribution of positive and negative sites. When the solution pH falls below 5.3, the useful groups present on the membrane surface, namely hydroxyl (–OH) and amino (–NH₂) groups, undergo protonation to form –OH₂⁺ and –NH₃⁺, respectively. This modification results in surface positively charged that electrostatically repels cationic mercury (Hg²⁺) species, thereby diminishing the efficiency of adsorption. Conversely, for pH values exceeding 5.3, the deprotonation of surface functional groups takes place, chief to the formation of negatively charged places (–O⁻ and –NH⁻) that facilitate strong electrostatic attraction and coordination with Hg²⁺ ions. The negative surface potential significantly augments mercury uptake over a synergistic mechanism that incorporates both electrostatic connections and chemical complexation, mediated by the formation of Hg–O and Hg–N bonds. As a result, the adsorption efficiency for Hg(II) markedly increases within the pH range above the pH_zpc, particularly in the vicinity of pH 6–8, where the membrane surface adopts a negative charge, and mercury predominantly exists as free Hg²⁺ or hydrolyzed Hg(OH)⁺ species. Hence, conducting the adsorption process at pH levels exceeding 5.3 creates an advantageous environment for the enhanced removal of mercury, ensuring robust binding affinity, elevated adsorption capacity, and chemical stability of the PVDF/CS-Al/Cu-LDH nanofiber membrane.

Fig. 5.

(a) Calculation of pH_zpc of PVDF/CS-Al/Cu-LDH, (b) Impact of solution pH of PVDF/CS-Al/Cu-LDH, (c) Impact of the adsorbent quantity of PVDF/CS-Al/Cu-LDH, (d) Impact of the original concentration of Hg(II) ions, (e) Impact of the interaction time, and (f) Impact of the temperature.

Batch experiments

Effect of pH

The influence of pH on the Hg(II) adsorption onto the PVDF/CS-Al/Cu-LDH is exemplified in Fig. 5b. The adsorption capacity (qₑ) exhibited a strong connection with the solution pH, demonstrating a progressive increase from 158.6 mg/g at a pH of 2 to a peak value of 343.2 mg/g at a pH of 6, surveyed by a minor decline at elevated pH levels⁴⁴. This pattern is closely related to the pH_zpc = 5.3 of the composite, suggesting that the surface charge of the adsorbent is a pivotal factor in mercury ion uptake. Below the pH_zpc of 5.3, the membrane’s surface retains a positive charge due to the protonation of –OH and –NH₂ functional groups into –OH₂⁺ and –NH₃⁺, resulting in significant electrostatic repulsion among the adsorbent surface and Hg²⁺ ions, which consequently diminishes adsorption efficiency. As the pH ascends beyond the pH_zpc, the functional groups on the surface undergo deprotonation to generate negatively charged –O⁻ and –NH⁻ sites, thereby enhancing electrostatic attraction and coordination with the positively charged Hg²⁺ ions. The optimal adsorption recognized at pH 6, marginally above the pH_zpc, can be accredited to improved electrostatic attraction and the formation of stable surface complexes between mercury ions and the active oxygen- and nitrogen-containing groups present in chitosan and Al/Cu-LDH. At this pH, Hg²⁺ species, counting Hg²⁺ and Hg(OH)⁺, are predominantly present, facilitating chemisorption through interactions of Hg–O and Hg–N without the precipitation of Hg(OH)₂. Conversely, at higher pH levels (> 7), the slight reduction in adsorption capacity is likely due to the generation of insoluble Hg(OH)₂ or competition among hydrolyzed mercury species, thereby restricting the availability of free Hg²⁺ ions for adsorption. Accordingly, pH 6 is recognized as the optimal condition for mercury removal, providing a favorable equilibrium among surface charge, ion speciation, and the obtainability of active sites, thus maximizing the adsorption efficiency of the PVDF/CS-Al/Cu-LDH nanofiber membrane.

Effect of adsorbent quantity

The influence of adsorbent quantity on the adsorption of Hg(II) ions onto the PVDF/CS-Al/Cu-LDH nanofiber membrane is illustrated in Fig. 5c. The findings substantiate that both the adsorption capacity (qₑ) and the removal efficiency are significantly affected by the quantity of adsorbent utilized. An increase in adsorbent dosage from 0.02 to 0.5 g results in a dramatic decline in adsorption capacity from 287.5 to 17.1 mg/g, while the removal efficiency escalates from 65.7 to 97.7%. The interplay between the concentration of Hg(II) ions in the solution and the presence of sites of activity helps clarify this opposite relationship. At lower dosages, the restricted number of active sites becomes rapidly saturated due to the elevated concentration of Hg(II), which yields a higher adsorption capacity per unit mass. Conversely, as the dosage increases, the aggregate number of available binding sites also escalates, thereby providing an increased surface area and active functional groups for mercury uptake, ultimately enhancing overall removal efficiency. However, the adsorption capacity per gram diminishes, as the Hg(II) ions are dispersed across a more extensive surface area, resulting in many adsorption sites remaining vacant. Furthermore, increased particle aggregation and the overlapping of active sites resulting from higher dosages may diminish effective surface area and impede mass transfer. Beyond the threshold of 0.4 g, the removal efficiency approaches a nearly stable value (~ 97%), indicating the attainment of equilibrium conditions wherein the majority of Hg(II) ions have been effectively captured, and additional quantities of adsorbent contribute negligibly to further removal improvements. As a result, an ideal adsorbent dosage of roughly 0.25–0.30 g is found, balancing high removal efficiency with efficient adsorbent consumption. These results affirm that the PVDF/CS-Al/Cu-LDH displays significant surface activity, abundant functional groups, and a pronounced affinity for Hg(II) ions, facilitating efficient mercury removal even at lower dosages⁴⁵.

Effect of Hg(II) concentration

The impact of the initial concentration of Hg(II) on the adsorption efficacy of the PVDF/CS-Al/Cu-LDH nanofiber membrane is depicted in Fig. 5d and detailed in the accompanying data set. Findings show that adsorption capacity (q_e) and removal efficiency are inversely related as Hg(II) concentration varies from 25 to 600 mg/L. As the concentration increases, adsorption capacity rises from 23.8 to 422 mg/g, while removal efficiency declines from 96.8 to 56.8%. This is due to the balance between available binding sites and Hg(II) ions during adsorption. At lower concentrations, more active sites are available, promoting interactions and high removal efficiency. In contrast, at higher concentrations, the increased number of Hg(II) ions leads to a stronger concentration gradient, improving mass transfer and allowing more ions to be adsorbed per unit mass, increasing q_e. However, when active sites become saturated, more Hg(II) results in competition for those limited sites, lowering removal efficiency. A noticeable decline beyond 300 mg/L indicates the saturation of surface-active sites like –OH and –NH₂, leading to chemisorption via coordination and electrostatic interactions. The rising q_e suggests that the adsorption method is characterized by monolayer chemisorption, consistent with the Langmuir isotherm model. An original concentration range of 200 to 300 mg/L is optimal, balancing high adsorption capacity with effective removal efficiency, highlighting the affinity and reactivity of the PVDF/CS-Al/Cu-LDH membrane toward Hg(II) ions⁴⁶.

Effect of contact time

The effect of contact duration on the Hg(II) adsorption by the PVDF/CS-Al/Cu-LDH nanofiber membrane shows a rapid adsorption increase. Adsorption capacity (q_t) rises quickly in the first few min, reaching equilibrium around 60 min, indicating an effective adsorption mechanism. In the first 30 min, q_t escalates from 56.0 mg/g at 5 min to 287.5 mg/g at 30 min, likely due to numerous active places on the membrane’s surface, with hydroxyl (–OH) and amino (–NH₂) groups, available for interaction with Hg(II) ions. As time developments, the adsorption rate slows due to active site occupation and increased electrostatic repulsion among adsorbed and free Hg(II) ions. After 60 min, equilibrium is reached near 424 mg/g, with no significant increase in q_t despite extended contact time (Fig. 5e). Kinetic analysis shows that the Hg(II) adsorption onto the PVDF/CS-Al/Cu-LDH membrane involves a biphasic process. It starts with ions adsorbing on the membrane’s surface, followed by a slower phase of intraparticle diffusion or chemisorption as ions penetrate the membrane. The observed equilibrium time of 60 min reflects the nanofiber composite’s strong affinity and swift mass transfer. This efficiency is due to the membrane’s high porosity and ample active sites from layered double hydroxides and chitosan in the PVDF matrix. The quick establishment of equilibrium highlights the system’s suitability for wastewater treatment, especially for fast removal of heavy metal contaminants⁴⁷.

Effect of temperature

The impact of temperature on the Hg(II) adsorption efficiency by the PVDF/CS-Al/Cu-LDH nanofiber membrane shows a significant increase in removal efficacy as temperature increases, from 64.6% at 20 °C to 98.6% at 45 °C, indicating an endothermic adsorption process (Fig. 5f). At lower temperatures, the efficiency is limited due to reduced molecular movement and limited Hg(II) ion diffusion towards active sites on the nanofiber membrane. Increased temperature enhances the kinetic energy and mobility of Hg(II) ions, accelerating their diffusion and improving interaction with functional groups in PVDF/CS-Al/Cu-LDH. Higher temperatures may also expand the polymeric matrix, revealing additional binding sites for metal ions. The continued increase in removal efficiency up to 45 °C suggests chemisorption, with elevated temperatures promoting the formation of stable coordination bonds between Hg(II) ions and donor atoms like oxygen and nitrogen. The endothermic nature implies that energy input helps overcome activation barriers for ion exchange and surface complexation. Notably, the lack of efficiency decline at high temperatures indicates the membrane’s thermal stability and structural integrity. In conclusion, temperature significantly enhances Hg(II) ion adsorption onto the PVDF/CS-Al/Cu-LDH nanofiber membrane by expediting diffusion, promoting chemisorption, and activating more surface sites, achieving 98.6% removal at 45 °C, indicative of the composite’s suitability for mercury remediation under elevated temperature conditions⁴⁸.

Adsorption isotherm

Employing numerous adsorption isotherm models Langmuir⁴⁹, Freundlich⁵⁰, Dubinin–Radushkevich (D–R)⁵¹, Temkin⁵², and Jossens to examine the adsorption of Hg(II) on the PVDF/CS-Al/Cu-LDH yields significant scientific insights. The Langmuir isotherm assesses monolayer adsorption on uniform surfaces with identical active sites, enabling calculations of maximum adsorption capacity and confirming the membrane’s effectiveness for metal ion uptake⁴⁹. In contrast, the Freundlich model describes heterogeneous surfaces, provided that insights into adsorption intensity and surface heterogeneity, reflecting the varied affinities from LDH layers, chitosan functional groups, and PVDF’s nanofibrous structure. The D–R isotherm differentiates among chemical and physical adsorption by estimating mean free energy, clarifying if Hg(II) uptake involves ion exchange, complexation, or physisorption (Table S4). The Temkin model inspects interactions between adsorbents and adsorbates, detailing how adsorption energy may decline linearly with increased surface coverage, relevant for interactions with functional groups like –OH, –COOH, and –NH₂. Lastly, the Jossens isotherm, based on statistical thermodynamics, evaluates the energetic distribution of adsorption sites, particularly beneficial for non-uniform composite materials like LDH–biopolymer membranes. Utilizing these models collectively enhances understanding of adsorption capacity, surface heterogeneity, energy, mechanisms, and interaction strength, elucidating how the PVDF/CS-Al/Cu-LDH membrane interacts with Hg(II) for its effective application in wastewater treatment and remediation of the environment (Table S5).

The operation of the Langmuir adsorption isotherm model offers substantial benefits in the assessment of the adsorption characteristics of the PVDF/CS-Al/Cu-LDH nanofiber membrane concerning Hg(II), as it facilitates an explicit mechanistic understanding of the adsorption procedure and allows for the extraction of essential quantitative limits. The optimized Langmuir model indicates a maximum monolayer adsorption capacity (q_m) of 422.68 mg.g⁻¹, illustrating the remarkably high affinity of the membrane for Hg(II) ions and enabling direct comparisons with previously documented adsorbents⁴⁹. The Langmuir constant ((K_L = 0.0199) L.mg⁻¹) further exemplifies the interaction strength among Hg(II) and the active places present on the surface of the composite, affirming strong binding over mechanisms like ion exchange and complexation. In addition, the computed dimension-less separation factor (R_L = 0.18), which falls within the interval (0 < R_L < 1), signifies that the adsorption is notably promising under the given experimental settings (Table S6). As a summary, the Langmuir model not only corroborates the evidence of monolayer adsorption on a comparatively uniform surface but also offers a dependable framework for evaluating adsorption favorability, affinity, and capacity, thereby establishing itself as an invaluable instrument for validating the efficacy and practical applicability of the developed membrane in the treatment of mercury-contaminated water (Fig. 6a). The adsorption of Hg(II) onto the PVDF/CS-Al/Cu-LDH nanofiber membrane can be better understood using the Freundlich adsorption isotherm model. The Freundlich model recognizes the possibility of multilayer adsorption and allows adsorption on heterogeneous surfaces, whereas the Langmuir model assumes homogeneity⁵⁰. This characteristic aligns more closely with the intricate architecture of the composite membrane, which consists of layered LDH, chitosan functional groups, and a porous PVDF matrix. The Freundlich constant (K_F = 44.99) (mg/g) (L/mg)^1/n serves as an indicator of the adsorption capacity and reveals a strong empathy for Hg(II), although the adsorption intensity factor (n = 2.42) within the range (1 < n < 10) substantiates the favorable and efficient nature of the adsorption method. Higher values of n correlate with increased surface heterogeneity and enhanced connections among the adsorbate and adsorbent. Unlike models predicated on monolayer adsorption, the Freundlich model accounts for the uneven energetic distribution of active sites, rendering it particularly appropriate for hybrid adsorbents characterized by diverse functional groups and binding sites. Consequently, the Freundlich model contributes essential insights into surface heterogeneity, the favorability of adsorption, and multilayer uptake dynamics, thereby complementing the Langmuir model and providing a more nuanced sympathetic of adsorption phenomena in complex composite materials employed for the removal of mercury.

Fig. 6.

(a) Adsorption isotherms, (b) Adsorption kinetics, and (c) Intraparticle diffusion.

The Dubinin–Radushkevich (D–R) adsorption isotherm model gifts a significant advantage in the analysis of the uptake of Hg(II) onto the PVDF/CS-Al/Cu-LDH nanofiber membrane. Unlike the Langmuir besides Freundlich models, the D–R model provides direct insights into the nature of the adsorption mechanism, differentiating among physical and chemical adsorption by approximating the mean adsorption energy (E_a)⁵¹. The D–R parameters yield a calculated adsorption capacity (mg.g⁻¹), reaffirming the substantial potential for Hg(II) adsorption by the membrane; concurrently, the constant (mol²/kJ²) facilitates the assessment of the adsorption energy. Notably, the derived mean free energy value (kJ.mol⁻¹) exceeds 8 kJ.mol⁻¹, suggesting that the adsorption process primarily proceeds via chemisorption rather than physical adsorption. This chemisorption process involves robust interactions, including ion exchange, complexation, and coordination with the functional groups of layered double hydroxide (LDH) and chitosan. Hence, the D–R model is particularly robust as it not only estimates the adsorption capacity but also substantiates the underlying adsorption mechanism, thereby demonstrating that the uptake of Hg(II) is driven by chemical bonding rather than the relatively weak van der Waals forces. This finding further corroborates the strong and irreversible binding characteristics observed in the composite membrane (Fig. 6a).

The Temkin adsorption isotherm model demonstrates particular efficacy in assessing the adsorption characteristics of Hg(II) onto the PVDF/CS-Al/Cu-LDH nanofiber membrane⁵². This model diverges from the Langmuir besides Freundlich models by factoring in the effect of connections among the adsorbate and the adsorbent, presupposing that the heat associated with adsorption diminishes progressively with a rise in surface coverage. Such a framework renders the model exceptionally applicable to adsorbents possessing chemically active practical groups, like –OH, –NH₂, and –COOH, where the energies of interaction are variable. Specifically, the Temkin binding constant (K_T = 0.45) L.mol⁻¹ serves as an indicator of the empathy of Hg(II) for the membrane surface. Concurrently, the Temkin heat parameter (b_T = 27.928) J.mol⁻¹ suggests that the adsorption dynamics entail moderate binding energy, indicative of a chemisorption process that is primarily governed by ion exchange and complexation rather than by weak physical forces. A notable advantage of using the Temkin model lies in its capacity to elucidate the energetics of adsorption, allowing researchers to determine whether the interactions between adsorbate and adsorbent are endothermic, uniform, or progressively weakened as adsorption sites become filled. Consequently, this model serves to augment the insights provided by the Langmuir, Freundlich, besides Dubinin-Radushkevich (D–R) isotherms by substantiating that the adsorption of Hg(II) is energetically promising and significantly modulated by the connections of the adsorbate, rather than occurring on a surface characterized by energetic uniformity.

The Jossens adsorption isotherm model, often referred to as the Elovich or statistical isotherm model, suggestions important advantages in the analysis of Hg(II) adsorption onto the PVDF/CS-Al/Cu-LDH nanofiber membrane⁵². This model diverges from traditional models like Langmuir, Freundlich, Temkin, otherwise D–R by elucidating the energetic heterogeneity of adsorption sites as well as the distribution of binding energies across complex composite adsorbents. The Jossens constants derived from the model, specifically K = 10.95 and J = 0.29, suggest that the membrane surface encompasses sites with varying affinities rather than exhibiting uniform characteristics, a finding that aligns with the nature of a multi-component hybrid adsorbent comprising LDH layers, chitosan functional groups, and a porous PVDF network. The observed value of J (< 1) further corroborates the notion that the adsorption mechanism is neither ideal nor homogeneous; instead, it is dictated by a heterogeneous energy distribution. This implies that sites with higher energy are occupied first, followed by those with progressively lower energy as the adsorption process progresses. Consequently, the Jossens model proves particularly relevant for systems characterized by chemisorption involving multiple binding mechanisms, such as ion exchange, complexation, and electrostatic attraction, as well as for materials that have been engineered with multifunctional surface chemistry. In conclusion, the utilization of the Jossens isotherm presents a significant benefit by facilitating the quantification of variations in adsorption energy across a surface. This capability permits a more profound analysis of site heterogeneity, competitive adsorption phenomena, and the actual behavior of surfaces, which simpler models fail to adequately address. Consequently, it serves to enhance the understanding provided by Langmuir, Freundlich, besides D–R models, confirming that the binding of Hg(II) transpires on a statistically heterogeneous surface, in alignment with the characteristics of the mixed LDH–biopolymer nanofiber architecture.

Adsorption kinetics

The application of various adsorption kinetic models, like Pseudo-First-Order⁵³, Pseudo-Second-Order⁵⁴, Intraparticle Diffusion⁵⁵, as well as Elovich models⁵⁶, facilitates a thorough and dependable assessment of the adsorption dynamics of Hg(II) onto the PVDF/CS-Al/Cu-LDH nanofiber membrane. Each model elucidates distinct facets of both the rate and mechanisms involved in the uptake process. Specifically, the Pseudo-First-Order model aids in determining if the adsorption kinetics are predominantly influenced by physisorption alongside interactions occurring at the external surface. In contrast, the Pseudo-Second-Order model is more adept at addressing scenarios where chemisorption and electron-sharing reactions predominate, thus providing precise predictions of the equilibrium adsorption capacity and substantiating the predominance of chemical bonding in the elimination procedure. The model of Intraparticle Diffusion elucidates whether the adsorption kinetics are restricted by mechanisms such as film or pore diffusion, or by a combination of sequential processes. This aspect is particularly crucial in the context of porous nanofibrous materials, where boundary layer diffusion typically precedes the internal diffusion into the LDH and chitosan networks. Furthermore, the Elovich model demonstrates to be essential in characterizing adsorption phenomena on heterogeneous surfaces that exhibit non-uniform energy distributions, which is indicative of the composite membrane’s intricate structure and the varying activation energies associated with its functional groups (–OH, –NH₂, –COOH). By integrating the insights gained from all four models, researchers can effectively differentiate between the aids of chemical and physical adsorption, ascertain the underlying rate-controlling mechanisms, and determine whether the adsorption procedure is ruled by single-step or multi-stage diffusion. This comprehensive approach provides robust mechanistic insights supporting the efficacy and practical utilization of the membrane for Hg(II) remediation efforts (Tables S4 and S5).

The Pseudo-First-Order kinetic model is applied to analyze Hg(II) adsorption onto PVDF/CS-Al/Cu-LDH nanofiber membranes. This model helps assess whether adsorption rates are primarily controlled by physisorption and external surface interactions instead of chemical bonds. The rate constant (K₁ = 0.0326 × 10^− 2 min^-1) indicates how quickly Hg(II) ions associate with the membrane’s surface sites. While this model effectively describes the initial rapid adsorption phase, comparing theoretical and empirical equilibrium adsorption values is vital to evaluate the model’s overall applicability (Table S7). A weak correlation between predictions and experimental values suggests that physisorption isn’t the sole mechanism at play; chemisorption or diffusion may also be significant. Further confirmation arises from using the Pseudo-Second-Order model and analyzing intraparticle diffusion. Ultimately, the Pseudo-First-Order model primarily helps identify the extent of physisorption, laying a foundational kinetic reference to ascertain whether adsorption is driven by weak surface interactions or stronger bindings⁵³.

The Pseudo-Second-Order kinetic model proposals compelling insights suggesting that the adsorption of Hg(II) onto PVDF/CS-Al/Cu-LDH nanofiber membrane is primarily influenced by chemisorption processes⁵⁴. This involvement of valence forces, electron sharing, or ion exchange supersedes mere physical interactions. The model exhibits a high degree of association with investigational findings, and the calculated equilibrium adsorption capacity (q_e = 423.2 mg.g⁻¹) closely aligns with the empirical data observed. This alignment corroborates that the adsorption kinetics are heavily reliant on the presence of active functional groups, specifically –OH, –NH₂, and –COOH, within the LDH–CS/PVDF matrix. Additionally, the determined rate constant (K₂ = 4.58 × 10⁻⁴ g.mg⁻¹min⁻¹) designates both the efficacy and rapidity of the chemisorption process, illustrating the membrane’s ability to swiftly establish robust interactions with Hg(II) ions over mechanisms like complexation and ion exchange. In comparison to the Pseudo-First-Order model, the enhanced fit of the Pseudo-Second-Order model substantiates the premise that chemical bonding is the principal adsorption mechanism, rather than surface-limited physical adsorption, thereby reinforcing the notable adsorption capacity and stability of Hg(II) immobilization on the surface of the composite membrane (Fig. 6b). The model of intraparticle diffusion, specifically the Weber–Morris model, characterizes the uptake of Hg(II) on the PVDF/CS-Al/Cu-LDH membrane⁵⁵. The fitted parameters reveal (K_i = 22.6 mg.g⁻¹min^1/2) and (X = 48.47 mg/g), suggesting a rapid internal mass transfer process that is accompanied by a significant film-diffusion resistance at the interface between the solution and membrane. Therefore, although pore diffusion plays a vital role in the complete kinetics, it does not constitute the exclusive rate-limiting factor; rather, a preliminary boundary-layer process occurs prior to diffusion into the micro- and mesopores of the LDH/CS/PVDF composite. This finding aligns with the observed multi-stage behavior of (q_t) as a function of (t^1/2) and the pronounced chemisorptive binding that ensues.

The kinetic model of Elovich effectively characterizes the Hg(II) adsorption on PVDF/CS-Al/Cu-LDH nanofiber membrane, designed for heterogeneous surfaces with a wide variety of activation energies. This reflects the multifunctional nature of the composite adsorbent. Parameters from the model α = 0.0125 mg.g⁻¹min⁻¹ for the initial adsorption rate and β = 145.11 g.mg⁻¹ for surface coverage indicate a rapid adsorption initiation due to numerous high-energy binding sites, including –OH, –NH₂, –COOH, and hydroxyl groups on the LDH surface. This is followed by a slower phase as binding places are filled, and diffusion occurs into inner layers. The Elovich model’s advantage over the Pseudo-First-Order besides Pseudo-Second-Order models lies in its recognition of chemically controlled, energetically non-uniform adsorption mechanisms. This complexity aligns with chemisorption processes like ion exchange, complexation, and surface coordination. Eventually, the Elovich model supports the view that Hg(II) adsorption occurs via heterogeneous chemisorption on a multilayered, multifunctional composite surface⁵⁶.

Diffusion mechanism

The intraparticle diffusion analysis for Hg(II) adsorption onto PVDF/CS-Al/Cu-LDH, as depicted in Fig. 6c, reveals three distinct linear segments, thereby substantiating the premise that the adsorption procedure is characterized by a multistep diffusion-controlled mechanism rather than a singular kinetic pathway. In the initial phase (Stage I), the sharply inclined slope indicates rapid adsorption occurring at the external surface or within the boundary layer, where Hg(II) ions are swiftly transported from the bulk phase to the membrane surface facilitated by strong electrostatic interactions and the high availability of active sites⁵⁵. The subsequent linear segment (Stage II) is indicative of the gradual adsorption process, which is predominantly influenced by intraparticle diffusion as Hg²⁺ ions infiltrate further into the internal pores of the LDH structure, chitosan matrix, and PVDF nanofiber assembly. During this interval, the rate of diffusion diminishes as adsorption sites become increasingly occupied and the concentration gradient weakens. The final segment (Stage III) represents the equilibrium stage, characterized by a significant reduction in the adsorption rate due to the majority of active sites being saturated or hindered spatially, culminating in a plateau approach toward maximum uptake. The observation that none of the linear segments intersect the origin underscores that intraparticle diffusion alone does not dictate the rate-limiting step; rather, it suggests that both film diffusion and pore diffusion collaboratively influence the kinetics of adsorption. This multi-stage response corroborates the notion that the uptake of Hg(II) onto the composite membrane proceeds through a rapid initial surface interaction, followed by a slower process of internal diffusion, ultimately achieving stabilization at equilibrium, which aligns with a heterogeneous and multilayer adsorption model (Fig. 6c).

Adsorption thermodynamics

The thermodynamic properties governing the adsorption of Hg(II) onto the PVDF/CS-Al/Cu-LDH nanofiber membrane were meticulously examined utilizing the Van’t Hoff and Arrhenius models, as depicted in Fig. 7a–b and summarized in Table S8. The Van’t Hoff plot demonstrating the association among ln K_c and 1/T revealed a linear correlation characterized by a positive slope, substantiating the conclusion that the adsorption process is endothermic in nature. An increase in temperature from 293 to 318 K corresponded with an development in the removal efficiency of Hg(II) ions, indicating that augmented thermal energy enhances the movement of ions and promotes their diffusion to the active places available for adsorption. The derived thermodynamic parameters ΔH^o = 102.15 kJ/mol, ΔS^o = 352.29 J/mol.K, and ΔG^o values ranging from − 1.26 kJ/mol at 293 K to − 10.09 kJ/mol at 318 K further suggest that the adsorption phenomenon is both spontaneous and influenced by temperature⁵⁷. The positive enthalpy change (ΔH^o) signifies an absorption of energy during the adsorption process, corroborating its endothermic classification. Meanwhile, the considerable positive entropy change (ΔS^o) designates a rise in disorder at the solid–liquid boundary, occurring as Hg(II) ions displace water molecules and engage with the practical groups existing on the membrane’s surface⁵⁸. Furthermore, the increasingly negative ΔG^o values with rising temperature suggest that the adsorption becomes progressively more thermodynamically favorable at elevated conditions. The Arrhenius plot illustrating ln k₂ vs. 1/T further reinforced these findings, yielding an activation energy (E_a) of 38.42 kJ/mol, a value consistent with typical chemisorption processes, which range from 20 to 80 kJ/mol. This parameter supports the idea that chemical connections, consisting of the sharing or exchange of electrons between the Hg(II) ions and the oxygen and nitrogen donor atoms of the hydroxyl (–OH) and amino (–NH₂) groups intrinsic to chitosan and Al/Cu-LDH, are primarily responsible for the adsorption mechanism. In summation, these results elucidate that the adsorption of Hg(II) onto the PVDF/CS-Al/Cu-LDH constitutes a spontaneous, endothermic, and chemisorptive phenomenon, accentuated by elevated temperatures which facilitate robust surface complexation, enhanced molecular dynamics, and improved adsorption efficacy⁵⁹.

Fig. 7.

(a) Van’t Hoff plot, and (b) Arrhenius plot.

Mechanism of interaction

The mechanism by which Hg(II) ions are adsorbed onto the PVDF/CS-Al/Cu-LDH nanofiber membrane is characterized by a multifaceted interplay of physical and chemical interactions that jointly facilitate effective removal. In the initial phase, Hg(II) ions are captured primarily via physisorption, which is influenced by van der Waals forces along with the extensive surface area provided by the porous PVDF nanofiber structure, thereby enhancing subsequent chemical interactions⁶⁰. The attendance of hydroxyl groups (–OH) on the Al/Cu-LDH surface promotes precipitation processes that lead to the formation of insoluble mercury hydroxide or oxide compounds (Hg(OH)₂), thereby immobilizing the Hg(II) ions on the membrane. Simultaneously, an ion exchange process occurs wherein Hg(II) ions replace interlayer ions such as carbonate (CO₃²⁻) or hydroxide (OH⁻) inside the LDH framework, resulting in the establishment of stable Hg–O–M (where M denotes either aluminum or copper) bonds. Furthermore, when the pH of the solution exceeds the point of zero charge (pH_zpc ≈ 5.3), the membrane surface acquires a negative charge, which fosters electrostatic attraction with the positively charged Hg(II) ions. Additionally, complexation reactions transpire between Hg(II) and the electron-donating functional groups inherent in chitosan (–NH₂, –OH) and LDH (–COOH, –OH), yielding robust coordination bonds. Collectively, these mechanisms comprising physisorption, precipitation, ion exchange, electrostatic interactions, and complexation enhance both the efficiency and stability of Hg(II) uptake. The chitosan and LDH components contribute a significant number of active sites, while the PVDF matrix provides necessary permeability and structural integrity, thereby enabling sustainable removal of mercury (Fig. S1).

Effect of salinity

Anions have the capacity to effect the elimination of Hg(II) ions by the PVDF/CS-Al/Cu-LDH nanofiber membrane adsorbent through mechanisms of competitive interaction. To evaluate this effect, an assortment of anions (CH₃COO⁻, SO₄²⁻, CO₃²⁻, and Cl⁻) was presented individually into a solution comprising Hg(II). Each anionic salt was added at a concentration of 0.1 M to 25 mL of a 200 mg/L Hg(II) solution, utilizing 0.02 g of the adsorbent, while maintaining a pH of 6 during a contact time of 100 min. The impact of these coexisting anions on the removal of Hg(II) is described in Fig. S2. The resultant elimination efficiencies for Hg(II) in the attendance of CH₃COO⁻, CO₃²⁻, and Cl⁻ were observed to be 94.6, 87.8, and 76.8%, respectively, signaling a moderate decline in effectiveness⁴⁴. This outcome suggests that these specific anions exhibit a limited interaction with the active places of the adsorbent. Conversely, the presence of SO₄²⁻ considerably diminished the adsorption performance of the PVDF/CS-Al/Cu-LDH nanofiber membrane for Hg(II), yielding an efficiency of 68.4%. This finding indicates that SO₄²⁻ possesses a more substantial competitive affinity for the adsorption sites on the nanofiber membrane relative to CH₃COO⁻, CO₃²⁻, and Cl⁻. This phenomenon may be attributed to the capacity of SO₄²⁻ to establish crosslink interactions with the positively charged sites of the PVDF/CS-Al/Cu-LDH nanofiber membrane, thereby facilitating a robust electrostatic attraction among SO₄²⁻ and the protonated amino groups present in the membrane. Nevertheless, despite the marked reduction in adsorption ability in the presence of SO₄²⁻, the sorbent retained a comparatively high adsorption capability of 68.4% for Hg(II).

Impact on actual water samples

Using a range of actual water matrices, including drinking water, industrial effluent, and saline water, the removal effectiveness of Hg(II) from PVDF/CS-Al/Cu-LDH nanofiber membrane was assessed. The effectiveness of removal in tap water was measured in the next experimental stage, and the removal efficiencies for wastewater and ocean were determined in the third phase. Using a standard concentration of 150 mg/L of Hg(II), the results showed notable differences in removal rates among the various water types examined: wastewater had a removal efficiency of 92.6%, seawater had an efficiency of 80.6%, and tap water had a removal rate of 96.8%. These results imply that the PVDF/CS-Al/Cu-LDH has a significant ability to remove Hg(II) in a variety of environmental settings⁶¹.

Reusability

In this investigation, 0.02 g of a composite nanofiber membrane made of PVDF/CS-Al/Cu-LDH was used to treat a 50 mL aqueous solution containing 100 mg/L of Hg(II) ions. The experiment was carried out for 100 min at a regulated pH of 6 in order to maximize the Hg(II) ions’ adsorption effectiveness. The Hg(II)-loaded nanofiber membrane was submerged in a different 50 mL solution (0.05 mol/L) of ethylenediaminetetraacetic acid (EDTA) after the adsorption procedure. To promote efficient interaction between the adsorbent and the EDTA, this mixture was continuously stirred for four hours at a constant temperature of 298.15 K. The resulting composite mixture was then cleaned with distilled water after the PVDF/CS-Al/Cu-LDH was separated from it using centrifugation. The membrane was made appropriate for reuse in later adsorption cycles after being vacuum-dried at 333 K. There were five replications of the adsorption and reactivation processes. One important feature that increases the sorbent’s potential for useful, commercial application is its notable capacity for reactivation^62,63. Retention capacities significantly decreased during the course of five consecutive cycles of adsorption-desorption, with percentages representing 97.4, 95.4, 93.8, 91.1, and 89.6% of the starting capacity, respectively (Fig. S3). The depletion of active sites and changes in the material’s particular geometric layout, which worsen with each cycle, are responsible for this decline in uptake. These findings show that the PVDF/CS-Al/Cu-LDH exhibits a significant capacity for efficient recycling. X-ray diffraction (XRD) investigations were performed both before and after the regeneration procedure to evaluate the stability of the PVDF/CS-Al/Cu-LDH. The analysis’s findings demonstrate that the adsorbent’s structural integrity was preserved even after five renewal cycles^64–66. For up to five cycles, the PVDF/CS-Al/Cu-LDH can operate at its best. This result suggests that the adsorbent’s stability is preserved for the course of the experimental evaluation⁶⁷.

Comparing various adsorbents

Table S9 offerings an assessment of the efficacy of the PVDF/CS-Al/Cu-LDH nanofiber membrane in adsorbing Hg(II) ion contaminants, compared with various sorbents in powder and membrane forms. This examination underscores the complexities inherent in comparing diverse adsorbents, which stem from their varying physical characteristics, including films, nanofibers, and powders, alongside the specific investigational conditions employed in each assessment.

Statistical analysis

ANOVA

The analysis of variance (ANOVA) results presented in Table S10 substantiate the statistical validity and significance of the quadratic response surface model formulated for the Hg(II) adsorption on the PVDF/CS-Al/Cu-LDH. This is illustrated by the exceptionally high model F-value of 61.11 and a corresponding p-value that approaches zero (p < 0.0001), signifying that the variability accounted for by the model is unlikely attributable to random error. Among the linear factors examined, pH (A), adsorbent dose (B), and contact time (C) each demonstrate statistically noteworthy individual effects on adsorption capacity (p < 0.05), with interaction time exhibiting the most substantial effect (F = 320.76, p < 0.0001); this reinforces the notion that the duration of interaction among Hg(II) ions and the nanofiber membrane plays a pivotal role in optimizing removal efficiency⁶⁸. In analyzing interaction terms, only the interaction between dose (B) and time (C) is found to be statistically significant (p = 0.0122), suggesting that the concurrent increase in dose and extension of contact time leads to a synergistic enhancement in adsorption. Conversely, the interactions AB and AC do not exhibit significance, implying that pH does not have a strong interaction with either dose or time within the range investigated. The highly significant quadratic terms A² (pH²) and C² (time²) (p = 0.0007 and p < 0.0001, individually) verify that the adsorption response adheres to a nonlinear trajectory, particularly regarding time, indicating the presence of an optimal operational zone. The non-significant lack of fit (p > 0.05) corroborates that the employed model effectively represents the experimental data without exhibiting systematic deviations. The high coefficient of resolve (R² = 0.9874), along with the adjusted R² (0.9713) and predicted R² (0.7989), reflects a robust alignment between observed and predicted values, with negligible concerns of model overfitting. Furthermore, the Adeq Precision value of 25.046 significantly surpasses the threshold of 4, ratifying a superior signal-to-noise ratio and the reliability of the model. The small standard deviation (20.59) and the acceptable coefficient of variation (C.V. = 8.91%) further reinforce the experimental precision and reproducibility of the findings. In conclusion, the outcomes of the ANOVA affirm that the established quadratic model is both statistically rigorous and highly predictive, thereby being well-suited for the optimization and control of processes related to Hg(II) adsorption utilizing the PVDF/CS-Al/Cu-LDH nanofiber membrane⁶⁹.

The summary statistics for the models under consideration clearly suggest that the quadratic model is the most fitting and statistically valid framework for characterizing the adsorption performance of Hg(II) on the PVDF/CS-Al/Cu-LDH nanofiber membrane. In difference, the linear and two-factor interaction (2FI) models demonstrate inferior statistical performance, evidenced by their R² values of 0.6580 and 0.6852, alongside relatively high standard deviations of 78.82 and 86.22, respectively. Moreover, their predicted R² values (0.4279 and − 0.0197) indicate inadequate predictive ability and a pronounced discrepancy between observed and estimated responses. Conversely, the quadratic model presents markedly superior statistical metrics, including the highest coefficient of determination (R² = 0.9874), an exceptionally high adjusted R² (0.9713), and a robust predicted R² (0.7989), all of which point to an excellent correlation between empirical and predicted values while exhibiting minimal overfitting. The significant reduction in standard deviation (20.59) and an extremely significant sequential p-value (< 0.0001) further substantiate that the quadratic model offers a markedly improved fit compared to models of lower order⁷⁰. Additionally, the predictive residual sum of squares (PRESS) for the quadratic model (47481.83) is substantially lower than those associated with the linear and 2FI models, representative enhanced predictive precision and model stability. Although the cubic model possesses the mathematical capacity to achieve perfect R² values, it is deemed “aliased”, thereby precluding statistical interpretation due to either insufficient data or model over specification. Subsequently, the quadratic model is posited as the most suitable model, offering high explanatory capability, dependable predictive power, and significant statistical relevance for the optimization of Hg(II) adsorption, as illustrated in Table 2.

Table 2.

Statistical evaluation of different Hg(II) adsorption models on an PVDF/CS-Al/Cu-LDH nanofiber membrane.

Source	Sum of square	Std. Dev.	Sequential p-value	Press	R²	Adjusted R²	Predicted R²
Linear	80761.85	78.82	0.0024	1.351E + 05	0.6580	0.5791	0.4279
2F1	74347.35	86.22	0.8334	2.408E + 05	0.6852	0.4963	-0.0197
Quadratic	2967.61	20.59	< 0.0001	47481.83	0.9874	0.9713	0.7989	Suggested
Cubic	0.0000	0.0000			1.0000	1.0000		Aliased

Modeling experimental design and response surface analysis

The externally normalized residuals are tightly matched with the reference diagonal line, according to the normal probability plot shown in Fig. S4a. The response surface model’s statistical presumptions are validated by this alignment, which implies that the errors resemble a normal distribution⁷¹. The lack of systematic curvature or significant deviations from linearity further implies that there are no substantial issues with non-normality, the necessity for transformation, or model misspecification. A common of the information points cluster in the central region of the plot, indicating a consistent variance and supporting the homoscedasticity of the residuals; only a minimal number of points are noted to be slightly displaced from the line, reflecting minor deviations rather than true outliers. The application of a color gradient scale, ranging from 26.66 to 424.2 mg/g, illustrates that the residuals are distributed randomly, irrespective of the response variable’s magnitude. This consistency suggests that the model demonstrates uniform performance across both low and high adsorption values. In conclusion, the residual analysis confirms that the quadratic model fits the experimental data well and is statistically sound, and devoid of bias, thereby rendering it suitable for making reliable predictions and optimizing Hg(II) adsorption through the PVDF/CS-Al/Cu-LDH nanofiber membrane⁷¹.

A strong agreement between the empirically measured adsorption capabilities and those predicted by the quadratic response surface model is shown in the Predicted versus Actual plot in Fig. S4b. The data points near proximity to the 45° reference line serves as proof of this. The nearly linear distribution of the points suggests that the model proves a high degree of accuracy in estimating the adsorption performance of the PVDF/CS-Al/Cu-LDH nanofiber membrane within the entire response range of 26.66 to 424.2 mg/g, without evidence of systematic under- or overestimation. The lack of curvature or random dispersion further substantiates the strength of the model’s assumptions and designates that the fitted polynomial equation successfully encapsulates the system’s true behavior without introducing bias⁷². Only minor deviations from the regression line are observed in a limited amount of data points, which are likely indicative of inherent experimental variability rather than limitations of the model; moreover, no significant outliers or influential anomalies are identified. The consistent distribution of points across low, medium, and high adsorption values affirms that the model exhibits substantial predictive capability throughout the entire design space, rather than being confined to a narrow concentration range. In sum, this diagnostic plot corroborates the reliability and predictive efficacy of the model, reinforcing its appropriateness for optimizing and scaling up the Hg(II) adsorption.

The Box–Cox plot obtainable in Fig. S4c assesses the necessity of a data transformation aimed at steadying alteration and enhancing the normality of the model. The findings demonstrate that no transformation is needed for the adsorption response data. The minimum point of the Box–Cox curve, which indicates the lowest residual sum of squares (Ln (Residual SS)), is located near λ = 1.0—this optimal λ value is well within the boundaries of the 95% confidence interval, as illustrated by the two vertical red lines. This outcome substantiates that the response data are appropriately scaled and conform to the assumptions of the quadratic model, without the need for transformations such as logarithmic, square-root, or inverse adjustments. The vertical green line at λ = 1 further validates that the existing model, derived from the untransformed experimental data, holds statistical integrity⁷². Additionally, the horizontal reference line (Ln SS ≈ 10.1156) indicates that no significant reduction in residual error can be attained by altering λ. The smooth and well-defined nature of the curve also implies the absence of heteroscedasticity or nonlinearity in variance. In summary, the Box–Cox diagnostic confirms that the fitted quadratic model is predicated on properly distributed and appropriately scaled response data, verifying that the experimental dataset does not exhibit skewness or issues related to non-constant variance, thus negating the need for further mathematical transformations to ensure reliable predictions and interpretations.

The plot of Residuals versus Predicted values presented in Fig. S4d corroborates the suitability and consistency of the fitted quadratic model. The externally studentized residuals exhibit a random distribution around the zero-reference line, devoid of any discernible trend, curvature, or funnel shape. This randomness indicates a lack of systematic errors, heteroscedasticity, or bias in the model. A majority of the residual values reside comfortably within the established control limits (± 4.81963), which illustrates that the discrepancy between the experimental and predicted adsorption values is minimal and consistently distributed throughout the prediction range. Furthermore, the absence of clustering or discernible patterns among the residuals supports the notion that the model displays constant error variance, demonstrating reliable performance across a spectrum of adsorption capacities (26.66–424.2 mg/g). Notably, only one observation falls outside the control limits, suggesting that this instance may denote a singular influential observation rather than indicating any structural inadequacy of the model. The symmetric distribution of residuals about zero further validates that the fitted response surface model adheres to the requisite assumptions of independence and randomness, thereby enhancing its dependability for predictive and optimization purposes⁷². Consequently, this diagnostic plot affirms that no corrective transformations of the data or additional regressors are necessary, thereby substantiating the robustness of the quadratic model in accurately representative the Hg(II) adsorption behavior.

The perturbation plot obtainable in Fig. S4e elucidates the sensitivity of the adsorption capacity (q_e) of the PVDF/CS-Al/Cu-LDH to fluctuations in three independent variables: pH (A), adsorbent dosage (B), and contact time (C), in relation to the reference point situated at the center of the enterprise space. A more pronounced slope in the response curve signifies a greater impact of that specific factor on the adsorption of Hg(II), whereas a flatter profile denotes a reduced sensitivity of the variable. Among the three parameters evaluated, the adsorbent dose (B, represented by the blue line) proves the most substantial effect on q_e, as demonstrated by its notable curvature. This finding proposes that even minor adjustments in the dosage of the adsorbent can lead to significant changes in adsorption capabilities. The pH (A, illustrated by the green line) exhibits a moderate inspiration, indicating that the adsorption capacity tends to increase as the pH approaches an optimal range, although it remains less impactful than the dosage. Conversely, contact time (C, denoted by the gray line) reflects minimal variation around the reference point, suggesting that q_e is relatively insensitive to small alterations in contact time once the adsorption process nears equilibrium. Furthermore, all three response curves demonstrate nonlinear characteristics, underscoring the relevance of quadratic terms within the model and validating the implementation of a second-order response surface design. In summary, the perturbation study indicates that the adsorbent dose is the primary determinant of Hg(II) uptake, surveyed by pH, while interaction time has a comparatively minor effect in proximity to the central operational zone⁷³.

The cube plot illustrated in Fig. S4f visually shows the synergistic effects of three independent variables pH (A), contact time (B), and adsorbent quantity (C) on the adsorption capacity (q_e). This graphical representation facilitates an understanding of how both extreme and intermediate settings of these factors affect the removal of Hg(II). The maximum adsorption capacity observed (439.192 mg/g) is reached under conditions of elevated pH (8), prolonged contact time (100 min), and minimal adsorbent quantity (0.02 g). This outcome substantiates the idea that optimal adsorption occurs when the surface is significantly deprotonated, which enhances electrostatic attraction, while the adsorbent remains below saturation limits due to low mass. In contrast, the lowest response recorded (42.3156 mg/g, extrapolated mathematically) manifests under conditions of low pH (2), brief contact time (5 min), and excessive adsorbent dose (0.5 g). This scenario exemplifies adverse adsorption conditions, which arise from surface protonation and insufficient interaction time despite the presence of surplus sorbent⁷⁴. The analysis further indicates that an increase in adsorbent dosage generally correlates with a decrease in adsorption per gram (q_e), while enhancements in either contact time or pH tend to elevate adsorption capacities. These observations corroborate the ANOVA results, wherein it is highlighted that dose exerts the most substantial negative impact, while time demonstrates the most pronounced positive effect. Moderate adsorption values noted along the edges of the cube (ranging from 117 to 267 mg/g) illustrate the nonlinear and interactive characteristics of the response. Notably, simultaneous fluctuations in pH and contact time yield more significant increases in adsorption than modulations of either variable in isolation. In summary, the cube plot reinforces the assertion that optimal adsorption is realized under conditions of high pH, extended contact time, and minimal sorbent mass. This supports the quadratic optimization model and confirms that the efficiency of adsorption is contingent upon the collective, rather than isolated, effects of the investigated variables.

Checking of model adequacy

The 3D interaction, contour, and desirability plots collectively elucidate the interactive belongings of pH (A) and adsorbent dosage (B) on the adsorption capacity (q_e) of Hg(II) as facilitated by the PVDF/CS-Al/Cu-LDH nanofiber membrane. These illustrations further support the optimization potential afforded by the response surface model. The 3D surface plot exhibits a curvilinear, ascending response trend, wherein q_e progressively increases with rising pH levels and decreasing adsorbent doses. This observation substantiates the significant synergistic interaction between deprotonated functional groups at elevated pH values and an increased availability of adsorption sites per gram at lower mass loadings. The associated contour plot reveals well-defined elliptical iso-response lines, thereby indicating a vital interaction between pH and dosage rather than independent effects⁷⁵. The region of maximum adsorption (350–424 mg/g) is notably concentrated within the pH range of 6 to 8 and doses falling below 0.1 g, aligning with the optimized solutions predicted by the model. Conversely, areas considered by low pH and high dosage yield minimal adsorption, credited to the existence of protonated surfaces and saturation of available sites. The desirability plot further distills this behavior into a dimensionless scale of optimization, where a desirability value approaching 1.0 is attainable only within a restricted area defined by elevated pH coupled with low dosage (D ≈ 0.994). This indicates that this specific region not only maximizes the adsorption response but also adheres to the established optimization criteria. The observed decline in desirability toward higher-dose and lower-pH areas corroborates the statistical prioritization of variable importance, reinforcing the conclusion that pH serves as a positive determinant, while dosage functions as a negative determinant in influencing q_e. Collectively, these graphical representations confirm the strength of the quadratic model, substantiate the existence of strong interactive effects, and delineate the most advantageous operational parameters for optimizing the approval of Hg(II) (Fig. 8a).

Fig. 8.

3D interaction, contour, and desirability.

The interaction plots in Fig. 8b illustrate the combined belongings of dosage and interaction duration on the Hg(II) adsorption capacity (q_e) of the PVDF/CS-Al/Cu-LDH. This indicates that adsorption efficiency relies heavily on the interaction between these factors rather than independent effects. The three-dimensional surface shows a significant increase in qₑ with longer interaction times, while higher dosages lead to a decrease in response. This suggests that longer contact times promote enhanced diffusion and surface complexation, whereas increased adsorbent mass reduces capacity per unit due to fewer active sites and surface aggregation. The contour plot reveals maximum adsorption capacity, over 400 mg/g, at low dosages (≤ 0.1 g) and prolonged times (≥ 80 min), while the minimum is under 100 mg/g at high dosages with short durations. The elliptical contour lines confirm the model’s reliability and statistical significance. The desirability plot shows that the peak score (D ≈ 0.994) aligns with low dosage and long contact time, affirming the model’s predictive abilities and underscoring optimal conditions for reduced material use and adequate interaction⁷⁵. The gradual decrease in desirability at high dosages and short times highlights the drawbacks of excessive dosage and insufficient equilibrium contact. Overall, these findings validate that while contact duration positively influences performance, dosage negatively affects it, indicating optimal performance through minimized adsorbent dosage and maximized contact time as exposed in Fig. 8b.

The collaborative effects of pH and contact duration on mercury ions (Hg(II)) adsorption (q_e) onto PVDF/CS-Al/Cu-LDH are shown by the three-dimensional interaction, contour, and desirability plots in Fig. 8c. The substantial synergistic effect of these factors on adsorption performance is demonstrated by this illustration. The surface plot indicates increased adsorption capacity with rising pH and extended contact time, displaying a continuously ascending curved surface. Prolonged exposure enhances interaction among Hg(II) ions and active –OH and –NH₂ groups, while higher pH promotes surface deprotonation, boosting electrostatic attraction and complex formation with Hg²⁺ ions. The contour plot shows optimal adsorption (≥ 350 mg/g) primarily within pH 6 to 8 and interaction times over 70 min, whereas low pH (2–3) and short contact times (< 20 min) lead to minimal adsorption (< 100 mg/g), suggesting a time-dependent chemisorption mechanism favored in alkaline conditions. The desirability plot normalizes the response, indicating peak desirability (D ≈ 0.990) at high pH and long contact times, aligning with the goal of maximizing adsorption capacity. The curvature and non-parallel contour lines confirm the interaction term’s statistical significance, with pH having a greater influence than contact time, shown by a steeper incline toward optimal conditions. Overall, the findings confirm that optimal Hg(II) uptake occurs at high pH and extended interaction time, supporting the chemisorption mechanism and the predictive validity of the quadratic model⁷⁵.

The desirability method and validation of the model

The optimization graph in Fig. S5a shows desirability functions for pH (A), contact time (B), and adsorbent dosage (C), along with the predicted response (q_e), to optimize Hg(II) adsorption on the PVDF/CS-Al/Cu-LDH membrane. Desirability approaches 1.0 with pH around 6, contact time of 80–100 min, and adsorbent dosage of about 0.02 g, indicating high sensitivity to these conditions. The predicted adsorption response curves illustrate that q_e increases at higher pH and longer contact duration but decreases with higher dosage, highlighting the negative impact of excessive sorbent use. The narrowed confidence bands in cyan confirm model precision and low prediction uncertainty. The optimal solution from the desirability algorithm is pH ≈ 6.2, time ≈ 91 min, and dosage ≈ 0.02 g, achieving a projected capacity of 424 mg/g and a desirability index of 0.994. This result indicates that the parameters effectively achieve the objectives, validating the model’s ability to identify conditions for optimal Hg(II) removal while minimizing sorbent usage and processing time⁷⁶.

The desirability bar plot obtainable in Fig. S5b delineates the individual and collective desirability scores derived from the numerical optimization of the Hg(II) adsorption process. This outcome substantiates that the chosen optimal conditions concurrently fulfill all established criteria with a substantial degree of statistical reliability. The three input variables—pH, contact time, and adsorbent dosage—attained desirability values of 1.0, 1.0, and 0.999997, respectively, signifying that the optimized parameters are situated virtually at the most advantageous positions within their permissible ranges. The response variable (q_e), prioritized due to its direct correlation with adsorption efficiency, reached a desirability score of 0.960913, demonstrating an exemplary equilibrium between maximum removal capacity and experimental limitations. The aggregate desirability value of 0.990081 signifies that the multi-criteria optimization methodology aptly identifies a singular operating condition that optimizes adsorption performance while maintaining all variables within practical and experimentally feasible limits. The elevated and consistent desirability scores across the parameters further corroborate the absence of conflicting influences during the optimization process and confirm the reliability and appropriateness of the quadratic RSM model for forecasting and enhancing Hg(II) uptake. Collectively, this plot affirms that the proposed solution epitomizes the most desirable and effective adsorption configuration among the 15 candidate solutions produced by the software, with negligible compromise on any variable⁷⁷.

Table S11 presents an overview of the numerical optimization criteria outlined for the response surface methodology, delineating the optimization objectives, the corresponding importance weights, and the resultant desirability scores for the independent variables (pH, contact time, and dosage) as well as the response variable (q_e). Each independent variable was restricted to its experimental range—specifically pH (2–6), contact time (5–100 min), and dosage (0.02–0.5 g)—and was allocated equal importance (weight = 1), yielding near-perfect individual desirability values (ranging from 0.999999 to 1.000000). This finding substantiates that the identified optimal point is firmly situated within the established practical and experimental thresholds. The adsorption capacity (q_e), designated as the variable to be maximized and assigned a higher weight of 5 due to its critical significance in the overall process efficacy, obtained a desirability score of 0.9609. This score signifies a commendable alignment between the predicted and target levels of response⁷⁷. The cumulative desirability score of 0.99008 illustrates the concurrent optimization of all factors, reinforcing the notion that the model adeptly identifies a singular operational condition that fulfills all stipulated criteria without diminishing any individual factor. The elevated composite desirability index validates that the chosen optimum yields a highly effective adsorption environment, effectively balancing maximum Hg(II) uptake against practical operational constraints, thus, the response surface approach optimization process’s prediction accuracy and robustness are confirmed.

The solution ramp plot in Fig. S5c shows the optimized conditions for maximum Hg(II) adsorption on the PVDF/CS-Al/Cu-LDH, identified through desirability-based numerical optimization. The optimal values are marked in red and blue; pH is 6, contact time is 100 min, and adsorbent dose is 0.0200 g, resulting in a predicted adsorption capacity (q_e) of 408.661 mg/g and a desirability index of 0.99. The linear ramps reflect the contribution of each factor to optimization; both pH and contact time positively affect outcomes as they reach their upper experimental limits, while the adsorbent dose is minimized for optimization. The steep slope for desirability concerning dose shows its negative impact on q_e, whereas the slopes for contact time and pH indicate their roles in improving adsorption efficiency. The high desirability indicates these conditions meet al.l constraints, demonstrating an optimal balance to maximize adsorption with practical feasibility. Thus, this plot visually confirms the optimization results, showing how the Response Surface Methodology (RSM) model achieves efficient adsorption with minimal material use and optimal performance⁷⁷.

Biological activity

Anticancer activity

Cell line MCF-7

Figures S6(a, b) collectively illustrate the cytotoxic and inhibitory effects of the PVDF/CS-Al/Cu-LDH nanofiber membrane and the PVDF/CS nanofiber membrane on MCF-7 human breast carcinoma cells, as evaluated via the MTT assay subsequent a 24-hour exposure period. In Fig. S6(a), a concentration-dependent decrease in cell viability is observed for both materials. Notably, the PVDF/CS-Al/Cu-LDH nanofiber membrane (represented by blue closed circles) demonstrates a more substantial and marked reduction in viability compared to the PVDF/CS nanofiber membrane⁷⁸. At lower concentrations (≤ 50 µg/mL), both membranes exhibit moderate cytotoxic effects; however, the PVDF/CS-Al/Cu-LDH nanofiber membrane significantly decreases cell viability beyond this threshold, approaching nearly complete cell death at 500 µg/mL. The calculated IC₅₀ value for the PVDF/CS-Al/Cu-LDH nanofiber membrane is 72.36 ± 2.86 µg/mL, suggesting a high efficacy, whereas the PVDF/CS nanofiber membrane presents a higher IC₅₀ of 122.36 ± 1.88 µg/mL, indicating a lower cytotoxic capability. Similarly, Fig. S6(b) depicts the percentage inhibition of MCF-7 cells, which rises proportionally with the concentration of both materials. The PVDF/CS-Al/Cu-LDH nanofiber membrane achieves higher inhibition percentages across all concentrations tested, exceeding 88% at approximately 300 µg/mL and nearing complete inhibition at 500 µg/mL. In contrast, the PVDF/CS nanofiber membrane necessitates higher concentrations to reach comparable levels of inhibition, which corresponds with its elevated IC₅₀. These results imply that the PVDF/CS-Al/Cu-LDH has better anticancer qualities, which are probably due to its increased surface reactivity, inherent photocatalytic capabilities, and capacity to cause oxidative stress and apoptosis in breast cancer cells^79,80.

Cell line HePG-2

The PVDF/CS-Al/Cu-LDH nanofiber membrane exhibits a markedly more rapid and pronounced decrease in cell viability in comparison to the PVDF/CS nanofiber membrane. At lower concentrations (≤ 50 µg/mL), both membranes demonstrate moderate cytotoxicity; however, the PVDF/CS-Al/Cu-LDH nanofiber membrane reveals a significant decline in viability at elevated concentrations, culminating in nearly complete cellular death at 500 µg/mL (Fig. S6c). The calculated IC₅₀ for the PVDF/CS-Al/Cu-LDH nanofiber membrane is 61.27 ± 1.49 µg/mL, which indicates a higher cytotoxic potential, whereas the PVDF/CS nanofiber membrane yields an IC₅₀ of 102.62 ± 2.84 µg/mL, suggesting a lower efficacy. Furthermore, as depicted in Fig. S6d, the inhibition percentage of HePG-2 cells exhibits a notable increase in relation to concentration for both materials⁸⁰. The PVDF/CS-Al/Cu-LDH nanofiber membrane consistently achieves superior inhibition rates across all tested concentrations, surpassing 87% at approximately 300 µg/mL and approaching complete inhibition at 500 µg/mL. In contrast, the PVDF/CS nanofiber membrane necessitates higher doses to reach comparable inhibition levels. These results elucidate that the PVDF/CS-Al/Cu-LDH nanofiber membrane demonstrates significantly enhanced anticancer activity against hepatocellular carcinoma cells when compared to the PVDF/CS nanofiber membrane, which may be ascribed to its increased surface reactivity, enhanced interactions with cells, and its potential to facilitate oxidative stress-mediated apoptosis.

Antioxidant activity

Figure S7 presents the DPPH radical scavenging activity of three different materials: the PVDF/CS-Al/Cu-LDH nanofiber membrane, the PVDF/CS nanofiber membrane, and ascorbic acid, which serves as the standard antioxidant for comparison⁸¹. The investigation encompasses a wide concentration range from 0 to 1000 µg/mL. Both varieties of nanofiber membranes displayed an increase in scavenging activity contingent upon the concentration. Importantly, the PVDF/CS-Al/Cu-LDH nanofiber membrane consistently surpassed the scavenging performance of the PVDF/CS nanofiber membrane across all concentrations tested. At lower concentrations (specifically those below 50 µg/mL), the PVDF/CS-Al/Cu-LDH nanofiber membrane demonstrated a marked antioxidant effect, which intensified significantly up to approximately 200 µg/mL, before reaching a plateau of around 84.2% scavenging efficiency at the highest concentration assessed. In contrast, the PVDF/CS nanofiber membrane exhibited a more gradual increase in scavenging activity, culminating in approximately 76.03% scavenging efficacy at the maximum concentration of 1000 µg/mL⁸². The recorded IC₅₀ values substantiate the enhanced antioxidant properties of the PVDF/CS-Al/Cu-LDH nanofiber membrane, which was quantified at 147.96 ± 5.42 µg/mL, in contrast to the PVDF/CS nanofiber membrane, which exhibited an IC₅₀ value of 228.3 ± 9.42 µg/mL. Ascorbic acid, indicated by blue diamonds, served as the positive control and displayed the highest radical scavenging potential, achieving over 90% inhibition at an approximate concentration of 200 µg/mL. The antioxidant enhancement in PVDF/CS-Al/Cu-LDH is not due to a single factor but rather a multi-mechanism synergy combining the hydrogen-donating capacity by -OH and -NH₂ from chitosan, the redox-active copper centers in LDH where electrons are transferred from the membrane surface to the radical, neutralizing it without requiring hydrogen donation, the high surface area of the nanofibrous architecture, and the π-conjugated electronic structure — all working together to achieve superior DPPH radical scavenging compared to PVDF/CS alone⁸¹.

Conclusion

This study successfully created and assessed a multifunctional PVDF/CS-Al/Cu-LDH electrospun nanofiber membrane, which exhibited exceptional efficacy in Hg(II) ions removal from aqueous environments while concurrently demonstrating notable biological activity. Structural, morphological, and surface characterizations corroborated the effective combination of Al/Cu-LDH and CS into the PVDF nanofibrous framework, leading to the creation of a highly porous and chemically active adsorbent enriched with hydroxyl and amino groups, capable of robust metal coordination. Adherence to the pseudo-second-order kinetic model, monolayer Langmuir isotherm fitting, and thermodynamic parameters revealing endothermic and spontaneous characteristics all demonstrated that chemisorption dominated the adsorption dynamics, with the membrane achieving a peak maximum adsorption capacity of 422.68 mg.g⁻¹. Optimization through the Box–Behnken experimental design established that influences such as pH, adsorbent quantity, and interaction duration significantly affect uptake efficiency; the model showcased remarkable predictive accuracy, attaining a desirability index of 0.99 under optimal conditions. XPS analysis confirmed that the mechanism of Hg(II) removal was chiefly facilitated by surface complexation and ion-exchange interactions involving the LDH layers and chitosan functionalities. Beyond its application in environmental remediation, the membrane also exhibited promising anticancer activity in contrast to MCF-7 and HePG-2 cancer cell lines and showcased pronounced antioxidant activity as demonstrated through DPPH radical scavenging assays. These multifaceted characteristics underline the material’s versatility, indicating its potential utility in advanced wastewater treatment methodologies as well as in biomedical and antimicrobial applications where concurrent removal of contaminants and bioactivity is advantageous. Future research directions may encompass real wastewater testing, investigation of membrane reusability in continuous-flow systems, and mechanistic studies elucidating the relationship between surface chemistry and biological interaction pathways.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Yasmeen A. S. Hameed , Jawza S. Alnawmasi: Data curation, formal analysis, methodology, and software; Ali Y. Alzahrani, Sahar Sallam: Investigation and writing—review & editing; Amnah S. Al Zbedy, Ibrahim S. S. Alatawi: formal analysis, investigation, writing—original draft. Hatem A. Al-Aoh, Nashwa M. El-Metwaly: Supervision and administration of research group.

Funding

Not applicable.

Data availability

All relevant data are within the manuscript and available from the corresponding author upon request.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.