Efficient removal of Congo Red Dye using a MgAl-LDH/cuttlebone composite adsorbent

Abstract

In this work, a novel, eco-friendly composite material composed of MgAl-layered double hydroxide and cuttlebone (LDH/CB) was synthesized for the efficient adsorption of Congo Red dye from aqueous solutions. The LDH/CB composite was synthesized via a one-step co-precipitation method, and its crystallinity was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The effects of key experimental parameters—including pH, contact time, and initial dye concentration—on the adsorption efficiency were systematically investigated. The adsorption kinetics followed the pseudo-first-order model, while the equilibrium data were well described by the Langmuir and D–R isotherm, with a maximum adsorption capacity of 380 mg/g. Thermodynamic analysis revealed that the adsorption process was spontaneous, exothermic, and primarily driven by electrostatic interactions. Moreover, the composite exhibited excellent reusability over five adsorption–desorption cycles, maintaining high stability in performance. These findings highlight the potential of the LDH/CB composite as an efficient and sustainable adsorbent for the treatment of dye-contaminated wastewater.

Background

Water scarcity and pollution are increasingly critical concerns worldwide, largely driven by the rapid growth of industrial sectors. Industries such as textiles, leather, plastics, and paper manufacturing contribute significantly to aquatic pollution through the discharge of synthetic dyes [1]. These dyes are typically non-biodegradable, persistent, and hazardous, threatening both aquatic biodiversity and public health [2–4]. Annual estimates suggest that several thousand tons of dyes are released into freshwater ecosystems, exacerbating long-term environmental damage [5]. Among these pollutants, Congo Red (CR) is of particular concern due to its azo structure, resistance to degradation, and potential carcinogenic effects [6, 7].

The environmental and health impacts of dye-laden effluents extend beyond aquatic organisms, affecting terrestrial ecosystems and entering the food chain through bioaccumulation [3, 7]. This has led to intensified efforts to develop efficient, sustainable methods for dye removal. A range of treatment strategies has been explored, including coagulation, membrane separation, advanced oxidation, and flocculation [2, 4, 8, 9]. Recent studies have also investigated biological approaches, such as enzymatic and microbial decolorization, showing promising performances for azo dye removal [10, 11].

Among the available treatment methods, adsorption has established itself as one of the most practical, economical, and environmentally sustainable options. Its appeal lies in its straightforward operation, strong efficiency even at low pollutant concentrations, and the possibility of adsorbent regeneration and reuse. For instance, activated carbon synthesized from walnut shells featuring an exceptionally high surface area (≈ 2347 m²/g) demonstrated outstanding performance in removing reactive dyes [12]. More broadly, recent reviews underscore that adsorption continues to be a preferential technology compared to alternatives that typically involve high costs or operational limitations [13]. In recent years, biosourced and waste-derived adsorbents have gained attention due to their low cost, availability, and alignment with circular-economy principles, while still offering competitive adsorption performance [14, 15]. A 2022 review documented the effectiveness of numerous low-cost biomasses as sorbents, including regeneration strategies and scalability pathways [16]. Furthermore, recent analyses highlight how advancements in regenerable, waste-derived adsorbents can enhance both the economic and environmental attributes of wastewater treatment processes [17]. Several studies have confirmed that low-cost materials such as agricultural wastes or natural biomasses (e.g., cactus or fungal biomass) can effectively remove textile dyes, thereby providing sustainable alternatives to activated carbon [18, 19].

In recent years, various adsorbents have been explored for the removal of Congo Red (CR) dye. For instance, NaOH-modified green coffee waste biochar demonstrated a maximum adsorption capacity of 91.54 mg/g for CR [20]. Similarly, activated biochar derived from Haematoxylum campechianum waste exhibited significant CR removal efficiency [21]. While these materials show promise, they often face limitations in terms of stability, reusability, and surface area. In contrast, our LDH/CB composite adsorbent offers enhanced structural stability, higher surface area, and improved adsorption kinetics, making it a more effective and innovative alternative for CR removal.

Recent studies have explored various LDH-based adsorbents for the removal of Congo Red (CR) dye from aqueous solutions. For instance, Mg/Ni/Al-Layered Double Hydroxide (LDH) has been utilized in both its as-prepared and thermally treated forms to effectively adsorb CR from textile effluents. The thermally treated Mg/Ni/Al-LDH exhibited enhanced adsorption capacities, reaching up to 90% removal efficiency under optimal conditions [22].

Additionally, the synthesis of MgAl-LDH using γ-Al₂O₃ as a crystal regulator resulted in a composite with a CR adsorption capacity of 233 mg/g, outperforming the 197 mg/g achieved by α-Al₂O₃-derived LDH [23]. Similarly, Zn-Al-Layered Double Hydroxides intercalated with nano zero-valent iron demonstrated significant CR removal, with the 5% nZVI/Zn-Al-CO₃ LDH composite achieving optimal adsorption performance.

These advancements underscore the potential of LDH-based materials in CR dye removal. In this context, our LDH/cuttlebone composite offers a novel approach, combining the structural stability and high surface area of LDHs with the natural porosity and biodegradability of cuttlebone. This integration aims to enhance adsorption efficiency and provide a sustainable solution for dye removal from aqueous environments.

This study focuses on the synthesis and application of a composite adsorbent made from MgAl-layered double hydroxide and cuttlebone (LDH/CB) using the co-precipitation method [24]. The combination leverages the complementary properties of LDH’s anion-exchange ability and CB’s porous framework to improve dye removal performance. LDHs, also referred to as anionic clays, consist of positively charged layers of metal hydroxides separated by interlayer anions and water molecules. Their structural versatility and non-toxic nature make them ideal hosts for molecular adsorption. Meanwhile, cuttlebone’s intrinsic porosity and mineral composition have been shown to enhance dye adsorption by offering extensive surface area and binding capacity [25–28].

In this work, we examine the adsorption behavior of Congo Red onto LDH/CB composites under varying conditions, including pH, initial dye concentration, and contact time. Structural and morphological analyses using techniques such as XRD, FTIR, SEM, and TEM will be conducted to elucidate the interaction of dye molecules with the composite, both within interlayer spaces and on external surfaces.

Methods

Chemicals

Aluminum chloride hexahydrate (AlCl₃,6H₂O, 99.99%),), magnesium chloride hexahydrate (MgCl₂,6H₂O, 99.999%), sodium hydroxide (NaOH, 98%),) were procured from Sigma–Aldrich (Germany), cuttlebone (CB). With the IUPAC name 1-napthalenesulfonic acid, 3,3-(4,4-biphenylenebis(azo)) bis (4-amino disodium) salt, Congo red (CR) is an anionic azo dye. The physicochemical properties and the molecular structure of the dye are shown in Table 1; Fig. 1 respectively. A stock solution of CR at a concentration of 1000 mg/L was prepared using double-distilled water. Working solutions, with concentrations ranging from 50 to 400 mg/L, were then obtained by appropriate dilution of the stock solution.

Table 1.

CR characteristics

Parameter	Values
Absorption maxima Synonyms Solubility in water Molecular weight Molecular formula	497 nm Direct red 28,C.I 22,120 1 g/30 mL 696.68 C32H22N6Na2O6S2

Fig. 1.

Molecular structure of CR

Preparation of MgAl-LDH/CB

To synthesize the MgAl-LDH/CB composite, we used a straightforward co-precipitation method under controlled pH conditions [29]. First, we prepared an aqueous solution containing MgCl₂·6 H₂O and AlCl₃·6 H₂O with a Mg²⁺ to Al³⁺ molar ratio of 2:1, maintaining the total metal ion concentration at 1 M. This solution was slowly added to a reactor containing distilled water and 1 gram of finely ground cuttlebone. To keep the pH stable at 9, a 2 M NaOH solution was added dropwise during the process under vigorous stirring. The entire reaction was carried out at room temperature (25 °C) in a nitrogen atmosphere to avoid contamination from atmospheric CO₂. Once precipitation was complete, the resulting solid was collected through several rounds of washing and centrifugation, and then left to dry at room temperature.

Structural characterization techniques

X-ray Diffraction (XRD) patterns were obtained using a BRUKER D8 ADVANCE diffractometer with Cu Ka radiation (λ = 0.154060 nm) at 40 kV and 30 mA, in continuous scanning mode. Data were recorded at 2θ range from 2° to 70°, with an angular step size of 0.019° and a counting time of 67.1 s per step in increments.

FTIR analysis was conducted using a Fourier Transform Spectrophotometer model IRAffinity-1 from SHIMADZU. The spectra of the samples, were recorded over a range of 4000 to 400 cm^− 1 range at a spectral resolution of 1 cm^− 1. Samples were prepared by pressing them into KBr disks.

SEM micrographs were taken using a Zeiss supra 55-VP scanning electron microscope operating at 10 kV. TEM images were captured with a Hitachi 7650 microscope at an acceleration voltage of 80 kV.

The point of zero charge (pH_pzc) of the material was determined using the mass titration method [30], a straightforward and reliable approach for assessing surface charge properties. In this procedure, a series of NaCl solutions (0.01 M) was prepared, and their pH was adjusted over a wide range using dilute HCl or NaOH. A fixed mass of the adsorbent was added to each solution, and the suspensions were stirred until equilibrium was reached. The final pH values were then recorded, and the difference between the initial and final pH (ΔpH) was plotted against the initial pH. The pH_pzc corresponds to the point where ΔpH = 0, i.e., where the surface carries no net charge. Below this value, protonation of surface groups results in a positively charged surface, which enhances the adsorption of anionic species such as Congo Red. Conversely, at pH values above the pH_pzc, deprotonation leads to a negatively charged surface, which may reduce adsorption efficiency. Overall, determining the pH_pzc is essential for understanding electrostatic interactions and optimizing the adsorption process.

Adsorption experiments

Batch adsorption experiments were performed using 50 mL Erlenmeyer flasks, each containing 50 mg of adsorbent and 25 mL of dye solutions with initial concentrations of 50, 100, 150, 200, 250, 300, 350 and 400 mg/L. The flasks were shaken in a water-bath shaker at a speed of 220 rpm for 24 h at a temperature of 25 °C. Dye concentration in the aqueous phase was measured with a double beam UV–vis spectrophotometer (UV-visible T60, PGI, Chine) at 497 nm. Prior to analysis, all samples were filtered using Whatman No. 1 filter paper. The uptake of CR at equilibrium, Q_e (mg/g), was determined by:

1

2

where C₀ and C_e (mg/L) are the initial concentration and the equilibrium concentration of dye, respectively, V (L) is the volume of the dye solution, and m (mg) is the weight of LDH/CB.

Adsorption studies were conducted at various pH values ranging from 3 to 10, contact times from 5 to 90 min, and adsorbent dosage (20–160 mg) were meticulously adjusted to achieve optimal adsorption. Under these optimized conditions, the adsorption capacity of LDH/CB for Congo red was assessed across various initial dye concentrations (50–400 mg/L). Furthermore, the adsorption capacity was examined using adsorption thermodynamics at temperatures of 25, 30, 35, 40, and 45 °C in a thermostatic bath. All the experimental trials were repeated thrice and all the data were expressed as mean values.

Discussion

Characterizations of LDH/CB

Figure 2 displays the X-ray diffraction patterns for the LDH samples. In the spectrum for LDH, reflections are observed at 003, 006, 110, and 113. These findings indicate that the synthesized LDH has well-organized layered structures [31, 32]. For LDH/CB, the X-ray diffraction patterns reveal no shift in the rays, which confirms that surface adsorption occurred without the intercalation of cuttlebone.

Fig. 2.

XRD diffractograms of (a) LDH/CB, (b) LDH and (c) cuttlebone CB

Table 2 presents the crystallographic data of the LDH/CB composite. After interaction with molecules from the cuttlebone, the co-precipitated phase exhibits significant changes in the X-ray diffraction (XRD) pattern compared to the original LDH material (Fig. 2). A notable broadening of the full width at half maximum (FWHM) is observed, indicating a reduction in crystallite size, as calculated using the Debye–Scherrer equation (Table 2). This co-precipitation between cuttlebone and LDH leads to an enhanced nanostructure of the composite, which may improve its physicochemical properties and expand its potential applications. Furthermore, these structural modifications are likely to increase the number of active sites and facilitate molecular interactions, thereby influencing the adsorption mechanism.

Table 2.

Crystallographic data of cuttlebone CB, LDH and LDH/CB composite

	d (nm)	2Ɵ (°)	FWHM (°)	Crytallite size D* (nm)
LDH/CB	0.754	11.72	0.524	15.5
LDH	0.759	11.65	0.150	56.4
Cuttlebone CB	0.340	26.18	0.168	51.1

The crystallite size was calculated by the Scherrer calculator using PANalytical X’Pert HighScore Plus software from the full-width at half-maximum (FWHM) intensity of the (003) reflection.

Figure 3 presents the FTIR spectrum of the LDH/CB composite, where several important vibrational bands are evident. A wide absorption band ranging from 3200 to 3700 cm⁻¹, with an intensity maximum at 3450 cm⁻¹, is primarily associated with the O–H stretching vibrations of hydroxyl groups. These vibrations originate from the layered hydroxide matrix (Mg/Al–OH or Al–OH) and water molecules either integrated within the structure or located in the interlayer space. Additional minor features around 2931 cm⁻¹ and 2841 cm⁻¹ are likely due to interactions between hydroxyl groups and carbonate species originating from the cuttlebone. The intense signal at 3450 cm⁻¹ reflects the strong presence of interlayer water. Moreover, the weak absorption peak at approximately 1646 cm⁻¹ is attributed to the bending motion of H–O–H, confirming the retention of water within the layered material [33–36].

Fig. 3.

FT-IR spectra of LDH/CB (a), LDH (b) and CB (c)

A weak band observed at around 1367 cm⁻¹ can be linked to the stretching vibration of carbonate ions (CO₃²⁻), which likely result from atmospheric CO₂ uptake during the washing process [37]. In the lower wavenumber region (1000–400 cm⁻¹), the FTIR spectrum reveals a series of overlapping bands. These include translational vibrations associated with Al–OH at approximately 675 cm⁻¹, and broader features in the 1000–700 cm⁻¹ interval that are attributed to OH groups and water molecules. Several distinct peaks—namely those at 557 cm⁻¹, 440 cm⁻¹, and again at 675 cm⁻¹—correspond to vibrational modes of Al–O, Mg–O, and mixed oxide bridges (O–Al–O and O–Mg–O), respectively. The sharp signal at 557 cm⁻¹, together with a shoulder above 3450 cm⁻¹, serves as a spectral signature typical of layered double hydroxides (LDHs) [36, 37]. Interestingly, the strong band observed at 1363 cm⁻¹ in the pristine material becomes noticeably weaker in the LDH/CB composite. This attenuation is attributed to the asymmetric ν₃ stretching mode of the interlayer anions [38], reflecting subtle structural changes induced by the incorporation of cuttlebone into the LDH matrix. The absorption bands detected in the 500–780 cm⁻¹ range are characteristic of metal–oxygen–metal (M–O–M) vibrations, where M represents Mg or Al [39, 40].

The surface morphology and internal structure of the LDH/CB composite were examined using SEM and TEM, as shown in Fig. 4. The images reveal that the MgAl-NO₃ component is composed of nanoscale, platelet-like spheres with diameters ranging from approximately 50 to 200 nm (panels c and d). These nanostructures are organized into a characteristic “sand rose” arrangement, a morphology frequently observed in hydrotalcite-type layered double hydroxides [41].

Fig. 4.

SEM micrographs of LDH/CB composite

Effect of pH

The solution pH is a crucial factor influencing the adsorption process, as it affects both the surface charge of the adsorbent and the ionization state of the dye molecules. To investigate this effect, experiments were carried out at pH values ranging from 3.0 to 12.0, using a dye concentration of 200 mg/L and 50 mg of adsorbent, with a fixed contact time of 2 h. Figure 5 illustrates how the removal efficiency and adsorption capacity vary with pH.

Fig. 5.

Plots of final pH against initial pH for the determination of pH_pzc of LDH/CB

Adsorption gradually increased at low pH, reached a maximum around pH 8.72, and then decreased under more alkaline conditions. This optimal pH closely matches the zero point of charge (pH_pzc = 8.72) of the LDH/CB composite. At pH values below the pH_pzc, the surface hydroxyl groups are protonated, giving the surface a net positive charge that promotes electrostatic attraction of the negatively charged CR molecules. Conversely, at pH values above the pH_pzc, the surface becomes negatively charged due to deprotonation, which reduces adsorption.

Furthermore, considering the pKa of CR (~ 4.0 for the sulfonic groups), the dye exists predominantly in its anionic form across the studied pH range (pH > pKa). Therefore, electrostatic interactions are the main driving force behind adsorption. The interplay between the adsorbent’s surface charge (pH_pzc) and the ionization state of CR (pKa) explains the observed maximum removal efficiency around pH 8.72.

The effect of pH on Congo Red adsorption by the LDH/CB composite shows that the removal efficiency remains consistently high (≈ 96.7–98.9%) over the entire tested pH range, demonstrating the stability and robustness of the material under different acidic and basic conditions. A slight enhancement is observed near neutral to slightly basic conditions (around pH 8.72), where both removal percentage and adsorption capacity reach their maximum values. At low pH, the decrease in adsorption can be attributed to protonation of the active surface sites by excess H⁺ ions, which reduces the electrostatic attraction toward the anionic dye molecules. Conversely, at strongly basic pH, the presence of excess OH⁻ ions and possible changes in surface charge may hinder dye adsorption. These results indicate that the LDH/CB composite can operate effectively over a broad pH range, with optimal performance in slightly basic conditions, making it suitable for practical wastewater treatment applications (Fig. 6.A).

Fig. 6.

(A) effect of pH solution, (B) effect of reaction time and (C) effect of adsorbent

Kinetics and effect of contact time

Figure 6.B show how contact time influence the adsorption behavior of the LDH/CB composite. During the initial 15 min, adsorption occurs rapidly, reflecting the abundance of active sites readily available for interaction with Congo Red molecules. As time progresses, fewer sites remain free, slowing the adsorption rate until it eventually levels off, reaching an almost complete dye removal of about 97.2% and a capacity near 390 mg/g. To make sure the system reaches equilibrium, a contact time of 30 min is recommended, which is practical for typical adsorption setups. The effect of varying the adsorbent dosage is presented in Fig. 6.C. Interestingly, increasing the dosage from 20 to 100 mg only slightly reduces the removal efficiency from 97 to 95.7%, but it significantly lowers the adsorption capacity from 400 mg/g down to 100 mg/g. This decrease happens because a higher adsorbent amount offers more active sites than needed for the dye molecules present, leaving many sites unused. Balancing efficiency and material use, a 20 mg dosage offers the best compromise for effective and economical dye removal.

Adsorption isotherms: effect of initial dye concentration

Figure 7.A depicts how the adsorption capacity and dye removal efficiency of the LDH/CB adsorbent respond to different starting concentrations of the dye, ranging between 50 and 500 mg/L. The tests were performed at 25 °C, using 20 mg of adsorbent and a 30-minute contact time. Up to 320 mg/L, the adsorbent demonstrates strong performance, removing over 97% of the dye. However, when concentrations increase beyond this point, the removal efficiency declines gradually, reaching approximately 76% at the highest concentration of 500 mg/L. This reduction is attributed to the limited number of adsorption sites available on the fixed adsorbent dosage, which becomes saturated as more dye molecules compete for binding. Figure 7.B further illustrates this trend: the adsorption capacity rises quickly at low equilibrium concentrations (C_e), then the increase slows down and eventually levels off, indicating site saturation.

Fig. 7.

(A) Initial dye concentration effect on the removal percentage and adsorption capacity of LDH/CB composite) and (B) Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm plots for adsorption of CR dye

To gain a clearer understanding of the adsorption process and to estimate the adsorbent’s maximum capacity, we analyzed the equilibrium data using four well-known isotherm models: Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R). The Langmuir model (Eq. 3) assumes adsorption occurs as a single layer on a uniform surface, with each site holding just one dye molecule and no interactions between neighboring molecules [42, 43]. In contrast, the Freundlich model (Eq. 4) accounts for multilayer adsorption on heterogeneous surfaces, where different sites have varying affinities and energies [44]. The Temkin model (Eq. 5) introduces the influence of indirect interactions between the adsorbate and the adsorbent, assuming that the heat of adsorption decreases linearly as the surface becomes more covered, offering insights into the distribution of adsorption energies. Lastly, the Dubinin–Radushkevich (D–R) model (Eq. 6) is particularly useful for distinguishing between physical and chemical adsorption, as it incorporates the adsorbent’s porosity and estimates the mean free energy of adsorption. Taken together, these models provide a comprehensive view of the adsorption mechanism, shedding light on surface heterogeneity, adsorption capacity, and the energetic characteristics of the process [45].

3

4

5

6

where q_m (mg/g) represents the maximum capacity for monolayer adsorption, K_L (L/mg) is the Langmuir equilibrium constant that pertains to the adsorption rate, and K_F [(mg/g)(L/mg)^1/n] and n are the Freundlich constants associated with adsorption capacity and intensity. K_t (L/g) is binding constant associated with maximal binding energy, B = RT/b_t) is a constant related to heat of adsorption, and q_D (mg/g) is D–R sorption effectiveness, respectively.

The isotherm fitting results obtained from our experiments are summarized in Table 3. According to Fig. 7.B, The Langmuir isotherm yielded a maximum adsorption capacity (q_m​) of 406 mg/g with a correlation coefficient of R² = 0.86, indicating a reasonably good fit and suggesting monolayer adsorption on a relatively homogeneous surface.

Table 3.

Langmuir, freundlich, Temkin and Dubinin-Radushkevich parameters

Fitting parameters	Langmuir	Freundlich	Temkin	Dubinin-Radushkevich
qm (mg/g)	406	-	-	-
qD (mg/g)	-	-	-	368.72
KF [(mg/g)(L/mg)1/n]	-	179	-	-
KL(L/mg)	0.56	-	-	-
1/n	-	0.20	-	-
R2	0.86	0.53	0.81	0.97
KT (L/g)	-	-	10.38	-
B (J/mol)	-	-	69.46	-
KD-R	-	-	-	3.37

In contrast, the Freundlich model showed a weaker correlation (R² = 0.53), implying that it describes the adsorption process less accurately. Nevertheless, the relatively high Freundlich constant (K_F = 179) reflects a strong affinity of the adsorbent for C_R. Moreover, the exponent 1/n = 0.20 < 1 indicates that the adsorption process is favorable.

The Temkin model (A = 10.38, B = 69. 46, R² = 0.81) considers adsorbate–adsorbent interactions. The moderate value of B suggests an intermediate interaction strength between C_R molecules and the active sites of the composite, pointing to a mechanism that is not purely physical.

The D–R isotherm provided the best fit (R² = 0.97), with a theoretical capacity of q_m = 368. 72 mg/g and a constant K_D−R = 3.37. The mean adsorption energy (E), calculated from E = (2 K_D−R) ^−0.5, was estimated at about 12.2 kJ/mol, which is characteristic of a chemisorption process.

Overall, the results indicate that the Langmuir and D–R models best describe the adsorption of CR, highlighting both a high adsorption capacity and the significant role of chemisorption, likely driven by specific interactions between CR and the functional groups of the LDH/CB composite. These findings are in good agreement with previous reports that emphasized the relevance of non-linear modeling and the complementary use of D–R and Temkin isotherms to clarify adsorption mechanisms.

To further assess the adsorbent’s performance, we compared our results with those reported in the literature for other adsorbents used in CR removal (see Table 4). The LDH/CB composite developed in this study exhibited a superior adsorption capacity, confirming its effectiveness as a promising material for treating dye-contaminated water.

Table 4.

Comparison of adsorption capacity of different adsorbents for Congo Red

Adsorbent	Qm (mg/g)	Reference
Graphene Oxide (GO)	200	[46]
Magnetic Nanocomposites	100	[47]
Clay-Polymer Composites	100	[48]
Biochar-based Composites	150	[49]
Carbon Nanotube (CNT)	300	[50]
LDH/CB	380	This study

Adsorption kinetics

In order to better understand the adsorption mechanism of Congo Red onto the LDH/CB composite, we analyzed the experimental kinetic data using three classical models: pseudo-first-order, pseudo-second-order, and intra-particle diffusion. The pseudo-first-order model is often used to describe physical adsorption, where the rate is linked to the availability of active sites. Meanwhile, the pseudo-second-order model assumes chemisorption dominates, with adsorption rate depending on the square of the number of vacant sites. Given the porous nature of the composite, the intra-particle diffusion model was also applied to examine whether internal diffusion within the adsorbent structure plays a significant role in the rate-limiting step.

7

8

9

In this context, q_t (mg/g) denotes the adsorption capacity at a given time, t (minutes) is the duration of the reaction, k₁ (min^− 1), k₂ (g/mg · min), and k_int (mg/g · min^0.5) represent the rate constants for the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models, respectively. Additionally, a signifies the constant associated with the thickness of the boundary layer.

Fig 8 shows the linear regression results for the three kinetic models, with the fitting parameters detailed in Table 5.

Fig. 8.

Experimental and nonlinear regression of adsorption kinetics with (A) pseudo-first-order, (B) pseudo-second-order, and (C) intra-particle diffusion models

Table 5.

Adsorption kinetic parameters for CR

Kinetic Model	Parameter	Value
Pseudo-first-order	k1 (min-1)	0.28
	qe (mg/g)	396.17
	R2	0.99
Pseudo-second -order	k2 (g/mg·min)	0.0015
	qe (mg/g)	414.52
	R2	0.87
	kint 1 (g/mg ⋅ min0.5)	9.4
	kint 2 (g/mg ⋅ min0.5)	0.08
	a1	260
	a2	392.14
	R2 1	0.96
	R22	0.88
Experimental qe (mg/g)		380

The kinetic study demonstrates that the adsorption of Congo Red onto the LDH/CB composite closely follows the pseudo-first-order model. The high correlation coefficient (R² = 0.99) and the calculated capacity (396.17 mg/g), in good agreement with the experimental value (380 mg/g), confirm that this model reliably describes the process. This indicates that adsorption is predominantly governed by physisorption, where the rate depends on the availability of active sites. In contrast, the pseudo-second-order model, although generally associated with chemisorption, shows a weaker fit (R² = 0.87) and overestimates the adsorption capacity, thus reducing its relevance. The intra-particle diffusion model, however, suggests a two-step mechanism: an initial rapid phase dominated by surface and external pore diffusion, followed by a slower phase related to internal diffusion. Overall, these findings highlight a complex adsorption process, where physisorption predominates but intra-particle diffusion also contributes at later stages.

Adsorption thermodynamics

Temperature has a decisive impact on adsorption, determining whether the process releases or absorbs heat. In our study, we examined temperatures ranging from 18 to 44 °C, with an initial dye concentration of 320 mg/L. As shown in Fig. 9.A, both the retention rate and adsorption capacity decrease as the temperature rises. The retention rate drops sharply, from about 100% at 293 K to just 45% at 313 K, highlighting a significant loss of efficiency at higher temperatures. In contrast, the adsorption capacity remains nearly unchanged (~ 311 mg/g to ~ 310 mg/g), a minimal variation with little practical significance. Overall, these findings indicate that adsorption is favored at lower temperatures, consistent with an exothermic mechanism [51, 52].

Fig. 9.

(A) Temperature effect on the removal percentage and adsorption capacity and (B) van’t Hoff plot of CR dye onto LDH/CB composite

The decrease in adsorption efficiency observed with increasing temperature can be explained by a reduction in the surface energy of the LDH/CB adsorbent, which in turn weakens the interactions between its active sites and the Congo Red molecules. To gain a deeper understanding of the adsorption process, key thermodynamic parameters—including the standard Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°)—were determined from the van’t Hoff plot (Fig. 9.B), with the results summarized in Table 6.

Fig. 10.

Recycle usability of LDH/CB adsorbent

Table 6.

Thermodynamic parameters for CR adsorption onto LDH/CB

Parameter		Value
ΔG° (kJ/mol)	291 K	-9.04
	298 K	-9.26
	300 K	-9.32
	306 K	-9.51
	309 K	-9.60
	317 K	-9.85
ΔH°(kJ/mol)		-14
ΔS°(J/mol ⋅ K)		30.58

10

11

12

where T (K) is the temperature, and R (8.314 J/mol · K) is the molar gas constant.

The ΔG° values were consistently negative across the studied temperature range (− 9.04 to − 9.85 kJ/mol), confirming that the adsorption of Congo Red onto LDH/CB is a spontaneous process. Interestingly, the slight increase in the magnitude of negativity with rising temperature suggests that the process becomes slightly more favorable at higher temperatures.

The negative enthalpy change (ΔH° = −14 kJ/mol) indicates that the adsorption is exothermic, highlighting that the interaction between the dye molecules and the LDH/CB surface is driven mainly by electrostatic attractions and surface complexation, rather than purely physical forces.

The positive entropy change (ΔS° = 30.58 J/mol · K) reflects an increase in randomness at the solid–liquid interface during adsorption. This can be attributed to the displacement of water molecules initially adsorbed on the surface and structural rearrangements occurring as Congo Red molecules occupy active sites.

Taken together, these thermodynamic findings reveal that the adsorption mechanism of Congo Red on LDH/CB is spontaneous, exothermic, and associated with increased interfacial disorder, demonstrating the combined influence of enthalpic and entropic contributions.

Reusability study

The practical application of the LDH/CB adsorbent in wastewater treatment depends heavily on its ability to be reused efficiently. To address this requirement, we conducted five consecutive adsorption–desorption cycles to evaluate the performance of our adsorbent at an initial dye concentration of 50 mg/L, with ethanol as the eluent and stirring for 1 h (Fig. 10). Despite observing a gradual decrease in the efficiency of dye removal with each cycle, the adsorbent maintained a significant adsorption capacity of 84.7% after the fifth cycle. This reduction is likely due to some Congo Red molecules remaining tightly bound and not fully removed during desorption. Overall, these results confirm the material’s resilience and reinforce its promise for long-term dye removal applications.

Fig. 11.

Molecular interactions between LDH/CB composite and CR

The Fig. 11 schematic representation of the adsorption mechanism of Congo Red (CR) onto the LDH/CB composite. The LDH (Layered Double Hydroxide) exhibits a layered structure enriched with metal cations and hydroxyl groups, providing positively charged surfaces and hydrogen-bonding sites. The CB matrix (cuttlebone, mainly composed of calcium carbonate) contributes additional active sites through its oxygen-containing groups (CO₃²⁻ and –OH). The CR molecule, being an anionic dye, carries sulfonate groups (–SO₃⁻) and aromatic rings, which facilitate multiple interactions with the composite.Three main interactions govern the adsorption process: (i) hydrogen bonding, established between –OH/CO₃²⁻ groups of LDH/CB and the polar functionalities (–NH₂, –SO₃⁻) of CR; (ii) electrostatic attraction, resulting from the interaction between the positively charged LDH layers and the negatively charged sulfonate groups of CR; and (iii) π–π interactions between the aromatic rings of CR and the carbon-based domains of the composite.The synergistic effect of LDH and CB enhances adsorption: LDH provides the dominant positively charged sites, while CB improves stabilization through its carbonate functions and porous structure. Overall, the adsorption of CR is driven by a combined network of electrostatic interactions, hydrogen bonding, and π–π forces, which accounts for the high capacity and stability observed experimentally.

Conclusions

This study demonstrated that LDH/CB is a highly effective material for the removal of Congo Red dye from aqueous solutions, achieving a maximum adsorption capacity of 380 mg/g, which can be attributed to its abundant active sites and favorable structural properties. The dye adsorption remained high across a wide pH range (3–10), and the equilibrium data fitted well with the Langmuir and D–R isotherm models. Kinetic analysis revealed that the process followed the pseudo-first-order model, while the thermodynamic parameters confirmed that adsorption is both spontaneous and exothermic, driven primarily by electrostatic interactions. Furthermore, reusability tests showed that the material maintained a satisfactory adsorption performance after five consecutive adsorption–desorption cycles, with only a slight decrease in efficiency, confirming its stability and regenerability. Overall, these findings highlight the strong potential of LDH/CB as a promising candidate for the efficient and sustainable treatment of dye-contaminated wastewater, owing to its high capacity, structural stability, and recyclability.

Abbreviations

LDH

Layered Double Hydroxide

CR

Congo Red

CB

Cuttlebone

XRD

X-ray diffraction

FTIR

Fourier-transform infrared

SEM

Scanning electron microscopy

V(L)

The volume of the dye solution

m(mg)

The weight of LDH/CB

C₀ (mg/L)

The initial concentration of dye

C_e (mg/L)

The equilibrium concentration of dye

q_e (mg/g)

The adsorbed quantity at equilibrium

q_m (mg/g)

The maximum monolayer adsorption capacity

q_t (mg/g)

The time-resolved adsorption capacity

K_F ((mg/g)(L/mg) ^(1/n))

The Freundlich constant associated with adsorption capacity

K_L (L/mg)

The Langmuir adsorption constant

n

The Freundlich constant associated with adsorption intensity (sans unité)

k₁ (min^− 1)

The pseudo-first-order rate constant

k₂ (g/mg ⋅ min)

The pseudo-second-order rate constant

k_int (mg/g ⋅ min^0.5)

The intra-particle diffusion rate constant

a

The constant related to boundary layer thickness

T (K)

Temperature

t (min)

The reaction time

T_{d f} (K)

The final decomposition temperature

T_{d i} (K)

The initial decomposition temperature

∆G°

(kJ/mol) Gibbs free energy

∆H°

(kJ/mol) Enthalpy

∆S°

(J/mol ⋅ K) Entropy

R (8.314 J/mol ⋅ K)

The universal gas constant

Author contributions

RC performed the experimental work, and was a major contributor in writing the manuscript. RC, SB, ZA, BH analyzed and interpreted the results. All authors read and approved the final manuscript.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.