Green synthesis of Mohr’s salt–modified keratin composite for selective removal of arsenate from polluted water

Abstract

Groundwater arsenic (As) contamination remains a major global public health concern, driving the need for sustainable and effective remediation materials. In this work, an iron-impregnated biocomposite (MSMK) was developed by valorizing poultry-derived keratin and modifying it with Mohr’s salt ((NH₄)₂Fe(SO₄)₂·6H₂O), which provides a stable ferrous iron source and limits the rapid oxidation typical of conventional iron salts. Under optimized conditions (pH 6.0, dosage 0.2 g, 25 °C, 1 mg/L), MSMK achieved a superior As(V) removal efficiency of 98.5%. Mechanistic interpretation, supported by a point of zero charge (pH_PZC) of 7.6, reveals that peak performance is governed by the robust electrostatic attraction of H₂AsO₄⁻ to a densely protonated surface. Textural analysis showed a specific surface area of 2.357 m² g⁻¹ and a total pore volume of 1.053 × 10⁻³ cm³ g⁻¹, while BJH pore size distribution identified a dominant pore diameter of 1.809 nm, indicating a predominantly microporous structure favorable for solute diffusion. Adsorption kinetics followed the non-linear pseudo-first-order model (R_adj² = 0.9646), and the negligible intercept obtained from the intra-particle diffusion model (C =  − 0.0039) suggested pore diffusion as the main rate-controlling step. Analysis of the equilibrium adsorption results indicated that the Freundlich isotherm provided the best fit (R_adj² = 0.9654), indicating adsorption on a heterogeneous surface iron-based sites (1/n = 0.285). Thermodynamic analysis confirmed that the adsorption process was spontaneous (ΔG⁰ =  − 6.41 kJ mol⁻¹) and exothermic (ΔH⁰ =  − 6.20 kJ mol⁻¹). Notably, the stable chelation between iron-oxyhydroxide species and keratinous ligands resulted in iron leaching levels below detection limits (BDL), far surpassing WHO safety standards. These findings establish MSMK as a stable, selective, and environmentally benign platform for advanced aqueous As remediation.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-43190-8.

Introduction

Water serves as the indispensable cornerstone of global ecological health and socio-economic advancement. Despite its criticality, the widespread contamination of groundwater by arsenic (As) has evolved into a catastrophic environmental challenge, particularly within the hydrogeological basins of South Asia and parts of Latin America¹. In the Indian subcontinent, data from the Central Ground Water Board (CGWB) indicates that contamination is particularly acute in the Indo-Gangetic plains. Specifically, in the state of Punjab, districts such as Amritsar and Gurdaspur have reported localized As hotspots reaching as high as 2 to 5 mg/L, significantly exceeding the Bureau of Indian Standards (BIS 10500:2012) limit of 0.01 mg/L².

The mobilization of As arise from both geogenic and anthropogenic sources. Geogenically, it is released through the chemical weathering of minerals like arsenopyrite (FeAsS), realgar, and orpiment. These natural processes are further exacerbated by anthropogenic activities, including intensive mining, the discharge of industrial effluents, and the historic use of arsenical pesticides in agriculture³. Under typical aerobic conditions, As predominantly exists as arsenate [As(V)], occurring as negatively charged oxyanions (H₂AsO₄⁻ and HAsO₄^2-). Due to their high mobility and stable aqueous chemistry, these species present significant remediation challenges⁴. Previously reported remediation strategies have recently transitioned toward “waste-to-wealth” paradigms, focusing on the valorization of proteinaceous residues like poultry feather keratin. Keratin is chemically distinguished by its high density of amino (–NH₂) and sulfur-rich thiol (–SH) functional groups, which offer a high-potential matrix for pollutant sequestration⁵. However, the native architecture of keratin often lacks the specific electrostatic affinity or coordination geometry required for high-capacity As(V) removal, necessitating targeted surface-level engineering⁶.

Recent reports suggest that impregnation with iron (Fe) significantly enhances As binding through the formation of stable bidentate inner-sphere complexes⁷. While various Fe precursors like ferric chloride or ferrous sulfate have been explored, their practical application is often hindered by poor coating stability, significant iron leaching, and reduced efficiency at high pollutant concentrations (> 1 mg/L).

The novelty of this work lies in the strategic utilization of Mohr’s Salt as a dual-action iron precursor and stabilizing agent. Unlike conventional iron salts, Mohr’s salt provides a unique ammonium-buffered environment during synthesis, which prevents the rapid, localized acidification that typically causes uneven coating in biochar or sand-based systems. This allows for the development of a highly stable Mohr’s salt-modified keratin (MSMK) biocomposite. As summarized in Table 1, MSMK addresses the research gaps found in existing systems by optimizing the Fe distribution for high-concentration “hotspots,” ensuring 98.5% removal efficiency without the secondary contamination risks associated with weaker physical bonding.

Table 1.

Comparative analysis of iron-based precursors and biopolymer matrices for Arsenic remediation.

Adsorbent matrix	Fe precursor	Mechanism of action	Limitation/research gap	References
Fe-coated sand	Ferrous Sulfate (FeSO4)	Surface precipitation	Weak physical bonding; significant iron leaching in field conditions	8
Fe modified biochar	Ferric Chloride (FeCl3)	Surface complexation	Energy-intensive synthesis; localized acidity reduces coating uniformity	9
Fe-keratin fibers	Fe Nitrate (Fe(NO3)3)	Electrostatic attraction	Susceptible to pH fluctuations; limited capacity for As(V) at > 1 mg/L	10
MSMK	Mohr’s Salt	Bidentate inner-sphere complexation	Optimized for 1–4 mg/L stabilized via ammonium-buffering	Present study

By utilizing the ammonium-buffered environment of Mohr’s salt, this study achieves a more robust and uniform Fe distribution, facilitating stable bidentate inner-sphere complexation even at the hotspots reported by the CGWB in Punjab.

Materials and methods

Reagents

Chicken feathers were collected from local slaughter houses and processed as a source of keratin. Mohr’s salt [(NH₄)₂Fe(SO₄)₂0.6H₂O, (≥ 99% purity analytical grade) from Sigma-Aldrich (USA). A standard arsenate (As(V)) solution (1000 mg/L as As(V)) was obtained from Merck (Germany) and diluted as required to prepare working solutions. Additional reagents, including sodium hydroxide, (NaOH ≥ 98%, analytical grade), hydrochloric acid (HCl 37% v/v, analytical grade) and ethanol (C₂H₅OH, 95% v/v, analytical grade) were also supplied by Sigma-Aldrich (USA). Deionized water was used throughout the experiments for solution preparation and glassware cleaning.

Fabrication of keratin

Extraction of keratin from chicken feathers

Chicken feathers were collected from local slaughterhouses in Kharar, Mohali, and subjected to a sequence of pre-treatments involving thorough washing with deionized water, detergent soaking, and ethanol immersion to eliminate impurities and residual fats. After drying, the feathers were finely cut and processed for keratin extraction. The solubilized keratin was precipitated by acidifying the solution to its isoelectric point, and the resulting protein precipitate was collected by centrifugation. The recovered keratin was then freeze-dried to obtain a stable powdered form, which was stored in airtight containers for later modification^11,12.

Synthesis of Mohr’s salt-modified keratin composite

Keratin was functionalized with Mohr’s salt to obtain MSMK composite. In a typical synthesis, keratin powder was dispersed in an aqueous solution of Mohr’s salt, prepared at a keratin to salt weight ratio of approximately 1:4. The salt solution was acidified with dilute 0.05 M H₂SO₄ to stabilize Fe²⁺ ions, and the pH was maintained at 4 using dilute NaOH or HCL. The reaction mixture was maintained under continuous stirring for 5 h at room temperature with a loose covering to minimize oxidation, followed by an aging step for 12 h to facilitate uniform deposition of Fe hydroxides on the keratin matrix. After filtration, the suspension was washed several times with deionized water until the pH of the filtrate became neutral. An additional ethanol wash was included to assist in removing residual ions and to promote efficient drying, as reported in related keratin–Fe composites¹³. The final material was dried at 50 °C overnight and stored for further use. The keratin-to-Mohr’s salt mass ratio (1:4) was optimized through preliminary experiments. Lower ratios (1:1 and 1:2) resulted in insufficient Fe loading and reduced adsorption efficiency. The selected ratio ensures effective coordination between Fe ions and the intrinsic nitrogen- and sulfur-containing functional groups of keratin, thereby maximizing the density of active sites for As(V) removal.

Characterization

The morphological, physicochemical and structural properties of the synthesized MSMK were comprehensively characterized using multiple analytical techniques.

Surface morphology and elemental composition were examined using Scanning electron microscopy (SEM, JSM-7610F Plus, JEOL) coupled with an energy-dispersive X-ray spectroscopy (EDX) detector. For morphological analysis, samples were sputter-coated with a thin layer of gold (Au) to enhance conductivity and prevent surface charging during SEM imaging. The bulk elemental composition (C, N, and S) of the MSMK composite was determined using a Thermo Finnigan combustion analyzer. A sample weight of 2.407 mg was analyzed using the K-Factor calibration method to ensure quantitative accuracy. The components were identified via gas chromatography with specific retention times of 0.733 min for Nitrogen (N) and 8.425 min for sulfur (S). Elemental mapping was also performed to confirm the uniform distribution of Fe species within the keratin matrix. The samples were first dispersed in ethanol, then drop-cast onto a silicon substrate and dried before imaging.

Phase identification was conducted by powder X-ray diffraction (PXRD) using a Bruker D8 Advance diffractometer (Bruker Corporation, Germany) operated with Cu-Kα radiation (λ = 1.5406 Å) and a Ni filter. Data were collected with a step size of 0.008° and a step time of 1 s per step, over the 2θ range of 10°–80°. Functional group analysis was conducted using Fourier-transform infrared spectroscopy (FTIR, Spectrum Two, PerkinElmer) in the range of 4500–400 cm⁻¹, with a spectral resolution of 4 cm⁻¹ and 32 scans per sample. Powdered samples were directly analyzed in transmission mode without palletization or further chemical treatment.

The point of zero charge pH_pzc of the keratin-Fe composite was determined using the pH drift method. Briefly, 50 mL of 0.01 M NaCl solutions were prepared with initial pH (pH_i) values ranging from 2.0 to 10.0, adjusted using 0.1 M HCl or NaOH. To each solution, 0.1 g of the adsorbent was added and agitated for 24 h at 25 °C to reach equilibrium. The final pH (pH_f) was measured, and the pH_pzc was identified as the point where delta (add sign of delta triangle) pH (pH_f − pH_i) equaled zero.

Batch adsorption experiments

Adsorption studies were performed to evaluate the As(V) removal efficiency of MSMK. A defined amount of adsorbent (0.2 g/L) was introduced into 100 mL of an aqueous As(V) solution (2 mg/L), and the process was followed by mixing the solution at 120 rpm for 30 min using a magnetic stirrer (Remi Instruments, India) at ambient temperature, in line with established methodologies. The efficiency of the material was quantified by calculating the removal efficiency (R%) and equilibrium adsorption capacity (q_e) using the following expressions:

1

2

where C₀ and Ct (mg/L) are the initial and residual As(V) concentrations at time t, Ce (mg/L) represents the equilibrium concentration, V (L) is the solution volume, and m (g) is the mass of the adsorbent.

After treatment, suspensions were centrifuged and passed through a 0.45 µm membrane filter. The remaining As(V) content in the supernatant was analyzed using inductively coupled plasma mass spectrometry (ICP–MS, Agilent Technologies, USA) under standard operating conditions. All measurements were performed in duplicate, with reproducibility greater than 95%.

To comprehensively evaluate the performance of MSMK, batch adsorption experiments were designed by varying a single parameter while maintaining the others constant. The influence of adsorbent dosage was examined over the range of 0.1–1.0 g/L, contact time between 10 and 240 min, initial As(V) concentration from 0.5 to 10 mg/L, pH values ranging from 2 to 10, and temperatures between 20 and 45 °C. In addition, the influence of co-existing species was examined using eleven commonly occurring ions in natural waters (phosphate, silicate, bicarbonate, sulfate, nitrate, chloride, fluoride, carbonate, calcium, magnesium, and sodium). These ions were introduced in controlled concentrations using a multi-element calibration standard (Agilent Technologies, ICP–MS grade, CRM traceable) to simulate realistic groundwater chemistry¹⁴. These parameters are essential for optimizing the adsorption process and ensuring effective and reproducible removal performance, maximum utilization of the active sites on the MSMK surface. Adsorbent MSMK, being a surface-modified Fe-based material, is expected to interact with As(V) primarily through electrostatic attraction, ligand exchange, and surface complexation mechanisms. Understanding these interactions provides insight into the removal process and helps evaluate the potential of MSMK for real-world As remediation applications.

To understand the adsorption mechanism and determine the maximum loading capacity, the equilibrium data were fitted to the Langmuir, Freundlich and Temkin isotherm models. Adsorption kinetics were analyzed using Pseudo-first-order (PFO) and Pseudo-second-order (PSO) models, while the rate-limiting step was investigated through the Weber-Morris intra-particle diffusion model. For thermodynamic analysis, batch experiments were conducted at three different temperatures. The thermodynamic parameters, including Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated using the Van’t Hoff equation.

The leaching stability of the keratin-Fe composite was evaluated by agitating 0.2 g of the material in 100 mL of deionized water at various pH levels (3.0–9.0) for 24 h. The concentration of leached Fe in the filtered supernatant was determined via (Atomic Absorption Spectroscopy).

Regeneration and reusability procedure

To assess MSMK’s durability, adsorption–desorption cycles were conducted. After each cycle, spent MSMK was recovered, rinsed thoroughly with deionized water, and regenerated using a mild alkaline solution (0.1 M NaOH). Following regeneration, the adsorbent was neutralized, oven-dried at 50 °C, and reused under the same experimental conditions.

Green chemistry assessment

The process utilizes chicken feathers as a renewable keratin source, operates in an aqueous medium, and employs Mohr’s salt as a benign Fe precursor. Synthesis proceeds under mild conditions without high-temperature calcination, hazardous reagents, or organic solvents, thereby reducing energy use and emissions. The sustainability of the MSMK preparation was evaluated using standard green-chemistry indicators, including Atom Economy (AE)¹⁵ E-factor¹⁶, and Process Mass Intensity (PMI)¹⁷ was calculated using the following expressions (3–5):

3

4

5

These metrics were calculated using the actual quantities used in the synthesis: Keratin and Mohr’s salt were combined in a molar ratio of ~ 1:4 under aqueous conditions, pH adjustment to 4 using 1 N NaOH, 6 h stirring, 12 h ageing, and 3–4 aqueous washing cycles.

Comparative performance assessment

MSMK’s As(V) removal efficacy, operational pH flexibility, and regenerability were benchmarked against a variety of Fe-modified adsorbents, including nanoparticles, biopolymer composites, and carbon-based hybrids reported in the literature^18,19.

Result and discussion

Extraction of keratin from chicken feathers

The extraction yielded a clean, white keratin powder with no visible impurities. The product was dry, uniform, and suitable for immediate use in the modification step.

Synthesis of Mohr’s salt-modified keratin composite

After treatment with Mohr’s salt, the keratin changed from white to a peach-colour powder, confirming interaction between iron species and the protein. The final material appeared uniform, clean, and stable, indicating successful functionalization. Previous studies employing FeCl₃ or FeSO₄ for biomaterial modification have shown improved adsorption capacity toward As^20,21, however, the present work is the first to explore Mohr’s salt as a stable Fe2⁺ precursor for keratin functionalization as shown in Fig. 1.

Fig. 1.

Schematic illustration of the interaction between Mohr’s salt and keratin.

Characterization

Comprehensive characterization of both Native Keratin (NK) and MSMK was done to understand the physicochemical transformations that enhance their applicability in environmental remediation. After washing and drying, the MSMK composite was obtained as a dry powder with an overall material yield of approximately 85–90%.

Surface morphology and elemental characterization of MSMK

SEM and EDX analyses morphological and compositional transformation of keratin after modification with Mohr’s salt. The SEM image of NK (Fig. 2a) exhibited a compact and irregular surface with no apparent porosity, indicating a dense structure that restricts the diffusion of adsorbate ions and limits the accessibility of functional sites. The results are at par with previous reports where Bianco et al.²² found that keratin extracted from chicken feathers showed similar aggregated and dense morphologies under SEM. This morphological constraint explains the relatively low affinity of NK toward contaminant ions. The corresponding EDX spectrum (Fig. 2b) showed major peaks of carbon (C) (51.00%), oxygen (O) (39.00%), sulfur (S) (8.57%), and a trace of sodium (Na) (0.43%), consistent with the cysteine-rich protein composition of feather keratin. The similar elemental composition was also reported by Bianco et al.²² confirming the presence of sulfur and trace elements typical of feather-derived keratin.

Fig. 2.

SEM and EDX analyses of (a, b) native keratin (NK) and (c, d) Mohr’s salt modified keratin (MSMK), (e–i) Elemental mapping of MSMK displaying uniform distribution of C, N, O, S, and Fe across the keratin matrix.

Following modification, the SEM micrograph of MSMK (Fig. 2c) revealed a distinctly rougher, porous structure composed of irregular sub-micron particles (150–400 nm) aggregated into larger clusters. Numerous interconnected pores (200–600 nm) were visible, along with a few wider channels approaching 1 µm. The EDX profile (Fig. 2d) confirmed the successful incorporation of Fe (2.80%), along with a relative decrease in C and enrichment in O (39.43%) and S (14.16%). These compositional changes indicate homogeneous Fe²⁺/Fe³⁺ integration within the keratin matrix and partial exposure of thiol and sulfonic groups, both of which are beneficial for As(V) binding. To supplement surface EDX data, bulk elemental composition was quantified via CHNS combustion analysis. The MSMK composite yielded 10.30% N and 3.61% S. The high N content confirms the structural integrity of the keratin protein matrix, while the S percentage validates the presence of essential thiol and disulfide motifs required for stable iron coordination. The resulting porous, Fe-rich microstructure therefore validates the morphological transformation observed in SEM highlighted in red circle and the development of new active centers capable of strong chemisorption. Compared with other Fe-functionalized biopolymers such as Fe–chitosan²³, Fe–cellulose²⁴, and Fe–alginate composites²⁵, MSMK demonstrates superior surface uniformity and structural integrity due to the inherent S chemistry of keratin that favors Fe–S coordination and uniform metal dispersion²⁶. Elemental mapping analysis of MSMK was conducted to investigate the spatial distribution of the principal elements on the surface of the adsorbent. The maps clearly show a uniform dispersion of C, N, O, S, and Fe, confirming that the Fe species introduced during modification are homogeneously distributed across the keratin matrix as shown in Fig. 2e–i. This uniformity is crucial for ensuring consistent adsorption behavior and supports the effectiveness of the modification protocol in functionalizing the surface without creating localized or uneven deposits. A similar uniform Fe distribution has been reported in magnetically recyclable wool keratin–Fe₃O₄ composites, where keratin stabilized the Fe species²⁷.

Chemical composition analysis

FTIR spectroscopy was used to study structural changes in keratin before and after modification with Mohr’s salt. In NK, the amide I band was centered at 1630 cm⁻¹, consistent with C=O (Fig. 3a) stretching of the peptide backbone, which is at par to the previously reported keratin spectra^28,29. After modification, this band shifted to 1636 cm⁻¹, suggesting coordination between Fe ions and the carbonyl groups. A significant new absorption band appeared at 472 cm⁻¹, assigned to Fe–O bending vibration, while bands at 560 cm⁻¹ and 1007 cm⁻¹ (Fig. 3b), were assigned to Fe–O and C-O stretching affected by iron coordination^30,31. The presence of these peaks, alongside Fe-S stretching features, confirms successful iron incorporation into the keratinous matrix³².

Fig. 3.

FTIR spectra of (a) NK (b) MSMK XRD patterns of (c) NK and (d) MSMK.

Compared with Fe-functionalized polysaccharides that mainly exhibit Fe–O interactions^23,25. MSMK displays combined Fe–O and Fe–S features unique to keratin’s sulfur-rich composition. This dual coordination mechanism through sulfur and peptide groups creates chemically diverse adsorption sites, distinguishing MSMK from conventional Fe–biopolymers²⁶.

Crystalline phase analysis

X-ray diffraction (XRD) was employed to investigate the crystalline and structural variations in keratin prior to and following modification. The diffractogram of NK exhibited characterization reflections at 2θ = 9.5°, 19.29° and 23.4° were indexed to the (001), (020) and (210) (Fig. 3c), which are attributed to the α-helix and β-sheet^33,34 crystalline planes of the polypeptide chains, respectively. After modification, the MSMK pattern (Fig. 3d) largely retained its semi crystalline nature but revealed the emerge, the peaks appearing at 2θ = 21.2°, 33.2° and 36.6° were indexed to the (110), (130) and (111) lattice planes of goethite (α- FeOOH), respectively, in accordance with JCPDS Card No. 29-0713. These results confirm the successful incorporation of crystalline Fe-oxyhydroxide species within the keratin matrix. Compared with conventional Fe–chitosan or Fe–cellulose composites, MSMK maintains a hybrid structural arrangement combining the inherent resilience of the protein backbone with dispersed Fe-based domains. This unique configuration favors heterogeneous surface complexation and improved ion accessibility, enhancing its potential for sustainable As(V) removal applications³⁵.

Elemental quantification and proof of capture

To authenticate the successful capture of As(V) and validate the proposed adsorption mechanism, EDS analysis was conducted on the spent MSMK adsorbent. As quantified in Table 2, As was detected on the post-adsorption surface at a concentration of 0.67 ± 0.33 wt%, while it was absent in the pristine material. The simultaneous presence of Fe (1.35 wt%) and O (45.15 wt%) supports the role of Fe-oxyhydroxide crystalline domains as the primary active sites for As(V) binding. The observed elemental ratios suggest an efficient interaction between As(V) species and surface-bound Fe³⁶. These findings corroborate the Freundlich isotherm results, confirming that the heterogeneous surface of Fe-modified keratin effectively immobilizes As(V) through stable chemical interactions, resulting in a removal efficiency of 98.5%.

Table 2.

Elemental composition of MSMK after As(V) adsorption.

Element	Mass %	Atom %
C	24.74 ± 0.30	31.24 ± 0.39
N	19.94 ± 0.67	21.60 ± 0.73
O	45.15 ± 0.72	42.80 ± 0.69
S	8.15 ± 0.33	3.86 ± 0.16
Fe	1.35 ± 0.73	0.37 ± 0.20
As	0.67 ± 0.33	0.14 ± 0.07

Surface area and pore structural evaluation

MSMK shows a Type IV nitrogen adsorption–desorption isotherm, characterized by a distinct H3 hysteresis loop in the high relative pressure region (P/P_o > 0.4), as shown in Fig. 4a, a characteristic feature of porous adsorbents containing slit-shaped pores. The calculated BET specific surface area of 2.357 m²/g and a Langmuir surface area of 3.572 m²/g suggest that while the external surface area is modest, the material possess significant internal structural complexity.

Fig. 4.

Nitrogen gas sorption analysis for MSMK: (a) Adsorption–desorption isotherm (b) Pore size distribution curve calculated from the BJH desorption branch.

The pore architecture was further quantified using the BJH desorption method and t-plot analysis (Fig. 4b). An average pore diameter of 1.787 nm was obtained, indicating that MSMK falls in the mesoporous range at the critical boundary between microporous (< 2 nm) and narrow mesoporous frameworks. This specific pore size distribution is a key factor in the previously discussed kinetic behavior.

Specifically, the near-zero intercept (C = − 0.0039) observed in the Intra-particle Diffusion (IPD) model is now physically validated by this pore data; the narrow 1.787 nm channels provide a consistent resistance that confirms pore diffusion as the primary rate-controlling step during As(V) sequestration. Furthermore, the presence of these defined micropores (1.302 m²/g) explains the high R² fit for the Freundlich isotherm, as the heterogeneous distribution of Fe sites within these narrow channels creates the varied bonding energies required for favorable oxyanion adsorption. A summary of the results is presented in Table 3.

Table 3.

Textural properties and pore structure parameters of the MSMK composite.

Analysis method	Physicochemical parameter	Unit	Value
Multi-point BET	Specific surface area (SBET)	m2/g	2.357
	C constant	—	7.251
Langmuir	Surface area (SLang)	m2/g	3.572
t-plot method	Micropore area (S micro)	m2/g	1.302
	External surface area (Sext)	m2/g	1.055
BJH desorption	Total pore volume (V total)	cc/g	1.053 × 10−3
	Pore diameter (Dv (d))	nm	1.809
Average pore	Mean pore diameter	nm	1.787

Influence of operational parameters on the adsorption process on the surface of MSMK

In the following sections, the effects of individual operational parameters on the adsorption performance of MSMK are presented in detail.

The adsorption performance of MSMK toward As(V) was evaluated by altering the adsorbent dosage between 0.05 and 0.25 g (Fig. 5a). At the lowest dose (0.05 g), MSMK achieved 70% removal, which increased sharply to 98.5% at 0.2 g. Further increasing the dose to 0.25 g did not enhance the removal efficiency, which remained constant at 98.5%, indicating that the As(V) concentration in solution became the limiting factor and that the active sites were sufficiently saturated at 0.2 g. This plateau phenomenon aligns with typical adsorption saturation behavior reported for Fe-functionalized biopolymers^37,38. The superior performance of MSMK at lower doses results from its structurally enriched surface, unlike NK, which remained largely inactive across doses. Compared with other Fe-loaded adsorbents that often require higher doses to achieve comparable efficiencies³⁹, MSMK demonstrates superior dose efficiency, highlighting its capability as a high-performance, low-mass green adsorbent.

Fig. 5.

Effect of various factors on As(V) removal efficiency (a) Adsorbent dose (b) contact time (c) pH (d) temperature (e) Initial concentration (f) competitive ions.

The contact-time study showed that MSMK attains rapid uptake of As(V), rising from 43% at 30 min to a removal of 98.5% at 120 min, with a modest decline to 90% at 150 min, as shown in Fig. 5b. This behavior reflects a two-stage kinetic process: a fast-initial adsorption onto readily available surface sites, followed by a slower approach to equilibrium as remaining sites become harder to access and may be intraparticle diffusion limits mass transfer. The small efficiency drop at extended contact time is consistent with partial reversibility or redox-mediated re-equilibration seen in Fe-bearing sorbents and can be minimized by selecting an optimal contact time (120 min) that balances uptake and stability^37,40. MSMK’s unusually rapid initial uptake and high equilibrium removal distinguish it from conventionally prepared Fe-loaded keratin and simple Mohr’s salt treated keratin reported in the literature: traditional Fe impregnation often produces surface aggregates or poorly dispersed Fe phases that slow access to active sites, whereas Mohr’s salt-based in-situ modification used here promotes finely dispersed Fe domains and strong Fe–S/Fe–O interactions with the keratin matrix, yielding faster kinetics and improved retention^13,39. Moreover, compared with nanoparticulate Fe systems which may show fast uptake but suffer from aggregation and recovery issues, MSMK provides both rapid sorption and practical stability for reuse⁴¹. Collectively, these points justify 120 min as the operational optimum and demonstrate MSMK’s kinetic and practical advantages over other Fe-modified adsorbents.

The influence of pH on As(V) uptake by MSMK was evaluated between pH 2.0 and 10.0. Removal efficiency peaked at 98.5% at pH 6.0, but remained modest (35–50%) under strongly acidic conditions and declined significantly at alkaline pH (Fig. 5c). This behavior is fundamentally explained by the measured pH_pzc of 7.6, which governs the surface charge density. At the optimal pH of 6.0 (pH < pH_pzc), the MSMK surface is positively charged due to the protonation of amine and Fe-hydroxyl groups. This creates a strong electrostatic attraction for the dominant As(V) species at this range, H₂AsO₄⁻, facilitating the formation of stable inner-sphere complexes⁴². As the pH increases toward 10.0 (pH > pH_pzc), the surface undergoes deprotonation and becomes negatively charged. The resulting electrostatic repulsion between the surface and the HAsO₄²⁻ ions, combined with hydroxyl ion competition, leads to the observed decrease in removal efficiency⁴³. Unlike conventional Fe-biopolymers, MSMK’s unique mixed Fe–O/Fe–S chemistry and keratinous donor sites broaden its effective range and promote stronger binding at near-neutral pH, distinguishing it from standard Fe-impregnated biochars⁴⁴.

The influence of temperature on As(V) removal by MSMK was evaluated between 15 °C and 55 °C (Fig. 5d). Peak efficiency (98.5%) was achieved at 25 °C, with high stability maintained up to 45 °C (98.2%). A decrease in removal to 90% at 55 °C, which implies that adsorption proceeds via an exothermic mechanism, where elevated thermal energy potentially facilitates partial desorption or weakens the coordination bonds between As(V) and the Fe–OH active sites⁴⁵. This decline may also be attributed to thermal agitation disrupting the electrostatic interactions at the keratin-iron interface. The robust performance observed between 25 and 45 °C highlights the material’s suitability for ambient water treatment without requiring external energy input. Conversely, the NK showed a negligible thermal response, confirming that the temperature-sensitive active sites are specifically associated with the Fe-functionalized domains⁴⁶.

The initial concentration of As(V) strongly influences the adsorption behavior of Fe-modified sorbents. In this study, MSMK achieved an exceptional removal efficiency of 98.5% at 1 mg/L (Fig. 5e), indicating a high affinity between As(V) species and the Fe–O/Fe–S active sites as well as the keratin-derived functional groups under low contaminant loading. This behavior aligns with the high low-concentration uptake reported for Fe-based biomaterials due to abundant unoccupied active sites⁴⁷. As the initial As(V) concentration increased, competition for surface sites intensified, and the decline in removal efficiency reflected the progressive saturation of active sorption domains on MSMK, consistent with trends observed for Fe-functionalized biopolymer adsorbents⁴⁸. The observed decline in removal percentage at concentrations exceeding 4 mg/L is attributed to active site saturation, as the experimental uptake aligns closely with the theoretical maximum capacity (q_max = 1.585 mg/g) derived from the Langmuir model. This indicates an efficient utilization of available Fe-oxyhydroxide and keratin-derived functional groups. This performance surpasses conventional Fe-loaded keratin materials, which often show sharper efficiency losses beyond 2–3 mg/L due to weaker Fe anchoring or less stable composite structures⁴⁹. At higher concentrations, the increased mass-transfer driving force accelerates initial uptake but also hastens surface saturation and pore filling, explaining the more pronounced decline beyond 4 mg/L. The initial concentration range (1–4 mg/L) was selected to align with the maximum contamination levels reported in groundwater hotspots⁵⁰ of the Indo-Gangetic plains. Testing at these provides a realistic assessment of the adsorbent’s capacity to handle the peak As loads often found in localized rural sectors of Punjab.

The adsorption selectivity of MSMK was further assessed under competitive conditions, revealing clear preferential uptake behavior compared with other iron-loaded keratin materials. As shown in Fig. 5f, MSMK exhibited the highest removal for Fe (99%), followed by Se, As, and Al, while Pb and U were removed to a lesser extent, indicating that ions with higher charge density and stronger binding affinity for keratin’s carboxyl, amino, and disulfide groups as well as Fe2⁺/Fe3⁺ sites derived from Mohr’s salt dominate adsorption. This strong performance arises from the uniform, in-situ incorporation of Fe achieved through Mohr’s-salt modification, which produces finely dispersed Fe domains capable of forming robust Fe–O and Fe–S inner-sphere complexes. In contrast, previously reported Fe-loaded keratin systems often rely on simple surface impregnation, leading to heterogeneous Fe deposition, limited accessibility of reactive sites, and weaker complexation strength, ultimately reducing their selectivity in multi-ion environments⁵¹. The superior competitive performance of MSMK, particularly toward Fe, As, and Se, underscores the enhanced coordination capability and higher affinity site density achieved through its novel synthesis route, which distinguishes it from conventional keratin–Fe composites that commonly exhibit poor discrimination between competing ions and diminished efficiency in real wastewater matrices⁵². Overall, the selective binding behavior of MSMK highlights not only its strong interaction potential but also its structural advantages over earlier Fe-loaded keratin adsorbents, confirming its suitability for practical multi-contaminant treatment scenarios.

Adsorption isotherms, kinetics, and thermodynamics

The adsorption kinetics of As(V) on MSMK were evaluated by applying PFO, PSO, and intraparticle diffusion models to the experimental data.

The PFO non-linear model exhibited the highest correlation (R²_adj = 0.9646), with the calculated qe (0.5258 mg/g) closely matching experimental observations, as illustrated in Fig. 6a. This suggests that the adsorption rate is primarily governed by the availability of Fe-active sites on the keratin surface. While the PSO model (Fig. 6b) also showed relevance R²_adj = 0.8905, it slightly overpredicted the equilibrium capacity, indicating a complex pathway often observed in modified bio-polymers⁵³. To authenticate the transport mechanism, the IPD model was applied as shown in Fig. 6c, the plot yielded a diffusion rate constant (k_id) of 0.0402 mg/g min^−0.5. Notably, the intercept (C) was found to be − 0.0039. In adsorption theory, an intercept value close to zero mathematically confirms that IPD is the primary rate-limiting step, with negligible boundary layer resistance⁵⁴. This indicates that the Fe-modification successfully enhanced the accessibility of internal pores for As(V) penetration. The comprehensive statistical data for these models are summarized in Table 4.

Fig. 6.

Kinetic and isotherm analyses for As(V) adsorption on MSMK: (a) non-linear PFO kinetic fit; (b) linear PSO kinetic fit; (c) non-linear IPD model; (d) Langmuir and Freundlich isotherm models (non-linear form); and (e) linear Temkin isotherm representing adsorption heat.

Table 4.

Comparison of kinetic, isotherm, and thermodynamic models applied to As(V) adsorption onto MSMK, along with fitted parameters, adjusted R², and χ² values.

Kinetic model	Equation used	Parameters	Statistical fit (R2 adj)
a. Kinetic models
PFO (Non-Linear)	)	K1 = 0.0154	0.9646 (Best Fit)
		Qe = 0.5258
PSO (Linear)	h =	K2 = 0.0196	0.9178
		Qe = 0.7005
		h = 0.096
IPD (Non-Linear)	+ C	kid = 0.042	0.8715
		C = − 0.0039
b. Isotherm model
Langmuir (Non-Linear)		Qmax = 1.585 ± 0.317	0.868
		RL = 0.009 ± 0.090
		KL = 10.204 ± 9.668
		χ2 = 0.0162
Freundlich (Non-Linear)		Kf = 1.614 ± 0.14	0.938 (Best Fit)
		1/n = 0.323 ± 0.064
		χ2 = 0.0075
		n = 3.09
Temkin (Linear)		B = 0.248 ± 0.063	0.831
		AT = 372.4
		χ2 = 0.0208
c. Thermodynamic
Van’t Hoff (Linear)	ΔG° = ΔH°—TΔS°	ΔH° = − 6.20
		ΔS° = 0.687
		ΔG° = -6.41

The adsorption equilibrium of As(V) onto MSMK was evaluated using Langmuir, Freundlich (Fig. 6d) and Temkin isotherm models (Fig. 6e), as summarized in Table 4, the experimental data were most accurately described by the Freundlich model (R²_adj = 0.938; χ² = 0.0075), indicating a heterogeneous surface mechanism. The favorable nature of the process (n = 3.096) is consistent with Fe-modified biopolymers previously reported for As(V) sequestration⁵⁵. Furthermore, the rigorous application of error functions like χ² ensures the statistical reliability of the model selection.

Thermodynamic evaluation of As(V) uptake by MSMK was performed based on the Van’t Hoff relationship. The obtained negative ΔG° value (− 6.41 kJ mol⁻¹) demonstrates the spontaneous and energetically favorable nature of the adsorption process. The negative enthalpy change (ΔH° = − 6.20 kJ/mol) indicates an exothermic mechanism, suggesting that adsorption is favored at lower temperatures. Furthermore, the positive entropy (ΔS° = 0.687 J/mol K)⁵⁶ reflects a slight increase in disorder at the solid–liquid interface⁵⁷. All calculated parameters and statistical fits are summarized in Table 4.

Q_e denotes the equilibrium adsorption capacity (mg g⁻¹), Q_max represents the maximum monolayer adsorption capacity (mg g⁻¹), k₁ is the pseudo-first-order rate constant (min⁻¹), and k₂ corresponds to the pseudo-second-order rate constant (g mg⁻¹ min⁻¹). K_L is the Langmuir equilibrium constant (L mg⁻¹), while R^L refers to the Langmuir separation factor (dimensionless). K_f indicates the Freundlich adsorption capacity (mg g⁻¹) (L mg⁻¹) ¹⁄ⁿ, and 1/n is the Freundlich intensity parameter (dimensionless). Ce represents the equilibrium concentration of As(V) in solution (mg L⁻¹), t denotes the contact time (min), χ² is the chi-square error function (dimensionless), and R² represents the correlation coefficient (dimensionless).

Proposed mechanism of surface adsorption

As(V) adsorption onto Mohr’s salt–modified keratin (MSMK) is suggested to occur predominantly through inner-sphere surface complexation involving As(V) species and Fe hydroxyl functional groups formed during the modification of keratin. Upon partial oxidation of Fe²⁺ to Fe³⁺, the MSMK surface becomes enriched with reactive ≡Fe–OH groups, which serve as primary coordination sites for As(V) binding. At pH 6, the predominant As(V) species (H₂AsO₄⁻) undergoes ligand-exchange with surface ≡Fe–OH groups, leading to the formation of stable inner-sphere ≡Fe–O–As complexes as shown in Eq. (2)⁵⁸.

6

The suggested adsorption mechanism agrees with the observed pH-dependent behavior, as increased proton competition under acidic conditions reduces As(V) uptake, while increased surface deprotonation and the associated electrostatic repulsion at higher pH reduce uptake of HAsO₄²⁻. The strong affinity of MSMK for As(V) therefore arises from the combined presence of abundant Fe hydroxyl sites and keratin-derived functional groups, both of which support inner-sphere coordination. Compared with previously reported Fe-loaded keratin and biopolymer systems, MSMK offers more uniformly distributed Fe active sites, strengthening As(V) complexation and enhancing overall adsorption efficiency⁵⁹.

Efficiency and regeneration

The desorption efficiency and reusability of MSMK were evaluated over five consecutive adsorption–desorption cycles. In the first regeneration cycle, MSMK exhibited a desorption efficiency of 98.5%, demonstrating that the majority of surface-bound As(V) could be effectively released without significant structural deterioration. With repeated cycling, a gradual decline in adsorption capacity was observed, retaining approximately 90% in the second cycle, 83% in the third, 75% in the fourth, and 63% after the fifth cycle (Fig. 7). This progressive reduction is attributed to partial blockage or irreversible occupation of Fe–O active sites, minor leaching of Fe²⁺/Fe³⁺ species, and possible structural relaxation of the keratin matrix, a trend consistent with previous findings on Fe-loaded biopolymer adsorbents⁶⁰.

Fig. 7.

Adsorption–desorption performance of MSMK over five consecutive cycles, demonstrating the material’s reusability and stability.

Despite this decline, MSMK demonstrates superior recyclability compared with earlier Fe-modified keratin and other iron-functionalized natural adsorbents, which typically retain only 40–60% adsorption efficiency after 4–5 cycles⁶¹. The improved reusability of MSMK likely arises from stronger Fe–O–As inner-sphere complexation and better anchoring of Fe species within the keratin network during Mohr’s salt modification, which enhances stability relative to conventional Fe-keratin and Fe–chitosan systems⁶².

Collectively, the results indicate that MSMK exhibits high structural robustness and favorable reusability, supporting its suitability as an economical and sustainable material for multiple As(V) adsorption cycles.

Stability and environmental safety

The structural integrity and chemical stability of the MSMK composite were evaluated to ensure the prevention of secondary metallic contamination. Following adsorption experiments across a broad pH spectrum (3.0–9.0), residual Fe levels in the treated effluent were quantified. The analysis confirmed that Fe concentrations remained consistently below the instrument’s detection limit, signifying negligible leaching of the Mohr’s salt-derived phases. This exceptional stability is attributed to the formation of strong coordinate covalent bonds between the Fe centers (Fe²⁺)⁶³ and the abundant N and S-containing ligands (amine and thiol groups) within the keratin matrix. Such stability ensures that the treated water complies with WHO drinking water standards, which mandate Fe levels below 0.3 mg/L to avoid aesthetic concerns⁶⁴.

Green chemistry assessment of MSMK synthesis

Product recovery (85–90%) and total solvent usage were incorporated into mass-balance calculations. The numerical outcomes of these indicators, summarized in Table 5, provide quantitative support for the environmental compatibility of the MSMK synthesis. Overall, the results demonstrate that the process aligns with green-chemistry principles by generating minimal solid waste, using benign aqueous media, and achieving favourable mass utilization efficiency compared with reported biopolymer–metal salt systems.

Table 5.

Summary of Green Chemistry Indicators for MSMK Synthesis.

Category	Indicator	Basis of calculations	Calculated results	Interpretation
Material Efficiency	Atom Economy (AE)	Fe incorporated from Mohr’s salt	44–47%	Typical for metal–biopolymer composites
Waste Generation	E-Factor (solids only)	Solid waste/product	1.0–1.4 g	Low solid waste generation
	Solvent-Inclusive E-Factor	Liquid + solid waste/product	12–15 g	Dominated by aqueous rinses
Mass Utilization	PMI (solids only)	Total input mass/product	13–16 g	Efficient use of benign reagents
	Solvent-Inclusive (incl.) PMI	Total mass incl. water/product	100–120 g	Typical for water-based materials; reducible

As summarized in Table 5, the MSMK synthesis demonstrates good alignment with green-chemistry criteria. The low solid-phase E-factor, moderate atom economy, and acceptable PMI values collectively indicate efficient use of reagents and minimal environmental burden.

Comparisons with other adsorbents

To demonstrate MSMK’s performance in practical water matrices, Table 6 compares its As(V) removal under competitive ion conditions with (i) previously reported Fe-loaded keratin adsorbents and (ii) representative Fe-modified biopolymer and engineered iron adsorbents. Primary comparisons with keratin-based materials highlight the synthesis-driven advantages of in-situ Mohr’s-salt modification (uniform Fe dispersion, Fe–S/Fe–O coordination), while secondary comparisons show MSMK’s competitive edge in dose efficiency and selectivity versus other Fe-based systems.

Table 6.

Comparative performance of MSMK and selected iron-based adsorbents.

A. MSMK vs Iron-loaded keratin adsorbents (direct comparison group)
Adsorbent	Fe-loading method	qmax (mg/g)	Removal with competing ions	Optimal dose/ contact time	Regeneration	Key advantage/drawback	References
MSMK- this study	In-situ Mohr’s-salt (Fe2+) treatment of feather keratin → controlled oxidation/anchoring (uniform Fe dispersion; Fe–S formation)	qmax = 1.62 mg/g	68.2% (mixed 12 ions; each 1–5 mg L−1; same dose/pH)	0.2 g; 120 min	63% retained after 5 cycles (0.1 M HCl regen)	Low optimal dose; sulfur-rich keratin enables Fe–S + Fe–O coordination; good cycling	–
Fe-keratin	Conventional Fe impregnation onto extracted keratin (surface loading)	qmax = 0.98 mg/g	Reported declines under multicomponent tests (authors report ~ 40–70% depending on competitor)	Dose and time vary by study; generally higher doses than MSMK reported	Reported drop to moderate retention after a few cycles (varies with eluent)	Simple impregnation → heterogeneous Fe deposition; some Fe leaching noted	49
Fe (III)-crosslinked keratin	Chemical crosslinking with Fe (III) to stabilize protein network and immobilize Fe	qmax = 1.25 mg/g	Moderate loss with competing ions; retention mid-range after regeneration (~ 50–75% reported)	Typically, moderate dose; contact times comparable (60–180 min)	Variable retention; regeneration reported adequate but declines with cycles	Crosslinking improves Fe anchoring but can reduce site accessibility	61
GA-gel / Fe-loaded feather keratin	Gelation (GA) + in-situ Fe loading within keratin gel network	qmax = 1.40 mg/g	Authors reported sensitivity to competitive anions (phosphate reduces uptake significantly)	Gel form —dosing differs; often lower mass per volume but different handling	Regeneration discussed; moderate stability	Good dispersion but gel form may complicate scaling and filtration	65
Keratin-Fe variants	Various: impregnation, crosslinking, in-situ precipitation	qmax = 1.30 mg/g	With mixed ions removal commonly drops to ~ 40–75% depending on ions/conc	Reported optimal doses vary widely (often higher than MSMK)	Regeneration outcomes vary; many reports progressive loss over cycles	Heterogeneous methods; many lack sulfur-stabilized Fe centers—prone to leaching / aggregation	26

B. MSMK vs. Other Iron based biopolymer/engineered adsorbents
Adsorbent	Max % As(V) removal, key conditions	% removal with competing ions	Limitations reported	References
Fe oxide nanoparticles/magnetite	> 90% in many controlled tests (q varies widely)	Often ~ 40–60% remaining in the presence of strong competitors (PO43−, CO32−); depends on conc.	High capacity, but aggregation, recovery & secondary release are concerns	26
Chitosan-coated bentonite	~ 91% under optimized lab conditions (As₀, dose per paper)	~ 40–50% in presence of Ca2+/Mg2+/HCO₃− typical of groundwater	Polysaccharide matrix; relies on oxygen/amine donors	66
Zr- or Zr–Fe loaded biochar—recent studies (Zhou/Liao/ others) 85–95% (single-ion under	85–95% (single-ion under ideal tests)	Strong decrease with PO43−/F−/CO32−; reported 40–60% depending on conc.	Highly selective for oxyanions, but phosphate competes strongly	67
La-impregnated activated alumina	~ 90% under ideal conditions	Retains ~ 40–60% with complex anions (carbonate/phosphate) per reports	Effective, but carbonate/phosphate reduces performance	68
Magnetic Fe₃O₄/graphene oxide composites	> 90% in controlled tests	~ 40–60% with phosphate/carbonate/bicarbonate; performance depends on GO functionalization	Fast kinetics & magnetic recovery; moderate anti-interference performance	69

The comparative data in Table 6A and B reveal that while many engineered adsorbents, such as Zr-loaded biochars and Magnetic GO composites, achieve high removal in single-ion systems, their efficiency is significantly compromised (dropping to 40–60%) in the presence of competing anions like phosphate and carbonate. In contrast, MSMK maintains a superior removal efficiency of 68.2% even within a highly complex 12-ion multicomponent matrix.

This enhanced selectivity constitutes the primary scientific novelty of this work. Unlike traditional Fe-loading methods that rely on surface precipitation or weak physical adsorption, the use of Mohr’s salt facilitates an in-situ coordination with the sulfur-rich protein framework of keratin. This results in the formation of stable Fe–S and Fe–O–C bonds, which provide specific ‘locking’ sites for As(V) oxyanions. Furthermore, the low-temperature synthesis (50 °C) of MSMK offers a more sustainable, green alternative to the high-temperature pyrolysis (> 700 °C) required for biochar systems, without sacrificing the structural integrity of the active functional groups.

Conclusion

A novel Mohr’s salt–modified keratin (MSMK) adsorbent was successfully developed, demonstrating a superior As(V) removal efficiency of 98.5% under optimized conditions (pH 6.0, 25 °C). The material’s performance is fundamentally governed by a point of zero charge (pH_PZC) of 7.6, which facilitates robust electrostatic attraction between the protonated surface and H₂AsO₄⁻ species at near-neutral pH. Structural and chemical characterization verified that the integration of crystalline Goethite (α-FeOOH) domains into the keratin matrix generated abundant ≡Fe–OH active sites, enabling selective inner-sphere complexation. Textural analysis revealed a predominantly microporous structure that dictates the adsorption kinetics, which aligned with the non-linear PFO model (R²_adj = 0.9646). The negligible intercept in the intra-particle diffusion model confirmed pore diffusion as the primary rate-controlling step. Equilibrium data followed the Freundlich isotherm (R²_adj = 0.938), while thermodynamic evaluation established the process as spontaneous (ΔG° = − 6.41 kJ/mol) and exothermic. MSMK exhibited strong tolerance toward competing ions and maintained over 60% efficiency after five regeneration cycles. Crucially, the coordination between Fe and the keratinous sulfur/nitrogen ligands ensured Fe leaching remained below detection limits (BDL), confirming the material’s environmental safety in accordance with WHO standards. Overall, the synergistic interaction between keratin-derived functional groups and Fe species renders MSMK a robust, environmentally benign, and scalable solution for As(V) remediation. Future efforts focusing on fixed-bed column studies and pilot-scale evaluations will be essential to transition this sustainable biocomposite from laboratory research to practical field applications.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Manju : Conceptualization, methodology, investigation, data curation, writing – original draft.Swati Sharma: Supervision, investigation, writing—review and editing.

Data availability

The datasets generated and analyzed during the current study, including raw data, processed results, and supporting experimental records, are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.