<p>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&#xa0;g, 25&#xa0;°C, 1&#xa0;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&#xa0;g⁻¹ and a total pore volume of 1.053 × 10⁻³&#xa0;cm³&#xa0;g⁻¹, while BJH pore size distribution identified a dominant pore diameter of 1.809&#xa0;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/<i>n</i> = 0.285). Thermodynamic analysis confirmed that the adsorption process was spontaneous (ΔG⁰ =  − 6.41&#xa0;kJ mol⁻¹) and exothermic (ΔH⁰ =  − 6.20&#xa0;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.</p>