Economic Design of a Novel Magnetic ZnO-Doped Biocomposite: An Integrated Advanced Ionic Theory and Statistical Physics Approach for Cr(VI) and Hg(II) Remediation

A previously unexplored magnetic biocomposite (CMC-HSDs/Fe3O4) was developed through the valorization of hydrophobic scleroprotein discards (HSDs). The synthesized material was evaluated for its efficacy in the adsorption of Cr(VI) and Hg(II) ions from contaminated aqueous systems. The physicochemical properties of the synthesized CMC-HSDs/Fe3O4 nanocomposite were characterized using XRD, FTIR, BET, TG/DTG, FESEM, EDX, and elemental mapping. Subsequently, a Box–Behnken experimental design was employed to model and optimize the adsorption process for Cr(VI) and Hg(II), focusing on the critical parameters of solution pH, adsorbent dosage, and interaction time. Kinetic data were best fitted to the pseudo-first-order (PFO) model. Equilibrium isotherm analysis revealed that Cr(VI) adsorption followed the Langmuir model, while Hg(II) adsorption was better fitted by the Freundlich model. Advanced ionic calculations elucidated a consistent multimolecular adsorption mechanism for both ions, characterized by temperature invariance and a preferential vertical geometry of the adsorbed species. Through a production cost of 25.56 USD/kg, the biosorbent demonstrates excellent reusability, retaining 88.60% efficiency for Cr(VI) and 85.69% for Hg(II) after five adsorption–desorption cycles. Based on a 50 mg/L influent concentration, projected treatment costs are ~$3.50/100 L for Cr(VI) and ~$1.22/100 L for Hg(II), underscoring the nanocomposite’s economic feasibility for industrial deployment in advanced tertiary wastewater remediation.