Mechanistic Elucidation on the Detoxification of Hexavalent Chromium Using a Magnetic "Anion Sponge" Fe₃O₄-pC₃N₄/PANI Composite with Redox Capability: Synergistic Design and Computational Study

Abstract

Water remediation of toxic hexavalent chromium (Cr(VI)) commonly involves singlefunction materials based on physical adsorption. In this research, Fe 3 O 4 -protonated C 3 N 4 /polyaniline (Fe 3 O 4 -pCN/PANI) magnetic composite was investigated for the removal of Cr(VI) from aqueous solutions by batch adsorption experiments and Monte Carlo simulations. As a result of hybridization of various components in the adsorbent material, a high Langmuir adsorption capacity (Q max = 971.88 mg.g -1 ), and a pseudosecond-order kinetic profile were recorded. The findings indicated that the composite's efficiency was the result of a multi-step pathway where electrostatic capture was followed by redox reduction, a process further confirmed by X-ray photoelectron spectroscopy (XPS), which evidenced the reduction of Cr(VI) to Cr(III) and its chelation, resulting in a stable system with significantly improved removal performance. A Monte Carlo (MC) simulation was performed to further understand the mechanisms of adsorption between adsorbates and Fe 3 O 4 -pCN/PANI adsorbent in an aqueous medium. As indicated by the negative adsorption energies, Cr(VI) species have strong affinity for the adsorbent surface, confirming the spontaneous and stable nature of the adsorption process. Further, since XPS analysis showed that adsorbed Cr(VI) was partially reduced to Cr(III), MC simulations were performed to examine Cr(III) adsorption behavior. As a result of these results, Fe 3 O 4 -pCN/PANI played a dual role in both Cr(VI) reduction and subsequent Cr(III) immobilization, further supporting Cr(III) adsorption stability.