Mechanistic elucidation of 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 single-function materials based on physical adsorption. In this research study, Fe₃O₄-protonated C₃N₄/polyaniline (Fe₃O₄–pCN/PANI) magnetic composite was investigated for the removal of Cr(VI) from aqueous solution by batch adsorption experiments and Monte Carlo simulations. As a result of the synergistic hybridization of its components, the Fe₃O₄–pCN/PANI composite exhibits a higher adsorption capacity than Fe₃O₄/pCN, PANI/pCN, and Fe₃O₄/PANI across all tested pH levels. It attains a maximum capacity of 79.02 mg g⁻¹ at pH 2, disclosing the strong synergy among the three components. Additionally, a high Langmuir adsorption capacity of 971.88 mg g⁻¹ and a pseudo-second-order kinetic profile are recorded. The findings indicate that Cr(VI) removal proceeds via a coupled adsorption–reduction pathway, where electrostatic capture is followed by redox reduction, as confirmed by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-mass spectrometry (ICP-MS) analyses. This indicates that detoxification is governed by the adsorption and reduction of Cr(VI) to Cr(III) and its chelation, resulting in a stable system with significantly improved removal performance. DFT calculations further confirm the thermodynamic favorability of the adsorption process and provide atomistic support for the dual adsorption–reduction mechanism. Regeneration experiments demonstrate the enhanced structural stability and regeneration capability of the prepared composite over multiple successive cycles, confirming its practical potential for real water remediation applications.