Preparation of phosphorus modified chitosan for selective adsorption of hexavalent chromium: Contribution of multiple hydrogen bonds

Adsorption remains a promising approach for the remediation of hexavalent chromium, Cr(VI), contaminations in both aqueous and soil environment, where adsorption performance, including capacity, affinity, and selectivity, is governed by the surface properties of the adsorbent. In this study, a phosphorus modified chitosan (PMC) adsorbent was synthesized via a facile reaction between chitosan and tetrakis(hydroxymethyl)phosphonium (THP), incorporating 5.5 wt% phosphorus as phosphonium groups. The resulting PMC exhibited superior Cr(VI) adsorption performance compared to pristine chitosan, demonstrating higher adsorption capacity (161.94 mg/g) and stronger affinity (Langmuir constant of 0.75 L/mg), enabling exceptional selectivity in the presence of competing anions of Cl − , NO 3 − , H 2 PO 4 − , and SO 4 2− . Despite its low regenerability (up to 50 % in the third cycle), PMC successfully applied in Cr(VI) contaminated soil remediation reaching fixation efficiency of 98.8 %. Detailed characterization via XPS, FTIR, and SEM-EDS revealed that Cr(VI) removal primarily occurs through ion exchange, with partial reduction (∼14 %) to Cr(III). Density functional theory (DFT) calculations further demonstrated that the adsorption mechanism is driven by strong electrostatic interactions facilitated by multiple hydrogen bonds between oxygen atoms in Cr(VI) anions and hydrogen atoms from both chitosan hydroxyl groups and THP moieties. These findings provide fundamental insights into the adsorption mechanism and offer valuable guidance for designing high-performance Cr(VI) adsorbents for practical applications in complex environmental matrices.