Amine-functionalized oyster shell waste for hexavalent chromium removal and phosphate recovery

Industrial wastewater contamination, particularly from hexavalent chromium [Cr(VI)] and phosphate (denoted PO₄ hereafter, collectively referring to the dissolved species H₂PO₄^-, HPO₄^2-, and PO₄^3-, whose relative proportions depend on solution pH), poses severe environmental risks and necessitates economical and sustainable remediation strategies. This study explores the potential to modify oyster shell particle (OSP) with amine-containing silane like N-[3-(trimethoxysilyl)propyl] ethylenediamine (AEAPS), for the simultaneous Cr(VI) and PO₄ removal. The AEAPS modification enhances the sorption capacity of OSP, with OSP-2.5AEAPS (OSP treated with 2.5 mL AEAPS) achieving maximum Cr(VI) and PO₄ sorption capacities of 22.19 and 110.23 mg·g^-1, respectively, at pH 5.5. Removal mechanisms involve electrostatic attraction, ligand exchange, Cr(VI) reduction, and Ca-P precipitation, as confirmed by Cr and phosphorus K-edge X-ray absorption near edge structure spectroscopy. Whereas Cr(VI) removal is primarily driven by electrostatic attraction between protonated amine groups and Cr(VI) oxyanions, followed by Cr(VI) reduction, leading to the formation of Cr(OH)₃ precipitates and Cr(III)-organic complexes, PO₄ removal is predominantly governed by Ca-P precipitation. Outperforming previously reported materials in both Cr(VI) and PO₄ removal, OSP-2.5AEAPS represents a promising biowaste-derived sorbent in which a low-cost oyster shell substrate is functionalized with only a small amount of chemical modifier. Moreover, calcium dissolution-induced structural alterations further enhanced PO₄ sorption efficiency and selectivity, emphasizing its potential for not only dual-anion removal but also PO₄ reclamation, contributing to circular economy approaches and sustainable resource management. This study highlights a waste-valorization-oriented sorbent that integrates a dual-function removal-recovery pathway, supporting sustainable wastewater management and circular economy principles.