New insights into adsorption of As(V) and Sb(V) from aqueous by HCO- (Fe3O4)x adsorbent: Adsorption behaviors, competition and mechanisms

Arsenic and antimony removal technology have received significant attention due to their considerable danger to aquatic ecosystems. In this study, we designed HCO-(Fe3O4)x adsorbents with substantial functional groups, ample pore spaces, and considerable surface area, specially tailored for the simultaneous adsorption of pentavalent arsenic (As(V)) and pentavalent antimony (Sb(V)). The adsorption performance of HCO-(Fe3O4)x was subjected variably to the Ce/Fe molar ratio (CFMR), solution pH, and co-existing anions (SO4 2 , PO4 3 , SiO3 2 ). The affinity order of As(V) and Sb(V) onto HCO-(Fe3O4)x in binary systems was pH-dependent. At a pH of 2.0, Sb (V) exhibited stronger adsorption than As(V), whereas the opposite trend occurred at pH levels above 4.0. Analysis of the kinetic data revealed the pseudo-second-order kinetic model was the most appropriate, indicating that chemical adsorption was the predominant mechanism. Notably, the formation of inner-sphere complexes led to a higher adsorption rate for As(V) compared to Sb(V), attributed to a lower energy barrier of 0.863 eV, facilitating the breakdown of the Fe-O bond in As(V)'s asymmetric tetrahedral structure. Various analyses (FTIR, XRD, and XPS) demonstrated that adsorption mechanisms involved multiple processes like hydrogen bonding, ligand exchange, electrostatic attraction, and complexation. The primary contributions for the adsorption of As(V) and Sb(V) were the inner-sphere complex of cerium, oxygen and arsenic atoms (Ce-O-As) and the outer-sphere complex of iron, oxygen and antimony atoms (Fe-O-Sb), respectively, indicating significant differences in the mechanisms. These findings underscored the potential of HCO-(Fe3O4)x in mitigating As(V) and Sb(V) water pollution, emphasizing its considerable promise as an effective remediation agent.