Valence-Dependent Adsorption of Sb(III) and Sb(V) on Spinel MFe2O4 Ferrites: Spectroscopic Insights into Surface Hydroxyl and Metal–Oxygen Interactions

Antimony (Sb) contamination in water poses significant environmental and health risks due to its high toxicity, persistence and complex redox behavior. Magnetic spinel ferrites (MFe2O4) have shown promise for Sb removal; however, the intrinsic influence of divalent metal species (M2+) in regulating Sb(III)/Sb(V) adsorption performance and interfacial mechanisms remains poorly understood. In this study, MnFe2O4, ZnFe2O4 and NiFe2O4 nanoparticles were synthesized and systematically evaluated to elucidate how M2+ governs Sb immobilization behavior. Batch adsorption experiments revealed pronounced M–dependent selectivity. MnFe2O4 exhibited the highest Sb(III) adsorption capacity (229.89 mg·g−1), whereas NiFe2O4 showed superior affinity toward Sb(V) (up to 257.07 mg·g−1). Adsorption kinetics for both Sb species followed pseudo-second-order models, indicating chemically controlled processes. Isotherm analyses indicated predominantly monolayer complexation for Sb(III), while Sb(V) adsorption displayed mixed adsorption characteristics, reflecting surface heterogeneity. Mechanistic investigations based on FTIR and XPS analyses suggest that Sb(III) immobilization is dominated by inner-sphere complexation with surface Fe–O/Fe–OH groups, whereas Sb(V) adsorption involves synergistic coordination with both Fe–O and M–O (Mn–O/Ni–O) functional groups. XPS analysis of Sb-loaded ZnFe2O4 revealed the coexistence of Sb(III) and Sb(V) species after Sb(III) adsorption, indicating surface-confined partial oxidation; the extent of solution-phase conversion was not independently quantified. Therefore, the redox process is interpreted as an interfacial phenomenon rather than bulk oxidation in solution. These results clarify that M2+ species influence Sb removal behavior by modulating the reactivity of surface functional groups and interfacial redox characteristics, rather than merely altering adsorption capacity. This work provides spectroscopic insight into M-dependent structure–activity relationships in spinel ferrites and offers a theoretical basis for the rational design of magnetic adsorbents for selective and efficient Sb remediation.