The oxidation and sorption mechanism of Sb on Δ-MnO₂

Manganese (Mn) oxide is considered as a potential regulator of the fate of many contaminants in soil due to its abundance and reactivity. The present study investigated the oxidation and sorption mechanisms of antimony (Sb) on δ-MnO₂, by combining batch equilibrium experiments, kinetic experiments and various spectroscopic techniques. The collective results confirmed the high Sb(III)-oxidizing capacity of Mn(IV) sites on δ-MnO₂ and the oxidation reaction was proton-driven. However, the oxidation process was gradually inhibited by the accumulation of manganese antimonate precipitates, the production of less reactive Mn(III), and the site-blocking effects of products Mn(II) and Sb(V) on the surface. Different from the direct sorption of Sb(V) by δ-MnO₂, the oxidation of Sb(III) destroyed the surface structure of δ-MnO₂ and exposed more reactive sites as evidenced by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The stronger Sb(V) sorption along with the oxidation of Sb(III), therefore, could be attributed to several binding modes, i.e., Mn(II) cation bridge, precipitation with Mn(II), along with inner-sphere complexations with surface Mn(IV) and Mn(III) sites. Sb(V) was sorbed at the edge sites of δ-MnO₂ forming monodentate mononuclear complex with a configuration of Mn-O(H)-Sb(V) as confirmed by supplementary X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (EXAFS) spectroscopy and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy techniques. The findings altogether help to contribute to a better understanding on the geochemical dynamics of Sb with δ-MnO₂ in subsurface environment.