Insight into the reactions of antimonite with manganese oxides: Synergistic effects of Mn(III) and oxygen vacancies.

Manganese oxides (MnO) are critical for determining the environmental behaviors and fate of antimonite (Sb(III)). However, little is known about the qualitative/quantitative connection between MnO structures and Sb(III) fate. Herein, the reactions of Sb(III) and six MnO with different structures were systematically investigated. The initial oxidation rates of Sb(III) (r) on six MnO decreased in the order of γ-MnO>δ-MnO>α-MnO>γ-MnOOH>MnO>β-MnO (pH=7.0), from 0.32 ± 0.04 to 11.17 ± 1.61 mmol/min/mol-Mn. The amounts of antimony retained (i.e., the sum of Sb(III) and antimonate (Sb(V))) on these MnO followed the same trend as that of oxidation. Oxidation of Sb(III) released Mn(II) and created more sites for adsorption. Outwardly, MnO with higher reduction potential (E) and specific surface area (SSA) favored faster Sb(III) oxidation. Inwardly, Mn(III) and oxygen vacancies (O) exhibited a synergistic effect on Sb(III) oxidation. Mn(III) can easier accept electron than Mn(IV) based on the change in Gibbs free energy calculation. O can adsorb free oxygen to form surface oxygen (O) which is much more reactive than lattice oxygen (O). Moreover, O is in close proximity to Mn(III) in high-valent MnO which facilitated the reactions between Sb(III) and Mn(III) through the enhancement of Sb(III) adsorption and electron transfer. O in low-valent MnO is adjacent to Mn(II), thus it showed weaker enhancement than that in high-valent MnO. Part of δ-MnO and almost all MnO were converted to γ-MnOOH during their reaction with Sb(III), while the other four MnO were barely changed. The results obtained provide mechanistic insight into the reactions occurring within Sb(III) and MnO, which are helpful for better understanding and prediction of the fate of Sb(III) in Mn-rich environments.