Arsenic adsorption on Mn3O4 surface: As(OH)3/AsO(OH)3 oxidation mechanism

Abstract

The adsorptive and oxidative properties of hausmannite (Mn3O4) are used to prevent the environmental impact of arsenic-rich tailings. Density Functional/plane-wave calculations have been performed to investigate the oxidation mechanism of arsenite to arsenate species on (001) hausmannite surfaces. The most stable cleavage plane is the (001) exposing Mn2+, Mn3+, and O2– ions in an asymmetric way. The arsenite, As(OH)3, adsorbs preferentially on Mn2+ in a monodentate-mononuclear form with a predicted adsorption energy of −34.2 kcal mol−1. The adsorption on Mn3+ adsorption sites is expected to be at least 6 kcal mol−1 higher in energy. The H2AsO4- species predominant in the environmental pH (pH = 4–8) prefer to adsorb on the Mn2+ adsorption sites in a bidentate binuclear mode with a predicted adsorption energy of –4.6 kcal mol−1, and the adsorption on Mn3+ is at least 4 kcal mol−1 higher in energy. The energy barrier for the As3+/As5+ oxidation on the Mn2+ adsorption sites is estimated to be at least 28 kcal mol−1 higher in energy than on the Mn3+ adsorption sites. Water plays a key role in the chemical reactivity of Mn3O4, promoting the oxidation of arsenite into arsenate through Mn(III)/Mn(II) reduction.