Effects of Mn average oxidation state on the oxidation behaviors of As(III) and Cr(III) by vernadite

Vernadite is a poorly crystalline phyllomanganate that widely distributed in natural environments, and plays a pivotal role in the geochemical transformations of heavy metal and other pollutants. Though many works have done about the reaction mechanisms between vernadite-like minerals and As(III)/Cr(III), the effects of some basic structure characteristics of the mineral, such as Mn average oxidation state (AOS), on the oxidation of As(III)/Cr(III) are not fully understood. In this study, vernadite samples with different Mn AOSs but almost the same particle sizes (Ver and Ver20) were synthesized, characterized, and their reactivities towards As(III)/Cr(III) oxidation and As(V)/Cr(VI) adsorption were compared. It is found that Ver and Ver20 have almost the same point of zero charge (PZC), but ver20 has increased contents of layer Mn(III) at particle edges and interlayer Mn(III) and Mn(II), thus greatly reduced Mn AOS. Decrease in Mn AOS greatly reduces the oxidation capacity and initial reaction rate (Kobs) of this mineral towards As(III) on mineral-water interfaces. The apparent As(III) oxidation amount and Kobs by Ver are 202 ± 8 mmol kg−1 and 0.3515 min−1 while that by Ver20 are 162 ± 7 mmol kg−1 and 0.0139 min−1. The oxidation capacity and Kobs for Cr(III) by these vernadie samples are also decreased but in a less extent compared to that in As(III) oxidation, e.g. Cr(III) oxidized by Ver and Ver20 are 2895 ± 47 and 1974 ± 109 mmol kg−1 and the Kobs are 0.0439 and 0.0169 min−1 respectively. Adsorption density of As(V) or Cr(VI) on Ver20 is ∼15-18% higher than that on Ver. These results suggest the more important role Mn AOS plays in As(III) oxidation than Cr(III) oxidation, and helps understand the geochemical behaviors of these toxic metals in natural environments mediated by Mn oxide minerals.