Arsenite oxidation by three types of manganese oxides

Article type: Research Article

Authors: FENG, Xiong-han | ZU, Yan-qun^; | TAN, Wen-feng | LIU, Fan

Affiliations: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China | College of Resources and Environment, Yunnan Agricultural University, Kunming 650201, China

Note: [] Corresponding author. E-mail: liufan@mail.hzau.edu.cn

Abstract: Oxidation of As(III) by three types of manganese oxides and the effects of pH, ion strength and tartaric acid on the oxidation were investigated by means of chemical analysis, equilibrium redox, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Three synthesized Mn oxide minerals, birnessite, cryptomelane, and hausmarnite, which widely occur in soil and sediments, could actively oxidize As(III) to As(V). However, their ability in As(III)-oxidation varied greatly depending on their structure, composition and surface properties. Tunnel structured cryptomelane exhibited the highest ability of As(III) oxidation, followed by the layer structured birnessite and the lower oxide hausmannite. The maximum amount of As(V) produced by the oxidation was in the order (mmol/kg) ofcryptomelane (824.2) > birnessite (480.4) > hausmannite (117.9). As pH increased from the very low value (pH 2.5), the amount of As(III) oxidized by the tested Mn oxides was firstly decreased, then negatively peaked in pH 3.0–6.5, and eventually increased remarkably. Oxidation of As(III) by the Mn oxides had a buffering effects on the pH variation in the solution. It is proposed that the oxidative reaction processes between As(III) and birnessite (or cryptomelane) are as follows: (1) at lower pH condition: (MnO_2)_x + H_3AsO_3 + 0.5H^+=0.5H_2AsO_4^-+0.5HAsO_4^{2-} +Mn^{2+}+(MnO_2)_{x-1} + H_2O; (2) at higher pH condition: (MnO_2)_x + H_3AsO_3 – 0.5H_2AsO_4+0.5HAsO_4^{2-} +1.5H^+ + (MnO_2)_{x-1}·MnO. With increase of ion strength, the As(III) oxidized by bimessite and cryptomelane decreased and was negatively correlated with ion strength. However, ion strength had little influence on As(III) oxidation by the hausmannite. The presence of tartaric acid promoted oxidation of As(III) by birnessite. As for cryptomelane and hausmannite, the same effect was observed when the concentration of tartaric acid was below 4 mmol/L, otherwise the oxidized As(III) decreased. These findings are of great significance in improving our understanding of As geochemical cycling and controlling As contamination.

Keywords: oxidation of As(III), As(V), Mn oxides, pH, ion strength, tartaric acid

Journal: Journal of Environmental Sciences, vol. 18, no. 2, pp. 292-298, 2006