Oxidation of As(Ⅲ) by three types of manganese oxides and the e ffects of pH, ion strength and tartaric acid on the oxi- dation were investiga ted by means of chemical analysis, equilibrium redox, X-ray diffraction(XRD) and transmission electron microscopy(TEM). Three synthesized Mn oxide minerals, bir nessite, cryptomelane, and hausmannite, 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, fol- lowed by the layer structured birnessite and the lower oxid e hausmannite. The maximum amount of As(V) produced by the oxidation was in the order(mmol/kg) of cryptomelane(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 th at the oxidative reaction processes between As(Ⅲ) and birnessite(or cryptomel ane) are as follows: (1) at lower pH condition: (MnO2)x + H3AsO3 + 0.5H+ 0 .5H2AsO4- + 0.5HAsO42- +Mn2++ (MnO2)x-1 + H2O; (2) at higher pH condition: (MnO2 )x + H3AsO3 0.5H2AsO4- + 0.5HAsO42- + 1.5H+ + (MnO2)x-1·MnO. With increa se of ion strength, the As(Ⅲ) oxidized by birnessite and cryptomelane decreased and was negatively correlated with ion strength. However, ion strength had litt le influence on As (Ⅲ) oxid- ation by the hausmannite. The presence of tart aric acid promoted oxidation of As(Ⅲ) by birnessite. As for cryptomelane and haus- mannite, the same effect was observed when the concentration of tartar ic acid was below 4 mmol/L, otherwise the oxidized As(Ⅲ) decreased. These findings are of great significance in improving our understanding of As geochem
Oxidation of AsⅢ by three types of manganese oxide minerals affected by goethite was investigated by chemical analysis, equilibrium redox, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Three synthesized Mn oxide minerals of different types, birnessite, todorokite, and hausmannite, could actively oxidize AsⅢ to AsⅤ, and greatly varied in their oxidation ability. Layer structured birnessite exhibited the highest capacity of AsⅢ oxidation, followed by the tunnel structured todorokite. Lower oxide hausmannite possessed much low capacity of AsⅢ oxidation, and released more Mn2+ than birnessite and todorokite during the oxidation. The maximum amount of AsⅤ produced during the oxidation of AsⅢ by Mn oxide minerals was in the order: birnessite (480.4 mmol/kg) > todorokite (279.6 mmol/kg) > hausmannite (117.9 mmol/kg). The oxidation capacity of the Mn oxide minerals was found to be relative to the composition, crystallinity, and surface properties. In the presence of goethite oxidation of AsⅢ by Mn oxide minerals increased, with maximum amounts of AsⅤ being 651.0 mmol/kg for birnessite, 332.3 mmol/kg for todorokite and 159.4 mmol/kg for hausmannite. Goethite promoted AsⅢ oxidation on the surface of Mn oxide minerals through adsorption of the AsⅤ produced, incurring the decrease of AsⅤ concentration in solutions. Thus, the combined effects of the oxidation (by Mn oxide minerals)-adsorption (by goethite) lead to rapid oxidation and immobilization of As in soils and sediments and alleviation of the AsⅢ toxicity in the environments.
FENG XionghanTAN WenfengLIU FanHuada Daniel RUANHE Jizheng
Todorokite commonly occurs in Earth surface environments. The factors governing formation of todorokite, such as reaction temperature, metal ions, dissolved O2 and pH, were investigated in this paper. Results showed that the forming rate of todorokite and its crystallinity decreased with falling reaction temperature, and the effect of temperature was more significant than that of other parameters. Nature of metal ions in the interlayer of buserite precursor and the structure of the buserite precursor obviously affected buserite transformation into todorokite. Weak bonding between the metal ions and MnO6 layer of buserite was favorable to todorokite formation. The rate of todorokite formation was promoted at a lower temperature with appropriate bubbling of O2. The pH in the system slightly influenced the todorokite formation, and todorokite could also be formed in a weak alkali medium or in a slightly acidic medium. Aged buserite pre-cursor more easily form todorokite than the unaged one.
CUI Haojie1, FENG Xionghan1, LIU Fan1, TAN Wenfeng1 & HE Jizheng2 1. College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
