Oxidation of As（Ⅲ） by three types of manganese oxides and the effects ofpH, ion strength and tartaric acid on the oxidation were investigated by means of chemical analysis, equilibrium redox, X-ray diffraction （XR...Oxidation of As（Ⅲ） by three types of manganese oxides and the effects ofpH, 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, bimessite, 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, 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） of cryptomelane （824.2） 〉 bimessite （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 biruessite（or cryptomelane） are as follows： （1） at lower pH condition： （MnO2）x＋ H3AsO3 ＋ 0.5H^＋=0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋Mn〉^2＋ （MnO2）x-1 ＋ H2O; （2） at higher pH condition： （MnO2）x ＋ H3AsO3 = 0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋ 1.5H^＋ ＋ （MnO2）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 hausmarmite. The presence of tartaric acid promoted oxidation of As（Ⅲ） by birnessite. As for cryptomelane and hansmannite, 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.展开更多
Cobalt （Co） exists in significant quantities in naturally occurring manganese （Mn） oxides and alters the growth of Mn oxide crystals. Four-layered Mn oxides, Na-buserite （Na-bus） and three Co-doped Na-buserite s...Cobalt （Co） exists in significant quantities in naturally occurring manganese （Mn） oxides and alters the growth of Mn oxide crystals. Four-layered Mn oxides, Na-buserite （Na-bus） and three Co-doped Na-buserite samples prepared from oxidation of Mn（OH）2 with 5%, 10%, and 20% Co/（Mn ＋ Co） molar ratios （5Co-Na-bus, 10Co-Na-bus, and 20Co-Na-bus）, were used to prepare todorokite, a common Mn oxide on the Earth＇s surface, using Mg2＋/Co2＋ ions as a template. The results showed that todorokites could be obtained by reflux treatment of Mg2＋-exchanged non-doped Na-buserite and three Co-doped Na-buserites at atmospheric pressure. However, the formation of todorokites was prohibited by reflux treatment of Co2＋-exchanged Na-bus, 5Co-Na-bus, and 10Co-Na-bus samples. Instead, todorokite was obtained by the reflux treatment of Co2＋-exchanged 20Co-Na-bus samples under atmospheric pressure. X-ray photoelectron spectroscopy analysis showed that doped Co existed as Co3＋ in the MnOs layers of doped Na-buserites. The amount of substituted Co3＋ in the MnO6 layers may play a key role in the conversion of buserite to todorokite using Co2＋ ions as a template.展开更多
文摘Oxidation of As（Ⅲ） by three types of manganese oxides and the effects ofpH, 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, bimessite, 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, 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） of cryptomelane （824.2） 〉 bimessite （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 biruessite（or cryptomelane） are as follows： （1） at lower pH condition： （MnO2）x＋ H3AsO3 ＋ 0.5H^＋=0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋Mn〉^2＋ （MnO2）x-1 ＋ H2O; （2） at higher pH condition： （MnO2）x ＋ H3AsO3 = 0.5H2AsO4^- ＋ 0.5HAsO4^2- ＋ 1.5H^＋ ＋ （MnO2）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 hausmarmite. The presence of tartaric acid promoted oxidation of As（Ⅲ） by birnessite. As for cryptomelane and hansmannite, 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.
基金Supported by the National Natural Science Foundation of China(Nos.41001139 and 40771102)
文摘Cobalt （Co） exists in significant quantities in naturally occurring manganese （Mn） oxides and alters the growth of Mn oxide crystals. Four-layered Mn oxides, Na-buserite （Na-bus） and three Co-doped Na-buserite samples prepared from oxidation of Mn（OH）2 with 5%, 10%, and 20% Co/（Mn ＋ Co） molar ratios （5Co-Na-bus, 10Co-Na-bus, and 20Co-Na-bus）, were used to prepare todorokite, a common Mn oxide on the Earth＇s surface, using Mg2＋/Co2＋ ions as a template. The results showed that todorokites could be obtained by reflux treatment of Mg2＋-exchanged non-doped Na-buserite and three Co-doped Na-buserites at atmospheric pressure. However, the formation of todorokites was prohibited by reflux treatment of Co2＋-exchanged Na-bus, 5Co-Na-bus, and 10Co-Na-bus samples. Instead, todorokite was obtained by the reflux treatment of Co2＋-exchanged 20Co-Na-bus samples under atmospheric pressure. X-ray photoelectron spectroscopy analysis showed that doped Co existed as Co3＋ in the MnOs layers of doped Na-buserites. The amount of substituted Co3＋ in the MnO6 layers may play a key role in the conversion of buserite to todorokite using Co2＋ ions as a template.
