| Manganese oxide is known to oxidize the variable valence elements to change their soil environmental behavior,such as Cr.Although many different manganese oxides have been shown to oxidize Cr(Ⅲ),the most common oxides are in high valence,and the main purpose is for thermodynamic properties and valence change of Cr.Despite considerable research,the mechanism of dynamic process,intermediate products and electron transformation are still unclear.Some recent studies have proven that Mn(Ⅲ) producing during the process of generating biogenic manganese oxides is an important source of Mn(Ⅲ) in environment,and it is possibly that Mn(Ⅲ) has some more crucial environmental impact for its high activity.Consequently,Mn(Ⅲ) appearing during the oxidation by mixed valence manganese oxides may also play an important role in nature, yet the detailed mechanism remains incompletely understood.For explaining the environmental behavior of Cr,it is significant to conduct extensive investigation of Cr(Ⅲ) oxidation by mixed valence manganese oxides to make clear of the dynamic process, reaction pathway,oxidation mechanism,the vital role Mn(Ⅲ) might play and its environmental impact.In this paper we conducted experiments to discuss the ideal reaction condition and give out evidence that strongly implicates Mn(Ⅲ) in the oxidation of Cr(Ⅲ) by birnessite synthesized at high pH.1,Mn(Ⅲ) affected the oxidation fate of Cr(Ⅲ) in a distinct way,and the leading part of it was not that remanined in the birnessite,but that presented after Mn(Ⅳ)→Mn(Ⅲ) electron transforming.It is also acceptable to conclude that the rate of reaction could be limited by the step of electron transform from Mn(Ⅳ) to Mn(Ⅲ).2,The birnessites which had been treated with Na4P2O7 would provide more adsorbable sites for Cr(Ⅲ),and there was only Mn(Ⅳ) took part in the oxidation once Mn(Ⅲ) appeared during the reaction was complexed by Na4P2O7 in solution.Meanwhile,the Mn realease amount demonstrated that Na4P2O7 in solution could inhibit the electron transform from Mn(Ⅲ) to Mn(Ⅱ),but it also promoted the electron transformation process of Mn(Ⅳ)→Mn(Ⅲ). 3,The two reaction pathways used to discuss the initial Kobs,the first of these pathways was postulated to be sensitive to Mn(Ⅲ)concentration,but the second one was thought to be independent of Mn(Ⅲ).The discussion indicated that the later one has less efficiency on reaction rate of Cr(Ⅲ) than the former one.This is the further evidence to testify Mn(Ⅳ) just has the greater oxidation capacity in thermodynamic equilibrium,but not for the oxidation rate in dynamic process.
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