Kinetics Study Of Cr（Ⅲ） Oxidation By δ-MnO₂

Chromium pollution is one of the most serious heavy metals pollution in the environments, its behavior and fate has recently received more and more concerns. Mn-oxides are the the most important natural oxidizers to transform Cr(Ⅲ) into Cr(VI).The kinetic process of Cr(Ⅲ) oxidation by Mn-oxides is not only the key factor to affect and determine the environmental behaviors of Cr(Ⅲ), but also the important information to reveal Cr reaction mechanism at the soil surface.This paper deals with the Cr(Ⅲ) oxidation by synthesized venadite and birnessite with both batch methods and stirred-flow methods under different status of concentration of Cr(Ⅲ) and mineral, solution pH, flowing velocity and ion strength. Oxidation kinetic model of stirred-flow method is established to obtain reaction rate constant (K). The main results can be showed as follows:1. XRD analyses demonstrated that the synthesized vernadite and birnessites are single phased δ-MnO2with good characteristic diffraction peaks. Manganese average oxidation states of both are nearly4.00. The specific surface areas wre230.8m2/g and11.46m2/g respectively.2. In the batch experiments, the oxidation of Cr(Ⅲ) by vernadite and birnessite follow the first kinetic equation at the initial stage. Results show that the apparent rate constant (k) becomes smaller in response to concentration of Cr(Ⅲ) solution, k becomes higher in response to concentration of vernadite suspension. k decreases with increase of reaction pH. k become larger in response to ion strength. The values of k by vernadite are between0.080min-1and0.425min-1, while the values of k by birnessite are between0.008min-1and0.072min-1。3. In the stirred-flow experiments, by establishment of the theoretical model and fittings of the experimental data by vernadite, it can be conducted that K follow the same law as that in the batch expriments with the function of the concentration of Cr(Ⅲ) and reaction pH. Also we can see that K increases with the flowing velocity. The values of K by vernadite are between0.477min-1and1.654min-1. While the theoretical model is not well fitted by birnessite.4. The concentration of Cr(Ⅲ) and manganese oxides, solution pH are the factors directly affecting kinetic process of Cr(Ⅲ) oxidation.While the ion strength and flowing velocity are the inderct ones. Meantime, the ratio between the concentration of manganese oxides and the concentration of Cr(Ⅲ) are the most important factor in affecting K.5. Bimessite has a higher oxidation capacity in chemical thermodynamics revealing a lower Gibbs free energy; while vernadite has a faster reaction rate in chemical kinetics revealing a larger reaction rate constant.6. The reaction mechanism of Cr(Ⅲ) oxidation by8-MnO2is that Cr(Ⅲ) is firstly absorbed on the surface of Mn-oxides, then enters into the MnO6octahedral vacant sites and forms complex with Mn(IV), nextly electrons transfer from Mn(Ⅳ) to Mn(Ⅲ) to Mn(Ⅱ), electrons transfer from Cr(VI) to Cr(Ⅲ), Cr(Ⅵ) and Mn(Ⅱ) are released to solution, a portion of Mn(Ⅱ) was absorbed to the surface of Mn-oxides.The absorption of Cr(Ⅲ) and releasing of Mn(Ⅱ) are both the rate determining steps.