Kinetics Of Cr(Ⅲ) Oxidation By H-Birnessites With Various Average Oxidation States

Chromium is a heavy metal that has always attracted extensive attention on its behavior and fate in environment. In order to understand the environmental behavior of Cr in soils, sediments, and water, it is significant to conduct extensive investigation on Cr(Ⅲ) oxidation characteristics by H-birnessites with various Mn average oxidation states (AOSs) to make clear of the dynamic process, reaction pathway, the oxidation mechanism of manganese oxides, and their environmental impacts.Kinetics of Cr(Ⅲ) oxidation by synthesized H-birnessites with various Mn average oxidation states (AOSs) were studied with batch technique and the stir-flow technique to elucidate dynamic process and reaction pathway. The results can be showed as follows:1.XRD analyses demonstrated that the synthesized H-birnessites are single phased hexagonal birnessite with good characteristic diffraction peaks.2.In the batch experiments, the oxidation of Cr(Ⅲ) by H-birnessites with various Mn average oxidation states complied with the pseudo first-order kinetic. The appropriate reaction pH was 4.4. The reaction system was stable without Cr(OH)3 precipitate. In the reaction process, the higher temperature, the greater of the apparent kinetic rate constant (Kobs), and the faster of the rate of oxidation reaction. When the temperature and pH were fixed, the higher intial concentration of Cr(Ⅲ), the slower of the rate of oxidation reaction, and the lower Kobs within the intial reaction period.3.In the batch experiments, The lower AOS (that mean the Mn average oxidation state was lower), the faster of the rate of oxidation reaction, and the greater of the oxidation ability. When the reaction condition was:temperature was 25℃,pH was 4.4, intial concentration of Cr(Ⅲ) was 33.28 mg/L, the AOS values of H-birnessites were 3.87, 3.95,4.00 respectively, the Kobs by the tested samples were:0.0589 min-1 (R2=0.9851),0.0533 min-1(R2=0.9793),0.0448 min-1 (R2=0.9537) respectively. However, the oxidation rate of Cr(Ⅲ) and oxidation capacity with manganite (y-MnOOH) which is only composed of Mn(Ⅲ) is far less than those of the three H-birnessites.Therefore, the AOS, the structure, the substructure, and the types of Manganese oxides are important factors in affecting the kinetics of Cr(Ⅲ) oxidation. 4. In the batch experiment, the results showed that the values of Ea for birnessites with AOS of 3.87,3.95,4.00 are 30.01kJ·mol-1,27.62 kJ·mol-1,22.54 kJ·mol-1 respectively. It can be seen from the values of Ea, the reaction process was controlled by diffusion. It was also acceptable to conclude that the rate of reaction could be limited by the step of the adsorption of reactant Cr(Ⅲ) or the release of product Cr(Ⅵ).5.In the batch experiment, at different temperatures, in the initial reaction period of oxidation of Cr(Ⅲ) by H-birnessites with various Mn average oxidation states, the release of Mn(Ⅱ) lagged behind the relase of Cr(Ⅵ).As the reaction went on, more and more Mn(Ⅱ) and Cr(Ⅵ), the ratios of Mn(Ⅱ) to Cr(Ⅵ) were closed to the theoretical value of the reaction stoichiometry.6.In the stir-flow experiment, by the establishment of the theoretical model and fittings of the experimental data, it can conducted that in the different intial concentration of Cr(Ⅲ), the kinetic rate constant (K) of the oxidation of Cr(Ⅲ) by the H-birnessite decreased as the increase of the intial concentration of Cr(Ⅲ).In the same intial concentration of Cr(Ⅲ), the result was:K (AOS 3.87)>K (AOS 4.00)>K (AOS 3.95). In comparison with the results of batch experiments, the results show:the lowest AOS, the highest K, and the fastest of the rate of oxidation reaction. It also was suggested that the reaction activity of Mn(Ⅲ) was stronger than that of Mn(IV).Meantime, compared to the value of KObs in batch experiments, the K in the stir-flow experiment was much greater. It indicated that the intial concentration of Cr(Ⅲ) and the ratio between the concentration of manganese oxides and the intial concentration of Cr(Ⅲ) are the two important factors in affecting kinetic process of Cr(Ⅲ) oxidation by H-birnessites with various Mn average oxidation states.