Thermodynamic investigation with chemical kinetic analysis on the reoxidation phenomenon of the Cr(iii) in air.

Abstract

In this paper, the reoxidation behaviours of CrOOH and Cr(OH)3 are investigated as the major reduction products of Cr(vi). The atmosphere oxidation of Cr(iii) is studied in the environment of soil without manganese and hydrogen peroxide. The influence of temperature and pH value on the oxidation rate of Cr(iii) is examined by Experiment methods in details. According to the experimental results, the oxidation of Cr(iii) is promoted in the environment with high pH value, however, the oxidation process is stable with temperature. The oxidation process of CrOOH and Cr(OH)3 are theoretically researched by thermodynamic calculation and density functional theory (DFT) simulation. The results of DFT indicate that both CrOOH and Cr(OH)3 are oxidized during the chemical adsorption process of O2 in alkaline environment. With the transformation from Cr(iii) to Cr(vi), the Cr–O covalent bond forms in the adsorption process. The crystal structure difference between CrOOH and Cr(OH)3 leads to the different oxidation reaction between O2 and Cr(iii). The significant alteration of oxidation process in (110), (310), (321) crystal planes is also observed indicating the crystal orientation dependence. Based on the chemical reaction kinetics, the chemical equivalent constant K of CrOOH is higher than Cr(OH)3, illustrating higher chemical stability in air. In summary, both experimental study and theoretical analysis on the reoxidation phenomenon of Cr(iii) reduced from Cr(vi) in natural environment demonstrate that not only the external factors such as temperature and pH value but also the crystal structure of Cr(iii) compound have dramatic influence on the oxidation process.