Study On Kinetics Of Chromium (Ⅵ) Reduction By Organic Acids And Thiers Impact Factors

Chromium (Cr) is widely used in industries, such as steel production, leather tanning, corrosion prevention, dye, tannery industry and galvanic. Aa a result of unproper disposal of the wastes containing chromium, chromium danger to human, animals and plants has become one of the most serious environmental contaminants. Chromium mainly exsits in two forms in natural environments: Cr⁶⁺ and Cr³⁺. These two forms exhibit completely different mobility and toxicity. Cr⁶⁺ is high toxicity and solubility. However, Cr³⁺ is much less toxic and has a strong affinity for soil particle surfaces, displaying immobility. So, Cr⁶⁺ is often reduced to Cr³⁺ by chemical reduce agents to control chromium contaminatnts. In the paper, kinetics of the reduction of Cr⁶⁺ by organic acids and its impact factors were studied in the homogenious and heterogeneous system.In chapter I The catalysis of Mn²⁺ on Cr⁶⁺ reduction by citrateacid, tartaric acid, oxalate acid, acetate acid, butyric acid and succinic and the reaction kinetics were studied through batch experiments at 25 °C and in a pH ranges of 4.0 to 5.0. Results showed that direct reduction of Cr⁶⁺ by citrate acid and oxalate acid was not observed at pH 4.5, but almost all initial Cr⁶⁺ was reduced within comparable time period when Mn²⁺ was added to the reaction system .The higher the initial Mn²⁺ concentration, the faster the Cr⁶⁺ reduction. A pseudo-first-order kinetics with respect to Cr⁶⁺ was used to describe the initial stage of the reaction when the concentrations of citrate acid and oxalate acid were in much excess over Cr⁶⁺. The plots of lnc(Cr⁶⁺) versus reaction time (t) deviated from the initial linear trend at the later stage of the reaction, suggesting that the pseudo-first order did not apply for the whole duration and some reaction intermediates might have accelerated Cr⁶⁺ reduction by citrate acid and oxalate acid greatly. The catalysis of intermediate products increased with the reaction progress and gradually reached stability. Then, plotting lnc(Cr⁶⁺) versus t was linear again and the rate constant was increased by 102 times to citrate acid and 393 times to oxalate acid as compared with that in the absence of Mn²⁺. The reduction of Cr⁶⁺ by tartaric acid took place faster in comparison with citrate acid and oxalate acid in absence of Mn²⁺. It is concoluded that the difference in reaction activity for different oganic acids resulted from α -OH of tartaric acid. Also, Mn²⁺ can accelerate the rate of Cr⁶⁺...