Pedosphere 16(5): 579--586, 2006
ISSN 1002-0160/CN 32-1315/P
©2006 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Catalysis of dissolved and adsorbed iron in soil suspension for chromium (Ⅵ) reduction by sulfide
LAN Ye-Qing, YANG Jun-Xiang and B. DENG
1 College of Science, Nanjing Agricultural University, Nanjing 210095 (China). E-mail: Langq102@yahoo.com.cn
2 Department of Civil and Environmental Engineering, University of Missouri-Columbil, Columbia, MO 65211 (USA)
ABSTRACT
      The kinetics of Cr(Ⅵ) reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe(Ⅱ) concentrations was examined using batch anaerobic experimental systems at constant temperature. The results showed that the reaction rate of Cr(Ⅵ) reduction was in the order of red soil < yellow-brown soil < chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr(Ⅵ) reduction. The reaction involved in the Cr(Ⅵ) reduction by Fe(Ⅱ) to produce Fe(Ⅲ), which was reduced to Fe(Ⅱ) again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr(Ⅵ) was reduced. The catalysis occurred because the one-step reduction of Cr(Ⅵ) by sulfide was slower than the two-step process consisting of rapid Cr(Ⅵ) reduction by Fe(Ⅱ) followed by Fe(Ⅲ) reduction by sulfide. In essence, Fe(Ⅱ)/Fe(Ⅲ) species shuttle electrons from sulfide to Cr(Ⅵ), facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe(Ⅱ)-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr(Ⅵ) concentration. The plots of lnc[Cr(Ⅵ)] versus reaction time were linear up to approximately 70% of Cr(Ⅵ) reduction, suggesting a first-order reaction kinetics with respect to Cr(Ⅵ). Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr(Ⅵ) reduction at a later stage of the reaction, resulting in deⅥation from linearity for the lnc[Cr(Ⅵ)] versus time plots.
Key Words:  catalysis, chromium, iron, soil suspension, sulfide
Citation: Lan, Y. Q., Yang, J. X. and Deng, B. 2006. Catalysis of dissolved and adsorbed iron in soil suspension for chromium (Ⅵ) reduction by sulfide. Pedosphere. 16(5): 579-586.
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