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The toxicity of chromium (Cr) to biota is related to its chemical forms and consequently to the redox conditions of soils. Hexavalent Cr [Cr(VI)] may undergo natural attenuation through reduction processes. In this study, the reduction kinetics of Cr(VI) in seven soils and its relationships with soil properties were investigated with laboratory incubation experiments. The results indicate that the reduction of Cr(VI) can be described by a first-order reaction. The reduction rates of Cr(VI) in the seven soils decreased in the order: Udic Ferrisols > Stagnic Anthrosols > Calcaric Regosols > Mollisol > Typic Haplustalf > Periudic Argosols > Ustic Cambosols. Simple correlation analysis revealed that the reduction of Cr(VI) in soils was positively related to organic matter content, dissolved organic matter content, Fe(II) content, clay fraction, and to the diversity index of the bacterial community but negatively correlated with easily reducible Mn content. Using stepwise regression, the reduction of Cr(VI) in soil could be quantitatively predicted by the measurement of dissolved organic matter content, Fe(II) content, pH, and soil particle size distribution, with a fitting level of 95.5%. The results indicated that the reduction of Cr(VI) in natural soils is not controlled by a single soil property but is the result of the combined effects of dissolved organic matter, Fe(II), pH, and soil particle size distribution.
Abbreviations:CEC, cation exchange capacity; DGGE, denaturing gradient gel electrophoresis; DOM, dissolved organic matter; Mn(ER), easily reducible Mn; PSD, particle size distribution; OM, organic matter.
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Chromium (Cr) is a natural element in the earth's crust and occurs in soils at concentrations of 10 to 150 mg kg-1 (Adriano, 2001). Anthropogenic sources, including ore refining, electroplating industry, tanning, paper making, steel production, and automobile manufacturing, contribute greatly to Cr pollution in the environment (Zayed and Terry, 2003). The lack of appropriate disposal facilities has led to severe Cr pollution in water and soils throughout the world (Loyaux-Lawniczak et al., 2001).
In the natural environment, the most stable forms of Cr are trivalent Cr [Cr(III)] and hexavalent Cr [Cr(VI)] (Rai et al., 1987). Trivalent Cr is generally considered nonbioavailable due to its low solubility in water at a normal pH range (4-9) (Rai et al., 1987). Trivalent Cr is an essential trace element for mammals...