Soil surface catalysis of Cr(VI) reduction by citric acid

The objective of this study was to investigate the catalytic effect of a red soil rich in Fe-oxides on the reduction of Cr(VI) by citric acid. Experimental results showed That the redox reaction would be greatly enhanced in the presence of the soil surfaces. The catalytic effect of the soil surface was very pronounced at low pH. Whereas with the increase in pH, the soil surfaces-catalyzed redox reaction was gradually inhibited because reactant adsorption might be a necessary step for the heterogeneous redox reaction to take place. Although more adsorbed Cr(VI) would be released into the soil solution when citric acid oxidised with phosphate, the concentration of the reduced product Cr(III) in the equilibrium solution was lower than that in the system with out phosphate, which further suggested that the reaction mainly occurred on the soil surfaces. The Fe(II)-Fe(III) redox couple has also been found to accelerate the reaction. Cr(VI) could be directly reduced by the Fe(II) ions produced during the citric acid-promoted reductive dissolution of Fe-oxides in the soil. In addition, the kinetic studies showed that the presence of Fe(III) ions, also released from the dissolution, process, could increase the Cr(VI) reduction rate by, the citric acid. Information obtained from natural soil in this study is important for remediation of Cr(VI)-contaminated soils by application of natural organic materials. (C) 2008 American Institute of Chemical Engineers.