Effect of microbial activity on the mobility of chromium in soils

The effect of microbial activity on the chemical state of chromium in a contaminated soil located in the Rhone-Alpes region (France), has been investigated. This soil contained 4700 mg kg(-1) Cr, with about 40% present in the soluble hexavalent form. Indigenous microbial activity was found to significantly reduce Cr(VI) to less mobile form (III) when the soil was incubated at 30degreesC in an aqueous medium containing glucose and nutrients. A Cr(VI)-reducing strain of Streptomyces thermocarboxydus was isolated from the contaminated soil. The strain was found to metabolize Cr(VI) in a similar manner as an exogenous inoculum of Pseudomonas fluorescens LB300, and to precipitate chromium as a Cr oxyhydroxide with a gammaCrOOH-like local structure. The Cr(VI)-reducing activity of S. thermocarboxydus was induced, or significantly accelerated, by the aggregation of bacterial cells or their adhesion to suspended solid particles, and was stimulated in pure culture by glycerol and was stimulated in pure culture by glycerol and chromate. (C) 2002 Elsevier Science