Chromium sorption and reduction in soil with implications to bioremediation

Chromium must be removed or made immobile to reduce its toxicity in contaminated soil. The objectives of this study were (i) to compare Cr (III) and Cr (VI) sorption in three soils, (ii) to determine the potential of Cr-tolerant microorganisms to sorb Cr (III and VI) or to reduce Cr (VI) to Cr (III), and (iii) to determine the effect of organic amendment on stimulating Cr (VI) reduction by soil microorganisms. Sorption of Cr (III) was soil-dependent; the maximum sorbed was 431 mmol kg(-1) in a clay loam soil. Sorption of Cr (VI) was concentration-dependent and independent of soil type. Maximum Cr (VI) sorbed was 63 mmol kg(-1) soil. Aspergillus niger cells sorbed more Cr (VI) than Cr (III). Binding of Cr (III) by A. niger depended on pH and was greatest at pH 9, but binding of Cr (VI) was not pH-dependent. Soil amendment with freeze-dried A. niger increased sorption of Cr (III) significantly. Organic amendment of Cr (VI) contaminated soil resulted in both direct sorption of Cr (VI) and reduction to Cr (III). Reduction was primarily by indigenous soil microorganisms, and inoculation with Cr tolerant pseudomonas maltophilia was of little benefit. Yeast extract and Bermuda grass were more effective than cow manure in removing Cr (BI) from solution after 15 days of incubation. Application of an organic amendment to reduce and bind Cr (BI) appears feasible as a technique to immobilize Cr (VI) in contaminated soils. Inoculation of Cr-tolerant microorganisms does not appear feasible or necessary to immobilize Cr (VI).