Bioremediation of a soil contaminated with radioactive elements

Some agricultural lands located in the Vromos Bay area, near the Black Sea coast, Southeastern Bulgaria, have been contaminated with radioactive elements (uranium, radium and thorium) and toxic heavy metals (copper, cadmium and lead) as a result of mining and mineral processing of polymetallic ores. Laboratory experiments carried out with soil samples from these lands revealed that an efficient remediation of the soils was achieved by an in situ treatment method based on the activity of the indigenous soil microflora. The treatment was connected with the dissolution of the contaminants in the upper soil horizons and their transfer into the deeply located soil horizons (mainly to the horizon B(2)) where they were immobilized as different insoluble compounds. The dissolution of contaminants was connected with the activity of both heterotrophic and chemolithotrophic aerobic microorganisms and the immobilization was due mainly to the anaerobic sulphate-reducing bacteria. The activity of these microorganisms was enhanced by suitable changes in the levels of some essential environmental factors such as water, oxygen and nutrient contents in the soil. On the basis of the above-mentioned laboratory results, the method was then applied under real field conditions in a heavily contaminated experimental plot of land located in the Vromos Bay area. Within 8 months of treatment, the contents of radioactive elements and toxic heavy metals in the soil were decreased below the relevant permissible levels. (C) 2001 Elsevier Science B.V. All rights reserved.