Decreasing the contamination and toxicity of a heavily contaminated soil by in situ bioremediation

•A new method for in situ bioremediation of heavily contaminated soils.•Real decreasing of the toxicity of the treated soil under field conditions.•Utilization of the wastewater after the soil treatment for electricity generation.

Experimental plot consisting of acidic leached cinnamonic forest soil heavily contaminated with radionuclides (mainly U and Ra) and non-ferrous metals (mainly Cu, Zn and Cd) was treated in situ under real field conditions using the activity of the indigenous soil microflora. This activity was enhanced by suitable changes of some essential environmental factors such as pH and water, oxygen and nutrient contents of the soil. The treatment was connected with solubilization and removal of contaminants from the top soil layers (horizon A) due to the joint action of the soil microorganisms and leach solutions used to irrigate the soils (mainly acidophilic chemolithotrophic bacteria and diluted sulphuric acid). The dissolved contaminants were transferred through the drainage soil effluents to the deeply located soil subhorizon B2 where they were precipitated as the relevant insoluble forms (uranium as uraninite, and the non-ferrous metals as the relevant sulphides) as a result of the activity of the sulphate-reducing bacteria inhabiting this soil subhorizon. The effluents from the subhorizon B2 containing much lower concentrations of inorganic contaminants but enriched in dissolved biodegradable organics were subjected to additional treatment in a constructed wetland for removing these residual contaminants. It was also demonstrated that an efficient treatment of the effluents from the subhorizon B2 could be achieved by a microbial fuel cell in which the cleanup was connected with electricity generation.