Field Scale Phytoremediation Experiments on a Former U Mining Site and Further Processing of the Plant Material

The remediation of large heavy metal/radionuclide (HM/R) contaminated areas by conventional ex-situ techniques (excavation or chemical treatment/-stabilization) is expensive, and low cost strategies with a long-term stabilization effect of such sites are a task of next years research. Phytoremediation can be an alterative solution and sustainable technology since low treatment costs make it feasible even for such sites. Field scale investigation is applied to an area of the former uranium mining in East Thuringia, Germany. On this area, a low grade uranium ore leaching dump was situated, which was later removed during the remediation process of the site. Now, an underground remains that is slightly to moderately contaminated with heavy metals and radionuclides (HM/R) with restrictions of land use. Concepts for remediation of HM/R-contaminated sites and for the subsequent utilization of the HM/R-loaded plant residues are developed in a joint project. Plant experiments with Triticale, Helianthus annuus, Brassica juncea and Sorghum bicolor were performed revealing the influence of biological additives (mycorrhiza, HM-resistant streptomyces) and soil amendment strategies (increasing pH and organic matter) on biomass production and plant tolerance to heavy metals. The addition of mycorrhiza and streptomyces (MS) had a significant effect on biomass, and decreased the bioavailable parts of HM/R (e.g. Ni, Sr, U) in soil in comparison to untreated polluted soil with all studied plants. In lysimeter experiments it was figured out, that the different soil improvement strategies, addition of MS and of calcareous top soil (MIX) resulted in a reduction of the concentration of contaminants in the seepage water, as well as of the seepage water rates and loads, thus decreasing the risk of groundwater contamination. Finally, the processing of the heavy metal loaded plant residues was investigated by different methods. Fermentation experiments carried out with Solanum tuberosum and Secale cereale achieved good results compared with industrial standards. Remaining plant parts can be processed in a biogas process, or combusted. Fermentation sludges and combustion ashes act as sinks for HM/R. As results of the project, phytoremediation is a convenient method for the stabilization and remediation of large sites with slight to medium contaminations, and a further utilization of the plant residues by fermentation and biogas production is feasible.