Review: Phytoremediation of potentially toxic elements in Swiss soils

Phytoremediation is a subprocess of bioremediation that aims to remediate contaminated soils in situ. Early research on phytoextraction of potentially toxic elements (PTE) concluded that remediation of heavily contaminated sites could take decades to centuries. Nevertheless, the field of research has grown over the last 30 years. This review examines the current state of research and commercialization of phytoextraction for PTE-polluted soils in Switzerland, Europe and North America. Field trials at sites contaminated with lead, copper or mercury did not show any reasonable short to medium-term results due to the low extraction efficiency or - in the case of Switzerland - due to un-suitable plants. On the other hand, for cadmium and zinc an extraction efficiency of up to 30% of the original soil concentration was achieved within one growth period. These two PTEs thus appear to be promising candidates for in-situ remediation through phytoextraction. In case of multiple contamination (sole contamination of Cadmium or Zinc can occur), a combination of different bioremediation strategies may be applicable. Nevertheless, the mitigation of a single contaminant can facilitate soil reuse. Some studies have investigated the potential for enhancing the efficiency of extraction through the introduction of chemical additives. While this approach may facilitate a higher uptake of PTE by the plants, the more effective chemicals also tend to exhibit a higher negative environmental impact. Consequently, their field application is controversial, and cannot be supported by the authors. Despite ongoing research efforts since the late 1980s/early 1990s, the number of private companies conducting commercial projects in the field of phytoextraction is still marginal. A search of the Swiss market revealed two companies with a focus on in-situ remediation of organic contaminants rather than on PTEs. Based on our literature study, we identified potential for the phytoextraction of cadmium and zinc. Especially from soils with contamination levels around the trigger value, where restrictions have been issued, remediation (below the guide value) within five years seems plausible. To enhance the extraction efficiency, methods to increase the bioavailability of PTEs in soils, such as lowering the pH, could be further investigated.