The potential of Lepidium sativum L. for phytoextraction of Hg-contaminated soil assisted by thiosulphate

Beata Smolinska 0 1 Sylwia Rowe 0 1 0 S. Rowe 25 Mill Park, Portglenone, Ballymena BT44 8LJ, Northern Ireland, UK 1 Responsible editor: Peter Schroeder 2 ) Department of Biotechnology and Food Sciences, Institute of General Food Chemistry, Lodz University of Technology , 4/10 Stefanowskiego Street, 90-924 Lodz, Poland Purpose The possibility of using higher plants to extract mercury from contaminated sites is dependent on both the concentration of Hg and its bioavailability. To increase the solubility of Hg in soil, some chemical compounds can be used. The aim of this study was to evaluate the effectiveness of Hg soil cleaning with the use of Lepidium sativum L. and sodium thiosulphate, as well as the leach ability of Hg from soil after phytoextraction. Materials and methods The experiment was conducted on soil artificially polluted by Hg, wherein sodium thiosulphate was tested as a phytoextraction promoter. The L. sativum L. plants were used for phytoextraction. The leaching of Hg was assessed by determination of Hg concentration in water extracts. All determinations of Hg in soil, plant and water extracts were analysed by CV-AAS method after acid mineralization. Results and discussion The result of the study showed that L. sativum L. accumulated Hg from contaminated soil mostly in belowground tissues. Even less than 8 % of Hg was translocated to the shoots of L. sativum L. Application of thiosulphate increased the total Hg accumulation over 238272 %, depending on both the Hg and thiosulphate concentrations in soil. After thiosulphate treatment, translocation of Hg to shoots of L. sativum L. increased even 10 times relative to unassisted process. Thiosulphate did not negatively affect plant biomass; however, the increased leaching of Hg after thiosulphate treatment was observed. Conclusions Lepidium sativum L. showed the potential of a non-hyperaccumulating plant that can be used during phytoextraction of Hg-contaminated soils in controlled conditions. Thiosulphate promoted the phytoextraction process by increasing the total Hg accumulation by whole plant and translocation of Hg to shoots of L. sativum L. Thiosulphatemobilized Hg in soil, which increased the Hg leaching. This constitutes the limitation of applying the technique in the field due to risk of Hg transferring to deeper layers of soil or water. Applying the technique in the field should be preceded by further investigations. 1 Introduction Soil contamination by mercury has become a serious problem in the world. Due to mercury toxicity and its physical and chemical properties, it constitutes a threat to the health of humans and wildlife, even in places which are not obviously contaminated. The risk is determined by both the likelihood of exposure and its chemical form. The increasing concentration of mercury in the environment is mostly ascribed to human activity, which includes a variety of industrial processes, for example, coal burning, disposal of Hg-containing products, mining, smelting and solid waste combustion. Mercury is often deposited in soils, where concentrations of this element can be large. In soil, mercury can be absorbed onto the solid-phase of organic matter or minerals (Evans 1989). However, a substantial fraction of Hg undergoes several transformations, including leaching, volatilisation, methylation or biological reduction (Moreno et al. 2005a). These processes increase the Hg mobility thus causing the spreading of the pollution. Soils contaminated by mercury constitute a danger for all living organisms because of the possibility of mercury absorption by microbes and plants, whilst at the same time, transferring Hg to the food chain (Kabata-Pendias and Pendias 1999). Therefore, the techniques of soil reclamation are still being searched. In methods of soil remediation adequate for Hg-polluted soils, physical and chemical treatments should be mentioned. Nevertheless, these techniques are relatively expensive and generate wastes that should be utilised. An alternative method, which is viewed as environmentally friendly is phytoremediation. Phytoremediation refers to the use of higher plants to decrease the toxicity of pollutants. One phytoremediation category is phytoextraction. This consists of the uptake of contaminants from soil or water by plant roots, their translocation and accumulation in plant shoots. The pollutants can then be removed by harvesting the aboveground tissues (Rafati et al. 2011; Ali et al. 2013; Wang et al. 2012). The main problem that occurs during phytoextraction is a low solubility of mercury in soil solution, which results in decreasing bioavailability of the metal. This problem can be solved by addition of chemical compounds straight to the soil. These chemicals promote the solubility of metals by the formation of watersoluble complexes and as a result, increase the metal bioavailability (Wang et al. 2011). The researchers have demonstrated that mercury can form water-soluble complexes with (...truncated)