Chelant-Enhanced Phytoextraction of Heavy Metal-Contaminated Soils and Its Environmental Risk Assessment

Abstract

The in situ phytoremediation, using plants to restore the deteriorated soils, is a promising technology in cleanup of polluted sites. The potential use of high-biomass plants was investigated for the EDTA or EDDS-enhanced phytoextraction of metals from contaminated soils. The approaches for enhancing the efficiency of chelant-assisted phytoextraction and the environmental risk (metals leaching) of applying EDTA and EDDS were also investigated. The results showed that, compared to the monocotyledon species (e.g., barley, wheat, sorghum, corn, vetiver grass, etc.), the dicotyledon species (i.e., mung bean, buckwheat, pea, sunflower oil, mustard, etc.) had a higher sensitivity to the EDTA or EDDS treatments, which could be reflected by the severer toxicity of seedlings and the higher concentrations of heavy metals in the shoots. Considering the three factors (the metal level of the shoot, the biomass of plants, and the growing seasons of plants), mustard and pea were more suitably used in the cool seasons, while corn, sunflower, mung bean, and buckwheat could be selected in the warm seasons. Vetiver grass might be suitably used in the phytostabilization of heavy metals in soils. There was a new approach that the increase of soil temperature could improve the efficiency of heavy metal accumulation of plants from soils by 10–40 times in comparison with the normal chelator application. This was the highest efficiency of chelator application reported in the literature up to now. Soil column leaching tests and a field study were carried out to investigate the leaching behavior of heavy metals in chelant-enhanced phytoextraction. Results showed that the soluble heavy metals in the contaminated soils after EDTA application could be persistent in the soils for a long time (at least 32 weeks), which could be easily leached down to the deep soils under the large rainfall percolation. On the contrary, EDDS application had a little effect on the movement of heavy metals in the soils. The results showed that the potential leaching associated with EDDS addition may be controlled under field conditions. The use of biodegradable chelators for enhancing phytoextraction of heavy metal-contaminated soils is generally regarded as an environmentally acceptable approach.