A Laboratory Assessment of Potentials and Limitations of Using EDTA, Rhamnolipids, and Compost-derived Humic Substances (HS) in Enhanced Phytoextraction of Copper and Zinc Polluted Calcareous Soils

Phytoextraction is an economically and environmentally attractive in-situ method for cleaning heavy metal polluted soil. Phytoextraction is a rather slow process, but it can be enhanced by the application of chelating agents such as the synthetic ethylenediaminetetraacetic acid (EDTA). However, EDTA is persistent, toxic, and can promote heavy metal leaching. Replacement of EDTA by natural, non-toxic compounds such as humic substances (HS) or rhamnolipids (bacterial-produced biosurfactants) might be environmentally attractive but before recommending such alternatives, their suitability must be assessed. Therefore, compost-derived HS and rhamnolipids were compared with EDTA as natural non-toxic alternatives in a multi-step batch extraction test. The test included 10 steps carried out on two Cu and Zn polluted calcareous soils using a solution: soil ratio of 10 (L/kg). In each step, soil was extracted with an extractant containing EDTA, HS, or rhamnolipids corresponding to 250 mmol DOC/kg of soil (3 g C/kg). By HS extraction, each step resulted in the release of similar to 0.29 mg Cu/L and similar to 0.19 mg Zn/L, which is considered to enhance plant uptake without leading to unacceptable leaching and toxification of the plants (and the environment), suggesting HS can enhance phytoextraction. In contrast, the EDTA and the rhamnolipid treatments were found to be unsuitable because the EDTA released Cu and Zn in concentrations that may be toxic to plants and can lead to leaching, whereas the rhamnolipids showed insufficient capacity to mobilize Cu (and Zn). However, future investigations in the field are needed to confirm these laboratory results.