Metal partitioning in plant-soil-water compartments under EDDS-assisted phytoextraction with Brassica carinata A. Braun

Although new easily degradable chelating agents have recently been proposed as alternatives to the more recalcitrant EDTA for metal phytoextraction with biomass species, there is still little information on their possible environmental side-effects. In this pot study, the fate of mobile Cu, Pb and Zn was evaluated in a mixture of contaminated pyrite waste and sand after one [S,S]-EDDS-assisted (ethylene diamine disuccinic acid) phytoextraction cycle with Brassica carinata A Braun. In comparison with untreated controls, doses of 1 mmol kg-1 of EDDS were repeated five times at 5- and 10-day intervals during cultivation, and 2.5 and 5 mmol kg-1 were applied once to the substrate one week before harvest. Although all EDDS treatments increased shoot Cu and Zn concentrations, their removal by the harvestable biomass worsened in the repeated treatments, due to impaired plant growth. In contrast, Pb was only taken up in treated plants. During cultivation, considerable Cu leaching occurred under EDDS, especially with repeated applications (20 mg Cu L-1 in leachate), but concern may also arise after single close to harvest applications, due to the high residual Cu bioavailability in the substrate (e.g., +68% with 5 mmol EDDS kg-1 vs. controls). Instead, bioavailable Zn and Pb seldom decreased, perhaps due to their colloidal re-absorption by virtue of the greater Fe- and Cu-EDDS affinity. Of the total mobile metal pool (bioavailable + taken up + leached), the bioavailable fraction in the substrate was still the largest, regardless of chelator application, whereas the plants themselves removed a negligible amount (<1%).