Water Leaching of Chelated Pb Complexes from Post-Phytoremediation Biomass

There is a pollution risk when disposing of post-remediation biomass from chelate-assisted metal phytoremediation. To assess this risk, we measured water extractable lead (Pb) in Brassica rapa tissues with ICP-MS, determined if chelated Pb was present with HPLC-MS, and identified Pb storage locations using electron microscopy with x-ray microanalysis. Ethylenediaminetetraacetic acid (EDTA) and ethylenediaminedisuccinic acid (EDDS) were used to enhance Pb movement from contaminated soil to above ground B. rapa tissues. With Pb-EDTA, 92% (+/-5) of Pb was water extractable from dried tissues and complexed as Pb-EDTA. Electron microscopy and x-ray microanalysis showed Pb stored in xylem vessels. After composting of plant tissues, 79 % (+/-2) of Pb was water extractable and complexed as Pb-EDTA. Total plant Pb accumulation was lower from soils amended with EDDS, but only 6.7 % (+/-0.3) of Pb was water extractable from dried tissues and 55%(+/-25) from wet tissues of plants grown in EDDS-amended soils. Pb-EDDS was detected in tissues but not at quantifiable levels. This work emphasizes the need for proper treatment and disposal of contaminated post-remediation plant tissues, especially when using EDTA. Composting of plant tissues containing Pb-EDTA was shown to significantly reduce waste material volume and slightly reduce the water extractable fraction, but further immobilization of Pb would be necessary to minimize transport risk. Amending Pb-contaminated soils with EDDS can result in plant biomass with a lower potential to leach Pb into groundwater, but the lower Pb accumulation with EDDS would require longer phytoremediation time compared with EDTA.