Uptake of metal-EDTA complexes by Brassica juncea: Implications for the free-ion activity model and phytoremediation design

The ability of plants to absorb and accumulate toxic metals poses potential human health and ecological concerns, but also provides an opportunity to remove metals from contaminated sites through phytoremediation. Predicting metal uptake for a particular soil system depends on soil chemistry and plant physiology. Although the activity of free metal ions is widely assumed to control metal uptake, recent studies suggest that direct uptake of metal complexes is important under certain circumstances. Ethylenediamine-tetraacetic acid (EDTA) is a synthetic chelating agent commonly used in plant fertilizers and in chelate-enhanced phytoextraction systems. This research was designed to quantify the contribution of metal-EDTA uptake to overall metal uptake, to investigate the phytotoxic effects of EDTA and metal-EDTA complexes, and to explore implications for phytoremediation and metal uptake models.; The results demonstrate that metal-EDTA complexes can contribute significantly to overall metal uptake, particularly when metal-EDTA concentrations are high. In hydroponic and sand-grown Brassica juncea (Indian mustard) exposed to 500 to 1000 muM CdEDTA2-, PbEDTA 2- or FeEDTA-, many plants had xylem sap metal-EDTA concentrations in excess of 80% of total metal concentrations, particularly for Pb and Cd. At solution concentrations of 50 muM metal-EDTA, only 17 to 40% of total Fe or Pb was present as FeEDTA- or PbEDTA2-. Hydroponic plants took up greater or equal quantities of PbEDTA2- and FeEDTA- than did sand-grown plants.; Metal-EDTA complexes most likely are taken passively up via the apoplastic pathway, a pathway that typically contributes less than 1% of transpirational flux into plants, according to uptake measurements of PTS, a fluorescent dye. However, xylem sap metal-EDTA concentrations were found to be one to two orders of magnitude higher than PTS concentrations, suggesting that previous estimates of apoplastic flow underestimated the potential contribution of metal-EDTA. Uptake data suggested a biphasic uptake regime: at relatively low xylem sap concentrations, solute uptake was strongly influenced by chemical properties of the solute and root cell wall, whereas at relatively high xylem sap concentrations, solute uptake was relatively non-selective, suggesting damage to root tissues.; Exposure to metal-EDTA solutions often caused visible damage to leaf tissues. Severe damage was associated with greater metal-EDTA uptake and metal accumulation in mature plants. Seedlings exposed to metal-EDTA solutions or solutions with excess EDTA (relative to the sum of micronutrient metal concentrations) exhibited inhibited root and shoot growth and decreased shoot water content. (Abstract shortened by UMI.)...