Localization and speciation of chromium in subterranean clover using XRF, XANES, and EPR spectroscopy

Optimization of phytoremediation and assessment of potential health hazards from metals in the environment requires an understanding of absorption, localization, and transport of the target metal by plants. The objectives of this study were to localize Cr and determine the oxidation state and possible complexation mode of Cr in intact plant tissue by means of XANES, synchrotron XRF microprobe spectroscopy, and EPR spectroscopy. Subterranean clover (Trifolium brachycalycinum) was grown hydroponically with Cr(VI) (0.04-2.0 mmol L-1) and compared with plants grown without Cr and with inorganic Cr(III) and various Cr(III)-organic sources. The uptake, translocation, and form of Cr in the plant were dependent on the form and concentration of supplied Cr. Chromium was found predominately in the +3 oxidation state, regardless of the Cr source supplied to the plant, though at high Cr(VI) treatment concentrations, Cr(VI) and Cr(V)were also observed. At low Cr(VI) concentrations, the plant effectively reduced the toxic Cr(VI) to less toxic Cr(III), which was observed both as a Cr(III) hydroxide phase at the roots and as a Cr(III)-organic complex in the roots and shoots. At low Cr(VI) treatment concentrations, Cr in the leaves was observed predominately around the leaf margins, while at higher concentrations Cr was accumulated at leaf veins.