Reduction of Cr(VI) to Cr(III) by wetland plants: Potential for in situ heavy metal detoxification

Reduction of heavy metals in situ by plants may be a useful detoxification mechanism for phytoremediation. Using X-ray spectroscopy, we show that Eichhornia crassipes (water hyacinth), supplied with Cr(VI) in nutrient culture, accumulated nontoxic Cr(III) in root and shoot tissues. The reduction of Cr(VI) to Cr(III) appeared to occur in the fine lateral roots. The Cr(III) was subsequently translocated to leaf tissues. Extended X-ray absorption fine structure of Cr in leaf and petiole differed when compared to Cr in roots. In roots, Cr(III) was hydrated by water, but in petiole and more so in leaf, a portion of the Cr(III) may be bound to oxalate ligands. This suggests that E. crassipes detoxified Cr(VI) upon root uptake and transported a portion of the detoxified Cr to leaf tissues. Cr-rich crystalline structures were observed on the leaf surface. The chemical species of Cr in other plants, collected from wetlands that contained Cr(VI)-contaminated wastewater, was also found to be Cr(III). We propose that this plant-based reduction of Cr(VI) by E crassipes has the potential to be used for the in situ detoxification of Cr(VI)-contaminated wastestreams.