Analytical Chemistry and Phytoextraction of Hexavalent Chromium with Portulaca Oleracea

Phytoextraction in the UAE desert soil (sandy, calcareous, less than 0.5% humus, and pH 7.9) has been studied. Twelve suspected polluted sites were investigated for contamination with eight heavy metals and sixteen local plants from the UAE desert were evaluated for their ability to accumulate heavy metals. The soil of Ajman industrial zone demonstrated high amounts of total chromium (1800 mg/kg) and of hexavalent chromium (97 mg/kg) which is a significant environmental threat. Portulaca oleracea (Purslane) has been shown to be the best candidate for Cr(VI) accumulation.

Total chromium concentration exceeded 4600 mg/kg in roots and 1400 mg/kg in stems confirming the role of P. oleracea as a Cr(VI) accumulator. More than 95% of the accumulated Cr(VI) was reduced to the less toxic Cr(III) within the plant.

The uptake of Cr(VI) by this plant has been investigated. The uptake of Cr(VI) increased as its concentration in soil increased between 50 and 400 mg/kg. The highest Cr(VI) uptake was observed at the high pH and low organic matter content of soil confirming the phytoextraction efficiency of P. oleracea in soils found in the UAE. The uptake of Cr(VI) increased in the presence of sulfate anion (suggesting that chromate uses the same carriers of sulfate in root cells) while nitrate and phosphate retarded the uptake. Potassium and ammonium ions, but not sodium ions, enhanced the uptake of Cr(VI) confirming the effect of accompanying cations. EDTA enhanced the translocation factor of chromium from roots to shoots in plants irrigated with either Cr(III) or Cr(VI). HPLC-MS analysis showed that ascorbic acid is the main antioxidant that reduced Cr(VI) to Cr(III) which is then mostly translocated to shoots after chelation with organic acids such as oxalate since glutathione and phytochelatins were not observed at significant levels in the tissues of plants exposed to Cr(VI).