Citric acid induced phytoextraction of heavy metals from uranium contaminated soils

English: Induced phytoextraction is an emerging soil remediation technology that makes use of soil amendments and high biomass crop species to accumulate and remove heavy metals from soils. The failure to rehabilitate an uranium trial mine on Rietkuil 307 in the Karoo Uranium Province of South Africa, has led to above normal concentrations of uranium (U), molybdenum (Mo) and arsenic (As) in topsoil in the vicinity of U ore stockpiles. A greenhouse evaluation was executed to assess the potential of citric acid to induce metal uptake in the shoots of Indian mustard [Brassica juncea (L.) Czern, cultivars: 211000 and 426308] and sunflower (Helianthus annuus L.) to decontaminate the low-level U (28 mg U kg-1 soil), Mo (4.1 mg Mo kg-1 soil) and As (8.9 mg As kg-1 soil) contaminated soil to background concentrations. A further greenhouse study was performed in a soil with high levels of metal concentrations; U (165 mg U kg-1 soil), Mo (125 mg Mo kg-1 soil) and As (49 mg As kg-1 soil), to assess the feasibility of phytoextraction on a high-level metal contaminated soil. Citric acid proved to be effective in enhancing shoot-U, -Mo and -As concentrations and the highest concentrations were observed in B. juncea 211000 (1788 mg U kg-1; 467 mg Mo kg-1; and 24 mg As kg-1) grown in the lowlevel contaminated soil. With a biomass yield of 5.51 tonnes per year for B. juncea 211000, it would take 9 to 14 years to decontaminate Mo and U on Rietkuil 307 soil to background concentrations. As a result of the low As solubility in soil and subsequent low shoot concentrations of the plant species, it is suggested that other measures for As remediation be investigated. Furthermore, plants grown in the high-level contaminated soil achieved considerably lower biomass yields and shoot-metal concentrations than in the low-level soil due to metal toxicity. Phytoextraction will, therefore, not be a feasible remediation technology on high-level U, Mo and As contaminated soils. These results suggest that citric acid induced phytoextraction may provide an environmentally friendly alternative for the decontamination of low-level U and Mo contaminated soils.