THE ROLE OF CITRIC ACID IN ENHANCED PHYTOEXTRACTION OF HEAVY METALS IN AN ANDOSOL BY Crotalaria juncea

A pot experiment was conducted to compare natural versus citric acid-assisted phytoextraction of heavy metals and their distribution in fractionated root zone soils using Crotalaria juncea. Seedlings of C. juncea were grown in an Andosol containing total concentrations of 9.2, 156.3, and 1480 mg kg(-1) Cd, Zn, and Mn respectively, and 20 mmol kg(-1) of citric acid were added to the pots. After 25 days of growth, the plants were harvested, and soil samples were fractionated into rhizoplane (RP), rhizosphere (RZ), and bulk soil (BK). C juncea exhibited a high bioaccumulation factor (2.04) as well as a high ability for hyperaccumulation of Cd. In comparison, the plant was less able to accumulate Zn and Mn in the shoots. The pH in the soil fractions followed the series: RP<RZ<BK. C. juncea decreased the concentration of 0.1 M HCl extractable heavy metals in BK but increased them in RP. Higher HCl extractable metals in RP may be associated with lower pH. Citric acid showed the greatest capacity in desorbing heavy metals in the Andosol compared to other organic acids such as oxalic, malic, and tartaric acids. The addition of citric acid did not significantly chancre the concentrations of Cd and Zn in roots and shoots hereas the concentration of Mn in the roots and shoots increased about 2- and 4-fold, respectively. The effect of citric acid on the phytoextraction of Mn probably corresponded to the increased mobility caused by citric-manganese complexation.