Effects of low molecular weight organic acids on Cu accumulation by castor bean and soil enzyme activities

Chelating agents have been considered as an important phytoremediation strategy to enhance heavy metal extraction from contaminated soil. A pot experiment was conducted to explore the effects of low molecular weight organic acids (LMWOAs) on the phytoremediation efficiency of copper (Cu) by castor bean, and soil enzyme activities. Results indicated that the addition of all the three kinds of LMWOAs (citric, tartaric, oxalic acids) did not decrease the biomass of castor bean, despite the fact they reduced the concentration of chlorophyll-a in leaves compared to the control. The Cu concentrations in the roots and shoots significantly increased by 6-106% and 5-148%, respectively, in the LMWOAs treatments so that the total accumulation of Cu by whole plants in all the LMWOAs treatments increased by 21-189% in comparison with the control. The values of the translocation factor (TF) and bio-concentration factor (BCF) of Cu in castor bean also rose following the addition of LMWOAs, indicating that the LMWOAs enhanced the uptake and transportation of Cu. Moreover, the application of LMWOAs did not significantly change the soil pH but significantly increased the activity of soil enzymes (urease, catalase, and alkaline phosphatase). The addition of exogenous LMWOAs increased the available Cu significantly in the soil, thus promoted the phytoextraction efficiency of Cu by castor bean. These results will provide some new insights into the practical use of LMWOAs for the phytoremediation of heavy-metal-contaminated soil employing castor bean.