Enhancement of the Dissolution and Removal of Soil Heavy Metals by Adding Reductants.

Paddy Soils polluted by heavy metals have caused serious impacts to the agricultural productivity in Taiwan. Dilution (i.e., mixing with clean soils) or soil washing with organic chelate-containing acidic solutions is alternative remediation technique currently accepted by Taiwan EPA to decrease the concentrations of heavy metal in these paddy soils. However, these techniques may deteriorate greatly soil qualities, such as soil structures, fertilities, and normal biofunctions. In addition, unaccepted amount of heavy metals are still found in the rice grown in those treated sites. Since our previous study found that heavy metals, particularly Cd, associated strongly with Fe-Mn oxides in paddy soils in Taiwan. The rapid establishment of an anaerobic condition for treated soils may lead to the dissolution of Fe-Mn oxides and release subsequently associated heavy metals. Therefore, in this study, a reductant (i.e., glucose) was applied to evaluate the efficiencies of reductive dissolution and removal of heavy metals from Changhua-Hemei and Taoyuan-Luzhu red soils. The results show that pH decreased rapidly from 6.7 to 5.1 and from 6.5 to 3.6 ;the Eh decreased from and +205 to -354 mV and from +337 to -147 mV, respectively, with the addition of reductant within 35 days. Dissolved and exchangeable Cd, Cu, Zn and Ni as defined by Tessier technique increased significantly with decreasing Eh in Changhua-Hemei soil. However, only did Cd and Zn have the same trend for Taoyuan-Luzhu soil. The change in dissolved and exchangeable Cr was insignificantly in both tested soils which may suggested the low association of Cr with Fe-Mn oxides. The results indicate that the addition of reductants to enhance the reductive dissolution of heavy metals is more efficient for specific metals, such as Cd. Since Cd has received much concern in rice production in Taiwan due to its high toxicity, the mild reduction technique may be developed to remediate the paddy soils polluted by Cd.