Organic Ligand and pH Effects on Isotopically Exchangeable Cadmium in Polluted Soils

Abstract

Organic ligands excreted by plant roots and pH changes in the rhizosphere are two factors that may modify the phytoavailability of Cd. The effect of these components on the quantity (isotopically exchangeable) and intensity (distribution between the solid and solution phases) of Cd in a polluted acidic and calcareous soil was examined in detail using a batch experimental system. The organic ligands included (0.25–5 mM) sodium tartrate, the free acid and sodium salt of citrate, histidine, and deoxymugineic acid (DMA). A reduction of pH was identified as the mechanism of Cd solubilization in the presence of some ligands. In other cases, however, organic ligands caused greater Cd solubilization than that because of pH changes alone. It was concluded that organic ligands might increase solution concentrations of Cd, in both soils, through a number of reaction mechanisms other than decreasing pH (e.g., varying the surface charge of the soil, cation exchange, aqueous metal complexation, etc.). In the acidic soil a reduction of pH from 5.7 to 3.8 only increased the quantity of isotopically exchangeable Cd (E value) by 17%, thus, indicating the existence of a recalcitrant nonlabile fraction of Cd in this soil. The organic ligands were also unable to significantly alter the E value of Cd. In contrast, the presence of organic ligands and decreases in pH elevated the E value of Cd in the calcareous soil by as much as 300%. It is postulated that conditions in the rhizosphere may not only increase the solution concentration of Cd, but also increase the quantity of phytoavailable Cd (L value).