The use of chloro-complexation to enhance cadmium uptake by Zea mays and Brassica juncea: testing a "free ion activity model" and implications for phytoremediation.

Maize and Indian mustard plants were studied to: 1) investigate the effect of Cl- complexation on Cd uptake from soil historically amended with sewage sludge (Cd 58 mg kg(-1)) and, 2) model the uptake of Cd by these plants with a Free Ion Activity Model (FIAM). Plants were treated with NaCl (50 to 300 mM in the soil pore water) along with controls using Na2SO4. Cadmium enhanced solubility in soil by Cl- generally reflected increases in Cd uptake by both plants. The free ion Cd2+ activity in soil solution, as modeled by WHAM-VI, remained almost unchanged despite the wide range of NaCl concentrations. Therefore, Na+ exchange for Cd2+ could not fully explain the differences in Cd content between the Cl- treatments because of the high buffering Cd2+ capacity in soiL Activities of Cd-chloro complexes showed the best correlations with the Cd concentrations in the plants compared to the activity of Cd2+. The FIAM showed a reasonable good fit for the plants when assuming competition by Cd2+ and CdCl+ for root sorption sites. Indirect evidence suggests that CaSO4 precipitation may have limited the formation of CdSO4 complexes and reduced Cd soil solubility. The implications of these results for phytoremediation are discussed.