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

Abstract

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.

KEY WORDS:

• Soil salinity
• phytoextraction
• maize
• Indian mustard
• Cadmium complexation
• FIAM

ACKNOWLEDGMENTS

This study was supported by CONACYT, Mexico, and Severn Trent Water Ltd as co-funders of the Biorem 11 (PASS) project under the Bioremediation Link Programme (UK BBSRC).