Evaluating a ‘biotic ligand model’ applied to chloride‐enhanced Cd uptake by Brassica juncea from nutrient solution at constant Cd2+ activity

Abstract

Evidence of chloride‐enhanced cadmium uptake by plants in soil experiments has been reported. However, it is still unclear whether this finding is due to increased rates of Cd2+ diffusion to plant roots or the direct uptake of complexes such as CdCl+. A controlled hydroponic experiment was undertaken to distinguish and quantify the uptake rates of free and inorganic‐complexed cadmium and to model the uptake of cadmium by Indian mustard plants with a ‘biotic ligand model’. Plants were treated with NaCl (0 to 200 mM) including equivalent Na2SO4 treatments. Cadmium speciation in solution was calculated using the WHAM‐VI model. Results of the current trials showed that higher Cl− concentrations in solution generally resulted in greater cadmium accumulation by plants than predicted by the Cd2+ activity. Activities of Cd–chloro complexes showed the best correlations with the cadmium concentrations in the plants compared with the activity of Cd2+. The biotic ligand model showed a reasonable good fit for the plants when assuming competition by Cd2+ and CdCl+ for sorption sites at root level. The relative values of the two reaction constants suggest that root affinity for Cd2+ is 3.4 times greater than for CdCl+. Nevertheless this clearly indicates a substantial role for chloro‐complexed cadmium accumulation.