Modeling of acute cadmium toxicity in solution to barley root elongation using biotic ligand model theory

Protons (H⁺) as well as different major and trace elements may inhibit cadmium (Cd) uptake in aquatic organisms and thus alleviate Cd toxicity. However, little is known about such interactions in soil organisms. In this study, the independent effects of the cations calcium (Ca^2 +), magnesium (Mg^2 +), potassium (K⁺), H⁺ and zinc (Zn^2 +) on Cd toxicity were investigated with 5-day long barley root elongation tests in nutrient solutions. The tested concentrations of selected cations and trace metal ions were based on the ranges that occur naturally in soil pore water. The toxicity of Cd decreased with increasing activity of Ca^2 +, Mg^2 +, H⁺ and Zn^2 +, but not K⁺. Accordingly, conditional binding constants were obtained for the binding of Cd^2 +, Ca^2 +, Mg^2 +, H⁺, and Zn^2 + with the binding ligand: logK_CdBL 5.19, logK_CaBL 2.87, logK_MgBL 2.98, logK_HBL 5.13 and logK_ZnBL 5.42, respectively. Furthermore, it was calculated that on average 29% of the biotic ligand sites needed to be occupied by Cd to induce a 50% decrease in root elongation. Using the estimated constants, a biotic ligand model was successfully developed to predict the Cd toxicity to barley root elongation as a function of solution characteristics. The feasibility and accuracy of its application for predicting Cd toxicity in soils were discussed.