Extension of a biotic ligand model for predicting the toxicity of metalloid selenate to wheat: the effects of pH, phosphate and sulphate

Abstract It has not been well understood that the influences of pH and accompanying anions on the toxicity of selenate (Se(VI)). The influences of pH and major anions on Se(VI) toxicity to wheat root elongation were determined and modeled based on the biotic ligand model (BLM) and free ion activity model (FIAM) concepts. Results showed that EC50[Se(VI)]T values increased from 162 to 251 μM as the pH values increased from 4.5 to 8.0, indicating that the pH increases alleviated the Se(VI) toxicity. The EC50{SeO42-} values increased from 133 to 203 μM while the EC50{HSeO4-} values sharply decreased from 210 to 0.102 nM with the pH increasing from 4.5 to 8.0. The effect of pH on Se(VI) toxicity could be explained by the changes of Se(VI) species in different pH solutions as SeO42- and HSeO4-were differently toxic to wheat root elongation. The toxicity of Se(VI) decreased with the increasing activities of H2PO4- and SO42- but not for NO3- and Cl- activities, indicating that only H2PO4- and SO42- had competitive effects with Se(VI) on the binding sites. An extended BLM was developed to consider effects of pH, phosphate and sulphate , and stability constants of SeO42-, HSeO4-, H2PO4- and SO42- to the binding sites were obtained: log K S e O 4 B L =3.45, log K H S e O 4 B L =5.98, log K H 2 P O 4 B L =2.05, log K S O 4 B L =1.85. Results implied that BLM performed much better than FIAM in the wheat root elongation prediction when coupling with toxic species SeO42- and HSeO4-, and the competitions of H2PO4- and SO42- for the binding sites while developing the Se(VI)-BLM.