An improved biotic ligand model (BLM) for predicting Co(II)-toxicity to wheat root elongation: The influences of toxic metal speciation and accompanying ions.

In order to explore the effects of pH and accompanying ions on divalent cobalt (Co(II)) toxicity to the wheat root elongation, an improved biotic ligand model (BLM) to predict Co(II) toxicity was developed in solution culture. The results showed that the Co(II)-toxicity decreased with the increases of K, Ca and Mg activities, and the activity of Na had no impact on the Co(II)-toxicity. High H activity reduced the Co(II)-toxicity by the competitive effect of H, while low H activity affected the toxicity by the change in the type of Co(II) in culture medium. Co and CoOH were toxic to wheat root elongation, and Co(OH) was not. On the basis of BLM theory, the conditional equilibrium constants for Co, CoOH, H, Mg, Ca, K were obtained: logK = 4.65, logK = 6.62, logK = 4.53, logK = 3.65, logK = 2.36 and logK = 2.17. Free Co and CoOH, and the competitions of K, Mg and Ca were suggested to be considered when developing the Co(II)-BLM.