Extension of biotic ligand model to account for the effects of pH and phosphate in accurate prediction of arsenate toxicity.

Biotic ligand model (BLM) was extended to predict the toxicity of inorganic arsenate (iAs(V)) to the luminescent bacteria, Aliivibrio fischeri. As the pH increased from 5 to 9, the HAsO form predominated more than the HAsO form did, and the EC[As] (50% effective iAs(V) concentration) decreased drastically from 3554 ± 393 to 39 ± 6 μM; thus, the HAsO form was more toxic to A. fischeri than HAsO. As the HPO activity increased from 0 to 0.44 mM, the EC{HAsO} values (50% effective HAsO activity) increased from 31 ± 6 to 859 ± 128 μM, indicating that the toxicity of iAs(V) decreased, owing to the competition caused by the structural similarity between iAs(V) and phosphate ions. However, activities of Ca, Mg, K, SO, NO, and HCO did not significantly affect the EC{HAsO} values. The BLM was reconstructed to take into account the effects of pH and phosphate, and the conditional binding constants for HPO, HPO, HAsO, and HAsO to the active binding sites of A. fischeri were obtained; 3.424 for logK, 4.588 for logK, 3.067 for logK, and 4.802 for logK. The fraction of active binding sites occupied by iAs(V) to induce 50% toxicity (f) was found to be 0.616.