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

Abstract

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 HAsO42- form predominated more than the H2AsO4 form did, and the EC50[As](T) (50% effective iAs(V) concentration) decreased drastically from 3554 +/- 393 to 39 +/- 6 mu M; thus, the HAsO(4)(2- )form was more toxic to A. fischeri than H2AsO4. As the HPO42- activity increased from 0 to 0.44 mM, the EC50 {HAsO42-} values (50% effective HAsO42- activity) increased from 31 +/- 6 to 859 +/- 128 pM, 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 Ca2+ , Mg2+ , K+ , SO42-, NO3-, and HCO3- did not significantly affect the EC50{HAsO42-} values. The BLM was reconstructed to take into account the effects of pH and phosphate, and the conditional binding constants for H2PO4-, HPO42-, H2AsO4-, and HAsO42- to the active binding sites of A. fischeri were obtained; 3.424 for logK(XH2PO4), 4.588 for logK(XPO4) , 3.067 for logK(XH2AsO4), and 4.802 for logK(XH2AsO4). The fraction of active binding sites occupied by iAs(V) to induce 50% toxicity (f(mix) 50% ) was found to be 0.616.