Antimony(III) binding to humic substances: Influence of pH and type of humic acid

Conditional distribution coefficients (D-om) for Sb(Ill) binding to three commercial humic acids (terrestrial, coal, and aquatic) were measured at environmentally relevant Sb(III)/DOC ratios and as a function of pH using an equilibrium dialysis method. Maximum binding of Sb(III) was observed around pH 6 for two of the humic acids. The third humic acid showed constant Dam values up to pH 6 and decreasing D-om values for pH > 6. Sb(III)/DOC ratio was found to be important for D-om (20times higher D-om for 60 times lower Sb(III)/DOC ratio). Moreover, Dam depends on the individual humic acid, suggesting that different functional groups are involved and/or different degrees of stabilization by chelation or H-bridges. Chemical modeling of Sb(III)-humics binding at different pH values is consistent with two binding sites involving (i) a phenolic entity forming a neutral complex and (ii) a carboxylic entity forming a negatively charged complex. Under environmentally relevant conditions, over 30% of total Sb(III) may be bound to natural organic matter.