ASSOCIATION OF TRIORGANOTIN COMPOUNDS WITH DISSOLVED HUMIC ACIDS

Sorption to dissolved organic matter (DOM) may influence significantly the fate and the effects of organic pollutants in the aquatic environ ment. To date, most studies on DOM-water partitioning have focused on neutral hydrophobic compounds. Very little is known on the binding of hydrophobic organic cations to DOM. In this study, the association of triorganotin compounds (TOTs) with dissolved Aldrich and Suwannee Rive r humic acids has been systematically investigated as a function of pH and sodium perchlorate concentration. The organotin compounds studied include the two widely used biocides tributyltin (TBT) and triphenylt in (TPT) as well as other trialkyltin compounds of various hydrophobic ities. Between pH 3 and pH 9, for both TBT and TPT, the overall DOM-wa ter distribution ratio (D-DOM) was strongly pH-dependent and exhibited a maximum close to the acidity constant (pK(a)) of the compounds. The observed pH dependence of D-DOM could be described successfully with a semiempirical discrete log K spectrum model. it was found that, over the whole pH range investigated, sorption was governed by complexatio n of the corresponding TOT cation (TOT+) by negatively charged ligands (i.e., carboxylate and phenolate groups) of the humic acids. The dete rmining factors of the TOT+ binding are postulated to be (i) complex f ormation between the tin atom and the deprotonated ligands and (ii) hy drophobic interactions. Significant differences in D-DOM of TBT were o bserved between Suwannee River and Aldrich humic acid. D-DOM values of TOTs determined for Aldrich humic acid were in the same range as part iculate organic matter-water distribution ratios reported in the liter ature for soils and sediments.