Association of Triorganotin Compounds with Dissolved Humic Acids

Abstract

Sorption to dissolved organic matter (DOM) may influence significantly the fate and the effects of organic pollutants in the aquatic environment. 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 River 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 triphenyltin (TPT) as well as other trialkyltin compounds of various hydrophobicities. Between pH 3 and pH 9, for both TBT and TPT, the overall DOM−water distribution ratio (DDOM) was strongly pH-dependent and exhibited a maximum close to the acidity constant (pKa) of the compounds. The observed pH dependence of DDOM 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 complexation of the corresponding TOT cation (TOT+) by negatively charged ligands (i.e., carboxylate and phenolate groups) of the humic acids. The determining factors of the TOT+ binding are postulated to be (i) complex formation between the tin atom and the deprotonated ligands and (ii) hydrophobic interactions. Significant differences in DDOM of TBT were observed between Suwannee River and Aldrich humic acid. DDOM values of TOTs determined for Aldrich humic acid were in the same range as particulate organic matter−water distribution ratios reported in the literature for soils and sediments.