Association of organotin compounds with aquatic and terrestrial humic substances

Organotin-humic substance (OT-HS) binding constants ( Kdoc) were determined by equilibrium-binding studies using dialysis techniques. The OT-HS binding was characterized by linear isotherms. The Kdoc values measured in this study were several orders of magnitude greater than would be predicted based only on nonpolar interactions with HS (as estimated by Kow). Binding of tri-n-butyltin (TBT) by Aldrich humic acid decreased with increasing salinity and pH, indicating that ionic interactions predominate in TBT-HS binding. Thus, the binding of OT by HS observed in this study occurs primarily through complexation, with secondary contributions from nonpolar interactions. The effects of the physicochemical properties of HS (e.g., molecular weight, % aromaticity, oxygen content) on TBT-HS binding was examined with HS from a range of sources, including a commercial humic acid and HS from aquatic and terrestrial sources. Although binding of organic contaminants by HS has been correlated with various structural features of HS, no meaningful relationship between any of the HS properties examined in this study and the extent of TBT-HS binding was found.