| Previous studies of the aqueous phase speciation and stability of organometallic compounds have largely been conducted in the absence of humic substances. Evidence however, suggests that the formation of stable humic substance-organometallic complexes exes is affected by both coordination and ligand binding at the metal center, and nonpolar interactions. Organotin-humic substance (OT-HS) binding constants (K{dollar}rmsb{lcub}doc{rcub}){dollar} were determined through equilibrium binding studies using the dialysis technique. Tributyltin (TBT) binding by Aldrich humic acid (HA) was characterized by linear isotherms and the extent of binding was independent of variations in TBT and Aldrich HA over the range of concentrations examined, with an average log K{dollar}rmsb{lcub}doc{rcub}{dollar} of approximately 6.0. The degree of TBT-HS binding was several orders of magnitude greater that what would be predicted based upon its K{dollar}rmsb{lcub}ow{rcub},{dollar} indicating that nonpolar interactions only partially characterize their association. A significant decrease in binding of TBT by Aldrich HA was observed with increasing salinity, indicating that ionic interactions predominate in TBT-HS binding. The drop in K{dollar}rmsb{lcub}doc{rcub}{dollar} above pH 7.3 suggests that at higher pH, TBT preferentially complexes with OH{dollar}sp-{dollar} over the ligand groups present on HS. Although the binding of organic contaminants by HS has been correlated with various structural features of HS, there was not a strong relationship between any of the HS properties measured and the extent of TBT-HS binding. Binding of other OT compounds by Aldrich HA followed a general trend of increasing binding with increases in the number and/or chain length of the C groups attached to the Sn center. Though published stability constants for TBT-organic ligand complexes are lacking, the stability constants for dimethyltin and several organic ligands suggest that TBT may form similar complexes with humic substances. Thus the binding of TBT (and presumably OTs in general) by Aldrich HA observed in this study is primarily through complexation with the Sn center, with minor contributions from nonpolar interactions. |
