| Abstract: |
Interactions of the pollutant tributyltin (TBT) with mineral surfaces affects its distribution and transport in aqueous systems. In the present work, model calculations are reported that quantify TBT adsorption onto pure-phase montmorillonite (sample SWy) under various pH and salinity conditions that are important from an environmental perspective. The pH level in the system is of substantial interest because it affects the speciation of TBT in solution as well as the surface properties of the solid phase, which are both important for adsorption reactions. The model is based on the generalized diffused layer model that includes >X- sites in order to account for the cation exchange effects of TBT attraction. The presence of >AlOH and >SiOH sites at the mineral surface was not considered separately during calculation. Instead, nonselective sites (>SOH) versus selective sites (>SsOH) were distinguished with respect to the sorptive sites on montmorillonite. The latter are characterized by a high affinity of TBT bonding. Both sorptive sites exhibit the same protolysis constants but different TBT binding constants [logKTBT/2 = −1.18 for (>SOH), logKTBT/1 = 3.98 for (>SsOH)]. LogKX/TBT for the cation exchange reaction was determined as between 3.05 and 4.14. The results indicate that the inclusion of selective sites during calculations is essential for quantifying pH-dependent TBT adsorption successfully. The parameters determined for the TBT adsorption onto pure-phase montmorillonite were subsequently used to calculate pH-dependent TBT adsorption onto a natural montmorillonite-rich sediment. |
