title: New model calculations of pH-depending tributyltin adsorption onto montmorillonite surface and montmorillonite-rich sediment.
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 >AIOH and >SiOH sites atthe 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 [logK(TBT/2) = -1.18 for (>SOH), logK(TBT/1) = 3.98 for (>SsOH)]. LogK(X/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.
authors.affiliation: Institut für Geologie und Mineralogie, Universität Erlangen-Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany. mhoch@geol.uni-erlangen.de
authors.forename: Marion
authors.initials: M
authors.lastname: Hoch
authors.forename: Rohan
authors.initials: R
authors.lastname: Weerasooriya
mesh_terms: D000327:Adsorption; D001546:Bentonite / Q000737:chemistry*; D019015:Geologic Sediments / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008962:Models, Theoretical; D014220:Trialkyltin Compounds / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014220:Trialkyltin Compounds; D014874:Water Pollutants, Chemical; D001546:Bentonite; C011559:tributyltin