Modeling interactions at the tributyltin-kaolinite interface

The generalized diffused double layer model (DLM) was used to quantify both, proton and TBT adsorption. Following intrinsic acidity and TBT binding constants resulting from the TBT/kaolinite system were used: >SOH ⇒ >SO− + H+, log K = −5.4; >SOH + H+ ⇒ SOH2+, log K = 4.6; >XNa + H ⇒ XH + Na+, log KX/H+ = −1.1; >SO− + TBT+ ⇒ >SOTBT, log K = 3.5; >XNa + TBT+ ⇒ Na+, log KX/TBT = 1.0. All surface-active variable charge sites on kaolinite, namely >AlOH and >SiOH are grouped and collectively refer to as >SOH in this paper. >XNa refers to ion exchange sites. Modeling of TBT adsorption onto kaolinite was conducted distinguishing selective (high affinity) sites (>SSOH) in addition to non-selective sites (>SOH). The inclusion of >SSOH was essential in order to quantify TBT adsorption successfully, while the inclusion of >XNa was optional. The reduction of surface coverage values by a 10-fold TBT adsorption modeling is in agreement with the results of molecular model calculations of the system. Parameters calculated for the monophase kaolinite were subsequently used to quantify the TBT adsorption onto kaolinite-rich sediment.