SORPTION OF ORGANOTIN BIOCIDES TO MINERAL SURFACES

The sorption of triorganotin biocides (TOTs) from aqueous solution to mineral surfaces was investigated in batch sorption experiments using homoionic clay minerals (kaolinites, montmorillonites, illites), and a luminum, iron, and silicon (hydr)oxides. The TOTs studied include the two most widely used organotin pesticides, triphenyltin (TPT) and trib utyltin (TBT), as well as shorter-chain trialkyltin homologues. In nat ural waters, these compounds are present predominantly as neutral TOT- OH species or as TOT+ cations (5.2 < pK(a) < 6.8). For all minerals in vestigated, sorption kinetics of TOTs were fast, and sorption was reve rsible. At clay minerals, sorption of TOTs was dominated by cation exc hange of the TOT+ species. Adsorption of TOTs at homoionic clays incre ased with decreasing selectivity coefficients of the exchangeable cati ons (Na+ > K+ approximate to Rb+ much greater than Cs+, Ba2+, Ca2+, Mg 2+). On a surface area basis, TOT sorption to montmorillonite and illi te was lower than to kaolinite, consistent with the surface charge den sities of the clays and the absence of TOT+ intercalation. Since the d ominating interaction of TOTs with all minerals was sorption of TOT+ c ations to negatively charged surface sites, =XO-, sorption was strongl y pH dependent, and sorption maxima occurred at the maximum overlap of TOT+ and =XO- concentrations. Thus, high TOT sorption to (hydr)oxide minerals occurred only if a significant fraction of negatively charged surface sites was present at pH values where TOT+ species predominate , i.e., to minerals exhibiting low pH(ZPC) Values such as silica. Cons istent with recently published data from marine and estuarine systems, our results demonstrate that sorption of TOT+ cations to minerals may significantly contribute to the overall sorption of TOTs to natural s olid matrices.