Sorption of organotin biocides to mineral surfaces

Abstract

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 aluminum, iron, and silicon (hydr)oxides. The TOTs studied include the two most widely used organotin pesticides, triphenyltin (TPT) and tributyltin (TAT), as well as shorter-chain trialkyltin homologues. In natural waters, these compounds are present predominantly as neutral TOT-OH species or as TOT+ cations (5.2 < pK(a) < 6.8). For all minerals investigated, sorption kinetics of TOTs were fast, and sorption was reversible. At clay minerals, sorption of TOTs was dominated by cation exchange of the TOT+ species. Adsorption of TOTs at homoionic clays increased with decreasing selectivity coefficients of the exchangeable cations (Na+ > K+ ≃ Rb+ > Cs+, Ba2+, Ca2+, Mg2+). On a surface area basis, TOT sorption to montmorillonite and illite was lower than to kaolinite, consistent with the surface charge densities of the clays and the absence of TOT+ intercalation. Since the dominating interaction of TOTs with all minerals was sorption of TOT+ cations to negatively charged surface sites, ≡XO-, sorption was strongly 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. Consistent with recently published data from marina and estuarine systems, our results demonstrate that sorption of TOT+ cations to minerals may significantly contribute to the overall sorption of TOTs to natural solid matrices.