Sorption and desorption behavior of organotin compounds in sediment-pore water systems

Sediments contaminated with organotin compounds (OTs), in particular triorg anotins (TOTs), are abundant in areas with high shipping activities. To ass ess the possible remobilization of these highly toxic compounds from such s ediments, a profound understanding of their sorption/desorption behavior is necessary. In this work the extent and reversibility of sorption of US to sediments has been investigated using contaminated freshwater harbor sedime nts and two certified OT containing marine sediments. Experiments conducted with perdeuterated OTs showed that sorption of OTs to sediments is a fast and reversible process involving primarily particulate organic matter (POM) constituents as sorbents. The organic carbon-normalized sediment-water dis tribution ratios (D-oc, expressed in L/kg(oc)) determined in the laboratory were consistent with in-situ D(oc)s obtained from OT concentrations measur ed in sediment and pore water samples from two dated sediment cores. For bo th butyl- and phenyltin compounds the log D-oc values were in the range of 4.7-6.1, and the following sequence was observed: D-oc (tri-OT) greater tha n or equal to D-oc (di-OT) greater than or equal to D-oc (mono-OT). However , the differences were much less pronounced than would have been expected f or hydrophobic partitioning of the corresponding compounds into POM. These results support our hypothesis from earlier work with dissolved humic acids that OT sorption to sediments occurs primarily by reversible formation of (innerspere) complexes between the tin atom and carboxylate and phenolate l igands present in POM. Because of the high D-oc values (i.e. log D-oc great er than or equal to 4) the diffusion of OTs from deeper sediments-to the su rface will be rather slow, and thus a major release from undisturbed sedime nts is not expected. However, because OTs readily desorb any resuspension o f contaminated sediments (e.g., by the tide, storms or dredging activities) will lead to enhanced OT concentrations in the overlaying water column. Fu rthermore, in contrast to polycyclic aromatic hydrocarbons (PAH) where larg e fractions may be tightly bound (in)to soot or other carbonaceous material s, OTs will be more readily bioavailable due to the fast and reversible sor ption/desorption behavior.