DETERMINANTS OF TBT ADSORPTION AND DESORPTION IN ESTUARINE SEDIMENTS

Tributyltin (TBT) has a low aqueous solubility and relatively high affinity for particulate matter, providing a direct and potentially persistent route of entry into benthic sediments. The need to determine the fate of TBT in such environments is consequently a priority as sediments become the major reservoir for this pollutant. A review of previously reported studies indicates highly variable behaviour in different sediments and environmental conditions, and requires explanation if the long-term threat of TBT is to be accurately assessed. In the present study, the influence of some major physico-chemical factors which control TBT partitioning is clarified. Batch testing methods using C-14-TBT and natural sediment/water systems have been validated, and partitioning found to exhibit a (Freundlich) dependence on TBT concentration. Partitioning is influenced by salinity (lowest K-d values occurring at low to intermediate salinities) and pH (highest K-d values existing at circum-neutral pH), and TBT appears to show characteristics of both metal ions and hydrophobic materials based on these parameters. Sediment characteristics also influence partitioning; K-d is positively correlated with total organic material, humic substances, Mn, and a high silt content. Although sediments are the major reservoir for TBT-particularly in organic-rich silts in sheltered areas-some desorption and loss does occur in association with declining inputs from leisure vessels. Local conditions modify the ultimate persistence and fate of this compound, however, and steady-state conditions can prevail even in small-boat dominated locations, where TBT legislation should be most effective.