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 14C-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 Kd values occurring at low to intermediate salinities) and pH (highest Kd 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; Kd 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.