Fate Of Tributyltin

The degradation of TBT in natural waters is primarily due to biological processes resulting in TBT half-lives ranging from 4 to 14 days. In upper waters with high nutrient levels, microalgae or bottom plants can be important in TBT degradation. Biological TBT degradation in sunlight resulted in the production of various hydroxybutyldibutyltins. TBT half-life in sunlit estuarine water supplemented with nutrients was 1 day. Fine-grained sediments can catalyze the non-biological dealkylation of TBT to form monobutyltin and inorganic tin. The half-lives of TBT added to fine-grained sediments ranged from 2 to 4 days. Biological, i.e. microbial, degradation of TBT added to sandy sediments resulted in longer half-lives, e.g. 13 days for one sandy area. Further work is required to assess the fate of TBT in anaerobic sediments and the TBT associated with paint chips in the sediments. INTRODUCTION The fate of tributyltin, like other toxic organics, is determined by various physical, chemical and biological processes. Photolysis and biological degradation act to modify TBT in coastal waters. Some of the TBT adsorbs to suspended particulates and these particulates can be carried to the bottom by sedimentation. Chemical and biological degradation can degrade TBT in sediment to various metabolic products. Since the fate of TBT has recently been reviewed (Maguire, 1987; Seligman et al., 1989), this article briefly discusses processes affecting the fate of TBT and describes in more detail the latest work on TBT degradation in both sediment and water. Fate of TBT in Water At TBT concentrations of 1 pg/liter or less, there was biological degradation resulting in TBT half-lives of 414 days (Francois et al., 1989; Hattori et al., 1987; Hinga et a1.,1987; Olson and Brinckman, 1986; Seligman et al., 1989; Thain et al., 1987). At high TBT concentrations (= 1 mgfliter) degradation was very slow, presumably due to inhibition of TBT-degrading microorganisms (Seligman et al., 1986). In the dark bacteria and fungi are important in TBT degradation. Recent studies have shown that photosynthetic organisms in natural waters play an important role in TBT degradation (Francois et al., 1989; Lee et al., 1989; Olson and Brinkman, 1986). These studies demonstrated higher TBT degradation in sunlit estuarine water compared with dark degradation rates (Table l), production of hydroxybutyldibutyltins in the light but not in the dark (Table 2), and metabolism of TBT to dibutyltin and hydroxylated metabolites by cultures of diatoms and dinoflagellates (Table 3). Maguire et al. (1984) found that the freshwater green algae, Ankistrodesmus falcatus, degraded TBT to dibutyltin, a small amount of butyltin and inorganic tin. The marine green algae (Dunaliella tertiolecta) and chrysophytes (Isochrysis galbana and Cricosphuera ricoco) showed a very limited ability to degrade TBT (Table 2). Both diatom cultures and sunlit natural waters with added TBT produced hydroxybutyldibutyltin compounds, which included delta-hydroxybutyldibutyltin, gammahydroxybutyldibutyltin, and beta-hydroxybutyldibutyltin (Table 2). There was a significant increase in TBT when nitrate was added to estuarine water before light exposure. The TBT half-life was 1 day in nitrate supplemented water. There was no increase in TBT degradation when nitrate was added to water kept in the dark. Eel grass (Zostera marina) found in intertidal areas can degrade TBT to diand monobutyltin (Francois et al., 1989). The half-lives of TBT in eelgrass under light and dark were 6.7 days and 13.8 days, respectively. Sunlight photolysis of TBT was relatively slow (half-life = 90 days, Maguire et al., 1983), since TBT absorbs in the ultra-violet region. Sterilization of estuarine water before adding TBT followed by incubation under sunlight resulted in no significant degradation of TBT after 6 days (Lee et al., 1989). Possibly, TBT found in the surface microlayer (Maguire and Tkacz, 1987) could be more rapidly degraded by direct photolysis. Fate of TBT in Sediment TBT adsorbed to suspended particulates in the water can enter the bottom by sedimentation processes. The TBT can gradually desorb from these particles (Unger et al., 1988). In areas Table 1 Degradation of Radiolabeled and Nonlabeled Tributyltin (TBT) Added to Estuarine Water from Skidaway River (Savannah, GA) (Data from Lee et al., 1989) Initial TBT Month/ TBT Concentration Treatment Temperature Half-life (Witer) (days) 0.5 (unlabeled) Light Sept./28-29"C 6 0.91 (unlabeled) Dark SeptJ28-29OC 9 1.5 (labeled) Light AugJ29"C 6 1.5 (labeled) Dark Aug.129"C 10 1.30 (unlabeled) Light Feb./lZ"C 7 1.60 (unlabeled) Dark Feb.ll2OC 11 1.5 (labeled) Light Jan./l 1°C 8 1.5 (labeled) Dark Jan./l 1 "C 13 0.4 (labeled) Light July/28 "C 3 0.4 (labeled) Dark July128 "C 7 512