Removal of tributyltin (TBT) from wastewater by microalgae

The biosorption and biodegradation of tributyltin (TBT) by free cells of a resistant microalgal species, Chlorella sorokiniana at two different densities, 1×10⁶ cells ml^-1 (low) and 1 × 10⁷ (high) were investigated. TBT in contaminated water was continuously removed by microalgal cells and reached a 54% and 74% removal at the end of 14 days treatment in low and high density cultures, respectively. The pattern of TBT removal in both densities was similar: with the decreases of TBT in water, TBT uptake by algal cells increased, reached a peak and declined to low levels towards the end of the treatment period. In the high density culture, total amounts of TBT uptake, including the adsorption on cell surface and absorption inside the cells, increased from 109 to 159μg TBT⁺from days 1 to 7, then declined to 95μg TBT⁺ at day 14. The proportions of TBT adsorbed on the cell surface decreased while that in the intracellular fraction increased with time, suggesting that the adsorbed TBT moved gradually into the cells for stepwise debutylation. The two degradation products, DBT (dibutyltin) and MBT (monbutyltin), started to appear in the intracellular fraction from day 3 and day 7onwards in high and low density cultures, respectively, with more MBT than that of DBT. These results indicate that C. sorokiniana was not only capable of removing TBT from contaminated water, TBT was absorbed and degraded into DBT which was then rapidly metabolized to MBT by intracellular enzymes.