Biosorption and biodegradation by microorganisms to remove tributyltin (TBT), a toxic and persistent contaminant, have attracted more and more interests in recent years but information related to microalgae is still inadequate. The present study aims to investigate the removal and degradation of TBT at 10, 50 and 100 mu g Sn-1 contamination levels by alginate-immobilized Chlorella vulgaris beads during six consecutive cycles (4 days each), and compare the performance with blank alginate beads (without algae). More than 90% of TBT was rapidly removed, within 1 day, by both algal and blank beads, irrespective to the spiked TBT concentrations and the number of cycles, indicating the initial removal was mainly by biosorption and alginate matrix provided many binding sites. For algal beads, TBT was mostly adsorbed onto alginate matrix with some on algal cell walls, and < 10% of the spiked TBT accumulated inside cells. The amounts of debutylated products, dibutyltin (DBT) and monobutyltin (MBT) in medium increased gradually with treatment cycles and with the spiked TBT concentrations. At the contamination levels of 50 and 100 mu g Sn l(-1), both DBT and MBT in algal bead treatments increased to > 5 mu g Sn in cycle 6 while little DBT and MBT were detected in blank beads throughout the cycles. The accumulation of DBT and MBT, same as TBT, inside cells was relatively small compared to that in medium and alginate matrix. For the high TBT level of 100 mu g Sn l(-1), 60% of the spiked TBT was biodegraded by algal beads at the end of the six cycles. These results suggested that the alginate immobilized alga, C. vulgaris was able to continuously detoxify TBT into DBT and MBT for six consecutive cycles even at the highest TBT contamination level. (c) 2006 Elsevier Ltd. All rights reserved.