Repeated use of two Chlorella species, C-vulgaris and WW1 for cyclic nickel biosorption

Two living Chlorella species were used to remove nickel from solution conta ining 30 mug Ni ml(-1) in 10 successive cycles. The present study also exam ined the continued viability of these two algal species after repeated expo sure to nickel. The two species of Chlorella were Chlorella vulgaris (comme rcially available) and WW1 (indigenous species isolated from domestic sewag e and was tentatively identified as Chlorella miniata). The nickel removal percentage of WW1 cells was maintained at around 85% in the first five cycl es, then declined slightly from the fifth cycle onwards, and finally achiev ed around 70% removal at the end of the 10th cycle. On the contrary, the re moval efficiency of C. vulgaris declined from 50 to 30% during the 10 cycle s of nickel bisorption. At the end of these 10 successive cycles, WW1 accum ulated a substantial amount of Ni2+ (the cumulative cellular Ni concentrati on was 0.92% dry w.), while the value was only 0.17% in the case of C. vulg aris. These results suggest that the local isolate, WW1, had more consisten t and satisfactory ability fdr removing Ni than the commercial C. vulgaris. Both algal species were still capable of dividing after each nickel treatm ent cycle, suggesting that the cells were not killed even when significant amounts of nickel were adsorbed/absorbed. However, Ni exposure adversely af fected the physiological activity of algal cells as reflected by the declin e in division rate and chlorophyll-a activity in both species. Such negativ e effects became more obvious as the number of cyclic treatments was increa sed. Nevertheless, WW1 cells appeared to recover from nickel treatment when re-cultivated in commercial medium for 2 weeks. (C) 2001 Elsevier Science Ltd. All rights reserved.