Removal of heavy metal by using microalgae

Removal of Hg, Pb and Cd in wastewater was investigated using microalgae th at were isolated in Thailand. Among 35 strains obtained, 20 strains exhibit ed high tolerance and 14 strains exhibited high removal ability of these he avy metallic ions. Of these, a cyanobacterium, Tolypothrix tenuis, exhibite d high level of tolerance to Hg, Pb and Cd and high Cd removal ability over a wide range of temperature and pH's. The Cd removal reached a plateau wit hin 30 min, and was the type that could be described by the Langmuir isothe rm equation. Thus, it was suggested to be due to adsorption of Cd onto the microalgal surface. The adsorption of Cd was not strongly affected by the p resence of Na, K, Ca, and Mg ions. Cadmium adsorption was greatly increased by pretreating the cells with 0.1 M NaOH. Experiments were also done to in clude removal of Cd from hard water that had high concentrations of Ca2+. L angmuir parameters were calculated and the amounts of Cd adsorbed were comp ared for different concentrations of Ca2+. Results showed that NaOH pretrea ted cells showed increased Cd adsorbing ability in the presence of Ca2+. Pr etreatment with NaOH was also found to be applicable to other cyanobacteria tested. These results suggested that biomass of NaOH treated filamentous c yanobacteria could be utilized as a novel biosorbent for Cd removal, even f rom hard water containing high concentrations of Ca2+ and/or Mg2+. Our resu lts suggested that NaOH-pretreated T tenuis might be applicable for removal of Cd from surface and ground water in certain outdoor situations.