Improvement of selective removal of heavy metals in cyanobacteria by NaOH treatment

In the freshwater cyanobacterium, Tolypothrix tenuis, treatment with 0.1 M NaOH increased its Cd-selective adsorption ability in the presence of Ca2+ or Mg2+. The selective adsorption was also achieved by other alkaline treatments. Energy-distributed spectroscopy analysis revealed that Cd2+ was found mainly on the surface of non-treated cells, whereas it was distributed throughout the cell after NaOH treatment. The alkaline treatment was effective in increasing the selective adsorption ability of the cyanobacterium for other bivalent heavy metals such as Cu2+, Pb2+ and Zn2+. The treatment was also applicable to Anabaena variabilis and Microcystis aeruginosa, which are typical cyanobacteria causing algal blooms. The main binding site of Cd2+ in NaOH-treated cells is assumed to be the carboxyl groups because the binding ability of the cells was diminished by the esterification of carboxyl groups. These results suggest that alkaline treatment of cyanobacteria is a useful technique for producing biosorbents having highly specific binding abilities for heavy metals.