The biosorption of Hg2+ by two strains of cyanobacteria, Spiridina platensis and Aphanothece flocculosa, was studied under a batch stirred reaction system. Essential process parameters, including pH, biomass concentration, initial metal concentration, and presence of co-ions were shown to influence the Hg2+ uptake. Hg2+ uptake was optimal at pH 6.0 for both strains. The maximum loading capacities per gram of dry biomass were found to be 456 mg Hg2+ for A. flocculosa and 428 mg Hg2+ for S. platensis. At an initial concentration of 10 ppm Hg2+, A. flocculosa was able to remove more than 98% of the mercury ion from solution. The biosorption kinetics of both strains showed that the metal uptake is bi-phasic, exhibiting a rapid initial uptake followed by a slower absorption process. The presence of dissolved Co2+, Ni2+, and Fe3+ were found to play a synergistic role for Hg2+ uptake by both strains. Regeneration of the biomass was examined by treating Hg2+-loaded samples with HCl and NH4Cl over four cycles of sorption and desorption. (c) 2007 Elsevier Ltd. All rights reserved.