Biosorption capacity and kinetics of cadmium(II) on live and dead Chlorella vulgaris

Pollution of aquatic environments with heavy metals from natural water is a serious problem because of the toxicity of heavy metals to humans, fish, and other live organisms. Cheap and environmentally friendly methods for removing heavy metals from water are therefore needed. Algae have emerged as a promising biosorbent to bioextract heavy metal ions by adsorption, and our objective was to evaluate the biosorption capacity and kinetics of cadmium ions by live and dead cells of the microalga Chlorella vulgaris. The biosorption of cadmium was assessed by varying the sorption parameters: use of dead or live material of C. vulgaris, contact time, initial metal ion concentration, and algal dosage. Cadmium ion removal was rapid with more than 95 % of total adsorption taking place in 5 min, and with equilibrium attained in 105 min. Chlorella vulgaris had high adsorption capacity for cadmium, with 96.8 and 95.2 % of the total amount of cadmium being removed by the dead algal and the live algal biomass, respectively. The biosorption capacity increased with increasing cadmium concentration, and the maximum adsorption capacity for cadmium at equilibrium was found to be 16.34 mg Cd(II) g(-1) biomass using live C. vulgaris cells and 16.65 mg Cd(II) g(-1) biomass using dead C. vulgaris cells. A positive correlation was found between the adsorption efficiency and (1) the concentration of Cd(II) until adsorption equilibrium of the live and dead C. vulgaris, and (2) with the adsorbent dosage of the live and dead C. vulgaris. The adsorption efficiency was consistently above 60 % in natural water. The kinetic data showed that a pseudo-first-order model described the sorption kinetics of Cd(II) ions by live algae better than a pseudo-second-order or an Elovich model, and use of dead algal cells was best modeled by a pseudo-second-order model. The results using both live and dead C. vulgaris fitted well to the Sips isotherm compared with other two-parameter (Langmuir, Freundlich) and three-parameter (Khan) isotherm models.