Equilibrium and Uptake Kinetics of Copper and Cadmium ions by the Microalgae, Chlorella vulgaris and Scenedesmus obliquus

Abstract

The removal of heavy metals from wastewater is an important step in a wastewater treatment process. Some metals are essential minerals, but others such as copper and cadmium are toxic when consumed. Microalgae have been identified as organisms that can tolerate and accumulate heavy metals. This tolerance offers a potential mechanism for metal removal that is cheaper and more efficient than current conventional methods. In this research, two species of microalgae, C. vulgaris and S. obliquus were exposed to water that was contaminated with copper and cadmium at concentrations of 0.25, 0.5, 0.75 and 2.5mg.L-1 . The binding capacity of the microalgae biomass to the metal ions increased linearly with the size of the ratio of the initial concentration of the metal ions to the initial concentration of the microalgae. The total metal ion removal from solution appeared to be independent of initial concentrations however, with C. vulgaris removing between 88% and 90% of the copper with all concentration combinations investigated. Based on equilibrium studies, the Freundlich Isotherm was concluded to best represent the experimental data. The linear Freundlich isotherms had R2 values greater than 0.98 for all experiments, and the Freundlich Isotherm assumptions relate more to the heterogeneity of the algae surface. The Langmuir isotherm showed a stronger linear correlation with R2 values greater than 0.99 in all cases, but did not produce realistic model parameters for the adsorption of copper by C. vulgaris. The adsorption kinetics of the metals were investigated and compared by fitting the Lagergren, Second-order model, the Elovich model and a kinetic expression derived from the Langmuir Isotherm to the experimental data. The second order model was found to fit experimental data more accurately than the Elovich and the Lagergren models when parameters were obtained by linearising the data. The Langmuir rate equation was approximated to the Lagergren model and the second order model, and based on the analysis the Lagergren model with parameters derived from the Langmuir Isotherm was most appropriate to model metal adsorption by S. obliquus. ii The current study analysed techniques used to compare the binding capacities of C. vulgaris and S. obliquus, and determined that both species were successful adsorbents for copper and cadmium. It was concluded from the rate expressions that C. vulgaris adsorbed the metal ions at a faster rate than S. obliquus, and has a higher capacity to both metal species. This would suggest that out of the two species of microalgae, C. vulgaris would be most suitable for metal removal for wastewater treatment purposes.