A comparative analysis of the adsorption kinetics of Cu2+ and Cd2+ by the microalgae Chlorella vulgaris and Scenedesmus obliquus

The ability of microalgae to adsorb heavy metals from their environment has been well documented, with numerous previous studies documenting their metal binding ability. This ability has brought interest of the water treatment industry as metal removal to ppb concentrations is an expensive undertaking using conventional methods. To date, few researchers have focussed on the kinetics of adsorption, with the majority of publications focussing primarily on equilibrium data and the production of adsorption isotherms. This current study investigates the adsorption of heavy metal ions onto the surface of microalgae biomass from dilute solutions, with the performance of adsorption of copper and cadmium ions by C. vulgaris and S. obliquus being compared by fitting the Lagergren and second-order kinetic models. Kinetic studies were able to fit the Lagergren model and the second-order model to reflect the experimental data closely. The Lagergren model required fitting using a non-linear least squares algorithm, whereas the second-order model reflected experimental data closely by obtaining parameters by linearising the model. C. vulgaris had a greater second-order uptake rate for both copper (0.33 g·mg−1·s−1) and cadmium (0.17 g·mg−1·s−1) compared to S. obliquus. The capacity of C. vulgaris was greater than that of S. obliquus 10.02 mg·g−1 for copper and 9.12 mg·g−1. When the metal ions were adsorbed from a binary metal solution, copper was preferably adsorbed compared to cadmium. Analysis of equilibrium data concluded that the Freundlich Isotherm was the more appropriate isotherm compared to the Langmuir Isotherm when representing the current experimental data. The value for adsorption at equilibrium was determined to be function of the initial concentration of adsorbate and adsorbent in solution.