Biosorption of Cd(II) by Nordic microalgae : tolerance, kinetics and equilibrium studies

The amount of heavy metals released into the environment has significantly increased. Industrial wastewaters, e.g. from mining or battery manufacturing, are often polluted with heavy metals such as Cd, Cr or Pb. These metals threat the environment and can cause health problems even at low concentrations. Therefore, their proper removal from industrial wastewater before its disposal is of paramount importance (Javanbakht et al. [1]). Here the ability of fourteen wild Nordic microalgal strains to remove cadmium (Cd(II)) from aqueous solutions has been studied. Three of the chosen strains, namely Chlorella vulgaris (13-1), Coelastrella sp. (3-4) and Scenedesmus obliquus (13-8), demonstrated high tolerance towards Cd(II) concentrations up to 2.5 mg L−1 and their sorption kinetics and equilibrium were studied. Metal sorption by Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) was described best by pseudo-second order kinetics, whereas the removal kinetics of Scenedesmus obliquus (13-8) was best fitted by the intraparticle diffusion model. Starting from an initial concentration of 2.5 mg L−1 Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) removed 72% and 82%, respectively, of the Cd(II) within only 24 h. Modeling their Cd(II) sorption equilibria revealed that the SIPS- and Dubinin-Radushkevich models were best suited for living microalgae, and the maximum adsorption capacity (qmax) was calculated. While Chlorella vulgaris (13-1) and Coelastrella sp. (3-4) were able to remove about 49 mg g−1 and 65 mg g−1 Cd(II), respectively, Scenedesmus obliquus (13-8) only removed around 25 mg g−1. Fourier-Transform Infrared Spectroscopy (FTIR) analyses of the biomass revealed the carboxylic moieties of the cell wall to be the key player in Cd(II) removal. This study demonstrates the high potential of Nordic microalgae to remove heavy metals at conditions relevant for an industrial tertiary wastewater treatment unit and will support the development of new, biobased, innovative technologies for the bioremediation of heavy metal polluted streams