Removal of cadmium (II), lead (II), nickel (II), and zinc (II) from synthetic medium by extremophile red alga Galdieria sulphuraria: Investigating single and mixed metal systems

Heavy metals (HMs) are extensively used in various industrial processes, and releasing them into the environment, is a significant concern due to their persistence and non-biodegradability nature. Therefore, there is a strong need for efficient, environment-friendly, and cost-effective methods for removing HMs from contaminated sites. The ability of the living cells of the extremophile red microalgal strain Galdieria sulphuraria (G. sulphuraria) to tolerate and remove Cd (II), Pb (II), Ni (II), and Zn (II) from acidic aqueous medium in both single and mixed metal systems was assessed in this study. HM ions were introduced at different concentrations to the Cyanidium medium in which G. sulphuraria was cultivated at an acidic pH of 2.5 for seven days. Growth, nutrient removal, and metal ion removal efficiency were assessed during seven-day incubation periods. Cd (II) and Pb (II) added medium inhibited growth on the initial days, whereas no initial growth inhibition was detected in the medium supplemented with Ni (II) and Zn (II). Moreover, the exponential growth phase begins on day one in the Zn (II) added medium. The highest removal efficiencies for Cd (II), Pb (II), Ni (II), and Zn (II) in a single metal system are 45.90 %, 25.15 %, 6.56 %, and 28.44 %, respectively, and these efficiencies are achieved at lower initial metal concentrations. The highest sorption capacity is 1.45, 0.53, 0.75, and 2.46 mg g(-1) of dry biomass for Cd (II), Pb (II), Ni (II), and Zn (II) in a single metal system, respectively. The experiment conducted in a mixed metal system observed a decline in removal efficiency for Cd (II), Pb (II), and Zn (II). In addition, higher removal efficiency was achieved in a mixed metal system when higher concentrations of HM were used. There was an effect in nitrogen removal with the presence of HM ions, where the presence of modest quantities of HMs does not significantly impact the removal of phosphorus. These results indicate that G. sulphuraria has the ability to remove HMs and nutrients simultaneously from aqueous solution, making a promising bioremediation solution for the combined treatment of industrial and municipal wastewater at acidic pH conditions.