Nickel removal performance and process characterization of indigenous Desmodesmus strains isolated from contaminated environments

Abstract

Metal contamination is a growing concern due to its toxicity, persistence, and accumulation in ecosystems. Thisproblem requires efficient remediation strategies. Unicellular algae are effective at removing metals from solu­tion. In this study, the nickel (Ni(II)) removal capacity of two indigenous Desmodesmus sp. strains, isolated fromcontaminated sites, was evaluated under varying conditions. Algae were cultured in mineral media and exposedto 55 mg/L Ni(II). Variables tested included initial pH, contact time, biomass concentration, light exposure, andlive vs. dead biomass. Maximum removal (>150 mg Ni(II)/g dry biomass) occurred at pH ~6.5–7, declining atpH < 4.5. Algal cultures increased the medium’s pH even during Ni(II) removal, surpassing pH 9, which pro­moted metal precipitation. Optimal removal conditions were an initial pH of 7, 0.3 g/L dry biomass, and 15 hcontact time. Fourier transform infrared (FT-IR) spectroscopy analysis revealed that functional groups present incell wall polysaccharides were mainly involved in Ni(II) binding. Additionally, scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) confirmed the surface deposition of Ni on algal cells.Although no intracellular Ni(II) was detected via Transmission Electron Microscope (STEM) combined with EDSanalysis, STEM images revealed structural alterations such as increased lipid droplets and starch granules. Theseresults suggest that adsorption and precipitation are the main mechanisms involved in Ni(II) removal. This studyprovides new insights into the remediation potential and metal removal mechanism of two newly isolatedDesmodesmus sp. strains, highlighting their potential applicability in bioremediation processes.