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

Metal contamination is a growing concern due to its toxicity, persistence, and accumulation in ecosystems. This problem 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 from contaminated sites, was evaluated under varying conditions. Algae were cultured in mineral media and exposed to 55 mg/L Ni(II). Variables tested included initial pH, contact time, biomass concentration, light exposure, and live vs. dead biomass. Maximum removal (>150 mg Ni(II)/g dry biomass) occurred at pH ∼6.5-7, declining at pH < 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 h contact time. Fourier transform infrared (FT-IR) spectroscopy analysis revealed that functional groups present in cell 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 EDS analysis, STEM images revealed structural alterations such as increased lipid droplets and starch granules. These results suggest that adsorption and precipitation are the main mechanisms involved in Ni(II) removal. This study provides new insights into the remediation potential and metal removal mechanism of two newly isolated Desmodesmus sp. strains, highlighting their potential applicability in bioremediation processes.