Removal of critical minerals from mining-impacted waters using microalgal biomass

Abstract

Rising global demand for critical minerals, driven by the transition toward renewable energy and sustainable technologies, is expected to increase mining and mineral processing activities, along with the volume of metal-contaminated water requiring treatment. Conventional treatment methods, such as chemical precipitation, are effective but involve high chemical consumption, secondary sludge generation, and associated disposal challenges. Microalgae offer a promising and sustainable alternative, removing metals through biosorption and bioaccumulation. Removal efficiency is strongly influenced by biological (strain, biomass state, cultivation conditions), physical (contact time, mixing rate), and environmental factors (pH, temperature, metal type, and metal concentration). Performance can be enhanced through biomass modification, including immobilization, co-culturing, drying, or chemical treatment. Additionally, metal desorption, biomass regeneration, and the repurposing of spent biomass, support the development of a circular treatment process for mining-impacted waters. This thesis investigates the removal of copper and nickel, two critical minerals of significance in Northern Ontario mining regions, using two laboratory strains (Chlorella vulgaris and Chlamydomonas reinhardtii) and two bioprospected strains (Coccomyxa sp. 228 and Scenedesmus sp. P918). Freeze-dried biomass demonstrated effective and rapid adsorption (<2 min), achieving maximum loading capacities of 6.98 and 6.41 mg gdcw-1 for copper and nickel, respectively (69.8% and 64.1% removal). Freeze-dried biomass also exhibited significantly higher removal than living microalgae at acidic conditions. Living biomass showed slower but sustained uptake, suggesting intracellular bioaccumulation, with C. vulgaris reaching the highest copper loading (7.6 mg gdcw-1); however, removal by living biomass was far more variable. In binary systems with freeze-dried biomass, copper adsorption was favored over nickel, indicating competitive binding. Adsorption behaviour was best described by the Sips model, with maximum theoretical loadings of 19.7 and 12.2 mg gdcw-1 for copper and nickel, respectively. Overall, this work demonstrates that microalgal biomass can provide rapid removal of copper and nickel from mining-impacted waters, supporting integration into mining and processing operations.