Integrated Biomass Harvesting and Metal Removal from Acid Mine Drainage Using Galdieria sulphuraria and Calcium Polysulfide

The extremophilic microalga Galdieria sulphuraria has shown considerable potential for the treatment of acid mine drainage (AMD) while producing biomass for value-added applications. However, biological metal removal alone is limited, and efficient biomass harvesting under highly acidic conditions remains a major challenge. This study developed an integrated treatment approach with potential sustainability benefits by cultivating Galdieria sulphuraria in AMD followed by simultaneous biomass harvesting and dissolved metal removal using calcium polysulfide (CaSx). Galdieria sulphuraria successfully grew in undiluted Berkeley Pit water (BPW), a representative AMD, demonstrating its adaptability to highly acidic and metal-rich conditions. During cultivation, only partial removal of dissolved metals was achieved, indicating that biological treatment alone was insufficient for effective AMD remediation. Conventional flocculants, ferric chloride and chitosan, achieved only moderate harvesting efficiencies (54–56%) and showed negligible contribution to metal removal. In contrast, CaSx achieved a maximum biomass harvesting efficiency of 98.7% after a 60min settling-time experiment. In a separate dosage experiment, Zn removal reached nearly 100% at 2.95 g L−1 of CaSx, while Mn removal increased with CaSx dosage and reached 78% at 7.36 g L−1. Control experiments with BPW in the absence of microalgae showed comparable metal removal, indicating that Zn and Mn removal using CaSx treatment was primarily driven by chemical processes. The enhanced treatment was consistent with sulfide-mediated precipitation of dissolved metals and the increase in solution pH, which promoted the formation of dense biomass aggregates. CHN, SEM, EDX, and XRD analyses consistently demonstrated that CaSx transformed the recovered material from predominantly organic microalgal biomass into a mineral-enriched biomass composite. This study demonstrates that integrating Galdieria sulphuraria cultivation with CaSx-assisted biomass harvesting and dissolved metal removal provides a potential approach for sustainable AMD remediation while generating a mineral-enriched biomass.