Dual pH-dependent functionality of Bacillus halotolerans for selective and differential flotation of chalcopyrite and pyrite in seawater

Freshwater scarcity is increasing reliance on saline water in flotation, where dissolved ions can compromise the selectivity of sulfide-mineral separation. This study evaluated the halotolerant bacterium Bacillus halotolerans as a bio-based modifier for chalcopyrite–pyrite flotation in a defined 6 g/L diluted synthetic-seawater matrix. Flotation tests conducted at pH 4–12 using 0, 50, and 100 mL of bacterial suspension were integrated with contact-angle measurements, SEM, FTIR, kinetic modelling, Fuerstenau upgrading analysis, and a bacterial-recovery assessment. Bacterial treatment increased mean copper concentrate grade between pH 6 and 10. At a dosage of 100 mL, mean grade increased by 31.7%, 42.6%, and 44.0% at pH 6, 8, and 10, respectively, while mean copper recovery increased by 4.9% at pH 6 and 15.5% at pH 8 but did not improve at pH 10. The most favorable combined separation response was obtained at pH 8, where the Fuerstenau selectivity factor reached α = 32.34. Bacterial conditioning decreased the pyrite contact angle from 73.73° to 51.80°, whereas chalcopyrite remained nearly unchanged at approximately 75.8°, supporting preferential modification of pyrite. FTIR analyses confirmed the adsorption of extracellular polymeric substances (EPS) and microbial organic compounds on mineral surfaces through the appearance of hydroxyl, amide, carboxylate, and polysaccharide-related bands. Under strongly alkaline conditions (pH 12), intensified carboxylate bands and the possible formation of oxidized surface species indicated the development of a more hydrophilic organic-rich layer on chalcopyrite, resulting in a marked depression of its flotation Kinetic analysis showed that bacterial conditioning produced a slower but more selective flotation regime, with the rate constant slightly decreasing from 0.739 to 0.712 min⁻¹, while the ultimate recovery increased from 70.27% to 76.07%. The first-order model with a rectangular distribution of floatability provided the best kinetic description, with Kₘ increasing modestly from 0.519 to 0.542 min⁻¹ under bacterial treatment. Collectively, the results demonstrate that Bacillus halotolerans exhibits a dual pH-dependent functionality, acting as a selective pyrite depressant under mildly alkaline conditions while promoting chalcopyrite depression under strongly alkaline conditions. This switching behavior highlights the potential of this halotolerant bacterium as a tunable bioreagent for selective and differential flotation processes in seawater-based mineral processing systems.