Semi-passive pilot scale bioreactors metal(loid) removal performance response to seasonal freeze-thaw cycle

Abstract Managing mine-contact water effectively and sustainably in (sub)arctic regions is crucial for expanding mining activities. The demand for cost-effective (semi-)passive water treatment that relies on natural chemical and biological processes and can withstand challenging weather conditions is increasing. This study investigated the ability of four pilot-scale bioreactors inoculated with locally sourced bacteria andaffected by a freeze-thaw cycle to effectively remove selenium and antimony. The bioreactors were operated at a Canadian subarctic mine for a year. Two duplicate bioreactors were installed in a heated shed maintained at 5°C over winter, while two other duplicates were installed outdoors and left to freeze. The removal rate of selenium and antimony was monitored weekly, while a genomic characterization of the microbial populations in the bioreactors was performed monthly. The bioreactors successfully removed selenium and antimony over the year, demonstrating their ability to manage freeze-thaw cycles. The overall percentage of selenium and antimony removal was similar in the outside and inside bioreactors, apart from the spring thawing period, when removal in the outdoors bioreactors was slightly lower. The dominant taxonomic groups of microbial populations were similar in all bioreactors, with slight variations observed in their relative abundance over time. The microbial population composition was consistent and re-established quickly after spring thaw in the outside bioreactors. This study demonstrated that the removal capacity of bioreactors inoculated with locally sourced bacteria was not largely affected by a freeze-thaw cycle, highlighting the strength of using local resources to design bioreactors in extreme climatic conditions.