Mn recycling in hypersaline wastewater: unnoticed intracellular biomineralization and pre-cultivation of immobilized bacteria.

Microbially induced manganese carbonate precipitation has been utilized for the treatment of wastewater containing manganese. In this study, Virgibacillus dokdonensis was used to remove manganese ions from an environment containing 5% NaCl. The results showed a significant decrease in carbonic anhydrase activity and concentrations of carbonate and bicarbonate ions with increasing manganese ion concentrations. However, the levels of humic acid analogues, polysaccharides, proteins, and DNA in EPS were significantly elevated compared to those in a manganese-free environment. The rhodochrosite exhibited a preferred growth orientation, abundant morphological features, organic elements including nitrogen, phosphorus, and sulfur, diverse protein secondary structures, as well as stable carbon isotopes displaying a stronger negative bias. The presence of manganese ions was found to enhance the levels of chemical bonds O-C=O and N-C=O in rhodochrosite. Additionally, manganese in rhodochrosite exhibited both + 2 and + 3 valence states. Rhodochrosite forms not only on the cell surface but also intracellularly. After being treated with free bacteria for 20 days, the removal efficiency of manganese ions ranged from 88.4 to 93.2%, and reached a remarkable 100% on the 10th day when using bacteria immobilized on activated carbon fiber that had been pre-cultured for three days. The removal efficiency of manganese ions was significantly enhanced under the action of pre-cultured immobilized bacteria compared to non-pre-cultured immobilized bacteria. This study contributes to a comprehensive understanding of the mineralization mechanism of rhodochrosite, thereby providing an economically and environmentally sustainable biological approach for treating wastewater containing manganese.