Calcium ion removal at different sodium chloride concentrations by free and immobilized halophilic bacteria.

Much attention has been paid to Ca ion removal by biomineralization due to the dangers of Ca on industrial processes and human health. However, Ca removal from hypersaline water by biomineralization is quite difficult due to there being few halophilic bacteria tolerating higher salinities. In this study, free and immobilized Virgibacillus massiliensis C halophilic bacteria exhibiting carbonic anhydrase activity were used to remove Ca ions from water at different NaCl concentrations. With increasing NaCl concentrations (10, 50, 100, 150 and 200 g/L), Ca ion concentrations in the presence of free bacteria and in two groups of immobilized bacteria for a period of 6 days sharply decreased from 1200 mg/L to 219-562 mg/L, 71-214 mg/L and 21-159 mg/L, respectively; Ca precipitation ratios were 55%-81%, 82%-94% and 87%-98%, respectively. The humic acid-like substances, protein, DNA and polysaccharide, released by the bacteria, promoted the Ca ion removal. The immobilized bacteria were able to be recycled and precultured, which would save industry costs and increase Ca ion removal efficiency. Biological processes for Ca ion removal include cell surface, intracellular and extracellular biomineralization. The biogenesis of calcium carbonate was proved by SEM-EDS, FTIR, XPS and stable carbon isotope values. This study provides insights into the effective removal of Ca ions by biomineralization in hypersaline water.