The bio-precipitation of calcium and magnesium ions by free and immobilized Lysinibacillus fusiformis DB1-3 in the wastewater

Biological processes have been widely employed in the treatment of industrial and municipal wastewater due to costs being lower than using physicochemical methods. The present investigation reports a simple and environmentally friendly method applying biological processes to precipitate calcium (Ca) and magnesium (Mg) ions as carbonates and phosphate using free and immobilized Lysinibacillus fusiformis DB1-3 bacteria isolated from a petroleum sample. The results show that the concentrations of Ca²⁺ and Mg²⁺ ions at different Mg/Ca molar ratios decreased by 93.58–91.46% and 62.55–58.29% when using free bacteria, and 95.56–94.86% and 58.22–70.25% when using immobilized bacteria. It took the free bacteria 30 days and the immobilized bacteria 6 days to remove Ca²⁺ and Mg²⁺ ions to such a degree. The Ca²⁺ and Mg²⁺ ions are biomineralized into calcite, Mg-rich calcite, aragonite and struvite by the bacteria. No minerals were detected in a control group without any bacteria. FESEM-EDS, FTIR, TG-DTG, DSC and stable carbon isotope analyses were performed to further prove the biogenesis of these minerals. Carbonic anhydrase (CA) and alkaline phosphatase (ALP) play important roles in the formation of these biominerals. The nanoparticles forming on the extracellular polymeric substances (EPS) and intracellular Ca and Mg inclusions were analyzed by HRTEM, SAED, STEM and elemental mapping. The fluorescence intensity illustrates that Ca²⁺ ions can enter the cell by diffusion but the presence of Mg²⁺ ions inhibits this process. This is one of the few studies in which Ca²⁺ and Mg²⁺ ions have been bio-precipitated by free and immobilized L. fusiformis bacteria in the treatment of wastewater polluted by these ions.