Iron and lead ion adsorption by microbial flocculants in synthetic wastewater and their related carbonate formation

Although microbial treatments of heavy metal ions in wastewater have been studied, the removal of these metals through incorporation into carbonate minerals has rarely been reported. To investigate the removal of Fe³⁺ and Pb²⁺, two representative metals in wastewater, through the precipitation of carbonate minerals by a microbial flocculant (MBF) produced by Bacillus mucilaginosus. MBF was added to synthetic wastewater containing different Fe³⁺ and Pb²⁺ concentrations, and the extent of flocculation was analyzed. CO₂ was bubbled into the mixture of MBF and Fe³⁺/Pb²⁺ to initiate the reaction. The solid substrates were analyzed via X-ray diffraction, transmission electron microscopy and energy dispersive spectroscopy. The results showed that the removal efficiency decreased and the MBF adsorption capacity for metals increased with increasing heavy metal concentration. In the system containing MBF, metals (Fe³⁺ and Pb²⁺), and CO₂, the concentrated metals adsorbed onto the MBF combined with the dissolved CO₂, resulting in oversaturation of metal carbonate minerals to form iron carbonate and lead carbonates. These results may be used in designing a method in which microbes can be utilized to combine CO₂ with wastewater heavy metals to form carbonates, with the aim of mitigating environmental problems.