Microbially induced metal carbonate precipitation using non-growing Sporosarcina pasteurii cells: a novel strategy for heavy metal removal

In the microbially induced calcium carbonate precipitation (MICP) process, urease-producing bacteria generate carbonate ions that facilitate the precipitation of heavy metals as carbonate salts. The study aimed to investigate heavy metal precipitation as carbonates by urease activity of Sporosarcina pasteurii. This study showed that S. pasteurii lacks resistance to heavy metals even at concentrations as low as 0.1 mM. Therefore, the cells were first grown in a metal-free, urea-containing medium and harvested at the end of the exponential phase. The washed cells, referred to as “non-growing cells,” were then added to urea solutions containing metals at concentrations of 5 and 10 mM of nickel, copper, lead, silver, and cadmium. The mixtures were incubated in a shaker incubator at 120 rpm for 24 h. Semi-quantitative X-ray diffraction (XRD) analysis revealed that, in lead-containing media, the precipitate consisted of 96.5% lead carbonate, whereas in cadmium-containing media, cadmium carbonate accounted for 83.8% of the precipitate. Based on the XRD results, the carbonate precipitates of Ni, Ag, and Cu exhibited an amorphous structure. Field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM–EDS) confirmed the presence of carbon, oxygen, and the corresponding metal elements in all precipitates. Inductively coupled plasma (ICP) spectroscopy showed that approximately 89.19% of nickel, 99.51% of copper, 99.97% of lead, 99.72% of silver, and 99.88% of cadmium were removed from the 10 mM metal–urea solutions following precipitation. Despite the absence of distinct XRD diffraction peaks for the Ni, Ag, and Cu precipitates, complementary FESEM-EDS and fourier-transform infrared spectroscopy (FTIR) analyses clearly confirmed the formation of their respective metal carbonates. FTIR spectra verified the presence of carbonate functional groups in all samples. Overall, nearly 90% of heavy metals were successfully precipitated using urease-producing non-growing cells, demonstrating an effective strategy for heavy metal removal without the need for metal-resistant microorganisms. Future studies should focus on evaluating the performance of this approach under diverse environmental conditions and assessing the long-term stability of the precipitated metal carbonates.