Effect of a harsh circular environment on self-healing microbial-induced calcium carbonate materials for preventing Pb2+ migration

Microbially induced carbonate precipitation (MICP) is increasingly being explored for Pb-contaminated water bodies and soil remediation. However, the Pb-related precipitate resulting from the MICP process can possibly leach acid over time when subjected to harsh environments, causing serious threats to human health. In this study, for the first time, self-healing microbial-induced calcium carbonate (MICC) materials are proposed and applied to prevent Pb2+ migration where the Pb-related precipitate is acid leached after spore germination, spore-vegetative cell transformation, urease secretion, and urea hydrolysis, thereby producing spore-containing precipitation. This process was repeated five times to explore the effect of a harsh circular environment on self-healing MICC materials. Results indicated that Pb immobilization would have deteriorated if the inosine and trace elements had not been intervened during spore germination and spore-vegetative cell transformation, respectively. The spores and vegetative cells provided extra nucleation sites for Pb2+ and minerals to attach. The extracellular polymeric substances (EPSs) combined Pb2+ with functional groups and chemical bonds to prevent their migration to surrounding environments. The scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) images also indicated that the cerussite mineral was precipitated prior to the calcite mineral because Pb2+ had more affinity to combine with CO32- and OH-. An immobilization efficiency of greater than 95% remained nearly the same after five cycles, while it reduced very quickly to less than 10% after three cycles when neglecting the self-healing MICC materials, thus highlighting their relative merits.