Different calcium sources affect the products and sites of mineralized Cr(VI) by microbially induced carbonate precipitation.

Microbially induced carbonate precipitation (MICP) is a common biomineralization method, which is often used for remediation of heavy metal pollution such as hexavalent chromium (Cr(VI)) in recent years. Calcium sources are essential for the MICP process. This study investigated the potential of MICP technology for Cr(VI) remediation under the influence of three calcium sources (CaCl, Ca(CHCOO), Ca(CHO)). The results indicated that CaCl was the most efficient in the mineralization of Cr(VI), and Ca(CHO) could significantly promote Cr(VI) reduction. The addition of different calcium sources all promoted the urease activity of Sporosarcina saromensis W5, in which the CaCl group showed higher urease activity at the same Ca concentration. Besides, with CaCl, Ca(CHCOO) and Ca(CHO) treatments, the final fraction of Cr species (Cr(VI), reduced Cr(III) and organic Cr(III)-complexes) were mainly converted to the carbonate-bound, cytoplasm and cell membrane state, respectively. Furthermore, the characterization results revealed that three calcium sources could co-precipitate with Cr species to produce CaCrO(CO), and calcite and vaterite were present in the CaCl and Ca(CHCOO) groups, while only calcite was present in the Ca(CHO) group. Overall, this study contributes to the optimization of MICP-mediated remediation of heavy metal contaminated soil. CaCl was the more suitable calcium source than the other two for the application of MICP technology in the Cr(VI) reduction and mineralization.