Removal of Cd2+ from wastewater by microorganism induced carbonate precipitation (MICP): An economic bioremediation approach

In recent decades, microbial-induced carbonate precipitation (MICP) technology has been widely used to treat heavy metal contamination in wastewater. However, the ammonia nitrogen produced by the MICP process and the high pH value of the treated wastewater is the critical drawbacks and challenges in the scientific domains. In this study, MICP was used to treat wastewater containing Cd2+. Thereafter, the treated wastewater is adjusted with phosphoric acid to make the suspension neutral and used for agricultural irrigation to improve soil fertility and promote plant growth by utilizing the ammonia nitrogen as a by-product. Results revealed that Urease - producing flora UB1 screened from soil have rich urea hydrolysis genes (UreA, UreB, UreC, and UreG) and effectively remove more than 90% of Cd2+ whose initial concentration is less than 0.5 mmol/L but its activity was inhibited at higher concentration (Cd2+ >0.5 mmol/L). Besides, cadmium carbonate (CdCO3) formed during the process of Cd2+ removal was confirmed by instrumental analysis (TEM and XRD simultaneously). The simulated irrigation experiment showed that MICP-treated sewage regulated by phosphoric acid can solve the purposes of efficient utilization of sewage water for irrigation and is firmly capable to improve soil fertility and enhance plant growth. This study provides new insights for solving the problems of ammonia nitrogen and high pH in MICP-treated wastewater.