Optimum conditions for microbial carbonate precipitation

Abstract

The type of bacteria, bacterial cell concentration, initial urea concentration, reaction temperature, the initial Ca2+ concentration, ionic strength, and the pH of the media are some factors that control the activity of the urease enzyme, and may have a significant impact on microbial carbonate precipitation (MCP). Factorial experiments were designed based on these factors to determine the optimum conditions that take into consideration economic advantage while at the same time giving quality results. Sporosarcina pasteurii strain ATCC 11859 was used at constant temperature (25 °C) and ionic strength with varying amounts of urea, Ca2+, and bacterial cell concentration. The results indicate that the rate of ureolysis (kurea) increases with bacterial cell concentration, and the bacterial cell concentration had a greater influence on kurea than initial urea concentration. At 25 mM Ca2+ concentration, increasing bacterial cell concentration from 106 to 108 cells mL−1 increased the CaCO3 precipitated and CO2 sequestrated by over 30%. However, when the Ca2+ concentration was increased 10-fold to 250 mM Ca2+, the amount of CaCO3 precipitated and CO2 sequestrated increased by over 100% irrespective of initial urea concentration. Consequently, the optimum conditions for MCP under our experimental conditions were 666 mM urea and 250 mM Ca2+ at 2.3 × 108 cells mL−1 bacterial cell concentration. However, a greater CaCO3 deposition is achievable with higher concentrations of urea, Ca2+, and bacterial cells so long as the respective quantities are within their economic advantage. X-ray Diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray analyzes confirmed that the precipitate formed was CaCO3 and composed of predominantly calcite crystals with little vaterite crystals.