Concept and experimental study for a new enhanced mineral trapping system by means of microbially mediated processes

The effectiveness of CO2 geological storage depends on a combination of physical and geochemical trapping mechanisms. Over time, the physical process of residual CO2 trapping and geochemical processes of solubility trapping and mineral trapping increase. According to the IPCC special report, mineral trapping is believed to be comparatively slow, potentially taking a thousand years or longer. This paper proposes a concept of enhanced mineral trapping system of CO2 geological storage by means of microbially mediated processes and describes some results of related preliminary microbial experiments. Environmental microorganisms have influences on groundwater chemistry and mineral precipitation and dissolution, and even accelerate mineral precipitation processes. The research objective is to accelerate mineral trapping by microbial activities. Especially, biomineralization of carbonate minerals acts an important role for the enhanced mineral trapping system. It is expected to reduce the time scale of mineral trapping from several thousand years to several decades (human life time). It is also expected to play a role for sealing capability. The goal of the research project is to extract optimum bacteria to form carbonate-forming habitats and to evaluate the possibility of bacteria-enhanced CO2 geological storage using numerical model. To develop the system, we plan to carry out the following four procedures; (1) looking for various kinds of carbonate-forming bacteria from natural samples, (2) extraction of bacteria for the condition of the system and further extraction of the bacteria with optimum performance of carbonate precipitation by laboratory experiments, (3) measurement the microbial kinetic parameters, such as specific growth rate of the bacteria by some laboratory experiments (4) performance assessment by hydro-bio-geochemical modeling using experimental data including microbial kinetic parameters. According to the research plan, we carried out literature investigation to make an inventory and preliminary laboratory experiments as the first step of looking for the optimum bacteria. Our target is biomineralization by moderately halophilic carbonate-forming bacteria, which few researchers have studied yet. Screenings of bacteria consists mainly of three steps as follows: 1) purchase of moderately halophilic bacteria, some of which are known carbonate-forming species, 2) selection of carbonate-formers from the isolated halophiles, and 3) selection of practically applicable carbonate-formers with higher performances. Preliminary batch culture experiments were performed with purchased different 6 species of moderately halophilic bacteria; Chromohalobacter marismortui, Halobacillus trueperi, Halomonas halophila, Marinobacter hydrocarbonoclasticus, Marinococcus halophilus, and Virgibacillus salexigens. Initial concentration of Ca2+ was set between 2.60 g/l and 2.75 g/l in the liquid cultures. Gradual decrease in Ca2+ concentration was observed for all microbial samples. After 98 days, Ca2+ concentration in the liquid culture of Mb. hydrocarbonoclasticus was 0.440 g/l. Ca2+ concentration of C. marismortui was 0.448 g/ l after 189 days. Decrease in Ca2+ concentration was also observed for other 4 microbial species, although its degree and decreasing rate varied among species. Meanwhile, decrease in Ca2+ concentration was not observed in abiotic liquid culture. Chemical analysis by EPMA was also carried out for some microbes after the incubation, and higher peak of calcium, oxygen, carbon and minor magnesium were detected. These results indicate that the decrease in Ca2+ concentration was caused by microbial carbonate precipitation. (C) 2011 Published by Elsevier Ltd.