1177 This can be a possible obstacle to the implementation of CCS in abandoned deep hard rock mines. Hence, the present study aims to enhance the CO2 mineralization efficiency with microbial enzyme, considering its application to ultramafic rocks in deep hard rock mines. 2. METHOD Basalt and peridotite are used for the experiment. The basalt and peridotite rocks were collected from quarries located in Nagasaki prefecture and Hokkaido, Japan, respectively. According to XRF analyses, the basalt contains 2.48% of Mg, 4.73% of Ca, 13.85% of Fe, whilst the peridotite contains 24.09% of Mg, 0.28% of Ca, and 6.23% of Fe. The rocks were first fragmented with a hammer, which were then powdered with a mortar. The experimental strains, Bacillus subtilis 168 (JCM 10629) was obtained from the Japan Collection of Microorganisms (RIKEN BRC, Tsukuba, Japan). Lysogeny Broth (LB) medium, consisting of 1% (m/v) tryptone, 0.5% (m/v) yeast extract, and 1% (m/v) NaCl, was used as the culture medium, adjusted to a pH of 7.0–7.3. Cultivation was performed at 37°C. Thereafter, microbial solution containing carbonic anhydrase (CA) was obtained by centrifuging the microbial suspension at 8000 rpm for 10 minutes. CA is a microbial enzyme that catalyzes the generation of bicarbonate ions when CO2 is dissolved in the water. Sealing tests were performed for the following three conditions: Case-1) the rock powders and water are put in a plastic container, which is then placed in a high-pressure vessel. After creating an anaerobic condition and injecting CO2 under 9 MPa, the vessel is sealed and kept at room temperature for one week, Case-2) instead of water, the microbial solution containing CA is used, Case-3) First, the mixed solution is put into the vessel in the same way as Case-1, and sealed. Then, after the elapse of one week, the microbial solution is added to the solution under an aerobic condition, followed by one-week sealing. Table 1 – Experimental conditions Case Rock mass (g) Water (g) Microbial solution Duration Temp 1 5 5 N/A 1 Room 2 1 N/A 9 2 37℃ 3 1 5 9 1+1 37℃ 3. RESULTS AND DISCUSSIONS Thermogravimetric analysis (TGA) was performed in order to quantify carbonate minerals precipitated during the experiment. The result clearly shows that Case-3, i.e., the two-stage injection method, can produce the largest amount of carbonate minerals, compared to the other two cases. It was deduced that the reason why Case-2 produces a less amount of carbonate minerals compared to Case-3 is the buffering effect of the microbial solution that hinders the dissolution of divalent cations from the rock. This suggests that the timing of microbial solution injection be delayed by a couple of weeks, in order for a sufficient amount of cations to be released from the rock in the carbonic water. 4. CONCLUSION The present study investigated the potential of carbonate precipitation with the chemical reaction between ultramafic rocks and carbonic water under the conditions of deep underground mines. The result shows that by using the microbial enzyme, i.e., carbonic anhydrase, it is possible to enhance the chemical reaction and precipitate carbonate minerals under low temperature conditions. Furthermore, it was suggested that the buffering effect of the microbial solution needs to be taken into account in order to maximize the effect of the microbial enzyme and realize efficient carbonate precipitation. ACKNOWLEDGEMENTS
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