Innovative method for CO2 fixation and storage

The concentration of CO2 in Earth’s atmosphere has been gradually increasing since the Industrial Revolution, primarily as a result of the use of fossil fuels as energy sources. Although coal and oil have been vital to the development of modern civilization, it is now recognized that atmospheric CO2 levels must be reduced to avoid the serious effects of climate change, including natural disasters. Consequently, there is currently significant interest in developing suitable methods for the fixation of CO2 in the air and in exhaust gases. The present work demonstrates a simple yet innovative approach to the chemical fixation of extremely low and very high CO2 concentrations in air, such as might result from industrial sources. This process is based on the use of aqueous solutions of the water-soluble compounds NaOH and CaCl2, which react with CO2 to produce the harmless solids CaCO3 (limestone) and NaCl (salt) via intermediates such as NaHCO3 and Na2CO3. The NaCl generated in this process can be converted back to NaOH via electrolysis, during which H2 (which can be used as a clean energy source) and Cl2 are produced simultaneously. Additionally, sea water contains both NaCl and CaCl2 and so could provide a ready supply of these two compounds. This system provides a safe, inexpensive approach to simultaneous CO2 fixation and storage.

www.nature.com/scientificreports/ and serious fires. CO 2 also dissolves in the oceans to form H 2 CO 3 , HCO 3 − and CO 3 2− , and there is approximately 50 times as much carbon dissolved in the oceans as exists in the atmosphere 5 . Conversely, all living organisms produce CO 2 during respiration, such that the rates of CO 2 consumption and production were balanced before human activities produced huge amounts of CO 2 . Certain CO 2 derivatives are used industrially 6 and in medicine 7 . The synthesis of methanol from CO 2 is particularly important because methanol is a primary raw material for the production of numerous other chemicals 8 . For example, our own group recently found that NaHCO 3 and Na 2 CO 3 accelerate glucose consumption in cultured cells 9,10 . These materials improve serum glucose levels in diabetes mellitus patients 11 . However, the rate of usage of CO 2 compounds in such applications is obviously much smaller than the rate of CO 2 production.
CaCO 3 can be used as a component of concrete, and CO 2 can also be reacted to generate important compounds such as methanol on an industrial scale 8 , although the CO 2 must first be captured and concentrated or fixed in some manner. CaCO 3 is also readily converted to CO 2 by reaction with HCl and other acids. Additionally, it should be noted that large amounts of CaCO 3 are produced naturally as coral or in the form of limestone. CO 2 can be captured from the ambient air or from flue gas via several techniques, including absorption 12 , adsorption [13][14][15][16][17][18] and membrane gas separation 14,19 . Absorption with amines is currently the dominant technology, while membrane and adsorption processes are still in the developmental stages with the construction of primary pilot plants anticipated in the near future. Recently, it was reported that an amine compound, spiroaziridine oxindole, fixed efficiently CO 2 under near ambient conditions and released CO 2 under mild conditions 17 . However, to the best of our knowledge, these methods alone cannot achieve the necessary worldwide reductions in atmospheric CO 2 .

Results and discussion
CaCO 3 precipitation. It is known that CO 2 is absorbed by alkaline solution 16 . In the present work, CO 2 was bubbled through an initially clear solution (Fig. 1a) containing 0.05 N NaOH and 0.05 M CaCl 2 to form an immediate white precipitate (Fig. 1b).
In other trials, varying the NaOH concentration between 0 and 0.5 N in the presence of 0.05 M CaCl 2 was found to generate a white precipitate above 0.2 N NaOH even in the absence of CO 2 . Because this precipitate resulted from the formation of Ca(OH) 2 , the 2NaOH + CaCl 2 + CO 2 → CaCO 3 + H 2 O + 2NaCl www.nature.com/scientificreports/ potential for CO 2 incorporation in the form of CaCO 3 was minimal under these conditions. Conversely, solutions with lower NaOH concentrations (from 0.05 to 0.1 N NaOH) together with 0.05 M CaCl 2 remained clear, while the addition of CO 2 bubbles produced a white precipitate (Fig. 2a). Under these conditions, CaCO 3 precipitation occurred in the presence of CaCl 2 , which means that high NaOH concentrations were reduced by the formation of a Ca(OH) 2 precipitate. However, prolonged bubbling with CO 2 decomposed the CaCO 3 precipitates to form Ca(HCO 3 ) 2 , which is water soluble. As the concentration of CaCl 2 was changed from 0 to 0.5 M, the amount of white precipitate was found to plateau at 0.05 M CaCl 2 (Fig. 2b).
One step CO 2 fixation. The CO 2 concentration in a 2-L bottle made of poly(ethylene terephthalate) (PET) was monitored to determine whether a solution containing 0.05 N NaOH and 0.05 M CaCl 2 reduced the level of CO 2 . These trials showed that the CO 2 reduction was clearly correlated with the time span over which the solution remained in the bottle and in contact with the internal atmosphere (Fig. 3a). Approximately 60% and 80% of the initial CO 2 was removed after 15-and 60-min treatments, respectively. After allowing the plastic bottle to sit overnight, the CO 2 in the bottle was completely removed. Thus, chemical fixation of CO 2 emission, regardless of volume/concentration of CO 2 could be efficiently captured and fixed by a solution containing 0.05 N NaOH and 0.05 M CaCl 2 . Laying the plastic bottle on its side increased the surface area of the solution and thus increased the CO 2 removal rate ( Fig. 3b). At a high CO 2 concentration of approximately 15%, the addition of 50 mL of a solution containing 0.05 N NaOH and 0.05 M CaCl 2 followed by vigorous shaking of the 2-L bottle for 30 s by hand reduced the CO 2 concentration to 10% (Fig. 3c). A further slight reduction of the CO 2 concentration was obtained by subsequently allowing the bottle to stand. The addition of 50 mL of a fresh solution also resulted in an additional slight reduction and a further addition of fresh solution after 24 h again reduced the CO 2 concentration (Fig. 3c). This slow reduction of the CO 2 level after the initial rapid removal is attributed to the presence of insufficient quantities of NaOH and CaCl 2 . The pH of the solution after 24 h and following the third addition was 6.5, while that of the initial fresh solution was 12.19. These results indicate that the NaOH in the solution was completely consumed. www.nature.com/scientificreports/ Two steps CO 2 fixation. In the above trials, a solution containing low concentrations of NaOH and CaCl 2 was used in a one step process. When using high NaOH concentrations (above 0.2 N), the CO 2 should first be treated solely with NaOH to prevent the formation of Ca(OH) 2 . This produces a solution of NaHCO 3 and Na 2 CO 3 to which CaCl 2 can be added after reducing the NaOH concentration to less than 0.1 N. The latter method is based on two steps and allows the use of high concentrations of NaOH and CaCl 2 .
Fog formation by absorbents. Because increasing the surface area of the highly concentrated NaOH solution is also important to ensuring efficient absorption of CO 2 , the generation of a fog can be beneficial. The formation of a fog greatly increases the liquid surface area and results in more rapid CO 2 removal in the plastic bottle (Fig. 4a). In experiments using a chimney model, when the chimney contained high CO 2 concentrations, the amounts of NaOH and CaCl 2 in the solution were insufficient to react with all the CO 2 at a gas flow rate of approximately 110 cm 3 /s (Fig. 4b). Thus, the solution could only capture a relatively small amount of the CO 2 in the chimney model.
Bubbling of CO 2 gas. The area over which the reagent solution interacted with CO 2 could also be increased by first passing the test gases through a porous stone to form bubbles. In these trials, a poly(vinyl chloride) pipe (40 mm in diameter and 50 cm in height) was partially filled with 250 mL each of aqueous solutions containing 0.1 N NaOH and 0.1 M CaCl 2 . Following this, the test gas was bubbled upwards through the solution at a flow www.nature.com/scientificreports/ rate of approximately 20 mL/s after passing through the porous stone at the bottom of the pipe. Under these conditions, the CO 2 contained in the air was completely absorbed by the solution (Fig. 5a). In trials using this same apparatus with a very high CO 2 concentration, the level was reduced from an initial value of 10-2.5% (Fig. 5b).
These data indicate that this concept could be employed to reduce high CO 2 levels in the exhaust streams from industrial operations such as thermal power plants and incinerators.
Diagram showing the proposed CO 2 fixation process. One means of producing NaOH on an industrial scale is the electrolysis of an aqueous NaCl solution. The products of this newly developed CO 2 fixation system based on NaOH and CaCl 2 are CaCO 3 and NaCl, and this NaCl could therefore be subsequently converted to NaOH, H 2 and Cl 2 via an electrolytic process. Thus, CO 2 could be captured using this system while simultaneously producing H 2 and Cl 2 (Fig. 6). Additionally, this process could potentially be integrated with existing generator systems based on atomic, thermal, solar, wind, hydro or wave power, and natural seawater could be used instead of an artificial NaCl solution in the electrolysis process. Conversely, the system presented in Fig. 6 is based on both CO 2 fixation and NaCl electrolysis. Because the efficient absorption of CO 2 with NaOH micro-droplets requires a large volume, while the electrolysis of a NaCl solution does not, a new CO 2 capture plant design was developed, as shown In Fig. 7. This plant is intended to continually capture CO 2 from the atmosphere or from exhaust gases. Using a large chamber equipped with spray nozzles, CO 2 can be captured efficiently by droplets of the NaOH solution. As indicated in the figure, this chamber could have various geometries. The cylindrical and meandering shapes would be applicable to either reclining or standing structures, while the other morphologies would be suitable only for a standing structure. This system could also be combined with the NaOH generating process described in the preceding section.
Recently, plastic waste has been shown to be a significant environmental pollutant, and micro-plastics have been found to affect marine organisms 20 . A small portion of the plastics that are used daily in human activities are recycled, while the remainder is simply treated as waste. Many of these materials could be incinerated but instead are typically sent to landfills. However, if a simple method of fixing CO 2 becomes available, this waste could be readily disposed of by burning without any environmental concerns and with the potential to generate energy. In addition, the current COVID-19 pandemic has resulted in vast quantities of waste materials potentially contaminated with the virus. It would be helpful to be able to burn contaminated plastic-based medical www.nature.com/scientificreports/ waste as a means of limiting the spread of infection. At present, chemical absorption using organic amines is typically employed to capture CO 2 emitted from thermal power plants, but liberating CO 2 from these complexes requires heat treatment that induces degradation. Because this treatment itself produces CO 2 , a new method that fixes CO 2 would be highly beneficial. The present method employing inorganic compounds generates a stable product, based on the neutralization of NaOH along with the formation of CaCO 3 and NaCl, both of which are harmless, stable natural compounds. This technique is applicable to thermal power plants, chemical plants, large ships, combustion operations, incinerators and automobiles. Under strict regulations for air pollution, exhaust of oxide of nitrogen (NO x ) and sulfur dioxide (SO 2 ) which have great influence on environment and human health from coal combustion 21,22 have been strongly prohibited by law. Contrary, there is no CO 2 emission control, and this resulted in accumulation of atmospheric CO 2 since the Industrial Revolution. Using this process, atmospheric CO 2 can be spontaneously fixed based on a simple apparatus at various locations to generate CaCO 3 . This newly developed and  www.nature.com/scientificreports/ facile system, which does not require organic chemicals, has minimal environmental impact and is completely sustainable, and so is expected to provide a means of reducing atmospheric CO 2 levels so as to mitigate climate change. At present, there is worldwide recognition that climate change has become a crisis 2 . Because humans "who are the most evolved organisms" 23,24 are responsible for this crisis, we have a moral duty to address the situation through global cooperation.
Milli-Q water was used throughout the experiments. CO 2 fixation. The reaction solution containing 0.05 N NaOH and 0.05 M CaCl 2 was prepared in a commercial 2-L plastic PET bottle or a commercially available 1.4-L octagonal plastic bottle and the bottles were allowed to stand or were shaken for the stated periods.
In the fog trials, approximately 4 mL of the solution was sprayed into a 2-L plastic PET bottle, after which the CO 2 concentration (in ppm) was measured using an RI-85 instrument (RIKEN). The chimney model was prepared by combining two 1-L paper milk boxes, after which air (at approximately 100 cm 3 /s) and CO 2 (approximately 10 cm 3 /s) were supplied into the lower box. A layer of gauze was placed between the two boxes and approximately 4 mL of the solution was sprayed into the middle part of the lower box. The CO 2 concentration (in %) was subsequently determined at the central point of the upper box using an XP-3140 instrument (COSMOS).