Scientific Reports volume 12, Article number: 1694 (2022 ) Cite this article
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.
Although Earth has undergone many periods of significant environmental change over time, the planet’s environment has been unusually stable for the past 10,000 years1. During this time, various natural systems regulated the Earth’s climate and maintained the conditions that enabled human development. However, these regulatory systems have been greatly disturbed, and the planet may be nearing a threshold beyond which unpredictable environmental changes may occur, such as increases in the mean global temperature2. To reduce atmospheric CO2 concentrations as a means of mitigating such effects, the so-called Paris Agreement was reached at the United Nations Climate Change Conference (COP20) in 2015. This agreement was based on the requirement to keep the increase in the mean global temperature below 2 °C relative to the temperature prior to the Industrial Revolution, and preferably less than 1.5 °C. At present, this goal is challenging based solely on the development of carbon-neutral energy systems. Even so, President Elect Joe Biden has stated that the United States of America will rejoin the Paris Agreement (rejoined historically today, January 20, 2021) and the current Prime Minister of Japan, Yoshihide Suga, has declared that Japan will achieve a carbon-neutral society by 2050. Additionally, the President of the People’s Republic of China, Xi Jinping, has declared that China will be carbon neutral by 2060. Even so, because the present atmospheric CO2 concentration is quite high, there are ongoing efforts to reduce the accumulated CO2 so as to prevent a climate change crisis. Climatologists have warned that a significant reduction in the level of CO2 in Earth’s atmosphere is required over the next decade2; therefore, it is necessary to immediately begin this process. The urgency of this work has been communicated by climate change activists such as Greta Thunberg, and “Fridays for Future” events have been held worldwide.
Although renewable energy sources, including solar radiation and wind, can result in reduced CO2 emissions, these alternative systems still require energy expenditure and may also involve CO2 production. Additionally, these renewable energy approaches do not remove CO2 that has already accumulated in the atmosphere, nor do they address the ongoing generation of CO2 from exhaust gases and industrial sources. Thus, even if a carbon-neutral society could be immediately achieved, the accumulated atmospheric CO2 would not be reduced. For these reasons, it is important to lower the CO2 level currently in Earth’s atmosphere and to develop practical means of doing so as soon as possible. For CO2 storage, geo-sequestration by injecting CO2 into underground geological formations, such as oil fields, gas fields, and saline formations, has been suggested3,4, although these systems are still projects for the future.
Plants consume large quantities of CO2 based on photosynthesis, in which CO2 and H2O are converted to carbohydrates using chlorophyll under sunlight. However, the planet’s largest forest, the Amazon, which greatly contributes to the removal of atmospheric CO2, is continually shrinking because of commercial development and serious fires. CO2 also dissolves in the oceans to form H2CO3, HCO3− and CO32−, and there is approximately 50 times as much carbon dissolved in the oceans as exists in the atmosphere5. Conversely, all living organisms produce CO2 during respiration, such that the rates of CO2 consumption and production were balanced before human activities produced huge amounts of CO2. Certain CO2 derivatives are used industrially6 and in medicine7. The synthesis of methanol from CO2 is particularly important because methanol is a primary raw material for the production of numerous other chemicals8. For example, our own group recently found that NaHCO3 and Na2CO3 accelerate glucose consumption in cultured cells9,10. These materials improve serum glucose levels in diabetes mellitus patients11. However, the rate of usage of CO2 compounds in such applications is obviously much smaller than the rate of CO2 production.
CaCO3 can be used as a component of concrete, and CO2 can also be reacted to generate important compounds such as methanol on an industrial scale8, although the CO2 must first be captured and concentrated or fixed in some manner. CaCO3 is also readily converted to CO2 by reaction with HCl and other acids. Additionally, it should be noted that large amounts of CaCO3 are produced naturally as coral or in the form of limestone.
CO2 can be captured from the ambient air or from flue gas via several techniques, including absorption12, adsorption13,14,15,16,17,18 and membrane gas separation14,19. 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 CO2 under near ambient conditions and released CO2 under mild conditions17. However, to the best of our knowledge, these methods alone cannot achieve the necessary worldwide reductions in atmospheric CO2.
It is known that CO2 is absorbed by alkaline solution16. In the present work, CO2 was bubbled through an initially clear solution (Fig. 1a) containing 0.05 N NaOH and 0.05 M CaCl2 to form an immediate white precipitate (Fig. 1b).
Photograph of CaCO3 precipitates. (a) A solution containing 0.05 N NaOH and 0.05 M CaCl2. (b) A solution treated with CO2 bubbles for 30 s at a flow rate of 2 cm3/s.
In other trials, varying the NaOH concentration between 0 and 0.5 N in the presence of 0.05 M CaCl2 was found to generate a white precipitate above 0.2 N NaOH even in the absence of CO2. Because this precipitate resulted from the formation of Ca(OH)2, the
potential for CO2 incorporation in the form of CaCO3 was minimal under these conditions. Conversely, solutions with lower NaOH concentrations (from 0.05 to 0.1 N NaOH) together with 0.05 M CaCl2 remained clear, while the addition of CO2 bubbles produced a white precipitate (Fig. 2a). Under these conditions, CaCO3 precipitation occurred in the presence of CaCl2, which means that high NaOH concentrations were reduced by the formation of a Ca(OH)2 precipitate. However, prolonged bubbling with CO2 decomposed the CaCO3 precipitates to form Ca(HCO3)2, which is water soluble. As the concentration of CaCl2 was changed from 0 to 0.5 M, the amount of white precipitate was found to plateau at 0.05 M CaCl2 (Fig. 2b).
CaCO3 precipitates. (a) Quantities obtained from 3 mL of 0–0.4 N NaOH mixed with 3 mL of 0.1 M CaCl2 in a plastic tube followed by exposure to CO2 bubbles for 10 s at a CO2 flow rate of 2 cm3/s. (b) Quantities obtained from 3 mL of 0–1.0 M CaCl2 mixed with 3 mL of 0.1 N NaOH followed by centrifugation at 3000 rpm for 10 min (LCX-100, TOMY, Tokyo, Japan). Note that the final CaCl2 concentration was 0.5 M although the initial concentration was 1.0 M. The tube mass was determined before and after CO2 precipitation using an ME 204 instrument (METTLER TOLEDO). The vertical axis represents the mass of the wet precipitate and the plotted values are the mean plus or minus one standard deviation based on five replicates.
The CO2 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 CaCl2 reduced the level of CO2. These trials showed that the CO2 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 CO2 was removed after 15- and 60-min treatments, respectively. After allowing the plastic bottle to sit overnight, the CO2 in the bottle was completely removed. Thus, chemical fixation of CO2 emission, regardless of volume/concentration of CO2 could be efficiently captured and fixed by a solution containing 0.05 N NaOH and 0.05 M CaCl2. Laying the plastic bottle on its side increased the surface area of the solution and thus increased the CO2 removal rate (Fig. 3b).
CO2 concentration changes in a bottle. (a) After the transfer of 10 mL of a solution containing 0.05 N NaOH and 0.05 M CaCl2 into a 2-L plastic PET bottle with a tight cap followed by standing for 15, 30 or 60 min. (b) After the transfer of 10 mL of this solution into a 1.4-L octagonal plastic bottle with a tight cap followed by standing or shaking for 5 min. (c) After the transfer of 50 mL of this solution into a 2-L plastic PET bottle with 15% CO2, followed by vigorous shaking for 30 s, then standing for various time spans. After 60 min, 50 mL of fresh solution was added with shaking for 30 s followed by standing for 24 h and shaking for 30 s. CO2 concentration in the gas phase was analyzed. All values are the means plus or minus one standard deviation based on four or five replicates.
At a high CO2 concentration of approximately 15%, the addition of 50 mL of a solution containing 0.05 N NaOH and 0.05 M CaCl2 followed by vigorous shaking of the 2-L bottle for 30 s by hand reduced the CO2 concentration to 10% (Fig. 3c). A further slight reduction of the CO2 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 CO2 concentration (Fig. 3c). This slow reduction of the CO2 level after the initial rapid removal is attributed to the presence of insufficient quantities of NaOH and CaCl2. 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.
In the above trials, a solution containing low concentrations of NaOH and CaCl2 was used in a one step process. When using high NaOH concentrations (above 0.2 N), the CO2 should first be treated solely with NaOH to prevent the formation of Ca(OH)2. This produces a solution of NaHCO3 and Na2CO3 to which CaCl2 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 CaCl2.
Because increasing the surface area of the highly concentrated NaOH solution is also important to ensuring efficient absorption of CO2, the generation of a fog can be beneficial. The formation of a fog greatly increases the liquid surface area and results in more rapid CO2 removal in the plastic bottle (Fig. 4a). In experiments using a chimney model, when the chimney contained high CO2 concentrations, the amounts of NaOH and CaCl2 in the solution were insufficient to react with all the CO2 at a gas flow rate of approximately 110 cm3/s (Fig. 4b). Thus, the solution could only capture a relatively small amount of the CO2 in the chimney model.
CO2 concentration changes obtained using a spray. A solution containing 0.05 N NaOH and 0.05 M CaCl2 was sprayed 10 times at 5-s intervals to provide a total volume of approximately 4 mL. (a) The solution was sprayed into a 2-L plastic PET bottle and (b) into a chimney model made from two milk boxes. In the latter case, the air and CO2 flow rates were 100 and 10 cm3/s, respectively. All values are the means plus or minus one standard deviation based on either six or ten replicates.
The area over which the reagent solution interacted with CO2 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 CaCl2. Following this, the test gas was bubbled upwards through the solution at a flow rate of approximately 20 mL/s after passing through the porous stone at the bottom of the pipe. Under these conditions, the CO2 contained in the air was completely absorbed by the solution (Fig. 5a). In trials using this same apparatus with a very high CO2 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 CO2 levels in the exhaust streams from industrial operations such as thermal power plants and incinerators.
CO2 concentrations above the solution in the pipe apparatus when bubbling (a) air and (b) 10% CO2 in air through the solution. All values are the means plus or minus one standard deviation based on either nine (a) or three (b) replicates.
One means of producing NaOH on an industrial scale is the electrolysis of an aqueous NaCl solution. The products of this newly developed CO2 fixation system based on NaOH and CaCl2 are CaCO3 and NaCl, and this NaCl could therefore be subsequently converted to NaOH, H2 and Cl2 via an electrolytic process. Thus, CO2 could be captured using this system while simultaneously producing H2 and Cl2 (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.
The figure shows proposed CO2 fixation process combined with the electrolysis of NaCl. 1: Carbon dioxide fixation apparatus, 10: reaction vessel, 11: reaction chamber, 12A: anode chamber, 12B: cathode chamber, 13A and 13B: partition wall, 20A and 20B: carbon dioxide fixing agent feeding units, 30: gas feeding unit, 31: insertion end point, 40A: Cl2 extraction portion, 40B: H2 extraction portion, 40C: air extraction portion, 50: liquid extraction portion, 51: filter, 121A: anode, and 121B: cathode. The original diagram was drawn by the author, and it was formally traced by Tsujimaru International Patent Office.
Conversely, the system presented in Fig. 6 is based on both CO2 fixation and NaCl electrolysis. Because the efficient absorption of CO2 with NaOH micro-droplets requires a large volume, while the electrolysis of a NaCl solution does not, a new CO2 capture plant design was developed, as shown In Fig. 7. This plant is intended to continually capture CO2 from the atmosphere or from exhaust gases. Using a large chamber equipped with spray nozzles, CO2 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.
The figure shows proposed CO2 fixation process. The spray chamber could potentially have several different geometries, including (a) cylindrical, (b) zig-zag, (c) meandering, and (d) spiral. Legend: 5: exit for the CO2 fixation solution, 6: filter, 7A: fixation solution, 10A: reaction chamber, 10a: gas entrance, 10b: reaction chamber, 10c: exit, 20, 21 and 22: nozzles, 70: water tank, 90a and 90b: sensors, and 200 and 201: pipes. The original diagram was drawn by the author, and it was formally traced by Matsushima Patent Office, using software “Hanako” add in “Ichitaro”.
Recently, plastic waste has been shown to be a significant environmental pollutant, and micro-plastics have been found to affect marine organisms20. 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 CO2 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 waste as a means of limiting the spread of infection. At present, chemical absorption using organic amines is typically employed to capture CO2 emitted from thermal power plants, but liberating CO2 from these complexes requires heat treatment that induces degradation. Because this treatment itself produces CO2, a new method that fixes CO2 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 CaCO3 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 (NOx) and sulfur dioxide (SO2) which have great influence on environment and human health from coal combustion21,22 have been strongly prohibited by law. Contrary, there is no CO2 emission control, and this resulted in accumulation of atmospheric CO2 since the Industrial Revolution. Using this process, atmospheric CO2 can be spontaneously fixed based on a simple apparatus at various locations to generate CaCO3. This newly developed and 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 CO2 levels so as to mitigate climate change. At present, there is worldwide recognition that climate change has become a crisis2. 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.
Reagent grade NaOH and CaCl2 were purchased from Wako-Junyaku Kogyo (Tokyo, Japan). Milli-Q water was used throughout the experiments.
The reaction solution containing 0.05 N NaOH and 0.05 M CaCl2 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 CO2 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 cm3/s) and CO2 (approximately 10 cm3/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 CO2 concentration (in %) was subsequently determined at the central point of the upper box using an XP-3140 instrument (COSMOS).
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The author thanks Hiroyuki Okada, President of Shinko-Sangyo Co. Ltd., Takasaki, Gunma, Japan, for financial support, Hideaki Kato, President of the Takasaki Denka-Kogyo, Co. Ltd., Takasaki, Gunma, Japan, for providing encouragement regarding the present work, and Edanz Group (https://en-author-services.edanz.com/ac) for editing a draft of this manuscript.
Present address: Bioscience Laboratory, Environmental Engineering, Co., Ltd., 1-4-6 Higashi-Kaizawa, Takasaki, Gunma, 370-0041, Japan
Research Laboratory, Gunma Agriculture and Forest Development, Takasaki, Gunma, 370-0854, Japan
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K.S. conceived, designed and carried out the study and also wrote the manuscript.
The author declares that the present data have been used to support applications to the Japan Patent Office (PTC/JP2019/03400, PTC/JP2019/045839, PTC/JP2019/045390, PTC/JP2019/048178, PTC/JP2020/02064, PTC/JP2020/02990, PTC/JP2020/029505, PTC/JP2020/002064, PTC/JP2020/031010, JP2021-321).
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Sorimachi, K. Innovative method for CO2 fixation and storage. Sci Rep 12, 1694 (2022). https://doi.org/10.1038/s41598-022-05151-9
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