COMPOSITIONS AND METHODS FOR CARBON DIOXIDE CAPTURE

Abstract

The present disclosure provides a carbon dioxide absorption composition, which includes a solution of an amino compound in a solvent comprising water and a glycol, as well as a method of using such composition to capture CO.sub.2. The present composition can be particularly advantageous for indoor use, with additional benefit for managing total volatile organic compounds and humidity to enhance overall indoor air quality.

Claims

1. A carbon dioxide absorption composition, comprising: a solution of an amino compound in a solvent comprising water, a glycol, and a surfactant.

2. The carbon dioxide absorption composition of claim 1, wherein the glycol is selected from the group consisting of ethylene glycol, diethylene glycol propylene, triethylene glycol, propylene glycol, di-propylene glycol, and a mixture thereof.

3. The carbon dioxide absorption composition of claim 1, wherein the amino compound comprises an alkylamine, an alkanolamine, an amino acid, or a combination thereof.

4. The carbon dioxide absorption composition of claim 1, wherein the amino compound is monoethanolamine or arginine.

5. The carbon dioxide absorption composition of claim 1, wherein the ratio of water to the glycol is about 0.5:1 to about 2:1 by volume.

6. The carbon dioxide absorption composition of claim 5, wherein the ratio of water to the glycol is about 1:1 by volume.

7. The carbon dioxide absorption composition of claim 1, wherein the amino compound has a concentration of about 500 mM to about 20000 mM in the solution.

8. The carbon dioxide absorption composition of claim 1, wherein the surfactant is selected from the group consisting of perfluorooctanoic acid (PFOA), sodium dodecyl sulfate (SDS), Triton X-100, and a combination thereof.

9. The carbon dioxide absorption composition of claim 1, wherein the relative proportion of the glycol to the surfactant enhances carbon dioxide absorption.

10. The carbon dioxide absorption composition of claim 1, wherein the surfactant has a concentration of about 200 mg/L to about 500 mg/L in the solution.

11. The carbon dioxide absorption composition of claim 1, wherein the amino compound is arginine and wherein the solvent comprises water and propylene glycol in a ratio of about 1:1 by volume.

12. A device for carbon dioxide absorption, comprising: the carbon dioxide absorption composition of claim 1; and a chamber in which the carbon dioxide absorption composition is placed.

13. (canceled)

14. A method of removing carbon dioxide in a gas phase, the method comprising contacting the gas phase with the carbon dioxide absorption composition of claim 1, so that the carbon dioxide in the gas phase reacts with the amino compound in the carbon dioxide absorption composition to produce a carbon dioxide-enriched composition, thereby reducing content of the carbon dioxide in the gas phase.

15. (canceled)

16. (canceled)

17. A method of improving indoor air quality, the method comprising: contacting the indoor air with the carbon dioxide absorption composition of claim 1, such that the carbon dioxide in the indoor air reacts with the amino compound in the carbon dioxide absorption composition to produce a carbon dioxide-enriched composition, thereby reducing content of the carbon dioxide in the indoor air.

18. The method of claim 17, wherein a relative humidity (RH) level of the indoor air is maintained within 30%-70%.

19. (canceled)

20. The method of claim 17, wherein a total volatile organic compounds (TVOCs) level of the indoor air is maintained below 10 ppm.

21. (canceled)

22. The method of claim 17, further comprising passing the indoor air through activated carbon.

23. (canceled)

24. The method of claim 14, further comprising subjecting the carbon dioxide-enriched composition to microwave irradiation, thereby producing a regenerated carbon dioxide absorption composition and releasing the absorbed carbon dioxide.

25. The method of claim 24, further comprising reusing the regenerated carbon dioxide absorption composition for reaction with the carbon dioxide in the gas phase or indoor air.

26. The method of claim 24, further comprising collecting the released carbon dioxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Further objects, features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the present disclosure. Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

[0011] FIG. 1A shows the reaction between CO.sub.2 and amine, presence and absence of water.

[0012] FIG. 1B shows the regeneration of amine.

[0013] FIG. 2A shows CO.sub.2 absorption during the CO.sub.2 absorption process for aqueous MEA, aqueous L-arginine, H.sub.2O-EG-Arg, and H.sub.2O-PG-Arg solution. Absorption 2 refers to the second cycle after the initial absorption-desorption cycle.

[0014] FIG. 2B shows TVOC concentration during the CO.sub.2 absorption process for aqueous MEA, aqueous L-arginine, H.sub.2O-EG-Arg, and H.sub.2O-PG-Arg solution. Absorption 2 refers to the second cycle after the initial absorption-desorption cycle.

[0015] FIG. 2C shows RH levels during the CO.sub.2 absorption process for aqueous MEA, aqueous L-arginine, H.sub.2O-EG-Arg, and H.sub.2O-PG-Arg solution. Absorption 2 refers to the second cycle after the initial absorption-desorption cycle.

[0016] FIG. 3A shows the effect of MW irradiation time on CO.sub.2 desorption for aqueous MEA solution.

[0017] FIG. 3B shows the effect of MW desorption time on volume of the solution for aqueous MEA solution.

[0018] FIG. 4 shows (a) absorption-desorption performance over ten cycles of aqueous MEA and H.sub.2O-PG-Arg solution. The percentage reduction of the amount of CO.sub.2 absorbed after each absorption process, as well as the cumulative reduction after ten cycles, is shown at the top of the figure. (b) absorption-desorption performance over ten cycles of aqueous MEA and H.sub.2O-PG-Arg solution with trendline.

[0019] FIG. 5 shows (a) Reaction rate constant over ten cycles of aqueous MEA and H.sub.2O-PG-Arg solution. The percentage decrease in the rate constant in each absorption process, as well as the cumulative reduction after ten cycles, is shown at the top of the figure. (b) Reaction rate constant over ten cycles of aqueous MEA and H.sub.2O-PG-Arg solution with trendlines.

[0020] FIG. 6 shows the experimental setup used for absorption and desorption of CO.sub.2 scrubbing solution (a) Two-neck round bottom flask absorption system (b) MW setup for solution regeneration.

[0021] FIG. 7A shows CO.sub.2 separation over time for aqueous MEA, aqueous Arg, H.sub.2O-EG-Arg, and H.sub.2O-PG-Arg solutions.

[0022] FIG. 7B shows the reaction kinetics for aqueous MEA, aqueous Arg, H.sub.2O-EG-Arg, and H.sub.2O-PG-Arg solution.

[0023] FIG. 8 shows the relative humidity profile for aqueous MEA and aqueous Arg solution. The slope of the line was determined by linear regression.

[0024] FIG. 9 shows the Arg crystals after desorption of aqueous Arg solution.

[0025] FIG. 10A shows TVOC for H.sub.2O-PG-based Arg solution with and without activated charcoal canister.

[0026] FIG. 10B shows relative humidity profile for H.sub.2O-PG-based Arg solution with and without activated charcoal canister.

[0027] FIG. 11A shows relative humidity inside the absorption test chamber over time for H.sub.2O-PG-Arg solution.

[0028] FIG. 11B shows TVOC levels inside the absorption test chamber over time for H.sub.2O-PG-Arg solution.

DETAILED DESCRIPTION

[0029] The present disclosure relates to compositions, devices, methods for absorbing carbon dioxide (CO.sub.2) from a gas phase, in particular indoor air.

[0030] Chemical absorption is one of the most promising technologies for CO.sub.2 capture due to its high selectivity and scalability opportunities. Various absorbing agents were reported, including aqueous alkali solutions, amines, ionic liquids, amine-functionalized adsorbents, and amino acid solutions, to chemically extract CO.sub.2 from air or gas streams. Among these methods, amine-based technology remains the most widely used. Researchers have shown that the interaction between CO.sub.2 and aqueous amines follows the zwitterionic mechanism. The nucleophilic lone pair on the nitrogen atom of the amine attacks the electrophilic carbon of CO.sub.2, forming zwitterionic adduct (RNH.sub.2.sup.+COO.sup.), which stabilizes into carbamate (RNHCOO.sup.) and an alkylammonium ion (RNH.sub.2H.sup.+) upon deprotonation (Reaction 1). As absorption progresses, the pH and absorption rate decrease due to the depletion of the CO.sub.2 scrubbing agent (CSA), weakening Reaction. This favors hydration reactions (Reactions 2-5), resulting in the formation of bicarbonate (HCO.sub.3.sup.) and carbonate (CO.sub.3.sup.2). Additionally, as the pH drops, carbamate formed in Reactions 1 and 2 decomposes into bicarbonate, as shown in Reaction 6. This process is reversible during desorption, as shown in Reactions 7-10. The full sequence of absorption and desorption reactions is outlined in FIGS. 1A-1B.

[0031] Alkanolamines are preferred over alkylamines for CO.sub.2 capture due to their higher boiling points and lower volatility, which reduces the emission of VOCs during absorption and desorption processes. Monoethanolamine (MEA) is used as a benchmark solvent due to its affordability, high water solubility, and rapid CO.sub.2 absorption rate. However, it still has drawbacks, such as volatility that can lead to higher TVOC emissions and may limit indoor use, along with a high energy demand for regeneration. Furthermore, it is prone to thermal and oxidative degradation, which results in increased viscosity, solution fouling, and corrosion of downstream equipment. These challenges highlight the need for alternative CSA for indoor CO.sub.2 capture. Aqueous amino acids are emerging as promising alternatives. Known for their high surface tension, they offer comparable CO.sub.2 absorption capacity with negligible vapor pressure and greater resistance to degradation. These properties make them well-suited for capturing CO.sub.2 in indoor applications, providing a viable path forward in overcoming the limitations of conventional absorbents like MEA. Arginine (Arg) specifically offers several advantages over traditional absorbents like MEA, including greater resistance to thermal and oxidative degradation. This allows the Arg solution to undergo multiple absorption-desorption cycles, reducing costs associated with frequent solution replacement. Its higher surface tension also minimizes evaporation losses during both CO.sub.2 capture and release, and its biodegradability makes Arg a more environmentally sustainable option. However, despite these promising attributes, only a limited number of studies have explored the role of Arg in CO.sub.2 capture. No research to date has specifically addressed CO.sub.2 capture from indoor air while simultaneously controlling TVOC and humidity levels. Maintaining optimal levels of both TVOCs and humidity is essential for safeguarding respiratory health and ensuring thermal comfort for occupants.

[0032] The present disclosure addresses the deficiencies of previous studies and provides efficient and affordable CO.sub.2 capture compositions, devices, methods. Advantageously, the present compositions and devices can be specifically designed for indoor use, with added functionality for managing TVOCs and humidity to enhance overall indoor air quality.

[0033] Concentration of CO.sub.2 in an indoor environment can be four to five times higher than the outdoor air. This higher indoor concentration of CO.sub.2 reduces the work efficiency of individuals working indoors and negatively impacts human health. However, the elevated concentration also makes it easier to capture CO.sub.2 from indoor air. In certain embodiments, the present disclosure demonstrated the performance of monoethanolamine (MEA) and L-arginine (Arg) solutions for indoor carbon dioxide (CO.sub.2) capture through experimental screening. The parameters evaluated include CO.sub.2 absorption and desorption capacity, absorption kinetics, and the impact on relative humidity (RH) and total volatile organic compound (TVOC) concentrations. In particular embodiments, two solvent formulations were studied: one utilizing pure water as the solvent and the other incorporating a water-glycol mixture. The aqueous Arg solution demonstrated minimal to no detectable increase in VOC levels and exhibited lower evaporation rates than the benchmark aqueous MEA solution. Microwave (MW) heating can be utilized to facilitate rapid CO.sub.2 desorption from saturated solutions. The regeneration efficiency, solvent loss, and energy consumption were found to be dependent on the MW desorption time. Optimizing the desorption resulted in faster and almost complete regeneration, minimized solvent loss, and reduced overall energy consumption. The incorporation of glycol minimized evaporation during absorption, decreased the likelihood of complete drying during desorption, and improved solution regeneration. Cyclic absorption-desorption experiments were conducted to evaluate the long-term stability and kinetic performance of the solutions. In representative studies, the water-PG-based Arg solution exhibited a promising performance, with only a 31.24% reduction in CO.sub.2 absorption and a 2.13% decrease in absorption kinetics after ten cycles. In contrast, the aqueous MEA solution showed much larger declines of 54.3% in CO.sub.2 absorption and 34.24% in kinetics. Additionally, the water-PG-based Arg solution resulted in lower volatile organic compound (VOC) levels and provided more effective control over relative humidity. These findings underscore the potential of the water-PG-based Arg solution for cyclic CO.sub.2 absorption and microwave-assisted regeneration processes.

[0034] In one aspect, the present disclosure provides a carbon dioxide absorption composition, comprising a solution of an amino compound in a solvent comprising water, a glycol, and a surfactant.

[0035] Typically, the amino compound of the present disclosure reacts with CO.sub.2 to form a soluble carbamate, thereby absorbing (or scrubbing) the CO.sub.2 into the solution. The absorbed CO2 can be released in a desorption process that includes hydrolysis of the carbamate and regeneration of the amino compound. Representative chemical reactions involved in the CO2 adsorption and desorption processes include those illustrated in FIGS. 1A-1B. The desorption process can be carried out, for example under heating or microwave irradiation.

[0036] Suitable amino compounds for the present composition include, but are not limited to, an alkylamine, an alkanolamine, an amino acid, or a combination thereof. The alkylamine refers to a compound having an amino group (NH.sub.2) attached to an alkyl group. The term alkyl as used herein, means a straight or branched chain saturated hydrocarbon. As non-limiting examples, the alkyl can be a C.sub.1-10alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl, 3-methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, and n-decyl. The alkanolamine refers to an alkylamine compound further substituted with one or more hydroxyl (OH) groups. The amino acid refers to compounds having both amino (NH.sub.2) and carboxylic acid (COOH) groups, or salts thereof. The amino acids as used herein can include natural or synthetic compounds, such as -amino acids, -amino acids, or -amino acids.

[0037] In some embodiments, the amino compound is an alkanolamine, such as monoethanolamine. In some embodiments, the amino compound is an amino acid, such as a natural amino acid. In some embodiments, the amino compound is arginine.

[0038] The glycol of the present composition refers to an organic compound with two hydroxyl (OH) groups attached to adjacent carbon atoms. For example, the glycol can include a carbon chain and two OH groups are attached to adjacent carbon atoms of the carbon chain. Suitable glycols can include, for example, ethylene glycol, diethylene glycol propylene, triethylene glycol, propylene glycol, di-propylene glycol, and a mixture thereof. In some embodiments, the glycol is ethylene glycol or propylene glycol. In some embodiments, the glycol is propylene glycol.

[0039] The present composition includes a solution of the amino compound dissolved in a solvent, which includes a mixture of water and the glycol. The ratio of the water to the glycol can be about 0.5:1 to about 2:1 by volume. In some embodiments, the ratio of water to the glycol is about 1:1 by volume.

[0040] In some embodiments, the amino compound has a concentration of about 500 mM to about 20000 mM in the solution, such as from about 500 mM to about 15000 mM, about 500 mM to about 10000 mM, about 500 mM to about 5000 mM, about 500 mM to about 2000 mM, about 500 mM to about 1000 mM, about 1000 mM to about 15000 mM, about 1000 mM to about 10000 mM, about 1000 mM to about 5000 mM, or about 1000 mM to about 2000 mM. In some embodiments, the amino compound has a concentration of about 500 mM, about 1000 mM, about 2000 mM, about 5000 mM, about 10000 mM, about 15000 mM, or about 20000 mM. In some embodiments, the amino compound is an amino acid having a concentration of about 500 mM to about 2000 mM. In some embodiments, the amino compound is an amino acid having a concentration of about 1000 mM. In some embodiments, the amino compound has a concentration of about 500 mM to about complete saturation in the solution (such as about 1000 mM or about 2000 mM). In some embodiments, the amino compound is a water miscible amine, which may have a concentration of about 1000 mM to about 10000 mM in the solution, such as about 2000 mM, about 5000 mM, or about 8000 mM. In some embodiments, the amino compound is a water miscible amine having a concentration of about 1000 mM.

[0041] Suitable surfactants can include, but are not limited to, cationic surfactants, anionic surfactants, amphoteric surfactants, zwitterionic surfactants, nonionic surfactants, and mixtures thereof. In some embodiments, the surfactant includes a nonionic surfactant, such as polyoxyalkylene alkyl ethers, polyoxyalkylene alkenyl ethers, polyoxyalkylene alkyl phenyl ethers, alkyl polyglucosides, fatty acid polyglycerine esters, fatty acid sugar esters, and fatty acid alkanolamides. In some embodiments, the surfactant includes an anionic surfactant, such as a carboxylate-, sulfonate-, or sulfate-based surfactant. In some embodiments, the surfactant includes sodium lauryl sulfate (SLS), sodium laureth sulfate (SLES), linear alkylbenzene sulfonates (LAS), sodium dodecyl sulfate (SDS), alpha olefin sulfonates (AOS), and sulfonated castor oil; the cationic surfactants include cetyltrimethylammonium bromide (CTAB), benzalkonium chloride (BAC), benzethonium chloride, dodecyltrimethylammonium chloride (DTAC), and cetrimonium chloride (CTAC); the non-ionic surfactants include polysorbates (Tween 20, Tween 80), sorbitan esters (Span 20, Span 80), alcohol ethoxylates, polyethylene glycol (PEG) derivatives, cocamide diethanolamine (Cocamide DEA), and cocamide monoethanolamine (Cocamide MEA); and the amphoteric surfactants include cocamidopropyl betaine (CAPB), lauryl betaine, sodium cocoamphoacetate, sodium lauryl amphopropionate, or a combination thereof. In some embodiments, the surfactant is perfluorooctanoic acid (PFOA), sodium dodecyl sulfate (SDS), Triton X-100, or a combination thereof.

[0042] In some embodiments, the surfactant has a concentration of about 200 mg/L to about 500 mg/L, such as about 200 mg/L to about 450 mg/L, about 200 mg/L to about 400 mg/L, about 200 mg/L to about 350 mg/L, or about 200 mg/L to about 300 mg/L. In some embodiments, the surfactant has a concentration of about 200 mg/L, about 250 mg/L, about 300 mg/L, about 350 mg/L, about 400 mg/L, about 450 mg/L, or about 500 mg/L. Suitable surfactant concentration can be measured by folds of the critical micelle concentration (CMC) of the surfactant in the solution. In some embodiments, the surfactant has a concentration that is about 4 to about 10 times the CMC, such as about 4 times the CMC, about 5 times the CMC, about 6 times the CMC, about 7 times the CMC, about 8 times the CMC, about 9 times the CMC, or about 10 times the CMC.

[0043] The effects of adding surfactants to aqueous amine solutions were previously studied. However, those studies do not examine the implications in a continuous operational setting. The present disclosure provides a deeper analysis of surfactant behavior under real process conditions, which is beyond the scope of earlier reports. Significantly, the present disclosure uncovers insights previously overlooked, making the findings herein uniquely valuable for practical applications.

[0044] In some embodiments, the present composition utilizes a 50 ml/50 ml (1:1) water-glycol mixture as a solvent, especially propylene glycol, leveraging the distinct properties of glycol to enhance performance. Glycol reduces solvent volatility, minimizing evaporative losses, which maintains solvent integrity over extended use. Additionally, glycols have lower specific heat capacities, meaning they require less thermal energy to reach the desorption temperature, improving energy efficiency.

[0045] The present composition can include one or more glycols in combination with one or more surfactants as described herein to enhance performance for continuous operation. The surfactant can act as a foaming agent to increase CO.sub.2 absorption when the composition contacts CO.sub.2. On the other hand, the glycol can act as a defoamer among its other roles. Further, the present disclosure demonstrates that the incorporation of surfactants and defoamers (such as glycols) can effectively control foam stability. Under continuous operating conditions, excessively stable foam can lead to overflow, causing chemical loss, operational disruptions, and potential safety hazards. Therefore, precisely tuning the foaming behavior is essential when surfactants are present. By adjusting the ratio of surfactants to defoamers, the foam's lifespan can be carefully managed, ensuring it remains transient and does not exceed a controlled level. In some embodiments, the relative proportion of the glycol to the surfactant enhances carbon dioxide absorption. For example, the present composition can include the glycol and the surfactant at a specific relative proportion to create a short-lived foam that enhances CO.sub.2 absorption from a gas phase, such as from indoor air. In some embodiments, a 1:1 ratio of water and glycol was found to be particularly effective, based on the solubility of the CO.sub.2-scrubbing chemicals and a fluorosurfactant concentration that is about 4 to about 10 times the critical micelle concentration (CMC) in water at 25 C.

[0046] While many surfactants can be used in the present carbon dioxide absorption composition, fluorosurfactants can be chosen in particular embodiments for their superior ability to reduce surface tension at lower concentrations, enhanced chemical stability over multiple adsorption-desorption cycles, and chemical inertness.

[0047] In some embodiments, the amino compound is arginine and the solvent comprises water and propylene glycol in a ratio of about 1:1 by volume.

[0048] In some embodiments, the composition includes arginine (e.g., L-arginine), water, and propylene glycol. In some embodiments, the ratio of water and propylene glycol is about 1:1 by volume. In some embodiments, the concentration of arginine (e.g., L-arginine) in water and propylene glycol is about 1 M. In some embodiments, the composition of arginine (e.g., L-arginine), water, and propylene glycol further includes a surfactant selected from perfluorooctanoic acid (PFO), sodium dodecyl sulfate (SDS), and Triton X100. In some embodiments, the composition includes about 1 M arginine (e.g., L-arginine) dissolved in a solvent of water and propylene glycol at a ratio of about 1:1 by volume, and a surfactant selected from perfluorooctanoic acid (PFO), sodium dodecyl sulfate (SDS), and Triton X100.

[0049] In some embodiments, the composition includes MEA, water, and propylene glycol. In some MEA composition, the ratio of water and propylene glycol can be about 1:1 by volume. In some embodiments, the MEA composition further includes a surfactant selected from perfluorooctanoic acid (PFO), sodium dodecyl sulfate (SDS), and Triton X100. In some embodiments, the composition includes MEA, water, and propylene glycol at a ratio of about 1:1 by volume, and a surfactant selected from perfluorooctanoic acid (PFO), sodium dodecyl sulfate (SDS), and Triton X100.

[0050] In some embodiments, the composition includes MEA, one or more glycols (such as propylene glycol), water, and one or more surfactants (such as fluorosurfactants). In some embodiments, the composition includes arginine (e.g., L-arginine), one or more glycols (such as propylene glycol), water, and one or more surfactants (such as fluorosurfactants).

[0051] In some embodiments, the composition includes arginine (e.g., L-arginine), water, ethylene glycol, and a surfactant. In some embodiments, the ratio of water and ethylene glycol is about 1:1 by volume. In some embodiments, the concentration of arginine (e.g., L-arginine) in water and ethylene glycol is about 1 M. In some embodiments, the composition includes about 1 M arginine (e.g., L-arginine) dissolved in a solvent of water and ethylene glycol at a ratio of about 1:1 by volume.

[0052] In another aspect, the present disclosure provides a device for carbon dioxide absorption, which comprises the carbon dioxide absorption composition as described herein and a chamber in which the carbon dioxide absorption composition is placed.

[0053] The present device can further comprise an inlet connected to the chamber for introducing a gas phase comprising carbon dioxide to react with the carbon dioxide absorption composition in the chamber, thereby reducing content of the carbon dioxide in the gas phase; and a channel connected to the chamber, through which the gas phase with reduced content of carbon dioxide exits the chamber.

[0054] The present device can further include other components and accessories, such as a power supply, a monitor, a housing, and a user interface. In some embodiments, the present device can be used to absorb indoor carbon dioxide.

[0055] In another aspect, the present disclosure provides a method of removing carbon dioxide in a gas phase. The method can comprise contacting the gas phase with the carbon dioxide absorption composition as described herein, so that the carbon dioxide in the gas phase reacts with the amino compound in the carbon dioxide absorption composition to produce a carbon dioxide-enriched composition, thereby reducing content of the carbon dioxide in the gas phase.

[0056] For example, the chemical reactions involved in the CO.sub.2 adsorption process include those illustrated in FIG. 1A. The contact between the gas phase and the carbon dioxide absorption composition can include any form of gas-liquid surface contact. For example, the gas phase can be flowed across the surface of the liquid composition or bubbled through the bulk volume of the liquid composition. Additionally, the gas phase can be introduced to the liquid composition repeatedly through recycling of the gas phase.

[0057] The gas phase can be present in an outdoor environment, an indoor environment, or an isolated space (e.g., a sealed container, a chamber with controlled input and output, or a flow path for a gas and/or a fluid). In some embodiments, the gas phase comprises air. The air can include natural air or artificially modified air for animal or human use. In some embodiments, the gas phase is indoor air. The term indoor as used herein refers to an enclosed or partially enclosed space for animal or human activities, such as the internal space of a building, a stadium, a laboratory, a room, a residence, an office, a hospital, a vehicle, a ship, a submarine, an aircraft, etc. The indoor air may have a CO.sub.2 level of about 400 ppm to about 5,000 ppm, or higher than 5,000 ppm. In some embodiments, the gas phase comprises more concentrated streams of CO.sub.2. For example, the gas phase with elevated CO.sub.2 content may include a gas product or by-product from a fossil fuel-based power plant, an exhaust gas from a vehicle, or a gas emitted from a source where a biomass is burned.

[0058] In another aspect, the present disclosure provides a method of improving indoor air quality. The method can comprise contacting the indoor air with the carbon dioxide absorption composition as described herein, such that the carbon dioxide in the indoor air reacts with the amino compound in the carbon dioxide absorption composition to produce a carbon dioxide-enriched composition, thereby reducing content of the carbon dioxide in the indoor air.

[0059] The composition and device as described herein may be particularly suitable for capturing (or scrubbing) CO.sub.2 from indoor air, providing an approach to effectively improve indoor air quality that has not been observed in the art. On the one hand capturing CO.sub.2 from indoor air is beneficial because the concentration of CO.sub.2 in indoor air is much higher than outdoor air. However, it can be also difficult to maintain comfort conditions of temperature and relative humidity (RH) in indoor air. In addition, introduction of additional volatile organic compounds (VOCs) in indoor air should be avoided. Advantageously, the present method may maintain comfort indoor conditions for the occupants while capturing CO.sub.2 from indoor air. Further, by capturing CO.sub.2 from indoor air, the CO.sub.2 levels is maintained below what is unhealthy for occupants.

[0060] In some embodiments, the indoor air has a CO.sub.2 level of about 400 ppm to about 5,000 ppm, or higher than 5,000 ppm.

[0061] In some embodiments, the method is carried out while a relative humidity (RH) level of the indoor air is maintained at a comfortable condition for the occupants, such as within 0-90%, within 30-70%, or within 40-60%. In some embodiments, the method is carried out while the relative humidity (RH) level of the indoor air is maintained within 30%-70%. In some embodiments, the method is carried out while the relative humidity (RH) level of the indoor air is maintained within 40%-60%.

[0062] In some embodiments, the method is carried out while a total volatile organic compounds (TVOCs) level of the indoor air is maintained below 10 ppm, such as below 8 ppm, below 5 ppm, below 2 ppm, or below 1 ppm. In some embodiments, the method is carried out while a total volatile organic compounds (TVOCs) level of the indoor air is maintained below 1 ppm. To further improve the indoor air quality, additional agents (such as activated carbon) may be used in the present device or method to absorb or remove the undesired components in the indoor air (such as organic compounds). For example, the additional agent can be placed in a separate unit or container in the indoor environment. Alternatively, the additional agent can be placed in a compartment or unit of the present device to used together with the present carbon dioxide absorption composition. In some embodiments, the present method for improving indoor air quality further comprises passing the indoor air through activated carbon. In particular, the use of activated carbon can facilitate reducing the TVOCs levels in the indoor air. For example, by passing the indoor air through activated carbon, the present method can be carried out while maintaining the TVOCs further below 1 ppm, such as below 0.5 ppm, below 0.2 ppm, or below 0.1 ppm (100 ppb). In some embodiments, the present method includes passing the indoor air through activated carbon, and the TVOCs level of the indoor air is maintained below 0.1 ppm, such as below 50 ppb, below 20 ppb, or below 10 ppb.

[0063] A CO.sub.2 desorption process may be utilized in the present methods to regenerate the CO.sub.2 absorption composition. For example, the chemical reactions involved in the CO.sub.2 desorption process include those illustrated in FIG. 1B. The CO.sub.2 desorption process can be facilitated by the application of external energy, such as heating and microwave irradiation. As an example, the carbon dioxide-enriched composition may be heated (e.g., by a hot plate or heating unit) or subjected to microwave (e.g., an industrial or house-hold equipment). In some embodiments, the carbon dioxide-enriched composition may be more effectively regenerated by microwave irradiation as compared to heating. In some embodiments, the present methods for removing CO.sub.2 or improving indoor air quality further comprises subjecting the carbon dioxide-enriched composition to microwave irradiation, thereby producing a regenerated carbon dioxide absorption composition and releasing the absorbed carbon dioxide.

[0064] In some embodiments, the method includes subjecting the carbon dioxide-enriched composition to microwave irradiation for about 10 to about 300 seconds, about 30 to about 270 seconds, about 50 to about 250 seconds, or about 60 to about 210 seconds. In some embodiment, a 1200-watt Panasonic microwave oven at heating power level 10 is utilized for irradiation, with an end temperature in the range about 90 to about 130 C. Typically, a 50 mL solution of the carbon dioxide-enriched composition is irradiated. Typically, the microwave irradiation process will release at least 70% of absorbed CO.sub.2 from the composition.

[0065] The present methods can be operated in a continuous manner, by which the CO.sub.2 adsorption-desorption processes are repeated in cycles. As used herein, each cycle or regeneration cycle refers to a complete sequence of CO.sub.2 adsorption and desorption according to the present methods, resulting in a regenerated carbon dioxide absorption composition. In some embodiments, the present method further comprises reusing the regenerated carbon dioxide absorption composition for reaction with the carbon dioxide in the gas phase or indoor air. Typically, the regenerated composition retains most of its capacity of CO.sub.2 absorption before regeneration. In some embodiments, the present composition can have less than 35% reduction in CO.sub.2 absorption capacity after 10 regeneration cycles. In some embodiment, the initial 3-4 cycles may exhibit a relatively greater drop in capacity, which is stabilized in the later cycles. As a result, the variations in the last four cycles may be minimal. By recycling and reusing the regenerated carbon dioxide absorption composition, the method can be carried out in continuous cycles at reduced cost.

[0066] In some embodiment, the present method can include carrying out a plurality of carbon dioxide absorption-desorption cycles, each of which can include the steps of: [0067] (A) contacting the gas phase or indoor air with the carbon dioxide absorption composition to produce a carbon dioxide-enriched composition and reduce the carbon dioxide content in the gas phase or indoor air; [0068] (B) subjecting the carbon dioxide-enriched composition to microwave irradiation, thereby producing a regenerated carbon dioxide absorption composition and releasing the absorbed carbon dioxide; and [0069] (C) recycling the regenerated carbon dioxide absorption composition to step (A) for reaction with the carbon dioxide in the gas phase or indoor air.

[0070] Typically, the desorption/regenerate process can be done with the release of CO.sub.2 into an environment where the released CO.sub.2 does not raise a safety concern. For example, the CO.sub.2 absorbed from indoor air by the present composition or device can be released in an outdoor environment. In some embodiments, the desorption process is carried out in an outdoor environment. Alternatively, the absorbed CO.sub.2 can be collected and used for production of other materials. For example, the CO.sub.2 collected from the present method may be used in making mineral water or jet fuel. Thus, the present composition, device, and method offers an effective way to recycle CO.sub.2, in particular CO.sub.2 from indoor air, for a useful purpose. In some embodiments, the present method further comprises collecting the released carbon dioxide, such as in a storage container, for useful applications.

[0071] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. As used herein, the terms have, has, having, include and including have the same meaning as the terms comprise and comprising. The terms comprise and comprising should be interpreted as being open transitional terms that permit the inclusion of additional components further to those components recited in the claims. The terms consist and consisting of should be interpreted as being closed transitional terms that do not permit the inclusion of additional components other than the components recited in the claims. The term consisting essentially of should be interpreted to be partially closed and allowing the inclusion only of additional components that do not fundamentally alter the nature of the claimed subject matter. As used herein, the singular forms a, an, and the include plural embodiments unless the context clearly dictates otherwise. The modifier about used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier about should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression from about 2 to about 4 also discloses the range from 2 to 4. The term about may refer to plus or minus 10% of the indicated number. For example, about 10% may indicate a range of 9% to 11%, and about 1 may mean from 0.9 to 1.1.

[0072] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference.

[0073] The present invention has been described in terms of one or more embodiments, and it should be appreciated that all possible equivalents, alternatives, variations, and modifications, aside from those expressly stated are within the scope of the invention.

EXAMPLES

[0074] This study explores the potential of various CO.sub.2 capture solutions, including aqueous MEA, aqueous Arg, and water-glycol-Arg solutions, to enhance indoor air quality by regulating CO.sub.2 levels, RH, and TVOC concentrations. Through experimental evaluations, the solutions' desorption capabilities, including regeneration efficiency, solvent stability, and visual integrity post-desorption, were analyzed using microwave irradiation at varying exposure times. A subsequent cyclic study was performed to assess the stability, safety, and long-term performance of aqueous MEA and water-PG-Arg solutions. This investigation advances our understanding of CO.sub.2 capture kinetics and positions Arg as a promising, sustainable alternative to traditional absorbents like MEA.

Example 1

Materials and Methods

Reagents

[0075] MEA (purity98%), L-arginine (purity98%), triethylene glycol (purity98%), dipropylene glycol (purity99%), diethylene glycol (purity99%), and ethylene glycol (purity99%) were procured from ThermoFisher Scientific. Propylene glycol (purity99.8%) was sourced from Millipore Sigma. All the solutions were prepared using Milli-Q water, and the CO.sub.2 gas (purity99.999 mol %) was obtained from the Airgas company.

Experimental Setup and Procedure

[0076] The CO.sub.2 scrubbing solutions (CSS) were formulated by adding 0.05 mol of CSA to 50 mL of solvent. For the aqueous CSS, Milli-Q water was used as the solvent, while the water-glycol-based CSS was prepared by mixing water and glycol in a 1:1 volumetric ratio based on the solubility of Arg in water.

[0077] The CO.sub.2 absorption setup consisted of a 100 mL two-neck round-bottom flask filled with the prepared CSS and connected to a condenser to minimize the loss of water and amine vapors. This setup was placed inside a sealed steel chamber measuring 323336 inches (FIG. 6), equipped with a 12 V fan to ensure thorough mixing of air within. CO.sub.2 was introduced from the top of the chamber until the concentration stabilized between 1000 and 1100 ppm.

[0078] The air inside was bubbled through the solution using a micro air pump during the CO.sub.2 absorption process. The experiments were performed under laboratory conditions at atmospheric pressure, with an initial relative humidity of 545%. Changes in C.sub.CO.sub.2 (ppm), RH (%), and TVOCs (ppb) were continuously monitored using Graywolf IQ-610 sensors and logged at 30-second intervals with an advanced sense meter. As per the guidelines from Graywolf Sensing Solutions, the TVOC sensor operates accurately within a relative humidity (RH) range of 0 to 90%, becoming unreliable beyond this threshold. Consequently, in our study, TVOC data were recorded until the RH reached 90%. The VOCs generated inside the chamber were collected by passing the air through 20 mL of Milli-Q water for 60 minutes.

[0079] The resulting solution (referred to as the TVOC solution) was analyzed using Triple Quadrupole Liquid Chromatography-Mass Spectrometry (LC-QqQ-MS) and Headspace Solid-Phase Microextraction Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS). Specifically, Agilent Technologies 1260 infinity LC system coupled with Agilent Technologies 6460 Triple Quad LC/MS (LC-MS/MS) was used and molecules were ionized using an Electrospray Ionization (ESI) source. GC-MS analysis was conducted on an Agilent 7890B gas chromatograph coupled with an Agilent 5977B Mass Selective Detector, utilizing Electronic Impact (EI) for ionization and fragmentation. Mode for qualitative analysis of LC-QqQ-MS: MS2 scan (i.e., Full scan). Mode for quantitative analysis of LC-QqQ-MS: MRM scan (i.e., Multiple reaction monitoring)].

[0080] In some studies, the absorption setup included a charcoal canister. For all the absorption experiments, the charcoal canister was excluded to independently assess each solution's performance. It was integrated into the system only after optimizing and selecting the best solution.

[0081] Once the CO.sub.2 absorption process was complete, water was added to replenish the solution that had evaporated due to vaporization. The solution was then transferred to a 500 mL conical flask, and a few small porcelain boiling chips were added to ensure smooth boiling and prevent bumping during the regeneration process. MW-assisted regeneration of the saturated scrubbing solution was performed. Initially, the temperature of the solution, along with the CCO.sub.2, RH, and TVOC values, were recorded. The solution was then heated using a Panasonic 1200 W MW oven at heating power level 10 for varying time periods. The temperature and volume of the solution were recorded right after heating, whereas other parameters, such as TVOC and RH, were measured five minutes later after they had stabilized. Rather than using one prolonged MW heating duration, the desorption process was broken into shorter sessions to ensure that the solution did not dry out completely.

[0082] In an alternative desorption setup, conventional thermal heating was used. Once the solution was loaded with CO.sub.2, water was added to replenish the volume lost due to vaporization. It was then transferred into a 100 ml three-neck round-bottom flask, with a condenser connected to the middle arm, a glass sparger on the left arm, and a thermometer on the right arm. First, the sand was preheated for 30 minutes by allowing it to stay on the hot plate at 2500 C. Then, the initial temperature of the sand and solution was noted, the two-neck round bottom flask was placed on it, and the chamber door was closed. Initial reading of CO.sub.2, RH, and TVOC was also recorded, and the micropump was turned on. This desorption experiment was continued until the CO.sub.2 concentration curves plateaued over a period of time. At this point, the final reading of the above-mentioned parameters was recorded, and the chamber was opened to note the final temperatures of the solution and the sand bath, thereby concluding the desorption process.

[0083] Energy consumption during the desorption process was measured using a P3 Kill-A-Watt meter, connected between the main power supply and the hot plate and microwave. For the micro air pump, energy use was calculated based on the voltage and current from the DC power supply, and the operation duration. The total energy consumed by each equipment was documented.

Quantitative Analysis and Kinetics

[0084] The CO.sub.2 loading (.sub.abs) is defined as the moles of CO.sub.2 absorbed per unit moles of CSA, while the CO.sub.2 unloading (.sub.des) represents the reverse process (Song et al., 2012). These values were calculated using the following formula based on the initial and final CO.sub.2 concentrations.

[00001]${\mathrm{CO}}_2\mathrm{loading}/\mathrm{removed}\left(\right)=\frac{\mathrm{Moles}\mathrm{of}{\mathrm{CO}}_2\mathrm{absorbed}/\mathrm{desorbed}}{\mathrm{Moles}\mathrm{of}{\mathrm{CO}}_2\mathrm{scrubbing}\mathrm{agent}}\left(\mathrm{mol}\right)$$\mathrm{Moles}\mathrm{of}{\mathrm{CO}}_2\mathrm{Abs}./\mathrm{Des}.\left(n_{{\mathrm{co}}_2}\right)=\frac{C}{1000000}\frac{1}{M_{{\mathrm{CO}}_2}}{}_{C{O}_2}1000V_c\left(\mathrm{mol}\right)$$C=\mathrm{Initial}\mathrm{Conc}.\mathrm{of}{\mathrm{CO}}_2(\mathrm{ppm})-\mathrm{Final}\mathrm{Conc}.\mathrm{of}{\mathrm{CO}}_2\left(\mathrm{ppm}\right)$$\mathrm{Moles}\mathrm{of}{\mathrm{CO}}_2\mathrm{scrubbing}\mathrm{agent}=\frac{\mathrm{Mass}}{\mathrm{Molecular}\mathrm{weight}}\left(\mathrm{mol}\right)$

[0085] Where C is the concentration gradient, M.sub.CO.sub.2=44 g/mol and pco2=1.9610.sup.3 g/ml are the molecular weight and density of CO.sub.2, respectively, and V.sub.c=623 liter is the chamber volume. The amount of CO.sub.2 absorbed or desorbed (in mg) depends on the moles of CO.sub.2 absorbed or desorbed (n.sub.CO.sub.2) and can be calculated using the following formula:

[00002]$\mathrm{Amount}\mathrm{of}{\mathrm{CO}}_2\mathrm{abs}./\mathrm{des}.=n_{C{O}_2}{M}_{C{O}_2}1000\left(\mathrm{mg}\right)$

[0086] Additionally, based on the CO.sub.2 absorption experimental data, the integrated rate law equation for a second-order reaction was used to model the kinetics of gas-liquid absorption reactions. The reaction rate constant (K) was determined from the slope of the linear regression plot of 1/C.sub.CO.sub.2 versus time.

Results and Discussion

[0087] The performances of aqueous MEA and aqueous Arg solutions in terms of CO.sub.2 absorption, kinetics, and their impact on RH levels and TVOC concentrations were evaluated. Although the aqueous Arg solution showed a lower capacity for CO.sub.2 absorption (=0.31 mol/mol) and slower kinetics (K=1.8010.sup.6 ppm.sup.1 min.sup.1) compared to the aqueous MEA solution (=0.40 mol/mol and K=2.5710.sup.6 ppm.sup.1 min.sup.1) (FIG. 2A and FIG. 7B), presumably due to Arg's larger molecular size (Suleman et al., 2020) affecting solution viscosity and hindering CO.sub.2 diffusion, it still presents notable advantages over MEA, particularly concerning VOC emissions and RH behavior. The aqueous MEA solution caused a significant increase in TVOC levels during the absorption process, as shown in FIG. 2B, whereas the aqueous Arg solution produced minimal to no detectable rise in TVOCs. Qualitative analysis of the VOCs emitted from the aqueous MEA solution using LC-QqQ-MS identified MEA as the primary volatile component, as its higher vapor pressure likely contributed to elevated TVOC levels, highlighting a potential drawback of using MEA for CO.sub.2 absorption. In contrast, the aqueous Arg solution demonstrated lower vapor pressure, as evidenced by the RH profiles. Analysis of the RH slopes revealed that the Arg solution took longer to reach 90% RH compared to the MEA solution, indicating a lower vaporization rate (FIG. 8). These findings indicate that, while the aqueous Arg solution has slightly lower CO.sub.2 absorption capacity and slower kinetics, it is a more environmentally sustainable option due to its minimal VOC emissions, especially indoors. To further evaluate the practical application of these solutions, MW regeneration experiments were conducted to assess their sustainability for continuous absorption-desorption cycles.

[0088] Microwave heating was employed for CO.sub.2 desorption due to its faster process, lower energy consumption, and simpler setup compared to conventional heating. The irradiation process was optimized to achieve full solution regeneration with minimal microwave irradiation time and solvent loss; to this end, a series of absorption-desorption studies were conducted using aqueous MEA solutions. The first MW irradiation time was adjusted by careful inspection of the solution during the MW session to avoid excessive boiling and bumping of the solution inside the MW chamber, while achieving maximum CO.sub.2 desorption possible. As shown in FIGS. 3A and 3B, it was observed that increasing the first MW session beyond 150 secs led to excessive solvent loss and worse CO.sub.2 desorption outcome. The optimum MW irradiation profile was found to be an initial MW session of 150 secs followed by another session of 60 seconds, achieving near-complete regeneration by desorbing 589 mg (99.5%) of CO.sub.2 from 592 mg absorbed, while limiting solvent loss to just 37 ml.

[0089] While the optimal MW irradiation times of 150 sec for the first session and 60 sec for the second session were effective for aqueous MEA, they proved insufficient for aqueous Arg solution. Only 436 mg (63.5%) of CO.sub.2 was desorbed from the 687 mg absorbed in the first session. Extending the second session to 90 seconds released an additional 82 mg, totaling 518 mg (75.4%), showing that the desorption remained incomplete. Prolonged MW irradiation also caused complete solvent evaporation, leaving behind solid Arg crystals, as shown in FIG. 9. To address this issue and maintain the solution in liquid form, a water-glycol-based Arg solution was formulated.

[0090] The choice of glycol with a higher boiling point was intended to manage the RH during the absorption process in view of its hygroscopic properties and to reduce complete evaporation of the solution during the desorption process. To formulate the water-glycol-based CSS, water and glycol were mixed in an optimized 1:1 volumetric ratio. A comprehensive evaluation of some commonly used glycol compounds was conducted, focusing on key properties such as low vapor pressure, appropriate viscosity, and high molecular weight, as shown in Table 2. Among these, the water-EG-Arg and water-PG-Arg solutions exhibited better Arg solubility and were thus chosen for further studies. Although pure glycol-Arg solutions were initially explored, they exhibited poor solubility of Arg in glycol and resulted in high viscosity of the solution, which hindered CO.sub.2 diffusion. Consequently, water was incorporated to enhance both the absorption performance and kinetics of the solution.

[0091] As illustrated in FIGS. 7A and 7B, the presence of glycol in the solution slowed the diffusion of CO.sub.2 from the gas to the liquid phase, reducing both the amount of CO.sub.2 absorbed and its reaction kinetics.

[0092] While the water-PG-Arg solution initially showed slightly lower CO.sub.2 absorption capacity (=0.24 mol/mol, K=1.1510.sup.6 ppm.sup.1 min.sup.1) than the water-EG-Arg solution (=0.26 mol/mol, K=0.9510.sup.6 ppm.sup.1 min.sup.1) (Table 1), it exhibited comparable kinetics and offered better RH control, taking longer to reach 90% RH during absorption (FIGS. 2A-2C). Qualitative analysis using LC-QqQ-MS and HS-SPME-GC-MS identified only propylene glycol among the emitted VOCs, with no trace of Arg in the case of water-PG-Arg solution. It is worth mentioning that PG, as a VOC, is odorless, non-toxic, and has a Workplace Environmental Exposure Limit (WEEL) of 3200 ppb, averaged over an 8-hour work shift, as set by the American Industrial Hygiene Association (AIHA) (New Jersey Department of Health, 2009). This limit is at least five times higher than the levels detected in our experiment. During desorption, the water-PG-Arg solution achieved 90% regeneration in a shorter time, reducing energy consumption by 29% (Table 3). Additionally, no visible color change was observed in the water-PG-Arg solution after the first desorption process, whereas the water-EG-Arg solution exhibited a shift from transparent to amber, indicating possible degradation.

TABLE-US-00001 TABLE 1 CO.sub.2 absorption and reaction kinetics of aqueous MEA and aqueous Arg solutions. Rate Constant (K) is based on second-order kinetics. Rate Initial Final Amt. of constant CO.sub.2 abs. TVOC at TVOC at CO.sub.2 abs. K 10.sup.6 time RH 54 5% RH 90% Solution (mg) (ppm.sup.1min.sup.1) (min) (ppb) (ppb) H.sub.2O (50 ml) + MEA (3 g) 860 2.57 819 285 601 H.sub.2O (50 ml) + Arg (8.71 g) 687 1.80 752 255 293 H.sub.2O (25 ml) + EG (25 ml) + 582 0.95 754 269 562 Arg (8.71 g) H.sub.2O (25 ml) + PG (25 ml) + 531 1.15 771 358 789 Arg (8.71 g)

TABLE-US-00002 TABLE 2 Summary table of properties of glycols solvents. Dynamic Boiling point Molecular Vapor pressure viscosity Organic ( C. at weight (mmHg at (cP at solvent 1 atm) (g/mol) 20 C.) 20 C.) Ethylene 196-198 62.06 0.12 21 Glycol (EG) Diethylene 245 106.12 0.01 37.2 glycol (DEG) Tri ethylene 285 150.17 <0.0075 48 glycol (TEG) Propylene 187 76.1 0.097 45 glycol (PG) Di-propylene 180-190 148.2 <0.01 4.32 glycol (DPG)

TABLE-US-00003 TABLE 3 Two absorption-desorption cycle comparisons between H.sub.2O-EG-Arg and H.sub.2O-PG-Arg solutions. Amount Energy of CO.sub.2 Run time consumed Solutions Cycles Process (mg) (min) (Whr) H.sub.2O (25 ml) + 1 Abs. 1 582 754 17.1 EG (25 ml) + Des. 1 591 5 140 Arg (8.71 g) 2 Abs. 2 521 759 17.3 Des. 2 556 4.5 130 H.sub.2O (25 ml) + 1 Abs. 1 531 771 17.6 PG (25 ml) + Des. 1 481 3.5 100 Arg (8.71 g) 2 Abs. 2 499 530 12.1 Des. 2 463 3.5 100

[0093] In subsequent cyclic test, the water-PG-Arg solution demonstrated improved stability with consistently lower TVOC emissions during absorption (FIG. 2B) and no color change after desorption processes. The water-EG-Arg solution, however, emitted notably higher TVOCs with an unpleasant pungent odor throughout the cyclic study, and its color gradually deepened to a darker red after each desorption process. This degradation was likely driven by elevated temperatures during desorption processes, reaching 156 C. in the first cycle and 176 C. in the second, suggesting potential side reactions and thermal degradation (Tables 5 and 6). In contrast, the water-PG-Arg solution exhibited lower temperature increases during desorption (Tables 7 and 16), attributed to the higher specific heat capacity of PG (Table 4). Additionally, PG's lower polarity and dielectric constant limited the absorption of MW energy, reducing excessive heat generation. Thus, the thermal stability, improved absorption performance, and reduced energy consumption of the water-PG-Arg solution highlight its potential for long-term CO.sub.2 capture applications. These findings support further investigation into its cyclic performance over multiple absorption-desorption processes.

TABLE-US-00004 TABLE 4 Dielectric properties and heat capacity of Ethylene glycol and propylene glycol. Dielectric Dielectric Specific heat capacity constants loss factor [cp] Compound [] [] (J/mol C. at 25 C.) Ethylene 41.2 5 149.8 Glycol Propylene 30.2 1 189.9 Glycol

[0094] The cyclic performances of aqueous MEA and water-PG-Arg solutions were evaluated over ten absorption-desorption cycles, as shown in FIG. 4, panels (a) and (b). The reaction rate constants for both solutions across these cycles are presented in FIG. 5, panels (a) and (b). Although the aqueous MEA solution initially exhibited higher CO.sub.2 absorption, its performance declined significantly over time, with a 54.3% reduction in CO.sub.2 absorption after ten cycles. In contrast, the water-PG-Arg solution demonstrated greater stability, with only a 31.24% decline over the same period, becoming increasingly more consistent as our study progressed. The steeper downward trend observed in the MEA solution (FIG. 4, panel (b)) highlights its limitations for long-term CO.sub.2 capture applications. Notably, during the third cycle of the aqueous MEA solution, 625 mg of CO.sub.2 was desorbed, exceeding the 573 mg absorbed, attributed to experimental error or due to residual CO.sub.2 from previous cycles. This behaviour was observed several times throughout the study. The effectiveness of the aqueous MEA solution declines over time because of oxidative and thermal degradation, leading to a reduced concentration of MEA available for CO.sub.2 absorption. The degradation products, such as ammonia, acetaldehyde, and acetone, may pose potential risks to indoor environments. As a result, the reaction rate constant of the aqueous MEA solution decreased by 34.24% after ten cycles, compared to only a 2.13% reduction for the water-PG-Arg solution. These findings highlight the superior stability and long-term viability of the water-PG-Arg solution for indoor CO.sub.2 capture applications.

[0095] FIGS. 11A-11B illustrates the RH levels and TVOC concentrations during 10 absorption cycles of the H.sub.2O-PG-Arg solution. Initially, the solution took longer to reach a RH of approximately 90% in the chamber; however, its ability to maintain RH progressively declined as the solution was used repeatedly. In contrast, TVOC concentrations were highest during the first cycle and they kept decreasing with each subsequent cycle. To understand this, TVOCs were sampled to analyze the compounds via LC-MS-QqQ and HS-SPME-GC-MS after the initial absorption process.

[0096] The analysis confirmed the presence of PG with no trace of Arg, based on the mass spectral data of water-PG-Arg solution and blank, which look similar. This analysis highlights Arg's stability across multiple cycles. However, the solution's declining ability to control RH during absorption can be attributed to PG evaporation during desorption, leading to a reduced mole fraction of PG in the resulting solutions, which can be attributed to a gradually lower TVOC generation.

[0097] To further enhance indoor air quality by capturing VOCs during absorption, a charcoal canister was integrated into the setup. FIGS. 10A-10B illustrate the system's performance with and without the charcoal canister.

[0098] As anticipated, the results indicate that activated charcoal effectively reduced TVOC level throughout the experiment while adsorbing some water vapor, stabilizing RH. It is worth mentioning that PG, as a VOC, is odourless, non-toxic, and has a Workplace Environmental Exposure Limit (WEEL) of 3200 ppb, averaged over an 8-hour work shift, as set by the American Industrial Hygiene Association (AIHA). This limit is at least twelve times higher than the levels recorded in our experiment (i.e., 271 ppb). This stability of VOCs within the chamber highlights the charcoal canister's efficacy in limiting PG escape into the air, further supporting the reliability of the water-PG-Arg solution in capturing CO.sub.2.

[0099] Tables 5-6 show experimental data of 2 absorption-desorption cycles of H.sub.2O-EG-Arg solution. Tables 7-16 show experimental data of 10 absorption-desorption cycles of H.sub.2O-PG-Arg solution. Tables 17-27 show experimental data of absorption-desorption cycles of the aqueous MEA solution.

TABLE-US-00005 TABLE 5 Experimental data of first absorption-desorption cycle of aqueous H.sub.2O-EG-Arg solution. Cycle 1 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1110 55 269 53 10 Final 633 100 12011 48 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 477 582 0.26 0.95 5 754 Desorption 1 Desorption step 1 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 387 properly in 600 213 Volume of solution (ml) 53 48 the chamber 5 Temperature ( C.) 22.7 103 54.8 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 600 properly in 688 88 Volume of solution (ml) 48 40 the chamber 8 Temperature ( C.) 54.8 110 58.9 Desorption step 3 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 688 properly in 763 75 Volume of solution (ml) 40 32 the chamber 8 Temperature ( C.) 58.9 118.9 60.7 Desorption step 4 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 763 properly in 830 67 Volume of solution (ml) 32 27 the chamber 5 Temperature ( C.) 60.7 133.9 62 Desorption step 5 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 830 properly in 895 41 Volume of solution (ml) 27 23 the chamber 4 Temperature ( C.) 62 156 80.2 Concentration of CO.sub.2 484 Amount of 591 Volume of the 30 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 140 CO.sub.2 unloading (mole 0.27 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00006 TABLE 6 Experimental data of second absorption-desorption cycle of aqueous H.sub.2O-EG-Arg solution. Cycle 2 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1118 66.4 1234 53 10 Final 691 100 3736 47 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 427 521 0.24 0.75 6 759 Desorption 1 Desorption step 1 Microwave desorption time - 90 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 379 properly in 615 236 Volume of solution (ml) 53 38 the chamber 15 Temperature ( C.) 23 106.7 55 Desorption step 2 Microwave desorption time - 90 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 615 properly in 717 102 Volume of solution (ml) 38 27 the chamber 11 Temperature ( C.) 55 111.8 65.1 Desorption step 3 Microwave desorption time - 90 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 717 properly in 834 117 Volume of solution (ml) 27 16 the chamber 11 Temperature ( C.) 56.1 176 68.3 Concentration of 455 Amount of 556 Volume of the 37 CO.sub.2 desorbed CO.sub.2 desorbed solution lost (ppm) (mg) (ml) Energy consumed 130 CO.sub.2 unloading (mole 0.25 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00007 TABLE 7 Experimental data of first absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 1 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1068 55.5 484 55 10 Final 612 89.5 2850 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 456 557 0.25 0.94 6 780 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 376 properly in 798 422 Volume of solution (ml) 55 25 the chamber 30 Temperature ( C.) 13 135.5 55 Concentration of 422 Amount of 515 Volume of the 30 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 70 CO.sub.2 unloading (mole 0.23 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00008 TABLE 8 Experimental data of second absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 2 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1083 53.7 530 55 10 Final 625 94.2 2715 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 458 559 0.25 0.99 6 781 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 387 properly in 787 400 Volume of solution (ml) 55 26 the chamber 29 Temperature ( C.) 22.7 104.5 53 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 787 properly in 827 40 Volume of solution (ml) 26 21 the chamber 5 Temperature ( C.) 53 123.8 51.3 Concentration of 440 Amount of 537 Volume of the 34 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 100 CO.sub.2 unloading (mole 0.24 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00009 TABLE 9 Experimental data of third absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 3 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1066 54.5 421 55 10 Final 634 97.1 1873 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 432 528 0.24 0.93 6 773 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 390 properly in 761 371 Volume of solution (ml) 55 28 the chamber 27 Temperature ( C.) 22.8 101.1 55.4 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 761 properly in 785 24 Volume of solution (ml) 28 21 the chamber 7 Temperature ( C.) 55.4 107.3 52.1 Concentration of 395 Amount of 482 Volume of the 34 CO.sub.2 desorbed CO.sub.2 desorbed solution lost (PPM) (mg) (ml) Energy consumed 100 CO.sub.2 unloading (mole 0.22 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00010 TABLE 10 Experimental data of fourth absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 4 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1089 54.9 330 55 10 Final 680 98.5 1398 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 409 499 0.23 0.94 6 682 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 370 properly in 760 390 Volume of solution (ml) 55 22 the chamber 33 Temperature ( C.) 23 100.4 50.4 Concentration of 390 Amount of 476 Volume of the 33 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 70 CO.sub.2 unloading (mole 0.22 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00011 TABLE 11 Experimental data of fifth absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 5 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1045 50.8 358 55 10 Final 610 100 1664 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 435 531 0.24 1.15 6 771 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 389 properly in 752 363 Volume of solution (ml) 55 25 the chamber 30 Temperature ( C.) 22.4 96.7 48.3 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 752 properly in 783 31 Volume of solution (ml) 25 19 the chamber 6 Temperature ( C.) 48.3 100 46.4 Concentration of 394 Amount of 481 Volume of the 36 CO.sub.2 desorbed CO.sub.2 desorbed solution lost (ppm) (mg) (ml) Energy consumed 100 CO.sub.2 unloading (mole 0.22 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00012 TABLE 12 Experimental data of sixth absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 6 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1077 52.8 331 55 10 Final 668 98.4 855 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 409 499 0.23 1.21 6 530 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 378 properly in 725 347 Volume of solution (ml) 55 27 the chamber 28 Temperature ( C.) 22.4 97.9 52.7 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 725 properly in 757 32 Volume of solution (ml) 27 21 the chamber 6 Temperature ( C.) 52.7 100 50.4 Concentration of 379 Amount of 463 Volume of the 34 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 100 CO.sub.2 unloading (mole 0.21 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00013 TABLE 13 Experimental data of seventh absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 7 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1035 58.1 335 55 10 Final 704 100 845 49 9 Concentration of Amount of CO.sub.2 loading Rate constant Volume of the Absorption CO.sub.2 absorbed CO.sub.2 absorbed (Mole CO.sub.2/ (K 10.sup.6) solution lost run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) (ml) (min) 331 404 0.18 1.10 6 603 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 367 properly in 708 341 Volume of solution (ml) 55 22 the chamber 33 Temperature ( C.) 22.8 97.3 46.7 Concentration of 341 Amount of 416 Volume of the 33 CO.sub.2 desorbed CO.sub.2 desorbed solution lost (ppm) (mg) (ml) Energy consumed 70 CO.sub.2 unloading (mole 0.19 during Desorption CO.sub.2/mole L-arg) (Wh)

TABLE-US-00014 TABLE 14 Experimental data of eight absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 8 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1098 52.9 349 55 10 Final 746 100 1197 49 9 Concentration Amount Rate of CO.sub.2 of CO.sub.2 CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) lost (ml) (min) 352 430 0.20 1.02 6 740 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 381 properly in 710 329 Volume of solution 55 18 the chamber 37 (ml) Temperature ( C.) 22.6 94.8 47 Concentration of 329 Amount of 402 Volume of 37 CO.sub.2 desorbed CO.sub.2 desorbed the solution (ppm) (mg) lost (ml) Energy consumed 70 CO.sub.2 unloading 0.18 during Desorption (mole CO.sub.2/ (Wh) mole L-arg)

TABLE-US-00015 TABLE 15 Experimental data of ninth absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 9 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1079 53.1 337 55 10 Final 747 97.2 1073 49 9 Concentration Amount Rate of CO.sub.2 of CO.sub.2 CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) lost (ml) (min) 332 405 0.18 1.24 6 460 Desorption 1 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 357 properly in 657 300 Volume of solution 55 20 the chamber 35 (ml) Temperature ( C.) 22.6 97.9 45.4 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 657 properly in 684 27 Volume of solution 20 13 the chamber 7 (ml) Temperature ( C.) 45.4 97.1 43 Concentration of 327 Amount of 399 Volume of 42 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed the solution (mg) lost (ml) Energy consumed 100 CO.sub.2 unloading 0.18 during Desorption (mole CO.sub.2/ (Wh) mole L-arg)

TABLE-US-00016 TABLE 16 Experimental data of tenth absorption-desorption cycle of H.sub.2O-PG-Arg solution. Cycle 10 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1054 54.9 314 55 10 Final 740 100 1048 49 9 Concentration Amount Rate of CO.sub.2 of CO.sub.2 CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole L-arg.) (ppm.sup.1 min.sup.1) lost (ml) (min) 314 383 0.17 0.92 6 556 Desorption 1 Desorption step 1 Microwave desorption time -150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to Conc. of CO.sub.2 (ppm) 361 mix 655 294 Volume of solution 55 23 properly in 32 (ml) Temperature ( C.) 22.8 95 the chamber 47.3 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 655 properly in 681 26 Volume of solution 23 13 the chamber 10 (ml) Temperature ( C.) 47.3 96.4 44.6 Concentration of 320 Amount of 391 Volume of 42 CO.sub.2 desorbed (ppm) CO.sub.2 desorbed the solution (mg) lost (ml) Energy consumed 100 CO.sub.2 unloading 0.18 during Desorption (mole CO.sub.2/ (Wh) mole L-arg)

TABLE-US-00017 TABLE 17 Experimental data of absorption-conventional desorption cycle of aqueous MEA solution. Concentration Solution Absorption of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1037 53.6 318 53 12 Final 318 100 22795 49 9 Concentration Amount Microair Volume of CO.sub.2 of CO.sub.2 CO.sub.2 loading pump power of the Absorption absorbed absorbed (Mole CO.sub.2/ consumption solution run time (ppm) (mg) Mole MEA) (Wh) lost (ml) (min) 719 878 0.41 19.9 7 871 Conventional Concentration Solution Solution Desorption of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) temperature (C.) Initial 361 55.5 330 53 24 Final 660 83.3 319 46 67 Concentration Amount CO.sub.2 Microair Volume of CO.sub.2 of CO.sub.2 unloading pump power of the Desorption desorbed desorbed (Mole CO.sub.2/ consumption solution run time (ppm) (mg) Mole MEA) (Wh) lost (ml) (min) 299 365 0.17 3.2 7 140 Energy consumed by hot plate during Desorption (Wh) 100 Additional Data Temperature set value of hot plate: 250 C, Sand preheating duration: 60 mins, Energy Consumed by hot plate during pre-heating: 50 Whr., Sand Temperature after pre-heating: 125 C, and Micro air pump power: 1.37 W.

TABLE-US-00018 TABLE 18 Experimental data of first absorption-desorption cycle of aqueous MEA solution. Cycle 1 Concentration Solution Absorption 1 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1080 58.5 285 53 12 Final 376 100 27090 45 9 Concentration Amount of CO.sub.2 of CO.sub.2 CO.sub.2 loading Rate constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole L-arg.) (1/ppm min) lost (ml) (mins) 704 860 0.40 2.57 8 819 Desorption 1 Desorption step 1 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 326 properly in 830 504 Volume of solution 53 27 the chamber 26 (ml) RH (%) 59.8 100 TVOC (ppb) 336 78611 Temperature (C.) 22.3 95.1 47 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 830 properly in 918 88 Volume of solution 27 19 the chamber 8 (ml) RH (%) 100 100 TVOC (ppb) 78611 78592 Temperature (C.) 47 93 43.1 Desorption step 3 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 918 properly in 980 62 Volume of solution 19 10 the chamber 9 (ml) RH (%) 100 100 TVOC (ppb) 78592 78615 Temperature (C.) 43.1 94 43 Concentration of CO.sub.2 654 Amount of 799 Volume of the 43 desorbed (ppm) CO.sub.2 desorbed solution lost (mg) (ml) Energy consumed 130 CO.sub.2 unloading 0.37 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00019 TABLE 19 Experimental data of second absorption-desorption cycle of aqueous MEA solution. Cycle 2 Concentration Solution Absorption 2 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1058 57.8 324 53 10 Final 515 100 18340 45 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 543 663 0.31 2.33 8 816 Desorption 2 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 363 properly in 890 527 Volume of solution 53 27 the chamber 26 (ml) RH (%) 57.9 100 TVOC (ppb) 310 67155 Temperature (C.) 22.2 93.6 55 Concentration of 527 Amount of 644 Volume of 26 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.30 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00020 TABLE 20 Experimental data of third absorption-desorption cycle of aqueous MEA solution. Cycle 3 Concentration Solution Absorption 3 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1053 61.1 343 53 10 Final 584 100 24576 45 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 469 573 0.26 2.04 8 720 Desorption 3 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 377 properly in 889 512 Volume of solution 53 25 the chamber 28 (ml) RH (%) 56.8 100 TVOC (ppb) 292 55984 Temperature (C.) 22.1 95.6 47.5 Concentration of 512 Amount of 625 Volume of 28 CO.sub.2 desorbed CO.sub.2 desorbed the solution (ppm) (mg) lost (ml) Energy consumed 70 CO.sub.2 unloading 0.29 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00021 TABLE 21 Experimental data of fourth absorption-desorption cycle of aqueous MEA solution. Cycle 4 Concentration Solution Absorption 4 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1061 59.2 307 53 10 Final 591 100 9271 45 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 470 574 0.27 1.8 8 611 Desorption 4 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 397 properly in 899 502 Volume of solution 53 26 the chamber 27 (ml) RH (%) 57.4 100 TVOC (ppb) 350 52773 Temperature (C.) 23 93.7 48.7 Concentration of 502 Amount of 613 Volume of 27 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.28 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00022 TABLE 22 Experimental data of fifth absorption-desorption cycle of aqueous MEA solution. Cycle 5 Concentration Solution Absorption 5 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1052 61.2 356 53 10 Final 549 100 14937 47 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole L-arg.) (1/ppm min) lost (ml) (mins) 503 614 0.28 2.11 6 570 Desorption 5 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 369 properly in 830 461 Volume of solution 53 25 the chamber 28 (ml) RH (%) 57.2 100 TVOC (ppb) 346 61771 Temperature (C.) 22.2 95.1 47 Desorption step 2 Microwave desorption time - 60 sec HPL - 10 Initial After 5 min wait Final Difference desorption Conc. of CO.sub.2 (ppm) 830 for air to 875 45 Volume of solution 25 18 mix 7 (ml) RH (%) 100 properly in 100 TVOC (ppb) 61771 the chamber 72019 Temperature (C.) 47 94.9 46.2 Concentration of 506 Amount of 618 Volume of 35 CO.sub.2 desorbed CO.sub.2 desorbed the solution (ppm) (mg) lost (ml) Energy consumed 100 CO.sub.2 unloading 0.29 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00023 TABLE 23 Experimental data of sixth absorption-desorption cycle of aqueous MEA solution. Cycle 6 Concentration Solution Absorption 6 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1045 61.2 371 53 10 Final 610 100 17751 45 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 435 531 0.25 1.77 8 598 Desorption 6 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 362 properly in 799 437 Volume of solution 53 23 the chamber 30 (ml) RH (%) 60.5 100 TVOC (ppb) 348 66449 Temperature (C.) 22.5 94.3 48 Concentration of 437 Amount of 534 Volume of 30 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.25 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00024 TABLE 24 Experimental data of seventh absorption- desorption cycle of aqueous MEA solution. Cycle 7 Concentration Solution Absorption 7 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1081 61 367 53 10 Final 694 100 6261 47 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 387 473 0.22 1.53 6 466 Desorption 7 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 367 properly in 780 413 Volume of solution 53 23 the chamber 30 (ml) RH (%) 57.8 100 TVOC (ppb) 366 58864 Temperature (C.) 22.5 93.5 48.3 Concentration of 413 Amount of 504 Volume of 30 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.23 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00025 TABLE 25 Experimental data of eighth absorption-desorption cycle of aqueous MEA solution. Cycle 8 Concentration Solution Absorption 8 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) PH Initial 1062 59.1 367 53 10 Final 697 100 16807 47 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 ofCO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 365 446 0.21 1.7 8 553 Desorption 8 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 361 properly in 766 405 Volume of solution 53 23 the chamber 30 (ml) RH (%) 59.5 100 TVOC (ppb) 460 78630 Temperature (C.) 22.6 94 45 Concentration of 405 Amount of 495 Volume of 30 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.23 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00026 TABLE 26 Experimental data of ninth absorption-desorption cycle of aqueous MEA solution. Cycle 9 Concentration Solution Absorption 9 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) pH Initial 1079 56.5 437 53 10 Final 705 100 10982 47 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 374 457 0.21 1.5 6 426 Desorption 9 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 378 properly in 760 382 Volume of solution 53 27 the chamber 26 (ml) RH (%) 58.9 100 TVOC (ppb) 481 78649 Temperature (C.) 22.8 94.7 46.7 Concentration of 382 Amount of 466 Volume of 26 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.22 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

TABLE-US-00027 TABLE 27 Experimental data of tenth absorption-desorption cycle of aqueous MEA solution. Cycle 10 Concentration Solution Absorption 10 of CO.sub.2 (ppm) RH (%) TVOC (ppb) volume (ml) PH Initial 1045 61.8 411 53 10 Final 723 100 8826 46 9 Concentration Amount CO.sub.2 Rate of CO.sub.2 of CO.sub.2 loading constant Volume of Absorption absorbed absorbed (Mole CO.sub.2/ (K 10.sup.6) the solution run time (ppm) (mg) mole MEA (1/ppm min) lost (ml) (mins) 322 393 0.18 1.69 7 375 Desorption 10 Desorption step 1 Microwave desorption time - 150 sec HPL - 10 Initial After 5 min wait Final Difference desorption for air to mix Conc. of CO.sub.2 (ppm) 343 properly in 664 321 Volume of solution 53 27 the chamber 26 (ml) RH (%) 59.5 100 TVOC (ppb) 369 78646 Temperature (C.) 22 96 48.6 Concentration of 321 Amount of 392 Volume of 26 CO.sub.2 desorbed CO.sub.2 the solution (ppm) desorbed lost (ml) (mg) Energy consumed 70 CO.sub.2 unloading 0.18 during Desorption (mole CO.sub.2/ (Wh) mole MEA)

CONCLUSION

[0100] This study evaluated seven CSS, focusing on their appearance, CO.sub.2 absorption-desorption performance, reaction kinetics, and effects on RH levels and TVOC concentrations. Aqueous MEIA showed high CO.sub.2 absorption and faster reaction kinetics but proved less suitable for indoor air capture due to elevated VOC emissions compared to aqueous Arg. Although the aqueous Arg solution performed better than aqueous MEA in terms of TVOC and RH control, it faced challenges with incomplete regeneration and significant solvent vaporization, limiting its use in continuous cycles. To address these issues, we introduced a water-PG-Arg solution, which demonstrated promising results by outperforming aqueous MEA in stability, safety, and long-term efficiency. The inclusion of PG effectively regulated RH during absorption and maintained the solution's liquid state during desorption. Overall, the water-PG-Arg solution exhibited consistent CO.sub.2 absorption and reliable cyclic performance, positioning it as a viable and sustainable solution for indoor CO.sub.2 capture. In our preliminary testing, the integration of an activated charcoal canister within the absorption setup further stabilized RH levels and reduced TVOC concentrations, as illustrated in FIGS. 10A-10B. As we progressed through the cyclic study, the water-PG-Arg solution's capacity to control RH was slowly impacted (FIG. 11A), presumably due to the loss of PG to evaporation primarily during the desorption processes, leading to a reduced mole fraction of PG in the resulting solutions, which can be attributed to a gradually lower TVOC generation (FIG. 11B) during the subsequent absorption processes.

Example 2

[0101] CO.sub.2 absorption was assessed with different surfactants. Foaming that occurs when the air is passed through the absorber is assessed in correlation with the use of the surfactant. One of the goals was to minimize foaming during this process in the optimization studies. Conditions that led to max absorption and least foaming have been identified (see Table 28). Least amount of foaming makes the process more sustainable, thus it is more desirable even though another formulation may show higher absorption but uncontrolled foaming.

Absorption

[0102] The CO.sub.2 absorption experiment was conducted using a 500 mL two-neck round-bottom flask containing a CO.sub.2 scrubbing solution. The flask was connected to a condenser to recover evaporated water and amine vapors. The entire setup was placed inside a closed steel chamber (dimensions: 323336 inches) equipped with an 8V fan to ensure air mixing. CO.sub.2 was injected from the top of the chamber until the concentration reached 1000-1100 ppm, after which air was bubbled through the solution using a micro air pump to facilitate absorption.

[0103] The experiment was conducted under lab conditions at atmospheric pressure, with an initial temperature of 255 C. and initial relative humidity of 555%. Throughout the process, changes in CO.sub.2 concentration (ppm), RH (%), and TVOCs (ppb) inside the chamber were measured using Graywolf IQ-610 sensors and logged every 30 seconds with a Graywolf Data Logger.

[0104] Multiple formulations of CO.sub.2 scrubbing solutions were tested, including solutions with L-arginine, MEA, with and without surfactants such as Perfluorooctanoic acid (PFOA) Sodium dodecyl sulfate (SDS), Triton X-100.

Results

[0105] The CO.sub.2 absorption performance was evaluated for different scrubbing solutions containing MEA (monoethanolamine) or L-arginine, with and without surfactants (PFOA, SDS, Triton X-100) in propylene glycol (PG) and water mixture. The key parameters analyzed were relative humidity (RH), total volatile organic compounds (TVOCs), and CO.sub.2 concentration before and after the absorption process. See Table 28.

TABLE-US-00028 TABLE 28 Change Change Initial Final in Initial Final in Initial Final Total CO2 Solution RH RH RH TVOC TVOC TVOC CO2 CO2 Absorbed MEA + PG + H2O 34 12 81 6 47 8 134 59 2166 331 2032 331 1065 10 917 21 148 20 MEA + PG + H2O + 55 2 87 2 32 2 324 33 2402 468 2078 501 1060 8 737 16 323 8 PFO MEA + PG + H2O + 55 2 88 1 33 1 329 48 2727 937 2398 953 1066 2 750 60 316 58 SDS MEA + PG + H2O + 63 5 88 1 25 4 391 98 1819 689 1428 777 1058 7 665 30 393 24 TritonX100 L Arginine + PG + 57 1 89 0 31 1 238 44 5158 1738 4920 1742 1045 10 923 13 121 10 H2O L Arginine + PG + 42 13 89 0 46 13 246 30 6131 1560 5884 1591 1060 7 744 15 316 10 H2O + PFO L Arginine + PG + 58 2 89 0 30 2 297 52 4787 1004 4490 1048 1055 4 808 52 247 50 H2O + SDS L Arginine + PG + 60 1 89 0 28 1 335 40 5019 556 4683 594 1039 12 774 17 265 25 H2O + TritonX100 MEA: Monoethanolamine SDS: Sodium Dodecyl Sulfate PFO: Perfluorooctanoic Acid PG: Propylene Glycol

Discussion

[0106] MEA+PG+H.sub.2O+Triton X-100 demonstrated the highest CO.sub.2 absorption among all tested solutions.

[0107] The highest CO.sub.2 absorption was observed with MEA+PG+H.sub.2O+Triton X-100, absorbing 39324 ppm, followed closely by MEA+PG+H.sub.2O+PFO (3238 ppm) and MEA+PG+H.sub.2O+SDS (31658 ppm).

[0108] Among the L-arginine-based solutions, L-arginine+PG+H.sub.2O+PFO exhibited the best absorption at 31610 ppm, followed by L-arginine+PG+H.sub.2O+Triton X-100 (26525 ppm) and L-arginine+PG+H.sub.2O+SDS (24750 ppm).

[0109] The control solutions (MEA+PG+H.sub.2O and L-arginine+PG+H.sub.2O) showed the lowest CO.sub.2 absorption at 14820 ppm and 12110 ppm, respectively, confirming the positive effect of surfactants on absorption performance.

[0110] Solutions that led to the least amount of foaming while achieving max absorption were also identified: MEA+PG+H.sub.2O+PFO and L Arginine+PG+H.sub.2O+PFO.

Desorption

[0111] A microwave heating method was used for efficient CO.sub.2 desorption from the scrubbing solution. The CO.sub.2-loaded solution was placed in a 500 mL conical flask and sealed within the desorption chamber. After a five-minute stabilization, initial temperature, CO.sub.2 concentration, RH, and TVOC levels were recorded.

[0112] Desorption was performed using a Panasonic 1200 W microwave oven at Power Level 10 for a set duration. Immediately after heating, the solution temperature and volume were measured, and the flask was covered to minimize vapor loss. After a five-minute waiting period, final readings of all parameters were taken.

[0113] This step was repeated multiple times without opening the chamber to ensure complete CO.sub.2 desorption while maintaining the solution volume, allowing for an efficient and repeatable recovery process.

TABLE-US-00029 Abbreviations HVAC Heating, Ventilation, and Air Conditioning system OSHA Occupational Safety and Health Administration NOAA National Oceanic and Atmospheric Administration C.sub.CO.sub.2 Concentration of CO.sub.2 CSS CO.sub.2 scrubbing solution CSA CO.sub.2 scrubbing agent MW Microwave TVOCs Total Volatile Organic Compounds RH Relative Humidity ppm parts per million Ppb parts per billion LC-MS-QqQ Liquid Chromatography Mass Spectroscopy triple quadruple HS-GC-MS Headspace Gas Chromatography Mass Spectroscopy Arg L-arginine MEA Monoethanolamine PG Propylene Glycol EG Ethylene Glycol

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