The fight against rising CO₂ emissions has become a global race, with researchers focusing on two key fronts: reducing dependence on fossil fuels by making renewable energy more accessible and efficient, and advancing direct air capture (DAC) and carbon capture and storage (CCS) technologies to remove CO₂ from the atmosphere. These efforts are not just about slowing emissions but also about reversing their impact—aiming to achieve net-zero CO₂ emissions by 2050 and preventing global temperatures from exceeding the critical 1.5°C threshold (Ma et al., 2022; Chen et al., 2023). According to the Global Monitoring Laboratory of the National Oceanic and Atmospheric Administration, the average global CO₂ concentration in 2023 was 420 ppm (0.042%), which is considered safe for humans (Global Monitoring Laboratory-NOAA, 2023). Nevertheless, indoor CO₂ concentrations, especially in workplaces like offices and conference halls, often surpass recommended limits due to occupant’s exhalation, placing increased demands on heating, ventilation, and air conditioning (HVAC) systems to maintain indoor air quality (Young et al., 2024). Prolonged exposure to elevated CO₂ levels can lead to health issues such as respiratory acidosis, increased heart rate, cognitive decline, and hypercapnia (Azuma et al., 2018; Jacobson et al., 2019). The Occupational Safety and Health Administration (OSHA) has established a maximum permissible exposure limit of 5,000 ppm over an 8-h workday (Young et al., 2024). However, relying solely on ventilation to meet this standard can significantly increase the system’s carbon footprint (Young et al., 2024). Additionally, research indicates that keeping CO₂ levels below specific thresholds enhances the occupants’ cognitive functions and productivity (Kuramochi et al., 2023). Therefore, capturing CO₂ is the only viable option to keep the CO₂ level within the acceptable limit in indoor spaces. Since the concentration of CO₂ in indoor air can be four to five times higher than that in outdoor air, capturing CO₂ directly from indoor air can be easier than from outdoor air. The benefits of capturing CO₂ from indoor air are twofold: first, it removes CO₂ from the atmosphere, and second, it improves the health and work efficiency of individuals working indoors.

Chemical absorption is one of the most promising technologies for CO₂ capture due to its high selectivity and scalability opportunities (Tsubaki et al., 2020). This process employs various absorbing agents, including aqueous alkali solutions (Zeman, 2007), amines (Kim et al., 2013), ionic liquids (Zhao and Baker, 2023), amine-functionalized adsorbents (Lee and Park, 2015), and amino acid solutions (Aronu et al., 2010; Wei et al., 2023), to chemically extract CO₂ from air or gas streams. Among these methods, amine-based technology remains the most widely used. Researchers have shown that the interaction between CO₂ and aqueous amines follows the zwitterionic mechanism. The nucleophilic lone pair on the nitrogen atom of the amine attacks the electrophilic carbon of CO₂, forming zwitterionic adduct (R-NH₂ ⁺-COO^-), which stabilizes into carbamate (R-NH-COO^-) and an alkylammonium ion (R-NH₂-H⁺) upon deprotonation (Reaction 1) (Masiren and Harun, 2017; Lv et al., 2015; Said et al., 2020). As absorption progresses, the pH and absorption rate decrease due to the depletion of the CO₂ scrubbing agent (CSA), weakening Reaction 1 (Lv et al., 2015). This favors hydration reactions (Reactions 2–5), resulting in the formation of bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) (Lv et al., 2015). Additionally, as the pH drops, carbamate formed in Reactions 1 and 2 decomposes into bicarbonate, as shown in Reaction 6 (Lv et al., 2015; Said et al., 2020). This process is reversible during desorption, as shown in Reactions 7–10 (Lv et al., 2015). The full sequence of absorption and desorption reactions is outlined in Figure 1:

Alkanolamines are preferred over alkylamines for CO₂ capture due to their higher boiling points and lower volatility (National Library of Medicine–Ethylamine, 2024; National Library of Medicine–Monoethanolamine, 2024), 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 CO₂ absorption rate (Chai et al., 2022). 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 (Chai et al., 2022; Custelcean, 2022). These challenges highlight the need for alternative CSA for indoor CO₂ capture. Aqueous amino acids are emerging as promising alternatives. Known for their high surface tension, they offer comparable CO₂ absorption capacity with negligible vapor pressure and greater resistance to degradation (Mai et al., 2023). These properties make them well-suited for capturing CO₂ in indoor applications (Mai et al., 2023; Sang Safidi and Luis, 2019), 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 CO₂ capture and release, and its biodegradability makes Arg a more environmentally sustainable option (Guo et al., 2018). However, despite these promising attributes, only a limited number of studies have explored the role of Arg in CO₂ capture (Mahmud et al., 2017; Wei et al., 2023). To the best of our knowledge, no research to date has specifically addressed CO₂ 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. This study aims to develop efficient and affordable CO₂ capture solutions specifically designed for indoor use, with added functionality for managing TVOCs and humidity to enhance overall indoor air quality.

This study explores the potential of various CO₂ capture solutions, including aqueous MEA, aqueous Arg, and water-glycol-Arg solutions, to enhance indoor air quality by regulating CO₂ 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 CO₂ capture kinetics and positions Arg as a promising, sustainable alternative to traditional absorbents like MEA.

The performances of aqueous MEA and aqueous Arg solutions in terms of CO₂ absorption, kinetics, and their impact on RH levels and TVOC concentrations were evaluated. Although the aqueous Arg solution showed a lower capacity for CO₂ absorption (α = 0.31 mol/mol) and slower kinetics (K ˝ = 1.80 × 10⁻⁶ ppm⁻¹ min⁻¹) compared to the aqueous MEA solution (α = 0.40 mol/mol and K ˝ = 2.57 × 10⁻⁶ ppm⁻¹ min⁻¹) (Figure 2A; Supplementary Figure S3), presumably due to Arg’s larger molecular size (Suleman et al., 2020) affecting solution viscosity and hindering CO₂ 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 Figure 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 (Supplementary Figure S4), as its higher vapor pressure likely contributed to elevated TVOC levels, highlighting a potential drawback of using MEA for CO₂ 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 (Supplementary Figure S5). These findings indicate that, while the aqueous Arg solution has slightly lower CO₂ 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.

Microwave heating was employed for CO₂ desorption due to its faster process, lower energy consumption, and simpler setup compared to conventional heating (McGurk et al., 2017). 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 CO₂ desorption possible. As shown in Figures 3A, B, it was observed that increasing the first MW session beyond 150 s led to excessive solvent loss and worse CO₂ desorption outcome. The optimum MW irradiation profile was found to be an initial MW session of 150 s followed by another session of 60 s, achieving near-complete regeneration by desorbing 589 mg (99.5%) of CO₂ from 592 mg absorbed, while limiting solvent loss to just 37 mL.

While the optimal MW irradiation times of 150 s for the first session and 60 s for the second session were effective for aqueous MEA, they proved insufficient for aqueous Arg solution. Only 436 mg (63.5%) of CO₂ was desorbed from the 687 mg absorbed in the first session. Extending the second session to 90 s 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 Supplementary Figure S6. To address this issue and maintain the solution in liquid form, a water-glycol-based Arg solution was formulated.

The choice of glycol with a higher boiling point was intended to manage the RH during the absorption process 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 (Taylor et al., 2024; Garg et al., 2019) was conducted, focusing on key properties such as low vapor pressure, appropriate viscosity, and high molecular weight, as shown in Supplementary Table S1. Among these, the water-EG-Arg and water-PG-Arg solutions exhibited better Arg solubility (Supplementary Figure S2) and were thus chosen for further studies.

While the water-PG-Arg solution initially showed slightly lower CO₂ absorption capacity (α = 0.24 mol/mol, K ˝ = 1.15 × 10⁻⁶ ppm⁻¹ min⁻¹) than the water-EG-Arg solution (α = 0.26 mol/mol, K ˝ = 0.95 × 10⁻⁶ ppm⁻¹ min⁻¹) (Table 1), it exhibited comparable kinetics and offered better RH control, taking longer to reach 90% RH during absorption (Figure 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 (Supplementary Figures S9, S10). It is worth mentioning that PG, as a VOC, is odorless, non-toxic, and has a Workplace Environmental Exposure Limit (WEEL) of 3,200 ppb, averaged over an 8-h 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% (Supplementary Table S2). Additionally, no visible color change was observed in the water-PG-Arg solution after the first desorption process (Supplementary Figure S7), whereas the water-EG-Arg solution exhibited a shift from transparent to amber, indicating possible degradation (Supplementary Figure S8).

In subsequent cyclic test, the water-PG-Arg solution demonstrated improved stability with consistently lower TVOC emissions during absorption (Figure 2B) and no color change (Supplementary Figure S7) 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 (Supplementary Figure S8). 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 (Supplementary Tables S4, S5). In contrast, the water-PG-Arg solution exhibited lower temperature increases during desorption (Supplementary Tables S6–S15), attributed to the higher specific heat capacity of PG (Supplementary Table S3). Additionally, PG’s lower polarity and dielectric constant limited the absorption of MW energy, reducing excessive heat generation (Sengwa, 2003; Gabriel et al., 1998). Thus, the thermal stability, improved absorption performance, and reduced energy consumption of the water-PG-Arg solution highlight its potential for long-term CO₂ capture applications. These findings support further investigation into its cyclic performance over multiple absorption-desorption processes.

The cyclic performances of aqueous MEA and water-PG-Arg solutions were evaluated over ten absorption-desorption cycles, as shown in Figures 4A, B. The reaction rate constants for both solutions across these cycles are presented in Figures 5A, B. Although the aqueous MEA solution initially exhibited higher CO₂ absorption, its performance declined significantly over time, with a 54.3% reduction in CO₂ 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 (Figure 4B) highlights its limitations for long-term CO₂ capture applications. 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 CO₂ absorption. The degradation products, such as ammonia, acetaldehyde, and acetone (Chanchey et al., 2011), 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 CO₂ capture applications.

This study evaluated seven CSS, focusing on their appearance, CO₂ absorption-desorption performance, reaction kinetics, and effects on RH levels and TVOC concentrations. Aqueous MEA showed high CO₂ 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 CO₂ absorption and reliable cyclic performance, positioning it as a viable and sustainable solution for indoor CO₂ 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 Supplementary Figures S11A, 11B. As we progressed through the cyclic study, the water-PG-Arg solution’s capacity to control RH was slowly impacted (Supplementary Figure S12A), 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 (Supplementary Figure S12B) during the subsequent absorption processes. Future studies will investigate the use of alternative CSA and less viscous solvents to develop improved CSS that can effectively manage relative humidity across multiple cycles while achieving even lower TVOC emissions.