pmc10152675743387Sep Purif TechnolSep Purif TechnolSeparation and purification technology1383-58661873-3794401252681192699710.1016/j.seppur.2024.131207HHSPA2052058ArticleCapturing and converting CO2 using amino acids as various commercially valuable nano-carbonateshttp://orcid.org/0000-0001-6199-1125LiQingyangabQianYongcXingMalcolmdhttp://orcid.org/0000-0002-5766-5666LiBingyuna*Biomaterials, Bioengineering & Nanotechnology Laboratory, Department of Orthopaedics, West Virginia University, Morgantown, WV 26506, United StatesDepartment of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, United StatesPathology and Physiology Research Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, WV 26505, United StatesDepartment of Mechanical Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, CanadaCorresponding author. bili@hsc.wvu.edu (B. Li).532025197202519720253611131207

Carbon dioxide (CO2) is the most significant greenhouse gas and one of the strategies to reduce CO2 emission is to convert CO2 into commercially valuable products. Currently, methods that can effectively capture and convert CO2 into carbonate nanomaterials, which have unique applications in various fields, have rarely been reported, and there are no universal methods that can capture and convert CO2 into different nano-carbonates. In this study, an innovative two-step strategy based on amino acids was developed to produce multiple different carbonate nanoparticles, including CaCO3, BaCO3, and Ag2CO3 nanoparticles with diameters of 70 nm, 50 nm, and 7 nm, respectively. Fundamentally important, the nuclear magnetic resonance data clearly demonstrated that it was the amino acids (e.g., glycine) that dictated the formation of carbonate nanoparticles. In the presence of amino acids, a competition in forming nanoparticles and microparticles was observed, and the formation of nanoparticles was proportional to the carbamate formed from CO2 reacting with amino acids. In the absence of amino acids, carbonate microparticles (~ 2 μm) were formed.

Carbon dioxideAmino acidNano-carbonateNanoparticleIntroduction

The emission of carbon dioxide (CO2), the most significant greenhouse gas, impacts the global climate because CO2 traps heat from the sun in the Earth’s atmosphere. Yet human activities, such as burning fossil fuels, have been continuing to elevate the atmospheric CO2 level, leading to multiple environmental crises, including severe weather events (hurricanes, droughts, heavy rains, and wildfires) and rising sea levels.

In response, the global community has been exploring various strategies, including carbon capture and storage (CCS) and carbon capture and utilization (CCU) that target mitigating and reducing CO2 emissions. CCS entails capturing CO2 emissions from major CO2 sources like power plants and industrial processes followed by their sequestration in deep underground formations. CCU, on the other hand, seeks not only to capture but also to convert CO2 into valuable products, thus potentially offsetting the costs associated with CO2 management. Among various CCU strategies, converting CO2 into stable minerals such as calcium carbonate (CaCO3) is emerging as a promising avenue, largely because such a CO2 conversion process is thermodynamically favored—the Gibbs free energy of carbonates is lower than that of CO2, making the reaction energetically favorable [1–3]. Calcium ion (Ca2+) and magnesium ion (Mg2+) are among the most extensively studied materials for CO2 mineralization due to their reactivity and availability. For example, Oeiyono et al. [4] used an MEA solvent to capture CO2 and subsequently mineralized the captured CO2 into CaCO3 particles by introducing CaO into the CO2-saturated MEA solution. Liu et al. [2] utilized CaO, MgO, and CaSiO3 to mineralize CO2 captured by amino acid salt or monoethanolamine (MEA) solvents, forming CaCO3 or MgCO3. Huang et al. [5] employed protonated amines (e.g., MEA, 3-amino-1-propanol, 2-amino-2-methyl-1-propanol, diethanolamine, and N,N-dimethylethanolamine) as reagents to extract Ca2+ from coal fly ash, which was then used to stabilize CO2 captured by the amines into CaCO3. To enhance the value of CaCO3, Madhav et al. [6] utilized polymers (polydopamine, polyvinyl alcohol, polyethylene glycol) to modify the formation of CaCO3 during CO2 capture and mineralization using CaCl2 and L-arginine as the solvent. These strategies are of great interest even though their mineralized products were typically large (micrometer or larger) along with wide size distributions.

Nanoparticles (particles with at least one dimension smaller than 100 nm) are well recognized for their dramatically altered properties from bulk materials [7–9]. Nanoparticles may be more valuable compared to their corresponding bulk size, and the nano-carbonate markets are quickly expanding. For instance, the global CaCO3 nanomaterials market was valued at US$ 8.4 billion in 2022 and was anticipated to grow at a compound annual growth rate (CAGR) of approximately 9.7 % to exceed US$ 17 billion by 2030 [10,11]. Several methods, including precipitation, emulsion, polymerization, and mechano-chemical methods have been studied to produce CaCO3 nanoparticles [12–16], and among them, few reports have shown the possibility of converting waste CO2 into CaCO3 nanoparticles. Unfortunately, these current methods require either additional external energy (e.g., heating the solvent to 90 °C [17]) or chelating agents (e.g., ethylenediaminetetraacetic acid [18] or L-Glutamic acid-N,N-diacetic acid tetrasodium salt [19]) that may create environmental concerns, and no studies so far have shown the production of carbonate nanoparticles at a large scale. No publications have been reported on converting CO2 into other nano-carbonates (e.g., BaCO3, Ag2CO3).

Here we report, for the first time, a straightforward, universal method that converts CO2 into carbonate nanoparticles. Our method is environmentally friendly and does not require any additional energy input. Nano-carbonates of various metal ions (e.g., CaCO3, BaCO3, and Ag2CO3) with sizes of 70 nm, 50 nm, and 7 nm were successfully produced. Equally importantly, the role of amino acids in the synthesis of carbonate nanoparticles was clearly determined and no amino acids were consumed during the synthesis.

Experimental sectionsMaterials

L-glycine (Gly, ≥98 wt%), L-asparagine (Asn, ≥99 wt%), L-alanine (Ala, ≥98 wt%), L-Cysteine (Cys, ≥98.5 wt%), sodium hydroxide (NaOH, ≥98 wt%), potassium hydroxide (KOH, ≥98 wt%), calcium chloride (CaCl2, ≥97 wt%), barium chloride (BaCl2, ≥99 wt%), silver nitrate (AgNO3, ≥99 wt%), deuterium oxide (D2O, ≥99.9 wt%), and 1,4-dioxane (≥99.5 wt%) were purchased from MilliporeSigma (Burlington, MA). CO2 (99.99 %) was purchased from Airgas (Radnor, PA). All chemicals were used as received.

CO<sub>2</sub> absorption and mineralization

Solvents for CO2 absorption were prepared by dissolving a base and/or an amino acid at a certain molar ratio in 25 mL of water and mixing in 50-mL plastic centrifuge tubes. The method for CO2 absorption experiments was detailed in our previous publications [20,21]. In brief, a 5-mL serological pipette was positioned at the base of each tube to serve as a CO2 gas input. Mass flow controllers were used to regulate the flow rate at 100 standard cubic centimeters per minute (sccm). After bubbling, metal ion (CaCl2, BaCl2, AgNO3) was added and the solvent was vortexed for 1 min to produce corresponding carbonates. Then precipitates were collected by centrifugation at 4,000 revolutions per minute (rpm) for 5 min and washed with deionized water. The centrifugation and washing were repeated four times to remove soluble chemicals from the solid products. The quantity of CO2 absorbed was determined by measuring the mass of the solvent at various time intervals during the bubbling process. The weight of the precipitates was obtained using an analytical balance after vacuum drying the washed samples for 72 h. In addition, the pH of the solvents was monitored with an Apera MP511 Benchtop pH Meter. Triplicate samples were run for each experiment. The CO2 loading and precipitates’ weight were quantified by calculating the ratios of captured CO2 (mol) to OH− (mol) and carbonate (mol) to OH− (mol), respectively.

Characterization<sup>13</sup>C nuclear magnetic resonance (<sup>13</sup>C NMR)

The 13C NMR spectra, aimed at identifying carbon species, were obtained using a 400 MHz Agilent NMR instrument. 500 μL solvent sample, 200 μL D2O (used as lock), and 15 μL 1,4 dioxane (used as a reference) were added in a 1-mL centrifuge tube, mixed thoroughly, and transferred into an NMR tube (S-5–600–7). 13C NMR experiments were conducted with a delay time of 1 s along with 256 scans. To investigate the potential presence of amino acid or carbamates within the precipitates, 10 mg precipitate was added in the mixed solvent of 700 μL D2O and 15 μL 1,4 dioxane, and the upper solvent was used for NMR analysis. In the 13C NMR spectra, the peak, corresponding to <di>HCO3−/CO23−, shifted from 168 ppm to 161 ppm. The spectral signals of carbamate were identified as peaks at 46 ppm (CH2), 165 ppm (C = O), and 180 ppm (C = O). The spectra also revealed shifts of peaks for Gly, from 182 to 174 ppm (C = O) and from 45 to 42 ppm (CH2). The peak corresponding to 1,4-dioxane at 67.19 ppm served asa reference but was excluded from the analysis to ensure a focus on the solutes of interest.

13C NMR spectroscopy also provides a means to quantify the concentration of each chemical species in the solvent based on the area of the respective NMR peaks. In our solvent system, amino acid can transform into carbamate. The percentages of carbamate and Gly/Gly− were calculated from the NMR spectra (Supplementary materials S.1).

Electron microscopy

The solid precipitates after vacuum drying were distributed onto a stainless-steel disc and were coated with gold using a Denton Desk V Sputter Coater for 240 sec at an 18-mA current in N2. The precipitates were imaged using a JEOL JSM-7600F Scanning Electron Microscope (SEM). Images were captured at an acceleration voltage of 5 kV and a working distance of ~ 15 mm. Transmission Electron Microscopy (TEM) was also used to visualize the precipitates using a JEOL JEM-2100 TEM at an operating voltage of 120 kV; samples were prepared by suspending the precipitates in ethanol and then depositing them onto a copper grid. ImageJ was employed to analyze the sizes and size distribution of the particles using the electron microscopy images.

X-ray Diffraction (XRD)

XRD analysis of the precipitates was carried out using a PANalytical X’Pert Pro diffractometer, equipped with a copper Kα radiation source. The operational settings were maintained at 40 kV and 25 mA, and utilized a LynxEye detector for enhanced sensitivity and resolution. Diffraction patterns were systematically recorded over a 2θ angle range from 5 to 80 degrees.

Statistical analysis

The statistical analyses were carried out utilizing JMP/V16 software by SAS Institute in Cary, NC. Determination of statistical significance, set at a p-value of less than 0.05, was achieved through an analysis of variance coupled with the application of Tukey’s HSD test.

Results

In this study, a universal method was developed to absorb and convert CO2 into carbonate nanoparticles or nano-carbonates (nano-CaCO3, nano-BaCO3, and nano-Ag2CO3) (Fig. 1). First, amino acid salt solvents were prepared by dissolving a base and an amino acid in water, and then CO2 was bubbled into the solvent for a certain time. Next, metal ions (Ca2+, Ba2+, Ag+) were added into the solvent to mineralize the absorbed CO2 into nano-carbonates.

Converting CO<sub>2</sub> into CaCO<sub>3</sub> nanoparticles using a two-step process

A two-step process was developed to convert CO2 into nano-CaCO3 (Fig. 2a). In the first step, CO2 was bubbled into a Gly salt solvent. In the second step, CaCl2 was added which led to the formation of solid precipitates (Supplementary materials S.2). SEM and TEM observations found that the precipitates were nanoparticles (Fig. 2a). NMR data (Fig. 2b) showed no signals of carbon-containing species within the precipitates and only Gly was shown in the solvent. Characterization of the precipitates found that the precipitates were pure calcite (JCPDS Card No. 86–2340), the most stable form of CaCO3 (Fig. 2c), and the precipitates had an average diameter of approximately 70 nm (Fig. 2d and Supplementary materials S.3).

Comparison between the conversion of CO<sub>2</sub> into CaCO<sub>3</sub> with and without amino acid

The role of amino acid in converting CO2 into CaCO3 using the two-step process was examined. It was found that, in the absence of Gly, micro-sized, dense CaCO3 particles (~2 μm) were formed (Fig. 3a); in comparison, CaCO3 nanoparticles (~70 nm) were formed in the presence of Gly (Fig. 3b). This finding suggests that Gly played a critical role in controlling the size of the final products.

NMR data (Fig. 3c) showed that, in the absenceof amino acids, <di>CO23−/HCO3− was formed within 2.5 min of CO2 bubbling and their peak position in the NMR spectra shifted slightly toward lower ppm with increasing CO2 bubbling time; the peak of CO23−/HCO3− disappeared immediately upon addition of CaCl2. In the presence of amino acid (i.e., Gly), by contrast, carbamate was first formed and <di>CO23−/HCO3− appeared at 20 min; upon addition of CaCl2, both carbamate and <di>CO23−/HCO3− disappeared and only the peaks for Gly/Gly− were observed (Fig. 3d). The percentage of carbamate increased sharply within the first 10 min and remained about the same until 20 min then dropped to 0 upon addition of CaCl2. In contrast, percentage of Gly decreased quickly within 10 min and then kept consistent at 20 min, and finally rose to 100 % upon the addition of CaCl2 (Fig. 3e). These NMR data clearly indicated that the chemical reactions during the two-step process were different between the solvents with and without amino acid (i.e., Gly), though XRD (Fig. 3f) confirmed that the converted solid products were pure calcites with or without amino acid.

Meanwhile, dramatic differences in pH change were observed between the two solvents with and without Gly (Fig. 3g). In the absence of Gly, the pH was 13.2 before CO2 bubbling which decreased with increasing CO2 bubbling time and dropped to 10.1 at 20 min. Upon addition of CaCl2, the pH immediately further dropped to 4.6. In comparison, in the presence of Gly, the pH was 11.5 before CO2 bubbling and it also decreased with increasing CO2 bubbling time and became 8.7 at 20 min; with the addition of CaCl2, the pH immediately dropped to 5.5.

In addition, the CO2 loading for the two solvents with and without Gly was almost identical; both were approximately 0.5 mol CO2/(mol OH−) after bubbling CO2 for 20 min (Fig. 3h).

Competition between the formation of nanoparticles and microparticles

It was further revealed that the morphology of particles changed dramatically with the increasing dose of Gly, as illustrated in Fig. 4a. It was proposed in Fig. 4b that microparticles tended to form in the absence of Gly, while nanoparticles would form in the presence of Gly. Both nanoparticles and microparticles were formed and co-existed at 0.0125 mol (Fig. 4a2) and 0.025 mol (Fig. 4a3) Gly. The percentage of nanoparticles increased with increasing Gly concentration in the range of 0 to 0.05 mol (Fig. 4c); note that the CO2 conversion at 0 to 0.05 mol Gly was about the same (Fig. 4d). The formation of nanoparticles directly correlated to the amount of carbamate formed; the more carbamate formation, the more formation of nanoparticles (Fig. 4c). It was clear that in the absence of amino acid (i.e., Gly), microparticles were formed and it was the presence of Gly that led to the formation of nanoparticles. At relatively lower concentrations of Gly, there was a competition in the formation of nanoparticles and microparticles.

Factors impacting CO<sub>2</sub> conversion as CaCO<sub>3</sub> nanoparticles

The use of both NaOH (Fig. 5a) and KOH (Fig. 5b) resulted in the formation of nanoparticles; KOH seemed to have resulted in a smaller size (~50 nm) along with more aggregation (Fig. 5b). The XRD spectra of the precipitates were identical for the use of both NaOH and KOH, demonstrating that the solid precipitates obtained were calcite (Fig. 5c). Furthermore, the CO2 loading (Fig. 5d) and pH profiles (Fig. 5e) of the two solvents were almost identical for both NaOH and KOH before and after CaCl2 was added. The CO2 loading increased with increasing CO2 bubbling time and reached about 0.5 mol CO2/(mol OH−) at 20 min; no significant changes in CO2 loading were observed before or after CaCl2 was added (Fig. 5d). The pH of the solvent decreased with increasing time from 11.5 at 0 min to 8.7 at 20 min, and a sharp drop in pH was observed immediately upon the addition of CaCl2; the pH dropped to about 5.5 and remained steady thereafter (Fig. 5e).

Besides Gly, three additional amino acids including Asn, Ala, and Cys were assessed; they were selected to represent amino acids with polar uncharged side chains, amino acids with hydrophobic side chains, and amino acids with a unique side chain like −SH, respectively. All the amino acids studied resulted in the formation of nano-CaCO3 using our two-step process (Fig. 6). Compared to the nano-CaCO3 formed from Gly (Fig. 2a), Asn and Ala formed similarly shaped nanoparticles, although the nanoparticles from Asn had more aggregation and were aggregated into large porous particles like those bone-like ones (Fig. 6a) and the nano-CaCO3 from Ala had a larger size distribution (Fig. 6b). The use of Cys led to the formation of ellipsoidal nanoparticles whose diameter was in the range of 20–70 nm and the ratio of length to diameter was between 1 and 3 (Fig. 6c). In addition, compared to doses of 0 and 0.05 mol, the 0.025 mol dose produced CaCO3 with a particle size intermediate between those obtained using 0 and 0.05 mol of Asn, Ala, and Cys, respectively (Fig. S4). XRD spectra data confirmed that all the nanoparticles were pure calcite (Fig. 6d). NMR data showed that all the amino acids studied including Gly had similar NMR spectra where carbamate was observed after bubbling CO2 for 20 min (Fig. 6e, 6f, and 6g).

SEM images revealed that the CaCl2 concentration significantly affected the aggregation of nano-CaCO3 (Fig. 7). Close examination of the precipitates found that, at lower CaCl2 concentrations (e.g., 0.0125 mol and 0.025 mol), the nano-CaCO3 aggregated into micro-sized porous particles which were about 4–6 μm at 0.0125 mol (Fig. 7a) and 1–2 μm at 0.025 mol (Fig. 7b). The shape and size of the nanoparticles formed at 0.05 mol (Fig. 7c) and 0.075 mol (Fig. 7d) were similar. Quantitative analysis showed that the molar ratio of CaCO3 produced to OH− added was approximately 0.25 mol CaCO3/(mol OH−) at 0.0125 mol CaCl2 and increased to about 0.5 mol CaCO3/(mol OH−) at 0.025–0.075 mol CaCl2 (Fig. 7e).

When keeping the molar ratio of Gly:NaOH:CaCl2 constant at 1:1:1, the size of the CaCO3 nanoparticles seemed to decrease slightly with increasing concentrations of the reactants (Fig. 8), suggesting that higher concentrations of reactants favor the formation of smaller nanoparticles. Interestingly, corn-shaped nanoparticles (~200 nm in diameter and 600 nm in length) were formed via aggregations at low reactant concentrations (i.e., Gly:NaOH:CaCl2 = 0.025 mol:0.025 mol:0.025 mol).

Converting CO<sub>2</sub> as CaCO<sub>3</sub> nanoparticles using simulated flue gas

A simulated flue gas (10 % CO2; 10 sccm CO2 and 90 sccm N2) was studied. After bubbling the solvent with the simulated flue gas for 20 min, the CO2 loading was 0.14 mol/mol OH− (Fig. 9a) and the particles were approximately 400 nm (Fig. 9b). After increasing the gas bubbling time to 120 min, the CO2 loading increased to 0.52 mol/mol OH− (Fig. 9a) and the particles formed were approximately 50 nm (Fig. 9c).

Converting CO<sub>2</sub> into other nano-carbonates (i.e., BaCO<sub>3</sub>, Ag<sub>2</sub>CO<sub>3</sub>) using a two-step process

Besides the nano-CaCO3, other carbonates like BaCO3 and Ag2CO3 were produced to demonstrate the universal use of the developed method. All experimental conditions were kept the same as those for nano-CaCO3 synthesis except that CaCl2 was replaced by BaCl2 and AgNO3 to produce nano-carbonates using the two-step method. It was shown (Fig. 10a and 10b) that both precipitates had at least one dimension smaller than 100 nm. Fig. 10a showed that the nano-BaCO3 were rods with diameters of approximately 50 nm (average from 100 particles; sizes varying from 30–90 nm) and ratios of diameter to length from 1:1 to 1:4. Fig. 10b showed that the obtained nano-Ag2CO3 were spherical shaped with small diameters of about 7 nm (varying from 5–10 nm). XRD spectra confirmed that those precipitates were BaCO3 (JCPDS No. 05–0378)[22] (Fig. 10c) and Ag2CO3 (JCPDS No. 31–1236) [23,24] (Fig. 10d).

Converting CO<sub>2</sub> into nanoparticles via a one-step process

The feasibility of a one-step process for CO2 conversion into nano-CaCO3 was also investigated where Gly, NaOH, and CaCl2 were mixed in water followed by CO2 bubbling. In the absence of Gly, CO2 loading was negligible (Fig. 11a), and the pH of the solution remained around 11.0 during the CO2 bubbling process (Fig. 11b). By contrast, Gly (0.05 mol) addition to the solvent before CO2 bubbling significantly enhanced CO2 loading, and with increasing CO2 bubbling time, CO2 loading increased almost linearly between 0 and 10 min followed by a slow increase that led to a CO2 loading of 0.34 mol/mol OH− at 20 min (Fig. 11a). Meanwhile, pH of the solvent with Gly decreased from 10.2 to 6.1 from time 0 to 20 min (Fig. 11b). After bubbling CO2 for 20 min, nano-CaCO3 was formed, and the majority of which aggregated into bigger particles with several hundred nanometers (Fig. 11c). XRD data showed calcite was the only product (Fig. 11d). With increasing CO2 bubbling time, the peaks of Gly in the NMR spectra shifted while no other new peaks were observed; no peaks for carbamate or CO23−/HCO3− were observed throughout the one-step process (Fig. 11e).

Discussion

We have developed innovative processes based on amino acids to convert CO2 into various nano-carbonates with average diameters of 7 nm, 50 nm, and 70 nm. Our method is energy-saving as it was conducted under ambient room conditions with no heat and pressure required. The production of nano-carbonates from CO2 is significant, because it not only converts CO2 into carbonates that are stable, but the smaller size and higher surface area make nano-carbonates more attractive compared to large-sized particles.

The global nanomaterials market, valued at approximately US$ 12.5 billion in 2023, is projected to grow at a 15.0 % CAGR from 2024 to 2030 [11]. Growth is mainly driven by increased demand in electronics and expanding applications in the medical industry. Carbonate (e.g., CaCO3, BaCO3, Ag2CO3) nanomaterials have been or potentially can be used as fillers, medicines, catalysts, and in various sectors such as industry (food, construction, paint, polymer), biomedical, environmental, and electronic fields. For example, CaCO3 nanoparticles have been widely used in plastics, rubber, paint, and paper industries as an enhancer and filler [25,26]. BaCO3 nanoparticles can be used in the manufacture of electronic ceramic materials such as high dielectric constant ceramics in capacitors and inductors [27,28]. Ag2CO3 nanomaterials can be used as antibacterial agents [29,30] and photocatalysts [31,32]. Moreover, the universal approach developed in this study can be applied to produce other carbonate nanoparticles like SrCO3, MgCO3, FeCO3, MnCO3, CuCO3, and so on. Nanoparticles like SrCO3 alone had a market value of US$ 220 million in 2023 which may reach US$ 340 million by 2030 [33,34].

This study clearly demonstrated that it was the amino acid (e.g., Gly) that dictated the formation of nano-carbonates. The formation of CaCO3 was used as an example to illustrate the nanoparticle formation mechanism. The sizes of the produced CaCO3 were dramatically different in the presence and absence of amino acid. In the absence of amino acid, microparticles (~ 2 μm) were formed while nanoparticles (~ 70 nm) were formed in the presence of Gly in the same process (Fig. 3a and 3b). Our NMR data clearly showed that in the presence of Gly, carbamate was formed first within minutes after CO2 bubbling and HCO3− appeared later, e.g., 20 min after CO2 bubbling (Fig. 3d). Meanwhile, the pH of the solvent decreased with increasing CO2 bubbling time (Fig. 3g). Our NMR findings indicated that Gly reacted with CO2 to form carbamate, which subsequently hydrolyzed into HCO3−, which further partially converted to CO23−; these findings, along with the pH change, were consistent with previous studies where amino acids were used for CO2 absorption [20,35–41]. Upon addition of CaCl2 in our two-step process, both carbamate and HCO3− disappeared and only the peaks for Gly/Gly− were observed (Fig. 3d). The pH of the solvent dropped significantly (Fig. 3g), and formation of pure nano-CaCO3 was confirmed via XRD (Fig. 2c and 3f). These results suggested that carbamate completely hydrolyzed and Ca2+ reacted with HCO3−/CO23− to form CaCO3 as shown below.

In the presence of amino acids:

In the absence of amino acids, HCO3−/CO23− was observed in the same two-step process during CO2 bubbling (Fig. 3c); with the addition of CaCl2, the pH of the solvent dropped sharply (Fig. 3g) and CaCO3 microparticles were formed immediately (Fig. 3a). These findings were consistent with the traditional method of production of CaCO3 through the following reactions:

In the absence of amino acids:

Meanwhile, at relatively low concentrations of Gly (i.e., molar ratio of Gly/OH− < 1) (Fig. 4a2 and 4a3), both nanoparticles and microparticles were formed, indicating the competition of the aforementioned two pathways (i.e., amino acid-driven nanoparticles and traditional microparticles) that led to the formation of nanoparticles and microparticles, respectively. The percentage of nanoparticles increased with increasing Gly concentration and such an increase correlated with the increase of carbamate formation from Gly reacting with CO2 (Fig. 4c). Therefore, it was clear that amino acids like Gly dictate the formation of nanoparticles.

Gly in the system can exist in three forms: remaining in the solvent, converted to carbamate, or adhered on CaCO3 nanoparticles. In this study, after forming CaCO3 nanoparticles, no carbamate was detected in the solvent and no Gly was detectable in the CaCO3 precipitates (Fig. 2b), therefore Gly remained in the solvent and no Gly was consumed upon the formation of CaCO3 nanoparticles. The Gly could be recycled and reused while additional steps may be necessary to address the buildup of Na+ and Cl− ions in the solution in the cyclic uses. Various methods for separating Na+ and Cl− are available [42] and we may investigate strategies to avoid the formation of such ions in the future.

Various experimental factors may influence the size and shape of the nanoparticles formed. These factors may include the Gly concentration (Fig. 4), different bases (Fig. 5), type of amino acids (Fig. 6), CaCl2 concentration (Fig. 7), and whole solvent concentration (Fig. 8). Among these, the dose of Gly appeared to be the most critical. Particle size decreased with an increase in Gly concentration and when it exceeded 0.05 mol, further changes in particle size were negligible. The chemical (e.g., Ca2+, Ba2+, Ag+) seemed to have played a major role in the formation of various carbonates of different nanoparticle sizes, since the CaCO3, BaCO3, and Ag2CO3 nanoparticles produced had different diameters of 70 nm, 50 nm, and 7 nm, respectively. It is possible to achieve similar sizes of different carbonate nanoparticles by optimizing the various factors mentioned above.

The method presented here was operated under mild reaction conditions and utilized readily available raw materials. By simply changing the salt used in the process, a variety of carbonate nanoparticles were produced. These advantages made the method highly suitable for industrial-scale production of various carbonate nanoparticles without requiring modifications to the procedure or additional energy input. Moreover, this method can utilize diluted CO2 concentrations (Fig. 9) to synthesize nanoparticles, making it a promising approach for converting CO2 from flue gases or ambient air. This conversion into value-added carbonate nanoparticles could help offset the costs associated with CO2 management, providing both economic and environmental benefits.

In this study, two methods have been developed for the conversion of CO2 into nano-CaCO3: two-step process (Fig. 2) and one-step process (Fig. 11). Gly played a critical role in controlling the morphology of calcite in both methods (Figs. 3 and 11). In the one-step process, the presence of Gly was essential as CO2 loading did not occur without it (Fig. 11a). Compared to the two-step process, the one-step approach offered greater convenience and was potentially more favorable for industrial applications because all reactants were added simultaneously at the beginning, eliminating the need to add CaCl2 during the process. However, this method resulted in particle aggregation (Fig. 11c) and lower CO2 loading (Fig. 11a). The reduced CO2 loading in the one-step process might be due to the observation of white precipitates when Gly, NaOH, and CaCl2 were mixed, theoretically resulting from the reaction between NaOH and CaCl2 that produced Ca(OH)2. It was clear that, in the two-step process, amino acids like Gly participated in the CO2 absorption process and resulted in the formation of nano-CaCO3. In the one-step process, although no formation of carbamate or HCO3− was observed, the shift of Gly peaks was similar to the two-step process (Fig. 11e), and the pH change was also similar (Fig. 11b). Therefore, it is believed that Gly also participated in the CO2 absorption in the one-step process although the detection of carbamate and bicarbonate formation via the NMR method was not possible.

Conclusions

Amino acid salt solvents have been studied to capture CO2 primarily for geological sequestration; this study, for the first time, developed innovative methods based on amino acid salt solvents to capture and convert CO2 into different types of nano-carbonates. It was found that when bubbling CO2 into solvents with NaOH only, HCO3− was formed, and CaCO3 microparticles (~ 2 μm) were formed upon the subsequent addition of CaCl2. Interestingly, when CO2 was bubbled into solvents with both Gly and NaOH at a 1:1 ratio, carbamate formed first before HCO3− formation and CaCO3 nanoparticles (~ 70 nm) were obtained when CaCl2 was added. NMR data clearly showed that the volume of nanoparticles formed was directly proportional to the amount of carbamate formed. When the ratios of Gly to NaOH were less than 1:1, both nanoparticles and microparticles were formed and a competition between two different reaction pathways seemed to have occurred. In the one-step process, when CaCl2 was added at the same time along with Gly and NaOH, CaCO3 nanoparticles were formed. However, they were more aggregated and the CO2 loading was relatively lower compared to the two-step process. Therefore, this study clearly demonstrated that amino acids were required for the formation of CaCO3 nanoparticles and amino acids resulted in different chemical reactions as compared to the solvents without amino acids. Furthermore, the method has been proven to be universal to produce various types of nano-carbonates including nano-CaCO3, nano-BaCO3, and nano-Ag2CO3.

Supplementary MaterialAcknowledgements

This material is based upon work supported by the Agriculture and Food Research Initiative Competitive Grant (no. 2020-68012-31881) from the USDA National Institute of Food and Agriculture and by the WV Higher Education Policy Commission under grant number RCG23-12 and the WVU Internal Grant. We also acknowledge use of the WVU Shared Research Facilities. The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, the funding agencies, or their institutions. The authors thank Suzanne Danley for proofreading.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Bingyun Li reports financial support was provided by USDA National Institute of Food and Agriculture. Bingyun Li reports financial support was provided by WV Higher Education Policy Commission. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.seppur.2024.131207.

Data availability

Data will be made available on request.

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Converting CO2 into various nano-carbonates using an amino acid-driven process.

a) an innovative two-step process capturing and converting CO2 into nano-CaCO3. b) NMR spectra of solvent and the precipitates after the two-step process. c) XRD spectra of the produced nano-CaCO3. d) size distribution from SEM images of the produced nano-CaCO3 (480 particles in 4 images). Experimental conditions: Gly (0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

SEM images of CaCO3 produced a) without and b) with Gly. NMR spectra c) without and d) with Gly. e) Gly and carbamate percentages at different CO2 bubbling times with Gly. f) XRD spectra without and with Gly. g) pH changes without and with Gly. h) CO2 loading without and with Gly. In the spectra in c and d, peaks 1 and 2 represent Gly/Gly−, peaks 3, 4, and 5 represent carbamate, and peak 6 represents CO32–/HCO−3. 1,4-dioxane was used as the control reference (ref). Experimental conditions: Gly (0 or 0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

a) precipitates produced using different doses of Gly (0 mol, 0.0125 mol, 0.025 mol, 0.05 mol, and 0.1 mol). b) Schematic diagram describing the evolution of particles with Gly dose. c) Percentage of nanoparticles produced at different doses of Gly, and carbamate amount at 20 min at different doses of Gly before adding CaCl2. d) CO2 loading after bubbling CO2 for 20 min. Experimental conditions: Gly (0–0.1 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

Nano-CaCO3 prepared from a) NaOH and b) KOH as the base. c) XRD spectra of nano-CaCO3. d) CO2 loading before and after CaCl2 addition. e) pH changes before and after CaCl2 addition. Experimental conditions: Gly (0.05 mol), KOH (0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

Nano-CaCO3 prepared from a) Asn, b) Ala, and c) Cys. d) XRD spectra of the nanoparticles prepared from various amino acids. 13C NMR spectra of solvent bubbled CO2 for 20 min with e) Cys, f) Ala, g) Asn, respectively. The peak numbers in the spectra are from molecular structures (insets). Experimental conditions: Asn (0.05 mol), Ala (0.05 mol), Cys (0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

Formation of CaCO3 nanoparticles at various doses of CaCl2 at a) 0.0125 mol, b) 0.025 mol, c) 0.05 mol, and d) 0.075 mol. a1, a2, b1, and b2 were at higher magnifications compared to a and b, respectively. e) Molar ratio of CaCO3 produced to NaOH added. Experimental conditions: Gly (0.05 mol), NaOH (0.05 mol), CaCl2 (0.0125–0.075 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min). *p < 0.05.

SEM images of CaCO3 nanoparticles formed at a) Gly:NaOH:CaCl2 = 0.025 mol:0.025 mol:0.025 mol, b) Gly:NaOH:CaCl2 = 0.050 mol:0.050 mol:0.050 mol, c) Gly:NaOH:CaCl2 = 0.075 mol:0.075 mol:0.075 mol, and d) Gly:NaOH:CaCl2 = 0.100 mol:0.100 mol:0.100 mol. Experimental conditions: Gly (0.025–0.100 mol), NaOH (0.025–0.100 mol), CaCl2 (0.025–0.100 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

Synthesis of nano-CaCO3 using diluted CO2 (10 %). a) CO2 loading when bubbling diluted CO2 for 20 min and 80 min. SEM of CaCO3 produced after bubbling diluted CO2 for b) 20 min and c) 80 min. d) XRD of nano-CaCO3. Experimental conditions: Gly (0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), diluted CO2 gas flow rate (100 sccm, CO2 10 scc L m & N2 90 sccm).

Two-step process converting CO2 into various carbonate nanoparticles. a-b) SEM of precipitates produced by BaCl2 and AgNO3, respectively. c-d) XRD of precipitates produced by BaCl2 and AgNO3, respectively. Experimental conditions: Gly (0.05 mol), NaOH (0.05 mol), BaCl2(0.05 mol), AgNO3(0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min).

Synthesis of nano-CaCO3 via the one-step process where a solvent with Gly, NaOH, and CaCl2 was mixed and followed by CO2 bubbling. a) CO2 loading vs. CO2 bubbling time with and without Gly. b) pH changes vs. CO2 bubbling time with and without Gly. c) SEM of nano-CaCO3 produced after bubbling CO2 for 20 min. d) XRD of nano-CaCO3. e) NMR vs. CO2 bubbling time with Gly. Experimental conditions: Gly (0 or 0.05 mol), NaOH (0.05 mol), CaCl2 (0.05 mol), CO2 gas flow rate (100 sccm), and CO2 bubbling time (20 min). *p < 0.05 compared to the control at the same CO2 bubbling time.