Amino Acid Salt Research Articles

New sterically hindered polyvinylamine-containing membranes for CO2 capture from flue gas

Amine-containing facilitated transport membranes can have both high permeability and selectivity due to the reversible reaction between CO 2 and amino groups for effective carbon capture from flue gas. We have developed an improved method for the synthesis of high-molecular-weight sterically hindered polyvinylamine (PVAm) as the new fixed-site carrier in thin-film composite membranes for CO 2 capture. Commercially available PVAm was N -monomethylated into poly- N -methyl- N -vinylamine (PVAm-CH 3 ) using the stepwise reductive amination with a highly polar fluorinated alcohol as the solvent to enhance the equilibrium shift to the formation of the imine intermediate. The method prevented over-alkylation, resulting in the increased yield of the target product. PVAm-CH 3 exhibited excellent CO 2 facilitation with a CO 2 permeability of 445.7 Barrer (1 Barrer = 3.349 × 10 −16 mol m m −2 s −1 Pa −1 ) and a CO 2 /N 2 selectivity of 70.3, which are above the Robeson 2008 upper bound. The PVAm-CH 3 solution retained a sufficiently high viscosity after incorporating the amino acid salt, 2-(1-piperazinyl)ethylamine sarcosinate (PZEA-Sar), as the mobile carrier for the membrane coating on nanoporous polyethersulfone (PES) substrates without any penetration issues. The resultant thin-film composite PVAm-CH 3 /PZEA-Sar membrane with a thickness of approximately 170 nm exhibited a superior CO 2 performance of 1071 GPU (1 GPU = 3.349 × 10 −10 mol m −2 s −1 Pa −1 ) and a CO 2 /N 2 selectivity of 183 at 57 °C and a feed gas pressure of 111.64 kPa (1.5 psig). This PVAm-CH 3 /PZEA-Sar membrane surpassed the latest redefined 2019 CO 2 /N 2 upper bound and outperformed other polymer-based membranes. Density functional theory calculations also demonstrated that PVAm-CH 3 showed a stronger preference, relative to PVAm, toward the more efficient bicarbonate pathway. Thus, the steric hindrance effect of PVAm-CH 3 enhanced the solubility of CO 2 in the polymer matrix and resulted in the higher CO 2 permeance. • Hindered polyvinylamine (PVAm-CH 3 ) synthesized using stepwise reductive amination. • The reaction conditions optimized for the highest N -methylation degree of 39%. • PVAm-CH 3 fixed-site carrier membrane exhibited excellent CO 2 facilitation. • The improved performance of PVAm-CH 3 was illustrated by DFT calculations. • A composite membrane showed 1071 GPU CO 2 permeance and 183 CO 2 /N 2 selectivity.

Multiple machine learning models for prediction of CO2 solubility in potassium and sodium based amino acid salt solutions

In this work, we developed artificial intelligence-based models for prediction and correlation of CO2 solubility in amino acid solutions for the purpose of CO2 capture. The models were used to correlate the process parameters to the CO2 loading in the solvent. Indeed, CO2 loading/solubility in the solvent was considered as the sole model’s output. The studied solvent in this work were potassium and sodium-based amino acid salt solutions. For the predictions, we tried three potential models, including Multi-layer Perceptron (MLP), Decision Tree (DT), and AdaBoost-DT. In order to discover the ideal hyperparameters for each model, we ran the method multiple times to find out the best model. R2 scores for all three models exceeded 0.9 after optimization confirming the great prediction capabilities for all models. AdaBoost-DT indicated the highest R2 Score of 0.998. With an R2 of 0.98, Decision Tree was the second most accurate one, followed by MLP with an R2 of 0.9.

Mass transfer and capture of carbon dioxide using amino acids sodium aqueous solution in microchannel

Capture and storage of CO2 are important to restrain the greenhouse effect. The amino acid salt solution as a new type of CO2 absorbent shows good application potential. Besides, microchannel has a unique enhancement to mass transfer, which is conducive to promote CO2 capture. The absorption performance of CO2 into aqueous solutions of sodium salts of l-threonine, l-valine, l-alanine and glycine in the microchannel was investigated. It was found that the CO2 absorption rate of four sodium amino acid aqueous solutions under the same condition follows in the order: ñ (sodium glycinate) > ñ (sodium l-alaninate) > ñ (sodium l-valinate) > ñ (sodium l-threoninate). The sodium glycinate aqueous solution has maximum CO2 absorption ability, thus the mass transfer and capture of CO2 absorbed into the sodium glycinate aqueous solution were studied systematically. The effects of operating conditions including gas flow rate, liquid flow rate, and sodium glycinate concentration on the specific surface area, overall volumetric mass transfer coefficient, overall mass transfer coefficient and absorption fraction of CO2 were explored. Furthermore, a new correlation was proposed to predict the volumetric mass transfer coefficient.

Innovative cycling reaction mechanisms of CO2 absorption in amino acid salt solvents

Amino acid salt (AAS) solvents with equimolar base (used to obtain the salt) to amino acid (AA) have been well studied for CO2 absorption, however, very little is known about the reactions when more base is applied. This study determined the reaction mechanisms of AAS solvents with base to AA ratio over equimolar and the potential benefits of such solvents for CO2 absorption. The CO2 loading capacity was found to be dictated by the molar concentration of the base and was approximately half of the base applied. The reaction mechanisms were investigated based on the compositions of carbamate, carbonate/bicarbonate, and AA using 13C-quantitative nuclear magnetic resonance (13C qNMR). An innovative cycling reaction mechanism of CO2 absorption was proposed and experimentally confirmed for AAS solvents with base/AA > 1. Besides all steps of the current widely accepted Zwitterion mechanism, this reaction pathway also contains two cycles: 1) The extra base (i.e., OH−) may react with the protonated AA generated during CO2 absorption to form deprotonated AA which further absorbs CO2 to form more carbamate. 2) The carbamate, as a product of reaction cycle 1, undergoes hydrolysis to yield bicarbonate and deprotonated AA, which further absorbs CO2. As a result, high CO2 loadings were achieved. Compared to the use of alkaline solutions like KOH alone for CO2 absorption, AAS solvents with base to AA ratio over equimolar resulted in lower pH and temperature while maintaining high CO2 loading. These innovative cyclic mechanisms will enable advanced CO2 management approaches using AAS solvents with base/AA > 1.

Mitigated carrier saturation of facilitated transport membranes for decarbonizing dilute CO2 sources: An experimental and techno-economic study

The CO2/N2 separation performances of facilitated transport membranes (FTMs) containing aminoacid salts as mobile carriers were characterized under dilute feed gases with 0.05–20% CO2. At a reduced CO2 partial pressure, the carrier saturation in the FTMs was mitigated, which enhanced both the CO2 permeance and CO2/N2 selectivity. The best FTM containing 2-(1-piperazinyl)ethylamine sarcosinate exhibited an uprising CO2 permeance from 1968 to 3822 GPU and an improved CO2/N2 selectivity from 249 to 472 with reducing CO2 content from 1% to 0.1%. The feasibility of this FTM is exemplified by designing a two-stage enriching membrane cascade to further remove 90% of the CO2 in a residual coal flue gas containing 1.75% CO2. Techno-economic analysis indicates a low capture cost of $83.8/tonne. The marginal costs beyond 90% capture are also evaluated for a variety of residual flue gases, indicating that the FTM-based capture from the coal or cement plant residual flue gas is more cost effective than direct air capture.

Recent advances in DFT assisted optimized energy, stability and distortions of optimized topologies of certain biopotent dimethyltin(IV) complexes

The generation, structural elucidation, reactivity, bonding, stability, optimized energy, distortions of optimized topologies of some non-bridged mononuclear and carboxylate bridged binuclear dimethyltin(IV) complexes of N-protected amino acids and 2-aminophenol/2-amino-4-methylphenol were studied experimentally and theoretically with the assistance of density functional theory (DFT). These complexes were synthesized by the reactions of Me2SnCl2 with sodium salts of N-protected amino acids and 2-aminophenol/2-amino-4-methylphenol in 1:1:1 ​molar ratio in refluxing dry THF. These complexes were characterized and their plausible structures were suggested with the assistance of spectroscopic [IR and NMR (1H,13C and 119Sn)] evidences, mass studies and DFT investigation. Mass spectral data indicated the tendency of the complexes to undergo dimerization on aging. The 119Sn NMR data demonstrated the presence of penta-coordinated tin centres in these complexes. DFT (B3LYP) was used to study the molecular orbitals, optimized bond lengths and bond angles, optimized energy, stability, distortions of optimized topologies and energy gap of representative complexes. One representative complex, Me2SnL(2)L’(2) was screened for its possible antimicrobial activities against gram negative bacteria, E. coli and polymorphic fungal pathogen, Candida albicans, respectively. The structure-activity relationship of this complex has also been investigated.

Isolation and Optimization of Laccase producing Marine Fungi Neurospora crassa AJAS1

The study includes isolation and screening of laccase enzyme producing fungus from marine soil sample collected from Kovalam beach (8.4004˚N, 76.9787˚E), Kerala. In this study, 12 fungal strains were isolated by serial dilution technique on different complex media. For primary screening of laccase production, plate assay was performed where the fungal isolates were cultivated on potato dextrose agar supplemented with tannic acid, ABTS and guaiacol as indicator substrates respectively. Out of the 12 isolates, 6 showed positive results for laccase production and were designated as FL1-FL6. Further, laccase enzyme assay was performed. The potential strain with maximum enzyme production was found to be FL6. The strain was optimised in submerged fermentation under different sets of environmental parameters. The fungal growth was maximum at 37˚C and pH 5 with starch, yeast extract, glutamine, KH2PO4 and 3% NaCl concentration as the carbon, nitrogen, amino acid, and mineral salt source respectively. Furthermore, the enzyme stability was also checked for varying temperature and pH. Antimicrobial potential of the strain FL6 was assessed using the Kirby Bauer well diffusion method where it showed good results against Gram-negative bacteria than the Gram-positive organisms. FL6 was morphologically identified as belonging to genus Neurospora using the lactophenol cotton blue stain.28s rRNA sequencing of the potential strain was performed and identified as Neurospora crassa AJAS1.

Spectroscopic study, theoretical calculations, and optical nonlinear properties of amino acid (glycine)-4-nitro benzaldeyhyde-derived Schiff base

Schiff base of amino acid salt: potassium 2-((4-nitrobenzylidene)amino)acetate 4 (C 9 H 7 KN 2 O 4 ), derived from the condensation reaction of amino acid (glycine) 1 and 4-nitro benzaldeyhyde 3 is synthesized and fully characterized by magnetic nuclear resonance ( 1 H and 13 C NMR), infrared (IR), mass, UV–visible spectroscopies and melting point. The optical nonlinear (ONL) properties of amino acid Schiff base salt 4 are studied using continuous wave (cw), visible, single transverse fundamental mode, low power, 473 nm laser beam. Ring diffraction patterns (RDPs) and Z-scan techniques are adopted to calculate the nonlinear index of refraction (NIR) together with testing the sample property of optical limiting (OLg). RDPs are simulated numerically using the integral of Fresnel-Kirchhoff and compared with experimental findings. • The Schiff base compound was synthesized and characterized by magnetic nuclear resonance ( 1 H and 13 C NMR). • We studied the nonlinear optical properties of Schiff base compound. • The nonlinear refractive index of this sample is determined using diffraction ring patterns and Z-scan techniques. . • Optical limiting threshold of Schiff base compound is found to be 9 mW.

CuAlOx catalyst for the batch-flow tandem synthesis of amino acid-derived furfurylamines

Various amino acid-based furfurylamine derivatives were synthesized by two-stage procedure, which includes the condensation of 5-hydroxymethylfurfural (or furfural) with amino acid salts in methanol followed by hydrogenation of obtained imines in a flow reactor over CuAlOx catalyst.

Production of cooling water by Ti3C2Tx MXene interlayered forward osmosis membranes for post-combustion CO2 capture system

Forward osmosis (FO) technology has the potential to replace the trim cooler and to supply cooling water for a post-combustion CO2 capture system. Herein, a high-performance MXene (Ti3C2Tx) interlayered FO membrane was fabricated. Thanks to the ability of MXene to adjust the structures of substrate and polyamide, the MXene interlayered FO (MXene-FO) membrane showed improved permeability and selectivity in a monoethanolamine (MEA) draw solution. The performance of MXene-FO membrane was investigated at different membrane orientations and temperatures with amino acid/salt (AAS) solutions (taurine/KOH and β-alanine/KOH) as draw solutions. Because the osmotic pressures provided by AAS solutions (104.4 ± 0.3 bar for 3 mol/L taurine/KOH and 153.7 ± 0.2 bar for 5 mol/L β-alanine/KOH solutions) are higher than that of 5 mol/L MEA solution (93.0 ± 0.1 bar), the MXene-FO membrane exhibited higher water fluxes in AAS solutions. Besides, the good performance of the MXene-FO membrane was shown with the presence of small specific amino acid fluxes and specific KOH fluxes. To investigate the feasibility of using saline water as the feed solution, 3.5 wt% NaCl solution was applied and the results indicated that the MXene-FO membrane maintained stable water fluxes and small specific solute fluxes. This work demonstrates the potential of nanomaterials interlayered FO membranes for cooling water production in a post-combustion CO2 capture system.

A Process Intensification Approach for CO2 Absorption Using Amino Acid Solutions and a Guanidine Compound

Environmentally friendly amino-acid salt solutions are used for the absorption of carbon dioxide from concentrated flue-gas streams via chemical absorption. Process intensification reduces operating and capital costs by combining chemical reactions and separation operations. Here, we present a new process-intensification approach that combines the CO2 capture and the amino-acid regeneration steps into a single process carried out in a slurry three-phase reactor. The absorbed CO2 precipitates as a solid carbonated guanidine compound. The cycle is completed by separation of the solid precipitate to strip the CO2 and regenerate the guanidine compound, while the liquid solution is recycled to the slurry reactor. The process was studied by modifying a model developed by the authors for a gas-liquid bubble column without the presence of the guanidine compound. The guanidine precipitation reaction was accounted for using kinetic parameters calculated by the authors in another study. The proposed model was implemented by modifying an existing computer code used for the simulation of gas-liquid bubble columns. The calculated results showed that the proposed cycle can significantly reduce energy, equipment, and operating costs and can make an important contribution to developing a competitive cost-effective large-scale process for CO2 capture.

CO2-selective membranes containing amino acid salts for CO2/N2 separation

In this paper, facilitated transport membranes comprising polyvinylamine (PVAm) as fixed carrier and different amino acid salts (AAS) as mobile carriers were synthesized for post-combustion capture. The AAS carriers were prepared by deprotonating alanine (Ala), lysine (Lys), and proline (Pro) with 2-(1-piperazinyl)ethylamine (PZEA). CO2 separation performances of the membranes with these AAS carriers were compared, and the AAS effectiveness to facilitate transport of CO2 was in the order of PZEA-Pro > PZEA-Lys > PZEA-Ala. The membrane comprising 35 wt% PVAm and 65 wt% PZEA-Pro at 57 °C rendered a promising CO2 permeance of 936 GPU and a CO2/N2 mixed gas selectivity of 193. When the PVAm was reduced to 15 wt%, i.e., its 20 wt% was replaced by the mobile carrier of PZEA-Sar (sarcosinate), the permeance further improved to 947 GPU with a remarkable CO2/N2 selectivity of 210. Moreover, thermal gravimetric analysis showed a good thermal stability of the membrane, and membrane stability testing also gave stable transport performance. Furthermore, spectroscopic ellipsometry analysis exhibited a uniform membrane selective layer and an excellent agreement on the membrane thickness of ca. 170 nm measured independently by SEM. In addition, the membranes presented in this paper surpassed both the Robeson upper bound and the modified upper bound, indicating a great potential for CO2 capture.

Investigating the core microbiota and its influencing factors in traditional Chinese pickles

Pickles are a type of traditional fermented food in Northeast China that exhibit a broad variety of preparations, flavors and microbial components. Despite their widespread consumption, the core microorganisms in various traditional pickles and the precise impact of ecological variables on the microbiota remains obscure. The present study aims to unravel the microbial diversity in different pickle types collected from household (12 samples) and industrial (10 samples) sources. Among these 22 samples tested, differences were observed in total acid, amino acid nitrogen, nitrite, and salt content. Firmicutes and Ascomycota emerged as the predominant microbial phyla as observed by Illumina MiSeq sequencing. Amongst these, the commonly encountered microorganisms were Lactobacillus, Weissella and yeast. Comparative analysis based on non-metric multidimensional scaling (NMDS), showed that the microbial community in the pickles was affected by external conditions such as major ingredients and manufacturing process. Correlation analysis further showed that the resident core microorganisms in pickles could adapt to the changing internal fermentation environment. The insights gained from this study further our understanding of traditional fermented foods and can be used to guide the isolation of excellent fermented strains.

Aqueous choline amino acid deep eutectic solvents.

We have investigated the structure and phase behavior of biocompatible, aqueous deep eutectic solvents by combining choline acetate, hydrogen aspartate, and aspartate amino acid salts with water as the sole molecular hydrogen bond donor. Using contrast-variation neutron diffraction, interpreted via computational modeling, we show how the interplay between anion structure and water content affects the hydrogen bond network structure in the liquid, which, in turn, influences the eutectic composition and temperature. These mixtures expand the current range choline amino acid ionic liquids under investigation for biomass processing applications to include higher melting point salts and also explain how the ionic liquids retain their desirable properties in aqueous solution.

Quality of walleye pollack frozen surimi by adding carboxylic acid salt in place of sugar compounds

Sugar compounds and polyphosphates are typically added to frozen surimi for the maintenance of its quality. To create healthier, low-calorie frozen surimi, frozen surimi using carboxylic acid and amino acid salts as preservatives in place of sugar compounds was prepared. After thawing, the surimi was salted, followed by preheating at 30°C for either 40 min or between 0 and 40 min, and then heating it at 90°C for 30 min. The breaking strength (BS) and breaking strain (bs) of the heat-induced gels of the surimi were then measured using a rheometer, and the gel strength (Gs = BS / bs) and jelly strength (JS = BS × bs) were calculated. The quality of the surimi was evaluated by assessing the physical properties of the heat-induced gels made from it, as well as the relationship between the BS and Gs during preheating for two-step heated gels. Results showed that the frozen surimi preparations made with 5% sodium gluconate and 3% sodium gluconate + 2% sodium glutamate, each with the addition of 0.1% Na3PO4, were comparable in quality to the typical preparation with 5.7% sugar compounds, remaining stable at -23°C for seven months or at -45°C for ten months. It was also found that, when comparing samples with equal BS values, the heat-induced gels of the 5% sodium gluconate and 3% sodium gluconate + 2% sodium glutamate preparations had slightly lower bs values as compared to typical frozen surimi with sugar compounds, thus exhibiting a slightly more brittle texture.

Assessment of the stability of amino acid complexes of heavy metals to biodegradation in the aquatic environment

Sodium salts of amino acids in solution have an alkaline reaction, which makes it possible to effectively bind metals into complex compounds, since in an alkaline medium, under the action of hydroxyl ions, the dissociation of amine groups is suppressed, and carboxyl groups remain dissociated, charging negatively, and behave like an anion.In our experiments, we used an amino acid-based reagent. When processing various environments (objects) contaminated with heavy metals (HM) with this reagent, HM amino acid complexes are formed. In our experiments, we used an amino acid-based reagent. With processing various environments (objects) contaminated with heavy metals (HM) with this reagent are formed HM amino acid complexes.Model experiments to assess the effect of microorganisms under aerobic conditions on the stability of amino acid complexes of heavy metals. After biodegradation of solutions with TM complexes, no more than 2-3% of heavy metals pass through the filter, which is ten times less than from untreated with the reagent solution.

Experimental research on chemical desorption based on CO2-rich absorption solutions

Although CO2 capture by chemical absorption can achieve highly efficient removal of CO2 from flue gas, the energy consumption of absorbent regeneration is high, and the CO2 after desorption need to be compressed, transported and sequestrated. To reduce energy consumption and sequestrate CO2 at low cost, an CO2 desorption process was developed, in which the CO2-rich solutions was regenerated and CO2 was sequestrated in the form of CaCO3. Five kinds of CO2-rich solutions, monoethanolamine (MEA), methyldiethanolamine (MDEA), potassium glycinate (PG), MDEA/MEA and MDEA/PG were investigated by adding calcium hydroxide. The optimum reaction condition, which CO2 loading was 0.621 mol/L and Ca(OH)2 dosage was 1:1 for desorption of CO2-rich solution, was determined. And the effects of three parameters (time, temperature and stirring rate) on CO2 desorption rate were further analyzed. The optimum reaction temperature for the amine absorbents and the amino acid salt solvents were 20℃ and 50℃, respectively. The optimum reaction time and standing time were 30 min, respectively, and the optimum stirring rate was 800 r/min. MEA exhibited the highest desorption rate (85.31%) among the five solutions. In addition, the results of multicycle-cycle experiments showed that the MEA lean solution desorbed by chemical desorption still had excellent absorption capacity. Compared with the conventional thermal desorption process, the chemical desorption process had great advantages for CO2-rich solutions regeneration and did not require further treatment of CO2.

Neural modeling and simulation of molecular separation using amino acid salt solutions

In this research paper, we studied absorption of carbon dioxide in amino acid solutions theoretically via the artificial intelligence (AI) method. The CO2 loadings in different solutions were predicted as a function of input parameters including amino acid parameters and operational parameters such as pressure and temperature. For the training of the data, we used differential evolution, and for the decision and prediction sections, we used the fuzzy interface system. The model can be abbreviated as differential evolution fuzzy interface system (DEFIS). Moreover, various methods and tuning parameters in the DEFIS were used to increase the accuracy of the model. Cluster influence range (CIR) was used for tuning the model, and different population sizes were used, including 9, 18, 27, and 36, with the purpose of observing the effects of these parameters on the training. The results from AI overlapped measured results meaning that we can use AI for optimization of the separation process for CO2 sorption in chemical solvents. The neural methodology indicated a reliable and robust prediction of CO2 solubility in different chemical solvents.

Evaluation of CO2 Absorption by Amino Acid Salt Aqueous Solution Using Hybrid Soft Computing Methods.

Amino acid salt (AAs) aqueous solutions have recently exhibited a great potential in CO2 absorption from various gas mixtures. In this work, four hybrid machine learning methods were developed to evaluate 626 CO2 and AAs equilibrium data for different aqueous solutions of AAs (potassium sarcosinate, potassium l-asparaginate, potassium l-glutaminate, sodium l-phenylalanine, sodium glycinate, and potassium lysinate) gathered from reliable references. The models are the hybrids of the least squares support vector machine and coupled simulated annealing optimization algorithm, radial basis function neural network (RBF-NN), particle swarm optimization–adaptive neuro-fuzzy inference system, and hybrid adaptive neuro-fuzzy inference system. The inputs of the models are the CO2 partial pressure, temperature, mass concentration in the aqueous solution, molecular weight of AAs, hydrogen bond donor count, hydrogen bond acceptor count, rotatable bond count, heavy atom count, and complexity, and the CO2 loading capacity of AAs aqueous solution is considered as the output of the models. The accuracies of the models’ results were verified through graphical and statistical analyses. RBF-NN performance is promising and surpassed that of other models in estimating the CO2 loading capacities of AAs aqueous solutions.

Insights into dual functions of amino acid salts as CO2 carriers and CaCO3 regulators for integrated CO2 absorption and mineralisation

Integrated CO2 absorption and mineralisation (IAM) is a promising technology for rapidly CO2 absorption and sequestration. However, the traditional IAM using alkanolamines as CO2 solvents and solid wastes as carbonation feedstocks still suffers from the utilization difficulty of low value products and environmental risks of amine degradation. In this study, the green solvent, amino acid salts (AAS), including potassium l-argininate (ArgK), potassium glycinate, potassium sarcosinate, potassium l-alaninate, potassium β-alaninate, were selected for IAM using rejected brine as the carbonation feedstock. Results shows that compared to amines (monoethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, piperazine, 2-amino-2-methy-1-propanol), the selected AASs in IAM were not only more effective CO2 carriers that passed CO2 to calcium ions to form CaCO3, but also better CaCO3 growth regulators to tune morphology properties of products. CO2 desorption efficiencies in CO2-loaded AASs (nearly 100 %) were much higher than those of in CO2-loaded amines especially than in MEA (38 %). ArgK stood out in regulating the morphology of crystal with smallest particle size. Then ArgK was selected for optimization experiments at various conditions (temperature, Ca2+/CO2 ratio, reactant concentration and chelating agent) to further investigate effects of operating conditions on CaCO3 properties and to provide some basic information for the polymorph and morphology regulation of CaCO3. Ethylene diamine tetraacetic acid (EDTA) as a chelating agent was found a notable influence on controlling the polymorph of CaCO3, where the amorphous CaCO3 and new structures of calcite appeared. However, the effect of EDTA on CO2 absorption step was not clear and required further investigation.