Determination of Dissolved CO 2 Concentration in Culture Media: Evaluation of pH Value and Mathematical Data

: Carbon dioxide is the most inﬂuential gas in greenhouse gasses and its amount in the atmosphere reached 412 µ mol / mol in August 2020, which increased rapidly, by 48%, from preindustrial levels. A brand-new chemical industry, namely organic chemistry and catalysis science, must be developed with carbon dioxide (CO 2 ) as the source of carbon. Nowadays, many techniques are available for controlling and removing carbon dioxide in di ﬀ erent chemical processes. Since the utilization of CO 2 as feedstock for a chemical commodity is of relevance today, this study will focus on how to increase CO 2 solubility in culture media used for growing microbes. In this work, the CO 2 solubility in a di ﬀ erent medium was investigated. Sodium hydroxide (NaOH) and monoethanolamine (MEA) were added to the culture media (3.0 g / L dipotassium phosphate (K 2 HPO 4 ), 0.2 g / L magnesium chloride (MgCl 2 ), 0.2 g / L calcium chloride (CaCl 2 ), and 1.0 g / L sodium chloride (NaCl)) for growing microbes in order to observe the di ﬀ erence in CO 2 solubility. Factors of temperature and pressure were also studied. The determination of CO 2 concentration in the solution was measured by gas analyzer. The result obtained from optimization revealed a maximum CO 2 concentration of 19.029 mol / L in the culture media with MEA, at a pressure of 136.728 kPa, operating at 20.483 ◦ C.


Introduction
Fossil fuels are broadly acknowledged as being the principal source of energy, and since the First Industrial Revolution the amount of carbon dioxide (CO 2 ) in the atmosphere has risen from 280 µmol/mol to 412 µmol/mol. The resulting CO 2 emissions contribute significantly to worldwide climate change [1]. Up to now, the deployment of cutting-edge low-carbon fossil-energy technologies was considered to be the ultimate solution. Preventing worldwide climate change can be achieved by taking two long-term emission objectives into account. First, CO 2 emissions have had to reach their highest point, and then in the second half of the century, the goal has had to be to strive to achieve net greenhouse gas neutrality, by balancing anthropogenic emissions by the sources with the removal by sinks [2]. Hence, it is crucial to decrease such anthropogenic emissions. Second, CO 2 can be captured and used as a significant feedstock to produce valuable commodities. As the world population increases, the need for energy supply rises at an exponential rate. Subsequently, to meet this demand, new and renewable energy sources are required. Along this line, treating CO 2 as a feedstock to many value-added chemicals and fuels addresses both emission-control and energy supply challenges [3].

Carbon Dioxide (CO2) Delivery System
A batch-typed glass (borosilicate) cylindrical reactor (Bio Gene ® , Australia) with a built-in motor, the total volume capacity of 5 L (D = 140 mm; h = 325 mm), equipped with a pressure gauge, (RS Components, Johor Bahru, Johor, Malaysia), pH (BOQU ® , Shanghai, China) and temperature probe (DPSTAR Manufacturing Sdn. Bhd., Kuala Lumpur, Malaysia) was employed for the carbon dioxide (CO2) absorption as shown in Figure 1. The reactor was connected to a pressurized gas mixture tank (Gaslink Industrial Gases Sdn. Bhd., Puchong, Selangor, Malaysia) through a flowmeter (HERO TECH ® , Puchong, Selangor, Malaysia) with a valve for controlling the flow rate of the mixture. The compositions of the gas mixture were 90% CO2 and 10% Nitrogen (N2). For providing a vacuum space inside the reactor, a vacuum pump (vacuubrand ® , Wertheim am Main, Baden-Württemberg, Germany) was mounted with a valve. The gas analyzer (Geotech Environmental Equipment Inc., Denver, CO, USA) was connected through one of the openings to determine the amount of CO2 in the headspace. The unwanted opening at the surface of the reactor was closed by a stopper. Additional CO2 diffuser (FunPetAqua.my, Kuala Lumpur, Malaysia) was employed to increase the CO2 absorption rate. All dimensions and components of the diffuser are described in Figure 2. All components were made of borosilicate except for the distributor head that was made of porous rubber.

Time Measurement of The Maximum Carbon Dioxide (CO2) Dissolved
An aqueous solution with the concentration of sodium hydroxide (NaOH) equal to 0.1 mol/L was prepared by dissolving sodium hydroxide pellets (Sigma-Aldrich, St. Louis, MO, USA) in 2 L of distilled water. Before absorption was conducted, the absorbent (NaOH) temperature was maintained at room temperature, 100 kPa pressure and pH = 11.0 by adding hydrochloric acid (HCl)

Time Measurement of The Maximum Carbon Dioxide (CO 2 ) Dissolved
An aqueous solution with the concentration of sodium hydroxide (NaOH) equal to 0.1 mol/L was prepared by dissolving sodium hydroxide pellets (Sigma-Aldrich, St. Louis, MO, USA) in 2 L of distilled water. Before absorption was conducted, the absorbent (NaOH) temperature was maintained at room temperature, 100 kPa pressure and pH = 11.0 by adding hydrochloric acid (HCl) (Sigma-Aldrich, St. Louis, MO, USA) or NaOH to adjust the pH value to a desirable number. All equipment, including tubes, fittings, and the headspace of the reactor, were sufficiently washed by a vacuum pump. After the conditions are met, the absorption was carried out by injecting the gas mixture into the absorbent via a sparger with a flow rate of 4.5 L/min controlled by a mass flow controller. The solution was mixed using a mechanical stirrer at a speed of 180 rpm for uniform reaction in an absorber. The variations of pH during the reaction were measured every 10 s. The gas mixture was bubbled into the absorbent until the pH value stops dropping. The time for the CO 2 component in the mixture to dissolved in the medium was taken and a graph for pH drop against time was drawn. The same experiment was repeated while using 0.1 mol/L monoethanolamine (MEA) (Sigma-Aldrich, St. Louis, MO, USA) solution to determine the pH drop for both absorbents.
Then the experiment was repeated by replacing the absorbent with culture media. Culture media composed of 3.0 g/L dipotassium phosphate (K 2 HPO 4 ), 0.2 g/L magnesium chloride (MgCl 2 ), 0.2 g/L calcium chloride (CaCl 2 ), and 1.0 g/L sodium chloride (NaCl) was prepared (Sigma-Aldrich, St. Louis, MO, USA). A total of 0.1 mol/L aqueous NaOH solution or 0.1 mol/L MEA solution was added to culture media to promote CO 2 absorption. It is important to determine media capability as CO 2 absorbent as culture media will help microbes to utilize CO 2 for producing the chemical commodity. The parameter is set as the previous run, until the CO 2 concentration is maximized, and then the gas mixture supply will be stopped. The graph of pH drop against time for both compositions was plotted to determine the significance of each composition. Then, all the experiments were repeated with the deployment of a CO 2 diffuser at the sparger to observe the change in time for pH drop.

Optimization through Surface Response Methodology
Additionally, three factors that affect CO 2 solubility (s), composition (X), pressure (p), and temperature (T) were also studied using three-factor, three-level Box-Behnken design (BBD). Each of these independent factors divided into three different levels as shown in Table 1. To describe the relationship between a set of parameters and output, the regression model was developed in BBD design and can be defined by Equation (1): where Y: response; β 0 : constant-coefficient; β i and β ii : linear and quadratic coefficients for the terms X i and X ii , respectively; β ij : coefficients which represent the interactions of X i and X j . Then, an analysis of variance (ANOVA) was used to determine whether the models are acceptable for analysis.

Analytical Methods
Dissolved carbon dioxide (CO 2 ) in culture media was determined by calculation based on readings obtained from a gas analyzer (Geotech Environmental Equipment Inc., Denver, CO, USA). Readings of pH and temperature were obtained by direct measurement using a portable pH (BOQU ® , Shanghai, China) and temperature probe (DPSTAR, Kuala Lumpur, Malaysia). Time was recorded using a stopwatch.

Model Description and Calculation
Dissolved carbon dioxide (CO 2 ) in culture media can be calculated using the information given by the gas analyzer. The reading given by the analyzer consists of the composition of air in the headspace. Since the value given by the analyzer is in percentage, some calculations need to be done. The percentage of CO 2 was multiplied by the volume of headspace to get the volume CO 2 in the headspace in m 3 . The volume of headspace was constant throughout the experiment. It was set at 2.5 × 10 −3 m 3 . By subtracting the total value of CO 2 supplied with the one in the headspace, the value of carbon dioxide aqueous, CO 2 (aq) can be obtained.

Relationship between Carbon Dioxide (CO 2 ) Concentration and pH
The pH of the absorbent decreased with increasing CO 2 concentration. A slight difference in the lowest value of pH was observed between the absorbent used; sodium hydroxide (NaOH), culture media, and culture media with either NaOH or monoethanolamine (MEA), but the difference was not significant except for MEA which was observed to take a longer time to achieve minimum pH. A significant difference was only in the time taken to achieve the minimum value of pH. The graph of pH against time was plotted for NaOH, MEA, and culture media in Figure 3. The pH decreased with time until the 18 min mark for NaOH and the final pH was 6.91. Meanwhile for MEA and culture media were t = 60 min; pH = 8.52 and t = 8 min; pH = 6.46 respectively. Finally, culture media with NaOH and MEA were t = 9 min; pH = 6.45 and t = 21 min; pH = 7.0 respectively.  Many factors can affect pH in the absorbent. The decrease in pH value in all absorbents was due to the increase of the concentration of dissolved inorganic carbon such as CO2(aq), HCO3 − , and CO3 2− present in CO2 that been dissolved in said absorbents [15]. Theoretically, NaOH will yield a higher CO2 solubility than culture media present because the total alkalinity of NaOH is higher due to NaOH is a strong alkaline. The mechanism of CO2 absorption in NaOH aqueous solution is summarized in the following [16]. All Na + and OH − was ionized in pure water, then aqueous CO2 reacts with OH − as expressed in the following equation: Since the behavior of culture media and MEA absorbents do not follow the trend of the other three absorbents, it not suitable to be included in the optimization process ( Table 1). The value of pH for culture media cannot be increased without the addition of other substances, thus, the comparison cannot be done as the initial pH value was way lower than other absorbents. For the case of MEA, the optimization process is used to find the best solution for a range of data, a large difference in data value can resulting in invalid optimization. Because of that, these two absorbents were not included in the optimization analysis. Additionally, the focus was always on culture media with the addition of NaOH and MEA.
Many factors can affect pH in the absorbent. The decrease in pH value in all absorbents was due to the increase of the concentration of dissolved inorganic carbon such as CO 2 (aq), HCO 3 − , and CO 3 2− present in CO 2 that been dissolved in said absorbents [15]. Theoretically, NaOH will yield a higher CO 2 solubility than culture media present because the total alkalinity of NaOH is higher due to NaOH is a strong alkaline. The mechanism of CO 2 absorption in NaOH aqueous solution is summarized in the following [16]. All Na + and OH − was ionized in pure water, then aqueous CO 2 reacts with OH − as expressed in the following equation: Both equations are reversible reactions with a high effect on pH changes. The reaction is continuous so every CO 2 (aq) that was present in the medium will instantaneously be consumed. Due to high alkalinity, reaction Equation (3) was the main reaction occur in the absorbent early on resulting in increasing of CO 3 2− , while OH − is rapidly consumed via both reactions. This explained the reason for the sudden change in pH at the early stage of the experiment. As CO 2 aerated through the medium, OH − will keep decreasing and CO 3 2− keep accumulating. This phenomenon will force a backward reaction of Equation (3) which will accelerate the forward reaction of Equation (2). The pH will keep dropping in this stage. At a certain point in the experiment, pH will stop dropping and remain constant due to the reaction at equilibrium. After all the reactions are at equilibrium, the overall reaction of NaOH with CO 2 can be written as Equation (4).
Meanwhile, for the culture media, the phenomenon of CO 2 dissolved can be explained by the reaction between CO 2 and water (because H 2 O is present in media). Firstly, aqueous CO 2 will react with water to form carbonic acid [17]: Then the H 2 CO 3 can lose one or both of its H + to form: The pH drops in media are due to the released hydrogen ions. However, this equation can operate in both directions depending on the current pH level, working as its buffering system. At a higher pH, this bicarbonate system will shift to the left, and CO 3 2− will pick up a free hydrogen ion [18]. But in the system, CO 2 was constantly added to the media, then increasing the value of dissolved CO 2 causing the reaction Equations (6) and (7) forced to be carried out from left to right. This increases H 2 CO 3 , which decreases pH. Additionally, based on Figure 2, it shows that culture media with NaOH was the fastest to reach the minimum or equilibrium pH, following by pure NaOH and media with MEA. When the pH reaches its equilibrium point, it indicates that no more CO 2 is being absorbed. With this information, it is concluded that the longer time is taken to reach equilibrium, the higher the CO 2 concentration in absorbent. As expected, culture media capability to absorb CO 2 is weaker than NaOH. However, when MEA was added to culture media, a higher amount of CO 2 was absorbed.

System Performance with Deployment of Carbon Dioxide (CO 2 ) Diffuser
One of the main factors in the CO 2 absorption rate is the surface area of the gas-liquid boundary [19]. In the next set of experiments, the factor was been investigated. CO 2 diffuser was mounted at the sparger at the bottom of the reactor where the CO 2 is aerated. This diffuser functions to turn the CO 2 bubble into a more refined form. Parameters used were based on the previous experiments. The result is shown in Figure 4.
Additionally, based on Figure 2, it shows that culture media with NaOH was the fastest to reach the minimum or equilibrium pH, following by pure NaOH and media with MEA. When the pH reaches its equilibrium point, it indicates that no more CO2 is being absorbed. With this information, it is concluded that the longer time is taken to reach equilibrium, the higher the CO2 concentration in absorbent. As expected, culture media capability to absorb CO2 is weaker than NaOH. However, when MEA was added to culture media, a higher amount of CO2 was absorbed.

System Performance with Deployment of Carbon Dioxide (CO2) Diffuser
One of the main factors in the CO2 absorption rate is the surface area of the gas-liquid boundary [19]. In the next set of experiments, the factor was been investigated. CO2 diffuser was mounted at the sparger at the bottom of the reactor where the CO2 is aerated. This diffuser functions to turn the CO2 bubble into a more refined form. Parameters used were based on the previous experiments. The result is shown in Figure 4. From Figure 4, the addition of the diffuser favorably affected the CO2 absorption rate by all absorbents. The final pH value were almost identical with previous run, while the time taken for pH to drop for NaOH (pH = 6.88, t = 15 min), MEA (pH = 8.49, t = 50 min), culture media (pH = 6,4, t = 6 min), culture media with NaOH (pH = 6.43, t = 7 min) and culture media with MEA (pH = 7, t = 17 min) were approximately decrease by 20% when using diffuser compared to the previous. By using a diffuser, CO2 is dissipated into countless small bubbles, which flows in the culture media in the form of a bubbling stream. This process is called atomization. The large number of bubbles scattered in culture media will increase the contact area of gas-liquid that will increase the absorption rate.
A simple relation between the contact surface area and mass transfer rate can be expressed in Equation (8) [20]: where m, mass transfer, jm, mass flow, and S, interfacial area, and it is shown that the relationship between mass transfer is directly proportional to the contact area. Additionally, a more detailed From Figure 4, the addition of the diffuser favorably affected the CO 2 absorption rate by all absorbents. The final pH value were almost identical with previous run, while the time taken for pH to drop for NaOH (pH = 6.88, t = 15 min), MEA (pH = 8.49, t = 50 min), culture media (pH = 6,4, t = 6 min), culture media with NaOH (pH = 6.43, t = 7 min) and culture media with MEA (pH = 7, t = 17 min) were approximately decrease by 20% when using diffuser compared to the previous. By using a diffuser, CO 2 is dissipated into countless small bubbles, which flows in the culture media in the form of a bubbling stream. This process is called atomization. The large number of bubbles scattered in culture media will increase the contact area of gas-liquid that will increase the absorption rate.
A simple relation between the contact surface area and mass transfer rate can be expressed in Equation (8) [20]: where m, mass transfer, j m , mass flow, and S, interfacial area, and it is shown that the relationship between mass transfer is directly proportional to the contact area. Additionally, a more detailed mathematical model of absorption rate with other parameters had been derived by Martinez I. et al. [21] using Fick's Law shown in Equation (9): where j g : diffusion mass flux; q m : mass flow rate; D g : gas diffusivity; w: mass fraction; ρ 1 : density of the liquid used. The assumption is as follows: symmetry is spherical, time-independent, constant liquid and gas density, and residence time of bubble much smaller than the dissolution time, the factor that affects the absorption rate can be defined by Equation (10): w ∞ : mass fraction before venting; w: gas mass fraction; v: kinematic viscosity of liquid; D g : diffusion coefficient of CO 2 gas in water; u g : gas injection speed; g: gravity acceleration; r 0 : radius of the bubble. The benefit of this simple mathematical formula is that the relations between different parameters are explicit. Gas diffusion from bubbles to the aqueous phase is measured by w ∞ , hence based on this equation, the mass of gas in a liquid is inversely proportional to the fourth power of bubble radius. This experiment only portrays the benefit of a simple diffuser. More complications, especially with made CO 2 diffusers for CO 2 absorption in the reactor can further increase the absorption rate of CO 2 .

Factors Affecting Carbon Dioxide (CO 2 ) Solubilities in Culture Media
CO 2 is one of the key factors in organic acid fermentation [22,23]. Organic acid had a wide application ranging from preservative agents for food to lab application [24]. Due to its benefit, the production of organic acid is widely studied. As mention earlier, one of the three factors that affect CO 2 solubilities is the composition of media. Many studies focusing on organic acid production have increased the CO 2 availability by the addition of chemicals such as magnesium carbonate (MgCO 3 ), sodium bicarbonate (NaHCO 3 ), or calcium carbonate (CaCO 3 ). Through the addition of such chemicals, the CO 2 solubility is greatly increased. Thus, studies on different chemicals such as sodium hydroxide (NaOH) and monoethanolamine (MEA) are also important.
The theoretical access to CO 2 solubilities in culture media is very limited. The effect of organic solutes on gas solubility can be rather complex. But parameters affecting the solubilities were well-studied. Three parameters affecting CO 2 solubilities are pressure, temperature, and media composition [25]. The effects of these parameters on the solubility of a gas in a pure solvent, expressed in a mathematical model were simplified in Table 2. Based on Table 2, composition (X), pressure (p), and temperature (T) had been identified as the three most important independent parameters affecting CO 2 solubility (s) in absorbent and thus are chosen as the inputs for the design.

Design of Experiment Analysis
In this study, a three-factor, three-level Box-Behnken design (BBD) was used to investigate the effects of composition (X), pressure (p), and temperature (T) the interactions of these factors on the CO 2 solubility (s) in absorbent measured by its concentration. A total of fifteen experimental samples were required for the BBD including three replicated experimental runs using the processing parameters at the center points (Table 3). The ANOVA analysis is performed as shown in Table 4 to determine the significance and adequacy of the regression models. The Model F-value of 135.76 implies the model is significant. There is only a 0.01% chance that an F-value this large could occur due to noise. p-values less than 0.05 indicate model terms are significant and all insignificant model terms had been reduced. Additionally, a graph in Figure 5 shows a good agreement between the actual data and the predicted values from the regression models. The Predicted R 2 of 0.9375 is in reasonable agreement with the Adjusted R 2 of 0.9897; i.e., the difference is less than 0.2. For adequate precision measures of the signal to noise ratio, a ratio greater than 4 is desirable. The ratio of 37.693 indicates an adequate signal. This model can be used to navigate the design space. This result shows that the regression model is statistically significant and adequate for the prediction and optimization of the CO 2 absorption process. Table 4. Results of ANOVA for the quadratic model of the CO 2 concentration in absorbent.

Degrees of Freedom
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Individual Effect of Experiment Parameters on Carbon Dioxide (CO2) Absorption
The significance of each parameter on CO2 solubility can be illustrated by the perturbation plot n Figure 6, where steep slope and curvature were obtained for both pressure and temperature Quadratic equations were derived to describe the relationships between the parameters and CO 2 solubility (s) as shown in Equations (11)-(13).

Individual Effect of Experiment Parameters on Carbon Dioxide (CO 2 ) Absorption
The significance of each parameter on CO 2 solubility can be illustrated by the perturbation plot in Figure 6, where steep slope and curvature were obtained for both pressure and temperature indicates that CO 2 solubility is sensitive to the parameters. The maximum value of CO 2 dissolved achieved with the addition of NaOH for culture media was lower than what been achieved for MEA. This is due to the MEA's rapid reaction with CO 2 in low partial pressure [27].
pressure. Also, the effect of temperature is more significant than the pressure (highest F-value). A slight increase in temperature will greatly reduce CO2 absorption. While to achieve more significant change by pressure, extremely high pressure needs to be applied [29]. The gas dissolves in liquid because of the interactions between its molecules and absorbent. This interaction will release heat when these new attractive interactions form in an exothermic process. Thus, additional heat will produce thermal energy that overwhelms the attractive forces between the gas and the absorbent molecules resulting in less CO2 dissolved in solution [28]. Figure 6. Perturbation plot comparing the response of CO2 solubility to changes in temperature and pressure in (i) pure NaOH, (ii) culture media with NaOH, and (iii) culture media with monoethanolamine (MEA).

Collective Effect of Experiment Parameters on Carbon Dioxide (CO2) Absorption
The F-value determine the relative importance of a parameter. From Table 4, the temperature has the highest F-value, which is 446.76, showing its importance in CO2 absorption in this case. Figures 7-9 are three-dimensional response surface and project contour for NaOH, culture media with NaOH, and culture media with MEA, respectively, which show the different experimental parameters and their effects on CO2 concentration in the absorbent. All graphs derived had curved rather than straight lines, indicating the strong interaction between parameters and the output.  Figure 6 shows the relationship between pressure and final CO 2 concentration which is directly proportional which was described quantitatively by Henry's law shown in Table 2; pressure. External pressure affects the concentration of gas molecules in space. When the partial pressure of the gas above the solution increases, it forces the gas molecules to solute in solution to maintain dynamic equilibrium [28]. Additionally, high temperature demotes CO 2 absorption as opposed to high pressure. Also, the effect of temperature is more significant than the pressure (highest F-value). A slight increase in temperature will greatly reduce CO 2 absorption. While to achieve more significant change by pressure, extremely high pressure needs to be applied [29]. The gas dissolves in liquid because of the interactions between its molecules and absorbent. This interaction will release heat when these new attractive interactions form in an exothermic process. Thus, additional heat will produce thermal energy that overwhelms the attractive forces between the gas and the absorbent molecules resulting in less CO 2 dissolved in solution [28].

Collective Effect of Experiment Parameters on Carbon Dioxide (CO 2 ) Absorption
The F-value determine the relative importance of a parameter. From Table 4, the temperature has the highest F-value, which is 446.76, showing its importance in CO 2 absorption in this case. Figures 7-9 are three-dimensional response surface and project contour for NaOH, culture media with NaOH, and culture media with MEA, respectively, which show the different experimental parameters and their effects on CO 2 concentration in the absorbent. All graphs derived had curved rather than straight lines, indicating the strong interaction between parameters and the output.

Process Optimization
It was observed that carbon dioxide (CO2) solubility (s) reaches the maximum values under low temperature operating in high pressure. However, there was a certain point in these parameters where a further change in their value will not affect the final CO2 solubility (s). Therefore, a balance value needs to be established to achieve the maximum result while optimizing the parameters. The purpose of this step is to observe the combination of the independent variable (i.e., absorbent, Figure 9. Effect of pressure, temperature and their interaction on CO 2 concentration in culture media with MEA (i) three-dimensional surface plot; (ii) projected contour plot.

Process Optimization
It was observed that carbon dioxide (CO 2 ) solubility (s) reaches the maximum values under low temperature operating in high pressure. However, there was a certain point in these parameters where a further change in their value will not affect the final CO 2 solubility (s). Therefore, a balance value needs to be established to achieve the maximum result while optimizing the parameters. The purpose of this step is to observe the combination of the independent variable (i.e., absorbent, temperature, and pressure) to get the maximum CO 2 solubility (s) simultaneously. The overall performance of the CO 2 absorption strongly depends on a wide range of experimental conditions. It is crucial to optimize the CO 2 absorption process with multiple inputs and multiple responses. Thus, a series of optimizing results proposed by response surface methodology (RSM) with multiple inputs and multiple responses are listed in Table 5. The optimal CO 2 solubility (s) can be obtained in the following conditions: culture media with MEA as absorbent; 20.483 • C of temperature; 136.728 kPa of pressure. The optimal CO 2 solubility (s) obtained at this condition was 19.029 mol/L. Additionally, a favorable value for each parameter can be chosen from the RSM. For example, the desired temperature is ≈ 22 • C, pressure must be around 140 kPa to obtain the optimal CO 2 solubility (s). Though from the optimization process, absorbents other than culture media with MEA was found to be undesirable.

Conclusions
The present work has investigated the features of sodium hydroxide (NaOH) aqueous solution and culture media to capture carbon dioxide (CO 2 ) at different compositions, pressure, and temperature. It was observed that the CO 2 solubility increases using monoethanolamine (MEA) compared to NaOH. CO 2 absorption was also favorable at high pressure and low temperature. Improvement of the absorption rate can be achieved by deploying a CO 2 diffuser. Using optimization software, the most optimized condition for the CO 2 absorption process was by using culture media added with MEA, at a pressure of 136.728 kPa operating at 20.483 • C. Besides, capturing CO 2 by using culture media will lead to the production of chemical commodities (such as succinic acid, formic acid, and acetic acid) by microbes that can be useful for industrial usage. This work serves as a base for further research on CO 2 absorption by culture media. Further studies such as the feasibility of this method by using gas mixture instead of pure CO 2 are necessary to demonstrate a more flexible process. The ultimate aim of this work is to produce value-added commodities utilizing CO 2 as the main carbon source with the help of suitable microbes.