Highlights

2.1. Soda ash carbonation

Soda ash carbonation is a commonly used method in industrial settings to make sodium bicarbonate (NaHCO₃) using Na₂CO₃ obtained from trona. Natural trona contains approximately 46% Na₂CO₃ and 35% NaHCO₃, is abundantly found in nature, and is extensively spread worldwide [43]. The techniques used in trona mining processes include the monohydrate and the sesquicarbonate phases [44]. The first stage of the process is the monohydrate process, which entails smashing the trona minerals, removing water and CO₂ using a calcination method to generate sodium carbonate, and then reacting the sodium carbonate with CO₂ to provide the end product, sodium bicarbonate. The reaction that yields Na₂CO₃ as a result of thermal decomposition of the trona is given by [45]:(1)

Theoretically, the temperature range of 57 °C to 100 °C is considered suitable for the conversion of trona into sodium bicarbonate [46]. The overall soda ash dissolution and carbonation reactions are as follows [34]:(2)

The forward reaction Eq. (6) is quite exothermic 47, 48, and crystallization reaction rate is affected by temperature, sodium carbonate concentration, and CO₂ flowrate 49, 50. The presence of CO₂ and vaporized water is also crucial for the conversion process. The rate of carbonation accelerates in direct proportion to the concentration of CO₂ in the gas phase and the relative humidity level which ranges from 50 to 70%. Nevertheless, both excessively high and excessively low levels of humidity impede the pace at which carbonation occurs 51, 52. Equilibrium between carbonate and bicarbonate ions in the aqueous solution also determines the stable solid phase in a saturated solution and the corresponding solubility concentrations of carbonate and bicarbonate. This equilibrium is strongly affected by the CO₂ partial pressure. At temperature < 80 °C (equilibrium constant (Ks) _NaHCO3 < 1) and ambient CO₂ partial pressure, solid bicarbonate (nahcolite) is the thermodynamically stable solid.

2.2. Solvay process

The Solvay process (also called ammonia-soda process) is another process that was originally designed for the production of sodium carbonate and is currently being used for the production of sodium bicarbonate in CO₂ utilization process. It involves the reaction of a concentrated sodium chloride solution with NH₃ and CO₂ to produce soluble ammonium bicarbonate. This ammonium bicarbonate then reacts with NaCl to yield soluble ammonium chloride (NH₄Cl) and a precipitate of NaHCO₃. The aqueous solution of NH₄Cl obtained may undergo a reaction with calcium hydroxide (Ca(OH)₂) in order to recover and reuse the NH₃[59].

In the conventional Solvay method, the chemical reactions involving CO₂ and ammoniated brine can be classified as primary and secondary. During the first phase, after the water dissociation in Eq. (7), CO₂ undergoes a chemical interaction with ammonia (NH₃) to produce carbamic acid. An additional reaction between carbamic acid and NH₃ produces ammonium carbamate [60]. The primary phase reaction is expressed as follows:(7)

Therefore, the overall reaction is given as:(12)

The primary reaction is rapid compared with the hydrolysis of the carbamate in the bulk of the solution. The secondary reaction entails the slow hydrolysis of carbamate to bicarbonate.(13)

The bicarbonate ions precipitate as sodium bicarbonate.(14)

The aforementioned phases include the process of mass transfer accompanied by a chemical reaction. The total yield of the reaction is influenced by the concentrations of NH₃ and CO₂. The absorption of CO₂ into strongly carbonated ammoniated brine solution is characterized by a relatively low rate of absorption [61]. As described in Eq. (12), the primary reaction rapidly consumes CO₂, and at a high carbonation time, the system is under diffusion control. Eq. (13) states that as CO₂ absorption rate increases, both the yield and the secondary reaction increase. The rate of reaction at any given moment, which is determined by the diffusion of the chemical reaction, can be expressed as:(15)

Sodium bicarbonate (NaHCO₃) is an essential intermediate compound in the Solvay carbonation process, in which its solubility is a key factor for the process to be successful. In order to maximize conversion, it is important to minimize the solubility of NaHCO₃ by optimizing the variables that might affect or decrease its solubility. The main parameters that affect the carbonation process include pH, temperature, gas flow rate, and ammonia concentration.

The pH level is the key factor for assessing the dominant carbon form in the ammoniated brine carbonation process. This is important to maximize both the CO₂ absorption capacity and the bicarbonate proportion. Fig. 1 illustrates the pH level impact on the relative quantity of bicarbonate ion (

$$

) present. The precipitation of sodium bicarbonate (NaHCO₃) is determined exclusively by the concentration of bicarbonate ions in the solution.

 

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Fig. 1

Another important factor is temperature, which has an impact on CO₂ capture efficiency, conversion rate, product solubility, and composition of the end products in the formation NaHCO₃ using the Solvay process. Increasing the temperature leads to an elevation in the solubility of the product, reducing the CO₂ capture efficiency and NaHCO₃ formation, resulting in alterations in the composition of the product. Moreover, increasing temperature accelerates the NH₃ escape, leading to a reduction in the pH of the ammoniated brine solution and eventually causing a decline in CO₂ absorption. On the other hand, at lower temperatures, the separation process becomes less effective due to the slower CO₂ reaction rates with aqueous ammonia. The optimum temperature is found to be 22°C for an ammoniated brine solution containing 2.5 M NH₃ and 4.3 M NaCl [63]. The rise in temperature from 10 °C to 50 °C results in an almost 50% decline in CO₂ removal efficiency and a reduction in Na removal efficiency, dropping from 45.2% to less than 5% in a brine reject brine solution [64]. Increasing gas flow rate with increasing temperature worsened the decline in carbon capture efficiency, which can be attributed to the shortened residence time of the gas [65].

Ammonia concentration had a greater impact required to absorb larger amounts of CO₂ 66, 67, 68. Ammonia does not participate in the main reaction of the Solvay process, but it does have a crucial function in the intermediate reactions. The presence of ammonia maintains the solution at an alkaline pH, which is essential for facilitating the precipitation of sodium bicarbonate.

2.3. Sodium sulfate carbonation process

The sodium sulfate carbonation route presents a promising strategy for capturing CO₂ emissions, with a capacity to capture 1.04 kg CO₂/kg of NaHCO₃[28]. Technoeconomic analysis demonstrates its cost-effectiveness, favoring the production of baking soda over soda ash [69]. Originally designed as an upgrade to the Solvay process, this method eliminates the need for calcareous material and enables the extraction of gaseous HCl from NaCl. The process itself is complex involving the transfer of heat, mass, and momentum in three phases, and includes intricate reversible chemical reactions and crystal formation. Additionally, the chemical reactions involved in the process are influenced by the compound's solubility in relation to temperature [70], resulting in the formation of ammonium sulfate as an intermediary product. The following equations illustrate the involved reactions [34].(16)

The carbonation stage of the ammoniated sodium sulfate process involves an overall chemical reaction Eq. (24), with ions exhibiting affinities due to the standard negative Gibbs energy [71]. However, a significant limitation arises from the co-precipitation of ammonium sulfate with sodium sulfate in the form of a double salt compound, potentially rendering the method less commercially viable. While simplified batch experiments demonstrated the ability to generate sodium carbonate and purified ammonium sulfate simultaneously [72], the continuous prototype scale revealed undesirable process limitations, particularly the risk of contamination and simultaneous precipitation of undesired compounds [34]. Nevertheless, a technique has been proposed for producing high-quality baking soda and purified ammonium sulfate as a byproduct, suitable for commercial use as a fertilizer 73, 74.

Controlling reaction parameters plays a crucial role in ensuring the successful crystallization of pure NaHCO₃. Among these parameters, the flow of NH₃ is particularly significant in the production process. Increasing the flow rate of a liquid, would result in a tremendous amount of liquid being dispersed over the packing surface, which would cause an increase in the interfacial area of the two phases. This leads to a higher liquid mass transfer that is strongly related to carbon capture efficiency in a liquid-phase controlled mass transfer 75, 76. Research findings suggest that higher NH₃ flow rates are favorable for CO₂capture, with excessive flow rates showing minimal effect on process performance. On the other hand, the molar flow rate of mirabilite is another important parameter that affects the carbon capture efficiency and production of NaHCO₃. The gradual increment of the molar flow rate of mirabilite from 5 kmol/h to 8 kmol/h, increased the CO₂ capture efficiency from 49.21% to 59.24% [77]. Then it gradually flattened, because the content of feed water remained unchanged, and the mirabilite solution became saturated. The results show that the use of saturated mirabilite is beneficial for the improvement of CO₂capture efficiency.

Another essential parameter in the production of sodium bicarbonate is crystallizing temperature. Studies showed that too a high crystallizing temperature is not conducive to CO₂ capture and NaHCO₃ crystallization, and the optimal reaction temperature is at 35.5 ^◦C [77]. It has been observed that lower absorption temperatures lead to faster absorption rates under experimental conditions. This phenomenon can be attributed to the inverse relationship between temperature and the solubility coefficient of Henry’s law, as well as the absorption driving force. The rise in the quasi-steady-state partial pressure of CO₂ with increasing absorption temperature contributes to this effect [78].

2.4. Sodium Hydroxide (NaOH)

Many studies have been conducted to reduce CO₂ emissions by employing NaOH 76, 79, 80. The CO₂ absorption capacity of NaOH is significantly greater than that of monoethanol amine (MEA) absorbent, in which the estimated absorbing capacity of NaOH is 1.11 tons CO₂/ton NaOH while MEA is approximately 0.72 tons CO₂/ton MEA [81]. Furthermore, NaOH is readily available and less expensive in comparison to MEA [82]. In recent times, there has been considerable interest in the application of NaOH for the production of NaHCO₃, a method that significantly reduces industrial CO₂ emissions [83].

In order to analyze the reaction between NaOH and CO₂ more thoroughly, the various stages of the absorption mechanism must be considered. In the first place, the aqueous solution of NaOH rapidly and fully ionizes into Na⁺ and OH⁻ions because of its high alkalinity.(25)

Injecting gaseous CO₂ into a NaOH solution produces an aqueous solution of CO₂ [84]. When aqueous CO₂ interacts with a hydroxide ion, it produces bicarbonates. At first, there are many hydroxide ions in the solution. As a result, the bicarbonate ions interact with hydroxide to form carbonates as expressed in Eqs. (27), (28). The carbonate ions react with sodium ions in the aqueous solution, resulting in the formation of Na₂CO₃, Meanwhile, the remaining hydroxide ion concentration is reduced. The loading curve and real-time changes in pH confirm this.(26)

Reaction of Eq. (27) is a second-order reaction, Eqs. (27), (28) are reversible chemical reactions that occur rapidly in high pH ranges [85]. Reaction Eq. (28) is carried out immediately after reaction Eq. (27) [86]. With a continuous input of CO₂, the concentration of carbonate ions will be higher while the concentration of hydroxide ion will be reduced, hence, the pH of the solution decreased. According to Le Chatelier's principle, the reverse reactions in Eq. (28) will dominate, thereby increasing the amount of 

The overall CO₂ absorption reaction with NaOH in an aqueous solution can be described as reaction Eq. (31),(31)

The pH level is an essential indication for evaluating the progression of the reaction in order to determine the predominant form of carbon and maximizing both the CO₂ absorption capacity and the bicarbonate proportion. A higher concentration of bicarbonate ions results in a proportional increase in the removal of sodium ions, as stated in Le Chatelier's principle [87].

Yoo et al. [82] investigated a concentration of 1 to 5 wt.% of NaOH solution to capture a gas mixture with 31.5 vol.% CO₂ and 68.5 vol.% N₂ in a Pyrex reactor. The CO₂ absorption process was conducted in a series of sequential reaction steps, with Na₂CO₃ and NaHCO₃ being formed. During the Na₂CO₃ production range, the reaction rate and capture efficiency exhibited a significant dependence on the concentration of NaOH. However, during the NaHCO₃production phase, these parameters remained constant regardless of the NaOH concentration. The absorbed CO₂ mass ratio that took part in the trona, NaHCO_3, and Na₂CO₃ production reactions was calculated to be 1:17: 20, respectively. In another study, Leventaki et al. [88] utilized a 3D-printer to develop a specialized reactor for assessing the CO₂ absorption capabilities of NaOH solutions within a concentration ranging of 1 to 8 w/w%. When the concentration of NaOH reaches 5 w/w% or greater, solid carbonate and bicarbonate compounds are produced as a result of saturation. They obtained absorption capacities ranging from 9.5 to 78.9 g CO₂/L at NaOH concentrations of 1 w/w% and 8 w/w%, respectively.

Apart from that, the effect of temperature on the CO₂ capture efficiency of the NaOH system was investigated by Kordylewski et al. [89]. From the result, it was shown that by increasing the absorption temperature from 25 to 61.5◦C, the overall CO₂ capture efficiency can be improved by approximately 5–20%. It was also stated that after the formation of Na₂CO₃ post absorption by NaOH, the carbonate can still capture CO₂ to form NaHCO₃ but with a relatively low CO₂capture efficiency of 4–5%.

2.5. Electrochemical conversion of CO₂

The electrochemical conversion of CO₂ into valuable compounds is receiving increasing interest due to its versatility, capacity to tackle emissions from dispersed sources (such as the ocean and atmosphere), and its compatibility with an electrified industrial facility 90, 91, 92. The process of electrochemical conversion of carbon dioxide has been successfully shown in a fuel cell, which was achieved by employing a specific catalyst that facilitated the conversion of carbon dioxide into carbonate ions that served as a means of transferring charge species [93]. Recently, a multi-compartment electrodialysis device utilizing an effective combination of anion exchange membranes (AEM) and cation exchange membranes (CEM) has successfully proven the simultaneous removal of carbon dioxide in a mineralized state. The captured CO₂ typically contains impurities that need to be removed before further processing. After the captured CO₂ is fed in to the electrochemical cell, the CO₂ is electrochemically converted to sodium bicarbonate (NaHCO₃) in the presence of water (H₂O) and a suitable electrolyte at the cathode. The electrolyte can be a solution of sodium salts, such as sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃). These sodium salts dissociate in the electrolyte, releasing sodium ions (Na⁺) that can participate in the electrochemical reaction at the cathode. During the electrochemical reduction of CO₂, the sodium ions (Na⁺) combine with the bicarbonate ions (

The electrochemical reaction of water splitting involves the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode which can be presented:(32)

By injecting CO₂ into the cathode gas supply, where it reacts with the hydroxide ions produced at the cathode, 

The selection of catalysts on the electrode surface, together with the composition and concentration of the electrolyte, plays a crucial role in defining the effectiveness of the process in terms of capturing, transporting, and releasing CO₂, as well as the kinetics involved [94].

3.1. Modified Solvay process

Numerous desalination facilities face the challenge of identifying effective methods to manage or dispose of the substantial quantities of concentrated brine generated during the desalination operations. El- Naas et al. [64] examined the viability of the Solvay process as an innovative method for handling reject brine and CCU. In another study, Mohammad et al. [99] employed response surface methodology to maximize the CO₂ capture by establishing the primary operating parameters using ammoniated saline. The CO₂ capture efficiency attained at a maximum of 86%, while the Na⁺ reduction efficiency reached 33% under optimum conditions.

Although the traditional Solvay Process has already provided a suitable process to treat the problems of brine and CO₂, this method still has some disadvantages. The biggest issue due to high volatility ammonia at any temperature which has an impact on environment and human health 100, 101. This limitation makes this procedure challenging to scale up. Researchers have investigated the use of stable alkanolamines as substitutes for ammonia in the traditional Solvay process, yielding promising findings. The interaction between amines and CO₂ follows either the carbamate or bicarbonate pathway, depending on the amine type. Carbamates are produced by primary and secondary amines, while bicarbonates result from tertiary and sterically hindered amines [102]. When carbamate hydrolysis occurs, CO₂ absorption capacity can exceed predictions. The presence of brine enhances CO₂ uptake as NaHCO₃formation increases. Equilibrium is reached when pH drops below 9 or when all amines converts to amine chloride, explaining the increased CO₂ absorption efficiency in the presence of NaCl [103]. Huang et al. [104] reported notable results, with monoethanol amine (MEA) and methylaminoethanol (MAE) emerging as effective substitutes for ammonia, demonstrating a NaHCO₃ yield of 0.0143 kg and an absorption capacity of 0.9 mole per mol MAE at specific concentrations. Dindi et al. 105, 106 evaluated the enhancement performance of several alkylamines, including 2‐ amino 2‐ methyl propanol (AMP), piperazine (PZ), MEA, and methyl diethanolamine (MDEA) for their capacity to improve the Solvay process by substituting ammonia. AMP has been recognized as the superior amine to achieve high precipitation yields under conditions of high brine concentration, medium amine concentration, and lower temperatures.

Although amines show promising results in the carbonation phase, a key limitation is the inability to regenerate amine-chloride formed during the process. Furthermore, the degradation of amines can lead to the release of harmful substances, potentially polluting the brine.

Other than amine solvents, different metal oxides such as CaO/ Ca(OH)₂, KOH, and Al₂O₃ were examined to modify the traditional Solvay process. El-Naas et al. 107, 108 substituted NH₃ in the conventional Solvay method by CaO, as described by Eq. (36).(36)

Under optimized conditions, a maximal 35% Na⁺ removal and 0.92 g CO₂absorption capacity per gram of CaO were achieved. Using the industrial actual reject brine, which has a salt level of 63 g/L, and under optimum circumstances, it was possible to obtain a sodium removal rate of 43% [109]. Lee et al. [110]precipitated Ca (OH)₂ from brine and applied it to the modified Solvay process. The CO₂ capture was 0.6 mol/ L brine, and the Na production rate was 45% under Ca(OH)₂ of 25 g/L and a temperature of 15 °C [110]. The conditions for the precipitation of NaHCO₃ were consequently improved as the result pH stability at lower temperatures (10 or 17.5 °C) and increased basicity. This is because CO₂dissolves in brine to form carbonic acid (H₂CO₃), which then decomposes into bicarbonate ions at a low pH (high temperature). However, Ca(OH)₂ may react with CO₂ to precipitate CaCO₃ during the process, interrupting the combination of HCO₃⁻and Na⁺ that can lead to clogging of equipment. Therefore, the reactor’s design or experimental parameters should be improved to increase the NaHCO₃ production.

KOH is another alkaline agent, capable of greatly enhancing the effectiveness of CO₂ absorption due to its high solubility in purified brine and its capacity to sustain at pH value of 13.6. The overall reaction of the KOH based modified Solvay method is illustrated in Eq.(37) [111].(37)

Mourad et al. [111] compared the CO₂ absorption capacity of CaO/Ca(OH)₂, KOH, and Al₂O₃. The KOH in the modified Solvay process could achieve a CO₂uptake of 0.31 g CO₂ CO₂/g _KOH, while ≈ 0.92 g of CO₂ was captured by 1 g of CaO. On the other hand, using Al₂O₃ is discouraged due to its inability to capture CO₂ effectively. This ineffectiveness is attributed to its limited solubility and relatively low initial pH level of ≈8.7 during the reaction [111]. In other studies, a response surface methodology (RSM) evaluation was implemented to predict and optimize the experimental factors, and there were two different predictions. The first predicted optimal conditions were gauge pressure of 2 bar, gas flow rate of 776 mL/min, and KOH of 30 g/ L. These conditions showed CO₂ uptake of 0.5 g CO₂ g/g KOH and a Na production rate of 45.6%. The second prediction was under a temperature of 10 °C, CO₂ gas flow rate of 848.5 mL/min, gauge pressure of 2.1 bar, and KOH concentration of 110 g/L. The results revealed an optimum CO₂ absorbance value of 0.58 g/g KOH and Na production rate of 44.1% 112, 113.

Dindi et al. [114] proposed metal oxide absorbent obtained by calcining Mg–Al layered double hydroxide (LDH) can be applied. LDHs are made of alternating layers of positively charged di or tri-valent cations and negatively charged balancing anions, which can remove chloride and generate hydroxide in the solution according to Eq.(41). The brine solution, which is a combination of NaCl and NaOH, is subsequently utilized to capture CO₂ in order to produce NaHCO₃. Furthermore, calcination allows the regeneration of the solvent. The regeneration process produces chemicals containing Cl^¯, including Cl₂ gas and HCl, as byproducts.(39)

The author concluded that the carbonated solution exhibited an approximate CO₂ absorption capacity of 0.082 g CO₂/g. The Na⁺ ions were reduced by 20%, and the process produced 0.044 kg of NaHCO₃ /kg carbonated solution. Nevertheless, further examination is necessary regarding the application of mixed metal oxides, wherein the primary objective is to identify circumstances that optimize CO₂ capture and NaHCO₃ production in the modified Solvay process.

In addition to reject brine, other industrial alkaline wastes have been studied to promote pH and form NaHCO₃ through reactions with CO₂ which can stabilize and safely dispose of these environmental wastes using modified Solvay process. The utilization textile dye bath effluent to convert industrial flue gas to sodium bicarbonate is investigated by Krishnaveni and K. Palanivelu [97]. The maximum sodium ion (Na⁺) removal efficiency of 38% is achieved under the optimal reaction parameters. However, the absence of individual separation stages results in low purity of the product, which restricts its industrial application.

3.2. Mineral carbonation

Apart from the Solvay Process, mineral carbonation proved to be successful in generating carbonated goods of better purity and improving the efficiency of carbonation. Indirect CO₂ carbonation technologies consist of two stages: an electrolysis stage, which involves the extraction of alkaline from brine, seawater, or industrial byproducts; and a carbonation stage, which involves the reaction of the alkaline with CO₂ produced from emission sources in order to yield minerals 115, 116, 117. In specific instances, alkaline solution derived from naturally occurring minerals are employed directly [118], while intermediate substances utilized in the carbonation process are procured externally [27].

The Capitol Aggregate San Antonio Cement Plant in San Antonio, Texas, USA developed SkyMine® employs an indirect mineral carbonation process to produce NaHCO₃. The plant utilized Sodium hydroxide NaOH solution that captures approximately 90% of CO₂ from a flue gas split stream and produces hydrochloric acid, bleach, and sodium bicarbonate. The NaOH is obtained by the process of electrolysis of a sodium chloride (NaCl) solution [24]. Fig. 2 illustrates the schematic diagram of the SkyMine® process 119, 120. The initiative has generated 1.4 kilotons of sodium bicarbonate annually from 7.5 kilotons of carbon dioxide since its inception [121]. The SkyMine® technique exhibits a 30% reduction in energy consumption when compared to the conventional amine procedure for carbon capture.

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Fig. 2

Lee et al. [117] performed a bench-scale experiment to test the efficiency of the mineral carbonation process using NaOH and obtained a CO₂ conversion rate of 95%. Furthermore, the resulting NaHCO₃ has 97% or above purity, showing that it is suitable for industrial use. In addition, a specialized high-ion conductive membrane was utilized in the saline water electrolysis stage, which is the highly energy-intensive phase in the mineral carbonation plant. This implementation resulted in a minimum 8% reduction in electrolysis energy consumption [122]. A mineral carbonation plant at a pilot size was established using the findings from evaluating the performance of a smaller unit at a bench scale. However, the mineral carbonation technology proposed in this study would need significant cost reductions to be a viable alternative to CCS technology for large-scale CO₂processing and greenhouse gas reduction.

In saline water containing high concentration of calcium and magnesium ions, the purpose of brine mineral carbonation by using pH-swing is to enhance the dissociation of CO₂ by introducing alkalinity and separate the calcium and magnesium ions to get high-purity carbonated products and avoid the precipitation competition between CaCO₃ and MgCO_3. When calcium and magnesium components are separated, sodium cations and chloride anions are the most prevalent ions in the remaining solution that can be used to precipitate sodium bicarbonate. Kang et al. [12] employed NaOH as a pH adjuster for separating calcium and magnesium ions in seawater-derived effluent, while CO₂was introduced in its ionic form using alkanolamine absorbents. The remaining NaCl solution was then subjected to react with CO₂ to precipitate NaHCO₃. According to the CO₂ balance evaluation, NaHCO₃ precipitation contributed the greatest CO₂ uptake compared with CaCO₃ precipitation and MgCO₃precipitation. Kim et al. [123] developed a two-stage indirect carbonation of seawater- based industrial wastewater, which involves the separation of cations and conversion in to value added products. In the first step, the aqueous NaOH solution was fully saturated with CO₂ and high-purity (92.2%) NaHCO₃ was produced. In the next step, wastewater was pretreated with Na₂SO₄ to remove the Ca²⁺ ions to ensure the high purity of the metal carbonates in the second mineral carbonation step. The pretreatment resulted in Ca²⁺ free wastewater, which was used to produce MgCO₃ ·3H₂O in the second carbonation step. In addition, because NaOH can be obtained by seawater electrolysis, this process was considered as a practical method to mineralize CO₂ as high-purity metal carbonates.

Moreover, other alkaline waste materials are utilized in mineral carbonation processes, which can help in reducing the overall process costs and minimizing negative environmental impacts. In a study by Na et al. [124], waste bittern from solar salt production was investigated for its potential use in magnesium (Mg) production and CO₂ capture through a two-step process. Initially, they recovered the dissolved Mg²⁺ from waste bittern as Mg(OH)₂ precipitates by adding NaOH. Subsequently, the dissolved sodium ions (Na⁺) in the residual waste bittern were recovered as sodium bicarbonate (NaHCO₃) by reacting with CO₂. They achieved >99% Mg precipitation from bittern at NaOH/Mg molar ratios of 2.70–2.75 and pH 9.5–10.0. Moreover, recovery of sodium bicarbonate precipitate resulted in the sequestration of 63 g CO₂ per liter of bittern. In another study, Villardi et al. [125] assessed the capacity of produced water, which is a byproduct of petroleum extraction, to reduce CO₂ and salinity requirements. The analysis revealed a sodium chloride conversion rate of 44.5% into sodium bicarbonate, which can potentially sequester 250,000 tons of CO₂ annually. Additional research is necessary to analyze the impact of utilizing various forms of sodium-rich alkaline industrial waste to improve pH levels and generate valuable goods, hence enhancing the sustainability of this process.

3.3. Electrochemical CO₂ conversion using saline wastes

Several studies investigated the utilization of saline wastes using electrochemical CO₂ capture technology with different electrolytes. Rau et al. [126] investigated the conversion of silicate minerals into NaHCO₃ as primary product, accompanied by silica, metal sulphate, and hydrogen in the presence of sodium sulfate electrolyte. The industrial deployment of electrolysis requires both flexibility and stability in order to effectively treat various saline wastes. Acosta-Santoyo et al. [127] combined electrolysis, reactive absorption, and crystallization to produce pure NaHCO₃, hypochlorite, and hydrogen using alkali generated in the chloralkaline process to fix carbon dioxide in highly saline wastewater. The results show over 90% CO₂ capture efficiency with an energy requirement of approximately 0.65 kWh per kilogram of CO₂, making it promising for sustainability, particularly with the potential use of green energy sources.

Dara et al. [128] proposed a new configuration of the electrochemical cell that utilizes a platinum catalyst integrated that facilitated the conversion of CO₂ into carbonate and bicarbonate salts. The electrochemical cell comprised five compartments, including the cathode, anode, acid, base, and saline feed solution, which were partitioned via cation and anion exchange membranes. It was found that possible catalyst poisoning was the cause of the substantial decline in cell efficiency that accompanied an increase in CO₂. The electrochemical cell, consisting of five compartments, exhibited commendable flexibility, exceptional stability, and enhanced process control. Fig. 3(a) depicts a diagram of the electrochemical cell. In their subsequent study, Dara et al. 128, 129 conducted a comparison between the rate at which carbonic acid is formed and the rate at which CO₂ is present without being converted into carbonic acid. It was shown that utilizing CO₂ directly results in a product production flow that is 40 times greater than when using carbonic acid. The limited ability of CO₂ to dissolve in water is addressed by employing a gas-fed electrode in the cathode compartment. This electrode facilitates the conversion of the gaseous mixture (CO₂ and oxygen) and salty water into valuable products. Fig. 3(b) shows the technical visibility of the approach for the production of bicarbonate salts and acids.

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Fig. 3

The energy cost serves as the primary performance measure, quantifying the amount of energy that is wasted in the process of CO₂ utilization. Thiruvenkatachari et al. [130] carried out economic evaluation of electrochemical CO₂ utilization processes to reveal their impact on the post combustion process integrated with a 600 MW power plant. The method comprises the generation of electrical energy and the synthesis of NaHCO₃ through a chemical reaction. This is achieved by establishing a pH difference between the electrodes using amine-CO₂, sodium salt brine waste, and alkaline fly ash (Ca(OH)₂) solutions. The integration process showed a reduction in CO₂ foot print of approximately 6.4% and 17.8% for MEA and AMP/PZ systems respectively. In another study, an electrochemical CO₂ mineralization approach also proposed for treating red mud waste from the aluminum industry [131]. The authors employed a system utilizes hydrogen-cycled membrane electrolysis to efficiently mineralize CO₂, recover carbon and sodium resources, and produce high-purity NaHCO₃. The system demonstrates high electrolysis efficiency, with an average efficiency of 95.3% and a purity of 99.4% for the obtained NaHCO₃ product. The process operates at a low electrolysis voltage of 0.453 V at 10 mA/cm², indicating potential for low energy consumption in industrial applications.

6.1. Solvent development

The primary objective in the development of novel solvents is to enhance energy efficiency, increase stability against oxidative and thermal decomposition, and reduce emissions [154]. It has been demonstrated that the modified Solvay route could potentially reduce total equivalent work by 70% and operation energy by 30% when compared to the traditional Solvay process [106]. For example; the thermodynamic study showed that development of Ca-based Solvay process emerged as more spontaneous and effective, demonstrating superior performance in CO₂ capture and energy consumption than the standard NH₃-based Solvay process. This finding underscores the significance of exploring innovative processing routes and materials in solvent development. One intriguing avenue for future research lies in the exploration of solvents capable of reducing the solubility of NaHCO₃ while maintaining high reaction rates. By investigating alternative chemical routes, optimizing CO₂ utilization, and leveraging thermodynamic analyses, researchers can further refine solvent development strategies to align with sustainability goals and industry demands.

6.2. Process intensification

Process intensification refers to the incorporation of several unit activities inside a process or the new development of process units, with the aim of reducing equipment size and cost, minimizing emissions, and enhancing productivity. Process intensification techniques can be categorized as intensified equipment and intensified methods 155, 156. The design and configuration of a reactor play a crucial role in enhancing the reaction kinetics and internal hydrodynamics of processes, while also reducing costs and increasing energy efficiency in the production of NaHCO₃. Researchers developed and investigated a novel Inert Particle Spouted Bed Reactor (IPSBR) to address CO₂ capture and reject brine management in the Solvay process [107]. The study reported an impressive CO₂capture efficiency of up to 97.7% with a gas-to-liquid volume ratio of 123 [107]. The increased effectiveness of CO₂ capture and ion removal was a result of the rotary movement exhibited by the particles within the reactor [157]. Validation of the results was performed using a bench-scale IPSBR, showing satisfactory results between experimental findings and modelling analysis 35, 158, 159.

According to different studies, bi-carbonation occurs at a smaller rate compared to carbonation during production of NaHCO₃[117]. To solve this, Shim et al. [18]developed distinctive continuous reactor design, as shown in Fig. 6, of an integrated a bubble column connected to structured packing bed reactor. The researchers investigated the effectiveness of CO₂ capture at room temperature reacted with NaOH solution generated through NaCl electrolysis. A maximum of 95% CO₂ capture and 97% NaHCO₃ generation were achieved. However, the process has drawbacks due to its high energy expenditure and complex reactor design [35]. Similarly, Lee et al. [117] employed a series connection of two columns to compensate for the comparatively sluggish pace of the bi-carbonation process. After carbonation occurred in the packed column, which is the initial reactor, bi-carbonation took place in the bubble column. However, the kinetics study which greatly contributes to increasing and boosting the reaction rate, has not been well examined. This can also facilitate the expansion of this method to the industrial scale.

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Fig. 6

Zhang et al. [160] has made additional progress by utilizing catalytically active nickel nanoparticles (NiNPs) within a tubular reactor with continuous plug-flow. The reactor, featuring a helically coiled tube, facilitated controlled and efficient mixing without the need for mechanical agitation. It allowed easy optimization, integration, and scalability for industrial applications, accommodating both homogeneous and heterogeneous reactions for continuous production.

Membrane technology is a viable alternative to packed columns which has recently played a crucial role in the advancement of modern CO₂ separation technology. Membrane contactors provide several benefits, including high interfacial area, operational flexibility, simple scaling, and independent gas and liquid operation [161]. Thus, they are able to offer a larger mass transfer and accomplish a quicker rate of mass transfer in the carbonation process [162]. Hwang and his colleagues introduced a novel approach employing hollow fiber modules with highly permeable membrane to directly utilize CO₂ from the flue gas, aiming to reduce initial expenditure by a minimum of 30% [163]. Nevertheless, the accumulation of substances on the membrane surface and the saturation of pores from post-combustion flue gas components inhibit the efficiency of the membrane devices. Further research is necessary to address this issue [164].

6.3. Application of Catalyst

A highly effective additive/catalyst could help to accelerate the reaction rate, lower temperature threshold, and promote yield. When choosing promoters for the precipitation process, it is widely acknowledged that chemical properties such as rapid absorption rate, the chemical effect on the aqueous solution on decreasing the solubility of NaHCO₃ and could increase its supersaturation tendency should be prioritized. Carbonic anhydrase (CA) is among the most promising catalysts capable of accelerating the CO₂ bi-carbonation reaction by a significant amount [165]. Fig. 7(a, b) illustrates the structure and CO₂ catalyzing mechanism of CA. It catalyzes the reversible hydration of CO₂ to 

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with very high rates [166]. When CO₂ molecule comes closer to the enzyme, it is strongly drawn and forms a strong bond with the hydroxyl group, resulting in the formation of a bicarbonate ion attached to the metallic ion. In addition, the 

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ion flows into the solution after it is removed from the active site by a water molecule. This occurs during the mass transfer limiting phase, which is the rate-determining step of the reaction [167]. During this phase, the metal-hydroxyl bond is regenerated through a proton transfer from the water molecule which driven by either the amino-acid chains present in the CA structure or the buffers in the surrounding solution. The proton is ultimately bonded to the buffered media, thereby completing the catalytic cycle of CA [168]. The study by Mourad et al. [169] investigated the rate of CO₂ absorption by potassium hydroxide (KOH) and concentrated brine with and without bovine carbonic anhydrase catalyst (BCA). Using an inert particle-spouted bed reactor, the researchers conducted semi-batch experiments to examine the enhanced CO₂ absorption in reject brine solutions with varying KOH concentrations exposing it to a continuous flow of a gas mixture containing 10% CO₂ and 90% air. They found that the utilization of 0.003 grams per liter of BCA catalyst leads to 181% significant enhancement in CO₂ capture in comparison to the scenario where catalyst is not used as shown in Fig. 7(c, d). On another research, the efficacy of BCA to enhance the rate of CO₂ water in high content of magnesium salt using NaOH was evaluated. The rates of CO₂ absorption were increased by approximately 600% [170].

 

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Fig. 7

Nevertheless, despite the remarkable potential of CA to absorb CO₂ and operate as a catalyst, its practical implementation in commercial facilities is impeded by reduced catalyst activating and strength, which prevents its sustained utilization. In addition, there are concerns about high production costs and the possible depletion of the CA during carbonation process. Consequently, other investigations are endeavoring to address these problems [171].

Organic promoters, such as alcohols and polyols, can enhance absorption rates by virtue of the hydroxyl groups in their molecules, which have the ability to establish hydrogen bonds. Czaplicka et al. [172] studied the improvement of CO₂capture using organic compounds such as neopentyl glycol, glycerol, isopropanol, methanol, pentaerythritol, and ethylene during the synthesis of CaCO₃ by carbonation method. The results revealed that found that neopentyl glycol showed the highest carbon capture efficiency (86.1%). On another study, Chiang et al. [173] indicates that adding glycerol to an aqueous solution of NaOH can increase the overall chemical absorption efficiency of CO₂ by over 90%. By increasing the rotation speed from 600 rpm to 1500 rpm, it was observed that the mass transfer coefficient increased by a factor of five and the absorption percentage and enhancement factor were doubled or tripled, respectively. Mun et al. [174] investigated the mineral carbonation process that results in the formation of sodium bicarbonate (NaHCO₃) by carbonating 3 g of an aqueous solution containing dissolved ethanol and sodium hydroxide (NaOH), with the ethanol concentration varying between 50.5% and 97%. The production of NaHCO₃ precipitates in this area increases parabolically with the ethanol concentration.

Another type of suitable organic promoter studied to enhance the rate of absorption in the modified Solvay process are amino acid salts 175, 176, 177. They are inexpensive, safe for the environment, have high absorption rates_, and lower vapor pressure [178]. Amino acids are organic compounds containing amine(−NH₂) and carboxyl (−COOH) functional groups, along with a side chain (R group) specific to each amino acid. There are 20 different types. Based on their stereochemistry, amino acids are defined by D and L enantiomers. The 20 amino acids are L-isomers, and their enantiomeric D-isomers are rarely found in nature [179]. Govindaraj et al. [176] employed amino acids (Glycine, L-arginine, and L-alanine) to improve the standard Solvay method and thereby boost the efficiency of Na⁺ recovery from textile dye bath effluent to produce NaHCO₃. Arginine showed highest performance in conversion efficiency while also lowering the process’s ammonia need. However , although adding amino acid salts improved K_G, the raising of NH₃ vaporization is the drawback of this application 175, 180. Further research is needed explore new catalysts and mixed solvents that can enhance the bicarbonation reaction and required to improve the process for high CO₂ capture from exhaust gases and production of pure NaHCO₃.

The addition of inorganic compound can also accelerate the CO₂ absorption rate. Recently, a very small amount (30 ppm) of nickel nanoparticles was utilized which resulted in carbon capture efficiency of 96.7%, in comparison with the case without catalyst [160]. The use of nickel nanoparticles led to a carbon dioxide gas utilization efficiency of 96.7% compared with only 87.8% without a catalyst. Although an inorganic heterogeneous catalyst can enhance the absorption rate, an analysis of solid particles with the presence of inorganic salts reveals the existence of small quantities of these metal ions in the final product, which is unfavorable for its future uses [181].

6.4. Renewable energy integration

The mismatch between energy supply and demand has resulted in significant emissions from power plants with increased energy production. To address this, a focus on capturing and storing greenhouse gas emissions is crucial, especially through renewable energy sources, for effective and economically viable emission reduction. The United Nations' thirteenth sustainable development goal aims for zero emission fuels by 2050, a target achievable through the utilization of renewable energy resources as highlighted in reference [182].

Bonaventura et al. [54] conducted an economic analysis of an integrated soda ash carbonation process facilitated by renewable energy sources for the synthesis of NaHCO₃. In terms of energy, utilizing solar energy for heating in the plant resulted in 3% decrease in economic efficiency. The study concluded that sodium bicarbonate production through this process is feasible, utilizing inexpensive and abundant trona as a raw material, solar energy for heating, and enabling carbon dioxide capture and reuse with minimal emissions. Furthermore, investigations reveal that by retrofitting the soda ash carbonation process to a 600 MW coal-fired power plant, a local minimum energy penalty of 3.99% at a 90% CO₂ capture efficiency and 99.9% CO₂ outlet purity could be achieved [57].

As mineral carbonation uses electrolysis for the production of alkali agents, the cost of electricity is a hindrance to near-term commercialization. Using a renewable source, such as solar energy, the cost of electricity for electrolysis could be reduced, thus the CCU process can be operated cheaply. For example, 1 kg of MEA costs US$2–4, and this “cost problem” can be overcome by electrolysis using renewable energy. On the other hands, 1 kg of NaOH cost US$ 0.2–0.3 [183]. However, substantial cost reductions and advance technology development are necessary for the mineral carbonation technology to become techno-economically feasible alternative to CCS technology for large-scale CO₂processing and greenhouse gas reduction.