KOH-Based Modiﬁed Solvay Process for Removing Na Ions from High Salinity Reject Brine at High Temperatures

: The traditional Solvay process and other modiﬁcations that are based on different types of alkaline material and waste promise to be effective in the reduction of reject brine salinity and the capture of CO 2 . These processes, however, require low temperatures (10–20 ◦ C) to increase the solubility of CO 2 and enhance the precipitation of metallic salts, while reject brine is usually discharged from desalination plants at relatively high temperatures (40–55 ◦ C). A modiﬁed Solvay process based on potassium hydroxide (KOH) has emerged as a promising technique for simultaneously capturing carbon dioxide (CO 2 ) and reducing ions from reject brine in a combined reaction. In this study, the ability of the KOH-based Solvay process to reduce brine salinity at relatively high temperatures was investigated. The impact of different operating conditions, including pressure, KOH concentration, temperature, and CO 2 gas ﬂowrate, on CO 2 uptake and ion removal was investigated and optimized. The optimization was performed using the response surface methodology based on a central composite design. A CO 2 uptake of 0.50 g CO 2 /g KOH and maximum removal rates of sodium (Na + ), chloride (Cl − ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) of 45.6%, 29.8%, 100%, and 91.2%, respectively, were obtained at a gauge pressure, gas ﬂowrate, and KOH concentration of 2 bar, 776 mL/min, and 30 g/L, respectively, and at high temperature of 50 ◦ C. These results conﬁrm the effectiveness of the process in salinity reduction at a relatively high temperature that is near the actual reject brine temperature without prior cooling. The structural and chemical characteristics of the produced solids were investigated, conﬁrming the presence of valuable products such as sodium bicarbonate (NaHCO 3 ), potassium bicarbonate (KHCO 3 ) and potassium chloride (KCl).


Introduction
The Solvay process is an effective approach for treating carbon dioxide (CO 2 ) produced from fossil fuel power plants and brine rejected from desalination industries in a single reaction [1]. Many factors affect the performance of the combined process, such as the reaction temperature, alkaline type, and solution pH. Among these variables, temperature has the greatest impact on the process because it controls the solubility of CO 2 and metal ions in the brine [1]. Generally, high temperatures have a negative effect on the solubility of CO 2 and decrease the precipitation of metal ions such as sodium (Na + ) ions, which are present in the brine at high concentrations compared with other ions such as calcium (Ca 2+ ), magnesium (Mg 2+ ), and chloride (Cl − ). The main objective of the traditional Solvay process is to produce insoluble sodium bicarbonate (NaHCO 3 ) via the reaction of the sodium chloride with CO 2 in the presence of ammonia (NH 3 ), according to Equation (1) [2]. NaCl + NH 3 + CO 2 + H 2 O ↔ NaHCO 3 + NH 4 Cl (∆G 20 C = −59.7 kJ/kmol) (1) Mohammad et al. [2] investigated the parametric sensitivity of the traditional Solvay process to optimize the conditions of the entire process. The response surface methodology (RSM) using Minitab 19.0 software was employed to optimize the responses under different ranges of temperature (13.2-46.8 • C), CO 2 gas flowrate (0.659-2.341 L/min), and NH 3to-sodium chloride (NaCl) molar ratio (1.7-3.3). All experiments were performed at atmospheric pressure. A maximum CO 2 capture efficiency of 86% and Na + removal efficiency of 33% were achieved at a temperature, gas flowrate and NH 3 :NaCl molar ratio of 19 • C, 1.54 L/min, and 3.3, respectively. Moreover, increasing the temperature from 13.2 • C to 46.8 • C caused a remarkable decrease in the CO 2 and Na + removal efficiency. Palitsakun et al. [3] investigated the impact of temperature and CO 2 flowrate on the performance of the traditional Solvay process based on NH 3 , obtaining the maximum NaHCO 3 amount of 20.67 wt. % at a temperature, CO 2 flowrate, and NaCl concentration of 20 • C, 0.1 L/min, and 3 M, respectively, using a 43 wt. % NH 3 solution. Moreover, a significant reduction in the CO 2 capture efficiency from~93% to 80% was observed when the temperature was increased from 20 • C to 38 • C.
Although NH 3 was used as a catalyst to enhance the reaction rate by increasing the pH, NH 3 is environmentally hazardous and its recovery is costly [2]. In this context, a calcium oxide (CaO)-based modified Solvay process (Equation (2)) was studied as an alternative by El-Naas et al. [4,5]. This process not only avoids the harmful impacts of the conventional Solvay process, but also achieves better performance in terms of CO 2 capture, Na + removal, and energy consumption. Furthermore, CaO could maintain a high pH value of 11.8. 2NaCl + 2CO 2 + Ca(OH) 2 ↔ 2NaHCO 3 + CaCl 2 (∆G 20 C = −56.0 kJ/kmol) (2) These authors employed RSM to optimize the entire process, achieving a maximum CO 2 uptake of 0.92 g CO 2 /g CaO and Na + removal of 35% under optimum conditions of 20 • C temperature, atmospheric pressure, 1 L/min CO 2 flowrate, and 20 g/L CaO (more than the solubility limit). Increasing the temperature from 20 • C to 50 • C led to a significant reduction in Na + removal from 35% to nearly 5%, resulting in less precipitation of NaHCO 3 [4].
Dindi et al. [6] examined the effectiveness of mixed metal oxides on CO 2 capture and reject brine management in a combined process. A CO 2 uptake of 0.082 g CO 2 /g carbonated solution and a Na + reduction of 20% in reject brine were achieved at 25 • C and 1 bar. Shim et al. [7] investigated the CO 2 capture efficiency at ambient temperature by reacting it with a sodium hydroxide (NaOH) solution produced from NaCl electrolysis. Up to 95% of CO 2 capture and 97% NaHCO 3 production were obtained. The entire process was conducted at ambient temperature and pressure. Recently, Ibrahim et al. [8] examined the use of steel-making bag house dust (BHD) as a potential source of CaO in the modified Solvay process. A maximum CO 2 uptake of (1 ± 0.04) g CO 2 /g BHD was achieved at ambient temperature and a pressure of 5 bar. Moreover, the precipitation of NaHCO 3 occurred at a low temperature of 15 • C.
Recently, Mourad et al. [9] investigated the effectiveness of potassium hydroxide (KOH) in the Solvay process. KOH was selected owing to its potential in directly capturing CO 2 from ambient air [10][11][12][13][14][15]. The overall reaction of the modified Solvay process based on KOH as the alkaline material can be described using Equation (3) [9].
This process was studied under the same optimized conditions of temperature, pressure, and gas flowrate previously reported by El-Naas et al. [4], where CaO was used as Sustainability 2021, 13, 10200 3 of 18 the alkaline material instead of NH 3 . The initial results revealed that KOH is a promising alkaline material, which can significantly accelerate CO 2 capture efficiency owing to its high solubility in treated brine and its ability to maintain a high pH value (∼13.6). The CO 2 uptake and Na + removal reached up to 0.31 g CO2/g KOH and 29%, respectively, at a temperature, pressure, gas flowrate, and KOH concentration of 20 • C, 1 atm, 1 L/min, and 70.57 g/L, respectively. Moreover, the main products formed in this reaction, such as NaHCO 3 and potassium chloride (KCl), have various industrial applications [9].
Based on the above literature review, temperature has been found to be a vital parameter that significantly affects the combined process in terms of metal solubility and CO 2 uptake. Notably, low temperature conditions are generally preferred to achieve high CO 2 capture and ion removal from reject brine. However, brine is commonly discharged from desalination plants at relatively high temperatures of 40-55 • C [16]. To the best of the authors' knowledge, no previous works have achieved the optimization of the KOHbased Solvay process to achieve high CO 2 capture and ion removal at high temperatures. Therefore, in this study, the optimal operating conditions (gas flowrate, pressure, and KOH concentration) were experimentally investigated to achieve the maximum ion removal and CO 2 uptake at relatively high temperatures (40-60 • C). Moreover, the structural and chemical characteristics of the precipitated solids were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy.

Materials
A gas mixture comprising 10% CO 2 and 90% air was obtained from Abu Dhabi Oxygen Company, UAE. KOH with a particle size of 5-7 mm and a purity of 85-100% was acquired from Scientific Progress Medical and Scientific Equipment Company, UAE. The real samples of reject brine were supplied by a multistage flash desalination unit at Abu Dhabi, UAE. The concentrations of Ca 2+ , Na + , potassium (K + ), Mg 2+ , and Cl − were 933.1 ± 3.3, 23,063.1 ± 7.4, 877.1 ± 4.1, 2675.8 ± 6.1, and 48,330 ± 4.2 ppm, respectively. The pH of the reject brine was 8.02 ± 0.01. The ion concentration in the reject brine was measured using inductively coupled plasma (ICP) spectrometry (Varian 710-ES ICP optical emission spectrometer). Cl − was analyzed using a HACH HQ40D portable multimeter equipped with a Cl − ion-selective electrode (HACH Intellical™ ISECL181). Additionally, the pH was measured using a HACH portable pH meter (Model HQ11D53000000). All measurements were determined by considering the average value of three collected reject brine samples.

Experimental Procedure
In this research, all experiments were conducted in a novel inert-particle spouted bed reactor (IPSBR) developed by El-Nass et al. [17,18]. Figure 1 shows a schematic diagram of the reactor [9]. The inert particles were composed of poly(methyl methacrylate) with an average density and diameter of 1020 kg/m 3 and 0.013 m, respectively. Particles with a volume fraction (volume of particles/volume of reject brine) of ∼6% were used. The internal diameter, height, and total working volume of the reactor were 0.078 m, 0.850 m, and 3000 mL [18][19][20][21]. In each experiment, a magnetic stirrer was used to mix a certain amount of KOH with 1 L of reject brine. Initially, the IPSBR was filled with the prepared solution and then exposed to a continuous flow of a CO 2 -air gas mixture via an orifice diameter of 2 mm from the bottom of the reactor [18]. The gas flowrate was controlled using a mass flow controller. To determine the amount of captured CO 2 , the outlet gas stream was continuously passed through a CO 2 gas analyzer. The pressure was controlled using a backpressure regulator valve at the outlet of the reactor. Each experiment was considered complete when the saturation point was reached, i.e., when the CO 2 concentration at the outlet was equal to the inlet value (10 vol. %). The pH of the mixture was measured before and after the reaction with CO 2 gas. At the end of each experiment, the collected mixture was filtered to separate the liquid from the produced solids using a Büchner setup. The ion Sustainability 2021, 13, 10200 4 of 18 concentration of the treated brine was measured using ICP spectrometry. The percentage of ion reduction and CO 2 uptake was calculated using Equations (4) and (5), respectively [4]. The characteristics of the solids were investigated using different analytical techniques, which is discussed in more detail in Section 4.
where X i is the initial ion concentration in the feed brine (mg/L) and X f is the final ion concentration in the treated brine (mg/L). CO 2 uptake value (g CO 2 /g KOH) = mass of CO 2 reacted (g) mass of KOH used (g) (5)  was equal to the inlet value (10 vol. %). The pH of the mixture was measured before and after the reaction with CO2 gas. At the end of each experiment, the collected mixture was filtered to separate the liquid from the produced solids using a Büchner setup. The ion concentration of the treated brine was measured using ICP spectrometry. The percentage of ion reduction and CO2 uptake was calculated using Equations (4) and (5), respectively [4]. The characteristics of the solids were investigated using different analytical techniques, which is discussed in more detail in Section 4.
where is the initial ion concentration in the feed brine (mg/L) and Xf is the final ion concentration in the treated brine (mg/L).

Experimental Design
To explore the effects of independent variables on the CO2 uptake and ion removal efficiency, a central composite design (CCD) was employed as an optimization tool for RSM using Minitab 19.0 software [20,22]. To determine the optimum conditions, four process-independent variables were considered, namely, temperature, gauge pressure, KOH concentration, and CO2 flowrate, and their influence on CO2 uptake and ion removal percentage (Ca 2+ , Na + , K + , Mg 2+ , and Cl − ) was investigated. Table 1 presents the levels established for the operating parameters. A total of 31 experimental runs was generated in a specific sequence using RSM based on the levels of each factor. After conducting all the

Experimental Design
To explore the effects of independent variables on the CO 2 uptake and ion removal efficiency, a central composite design (CCD) was employed as an optimization tool for RSM using Minitab 19.0 software [20,22]. To determine the optimum conditions, four process-independent variables were considered, namely, temperature, gauge pressure, KOH concentration, and CO 2 flowrate, and their influence on CO 2 uptake and ion removal percentage (Ca 2+ , Na + , K + , Mg 2+ , and Cl − ) was investigated. Table 1 presents the levels established for the operating parameters. A total of 31 experimental runs was generated in a specific sequence using RSM based on the levels of each factor. After conducting all the experiments, all responses were inserted in the RSM for optimization and predicting the Sustainability 2021, 13, 10200 5 of 18 ability of the entire process to achieve a good level of CO 2 capture and ion removal at a relatively high temperature of 50 • C.

Results and Discussion
In the following sections, the investigation of the impact of the operating parameters and their interactions on CO 2 uptake and ion removal is described. The relation between the independent variables, i.e., temperature, pressure, flowrate, and KOH concentration, and responses can be represented using three-dimensional (3D) plots (Figures 2-5).

Impact of Independent Factors on CO 2 Uptake
Figure 2a reveals a significant linear relation between pressure and CO 2 uptake. The CO 2 uptake of nearly 0.76 g CO 2 /g KOH was achieved at a maximum gauge pressure of 3 bar, followed by a decrease to almost half the maximum value, around 0.38 g CO 2 /g KOH, at a gauge pressure value of 1.5 bar over the whole range of concentration. These experiments were performed at a constant temperature and gas flowrate of 30 • C and 1000 mL/min, respectively. This considerable effect of the pressure on CO 2 uptake was expected; as the pressure increased, more CO 2 was solubilized, shifting the reaction (Equation (3)) toward the product side. The 3D plot shown in Figure 2b reveals that the maximum CO 2 uptake was obtained at a maximum gas flowrate of 1600 mL/min and a low temperature of 10 • C, owing to the high solubility of CO 2 gas at low temperatures [1,4,8]. Furthermore, CO 2 uptake still reached a value of up to 0.53 g CO 2 /g KOH at a temperature of 30 • C and a low gas flowrate of nearly 400 mL/min. This observation confirms the effect of gas flowrate on CO 2 uptake, where a low gas flowrate resulted in a high residence time and hence high CO 2 capture [4,8]. This result reflects the novelty of the IPSBR [18][19][20], which can still operate under high feed-gas flowrate and achieve high CO 2 uptake under certain conditions. Figure 2c demonstrates the impact of the interaction between temperature and pressure on the CO 2 uptake. As a general conclusion, the uptake increases with the pressure at a constant temperature. This is for the studied ranges of temperature (10-50 • C) and gauge pressure (1-3 bar). These results were achieved under a constant gas flowrate of 1000 mL/min and a KOH concentration of 70 g/L. Figure 3a,b present the impact of independent parameters on Na + reduction. Figure 3a shows good Na + removal over the range of temperatures. These data were measured under a constant gauge pressure of 2 bar and a KOH concentration of 70 g/L. Further, Figure 3b depicts the effect of the interaction between temperature and KOH concentration on the response, which was evaluated under a constant gauge pressure of 2 bar and a gas flowrate of 1000 mL/min. The maximum Na + removal efficiency was achieved at a low temperature and a high concentration of 110 g/L, which was beyond the stoichiometric value (70.57 g/L). Moreover, a good Na + removal efficiency can still be achieved at high temperatures and low KOH concentrations (30 g/L).  Figure 3b depicts the effect of the interaction between temperature and KOH concentration on the response, which was evaluated under a constant gauge pressure of 2 bar and a gas flowrate of 1000 mL/min. The maximum Na + removal efficiency was achieved at a low temperature and a high concentration of 110 g/L, which was beyond the stoichiometric value (70.57 g/L). Moreover, a good Na + removal efficiency can still be achieved at high temperatures and low KOH concentrations (30 g/L).

Impact of Independent Factors on Na + and Cl − Removal from Reject Brine
(a) (b) Figure 3. Surface response plots of the effect of (a) gas flowrate and temperature, and (b) temperature and KOH concentration, on Na + reduction in reject brine. Figure 4a illustrates the influence of gas flowrate and temperature on Cl − reduction at a constant gauge pressure of 2 bar and a KOH concentration of 70 g/L. The figure depicts that the maximum Cl − removal is obtained under a maximum temperature of 50 °C and low gas flowrate of 400 mL/min. At a low gas flowrate, the gas residence time increased, enhancing the reaction rate and enabling a high removal under relatively high temperature conditions. Notably, owing to the significant effect of the residence time on the overall process performance and reaction rate, the maximum Cl − removal efficiency was obtained at a relatively low KOH concentration of 50 g/L and a low feed-gas flowrate of 400 mL/min, as illustrated in Figure 4b. These data were obtained at a temperature of 30 °C and a gauge pressure of 2 bar.  Figure 4a illustrates the influence of gas flowrate and temperature on Cl − reduction at a constant gauge pressure of 2 bar and a KOH concentration of 70 g/L. The figure depicts that the maximum Cl − removal is obtained under a maximum temperature of 50 • C and low gas flowrate of 400 mL/min. At a low gas flowrate, the gas residence time increased, enhancing the reaction rate and enabling a high removal under relatively high temperature conditions. Notably, owing to the significant effect of the residence time on the overall process performance and reaction rate, the maximum Cl − removal efficiency was obtained at a relatively low KOH concentration of 50 g/L and a low feed-gas flowrate of 400 mL/min, as illustrated in Figure 4b. These data were obtained at a temperature of 30 • C and a gauge pressure of 2 bar.

Impact of Independent Factors on Ca 2+ and Mg 2+ Removal from Reject Brine
The 3D plots of the impact of temperature and pressure on the Ca 2+ reduction efficiency are shown in Figure 5a. The figure reveals that high Ca 2+ removal values (~98%) were obtained at a high temperature value of 50 °C and a gauge pressure of 1 bar. These results, which were obtained at a constant feed-gas flowrate of 1000 mL/min and a KOH concentration of 70 g/L, can be explained by the high reactivity of Ca 2+ with CO2 gas in high-pH media [23]. The interaction effect of gas flowrate and KOH concentration on Mg 2+ perature of 30 °C and gauge pressure of 2 bar. The maximum Mg 2+ reduction efficiency of approximately 95% was achieved at a high KOH concentration and high feed-gas flowrate of 110 g/L and 1600 mL/min, respectively. Upon decreasing the gas flowrate to 400 mL/min at the same KOH concentration of 110 g/L, the Mg 2+ removal efficiency was reduced to almost 68%. Thus, a high gas flowrate and high KOH concentration are essential for enhancing Mg 2+ reduction, owing to an improvement in the reaction rate.

Impact of Independent Factors on Ca 2+ and Mg 2+ Removal from Reject Brine
The 3D plots of the impact of temperature and pressure on the Ca 2+ reduction efficiency are shown in Figure 5a. The figure reveals that high Ca 2+ removal values (∼98%) were obtained at a high temperature value of 50 • C and a gauge pressure of 1 bar. These results, which were obtained at a constant feed-gas flowrate of 1000 mL/min and a KOH concentration of 70 g/L, can be explained by the high reactivity of Ca 2+ with CO 2 gas in high-pH media [23]. The interaction effect of gas flowrate and KOH concentration on Mg 2+ reduction is clearly shown in Figure 5b. These experimental runs were operated at a temperature of 30 • C and gauge pressure of 2 bar. The maximum Mg 2+ reduction efficiency of approximately 95% was achieved at a high KOH concentration and high feed-gas flowrate of 110 g/L and 1600 mL/min, respectively. Upon decreasing the gas flowrate to 400 mL/min at the same KOH concentration of 110 g/L, the Mg 2+ removal efficiency was reduced to almost 68%. Thus, a high gas flowrate and high KOH concentration are essential for enhancing Mg 2+ reduction, owing to an improvement in the reaction rate.

Optimization of Responses under Relatively High Temperature Conditions
Brine is commonly rejected from desalination plants at relatively high temperatures of 40 • C to 55 • C [16]. Therefore, the maximum ion reduction that could be reached at 50 • C was predicted using the response optimizer in Minitab software. Based on the responses of 31 experimental runs, the developed RSM model predicted that the maximum ion reduction can be achieved at a KOH concentration, gauge pressure, and gas flowrate of 30 g/L, 2.0 bar, and~776 mL/min, respectively, under a constant temperature of 50 • C. Table 2 summarizes the predicted optimum conditions, confidence interval (CI), fitted responses, and desirability function. Additionally, a high desirability value of almost 0.85 confirms the optimal performance of the studied parameters [20]. In addition, Figure 6 demonstrates the response optimization plot obtained via Minitab software. It is noticed that a high KOH concentration has a negative effect on the Na + , Cl − , and Mg 2+ reduction. However, it can positively affect the reduction in Ca 2+ ions. The figure also shows a negligible influence of pressure on the Na + removal percentage. It is worth noting that the optimum reduction for all ions can be achieved at a high temperature (50 • C). Moreover, the figure reveals that a low gas flowrate has a good impact on ions removal, except for Ca 2+ ions, on which it Sustainability 2021, 13, 10200 9 of 18 has a negative impact. Therefore, the optimum reduction for all ions can be achieved at an intermediate gas flowrate value of 776 mL/min.

Experimental Validation of the Predicted Optimal Responses
The predicted operating conditions and corresponding responses provided in Table  2 were experimentally verified. Na + , Cl − , Mg 2+ , and Ca 2+ show a reduction efficiency of up to 45.6%, 29.8%, 91.2%, and 100%, respectively. All the obtained results were within the 95% CI range (Table 2), confirming the ability of the model to predict the performance of the process at several operating parameters. Notably, under the same operating conditions, CO2 uptake reached up to 0.5 g CO2/g KOH, which still constitutes a good level of CO2 capture. The CO2 uptake was expected to decrease when the temperature was increased from 10 °C to 50 °C. However, owing to the effect of the linear relation between pressure and temperature on CO2 uptake (Figure 2c), the effect of pressure was a good CO2 uptake.
The results illustrated in Figure 6 show that a high Na + removal efficiency can be achieved at a relatively high temperature of 50 °C , and under certain operating conditions of gas flowrate, KOH concentration, and pressure. This finding was also confirmed by the validated results. This result can be explained by the so-called hot potassium carbonate (K2CO3) process (hot pot process), according to which K2CO3 is mainly used to remove

Experimental Validation of the Predicted Optimal Responses
The predicted operating conditions and corresponding responses provided in Table 2 were experimentally verified. Na + , Cl − , Mg 2+ , and Ca 2+ show a reduction efficiency of up to 45.6%, 29.8%, 91.2%, and 100%, respectively. All the obtained results were within the 95% CI range (Table 2), confirming the ability of the model to predict the performance of the process at several operating parameters. Notably, under the same operating conditions, CO 2 uptake reached up to 0.5 g CO 2 /g KOH, which still constitutes a good level of CO 2 capture. The CO 2 uptake was expected to decrease when the temperature was increased from 10 • C to 50 • C. However, owing to the effect of the linear relation between pressure and temperature on CO 2 uptake (Figure 2c), the effect of pressure was a good CO 2 uptake.
The results illustrated in Figure 6 show that a high Na + removal efficiency can be achieved at a relatively high temperature of 50 • C, and under certain operating conditions of gas flowrate, KOH concentration, and pressure. This finding was also confirmed by the validated results. This result can be explained by the so-called hot potassium carbonate (K 2 CO 3 ) process (hot pot process), according to which K 2 CO 3 is mainly used to remove large amounts of CO 2 from ammonia plants [24][25][26] via the reaction described in Equation (6) [25]. In this process, the entire system is operated at high temperatures (110-116 • C) to increase the solubility of K 2 CO 3 . Additionally, a relatively high CO 2 pressure (2-6 bar gauge) is required to keep CO 2 soluble under such high temperature conditions [25]. HSC Chemistry 6.1 software [27] was used to perform a thermodynamic analysis via Equation (6). Table 3 illustrates the calculated thermodynamic properties. The analysis indicates that the reaction is spontaneous for the whole temperature range (0 to 100 • C), as indicated by the negative ∆G. Note that the KOH concentration affects the absorption of CO 2 . An excessive amount of KOH yields K 2 CO 3 , which competes against the formation of KHCO 3 [28]. As shown using Equation (7), the addition of KOH at amounts equal to or less than the stoichiometric KOH amount (70.57 g/L) would produce bicarbonate ions (HCO 3 − ). Alternatively, according to Equation (8), an excess amount of KOH would yield carbonate ions (CO 2− 3 ), which are less soluble than HCO 3 − . Therefore, at low KOH concentrations (30 g/L), the formation reaction of NaHCO 3 and KHCO 3 is more favorable, resulting in a greater ion reduction.
In summary, the hot pot process and its associated reactions can explain the realization of good CO 2 capture and Na + reduction at a relatively high temperature, gauge pressure, gas flowrate, and KOH concentration of 50 • C, 2 bar, 776 mL/min, and 30 g/L, respectively. A high temperature ensures a high solubility of the produced K 2 CO 3 , and simultaneously, a high pressure maintains CO 2 gas solubility in the solution, facilitating the reaction described in Equation (6) to proceed. Further, a moderate gas flowrate enhances ion removal because it increases the residence time. Moreover, a low KOH concentration favors the formation of HCO 3 − , resulting in the formation of KHCO 3 and NaHCO 3 . At 50 • C, both KHCO 3 and NaHCO 3 are soluble. However, owing to its high lattice energy, KHCO 3 has higher solubility than NaHCO 3 [29]. This causes the treated solution to be oversaturated with HCO 3 − ; therefore, KHCO 3 starts to compete against NaHCO 3 , resulting in a high precipitation of NaHCO 3 . This observation was also confirmed by the high precipitation rate of solids when the mixture was collected from the reactor at 50 • C before the cooling step. The collected mixture was then cooled to allow the precipitation of the rest of the solids, whose characterization confirmed the presence of KHCO 3 and NaHCO 3 . More details about the characterization of the solids are described in Section 4.

Evaluation of the Optimum Responses at 40 • C and 60 • C
Additional experimental runs were conducted at 40 • C and 60 • C, and the results were compared with those obtained at 50 • C. As shown in Figure 7, CO 2 uptake reached 0.37 g CO 2 /g KOH at 40 • C, 0.50 g CO 2 /g KOH at 50 • C, and 0.57 g CO 2 /g KOH at 60 • C. These findings indicate an improvement in CO 2 absorption owing to the same reasons given in Section 3.4.2, Equation (6). Figure 8 shows the average ion removal efficiency at different temperatures for three different experimental replicates. Almost the same reduction efficiencies of 91.2% and 100% for Mg 2+ and Ca 2+ , respectively, were observed in the entire temperature range. At 40 • C, the Na + and Cl − reduction efficiencies were 40.8% and 21.47%, respectively. The removal of Na + and Cl − increased to nearly 45.6% and 29.84%, respectively, at 50 • C, and to 48.1% and 30.2%, respectively, at 60 • C. Notably, the K + reduction efficiency reached up to 79% in the entire temperature range. Although KOH was used as the main alkaline material at high concentrations, a high reduction efficiency was still achieved in the treated brine, confirming the high efficiency of the reaction under the optimized conditions. Table 4 summarizes the main findings from the current research and compares them with recent literature studies for the combined process. It can be noticed that the temperature variation significantly affects the combined process in terms of metal solubility and CO 2 capture efficiency. Table 4 also shows that a low temperature is preferred to achieve high CO 2 capture and ion removal from reject brine. However, brine is usually discharged at a relatively high temperature range from 40 • C to 50 • C. The findings of the current work reveal that CO 2 and reject brine can be treated under high temperature, as presented in Table 2. This demonstrates the novelty of the KOH-based modified Solvay process. respectively, at 50 °C , and to 48.1% and 30.2%, respectively, at 60 °C. Notably, the K + reduction efficiency reached up to 79% in the entire temperature range. Although KOH was used as the main alkaline material at high concentrations, a high reduction efficiency was still achieved in the treated brine, confirming the high efficiency of the reaction under the optimized conditions. Table 4 summarizes the main findings from the current research and compares them with recent literature studies for the combined process. It can be noticed that the temperature variation significantly affects the combined process in terms of metal solubility and CO2 capture efficiency. Table 4 also shows that a low temperature is preferred to achieve high CO2 capture and ion removal from reject brine. However, brine is usually discharged at a relatively high temperature range from 40°C to 50 °C. The findings of the current work reveal that CO2 and reject brine can be treated under high temperature, as presented in Table 2. This demonstrates the novelty of the KOH-based modified Solvay process.
Temperature   Figure 7. Effect of high temperature on the CO2 uptake at a gauge pressure, KOH concentration, and gas flowrate of 2 bar, 30 g/L, and 776 mL/min, respectively.   The structural properties of the collected samples were examined using an X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). All measurements were performed using a tube current of 30 mA and a target voltage of 40 kV. The scanning range was set at 2θ values of 5-70 • to cover all significant diffraction peaks at a scan speed of 2 • /min. The XRD spectra of the three samples are shown in Figure 9. The XRD diffraction peaks of the collected solids were identified according to the theoretical intensity of the three most intense peaks for the mineral in a pure state [30,31]. The characterization of the collected solids confirmed the recovery of brine salts in the form of CaCO 3 , KCl, KHCO 3 , K 2 CO 3, Mg(OH) 2 , and NaHCO 3 . A clear change in the structural properties was observed when the reaction temperature was increased. The reaction conditions significantly change the intensity of the XRD peaks of the collected solids. The produced solids have many applications in various industrial fields. NaHCO 3 can be used as a pH buffering agent [4,32]. KCl is used in the production of crop fertilizers (95% of the fertilizers in the USA are supplied in the form of KCl [33]) and has medical applications [34]. KCl can also be reused in the production of KOH through the electrolysis process [35], according to Equation (9), which confirms the sustainability of this promising alkaline (KOH) in the modified Solvay process. K 2 CO 3 and/or bicarbonate KHCO 3 are largely used as cleaning and emulsifying agents [36].
2 KCl + 2 H 2 O → 2 KOH + Cl 2 + H 2 (9) 4.2. SEM Analysis of Solids Obtained at 40 • C, 50 • C, and 60 • C The surface morphology, texture, and shape of the collected solid samples were characterized using SEM in three different areas. Before the SEM analysis, the samples were coated with a thin gold film, and then average energy dispersive X-ray spectroscopy (EDS) analysis was performed to detect the main elements in the sample. The SEM micrographs of the cross-sections of the samples obtained under the optimum conditions exhibited clear morphological structures (Figure 10). A specific morphological structure was observed at each temperature; for example, under the optimum conditions (KOH concentration = 30 g/L, gauge pressure = 2 bar, and gas flowrate = 776 mL/min) and 40 • C, elongated rod agglomerates with spheroidal particles were visible (Figure 10a). The elongated rods correspond to NaHCO 3 crystals [37]. When the reaction temperature was increased up to 50 • C (Figure 10b), a higher number of spheroidal particles and a lower concentration of elongated rods were detected in the general structure. Figure 10c shows the morphology of the samples at 60 • C. In this figure, the elongated rods completely disappeared; instead, uniform spheroidal particles and smaller shell-shaped crystals were detected. The production of more KHCO 3 and NaHCO 3 can explain the change in the morphology of the samples from elongated rods to spheroidal particles [38,39]. These findings were also confirmed by the average particle size, which reached a minimum value of 15-25 µm at a high temperature of 60 • C ( Figure 11). EDS analysis was also conducted, which confirmed the results obtained using XRD analysis and showed that the main elements in the collected solids were Na, Ca, Mg, C, O, K, and Cl (Figure 10a-c).

FTIR Spectroscopy Analysis of Solids
Obtained at 40 • C, 50 • C, and 60 • C The collected solid samples were examined using FTIR analysis by employing an IRTrace-100 FTIR spectrophotometer (Shimadzu, Kyoto, Japan) to investigate the presence of functional groups. The spectra were recorded in a wavenumber range of 500-4000 cm −1 using a spectral resolution of 4 cm −1 and 34 scans. Figure 12 shows the spectra of the solids obtained at 40 • C, 50 • C, and 60 • C. Many absorption bands with a clear difference were observed between the samples. In all the spectra, a band appearing at 3250-3750 cm −1 can be attributed to the OH groups and a peak at 2750-3000 cm −1 corresponded to the C-H stretching. An absorption band attributed to CO 2− 3 was clearly detected at 1250-1750 cm −1 . the production of KOH through the electrolysis process [35], according to Equation (9), which confirms the sustainability of this promising alkaline (KOH) in the modified Solvay process. K2CO3 and/or bicarbonate KHCO3 are largely used as cleaning and emulsifying agents [36].

SEM Analysis of Solids Obtained at 40 °C, 50 °C, and 60 °C
The surface morphology, texture, and shape of the collected solid samples were characterized using SEM in three different areas. Before the SEM analysis, the samples were coated with a thin gold film, and then average energy dispersive X-ray spectroscopy (EDS) analysis was performed to detect the main elements in the sample. The SEM micrographs of the cross-sections of the samples obtained under the optimum conditions exhibited clear morphological structures (Figure 10). A specific morphological structure was observed at each temperature; for example, under the optimum conditions (KOH concentration = 30 g/L, gauge pressure = 2 bar, and gas flowrate = 776 mL/min) and 40 °C, elongated duction of more KHCO3 and NaHCO3 can explain the change in the morphology of the samples from elongated rods to spheroidal particles [38,39]. These findings were also confirmed by the average particle size, which reached a minimum value of 15-25 µ m at a high temperature of 60 °C (Figure 11). EDS analysis was also conducted, which confirmed the results obtained using XRD analysis and showed that the main elements in the collected solids were Na, Ca, Mg, C, O, K, and Cl (Figure 10a The collected solid samples were examined using FTIR analysis by employing an IRTrace-100 FTIR spectrophotometer (Shimadzu, Kyoto, Japan) to investigate the presence of functional groups. The spectra were recorded in a wavenumber range of 500-4000 cm −1 using a spectral resolution of 4 cm −1 and 34 scans. Figure 12 shows the spectra of the

Conclusions
The optimization of the KOH-based modified Solvay process was performed under different operating conditions to obtain the maximum CO 2 capture and ion recovery from reject brine at a relatively high temperature of 50 • C. An RSM-based CCD approach using Minitab 19.0 was implemented to predict and optimize the effects of four important factors on the responses, namely, KOH concentration (30-110 g/L), gauge pressure (1-3 bar), temperature (10 • C-50 • C), and CO 2 gas flowrate (400-1600 mL/min). The responses of 31 experimental runs were properly represented using 3D surface response plots. A response optimizer was used to predict the optimum conditions to achieve the maximum responses by holding the temperature at 50 • C. The predicted optimum conditions were obtained at a gauge pressure, gas flowrate, and KOH concentration of 2 bar, 776 mL/min, and 30 g/L, respectively. The predicted responses under these conditions were experimentally validated, and the results showed a good agreement with the predicted values. The experimental results showed that under the optimized conditions and at 50 • C, a maximum CO 2 uptake of ∼0.50 g CO 2 /g KOH and maximum reduction efficiencies of Na + , Cl − , Ca 2+ , and Mg 2+ of 45.6%, 29.8%, 100%, and 91.2%, respectively, were achieved. Under these operating conditions, the reaction rate was relatively high, and a new reaction occurred. This can be explained by the hot pot process of K 2 CO 3 , during which the pressurized CO 2 reacts with soluble K 2 CO 3 at high temperatures to form KHCO 3 . The solid products were characterized using SEM, XRD, and FTIR analysis. All the results confirmed the production of valuable products with diverse industrial applications, such as NaHCO 3 , KHCO 3 , K 2 CO 3 , and KCl.