In a previous work, the overall performance of modified Solvay process was investigated in the absence of ammonia, where carbon dioxide and brine treatments were accomplished in a single reaction and in the presence of calcium hydroxide (Ca(OH)₂). In this study, the impact of alkaline and amphoteric oxides, namely potassium hydroxide (KOH) and aluminum oxide (Al₂O₃), on the pH level, CO₂ capture capacity, ions reduction, and type of produced solids was investigated at a temperature of 20°C and a gas flow rate of 1 L/min. At the stoichiometric ratio and less than the solubility limit, the KOH/brine mixture achieved a CO₂ uptake of 0.31 g CO₂/g KOH. In comparison, about 0.92 g of CO₂ was captured by 1 g of CaO when CaO was added to the brine in an amount that was more than the solubility limit but less than the stoichiometric ratio. Moreover, the percentage of the ions reduction for the KOH and CaO was almost the same except for the sulfate ions, as the best removal was for the CaO. The poorest CO₂ capture and ions removal occurred with the Al₂O₃. X‐ray diffraction was used to identify most of the solid products, and the obtained results proved that KOH is a promising alkaline for the combined process. In addition, potassium chloride crystals were produced when KOH was utilized, which is a very valuable product, and it can also be easily separated. Although Al₂O₃ showed no reactivity, it revealed good results in terms of magnesium ions recovery and could be considered as a coagulant for recovering magnesium ions in the reject brine solution.

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 CO₂. These processes, however, require low temperatures (10–20 °C) to increase the solubility of CO₂ 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 (CO₂) 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 CO₂ gas flowrate, on CO₂ uptake and ion removal was investigated and optimized. The optimization was performed using the response surface methodology based on a central composite design. A CO₂ uptake of 0.50 g CO₂/g KOH and maximum removal rates of sodium (Na⁺), chloride (Cl⁻), calcium (Ca²⁺), and magnesium (Mg²⁺) 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 (NaHCO₃), potassium bicarbonate (KHCO₃) and potassium chloride (KCl).