Since the Industrial Revolution, the usage of fossil fuels such as petroleum and coals has resulted in a large amount of carbon dioxide emitted into the atmosphere. Normally, the average temperature of the Earth is kept constant by the greenhouse effect. However, when the concentration of carbon dioxide in the atmosphere is too high, the thermal equilibrium between solar radiation and Earth radiation energy is disrupted and the temperature of the Earthâs surface rises. Nature and ecology have an ability to cope with temperature changes up to a certain degree. However, climate change results in serious environmental problems such as changes in the ecology and coast lines, floods and many others. These happen when normal greenhouse effects adapt to the changing climate. To prevent such events, many developed nations are developing technologies that can prevent carbon dioxide emissions into the atmosphere. While there are other types of greenhouse gases that have the potential to trigger serious greenhouse effects or climate change, due to the massive amounts emitted, carbon dioxide is thought to be the main cause of climate change and global warming. In order to prevent carbon dioxide produced from industrial facilities using fossil fuel from being emitted into the atmosphere, various technologies, commonly called Carbon Capture and Storage (CCS) technologies, have been invented. In the well-known wet absorption process, carbon dioxide in flue gas is separated using a wet absorbent such as alkanolamines or metal hydroxides in an absorber. When this absorbent is saturated with carbon dioxide, it is transported to a desorber where captured carbon dioxide is separated by the heat of hot steam. Separated carbon dioxide is then compressed and transported to a storage site, resulting in the isolation of carbon dioxide from the atmosphere. However, there are some drawbacks associated with wet absorption. First, a large amount of energy is required to operate the whole process. In particular, hot steam is required in the desorption process to separate carbon dioxide from the absorbent solution. The energy used in this process is called the regeneration energy, which represents 70% to 80 % of the total CCS process operation energy. The process cost is high, requiring part of the energy production in the facilities to be provided to the CCS process. Second, many nations lack the storage sites for this process. In Europe, underground storage of carbon dioxide is not feasible since there are no massive underground energy sources such as petroleum or natural gases. Also, the main component of the crust in this continent are limes that react with carbon dioxide to form calcium bicarbonate, which is soluble to water and thus is potentially weakened upon reaction with CO2
. For some nations located in the circum-Pacific orogenic zone, the high level of geological activities makes difficult to guarantee the stability of stored carbon dioxide. Hence, a new way of treating carbon dioxide is needed, preferably by converting carbon dioxide into useful materials. This concept is commonly referred to as Carbon Capture and Utilization (CCU). CCU can be divided into two processes: organic and inorganic utilization processes. In organic utilization, carbon dioxide is converted to organic substances such as diesel, plastics and other organic chemicals. However, plenty of energy is required to trigger the reactions involved, since carbon dioxide is very stable. Although catalysts may be used, their cost is too high to make it feasible for commercialization. In inorganic utilization, inorganic substances such as metal carbonates are produced. When the wet absorption process is applied to the inorganic utilization process, a massive amount of carbon dioxide capture and conversion can be achieved, due to the absorption characteristics of absorbents and carbon dioxide. When carbon dioxide is absorbed in alkanolamine-type absorbent solutions, it undergoes the following reactions for primary and secondary amines:
RRâNH + CO2 â RRâNH+COO-(zwitterion) -------------------- (1)
RRâNH+COO- + RRâNH â RRâNCOO- + RRâNH2+---------------- (2)
2RRâNH + CO2 â RRâNCOO-(carbamate) + RRâNH2+ -------- (3) <overall reaction>
For ternary amines:
CO2 + R3N + H2O â HCO3-(bicarbonate) + R3NH+-------------------- (4)
As shown in the chemical equations above, carbon dioxide exists in the form of reactive ionic CO2 (ri-CO2), which reacts with metal cations. Therefore, this allows for large scale CO2 capture and the formation of metal carbonate. Moreover, this process requires no heat to separate the captured carbon dioxide from the absorbent solutions, since most of the CO2 is removed when it precipitates in the form of metal carbonate salts. These metal carbonate salts can be used for various industrial applications such as cement, paper and others. The inorganic utilization process has received plenty of attention. However, most of the studies have used metal cations from natural sources such as limestone; or from some construction materials such as waste concrete. Upon consideration of the economic feasibility, it seems inappropriate to use natural resources to treat waste gas. On the other hand, waste concrete contains plenty of aggregates that can be reused after separation. Hence, the pretreatment process is complicated and has to be applied before the metal cation sources in the inorganic utilization process can be used. In this study, seawater-based industrial wastewater was utilized as the metal cation source. In the refined salt production process, concentrated wastewater is produced as a byproduct. When released to the near shore without any treatment, this wastewater may negatively impact the near shore ecology. Hence, when metal cations contained in this wastewater are utilized, wastewater treatment and metal cation capture along with carbon dioxide conversion can be achieved simultaneously. In this study, we considered both metal cation separation and chemical conversion of carbon dioxide. In the former, calcium and magnesium cations contained in the wastewater were separated by using sodium hydroxide that can be obtained from the electrolysis of seawater. Calcium and magnesium components are separated in the form of calcium hydroxide and magnesium hydroxide, respectively. The major cationic component of the remaining solution after the Ca/Mg separation are sodium ions.
After cation separation, carbon dioxide is captured using an absorbent, i.e., monoethanolamine (MEA) solution. When this absorbent is saturated, calcium and magnesium hydroxides are added, resulting in the formation of calcium and magnesium carbonates, respectively. To form magnesium carbonate, the pH of the solution is controlled using sodium hydroxide to enhance the reaction. After Ca/Mg separation, sodium bicarbonate was produced from the remaining solution by applying the steric hindrance effect of alkanolamines.