Human activity has caused the CO2 levels in the atmosphere to increase to dangerous levels, resulting in changes in the earth’s climate. Everyday new CO2 emissions are being released from various sources and adding to this problem. Carbon intensive industrial plants, such as coal<!–[if supportFields]> XE “coal” <![endif]–><!–[if supportFields]><![endif]–>-fired power plants, contribute a large portion of these waste gas emissions. Wang et al. (2011) have investigated the use of magnesium and calcium ions to react with the emitted CO2 to form a carbonate precipitate. The carbonate is a very stable substance that sequesters the carbon and keeps it from separating and mixing into the atmosphere. The authors propose the use of seawater as the source for the magnesium and calcium ions, particularly waste seawater from desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> plants with high ion concentrations. They determined the optimal conditions to push this reaction to form the most carbonate precipitant. —Anna Fiastro
Wang, W., Hu, M., Ma, C. 2011. Possibility for CO2 sequestration<!–[if supportFields]> XE “sequestration” <![endif]–><!–[if supportFields]><![endif]–> using seawater. Bioinformatics and Biomedical Engineering 4, 1–4.
Wang et al. focused on the mixing of salt water with the CO2 emissions from coal<!–[if supportFields]> XE “coal” <![endif]–><!–[if supportFields]><![endif]–> power plants. They used various equations to calculate the possible carbonate precipitation under different conditions and carbon emissions. They found that the pressure of the carbon containing gas and the acidity of the salt solution were the two driving factors of the reaction, and determined the optimal partial pressure and pH range.
For this reaction to happen the CO2 from the gas must be absorbed into the liquid. By increasing the pressure of the gas, more CO2 passes into the liquid and is available to form carbonate ions. The atmospheric pressure allows the ocean to take up CO2 from the atmosphere naturally, but this is a slow process. Increasing the pressure to more than 1 atmosphere speeds up the formation of carbonate. Emissions from most industrial plants are in a gas form that has a partial pressure several times higher than that of the atmosphere. Therefore, the mixing of this gas with seawater should accelerate the process.
Wang et al. established that an enhanced alkaline solution would also lead to increased carbonate precipitation. Increased pH drives the buffer equilibrium from CO2 towards the formation of carbonate ions (CO32-). These ions then react to form the carbonate, which is precipitated out, sequestering the carbon in a stable condition. The more basic the solution, the more carbonate ions there are to form carbonate. There is however a threshold for this trend, where the pH is too high and unwanted precipitates are formed. Seawater does not have the optimal pH to push this reaction; however increasing its pH is a very difficult. The authors propose several ways to increase pH, including electrolytically, but with the technology available today, they are all expensive processes.
Pressure and pH cause more carbonate ions to be present to react with other positive ions to form the carbonate solids. The author’s analysis of the various cations in seawater found that magnesium and calcium are abundant enough and strong enough cations to precipitate carbonate anions. The condensed seawater that comes from desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> plants as well as underground brine offer high concentrations of these ions to react with the carbonate ions.
Finally, Wang et al. applied these ideas to an existing coal<!–[if supportFields]> XE “coal” <![endif]–><!–[if supportFields]><![endif]–>-fired power plant. When the pressure of the gas and pH of the solution where known, the amount of precipitate could be calculated and the amount of carbon sequestered could be predicted. The addition of alkaline seawater to emissions seems to be a promising method of carbon capture and storage<!–[if supportFields]> XE “carbon capture and storage (CCS)” <![endif]–><!–[if supportFields]><![endif]–> in the form of carbonate precipitate.