Sequestration of New CO2 Emissions by Reacting with Seawater

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, 14.

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.

The Effects of Injecting CO2 into Deep Bethypelagic Layers of the Ocean

In the face of increasing CO2 levels in the atmosphere one approach to reducing new CO2 emissions is carbon capture and storage.  Yamada et al. (2010) examine the technique of dissolution; injecting CO2 into deep layers of the ocean.  The limited mixing of these deep waters would prevent the CO2 from entering the atmosphere for a long time, but the CO2 could affect the prokaryotic populations at these depths and their associated nutrient cycles.  The research looked at the effects of increased CO2 on these populations by capturing samples of them in water samples from deep in the Pacific Ocean and conducting laboratory experiments on them, increasing CO2 levels and evaluating the effects.—Anna Fiastro
Yamada, N., Tsurushima, N., Suzumura, P., 2010. Effects of Seawater Acidification by Ocean CO2 Sequestration on Bathypelgic Prokaryote Activities. Journal of Oceanography. Vol 66, p 571-580.

The plan for dissolution is to inject CO2 into the benthypelagic zone, which ranges from 1000 to 3000 meters from the surface.  This is an important area for the regeneration of nutrients and organic material.  The layers of the ocean are separated by temperature and salinity gradients that prevent mixing. Due to limited mixing of the layers of the ocean it is thought that the CO2 would not move up and not be introduced into the atmosphere.  The CO2 would dissolve into the surrounding water and remain at depth, causing a decrease in the pH, also known as acidification, but only locally.  It is important to look at the effects of these elevated CO2 levels on the systems that operate in these layers, specifically the prokaryotes who are responsible for these nutrient cycles.
          Yamada et al. took water samples from two different locations in western North Pacific at 2000 meters deep, which were used in experiments within 10 days of sampling.  CO2 injection conditions were simulated by bubbling air containing different concentrations of CO2 though the tanks containing the samples.  The pH, total cell count, and heterotrophic prokaryotic production rates were monitored in each sample.  Although there was variation between the sites, thought to be due to seasonal differences, clear results were obtained.  The bubbling of CO2 increased the acidity of the water (decreased the pH).  The total cell counts remained relatively constant independent of pH, but the heterotrophic prokaryotic production rates decreased with increasing acidity.  Another way to say this is that with more CO2 in the water, productivity of the organisms living in it went down. 
It seems counter-intuitive that total cell count would remain the same while productivity went down.  In order to further examine this, the researchers looked at the direct viable count, or the number of thriving prokaryotic cells capable of growth.  This was shown to decrease with acidification, explaining the decreased productivity rates.
Another trial was run in which acidification was simulated by adding a chemical buffer.  This showed similar results to the CO2 bubbling method.  As pH decreased and acidity increased, prokaryotic growth and production were lowered.
In these experiments, acidification suppressed bacterial activity more than Archaea activity.  The significance of this is not fully understood, and further research is necessary to look at the life histories of different types of Archaea to better understand their reaction to changing pH levels.

Sequestration of CO2 in Geological Formations as Carbonate Minerals.

Atmospheric carbon dioxide concentrations have been steadily increasing over the past century causing detrimental effects on the earth’s climate.  In addition to efforts to decreased future carbon emissions, the capture and storage of current CO2 in the atmosphere is an important component of a long-term solution to for reducing CO2 concentrations.  One method proposed for this is geological CO2 storage.  This is a process in which CO2 emissions are pumped into geological formations instead of into the earth’s atmosphere.  Since the CO2 being inserted in to the rock is buoyant, when compared to the rock and surrounding water, there are different trapping mechanisms to insure that the CO2 remains at depth and does not resurface to be released into the air.  The focus of this paper by Matter and Kelemen (2009) is “mineral tapping” in which dissolved CO2 reacts with water and the minerals of the surrounding rock to form solid carbonate that will remain in place.  This is a long-term storage solution for large quantities of CO2.  The success of this solution, however, depends on the type of physical and chemical properties of the location chosen for injection.—Anna Fiastro
Matter, J., Kelemen, P., 2009. Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation. Nature geoscience. doi:10.1038/NGEO683.
The emissions are pumped to depths of over 800 meters where the combination of temperature, pressure, and salinity in addition to the pH of the location induced a fluid-rock reaction that causes carbonate mineral formation.  Early studies examined deep aquifers in sedimentary rock because of the porous nature of the rock.  It was thought that space was a necessary characteristic of the host rock because it offered a place to deposit the carbonate mineral product.  Sedimentary rock includes sandstone, siltstone, shale, and limestone, however these types of rock have very low mineral trapping potential.  This is seen in prediction models run in various labs, and in field observations of natural CO2 reservoirs leeching into rock.
          The field of research then looked towards aquifers containing ‘basic’ silicate minerals, such as olivine, serpentine, pyroxenes, plagioclase, and basaltic glass. It was found that silicate minerals buffer the pH in these reactions making them essential for enhancing mineral storage. It has been shown in laboratory experiments and in natural analogues that these types of rock react rapidly to form carbonate minerals.  These types of rock are also commonly found all around the world and on every continent.  This means that their capacity for CO2 storage in carbonate is enormous.
The original concern with mineral trapping was the need for space.  The reactions are often self-limiting because they fill in empty space and can create boundaries between the unreacted CO2 and fluid.  As was mentioned earlier, this was the advantage of sedimentary rock originally being examined.  The porous nature of the rock is important to ensure ample room for product creation. It was thought that optimal rock containing silicate minerals would not be porous enough to have a continued reaction and convert all of the CO2 to carbonate.  As a solution to this, it is hypothesized that the crystallization can fracture the rock to increase permeability.  This has been proven to occur in both laboratory simulations as well as naturally occurring systems.  The fracturing that occurs creates more space for the carbonate product to be deposited and allowed the reactants to continually come in contact with each other forming more carbonate.
Another aspect that makes this process a favorable solution to CO2 capture is the self-heating cycle that occurs.  Heat is given off from the initial reaction and remains to speed up the continued reaction of more and more CO2, increasing the overall reaction rate.  With continued reactions taking place, the elevated temperature is maintained and so is the speedy reaction rate.  This results in more and more CO2 being sequestered.
The combination of silicate minerals, fracturing and excess heat allow for large quantities of carbon dioxide to be captured and deposited in underground aquifers as carbonate minerals.  This is a solution to increased CO2 levels that is being examined further.