Web Date: April 3, 2014
Microbial Reactor Could Help Capture Carbon Sustainably
A new reactor powered in part by microbes can generate hydrogen gas while producing solutions of hydrochloric acid and sodium hydroxide for use in carbon dioxide sequestration (Environ. Sci. Technol. Lett. 2014, DOI: 10.1021/ez500073q). The reactor’s developers think the device could lower the cost of one method of sequestering the greenhouse gas.
To combat climate change by reducing atmospheric levels of CO2, some scientists have proposed capturing the CO2 released from power plants. Several industrial projects are now underway. One proposed strategy, called mineral sequestration, converts CO2 into solid carbonate minerals by reacting the gas with natural deposits of silicate minerals. In nature this reaction occurs very slowly, but acid and base treatment steps can enhance the rate of carbonate formation.
Unfortunately, producing the acids and bases for these steps on a sufficiently large scale requires energy, the production of which would generate more CO2. To make mineral sequestration sustainable, researchers want a carbon-neutral way to make acids and bases.
Bruce E. Logan at Pennsylvania State University and his team, including Xiuping Zhu, thought a microbial reactor could do just that. They have spent years designing devices that harness electrochemically active microbes to produce energy. For example, they’ve built a microbial electrolysis cell in which bacteria break down organic matter and transfer electrons to an anode. With the addition of a small amount of electricity, an inorganic catalyst at the cathode reduces protons in water to produce hydrogen gas.
Because the electrolysis cell produces acid and base in the form of protons and hydroxide ions, Zhu realized that they could modify the reactor design to harvest acidic and basic solutions for CO2 sequestration.
To do this, the team introduced an ion exchange membrane between the anode and cathode compartments of the cell. In the anode compartment, the microbes release protons as they chew up acetate or other organic molecules. Dissociation of water at the membrane helps maintain a neutral pH in the anode compartment, generating an acidic solution in a separate chamber on the other side of the membrane. Meanwhile, in the cathode compartment, water reduction produces hydrogen gas and hydroxide ions, yielding a basic solution. Both the gas and the basic solution are pumped out to a collection vessel.
But to produce amounts of acid and base sufficient for mineral sequestration, the cell needs to operate at a higher voltage than that generated by the microbes alone. To augment this voltage without relying on external power sources, the team incorporated a stack of membranes that generate electricity from the energy released when saltwater and freshwater mix, a process called reverse electrodialysis.
The team built a prototype reactor with a 28-mL anode chamber that they filled with a 0.8 g/L solution of acetate as food for the bacteria. The prototype produced 15 mL of 30 mM hydrochloric acid and 15 mL of 73 mM sodium hydroxide, as well as 10 mL of hydrogen gas. The researchers used the acid to dissolve serpentine, a silicate mineral. Then they used the base to increase the solution’s pH so it could absorb about 9 mL of CO2, eventually precipitating about 12 mg of the resulting carbonate minerals.
Based on costs to mine and process the serpentine mineral, as well as to pump solutions through the reactor, the researchers estimate the reactor’s operating costs at $25 per metric ton of captured CO2. They point out that hydrogen generated by the system could help offset costs further. Cost estimates for a more common sequestration approach start around $65 per ton (Energ. Econ. 2011, DOI: 10.1016/j.eneco.2010.11.004).
Jennifer Wilcox, an expert on carbon sequestration at Stanford University, says the system’s design is sound. But she says the team’s cost estimate doesn’t account for the full costs of mineral sequestration, including transporting the silicate minerals and disposing of the resulting carbonate rocks. When it comes to capturing gigatons of CO2, “there will be no silver bullets,” she says.
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