Energy futuresBacteria will help keep CO2 safely sequestered

A dollop of bacterial gloop, mixed with a splash of urea and poured into an underground aquifer, is the latest recipe for a secure carbon dioxide storage site.

With the world still heavily reliant on fossil fuels to meet its energy needs, carbon sequestration technologies could help reduce greenhouse gas emissions. One of the big challenges to making this a reality is ensuring that the CO₂ stays locked away underground.

Helen Knight writes that one way of doing that is physically to trap the gas by pumping it into permeable rock strata beneath a layer of impermeable “cap rock.” An alternative technique, called solubility trapping, pumps the CO₂ into brine held within the rock, to create an underground reservoir of carbonated water. This captured CO₂ can also react with minerals in the surrounding rock to form carbonates that hold carbon in solid form.

Now Andrew Mitchell at Aberystwyth University in the United Kingdom and colleagues at Montana State University in Bozeman think microbes could help these rocks hold carbon even more securely.

Knight writes that the team created a test chamber designed to mimic the high-pressure conditions within an underground saline reservoir at the Powder river basin in Wyoming, which has been identified as a potential CO₂ storage site.

They then introduced Bacillus subtilis bacteria into the saline-filled chamber and added supercritical CO₂ — followed by urea for the bugs to feed on.

They found that the bacteria formed a biofilm across the surface of the chamber that acted as a seal. What is more, as the bacteria munched on the urea, they increased the pH of the saline, encouraging the CO₂ to react with calcium in the rock to form calcium carbonate on the walls of the reservoir. “The CO₂ becomes trapped in a more stable form, while the minerals themselves also help to plug pores in the ground, providing another way to reduce CO₂ leakage,” says Mitchell.

The combination of biofilm and mineral deposit reduced the permeability of the rock by 95 percent, Mitchell says.

Finally, the change in pH also caused the saline solution to absorb more CO₂, increasing the amount of gas the reservoir can safely store by over 30 percent, says Mitchell. “It allows you to pump lots more CO₂ into the water by changing the chemistry,” he says.

If the urea were obtained from waste-water treatment plants, it could also help to further reduce emissions, Mitchell says. “Waste water is essentially surface carbon, so it could be a way of using up some of that waste carbon.”

Stuart Haszeldine, a carbon capture and storage researcher at the University of Edinburgh, says the use of microbes is interesting and worth pursuing, but the experiments will need to be scaled up considerably before they are ready for commercial use.

-Read more in Andrew C. Mitchell et al., “Microbially Enhanced Carbon Capture and Storage by Mineral-Trapping and Solubility-Trapping,” Environmental Science & Technology (14 June 2010) (DOI: 10.1021/es903270w)