Besides spewing carbon dioxide and other pollutants into the air, coal-fired plants also produce solid waste that poses environmental problems. Coal ash is the residue produced when coal is burnt. Coal plants in the US and elsewhere are required to capture and then either recycle or store their ash. To store the material, plants typically mix it with water and store it in waste ponds, many of which are unlined.
The composition of coal ash varies but it can contain toxic elements such as arsenic, mercury, chromium, selenium, and other metals. According to a study released March 4 by advocacy groups including the Environmental Integrity Project, toxic chemicals leaching from coal-ash storage ponds contaminate groundwater at 91% of coal plants in the US. And when there’s a structural failure in these storage areas, or a natural disaster, the ash slurry can spill into nearby bodies of water.
But a team at North Carolina State University thinks that seeding coal-ash ponds with microbes could help contain this toxic waste. With the right food supply, these microbes create calcium carbonate, which precipitates out of the water to stiffen the sludge and trap toxic metals (J. Geotech. Geoenviron. Eng. 2019, DOI: 10:1061/(ASCE)GT.1943-5606.0002036).
North Carolina State University civil engineer Brina Montoya moved to the state in 2012. She says she was struck by the coal industry’s storage problem—particularly when 2014 floods triggered a leak of about 74,000 metric tons of coal ash from a Duke Energy plant into North Carolina’s Dan River. Remediating coal-ash ponds is expensive. “There are not a lot of options available today,” Montoya says. “To really address this you have to take the water out of the ponds, dig them up, and put the waste in a lined landfill.”
Montoya began to think about ways to prevent future disasters without the need to completely rebuild storage areas. And that’s when she thought about biocementation, which is the biological production of calcium carbonate, the main component in cement. The requirements for biocementation are simple. Bacteria need a taste for nitrogen and the genes to make the enzyme urease. To spur biocementation, scientists fertilize an area with urea and make sure there is a sufficient supply of calcium. When microbes metabolize urea with urease, they produce ammonia and carbonate. If there is calcium in the environment, calcium carbonate precipitates out.
In her previous research, Montoya had studied biocementation as a way to stabilize sandy soils in earthquake-prone areas. Perhaps a similar process could stabilize coal-ash ponds, she thought.
Just about every ecosystem houses some naturally occurring bacteria that can carry out biocementation. But for lab studies, the researchers used a biocementation star, the model microbe Sporosarcina pasteurii. It makes lots of urease, and it’s hardy—even civil engineers can work with it, Montoya says.
Her lab got coal-ash samples from three sites in the northeastern US, and studied how biocementation changed the samples’ mechanical properties. They placed the samples in plastic tubes fitted with equipment for measuring viscosity and other parameters, then added bacteria, urea, and calcium. As the ingredients flowed down through the coal ash, the slurries stiffened up. Surprisingly, she says, the process didn’t work with one of the three samples—a sign of variation in these waste materials that could mean engineers would need to tailor the process to a given waste pond.
“So far we’re focusing on whether this can work at all—can we get the ash stiffer or less compressible?” she says. Stiffer coal ash is less likely to spill out if there’s a structural failure or a flood at a storage facility.
Next, Montoya plans to examine whether biocementation can chemically immobilize toxic metals in coal ash. Previous research has shown that the process can trap radioactive strontium ions, which get swapped in for calcium because they carry the same +2 charge. Montoya also is exploring whether biocementation could trap other divalent cations commonly found in coal ash, such as lead.
Rick Colwell, a biogeochemist at Oregon State University, has participated in some field studies of biocementation, including the work on strontium. He says it’s possible that techniques like Montoya’s could both physically and chemically contain coal-ash waste, but if this were done in the field, it would not be possible to use the model organism. In many states, introducing non-native microbes to the environment is illegal. So the team would need to prove not only that the process works, but also that the addition of urea and calcium to coal ash ponds in the real world can fertilize naturally occurring bacteria to kick off biocementation.