Testing limestone’s ability to capture carbon from air

• Link to: Northwestern Now Story

EVANSTON, Ill. --- A Northwestern University-led team of interdisciplinary researchers has received $5.1 million from the U.S. Department of Energy (DOE) to test the efficacy of enhanced weathering, a relatively new strategy to remove carbon dioxide (CO₂) from the ever-warming atmosphere.

Sprinkling crushed rock onto farmland could remove billions of tons of CO₂ from the atmosphere annually — and successfully trap it for tens of thousands of years. But researchers still need to perform more in-depth research and analysis to determine whether enhanced weathering represents a plausible decarbonization pathway.

With the new grant, the team will distribute crushed limestone on a downstate Illinois farm and analyze its ability to remove atmospheric CO₂. Farmers already employ limestone to control soil pH and improve crop yields, but little research has been done to optimize existing practices to promote CO₂ drawdown.

Kicking off in January 2025, the project is part of a $58.5 million DOE investment into the development of commercially viable carbon-removal technologies.

By collecting and analyzing data from water and soils over two growing seasons, the researchers aim to accelerate the carbon-removal process and develop enhanced weathering protocols that farmers throughout the Midwest and other regions can readily adopt. In addition to providing a safe and durable decarbonization solution, researchers expect enhanced weathering will empower farmers to generate additional income by selling carbon offset credits.

“Within the past few years, interest in enhanced rock weathering has increased exponentially,” said Northwestern’s Andrew D. Jacobson, who is the principal investigator on the grant. “The decarbonization industry is rapidly expanding, and Northwestern is ideally positioned to take a lead role to hone enhanced weathering. We are motivated to test enhanced weathering’s potential with the highest levels of scientific rigor and quality.”

An expert in geochemistry, Jacobson is a professor of Earth and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences. Along with Northwestern professors Brad Sageman and Louise Egerton-Warburton, Jacobson will complete the project in partnership with carbon-removal startup Silicate Carbon and Frank McDermott, a geology professor at University College Dublin.

Accelerating weathering from millennia to decades

Proponents of enhanced rock weathering are inspired by Earth’s natural long-term carbon cycle. Rock weathering converts CO₂ into bicarbonate, an ionic form of carbon that is relatively stable in water over geologic timescales. Bicarbonate ions filter down through soil and are eventually washed into streams, rivers and, finally, the ocean, where they form solid calcium carbonate.

“Once carbon is in its mineral form, it can’t escape,” said Sageman, an expert on Earth’s ancient climate and professor of Earth and planetary sciences at Weinberg. “We want to capitalize on the Earth’s natural feedback system. But this process happens over geological timescales. In order for it to make a difference during our lifetimes, we have to accelerate the process from millennia to decades.”

By grinding up rocks — rather than allowing rocks to naturally weather over long timescales — researchers increase the surface area that’s available for chemical reaction. This should accelerate the timescale of CO₂ drawdown.

The Northwestern team is collaborating with Silicate Carbon, an Ireland-based company testing the potential of enhanced weathering at Schott Farms in Buckingham, Illinois, a small village outside Kankakee. Last year, Silicate began systematically covering more than 40 acres of the soy and corn farm with fine dust from waste concrete. After multiple false starts — interrupted by a lightning strike and failed solar panels — the team finally began collecting preliminary data from water runoff and emitted gas this past summer.

“It’s amazing work, but we have a shoestring budget,” said Maurice Bryson, CEO of Silicate. “We’ve been doing it with the most minimal budget you could possibly imagine. We are using donated equipment and sheer grit and determination. Now, with the DOE funding, we will be able to perform research at a much deeper level. It gives us the right people, the right resources and a detailed plan of how to address multiple questions.”

A first for limestone

The DOE-funded study will focus mostly on limestone but will also test other geologic materials, such as basalt and wollastonite. To date, most enhanced weathering strategies have employed basalt, which researchers think weathers faster than other silicate rock types, such as granite. However, limestone weathers orders of magnitude faster than basalt, thus increasing the likelihood that CO₂ drawdown rates can be confidently measured.

“Most teams around the world are using basalt, which is an igneous rock,” Bryson said. “We know basalt rock weathering is a natural carbon sink over geological timescales, but it is not clear whether it can be a sink for CO₂ over human timescales. The potential of carbonates, like limestone, however, to rapidly draw down atmospheric CO₂ is very clear, and something that we have been testing for the past three years.

“To my knowledge, no one else has studied limestone from the weathering perspective yet. I mean, farmers have used limestone for forever to change soil pH precisely because it has rapid dissolution kinetics, so that’s nothing new. But from the enhanced weathering perspective of trying to draw down CO₂ and make dissolved bicarbonate, nobody else has attempted that.”

Erich Schott, who owns Schott Farms, already adds limestone to his fields every five years or so. When soil is too acidic, limestone balances the pH to make nutrients more readily available for his crops. As a part of the new study, researchers will explore whether limestone should be added at more frequent intervals and/or at higher volumes.

Schott feels “cautiously optimistic” about the enhanced weathering project happening outside his window. The farm has been in his family since in the 1850s. During his time running the farm, he’s been open-minded about experimenting with new farming techniques. After the State of Illinois legalized marijuana, for example, Schott tried growing hemp fibers as a possible alternative for the synthetic fibers used in concrete. Now, he views enhanced weathering as another opportunity to explore potential solutions for long-existing problems.

“It’s a bit of a pain but nothing traumatic,” Schott said about the project. “If you don’t try different ideas, you will miss any opportunity to learn. If it works, that would be great news. But, if not, we tried.”

Measuring soil, plant health

Similarly, this isn’t the first enhanced weathering study the Northwestern scientists have attempted. Funded by the Paula M. Trienens Institute for Sustainability and Energy, Jacobson also leads a two-year demonstration project to test the CO₂ removal efficacy of basalt.

Located inside a greenhouse at the Chicago Botanic Garden, the project comprises dozens of mesocosms (experimental systems that simulate natural conditions). The research team uses these systems to examine the effects of temperature, crop type and other variables relevant to Midwestern agriculture. As the plants grow, the researchers measure bicarbonate, pH and dissolved ions in water runoff, as well as crop yields and gases emitted from the soil.

The team also brings soil and crop samples back to the laboratory for further testing. In particular, important questions surround how enhanced weathering might affect the microbiomes of agricultural soil.

After these experiments conclude, the researchers plan to have results that could guide the effort to optimize and predict CO₂ removal. While enhanced weathering alone cannot reverse climate change, it could be yet another tool to slow warming.

“Climate change is here, and the world is not adjusting its behavior just yet,” Bryson said. “So, we have to think of new solutions that can help reduce the effects of burning fossil fuels. It’s going to take a lot of work, but it’s a challenge that we absolutely must pursue.”