Testing limestone’s ability to capture carbon from air

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.”

Climate change is here, and the world is not adjusting its behavior just yet.”

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.”