Enzymes boost ‘rock weathering’ to trap CO₂ in soil

Spreading powdered basalt on farmland traps CO₂ from the atmosphere, a technology that is cheap, requires no huge steel reactors and, with a biotech boost, may even reduce fertilizer use.

In a quiet field in rural England, enzymes are bringing biotech ingenuity to the battle against climate change. The site in Oxfordshire is testing whether spreading an enzyme — carbonic anhydrase — on the field can boost a natural geological process that draws carbon dioxide from the atmosphere and traps it in the ground.

Biotech enzymes and crushed basalt spread on fields can speed up a reaction between rocks, water and air that removes carbon dioxide from the atmosphere. Credit: FabricNano

That natural process is called weathering. It occurs because CO₂ dissolves in water to form carbonic acid. When that water meets certain types of rocks, the acid gradually breaks down minerals such as silicates. Crucially, these reactions also convert CO₂ into bicarbonate and carbonate ions that largely remain in the ground or the ocean, permanently removing the CO₂ from the air.

Various companies are deploying faster forms of weathering to tackle climate change. In an approach called enhanced rock weathering, they scatter finely ground silicate rocks such as basalt onto farmland. This enhanced weathering happens much more quickly than natural weathering, offering a higher rate of CO₂ removal. For every tonne of CO₂ that the companies sequester in this way, they can sell a ‘carbon credit’ — a token that customers can buy to offset their own CO₂ emissions.

Carbonic anhydrase could accelerate rock weathering’s action because it speeds up the interconversion between dissolved CO₂ and bicarbonate. That could accelerate CO₂ sequestration, and it may also enable companies to use somewhat larger grains of rock in their products, which are cheaper and easier to source. Farmers, too, could benefit because weathering releases minerals from the basalt that help to fertilize the fields.

The field trials happening now are the first large-scale trials to deploy carbonic anhydrase in enhanced rock weathering, says Charlie Hendry-Smith, the carbon removal development manager at resource management company Veolia. The multinational company, which is headquartered in Aubervilliers, France, has partnered with London-based enzyme engineering company FabricNano to run the experiments in Oxfordshire and at several other UK sites.

If the trials prove successful, it could help enhanced rock weathering to become a key technology in a growing palette of methods that suck CO₂ from the air for long-term storage. The United Nations’ Intergovernmental Panel on Climate Change says that large-scale carbon dioxide removal will be necessary to limit global warming to well below 2 °C above preindustrial levels — the goal set by the 2015 Paris Agreement on climate change.

In June an international team of climate researchers concluded that we will need to remove roughly 7–9 billion tonnes of CO₂ per year by 2050 to meet the Paris climate goals. Reaching that target will require not only conventional forms of CO₂ removal such as tree planting, but also novel technologies such as enhanced rock weathering. Last year, global management consultancy McKinsey estimated that a carbon dioxide removal industry of that magnitude could be worth up to $1.2 trillion by 2050.

Enhanced rock weathering is seen as a promising option because of its simplicity, the longevity of its carbon storage, and its potential benefits to agriculture. To make the weathering happen faster, the method tends to use rock grains roughly 1 mm or smaller, which have a high active surface area available for the reaction to take place. “Unlike with other processes, you don't need to develop large capital infrastructure with lengthy permitting challenges and development costs,” says Hendry-Smith. “This uses existing supply chains, and that makes it a lot easier to scale.”

If large countries with warm, wet climates spread basalt on 40% of their agricultural land, enhanced rock weathering could remove roughly 2 billion tonnes of CO₂ every year, according to research led by David Beerling of the University of Sheffield, where he is the director of the Leverhulme Centre for Climate Change Mitigation. Even better, Beerling’s team has found that the minerals released during weathering improve soil fertility and crop yields.

Beerling says that the ease of spreading rock on farmland and the benefits to the farmer mean that it could be more readily adopted than other forms of carbon dioxide removal such as direct air capture, an industrial process that uses solvents to absorb CO₂ from the air.

Several companies, including Lithos Carbon of San Francisco and Undo in London, are already testing enhanced rock weathering at large scales, collectively spreading thousands of tonnes of rock last year. But none currently uses enzymes to assist the process.

Veolia and FabricNano hope that the biotech addition will give their approach the edge. The carbonic anhydrase enzyme is ubiquitous in nature, and there is already evidence that it is involved in natural rock weathering to some degree. In laboratory experiments, FabricNano has found that the enzyme enables faster dissolution of silica and other minerals, even when using larger grains of basalt. This is an advantage because bigger granules are more widely available, so the rocks can be sourced from quarries close to trial sites, reducing the costs and carbon emissions associated with transporting the material.

How tightly the enzyme binds to the rocks is crucial because it needs to keep working while it’s out in the field, come rain or shine. FabricNano has studied a wide range of carbonic anhydrase variants to identify those best suited to stick to the rock and remain chemically active. “We’re the first to look at how this enzyme sticks to rocks,” says Grant Aarons, CEO of FabricNano.

The company screened recombinant carbonic anhydrases, including some sourced from biotech giant Novonesis, headquartered in Lyngby, Denmark, as well as enzymes extracted from natural sources. They studied how a range of factors — including pH, temperature, concentration and the buffer solution used to deliver the enzyme — affect enzyme loading, catalytic activity and stability. The company then selected a few of the best carbonic anhydrases for the Veolia trials, including enzymes isolated from baker’s yeast and yogurt. (Novonesis declined to be interviewed for this article and did not respond to questions about its work with carbonic anhydrase.)

Aarons says their lab experiments found no significant loss in enzyme activity after it was stuck to basalt for several months. That is probably because binding to the basalt helps to improve enzymes’ thermal stability, and it may also offer some protection from degradation by soil microbes and proteases.

The Veolia trials, which began in April, are testing carbonic anhydrase applied at different concentrations. In some plots, basalt grains were preloaded with enzyme before being spread on the field; in others, the enzyme was sprayed on afterwards. Soil sampling will help to assess how much CO₂ is being locked into the ground, and the initial trials will conclude once crops growing in the fields are harvested later this year.

Veolia calculates that each tonne of basalt — teamed with micrograms to milligrams of enzyme, depending on the concentration of the applied solution — should be able to sequester about 300 kg of CO₂ when deployed on farmland. That amount of removal far outweighs the CO₂ emissions from transporting and spreading the basalt and enzymes.

The company is already considering further trials at both agricultural and non-agricultural sites, including former quarries and landfills. Although the process will initially cost $380–$510 per tonne of CO₂ removed — only a little cheaper than running a direct air capture plant — Veolia hopes that will fall to around $190 per tonne of CO₂ in the next 5 to 8 years. And if the weathered basalt does indeed improve crop yields, it could even reduce conventional fertilizer use.

But researchers who have previously studied carbonic anhydrase catalysis are cautious about the potential of this approach. “It may work in certain soils, but in my opinion, it won’t really be the magic bullet that we’re looking for,” says Dimitar Epihov at the University of Sheffield, who collaborates with Beerling’s team. Research led by Derek Bell, a PhD student supervised by Epihov and Beerling, has found that although carbonic anhydrase seems to accelerate basalt weathering in alkaline conditions, it is not effective in the acidic conditions found in most agricultural soils.

Ian Power, a geoscientist at Trent University in Peterborough, Ontario, looked at the impact of bovine carbonic anhydrase on the reaction between CO₂ and brucite, a magnesium hydroxide mineral, to form magnesium carbonate. The enzyme helped to speed up the process, but probably not enough for it to be economically worthwhile. “Unless it’s creating a massive improvement, I think it might be difficult to justify the expense,” Power says.

Craig Storey, a geologist at the University of Portsmouth, UK, is investigating whether carbonic anhydrase can accelerate similar geochemical reactions that would trap CO₂ deep underground in volcanic rock formations — as yet without success. But he has spoken with FabricNano about their work and notes that “they say they have demonstrated carbonate formation in the lab. I was quite surprised by that. But it’s exciting, if that's the case.”

FabricNano and Veolia are not alone is seeing the potential of carbonic anhydrase in climate tech. Last year, for example, engineering firm Saipem in Milan, Italy, launched a system that uses carbonic anhydrase supplied by Novonesis to help capture concentrated streams of CO₂ from industrial plant emissions.

In Sweden, Io Antonopoulou, a chemical engineer at Luleå University of Technology, is deploying carbonic anhydrase to speed up the absorption of CO₂ using byproducts of the pulp and paper industry. The reaction produces a bicarbonate-rich solution that could be injected into certain underground rock formations to sequester the carbon.

Although Antonopoulou’s team had some success in drawing CO₂ from the air, results were much improved when they used a 20% CO₂ stream, similar to the exhaust flue gas from industrial processes. In this case, adding carbonic anhydrase enabled up to a fivefold increase in the amount of CO₂ captured. “Flue gases are more ideal for this approach, rather than taking CO₂ from the air,” says Antonopoulou.

She also notes that by performing this process within an industrial reactor, rather than on open land, the efficiency of the process could be improved by fine-tuning the concentration of the solution, the CO₂ flow rate and other factors. Her team is collaborating with several industrial partners, including pulp and paper company Billerud in Solna, Sweden, to develop the technology.

Meanwhile, Beerling’s team is focusing on an alternative to enzyme acceleration. They are instead using synthetic chelator molecules to scavenge iron from basalt and speed up its breakdown. This creates iron-starved conditions that also spur soil microbes to make more of their own iron-harvesting molecules, known as siderophores, which attack the basalt as well. “What’s elegant about this is you’re manipulating the native soil microbiome,” Beerling says. “It’s like a double whammy.”

Still, FabricNano’s Aarons is optimistic about carbonic anhydrase. He acknowledges that the enzyme’s activity may vary depending on the chemistry of soils and basalts in different locations, and he hopes to build up a library of carbonic anhydrase formulations that could be deployed on the basis of the specific local conditions. “We’ll be able to do quick studies pairing enzymes with rocks, to know which enzyme works well, on which rock, from which quarry — and then we can start selling it,” he says.