Permanent CO2

removal in natural minerals

Sunstones pioneers technology that permanently removes
atmospheric CO2 by converting it into stable underground minerals

How CO2 mineralization works

For billions of years, the Earth has naturally regulated atmospheric CO2 by converting it into stable carbonate minerals such as limestone. This process occurs when rainwater containing CO2 reacts with metal-rich rocks, forming solid minerals — a natural and permanent form of carbon storage.

This mineralization process balances natural CO2 emissions from volcanoes, but it cannot keep up with today’s much larger human-made emissions. As a result, CO2 is accumulating in the atmosphere leading to global warming.

To avoid severe climate change, we must both eliminate future fossil emissions and actively remove some of the excess CO2 already in the atmosphere. At Sunstones, we accelerate nature’s own mineralization process to deliver permanent CO2 removal.

See the 4 steps of CO2 mineralization below.

Step 1: CO2 capture

CO2 capture by photosynthesis

Plants absorb CO2 from the atmosphere through photosynthesis, but this CO2 is released again as plants decompose or move through the food chain — a natural part of the Earth’s biological carbon cycle. Fossil fuels, however, add vast amounts of additional CO2 to this cycle, driving global warming.

To counter this, Sunstones works to permanently remove CO2 from the cycle. We capture biogenic CO2 — for example from biogas plants — and convert it into stable underground minerals using proven mineralization technology.

Our goal is to deliver lasting climate benefits in close collaboration with agriculture and the biogas sector. We also support direct removal of CO2 from the atmosphere and oceans through technologies such as Direct Air Capture and Direct Ocean Capture — helping to reduce greenhouse gases, one tonne of CO2 at a time.

Step 2: Water

WATER-POWERED CARBON STORAGE

To mineralise CO2, the gas is first dissolved in water — effectively creating “sparkling water” that can be pumped underground using standard water technology. This approach eliminates the need for energy-intensive gas compression, reduces leakage risk, and ensures the CO2 remains underground. Because CO2-rich water is denser than regular water, it naturally sinks deeper, where it can be permanently mineralised.

Sunstones’ technology circulates water from deep underground to the surface, where CO2 is dissolved into it before being returned below ground. The water used is saline and not suitable for drinking, ensuring that the process does not compete with valuable freshwater resources.

Step 3: Volcanic ash

Volcanoes provide the final ingredients for mineralisation

Beneath large parts of Denmark lie geological layers of volcanic ash, deposited around 55 million years ago when eruptions in the Atlantic spread ash across what was then a shallow sea. In some locations, these ash layers are exposed along the Limfjord coast, where Ice Age glaciers pushed them to the surface. In most areas, however, they remain buried several hundred metres below the surface.

Research shows that these deep ash formations are particularly well suited for CO2 mineralisation. Their volcanic origin makes them rich in metals such as calcium, iron, and magnesium — essential components in the mineralisation process. When CO2-rich water seeps through the ash, these metals are gradually released into the water, creating “hard” water similar to that found in drinking water across much of Denmark.

Step 4: Minerals

TURNING CO2 INTO LASTING MINERALS

As the water slowly flows through the deep ash layers, it becomes enriched with naturally occurring metals, increasing its hardness. These metals then begin to react with the dissolved CO2, transforming it into stable carbonate minerals — similar to the limescale deposits formed by hard water in our kitchens and bathrooms. This process permanently binds the CO2 in mineral form, where it remains stable for millions of years.

Mineral formation takes place within the many pore spaces of the ash layers, which provide ample room for the process to occur. Our research shows that mineralisation is confined to a limited area — typically within a few hundred metres of the water injection. As a result, the entire process, from water injection to permanent CO2 binding, occurs locally and discreetly — for example, beneath a farming field next to a biogas plant.

Throughout the process, conditions are carefully monitored and documented through continuous water sampling.

About

Why

We are Sunstones

Sunstones is a Danish climate technology company founded in 2023 by scientists, united by a clear mission: to enable permanent CO2 removal at scale in a safe, natural and cost-effective manner. We develop, build and operate CO2 mineralisation systems in close collaboration with biogas facilities, combined heat and power plants and, in the future, Direct Air Capture solutions. Our customers are primarily organisations seeking to remove both current and historical CO2 emissions.

Through CO2 mineralisation, Sunstones delivers a solution that combines natural processes with technological innovation and local integration. By transforming CO2 to mineral form, we make it possible to remove carbon for good.

Our purpose

Human emissions of fossil CO2 have raised the concentration of greenhouse gases in the atmosphere to levels not seen for millions of years. The long-term implications are well understood: accelerating climate change and rising sea levels that will affect generations to come.

According to the Intergovernmental Panel on Climate Change (IPCC), stabilising the climate requires not only a rapid reduction of future emissions, but also the large-scale removal of CO2 that has already been released. At Sunstones, we are developing technologies designed to address exactly this challenge.

We recognise that no single solution can solve the climate crisis. However, by working alongside other technologies and stakeholders, we can take the first critical steps toward reversing the flow of CO2 — delivering meaningful, durable climate impact.