Enhanced Rock Weathering: How it Works and How it Compares to Direct Air Capture

By Prof. Don MacElroy, Chairman of the NEG8 Carbon Advisory Board

The drive towards net-zero carbon emissions has given rise to several carbon dioxide removal technologies.

We have outlined the advantages and disadvantages of some of these (Biochar and Forestation) and here we give an overview of another approach which has actually been known for many years, namely weathering.

ERW was first proposed over 20 years ago, while a more recent, related approach simply referred to as ex situ Mineral Carbonation (MC) – a more intensive and industrialized form of ERW and which can involve greater energy demands – will not be discussed here but will be covered at a later time.

There are two main approaches to ERW, land-based and marine-based. In this article, we’ll be focusing on land-based ERW.

What is Enhanced Rock Weathering?

Enhanced rock weathering (ERW) is a method for capturing carbon dioxide (CO₂) by speeding up a process that happens naturally over a long period. It is like nature’s own version of carbon capture but sped up by crushing the rocks and spreading them where they can be most effective on open land.

How Does Enhanced Rock Weathering Work?

In nature, certain types of rocks—like basalt or olivine—gradually break down when they come into contact with rainwater and CO₂ from the air, and this process happens over 10,000s to 100,000s of years.

This chemical reaction locks the CO₂ into the rock in a solid mineralised form and therefore it cannot go back into the atmosphere. Over time, that locked-away carbon gets washed into rivers and eventually ends up as a carbonate on the ocean floor.

On a per hectare basis, natural weathering removes only a few tonnes of CO₂ per year, depending on rock type, rainfall, temperature, and surface area exposed.

On the other hand, enhanced rock weathering aims to speed this up by grinding the rock into a fine powder and spreading it over farmland. This increases the surface area exposed to air and water, helping the rocks absorb CO₂ more quickly.

Ultimately, with ERW, CO₂ removal can occur over months to years instead of millennia.

Benefits of Enhanced Rock Weathering

Along with pulling CO₂ out of the air to help fight climate change, ERW has several other benefits.

Long-term CO₂ storage

ERW captures carbon dioxide as stable carbonate minerals, locking it away for thousands to millions of years.

Soil improvement

Silicate rocks like basalt can remineralise soil and so enhance soil fertility which helps crops grow better and reduces the need for fertilisers.

Additional climate benefits

ERW may reduce emissions of other greenhouse gases such as nitrous oxide.

Scalable potential

With the right rock types and conditions, the IPCC estimates that ERW could remove up to 2 to 4 gigatonnes of CO₂ per year by 2050 (The European Parliament’s report).

Natural process

ERW builds on processes already happening in nature, offering a less intrusive method than many industrial alternatives.

Challenges of Enhanced Rock Weathering

Some potentially harmful outcomes are associated with poorly managed ERW applications. These include:

Verifiability

ERW verifiability as an approach to permanent storage of CO₂ is a possible issue requiring investigation (see I. M. Power et al, Frontiers in Clim. 6 (2025) 1510747). Processing via ex situ MC can provide a fully verifiable approach but remains to be fully developed technically and assessed economically.

Eco-toxicity and environmental damage

The effects of ERW distribution on land, waterways and fauna neighbouring the treated fields is largely unknown.

Nevertheless, there is a risk of heavy metal leaching, e.g. chromium and nickel, especially with certain rock types, notably powdered ultramafic rocks and olivine or serpentine rocks. However, no issues have been observed where basaltic rock powders were used. (As noted by J. Kierczak et al., Sci. Tot. Environ., 755 (2021) 142620, and J.P.M. Vink and P. Knops, Minerals, 13 (2023) 235)

Ecological impacts

Silicate particulates in the effluent run-off from ERW treated land could affect aquatic and marine life and biodiversity. Moreover, the effects of pH changes in tropical peatlands may cause environmental damage.

Health concerns

Since the main component of the typical rocks used in ERW is silica, if the rock powders contain high levels of crystalline silica then health issues arise such as silicosis and other related health issues.

Infrastructure demands

The transport networks and other infrastructure, which may be needed with large-scale ERW and the concomitant mining operations, could be environmentally harmful.

Emissions from energy usage and transport

Mining, crushing, and transporting the rock used in ERW from quarries and mining areas over large distances can produce notable emissions and demand high energy usage if not managed carefully.

Enhanced Rock Weathering (ERW) Compared to Direct Air Capture (DAC)

Enhanced Rock Weathering and Direct Air Capture (DAC) are both methods for removing carbon dioxide from the atmosphere, each with distinct benefits and drawbacks.

ERW is relatively low-cost, passive, and can improve soil health, but it depends heavily on suitable land, weather conditions, and long timescales for measurable CO₂ removal.

There are also concerns around a potential toxic effects on the environment and the lack of certifiable verification of permanent CO₂ storage.

ERW uses less energy than DAC per tonne of CO₂ removed, depending on how finely you crush the rocks. (ERW ~0.2–3.5 MWh per tCO₂ vs DAC ~1 – 2.4 MWh per tCO₂) However, DAC can capture CO₂ more quickly and in a controlled setting, while ERW is slower and depends on soil conditions, weather, and land availability.

DAC uses engineered machines to capture CO₂ directly from the air using a chemical process. (See: How Does Direct Air Capture Work?) The captured CO₂ can then be permanently stored underground or used in climate-neutral carbon products, such as Sustainable Aviation Fuel or re-used in storing CO₂ in concrete.

Furthermore, in some DAC systems, water is a by-product of the carbon capture process. For example, in NEG8 Carbon’s DAC technology, between 1 and 2,5 tonnes of water is produced for every tonne of CO₂ captured.

DAC can be deployed almost anywhere, and it allows for precise monitoring and permanent storage, but it comes at a higher financial and energy cost.

For more, see: The Advantages of Direct Air Capture

Enhanced Rock Weathering vs Direct Air Capture

Relevance to Ireland

Ireland has notable basalt resources, especially in the northeastern counties (County Antrim and the Antrim-Derry plateau in particular), and olivine may be a by-product of any mining activity that may be undertaken in the future.

There are also regions with metamorphic rock where olivine can be found, for example possibly in the Ox mountains in County Sligo and other areas with ultramafic rocks, including other western regions such as Dawros, Co Galway and Ballyconnell County Leitrim.

These natural resources lend themselves to enhanced rock weathering activity.

Case Study Example

Based on the observations from this broad range of studies, the following example provides estimates for a case study relating to basaltic rock treatment of arable cropland in Ireland:

1. The arable land area in Ireland is 436,000 ha and assuming the mining conditions for basaltic rock have been established within the country (none exist at the present time) and further assuming:

(i) Mined rock ground to an average 50 microns in diameter

(ii) A CDR rate of 5 tonnes CO₂/ha/yr in the first year of spreading with 50 tonnes of ground rock spread per hectare (i.e. a total rock requirement of 21.8 Mtonnes)

(iii) An average 275 km travel distance to transport the ground rock materials to individual sites for spreading

2. Based on these conditions and assuming the rock preparations are completed and resulting powder is spread in the first year of treatment, the capture rates and concomitant emissions are as follows:

(a) Capture rate by weathering in year 1: 2.18 Mtonnes of CO₂/yr

(b) Process emissions:
(i) Mining/quarrying of suitable rock = 81,450 Tonnes CO₂
(ii) Rock crushing, grinding and sizing = 171,800 Tonnes CO₂ (the millling power demand for all the rock required is 66 MW if completed in year 1 at a cost of €87M (at €0.15/kWh)
(iii) Powder loading, transport and spreading on site = 1.133 Mtonnes CO₂ (transport results in the highest CO₂ emissions at 1.032 Mtonnes of CO₂).

3. The net capture in year 1 is therefore 0.79 Mtonnes CO₂. However this will increase to ~ 2 Mtonnes in year 2 and the distributed powder will continue to absorb CO₂ over the next 8 year with a net total CDR of ~ 10-11 Mtonnes over the lifetime of the treatment i.e. ~1 Mtonnes/yr on average.

This can be increased by replenishing consumed materials in an appropriately managed way and/or by reducing the powder particle size to ~ 25 microns. Increasing the grinding emissions from 116 ktonnes to 164 ktonnes CO₂ but also increasing the CDR rate to ~ 2-4 Mtonnes/yr with repeated treatment over a period of 5-10 years.

The above example clearly demonstrates the possible value of ERW to Ireland and Irish agriculture however this needs to be borne in mind with the caveats noted earlier.

Furthermore, as mentioned in the ‘Challenges of ERW’ above, verifying the permanent storage of CO₂ using ERW remains an issue that needs further development.

Enhanced Rock Weathering in More Detail

For the more technical among us, here we go deeper into how ERW works.

Weathering of exposed rock and mineral surfaces can arise from a number of possible physicochemical or biological interactions between solid surface materials and the local environment.

The physical aspects of weathering primarily relate to the mechanical forces which rocks may be subject to as freezing water expands or as intrapore pressures increase sufficiently, due to expanding fluids under depressurization, resulting in stress fractures and rock deformation.

This overview will primarily outline the chemical aspects of weathering specifically from the perspective of enhanced rock weathering (ERW).

The ultimate benefits of ERW depend on two primary issues:

1. Rock origin and the processing conditions required for successful weathering.

2. Possible negative impacts which may arise from interactions with the environment and unavoidable emissions inherent in the initial phases of mineral preparation.

General Approach

To elucidate the general principals underlying ERW the work of J. Jerden et al, Appl. Geochem., 167 (2024) 106002, provides a useful introduction. This paper presents a model for ERW that accounts for biogeochemical and life-cycle variables.

The major processes arising in the application of ERW are:

Pathways to CO₂ emissions from ERW
(a) Mining/quarrying of suitable rock;
(b) Rock loading, transporting to comminution facility;
(c) Rock grinding and sizing;
(d) Powder transport to spreading site;
(e) Powder spreading.

Pathway to CO₂ capture with ERW
(f) Powder weathering.

The pathway of major interest in the above is (f) Powder weathering. This pathway entails the following process steps:

Atmospheric CO₂ is dissolved in rainwater to produce the weak carbonic acid H₂CO₃. The carbonic acid then reacts with the silicate minerals (e.g. components of basalt such as olivine) that have been spread, in powder form, on the soil.

These weathering reactions in turn produce carbonate species which can precipitate or are dissolved in run-off water and are carried by effluent flows to the oceans and these carbonates reside on land or in the sea for many thousands or millions of years.

The extent of CO₂ captured by the weathering process depends on cation (e.g. the divalent ions Mg²⁺, Ca²⁺) release rates from the rocks which are balanced by combination with the carbonate species (e.g. the bicarbonate HCO₃– or carbonate CO₃^2- anions).

For example, for basaltic examples a common rock component is Forsterite (Fo90) (Mg_1.8Fe_0.2SiO₄), which is most prevalent in olivine, and a widely available material under discussion as a basis for ex situ MC, in situ MC (e.g. Carbfix) as well as ERW, the following reaction takes place:

Mg_1.8Fe_0.2SiO₄ +3.6 CO₂(aq)+2.0 H₂O
→ SiO₂(aq)+0.2 Fe(OH)₂ +1.8 Mg²⁺+3.6HCO₃^–

Note that for minerals containing predominantly divalent ions (e.g. Mg²⁺, Fe²⁺) a much higher proportion of CO₂ is consumed than for monovalent ions such as Na⁺. A variety of possible reactions can take place depending on rock composition some of which are listed by Jerden et al.

The Stella ERW Model

In their paper Jerden et al provide details of the Stella ERW model which they demonstrate can provide a useful predictive tool for the behaviour of ERW systems.

In addition to accounting for carbon dioxide removal (CDR) by rock weathering, the Stella ERW model also takes into consideration CO₂ emissions from rock extraction, rock loading, transportation, crushing, milling, and spreading.

Given the significant benefits and risks of ERW, it is essential that the principal variables arising in the ERW process are cooperatively optimised.

The weathering process itself involves dissolution of the specific rock components to form carbonate. The rate of dissolution and hence the rate of CO₂ fixation is linearly proportional to the surface area of the powder, and hence inversely proportional to the effective particle size, and exponentially dependent on temperature through the reaction kinetics.

This rate is also a function of the activity of the hydrogen ion (pH) in the aqueous medium in which the rock powder is immersed. While pH is dependent on soil conditions it will also vary during the weathering process. Increases in pH with time arise as alkalinity (mainly bicarbonate) is produced during rock weathering (see for example the reaction above).

And model predictions demonstrate, in agreement with experimental observations, that at low initial pH the dissolution rate initially drops but as the system pH passes from acidic to neutral conditions (pH = 6 to 7) the dissolution rate rises again. The Stella ERW accounts for this effect using an empirical factor that scales pH to the mass of weathered rock with each time step.

A summary of observed CDR rates is provided by Jerden et al. The rock types with the highest values include olivine and basalt. Typical values for CDR range from 1 to 7 tCO₂/ha/yr for both olivine powders and basalts subject to soil conditions (pH), powder particle size and application rates. The model predictions are in fair agreement with observations and demonstrate that higher pH, higher soil temperatures and smaller rock powder particles lead to significantly enhanced CDR rates.

Conclusion

Enhanced rock weathering lets the earth do the work, spreading crushed rocks to lock carbon away, whereas Direct Air Capture removes CO₂ straight from the air fast and at scale, yet with the added challenge of reducing energy needs.

For more:

• Point Source vs Direct Air capture
• Biochar Carbon Sequestration vs Direct Air Capture
• Forestation vs Direct Air Capture
• Accelerated Carbonation Technology Plus Direct Air Capture

Interested in NEG8 Carbon’s CO₂ capture technology?

Contact the NEG8 Carbon Team