Enhanced Rock Weathering: A Dual Opportunity for Climate and Agriculture

Enhanced rock weathering (ERW) is emerging as a powerful and rapidly advancing technology in the fight against climate change. By leveraging natural geological processes, ERW not only offers a permanent solution for carbon dioxide (CO₂) removal (CDR) but also brings significant benefits to agricultural lands, presenting a scalable opportunity for carbon credits. Current innovations in ERW focus on scaling field applications, optimizing the selection of materials (including health and safety risk assessment), and integrating these processes into climate-smart agriculture and fertilizer development. 

Understanding the ERW Process

ERW deployment involves intentional spreading of finely crushed silicate and alkaline minerals, onto croplands and soils. This accelerates the natural weathering process, where these minerals react with atmospheric CO₂. The CO₂ is then converted into stable carbonates, effectively sequestering carbon for decades to millennia.

The overarching mechanism involves the reaction between the carbonic acid system and the dissolution of silicate/alkaline minerals in the soil. According to Henry’s Law, there is an equilibrium between gaseous CO2 in the soil or atmosphere and dissolved CO2 in pore water (Eq. 1). In addition, the carbonic acid system constantly responds to shifts in solution chemistry (Eq. 2).

 CO2(g) ⟷ CO2(aq): KH = CO2(aq)/CO2(g) [Eq. 1]

CO2(aq) + H2O(l)⟷ H2CO3(aq) ⟷ H+(aq) + HCO3-(aq) ⟷ 2H+(aq) + CO32-(aq) [Eq. 2]

As the silicate/alkaline mineral dissolves, it provides excess base cations, which removes H+ from soil solution, generating alkalinity. The resulting alkalinity shifts the balance of the carbonic acid system away from CO2(aq) and towards the HCO3- and CO32-, causing disequilibrium between dissolved CO2(aq) and gaseous CO2(g) in the atmosphere or soil. More CO2 in the atmosphere or soil then dissolves into pore water, re-establishing equilibrium and leading to CDR.

The overall reaction can be illustrated by the carbonic acid weathering of wollastonite, which can consume up to 2 moles of CO2 per mole of CaSiO3:

CaSiO3(s) + 2CO2(g) + 3H2O(l) ⟶ Ca2+(aq) + 2HCO3-(aq) + H4SiO4(aq) [Eq. 3]

The amount of CO2 sequestered depends on the type of minerals in the rock and the cations and anions produced during dissolution. Following the reaction in Eq. 3, the number of moles of CO2 consumed per mole of common rock and silicate minerals used in ERW deployments are summarized below. 

This information can be used to calculate the maximum potential CDR, which is defined as the maximum amount of CDR that could occur based on the mineral composition and chemistry. Maximum potential CDR is usually greater than potential CDR and net CDR, which are other terms used in CDR quantification. Potential CDR accounts for actual dissolution of the silicate mineral while net CDR accounts for transient cation or carbon losses, permanent losses, and process emissions associated with silicate mineral crushing, transport, and spreading.

Measuring, Reporting and Verifying Carbon Removal (MRV)

Quantification of CDR in ERW involves either tracking excess base cations in waters, monitoring their movement through soil, or following carbon directly by monitoring dissolved inorganic carbon (DIC) export or changes in soil CO2 efflux.

In addition, the durability or permanence of the sequestered CO2 is important. Weathering products are transported through soil and into downstream systems like groundwater, rivers, and ultimately the ocean. Removed CO2 stored as carbonate mineral in downstream systems is site-specific while those stored as DIC in groundwater and ocean are considered permanent.

A robust but evolving Measurement, Reporting, and Verification (MRV) framework for ERW deployment was recently published by Cascade Climate. This framework covers various DIC fluxes in the near-field zone (the upper part of the soil profile) and the far-field zone (the lower vadose zone, groundwater, rivers, and the ocean).

Scalable Carbon Credit Opportunity in Croplands

The integration of ERW into agricultural practices opens up a significant opportunity for scalable carbon credit generation. Croplands, being vast and globally distributed, offer immense potential for CO₂ sequestration through ERW.

Source: Weathering Potential Explorer.