ERW Secondary Mineral Formation & Cation Retention Model

As silicate minerals weather, chemical elements remain in solution and are transported through the soil profile, others may be incorporated into newly formed secondary minerals, in which the base cations Ca²⁺, Mg²⁺, Na⁺ or K⁺ can occupy exchange sites. Part of the cation inventory can therefore remain associated with the solid phase rather than appearing immediately in the porewater, delaying alkalinity export, which causes a delay in alkalinity export.

The CDRexperts ERW Secondary Mineral & Cation Retention Model evaluates the potential magnitude of this effect for a specific feedstock.

Using the measured mineralogical and chemical composition of the rock, the model estimates which secondary mineral phases could form during weathering and calculates the maximum amount of base-cation retention that could be associated with their newly formed cation exchange capacity. The model is based on conservative assumptions to answer the question:

How large could secondary-mineral and cation-exchange retention become for a specific feedstock?

A maximum-retention model.

The feedstock primary mineralogy determines which phases weather into what type of secondary mineral. A plagioclase-rich basalt, for example, presents a different secondary-mineral formation potential from a feedstock dominated by pyroxenes, glass or pre-existing alteration phases. The CEC component is then used to calculate a maximum-retention bounding scenario.

The model assumes complete weathering of the modelled reactive mineral assemblage and full formation of the corresponding secondary phases. High-end CEC assumptions can be used for exchangeable clay minerals, together with a base-saturation sensitivity parameter.

That is useful because:

• If the maximum possible effect is already small compared with the feedstock's total CDR potential, a project may have little reason to invest immediately in much more complex modelling of this process.

• If the upper bound is substantial, secondary-mineral and cation-exchange behaviour may deserve closer attention in the project's MRV strategy.

What you’ll need to provide:

• Quantitative XRD mineralogy of the feedstock

• XRF or equivalent bulk chemical composition of the same sample as above

What you‘ll receive:

• Feedstock-specific secondary-mineral formation assessment

• Maximum CDR capacity of the rock (i.e. CDRmax)

• Maximum newly formed cation exchange capacity

• Maximum base-cation retention expressed as charge equivalents

• CO₂-equivalent maximum retention in kg or t CO₂ per tonne of rock

• Retention as a percentage of the measured feedstock CDR potential

• Identification of the minerals and assumptions controlling the result

• Project-specific scientific interpretation and technical report

The underlying mineral-reaction framework and calculation model remain proprietary to CDRexperts. Clients receive the project-specific results, assumptions, sensitivity analyses and scientific interpretation required to use the assessment in project development and MRV decision-making.

What the model does and does not provide:

• The calculated value is not a prediction of CDR loss. Cation retention as CEC is actually a good thing for soil health and nutrient management. How much is lost by plant uptake needs its own measurement and/or modelling.

• It is also not a kinetic model and does not assign a residence time to cations held on exchange sites. CEC describes the capacity of a mineral surface to hold exchangeable cations. It does not, by itself, determine how long those cations will remain there.

• Actual behaviour in the field will depend on factors including soil pH, existing CEC, exchangeable acidity, base saturation, hydrology, biological uptake, solution chemistry and continued mineral reactions.

• The purpose of the model is therefore to establish a scientifically defensible upper bound.

• More complex geochemical or surface-charge modelling can subsequently be applied when the size of the potential effect justifies it.

Assess your ERW feedstock with CDRexperts

CDRexperts now offers project-specific assessments using the ERW Secondary Mineral & Cation Retention Model.

If you already have XRD and XRF data for a potential or existing ERW feedstock, we can evaluate its secondary-mineral formation potential and calculate a conservative upper bound for CEC-related base-cation retention.

The assessment can be delivered as a standalone feedstock evaluation or integrated into broader ERW MRV, sampling and CDR quantification work.

Working with an ERW feedstock and want to know whether secondary-mineral formation could materially affect your CDR accounting?

Contact CDRexperts at contact@CDRexperts.org to discuss your mineralogical data and project requirements.