Evaluating the performance of ion-exchange resin devices as an MRV tool in mesocosm-scale weathering experiments

Despite increasing scientific interest in Enhanced Rock Weathering (ERW) and a rapid growth of commercial operations in this field, reliable and affordable methods for Monitoring, Reporting and Verification (MRV) remain a critical challenge for upscaling. Soil-based MRV methods provide an upper-bound estimate for Carbon Dioxide Removal (CDR) established through the quantification of feedstock weathering rates. However, robust CDR calculations require accounting for various cation loss pathways, such as plant uptake and soil sorption, necessitating extensive and costly measurements, each with the potential to introduce uncertainty into the CDR estimate. Porewater-based MRV methods, which aim to quantify the export of CDR-relevant aqueous-phase weathering products, also present challenges. These methods typically rely on specialized water extraction instruments that can be expensive to procure, install and maintain, and may not function reliably under low soil moisture conditions. Furthermore, solute flux calculations derived from discrete porewater samples are dependent on accurate water balance estimates, and often involve interpolating data gaps, potentially introducing uncertainty. As commercial ventures continue to explore ERW, developing reliable and scalable MRV methodologies is essential for ensuring the credibility and widespread adoption of this CDR strategy.

We propose the use of ion-exchange resin devices, known as Self-Integrating Accumulators (SIAs), as a potential improvement for MRV of ERW. Originally developed to determine nutrient leaching in soils¹, SIAs offer the potential for cost-effective and robust time-integrated measurements of subsoil cation and anion fluxes relevant to CDR quantification. The performance of SIAs adapted for this purpose was evaluated in a series of mesocosm-scale weathering experiments, primarily designed to assess the adsorption efficiency and recovery rates of major ions used for MRV.

Preliminary experiments, utilizing basaltic feedstock (applied at an equivalent rate of 200 t/ha) and agricultural soil sourced from western Germany, demonstrated high (>90%) major ion adsorption efficiencies by SIA devices, despite the induction of strong weathering fluxes. The second phase of this research employed a lower application rate (100 t/ha) to more closely simulate feasible operational deployments. We present here initial findings from this series of experiments, aiming to demonstrate the applicability of SIAs for large-scale ERW deployments, which we believe could significantly improve the accuracy, cost-effectiveness, and efficiency of MRV.

References

¹Bischoff, Wolf-Anno. 2007. “Development and Applications of the Self-Integrating Accumulators: A Method to Quantify the Leaching Losses of Environmentally Relevant Substances.” PhD Thesis, Berlin, Germany: Technische Universität Berlin. https://depositonce.tu-berlin.de/items/64e16848-244b-4b50-920c-8d835efda918.