Lime carbonation direct air capture (DAC) systems remove atmospheric carbon dioxide (CO2) by carbonating calcium hydroxide (Ca(OH)2) to produce calcium carbonate (CaCO3), which can release CO2 for durable storage. Accurate and precise measurement of generated CaCO3 is essential in quantifying CO2 removed from the atmosphere, and for optimizing the carbonation process. Methods for measurement of carbonate content are well established, but have yet to be applied to materials produced by this system (i.e., almost solely Ca(OH)2 and CaCO3). Five carbonate content analysis techniques (loss on ignition, LOI; thermogravimetric analysis, TGA; combustion analysis of carbon via infrared absorption, CAC-IR; volumetric calcimetry; and quantitative Fourier transform infrared spectroscopy, FTIR) were investigated for their measurement accuracy and precision over a range of carbonate contents. Sample throughput and levelized cost of analysis were considered in addition to accuracy and precision. LOI and CAC-IR proved favorable against equal consideration of the four factors. Weighting for accuracy and precision, LOI was favorable. Standard operating procedures, including established accuracy and precision levels, for viable carbonate content quantification techniques should be developed, tested, and presented to assure carbon credit buyers, the scientific community, and the public on the validity of carbon credits generated by lime carbonation DAC.

Soil porewater chemistry underpins the quantification of carbon dioxide removal (CDR) in enhanced rock weathering (ERW), where alkalinity and dissolved ion measurements are used within monitoring, reporting and verification (MRV) frameworks. The reliability of these measurements depends on the extraction method, with many in-field techniques constrained by soil moisture availability. This dependence can limit data continuity, reduce comparability across sites and seasons and introduce uncertainty into CDR-relevant interpretations. Here, we present and evaluate a new soil porewater extraction method, SATuration–Centrifugation (SAT-C), designed to obtain porewater from a defined soil volume independent of moisture conditions. The method combines saturation of intact soil cores using deionized water with centrifugation to recover porewater for chemical analysis. We compare SAT-C with conventional sampling using soils collected from grassland field sites, including sites amended with crushed basalt as part of ERW field trials. Across sites, SAT-C porewater chemistry shows strong agreement with rhizon samples for major ions, and bicarbonate concentrations inferred from charge balance are consistent with measured alkalinity. These initial results show that SAT-C provides a robust and reproducible approach for soil porewater extraction, with the potential to improve sampling reliability across variable conditions and thereby support more reliable carbon accounting in ERW.

A holistic, transparent life cycle assessment (LCA) of coal ash as a feedstock for indirect carbon mineralization is presented, enabling the tracking of energy and material inputs and outputs with a cradle-to-gate system boundary. We analyze the carbon storage and coproduct (e.g., rare earth element and supplementary cementitious material) recovery potential of high and low carbon storage potential coal ashes. The LCA allows for the variation of grid carbon intensity; ash feedstock elemental composition, particle size distribution and moisture content; CO2 source and associated separation and compression energy; extent of reagent electrochemical regeneration; and ash transport distance. We find three plausible scenarios to achieve net removal with coal ash mineralization: (1) high efficiency reagent regeneration with a clean power system, (2) feedstock-constrained deployment with coproduct displacement via avoided emissions burden and (3) reduced chemical intensity, sacrificing some mineral revenue for lower energy and reagent use.

We demonstrate the potential of deploying rigorous, data-driven monitoring, reporting, and verification (MRV) techniques across small-scale, distributed biochar production projects operating in India, Kenya, the United States, and Canada. Conventionally, such rural and decentralised systems are often perceived as “low-tech” or “artisanal” and insufficiently robust for carbon markets that have traditionally favoured large, centralised, industrial carbon dioxide removal (CDR) facilities. This paper challenges that assumption by presenting a suite of digital MRV tools – being co-developed with local community partners – that provide visibility into reactor operation, production events, and data quality, even under resource-constrained conditions. Automated validation layers can detect and flag errors and may eventually reduce intensive human-based checking, while human-centred interfaces support accurate data entry by operators with varied technical backgrounds. Active dialogues with various registries have contributed to the ongoing development of new guidelines around how decentralised biochar carbon removal should be implemented and verified, the first version already available for public view by Puro.earth, Isometric, and Rainbow. Lowering the operational and evidentiary barriers to certification expands access to carbon markets for rural and underserved communities, potentially broadening participation in carbon removal markets while supporting the production of registry-aligned carbon removal credits at geographically distributed scales.

The rapid rise in atmospheric carbon dioxide (CO2) levels has led to severe global warming issues in the twenty-first century. To address this issue, direct air capture (DAC) is one of the most promising negative-emission technologies for achieving a carbon-neutral environment in the near future. Metal–organic frameworks (MOFs), with their high porosity and tunability in enhancing adsorption capabilities, have gained prominence as a promising sorbent candidate for CO2 capture. This review article aims to: a) briefly outline the most recent developments of MOFs directly targeted for DAC applications; b) provide a comprehensive review of the various classes of MOFs, such as hydroxy-rich, amine-functionalized and MOFs with open-metal sites that can enhance compelling DAC efficiency and c) provide insights into the key challenges and future perspectives in the development of an MOF material that is cost-effective, high-performing and with great durability in the pathway of achieving carbon neutrality toward real-life DAC applications.