This week’s publication highlights relate to direct air capture, biochar and ocean alkalinity enhancement.
Solid Sorbents for Direct Air Capture: A Technological and Environmental Perspective
Abstract
Direct air capture (DAC) is a pivotal technology for achieving net-zero emissions, yet its scalability is constrained by energy intensity and material limitations. This work critically examines the current landscape of solid sorbents for DAC, focusing on their performance, durability, and environmental impact. Key sorbent classes — amine-functionalized materials, carbonates, zeolites, and metal-organic frameworks — are evaluated in terms of CO₂ uptake, energy requirements, and life cycle emissions. A novel exergetic efficiency metric is introduced, incorporating sorbent degradation to better reflect real-world performance. Structured supports such as laminates and monoliths are discussed for their role in enhancing mass transfer and reducing pressure drop, though often at increased cost and environmental burden. Life cycle assessment (LCA) results highlight that energy consumption dominates DAC’s carbon footprint, with sorbent-related impacts becoming significant only for short-lived or energy-intensive materials. Emerging materials like hydroxylated activated carbon, along with alternative processes such as moisture swing adsorption and electrochemical DAC, offer promising pathways to reduce energy demand and improve sustainability. The work underscores the need for integrated assessments that link sorbent properties, process design, and environmental metrics from early development stages. Future research should prioritise sorbent longevity, comprehensive kinetic data, and inclusion of support structures in LCA models to enable cost-effective and climate-positive DAC deployment.
Mennitto, R. et al. (2025) Solid Sorbents for Direct Air Capture: A Technological and Environmental Perspective 50 (101195) Current Opinion in Chemical Engineering.
Read the full paper here: Solid Sorbents for Direct Air Capture: A Technological and Environmental Perspective I Current Opinion in Chemical Engineering.
Is Soil Sampling Appropriate for Quantitative Carbon Accounting for Biochar? An Experimental Investigation to Assess Soil Carbon Accumulation
Abstract
Biochar, a major CDR method with significant co-benefits to agriculture, is listed as a sustainable agricultural method for SCA in sustainable biofuel regulations. In Europe, this is accounted via the esca factor (REDII-IR), while at international level this is considered through the Fsca factor. Fsca is analogous to esca in REDII, with similar, even if not identical, requirements (ICAO, for SAF). RED-II requires soil sampling to quantitatively assess the SCA from biochar addition: instead, ICAO CORSIA, as well as the draft incoming EU-CRCF (for voluntary carbon removals), require full characterization of biochar, incorporation in soil and third-party auditing during deployment (ICAO), but not necessarily soil sampling. This study presents experimental evidence evaluating the adequacy of current soil sampling protocols for the quantitative accounting of carbon saving/removals from biochar application to soil. The findings demonstrate that these protocols have intrinsic limitations, even when applied within a narrowly defined (75 m2), homogeneous, and controlled area. Key issues include the arbitrary selection of sampling locations, the limited quantity of material analysed by standard laboratory instrumentation, and the statistically insignificant variation observed in SOC and BD measurements. Measured SOC figures were inconsistent with the amount of carbon introduced through biochar amendment: the SOC content of the biochar-amended soil plot was larger than the one actually introduced and thus expected to be retrieved via analytics. This observation is attributed to the spatial heterogeneity of soil characteristic, and statistical significance of measured samples, in addition to the physical challenge of blending homogeneously a solid amendment (biochar) in a the solid soil phase, a limitation that cannot be entirely overcome even when employing conventional and appropriate tillage methods.
These results also raise broader concerns regarding the use of conventional soil sampling protocols for establishing SOC baselines in other (i.e. non biochar-based) carbon farming approaches. The observed high variability in carbon stock measurements hardly matches the precision required for assigning economic value. To address these shortcomings, an integrated approach combining rigorous experimental design with validated modelling frameworks is necessary to ensure scientifically robust and quantitatively defensible allocation of greenhouse gas (GHG) mitigation benefits and carbon savings/credits.
Chiaramonti, D. et al. (2026) Is Soil Sampling Appropriate for Quantitative Carbon Accounting for Biochar? An Experimental Investigation to Assess Soil Carbon Accumulation 205 (108537) Biomass and Bioenergy.
Read the full paper here: Is Soil Sampling Appropriate for Quantitative Carbon Accounting for Biochar? An Experimental Investigation to Assess Soil Carbon Accumulation I Biomass and Bioenergy.
Highly Efficient and Regenerable Amine-Impregnated Adsorbents: Mechanic Insights into Glycerol Modification for Enhanced Direct Air Capture
Abstract
Amine-impregnated adsorbents play a crucial role in direct air capture (DAC) technologies, however, challenges remain in balancing high amine loading, enhanced amine efficiency and long-term stability against urea formation. This study introduces a highly efficient and regenerable GI-PEI@MSF adsorbent, synthesized by impregnating glycerol-modified polyethyleneimine (GI-PEI) onto a mesoporous silica foam (MSF) support with ultra-large pore structure derived from coal fly ash (CFA). The 30 %GI-PEI@MSF adsorbent exhibited a remarkable CO2 uptake of 9.70 mmol·gPEI−1 with an enhanced amine efficiency of 115.9 %, while maintaining excellent cyclic stability under harsh regeneration conditions. Hydroxyl-rich GI formed the intermolecular hydrogen bonds with PEI and facilitated the PEI dispersion, enabling a lower optimal adsorption temperature and faster adsorption kinetics, which are critical for lowering DAC energy demands. Characterization and Density Functional Theory (DFT) analysis provided molecular-level insights into the dual-functionality mechanism of GI modification: The secondary hydroxyl groups of GI, acting as Bronsted bases, can partially replace the primary or secondary amines of PEI in CO2 adsorption, thereby enhancing the amine efficiency and inhibiting the urea formation. These findings demonstrate the feasibility of CFA recycling and GI modification for enhanced CO2 adsorption, providing a promising strategy for scalable and cost-effective DAC applications.
Yan, F. et al. (2025) Highly Efficient and Regenerable Amine-Impregnated Adsorbents: Mechanic Insights into Glycerol Modification for Enhanced Direct Air Capture 520 (166450) Chemical Engineering Journal.
Read the full paper here: Highly Efficient and Regenerable Amine-Impregnated Adsorbents: Mechanic Insights into Glycerol Modification for Enhanced Direct Air Capture I Chemical Engineering Journal.
Assessing the Effectiveness of Ocean Alkalinity Enhancement on Carbon Sequestration and Ocean Acidification
Abstract
As atmospheric carbon dioxide (CO2) levels continue to rise, increasing attention is focussed mitigation techniques that could enhance the natural draw down of CO2. One such mitigative intervention is ocean alkalinity enhancement (OAE). OAE involves dissolving alkaline materials into ocean surface waters to increase its natural CO2 buffering capacity. Limestone and lime have received the most attention given their widespread availability. Here, we address the order one policy-relevant question of whether OAE represents a viable CO2 removal solution to global warming. We use the UVic Earth System Climate Model to explore the potential of OAE interventions under representative concentration pathways (RCPs) 2.6, 4.5, 6.0, and 8.5. For each RCP, we undertake three OAE interventions. First, we assume that the global annual production of limestone is crushed and uniformly distributed across and immediately disassociates in the surface waters of the global ocean. Second, we assume that the global production of limestone is converted to lime with the CO2 released in this process being added to the atmosphere. In the third intervention, we repeat the second intervention but sequester the CO2 arising from lime production. Our results suggest that CaCO3-based OAE interventions have little potential for mitigating global warming given the restraint of the current world’s output of limestone from mining.
Martin, K. et al. (2025) Assessing the Effectiveness of Ocean Alkalinity Enhancement on Carbon Sequestration and Ocean Acidification. Facets.
Read the full paper here: Assessing the Effectiveness of Ocean Alkalinity Enhancement on Carbon Sequestration and Ocean Acidification I Facets.
From Semi-Arid Biomass to Carbon Sequestration: Modelling the Impact of Pyrolysis Atmosphere and Soil Temperature on the Carbon Removal Potential of Agava-based Biochar
Abstract
Biochar has emerged as a promising carbon dioxide removal strategy due to its long-term stability and agronomic co-benefits. However, certification bodies often overlook soil temperature variability, and bench-scale experiments typically assume inert atmospheres unlike those in industrial systems. This study investigates the effects of pyrolysis temperature (350–575 °C) and atmosphere (nitrogen (N2) and recirculated pyrolysis gas (PyGR)) on the carbon removal potential of Agave wercklei biochar using a temperature-dependent modeling approach. Sensitivity analyses confirmed that pyrolysis temperature is the most influential variable affecting carbon permanence. Partial derivatives showed that increasing pyrolysis temperature leads to higher carbon sequestration, especially under PyGR. In contrast, increasing soil temperature had a negative effect on permanence, though this impact became less significant at high pyrolysis temperature. Overall, PyGR-based biochars produced at temperatures ≥500 °C demonstrated superior carbon removal performance. The estimated carbon sequestration for PyGR-based biochars in this temperature ranged in 0.35–0.49 tCO2eq tbiomass−1, with maximum values reaching 0.58 tCO2eq tbiomass−1 under optimal conditions. This higher performance also translated into greater economic return for semi-arid context, with an average carbon credit value of 55.92 € tbiomass−1 and peak values near 65 € tbiomass−1. In contrast, N2-based biochars under similar pyrolysis temperatures exhibited a range in 0.28–0.46 tCO2eq tbiomass−1, reaching a maximum of 0.47 tCO2eq tbiomass−1, with average credit potential estimated at 41.36 € tbiomass−1. These results also highlight the importance of using realistic pyrolysis atmospheres in bench-scale tests and considering soil temperature to improve carbon permanence estimates carbon storage scenarios.
Da Silva, S. et al. (2025) From Semi-Arid Biomass to Carbon Sequestration: Modelling the Impact of Pyrolysis Atmosphere and Soil Temperature on the Carbon Removal Potential of Agava-based Biochar 203 (108283) Biomass and Bioenergy.
Read the full paper here: From Semi-Arid Biomass to Carbon Sequestration: Modelling the Impact of Pyrolysis Atmosphere and Soil Temperature on the Carbon Removal Potential of Agava-based I Biomass and Bioenergy.