This week’s publication highlights relate to direct air capture, enhanced rock weathering and ocean alkalinity enhancement.
Review: Scenario-Specific Applications of Direct Air Capture Technology and System Optimization Approaches
Abstract
As an indispensable negative emission solution within carbon neutrality strategies, the transition of direct air capture (DAC) technology from laboratory research to engineering deployment faces dual challenges of technical adaptability and system integration efficiency. This paper systematically analyzes the performance characteristics of mainstream technical pathways—including absorption, adsorption, and membrane separation methods—revealing high energy consumption and cost bottlenecks in DAC systems. It further identifies inadequate material cycling stability and poor energy supply compatibility as barriers to large-scale implementation. Building on this analysis, the study pioneers a multi-scenario application framework for industrial zones, urban areas, and remote regions, elucidating technology deployment across geographically distinct contexts. For system performance optimization, it establishes a cross-scale enhancement pathway spanning molecular-level interface modification, equipment-layer energy efficiency upgrades, and multi-energy supply systems. This is achieved through adsorbent modification (e.g., hydrophobic treatment of physical adsorbents, amine group dispersion reinforcement, and intelligent screening of porous materials), cascading waste heat utilization strategies, and multi-energy system integration. The work provides a theoretically rigorous and practically viable decision-support framework to advance DAC technology from unit innovation to system integration, offering critical guidance for accelerating the engineering implementation of negative emission technologies.
Zhou, Z. et al. (2026) Review: Scenario-Specific Applications of Direct Air Capture Technology and System Optimization Approaches 226 (116270) Renewable and Sustainable Energy Reviews.
Read the full paper here: Review: Scenario-Specific Applications of Direct Air Capture Technology and System Optimization Approaches I Renewable and Sustainable Energy Reviews.
Techno-Economic Assessment of Direct Air Capture Integrated with Heating Tower Heat Pump
Abstract
Direct Air Capture (DAC) technologies offer a promising means of mitigating climate change by removing CO2 from the atmosphere, but their widespread adoption has been hindered by high energy consumption and operational costs. To address this challenge, this study proposes an innovative approach that integrates DAC with an existing heating tower heat pump (HTHP), capitalizing on their structural and functional similarities. Structurally, both systems share key components, including fans, pumps, and towers. Functionally, while the HTHP absorbs heat from the ambient air, the DAC system captures atmospheric CO2, and both rely on regeneration processes to sustain operational efficiency. The integrated system uses a sodium carbonate (Na2CO3) solution to absorb both heat and CO2, enabling efficient integration of DAC and HTHP functions. Aspen Plus simulations show that the proposed system requires only 2.37 GJ/tCO2 for CO2 capture, representing a 73.1% reduction compared with a conventional DAC process (8.81 GJ/tCO2). The operating expenditure is correspondingly reduced to $39.5/tCO2, which is 68.4% lower than that of the conventional DAC system ($120/tCO2). If a carbon capture and direct utilization integrated technology is applied instead of conventional thermal desorption, the energy requirement can be further reduced to 0.5 GJ/tCO2. These findings demonstrate that integrating DAC with the HTHP system enables low-energy and low-cost carbon removal, offering a feasible and scalable pathway for large-scale CO2 capture.
Cai, W. et al. (2025) Techno-Economic Assessment of Direct Air Capture Integrated with Heating Tower Heat Pump. Energy & Fuels.
Read the full paper here: Techno-Economic Assessment of Direct Air Capture Integrated with Heating Tower Heat Pump I Energy & Fuels.
Riverine Photosynthesis Influences the Carbon Sequestration Potential of Enhanced Rock Weathering
Abstract
As climate mitigation efforts lag, dependence on anthropogenic CO2 removal increases. Enhanced rock weathering (ERW) is a rapidly growing CO2 removal approach. In terrestrial ERW, crushed rocks are spread on land where they react with CO2 and water, forming dissolved inorganic carbon (DIC) and alkalinity. For long-term sequestration, these products must travel through rivers to oceans, where carbon remains stored for over 10,000 years. Carbon and alkalinity can be lost during river transport, reducing ERW efficacy. However, the ability of biological processes, such as aquatic photosynthesis, to affect the fate of DIC and alkalinity within rivers has been overlooked. Our analysis indicates that within a stream-order segment, aquatic photosynthesis uptakes 1%–30% of DIC delivered by flow for most locations. The effect of this uptake on ERW efficacy, however, depends on the cell-membrane transport mechanism and the fate of photosynthetic carbon. Different pathways can decrease just DIC, DIC and alkalinity, or just alkalinity, and the relative importance of each is unknown. Further, data show that expected river chemistry changes from ERW may stimulate photosynthesis, amplifying the importance of these biological processes. We argue that estimating ERW’s carbon sequestration potential requires consideration and better understanding of biological processes in rivers.
Neumann, R. et al. (2025) Riverine Photosynthesis Influences the Carbon Sequestration Potential of Enhanced Rock Weathering. Frontiers in Climate.
Read the full paper here: Riverine Photosynthesis Influences the Carbon Sequestration Potential of Enhanced Rock Weathering I Frontiers in Climate.
Mineral Formation during Shipboard Ocean Alkalinity Enhancement Experiments in the North Atlantic
Abstract
Ocean alkalinity enhancement (OAE) is a carbon dioxide (CO2) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO2. Increasing seawater alkalinity leads to an increase in the saturation state (Ω) with respect to several minerals, which may trigger mineral precipitation, consuming the added alkalinity and thus decreasing the overall efficiency of OAE. To explore mineral formation due to alkalinity addition, we present results from shipboard experiments in which an aqueous solution of NaOH was added to unfiltered seawater collected from the surface ocean in the Sargasso Sea. Alkalinity addition ranged from 500 to 2000 µmol kg−1, and the carbonate chemistry was monitored through time by measuring total alkalinity (TA) and dissolved inorganic carbon (DIC), which were used to calculate Ω. The amount of precipitate and its mineralogy were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant precipitate phase is aragonite with trace amounts of calcite and magnesium hydroxide (MgOH2, i.e., brucite). Aragonite crystallite size increases and its micro-strain decreases with time, consistent with Ostwald ripening. The precipitation rate (r) in our experiments and those of other OAE-related calcium carbonate precipitation studies correlate with the aragonite saturation state (ΩA), and the resulting fit of log10(r) = n × log10 (ΩA−1) + log10 (k) yields a reaction order n=2.15 ± 0.50 and a rate constant k=0.20 ± 0.10 µmol h−1. The reaction order is comparable to that derived from previous studies, but the rate constant is 1 order of magnitude lower, which we attribute to the fact that our experiments are unseeded compared with previous studies that used aragonite seeds which act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlated with the initial Ω. Mineral precipitation occurred in a runaway manner, decreasing TA to values below those of seawater prior to alkalinity addition.
This study demonstrates that the highest risk of mineral precipitation is immediately following alkalinity addition and before dilution and CO2 uptake by seawater, both of which lower Ω. Aragonite precipitation will decrease OAE efficiency because aragonite is typically supersaturated in surface ocean waters. Thus, once formed, aragonite essentially permanently removes the precipitated alkalinity from the CO2 uptake process. Runaway mineral precipitation also means that mineral precipitation following OAE may not only decrease OAE efficiency at sequestering CO2 but could also render this approach counterproductive. As such, mineral precipitation should be avoided by keeping Ω below the threshold of precipitation and quantifying its consequences for OAE efficiency if it occurs. Lastly, in order to be able to quantitatively determine the impact of mineral precipitation during OAE, a mechanistic understanding of precipitation in the context of OAE must be developed.
Hashim, M. et al. (2025) Mineral Formation during Shipboard Ocean Alkalinity Enhancement Experiments in the North Atlantic 22 Biogeosciences.
Read the full paper here: Mineral Formation during Shipboard Ocean Alkalinity Enhancement Experiments in the North Atlantic I Biogeosciences.
Observation of Decadal Natural Ocean Alkalinity Enhancement in the South China Sea
Abstract
Ocean alkalinity enhancement (OAE) is a process of artificially increasing the alkalinity of seawater, chemically allowing the ocean to permanently absorb more carbon dioxide (CO2) from the atmosphere and reverse some of the chemical changes resulting from CO2-induced acidification. However, the long-term real impacts of OAE on ocean carbonate chemistry remain unexplored. This study examined a 26-year time-series study in the South China Sea as part of the Joint Global Ocean Flux Study, showing that the total alkalinity of the surface seawater increased annually by 0.56 μmol kg−1. Consequently, seawater increased its CO2 absorption by 28% and reversed the declines in seawater pH by 14% and calcium carbonate saturation state by 22%. The South China Sea provides a regional example of the consistency between the theory and field observations of OAE.
Lui, H. (2025) Observation of Decadal Natural Ocean Alkalinity Enhancement in the South China Sea 52 (17) Geophysical Research Letters.
Read the full paper here: Observation of Decadal Natural Ocean Alkalinity Enhancement in the South China Sea I Geophysical Research Letters.