Moisture-Driven Air Capture System

• Moisture-driven air capture is a process where changes in water activity shift CO2 adsorption and desorption, enabling direct air capture.
• It utilizes various sorbents—from humidity-swing resins to aqueous concentration methods—to achieve controlled CO2 uptake and release.
• Demonstrations reveal performance tradeoffs, emphasizing the need for optimized water management and material durability in scalable systems.

Searching arXiv for papers on moisture-driven direct air capture, humidity swing sorbents, and related system studies. A moisture-driven air capture system is a direct air capture configuration in which the controlling process variable is water activity rather than high-temperature regeneration. Across the literature, the defining operational motif is that a sorbent or alkaline capture medium changes its carbon affinity when it is dried, wetted, diluted, concentrated, or exposed to water vapor, so that one humidity state favors atmospheric CO$_2$ uptake and another favors release (Shi et al., 2017, Rinberg et al., 2021, Sinyangwe et al., 24 Jun 2026). The topic spans several material and process families, including quaternary-ammonium anion-exchange resins, activated-carbon-supported bicarbonate or carbonate salts, charged polymers, diamine-appended metal-organic frameworks whose adsorption mechanism changes under humid direct-air-capture conditions, and aqueous alkalinity concentration swings implemented through desalination-style concentration hardware (Tomaraei et al., 5 Aug 2025, Mendez-Lozoya et al., 6 Aug 2025, Marshall et al., 2024, Rinberg et al., 2021). In this broader sense, a moisture-driven air capture system is best understood as a humidity-responsive or water-content-driven carbon capture process in which capture, regeneration, delivery, or outgassing is coupled to hydration-state-dependent chemistry, adsorption morphology, or solution speciation rather than to conventional thermal swing alone (Shi et al., 2017, Flory et al., 4 Aug 2025).

1. Concept and operating logic

The classic moisture-swing logic is that the sorbent is in a capture-favorable state when relatively dry and in a release-favorable state when relatively wet. In charged polymer systems, the dry material is described as favoring CO$_2$ uptake and the wet material as favoring CO$_2$ release, with regeneration accomplished by changing water activity rather than adding heat or applying vacuum (Yogaganeshan et al., 15 Aug 2025). In quaternary-ammonium ion-exchange systems, the operating description is similarly stated as “drying-induced CO2 uptake” and “hydration-induced CO2 release” (Tomaraei et al., 5 Aug 2025). Earlier work on carbonate-containing nanoporous materials formulates the same principle as a system that “absorbs CO2 from the air when the surrounding is dry, whereas desorbs CO2 when wet” (Shi et al., 2017).

This logic also appears in water-mediated liquid processes, but with the wet and dry states expressed as dilution and concentration rather than as sorbent hydration alone. In Alkalinity Concentration Swing, a dilute alkaline solution exposed to air absorbs atmospheric CO$_2$, while concentrating that same solution by removing water raises its equilibrium $p_{CO_2}$ and causes outgassing; dilution then restores a capture-ready state (Rinberg et al., 2021). A plausible implication is that “moisture-driven” is not restricted to solid sorbents. In the literature surveyed here it includes both humidity-swing solids and aqueous concentration-swing systems, provided that changing water content is the swing variable.

Several practical system embodiments follow from this logic. One embodiment uses a moving belt or loop that cyclically transports quaternary-ammonium anion-exchange resin between ambient air and an alkaline aqueous medium, so that the resin captures CO$_2$ while drying and releases it when immersed (Flory et al., 4 Aug 2025, Flory et al., 6 Aug 2025). Another uses a packed bed of macroporous ion-exchange resin and regenerates it by vacuum-driven water-vapor stripping rather than external heat, so that water loading directly controls CO$_2$ affinity (Sinyangwe et al., 24 Jun 2026). In all of these cases, the air-capture step and the regeneration step are linked by controlled changes in hydration state.

2. Chemical and thermodynamic basis

The moisture-swing mechanism is commonly expressed through carbonate, bicarbonate, and hydroxide interconversion. In charged polymer systems the key chemistry is given or implied as

$\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$

followed, under more alkaline conditions, by

$$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$

with the moisture-swing overall equilibrium represented as

$$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$

(Yogaganeshan et al., 15 Aug 2025). In the quaternary-ammonium resin literature, the hydration-state-dependent formulation is written as

$_2$0

$_2$1

and

$_2$2

(Tomaraei et al., 5 Aug 2025). These equations encode the defining claim that hydration shell size changes the free-energy balance among reactive anionic states.

A distinct but related thermodynamic treatment appears in the nanoporous carbonate system of “A Carbon Dioxide Absorption System Driven by Water Quantity” (Shi et al., 2017). There the key humidity-sensitive hydrolysis equilibrium is written as

$_2$3

and, more explicitly,

$_2$4

(Shi et al., 2017). The paper’s central claim is that as the number of water molecules decreases in confined nanopores, the free energy of carbonate ion hydrolysis is reduced (Shi et al., 2017). Quantitatively, it reports that with fewer than about 5 surrounding water molecules the reaction energy is negative, while at high water numbers the energy approaches a plateau of about $_2$5 (Shi et al., 2017). This establishes a mechanistic basis for dry-state activation in confined systems.

In the aqueous alkalinity-swing framework, the governing chemistry is expressed through dissolved inorganic carbon,

$_2$6

together with Henry’s law and carbonate equilibria (Rinberg et al., 2021). The concentration swing produces a higher equilibrium outgassing pressure because, to leading order, the maximum outgassing pressure scales as

$_2$7

where $_2$8 is the concentration factor (Rinberg et al., 2021). This suggests that “water-mediated direct air capture” and “moisture swing” are thermodynamically analogous in the limited sense that both rely on water-content-induced shifts in carbonate speciation.

Humidity can also change adsorption mechanism rather than only equilibrium position. In humid direct-air-capture experiments on diamine-appended $_2$9, adsorbed water shifts the equilibrium adsorbed morphology from cooperative ammonium-carbamate chains to predominantly non-cooperative CO$_2$0 species, explaining the disappearance of the anomalous dry-air induction effect and the transition from dry type-V-like stepped adsorption to humid type-I-like adsorption behavior (Marshall et al., 2024). The proposed physical mechanism is electrostatic screening by water, discussed through Coulomb’s law,

$_2$1

with increasing $_2$2 weakening the ionic interaction that stabilizes cooperative chains (Marshall et al., 2024). This is not a moisture swing in the resin sense, but it demonstrates that humidity can be a first-order control variable for DAC adsorption physics.

3. Sorbent classes and structural organization

Moisture-driven air capture has been implemented in several sorbent classes with different structural consequences for water, ion, and CO$_2$3 transport. One major class is the strong-base anion-exchange resin bearing quaternary ammonium groups. Amberlite IRA900 is described as a macroporous anion exchange resin consisting of a styrene-divinylbenzene crosslinked matrix functionalized with trimethylammonium groups (Sinyangwe et al., 24 Jun 2026), while Purolite A501 is described as a crosslinked polystyrene backbone functionalized with quaternary ammonium groups (Flory et al., 6 Aug 2025). In direct-contact biological delivery studies, the measured ion exchange capacities of A501, HPR-4800, and IRA-900 were around $_2$4–$_2$5, with A501 chosen for biocompatibility, alkaline stability, and rapid CO$_2$6 delivery kinetics (Flory et al., 6 Aug 2025).

A second class is the charged polymer or anion-exchange membrane used as a moisture-swing sorbent. Structural characterization of Fumasep FAA-3 and IRA 900 shows that both possess short-range molecular order, humidity changes that order, and water causes swelling and reorganization (Yogaganeshan et al., 15 Aug 2025). For Fumasep FAA-3, WAXS features near $_2$7 were reported, with the $_2$8 feature shifting to smaller length scales with increasing RH, while a distinct perpendicular SAXS hump appears at 95% RH corresponding to roughly $_2$9–$_2$0 (Yogaganeshan et al., 15 Aug 2025). For IRA 900, a WAXS peak at $_2$1 corresponds to $_2$2, and a SAXS plateau around $_2$3 corresponds to about $_2$4, indicating larger-scale clustering of polymer chains (Yogaganeshan et al., 15 Aug 2025). AFM, FIB-SEM, and TEM revealed clusters at $_2$5, pores of $_2$6–$_2$7, and stacked substructures around $_2$8 in IRA 900 (Yogaganeshan et al., 15 Aug 2025). The authors argue that pores in the nanoporous regime might facilitate bulk water, ion, and gas transport during moisture-mediated DAC (Yogaganeshan et al., 15 Aug 2025).

A third class is activated carbon or nanostructured graphite carrying bicarbonate or carbonate salts. Activated carbon impregnated with potassium bicarbonate, denoted AC-KHCO$_2$9, was examined spectroscopically as a moisture-swing material (Mendez-Lozoya et al., 6 Aug 2025). Related atomistic simulations on realistic activated carbons doped with K$p_{CO_2}$0CO$p_{CO_2}$1 show that potassium carbonate clusters act as extra adsorption sites for both CO$p_{CO_2}$2 and water, shifting adsorption onset pressures to lower values and promoting the formation of a hydrogen bond network within activated-carbon pores (Dongen et al., 7 Oct 2025). The study is explicit that it does not simulate a full moisture-swing DAC cycle or reactive carbonation, but it demonstrates the structural preconditions for humidity-responsive behavior in supported carbonate systems (Dongen et al., 7 Oct 2025).

A fourth class is the humid-DAC-responsive MOF. In diamine-appended $p_{CO_2}$3, humidity changes the adsorbed morphology rather than simply adding competitive water (Marshall et al., 2024). In broader humid-air MOF design, water-stable frameworks and hydrophobically encapsulated MOFs are treated as essential because water can hydrolyze the framework, occupy adsorption sites, or alter amine-based CO$p_{CO_2}$4 binding chemistry (Shi et al., 2022). In the DAC-screening context, the ODAC23 dataset was built precisely because most MOFs bind H$p_{CO_2}$5O more favorably than CO$p_{CO_2}$6, making water-aware screening indispensable (Sriram et al., 2023).

4. Process architectures and demonstrated systems

The most direct system demonstration is the moving-sorbent architecture that alternates exposure to air and immersion in an aqueous medium. One laboratory-scale system delivering $p_{CO_2}$7 per day was demonstrated in a laminar flow hood, and a small pilot-scale system that could deliver $p_{CO_2}$8 daily was operated outdoors in a $p_{CO_2}$9 raceway pond (Flory et al., 4 Aug 2025). The system uses strong-base anion exchange resin contained in elongated nylon mesh tube packets. Compared with a single larger mesh bag, these packets reduced drying and CO$_2$0 loading time by about 4-fold, with drying before loading beginning after $_2$1 rather than $_2$2, and time to 90% of capacity dropping from $_2$3 to $_2$4 under $_2$5, 15% RH, and $_2$6 (Flory et al., 4 Aug 2025). In the lab-scale 1g system, about $_2$7 were delivered over 44 h with the sorbent belt, compared with about $_2$8 background uptake, corresponding to a net sorbent-driven delivery of $_2$9 (Flory et al., 4 Aug 2025). In the outdoor pilot, a seven-day trial gave a measured delivery of $_2$0, compared with a model prediction of $_2$1, a difference of about 3% (Flory et al., 4 Aug 2025).

A biologically integrated version of this system used Purolite A501 resin packets to deliver air-captured CO$_2$2 directly into alkaline cultivation media for cyanobacteria and microalgae (Flory et al., 6 Aug 2025). At flask scale, daily 30 min immersion supported rapid growth of engineered Synechocystis with a biomass growth rate of about $_2$3 (Flory et al., 6 Aug 2025). A bench-scale 12 L system delivered about $_2$4 into abiotic medium and about $_2$5 in the presence of Synechocystis (Flory et al., 6 Aug 2025). A small pilot-scale system installed in a $_2$6 outdoor raceway pond in Mesa, Arizona, delivered about $_2$7 into abiotic alkaline cultivation medium (Flory et al., 6 Aug 2025). These demonstrations establish that moisture-driven air capture can be coupled directly to a receiving liquid phase rather than producing only a concentrated gas stream.

A distinct process architecture is vacuum moisture swing direct air capture, or VMS-DAC (Sinyangwe et al., 24 Jun 2026). In this packed-bed cycle, vacuum is used to remove residual air, induce water evaporation from a reservoir, pull water vapor through the bed to trigger CO$_2$8 release, evacuate released CO$_2$9, and condense the water vapor downstream (Sinyangwe et al., 24 Jun 2026). The cycle comprises pressurization, CO$\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$0 sorption from ambient air, air evacuation, vapor stripping, and final desorption (Sinyangwe et al., 24 Jun 2026). Optimized VMS operation at 20 percent relative humidity achieves CO$\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$1 productivities of $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$2 to $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$3, with electrical energy required for gas and vapor flow and CO$\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$4 compression to $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$5 ranging from $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$6 to $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$7, and a representative point of about $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$8 at $\mathrm{CO_2 + OH^- \rightarrow HCO_3^-}$9 (Sinyangwe et al., 24 Jun 2026). Water losses of $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$0 to $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$1 are reported, while water processed is much larger, $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$2–$$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$3, because most is internally condensed and recycled (Sinyangwe et al., 24 Jun 2026).

At the other end of the design spectrum, Alkalinity Concentration Swing proposes a non-thermal water-mediated DAC process in which a dilute alkaline solution absorbs atmospheric CO$$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$4, then a desalination technology such as reverse osmosis or capacitive deionization concentrates the solution, raising its outgassing pressure and allowing CO$$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$5 extraction (Rinberg et al., 2021). The paper estimates, for example, that $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$6 to $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$7 with $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$8 gives $$\mathrm{HCO_3^- + OH^- \rightarrow CO_3^{2-} + H_2O}$$9, $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$0, and $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$1 (Rinberg et al., 2021). This is a moisture-driven air capture system in the sense that changing water content by concentration and dilution is itself the regeneration mechanism.

5. Characterization, modeling, and control frameworks

The field combines operando spectroscopy, adsorption theory, packed-bed process modeling, and atomistic simulation. A particularly direct diagnostic advance is the use of in situ surface-enhanced Raman spectroscopy to track moisture-swing speciation (Mendez-Lozoya et al., 6 Aug 2025). Ni-coated Ag nanowires were employed as SERS substrates, giving a reported enhancement factor of $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$2 (Mendez-Lozoya et al., 6 Aug 2025). In IRA900-HCO$$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$3, the bicarbonate peak at $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$4 decreases while the carbonate peak at $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$5 increases during humidification in both air and N$$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$6, providing operando evidence for humidity-dependent interconversion between bicarbonate and carbonate species (Mendez-Lozoya et al., 6 Aug 2025). In AC-KHCO$$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$7, humidification caused strong growth of the carbonate peak near $$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$8–$$\mathrm{2HCO_3^- \leftrightarrow CO_3^{2-} + CO_2 + H_2O}$$9, decline of the bicarbonate peak near $_2$00–$_2$01, and growth of OH/water bands in the $_2$02–$_2$03 region (Mendez-Lozoya et al., 6 Aug 2025). This makes speciation directly observable rather than inferred only from gas uptake.

For humid DAC adsorbents whose mechanism changes with water, lattice and kinetic theories have been developed. In diamine-appended $_2$04, adsorption lanes are treated as approximately independent 1D lattices with single-site, chain-end, and chain-interior states (Marshall et al., 2024). The equilibrium constants are written as

$_2$05

and the grand free energy is

$_2$06

with loading obtained from

$_2$07

(Marshall et al., 2024). Humidity is incorporated through a water-dependent rescaling of the single-site entropic volume,

$_2$08

with $_2$09 and $_2$10, so that pore-filling water has a much stronger thermodynamic effect than lower-RH site-based water adsorption (Marshall et al., 2024).

For VMS-DAC, the process model is a cyclic 1D packed-bed formulation with moisture-dependent sorption equilibrium (Sinyangwe et al., 24 Jun 2026). Water sorption is represented by a GAB isotherm,

$_2$11

and CO$_2$12 equilibrium by a moisture-dependent isotherm,

$_2$13

so that higher water loading suppresses CO$_2$14 affinity directly (Sinyangwe et al., 24 Jun 2026). Sorption kinetics are represented by a linear driving force model,

$_2$15

with measured baseline coefficients at $_2$16 of $_2$17 for CO$_2$18 sorption, $_2$19 for CO$_2$20 desorption, $_2$21 for water desorption during drying, and $_2$22 for water sorption during humidification (Sinyangwe et al., 24 Jun 2026).

In MOF screening, the ODAC23 dataset was created because humid-air DAC requires explicit treatment of CO$_2$23, H$_2$24O, co-adsorption, defects, and framework relaxation (Sriram et al., 2023). The dataset includes 8,412 total MOFs and more than 38M DFT calculations (Sriram et al., 2023). Screening used the criteria that the adsorption energy of CO$_2$25 is $_2$26 and more favorable than that for H$_2$27O (Sriram et al., 2023). A total of 135 pristine and 107 defective MOFs satisfied these criteria (Sriram et al., 2023). This underlines a recurring point across the literature: humid DAC cannot be inferred from dry CO$_2$28 affinity alone.

6. Performance, tradeoffs, and unresolved challenges

The central tradeoff in moisture-driven air capture is that water is simultaneously the regeneration lever and a process burden. In the mixed-gas sorption study of IRA900-HCO$_2$29 and AC-KHCO$_2$30, RH cycling between 20% and 95% at $_2$31 gave water loading swings of $_2$32 and $_2$33, respectively, while the CO$_2$34 loading swings were $_2$35 and $_2$36 (Mendez-Lozoya et al., 6 Aug 2025). Expressed as mass ratios, the cycled water per unit CO$_2$37 separated was 11:1 wt/wt for IRA900 and 13:1 wt/wt for AC-KHCO$_2$38 (Mendez-Lozoya et al., 6 Aug 2025). In the direct-delivery system using A501 packets, water uptake of the sorbent contained in hydrophilic mesh packets reached about 330 wt.%, which the authors identify as a dominant cause of long drying times and high water use (Flory et al., 4 Aug 2025).

The economic and energetic implications are substantial. In the direct-delivery system, the practical scenario is reported as $_2$39 to extract CO$_2$40 from solution, purify it, and compress it to 15 MPa (Flory et al., 4 Aug 2025). An aspirational scenario assuming a sorbent capacity of $_2$41, water uptake of 50 wt.%, and drying/loading within 1 h gives $_2$42 to reach 15 MPa (Flory et al., 4 Aug 2025). The same study reports that the aspirational moisture-driven process uses up to 87% less energy than thermal and/or vacuum swing DAC, but that present hydrophilic mesh packets lead to water use far above thermodynamic limits (Flory et al., 4 Aug 2025). These claims are scenario-dependent rather than demonstrated plant-level results, but they define explicit material targets.

Durability remains a major limitation. In the cultivation-integrated system, exopolysaccharides and other excreted products fouled the sorbent beads, reducing capacity to 25%, partially restoring to 70% after a wash protocol, while delivery kinetics remained 3–4 fold slower (Flory et al., 6 Aug 2025). Analysis after over 300 days of outdoor wet and dry cycling showed significant mechanical fracturing, with infrared spectroscopy and thermogravimetric analysis indicating significant loss of NR$_2$43 functional groups necessary for CO$_2$44 capture (Flory et al., 6 Aug 2025). The work attributes degradation to repeated wet/dry cycling, alkaline exposure, oxygen, UV exposure, and the biological environment (Flory et al., 6 Aug 2025). This is a direct caution against assuming that chemically plausible moisture-swing resins are automatically durable under outdoor process conditions.

Humidity itself can be either beneficial or harmful depending on the material class. In diamine-appended MOFs, higher RH improves initial capture dynamics and erases the dry-air induction effect, with 50–75% RH predicted to be nearly 100% non-cooperative singles at long times (Marshall et al., 2024). In moisture-swing resins, by contrast, high humidity is the release state, so very humid climates can suppress capture performance. In VMS-DAC, productivity at a fixed energy benchmark of $_2$45 falls from $_2$46 at 20% RH to $_2$47 at 80% RH, while water loss decreases from $_2$48 to near zero $_2$49 (Sinyangwe et al., 24 Jun 2026). This suggests that “moisture-driven” does not imply one universal climate preference; the favorable humidity window depends on whether humidity is intended to promote uptake, release, or both in sequence.

A common misconception is that water merely helps or hurts capture empirically. The literature is more specific. Water can reduce the free energy of carbonate hydrolysis in confinement (Shi et al., 2017), shift adsorbed morphology from cooperative chains to singles (Marshall et al., 2024), stabilize carbonate relative to bicarbonate in quaternary-ammonium materials (Tomaraei et al., 5 Aug 2025), dominate transport and morphology in charged polymers (Yogaganeshan et al., 15 Aug 2025), and overwhelm DAC selectivity in most MOFs unless water is screened explicitly (Sriram et al., 2023). Moisture-driven air capture is therefore not a single reaction scheme but a family of water-coupled transport and equilibrium phenomena.

7. Research directions and interpretive synthesis

The field increasingly treats moisture-driven air capture systems as hierarchical transport-reactive media rather than as isolated sorbent chemistries. Structural characterization of FAA-3 and IRA 900 suggests that multiscale morphology, accessible pores, clustering, swelling, and local ordering control the accessibility and transport of water, ions, and CO$_2$50 (Yogaganeshan et al., 15 Aug 2025). The authors explicitly suggest that next-generation moisture-swing polymers should combine fixed cations, controlled hierarchical porosity, nanoscale ionic clustering or stacking, reversible humidity-induced domain reorganization, and mechanical robustness over many wet-dry cycles (Yogaganeshan et al., 15 Aug 2025). This suggests that the appropriate design objective is not maximum basicity alone, but a balance among ion density, nanoconfinement, water uptake, transport connectivity, and durability.

Across system studies, the most consistently repeated material targets are higher effective CO$_2$51 capacity, lower water uptake, faster wet/dry kinetics, and better long-term stability. The direct-delivery system states an aspirational target of $_2$52, 50 wt.% water uptake, and drying/loading within 1 h (Flory et al., 4 Aug 2025). The cultivation-integrated system argues that avoiding direct contact between sorbent and biomass, for example by delivering CO$_2$53 into a media recycle stream after biomass harvest, is likely necessary to preserve abiotic performance (Flory et al., 6 Aug 2025). VMS-DAC identifies slow water sorption as a key kinetic bottleneck and suggests structured contactors and alternative moisture-swing sorbents as next steps (Sinyangwe et al., 24 Jun 2026). The SERS work points toward operando speciation monitoring as a practical route for optimization and quality control (Mendez-Lozoya et al., 6 Aug 2025).

A broader synthesis is that moisture-driven air capture now spans three technically distinct but chemically connected paradigms. The first is the classic humidity-swing solid sorbent in which dry capture and wet release are produced by hydration-state-dependent carbonate chemistry (Shi et al., 2017, Tomaraei et al., 5 Aug 2025, Mendez-Lozoya et al., 6 Aug 2025). The second is the water-managed cyclic process in which humidity control is implemented through contactor design, moving sorbents, or vacuum-enabled water-vapor stripping (Flory et al., 4 Aug 2025, Flory et al., 6 Aug 2025, Sinyangwe et al., 24 Jun 2026). The third is the water-mediated adsorption or aqueous concentration swing in which water changes adsorption morphology or solution speciation even if the process is not a literal “dry capture, wet release” loop (Marshall et al., 2024, Rinberg et al., 2021). In all three cases, the common principle is that water is not an external nuisance variable. It is a primary thermodynamic and kinetic control parameter for low-temperature CO$_2$54 separation from air.