Unravelling the Water Adsorption Mechanism in Hierarchical MOFs: Insights from In Situ Positron Annihilation Lifetime Studies

Understanding the Mechanism and Ascertaining the Sorption Sites for the Highly Efficient and Selective Sorption of U(VI) and Th(IV) by Graphene/Zeolitic Imidazolate Framework-90 Composites: An Experimental and Theoretical Approach

Efficient removal of radionuclides from aqueous solutions is of utmost importance for public and environmental safety. The development of efficient adsorbents and a deeper understanding of the adsorption mechanism of radionuclides are crucial for designing superior adsorbents. In view of this, the present investigation deals with the understanding of the highly efficient sorption of U(VI) and Th(IV) from an aqueous acidic feed using a multilayer graphene/Zeolitic Imidazolate Framework-90 (ZIF-90G) composite. This is the first ever report on utilization of positron annihilation spectroscopy in order to ascertain the sites of sorption in the three-dimensional pore network of ZIF-90G. The interaction of ortho-positronium with U(VI) in the pore network revealed that the sorption proceeded throughout the pore network with preferential adsorption at the aperture site. The exothermic spontaneous sorption of U(VI) and Th(IV) was found to be maximum in the aqueous feed with pH ranging from 3-4, predominantly following a Langmuir isotherm with pseudo-second-order rate kinetics. In the XPS spectra, the appearance of peaks at ∼392.4 eV (U 4f5/2) and ∼381.9 eV (U 4f7/2) for the U(VI) sorbed sample and peaks at ∼344.01 eV (Th 4f5/2) and ∼334.81 eV (Th 4f7/2) for the Th(IV) sorbed sample evidenced the effective sorption, while the red shift in the O 1s spectrum (∼531.7 eV and ∼534.4 eV for ZIF-90G) indicated the coordination of metal ions through chelation of carboxaldehyde functionalities. Elemental mapping using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy also evidenced the uniform sorption of the U(VI) and Th(IV). Density functional theory and X-ray absorption spectroscopy have been employed for the further understanding of the mechanism at the molecular level.

Uncovering the Dynamic CO2 Gas Uptake Behavior of CALF-20 (Zn) under Varying Conditions via Positronium Lifetime Analysis

Carbon dioxide (CO2) is a major greenhouse gas contributing to global warming. Adsorption in porous sorbents offers a promising method for CO2 capture and storage. The zinc-triazole-oxalate-based Calgary framework 20 (CALF-20) demonstrates high CO2 capacity, low H2O affinity, and low adsorption heat, enabling energy-efficient and stable performance over multiple cycles. This study examines CO2 adsorption mechanism in CALF-20 using positron annihilation lifetime spectroscopy (PALS), in situ powder X-ray diffraction (PXRD), and gas adsorption experiments under varying temperatures and humidity levels. Variable-temperature PALS experiments demonstrate that CO₂ molecules are spatially localized within the CALF-20 cages, leaving temperature- and pressure-dependent gaps. CO2 begins at cage centers, forming 1D chains, and ultimately adheres to pore walls. Interestingly, positronium intensity correlates with the Langmuir-Freundlich isotherm, reflecting gas uptake behavior. Moreover, under pure relative humidity (RH), water molecules form isolated clusters or small oligomers at low RH, transitioning to hydrogen-bonded networks above 35 %RH, significantly altering free volumes. In humid CO₂ conditions, competitive interactions arise: CO₂ initially disrupts water propagation, but higher RH leads to extensive water networks filling the framework. The synergy between in situ-PALS, in situ-PXRD, and gas adsorption techniques provides comprehensive insights into CALF-20's potential for efficient CO2 capture under varying conditions.

Enhancement of Water Productivity and Energy Efficiency in Sorption-based Atmospheric Water Harvesting Systems: From Material, Component to System Level

To address the increasingly serious water scarcity across the world, sorption-based atmospheric water harvesting (SAWH) continues to attract attention among various water production methods, due to it being less dependent on climatic and geographical conditions. Water productivity and energy efficiency are the two most important evaluation indicators. Therefore, this review aims to comprehensively and systematically summarize and discuss the water productivity and energy efficiency enhancement methods for SAWH systems based on three levels, from material to component to system. First, the material level covers the characteristics, categories, and mechanisms of different sorbents. Second, the component level focuses on the sorbent bed, regeneration energy, and condenser. Third, the system level encompasses the system design, operation, and synergetic effect generation with other mechanisms. Specifically, the key and promising improvement methods are: synthesizing composite sorbents with high water uptake, fast sorption kinetics, and low regeneration energy (material level); improving thermal insulation between the sorbent bed and condenser, utilizing renewable energy or electrical heating for desorption and multistage design (component level); achieving continuous system operation with a desired number of sorbent beds or rotational structure, and integrating with Peltier cooling or passive radiative cooling technologies (system level). In addition, applications and challenges of SAWH systems are explored, followed by potential outlooks and future perspectives. Overall, it is expected that this review article can provide promising directions and guidelines for the design and operation of SAWH systems with the aim of achieving high water productivity and energy efficiency.

Revisiting Metal-Organic Frameworks Porosimetry by Positron Annihilation: Metal Ion States and Positronium Parameters

Metal-organic frameworks (MOFs) stand as pivotal porous materials with exceptional surface areas, adaptability, and versatility. Positron Annihilation Lifetime Spectroscopy (PALS) is an indispensable tool for characterizing MOF porosity, especially micro- and mesopores in both open and closed phases. Notably, PALS offers porosity insights independent of probe molecules, which is vital for detailed characterization without structural transformations. This study explores how metal ion states in MOFs affect PALS results. We find significant differences in measured porosity due to paramagnetic or oxidized metal ions compared to simulated values. By analyzing CPO-27(M) (M = Mg, Co, Ni), with identical pore dimensions, we observe distinct PALS data alterations based on metal ions. Paramagnetic Co and Ni ions hinder and quench positronium (Ps) formation, resulting in smaller measured pore volumes and sizes. Mg only quenches Ps, leading to underestimated pore sizes without volume distortion. This underscores the metal ions' pivotal role in PALS outcomes, urging caution in interpreting MOF porosity.