Abstract Uranium (U), a key nuclide in nuclear energy development and carbon neutrality strategies, exhibits environmental adsorption and transport behaviors that directly affect the safety of nuclear waste geological disposal and the long-term risk assessment of contaminated sites. The sandy vadose zone in arid regions of Gansu, China, is a representative medium for uranium contamination; however, its vertical migration and retention mechanisms remain poorly quantified. In this study, laboratory column experiments combined with Hydrus-1D simulations were conducted to investigate the coupled effects of water flow and experimentally determined linear adsorption on uranium transport in a homogeneous sandy vadose zone. Uranium migrated rapidly downward by approximately 8 cm within the first 20 days. Scenario simulations (20–50 years) suggest that, under evaporation-driven negative pressure gradients, migration depth stabilizes, with most uranium retained within the top 7.9 cm. Sensitivity analysis shows that the dispersion coefficient (DL) and residual water content (θr) are the most influential intrinsic parameters, while the precipitation-evaporation balance is the key external driver controlling long-term retention. Furthermore, rainfall-induced wetting-drying cycles may promote a shift of uranium toward relatively less mobile fractions under the Tessier extraction scheme. This study suggests that, under evaporation-dominated climatic conditions and the assumptions of the present conceptual model, uranium transport exhibits a trend of rapid initial migration followed by long-term shallow retention, providing quantitative support for scenario-based assessment of radionuclide transport in homogeneous sandy vadose zones.
Abstract In lake ecosystems, the carbonate–organic matter interaction forms a selective protective effect on specific organic matter. The key to understanding the selective protection mechanism of carbonates is to clarify the compositional characteristics of organic matter under carbonate regulation. In this study, sediment from Daru Co Lake on the Qinghai–Tibet Plateau was used as the research object, and the molecular structural characteristics of organic matter buried by carbonates were determined using high-resolution mass spectrometry and X-ray photoelectron spectroscopy (XPS). The results showed that, over the nearly 100 year period (1928–2015), the main components of organic matter buried by carbonates included not only the classically refractory macromolecule lignin (32.2%) but also labile active substances, such as proteins (29.1%) and lipids (18.8%). This results suggests that, in carbonate lake environments, carbonates can actively capture labile organic matter and form stable associations, allowing labile organic matter to undergo long-term geological sequestration under the protection of inorganic minerals instead of being inevitably rapidly mineralized.
Abstract Soil organic matter is redox-active, capable of responding to redox fluctuations, and acts as a biogeobattery to facilitate biogeochemical cycling. However, whether the electron-transfer capacities of mineral-associated organic matter (MAOM) are reversible under controlled chemical redox-switching conditions remains unclear. Mediated electrochemical reduction/oxidation (MER/MEO) analysis showed that MAOM from the transition zones (e.g., between wetland and grassland ecosystems) exhibited the highest electron-accepting and -donating capacities (EAC and EDC) and reversible electron transfer capacity under varying redox states. Specifically, when conditions shift from oxidizing to reducing, the EAC of MAOM decreased in tandem with an equivalent increase in EDC. Conversely, when conditions revert to oxidizing, EAC increased while EDC decreased, demonstrating a fully reversible electron transfer behavior. Multiple complementary analyses reveal that MAOM in the transition zone was enriched in redox-active metastable phases (RAMPs), including redox-active carboxyl functional groups and mineral phases. Mineral phases were the major source of total electron exchange capacity (EEC), whereas organic matter functioned as an important modulator of coupled redox dynamics. MAOM features co-occurrences of iron–carbon (Fe–C) covalent bonding and abundant carboxyl functional groups, as well as mixed-valence iron phases (e.g., green rust and magnetite) and poorly crystalline iron (oxyhydr)oxide minerals (e.g., ferrihydrite and goethite), all of which contribute to electron transfer reversibility. These results suggest that MAOM contains RAMPs that support electron transfer reversibility under varied redox conditions, thereby influencing the overall biogeochemistry and persistence of the soil ecosystem.
Abstract The chemical evolution of CH4:CO2:NH3 astrophysical ice analogs was investigated under irradiation by 538 MeV 64Ni24+ ions at 14 K, simulating the effects of heavy cosmic rays in dense interstellar environments. Molecular changes were monitored in situ using Fourier transform infrared (FTIR) spectroscopy. A progressive depletion of the precursor species was observed with increasing ion fluence, accompanied by the formation of a diverse set of products, including CO, H2O, C2H6, C2H4, CH3OH, H2CO, HCOOH, HCOO–, OCN–, and N2O. The evolution of column densities revealed an initial rapid growth phase followed by saturation, reflecting the competition between formation and destruction processes. Destruction and formation cross sections were derived assuming first-order kinetics, and the corresponding radiochemical yields were determined. CO exhibited the highest formation yield, indicating that CO2 dissociation is one of the dominant irradiation-induced processes, while the efficient production of CH3OH, H2CO, OCN–, and N2O demonstrated the simultaneous operation of carbon-, oxygen-, and nitrogen-bearing reaction pathways. An atomic budget analysis indicated that a significant fraction of dissociated atoms was not accounted for in the observed infrared-active species, suggesting the formation of more complex or refractory compounds. These findings demonstrate the key role of heavy-ion processing in driving chemical complexity in interstellar ices, promoting the formation of oxygen- and nitrogen-bearing organic species of astrochemical and prebiotic relevance.
The reactions of atomic carbon in its ground electronic state configuration, C(3P), are potentially important processes in astrochemistry due to the large abundance of C(3P) atoms in the interstellar medium (ISM) and its high overall reactivity towards a wide range of molecules. Although benzene, C6H6, has not been detected in the dense ISM, its presence at high abundance levels is inferred through the detection of functionalized derivatives. Here we present a combined experimental and astrochemical modeling investigation of the gas-phase C(3P) + C6H6 reaction. Experimentally, rate constants were determined over the 50-296 K range using the Laval nozzle technique coupled with pulsed laser photolysis and laser induced fluorescence for C(3P) generation and detection respectively. Product yields of atomic hydrogen, H(2S) were also measured at 177 and 296 K to provide some information on the product channels of the reaction. The measured rate constants are very large, between 3.3 and 5.7 x 10-10 cm3 s-1 indicating the fast, barrierless nature of the reaction, while the H-atom yields are all below 10 % when compared to the C(3P) + C2H4 reference reaction. As the C(3P) + C6H6 reaction is not currently included in astrochemical databases, its influence on the simulated abundances of C6H6 and related species was tested using a gas-grain model of dense interstellar clouds. The C(3P) + C6H6 reaction is shown to be the main loss process for interstellar benzene, while different hypotheses regarding the product channels are discussed in the context of observations to shed light on the preferred formation pathways.
Space weathering causes physical and spectral changes on the surfaces of airless bodies. However, our understanding of how space weathering operates in the presence of volatile ices is in its early stages. Electron irradiation of ice-coated surfaces is expected in astrophysical environments including the early solar system, volatile ice-rich permanently shadowed regions of the Moon and Mercury, and other airless bodies like asteroids. A recent study suggests that anomalous oxygen isotope exchange occurs between water-ice and underlying surfaces when exposed to electron irradiation at extremely low temperatures (10 K). To delve deeper into the physical processes underlying isotopic exchange, we employ nanoscale atomic force microscopy-based infrared (AFM-IR) spectroscopy to identify Si-O bond formation resulting from the electron irradiation of H2O ice coated silicon targets. Experimental variables include electron energy, amount and timing of water-ice deposition, and surface area exposed to the electron beam. AFM-IR point spectra, surface topography and IR absorption mapping reveal that the degree of surface oxidation is dependent upon experimental conditions. Scanning electron microscopy and (scanning) transmission electron microscope imaging confirm the formation of thicker SiO x in regions of enhanced interaction between electron irradiation, water-ice, and the silicon substrate. In summary, we find that electron irradiation with energies as low as 1 keV/electron can break the chemical bonds of refractory solids like Si under these simulated cold astrophysical conditions. These results suggest that cosmic rays may play a more significant role than previously thought in the chemical evolution of dust grains in cold astrophysical and protoplanetary environments.
Although energetic and nonenergetic processing of interstellar dust grain surfaces and ice is thought to be one of the dominant mechanisms for the extraterrestrial synthesis of prebiotic molecules, the role that "ion-ice" chemistry (bulk ion-neutral and ion-ion reactions within the dust-grain ice mantle) plays is still a largely unexplored topic. In this work, we present the results of our study to investigate the astrochemical importance of ions produced within dust grain ice mantles via irradiation by VUV photons, though the methods are applicable to other kinds of ionizing radiation. To this end, we present new models of the chemical evolution of a pure oxygen ice irradiated by UV photons, using a chemical network that explicitly includes bulk reactions involving secondary ions. A comparison of our calculations with previous data suggests that ion-neutral and ion-ion reactions may play a critical role in understanding the chemistry of interstellar ice.
