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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Fluorination of Lithium‐Excess Transition Metal Oxide Cathode Materials

Abstract Fluorination of Li‐ion cathode materials is of significant interest as it is claimed to lead to significant improvements in long‐term reversible capacity. However, the mechanism by which LiF incorporates and improves performance remains uncertain. Indeed, recent evidence suggests that fluorine is often present as a coating layer rather than incorporated into the bulk of the material. In this work, first‐principles calculations are used to investigate the thermodynamics of fluorination in transition metal oxide cathodes to determine the conditions under which bulk fluorination is possible. It is found that unlike classic well‐ordered cathodes, which cannot incorporate fluorine, disordered rock salt‐structured materials achieve significant fluorination levels due to the presence of locally metal‐poor, lithium‐rich environments that are highly preferred for fluorine. As well as explaining the fluorination process in known materials, this finding is encouraging for the development of new disordered rock salt lithium‐excess transition metal oxides, a promising new class of Li‐ion battery cathode materials that offer superior practical capacity to traditional layered oxides. In particular, it is found that bulk fluorination may serve as an alternative source of Li‐excess in these compounds that can replace the conventional substitution of a heavy redox‐inactive element on the transition metal sublattice.

Richards, William D.↗
DOE OSTI2021

Integrating computation and experiment to investigate photoelectrodes for solar water splitting at the microscopic scale.

CONSPECTUS: Photoelectrochemical water-splitting is a promising and sustainable way to store the energy of the sun in chemical bonds and use it to produce hydrogen gas, a clean fuel. The key components in photoelectrochemical cells (PECs) are photoelectrodes, including a photocathode that reduces water to hydrogen gas and a photoanode that oxidizes water to oxygen gas. Materials used in photoelectrodes for PECs must effectively absorb sunlight, yield photogenerated carriers, and exhibit electronic properties that enable the efficient shuttling of carriers to the surface to participate in relevant water-splitting reactions. Discovering and understanding the key characteristics of optimal photoelectrode materials is paramount to the realization of PEC technologies. Oxide-based photoelectrodes can satisfy many of these materials requirements, including stability in aqueous environments, band edges with reasonable alignment with the redox potentials for water splitting, and ease of synthesis. However, oxide photoelectrodes generally suffer from poor charge transport properties and considerable bulk electron-hole separation, and they have relatively large band gaps. Numerous strategies have been proposed to improve these aspects and understand how these improvements are reflected in the photoelectrochemical performance. Unfortunately, the structural and compositional complexity of multinary oxides accompanied by the inherent complexity of photoelectrochemical processes makes it challenging to understand the individual effects of composition, structure, and defects in the bulk and on the surface on a material's photoelectrochemical properties. The integration of experiment and theory has great potential to increase our atomic-level understanding of structure-composition-property relationships in oxide photoelectrodes. In this Account, we describe how integrating experiment and theory is beneficial for achieving scientific insights at the microscopic scale. We highlight studies focused on understanding the role of (i) bulk composition via solid-state solutions, intercalation, and comparison with isoelectronic compounds, (ii) dopants for both the anion and cation and their interactions with oxygen vacancies, and (iii) surface/interface structure in the photocurrent generation and photoelectrochemical performance in oxide photoelectrodes. In each instance, we outline strategies and considerations for integrating experiment and theory and describe how this integration led to valuable insights and new directions in uncovering structure-composition-property relationships. Our aim is to demonstrate the unique value of combining experiment and theory in studying photoelectrodes and to encourage the continued effort to bring experiment and theory in closer step with each other.

Wang, Wennie↗
DOE OSTI2021

Enhancing the performance of lithium oxygen batteries through combining redox mediating salts with a lithium protecting salt

Li–O 2 batteries have recently emerged to meet nowadays elevated electric energy demands. Redox mediators (RMs) for solution-inducing decomposition of discharge products are one approach to increase energy efficiency and reduce high overpotentials in these batteries. However, multiple obstacles hinder their usage such as redox shuttling, capacity fading, electrolyte degradation, etc. Herein, we present a new chemistry based on a combination of LiNO 3 , TEGDME and an ionic liquid that enables LiI (1 M) to lower the charge potential (3.5V) with a long cycle life of 270 cycles. 0.1 M LiI increases the cyclability up to 500 with a slightly increased charge potential (~4V) for a fixed capacity of 1000 mAh/g. Up to 100 cycles, this battery system retained ~95% Li 2 O 2 capacity with a ~0.8 V charge-discharge polarization gap. The addition of LiNO 3 to the electrolyte provides a protective solid electrolyte interface (SEI) on anode that works in synergy with the LiI RM. Moreover, we found that this electrolyte blend results in domain formation of ionic and neutral species enhancing the discharge and charge processes. Finally, DFT calculations provide a better understanding of the role of the anode SEI layer and the Li 2 O 2 decomposition promoted by the LiI during charge on the cathode.

25 ENERGY STORAGE↗
DOE OSTI2024

Moderate active Fe 3+ doping enables improved cationic and anionic redox reactions for wide-voltage-range sodium storage

Layered metal oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high theoretical specific capacity and wide Na + diffusion channels. However, the irreversible phase transitions and cationic/anionic redoxes cause fast capacity decay. Herein, P2-type Na 0.67 Mg 0.1 Mn 0.8 Fe 0.1 O 2 (NMMF-1) cathode material with moderate active Fe 3+ doping has been designed for sodium storage. Uneven Mn 3+ /Mn 4+ distribution is observed in NMMF-1 and the introduction of Fe 3+ is beneficial for reducing the Mn 3+ contents both at the surface and in the bulk to alleviate the Jahn–Teller effect. The moderate Fe 3+ /Fe 4+ redox can realize the best tradeoff between capacity and cyclability. Therefore, the NMMF-1 demonstrates a high capacity (174.7 mAh g -1 at 20 mA g -1 ) and improved cyclability (78.5% over 100 cycles) in a wide-voltage range of 1.5–4.5 V (vs. Na + /Na). In-situ X-ray diffraction reveals a complete solid-solution reaction with a small volume change of 1.7% during charge/discharge processes and the charge compensation is disclosed in detail. This study will provide new insights into designing high-capacity and stable layered oxide cathode materials for SIBs.

25 ENERGY STORAGE↗
DOE OSTI2020

Redox-active dinuclear oxorhenium(V) pyrazolate complexes

Four new structurally similar dinuclear oxorhenium(V) complexes, [{Re(O)X(PPh 3 )} 2 (μ-O)(μ-4-x'-pz) 2 ], where pz = pyrazolate anion, X = X' = Cl (1) and Br (4), X = Cl, X' = Br (2), and X = Br, X' = Cl (3), have been synthesized and characterized. Little variation in spectroscopic features – 1 H NMR, IR, UV–Vis – exists among the four complexes. All complexes possess a bent Re-O-Re core as well as distorted octahedral coordination geometry around the rhenium centers. Finally, a reversible one-electron electrochemical process is observed at approximately 0.84 V vs. Fc + /Fc in all four complexes; however, changing the terminal halide from chloride to bromide slightly destabilizes the oxidized Re(VI) center.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Redox Potential of Intermittently Wet Soil, Old Woman Creek National Estuarine Research Reserve, Huron, OH, 2023-05-10 to 2023-12-15

This dataset contains collected reduction-oxidation (redox) potential measurements of the underlying soil at various depths within a wetland, referred to as The Cove, at Old Woman Creek Estuarine Research Reserve in Huron, OH. Redox potential was measured in the sediments of a coastal wetland to assess how redox potential varies over time with changes in hydrological events. Measurements were collected by Campbell Scientific CR1000X dataloggers paired with a PaleoTerra redox probe and reference electrodes. Measurements were collected at 3 different locations within The Cove at multiple depths into the underlying soil of the wetland and were collected every 10 minutes. The Redox_Datafile.csv contains the recorded measurements of the redox probes, and the RefElecDataFile.csv contains the background-noise measurements collected by the reference electrodes. Redox_InstallMethods describes the installation methods of the datalogger and associated probes.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Fast oxygen redox enabled by flexible Al–O bonds in P2-type layered oxides for sodium batteries

Sodium-ion batteries (SIBs) exhibit significant potential for large-scale energy storage systems due to the abundance and low cost of sodium resources. Triggering lattice oxygen redox (LOR) in P2-type transition metal oxides is considered a promising approach to enhance energy density in SIB cathodes, providing high operating potential and substantial capacity. However, irreversible phase transitions associated with LOR, particularly from prisms (P-type stacking) to octahedrons (O-type stacking), lead to severe structural distortions and sluggish Na + diffusion kinetics. In this work, an Al-substitution strategy is proposed to suppress the formation of O-type stacking and instead promote the formation of a beneficial Z phase. Furthermore, the flexible Al-O bonds accommodate asymmetric variations in their occupied states during the sodiation process, mitigating local structural distortions through Al-O bond contraction. Stabilization of the local structure ensures the maintenance of a robust Na + diffusion pathway. As a result, the Al-substituted cathode achieves a low Na + diffusion barrier of 0.47 eV and delivers a capacity of 86 mAh/g even at a high current density of 1 A/g within 1.5–4.5 V, maintaining 62.5% capacity retention over 100 cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Diversity in Photoprotection and Energy Balancing in Terrestrial and Aquatic Phototrophs

The evolution of oxygenic photosynthesis enabled organisms to use sunlight as an energy source, allowing them to colonize new niches. At the same time, life (as we know it) places severe constraints on photosynthesis. For example, the initial reactions of photosynthesis involve highly energetic intermediates that, if not controlled, can generate highly toxic side products (especially reactive oxygen species, ROS), that can damage other essential components of the organisms it powers. Photosynthesis must therefore be tightly regulated to balance the need for efficient energy conversion with the necessity of avoiding photodamage (Gust D, Kramer D, Moore A, Moore T, Vermaas W, Mater Res Bull 33:383–389, 2008). A related constraint on photosynthesis is the need to precisely balance how much energy is stored in ATP and NADPH to precisely meet biochemical demands. If this balancing does not occur, the system will fail, leading to photodamage (Kramer DM, Evans JR, Plant Physiol 155:70–78, 2011). Consideration of these requirements is essential for efforts to improve the efficiency of photosynthesis by introducing CO 2 concentrating mechanisms, altering metabolism or biosynthetic pathways to shunt energy to alternative products (Kramer DM, Evans JR, Plant Physiol 155:70–78, 2011). These balancing processes must be extremely robust to contend with the rapid and unpredictable fluctuations in environmental conditions and metabolic demands that occur in nature. A large body of work has come from model systems, especially terrestrial higher plants and the green alga Chlamydomonas reinhardtii, leading to a model for the regulation of light reactions that involves 1) sensing of the pH gradient component of the thylakoid proton motive force (pmf), and 2) the redox state of the plastoquinone- and stromal pools. Over the short term, these sensors trigger regulation of light capture by altering the activity of ATP synthase leading to adjustments in lumen pH, which fine tunes light capture through nonphotochemical quenching (NPQ) and control of electron flow by adjusting the rate of PQH 2 oxidation at the b 6 f complex. Simultaneously, this system controls the balance of ATP/NADPH by adjusting electron flux to linear and cyclic electron flow pathways to balance ATP/NADPH. This integrated “pmf paradigm” model explains much of the existing data on plants and green algae, but may not extend to other diverse organisms. This review considers how advances in our understanding of photosynthesis over the past 7–8 years, particularly in the discoveries of diverse biochemical/biophysical mechanisms in aquatic photosynthetic species, affects the view of energy balance, including the shunting of electrons to O 2 through the flavodiiron proteins (FLV), the plastid terminal oxidase, the dissipation of electric field by ion movements, and the activation of alternative electron sinks. Furthermore, we will introduce the basic model that has been developed for higher plant chloroplasts, then contrast these with selected aquatic systems, focusing on how the differences impact the needs to re-balance both energy input and its partitioning into energy currencies.

Kanazawa, Atsuko↗
DOE OSTI2024

Thiol-based pathways in the thylakoid lumen and their role in photoprotection

The long-term goal of this research is to elaborate the catalysis of thiol-disulfide transactions in the thylakoid lumen, a compartment required for transducing energy via photophosphorylation. The molecular identity of the redox players catalyzing thiol-disulfide exchanges, their relevant targets of action in vivo and how thiol-disulfide chemistry in general controls photosynthesis are the experimental questions this research aims to address. Through previous work, we documented the requirement of catalyzed disulfide formation and reduction for the biogenesis of two photosynthetic enzymes, namely Photosystem II and the cytochrome b6f complex. We established that dedicated trans-thylakoid pathways operate in these processes by delivering reducing and oxidizing power to cysteine-containing subunits of the photosynthetic complexes, which are localized in the lumen compartment. Here we focused on CCDA, CCS4 and CCS5/HCF164, the components of the disulfide-reducing pathway which maintains the heme binding cysteines of apoforms of plastid cytochrome c in the reduced form prior to covalent heme ligation. We showed that CCS4 and CCS5 operate in functionally redundant pathways for the supply of reducing power and postulate a role for CCS4 in recruiting the source of reductants to CCDA or controlling the CCDA-dependent transduction of reducing equivalents. We also demonstrate the disulfide reducing pathway operates to counter thiol oxidation of the heme-binding cysteines by disulfide forming enzyme LTO1 in the thylakoid lumen.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

The Kinetics of Electron Transfer from CdS Nanorods to the MoFe Protein of Nitrogenase

Combining the remarkable catalytic properties of redox enzymes with highly tunable light absorbing properties of semiconductor nanocrystals enables the light-driven catalysis of complex, multielectron redox reactions. This work focuses on systems that combine CdS nanorods (NRs) with the MoFe protein of nitrogenase to drive photochemical N 2 reduction. We used transient absorption spectroscopy (TAS) to examine the kinetics of electron transfer (ET) from CdS NRs to the MoFe protein. For CdS NRs with dimensions similar to those previously used for photochemical N 2 reduction, the rate constant for ET from CdS NRs competes with other electron relaxation processes, such that when a MoFe protein is bound to a NR, about one-half of the photoexcited electrons are delivered to the enzyme. The NR-MoFe protein binding is incomplete with more than one-half of the NRs in solution not having a MoFe protein bound to accept electrons. The quantum efficiency of ET (QE ET ) in these ensemble samples is similar to previously reported efficiencies of product (NH 3 and H 2 ) formation, suggesting that the enzyme utilizes the delivered electrons without major loss pathways. Our analysis suggests that QE ET , and therefore the photochemical product formation, is limited at the ensemble level by the NR-MoFe protein binding and at the single-complex level by the competitiveness of ET. We characterized ET kinetics for several CdS NRs samples with varying dimensions and found that for CdS NRs with an average diameter of 4.2 nm the ET efficiency dropped to undetectable levels, defining a maximum NR diameter that should be used to photochemically drive the MoFe protein. The work described here provides insights into the design of systems with increased control of photochemical N 2 reduction catalyzed by the MoFe protein of nitrogenase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Hydration of a Side-Chain-Free n-Type Semiconducting Ladder Polymer Driven by Electrochemical Doping

Here, we study the organic electrochemical transistors (OECTs) performance of the ladder polymer, poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance (eQCM) gravimetry, in-operando atomic force microscopy (AFM), and both ex-situ and in-operando grazing incidence wide-angle x-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, inoperando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized, and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Realizing High Capacity and Zero Strain in Layered Oxide Cathodes via Lithium Dual-Site Substitution for Sodium-Ion Batteries

Sodium-ion batteries have garnered unprecedented attention as an electrochemical energy storage technology, but it remains challenging to design high-energy-density cathode materials with low structural strain during the dynamic (de)sodiation processes. In this paper, we report a P2-layered lithium dual-site-substituted Na 0.7 Li 0.03 [Mg 0.15 Li 0.07 Mn 0.75 ]O 2 (NMLMO) cathode material, in which Li ions occupy both transition-metal (TM) and alkali-metal (AM) sites. The combination of theoretical calculations and experimental characterizations reveals that Li TM creates Na–O–Li electronic configurations to boost the capacity derived from the oxygen anionic redox, while Li AM serves as LiO 6 prismatic pillars to stabilize the layered structure through suppressing the detrimental phase transitions. As a result, NMLMO delivers a high specific capacity of 266 mAh g –1 and simultaneously exhibits the nearly zero-strain characteristic within a wide voltage range of 1.5–4.6 V. Our findings highlight the effective way of dual-site substitution to break the capacity–stability trade-off in cathode materials for advanced rechargeable batteries.

25 ENERGY STORAGE↗
DOE OSTI2025

Technology for Electrically Enhanced Thermochemical Hydrogen (TEETH)

This is the Final Technical Report for the TEETH project. The TEETH concept couples high-temperature solar-thermochemical water splitting (TCWS) with electrochemical H 2 pumping through a proton conducting membrane (PCM) and capitalizes on the benefits of the individual technologies to synergistically providing new benefits. That is, TEETH is a coupled thermochemical/electrochemical process to produce H 2 from steam using solar energy. This approach is thermodynamically equivalent to other hybrid electrolytic processes but is unique in that the equilibrium of the reaction is driven forward by close coupling an electrically driven proton-conducting-membrane to the H 2 -producing reoxidation step. The process uniquely provides and benefits from the necessary H 2 /steam separation and, also unlike other hybrid processes, benefits thermodynamically from the use of readily generated high pressure steam. The concept also satisfies the objectives of previous concepts: 1) decreasing the reduction enthalpy (the reduction temperature), of the working metal-oxide (MO); 2) eliminating the need for a windowed receiver; and 3) widening the scope of material candidates, while also obviating the need for electrical connections to the working MO and avoiding the use of aqueous electrolytes and hydrated redox species (there is no liquid phase), without increasing mechanical complexity.

08 HYDROGEN↗
NASA NTRS1991

Biologically controlled minerals as potential indicators of life

Minerals can be produced and deposited either by abiotic or biologic means. Regardless of their origin, mineral crystals reflect the environment conditions (e.g., temperature, pressure, chemical composition, and redox potential) present during crystal formation. Biologically-produced mineral crystals are grown or reworked under the control of their host organism and reflect an environment different from the abiotic environment. In addition, minerals of either biologic or abiotic origin have great longevities. For these reasons, biologically produced minerals have been proposed as biomarkers. Biomarkers are key morphological, chemical, and isotopic signatures of living systems that can be used to determine if life processes have occurred. Studies of biologically controlled minerals produced by the protist, Paramecium tetraurelia, were initiated since techniques have already been developed to culture them and isolate their crystalline material, and methods are already in place to analyze this material. Two direct crystalline phases were identified. One phase, whose chemical composition is high in Mg, was identified as struvite. The second phase, whose chemical composition is high in Ca, has not been previously found occurring naturally and may be considered a newly discovered material. Analyses are underway to determine the characteristics of these minerals in order to compare them with characteristics of these minerals in order to compare them with characteristics of minerals formed abiotically, but with the same chemical composition.

Schwartz, D. E.↗
NASA NTRS1997

The Galim LL/EH Polymict Breccia: Evidence for Impact-Induced Exchange Between Reduced and Oxidized Meteoritic Material

Galim is a polymict breccia consisting of a heavily shocked (shock stage S6) LL6 chondrite, Galim (a), and an impact-melted EH chondrite, Galim (b). Relict chondrules in Galim (b) served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt. Many of the reduced phases typical of EH chondrites (e.g., Si-bearing metallic Fe-Ni; Ti-bearing troilite) are absent. Galim (b) was probably shock-melted while in contact with a more oxidized source, namely, Galim (a); during this event, Si was oxidized from the metal and Ti was oxidized from troilite. Galim (a) contains shock veins and recrystallized, unzoned olivine. The absence of evidence for reduction in Galim (a) may indicate that the amount of LL material greatly exceeded that of EH material; shock metamorphism may have taken place on the LL parent body. Shock-induced redox reactions such as those inferred for the Galim breccia appear to be restricted mainly to asteroids because the low-end tail of their relative-velocity distribution permits mixing of intact disparate materials (including accretion of projectiles of different oxidation states), whereas the peak of the distribution leads to high equilibration shock pressures (allowing impact-induced exchange between previously accreted, disequilibrated materials). Galim probably formed by a two-stage process: (I) accretion to the LL parent body of an intact EH projectile at low relative velocities, and (2) shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities.

Rubin, Alan E.↗
DOE OSTI2025

Uncovering the Critical Role of Ni on Surface Lattice Stability in Anionic Redox Active Li 1.2 Ni 0.2 Mn 0.6 O 2

Anionic redox reaction (ARR) can provide extra capacity beyond transition metal (TM) redox in lithium-rich TM oxide cathodes. Practical ARR application is much hindered by the structure instability, particularly at the surface. Oxygen release has been widely accepted as the ringleader of surficial structure instability. However, the role of TM in surface stability has been much overlooked, not to mention its interplay with oxygen release. Herein, TM dissolution and oxygen release are comparatively investigated in Li 1.2 Ni 0.2 Mn 0.6 O 2 . Ni is verified to detach from the lattice counter-intuitively despite the overwhelming stoichiometry of Mn, facilitating subsequent oxygen release of the ARR process. Intriguingly, surface reorganization occurs following regulated Ni dissolution, enabling the stabilization of the surface and elimination of oxygen release in turn. Accordingly, a novel optimization strategy is proposed by adding a relaxation step at 4.50 V within the first cycle procedure. Battery performance can be effectively improved, with voltage decay suppressed from 3.44 mV/cycle to 1.60 mV/cycle, and cycle stability improved from 66.77% to 90.01% after 100 cycles. This work provides new perspectives for clarifying ARR surface instability and guidance for optimizing ARR performance.

25 ENERGY STORAGE↗
DOE OSTI2025

NiAl–MoO 2 S 2 Nanoparticles: Structural Evolution and Mechanistic Insights into High-Performance Selenium Oxyanion Removal across Diverse pH Conditions

Advancing sorbent materials for the selective removal of toxic oxyanions from water requires synthetic control, tunable chemistry, and an atomic-level understanding of structure–function relationships. Here, we report the synthesis and detailed characterization of NiAl–MoO 2 S 2 , a novel layered double hydroxide (LDH) nanomaterial designed for the efficient sequestration of selenium oxoanions (SeO 3 2– and SeO 4 2– ) from complex aqueous environments. The material is synthesized through a room-temperature ion-exchange process, wherein interlayer NO 3 – anions in NiAl–LDH are replaced with MoO 2 S 2 2– clusters, forming high-surface-area, flower-like nanoparticles. Comprehensive structural analysis using the synchrotron X-ray pair distribution function, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy reveals a distinct chemical transformation of intercalated [MoO 2 S 2 ] 2– into [Mo 2 O 2 S 6 ] 2– -like clusters, generating redox-active interlayers that drive selenium capture. This tailored interfacial chemistry underpins the material’s exceptional sorption performance, achieving distribution coefficients (K d ) ≥ 10 6 mL/g and maximum capacities of 343 mg/g for SeO 4 2– and 514 mg/g for SeO 3 2– , outperforming state-of-the-art inorganic sorbents. Importantly, NiAl–MoO 2 S 2 maintains high selectivity and capacity across acidic, neutral, and alkaline pH, efficiently removing selenium from ppm to sub-10 ppb trace levels, even in the presence of competing ions typical of natural and industrial waters. The selenium uptake proceeds via reductive precipitation coupled with the oxidation of molybdenum and sulfide within the LDH framework. This study highlights the power of strategic synthetic modification and interlayer functionalization in LDHs to unlock new structural motifs and redox chemistries, offering a scalable route to advanced materials for environmental remediation.

Adsorption↗
DOE OSTI2018

Relating soil gas to weathering using rock and regolith geochemistry

Weathering-induced fracturing (WIF) has been posited to be a mechanism that develops secondary porosity when mineral reaction fronts separate over depth intervals in regolith, and, in particular, when oxidation (which can promote porosity development through volume expansion) occurs deeper than dissolution (which grows porosity through material removal). If this is true, then the protolith’s capacity to reduce O2 (for example, the Fe(II) content) and O2 availability should affect WIF. This study explores the hypothesis that if the ratio of pO2 to pCO2, in soil water, R’(aq), is greater than the ratio of the capacity of the protolith to consume O2 and CO2, R0, then WIF is more likely to occur, and regolith will become thicker. We evaluated this hypothesis by measuring the bulk geochemistry of regolith and rock and monitoring soil gas at three sites, encompassing a wide range of FeO concentrations and regolith thickness: a Pennsylvania (PA) diabase (10.15%; 3.8 m), a Virginia (VA) diabase (10.49%; 1.4 m), and a VA granite (1.45%; 20 m). We inferred soil water O2 concentrations from calculated equilibrium with the measured soil gas pO2. We observed WIF in the VA granite and PA diabase where R’(aq) > R0, while at the site that lacked WIF – VA diabase – the R’(aq) < R0 particularly during the wet season. In the VA diabase, the presence of swelling clays (smectite) limits the ability of the oxidant (O2) to diffuse deeper into the weathering profile during the wet season and microbially accelerated iron oxidation rapidly consumes O2, limiting O2 availability for WIF. Smectite has little to no observable effect on O2 consumption in the PA diabase because the PA diabase is more fractured. A compilation of dissolved soil gas oxidation ratios, the stoichiometric ratio of O2 consumed to CO2 produced, shows that for unsaturated conditions, the mean is -1.45 ± 0.88, which is consistent with aerobic root and microbial respiration and the oxidation of organic matter. For near-saturated conditions, the mean oxidation ratio of the compilation is -3.46 ± 1.79, which is consistent with Fe redox and microbial metabolism under reducing conditions. The consistency between the VA and PA data presented here and the compilation suggests that soil-water surplus drives coupled Fe-redox reactions that may act as a negative feedback, limiting O2 supply and WIF under wetter soil moisture conditions. We defined Rz, the ratio of O2 consumption to CO2 consumption during weathering for each depth interval, z. For all profiles, R’(aq) > Rz near the surface but R’(aq) approaches Rz in the saprolite. We suggest that R’(aq) > Rz in the soil reflects consumption of O2 and production of CO2 due to biotic processes whereas R’(aq) approaching Rz suggests that low fluxes at depth are at least partly dictated by rock and regolith composition, notably tortuosity of pores. In the VA diabase, we observed R’(aq) < Rz occasionally during the wet season in the lowermost soil and saprolite. Thus, at times the O2 availability may be less than the O2 consumption at that depth, consistent with Fe(II) loss and a lack of WIF. Mass-balance calculations show Fe loss in the VA diabase. The influence of rock composition on aqueous O2/CO2 concentrations in saprolite is consistent with the hypothesis that the protolith’s capacity to consume O2 and CO2 has some effect on oxidation and acid consumption deep in the weathering profile.

54 ENVIRONMENTAL SCIENCES↗