Search NASA⌕ Search

SEARCH · Search NASA

Results for “aqueous”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Thermodynamic analysis of hydrogel swelling in aqueous sodium chloride solutions

Here, a comprehensive thermodynamic model is presented for swelling and salt partitioning of poly (n-isopropyl acrylamide) hydrogels in aqueous sodium chloride (NaCl) solutions at 298 K. Together with a Helmholtz energy expression for network elastic energy, a modified electrolyte Nonradom Two-Liquid activity coefficient model is used to represent both the long-range electrostatic interactions and the short-range van der Waals interactions present in the gel phase. The model parameters include one network parameter per hydrogel system plus two binary parameters per interaction pair in the polymer–solvent-salt systems. With semiquantitave agreement with experimental data for both hydrogel swelling and salt partitioning up to salt saturation, the analysis suggests the ion hydration between NaCl and water is the root cause for hydrogel deswelling in aqueous NaCl solution at high NaCl concentrations, i.e., NaCl weight fraction >0.03. On the other hand, the strong repulsive interaction between NaCl and hydrogel polymer is the dominant factor for salt partitioning.

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

The apparent reversal of the Law of Mass Action in concentrated multicomponent aqueous solutions

Although it is well known that aqueous electrolyte solutions behave non-ideally, few are so non-ideal that adding an electrolyte to a saturated solution with a common ion enhances rather than depresses solubility. Nonetheless, this apparent reversal of the Law of Mass Action (LMA) has been observed in simple nitrate solutions at high concentrations. A leading hypothesis is that ion clusters are formed and stabilized by ions having different charge densities. We report the present study examines this concept, in part by reviewing relevant data from multicomponent aqueous solutions containing sodium nitrate (NaNO 3 ), sodium nitrite (NaNO 2 ), sodium hydroxide (NaOH), and sodium aluminate (NaAl(OH) 4 ) - the major constituents in alkaline nuclear waste. Here, NaOH and NaAl(OH) 4 did not enhance the solubility of NaNO 3 or NaNO 2 , whereas NaNO 2 and NaNO 3 enhanced rather than depressed the solubility of each other despite each having the sodium cation (Na + ) in common. Solutions evaluated in this study have more than 20 molal total Na + concentration, and most have less than one mole of water per mole of ion. Thus, this reversal of the LMA occurs in solutions where there is not enough water to fully hydrate the ions, pointing to the importance of ion cluster formation. Within the composition range of the solutions analyzed here, this reversal of the LMA occurs regardless of NaAl(OH) 4 and NaOH concentrations. Elevated temperatures also result in the reversal of the LMA in the subsystems NaNO 2 -NaNO 3 -H 2 O and NaOH-NaNO 3 -H 2 O, consistent with spectroscopic and computational studies showing enhanced interactions at higher temperatures. Although speciation in these highly concentrated electrolyte solutions is not well understood, the stabilization of ion clusters by: (i) optimizing charge density around the ions through different combinations of mixed cations and anions in solution; and (ii) high temperatures where ions experience prolonged contact, offers important clues for future research into how these species control solubility.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Polyvinyl alcohol coating induced preferred crystallographic orientation in aqueous zinc battery anodes

The development of rechargeable aqueous zinc batteries is mainly hindered by the Zn anode, which suffers from dendrite growth, corrosion, hydrogen evolution, and surface passivation. Herein, a thin polyvinyl alcohol (PVA) coating layer on Zn anode has enabled dendrite-free, long-life aqueous Zn batteries by effectively regulating the interfacial ion diffusion and inducing the homogeneous Zn nucleation and deposition of stacked plates with preferentially crystallographic orientation along (002) Zn planes. The PVA@Zn anode achieved an ultralong cycle lifespan of thousands of hours at 0.25 and 1 mA cm -2 . Outstanding durability under a deep cycling capacity (5 mA h cm -2 ), high current density (10 mA cm -2 ), and long duration conditions were achieved. The superior cyclability of PVA@Zn anode was also demonstrated in PVA@Zn//V 2 O 5 full cells. In conclusion, the insights of PVA induced Zn deposition with preferred crystal orientation and interfacial regulation shed light on the future development of stable Zn anodes.

Crystallographic orientation↗
DOE OSTI2022

Effects of oxygen on the adsorption/oxidation of aqueous Sb(III) by Fe-loaded biochar: An X-ray absorption spectroscopy study

Fe-loaded biochar (FeBC) has been considered for Sb(III) adsorption, but the effects of oxygen (O 2 ) on the adsorption need further investigation. Liquid-/solid-phase analyses were conducted to investigate the role of O 2 in the Sb(III) adsorption by FeBC. The adsorption was best described by the pseudo-second-order (PSO) model for kinetic results and by the Langmuir model for thermodynamic results. More than 96.8 % of Sb(III) was adsorbed by FeBC, and available O 2 increased the liquid-phase Sb(III) oxidation efficiency by 2.1–7.5 times. The peak changes at ~1640 and 3450 cm -1 in FTIR spectra indicated the occurrence of inner-sphere complexation between Sb(III)/Sb(V) and hydroxyl (–OH)/carboxyl (–COOH) groups in FeBC under aerobic and anaerobic conditions. Fe/Sb X-ray absorption spectroscopy (XAS) analysis results showed aqueous Sb(III) complexed to the edge-sharing Fe(III)-O-Fe(III) in FeBC. Regardless of whether O 2 was available or not, solid-phase edge-sharing Fe(III)-O-Sb(V) complexes (~3.05 Å), which had lower toxicity and migration ability than aqueous Sb(III), formed through a ligand-to-metal charge-transfer (LMCT) process. More than 91 % of adsorbed Sb(III) was oxidized to edge-sharing Fe(III)-O-Sb(V) complexes in 3 h. Additionally, the Sb(V) from liquid-phase oxidation could also directly complex to the Fe(III)-O-Fe(III) and form edge-sharing Fe(III)-O-Sb(V) complexes. Here these results provide evidence to inform further FeBC application for the Sb-contaminated water treatment.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Carbon dioxide capture with aqueous amino acids: Mechanistic study of amino acid regeneration by guanidine crystallization and process intensification

CO 2 capture from powerplant-generated flue gas via a phase-changing process involving absorption with aqueous amino acids (e.g., glycine or sarcosine) and bicarbonate crystallization with bis-iminoguanidines (e.g., glyoxal-bis-iminoguanidine or GBIG) is investigated in this paper. This process is of high interest due to its potential to decrease the energy penalty for CO 2 capture by significantly reducing the solvent regeneration energy typically associated with aqueous amine solvents. A critical step in the proposed CO 2 capture mechanism is the regeneration of the amino acid by removal of protons and bicarbonate ions from solution through crystallization of GBIGH 2 2+ bicarbonate salt. Here, we investigated the thermodynamics and kinetics of glycine regeneration by crystallization of GBIGH 2 2+ (HCO 3 – ) 2 (H 2 O) 2 . A theoretical model was developed and compared to experimental data to simulate and predict the glycine regeneration and determine its reaction mechanism. This combined experimental and theoretical study led to the conclusion that, while the GBIGH 2 2+ bicarbonate crystallization step provides most of the thermodynamic driving force for the glycine regeneration, the rate-limiting step is the protonation of GBIG prior to crystallization. The CO 2 loading and amino acid regeneration steps were combined into a single, intensified process using a bubble column reactor. The CO 2 loading capacity of GBIG was experimentally determined to be roughly 1.36 mol CO 2 per mol GBIG. These results provide the fundamental basis for developing an effective carbon capture technology with phase-changing amino acid/guanidine absorbents.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Solvent-driven aqueous separations for hypersaline brine concentration and resource recovery

Solvent-driven separation processes can extract water and high-value minerals from high salinity or contaminated brines, simultaneously reducing the environmental impact of brine disposal and enabling resource recovery. The efficient dewatering of hypersaline brines is essential for the sustainable minimal and zero liquid discharge processing of industrial wastewaters. Fractional crystallization can selectively extract ions from contaminated waste streams, allowing critical materials to be recycled, including transition and lanthanide metals required for renewable energy generation and storage. Mass transfer in solvent-driven water extraction occurs across a liquid–liquid interface, eliminating the scaling and fouling of membrane and heat exchanger surfaces and limiting the need for extensive pretreatment. Solvent-driven fractional crystallization can leverage sequential treatment and control of process conditions to rapidly recover salts without requiring evaporation of water. Despite promising applications, the principles and potential of solvent-driven aqueous separations remain poorly understood. This critical review explores the opportunities presented by solvent-based aqueous separations from the molecular to process scale, evaluating the chemistry of solvation and system design in the broader context of desalination, resource recovery, water softening, and mineral production.

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

An ab initio free energy study of the reaction mechanism and rate-limiting steps of CO 2 capture by aqueous glycine

Aqueous amino acids are promising absorbents for direct air capture (DAC) of CO 2 . Herein, we investigate the possibility of kinetic control of CO 2 absorption with aqueous anionic glycine (GLY - ) by employing extensive ab initio molecular dynamics simulations, free energy analysis, and reaction rate theory. We find that first GLY - binds to CO 2 by overcoming a barrier (7.4 kcal/mol) to form a zwitterion intermediate, which then releases a proton by overcoming a similar barrier. Despite the similarity in the barrier, zwitterion formation appears to be the rate-limiting step because it is two orders of magnitude slower (microseconds) than the proton release step. This is predominantly due to stronger nonequilibrium solvent effects for the former that cause many barrier-recrossing events and effectively slow down the reaction rate. Such a detailed fundamental understanding of the amino acid-based CO 2 -absorption mechanism and rates is key to improving the kinetic efficiency of DAC technology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Mechanisms of proton transport in aqueous acid solutions

The fundamental understanding of proton transport, specifically the Grotthuss-like mechanism, is critical for many technological applications. The present study presents a comprehensive analysis of proton transport in aqueous solutions of sulfuric and phosphoric acids using quasielastic neutron and light scattering, broadband dielectric spectroscopy, rheology, pulsed-field gradient-NMR measurements, and ab initio molecular dynamic simulations. Results show that in all systems, proton transport occurs through short “jumps” of ~0.5 Å that are faster than structural relaxation. However, proton hopping appears to be coupled to structural relaxation in aqueous solutions of sulfuric acid, while these processes are decoupled in phosphoric acid. Neutron scattering indicates that all protons have the same fast mobility in phosphoric acid systems, while at least one proton per sulfuric molecule remains slower than other protons. Analysis reveals correlated proton jumps, but these correlations suppress conductivity, suggesting that expected Grotthuss-like enhancement of conductivity is unlikely in bulk liquids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Intercalation-Induced Amorphization Boosts Aqueous Magnesium-Ion Storage

The design of aqueous battery cathode materials that can store divalent ions with high capacity and satisfactory reversibility is of great technical importance and challenge. Here, we report that divalent Mg 2+ storage is facilitated by an intercalation-induced amorphization of vanadate electrode materials. Electrokinetic analyses and in situ synchrotron X-ray diffraction and absorption spectroscopy collectively demonstrate that vanadate layered materials (Li–V 3 O 8 ) undergo a structural transformation to amorphization induced by Mg 2+ intercalation, and a reversible restoration of crystalline structure upon Mg 2+ deintercalation. Debye scattering simulations suggest that intercalation-induced turbostratic disorder, especially random rotations, translational shifts, oscillatory motions, or varied interlayer spacing of adjacent V–O molecular layers, could be responsible for the observed amorphization. The highly distorted local structure, in turn, facilitates Mg 2+ intercalation across the vanadate electrode materials, responsible for nearly 3/7 of the total Mg 2+ ions intercalated. The study presented reveals an intriguing relationship between ion transport and the reversible amorphization-to-crystallization dynamics it induces, opening a paradigm for designing advanced aqueous battery electrodes.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Speciation and Aqueous Dissolution of Macronutrients in Fire Ash: Variation across Ecosystems and the Effects on Nutrient Cycling

This study investigated the speciation and aqueous dissolution of macronutrients in fire ash from diverse ecosystems and speciation of ash and smoke from laboratory burning, exploring the variations and their causes. The speciation of phosphorus (P), calcium (Ca), and potassium (K) in fire ash from five globally distributed ecosystems was characterized by using X-ray absorption spectroscopy and sequential fractionation. Aqueous dissolution of the macronutrients was measured by batch experiments at acidic and alkaline pHs. The results showed that P existed mainly as Ca phosphates, Ca as double carbonates, calcite, and sulfates, and most K was associated with Ca carbonates. Mineralogy and the relative abundance of the species were primarily controlled by elemental stoichiometry and fire temperature. Differences in Ca and P speciation existed between ash and smoke from laboratory burning, possibly caused by the temperature difference and/or mass fractionation during burning. The rates, extents, and pH dependencies of macronutrient dissolution differed among macronutrients and depended on their speciation, with K being highly soluble and the P and Ca regulated by solution pH. The variability in ash macronutrient chemistry and ecosystemspecific fire ash loads resulted in varying loads and availability of individual macronutrient from fire among ecosystems. This study provides a mechanistic understanding of how fires transform the chemistry of macronutrients and affect macronutrient returns to soils across different ecosystems, which is essential for evaluating the disturbance to ecosystem nutrient cycling by fires.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Liquefying Lignins: Determining Phase-Transition Temperatures in the Presence of Aqueous Organic Solvents

Under appropriate conditions of temperature and solvent composition, single-phase aqueous organic solvents are capable of liquefying lignin to form a lignin-rich liquid phase separate from the solvent phase, creating liquid–liquid equilibrium. The extent to which this phenomenon occurs when Kraft lignin is combined with the solvents methanol, ethanol, isopropanol, and acetone was determined, with solid–liquid (SL) to liquid–liquid (LL) phase-transition temperatures being measured over the range of applicable organic–water compositions. Lignins tested from a variety of biomass sources were all discovered to form this liquid–liquid equilibrium with aqueous ethanol solutions. Furthermore, the formation of such phase behavior between lignin and economical, green solvents creates new opportunities for the large-scale purification and fractionation of raw bulk (i.e., technical) lignins.

09 BIOMASS FUELS↗
DOE OSTI2021

Corrosion Prevention of Additively Manufactured Aluminum Packing Devices Developed for Process Intensification of CO 2 Capture by Aqueous Amines

Corrosion of additively manufactured (i.e., 3D printed) aluminum packing devices, developed to enhance heat and mass transfer in CO 2 absorption columns using amine-CO 2 scrubbing solutions, is investigated in this paper. The aluminum structures printed by selective laser melting with Al-10Si-0.3Mg feed powder are susceptible to corrosion in aqueous monoethanolamine, both fresh and used CO 2 -saturated solutions. The efficacy of corrosion-protective surface layers of aqueous polyether–ketone–ketone (PEKK) dip coating to provide amine corrosion resistance to the 3D printed aluminum structure is also evaluated. PEKK nanothick layers were coated on the corrugated surface of a 3D printed structure, identical to that of the intensified device, yielding 27 times higher corrosion resistance than the uncoated Al alloy surface. Finally, a systematic long-term electrochemical corrosion analysis revealed that a multilayer PEKK dip coating protocol enables protection of the 3D printed Al alloy surface in various CO 2 –monoethanolamine solutions.

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

Predicting Multicomponent Aqueous Phase Adsorption Equilibria of Organic Acids Using the Generalized Brunauer–Emmett–Teller Isotherm Model

Here, to support process development of adsorptive separation of organic acids, this work presents a thermodynamic modeling methodology to predict multicomponent aqueous phase organic acid adsorption equilibria from single-component adsorption isotherms using the generalized Brunauer–Emmett–Teller isotherm model (gBET). With the organic acid fugacities rigorously accounting for the aqueous phase organic acid solution chemistry and solution nonideality, gBET precisely represents the single-component adsorption isotherms and accurately predicts the binary and ternary mixed-acid adsorption equilibria for the ranges of initial pH (∼3–7), acid concentration (100–400 mmol/L), and temperature (298.15–328.15 K) with less than 10% average absolute relative deviation. In addition, gBET with pH-independent parameters provides insights into the underlying adsorption phenomena, including the adsorbate loadings and compositions in the monolayer and subsequent layers under varying initial pH, temperature, concentration, and composition. The gBET model predictions outperform the predictions from the overloading model and the Ideal Dilute Solution Theory.

09 BIOMASS FUELS↗
DOE OSTI2020

Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases

Isolating isomorphic compounds of tetravalent actinides (i.e., Th IV , U IV , Np IV , and Pu IV ) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M IV 6 O 4 (OH) 4 ] 12+ building block. Prior studies have shown the predominance and coexistence of smaller species for Th IV (monomers, dimers, and hexamers) and larger species for U IV , Np IV , and Pu IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th IV (and Zr IV ) in its isolated solidphase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U 2 ) formulated as U 2 (OH) 2 (SO 4 ) 3 (H 2 O) 4 and a monomer-linked hexamer framework (U-U 6 ) as (U(H 2 O) 3.5 ) 2 U 6 O 4 (OH) 4 (SO 4 ) 10 (H 2 O) 9 . These structures are similar to those previously described for ThIV. Moreover, cocrystallization of monomer and dimer and of dimer and monomer–hexamer phases for both Th IV (prior) and U IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U 2 and U-U 6 crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV–vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. Furthermore, this study describes a bridge across the first half the actinide series, highlighting U IV similarities to Th IV , in addition to the previously known similarities to the transuranic elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Complexation of Lanthanides and Heavy Actinides with Aqueous Sulfur-Donating Ligands

The separation of trivalent lanthanides and actinides is challenging because of their similar sizes and charge densities. S-donating extractants have shown significant selectivity for trivalent actinides over lanthanides, with single-stage americium/ lanthanide separation efficiencies for some thiol-based extractants reported at >99.999%. While such separations could transform the nuclear waste management landscape, these systems are often limited by the hydrolytic and radiolytic stability of the extractant. Progress away from thiol-based systems is limited by the poorly understood and complex interactions of these extractants in organic phases, where molecular aggregation and micelle formation obfuscates assessment of the metal–extractant coordination environment. Because S-donating thioethers are generally more resistant to hydrolysis and oxidation and the aqueous phase coordination chemistry is anticipated to lack complications brought on by micelle formation, we have considered three thioethers, 2,2'-thiodiacetic acid (TDA), (2R,5S)- tetrahydrothiophene-2,5-dicarboxylic acid, and 2,5-thiophenedicarboxylic acid (TPA), as possible trivalent actinide selective reagents. Formation constants, extended X-ray absorption fine structure spectroscopy, and computational studies were completed for thioether complexes with a variety of trivalent lanthanides and actinides including Nd, Eu, Tb, Am, Cm, Bk, and Cf. TPA was found to have moderately higher selectivity for the actinides because of its ability to bind actinides in a different manner than lanthanides, but the utility of TPA is limited by poor water solubility and high rigidity. While significant competition with water for the metal center limits the efficacy of aqueous-based thioethers for separations, the characterization of these solution-phase, S-containing lanthanide and actinide complexes is the most comprehensively available in the literature to date. Here, this is due to the breadth of lanthanides and actinides considered as well as the techniques deployed and serves as a platform for the further development of Scontaining reagents for actinide separations. Additionally, this paper reports on the first bond lengths for Cf and Bk with a neutral S donor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Transient Radiation-Induced Berkelium(III) and Californium(III) Redox Chemistry in Aqueous Solution

Despite the significant impact of radiation-induced redox reactions on the accessibility and lifetimes of actinide oxidation states, fundamental knowledge of aqueous actinide metal ion radiation chemistry is limited, especially for the late actinides. A quantitative understanding of these intrinsic radiation-induced processes is essential for investigating the fundamental properties of these actinides. We present here a picosecond electron pulse reaction kinetics study into the radiation-induced redox chemistry of trivalent berkelium (Bk(III)) and californium (Cf(III)) ions in acidic aqueous solutions at ambient temperature. New, and first of a kind, second-order rate coefficients are reported for the transient radical-induced reduction of Bk(III) and Cf(III) by the hydrated electron (eaq−) and hydrogen atom (H•), demonstrating significant reactivity (up to 1011M-1s-1) indicative of a preference of these metals to adopt divalent states. Additionally, we reportthe first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by reaction with hydroxyl (•OH) and nitrate (NO3•) radicals, which also exhibited fast reactivity (ca. 108 M-1s-1). Transient Cf(II), Cf(IV), and Bk(IV) absorption spectra are also reported. Overall, the presented data highlight the existence of rich, complex, intrinsic late actinide radiation-induced redox chemistry that has the potential to influence the findings of other areas of actinide science.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Importance of Nuclear Quantum Effects on Aqueous Electrolyte Transport under Confinement in Ti 3 C 2 MXenes

Protons display a high chemical activity and strongly affect the charge storage capability in confined interlayer spaces of two-dimensional (2D) materials. As such, an accurate representation of proton dynamics under confinement is important for understanding and predicting charge storage dynamics in these materials. While often ignored in atomistic-scale simulations, nuclear quantum effects (NQEs), e.g., tunneling, can be significant under confinement even at room temperature. Here, using the thermostatted ring polymer molecular dynamics implementation of path integral molecular dynamics (PIMD) in conjunction with the ReaxFF force field, density functional tight binding (DFTB), and NequIP neural network potential simulations, we investigate the role of NQEs on proton and water transport in bulk water and aqueous electrolytes under confinement in Ti 3 C 2 MXenes. Although overall NQEs are relatively small, especially in bulk, we find that they can alter both quantitative values and qualitative trends on both proton transport and water self-diffusion under confinement relative to classical MD predictions. Therefore, our results suggest the need for NQEs to be considered to simulate aqueous systems under confinement for both qualitative and quantitative accuracy.

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

Ten-Fold Solvent Kinetic Isotope Effect for the Nonradiative Relaxation of the Aqueous Ferrate(VI) Ion

Hypervalent iron intermediates have been invoked in the catalytic cycles of many metalloproteins, and thus, it is crucial to understand how the coupling between such species and their environment can impact their chemical and physical properties in such contexts. In this work, we take advantage of the solvent kinetic isotope effect (SKIE) to gain insight into the nonradiative deactivation of electronic excited states of the aqueous ferrate-(VI) ion. Here we observe an exceptionally large SKIE of 9.7 for the nanosecond-scale relaxation of the lowest energy triplet ligand field state to the ground state. Proton inventory studies demonstrate that a single solvent O-H bond is coupled to the ion during deactivation, likely due to the sparse vibrational structure of ferrate-(VI). Such a mechanism is consistent with that reported for the deactivation of f-f excited states of aqueous trivalent lanthanides, which exhibit comparably large SKIE values. This phenomenon is ascribed entirely to dissipation of energy into a higher overtone of a solvent acceptor mode, as any impact on the apparent relaxation rate due to a change in solvent viscosity is negligible.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗