Radiolytic evaluation of a new technetium redox control reagent for advanced used nuclear fuel separations.

Technetium is a problematic radioisotope for used nuclear fuel (UNF) and subsequent waste management owing to its high environmental mobility and coextraction in reprocessing technologies as the pertechnetate anion (TcO4-). Consequently, several strategies are under development to control the transport of this radioisotope. A proposed approach is to use diaminoguanidine (DAG) for TcO4- and transuranic ion redox control. Although the initial DAG molecule is ultimately consumed in the redox process, its susceptibility to radiolysis is currently unknown under envisioned UNF reprocessing conditions, which is a critical knowledge gap for evaluating its overall suitability for this role. To this end, we report the impacts of steady-state gamma irradiation on the rate of DAG radiolysis in water, aqueous 2.0 M nitric acid (HNO3), and in a biphasic solvent system composed of aqueous 2.0 M HNO3 in contact with 1.5 M N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA) dissolved in n-dodecane. Additionally, we report chemical kinetics for the reaction of DAG with key transients arising from electron pulse radiolysis, specifically the hydrated electron (eaq-), hydrogen atom (H˙), and hydroxyl (˙OH) and nitrate (NO3˙) radicals. The DAG molecule exhibited significant reactivity with the ˙OH and NO3˙ radicals, indicating that oxidation would be the predominant degradation pathway in radiation environments. This is consistent with its role as a reducing agent. Steady-state gamma irradiations demonstrated that DAG is readily degraded within a few hundred kilogray, the rate of which was found to increase upon going from water to HNO3 containing solutions and solvents systems. This was attributed to a thermal reaction between DAG and the predominant HNO3 radiolysis product, nitrous acid (HNO2), k(DAG + HNO2) = 5480 ± 85 M-1 s-1. Although no evidence was found for the radiolysis of DAG altering the radiation chemistry of the contacted DEHiBA/n-dodecane phase in the investigated biphasic system, the utility of DAG as a redox control reagent will likely be limited by significant competition with its degradation by HNO2.

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl2•- and SO4•-).

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for the accurate interpretation of their physical and chemical properties. Here, we explore the transient interactions between trivalent californium ions (Cf3+) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl2•-) and sulfate (SO4•-) radical anions. Chemical kinetics, as measured using integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf3+ ions. The derived and ionic strength-corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf3+ + Cl2•-) = (8.28 ± 0.61) × 105 M-1 s-1 and k(Cf3+ + SO4•-) = (9.50 ± 0.43) × 108 M-1 s-1 under ambient temperature conditions (22 ± 1 °C).

Kinetics of the Temperature-Dependent eaq - and ⋠OH Radical Reactions with Cr(III) Ions in Aqueous Solutions.

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (eaq - ) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr3+ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)2+ , did with a rate coefficient of k= (7.2±0.3)×108  M-1 s-1 at 25 °C. The oxidation of Cr(OH)2+ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.

Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling.

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes.

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq-), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq- reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq-), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC-1 s-1. kDOM,eaq- measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol-1 and that kDOM,eaq- could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq- exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq- probe revealed that continued eaq- exposure abates DOM chromophores and eaq- scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq- scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation.

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙+) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙+ from 10-40 °C, giving the activation energy (Ea = 17.43 ± 1.64 kJ mol-1) and pre-exponential factor (A = (2.36 ± 0.05) × 1013 M-1 s-1). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [LnIII(TODGA)3(NO3)3] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙+, relative to the non-complexed ligand: k([LnIII(TODGA)3(NO3)3] + RH˙+) = (8.99 ± 0.93) × 1010, (2.88 ± 0.40) × 1010, and (1.53 ± 0.34) × 1010 M-1 s-1, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations-based on a model [LnIII(TOGDA)]3+ complex system-indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [LnIII(TEGDA)3(NO3)3], toward electrophilic attack is for the coordinated nitrate (NO3-) counter anions. Therefore, it is possible that radical reactions with the complexed NO3- counter anions dominate the differences in rates seen for the [LnIII(TODGA)3(NO3)3] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

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 a significant reactivity (up to 1011 M-1 s-1) indicative of a preference of these metals to adopt divalent states. Additionally, we report the first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by a reaction with hydroxyl (•OH) and nitrate (NO3•) radicals, which also exhibited fast reactivity (ca. 108 M-1 s-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.

Influence of uranyl complexation on the reaction kinetics of the dodecane radical cation with used nuclear fuel extraction ligands (TBP, DEHBA, and DEHiBA).

Specialized extractant ligands - such as tri-butyl phosphate (TBP), N,N-di-(2-ethylhexyl)butyramide (DEHBA), and N,N-di-2-ethylhexylisobutryamide (DEHiBA) - have been developed for the recovery of uranium from used nuclear fuel by reprocessing solvent extraction technologies. These ligands must function in the presence of an intense multi-component radiation field, and thus it is critical that their radiolytic behaviour be thoroughly evaluated. This is especially true for their metal complexes, where there is negligible information on the influence of complexation on radiolytic reactivity, despite the prevalence of metal complexes in used nuclear fuel reprocessing solvent systems. Here we present a kinetic investigation into the effect of uranyl (UO22+) complexation on the reaction kinetics of the dodecane radical cation (RH˙+) with TBP, DEHBA, and DEHiBA. Complexation had negligible effect on the reaction of RH˙+ with TBP, for which a second-order rate coefficient (k) of (1.3 ± 0.1) × 1010 M-1 s-1 was measured. For DEHBA and DEHiBA, UO22+ complexation afforded an increase in their respective rate coefficients: k(RH˙+ + [UO2(NO3)2(DEHBA)2]) = (2.5 ± 0.1) × 1010 M-1 s-1 and k(RH˙+ + [UO2(NO3)2(DEHiBA)2]) = (1.6 ± 0.1) × 1010 M-1 s-1. This enhancement with complexation is indicative of an alternative RH˙+ reaction pathway, which is more readily accessible for [UO2(NO3)2(DEHBA)2] as it exhibited a much larger kinetic enhancement than [UO2(NO3)2(DEHiBA)2], 2.6× vs. 1.4×, respectively. Complementary quantum mechanical calculations suggests that the difference in reaction kinetic enhancement between TBP and DEHBA/DEHiBA is attributed to a combination of reaction pathway (electron/hole transfer vs. proton transfer) energetics and electron density distribution, wherein attendant nitrate counter anions effectively 'shield' TBP from RH˙+ electron transfer processes.

Radiation-induced effects on the extraction properties of hexa-n-octylnitrilo-triacetamide (HONTA) complexes of americium and europium.

The candidate An(iii)/Ln(iii) separation ligand hexa-n-octylnitrilo-triacetamide (HONTA) was irradiated under envisioned SELECT (Solvent Extraction from Liquid waste using Extractants of CHON-type for Transmutation) process conditions (n-dodecane/0.1 M HNO3) using a solvent test loop in conjunction with cobalt-60 gamma irradiation. The extent of HONTA radiolysis and complementary degradation product formation was quantified by HPLC-ESI-MS/MS. Further, the impact of HONTA radiolysis on process performance was evaluated by measuring the change in 243Am and 154Eu distribution ratios as a function of absorbed gamma dose. HONTA was found to decay exponentially with increasing dose, affording a dose coefficient of d = (4.48 ± 0.19) × 10-3 kGy-1. Multiple degradation products were detected by HPLC-ESI-MS/MS with dioctylamine being the dominant quantifiable species. Both 243Am and 154Eu distribution ratios exhibited an induction period of ∼70 kGy for extraction (0.1 M HNO3) and back-extraction (4.0 M HNO3) conditions, after which both values decreased with absorbed dose. The decrease in distribution ratios was attributed to a combination of the destruction of HONTA and ingrowth of dioctylamine, which is capable of interfering in metal ion complexation. The loss of HONTA with absorbed gamma dose was predominantly attributed to its reaction with the n-dodecane radical cation (R˙+). These R˙+ reaction kinetics were measured for HONTA and its 241Am and 154Eu complexes using picosecond pulsed electron radiolysis techniques. All three second-order rate coefficients (k) were essentially diffusion limited in n-dodecane indicating a significant reaction pathway: k(HONTA + R˙+) = (7.6 ± 0.8) × 109 M-1 s-1, k(Am(HONTA)2 + R˙+) = (7.1 ± 0.7) × 1010 M-1 s-1, and k(Eu(HONTA)2 + R˙+) = (9.5 ± 0.5) × 1010 M-1 s-1. HONTA-metal ion complexation afforded an order-of-magnitude increase in rate coefficient. Nanosecond time-resolved measurements showed that both direct and indirect HONTA radiolysis yielded the short-lived (<100 ns) HONTA radical cation and a second long-lived (µs) species identified as the HONTA triplet excited state. The latter was confirmed by a series of oxygen quenching picosecond pulsed electron measurements, affording a quenching rate coefficient of k(3[HONTA]* + O2) = 2.2 × 108 M-1 s-1. Overall, both the HONTA radical cation and triplet excited state are important precursors to the suite of measured HONTA degradation products.

Does addition of 1-octanol as a phase modifier provide radical scavenging radioprotection for N,N,N',N'-tetraoctyldiglycolamide (TODGA)?

To mitigate third phase formation in next generation used nuclear fuel reprocessing technologies, the addition of 1-octanol has been trialed. However, contradictory reports on the radiolytic effect of 1-octanol incorporation on separation ligand degradation need to be resolved. Here, 50 mM N,N,N',N'-tetraoctyldiglycolamide (TODGA) dissolved in n-dodecane was gamma irradiated in the presence and absence of 1-octanol (2.5-10 vol%) and a 3.0 M HNO3 aqueous phase. Radiation-induced TODGA degradation exhibited pseudo-first-order decay kinetics as a function of absorbed gamma dose for all investigated solution and solvent system formulations. The addition of 1-octanol afforded diametrically different effects on the rate of TODGA degradation depending on solvent system formulation. For organic-only irradiations, 1-octanol promoted TODGA degradation (d = 0.0057 kGy-1 for zero 1-octanol present vs.∼0.0073 kGy-1 for 7.5-10 vol%) attributed to a favourable hydrogen atom abstraction reaction free energy (-0.31 eV) and the ability of 1-octanol to access a higher yield of n-dodecane radical cation (RH˙+) at sub-nanosecond timescales. This was rationalized by determination of the rate coefficient (k) for the reaction of 1-octanol with RH˙+, k = (1.23 ± 0.07) × 1010 M-1 s-1. In contrast, irradiation in the presence of 1-octanol and a 3.0 M HNO3 aqueous phase afforded significant radioprotection (d = 0.0054 kGy-1 for zero 1-octanol present vs.≤ 0.0044 kGy-1 for >2.5 vol%) that increases with 1-octanol concentration, relative to the single phase, organic-only solutions. This effect was attributed to the extraction of sufficiently high concentrations of HNO3 and H2O into the organic phase by TODGA and 1-octanol as adducts which interfere with the hydrogen atom abstraction process between the 1-octanol radical and TODGA. Our findings suggest that the addition of 1-octanol as a phase modifier will enhance the radiation robustness of TODGA-based separation technologies under envisioned solvent system conditions in the presence of aqueous HNO3.

Effect of Ionizing Radiation on the Redox Chemistry of Penta- and Hexavalent Americium.

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium's inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am(VI) to Am(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO3 radiolysis, i.e., HNO2, H2O2, and HO2•, which dictated its practical lifetime under acidic conditions. In contrast, Am(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO3•, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am(IV). Our multiscale modeling calculations predict that the lifetime of Am(V) is dictated by its rate of disproportionation, 2AmO2+ + 4Haq+ → AmO22+ + Am4+ + 2H2O, with a fourth-order dependence on [Haq+] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10-6 M-5 s-1. This disproportionation initially produces Am(IV) and Am(VI) species, but the lack of any spectroscopic evidence in our study for Am(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am(VI)/Am(V) irradiations is Am(III), which shows great stability in an irradiation field.

The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis.

Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p Ka = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone Degradation in Aqueous Phase Advanced Oxidation Processes.

Advanced oxidation processes (AOPs) that produce highly reactive hydroxyl radicals are promising methods to destroy aqueous organic contaminants. Hydroxyl radicals react rapidly and nonselectively with organic contaminants and degrade them into intermediates and transformation byproducts. Past studies have indicated that peroxyl radical reactions are responsible for the formation of many intermediate radicals and transformation byproducts. However, complex peroxyl radical reactions that produce identical transformation products make it difficult to experimentally study the elementary reaction pathways and kinetics. In this study, we used ab initio quantum mechanical calculations to identify the thermodynamically preferable elementary reaction pathways of hydroxyl radical-induced acetone degradation by calculating the free energies of the reaction and predicting the corresponding reaction rate constants by calculating the free energies of activation. In addition, we solved the ordinary differential equations for each species participating in the elementary reactions to predict the concentration profiles for acetone and its transformation byproducts in an aqueous phase UV/hydrogen peroxide AOP. Our ab initio quantum mechanical calculations found an insignificant contribution of Russell reaction mechanisms of peroxyl radicals, but significant involvement of HO2• in the peroxyl radical reactions. The predicted concentration profiles were compared with experiments in the literature, validating our elementary reaction-based kinetic model.

UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse.

A sequential combination of membrane treatment and UV-based advanced oxidation processes (UV/AOP) has become the industry standard for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addition, persulfate (S2O82-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH2Cl and S2O82- on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and solution pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D degradation rate. At the optimal ratio, the degradation rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to the solution pH due to a disproportionation of NH2Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride transformed HO• and SO4•- to Cl2•- that is less-reactive with 1,4-D, while the presence of dissolved O2 promoted gaseous nitrogen production. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.

Kinetics of the Autoreduction of Hexavalent Americium in Aqueous Nitric Acid.

The rate of reduction of hexavalent 243Am due to self-radiolysis was measured across a range of total americium and nitric acid concentrations. These so-called autoreduction rates exhibited zero-order kinetics with respect to the concentration of hexavalent americium, and pseudo-first-order kinetics with respect to the total concentration of americium. However, the rate constants did vary with nitric acid concentration, resulting in values of 0.0048 ± 0.0003, 0.0075 ± 0.0005, and 0.0054 ± 0.0003 h-1 for 1.0, 3.0, and 6.5 M HNO3, respectively. This indicates that reduction is due to reaction of hexavalent americium with the radiolysis products of total americium decay. The concentration changes of Am(III), Am(V), and Am(VI) were determined by UV-vis spectroscopy. The Am(III) molar extinction coefficients are known; however, the unknown values for the Am(V) and Am(VI) absorbances across the studied range of nitric acid concentrations were determined by sensitivity analysis in which a mass balance with the known total americium concentration was obtained. The new extinction coefficients and reduction rate constants have been tabulated here. Multiscale radiation chemical modeling using a reaction set with both known and optimized rate coefficients was employed to achieve excellent agreement with the experimental results, and indicates that radiolytically produced nitrous acid from nitric acid radiolysis and hydrogen peroxide from water radiolysis are the important reducing agents. Since these species also react with each other, modeling indicated that the highest concentrations of these species available for Am(VI) reduction occurred at 3.0 M HNO3. This is in agreement with the empirical finding that the highest rate constant for autoreduction occurred at the intermediate acid concentration.

Oxidative remediation of 4-methylcyclohexanemethanol (MCHM) and propylene glycol phenyl ether (PPh). Evidence of contaminant repair reaction pathways.

A large spill of 4-methylcyclohexanemethanol (MCHM) and propylene glycol phenyl ether (PPh) into the Elk River near Charleston, West Virginia on January 9, 2014 led to serious water contamination and public concerns about appropriate remediation. To assess the feasibility of advanced oxidation processes (AOPs) for remediation of waters contaminated with these compounds, we induced hydroxyl radical (HO˙) reactions using time-resolved and steady-state radiolysis methods. Detailed product analyses showed initial HO˙ attack was at the benzene ring of PPh, and occurred through H-atom abstraction reactions for MCHM. Pulse radiolysis and steady state radiolysis experiments conducted using pure compound solutions, mixtures of the compounds and real water solvents allowed us to obtain mechanistic insights of hydroxyl radical attack and establish the fate of the compounds using AOP remediation technologies. These results demonstrate that hydroxyl radical induced oxidization of PPh can lead to "repair-type" reactions, which regenerates this contaminant. The study further highlights the importance of such counterproductive reactions for the quantitative estimate of the required amount of oxidant in any large-scale treatment approaches.

Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter.

We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.

Experimental and theoretical studies on aqueous-phase reactivity of hydroxyl radicals with multiple carboxylated and hydroxylated benzene compounds.

In this study, we shed light on the initial addition of hydroxyl radicals (HO˙) to multiple carboxylated and hydroxylated benzene compounds in aqueous-phase advanced oxidation processes (AOPs). We analyze the experimentally measured transient spectra near neutral pH using quantum mechanical-based time-dependent density functional theory (TD-DFT). The ab initio DFT method was first used to find and optimize aqueous-phase transition state structures, then the TD-DFT was used to analyze molecular orbitals (MOs) of the optimized transition state structures to reveal the functional groups that are responsible for the individual absorption peaks. The initial addition of HO˙ to the benzene ring produced hydroxycyclohexadienyl radicals. Then, HO-adducts are generated from dimerization or disproportionation of hydroxycyclohexadienyl radicals and represent their transient spectral peaks at approximately 350 nm and 250 nm. As reaction proceeds, the HO-adducts are decreased depending on the subsequent reactions. These investigations into the experimental transient spectra coupled with the theoretical analysis using the TD-DFT enable us to visualize an initial transformation of organic compounds induced by the aqueous phase HO˙ oxidation. Moreover, the experimental reaction rate constants and the theoretically calculated aqueous phase free energies of activation provide quantitative insights into the addition of HO˙ to multiple carboxylated and hydroxylated benzene compounds.

Development of linear free energy relationships for aqueous phase radical-involved chemical reactions.

Aqueous phase advanced oxidation processes (AOPs) produce hydroxyl radicals (HO•) which can completely oxidize electron rich organic compounds. The proper design and operation of AOPs require that we predict the formation and fate of the byproducts and their associated toxicity. Accordingly, there is a need to develop a first-principles kinetic model that can predict the dominant reaction pathways that potentially produce toxic byproducts. We have published some of our efforts on predicting the elementary reaction pathways and the HO• rate constants. Here we develop linear free energy relationships (LFERs) that predict the rate constants for aqueous phase radical reactions. The LFERs relate experimentally obtained kinetic rate constants to quantum mechanically calculated aqueous phase free energies of activation. The LFERs have been applied to 101 reactions, including (1) HO• addition to 15 aromatic compounds; (2) addition of molecular oxygen to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3) disproportionation of 10 peroxyl radicals, and (4) unimolecular decay of nine peroxyl radicals. The LFERs correlations predict the rate constants within a factor of 2 from the experimental values for HO• reactions and molecular oxygen addition, and a factor of 5 for peroxyl radical reactions. The LFERs and the elementary reaction pathways will enable us to predict the formation and initial fate of the byproducts in AOPs. Furthermore, our methodology can be applied to other environmental processes in which aqueous phase radical-involved reactions occur.