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.

Dataset and kinetic model reaction compilation for the radical-induced degradation of formic acid and formate in aqueous solution.

This article contains the individual datasets and complete reaction kinetics compilation for the formic acid/formate component of the kinetic model described in "Radiolytic Degradation of Formic Acid and Formate in Aqueous Solution: Modeling the Final Stages of Organic Mineralization under Advanced Oxidation Process Conditions" [1]. Gamma irradiation data were collected for aqueous sodium formate solutions under pH = 1.5 and 9.0 conditions. To determine the optimum conditions necessary to effectively mineralize formic acid/formate in an Advanced Oxidation Process utilized for water treatment, several solution compositions were evaluated: air, nitrogen, and nitrous oxide saturation. Data were collected by a combination of high performance liquid chromatography and gas chromatography. These measured values were used to construct a kinetic computer model, by combining with published literature rate coefficients and optimizing specific important rate coefficients to afford the best agreement with experimental data.

Radiolytic degradation of formic acid and formate in aqueous solution: modeling the final stages of organic mineralization under advanced oxidation process conditions.

The successful use of advanced oxidation processes to treat aqueous solutions containing undesirable organic species requires the degradation of these species to lower molecular weight, lower hazard compounds. Safe application of this technology requires a thorough understanding of the mechanisms of degradation. These oxidative transformations are mainly initiated by the reactions of reactive oxygen species, particularly hydroxyl radicals. These react with organic molecules to generate carbon-centered radicals. In the presence of dissolved oxygen, the carbon-centered radicals are next converted to peroxyl radicals, which then decay to lower molecular weight species by multiple mechanistic pathways. Formic acid and its conjugate base formate are the last stable chemical species produced immediately before the complete mineralization of any organic molecule undergoing oxidative degradation in aqueous solution. Once understood, the radical-induced chemistry of formic acid/formate under these conditions has wide applicability in all advanced oxidation technologies. To develop this quantitative knowledge, we have performed a series of 60Co gamma irradiation studies on aqueous formic acid/formate over different pH and solution conditions. The measured species concentration changes, as a function of applied dose, are compared with the predictions of a kinetic computer model constructed from literature reactions and reported rate coefficients. The excellent agreement found between the results and modeling gives confidence in the mechanism presented here and provide the first complete computer model for the radiolytic degradation of formic acid in water.

Gamma radiolysis of hydrophilic diglycolamide ligands in concentrated aqueous nitrate solution.

The radiation chemistry of a series of hydrophilic diglycolamides (DGAs: TEDGA, Me-TEDGA, Me2-TEDGA, and TPDGA) has been investigated under neutral pH, concentrated aqueous nitrate solution conditions. A combination of steady-state gamma and time-resolved pulsed electron irradiation experiments, supported by advanced analytical techniques and multi-scale modeling calculations, have demonstrated that: (i) the investigated hydrophilic DGAs undergo first-order decay with an average dose constant of (-3.18 ± 0.23) × 10-6 Gy-1; (ii) their degradation product distributions are similar to those under pure water conditions, except for the appearance of NOx adducts; and (iii) radiolysis is driven by hydroxyl and nitrate radical oxidation chemistry moderated by secondary degradation product scavenging reactions. Overall, the radiolysis of hydrophilic DGAs in concentrated, aqueous nitrate solutions is significantly slower and less structurally sensitive than under pure water conditions, similar to their lipophilic analogs. Acid hydrolysis, not radiolysis, is expected to limit their useful lifetime. These findings are promising for the deployment of hydrophilic DGAs as actinide aqueous phase stripping and hold-back agents, due to the presence of high concentrations of nitrate in envisioned large-scale process conditions.

Effect of chemical environment on the radiation chemistry of N,N-di-(2-ethylhexyl)butyramide (DEHBA) and plutonium retention.

N,N-di-(2-ethylhexyl)butyramide (DEHBA) has been proposed as part of a hydro-reprocessing solvent extraction system for the co-extraction of uranium and plutonium from spent nuclear fuel, owing to its selectivity for hexavalent uranium and tetravalent plutonium. However, there is a critical lack of quantitative understanding regarding the impact of chemical environment on the radiation chemistry of DEHBA, and how this would affect process performance. Here we present a systematic investigation into the radiolytic degradation of DEHBA in a range of n-dodecane solvent system formulations, where we subject DEHBA to gamma irradiation, measure reaction kinetics, ligand integrity, degradation product formation, and investigate solvent system performance through uranium and plutonium extraction and strip distribution ratios. The rate of DEHBA degradation in n-dodecane was found to be slow (G = -0.31 ± 0.02 µmol J-1) but enhanced upon contact with the oxidizing conditions of the investigated solvent systems (organic-only, or in contact with either 0.1 or 3.0 M aqueous nitric acid). Two major degradation products were identified in the organic phase, bis-2-ethylhexylamine (b2EHA) and N-(2-ethylhexyl)butyramide (MEHBA), resulting from the cleavage of C-N bonds, and could account for the total loss of DEHBA up to ∼300 kGy for organic-only conditions. Both b2EHA and MEHBA were also found to be susceptible to radiolytic degradation, having G-values of -0.12 ± 0.01 and -0.08 ± 0.01 µmol J-1, respectively. Solvent extraction studies showed: (i) negligible change in uranium extraction and stripping with increasing absorbed dose; and (ii) plutonium extraction and retention exhibits complex dependencies on absorbed dose and chemical environment. Organic-only conditions afforded enhanced plutonium extraction and retention attributed to b2EHA, while acid contacts inhibited this effect and promoted significant plutonium retention for the highest acidity. Overall it has been demonstrated that chemical environment during irradiation has a significant influence on the extent of DEHBA degradation and plutonium retention.

31P NMR study of the activated radioprotection mechanism of octylphenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and analogues.

We report a 31P NMR investigation into the activated radioprotection mechanism of octylphenyl-N,N-diisobutyl-carbamoylmethyl phosphine oxide (CMPO) and analogues in the presence of a gamma radiation field. CMPO exhibits significantly enhanced radiation resistance in the presence of high nitric acid concentrations, compared to other ligands proposed for recovery of the trivalent actinides from spent nuclear fuel. The fundamental mechanism behind this activated radioprotection has been investigated using 31P NMR and other supporting analytical techniques (GCMS and LCMS) in conjunction with systematic gamma irradiation studies, covering solvent system formulation and structural effects through the use of the CMPO analogues, dioctylphenylphospine oxide (DOPPO) and trioctylphosphine oxide (TOPO). These experiments have demonstrated that the acid-dependent, radioprotection mechanism requires a protonated phenyl-phosphine oxide motif to activate. Further, contacting these three ligand loaded organic phases with a range of mineral acids (nitric, sulfuric, hydrochloric, and perchloric acids) shows specificity for nitric acid (HNO3), and the formation of a distinct [ligand·HNO3] complex for CMPO and DOPPO, as identified by 31P NMR, and predicted by DFT calculations. We propose that this complex is capable of sequential n-dodecane excited state quenching through the conjugated aromatic functionalities on the constituent CMPO and DOPPO molecules.

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.

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.

Electrochemical oxidation of ²4³Am(III) in nitric acid by a terpyridyl-derivatized electrode.

Selective oxidation of trivalent americium (Am) could facilitate its separation from lanthanides in nuclear waste streams. Here, we report the application of a high-surface-area, tin-doped indium oxide electrode surface-derivatized with a terpyridine ligand to the oxidation of Am(III) to Am(V) and Am(VI) in nitric acid. Potentials as low as 1.8 volts (V) versus the saturated calomel electrode were applied, 0.7 V lower than the 2.6 V potential for one-electron oxidation of Am(III) to Am(IV) in 1 molar acid. This simple electrochemical procedure provides a method to access the higher oxidation states of Am in noncomplexing media for the study of the associated coordination chemistry and, more important, for more efficient separation protocols.

Gamma-radiolytic stability of new methylated TODGA derivatives for minor actinide recycling.

The stability against gamma radiation of MeTODGA (methyl tetraoctyldiglycolamide) and Me2TODGA (dimethyl tetraoctyldiglycolamide), derivatives from the well-known extractant TODGA (N,N,N',N'-tetraoctyldiglycolamide), were studied and compared. Solutions of MeTODGA and Me2TODGA in alkane diluents were subjected to (60)Co γ-irradiation in the presence and absence of nitric acid and analyzed using LC-MS to determine their rates of radiolytic concentration decrease, as well as to identify radiolysis products. The results of product identification from three different laboratories are compared and found to be in good agreement. The diglycolamide (DGA) concentrations decreased exponentially with increasing absorbed dose. The MeTODGA degradation rate constants (dose constants) were uninfluenced by the presence of nitric acid, but the acid increased the rate of degradation for Me2TODGA. The degradation products formed by irradiation are also initially produced in greater amounts in acid-contacted solution, but products may also be degraded by continued radiolysis. The identified radiolysis products suggest that the weakest bonds are those in the diglycolamide center of these molecules.

Supercritical fluid extraction and separation of uranium from other actinides.

The feasibility of separating U from nitric acid solutions of mixed actinides using tri-n-butylphosphate (TBP)-modified supercritical fluid carbon dioxide (sc-CO2) was investigated. The actinides U, Np, Pu, and Am were extracted into sc-CO2 modified with TBP from a range of nitric acid concentrations, in the absence of, or in the presence of, a number of traditional reducing and/or complexing agents to demonstrate the separation of these metals from U under sc-CO2 conditions. The separation of U from Pu using sc-CO2 was successful at nitric acid concentrations of less than 3M in the presence of acetohydroxamic acid (AHA) or oxalic acid (OA) to mitigate Pu extraction, and the separation of U from Np was successful at nitric acid concentrations of less than 1M in the presence of AHA, OA, or sodium nitrite to mitigate Np extraction. Americium was not well extracted under any condition studied.

Determination of CMPO using HPLC-UV.

Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) is an extractant proposed for selective separation of radionuclide metals from used nuclear fuel solutions using solvent extraction. Radiolysis reactions can degrade CMPO and reduce separation performance and hence methods for measuring the concentration of CMPO are needed. A novel high performance liquid chromatography (HPLC) method was developed for measuring CMPO in dodecane that featured a low pH buffer, octanol as a co-solvent with 2-propanol, and ultraviolet (UV) detection. Validation data indicated that the HPLC-UV method for CMPO determination provided good linearity, sensitivity, accuracy and precision. Method performance was evaluated using CMPO samples that had undergone radiolysis, and the results showed a decrease in CMPO concentration and the appearance of degradation products. The degradation products were identified using electrospray ionization mass spectrometry, which also showed formation of CMPO-nitric acid complexes that account for the apparent loss of CMPO in an acidic environment, independent of irradiation.

Bismuth coordination chemistry with allyl, alkoxide, aryloxide, and tetraphenylborate ligands and the {[2,6-(Me2NCH2)2C6H3]2Bi}+ cation.

A series of bis(aryl) bismuth compounds containing (N,C,N)-pincer ligands, [2,6-(Me(2)NCH(2))(2)C(6)H(3)](-) (Ar'), have been synthesized and structurally characterized to compare the coordination chemistry of Bi(3+) with similarly sized lanthanide ions, Ln(3+). Treatment of Ar'(2)BiCl, 1, with ClMg(CH(2)CH═CH(2)) affords the allyl complex Ar'(2)Bi(η(1)-CH(2)CH═CH(2)), 2, in which only one allyl carbon atom coordinates to bismuth. Complex 1 reacts with KO(t)Bu and KOC(6)H(3)Me(2)-2,6 to yield the alkoxide Ar'(2)Bi(O(t)Bu), 3, and aryloxide Ar'(2)Bi(OC(6)H(3)Me(2)-2,6), 4, respectively, but the analogous reaction with the larger KOC(6)H(3)(t)Bu(2)-2,6 forms [Ar'(2)Bi][OC(6)H(3)(t)Bu(2)-2,6], 6, in which the aryloxide ligand acts as an outer sphere anion. Chloride is removed from 1 by NaBPh(4) to form [Ar'(2)Bi][BPh(4)], 5, which crystallizes from THF in an unsolvated form with tetraphenylborate as an outer sphere counteranion.

Free-radical chemistry of disinfection byproducts. 3. Degradation mechanisms of chloronitromethane, bromonitromethane, and dichloronitromethane.

Halonitromethanes (HNMs) are byproducts formed through ozonation and chlorine/ chloramine disinfection processes in drinking waters that contain dissolved organic matter and bromide ions. These species occur at low concentration but have been determined to have high cytotoxicity and mutagenicity and therefore may represent a human health hazard. In this study, we have investigated the chemistry involved in the mineralization of HNMs to nonhazardous inorganic products through the application of advanced oxidation and reduction processes. We have combined measured absolute reaction rate constants for the reactions of chloronitromethane, bromonitromethane, and dichloronitromethane with the hydroxyl radical and the hydrated electron with a kinetic computer model in an attempt to elucidate the reaction pathways of these HNMs. The results are compared to measurements of stable products resulting from steady-state (60)Co gamma-irradiations of the same compounds. The model predicted the decomposition of the parent compounds and ingrowth of chloride and bromide ions with excellent accuracy, but the prediction of the total nitrate ion concentration was slightly in error, reflecting the complexity of nitrogen oxide species reactions in irradiated solution.

Determination of arrhenius and thermodynamic parameters for the aqueous reaction of the hydroxyl radical with lactic acid.

Lactic acid is a major component of the TALSPEAK process planned for use in the separation of trivalent lanthanide and actinide elements. This acid acts both as a buffer and to protect the actinide complexant from radiolytic damage. However, there is little kinetic information on the reaction of water radiolysis species with lactic acid, particularly under the anticipated process conditions of aerated aqueous solution at pH approximately 3, where oxidizing reactions are expected to dominate. Here we have determined temperature-dependent reaction rate constants for the reactions of the hydroxyl radical with lactic acid and the lactate ion. For lactic acid this rate constant is given by the following equation: ln k(1) = (23.85 +/- 0.19) - (1120 +/- 54)/T, corresponding to an activation energy of 9.31 +/- 0.45 kJ mol(-1) and a room temperature reaction rate constant of (5.24 +/- 0.35) x 10(8) M(-1) s(-1) (24.0 degrees C). For the lactate ion, the temperature-dependent rate constant is given by ln k(2) = (24.83 +/- 0.14) - (1295 +/- 42)/T, for an activation energy of 10.76 +/- 0.35 kJ mol(-1) and a room temperature value of (7.77 +/- 0.50) x 10(8) M(-1) s(-1) (22.2 degrees C). These kinetic data have been combined with autotitration measurements to determine the temperature-dependent behavior of the lactic acid pK(a) value, allowing thermodynamic parameters for the acid dissociation to be calculated as DeltaH(o) = -10.75 +/- 1.77 kJ mol(-1), DeltaS(o) = -103.9 +/- 6.0 J K(-1) mol(-1) and DeltaG(o) = 20.24 +/- 2.52 kJ mol(-1) at low ionic strength.

Tributylphosphate extraction behavior of bismuthate-oxidized americium.

Higher oxidation states of americium have long been known; however, options for their preparation in acidic solution are limited. The conventional choice, silver-catalyzed peroxydisulfate, is not useful at nitric acid concentrations above about 0.3 M. We investigated the use of sodium bismuthate as an oxidant for Am (3+) in acidic solution. Room-temperature oxidation produced AmO 2 (2+) quantitatively, whereas oxidation at 80 degrees C produced AmO 2 (+) quantitatively. The efficacy of the method for the production of oxidized americium was verified by fluoride precipitation and by spectroscopic absorbance measurements. We performed absorbance measurements using a conventional 1 cm cell for high americium concentrations and a 100 cm liquid waveguide capillary cell for low americium concentrations. Extinction coefficients for the absorbance of Am (3+) at 503 nm, AmO 2 (+) at 514 nm, and AmO 2 (2+) at 666 nm in 0.1 M nitric acid are reported. We also performed solvent extraction experiments with the hexavalent americium using the common actinide extraction ligand tributyl phosphate (TBP) for comparison to the other hexavalent actinides. Contact with 30% tributyl phosphate in dodecane reduced americium; it was nevertheless extracted using short contact times. The TBP extraction of AmO 2 (2+) over a range of nitric acid concentrations is shown for the first time and was found to be analogous to that of uranyl, neptunyl, and plutonyl ions.

A pulse radiolysis investigation of the reactions of tributyl phosphate with the radical products of aqueous nitric acid irradiation.

Tributyl phosphate (TBP) is the most common organic compound used in liquid-liquid separations for the recovery of uranium, neptunium, and plutonium from acidic nuclear fuel dissolutions. The goal of these processes is to extract the actinides while leaving fission products in the acidic, aqueous phase. However, the radiolytic degradation of TBP has been shown to reduce separation factors of the actinides from fission products and to impede the back-extraction of the actinides during stripping. As most previous investigations of the radiation chemistry of TBP have focused on steady state radiolysis and stable product identification, with dibutylphosphoric acid (HDBP) invariably being the major product, here we have determined room temperature rate constants for the reactions of TBP and HDBP with the hydroxyl radical [(5.00 +/- 0.05) x 10(9), (4.40 +/- 0.13) x 10(9) M(-1) s(-1)], hydrogen atom [(1.8 +/-0.2) x 10(8), (1.1 +/- 0.1) x 10(8) M(-1) s(-1)], nitrate radical [(4.3 +/- 0.7) x 10(6), (2.9 +/- 0.2) x 10(6) M(-1) s(-1)], and nitrite radical (<2 x 10 (5), <2 x 10(5) M(-1) s(-1)), respectively. These data are used to discuss the mechanism of TBP radical-induced degradation.

Free radical chemistry of disinfection byproducts. 2. Rate constants and degradation mechanisms of trichloronitromethane (chloropicrin).

Absolute rate constants for the free-radical-induced degradation of trichloronitromethane (TCNM, chloropicrin) were determined using electron pulse radiolysis and transient absorption spectroscopy. Rate constants for hydroxyl radical, *OH, and hydrated electron, e(aq)-, reactions were (4.97 +/- 0.28) x 10(7) M(-1) s(-1) and (2.13 +/- 0.03) x 10(10) M(-1) s(-1), respectively. It appears that the *OH adds to the nitro-group, while the e(aq)- reacts via dissociative electron attachment to give two carbon centered radicals. The mechanisms of these free radical reactions with TCNM were investigated, using 60Co gamma irradiation at various absorbed doses, measuring the disappearance of TCNM and the appearance of the product nitrate and chloride ions. The rate constants and mechanistic data were combined in a kinetic computer model that was used to describe the major free radical pathways for the destruction of TCNM in solution. These data are applicable to other advanced oxidation/reduction processes.