Substituted chloromethyl radicals and anions are potential intermediates in the reduction of substituted chlorinated methanes (CHxCl3-xL, with L- = F-, OH-, SH-, NO3-, HCO3- and x = 0−3). Thermochemical properties, (298.15 K), S°(298.15 K,1 bar), and ΔGS(298.15 K, 1 bar), were calculated by using ab initio electronic structure methods for the substituted chloromethyl radicals and anions: CHyCl2-yL• and CHyCl2-yL-, for y = 0−2. In addition, thermochemical properties were calculated for the aldehyde, ClHCO, and the gem-chlorohydrin anions, CCl3O-, CHCl2O-, and CH2ClO-. The thermochemical properties of these additional compounds were calculated because the nitrate-substituted compounds, CHyCl2-y(NO3)• and CHyCl2-y(NO3)-, were not stable, with all levels of ab initio theory leading to highly dissociated complexes. On the basis of these thermochemical estimates, the overall reaction energetics (in the gas phase and aqueous phase) for several mechanisms of the first electron reduction of the substituted chlorinated methanes were predicted. In almost all of the cases, the thermodynamically most favorable pathway resulted in loss of Cl-. The exception was for the reduction of the nitrate-substituted chlorinated methanes CHxCl3-x(NO3). On reduction, these compounds were shown to readily decompose into a Cl- anion, NO2• gas, and an aldehyde. In addition, the results of this study suggest that a higher degree of chlorination corresponds to a more favorable reduction. Relative to the nonsubstituted chlorinated methanes, the thermodynamic results suggest the CHxCl3-xF, CHxCl3-xOH, and CHxCl3-x(HCO3) compounds are moderately more difficult to reduce, the CHxCl3-xSH compounds are moderately less difficult to reduce, and the CHxCl3-x(NO3) compounds are substantially more favorable to reduce. These results demonstrate that ab initio electronic structure methods can be used to calculate the reduction potentials of organic compounds to help identify the potentially important environmental degradation reactions.
'The program at Florida State University was funded to collaborate with Dr. A. Felmy (PNNL) on speciation in high level wastes and with Dr. D. Rai (PNNL) on redox of Pu under high level waste conditions. The funding provided support for 3 research associates (postdoctoral researchers) under Professor G. R. Choppin as P.I. Dr. Kath Morris from U. Manchester (Great Britain), Dr. Dean Peterman and Dr. Amy Irwin (both from U. Cincinnati) joined the laboratory in the latter part of 1996. After an initial training period to become familiar with basic actinide chemistry and radiochemical techniques, they began their research. Dr. Peterman was assigned the task of measuring Th-EDTA complexation prior to measuring Pu(IV)-EDTA complexation. These studies are associated with the speciation program with Dr. Felmy. Drs. Morris and Irwin initiated research on redox of plutonium with agents present in the Hanford Tanks as a result of radiolysis or from use in separations. The preliminary results obtained thus far are described in this report. It is expected that the rate of progress will continue to increase significantly as the researchers gain more experience with plutonium chemistry.'
Three simulated waste solutions representing wastes from tanks SY-102 (high nitrate, modified to exceed guidance limits), AN-107, and AY-102 were supplied by PNNL. Out of three solutions tested, both optical and electrochemical results show that carbon steel samples corroded much faster in SY-102 (high nitrate) than in the other two solutions, AN-107 and AY-102, with lower ratios of nitrate to nitrite. The effect of the surface preparation was not as strong as the effect of solution chemistry. In areas with pristine mill-scale surface, no corrosion occurred even in the SY-102 (high nitrate) solution, however, corrosion occurred in the areas where the mill-scale was damaged or flaked off due to machining.
This document is the user`s manual and technical reference for the Soil Chromium Attenuation Model (CHROMAT{trademark}), a computer code designed to calculate both the dissolved Cr concentration and the amount of Cr attenuated in soils as a result of the geochemical reactions that occur as Cr-containing leachates migrate through porous soils. The dissolved Cr concentration and the amount of Cr attenuated are calculated using thermodynamic (mechanistic) data for aqueous complexation reactions, adsorption/ desorption reactions, and precipitation/dissolution reactions involving both CR(III) and Cr(VI) species. Use of this mechanistic approach means that CHROMAT{trademark} requires a minimum amount of site-specific data on leachate and soil characteristics. CHROMAT{trademark} is distributed in executable form for IBM and IBM-compatible personal computers through a license from the Electric Power Research Institute (EPRI). The user interacts with CHROMAT{trademark} using menu-driven screen displays. Interactive on-line help options are available. Output from the code can be obtained in tabular or graphic form. This manual describes the development of CHROMAT{trademark}, including experimental data development in support of the model and model validation studies. The thermodynamic data and computational algorithm are also described. Example problems and results are included.
Abstract The aqueous solubility of BaSeO 4 (cr) was studied at 23 ± 2 ℃ as a function of Na 2 SeO 4 concentrations (0.0001 to 4.1 mol kg – 1 ) and equilibration periods (3 to 596 d). The equilibrium, approached from both the under- and over-saturation directions, in this system was reached rather rapidly (≤3 d). The SIT and Pitzer's ion-interaction models were used to interpret these data and the predictions based on both of these models agreed closely with the experimental data. Thermodynamic analyses of the data show that BaSeO 4 (cr) is the solubility-controlling phase for Na 2 SeO 4 concentrations <0.59 mol kg – 1 . The log 10 K 0 value for the BaSeO 4 (cr) solubility product (BaSeO 4 (cr) ⇌ Ba 2+ + SeO 4 2 – ) calculated by the SIT and Pitzer models were very similar (− 7.32 ± 0.07 with Pitzer and − 7.25 ± 0.11 with SIT). Although the BaSeO 4 (cr) solubility product and Ba concentrations as a function of Na 2 SeO 4 concentrations predicted by both the SIT and Pitzer models are similar, the models required different sets of fitting parameters. For examples, 1) interpretations using the SIT model required the inclusion of Ba(SeO 4 ) 2 2 – species with log 10 K 0 = 3.44 ± 0.12 for the reaction (Ba 2+ + 2SeO 4 2 – ⇌ Ba(SeO 4 ) 2 2 – ), whereas these species are not needed for Pitzer model, and 2) at Na 2 SeO 4 concentrations >0.59 mol kg – 1 it was also possible to calculate the value for log 10 K 0 for the solubility product of a proposed double salt (Na 2 Ba(SeO 4 ) 2 (s) ⇌ 2Na + + Ba 2+ + 2SeO 4 2 – ) which for the SIT model is − (8.70 ± 0.29) whereas for the Pitzer model it is − (9.19 ± 0.19). The ion-interaction/ion-association parameters hitherto unavailable for both the SIT and Pitzer models required to fit these extensive data extending to as high ionic strengths as 12.3 mol kg – 1 were determined. The model developed in this study is consistent with all of the reliable literature data, which was also used to extend the model to barium concentrations as high as 0.22 mol kg – 1 and pH ranging from 1.4 to 13.8, in addition to selenium concentrations as high as 4.1 mol kg – 1 .
The presence of different anionic species in natural waters can significantly alter the degradation rates of chlorinated methanes and other organic compounds. Favorable reaction energetics is a necessary feature of these nucleophilic substitution reactions that can result in the degradation of the chlorinated methanes. In this study, ab initio electronic structure theory is used to evaluate the free energies of reaction of a series of monovalent anionic species (OH-, SH-, NO3-, HCO3-, HSO3-, HSO4-, H2PO4-, and F-) that can occur in natural waters with the chlorinated methanes, CCl4, CCl3H, CCl2H2, and CClH3. The results of this investigation show that nucleophilic substitution reactions of OH-, SH-, HCO3-, and F- are significantly exothermic for chlorine displacement, NO3- reactions are slightly exothermic to thermoneutral, HSO3- reactions are slightly endothermic to thermoneutral and HSO4-, and H2PO4- reactions are significantly endothermic. In the case of OH-, SH-, and F- where there are limited experimental data, these results agree well with experiment. The results for HCO3- are potentially important given the near ubiquitous occurrence of carbonate species in natural waters. The calculations reveal that the degree of chlorination, with the exception of substitution of OH-, does not have a large effect on the Gibbs free energies of the substitution reactions. These results demonstrate that ab initio electronic structure methods can be used to calculate the reaction energetics of a potentially large number of organic compounds with other aqueous species in natural waters and can be used to help identify the potentially important environmental degradation reactions.
Abstract The solubility of amorphous UO 2 ·xH2O (I) is studied in dilute solutions in the pH range 2‐12 using Fe powder and Eu 2+ to minimize the possibility of oxidation of U(IV) to U(VI) during the experiments.
Shale formations play fundamental roles in large-scale geologic carbon sequestration (GCS) aimed primarily to mitigate climate change and in smaller-scale GCS targeted mainly for CO2-enhanced gas recovery operations. Reactive components of shales include expandable clays, such as montmorillonites and mixed-layer illite/smectite clays. In this study, in situ X-ray diffraction (XRD) and in situ infrared (IR) spectroscopy were used to investigate the swelling/shrinkage and H2O/CO2 sorption of Na(+)-exchanged montmorillonite, Na-SWy-2, as the clay is exposed to variably hydrated supercritical CO2 (scCO2) at 50 °C and 90 bar. Measured d001 values increased in stepwise fashion and sorbed H2O concentrations increased continuously with increasing percent H2O saturation in scCO2, closely following previously reported values measured in air at ambient pressure over a range of relative humidities. IR spectra show H2O and CO2 intercalation, and variations in peak shapes and positions suggest multiple sorbed types of H2O and CO2 with distinct chemical environments. Based on the absorbance of the asymmetric CO stretching band of the CO2 associated with the Na-SWy-2, the sorbed CO2 concentration increases dramatically at sorbed H2O concentrations from 0 to 4 mmol/g. Sorbed CO2 then sharply decreases as sorbed H2O increases from 4 to 10 mmol/g. With even higher sorbed H2O concentrations as saturation of H2O in scCO2 was approached, the concentration of sorbed CO2 decreased asymptotically. Two models, one involving space filling and the other a heterogeneous distribution of integral hydration states, are discussed as possible mechanisms for H2O and CO2 intercalations in montmorillonite. The swelling/shrinkage of montmorillonite could affect solid volume, porosity, and permeability of shales. Consequently, the results may aid predictions of shale caprock integrity in large-scale GCS as well as methane transmissivity in enhanced gas recovery operations.
