Pressure-induced phase transition of Lu2Ti2O7and Lu1.5Ce0.5Ti2O7+xpyrochlores.

This study utilizes both experimental and computational approaches to investigate the performance of Lu2Ti2O7(LTO) and Lu1.5Ce0.5Ti2O7+x(LCTO) pyrochlores under high pressure. The structural changes of LTO and LCTO pyrochlores were characterized usingin-situsynchrotron x-ray diffraction (SXRD) andin-situRaman spectroscopy at pressures up to 44.6 GPa. The kinks inP-aandP-Vcurves at around 5 GPa are mainly attributed to the interaction between the pressure medium and the isostructural changes. The onset pressures for transitioning from the cubic pyrochlore phase (Fd-3 m) to the monoclinic phase (P21) are observed at 32.5 GPa and 38.1 GPa, respectively. It is important to note that at the highest measured pressures, the phase transition remains incomplete. This partial transition is likely the result of oriented disorder among cations and anions under high pressure. In addition, introducing Ce as a dopant significantly enhances structural stability. This can be explained by the larger ionic radius of Ce, which hinders the disordering process.

Update to the Newly Developed Expression for the Stability Ratio of Colloidal Dispersions.

The colloidal stability, one of the basic and important properties of a colloidal dispersion, is commonly evaluated in terms of the stability ratio. In this study, a recently developed expression for the stability ratio is updated, by reformulating the fraction of successful collisions leading to secondary minimum coagulation. The updated formula reinterprets the statistical meaning of the fraction of successful collisions leading to primary or secondary minimum coagulation, ensuring that the total fraction of successful collisions is always less than or equals to 1. It was shown to be superior to the available expressions in accounting for the contribution of the primary and secondary minimum coagulations on the stability ratio. It can well interpret the stability of colloidal dispersions of spherical particles; moreover, it is of great potential to be applied to colloidal dispersions of plate-like particles. In addition, this formula is found to be consistent with the concept of the critical coagulation concentration and well interpret the effects of particle size, counterion valence, surface potential, and Hamaker constant on the colloidal stability.

Study of natural attenuation after acid in situ leaching of uranium mines using isotope fractionation and geochemical data.

Acid in situ leaching (AISL) is a subsurface mining approach suitable for low-grade ores which does not generate tailings, and has been adopted widely in uranium mining. However, this technique causes an extremely high concentration of contaminants at post-mining sites and in the surroundings soon after the mining ceases. As a potential AISL remediation strategy, natural attenuation has not been studied in detail. To address this problem, groundwater collected from 26 wells located within, adjacent, upgradient, and downgradient of a post-mining site were chosen to analyze the fate of U(VI), SO42-, δ34S, and δ238U, to reveal the main mechanisms governing the migration and attenuation of the dominant contaminants and the spatio-temporal evolutions of contaminants in the confined aquifer of the post-mining site. The δ238U values vary from -0.07 ‰ to 0.09 ‰ in the post-mining site and from -1.43 ‰ to 0.03 ‰ around the post-mining site. The δ34S values were found to vary from 3.3 ‰ to 6.2 ‰ in the post-mining site and from 6.0 ‰ to 11.0 ‰ around the post-mining site. Detailed analysis suggests that there are large differences between the range of isotopic composition variation and the range of pollutants concentration distribution, and the estimated Rayleigh isotope fractionation factor is 0.9994-0.9997 for uranium and 1.0032-1.0061 for sulfur. The isotope ratio of uranium and sulfur can be used to deduce the migration history of the contaminants and the irreversibility of the natural attenuation process in the anoxic confined aquifer. Combining the isotopic fractionation data for U and S with the concentrations of uranium and sulfate improved the accuracy of understanding of reducing conditions along the flow path. The study also indicated that as long as the geological conditions are favorable for redox reactions, natural attenuation could be used as a cost-effective remediation scheme.

Revisiting the Theory of Coagulation of Colloidal Dispersions: An Improved Expression for the Stability Ratio.

The stability of a colloidal dispersion has long been expressed in terms of the stability ratio. Based on the available theories of coagulation of colloidal dispersions, a novel expression, complying with the classical definition, is developed for the stability ratio. It accounts for the contributions of both primary and secondary minimum coagulations to the overall rate of coagulations. In addition, it can also be regarded as the result of a combination of the kinetic theory of an ideal gas and the Smoluchowski theory with Fuchs' correction, considering the interaction between identical spherical particles and their surfaces immersed in a symmetrical electrolyte solution. The agreement with experimental data suggested that it is superior to the classical ones in describing the weak dependence of the stability ratio on the particle size and the valence of the counterion, by emphasizing the importance of the secondary minimum coagulation in dispersion stability and the complementation between the two modes of coagulation.

Development and application of an advection-dispersion model for data analysis of electromigration experiments with intact rock cores.

An advection-dispersion model was developed for interpreting the experimental results of electromigration in granitic rock cores. The most important mechanisms governing the movement of the tracer ions, i.e. electromigration, electroosmosis and dispersion were taken into account by the advection-dispersion model, but the influence of aqueous chemistry was ignored. An analytical solution in the Laplace domain was derived and then applied to analyze the measured results of a series of experiments, performed in an updated experimental device using different applied voltages. The modelling results suggested that both studied tracers, i.e. iodide and selenite, are effectively non-sorbing in the intact rock investigated. The effective diffusivities and formation factors evaluated from the model were also found to be in good agreement with data reported in literature and the associated uncertainties are much smaller than those obtained from the classical ideal plug-flow model, which accounts only for the dominant effect of electromigration on ionic transport. To explore further how the quality of parameter identifications would be influenced by neglect of aqueous chemistry, a reactive transport model was also implemented, which may be regarded as a multi-component version of the advection-dispersion model. The analysis showed that the advection-dispersion model works equally well as the reactive transport model but requires much less computational demand. It can, therefore, be used with great confidence to interpret the experimental results of electromigration for studies of diffusion and sorption behavior of radionuclides in intact rock cores.

A modification of the electromigration device and modelling methods for diffusion and sorption studies of radionuclides in intact crystalline rocks.

To determine the diffusion and sorption properties of radionuclides in intact crystalline rocks, a new electromigration device was built and tested by running with I- and Se(IV) ions. By introducing a potentiostat to impose a constant voltage over the studied rock sample, the electromigration device can give more stable and accurate experimental results than those from the traditional electromigration devices. In addition, the variation in the pH of the background electrolytes was minimised by adding a small amount of NaHCO3 as buffers. To interpret the experimental results with more confidence, an advection-dispersion model was also developed in this study, which accounts for the most important mechanisms governing ionic transport in the electromigration experiments. Data analysis of the breakthrough curves by the advection-dispersion model, instead of the traditional ideal plug-flow model, suggest that the effective diffusivities of I- and Se(IV) are (1.15 ± 0.06) × 10-13 m2/s and (3.50 ± 0.86) × 10-14 m2/s, respectively. The results also show that I- is more mobile than Se(IV) ions when migrating through the same intact rock sample and that their sorption properties are almost identical.

Counterion-only electrical double layers: An application of density functional theory.

Within the framework of density functional theory, a self-consistent approach of weighted correlation approximation is developed to give an accurate account of the cross correlations between the Coulombic interaction and the hard-sphere exclusion in the counterion-only electrical double layers. Application of the approach to the cases of practical interest, against the Monte Carlo simulations, shows that it is excellent in describing the structural properties and the pressures of the confined solutions involving both mono- and divalent counterions between two planar charged walls. In particular, the study suggests that the relative importance of electrostatic correlations in comparison to the effects of ionic excluded volume and direct Coulomb interactions depends on the valency of the counterions and the surface charge density. In a clay system with mixed counterions, the competition between the mono- and divalent ions results in a large swelling when the fraction of surface charge compensated by monovalent counterions is greater than 30%. In the opposite situation involving mostly divalent counterions, a limited swelling is found and the attraction between the clay particles favors the formation of stacks incorporating a water layer of about 1.0 nm. These findings are consistent with experimental observations, giving insight into some mechanisms governing the stability of colloidal clay in salt-free or dilute solutions.

A systematic comparison of different approaches of density functional theory for the study of electrical double layers.

Based on the best available knowledge of density functional theory (DFT), the reference-fluid perturbation method is here extended to yield different approaches that well account for the cross correlations between the Columbic interaction and the hard-sphere exclusion in an inhomogeneous ionic hard-sphere fluid. In order to quantitatively evaluate the advantage and disadvantage of different approaches in describing the interfacial properties of electrical double layers, this study makes a systematic comparison against Monte Carlo simulations over a wide range of conditions. The results suggest that the accuracy of the DFT approaches is well correlated to a coupling parameter that describes the coupling strength of electrical double layers by accounting for the steric effect and that can be used to classify the systems into two regimes. In the weak-coupling regime, the approaches based on the bulk-fluid perturbation method are shown to be more accurate than the counterparts based on the reference-fluid perturbation method, whereas they exhibit the opposite behavior in the strong-coupling regime. More importantly, the analysis indicates that, with a suitable choice of the reference fluid, the weighted correlation approximation (WCA) to DFT gives the best account of the coupling effect of the electrostatic-excluded volume correlations. As a result, a piecewise WCA approach can be developed that is robust enough to describe the structural and thermodynamic properties of electrical double layers over both weak- and strong-coupling regimes.

Solute transport in a single fracture involving an arbitrary length decay chain with rock matrix comprising different geological layers.

A model is developed to describe solute transport and retention in fractured rocks. It accounts for advection along the fracture, molecular diffusion from the fracture to the rock matrix composed of several geological layers, adsorption on the fracture surface, adsorption in the rock matrix layers and radioactive decay-chains. The analytical solution, obtained for the Laplace-transformed concentration at the outlet of the flowing channel, can conveniently be transformed back to the time domain by the use of the de Hoog algorithm. This allows one to readily include it into a fracture network model or a channel network model to predict nuclide transport through channels in heterogeneous fractured media consisting of an arbitrary number of rock units with piecewise constant properties. More importantly, the simulations made in this study recommend that it is necessary to account for decay-chains and also rock matrix comprising at least two different geological layers, if justified, in safety and performance assessment of the repositories for spent nuclear fuel.

A model to describe the performance of the UASB reactor.

A dynamic model to describe the performance of the Upflow Anaerobic Sludge Blanket (UASB) reactor was developed. It includes dispersion, advection, and reaction terms, as well as the resistances through which the substrate passes before its biotransformation. The UASB reactor is viewed as several continuous stirred tank reactors connected in series. The good agreement between experimental and simulated results shows that the model is able to predict the performance of the UASB reactor (i.e. substrate concentration, biomass concentration, granule size, and height of the sludge bed).

Weighted correlation approach: an extended version with applications to the hard-sphere fluid.

The purpose of this study is to extend the weighted correlation approach (WCA) for inhomogeneous fluids. It now introduces a generic expression to evaluate the single-particle direct correlation function in terms of a series of pair direct correlation functions weighted by different correlation-weight functions and adjustable weight factors. When applied for practical use, however, approximations of the pair direct correlation functions have to be made, together with appropriate definitions of the weighted densities and the choices of the correlation-weight functions. The WCA approach would, then, not only help us to connect and compare different strategies and their underlying assumptions in the density functional approaches, but also enable us to propose and apply density functional theory methods to predict the density profile of, e.g., the hard-sphere fluid confined between a pair of parallel planar hard walls. Numerical results of the extended WCA approach, against the Monte Carlo (MC) simulations in a range of surface separations and bulk densities, suggest that it is capable of representing the fine features of the hard-sphere density distributions. The WCA results also agree well with the calculations from the fundamental measure theory. In addition, the thermodynamic self-consistency of the WCA approach is confirmed by its fairly good agreement with the MC fitted data for the surface tension of a hard-sphere fluid at a planar hard wall. All these tests show that a pure WCA approach can be constructed to investigate the states of ionic hard-sphere fluids.

The weighted correlation approach for density functional theory: a study on the structure of the electric double layer.

Within the framework of density functional theory, a weighted correlation approach is developed in order to obtain the density distributions of an inhomogeneous fluid. It results in a formally exact expression, by means of the concept of a weighted pair correlation function, used to evaluate the change of the single-particle direct correlation function of the system relative to that of a reference state. When applying the approach for practical use, however, an approximation of the pair correlation function has to be made, along with an appropriate definition of a weight function. Noticeably, combining this approach with fundamental measure theory gives rise to a new method, which we call the FMT/WCA-k(2) approach, for studying the structural and thermodynamic properties of a charged hard-sphere fluid subjected to a spatially varying external potential. Application of the FMT/WCA-k(2) approach in a range of electrolyte concentrations and surface charge densities, against the Monte Carlo simulations, shows that it is superior to the typical approaches of density functional theory in predicting the ionic density profiles of both counter-ions and co-ions near a highly charged surface. It is capable of capturing the fine features of the structural properties of the electric double layers, to well reproduce the layering effect and the charge inversion phenomenon, also in strongly coupled cases where divalent counter-ions are involved. In addition, it is found that the FMT/WCA-k(2) approach even has an advantage over the anisotropic, hyper-netted chain approaches in giving better agreement with the Monte Carlo results.

A novel method to describe the interaction pressure between charged plates with application of the weighted correlation approach.

Based on the Euler-Lagrange equation for ion density distribution in an inhomogeneous, charged, and hard-sphere fluid, a novel method is proposed to determine the interaction pressure between charged plates. The resulting expression is a sum of distinct physical contributions to the pressure, which involves different contributions to the single-particle direct correlation function. It can, therefore, be conveniently used in any density functional approach to facilitate analysis of the pressure components. In this study, the so-called fundamental measure theory (FMT)∕weighted correlation approach (WCA) approach is applied to estimate both the hard-sphere and the electric residual contributions to the single-particle direct correlation function, upon the calculation of the ionic density profiles between charged plates. The results, against the Monte Carlo simulations, show that the FMT∕WCA approach is superior to the typical FMT∕mean spherical approximation approach of the density functional theory in predicting the interaction pressure between charged plates immersed in an electrolyte solution upon various conditions in the primitive model. The FMT∕WCA approach can well capture the fine features of the pressure-separation dependence, to reproduce not only the shoulder shape and the weak attractions in monovalent electrolytes but also the strongly oscillatory behavior of pressure in divalent electrolytes where pronounced attractions are observed. In addition, it is found that the FMT∕WCA approach even has an advantage over the anisotropic, hyper-netted chain approach in that it agrees with the Monte Carlo results to a very good extent with, however, much less computational effort.

A novel approach to determine the critical coagulation concentration of a colloidal dispersion with plate-like particles.

The critical coagulation concentration (ccc) of counterions is commonly described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on the basis of a static force balance. It can, however, also be estimated from a kinetic point of view by studying the process of colloidal coagulation, or from a dynamic point of view by considering colloidal transport in nonequilibrium systems where other processes such as diffusion and the influence of gravity come into play. In particular, in a test tube where colloidal expansion takes place, the ccc can be interpreted as the electrolyte concentration below which expansion of colloids would always lead to full access to the entire volume of the test tube and above which a sharp boundary is established between a colloidal gel and pure water. On the basis of this perception and the dynamic force balance model that we developed to describe colloidal expansion in a test tube, accounting for the effects of particle diffusion and gravity in contrast to the DLVO theory, we propose an alternative way to assess the ccc of counterions. We also derive an approximate expression for the case of homointeraction at constant charge for montmorillonite. The estimated ccc values agree quite well with those observed experimentally for both Na(+) and Ca(2+) counterions for montmorillonite dispersions, at pH approximately 6.5. This is in contrast to the DLVO theory, which overpredicts the ccc by about 2 orders of magnitude. In addition, the detailed analyses suggest that the ccc of counterions decreases with increasing surface area and with the thickness of the particles. For montmorillonite, the ccc is nearly independent of the surface charge density of the particles for the range of typical charge densities.

A dynamic force balance model for colloidal expansion and its DLVO-based application.

A force balance model that describes the dynamic expansion of colloidal bentonite gels/sols is presented. The colloidal particles are assumed to consist of one or several thin sheets with the other dimensions much larger than their thickness. The forces considered include van der Waals force, diffuse double layer force, thermal force giving rise to Brownian motion, gravity, as well as friction force. The model results in an expression resembling the instationary diffusion equation but with an immensely variable diffusivity. This diffusivity is strongly influenced by the concentration of counterions as well as by the particle concentration in the colloid gel/sol. The properties of the model are explored and discussed, exemplified by the upward expansion of an originally highly compacted bentonite tablet in a test tube. Examples are presented for a number of cases with ionic concentrations varying between very dilute waters up to several molar of counterions. The volume fraction of particles ranges from 40% to very dilute sols.