Sludge Treatment Evaluation: 1992 Technical progress

This report documents Fiscal Year 1992 technical progress on the Sludge Treatment Evaluation Task, which is being conducted by Pacific Northwest Laboratory. The objective of this task is to develop a capability to predict the performance of pretreatment processes for mixed radioactive and hazardous waste stored at Hanford and other US Department of Energy (DOE) sites. Significant cost savings can be achieved if radionuclides and other undesirable constituents can be effectively separated from the bulk waste prior to final treatment and disposal. This work is initially focused on chemical equilibrium prediction of water washing and acid or base dissolution of Hanford single-shell tank (SST) sludges, but may also be applied to other steps in pretreatment processes or to other wastes. Although SST wastes contain many chemical species, there are relatively few constituents -- Na, Al, NO[sub 3], NO[sub 2], PO[sub 4], SO[sub 4], and F -- contained in the majority of the waste. These constituents comprise 86% and 74% of samples from B-110 and U-110 SSTS, respectively. The major radionuclides of interest (Cs, Sr, Tc, U) are present in the sludge in small molal quantities. For these constituents, and other important components that are present in small molal quantities, the specific ion-interaction terms used in the Pitzer or NRTL equations may be assumed to be zero for a first approximation. Model development can also be accelerated by considering only the acid or base conditions that apply for the key pretreatment steps. This significantly reduces the number of chemical species and chemical reactions that need to be considered. Therefore, significant progress can be made by developing all the specific ion interactions for a base model and an acid dissolution model

Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of alpha-Fe2O3

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two-dimensions and of finite size in the third dimension (slabs) which is significantly faster than existing methods. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated, and solvated surfaces of alpha-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on alpha-alumina by Manassidis et al. [Surf. Sci. Lett. 285, L517, 1993]. Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.Comment: 23 pages, REVTeX (LaTeX), 8 figures not included, e-mail to [email protected], paper accepted in Surface Scienc

GMIN: A computerized chemical equilibrium model using a constrained minimization of the Gibbs free energy

This document is a user's manual and technical reference for the computerized chemical equilibrium model GMIN. GMIN calculates the chemical composition of systems composed of pure solid phases, solid-solution phases, gas phases, adsorbed phases, and the aqueous phase. In the aqueous phase model, the excess solution free energy is modeled by using the equations developed by PITZER and his coworkers, which are valid to high ionic strengths. The Davies equation can also be used. Activity coefficients for nonideal soild-solution phases are calculated using parameters of polynomial expansion in mole fraction of the excess free energy of mixing. The free energy of adsorbed phase species is described by the triple-layer site-binding model. The mathematical algorithm incorporated into GMIN is based upon a constrained minimization of the Gibbs free energy. This algorithm is numerically stable and reliably converges to a free energy minimum. The data base for GMIN contains all standard chemical potentials and Pitzer ion-interaction parameters necessary to model the system Na-K-Ca-Mg-H-Cl-SO{sub 4}-CO{sub 2}-B(OH){sub 4}-H{sub 2}0 at 25{degrees}C

Chemical speciation of strontium, americium, and curium in high level waste: Predictive modeling of phase partitioning during tank processing. Annual progress report, October 1996--September 1997

'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.

Interfacial reactivity of radionuclides: emerging paradigms from molecular-level observations

Over the past few decades an increasing array of molecular-level analytical probes has provided new detailed insight into mineral and radionuclide interfacial reactivity in subsurface environments. This capability has not only helped change the way mineral surface reactivity is studied but also how field scale contaminant migration problems are addressed and ultimately resolved. Here we review relatively new interfacial reactivity paradigms and assess their implications for future research directions. Specific examples include understanding the following: the role of site-to-site electron conduction at mineral surfaces and through bulk mineral phases and the effects of local chemical environment on the stability of intermediate species in oxidation-reduction reactions and the importance of mechanistic reaction pathways for defining possible reaction products and thermodynamic driving force. The discussion also includes examples of how detailed molecular/microscopic characterization of field samples has changed the way complex contaminant migration problems are conceptualized and modeled

Predictive geochemical modeling of interactions between uranium-mill-tailings solutions and sediments in a flow-through system: model formulations and preliminary results

An equilibrium thermodynamic conceptual model consisting of minerals and solid phases was developed to represent a soil column. A computer program was used as a tool to solve the system of mathematical equations imposed by the conceptual chemical model. The combined conceptual model and computer program were used to predict aqueous phase compositions of effluent solutions from permeability cells packed with geologic materials and percolated with uranium mill tailings solutions. Initial calculations of ion speciation and mineral solubility and our understanding of the chemical processes occurring in the modeled system were used to select solid phases for inclusion in the conceptual model. The modeling predictions were compared to the analytically determined column effluent concentrations. Hypotheses were formed, based on modeling predictions and laboratory evaluations, as to the probable mechanisms controlling the migration of selected contaminants. An assemblage of minerals and other solid phases could be used to predict the concentrations of several of the macro constituents (e.g., Ca, SO/sub 4/, Al, Fe, and Mn) but could not be used to predict trace element concentrations. These modeling conclusions are applicable to situations where uranium mill tailings solutions of low pH and high total dissolved solids encounter either clay liners or natural geologic materials that contain inherent acid neutralizing capacities. 116 references, 22 figures, 6 tables

Estimating the hydrogen ion concentration in concentrated NaCl and Na{sub 2}SO{sub 4} electrolytes

Combination glass electrodes were tested for determining H{sup +} concentrations in concentrated pure and mixed NaCl and Na{sub 2}SO{sub 4} solutions, as well as natural brine systems. NaCl, Na{sub 2}SO{sub 4}, and mixtures of NaCl and Na{sub 2}SO{sub 4} solutions were analyzed. Correction factors for estimating pC{sub H}{sup +} (negative logarithm of H{sup +} concentration) were determined from measured/observed pH values. Required Gran-type titrations were done with HCl and/or NaOH. The titration method is described and a step-by-step procedure provided; it has been used previously for determining pC{sub H}{sup +} values of synthetic chloride-dominated brines. Precautions are required to determine correction factors for electrolytes that react with H{sup +} or OH{sup {minus}} [sulfate brines for titration with acid; magnesium brines for titration with base because of precipitation of Mg(OH)2]. Correction factors A (pC{sub H}{sup +} = pH{sub ob} + A) from HCl titrations were similar to those from NaOH titrations where the concentration of free H{sup +} was calculated using a thermodynamic model. These values should be applicable to solns with a very large range in measured pH values (2 to 12). Because a large number of solns were titrated with HCl and the A values are similar for HCl and NaOH titrations, the A values for NaCl and Na2SO4 solns were fit as a function of molality to allow extrapolation. For NaCl solns 0 to 6.0 M, A can be obtained by multiplying the molality by 0.159. For Na2SO4 solns 0 to 2.0 M, the values of A can be obtained from (0.221 {minus} 0.549X + 0.201X{sup 2}), where X is the molality of Na{sub 2}SO{sub 4}. Orion-Ross electrode evaluations indicated that the A values did not differ significantly for different electrodes. Results suggest that the data in this report can be used to estimate A values for different NaCl and Na{sub 2}SO{sub 4} solns even for noncalibrated electrodes

MININR: a geochemical computer program for inclusion in water flow models - an application study

MININR is a reduced form of the computer program MINTEQ which calculates equilibrium precipitation/dissolution of solid phases, aqueous speciation, adsorption, and gas phase equilibrium. The user-oriented features in MINTEQ were removed to reduce the size and increase the computational speed. MININR closely resembles the MINEQL computer program developed by Westall (1976). The main differences between MININR and MINEQL involve modifications to accept an initial starting mass of solid and necessary changes for linking with a water flow model. MININR in combination with a simple water flow model which considers only dilution was applied to a laboratory column packed with retorted oil shale and percolated with distilled water. Experimental and preliminary model simulation results are presented for the constituents K/sup +/, Na/sup +/, SO/sub 4//sup 2 -/, Mg/sup 2 +/, Ca/sup 2 +/, CO/sub 3//sup 2 -/ and pH

The chemistry of sludge washing and caustic leaching processes for selected Hanford tank wastes

A broad-based study on washing and caustic leaching of Hanford tank sludges was performed in FY 1995 to gain a better understanding of the basic chemical processes that underlie this process. This approach involved testing of the baseline sludge washing and caustic leaching method on several Hanford tank sludges, and characterization of the solids both before and after testing by electron microscopy, X-ray diffraction, and X-ray absorption spectroscopy. A thermodynamically based model was employed to help understand the factors involved in individual specie distribution in the various stages of the sludge washing and caustic leaching treatment. The behavior of the important chemical and radiochemical components throughout the testing is summarized and reviewed in this report