In this study, solubility measurements were conducted for sodium polyborates in MgCl2 solutions at 22.5 ± 0.5 °C. According to solution chemistry and XRD patterns, di-sodium tetraborate decahydrate (borax) dissolves congruently, and is the sole solubility-controlling phase, in a 0.01 mol/kg MgCl2 solution: {equation presented} However, in a 0.1 mol/kg MgCl2 solution borax dissolves incongruently and is in equilibrium with di-sodium hexaborate tetrahydrate: {equation presented} In this study, the equilibrium constant (log K0) for Reaction 2 at 25 °C and infinite dilution was determined to be -16.44 ± 0.13 (2σ) based on the experimental data and the Pitzer model for calculations of activity coefficients of aqueous species. In accordance with the log K0 for Reaction 1 from a previous publication from this research group, and log K0 for Reaction 2 from this study, the equilibrium constant for dissolution of di-sodium hexaborate tetrahydrate at 25 °C and at infinite dilution, {equation presented} was derived to be -45.42 ± 0.16 (2σ). The equilibrium constants determined in this study can find applications in many fields. For example, in the field of nuclear waste management, the formation of di-sodium hexaborate tetrahydrate in brines containing magnesium will decrease borate concentrations, making less borate available for interactions with Am(III). In the field of experimental investigations, based on the equilibrium constant for Reaction 2, the experimental systems can be controlled in terms of acidity around neutral pH by using the equilibrium assemblage of borax and di-sodium hexaborate tetrahydrate at 25 °C. As salt lakes and natural brines contain both borate and magnesium as well as sodium, the formation of sodium hexaborate tetrahydrate may influence the chemical evolution of salt lakes and natural brines. Di-sodium hexaborate tetrahydrate is a polymorph of the mineral ameghinite [chemical formula Na2B6O10·4H2O; structural formula NaB3O3(OH)4 or Na2B6O6(OH)8]. Di-sodium hexaborate tetrahydrate could be a precursor of ameghinite and could be transformed when borate deposits are subject to diagenesis.
The stability constant of FeB(OH)4+ is expected to find applications in many areas of study. For instance, FeB(OH)4+ may have played an important role in transport of ferrous iron in reducing water bodies at the surface of the primitive Earth. In the nearfield of geological repositories, the formation of FeB(OH)4+ can sequestrate soluble borate, lowering borate concentrations available to the formation of the Am(III)-borate aqueous complex.
Gautier et al. (2014) recently published their determination of hydromagnesite solubility constant and hydromagnesite growth kinetics. Although their raw data appear to be of high quality, there is an oversight in their calculations of the hydromagnesite solubility constants given the solution compositions in their experiments. The oversight lies in the fact that they did not consider the constraint of simultaneous equilibrium with brucite. This oversight causes their newly calculated equilibrium constant for hydromagnesite to be discordant with the literature values (Königsberger et al., 1992; Xiong, 2011).
This report is a summary of the international collaboration and laboratory work funded by the US Department of Energy Office of Nuclear Energy Spent Fuel and Waste Science & Technology (SFWST) as part of the Sandia National Laboratories Salt R&D work package. This report satisfies milestone levelfour milestone M4SF-17SN010303014. Several stand-alone sections make up this summary report, each completed by the participants. The first two sections discuss international collaborations on geomechanical benchmarking exercises (WEIMOS) and bedded salt investigations (KOSINA), while the last three sections discuss laboratory work conducted on brucite solubility in brine, dissolution of borosilicate glass into brine, and partitioning of fission products into salt phases.
In this study, solubility measurements on tri-calcium di-citrate tetrahydrate [Ca3[C3H5O(COO)3]2•4H2O, abbreviated as Ca3[Citrate]2•4H2O] as a function of ionic strength are conducted in NaCl solutions up to I = 5.0 mol•kg–1 and in MgCl2 solutions up to I = 7.5 mol•kg–1, at room temperature (22.5 ± 0.5°C). The solubility constant (log K$0\atop{sp}$) for Ca3[Citrate]2•4H2O and formation constant (logβ$0\atop{1}$) for Ca[C3H5O(COO)3]–Ca3[C3H5O(COO)3]2•4H2O (earlandite) = 3Ca2+ + 2[C3H5O(COO)3]3– + 4H2O (1) Ca2+ + [C3H5O(COO)3]3– = Ca[C3H5O(COO)3]– (2) are determined as –18.11 ± 0.05 and 4.97 ± 0.05, respectively, based on the Pitzer model with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media.
For this study, the interactions of lead with citrate and ethylenediaminetetraacetate (EDTA) are investigated based on solubility measurements as a function of ionic strength at room temperature (22.5 ± 0.5°C) in NaCl and MgCl2 solutions. The formation constants (log β10 ) for Pb[C3H5O(COO)3]– (abbreviated as PbCitrate–) and Pb[(CH2COO)2N(CH2)2N(CH2COO)2)]2– (abbreviated as PbEDTA2–) Pb2+ + [C3H5O(COO)3]3– = Pb[C3H5O(COO)3]– (1) Pb2+ + (CH2COO)2N(CH2)2N(CH2COO)2)4- = Pb[(CH2COO)2N(CH2)2N(CH2COO)2)]2– (2) are evaluated as 7.28 ± 0.18 (2σ) and 20.00 ± 0.20 (2σ), respectively, with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media. Based on these parameters, the interactions of lead with citrate and EDTA in various low temperature environments can be accurately modelled.
In this study, solubility measurements on di-calcium ethylenediaminetetraacetic acid hydrate [Ca2C10H12N2O8·7H2O(s), abbreviated as Ca2EDTA·7H2O(s)] as a function of ionic strength are conducted in NaCl solutions up to I = 4.4 mol·kg− 1 and in MgCl2 solutions up to I = 7.5 mol·kg− 1, at room temperature (22.5 ± 0.5 °C). The solubility constant (logKsp0) for Ca2EDTA·7H2O(s) and formation constant (logβ10) for CaEDTA2 −, Ca2EDTA · 7H2O(s) = 2Ca2 + + EDTA4 − + 7H2O (1) Ca2 + + EDTA4 − = CaEDTA2 − (2)are determined as − 15.57 ± 0.10 and 11.50 ± 0.05, respectively, based on the Pitzer model with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media. The solubility measurements and thermodynamic modeling indicate that Ca2EDTA·7H2O(s) could become a solubility-controlling phase for EDTA in geological repositories for nuclear waste when the inventories of EDTA reach the saturation concentrations for Ca2EDTA·7H2O(s). The model developed in this work would also enable researchers to calculate the optimal EDTA concentrations to be used for remediation of soils contaminated with heavy metals, and to calculate the maximum EDTA concentrations that could be present in soils after an ETDA washing technology has been applied.
Borate is present in natural groundwaters and borate is also released into groundwaters when borosilicate glass, waste form for high level nuclear waste, is corroded. Borate can form an aqueous complex, AmHB4O7 2+, with actinides in +III oxidation state. In this work, we present our evaluation of the equilibrium constant for formation of AmHB4O7 2+ and the associated Pitzer interaction parameters at 25°C. Using Nd(III) as an analog to Am(III), solubility data of Nd(OH)3(s) in NaCl solutions in the presence of borate ion from the literature, is used to determine Am(III) interactions with borate. The log10K for the formation reaction is 37.34. This evaluation is in accordance with the Waste Isolation Pilot Plant (WIPP) thermodynamic model in which the borate species include B(OH)3(aq), B(OH)4 -, B3O3(OH)4 -, B4O5(OH)4 2-, and NaB(OH)4(aq). The WIPP thermodynamic database uses the Pitzer model to calculate activity coefficients of aqueous species. In addition, the equilibrium constant for dissolution of AmB9O13(OH)4(cr) at 25°C is evaluated from the solubility data on NdB9O13(OH)4(cr) in NaCl solutions, again using Nd(III) as an analog to Am(III). The log10K for the dissolution reaction is -79.30. In the evaluation for log10K for the dissolution reaction, AmHB4O7 2+ is also considered. The equilibrium constant and Pitzer parameters evaluated by this study will be important to describe the chemical behavior of Am(III) in the presence of borate in geological repositories.
ANS IHLRWM 2017 - 16th International High-Level Radioactive Waste Management Conference: Creating a Safe and Secure Energy Future for Generations to Come - Driving Toward Long-Term Storage and Disposal
The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy geological repository for the permanent disposal of defense-related transuranic (TRU) waste. Industrial-grade MgO consisting mainly of the mineral periclase is the only engineered barrier certified by U.S. EPA for emplacement in the WIPP in the U.S. An Mg(OH)2-based engineered barrier consisting mainly of the mineral brucite is to be employed in the Asse repository in Germany. The WIPP is located in a bedded salt formation, and the Asse repository is located in a domal salt formation. Colloids would facilitate transport of contaminants including actinides. The regulator for the WIPP, U.S. Environmental Protection Agency (EPA), expressed its interest that possible formation of mineral colloids by MgO and its hydration and carbonation products under the WIPP-relevant conditions be evaluated. In this presentation, we report a systematic experimental study to address U.S. EPA's interest. We evaluated the possible formation of mineral colloids by using two approaches. In the first approach, as the hydration products, Mg(OH)2 (brucite), and(Mg)3Cl(OH)5′4H2O (phase 5), and the carbonation product, (Mg)5(CO3)4(OH)2•4H2O (hydromagnesite), contain magnesium, should mineral fragment colloids exist, magnesium concentrations in solution samples from MgO hydration and carbonation experiments would show a dependence on ultrafiltration, i.e., a decrease in magnesium concentrations as a function of ultrafiltration with decreasing molecular weight (MW) cut-offs. Therefore, we investigated magnesium concentrations from solutions samples in hydration and carbonation experiments as a function of ultrafiltration. We ultrafiltered solutions with a series of MW cut-off filters at 100 κD, 50 κD, 30 κD and 10 κD. Our results demonstrate that the magnesium concentrations remain constant with decreasing MW cut-offs, implying the absence of mineral fragment colloids. In the second approach, because Cs+ is easily absorbed by colloids, we spiked MgO hydration and carbonation experiments under the WIPP-relevant conditions with Cs+. Then, we ultra-filtered solutions with a series of MW cut-off filters at 100 κD, 50 κD, 30 κD and 10 κD. The concentrations of Cs do not change as a function of MW cut-offs, indicating the absence of colloids from MgO hydration and carbonation products. Therefore, both approaches demonstrate that the absence of mineral fragment colloids from MgO hydration and carbonation products.
In this study, solubility constants of hydroxyl sodalite (ideal formula, Na8[Al6Si6O24][OH]2·3H2O) from 25 °C to 100 °C are obtained by applying a high temperature Al—Si Pitzer model to evaluate solubility data on hydroxyl sodalite in high ionic strength solutions at elevated temperatures. A validation test comparing model-independent experimental data to model predictions demonstrates that the solubility values produced by the model are in excellent agreement with the experimental data. The equilibrium constants obtained in this study have a wide range of applications, including synthesis of hydroxyl sodalite, de-silication in the Bayer process for extraction of alumina, and the performance of proposed sodalite waste forms in geological repositories in various lithologies including salt formations. The thermodynamic calculations based on the equilibrium constants obtained in this work indicate that the solubility products in terms of mΣAl×mΣSi for hydroxyl sodalite are very low (e.g., ∼10–13 [mol·kg–1]2 at 100 °C) in brines characteristic of salt formations, implying that sodalite waste forms would perform very well in repositories located in salt formations. The information regarding the solubility behavior of hydroxyl sodalite obtained in this study provides guidance to investigate the performance of other pure end-members of sodalite such as chloride- and iodide-sodalite, which may be of interest for geological repositories in various media.
Abstract In this study, solubility measurements of lead carbonate, PbCO3(cr), cerussite, as a function of total ionic strengths are conducted in the mixtures of NaCl and NaHCO3 up to I = 1.2 mol kg-1 and in the mixtures of NaHCO3 and Na2CO3 up to I = 5.2 mol kg-1, at room temperature (22.5 ± 0.5 °C). The solubility constant (log Kos) for cerussite was determined as -13.76 ± 0.15 (2σ) with a set of Pitzer parameters describing the specific interactions of PbCO3(aq), Pb(CO3)2-2, and Pb(CO3)Cl- with the bulk-supporting electrolytes, based on the Pitzer model. The model developed in this work can reproduce the experimental results including model-independent solubility values from the literature over a wide range of ionic strengths with satisfactory accuracy. The model is expected to find applications in numerous fields, including the accurate description of chemical behavior of lead in geological repositories, the modeling of formation of oxidized Pb-Zn ore deposits, and the environmental remediation of lead contamination.
The dissociation constants of oxalic acid (Ox), and the stability constants of Am3+, Cm3+ and Eu3+ with Ox2– have been determined at 25 °C, over a range of concentration varying from 0.1 to 6.60 m NaClO4 using potentiometric titration and extraction techniques, respectively. The experimental data support the formation of complexes, M(Ox)n3 – 2n, where (M = Am3+, Cm3+ and Eu3+ and n = 1 and 2). The dissociation constant and the stability constant values measured as a function of NaClO4 concentration were used to estimate the Pitzer parameters for the respective interactions of Am3+, Cm3+ and Eu3+ with Ox. Furthermore, the stability constants data of Am3+ –Ox measured in NaClO4 and in NaCl solutions from the literature were simultaneously fitted in order to refine the existing actinide–oxalate complexation model that can be used universally in the safety assessment of radioactive waste disposal. The thermodynamic stability constant: log β0101 = 6.30 ± 0.06 and log β0102 = 10.84 ± 0.06 for Am3+ was obtained by simultaneously fitting data in NaCl and NaClO4 media. Additionally, log β0101 = 6.72 ± 0.08 and log β0102 = 11.05 ± 0.09 for the Cm3+ and log β0101 = 6.67 ± 0.08 and log β0102 = 11.15 ± 0.09 for the Eu3+ were calculated by extrapolation of data to zero ionic strength in NaClO4 medium only. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and CΦ determined in this work. The thermodynamic model developed in this work will be useful in accurately modeling the potential solubility of trivalent actinides and early lanthanides to ionic strength of 6.60 m in low temperature environments in the presence of Ox. Furthermore, the work is also applicable to the accurate modeling transport of rare earth elements in various environments under the surface conditions.