Applied Geochemistry (v.55, #C)

Preface: SI: Geochemical speciation codes and databases by Dmitrii A. Kulik; Wolfgang Hummel; Johannes Lützenkirchen; Grégory Lefèvre (1-2).

Display OmittedThe current status of the software package JESS (Joint Expert Speciation System), which has been developed over the last 30 years, is described. Chemical speciation models of seawater and of metal-ion complexation in human blood plasma are used as large equilibrium systems to explore the present capabilities of the code and database. Strengths of JESS are considered to be (a) the power and flexibility of its command-driven programs, (b) the size, generality and openness of its reaction database, (c) its automatic facility to achieve thermodynamic consistency, and (d) its ability to partition chemical reactions to model kinetic constraints. A special feature of JESS is its ability automatically to generate a complete, stand-alone FORTRAN program for any particular chemical equilibrium model that has been developed. Weaknesses of JESS include the lack of a graphical user interface, the resulting effort required in familiarisation, and certain limitations regarding pressure and temperature corrections in concentrated solutions. However, the most troublesome issues – common to all ‘ion-association’ frameworks – are due to inadequacies in the thermodynamic data available from the literature and to persistent deficiencies in the fundamental theory of concentrated electrolyte solutions. Accordingly, work on JESS has commenced to construct a global parameterisation facility using both reaction data and the physicochemical properties of strong electrolytes in aqueous solution (including solubilities) intended to improve model function testing and to provide even better mechanisms for data harmonisation.

This paper describes four programs for Windows, designed to obtain the thermodynamic properties of aqueous species from experimental data and reporting them in the Unitherm database format (HCh software package). Programs OptimA and OptimS allow users to derive the standard Gibbs free energies of aqueous species from the results of chemical experiments (e.g., potentiometry or solubility) and from ultraviolet–visible (UV–Vis) absorption spectra, respectively; programs OptimB and OptimC enable optimization of the parameters of the revised Helgeson–Kirkham–Flowers equation of state and the modified Ryzhenko–Bryzgalin electrostatic model, respectively, for an aqueous species from its standard Gibbs free energy or stability constant as a function of temperature and pressure.

GEMSFITS: Code package for optimization of geochemical model parameters and inverse modeling by George D. Miron; Dmitrii A. Kulik; Svitlana V. Dmytrieva; Thomas Wagner (28-45).

A chemical kinetics algorithm for geochemical modelling by Allan M.M. Leal; Martin J. Blunt; Tara C. LaForce (46-61).
A chemical kinetics algorithm is presented for geochemical applications. The algorithm is capable of handling both equilibrium- and kinetically-controlled reactions in multiphase systems. The ordinary differential equations (ODEs) are solved using an implicit multistep backward differentiation formula (BDF) algorithm to ensure efficiency and stability when integrating stiff ODEs. An adaptive control scheme of the time step is adopted to guarantee small steps in steeper regions and large steps in smoother regions of the integration. Analytical derivatives of the reaction rates and species activities are used to permit the use of larger time steps, and to increase the robustness of the calculations. The chemical equilibrium calculations are performed using a Gibbs free energy minimisation algorithm, which is based on a trust-region interior-point method adapted with a watchdog strategy that yields quadratic rates of convergence near the solution. The chemical kinetics algorithm is applied to geochemical problems relevant to carbon storage in saline aquifers. The calculations assume aqueous, gaseous and mineral phases, where the kinetics of the water–gas–rock interactions are investigated. The results allow us to estimate the time frames at which brine of different salinities and supercritical CO2 attains equilibrium with a carbonate rock, as well as the amount of carbon dioxide trapped by solubility and mineralisation mechanisms.

Databases distributed with PHREEQC may give widely different results for concentrated solutions. Only the database that uses Pitzer’s interaction coefficients provides both correct solubilities and mean activity coefficients. The applicability of this database for predicting scaling by mineral precipitation is extended by fitting interaction coefficients for the Na–K–Mg–Ca–Ba–Cl–CO2–HCO3–SO4–H4SiO4 system from isopiestic and solubility data at high temperatures. The pressure dependence of equilibrium constants is calculated from the reaction volume, in which the apparent molar volume of the solutes is derived from density measurements. The apparent volumes are a function of temperature, pressure, and ionic strength, and incorporate complicated changes of the partial molar volumes of the water molecules.Fugacity coefficients for CO2 can be obtained reliably with the Peng–Robinson equation of state for gases. The CO2 ion interaction parameters given by Harvie et al. (1984) for 25 °C are also valid for calculating the CO2 solubility at high temperatures, pressures and salinities.PHREEQC input files are available for download, comparing experimental and calculated solubilities of (Na, K, Mg, Ca, Ba) minerals of chlorides, sulfates and carbonates, and of amorphous silica and CO2 in concentrated solutions.

Research on the solubility of hazardous substances in saturated salt solutions is an ongoing task in Germany. Several institutions deliver contributions in line with their respective expertise. Scientific studies ultimately yield thermodynamic data which are used for thermodynamic equilibrium modelling. In order to join forces and render thermodynamic equilibrium calculations comparable it was decided to setup a common thermodynamic reference database (THEREDA) from which ready-to-use parameter files for commonly used geochemical codes should be created.It is the objective of this paper to explain how THEREDA is designed from a data management point-of-view, both conceptionally and technically. Data tables and mutual dependencies are described that allow managing administration of data for aqueous solution, solids, solid solutions, and surfaces. Moreover, quality assurance, traceability, consistency, and efficient, long-term maintenance are major topics shaping the database structure. Finally, robust and flexible human interfaces (to editors as well as end-users) are implemented. This paper is not aimed at giving an account of the model definitions, system selections, evaluation schemes, and thermodynamic data themselves stored in THEREDA, which represent the actual scientific work done by many more scientists within the project. However, this methodological guide to THEREDA has its own merits as it helps to bring thermodynamic data to work. Its specific implementation may serve as a useful example for similar projects going far beyond waste disposal.

ThermoChimie database developments in the framework of cement/clay interactions by P. Blanc; P. Vieillard; H. Gailhanou; S. Gaboreau; N. Marty; F. Claret; B. Madé; E. Giffaut (95-107).
We present here the global results of an eight-year project, where we attempted to complete the ThermoChimie database for cement minerals, zeolites and clay minerals. This work reports the methodology adopted to ensure the consistency between the three groups of minerals. Our first step concerns the relations between the three groups of minerals from field observations and experimental results gathered from the literature. A summary of the selection process is then given for clay and cement minerals and a specific focus is proposed for zeolites, with examples of analcime and mordenite. The global consistency of the selection is investigated with respect to the main chemical tendencies found for cement/clay interactions, especially considering the “alkalinity” of the minerals. Consistency is checked by establishing predominance diagrams in the chemical sub-systems of interest for the three groups of minerals, then comparing the phase relations with respect to experimental results or field observations. The methodology is illustrated using the case of gismondine and zeolite P(Ca).

A database of dissolution and precipitation rates for clay-rocks minerals by Nicolas C.M. Marty; Francis Claret; Arnault Lassin; Joachim Tremosa; Philippe Blanc; Benoit Madé; Eric Giffaut; Benoit Cochepin; Christophe Tournassat (108-118).
Many geoscientific fields use reactive transport codes to set up and interpret experiments as well as to understand natural processes. Reactive transport codes are also useful to give insights in the long term evolution of systems such as radioactive waste repositories or CO2 storage sites, for which experiments cannot reach the targeted time scale nor the dimension of those systems. The consideration of kinetic reaction rates is often required to reproduce correctly the geochemical and transport processes of interest. However, kinetic data are scattered in the literature, making data and selection a tedious task. Kinetic parameters on a single system are also highly variable depending on data choice, interpretation and chosen kinetic modelling approaches, thus making inter-comparison of modelling studies difficult. The present work aims at proposing a compilation of kinetic parameters to overcome part of above cited problems. The proposed selection was done (i) to ensure consistency of data selection criteria and data treatment and (ii) to ease the use of common kinetic parameters that are independent of the chosen geochemical modelling code. For those two reasons, the kinetic formalism of the transition state theory (TST) was chosen. The selection of minerals is currently limited to those present in clay rich rocks and cements, reflecting the effort made at predicting the evolution of radioactive waste underground storage systems. Still, the proposed compilation should also be useful for other applications such as CO2 sequestration.

Aqueous species equilibrium constants and activity models form the foundation of the complex speciation codes used to model the geochemistry of geothermal energy production, extremophilic ecosystems, ore deposition, and a variety of other processes. Researchers have shown that a simple three species model (i.e., Na+, Cl, and NaCl(aq)) can accurately describe conductivity measurements of concentrated NaCl and KCl solutions at elevated temperatures and pressures (Sharygin et al., 2002). In this model, activity coefficients of the charged species (e.g., Na+, K+, Cl) become sufficiently low that the complexes must redisocciate with increasing salt concentration in order to meet equilibrium constant constraints. Redissociation decreases the proportion of the elements bound up as neutral complexes, and thereby increases the true ionic strength of the solution. In this contribution, we explore the consequences of the redissociation phenomenon in albite–paragonite–quartz (APQ) buffered systems. We focus on the implications of the redissociation phenomenon for mineral solubilities, particularly the observation that, at certain temperatures and pressures, calculated activities of charged ions in solution remain practically constant even as element concentrations increase from <1 molal to 4.5 molal. Finally, we note that redissociation has a similar effect on pH, and therefore aqueous speciation, in APQ-hosted systems. The calculations and discussion presented here are not limited to APQ-hosted systems, but additionally apply to many others in which the dominant cations and anions can form neutral complexes.

Treatment of multi-dentate surface complexes and diffuse layer implementation in various speciation codes by Johannes Lützenkirchen; Remy Marsac; Dmitrii A. Kulik; Timothy E. Payne; Zhengrong Xue; Silvia Orsetti; Stefan B. Haderlein (128-137).
Display OmittedSpectroscopic studies and atomistic simulations of (hydr)oxide surfaces show that ionic aqueous adsorbates can bind to one, two, three, or four surface oxygen atoms (sites), forming multi-dentate species in surface complexation reactions. The law of mass action (LMA) for such reactions can be expressed in several alternative scales of surface concentration (activity). Unlike for mono-dentate surface complexes, the numerical value of the equilibrium constant is not independent of the choice of the surface concentration scale. Here, we show in a number of examples that the different formalisms implemented in popular speciation codes (MINEQL, MINTEQ, PHREEQC, and ECOSAT) yield different results for the same systems when the same parameters are used. We conclude that it is very important to generate general equations to easily transfer stability constants between the different concentration scales. It is of utmost importance for application of these models to reactive transport that the implementation in both the model fitting and speciation codes, and in the transport codes, is transparent to users.We also point to the problem that the implementation of the diffuse layer formalism in the various codes is not necessarily generally applicable. Thus, codes like VisualMinteq or MINEQL involve the Gouy–Chapman equation, which is limited to symmetrical (z:z) electrolytes, while PHREEQC and ECOSAT use general equations. Application of the former two to environmental problems with mixed electrolytes will therefore involve an inconsistency.

Physicochemical modeling of formation of Ag–Au–Hg solid solutions: Kyuchyus deposit (Yakutia, Russia) as an example by Konstantin V. Chudnenko; Galina A. Palyanova; Galina S. Anisimova; Stepan G. Moskvitin (138-151).
Display OmittedThe «Selektor-C» software package and standard thermodynamic functions of ternary Ag–Au–Hg solid solutions were used for developing physicochemical models in natural processes with participation of gold, silver and mercury. On the example of the Kyuchyus Au–Sb–Hg deposit we have worked out hypogene and hypergene models of formation of native gold, including mercuric gold. We obtained thermodynamic evidence that the Kyuchyus deposit ores formed with the origin of electrum at the early main productive quartz–sulfide stage and ternary Au–Ag–Hg solid solutions at the late non-substantive Au-bearing stages.

Uranium is a common contaminant of concern in the aquifers of nuclear waste management facilities around the world. The Electrochemical Plant (ECP) (Krasnoyarsk Krai, Russia) and the Angarsk Electrolysis Chemical Complex (AEСC) (Irkutsk Region, Russia) have produced enriched uranium and related nuclear wastes (sludge materials) since the 1960s. The results of the detailed sampling of groundwater and waste solutions near the storage sites of radioactive waste are presented elsewhere (Gaskova et al., 2011, Boguslavskiy et al., 2012). A number of experimental studies were conducted in the laboratory (a) to investigate the uranium-containing sludge leaching by regional groundwater and (b) to understand the ability of host sediments to retain major and minor components from drainage solutions as a function of the water/rock ratio (indicating the timescale over which the behavior is observed). The geochemical modeling code “HCh” was applied to simulate these processes and to predict the long-term environmental impact of residual uranium. The experimental and thermodynamic results indicate that U leaching from the sludge materials is less dependent on the specific solubility of the sludge sediments and more dependent on the solid uranium species in this sludge. The model simulation of uranium sorption in dynamic experiments was successful only in the case of the permanent filtration of less mineralized groundwater in fine-grained clay rocks. To more accurately describe the potential future release of U from the residual waste sites, a series of batch reactor experiments needs to be conducted.

We studied the effects of evaporation and groundwater flow on the formation of salt minerals in the Sabkha of Oum El Khialate in South East Tunisia, which contains large amounts of sulfate sodium mineral deposits. Due to the fact that there are no important surface water bodies present in this sabkha, transport of solutes is dominated by advection rather than mixing in lakes. For our study we used both analytical conservative and numerical reactive transport models. Results showed that salinity varies with distance and may reach very high levels near a watershed where the groundwater flux is zero. As a consequence, reactive transport simulations results showed that more minerals precipitate and water activity decreases values near this watershed. Model results also showed that a sequence of precipitating minerals could be deduced after 140,000 years. From the boundary of the sabkha towards the watershed the mineral sequence was dolomite, gypsum, magnesite, bloedite, halite and mirabilite. It was found that the amounts as well as the mineral precipitation distribution strongly depend on salinity and rates of inflowing water.

Aqueous electrolyte solution modelling: Some limitations of the Pitzer equations by Darren Rowland; Erich Königsberger; Glenn Hefter; Peter M. May (170-183).
Despite intense efforts, general thermodynamic modelling of aqueous electrolyte solutions still presents a difficult challenge, with no obvious method of choice. Even though the Pitzer equations seemingly provide a well-established theoretical framework applicable to many chemical systems over a wide range of temperatures and pressures, they are not as widely adopted as their early promise might have suggested. This is strikingly illustrated by the simultaneous appearance in the literature of numerous, different (and potentially incompatible) Pitzer models alongside a proliferation of alternative theoretical approaches with inferior capabilities.To better understand this problem, the ability of the Pitzer equations to represent the physicochemical properties of aqueous solutions has been systematically investigated for exemplar electrolyte systems. Pitzer ion-interaction parameters have been calculated for selected systems by least-squares regression analysis of published solution data for activity coefficients, osmotic coefficients, relative enthalpies, heat capacities, volumes and densities to high temperatures and pressures. Although satisfactory fits can be achieved when the ranges of conditions are carefully chosen and when sufficient data are available to constrain the regression, the fits obtained tend otherwise to be unsatisfactory. The Pitzer equations do not cope well with gaps and other deficiencies in the regressed data. Profound difficulties, poorly recognized hitherto, can also arise because of variation in the sensitivity of the Pitzer functions to values for different physicochemical properties when these are combined. Given the dimensionality of numerous related thermodynamic properties, all changing as functions of composition, temperature and pressure, these problems are difficult to detect, let alone address, especially in multicomponent systems. The growing practice of improving fits simply by adding basis functions (thereby increasing the number of adjustable parameters) should be deprecated because it increases the likelihood of error propagation, introduces subjectivity, makes independent verification difficult and has deleterious implications for both automated data processing and for consistency between thermodynamic models.

Use of redundant data to reduce estimation errors in geochemical speciation by F. De Gaspari; M.W. Saaltink; J. Carrera; L.J. Slooten (184-191).
Speciation is the process of evaluating the concentrations of all the species in a chemical system from equilibrium conditions and measured data such as total concentrations of components, electrical conductivity, pH, redox potential or gas partial pressure. It is essential for analyzing geochemical data and defining the chemical composition of waters for geochemical modeling problems like evaluating the chemical composition of evaporating, diluting, mixing waters or reactive transport. We present an algorithm that reduces estimation errors in chemical speciation calculations by means of the use of redundant data. Redundant data are measurements and assumptions that exceed the minimum required, and therefore are not strictly necessary, to speciate a water sample. The proposed method was compared with the classical speciation algorithm on two synthetic examples. Our results show that using redundant data improves speciation results reducing the estimation error between computations and measurements. In fact, the larger the amount of redundant data, the better the speciation in terms of errors of the estimated concentrations.

A modelling exercise on the importance of ternary alkaline earth carbonate species of uranium(VI) in the inorganic speciation of natural waters by Thomas Vercouter; Pascal E. Reiller; Eric Ansoborlo; Laureline Février; Rodolphe Gilbin; Claire Lomenech; Violaine Philippini (192-198).
Predictive modelling of uranium speciation in natural waters can be achieved using equilibrium thermodynamic data and adequate speciation software. The reliability of such calculations is highly dependent on the equilibrium reactions that are considered as entry data, and the values chosen for the equilibrium constants. The working group “Speciation” of the CETAMA (Analytical methods establishment committee of the French Atomic Energy commission, CEA) has organized a modelling exercise, including four participants, in order to compare modellers’ selections of data and test thermodynamic data bases regarding the calculation of U(VI) inorganic speciation. Six different compositions of model waters were chosen so that to check the importance of ternary alkaline earth carbonate species of U(VI) on the aqueous speciation, and the possible uranium solid phases as solubility-limiting phases. The comparison of the results from the participants suggests (i) that it would be highly valuable for end-users to review thermodynamic constants of ternary carbonate species of U(VI) in a consistent way and implement them in available speciation data bases, and (ii) stresses the necessary care when using data bases to avoid biases and possible erroneous calculations.