Two cation exchange models for direct and inverse modelling of solution major cation composition in equilibrium with illite surfaces
Introduction
The objective of the present study was to provide an up-to-date cation exchange constant database for Na, K, Ca and Mg on illite surfaces. Cation exchange databases are easily found in chemical reaction databases provided with speciation calculation codes, such as PHREEQC2 (Parkhurst and Appelo, 1999). However, (i) these databases contain no or few references to published results and (ii) the tabulated selectivity coefficients should be considered for montmorillonite only.
Knowledge of reliable cation exchange selectivity coefficients for major cations on illite is of importance in the context of nuclear waste deep disposal studies in order to simulate the porewater solute composition. The exchanger composition provides information about the relative amount of Na+, K+, Ca2+ and Mg2+ in solution:where X− represents one mole of the exchanger. The exchanger composition, an easily measurable parameter in compacted clayey rocks, is then an “image” of the porewater major cation composition, which cannot be directly characterized by water extraction and concentration measurements due to the low water content of these rocks (Bradbury and Baeyens, 1998, Pearson et al., 2003, Gaucher et al., 2006). The following calculation shows how to obtain the Na+ concentration assuming that (i) the exchanger composition is known, (ii) the sum of the equivalent concentrations of Na+, K+, Ca2+ and Mg2+ is equal to the sum of the equivalent concentrations of Cl− and (i.e. neglecting H+ and other minor cation contributions) and (iii) the exchange selectivity coefficients are known.
In the following, the Gaines and Thomas convention is used to express the equilibrium equation with the exchanger phase (e.g. Sposito, 1981). According to this convention, the activity of an exchanged species is equal to the equivalent fraction occupied by the cation on the exchanger phase, times an activity coefficient. This activity coefficient is here set equal to 1 for the analysis that follows (Eqs. (1), (2), (3), (4), (5), (6), (7), (8)).
Exchange reaction equilibria then lead to the following equations:where values in brackets are representative of species activities and Ei is representative of the equivalent fraction on the exchanger of surface species i. The concentrations of each solution species as a function of the Na+ concentration are then:where γi is the activity coefficient of the species i in solution. These coefficients can be obtained using Debye-Hückel, Davies or Pitzer equations.
By combining Eqs. (4), (5), (6) with the electroneutrality relationship:where values in square brackets are concentrations, one obtains:Bicarbonate concentrations are neglected in the electroneutrality relationship, because they were found to represent a minor fraction (∼2%) of the anionic charge in the considered clayey systems (e.g. Gaucher et al., 2006, for a Callovian–Oxfordian formation with total normality about 0.05–0.1 mol/L, or Pearson et al., 2003 for an Opalinus Clay formation). Second order equations similar to Eq. (8) can be derived for K, Ca and Mg. One obtains five second order equations with 10 unknown parameters (the five concentrations and the associated activity coefficients). Debye-Hückel, Davies or Pitzer (for concentrated solutions) equations give five other independent equations for the activity coefficients. Hence, the mathematical system is fully constrained (10 unknowns with 10 independent equations) and the Na, K, Ca and Mg concentrations can be calculated, based on the exchanger composition and the Cl− and concentrations (and possibly other anions that contribute significantly to the total anion equivalent concentration). However, a good precision on the selectivity coefficient value is needed to obtain reliable concentration values. The present work aims at obtaining these selectivity coefficients for Na, K, Ca and Mg on illite.
Data collected from the literature are widely used in the following work together with new exchange data obtained in our laboratories. Data from the literature always originated from exchange experiments where both the solution and the exchanger composition were measured. New exchange data were obtained in the framework of the French Nuclear Waste Management Agency (ANDRA) investigation program on the clayey formation surrounding the French Underground Laboratory (URL) in Bure (Meuse Haute-Marne, France).
Section snippets
Chemicals
All solutions and suspensions were prepared with Millipore Milli-Q 18 MΩ water. NaCl, KCl, MgCl2 and CaCl2 solutions were prepared from analytical grade salts.
Clay material preparation
IMt-2 clay sample material was obtained from the Source Clays Repository (http://www.clays.org/sourceclays/SourceClaysCCM.html). After saturating the suspension with NaCl (0.5 mol/L), it was successively treated with a H2O2/HNO3 mixture (pH ∼2) to remove carbonate and organic matter impurities, then with a mixture of sodium
Experimental results
The complete set of data obtained in this study and from the literature is given in the table of Electronic Annex 1. It can be seen from this table that the Kv values calculated for divalent–monovalent cation exchange on the basis of the data reported by Thellier and Sposito (Thellier and Sposito, 1988, Thellier and Sposito, 1989b) are different from the values reported by these authors by a factor 2, probably due to a misuse of equivalent/kg instead of mol/kg for divalent cations in Thellier’s
Multi-site modelling approach
A multi-site modelling approach for cation exchange processes on illite has been shown to be successful in predicting Cs+ sorption under various conditions of pH and ionic strength (e.g. Brouwer et al., 1983, Poinssot et al., 1999, Zachara et al., 2002, Liu et al., 2004) or to explain the variation in CEC measurements as a function of ionic strength with the 22Na isotope dilution technique (Baeyens and Bradbury, 2004).
A similar approach was therefore adopted here in order to fit both the
One-site modelling approach
The uncertainty on the concentration prediction capability of the two-site model presented above leads us to model the data in a different way: the changes in selectivity coefficient are interpreted here as a continuous change in surface species activity coefficient. The use of a one-site model for cation-exchange on illite represents a departure from the approach used in all recent studies. Although questionable from a theoretical point of view, it will be shown in the following that this
Application to natural and engineered systems
Natural systems with a clay fraction constituted only by illite are not common. Here, we consider the example of illite rich layers in the Callovian–Oxfordian formation for the calculation of exchanger selectivity coefficients towards Na, Ca, Mg and K. Data on exchanger composition and ionic strength were obtained from Gaucher et al. (Gaucher et al., 2004) and are given in Table 4. Selectivity coefficients in the Vanselow convention were calculated using the model described in this paper (Table
Conclusions
We have shown that our one-site model considering the ionic strength effect and variations in the surface species activity coefficients is efficient for the determination of water major cation composition for a wide range of exchanger compositions. For natural systems, this model still needs to be refined in order to consider the presence of smectite and illite/smectite rather than pure illite. Moreover, the compaction effect deserves further study as a similar effect of compaction is expected
Acknowledgments
This research was funded by the French National Radioactive Waste Management Agency (ANDRA) and the French Geological Survey (BRGM) in the framework of the BRGM-ANDRA scientific partnership (THERMOAR project under the coordination of Dr. E. Jacquot and Dr. E. Gaucher). H.G. acknowledges ANDRA for her Ph.D. financial support, under the supervision of Dr. N. Michau. Dr. C.A.J. Appelo, two anonymous reviewers and Pr. Sposito, Associate Editor, are gratefully acknowledged for their constructive
References (37)
- et al.
Potassium–calcium and potassium–magnesium exchange equilibria in an acid savanna soil from northern Nigeria
Geoderma
(2006) - et al.
A Physicochemical characterisation and geochemical modelling approach for determining porewater chemistries in argillaceous rocks
Geochim. Cosmochim. Acta
(1998) - et al.
ANDRA underground research laboratory: interpretation of the mineralogical and geochemical data acquired in the Callovian–Oxfordian Formation by investigative drilling
Phys. Chem. Earth
(2004) - et al.
Modelling the porewater chemistry of the Callovian-Oxfordian formation at a regional scale
C.R. Geosci.
(2006) - et al.
A cation exchange model to describe Cs+ sorption at high ionic strength in subsurface sediments at Hanford site, USA
J. Contam. Hydrol.
(2004) - et al.
Experimental and modelling studies of caesium sorption on illite
Geochim. Cosmochim. Acta
(1999) - et al.
The titration of clay minerals. Part II. Structural-based model and implications for clay reactivity
J. Colloid Inter. Sci.
(2004) - et al.
The titration of clay minerals. Part I. Discontinuous backtitration technique combined to CEC measurements
J. Colloid Inter. Sci.
(2004) - et al.
Sorption of Cs+ to micaceous subsurface sediments from the Hanford site, USA
Geochim. Cosmochim. Acta
(2002) - et al.
Cation exchange capacity measurements on illite using the sodium and cesium isotope dilution technique: effects of the index cation, electrolyte concentration and competition: modeling
Clays Clay Miner.
(2004)
Geochemical reaction modeling
Cesium and Rubidium ion equilibria in illite clay
J. Phys. Chem.
Fe(II)–Na(I)–Ca(II) cation exchange on montmorillonite in chloride medium; evidence for preferential clay adsorption of chloride–metal ion pairs in seawater
Aquat. Geochem.
The thermodynamics of ternary cation exchange systems and the subregular model
Soil Sci. Soc. Am. J.
Determination of cation exchange capacity and exchangeable cations in soils by means of cobalt hexamine trichloride. Effects of experimental conditions
Agronomie
Sorption of cesium on illite: non-equilibrium behaviour and reversibility
Geochim. Cosmochim. Acta
Evidence for calcium-chloride ion pairs in the interlayer of montmorillonite and implications on hydration state. A XRD profile modelling approach
Clays Clay Miner.
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2020, Applied Clay ScienceCitation Excerpt :A fine (<2 μm) montmorillonite fraction was extracted from Bentonite MX-80 by 5 g/l suspension centrifugation at 1000 rpm and it was further exchanged with Na by following the standard procedure: contact with 0.5 mol/l NaCl solution, 6 cycles of centrifugation at 20000 rpm and subsequent washing/re-dispersion in deionized water and suspension drying at 323 K under air. The clay fraction from Illite du Puy was purified by an elutriation procedure and further transformed to a Na-exchanged form in the same way as a montmorillonite sample (Tournassat et al., 2007). Kaolinite was received from Fluka Analytical (Sigma Aldrich) in form of fine powder (<150 μm) and analysed without any purification.