Bart Baeyens

A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part I: Titration and sorption measurements

Abstract In this paper experimental investigations into the acid/base titration characteristics of Na-montmorillonite and the sorption behaviour of Ni and Zn under a wide variety of conditions are presented. From these measurements the dominant sorption mechanisms could be deduced. In the following paper (Bradbury and Baeyens, 1997) the titration and sorption data are analysed to determine the parameters in cation exchange and surface complexation based models which together provide a quantitative description of the titration and sorption data. A conditioning procedure was applied to the SWy-1 Na-montmorillonite starting material in order to remove background metal impurities, soluble salts and sparingly soluble minerals which could influence titration and sorption measurements. The purified clay, in the homo-ionic Na form, was thoroughly physico-chemically characterised before carrying out batch titration measurements on suspensions in 0.1 and 0.5 M NaClO 4 . The influence of background impurities, not removed by the conditioning, and cation exchange processes on the form of the titration curves is discussed. Titration data can be analysed to yield site capacities and protonation/deprotonation constants for the amphoteric surface hydroxyl groups (≡SOH). The acid endpoint in the titration data was used to estimate an ≡SOH site capacity of 0.08 mol kg −1 . The sorption of Ni and Zn on conditioned Na-montmorillonite was studied at trace concentrations as a function of pH over a range from ∼ 3 to ∼ 10 to produce so-called “sorption edges”. In the case of Ni, such measurements were carried out as a function of the NaClO 4 background electrolyte concentration. In addition, sorption isotherms were determined for both nuclides at several fixed pH values in 0.1 M NaClO 4 . From the form of the “edges” it was deduced that two main sorption mechanisms were controlling the uptake of Ni and Zn onto the clay mineral; a pH-independent component, identified as cation exchange on the permanent charge sites, and a pH-dependent one, interpreted as surface complexation on the amphoteric surface hydroxyl groups. The non-linearity of the sorption isotherms indicated that at least two different ≡SOH type sites were contributing to the overall sorption on Na-montmorillonite.

A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part II: modelling

Abstract Titration and sorption edge/isotherm measurements, carried out under a wide variety of conditions on Na-montmorillonite, were reported in Part I (Baeyens and Bradbury, 1997). These data are modelled here in terms of cation exchange and surface complexation mechanisms using a computer code called MINSORB. This code allowed the uptake of radionuclides by both mechanisms to be calculated simultaneously; also taking into account competitive reactions from other cations present. A stepwise iterative fitting/modelling procedure is described. For the case of Na-montmorillonite it is demonstrated that an electrostatic term in the surface complexation model is not required. From the modelling of the titration results, values for site capacities and protonation/deprotonation constants were deduced. These values were then fixed and used in all further surface complexation modelling of the sorption measurements. The main study was carried out with Ni, but impurity cations present in the system, particularly Zn, had to be examined in addition due to their competitive effects on Ni sorption. The surface complexation behaviour of Ni and Zn was investigated in detail to give intrinsic surface complexation constants on two of the ≡SOH type sites included in the model. The sorption of Mn is also considered, though in less detail, and estimated surface complexation constants for this element are presented. Cation exchange was included in all of the calculations. Measured selectivity coefficients for NiNa and ZnNa exchange reactions are given. The model, with the derived parameters, allowed all the experimental data from titration measurements through sorption edges to sorption isotherms to be quantitatively described.

Experimental and modelling studies of caesium sorption on illite

Abstract A natural illite (illite du Puy) was purified and converted to the homo-ionic Na form. The conditioned Na–illite was characterised in terms of its mineralogy, chemical inventory, and surface properties. The structural formula was determined from EDS analyses (SEM/TEM) and bulk chemistry. A cation exchange capacity of 127 mEq/kg was determined by the Na isotope dilution method at neutral pH. The sorption of Cs was measured as a function of NaClO 4 background electrolyte concentration (1.0, 0.1 and 0.01 M), Cs concentration and pH in the range ≈3 to ≈10. Before obtaining these measurements the kinetics of Cs uptake were determined at initial concentrations of 2 × 10 −8 M and 7 × 10 −5 M, representing the extremes of the range investigated, and was found to be concentration dependent. The supernatant solutions after centrifugation were analysed for major cations in all of the sorption tests. A two-site cation exchange model was developed to describe the sorption of Cs over the whole range of experimental conditions. The two-site types were termed frayed edge sites, FES (high affinity/low capacity) and type II sites (low affinity/high capacity). At low NaClO 4 concentrations, Cs sorption decreased at pH values less than neutral. This was interpreted in terms of competitive effects from H, and K released by the partial dissolution of illite, which cannot be avoided at low and high pH values. Selectivity coefficient values for Cs–Na, Cs–K, K–Na, and H–Na exchange equilibria on the FES sites, and Cs–Na exchange on the type II sites are given for illite together with the corresponding site capacities.

A generalised sorption model for the concentration dependent uptake of caesium by argillaceous rocks

A three-site cation exchange model is proposed to describe the concentration dependent uptake of Cs on natural argillaceous rock systems. Major premises in the model are that the sorption of Cs is dominated by the illite mineral component in the rock and that there is a fixed relationship between the site capacities of the three site types denoted as frayed edge, type II and planar sites. The definition of a “reference illite” with a cation exchange capacity of 0.2 equiv. kg-1 allows the three site capacities to be fixed in the model calculations over the weight fraction of illite in the argillaceous rocks. Up to Cs equilibrium concentrations of ∼10-3 M sorption occurs predominantly on the frayed edge and type II sites (higher affinity sites), with the planar site type playing only a minor role. Competition with Cs for sorption on the former two site types arises predominantly from monovalent cations such as K, Rb and NH4 which have low hydration energies. H and Na (except at high concentrations) are considerably less competitive and bivalent cations such as Mg, Ca and Sr are effectively non-competitive. A consistent set of selectivity coefficients for Cs with respect to K, Rb, NH4 and Na was derived from analyses and modelling of a wide range of Cs sorption data available in the open literature on pure illites from many different sources. The model was tested against four Cs sorption isotherm data sets determined on argillaceous rocks: Boom clay, Oxford clay, Palfris marl and Opalinus clay. The water chemistries and illite contents given in these experiments allowed the Cs sorption isotherms to be predicted. It is concluded that the Cs sorption model presented here, in which there are no free parameters, can be used to predict the uptake of Cs at equilibrium concentrations below ∼10-3 M to within a factor of 2 to 3 in natural argillaceous rock systems.

Structural evidence for the sorption of Ni(II) atoms on the edges of montmorillonite clay minerals: A polarized X-ray absorption fine structure study

The nature of surface complexes formed on Ni uptake onto montmorillonite (a dioctahedral smectite) has been investigated over an extended time period by polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy. Self-supporting films of Ni-sorbed montmorillonite were prepared by contacting Ni and montmorillonite at pH 7.2, high ionic strength (0.3 M NaClO4), and low Ni concentration ([Ni]initial = 19.9 μM) for 14- and 360-d reaction time. The resulting Ni concentration on the clay varied from 4 to 7 μmol/g. Quantitative texture analysis indicates that the montmorillonite particles were well orientated with respect to the plane of the film. The full width at half maximum of the orientation distribution of the c* axes of individual clay platelets about the normal to the film plane was 44.3° (14-d reaction time) and 47.1° (360-d reaction time). These values were used to correct the coordination numbers determined by P-EXAFS for texture effects. Ni K-edge P-EXAFS spectra were recorded at angles between the incident beam and the film normal equal to 10, 35, 55, and 80°. Spectral analysis led to the identification of three nearest cationic subshells containing 2.0 ± 0.5 Al at 3.0 A and 2.0 ± 0.5 Si at 3.12 A and 4.0 ± 0.5 Si at 3.26 A. These distances are characteristic of edge-sharing linkages between Al and Ni octahedra and of corner-sharing linkages between Ni octahedra and Si tetrahedra, as in clay structures. The angular dependence of the Ni-Al and Ni-Si contributions indicates that Ni-Al pairs are oriented parallel to the film plane, whereas Ni-Si pairs are not. The study reveals the formation of Ni inner-sphere mononuclear surface complexes located at the edges of montmorillonite platelets and thus that heavy metals binding to edge sites is a possible sorption mechanism for dioctahedral smectites. Data analysis further suggests that either the number of neighboring Al atoms slightly increases from 1.6 to 2 or that the structural order of the observed surface complexes increases from 0.01 A2 to 0.005 A2 with increasing reaction time. On the basis of the low Ni-Al coordination numbers, it appears that over an extended reaction time period of 1 yr the diffusion of Ni atoms in the octahedral layer is not the major uptake mechanism of Ni onto montmorillonite.

Modelling the sorption of Zn and Ni on Ca-montmorillonite

In some previous work titration and Ni/Zn sorption edge/isotherm measurements carried out under a wide variety of experimental conditions on purified Na-montmorillonite were modelled in terms of cation exchange and surface complexation mass action equations. A major objective of the experimental/modelling programme is to understand and predict sorption in commercial bentonite systems. Since montmorillonite is the dominant clay mineral in bentonite and is often present in a mixed Na/Ca form, a natural extension to the previous investigations was to study Ni/Zn sorption on a conditioned Ca-montmorillonite. An important open question was whether the same basic parameters such as site types, site capacities, and acidity constants could be used for both materials and to see to what extent the Ni and Zn surface complexation constants were influenced by the form of the montmorillonite. Sorption edges for Ni and Zn at different Ca(NO3)2 background electrolyte concentrations, together with sorption isotherms measured over a range of pH values, are presented and modelled using the MINSORB code. The parameters characterising the sorption of Ni and Zn on Na- and Ca-montmorillonite systems are compared. Finally, examples are given that illustrate how the modelling can provide insight into the sorption processes.

A PHYSICOCHEMICAL CHARACTERISATION AND GEOCHEMICAL MODELLING APPROACH FOR DETERMINING POREWATER CHEMISTRIES IN ARGILLACEOUS ROCKS

Abstract Argillaceous sediments are being investigated in many countries as potential host rock formations for the disposal of high level radioactive waste. Quantification of the water chemistry in such formation is often difficult because the groundwater flow rates are so low that reliable samples for chemical analysis cannot be obtained. A physicochemical characterisation procedure for rock samples combined with geochemical modelling is described which allows porewater composition to be determined in low porosity/permeability clay rich systems. The methodology was applied to core samples of Opalinus clay within the framework of an international investigation being carried out at Mt. Terri, Canton Jura, Switzerland.

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Abstract Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO3)2), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 A in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide– or a Ni-phyllosilicate–like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 A in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 A in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.

Redox properties of structural Fe in clay minerals. 1. Electrochemical quantification of electron-donating and -accepting capacities of smectites

Clay minerals often contain redox-active structural iron that participates in electron transfer reactions with environmental pollutants, bacteria, and biological nutrients. Measuring the redox properties of structural Fe in clay minerals using electrochemical approaches, however, has proven to be difficult due to a lack of reactivity between clay minerals and electrodes. Here, we overcome this limitation by using one-electron-transfer mediating compounds to facilitate electron transfer between structural Fe in clay minerals and a vitreous carbon working electrode in an electrochemical cell. Using this approach, the electron-accepting and -donating capacities (Q(EAC) and Q(EDC)) were quantified at applied potentials (E(H)) of -0.60 V and +0.61 V (vs SHE), respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2) having different total Fe contents (Fe(total) = 2.3 to 21.2 wt % Fe) and varied initial Fe(2+)/Fe(total) states. For every SWa-1 and SWy-2 sample, all the structural Fe was redox-active over the tested E(H) range, demonstrating reliable quantification of Fe content and redox state. Yet for NAu-1 and NAu-2, a significant fraction of the structural Fe was redox-inactive, which was attributed to Fe-rich smectites requiring more extreme E(H)-values to achieve complete Fe reduction and/or oxidation. The Q(EAC) and Q(EDC) values provided here can be used as benchmarks in future studies examining the extent of reduction and oxidation of Fe-bearing smectites.

Cation exchange capacity measurements on illite using the sodium and cesium isotope dilution technique: Effects of the index cation, electrolyte concentration and competition: Modeling

The isotope dilution technique using Na and Cs as index cations was used to determine the cation exchange capacity (CEC) of illite du Puy as a function of background electrolyte composition. The work showed, in accord with previous studies, that the CEC values were in the order Cs-CEC > Na-CEC. Sodium is commonly chosen as the index cation in CEC determinations using the isotope dilution method. The experimentally measured Na-CEC values for Na-illite increased from ∼75 to ∼200 meq kg−1 for NaClO4 concentrations in the range 5.6 × 10−4 to 1.25 × 10−2 M. Cesium CEC determinations showed a much less pronounced trend over a CsNO3 concentration range from 10−3 to 10−2 M. A reference Cs-CEC value of 225 meq kg−1 was chosen. Careful chemical analyses of the supernatant solutions revealed that Ca and Mg at the (sub)umolar level were present in all the determinations, despite the extensive conditioning procedures used. Competition between (Ca + Mg) and Na for the exchange sites was put forward as an explanation for the variation of Na-CEC values. This hypothesis was confirmed in a series of single (45Ca) and double (45Ca plus 22Na) labeling experiments. Calcium-sodium selectivity coefficients (