Radionuclide Interaction with Clays in Dilute and Heavily Compacted

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Radionuclide Interaction with Clays in Dilute and Heavily Compacted Systems: A Critical Review Andrew W. Miller* and Yifeng Wang Sandia National Laboratory, Albuquerque, New Mexico 87123, United States S Supporting Information *

ABSTRACT: Given the unique properties of clays (i.e., low permeability and high ion sorption/exchange capacity), clays or clay formations have been proposed either as an engineered material or as a geologic medium for nuclear waste isolation and disposal. A credible evaluation of such disposal systems relies on the ability to predict the behavior of these materials under a wide range of thermal−hydrological−mechanical−chemical (THMc) conditions. Current model couplings between THM and chemical processes are simplistic and limited in scope. This review focuses on the uptake of radionuclides onto clay materials as controlled by mineral composition, structure, and texture (e.g., pore size distribution), and emphasizes the connections between sorption chemistry and mechanical compaction. Variable uptake behavior of an array of elements has been observed on various clays as a function of increasing compaction due to changes in pore size and structure, hydration energy, and overlapping electric double layers. The causes for this variability are divided between “internal” (based on the fundamental structure and composition of the clay minerals) and “external” (caused by a force external to the clay). New techniques need to be developed to exploit known variations in clay mineralogy to separate internal from external effects.

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INTRODUCTION Clay minerals are ubiquitous in the environment and have been intensely studied for decades. More recently, clays have grown in importance as their low permeability, high sorption capacity, and swelling capability make them ideal materials for natural and engineered barriers for nuclear waste isolation or CO2 sequestration. In describing these systems, a conceptual model is required that can encompass and describe the relative impacts from coupled processes. These processes include the geomechanical processes related to stresses, pressures, and temperatures at depth, as well as the hydrological processes of water/fluid flow, and the geochemical processes of aqueous and surface complexation. Such a conceptual model is ambitious, as each of these disciplines has individualized limitations; the combination of processes creates new realms of interdisciplinary inquiry based on overlapping or interacting processes. The focus of this review is to consider how chemical reactions are controlled by mineral composition, structure, and material texture (e.g., pore size distribution), with an emphasis on the effect of geomechanical compaction under conditions relevant to nuclear waste disposal. Chemical reactions to be considered include radionuclide sorption/ion exchange to individual clay samples. From a geochemical perspective, the degree of compaction ranges from the single clay particle or dilute suspensions up to heavily compacted clay columns. This range of systems allows for a broad range of chemical variability based on the degree of particle−particle interaction even though the chemistry of the solid phase may be consistent throughout. This review begins with a general discussion of chemistry in physically confined environments. This general © 2012 American Chemical Society

discussion will then be applied to the more specific case of radionuclide sorption to clays. Many aspects of sorption will be considered including acid/base chemistry of clays, surface charge, and models that have been derived to describe these interactions. The review will also cover descriptions and experiments involving sorption and diffusion in highly compacted clays, and will end with several perspectives on the state of the science and future directions. Specifically not covered in this review are the feedback aspects of thermal− hydrological−mechanical−chemical (THMc) modeling (e.g., porewater composition affecting swelling pressure, which affects pore structure which can in turn affect porewater composition, etc.).

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BACKGROUND The incorporation of chemical reactions to a THMc model is an explicit attempt at joining the physical and chemical realms. At their simplest, clays are chemically mixed metal oxides. They contain both a metal or metalloid (Al, Mg, or Si) bound to either oxygen or hydroxide. Yet there is much to distinguish clays from simple metal oxides such as quartz or iron oxides (Table 1). Simple metal oxides are simple in chemical composition. Ignoring any impurities, there are only two or three atomic species controlling the chemical behaviors, including the metal, oxygen, and hydrogen. To a certain extent, Received: Revised: Accepted: Published: 1981

September 19, 2011 January 16, 2012 January 18, 2012 January 18, 2012 dx.doi.org/10.1021/es203025q | Environ. Sci. Technol. 2012, 46, 1981−1994

Environmental Science & Technology

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complexity leads to behaviors unobserved in simple metal oxides such as ion exchange. Many experimental techniques used to probe clays are taken directly from the study of the simple metal-oxides. Despite furthering knowledge of clay minerals, this application of experimental techniques is potentially hindered by the differences between simple metal oxides and clays. The relationships discussed so far are all ‘internal’ in nature; they arise from the differences in chemical compositions and crystallographic structures of individual clay particles. THMc modeling also needs to consider “external” effects. These effects represent interaction among clay particles or forces acting on aggregates of clay particles which change the chemical relationships between mineral and solution phases. For example, these processes could include hydrothermal mineral phase alterations,3,4 or compaction and mineral dissolution/alteration.5 The connection between “external” effects and chemistry is probably the weakest connection in THMc modeling. However, there is broader evidence that external forces may lead to alterations in surface chemical reactions. For example, recent studies on nanoporous materials indicate that the mineral−water interface in confined nanopores can exhibit different chemical behaviors from unconfined surfaces.6,7 Nanopores account for a significant portion of total porosity in geologic materials, especially in clay formations.7 For instance, the porosity of a Georgia kaolinite was dominated by pores smaller than 10 nm.8 Figure 2 shows a plot conceptualizing the relationship between surface chemical behaviors and confined space, or between internal and external effects. The x-axis shows a continuum between Al−O and Si−O endmembers with clay minerals being a mixture of the two. For some clays it makes more sense to have a Mg−O endmember instead of Al−O. For the purposes of this discussion either are viable options, but there is more experimental data for nanoporous Al−O than Mg−O. The y-axis shows a continuum between constrained space and infinite space. Location within this space relates to the chemical behavior for the conditions given. For example, the Al−O endmember in infinite space can be thought of as an Al−O particle in a stirred batch solution. In this case, the space between individual solid particles is large enough that particle− particle interactions are negligible and space is “infinite”. By contrast, nanoporous Al−O have been shown to have macroscopic chemical behaviors quite different from that of well dispersed non-nanoporous particles.6,7 Despite having an identical chemical nature, constraining space in the nanoporous regime alters observable chemistry. That is why these two areas are separated in the figure. Similar arguments hold for Si−O phases9,10 (see also 11 for a broader view of nanoporous materials in earth systems). The difference in this chemical behavior has yet to be definitively understood;12 however, the physical structure of nanoporous materials is quite different from the idealized flat surface plane from which most surface complexation theory is derived. The surfaces of nanoporous materials are heavily curved, and pore spaces are small enough that the surface charge distributions into solution can overlap. Clays also deviate from the ideal flat surface plane model with the formation of an interlayer space between clay particles. Like nanoporous materials, the interlayer space is small enough that surface charge domains can overlap. This interlayer space only accounts for a certain percentage of the surface charge. The balance is on surfaces exposed directly to solution, and may follow flat plane surface complexation theory. Because of the

Table 1. Physical and Chemical Properties of Simple Metal Oxides and Clays Traditional metal-oxide Physical

Chemical

Crystal shape (cubic, tetrahedral, etc.) Imperfections (pits, etchings, etc.)

Homogenous (2 or 3 species make up crystal ignoring imperfections)

Clays Layered (1:1 vs 2:1) Imperfections (stack irregularities, etchings, etc.) Morphological variations (fibrous, platy, etc.) Heterogeneous mixture (Al, Si, and Mg-oxides) Lattice substitutions

the variation in observed chemical behaviors stems from the different crystallographic orientations of terminating surfaces. When a crystal is cleaved, there are local alterations in the number of bonds exposed to solution based on the physical structure that is created. Pits, edges, corners, and any other physical microcharacteristic of the crystal give rise to surface heterogeneity that may also play a role in interfacial sorption. Clays are much more complex than simple oxides, both compositionally and structurally. They contain mixtures of Al and/or Mg with Si, and these species are separated into tetrahedral and octahedral sheets (see Figure 1A). Within these

Figure 1. (A) Visual conceptualization of clay layering. T sheets are tetrahedral Si−O sheets, O sheets are octahedral Al−O (or Mg−O) sheets. This picture shows a 2:1 clay; a 1:1 clay would only have 1 T sheet and 1 O sheet comprising a TO layer. As pictured there are cations in the interalyer space denoted by the circles with + symbols. In certain clays the interlayer is only filled with water. Also shown are the physical locations of the basal and edge surfaces. (B) Visual conceptualization of clay stacks under various degrees of compaction (adaptd from ref 1). (1) TOT clay layer, (2) interlayer water, (3) diffuse double layer water, (4) free water. Crystallographic images of clay stacking available in ref 2.

sheets, there are often a significant amount of substitutions where a central ion is replaced by another species such that an individual sheet is not pure Si−O or Al−O. These layers can be bound together in a variety of ways giving rise to different structures and morphologies of clay minerals. For example, the individual 2:1 layers in sepiolite invert in a regular pattern forming fibers and large void spaces filled with water. Other clays, for example kaolinite, form continuous sheets leading to a voidless, platy morphology. This chemical and physical 1982

dx.doi.org/10.1021/es203025q | Environ. Sci. Technol. 2012, 46, 1981−1994


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Figure 2. Chemical behaviors as a function of chemical and physical environments.

interlayer space, dispersed clays are slightly lower on the space axis; nanoconfined environments are possible even in dilute systems due to internal clay mineral properties. Furthermore, as clays are compacted, the amount of nanoconfinement is likely to increase. Clays have been found to have a dual porosity nature.13−15 The two different porosities change independently as a function of increasing compaction. The interaggregate pore space decreases with increasing compaction, while the intraaggregate pore space does not.13 This means that as compaction increases, the pore space is increasingly dominated by the smaller intra-aggregate pores ( 9.58 Furthermore, ionic mixtures can cause deviation from these general behaviors. When ionic strength is dominated by sodium, uranium sorption to smectite follows the actinide trend described above, but when ionic strength is dominated by 1985



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Table 2. Summary of Results for Batch Ion Complexation Studies Involving Nuclear Waste Specific Clays and Ions

that Se reduction is related to the presence of pyrite, but it is still unclear what is causing the reduction of iodate. Understanding iodine* (iodine present in an unspecified redox state: iodide/iodate/elemental iodine, collectively69) behavior has proven to be especially daunting. Many challenges relate to experimental artifacts (e.g., radiotracer purity, oxic state of clay samples) and to conflicting information on uptake mechanisms. Besides sorption, isotopic exchange and uptake in carbonate minerals have also been shown to be important.69 I− and IO3− are often studied simultaneously as the oxidation state under field and laboratory conditions is not always clear.67,70,71 Small amounts of iodate sorption to kaolinite have been observed, while no sorption of iodide could be measured.62 Significant amounts of iodide sorb to illite (Kd = 27.70 ± 11.69 mL/g), while there was little to negative sorption on montmorillonite or vermiculite.63 Iodide sorption to CallovoOxfordian argillites (mostly illite and interstratified illite/ smectite) was only observed at low initial iodide concentrations, and over periods of time extending up to 3 months.72 Furthermore, careful control of oxidation state has shown that iodide sorption to clays is probably negligible. An anoxic clay core was removed from the Callovo-Oxfodian formation, and I− sorption experiments were performed under an inert atmosphere.73 No significant iodide uptake was observed. In iodine* diffusion experiments, small Kd values (∼0.001−2.9 mL/g) are generally required to fit the data;72,74,75 iodine* is visually retarded relative to chloride. Current data makes envisioning a consistent description of iodine* in clay-based systems difficult. Generally, iodate appears more reactive than iodide, but predicting iodine* speciation in a given clay is currently beyond our understanding.

calcium, the ionic strength dependence at low pH is diminished.57 When considering a single sorbent and a range of sorbates including transition metals and actinides, linear free energy relationships exist between the aqueous hydrolysis constant of the sorbate and the complexation constant to a strong edge site.52,59 This suggests some level of thermodynamic consistency across a range of experimental approaches. Anion interactions with clays are less understood and less consistent than the cationic relationships. The anion discussion in this review will be limited to common anions and oxyanions in nuclear waste, including I−, IO3−, SeO32−, and TcO4−. Relative to cations, anion uptake to any clay mineral is weak and much slower which is consistent with charge repulsion between the fixed negative charge and the anions.22,60−64 For example, selenite required a minimum of 5 days to reach equilibrium with illite and smectite,65 and iodide required at least 7 days.63 With traditional metal oxides, anion sorption decreases with increasing pH; this should also be true for the amphoteric edge sites in clays. Iodide has been shown to have a weak pH dependence consistent with this model.63 Yet, selenium(IV) sorption to montmorillonite and kaolinite increases slightly as a function of pH until the pH is circumneutral. Above that pH sorption drops to near zero.66 Selenium(IV) sorption to smectite decreases nearly linearly with pH while sorption to illite is constant as a function of pH.65 Pertechnetate exhibits a maximum of ∼15% sorption to bentonite at neutral pH, and a weak pH dependence in general.66 There remains some debate about whether observed anion uptake is truly a sorption process. Both selenium and iodide/iodate have been shown to undergo redox transformations when contacted with certain clays.67,68 It appears 1986



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All of this information is summarized as Kd values in Table 2; similar tables summarizing more references are available in the Supporting Information. Kd values have known limitations; among them are the lack of validity for changing chemical conditions and spatial heterogeneity of values within natural systems. Regardless, they still represent general indications of sorption preference, and are commonly used in waste repository performance assessments. Before proceeding to discuss diffusion experiments, the modeling methods used to describe internal chemical behaviors will be addressed. Modeling Techniques. To complement the three main variables studied in titrations and batch experiments, there are three main components included in clay system conceptual models, including surface charge, ion complexation, and connections to internal structure. Models describing all three components have been constructed with varying complexities. An extension of the model described earlier linking zeta potential to surface charge37 is to consider clay particles not as a series of planes, but instead as a porous solid where the fixed charge is located at a variety of depths within the solid.76 This creates a potential curve extending from the mineral surface both into solution and into the solid phase, a double−double layer. This model has successfully described charging behaviors of kaolinite and montmorillonite.77 The addition of interior negative charge allows for the description of decreasing isoelectric point values as a function of increasing ionic strength, a common experimental observation. A similar porous medium approach has also been generalized such that application can be extended to any microporous material.78 Earlier ion complexation models took the form of hard−soft acid−base models79−81 (HSAB; A more comprehensive evolutionary description of clay complexation models is given in 82). These models were based on fundamental parameters such as electronegativity, ionization potentials, and ionic radii. More recently, surface complexation models (SCM) have dominated descriptions of ion sorption to clays. SCM have been described by others,47,51,83,84 but a brief introduction is given here. Surface complexation modeling based on simple metal-oxides treats mineral surfaces as amphoteric. For example, an aluminum oxide surface could be described using the reactions in eqs 1 and 2 above. The surface protonation state is pH dependent, as are the charge and ion complexation reactions. This idea was generalized and applied to mineral mixtures where individual component contributions are averaged into a single set of complexation constants (GC, generalized composite approach83,86). The major extension of this model required for clays is the addition of a generic ion exchange site (generally denoted ≡X). The complexation behavior of this site is largely pH independent, and represents the contribution to particle charge from isomorphous substitutions. For clays, it is generally sufficient to use three surface sites: an amphoteric Al site, a neutral or negative Si site, and an exchange site. A slightly less common method is to use the GC approach, and simply assign a series of generic sites which do not necessarily correspond to the physical sites present in clays. This often requires a “strong” and “weak” generic site in order to obtain sufficient fits. The clay surface can be mathematically fit with either of these structurally independent models. This type of modeling has been applied to a wide array of clays, under broad sets of aqueous conditions and for a wide range of metal sorbates. A major distinguishing factor between models is the assumption made regarding surface charge distribution. These

assumptions range from a nonelectrostatic model (charge not distributed into solution51,52,55,65,87), to the constant capacitance model (charge decays linearly into solution41,44,56), to the diffuse double layer model (partial charge neutralization in one layer due to sorbed cations and exponentially decaying charge in the second layer37,50,66), to the triple layer model88 (TLM; inner sphere complexation in the first layer, outer sphere complexation in the second and a diffuse swarm in the third53,57), and finally to the MUSIC model89 (a model based on bonding principles and the physical structure of a mineral surface90). Despite the importance of charge in clay systems, all of these models are capable of being fit to experimental data. The use of such an array of models precludes intermodel comparison of complexation constants. Obtaining the linear free energy relationships described earlier52 required remodeling of existing data using a consistent approach. Since all of these models are capable of simulating data, model appropriateness is based on “softer” considerations such as ease of use. The relative popularity of the nonelectrostatic and constant capacitance models is unsurprising as they are conceptually the simplest, and the easiest to implement. In a study which transcends the molecular to observable scales, molecular dynamic simulations were used to test assumptions associated with the use of triple layer models;91 this model was further extended to describing electrokinetic data. The use of molecular dynamics allowed for constraint of TLM and Stern Model parameters, and also showed significant agreement between the output of the different modeling techniques. Of interest in this review are models which are capable of connecting internal and external processes. Due to linkages to underlying structure, it would appear that the MUSIC model is the most likely candidate.90 When applied to montmorillonite, this model used 27 total site types corresponding to individual binding environments based on crystallographic representation of a montmorillonite platelet. The model constrains many fitting parameters through structural arguments, and has fewer fitting parameters than many structurally independent models. Unambiguous parametrization or experimental confirmation of parameters in the MUSIC model are currently overwhelming, and it remains to be seen is if this internally based model would be sufficiently robust to respond to an external force. In other words, if compaction puts montmorillonite platelets in close proximity such that the 27 possible sites start to interact, does the model still describe behavior? Another major shortcoming of models currently in the literature is charge interaction. By definition, structurally independent models cannot describe local spatial charge variations present on a clay particle. Even in the structurally grounded MUSIC model, equations do not exist describing positive/negative charge interaction between edge and basal plane locations in the pore and interlayer space. Some have suggested that titration techniques are not sensitive enough to distinguish edge and basal behaviors.28 More direct instrumental techniques with resolution to separate basal plane behavior from edge site behavior may be required to distinguish these different binding environments.92,93

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EXTERNAL STRUCTURE−FUNCTION This section reviews how chemistry proceeds when reactive environments are externally constrained. To maintain focus on nuclear waste repositories, much of the discussion will be centered on nuclear-waste-relevant, diffusion-based studies in compacted clays. 1987



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Table 3. Summary of Results for Diffusion Studies Involving Nuclear Waste Specific Clays and Ionsa

a Diffusion coefficients in italics are apparent diffusivities (Da) while non-italicized values are effective diffusivities (De). Da = De/(ε + ρKD), where ε is the accessible porosity, ρ is the dry bulk density, and Kd is the distribution coefficient.

Diffusion Experiments and Modeling Results. Several types of diffusion experiments exist. Through-diffusion experiments consist of up- and downgradient reservoirs and are separated by a compacted clay column. A diffusive gradient is imposed across the column by maintaining a semiconstant tracer concentration in the upgradient reservoir, while the downgradient reservoir is kept nearly tracer free. Tracer concentration is monitored in both reservoirs. These experiments continue until the diffusive flux reaches steady state. Indiffusion experiments consist of a clay column connected to a tracer reservoir and tracer flux is determined by monitoring concentration in the single reservoir. Out-diffusion experiments are performed at the end of a through-diffusion experiment. Once steady state is established via through-diffusion, the reservoirs are filled with tracer free electrolyte, and the ion of interest diffuses back out of the clay. In another variation, a clay column is compacted with an embedded ion source. The column is saturated, and the ions migrate outward from the source. Generally the column is sliced, and the ion is extracted from the solid for analysis. These different types of experiments simulate different waste release scenarios, and the array of experiments allows for better fitting of model parameters. Diffusion experiments are further distinguished by the use of reactive or nonreactive tracers. Diffusion behaviors in clay roughly divide into the nonreactive tritiated water (HTO), weakly interactive ions (ion exchange and/or outer sphere surface complexation), and strongly interactive ions (inner sphere surface complexation). The ultimate nonreactive tracer, HTO, is used to determine pore volumes, effective diffusion coefficients, and geometrically related factors such as tortuosity or capacity factors.75,94−96

Porosities and clay textures in relation to porewater chemistry significantly affect HTO diffusion.95,97 Specifically, effective diffusion coefficients increase in the order Na-montmorillonite < Ca-montmorillonite < Ca-illite < Na-illite ≤ kaolinite. This behavior is physically interpreted through changing particle sizes between clay minerals (larger particle sizes allow for lower tortuosity), and through the differences in hydration energy between Ca and Na. So, both chemical and physical internal effects can potentially change HTO diffusion. In another study, sodium and HTO diffusion was measured through Na-, Ca-, and Cs-bentonites;19 HTO fluxes were larger in Cs-bentonite, while sodium fluxes were larger in the Ca- and Na-bentonites. Through HRTEM characterization, the authors attribute this to the formation of a “gel-like phase” in the Ca- and Na-bentonite that is lacking in the Cs form. In this study, an internal morphological change induced by cation interaction with the clay particle creates larger scale differences in diffusional properties. HTO diffusion coefficients can also decrease with increasing compaction due to changing pore structures.75,98 Thus an external change can alter diffusion properties. Relevant results from diffusion experiments are summarized in Table 3; similar tables summarizing more references are available in the Supporting Information. HTO also acts as an ideal benchmark to explore variations in ion diffusion. Due to the fixed surface charges and overlapping charge regimes in compacted clays, traditionally “nonreactive” species appear to be quite reactive. A long-observed phenomenon is that monovalent alkali cations have larger fluxes, and halides have smaller fluxes relative to HTO in clays with significant CEC. The former is often explained through surface diffusion99,100 while the latter is often explained through 1988



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anion exclusion.1,101 Surface diffusion is conceptualized as cation transfer via the cation exchange sites. Recent advances in surface diffusion understanding have included the separation of diffusional fluxes into different components. For example cation flux in compacted montmorillonite has been mathematically separated between interlayer and porewater fluxes.101,102 Using this separation to describe experimental data it was found that the majority of sodium and strontium diffusion (both weakly interactive) is occurring through the interlayer space. More recently, a similar analysis was completed for a wider range of interacting elements including cesium, rubidium, and cobalt.103 Using this analysis, a scaling factor was derived based on the bulk density, Kd value, and the pore pathway porosity. Using this scaling factor it appears that diffusion rates are intrinsically determined by the nature of the cation, and not the nature of the clay. It remains to be seen if a similar analysis will hold with more strongly interactive ions (e.g., the actinides). Anion diffusion behavior varies with bulk density and solution conditions. For example, chloride diffusion coefficients were nearly an order of magnitude lower when bulk density was increased from 1300 to 1900 kg·m−3.1 Linear relationships between effective diffusion coefficients and diffusion accessible porosity in log−log space suggest that changes in porosity with increased compaction are the major factor in changing diffusion coefficients (Figure 1B). Diffusion coefficients also increase with increasing ionic strength suggesting a compression of the double layer which allows the anion to use more of the total pore space. Taking this one step further, iodide and chloride diffusion in compacted bentonites have fast and slow components.104 The fast component was assumed to be intralayer space diffusion, and is affected by changes in bulk density. The slow component was assumed to be interparticle diffusion, and is largely independent of compaction up to 1800 kg·m−3. This is consistent with the dual porosity nature of clays. However, an alternative explanation to the dual component diffusion is that it is an experimental artifact caused by spatially heterogeneous accessible porosities within a compacted clay column.105 When more reactive ions are considered, there is the added complexity of deconvoluting the diffusion and reactive behaviors. In stark contrast to the surface complexation/ion exchange models discussed earlier, reactions in diffusion experiments between weakly and strongly interacting ions with clays are most often interpreted with Kd values. Commonly studied species are elements with isotopes of significant concentration in nuclear waste as well as relatively long-half-lives, including several fission products (Cs, Se, Sr, I, Tc67,72,74,85,94,106,107) and actinides (U, Np, Am108,109). Kd values must be fit to data using a conceptual framework. The most common approach is to use a variation of Fick’s Law, with sorption reactions being described by the Kd value (eq 4), δC δ ⎛ εD δC ⎞ (ε + ρ bKD) = ⎜ ⎟ δt δx ⎝ τ2 δx ⎠

representing interlayer and pore diffusion.110,111 Using these models, the basic conclusions of many authors are that as compaction increases, porosity decreases, tortuosity increases, and Kd can either increase or decrease depending on the element. The Kd value for Cs decreases as a function increasing compaction (Kunipia-F108), and also increases with increasing compaction (Bentonite MX-80;106 Volclay KWK107). The behavioral differences are most likely related to Na concentrations in the background electrolyte and that Cs tends to interact with clays through ion exchange, and this uptake is nonlinear as a function of Cs and Na concentrations. Kd values decreased when the column was saturated with distilled water, and increased when saturated with a sodium rich artificial pore water or swamping NaClO4 solutions. Using a previously developed hydration based model,82 the preferential uptake of Cs as a function of increasing compaction has been attributed to the coupled processes of hydration/dehydration of the Na and Cs ions.107 Coupling hydration energies with the restricted water activity within compacted interlayer spaces, it is thermodynamically favored to have Cs in the interlayer and Na in the pore fluid. As the system is less compressed, the interlayer and pore waters are more similar, and this energetic favoritism relaxes leading to less Cs occupying exchange sites in the interlayer space. Similar arguments have been cited which relate the solvent structuring ability of individual cations to surface charge development on colloidal silica.10 For higher valence species, Kd values generally decrease as a function of increasing compaction, with some notable exceptions. Values for Sr follow this trend,106,108 while values for Co are large and constant as a function of compaction.106 Kd values decrease slightly for Am, while for Np they start at a value of 0.42 m3/kg at 200 kg/m3 decrease to 0.1 at 1200 kg/m3 and increase to 0.27 at 2000 kg/m3.108 Also, both I and Tc have decreasing Kd values as a function of increasing compaction.108 Chemical interpretations of these complex behaviors have yet to be posited. There remains a possibility that using Kd based interpretations may be leading to the somewhat contradictory and variable evidence of individual ion behaviors as a function of compaction. Kd values are a high-level engineered approach at describing another set of fundamental processes. Models that can explicitly describe the underlying processes may be able to dispel the black box surrounding Kd usage.78,112,113 Many of these models are quite recent, and application to large data sets under varying conditions has yet to be completed. One approach combined geometrical effects with charge dissemination (Donnan) with SCM/IX with diffusion.94 Here Cs, Sr, and I are all assumed to interact with the clay, and the reactions are described with a combined SCM/IX model that distributes charge into the pore space using a diffuse double layer/Donnan approach. The model is applied to a radial diffusion experiment at 7 MPa. Despite the cationic/anionic/neutral characteristics of the tracers involved, as well as the variable reactivity, diffusion coefficients within the porous medium were derived from diffusion in bulk water in a systematic way. Similar application of this and other models to variably compacted systems may help explain the Kd behaviors observed when applying Fick’s Law.

(4)

where ε is the accessible porosity, ρb is the dry bulk density, Kd is the sorption distribution coefficient, C is the aqueous concentration, t is time, x is a spatial coordinate, D is the diffusion coefficient in bulk water, and τ is the tortuosity. The parenthetical term on the left-hand side of the equation is referred to as the rock capacity factor. Some authors have extended this analysis to include a dual porosity formulation of Fick’s law, with the two different porosities conceptually

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PERSPECTIVES While great strides have been made in understanding radionuclide behavior in clays, there are still several frontiers remaining.114 In terms of batch scale dilute system observa1989



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are tailored to what is known about clay minerals, and what can be exploited experimentally. One potential application of this idea is to perform more interclay comparison studies with a broader range of clay minerals. Many nuclear waste repository studies focus on illite and montmorillonite as these are the major minerals for proposed clay repositories. Kaolinite is also included as a simpler system with smaller fixed charge. Clays as a class of minerals exhibit a much broader range of chemical and physical properties with differing contributions of edge and basal surface charge. Studies involving less directly applicable clay minerals may lead to an eventual deconvolution of basal and edge charge properties. To date, there is no clear evidence that increasing clay compaction will lead to fundamentally different uptake properties of radionuclides. However, ions with experimental evidence at a range of compactions tend to be ions with simpler solution, redox, and surface chemistry. In other words, the ions studied are less chemically active than many of the ions relevant to waste disposal. These less chemically active ions may not be the best probe to examine compaction effects on chemistry. Many of the observed behaviors of lower valency ions are related to the hydration state and energy of the ion in solution (e.g., the Na/Cs exchange in 107). Additional ligands become more common with higher valence ions, and it is unknown how these different ligands will affect the observed chemistry. Moving forward two possibilities exist: (1) the overall reactivity of the ion will not matter, and chemistry will be independent of compaction, (2) the more reactive ions will show more complex behavior that is attributable to effects of compaction. The first of these is consistent with the clay data to date; the second of these is consistent with much of the observed reactivity in constrained spaces discussed in the Background. Determining which of these paths will dominate requires diffusion experiments with strongly interacting ions. The time scales needed is the largest hurdle to such experiments. Electromigration experiments can speed the diffusional process, but may cause redox concerns as well.122,123 Spectroscopic/ microscopic techniques may also prove useful to measure diffusion over the small distances involved. Making a Capital C. THMc modeling has typically used the lower-case “c” as the chemistry that is invoked is only a small subset of those involved with repository performance. This review has focused on the connection between the mechanical (compaction) and the chemical. Conceptual representation of the THMc construct depicts interrelations between hydrologic and thermal processes as well. The ideal model is one of a fully coupled multiprocess system. In the simplest of terms, equations are needed with a description of, for example, a mechanical process to the left of the equal sign and a description of a chemical process to the right. This modeling process mirrors that of nanoscience; there the size of the particle can determine chemical reactions, and in compacted clay systems the size of the pore surrounding the particle may partially determine chemical behavior. As with nanoscience, the actuality will likely be significantly more complex due to the demonstrated interrelation and nonlinear relationship between pore size distribution, compaction, and porewater/surface chemistry.

tions, there is a large body of results and model descriptions of both weakly and strongly interacting elements with several major clay minerals. The interactions of the studied elements range from purely electrostatic to inner sphere edge complexation. These models are quite robust, including predictive capability when sorption to mixed mineral clay rocks was blindly predicted from individual mineral studies.115 However, as systems become more compacted, the range of chemical behaviors studied shrinks considerably. In diffusion studies, the majority of the published work focuses on weakly interacting elements (electrostatic effects and ion exchange type interactions). This is a pragmatic response to the fact that elements with strong interactions (surface complexation to edge sites) could take years for lab scale tests and decades for larger in situ tests. This pragmatic limitation greatly inhibits experimental work testing hypotheses related to chemical variability as a function of compaction. Elements that interact more strongly with clay minerals also tend toward having a larger number of relevant aqueous species as well as potentially having multiple relevant oxidation states. Currently there are three main hypotheses regarding the observed Kd variability as a function compaction, including porewater composition, effective surface area variability, and surface charge heterogeneity. Experimental extraction of porewater typically proceeds through high pressure techniques or through long-term equilibration and monitoring of pore solutions exuded from cores or boreholes.116 These techniques introduce many artifacts, including the logic loop of how to study chemistry as a function of compaction when the sampling procedure requires compaction. Alternatively, models based on first principles can ostensibly describe the reactivity in the pore spaces of compacted clay. These models abound in the literature (e.g., refs 117−119, etc.). However, they remain difficult to directly test as they predict a porewater distribution that is problematic to measure. These models are better tested by predicting more directly observable effects that occur in compacted diffusive columns. Although predicting diffusion column results is quite valuable, better experimental methods capable of measuring porewater chemistry would be equally valuable. Another major experimental limitation is the determination of the effective surface areas for clay/radionuclide interactions. One method to test for reactive surface area is to perform batch sorption experiments with crushed rock, and with intact rocks via diffusion experiments (e.g., refs 120 and 121). This technique has shown that for ion exchange uptake mechanisms in clays, the effective specific surface area is identical for crushed and intact materials; the same ion exchange model constants could describe data from both types of experiments. This implies that the number of exchange sites does not change due to clay mineral proximity. What remains unknown is if this same similarity of effective surface area will be seen with ions that interact with clays through other mechanisms such as complexation with frayed edge sites. Batch sorption experiments have been completed for a range of such ions, but the required time commitment of diffusion experiments has left this hypothesis currently untestable. And finally, basal and edge charge interactions on an individual clay particle are not well understood. These interactions may lead to unique charge environments in compacted systems which can alter observed chemistry. New experimental methods are needed to probe surface charge that 1990



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(12) Goyne, K. W.; Zimmerman, A. R.; Newalkar, B. L.; Komarneni, S.; Brantley, S. L.; Chorover, J. Surface charge of variable porosity Al2O3(s) and SiO2(s) adsorbents. J. Porous Mater. 2002, 9, 243−256. (13) Lloret, A.; Villar, M. V.; Sanchez, M.; Gens, A.; Pintado, X.; Alonso, E. E. Mechanical behaviour of heavily compacted bentonite under high suction changes. Geotechnique 2003, 53, 27−40. (14) Musso, G.; Morales, E. R.; Gens, A.; Castellanos, E. The role of structure in the chemically induced deformations of FEBEX bentonite. Appl. Clay Sci. 2003, 23, 229−237. (15) Keller, L. M.; Holzer, L.; Wepf, R.; Gasser, P. 3D geometry and topology of pore pathways in Opalinus Clay: Implications for mass transport. Appl. Clay Sci. 2011, 52, 85−95. (16) Coppin, F.; Berger, G.; Bauer, A.; Castet, S.; Loubet, M. Sorption of lanthanides on smectite and kaolinite. Chem. Geol. 2002, 182, 57−68. (17) Tournassat, C.; Greneche, J.-M.; Tisserand, D.; Charlet, L. The titration of clay minerals I. Discontinuous backtitration technique combined with CEC measurements. J. Colloid Interface Sci. 2004, 273, 224−233. (18) Chipera, S. J.; Bish, D. L. Baseline studies of the clay minerals society source clays: Powder X-ray diffraction analyses. Clays Clay Miner. 2001, 49, 398−409. (19) Melkior, T.; Gaucher, E. C.; Brouard, C.; Yahiaoui, S.; Thoby, D.; Clinard, C.; Ferrage, E.; Guyonnet, D.; Tournassat, C.; Coelho, D. Na+ and HTO diffusion in compacted bentonite: Effect of surface chemistry and related texture. J. Hydrol. 2009, 370, 9−20. (20) Dultz, S.; Riebe, B.; Bunnenberg, C. Temperature effects on iodine adsorption on organo-clay minerals II. Structural effects. Appl. Clay Sci. 2005, 28, 17−30. (21) Bektas, N.; Agim, B. A.; Kara, S. Kinetic and equilibrium studies in removing lead ions from aqueous solutions by natural sepiolite. J. Hazard. Mater. 2004, 112, 115−122. (22) Fuhrmann, M.; Bajt, S.; Schoonen, M. A. A. Sorption of iodine on minerals investigated by X-ray absorption near edge structure (XANES) and I-125 tracer sorption experiments. Appl. Geochem. 1998, 13, 127−141. (23) Sanchez del Rio, M.; Suarez, M.; Garcia-Romero, E.; Alianelli, L.; Felici, R.; Martinetto, P.; Dooryhee, E.; Reyes-Valerio, C.; Borgatti, F.; Doyle, B.; Giglia, A.; Mahne, N.; Pedio, M.; Nannarone, S. Mg Kedge XANES of sepiolite and palygorskite. Nucl. Instrum. Methods Phys. Res., B 2005, 238, 55−60. (24) Schlegel, M. L.; Manceau, A.; Charlet, L.; Hazemann, J. L. Adsorption mechanisms of Zn on hectorite as a function of time, pH, and ionic strength. Am. J. Sci. 2001, 301, 798−830. (25) Chen, H.; Wang, A. Q. Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay. J. Colloid Interface Sci. 2007, 307, 309−316. (26) Thompson, H. A.; Parks, G. A.; Brown, G. E. Dynamic interactions of dissolution, surface adsorption, and precipitation in an aging cobalt(II)-clay-water system. Geochim. Cosmochim. Acta 1999, 63, 1767−1779. (27) Ma, C.; Eggleton, R. A. Surface layer types of kaolinite: A high resolution transmission electron microscope study. Clays Clay Miner. 1999, 47, 181−191. (28) Brady, P. V.; Cygan, R. T.; Nagy, K. L. Molecular controls on kaolinite surface charge. J. Colloid Interface Sci. 1996, 183, 356−364. (29) Krekeler, M. P. S; Guggenheim, S. Defects in microstructure in palygorskite-sepiolite minerals: A transmission electron microscopy (TEM) study. Appl. Clay Sci. 2008, 39, 98−105. (30) Drits, V. A. Structural and chemical heterogeneity of layer silicates and clay minerals. Clay Miner. 2003, 38, 403−432. (31) Johnston, C. T. Probing the nanoscale architecture of clay minerals. Clay Miner. 2010, 45, 245−279. (32) Alkan, M.; Demirbas, O.; Dogan, M. Electrokinetic properties of kaolinite in mono- and multivalent electrolyte solutions. Microporous Mesoporous Mater. 2005, 83, 51−59. (33) Sposito, G. On points of zero charge. Environ. Sci. Technol. 1998, 32, 2815−2819.

ASSOCIATED CONTENT

S Supporting Information *

Extended versions of Tables 2 and 3 are given, with summaries of more data from a larger set of references. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 505.844.2910; fax: 505.844.2348; e-mail: andmill@ sandia.gov. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Gracious thanks are extended to four anonymous reviewers who greatly improved and narrowed the scope of this review. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This work is supported by DOE Sandia Laboratory-Directed R&D Program and DOE Used Fuel Disposition Program.

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REFERENCES

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Radionuclide Interaction with Clays in Dilute and Heavily Compacted

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