Charging Behavior of Clays and Clay Minerals in Aqueous Electrolyte Solutions — Experimental Methods for Measuring the Charge and Interpreting the Results

We discuss the charging behavior of clays and clay minerals in aqueous electrolyte solutions. Clay platelets exhibit different charging mechanisms on the various surfaces they expose to the solution. Thus, the basal planes have a permanent charge that is typically considered to be independent of pH, whereas the edge surfaces exhibit the amphoteric behavior and pH-dependent charge that is typical of oxide minerals. Background electrolyte concentration and composition may affect these two different mechanisms of charging in different ways. To guide and to make use of these unique properties in technical application, it is necessary to understand the effects of the various master variables (i.e. pH and background salt composition and concentration). However, how to disentangle the various contributions to the charge that is macroscopically measurable via conventional approaches (i.e. electrokinetics, potentiometric titrations, etc.) remains a challenge. The problem is depicted by discussing in detail the literature data on kaolinite obtained with crystal face specificity. Some results from similar experiments on related substrates are also discussed. As an illustration of the complexity, we have carried out extensive potentiometric mass and electrolyte titrations on artificial clay samples (Na-, Ca-, and Mg-montmorillonite). A wide variety of salts was used, and it was found that the different electrolytes had different effects on the end point of mass titrations. In the case of a purified sample (i.e. no acid-base impurities), the end point of a mass titration (the plateau of pH achieved for the highest concentrations of solid), in principle, corresponds to the point of zero net proton and hydroxide consumption, at which in ideal systems, such as oxide minerals, the net proton surface charge density is zero. To such concentrated (dense) suspensions of clay particles, aliquots of salts can be added and the resulting pH indicates the specificity of a given salt for a given clay particle system. In the experimental data, some ambiguity remains, which calls for further detailed and comprehensive studies involving the application of all the available techniques to one system. Although, right now, the overall picture appears to be clear from a generic point of view (i.e. concerning the trends), clearly, in a quantitative sense, huge differences occur for nominally identical systems and only such a comprehensive study will allow to proof the current phenomenological picture and allow the next step to be taken to understand the fine details of the complex clay-electrolyte solution interfaces

Comment on "Kinetics and thermodynamics of Eu(III) adsorption onto synthetic monoclinic pyrrhotite" by Y. Zhu et al., Journal of Molecular Liquids, 218 (2016), 565-570

International audienceThe paper of Zhu et al. [1] is of great interest, since it reported adsorption data of Eu(III) onto pyrrhotite, an uninvestigated solid surface to date. We are of the opinion that the system is of relevance in the context of nuclear waste disposal in deep repositories. In the study, Eu(III) is used as an analogue for trivalent actinides and pyrrhotite might be a relevant mineral under reducing conditions. Unfortunately, for readers interested in using the uptake data for developing a surface complexation model, some crucial information concerning experimental conditions is missing and some statements in the text require some clarification. We aim here to raise some relevant questions in a constructive way, to achieve a correct understanding of this interesting data

Experimental Data Contributing to the Elusive Surface Charge of Inert Materials in Contact with Aqueous Media

International audienceWe studied the charging of inert surfaces (polytetrafluoroethylene, i.e., PTFE; graphite; graphene; and hydrophobic silica) using classical colloid chemistry approaches. Potentiometric titrations showed that these surfaces acquired less charge from proton-related reactions than oxide minerals. The data from batch-type titrations for PTFE powder did not show an effect of ionic strength, which was also in contrast with results for classical colloids. In agreement with classical colloids, the electrokinetic results for inert surfaces showed the typical salt level dependence. In some cases, the point of zero net proton charge as determined from mass and tentatively from acid-base titration differed from isoelectric points, which has also been previously observed, for example by Chibowski and co-workers for ice electrolyte interfaces. Finally, we found no evidence for surface contaminations of our PTFE particles before and after immersion in aqueous solutions. Only in the presence of NaCl-containing solutions did cryo-XPS detect oxygen from water. We believe that our low isoelectric points for PTFE were not due to impurities. Moreover, the measured buffering at pH 3 could not be explained by sub-micromolar concentrations of contaminants. The most comprehensive explanation for the various sets of data is that hydroxide ion accumulation occurred at the interfaces between inert surfaces and aqueous solutions

Site-Specific Retention of Colloids at Rough Rock Surfaces

The spatial deposition of polystyrene latex colloids (<i>d</i> = 1 μm) at rough mineral and rock surfaces was investigated quantitatively as a function of Eu­(III) concentration. Granodiorite samples from Grimsel test site (GTS), Switzerland, were used as collector surfaces for sorption experiments. At a scan area of 300 × 300 μm², the surface roughness (rms roughness, <i>Rq</i>) range was 100–2000 nm, including roughness contribution from asperities of several tens of nanometers in height to the sample topography. Although, an increase in both roughness and [Eu­(III)] resulted in enhanced colloid deposition on granodiorite surfaces, surface roughness governs colloid deposition mainly at low Eu­(III) concentrations (≤5 × 10^–7 M). Highest deposition efficiency on granodiorite has been found at walls of intergranular pores at surface sections with roughness <i>Rq</i> = 500–2000 nm. An about 2 orders of magnitude lower colloid deposition has been observed at granodiorite sections with low surface roughness (<i>Rq</i> < 500 nm), such as large and smooth feldspar or quartz crystal surface sections as well as intragranular pores. The site-specific deposition of colloids at intergranular pores is induced by small scale protrusions (mean height = 0.5 ± 0.3 μm). These protrusions diminish locally the overall DLVO interaction energy at the interface. The protrusions prevent further rolling over the surface by increasing the hydrodynamic drag required for detachment. Moreover, colloid sorption is favored at surface sections with high density of small protrusions (density (<i>D</i>) = 2.6 ± 0.55 μm^–1, asperity diameter (ϕ) = 0.6 ± 0.2 μm, height (<i>h</i>) = 0.4 ± 0.1 μm) in contrast to surface sections with larger asperities and lower asperity density (<i>D</i> = 1.2 ± 0.6 μm^–1, ϕ = 1.4 ± 0.4 μm, <i>h</i> = 0.6 ± 0.2 μm). The study elucidates the importance to include surface roughness parameters into predictive colloid-borne contaminant migration calculations

Sorption and redox speciation of plutonium at the illite surface under highly saline conditions

International audienceNatural groundwater may contain high salt concentrations, such as those occurring at several potential deep geological nuclear waste repository sites. Actinide sorption to clays (e.g. illite) under saline conditions has, however, been rarely studied. Furthermore, both illite surface and ionic strength may affect redox speciation of actinides like plutonium. In the present study, Pu sorption to illite is investigated under anaerobic conditions for 3 6, overall Pu uptake is largely insensitive to m(Nacl) due to the prevalence of strongly adsorbed Pu(IV). By applying appropriate corrections to the activity coefficients of dissolved ions and using the 2-site protolysis non-electrostatic surface complexation and cation exchange (2 SPNE SC/CE) model, experimental data on Pu sorption to illite as a function of pH, Eh and m(Nacl) can be very well reproduced. (C) 2016 Elsevier Inc. All rights reserved