Montmorillonite colloids: I. Characterization and stability of dispersions with different size fractions

Abstract

Bentonite is planned to be used as a technical barrier in the final storage of spent nuclear fuel and high level vitrified waste. In contact with ground water of low ionic strength, montmorillonite colloids may be released from the bentonite buffer and thereby enhance the transport of radionuclides (RNs) sorbed. In the present case, clay colloids represent aggregates of several clay mineral layers. It is of major importance to determine RN sorption properties for different sizes of montmorillonite aggregates, since size fractionation may occur during particle transport in natural media. In this study, a protocol for size fractionation of clay aggregates is developed, by sequential and direct centrifugation, in the presence and absence of organic matter. Seven colloidal fractions of different mean aggregate sizes are obtained ranging, when considering the mean equivalent hydrodynamic sphere diameter (ESD), from~960 nm down to~85 nm. Applying mathematical treatments (Jennings and Parslow, 1988) and approximating the clay aggregates to regular disc-shaped stacks of clay mineral sheets result in mean surface diameters varying from~1.5 μm down to~190 nm. All these colloidal fractions are characterized by XRD, IC and ICP-OES where they are found to have the same chemical composition. The number of edge sites (aluminol and silanol) is estimated (in mol/kg) for each colloidal fraction according to (Tournassat et al., 2003). It is calculated from the mean particle sizes obtained from AsFlFFF and PCS measurements, where the clay aggregates are approximated to regular disc-shaped stacks of clay mineral sheets. The estimated number of edge sites varies significantly for the different clay dispersions. In addition, stability studies using the various clay colloidal fractions are performed by the addition of NaCl, CaCl 2 or MgCl 2 , in the presence or absence of organic matter, where no difference in stability is found.

Bentonite Nanoparticle Stability and the Effect of Fulvic Acids: Experiments and Modelling

“…Taking the intersection at W = 1 as calcium critical coagulation concentration in presence of fulvic acids (Ca/FA-CCC) for pH 10, only a marginal increase can be detected, which is within the analytical uncertainty. This observation is in line with measurements of Norrfors et al [19], who could not determine a change in colloid stability by addition of GoHy-573FA to MX-80 bentonite colloids. However, at lower pH values, an increase in the Ca/FA-CCC to values ≤2 mmol L −1 could be determined, and therefore 2 mmol L −1 was taken as Ca/FA-CCC.…”

“…It is determined by means of the stability ratio W, which is defined as the fastest coagulation rate r fast of single colloid to doublet formation in the system of interest (purely diffusion-controlled aggregation) to the actual coagulation rate of doublet formation r actual [17,18]. A stability ratio is colloid-size-independent [19]. The critical coagulation concentration (CCC) refers to the [Me + ] or [Me 2+ ] concentration at the transition of fast, purely diffusion-controlled to slow coagulation.…”

Montmorillonite colloids: II. Colloidal size dependency on radionuclide adsorption

“…These parameters should therefore be included in reactive transport modelling. In a recent study on montmorillonite colloidal stability performed at various ionic strengths (Norrfors et al, 2015), no dependency on the montmorillonite size could be observed. Based on this study, hydrodynamic induced size fractionation and the potential formation of different colloid size fractions will not influence their stability under the same geochemical conditions but some colloids may be able to migrate further due to e.g.…”

“…Given the same density of all clay particles, the amount of edge sites are proportional to the aspect ratio. This model was further developed by the authors in a previous work (Norrfors et al, 2015) where montmorillonite colloids were successfully separated into fractions with varying mean colloid sizes. These colloids were used in the adsorption study presented in this work.…”

Crystalline Swelling Process of Mg-Exchanged Montmorillonite: Effect of External Environmental Solicitation

“…Indeed, it plays an important role in many geological processes, the phenomena of the petroleum migration, greenhouse gas sequestration, oilfield [3], and in engineering. Moreover, for last decades, the smectites were widely used as crucial components for elaboration of natural barriers to isolate the hazardous wastes and for the removal of heavy metal cations from various effluents of industrial and wastewater treatment [4][5][6][7][8][9][10][11][12] and also proposed as geotechnical barriers in many nuclear waste disposal concepts in order to retard the potential transport of radionuclides towards the biosphere [13][14][15].…”