Atomic force microscopy study of montmorillonite dissolution under highly alkaline conditions

Abstract

Montmorillonite dissolution under highly alkaline conditions (pH = 13.3; I = 0.3 M) was investigated by bulk dissolution methods and in situ atomic force microscopy (AFM). In bulk dissolution experiments, initial SiO2 concentrations were high, and a steady state was reached after 136 h. The dissolution rates derived from the edge surface area (ESA) at the steady-state condition at 30, 50 and 70°C were 3.39 x 10−12, 1.75 × 10−11 and 5.81 × 10−11 mol/m2 s, respectively. The AFM observations were conducted under three conditions: (Run I) short-term in situ batch dissolution at RT; (Run II) long-term in situ flow-through dissolution at RT; and (Run III) long-term batch dissolution at 50°C. The observed reductions in montmorillonite particle volume for Runs I and II were due primarily to edge-surface dissolution. The ESA-based dissolution rate for Run I (10−9 mol/m2 s) was three orders of magnitude faster than that for Run II (10−12 mol/m2 s). The rate obtained for Run II corresponded to the rate at the steady-state conditions in the bulk dissolution experiments. A small number of etch pits developed in Run III slightly increased the ESA of montmorillonite since most of the montmorillonite particles were separated into monolayers lacking three-dimensional periodicity. The ESA-based dissolution rate for Run III was 2.26 × 10−11 mol/m2 s. Dissolution rates based on long-term AFM observations could be directly compared with steady-state rates obtained from bulk dissolution experiments. The AFM observations indicated that dissolution occurred at edge surfaces; therefore, the ESA should be used to calculate the dissolution rate for montmorillonite under alkaline conditions. Dissolution rates of individual particles with different morphologies estimated by AFM were similar to rates estimated from bulk dissolution experiments.

Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25°C

“…However, we cannot separate this value into basal and edge surface area contributions because the surface areas of dry samples consist of large aggregates that may not be representative of reactive area of clay particles dispersed in solution. Nevertheless, the morphology (shape, diameter, thickness) of single clay particles in our K-montmorillonite is comparable to the morphology of the smectites studied by Tournassat et al (2003), Metz et al (2005b) and Yokoyama et al (2005). Therefore, it is reasonable to use the average ESA of 6.5 m 2 g À1 estimated in these studies to roughly approximate the ESA of our sample.…”

“…Instead, they suggested using AFM measurements of the specific edge surface area (ESA) as a proxy for the reactive surface area of smectite (5.5 m 2 g À1 , SAz-1). Yokoyama et al (2005) determined the ESA of Kunipa smectite by AFM on single plates, obtaining 5.4 m 2 g À1 . Tournassat et al (2003) measured lateral surface area of smectites by gas adsorption and AFM (8.5 m 2 g À1 , MX-80 Wyoming bentonite) and stated that edge surface area does not depend on the number of smectite platelet stacked in a particle, in dry state or in aqueous suspension, as the stacking does not modify the edge surface.…”

“…Sample degassing may produce different results and there is no correlation between reactive surface area in smectites and BET surface area (Tournassat et al, 2003;Metz et al, 2005b). The direct observation of the dissolution reaction on the crystal edges on phyllosilicates (Bosbach et al, 2000;Bickmore et al, 2001;Yokoyama et al, 2005;Kuwahara, 2006) highlights the importance of normalizing dissolution rates to edge surface area, determined by AFM (Tournassat et al, 2003;Metz et al, 2005b;Yokoyama et al, 2005). Alternatively, mass normalization-as shown in Fig.…”

Using a reactive transport model to elucidate differences between laboratory and field dissolution rates in regolith

“…The ability to successfully model mineral dissolution in soils will be important for predicting how variations in temperature and precipitation due to global climate change will affect soil chemistry into the future (e.g., Williams et al, 2003). Predictions of mineral dissolution rates are also necessary to understand numerous other societal issues including maintenance of water quality (e.g., Malmströ m et al, 2000), nuclear waste disposal (e.g., Yokoyama et al, 2005), and geologic carbon sequestration (e.g., Carroll and Knauss, 2005). However, researchers find that mineral dissolution rates calculated from field data, including rates calculated from aquifers, soil pore waters, soil chemistry, and stream chemistry, are two to five orders of magnitude slower than experimental mineral dissolution rates observed under far-from-equilibrium conditions (e.g., White et al, 1996;Oliva et al, 2003;White et al, 2005;Zhu, 2005;White et al, 2008).…”

A preliminary report of development of high resolution phase shift interferometry for the assessment of nuclear waster disposal

“…We are able to observe the calcite dissolution within a few minutes. The dissolution rates of smectite are about 4 orders of magnitude slower than those of calcite (Yokoyama et al, 2005), so we anticipate that a few days will be necessary to observe smectite dissolution.…”