Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions
Introduction
Clay minerals are distinguished from other colloidal materials by the highly anisometric and often irregular particle shape, the broad particle size distribution, the flexibility of the layers, the different types of charges (permanent charges on the faces, pH-dependent charges at the edges), the heterogeneity of the layer charges, see below), the pronounced cation exchange capacity, the disarticulation (in case of smectites) (Fig. 1) and the different modes of aggregation [1], [2], [3], [4], [5].
Montmorillonites (from Montmorillon, a town in the Poitou area, France) are the most abundant minerals within the smectite group of 2:1 clay minerals. They are the determinative components in bentonites. Montmorillonite particles may be as large as 2 μm and small as 0.1 μm in diameter with average sizes of ∼0.5 μm. The particles are of irregular shape. They can be compact but, mostly, they are foliated and look like paper sheets torn into smaller pieces. The particles are never true crystals but are more like assemblages of silicate layers. These assemblages a few silicate layers contain coherent domains of equally spaced silicate layers. The domains are separated by zones of silicate layers in different distances. The contour lines of the particles are of irregular shape, the edges are frayed and layers or thin lamellae of a few layers protrude out of the packets and enclose wedge-shaped pores [3], [6], [7], [8], [9], [10], [11].
The permanent charges of the silicate layers result from isomorphous substitutions. However, the degree of substitution changes from layer to layer within certain limits so that the interlayer cation density also varies from interlayer space to interlayer space and may also vary in directions parallel to the layers (heterogeneous charge distribution). The distribution of the interlayer cation density can easily be determined by the alkylammonium method [12], [13]. The average layer charge of montmorillonites varies between 0.2 and 0.4 eq/formula unit (Si, Al)4O10 but most montmorillonites have layer charges around 0.3 eq/formula unit. This charge density corresponds to a surface charge of 0.10 Cm−2[3], [4]. On the basis of the high layer charge densities one calculates high surface potentials for the isolated particles, e.g. ∼200 mV for a salt concentration of 10−3 mol/l and a surface charge density of 0.10 Cm−2. Thus, high critical salt concentrations for face/face coagulation are expected [14].
The edges of the layers show aluminol and silanol groups and are positively charged at low pH and negatively charged at higher pH. The position of the point of zero charge of the edges is still uncertain but there are several colloid chemical arguments that it must be near pH∼5 for montmorillonites [3], [15], [16]. Keren and Sparks [17] by potentiometric titration experiments found the p.z.c. of the edges of pyrophyllite (no permanent charges!) at pH 4.2.
An outstanding property of dispersed montmorillonite particles is delamination into the single silicate layers or thin packets of them when the counterions are alkali cations, preferentially lithium and sodium, and the salt concentration is sufficiently small (approx. <0.2 mol/l for sodium ions) (Fig. 1) [3], [4], [18], [19], [20], [21], [22], [23].
Approximately 10 million tons of bentonites are used per year in innumerable applications [3], [4], [24]. Important uses are listed in Table 1. The colloidal state of the montmorillonite particles in the bentonite dispersion is decisive in many practical applications. Therefore, studies on the coagulation of montmorillonite dispersions were started many decades ago [25], [26], [27], [28], [29], [30], [31], [32], [33].
The coagulation of montmorillonite dispersions can be described by the DLVO theory, see for instance [1], [17], [30], [34], [35], [36], [37], [38], [39], [40]. A different and, for a colloid scientist, strange opinion was put forward by Low and co-workers, see for instance [41], [42], [43]. The electrostatic interactions and the hydration of the counterions (interlayer cations) were considered being of minor importance, and non-specific interactions with the clay mineral surface (whatever this term means in these papers) is seen as the most important cause of swelling. Indeed, the structure of water in the interlayer space differs from bulk water [36], [44], [45], [46], [47], [48], [49] and is a decisive factor, but, as discussed by Delville and Laszlo [36], it is the behavior of the water molecules in the interlamellar electrostatic force field which drives the swelling. Recently, Quirk and Marcelja [39] proved that the Poisson–Boltzmann and the DLVO double layer theory satisfactorily described the swelling of Li+ montmorillonite in water and 10−4 to 1 M LiCl solutions at swelling pressures of 0.05–0.9 MPa. The DLVO theory with a 0.55-nm thick Stern layer indicated Stern potentials of −58 mV (1 M LiCl) to −224 mV (10−4 M LiCl) and a Gouy plane charge of 0.038 Cm−2 which is approximately 30% of the layer charge.
Section snippets
Materials and methods
Bentonite from Wyoming (Greenbond, M40 and M40A) and from Bavaria (Süd–Chemie Co., M 47) were purified by removal of iron oxides (reduction by dithionite and complexing Fe2+ by citrate) and humic materials (oxidation with H2O2) [50]. The <2-μm fraction of the sodium montmorillonite was then separated by sedimentation. The dialysed samples were freeze-dried. Usually, the freeze-dried samples were redispersed in water by intense shaking and ultrasound dispersion. The pH of the dispersions was
Coagulation with sodium, calcium and aluminium salts
The cK value of sodium, calcium and aluminium chloride was 5, 0.4 and 0.08 mmol/l for 0.025% sodium montmorillonite dispersions. Other montmorillonites showed similar cK values of approximately 5–10 mmol/l sodium chloride. Even the more highly charged beidellite (Unterrupsroth, Germany, sample B 18/4, mean layer charge of 0.39 eq/unit) coagulated at ∼6 mmol/l. The cK value increased with the solid content. The 0.5% dispersions of sodium montmorillonite (Wyoming) were coagulated by 20 mmol/l
Coagulation by inorganic salts
The critical coagulation concentration of 5–10 mmol/l sodium counterions for sodium montmorillonite dispersions is extremely low, compared with the usual values between 25 and 500 mmol/l [5], [14], [56], [57]. Decades ago, this observation was explained by the interaction of positive edge charges with negative surface charges producing T-type contacts and aggregation in cardhouse type arrangements [1], [58], [59], [60]. However, pH≈6.5 is near or, more likely, above the p.z.c. of the edges i.e.
Conclusion
Coagulation and flocculation of sodium montmorillonite dispersions (soda activated bentonites) reveal a diversity of destabilization processes. The anisometric shape and charge distribution of the montmorillonite particles cause very low critical coagulation concentrations of inorganic salts. Coagulation occurs between edges (−) and faces (−). The influence of the counterion valence corresponds to the DLVO theory. Adsorption of multivalent anions, especially of several phosphates increases the
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