Kinetic Studies on Montmorillonites and Nontronite by the Acid-Dissolution Technique

Abstract

Several montmorillonites and a nontronite were digested in hydrochloric acid at constant temperature for various periods of time. Soluble iron, aluminum, magnesium, and in two samples titanimn, were determined for each digestion period. Rate of solution curves were obtained by plotting the logarithm of the percent of the residual ions against time. In general, the solution curves were straight lines or curves that could be resolved into two straight lines. From the interpretation of the dissolution curves, it was possible to determine the amount of the ions in octahedral and tetrahedral coordination. For some of the samples, the rate curves also gave the amount of iron and/or aluminum extraneous to the clay lattice. The amount of silicon in the clay lattice was calculated using the experimentally determined octa, hedral cations, substituted tetrahedral cations if any, and the exchangeable cations assuming 44 charges per unit cell. The distribution of cations in lattice layers indicates that the eight montmorillonites are substantially dioctahcdral. Two of the samples showed the presence of iron in tetrahedral coordination while all the samples showed extraneous silicon. The presence of extraneous silicon, iron, and aluminum in purified samples indicates that formulas derived from bulk chemical analyses can be inaccurate. The dissolution of the ions from the clay lattice determined at several different temperatures in one sample and at two acid concentrations in another sample is in agreement with chemical kinetic principles and is a first-order reaction. The rate constant for the acid dissolution reaction increased proportionately with the acid concentration. The activation energies were determined for the solution of iron and aluminum in montmorillonite from Polkville, Mississippi, and nontronite from Garfield, Washington, using Arrhenius' Law. In both samples the activation energies (17 to 18 kilocalories per mole) were the same. In the nontronite sample the activation energies were identical for what have been interpreted as octahedral and tetrahedral ions.

Dissolution of Smectites in Hydrochloric Acid: II. Dissolution Rate as a Function of Crystallochemical Composition

“…With regard to the greater ionic radius of trivalent iron and bivalent magnesium, when compared with aluminum, the octahedra of the layer are flattened, more distorted (Donnay et al, 1964) and the tension in the structure is increasing. This can support the more rapid release of central atoms from the structure despite the nearly constant apparent activation energies for the samples within the whole range of substitutions (Osthaus, 1956;Turner, 1964;Gregor et al, 1966).…”

“…This is a linear dependence until E remains constant and thus the value t05 may be read for the temperature of 96~ as it is shown in Figure 1 Turner (1964) using the analytical data of Osthaus (1954Osthaus ( , 1956 and Turner (1964).…”

“…The kinetics and mechanism of dissolution of smecrites in HCI have been described by various authors such as Brindley and Youell (1951), Kar~ulin and Stubi6an (1954), Osthaus (1954Osthaus ( , 1956), Packter (1955), Granquist and Gardner-Sumner (1959), Fahn (1963), Turner (1964), Ci6el et al (1965), and many others.…”

Dissolution of Hectorite in Inorganic Acids

“…The kinetics of acid reaction of clay minerals have been investigated by Osthaus (1956), 1216el and NovS.k (1977), Ci6el et al (1990, Luca and MacLachlan (1992) and Tk~6 et al (1994) for montmorillonites and nontronites: by Corma et al (1987Corma et al ( , 1990 for palygorskite; by Cetisli and Gedikbey (1990) for sepiolite; and by Gastuche and Fripiat (1962) for montmorillonite, glauconite, biotite and kaolinite. Nov~k and (~f~el (1978) found that the dissolution rate of dioctahedral smectites in 6 M HC1 increases with increasing octahedral Fe and Mg substitution.…”

Acid-Treated Montmorillonites—A Study by 29Si and 27Al MAS NMR

“…kinetic parameters (Q(6el & Novfik, 1977;Novfik & Ci6el, 1978;Corma et al, 1990;Cetisli & Gedikbey, 1990;Gtiler & Sarier, 1990), or characteristics of reactants and reaction products (t~f6el et al, 1965;Fahn, 1973;Garcia et al, 1989;Komadel et al, 1990). Osthaus (1956) proposed a method for calculation of structural formulae of smectites from their acid dissolution data. This method is based on the assumption that the rate of dissolution of tetrahedral cations is significantly lower than that of octahedral cations.…”