Ion Exchange on Mixed Ionic Forms of Montmorillonite at High Ionic Strengths

Abstract

Adsorption equilibria of inorganic ions between water and minerals in formations control brine composition and, through these, affect many aspects of the chemistry of surfactant solutions. For example, alkaline earth ions influence phase and interfacial properties of aqueous-hydrocarbon systems containing surfactants. Precipitation of surfactants may occur in the presence of alkaline earth ions. Thus information concerning the distribution of alkaline earth and alkali metals between solutions and minerals is basic to the prediction of behavior of surfactant solutions in reservoirs. Because many clays have large cation exchange capacity, they may dominate adsorption properties of those formations in which they are present. Sodium and calcium are very common and representative of the ions under consideration. This paper summarizes studies of the distribution of these ions between a common clay, montmorillonite, as well as several other clays, and a series of solutions of constant total ionic strength (I) with varying ionic strength fraction of sodium. Distribution coefficients D for Na(I) and Ca(II) were determined by batch equilibrations using isotope dilutions with radioactive tracers. Equilibrium quotients (K/T) for the exchange of sodium and calcium were then calculated and the effects of solution composition, of solution phase activity coefficients, of ionic strength, of degree of purification, and of source of clay were investigated. Equilibrium quotients with adjustment for solution-phase activity coefficients did not vary greatly with I, except at low loading of sodium on the calcium form of montmorillonite, where D _Na became anomalously high. Values of K/T for illite and attapulgite were within an order of magnitude of those for montmorillonite.

Research sponsored by Fossil Fuel Extraction, the U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation.