No abstract

The link between buffer capacity and the ability of clays to retain Pb in interactions between Pb contaminant leachate and the clays, has been studied, using a natural clay from Quebec and laboratoryprepared clays (kaolinite, illite, and smectite). The retention of Pb in the clay suspensions as they received increasing amounts of acid was investigated, and the results examined in terms of the buffer capacity of the clays. As the clays receive increasing amounts of acid (e.g. akin to addition of acid leachate), high amounts of Pb can be retained if the buffer capacity prevents the pH from dropping to values where precipitation mechanisms are not operative. The high carbonate content in the illite gave it a higher buffer capacity than the smectite and natural clay, and permitted it to retain high amounts of Pb. High Pb uptake by the clays can affect the buffer capacity and the Pb retention capacity.

A montmorillonite suspension was placed in a miniature compression chamber bounded on one side by a cellulose membrane supported by a water saturated porous Alundum disk, and on the other by a rubber film. Changing the ambient pressure on the film caused water to move across the membrane until the swelling pressure of the suspension equalled the applied pressure. Existing theory, based upon the Gouy‐Chapman description of the diffuse double layer and upon its anticipated osmotic activity, was used to predict swelling pressures for different suspensions. During initial compression, measured swelling pressures of both Na‐ and Ca‐montmorillonite exceeded predicted values. Subsequent decompression and recompression cycles were reversible except for a slight but measurable hysteresis, but the position of the reversible portion of the curve depended upon the terminal pressure of the initial compression. When the latter was large (25 to 50 atm.), measured swelling pressures of decompression‐recompression cycles for Na‐montmorillonite agreed quantitatively with theory over the range 0.2 to 50 atm. Under similar conditions, Ca‐montmorillonite exhibited pressures appreciably less than predicted by theory.

Pressures exerted against a confining restraint by Na-montmorillonite in equilibrium with NaCl solutions were measured. The swelling pressures decreased with increasing NaCl concentration as predicted theoretically, but the rate of decrease was lower than calculated so that the ratio of observed to calculated pressure increased with increasing salt concentration.These larger swelling pressures could result from a higher activity of the ions between interacting clay particles compared with the ions in the neutral salt of the dialysis solution. It is then not necessary to postulate a non-osmotic force of repulsion.