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Bentonite Instability and Its Influence on Activation Energy Measurements¹

ABSTRACT

A time-dependent temperature hysteresis in a-c conductance activation energy plots for dilute, neutral, and salt-free Na⁺-bentonite suspensions is reported. Fresh suspensions prepared by washing with NaOAc or NaCl had activation energies of 9,398 and 5,789 cal/mole initially which gradually decreased to 3,651 and 3,422 cal/mole, respectively, after several days.

A-c conductances, concentrations of free Na⁺ in solution, pH, and dissolved silica were monitored in suspensions aged at 30C. Conductance increases of 2–3 fold were observed, with the concentration of free Na⁺ released from the exchange sites increasing to 5.4 and 34 ppm, respectively, for the NaCl and NaOAc preparations. Dissolved silica in solution increased to 3–4 ppm over the same period indicating that degradation of the lattice had occurred. Results are interpreted in terms of the hydrolysis of the exchangeable Na⁺ ions as predicted by Donnan and diffuse double layer theory followed by an acid attack of the lattice. Free salts added to the suspensions were effective in suppressing the hydrolysis and stabilizing the activation energies.

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana 61801. Supported in part by funds from the Ill. Ag. Ex. Sta.

² Graduate Research Assistant and Professor of Soil Chemistry, respectively.

Received for publication February 12, 1971. Accepted for publication May 4, 1971.