Mechanism of NH3 Adsorption by Montmorillonite and Kaolinite*

D. W. James and M. E. Harward
Oregon State University, Corvallis, Oregon
* Technical paper no. 1604, Oregon Agricultural Experiment Station, Corvallis, Oregon. This paper includes portions of a thesis submitted by the senior author in partial fulfillment of the requirements for the Doctor of Philosophy degree at Oregon State University. Appreciation is expressed to J. L. Young U.S.D.A., A.R.S., Corvallis, for helpful discussions and exchange of ideas during the conduct of these studies.
Formerly Instructor, Oregon State University, now Assistant Soil Scientist, Washington State University, Irrigation Experiment Station, Prosser, Washington, and Associate Professor of Soils, Oregon State University, Corvallis, Oregon, respectively.

Abstract: Dry, homoionic systems of two montmorillonites and a kaolinite were treated with NH3 and then evacuated exhaustively to eliminate any physically adsorbed ammonia. It was concluded that the NH3 retained by the minerals was associated with the exchangeable cations. The mechanism of this adsorption process is the same as that which leads to the formation of inorganic ammines, namely, the mutual attraction between unshared electrons in the NH3 molecules and ionic centers of positive charge. This sorption process is reversible at elevated temperatures and is described by the equation, nNH3 + CX ⇄ (nNH3 · C)X, where n is the number of NH3 molecules associated with each exchangeable cation, C, on exchange complex X. Accordingly, n is a coordination number and has the following values for the cations indicated: NH4, 0; K, 0; H, 1; Ca, 2; Mg, 2; Alin, 3; Alex, 6. Here in and ex refer to interior and exterior exchange sites respectively. The retention of NH3 by cations which saturate the exchange complex of clays was shown to be analogous to the formation of ammoniates by reaction of NH3 with inorganic salts.

A reduced retention of NH3 by the Mg and Al 1:1 systems was ascribed to the possibility that exchangeable Mg and Al on kaolinite occur as hydroxy complexes.

A significant amount of NH3 was adsorbed to the weak acid hydroxyl groups on the broken clay edges, Relative to exchange capacities this phenomenon was greater in magnitude on the 1:1 than on the 2:1 clays.

Energies of NH3 adsorption on CaCl2, Ca-bentonite and NH4-bentonite were determined. Values of ΔH as a function of temperature were shown to be different for the Ca and NH4-clay systems. The data indicated stronger energies of retention of NH3 by the Ca-clay than for the NH44 clay. The thermodynamic data thus support the mechanisms proposed here.

Clays and Clay Minerals; 1962 v. 11; no. 1; p. 301-320; DOI: 10.1346/CCMN.1962.0110131
© 1962, The Clay Minerals Society
Clay Minerals Society (www.clays.org)