Sorption of NH3 on Montmorillonitic Clay

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Abstract
Various dry minerals were permitted to react with anhydrous ammonia under controlled conditions. When the pressures of ammonia prevailing in the sorbing system were plotted against the quantity of ammonia sorbed, typically parabolic or hyperbolic sorption isotherms were obtained with ammonium bentonite, potassium bentonite, and finely ground hydrogen bentonite. Unground hydrogen bentonite gave a complex ammonia sorption curve, in which there was a short parabolic section initially, followed by an inflection and finally a second parabolic or hyperbolic sorption section. Concurrent x‐ray studies indicated that the inflection and subsequent portion of the sorption curve were accompanied by widening of the c axis spacing. The sorption and concomitant x‐ray data permitted the view that the sorption of ammonia by unground hydrogen montmorillonite occurred on exterior planar surfaces, interplanar surfaces, broken bonds, corners, and edges. Data on the thermal decomposition of ammonium bentonite were consistent with this view. It was found that sorbed ammonia occupied base exchange reactive spots; physical trapping of ammonia molecules in the interplanes would not account for the lattice expansion. The data also appeared consistent with the theory that both lattice hydroxyl ions and polarization of water molecules may give rise to base exchange capacity on grinding some clay minerals.