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Characteristics of Ammonia Adsorption by Homo-Ionic Clays¹

ABSTRACT

Adsorption of ammonia by aluminum-saturated bentonite and kaolinite was described by Langmuir's adsorption equation. The inverse values of the slopes of the two straight line segments of each adsorption isotherm approximate the permanent charge and the total charge or cation exchange capacity at a particular pH (above 6), respectively.

Ammonia was presumably adsorbed as NH₄⁺ to the full extent of the permanent charge and in addition ammonia appeared to be chemisorbed at pH-dependent sites. Exchangeable Al³⁺ as well as hydroxy aluminum compounds may be precipitated as Al(OH)₂ provided that the concentration of ammonia is high enough.

For homo-ionic calcium, magnesium, sodium and potassium clays Langmuir's adsorption equation did not characterize chemisorption of ammonia from aqueous ammonia solutions satisfactorily. The S-shaped ammonia adsorption isotherms obtained do not necessarily support the theory of physical multilayer adsorption.

¹ Contribution from the Dep. of Agronomy, Research Div., Virginia Polytechnic Inst., Blacksburg, Va.

² Graduate Research Assistant, presently Chief Professional Officer in the Chemistry Section, Agricultural Research Inst., Glen, O.F.S. Republic of South Africa, and Associate Professor of Agronomy, respectively.

Received for publication May 10, 1966. Accepted for publication January 3, 1967.