Chemisorption of ammoniacal nitrogen by clays

A desorption technique was employed to evaluate chemisorption of ammonia by clays under moist conditions. In equilibria involving ammonia, all factors favoring volatilization losses of gaseous ammonia would undoubtedly exert an influence on the ultimate equilibria which are reached. Investigation of the relationship between pH and ammonia equilibria in soils revealed that an increase in the concentration of ammonium ions, applied as ammonium sulfate, stimulated ammonia losses from fine-textured soils at pH's ranging from 4.5 to 7.1. Ammonia volatilization was directly related to the initial pH of the soil and increased with an increase in pH. It was postulated that ammonia may be volatilized, even from acid soils, due to the equilibrium NH₄⁺ + OH⁻ ⇌ NH₃ + H₂O. By using initial concentrations of ammonium ions from ammonium sulfate applied, initial pH of an aqueous suspension of the soil and a value K_b = 1.80 x 10⁻⁵, values of ammonia losses were predicted. The linear relationship found between predicted and measured amounts of ammonia losses from acid as well as neutral soils, was interpreted as evidence of the existence of the proposed volatilization mechanism in soils.

In attempting to evaluate the characteristics of ammonia retention by homo-ionic bentonites and kaolinites under moist conditions, it was found that base-saturated clays did not chemisorb ammonia in a fashion which could be described by Langmuir's adsorption isotherms. The S-shaped curves did not necessarily reflect polymolecular sorption. In strong contrast, chemisorption of ammonia by Al-saturated clays, followed Langmuir's equation almost perfectly. Evaluation of the inverse values of the differential slopes of two straight line segments obtained in the desorption isotherms corresponded closely to values obtained for the permanent charge (CEC) and the total charge (total CEC) developed at a specific pH higher than 6 respectively. It was shown that if sufficient ammonia is applied, exchangeable Al⁺⁺⁺ of Al-compounds may be precipitated as Al(OH)₃. An attempt to evaluate pseudo-equilibrium constants for aluminum-clay ammonia equilibria by using Langmuir's adsorption equation, was unsatisfactory. Chemical kinetic studies indicated that ammonia volatilization reactions could be described by a fast reaction and a slow reaction. The slow reaction represented chemisorption of ammonia on the permanent charge exchange sites. Extrapolation of the slow reaction to zero time yielded values corresponding closely to the CEC permanent charge.

Investigation of the effects of CO₂, the NH₄⁺ concentration is increased, resulting in more effective replacement of basic cations for "permanent charge" exchange spots. The lower pH and higher effective NH₄⁺ concentration resulted in higher retention of NH₄⁺ in Na⁺- and K⁺-clays. The most striking retention of NH₄⁺ was obtained in Mg- and particularly in Ca-bentonites. It is believed that the Ca and Mg ions, exchanged by NH₄⁺ ions, are effectively removed from the soil solution as a result of the precipitation of these compounds which are sufficiently insoluble to ensure conservation of ammonia as chemisorbed ammonium ions.