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Equilibrium and Kinetic Study of Ammonium Adsorption and Fixation in Sodium-Treated Vermiculite

USDA-ARS, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038

W. Doral Kemper

USDA, ARS/NPS, Beltsville, MD 20520

* Corresponding author ( sshen{at}arserrc.gov).

ABSTRACT

Ammonium fixation in vermiculite affects the movement of N in many soils. The effects of particle size, solution concentration, pH, and associated anions on NH⁺₄ fixation in vermiculite are also important information for reducing N leaching from soils. In this study, the retention of NH⁺₄ on the exchangeable and nonexchangeable sites of Montana vermiculite was determined in batch experiments. In the NH⁺₄-K⁺ exchange isotherm, the exchangeable sites of the vermiculite exhibited a preference for K⁺ to NH⁺₄, while the nonexchangeable sites preferred NH⁺₄ to K⁺. The nonexchangeable sites of the sand fraction had a higher preference for NH⁺₄ at lower solution NH⁺₄ concentration and a lower preference at higher NH⁺₄ concentration. An opposite case was observed for the NH⁺₄ concentration effect on the preference of nonexchangeable sites in the clay fraction. The retention isotherm of total NH⁺₄ in the vermiculite exhibited S-shape curves that can be described by the "two-surface" Langmuir-Freundlich equation. In the kinetic study, the clay fraction adsorbed the largest amount of exchangeable NH⁺₄, but the silt fraction fixed the most NH⁺₄ on its nonexchangeable sites. The retention of NH⁺₄ in vermiculite increased with solution NH⁺₄ concentration. Ammonium adsorption on the exchangeable sites increased at low solution pH, while NH⁺₄ fixation was unaffected by pH change. The effect of associated anions was insignificant, except they caused a pH difference in solution.

Reference of brand or firm names does not constitute an endorsement by the U.S. Department of Agriculture over others of a similar nature not mentioned.

Received for publication July 3, 1996.

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