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

Abstract

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.