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

Abstract

AbstractAmmonium 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.