Equilibrium and kinetic study of ammonium adsorption and fixation in sodium-treated vermiculite

Ammonium fixation in vermiculite affects the movement of N in many soils, The effects of particle size, solution concentration, pH, and associated anions on NH4+ fixation in vermiculite are also important information for reducing N leaching from soils, In this study, the retention of NH4+ on the exchangeable and nonexchangeable sites of Montana vermiculite was determined in batch experiments, In the NH4+-K+ exchange isotherm, the exchangeable sites of the vermiculite exhibited a preference for K+ to NH4+, while the nonexchangeable sites preferred NH4+ to K+. The nonexchangeable sites of the sand fraction had a higher preference for NH4+ at lower solution NH4+ concentration and a lower preference at higher NH4+ concentration, An opposite case was observed for the NH4+ concentration effect on the preference of nonexchangeable sites in the clay fraction. The retention isotherm of total NH4+ 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 NH4+, but the silt fraction fired the most NH4+ on its nonexchangeable sites. The retention of NH4+ in vermiculite increased with solution NH4+ concentration. Ammonium adsorption on the exchangeable sites increased at low solution pH, while NH4+ fixation was unaffected by pH change. The effect of associated anions was insignificant, except they caused a pH difference in solution.