EQUILIBRIUM AND KINETIC-STUDY OF AMMONIUM ADSORPTION AND FIXATION IN SODIUM-TREATED VERMICULITE

Ammonium fixation in vermiculite affects the movement of N in many soi ls, The effects of particle size, solution concentration, pH, and asso ciated anions on NH4+ fixation in vermiculite are also important infor mation for reducing N leaching from soils, In this study, the retentio n of NH4+ on the exchangeable and nonexchangeable sites of Montana ver miculite was determined in batch experiments, In the NH4+-K+ exchange isotherm, the exchangeable sites of the vermiculite exhibited a prefer ence for K+ to NH4+, while the nonexchangeable sites preferred NH4+ to K+. The nonexchangeable sites of the sand fraction had a higher prefe rence for NH4+ at lower solution NH4+ concentration and a lower prefer ence at higher NH4+ concentration, An opposite case was observed for t he NH4+ concentration effect on the preference of nonexchangeable site s in the clay fraction. The retention isotherm of total NH4+ in the ve rmiculite 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 si lt fraction fired the most NH4+ on its nonexchangeable sites. The rete ntion of NH4+ in vermiculite increased with solution NH4+ concentratio n. Ammonium adsorption on the exchangeable sites increased at low solu tion pH, while NH4+ fixation was unaffected by pH change. The effect o f associated anions was insignificant, except they caused a pH differe nce in solution.