KINETICS OF AMMONIUM ADSORPTION AND DESORPTION BY THE NATURAL ZEOLITECLINOPTILOLITE

The kinetics of NH4+ adsorption and desorption were investigated on th e natural zeolite clinoptilolite to ascertain its ability to adsorb an d release the important plant nutrient N in its NH4+ form at various p H values and initial NH4+ concentrations. Kinetics of NH4+ adsorption were evaluated on the samples using solutions containing 140.1, 280.2, 560.4, and 840.6 mg L-1 of NH4+-N at pH 4, 5, 6, and 7, equilibrated for 5, 10, 15, 20, 30, 45, 60, 75, 90, and 120 min. Samples for NH4+ d esorption were equilibrated with 70.1, 280.2, 560.4, and 1401 mg L-1 N H4+-N solution at pH 4, 5, 6, and 7 for 2.5 h, and adsorbed NH4+ extra cted with 2 M KCI for 5, 10, 20, 30, 45, 60, 90, 120, 150, 180, and 30 0 min. Equilibrium time for NH4+ adsorption ranged from 60 min for 140 .1 mg L(-)1 initial NH4+-N concentration at pH 4 to 120 min for 840.6 mg L-1 initial NH4+-N concentration at pH 7. Desorption was nearly com plete in 150 min for low initial NH4+ concentrations and 200 min for h igh initial NH4+ concentrations. Amounts of NH4+ sorbed increased with increasing pH and initial NH4+ concentrations. Models evaluated inclu ded the first-order kinetics, modified Freundlich, parabolic diffusion , Elovich, and heterogeneous diffusion. All the models adequately desc ribed the NH4+ adsorption process, with r(2) values ranging from 0.955 to 0.999. With the exception of first-order kinetics, they also descr ibed the desorption process well, with r(2) values ranging from 0.897 to 0.999, for all pH and initial NH4+ concentrations. Reaction rate co efficients (k) were calculated from the modified Freundlich model and ranged from 0.134 to 0.193 min(-1) for the adsorption process, and 0.1 29 to 0.226 min(-1) for the desorption process. The models indicated t hat NH4+ adsorption and desorption by the zeolite was diffusion contro lled. Data from this study indicated the potential use of the tested n atural zeolite as an NH4+ adsorbent and a controlled-release NH4+ fert ilizer.