Ion-exchange process for ammonium removal and release using natural Iranian zeolite

In this study, the ability to remove ammonium (NH4+) from aqueous solutions with different Na+ concentrations (0.03, 0.1, and 0.3 M) by natural Iranian zeolite in millimeter and nanometer particle sizes was determined and the equilibrium isotherms were characterized. The effect of ionic strength on NH4+ release from Iranian zeolite was also evaluated and several kinetic models were tested. It was demonstrated that the initial NH4+ and Na+ concentrations had significant effect on the amount of NH4+ exchanged. A decrease in Na+ concentration resulted in greater amounts of NH4+ being exchanged and therefore resulted in greater removal efficiency. In addition, three parameter isotherm models (Redlich-Peterson and Langmuir-Freundlich) proved a better fit in terms of the coefficient of determination (R-2) and standard error of estimate (SEE) than two-parameter isotherm models (Langmuir and Freundlich), with the Langmuir model providing a better description of ion-exchange processes than the Freundlich model. NH4+ release was rapid for the first 60 min, but slowed thereafter, and also displayed a concomitant increase with the ionic strength of the solution. The NH4+ release data were successfully described by Elovich, power function, and pseudo second-order models, while the best fit was generally found between the experimental data and the Elovich model with R-2 and SEE ranging from 0.866 to 0.958 and 0.043 to 0.194, respectively. Based on the results, it can be suggested that the natural Iranian zeolite is a suitable ion-exchanger for NH4+ ion removal and consequently has potential as a controlled-release NH4+ fertilizer. (c) 2010 Elsevier B.V. All rights reserved.