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

► Initial NH4+ concentration, solution Na+ concentration, and zeolite particle size, influenced the removal of NH4+ ion by natural Iranian zeolite. ► The NH4+ ion-exchange capacity of natural zeolite increased with decreasing particle size and Na+ concentration. ► The NH4+ ion exchange data was fitted to the Freundlich, Langmuir, Redlich-Peterson, and Langmuir-Freundlich models. ► An increase in ionic strength resulted in greater amounts of NH4+ being released by Iranian zeolite. At all ionic strengths the release kinetics were best fitted to Elovich model.