Geochemical modeling to constrain the parameters influencing ammonium exchange from NH4-rich solutions and natural zeolites mixtures

Natural zeolites are more and more frequently employed to remove excess ammonium in wastewater, since their capital cost is not excessive and their cation exchange capacity is elevated. Recently, natural zeolites have been also applied as fertilizer in orthoculture, after an exchange step within a ammonium rich solution. Despite of all this applications, many are the unknown parameters which can affect the cation exchange capacity of natural zeolites from an ammonium rich solution. For this reason a series of numerical scenarios have been performed to quantify possible side effects. The geochemical code PHREEQC-2, was used in batch mode to evaluate the variation of the equilibrium exchange between ammonium and other cations (present onto the exchanger site of natural zeolites) induced by selected master parameters. The master parameters which were varied in a range of two order of magnitude, are: temperature (-10 to 90 °C), ammonium concentration (2 to 200 mM/L), bulk cation exchange capacity of the matrix (1 to 100 meq/100g) and the composition of the exchanger sites. The latter two parameters are dependent from the type and amount of zeolites present in the original rock/sediment matrix. Results are important to understand what is the best parameters combination to enhance ammonium exchange onto natural zeolites and to provide optimal ammonium concentrations for agricultural purposes