Titration Calorimetry as a Tool to Estimate Optimum Retention Time for Ammonium Removal by Zeolites.

Zeolite minerals have been extensively studied for the purpose of removing ammonium (NH₄⁺) from solution.  In the context of zeolites serving as ammonium filters (for example, waste water treatment), sorption kinetics are very important since the passing solution will only interact with the zeolite mineral for a finite time (i.e. retention or contact time).  The objectives of this study were to investigate the effect of retention time on NH₄⁺ sorption by zeolites in a flow-through setting and determine if isothermal titration calorimetry (ITC) could provide any useful information in regard to determining optimum retention time.  Flow-through cells possessing  contact times > 100 sec displayed greater NH₄⁺ sorption than batch systems, suggesting that maintaining high NH₄⁺ concentration in solution, removal of exchange products, and sufficient reaction time are critical to maximizing NH₄⁺ removal by zeolites.  Within 100 sec after NH₄⁺ addition, endothermic heat responses indicated that NH₄⁺ -K⁺ exchange had peaked and this was followed by significant heat rate reduction for 50 min.  This confirmed findings of an initial fast NH₄⁺ -K⁺ exchange followed by a slower one and suggests the 100 sec period of rapid reaction is an indicator of the minimum flow through retention time required to optimize NH₄⁺ sorption to zeolites used in this study.