Hi,
had some work to do and was unable to quickly react on this interesting discussion.
As a mineral ion-exchanger (clinoptilolithe as a cation exchanger) the zeolite simply binds cations on its surface. However, every particle is surrounded by a dead layer where particles approach the surface via diffusion. The thickness of this dead layer is largely determined by the current/water flow. That means, the zeolite doesn't catch cations, but the latter approach the adsorptive surface and are bound to the surface after contact. Thereby, the charged environment quickens this binding process.
To get back to the hobby: if one puts zeolite into a pot filter, the surface might get saturated with ammonia, which is kept in an equilibrium as sodium and potassium compete with ammonia. Although ammonia is preferentially adsorbed, the high sodium and potassium concentration might fully displace ammonia. That may happen. But this is hypothetically, as we do not know the binding constants for ammonia, sodium and potassium. It even occured that the calcium concentration in seawater was significantly lower after zeolite was applied to the system. That means, although calcium is not preferentially adsorbed, the high concentration of the salt solution has great impact on the adsorption characteristics of the zeolite.
Assuming that the fresh zeolite is saturated with ammonia, AOB can settle on the surface and utilize the ammonia to produce nitrite. But as the establishement of NOB populations characteristically show a lag phase (because they're ammonia-sensitive and toxified by high ammonia concentrations), these NOB would settle on the zeolite after AOB have already developed. Thus, they would potentially occur when almost all adsorbed ammonium is already oxidized to nitrite. This nitrite would then be released to the water and washed off the zeolite filter without being further oxidized to nitrate within the zeolite filter. But this would mean that the zeolite filter would as work as good as a wet/dry filter filled with bioballs and would strongly increase the nitrate concentration in the water.
So, I would suppose something else is happening.
When bacteria settle on a given surface they release strong organic glues to the surface to attach themselves. As these biofilms may break off the substrate, the glue would still stick to the surface and would clog the pores. Consequently, the ion-exchange capacities of the zeolite would strongly decrease with time or even would approach zero. However, if the zeolite grains scratch against each other due to the strong current in the filter, and rub off the surface which is thereby regularily removed to a degree, that ammonia could newly be adsorbed, and new AOB settle on the surface.
I know, some of you are really bored by all these theoretical approaches to the truth, while we're simply speculating about things which might happen or might not happen. This is off course non-scientific, however, as there's great knowledge within this forum, we might rule out specific issues and might have the chance to proof certain hypotheses by experiments in the future.
