The second stage of ion release occurred from day 8 onward,
where no SO4
2P ions were detected in the percolating
solutions and the molar ratio drops to zero. On the
Research article
4 Environmental Geology 34 (1) April 1998 7 Q Springer-Verlag
other hand, ammonium ions were continuously released
at an approximately constant rate. A similar trend was
observed by Garcia Hernandez and others (1993b) for potassium
ions, and by Garcia Hernadez and others (1994)
for ammonium ions.
The second stage of ammonium ions releases by acidic
deionized water is a cation-exchange reaction as follows:
NH4 c -philippsite cHcpHc-phillipsitecNH4 c
In the above exchange reaction the process supplies low
concentrations of ammonium ions to a minimum of 70
days. The total amount of occluded ammonium ions in
the Jordanian phillipsite tuff was 29.6 cmol/kg, however,
the leaching rate for the same ions was calculated to be
0.35 ymol/day. Considering the CEC of the phillipsite tuff
sample, and assuming a complete saturation of the exchange
sites by NH4 c , this ion would be released completely
by 635 days. Therefore, the Jordanian phillipsite
tuff, when saturated with ammonium ions, shows promising
properties as a slow-release fertilizer.