It was necessary to investigate the effect of contact time on
ammonium nitrogen adsorption; therefore, adsorption equilibrium
time for the whole experiment could be determined. Fig. 2 shows
the ammonium uptake by the synthesized ceramic adsorbent with
contact times from 0 to 24 h. The initial concentration of the
ammonium nitrogen was 10,000 mg L1, and the adsorbent dosage
was 20 g L1.
The results revealed that the removal of ammonium by the
ceramic adsorbent was a very rapid process within the first
30 min, and the ammonium uptake gradually decreased during
the following 450 min. The adsorption capacity was almost unchanged
from 480 min to 1440 min. Therefore, 480 min after initial
reaction, the adsorption of ammonium nitrogen reached equilibrium.
During the first 30 min, the most readily available adsorption
site and high concentration gradient might lead to the rapid
ammonium adsorption, and this explanation has also proposed
by some other studies [16,19]. Since the ammonium adsorption
was a very fast process, the equilibrium time reported by many
other researches was within 60 min [22–24], while the adsorption
equilibrium time was 480 min in present study. The longer equilibrium
time could be ascribed to the very high concentration of the
initial ammonium nitrogen taking a long diffusion time to achieve
complete equilibrium. In addition, Karadag [25] proved that for
lower initial concentration of the NHþ4
, the equilibrium time was
lower than initial higher concentrations because of increased competition
for the active sites with increasing NHþ4
concentration.
Comparing their low initial concentration of ammonium nitrogen
[22–24], in the present study, the high ammonium nitrogen
concentration of 10,000 mg L1 was 100 times higher, but the
equilibrium time was only 8 times more than that of the lower
concentration. This indicated that the ceramic adsorbent had
effective performance on the ammonium nitrogen adsorption.
The time of 480 min was chosen to ensure equilibrium adsorption
in the following experiments.