The
Malaysian Journal of Analytical Sciences Vol 11 No 1 (2007): 219 – 228
KINETICS AND THERMODYNAMIC FOR SORPTION OF ARSENATE
BY LANTHANUM-EXCHANGED ZEOLITE
Md Jelas Haron*, Saiful Adli Masdan, Mohd Zobir
Hussein , Zulkarnain Zainal and Anuar
Kassim
Department of
Chemistry, Faculty of Science, Universiti Putra Malaysia,
43400 Serdang,
Selangor, Malaysia
*Corresponding
author: mdjelas@fsas.upm.edu.my
Abstract
Zeolites
are crystalline, hydrated aluminosilicate containing exchangeable alkaline and
alkaline earth cations in their structural frameworks. Since
zeolites have permanent negative
charges on their surfaces, they have no
affinity for anions. However recent
studies have shown that modification of zeolites with certain surfactants or
metal cations yield sorbents with a strong affinity for many anions. In
this paper, modification of zeolites (zeolite A, X and ZSM 5) were
performed by exchange of naturally occurring cations with lanthanum ion that
forms low solubilit y arsenate salt. The
exchanged zeolites were used to sorb arsenate from aqueous solution.
Among parameters investigated were effect of pH, arsenate initial
concentrations, contact time and temperature. The maximum exchanged capacity of
La(III) ion was obtained when using solution with initial pH of 4. Zeolite X gives the highest La(III)
exchanged capacity compared to other
zeolites . The results showed that
As(V) sorption by La-zeolites occurred
at about pH 6.5
and increased as pH increased and
reaching maximum at equilibrium pH about 7.8. On the other hand, almost no arsenate
sorption occurred on unexchanged zeolites. This indicates that
La(III) ion on the exchanged zeolites
is taking part on the As(V)
sorption via surface precipitation. The
results also showed
that the sorption capacities
increased with increasing initial
As(V) concentrations. The sorption followed Langmuir model with maximum
sorption capacities of 0.41, 0.21 and 0.19 mmol/g
at 25°C for La exchanged zeolite
X (La-ZX) , La exchanged
zeolite ZSM 5 (La -ZSM) and La exchanged
zeolite A (La-ZA), respectively. The amo unts
of sorption of
As(V) by La
exchanged zeolite increased as
temperature increased from 25 to 70°C indicating that the process is endothermic. The free energy changes ( ΔG ) for the sorption at 25 C were -10.25, -9.65 and -8.49 kJ/mol for La -ZX, La
-ZSM and La -ZA, respectively. The
negative values of ΔGo meant that the sorption of As(V) ions on La
-exchanged zeolite was spontaneous, perhaps because the La(III) had high
affinity towards the arsenic ion as indicated by a low Ksp value of
of lanthanum arsenate. A slightly
positive entropy change for sorption of As(V) ion on La–exchanged zeolite could
be due to fixation of the ions on the La(III) exchange sites that was randomly
distributed on the sorbents. The
kinetics study showed that the As(V) sorption followed first order kinetic
model. The first-order kinetic constants
for the sorption are 2.77x10-3, 2.25x10-3 and 1.60x10-3 min-1
for La -ZX, La -ZSM and La-ZA, respectively.
Keywords: La-exchanged
zeolite, arsenate sorption, thermodynamic and kinetics
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