Kinetics and thermodynamic for sorption of arsenate by lanthanum-exchanged zeolite

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 ZSM5) were performed by exchange of naturally occurring cations with lanthanum ion that forms low solubility 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 ZSM5 (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 (DG°) 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 DG° 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.

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