Kinetics and Thermodyanamic of Sorption of Chromate by HDTMA–Exchanged Zeolite

Zeolites are hydrated crystalline aluminosilicate containing exchangeable alkaline and alkaline earth cations in their structural frameworks. Since zeolite 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 ERIONITE (E), COWLESITE (C), WILLHENDERSONITE (W) were performed by exchange of naturally occurring cations with HDTMA-Br ion. The exchanged zeolites were used to sorbs chromate from aqueous solution. Among parameters investigated were effect of pH, chromate initial concentrations, contact time and temperature. The maximum exchange capacity of HDTMA exchanged zeolite was obtained when using solution with initial pH of 5. Zeolite (C) gives highest HDTMA exchanged capacity compared to other zeolites. The results showed that Cr(VI) sorption by surfactant modified zeolites occurred at about pH 5 and increased as pH increased and reaching maximum at equilibrium pH increased and reaching maximum at equilibrium pH about 7. On the other hand, almost no chromate sorption occurred unexchanged zeolites. This indicates that HDTMA ion exchanged zeolites is taking part on Cr (VI) sorption via surface precipitation. The results also showed that the sorption capacities of 0.41, 0.21, and 0.19 mmol/g at 25^oC for HDTMA – exchanged zeolite (ERI- SMZ) HDTMA-E, HDTMA – exchanged zeolite(COW-SMZ) HDTMA–C and HDTMA – exchanged zeolite (WILL- SMZ) HDTMA-W .The amounts of sorption of Cr(VI) by HDTMA exchanged zeolite increased as temperature increased from 15^oC to 35^oC indicating that the process is endothermic. The free energy change (ΔG^o) for the sorption at 25^oC were -1.01, -24.06 and -24.56 KJ/mol for HDTMA- E, HDTMA-C and HDTMA-W, respectively. The negative values of (ΔG^o) meant that the sorption of Cr(VI) ion on HDTMA–exchanged zeolite was spontaneous perhaps because the HDTMA-Br had high affinity towards the chromium ion as indicated by a low Ksp value of HDTMA chromate. A slightly positive entropy change for sorption of Cr (VI) ion on HDTMA– exchanged Zeolite could be due to fixation of the ions on the HDTMA exchanged sites that was randomly distributed on the sorbents. The kinetic study showed that the Cr (VI) sorption followed first–order kinetic constants for the sorption are 11.7 X 10-3, 56.9 X 10-3 and 8.1 X 10-3min-1 for HDTMA-E, HDTMA-C, and HDTMA- W respectively.