Title of Thesis
Effect of Heat Treatment on the Sorption
Properties of Al (III) Phosphate
National Centre for Excellence in Physical
Chemistry / University of Peshawar, Peshawar
|Number of Pages|
|Keywords (Extracted from title, table of contents and
abstract of thesis)|
ZrP = Zirconium phosphate, TiP= Titanium phosphate, sorption, ion exchange, activation, kinetics, dissolution,
r = amount sorbed, Cc = equilibrium concentration, PZC = point of zero charge
The Aluminum (III) phosphate samples were synthesized and were treated at 105 and 4000C respectively for 24 hours. XRD, FTIR, aluminum to phosphate ratio, point of zero charge (PZC) and hydrolytic dissolutions were carried out for the characterization of the non-activated and activated AIP04. XRD showed the AIPO4 to be amorphous before and after heat treatment, FTIR showed a deduction in framework water molecules with activation, while the PZC was observed to rise with activation. However, the hydrolytic dissolution was almost equal for both non-activated and activated
Potentiometric titrations were performed for both the adsorbents in the presence of different concentrations of metal cations at 293 to 323 K, which showed a decrease in the ion exchange capacity of Al (111) phosphate
with heat treatment. pKa values calculated from Gaines-Thomas plot were found to be greater in case of activated
AIPO4 showing that the activated AIPO4 is more weakly acidic type than the non-activated AIPO4. Further the pKa values were observed to decrease with concentration for both the
forms of AIPO4.
Sorption/ desorption studies showed the ion exchange mechanism in case of non-activated
AIPO4 while non-specific sorption mechanism i.e., sorption inside the pores was proposed in case of activated
AIPO4. Both sorption and desorption capacities were larger in case of activated
AIPO4. The selectivity observed in case of both the non-activated and activated AIPO4 was found to be in the order, Pb2+> CU2+> Zn2+> Cd2+> CO2+ which
was in the order of the hydrolysis constants (pK) of the metal cations in aqueous system. Hydrolytic dissolution in the presence of metal cations showed an increase in hydrolytic stability with heat treatment. The dissolution was also noted in both the cases to increase with temperature while with pH the dissolution of the activated solid increased from pH 3 to 5 and decreased above pH 5. However, in non-activated case the dissolution was found to decrease with pH. Further, the sorption equilibrium was established earlier in case of non-activated as compared to the activated form of the AIPO4. The rate constants calculated from the first order Lagregren's plots, were observed to decrease for both the solid while activation energy, enthalpy, entropy and free energy of activation were observed to increase with activation.
A new equation was developed from the proposed ion exchange mechanism in case of non-activated AIPO4 and was applied successfully. The stoichiometry of surface -hydrogen ions release to metal ions uptake was observed to lie between 1 to 2. The constant pK (-log K) values calculated from the intercept of the new equation increased with pH and decreased with temperature of the system and lied in the increasing order for the metal cation, Pb2+> Cu2+> Zn2+> Cd2+> Co2'.
⌂Go, ⌂Ho and ⌂So were calculated from the constants of new equation and were found in the range cited in the literature for ion exchange. In contrast to the new equation the Freundlich equation was found applicable both in case of non-activated and activated AIPO4. The Freundlich constant K values increased with heat treatment and were found to be in the same increasing order as observed
in case of sorption for the solids. The ⌂Ho, ⌂So and ⌂Go values were positive and were observed to decrease with pH before and after heat treatment of AIPO4. However, these values decreased with heat treatment of
AIPO4. The isosteric heats of sorption decreased with increase in sorption for both
non-activated and activated AIPO4. The DH values were also larger for all the metal ions sorption on activated AIPO4 in comparison to the non-activated AIPO4.
FTIR spectrometry was employed for the confirmation of the proposed mechanisms, which also supported the conclusions about the proposed mechanism for non-activated and activated AIPO4. The FTIR studies showed no sign of precipitation, co-precipitation of the corresponding metal phosphates and changes in lattice structure of non-activated and activated
AIPO4 except a decrease in lattice water molecules with heat treatment.