Preparation of mesoporous alumina and in situ ATR-FTIR investigation of phosphate at the alumina-water interface

Abstract: The aim of this study was to synthesize aluminum oxide with high surface
area and then investigate phosphate adsorption on the aluminum oxide
surface. The method we used to probe phosphate surface complexes at
aluminum oxide phase was in situ Attenuated Total Reflection Fourier
Transform Infrared (FTIR) spectroscopy, which is very sensitive to the
coordination environment and protonation state of phosphate complexes, and
therefore a useful tool for characterizing phosphate surface species at the
molecular scale.

Mesoporous alumina material had been synthesized using PEG as structure
directing reagent and dodecylamine as co-template. XRD, SEM, N2
adsorption/desorption technique and zeta potential charge measurement were
used to characterize the general properties of the synthesized material.
The results showed that the aluminum oxide material is £f-Al2O3 with a high
surface area viz. 329 m2g-1. The deposited alumina on the crystal surface
was a film assembled with small alumina particles about 20-30 nm and the
thickness of the film was around 3-4 ƒÝm. The zeta potential was affected
by
electrolyte concentration and the adsorbed phosphate on the surface of the
alumina film could significantly influence the charge as well as the PZC
value and make PZC to occur at pH values less than that without phosphate.
Phosphate adsorption on alumina surface was investigated by in situ ATR-
FTIR spectroscopy. Spectroscopic studies on these systems have shown that
phosphate forms inner-sphere complexes at the solid¡Vliquid interface. In
higher pH value (9.0) only one complex exist in the water-alumina
interface, however, a mixture of different phosphate complexes formed at
lower pH value (4.05 and 4.96). The adsorption isotherms and kinetics at
acid and base environment were also studied. The results show that the
amount of phosphate adsorbed on aluminum oxide increased with increasing
concentration but the increase gradually became smaller at higher
concentrations. The adsorption data were evaluated with the Langmuir and
Freundlich isotherm models. It was indicated that the adsorption data fits
the Langmuir isotherm better than the Freundlich isotherm at lower [PO4]
concentration, however, it fits the Freundlich isotherm better than the
Langmuir isotherm at higher [PO4] concentration both at pH 9.0 and 4.15.
Adsorption kinetics show that both adsorption at 9.0 and 4.96 have similar
characteristics except at 5ƒÝM, with a fast adsorption between t = 0 and 10
min, and a slower adsorption at longer times. Finally, seven possible
molecular symmetries are proposed for this Al-P complex with C2v or lower
symmetry.