Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 326

impregnated mesoporous materials showed fast adsorption kinetics as well as high
adsorption capacities, as compared with conventional AA. The uniform mesopores of
the Al-impregnated mesoporous materials increased the diffusion rate in the adsorption
process, which in turn caused the fast adsorption kinetics. High phosphate adsorption
capacities of the Al-impregnated mesoporous materials were attributed to not only an
increase in the surface hydroxyl density on Al oxide (due to well-dispersed impregnation
of Al components) but also a decrease in stoichiometry of surface hydroxyl ions to
phosphate by the formation of monodentate surface complexes (Shin et al., 2004). Danis
et al. (1998) studied alumina pillared montmorillonite (AIPMt) and mesoporous alumina
aluminum phosphates (AAPs) interaction with aqueous phenols (2,4-dichlorophenol,
2,4,6-trichlorophenol and pentachlorophenol) solutions, at a concentration range
between 25 and 250 g/L, in batch equilibrium experiments. They found that the
removal of chlorophenols by the adsorbents increased with an increase in chlorine
substitution in their molecules. In the case of pentachlorophenol, the increased affinity
allowed adsorption to occur much more efficiently than in the case of other compounds.
AIPMt material adsorbed 26.3% of 2,4-dichlorophenol, 75.6% of 2,4,6-trichlorophenol
and 95.2% of pentachlorophenol at equilibrium. The adsorption of chlorophenols on
mesoporous AAPs was much less pronounced as compared to clays but increased with
the ratio of P/Al, as the surface acidity of those solids also increased. The AAP mixture
with a P/Al ratio = 0.6 adsorbed 14.8% of 2,4-dichlorophenol, 27.1% of 2,4,6-
trichlorophenol and 58.3% of pentachlorophenol. The amounts of chlorophenols
decomposed during the treatment increased in AAPs, especially in those with a higher
P/Al ratio (= 0.6) (Danis et al., 1998).
Papelis et al. (1995) studied the uptake rates of cadmium and selenite by porous
aluminas using three porous transition aluminas. All of these adsorbents were differed in
size and pore structure, CP-5 and CP-100 being the smallest and largest particles,
respectively, both exhibiting some microporosity, and C-33 being intermediate-size,
mesoporous particles. The rate data were interpreted with a diffusion model, assuming
solute diffusion in a sphere from a limited volume. The diffusion model was in fair
agreement with the rate data, suggesting that cadmium and selenite uptake is controlled
by intraparticle mass transfer. Solute uptake by the smaller particles (CP-5) was
considerably faster than that by the larger particles (CP-100). The measured apparent
diffusivities for both adsorbates and all adsorbents were orders of magnitude lower than
bulk aqueous diffusivities, in accordance with expectations for highly retarded sorption.
The measured effective diffusivities were substantially lower than aqueous molecular
diffusivities, suggesting the presence of strong hindrance effects in these microporous
adsorbents (Papelis et al., 1995). Vaudry et al. (1996) synthesized alumina mesophases
by reacting aluminum alkoxides and carboxylic acids with controlled amounts of water
in low-molecular-weight alcoholic solvents. Calcination of these materials yields
aluminas that are thermally stable to 1073 K and contain randomly ordered pores.
Specific surface areas as high as 710 m 2 /g and narrow pore size distributions centered at