Nanoscale Porous Materials for Water Treatment: Advances and Challenges - Nanotechnologies for Water Environment Applications - page 315

based NPM. Each of these NPM has been subdivided into several different subgroups
based on the host material. Table 11.2 shows some of the NPM and their properties.
11.2.1 Inorganic Nanoporous Materials
This covers one of the oldest materials which have been one of the core parts of
environmental research for decades, due to their extremely good catalytic properties,
large surface areas, excellent redox properties and high reactivity. This includes those
NPM made of a variety of inorganic materials (e.g., silica, alumina, iron, silver). Here,
we subdivide them based on the type of materials used in the environmental research.
11.2.1.1 Zeolite
Zeolites (Greek, zein , "to boil"; lithos , "a stone") are the largest and the oldest
group of microporous materials widely known as "molecular sieves." The term was
originally introduced in the 18 th century by Axel Fredrik Cronstedt, Swedish
mineralogist who observed rapid movement of a stone while heating caused by rapidly
evaporating water. There are about 48 types of natural zeolites. However, due to the
difficulty in getting their pure forms and obtaining novel properties, a variety of zeolites
(> 150 types) have been synthesized.
The most known synthesized zeolites are silicate-1, ZSM-5, zeolite beta and
zeolites XY and A. Figure 11.1 shows a typical TEM and XRD patterns of zeolite. In
general, they are crystalline inorganic polymers, consisted of a 3-dimentional
arrangement of [SiO 4 ] 4- and [AlO 4 ] 5- polyhydra connected through their oxygen atoms to
form large negative charged lattices with Bronsted and Lewis acid sites. In this way,
they are basically hydrated alumino-silicate minerals of the alkaline and alkaline-earth
metals with an "open" structure that can accommodate a wide variety of cations, such as
Na + , K + , Ca 2+ , Mg 2+ and others. These positive ions are rather loosely held and can
readily be exchanged for others in a contact solution. Thus, they have excellent ability to
selectively sort molecules based primarily on a size exclusion process. This selection
process is controlled by dimension of the tunnels which exist underneath the pores.
Hence, they are of great interest in a wide range of research due to their unique
adsorption, ion-exchange, molecular sieve, and catalytic properties. Zeolites also belong
to one of the most developed field on microporous research, having established
organization like International zeolite association. Hence, this is a huge field itself,
numbers of excellent reviews have summarized its outstanding properties on vast
research fields (Savage and Diallo, 2005; Logar and Kaui, 2006). Here, we have
summarized some of the most recent applications of zeolites for water treatment.
Generally, zeolites have a pore size < 2 nm, and it is conventionally called
microporous materials. However, sometimes the targeted contaminants sizes exceed the