Civil Engineering Reference
In-Depth Information
16.4.1 Zeolites
Zeolites are microporous crystalline solids with well-defi ned structures.
Generally they contain silicon, aluminium and oxygen in their framework
and cations, water and/or other molecules within their pores. Many occur
naturally as minerals and they are extensively mined in many parts of the
world. Others are synthetic and are made commercially for specifi c uses.
Synthetic zeolites are usually made from silicon-aluminium solutions or
coal fl y ash, and are used as sorbents in column fi lters. Zeolites are widely
used as ion-exchange beds for domestic and commercial water purifi cation,
and water softening where alkali metals such as sodium or potassium prefer
to exchange out of the zeolite, being replaced by the 'hard' calcium and
magnesium ions from the water. Commercial wastewater containing heavy
metals can also be cleaned up using such zeolites. Zeolites are generally
used for the removal of metal contaminants. Natural zeolites from Mexico
and Hungary have been shown to reduce arsenic from drinking water
sources to levels deemed acceptable by the World Health Organization
(Elizalde-González
et al.
, 2001).
Zeolites made from coal fl y ash can adsorb a variety of heavy metals
including lead, copper, zinc, cadmium, nickel, and silver from wastewater.
Under some conditions, fl y ash zeolites can also adsorb chromium, arsenic
and mercury. NaP1 zeolites (Na
6
Al
6
Si
10
O
32
, 12H
2
O) have a high density of
Na (I) ion exchange sites. They can be inexpensively synthesized by hydro-
thermal activation of fl y ash with low Si/Al ratio at 150°C in 1.0-2.0 M NaOH
solutions. NaP1 zeolites have been evaluated as ion exchange media for the
removal of heavy metals from acid mine wastewaters. Alvarez-Ayuso
et al.
(2003) reported the successful use of synthetic NaP1 zeolites to remove Cr
(III), Ni (II), Zn (II), Cu (II) and Cd (II) from metal electroplating waste-
water. The adsorptive capacity of zeolites is infl uenced by several factors
including their composition, the water pH, and the concentrations and types
of contaminants. For example, the water pH infl uences whether the ash
surface is positively or negatively charged. Also, because lead and copper are
more easily adsorbed by fl y ash, high concentrations of these metals decrease
the amount of cadmium and nickel removed (Wang
et al.
, 2004).
Zeolite-silver compound has been proven effective against microorgan-
isms, including bacteria and mould. Additionally, the silver in this compound
provides residual protection against regrowth of these biological contami-
nants. Zeolites do not adequately remove organic contaminants. Also, air
moisture contributes to zeolites' saturation and makes them less effective.
Zeolites were reported to result in high fl ux reverse osmosis nanocomposite
membrane (the utilities of having a membranous structure have been dis-
cussed in the last section) without compromise in selectivity (Jeong
et al.
,
2007).
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