Environmental Engineering Reference
In-Depth Information
viscosity, p the pressure, and γ the surface tension. This equation has been shown to provide
reasonable estimates of the bubble dynamics under an acoustically driven ield but only up
to inal stages before it collapses. Intricate measurements have shown that during the critical
inal stage of implosion, the bubble wall velocity exceeds the predicted speed of sound of the
gas contained inside the bubble, and the Rayleigh-Plesset equation is no longer applicable.
3
2
1
R
RR
2
γ
 
2
RR
+
R
= −− −
p
PPt
()
4
µ
g
0
ρ
Controlled hydrodynamic cavitation is a process that seeks to optimally produce cavita-
tion and harness the kinetic energy that is imparted to the luid without any damage to
equipment and has been demonstrated applicable to bacteria eradication, removal of dis-
solved gases, precipitation of certain inorganic salts, creation of hydroxyl radicals, and the
formation of stable emulsions.
6.4 Advanced Oxidation Process (AOP)
The use of photocatalytic nanocoated structures such as the anatase form of titanium diox-
ide (TiO 2 ) exposed to UV light and in combination with air or water begins a reaction
at the illuminated TiO 2 surface-water boundary where the water molecules start to split
into H + and OH groups. When these outer electrons on TiO 2 atoms are excited by com-
mon electromagnetic radiation such as UV from the sun, it can move electrons into higher
stable outer orbitals, only then to subsequently remit their energy in the form of photons
again on their way back down to lower stable orbitals in a renewable and catalytically
self-sustaining way that does not consume the photocatalytic surface. During this stage,
photons are reemitted but at lower energy packet values, which are then accompanied by
a new set of electrochemical processes producing the mysterious behaviors found in AOP,
described elsewhere in this topic (see Chapter 24).
The OH groups (hydroxyl radicals) produced are a form of very reactive free radicals
that attack almost any substance but are particularly effective in reducing organic mate-
rial such as the cell membranes in bacteria. In this way, it is possible to disinfect water that
has been contaminated with not only bacteria but also viruses, molds and lichens, and
pathogens of every type in order to achieve potable water anywhere by removing most
of the contaminants commonly found in water, including dissolved metals, and metasta-
ble compounds. There are in fact at least ive separate reactions: photocatalytic oxidation,
photocatalytic reduction, photoadsorption, photolysis, and photodisinfection, all working
together to treat contaminants in water including heavy metal ions.
Pollack, whose detailed observations describing the relationship of just how matter inter-
acts with light, and between the substrate and the boundary interfaces in contact with the
water molecule itself, is transforming the way we understand water. A new form of water
called by many “the fourth state of water” is forcing us to reconsider and truly understand
how water interacts and behaves with biological organisms at the smallest scales. We are
only now beginning to truly understand and recognize how to harness this capacity and
to implement it in ingenious manners for the use of nanotechnologies in water treatment,
and certainly graphene in combination with TiO 2 will be further changing it as well.
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