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The advanced oxidation process shows the ability to
remove toxic pollutants in waste waters. Following by the AOP technique
development, the most appropriate solution about specific water treatment
problems are available in this age.

Base on this mechanism, the hydroxyl radical could be
produce efficiently.

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•?+O3 ? O2 +O3 •?
;  O3 •?+H+
? HO3 • ;  HO3
• ? HO•+O2

By referring of ozone chemistry in alkaline solution,
the ozone water system had been designed by J. Hoignè et al for the water treatment. In this
process,  the hydroxyl radicals could be formed by the
decomposition of O3 in aqueous solution:

In the photocatalysis process, the semiconductor metal
oxide is used as catalyst and oxidizing agent with oxygen, and TiO2
in anatase is the best option because of high stability, superior performance
and low cost. When TiO2 absorb the radiation, the oxygen will have
dissolved and the metal will be reduced, the superoxide radical ion O2•?
formed, and remaining holes are capable to reactive hydroxyl radical by
oxidizing absorbed H2O or HO-, therefore this mechanism
involve in following reaction which is available to produce high concentration
of H2O and HO- (ref10).

Base on this mechanism, an improvement of Fenton
process known as photo-Fenton are present for the abatement of organic
pollution, according to the experimental data, the photo-Fenton process could
save 20% of energy to purify the same volume
of selected waste water10.

?Fe(aq)2++HO•(aq) (under UV-VIS light)

The economical and environmentally safety are the
advantages of this process. To accelerate the Fenton process, the UV-VIS light (wavelength
higher then 300nm) has been used and the photolysis of Fe3+
complexes allows Fe2+ regeneration:

? Fe3+(aq)+OH-(aq)+OH•(aq)

Another technique of AOP is the Fentone process. The
toxic compounds in the waste water such as phenols and herbicides which involve
in this process could be destroys by hydroxyl radical which play an important
part as strong oxidize reagent. To produce hydroxyl radical, the Fenton’s
reagents Fe2+ and H2O2 has been used:  

Generally, the AOP could be characterized by
production of hydroxyl radicals, which exhibits the same chemical feature, but
different reacting systems compared to outdated techniques. In the AOP, the
Chemical Oxygen Demand (COD), which is a measurement of the oxygen required to
oxidize soluble and organic matter in water11, represents the
effectiveness of AOP applications. The waste with relatively small COD contents
(? 5.0g/l) could be treated efficiently. However, the waste with large COD
waste needs very large amount of reactant, those waste needs to be treated by
wet oxidation or incineration. The wet oxidation is a process which oxidize the
pollutant at elevated temperature (130-300?) and pressure (0.5-20MPa) by oxygen
or air, hence the waste in water with high COD (approximately 20g/l) will be
combusted and oxidized10.

The Advanced Oxidation Processes (AOP) could achieve
the complete abatement and through mineralization of reactive pollutants by
exploiting the high reactivity of hydroxyl radical in oxidation process. According
to specific feature of different AOP, the application of water pollution
abatement will be different. There are two methods for toxic pollutants
separations which been applicated in last two decades: phase separation
techniques, including absorption and stripping, and contaminants destruction,
including chemical oxidation or reduction. In fact, the chemical destruction
method can provide a better solution to the problem of pollutant abatement. The
contaminants in this method would be mineralizes to carbon dioxide, water and
inorganic substance, and those compounds would be transfer into harmless
product. According to the “Second order
rate constants for ozone and hydroxyl radical for a variety of compounds”
by R. Andreozzi et al, the pollutants which are not effective through
biological treatment may also be difficult to be completely mineralized. Therefore,
an enhancement process of water purification had been developed since last
three decades, which is called the Advanced Oxidation Process (AOP)10.

According to the estimation, the world population will
increase additional two billion people by the year 2030, as a result the water
resources will became a critical and difficult challenge in the 21st
century9, and water purification is one of the best options to ease
the water scarcity.

Figure 1ref7

According to the phase diagram of water (Figure 1)
ref7, while temperature was 282.32 kelvins and the pressure was nearly 0
Pa at this phase diagram, the liquid water will sublime to the gaseous state
instantly under this condition. After the gaseous water forms, those vapors
will change to solid stat water in the low background temperature. At the
surface of Earth, the pressure at sea level is 1 atm (about 10E5 Pa) and the
average temperature is 331.24 kelvins8, the water is present the
liquid state. In general, the difference of temperature and pressure between
outer space and ground makes the water present different phenomenon.

If someone open a bottle of water in the outer space,
the water will boil almost instantly, then those water drops in gaseous state
will freeze immediately in the outer space4. The temperature of
outer space near the Earth is 283.32 kelvins5, because of the low
density and pressure in the outer space, the pressure at 100 kilometers of
altitude is 3.2×10-2Paref6.

In the biomolecular structure, the hydrogen bonds of
water make the shape and control the structure to perform versatility. In
liquid water cluster, each water molecule can form up to four tetrahedrally
directed hydrogen bonds with adjacent water molecules, and each of the molecules
can also establish dipole and induced dipole interactions with other molecules.
Therefore, the water cluster give liquid water a heterogeneous character which
can change with various physical and environmental conditions, and the
structure of adjacent water molecules and biomolecular group, as well as
secondarily linked molecules and distant parts of the biomolecules, could be
affected by the replacement of any links around each water molecule. The place exchange
of a water molecule with adjacent water molecules on the timescale about 1-100
picoseconds, and this exchange mostly depends on the local topography and
exposure to competing water molecules. As a result, this exchange process
allows the surface shape of a biomolecule to change but would not change other
effects of hydration significantly, such as net direction and strength of the
hydrogen bonding3.

The thermal properties of water protect the creature
on Earth as well. Because of the high specific heat capacity of water (4.2 KJg-1?-1), the organism will gain or lost the
temperature in a broad range to prevent damage from the abrupt temperature
changes. The water has high latent heat of vaporization as well, which means
water needs a lot of energy to evaporate, which keeps the water in the creature
without evaporating. Because of the high latent heat of fusion, water could
resist the freezing which protect the organism made by cells2.

The dipole of water molecules means these molecules
have regions with positive and negative charge. In this way, these charges
attract the charged solutes in solution. Furthermore, the water molecules form
a layer around each solute particle and stabilize the solution. The other
important property for water is its adhesive property. The water molecules can
stick to other polar substance, which means that water can be transferred to
the whole body of creatures. For example, the tall plants like trees, their
xylem is filled up with the water, because of the adhesive property, water
could be transfer to the top of the tree by xylem2.

 The hydrogen
bond results from electrostatic force. To be more specific, an electropositive
atom (typically hydrogen) exhibits strong electrostatic interaction with the
strong electronegative atoms (typically fluorine, nitrogen and oxygen). Because
water is in liquid state at room temperature, which ranges from 59° to 77°F
(15° to 25°C) (ref.1), the hydrogen bonds could be formed between up
to pairs of water molecules. Hence marine environment is available for
organisms to live in, and the liquid environment could be present inside the

In this discussion about model of water, the following
topics will be covered: the biological importance of water structure, the
result that an astronaut open a bottle of water in the outer space, and a water
purification technique named Advanced Oxidation process (AOP).

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