31
H
2
O + e
-
→ HO•+ •H + e
-
2.33 H
2
O + e
-
→ H
2
O
+
+ 2 e
-
2.34 H
2
O + H
2
O
+
→ H
2
O
+
+ HO•
2.35
Such radicals could react between themselves resulting in the production of H
2
O
2
, H
2
or H
2
O. HO• + HO• → H
2
O
2
2.36 H
• + H• → H
2
2.37 HO
• + H• → H
2
O 2.38
Furthermore, the degradation of organic compounds could be performed by: a direct reactions with highly reactive species
HO•, b indirect reaction over radicals formed from stable molecules H
2
O
2
, and c direct reactions with stable molecules.
2.3.5 Others AOPs
This subchapter contains review of other AOPs, such as: ultrasound or ultrasonic irradiation, radiolysis of water and electrochemical processes.
A. Ultrasound
.
Ultrasonic processes as wastewater treatment method generate free radicals such hydroxyl radical upon the action of ultrasonic waves on liquid. The applied
frequency ranges from 20 – 40 kHz. Ultrasound produces the chemical effect through
several different physical mechanisms and the most important nonlinear acoustic process for sonochemistry is cavitation.
32
Water irradiation using ultrasound causes decomposision of the water molecules in to extremely reactive radical
HO• and H•, shown in equation below:
H
2
O + ultrasound → HO• + H•
2.39
Further the reactive radical species could react with organic pollutant present in the water through oxidation or reduction.
B. Water Radiolysis
Water radiolysis processes involve high energy ionizing radiation ranged from keV to MeV to irradiate of dilute aqueous solution resulting in the excitation and
ionization of water molecules. It is well known that radical species are very reactive to degrade organic pollutants present in water.
C. Electrochemical Processes
The electrochemical processes can occur by direct electron transfer reaction of reduction or oxidation of organic pollutant, or by chemical reaction of the pollutant
with previously electrogenerated species. Mechanism of the reaction generally viewed as a direct anodic oxidation of organic pollutant involving its reduction by
direct electron transfer from organic molecule to the electrode to form a radical cation that readily deprotonates.
RH + electrolysis →R• + H
+
+ e
-
2.40
33
2.4 Degradation Intermediate
Oxidation of an organic compounds such as amines by hydroxyl radical may proceed through abstraction of hydrogen atoms leading to the formation of carboxylic
acids which are further degraded to smaller fragments and eventually to CO
2
and H
2
O when enough hydroxyl radicals are generated in the reaction medium.
The electrophilic attack of the hydroxyl radical may also cause a cleavage of C-N bond. Under neutral or acid condition the amino functional group is protonated
to a certain extent, which might deactivate the α-CH bond. Hence, a further located C-atom of the amine is oxidized. In contrast with this, in alkaline condition a
competitive direct electrophilic attack at the free electron pair of the nitrogen atom also can take place. This is due to the fact that the amino function is unprotonated. As
a result, steric screening affects the direct electrophilic attack of the hydroxyl radical at the free electron pair of the nitrogen. Reaction scheme for degradation of a
secondary amine by hydroxyl radical is showed in Figure 2.3. It is based on the electrophilic attack of hydroxyl radical which leads to hydrogen abstraction inducing
a cleavage of the C-N bond. Subsequently, the organic nitrogen is transformed to CO
2
, NH
4 +
, NO
2 -
and NO
3 -
.
2.5 Biological Oxidation
Biological treatment is a method to remove contaminants in the wastewater by biological activity. Primarily, biological treatment is used to remove biodegradable
organic substances colloidal or dissolved in the wastewater. These substances are absorbed, fragmented and metabolized by the bacteria leading to biomass growth as
formation of metabolic product. Pretreatment may be required for contaminants which are toxic to the microorganism. Biological treatment can also remove nutrients
nitrogen and phosphorous in the wastewater. The removal of carbonaceous biochemical oxygen demand is accomplished biologically using a variety of
