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formation of stable Fe
3+
-complexes between free Fe
3+
ions and some aliphatic acid formed as byproducts of degradation Equation 2.12.
H
2
O
2
+ hv → 2 HO•
2.10 FeOH
2+
+ hv → Fe
2+
+ HO•
2.11 Fe
3+
L
-
+ hv → Fe
2+
+ L• 2.12
Second benefit of the presence UV light in the photo-Fenton Processes is to achieve the complete mineralization due to the degradation of some hydroxyl radical
persistent byproduct, such oxalic acid and acetic acid, by UV light.
2.3.2 UV-based Processes
The “UV-based Processes” are considered as all processes that apply UV light either for degradation of organic pollutant or for the initiation of oxidation
mechanisms by the irradiation of some powerful oxidants or photo-catalyst. UV- based processes could be classified into UV photolysis, photochemical processes and
photocatalytic.
A. UV Photolysis
Investigations regarding UV light were begun since Isaac Newton observed the diffraction of white beam when passing through a prism. At the beginning of 19
th
century, the radiant energy beyond two ends of spectra of visible light was discovered. One of those is identified as infrared and another as ultraviolet region.
Furthermore, it was shown that invisible chemically active irradiation beyond violet end of spectrum were the subject of laws of interference. Further investigation
23
indicated the fact that irradiation with visible VIS, infrared IR or ultraviolet UV light has characteristic of the same electromagnetic irradiation, but they differ in
respect of its frequency, and what was discovered latter, pertaining to energy.
Table 2.4 Radiation type and pertaining energy; 1 Einstein = 1 mol of photons.
Radiation Wavelength nm
Energy range kJ Einstein-1
IC 780
155 VIS
780 – 400
155 – 300
UV-A 400
– 315 300
– 377 UV-B
315 – 280
377 – 425
UV-C 280
– 100 425
– 1198
. The primary usage of UV radiation in the earlier period was for disinfection,
but with the development of reaction mechanism, UV radiation nowadays establishes the usage for oxidation purpose as well. UV-C is mostly used for oxidation processes.
While the most common UV-C wavelength is 254 nm that could be achieved by low- pressure vapor mercury lamp invented by Hewith at 1901.
At room temperature, most molecules reside in their lowest-energy electronic state, i.e. “ground state”. When molecules are exposed to UV radiation, they get
transferred to a state with higher energy, i.e. “exited state”. The molecules in the “exited state” have very short lifetime 10
-9
– 10
-8
s, after which it returns to “ground state” by one or several mechanism fluorescence phosphorescence or decompose to
yield a different molecule. The mechanism of direct photolysis is expressed below:
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M + h υ → M
2.13 M → M
2.14 M → Product
2.15
UV radiation is generally used in combination with some powerful oxidant or photocatalyst. The efficiency of its separate use depends on limitations such as:
a. Water solution should be treated in a way to achieve the highest possible
UV transmission, i.e. turbidity should be as low as possible. b.
Very high concentration of hydroxyl radical could inhibit mineralization reaction of organic contaminant present in water.
c. Water solution should be free of heavy metals and oil.
d. Costs of UV radiation are higher than Fenton dark process.
B. Photochemical Processes
