24
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
Photochemical processes use combination of UV light and some powerful oxidant such hydrogen peroxide H
2
O
2
and or Ozone O
3
. Application of UV H
2
O
2
has been investigated for water purification. This purification involves hydroxyl radical generation through direct photolysis of H
2
O
2
. It is well known that hydroxyl radical is a very reactive species that could degrade the organic contaminant. Success
of this application depends on the initial concentration of organic contaminant in water and presence of “scavenger” such organic or inorganic compound which could
inhibit or even stop the treatment process. UVH
2
O
2
process is in use: a.
Removal of micro- and macro- pollutants from drinking water. b.
Treatment of low concentration organic toxic compounds present in ground water.
c. Treatment of smaller volume of highly recalcitrant pollutants in order to
achieve their detoxification and faster degradation. d.
To control of exhaust gases in the case of volatile organic compound.
25
Quantity of energy required for direct photolysis of hydrogen peroxide is very high, and theoretically two hydroxyl radicals could be generated per absorbed energy
quantum. In practice, the highest quantum yield for generation of hydroxyl radical is 0.5 mol of H
2
O
2
per Einstein. Generation of hydroxyl radical by UV radiation can be expressed as follows:
H
2
O
2
+ h υ → 2 HO•
2.16
While the scavenger mechanism of H
2
O
2
and hydroxyl radical which influences the overall process efficiency can be expressed as shown below:
H
2
O
2
↔ HO
2 -
+ H
+
2.17 HO• + H
2
O
2
→ HO
2
• + H
2
O 2.18
HO• + HO
2 -
→ HO
2
• + HO
-
2.19 HO
2
• + HO• → H
2
O + O
2
2.20
Important parameters of the UVH
2
O
2
process are UV lamp characteristic, reactor configuration, pH of solution and initial concentration of H
2
O
2
. While some limitation on UV H
2
O
2
process which should be taken is the presence of iron and potassium salts in treated water resulting with reduction of UV radiation. This salt
could be avoided by adjusting pH solution to the value where those salts can precipitate. Furthermore another limitation is related to the large quantities of
suspended particle resulting with increased turbidity. This problem could be solved by filtration as pretreatment of such wastewater.
Like hydrogen peroxide, ozone is also widely used as an oxidant in the photochemical process. In addition, ozone is even better oxidant than hydrogen
peroxide due to the significantly higher value of molar absorption coefficient at 254 nm, typically the wave length for UV-C radiation. Moreover, the rate of ozone
photolysis is almost 1000 time higher than hydrogen peroxide.
26
The UVO
3
process is based on the fact that by the decomposition of ozone under UV radiation two hydroxyl radicals are generated which rather form hydrogen
peroxide than react with organic matter present in the water, shown by the equation below:
O
3
+ H
2
O + h υ → H
2
O
2
+ O
2
2.21
Furthermore, the H
2
O
2
formed can decompose under UV radiation to hydroxyl radicals that react with organic matter presence in the water, equation 2.13. There
are several mechanisms for the degradation of organic pollutant in water: direct photolysis, hydroxyl radical attack generated from different source, and direct ozone
attack. There is also combination of these two binary systems UVH
2
O
2
and UVO
3
as called UV H
2
O
2
O
3
. Furthermore the process could enable complete mineralization of organics presence in water.
C. Photocatalytic Process
