10.2.1.1 Processes by Which Hydroxyl Radicals Are Generated
and Their Subsequent Use in Advanced Water Treatment
It is well-known that hydroxyl radicals have a short lifetime, they are just produced
through various techniques such as via the mixing of oxidizing operators (e.g., H 2 O 2
and O 3 ), light (e.g., bright light or ultrasound), and salts (e.g., Fe
2+ ). The practices
for the generation of hydroxyl radicals are summarized below.
H 2 O 2 /UV Process
One of the most effective techniques in treatment of wastewaters containing toxic
organic pollutants is the combination of hydrogen peroxide with ultraviolet photolysis (H 2 O 2 /UV). This process involves two types of reactions simultaneously—
photolysis and oxidation.
The degradation of water contaminants depends on the molecular structure and
the wavelength of irradiated UV light. Generally, in order to meet these objectives,
two types of mercury-containing UV lamps are used:
(a) Low-pressure (LP) UV lamps, emitting irradiation with a wavelength of
253.7 nm.
Braun et al. (1991) reported that the low-pressure mercury light is a monochromatic UV source that has a solid outflow of unreversed resonance line at
253 nm and 184 nm alongside other significantly weaker lines. UV radiation
likewise gives the vitality to start the decay of ozone, which prompts the
arrangement of two hydroxyl radicals (OH). The mercury outflow at 253.7 nm
delivered specifically by low-weight circular segments has been accounted for
degradation of chemicals in water. Numerous analyses have been conducted to
decompose chlorinated hydrocarbons such as 85% tetrachloroethene, 55%
trichloroethene, 45% 1,1,1-trichloroethene with a low-weight mercury light at
253.7 nm (Lee and Lee 2005).
(b) Medium-pressure (MP) UV lamps, emitting over a wavelength range of
200 to 300 nm.
The MP UV lamps are loaded with mercury vapor and work at a pressure of ca
1 atm. With the increase of pressure in the discharge tube, two major changes occur:
• Due to the growing number of collision with the energetic electrons, the gas
temperature increases.
• The high temperature becomes localized at the center of the discharge and a
temperature gradient is developed towards the walls, which are much cooler.
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R. K. Sharma et al.
and Their Subsequent Use in Advanced Water Treatment
It is well-known that hydroxyl radicals have a short lifetime, they are just produced
through various techniques such as via the mixing of oxidizing operators (e.g., H 2 O 2
and O 3 ), light (e.g., bright light or ultrasound), and salts (e.g., Fe
2+ ). The practices
for the generation of hydroxyl radicals are summarized below.
H 2 O 2 /UV Process
One of the most effective techniques in treatment of wastewaters containing toxic
organic pollutants is the combination of hydrogen peroxide with ultraviolet photolysis (H 2 O 2 /UV). This process involves two types of reactions simultaneously—
photolysis and oxidation.
The degradation of water contaminants depends on the molecular structure and
the wavelength of irradiated UV light. Generally, in order to meet these objectives,
two types of mercury-containing UV lamps are used:
(a) Low-pressure (LP) UV lamps, emitting irradiation with a wavelength of
253.7 nm.
Braun et al. (1991) reported that the low-pressure mercury light is a monochromatic UV source that has a solid outflow of unreversed resonance line at
253 nm and 184 nm alongside other significantly weaker lines. UV radiation
likewise gives the vitality to start the decay of ozone, which prompts the
arrangement of two hydroxyl radicals (OH). The mercury outflow at 253.7 nm
delivered specifically by low-weight circular segments has been accounted for
degradation of chemicals in water. Numerous analyses have been conducted to
decompose chlorinated hydrocarbons such as 85% tetrachloroethene, 55%
trichloroethene, 45% 1,1,1-trichloroethene with a low-weight mercury light at
253.7 nm (Lee and Lee 2005).
(b) Medium-pressure (MP) UV lamps, emitting over a wavelength range of
200 to 300 nm.
The MP UV lamps are loaded with mercury vapor and work at a pressure of ca
1 atm. With the increase of pressure in the discharge tube, two major changes occur:
• Due to the growing number of collision with the energetic electrons, the gas
temperature increases.
• The high temperature becomes localized at the center of the discharge and a
temperature gradient is developed towards the walls, which are much cooler.
226
R. K. Sharma et al.