T E C H N I C A L
B R I E F
Use of UV
Oxidation in Water Purification
FOR THE PRODUCTION OF HIGH-PURITY WATER
Introduction
Ultraviolet (UV) oxidation is an
extremely important purification
technology used in the production
of high-purity water for the chemical,
food and beverage, pharmaceutical
and semiconductor industries. When
strategically combined with other
purification technologies in a complete water system, UV oxidation
provides unique benefits in the reduction of dissolved organics and
microorganisms. This article outlines
the key principles and practical
applications of UV technology.
Principles and
Production of
UV Energy
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Light is electromagnetic radiation or
radiant energy which travels in the
form of waves. The electromagnetic
spectrum consists of radiant energy
in specific wavelengths, typically
measured in nanometers:
⁄ nm =10 –9 m = 10 angstroms
The relationship between energy
and wavelength can be expressed
as follows:
E = hν = hc/λ
where:
E = energy
h = Planck’s constant
ν = frequency of radiation
c = velocity of light
λ = wavelength of radiation
Therefore, the energy carried in a
beam of radiation is inversely related
to its wavelength.
UV radiation is found in the electromagnetic spectrum between the
wavelengths of visible light and
X-rays. The UV region is further
divided into four smaller regions:
UV-A (long-wave)
UV-B (medium-wave)
UV-C (short-wave)
UV-vacuum
315 – 400
280 – 315
200 – 280
100 – 200
nm
nm
nm
nm
The hardware used to generate UV
radiation in a water purification system includes a low-pressure, mercuryvapor lamp, a ballast and a power
supply1 (see Figure 1). The lamp
consists of a sealed quartz tube with
electrodes (cathodes) on each end.
The lamp tube contains a small
amount of mercury and an inert gas,
such as argon or neon, at a very low
pressure. The power supply energizes
the ballast which regulates the current
to the lamp. Electrical current from
the ballast pre-heats the lamp cathodes. Electrons “boil” off the cathodes
and vaporize the mercury in the
lamp, and the current flows through
the conductive, ionized mercury
vapor. The electrical energy added
to the mercury atoms causes specific
electron transitions that result in the
release of energy as UV radiation.
Some energy is also released as
visible light and heat.
UV radiation penetrates specific
materials to different degrees
depending on the wavelength of the
energy. Grade 2⁄9 quartz allows
efficient transmission of UV radiation
The Ultraviolet (UV)
Photo-oxidation Chamber
at 254 nm (UV254). Grade 2⁄4
quartz permits efficient transmission
of UV radiation at both 254 nm and
⁄85 nm (UV185/254). The materials of
construction (i.e., type of quartz)
determine whether a lamp can be
used for germicidal purposes or
Total Organic Carbon (TOC) reduction. After extended operation of the
lamp, mercury film accumulates on
the interior surface of the quartz tube
and depletes the mercury available
for ionization. Therefore, UV lamps
should be replaced periodically
according to manufacturer maintenance guidelines.
Light Penetrates Water
Light Reflects Off
Polished Surface
and Penetrates
Water Again
Ultrapure Quartz
Sleeve
316 L Electropolished
Stainless Steel
Housing
Low Pressure Mercury
Vapor Lamp with
Ultrapure Quartz
Sleeve
H 2O
Figure 1
The decomposition of a substance
depends on the total energy
delivered to that substance and
is a function of the following:
• The type, structure and concentration of the molecule
• The conditions and composition of
the water (oxidizers, catalysts)
• The geometry and configuration of
the UV lamp and reaction chamber
• The UV dosage, where:
UV intensity (µW/cm2) x
residence time (seconds) =
UV dosage (µW-sec/cm2)
Role of UV Oxidation
in Water Purification
Molecules are composed of atoms
joined together by chemical bonds.
The strength of the chemical bonds
in a molecule can be described by
a bond dissociation energy, which is
the minimum energy required to break
a particular bond2 (see Table ⁄).
When a molecule is exposed to UV
radiation, it gains energy. If the energy absorbed exceeds the bond dis-
Two UV wavelengths, UV254 and
UV185, are useful in water purification. UV254 is effective in breaking
bonds between carbon, nitrogen,
and hydrogen atoms. As a result,
UV254 can destroy living microorganisms by disrupting their DNA. The
destruction of various organisms is
UV dosage-dependent. UV254 also
Dissociation Energies for Interatomic Bonds in Organic Substances
Bond
H 2O
sociation energy, the chemical bonds
will be broken and the molecule subsequently decomposed.
Dissociation
Energy (k cal)
Maximum
Wavelength for
Dissociation (nm)
Possibility of
Dissociation with
184.9-nm UV (154 k cal)
C–C
82.6
346.1
Yes
C=C
–C
C=
145.8
196.1
Yes
199.6
143.2
No
C – CI
81.0
353.0
Yes
C–F
116.0
246.5
Yes
C–H
98.7
289.7
Yes
C–N
72.8
392.7
Yes
C=N
147.0
194.5
Yes
C–O
85.5
334.4
Yes
C = O (aldehydes)
176.0
162.4
No
C = O (ketones)
179.0
159.7
No
N–N
52.0
549.8
Yes
N=N
60.0
476.5
Yes
N – N (NH)
85.0
336.4
Yes
102.2
280.3
Yes
S–H
83.0
344.5
Yes
S–N
115.2
248.6
Yes
S–O
119.0
240.3
Yes
N – N (NH3)
Table 1
→
HCOOH + H2 O ←
HCOO– + H3O+
→ HCO
CO + H O ←
exhibits good penetration in air and
water making it useful in disinfecting
process water streams.
6)
UV185 carries more energy than the
longer UV254 wavelength. It is important to note that a UV lamp which
emits both UV185 and UV254 must be
used to oxidize organics and reduce
TOC in product water. UV185 not
only breaks organic bonds, but also
generates chemical species called
free radicals. Free radicals are shortlived, highly reactive molecules or
atoms which can rapidly oxidize
many organic and some inorganic
molecules.3 Of particular interest is
one of the most powerful oxidizing
species, the hydroxyl radical (OH•),
which can be formed via different
pathways including:
The ions generated cause an
increase in the conductivity of the
water. Additionally, the previously
neutral organic molecules are ionized
and removed via ion-exchange
resins.4 The use of ion-exchange
resins downstream of UV oxidation
lamps allows the continuous production of ultrapure water with significantly reduced organic content.
1)
H2 O + hν → H• + OH•
2)
1.5 O2 → O3 →
7)
2
–
3
+ H+
Practical Uses and
Benefits of Organicfree Ultrapure Water
Water with low organic levels
(approximately ⁄ µg/L as TOC) is
•
•
•
•
•
•
•
•
•
•
HPLC
Ion chromatography
LC-MS
GC-MS
Solid phase extraction
BOD/COD
TOC analysis
UV spectroscopy
Dynamic surface tension
Rinse of semiconductor materials
The use of UV oxidation within a
water system improves system performance by allowing a rapid rinse to
low organic levels (Figure 2) and by
extending the useful life of consum-
Rinse Down to Quality Effect of UV on Rinse Time
1.5O2
80
UV at
185 nm
TOC, ppb
+H2O
UV at
254 nm
Initial
Start-up
STD
60
O3
UV at
254 nm
2
essential for the preparation of blanks,
standards and calibration solutions.
Organic-free water is required for
maximum consistency and sensitivity
in applications such as:
40
20
UV
O2 + O•
0
H2O2 + O2
+ H2 O
0
UV at
254 nm
UV at
254 nm
10
20
30
Rinse time, minutes
40
60
30
After
100 Liters
2 OH•
The OH• created may freely react
with organic molecules to partially
ionize or fully oxidize them to CO2
and water as demonstrated in the
oxidation of methanol:
TOC, ppb
STD
20
10
UV
0
0
30
4)
HCHO + 2OH• →
HCOOH + H2 O
20
5)
HCOOH + 2OH• →
CO2 + 2H2 O
The products of the above reactions
ionize as follows:
TOC, ppb
CH3OH + 2OH• →
HCHO + 2H2 O
3)
50
10
20
30
Rinse time, minutes
40
50
60
After
300 Liters
STD
10
UV
0
0
Figure 2
10
20
30
Rinse time, minutes
40
50
60
able purification cartridges (Figure 3). No chemicals are added to the water,
and no undesirable chemical by-products are generated. Overall, UV purification technology is an effective, low-maintenance, environmentally safe process.
The benefits of UV oxidation technology in water purification systems are numerous and significant. A clear understanding of the processes involved is needed
to select and properly incorporate UV technology into a water system to produce superior water quality and optimal results in laboratory applications.
For Further Information
In the U.S. and Canada, call toll free
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Effect of UV on Cartridge Life
AUSTRALIA
Tel. 1 800 222 111
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TOC, ppb
120
80
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UV
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0
200
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Water used, liters
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Figure 3
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REFERENCES
1. Foley, A., Technical Note, “Ultraviolet Light Technology,” Millipore Corporation,
April 1999.
2. Srikanth, B., “Ultraviolet Light: Recent Advancements in UV Technology Yield
Enhanced TOC Reduction Performance,” Ultrapure Water, July/August, pp. 40-46,
Tall Oaks Publishing, Inc.,1998.
®
3. Technical Brief TB050, “Production of Ultra-Low Organics Water with the Milli-Q
UV Plus Water System,” Millipore Corporation, 1991.
4. Neta, P.1, J. Grodkowski1, A. Ross2, “Rate Constants for Reactions with Aliphatic
Carbon-Centered Radicals in Aqueous Solution,” 1. National Institute of Standards
and Technology, Chemical Kinetics and Thermodynamics Division, Gaithersburg, MD
20899. 2. University of Notre Dame, Radiation Chemistry Data Center, Radiation
Laboratory, Notre Dame, IN 46556. Research summary available at
www.rcdc.nd.edu/compilations/hydroxyl, October, 1999.
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