Environmental Engineering Reference
In-Depth Information
5.9 ENVIRONMENTAL PHOTOCHEMICAL REACTIONS
Solar radiation is the most abundant form of energy on earth. All regions of the sun's
spectrum (UV, visible, and infrared) reach the earth's atmosphere. However, only a
small fraction of it is absorbed by water or land, while the rest is reflected, absorbed,
or dissipated as heat. Several reactions are initiated in the natural environment as a
result of the absorbed radiation. The term
photochemistry
refers to these transforma-
tions. Photochemistry is basic to the world we live in. Plants depend on solar energy
for photosynthesis. Photolytic bacteria draw solar energy for conversion of organic
molecules to other products. Absorption of photoenergy by organic carbon in natural
waters leads to the development of color in lakes and rivers. Redox reactions in the
aquatic environment are initiated by absorption of light energy. Reactive free radi-
cals (OH
•
,NO
3
, and HO
2
)
are formed by photochemical reactions in the atmosphere
where they react with other species.
There are two fundamental laws in photochemistry:
1. Grotthus-Draper law:
Only light absorbed by a system can cause chemical
transformations.
2. Stark-Einstein law:
Only one quantum of light is absorbed per molecule of
absorbing and reacting species that disappear.
There are two important laws that are derived from the Grotthus-Draper law. The
first is called the
Lambert
'
s law
, which states that the fraction of incident radiation
absorbed by a transparent medium is independent of the intensity of the incident light,
and that successive layers in the medium absorb the same fraction of incident light.
The second is called the
Beer
'
s law
, which states that absorption of incident light is
directly proportional to the concentration of the absorbing species in the medium. By
combining the two laws we can express the ratio of change in absorption to the total
incident radiation as follows:
d
I
I
= α
ν
C
i
d
z
.
(5.185)
α
v
is proportionality constant
(m
2
/mole).d
z
istheincrementinthicknessofthemediumperpendiculartotheincident
radiation.
The above equation is called the
Beer-Lambert law
.
ν
is the frequency of the incident light. If
I
=
I
0
at
z
=
0, we can obtain
upon integration
log
I
0
I
= ε
ν
C
i
Z
,
(5.186)
ε
v
= α
v
/
2.303iscalledthe
molarextinctioncoefficient
(m
2
/mole).Itisspecific
to a specific frequency
where
.
C
i
is expressed in mole/m
3
, and
Z
is expressed in m. If
C
i
is in mole/dm
3
, then
Z
is expressed in mm. log
(I
0
/I)
is
called the absorbance
A
λ
.
I
is expressed in J/m
2
s. The amount of light absorbed by
the medium is
ν
, or wavelength
λ =
1
/
ν
10
−ε
v
C
i
Z
)
.
I
abs
=
I
0
−
I
=
I(
1
−
(5.187)
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