2. XENOBIOTICS
Environmental chemicals (air, soil, water)
Derived from Greek word:
xenon = a stranger
+
bios = life
i.e., stranger to life
eg: food additives, pesticides, dyes, detergents,
polymers, industrial wastewater, CO, SO, NO, metals,
particulate matter, etc.
3. PHOTODEGRADATION
One of the possible routes for the elimination of
xenobiotics from the environment.
Photodegradation is the oxidation of organic
compounds in exoergic processes, in which the
absorption of light energy is the driving reaction
force.
4. Photodegradation is likely to be the predominant
elimination process in the atmosphere where
biodegradation is normally unlikely.
In water and soil, biodegradation process is the likely
pathway for elimination.
When biodegradation process in water and soil
become too slow, photodegradation become more
effective route for degradation.
5. MECHANISM OF PHOTOCHEMICAL
DEGRADATION
Excited molecule is unstable in presence of O2 or H2O and get
spontaneously decomposed
Molecule gets excited from the ground state
Transfer of energy to electrons in the molecule
Absorption of photons (290-700 nm wavelength)
6. TYPES OF PHOTODEGRADATION
1. Direct photochemical degradation
2. Advanced oxidation process
3. Indirect photochemical oxidation
7. DIRECT PHOTOCHEMICAL DEGRADATION
The chemical rearrangements and reactions such as
isomerisation, bond cleavage and photolysis that result
from absorption of photon directly.
In isomerisation, the absorption of light energy leads to
bond breakage and rearrangement, leading to a change
in conformation of the molecule.
In direct photolysis, the excited state formed directly by
absorption of light energy undergoes bond cleavage
leading to degradation of large polymer into smaller ones.
8. ADVANCED OXIDATION PROCESS
A set of processes involving the production of very
reactive oxygen species able to destroy a wide
range of organic compounds.
Efficient method to remove organic contamination
not degradable by means of biological processes.
Driven by external energy sources such as electric
power, ultraviolet radiation (UV) or solar light, so
these processes are often more expensive.
9. AOP Mechanism
1. Formation of strong oxidants (e.g. hydroxyl
radicals).
2. Reaction of these oxidants with organic compounds
in the water (Kommineni et al. 2008) producing
biodegradable intermediates.
3. Reaction of biodegradable intermediates with
oxidants referred to as mineralisation (i.e. production
of water, carbon dioxide and inorganic salts).
10. INDIRECT PHOTOCHEMICAL OXIDATION
Also known as secondary photochemical reaction.
The chemical changes brought on by photosensitizers,
the molecules excited by the initial absorption of light.
Photosensitizers has a short lifespan and it tends to
transfer energy to a receptor molecule.
Any photosensitized reaction involves the transfer of
energy, hydrogen atoms, protons or electrons and the
result of these charge transfers are reactions that
cause bond breakage or oxidation.
11. AN EXAMPLE FOR
PHOTOCHEMICAL DETOXINATION
CYANIDE DEGRADATION
Used in previous metal processing, for example to
recover gold-mill tailing.
Cyanide often is complexed as hexacyanoferrate (11)
and hexacyanoferrate (111).
These hexacyanoferrate complexes may undergo
photolysis releasing free cyanide ions.
12. Cyanide ions themselves can be oxidised by alkaline
chlorination or with H2O2.
Photocatalytic oxidation of cyanide ions in alkaline
aqueous solutions is done using semi conductor
powders such as TiO2, ZnO and CdS yielding
nontoxic cyanate ion CNO-
Removal of cyanide from water is achieved by solar
irradiation using ZnO as photocatalyst. Optimal rate
of oxidisation of cyanide is achieved at ph 11.
13. The intermediate product is cyanate but after 2 hrs
80% of carbon atoms are converted to carbonate .
In the presence of rhodium loaded cadmium sulfide
in alkaline sulfide media, illumination with visible light
results in the transformation of cyanide to much less
toxic thiocyanate with a quantum efficiency of more
than 25%.
14. PHOTODEGRADATION IN AIR
For photodegradation in atmosphere, secondary
photochemical reactions prevail over transformation
of the molecule by photolysis.
OH radical and ozone are essentially two species
which cause the degradation of molecule in air by
oxidation.
OH radical is more important reactant because of high
reactivity whereas ozone reacts with aromatic and
olefinic compounds.
15. IMPORTANCE OF PHOTODEGRADATION
1. Pharmaceuticals
• Pharmaceuticals have deleterious effects on aquatic
organisms including toxicity, endocrine disruption,
genetic damage.
2. Pesticides and Herbicides
• Additional modalities are implemented to enhance
their photodegradation, including the use of
photosensitizers, photocatalysts (e.g. TiO2, ), and the
addition of reagents such as H2O2 that would
generate hydroxyl radicals that would attack the
pesticides.
16. 3. Polymers
• Common synthetic polymers that can be attacked
include polypropylene and LDPE, where tertiary
carbon bonds in their chain structures are the centres
of attack.
• UV rays interact with these bonds to form free
radicals, which then react further with oxygen in the
atmosphere, producing carbonyl groups in the main
chain.
17. • The exposed surfaces of products may then
discolour and crack, and in extreme cases, complete
product disintegration can occur.
Effect of UV exposure
on polypropylene rope
Photodegradation of a
plastic bucket used as an
open-air flowerpot for some
years
18. REFERENCES
Campbell I, Hulpke H, et. al.; An Assessment of Test
Methods for Photodegradation of Chemicals in the
Environment; Ecology and Toxicology Centre, Belgium.
Hamulski Matthaus; Photodegradation of Water
Pollutants.
Martin M. Halmann –Photodegradation of Water
Pollutants.
Leighton (1961), Photochemistry of Air Pollution,
Academic Press, New York.
Niles, G P, Zabik M J ; 1974; Photochemistry of Bioactive
