Photodegradation

Photo-degradation is the alteration of materials by light. Typically, the term refers to the combined action of sunlight and air. Photo-degradation is usually oxidation and hydrolysis. Often photodegradation is avoided, since it destroys paintings and other artifacts. It is however partly responsible for remineralization of biomass and is used intentionally in some disinfection technologies.

Where photodegradation is important

Pesticides

The photodegradation of pesticides is of great interest because of the scale of agriculture and the intensive use of chemicals. Pesticides are however selected in part not to photodegrade readily in sunlight in order to allow them to exert their biocidal activity. Thus, additional modalities are implemented to enhance their photodegradation, including the use of photosensitizers, photocatalysts (e.g., titanium dioxide), and the addition of reagents such as hydrogen peroxide that would generate hydroxyl radicals that would attack the pesticides.

Pharmaceuticals

The photodegradation of pharmaceuticals is of interest because they are found in many water supplies. They have deleterious effects on aquatic organisms including toxicity, endocrine disruption, genetic damage.>

Mechanism of photodegradation

Many organic chemicals are thermodynamically unstable in the presence of oxygen, however, their rate of spontaneous oxidation is slow at room temperature. In the language of physical chemistry, such reactions are kinetically limited. This kinetic stability allows the accumulation of complex environmental structures in the environment. Upon the absorption of light, triplet oxygen converts to singlet oxygen, a highly reactive form of the gas, which effects spin-allowed oxidations. In the atmosphere, the organic compounds are degraded by hydroxyl radicals, which are produced from water and ozone.

Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290-700 nm (at the surface of the Earth). The energy of an absorbed photon is transferred to electrons in the molecule and briefly changes their configuration (i.e., promotes the molecule from a ground state to an excited state). The excited state represents what is essentially a new molecule. Often excited state molecules are not kinetically stable in the presence of O₂ or H₂O and can spontaneously decompose (oxidize or hydrolyze). Sometimes molecules decompose to produce high energy, unstable fragments that can react with other molecules around them. The two processes are collectively referred to as direct photolysis or indirect photolysis, and both mechanisms contribute to the removal of pollutants.

The United States federal standard for testing plastic for photo-degradation is 40 CFR Ch. I (7–1–03 Edition)PART 238

Protection against photodegradation

Photodegradation of plastics and other materials can be inhibited with additives, which are widely used. These additives include antioxidants, which interrupt degradation processes. Typical antioxidants are derivatives of aniline. Another type of additive are UV-absorbers. These agents capture the photon and convert it to heat. Typical UV-absorbers are hydroxy-substituted benzophenones, related to the chemicals used in sunscreen.

See also: Biodegradable plastic

References

Sources

• Walter Simmler "Air, 6. Photochemical Degradation" in Ullmann's Encyclopedia of Industrial Chemistry 2011, Wiley-VCH, Weinheim.
• Rainer Wolf, Bansi Lal Kaul "Plastics, Additives" in Ullmann's Encyclopedia of Industrial Chemistry 2000, Wiley-VCH, Weinheim.