Photodegradation

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Photo-degradation is the alteration of a molecule by photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. However, other forms of electromagnetic radiation can cause photo-degradation. Photo-degradation includes photodissociation, the breakup of molecules into smaller pieces by photons. It also includes the change of a molecule's shape to make it irreversibly altered, such as the denaturing of proteins, and the addition of other atoms or molecules. A common photo-degradation reaction is oxidation. This type of photo-degradation is used by some drinking water and waste water facilities to destroy pollutants. Photodegradation in the environment is part of the process by which ambergris evolves from its fatty precursor.

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

Photo-degradation also destroys paintings and other artifacts.

Sunlight breaks down contaminants[edit]

Organic chemicals are thermodynamically unstable in the presence of oxygen. However the kinetics of spontaneous oxidation reactions are slow because the direct reaction of oxygen with most organics is spin-forbidden. This kinetic stability allows the accumulation of complex environmental structures in the environment. These structures can range in scale from complex living organisms to single molecules.

The degradation of organic structures (molecules) is part of natural attenuation and is often accelerated by chemical processes associated with exposure to sunlight. Photochemical reactions in the environment 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 (e.g. 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.

There are several classes of pollutants that are known to photo-degrade. One of the most interesting are pharmaceuticals associated with human or veterinary uses. They are justifiable objects of research because these materials collectively are designed to interact with the human ecosystem at low levels, where the activity of many other classes of pollutant is incidental to their original function. The occurrence of pharmaceuticals in the hydrosphere is widespread. Recent reports indicate that many of these compounds persist in the aquatic systems in amount ranging from ng to µg/l (more information required). Several factors contribute to these levels; population growth, growth in use for veterinary applications and improved analytical techniques. Many of the pharmaceuticals are not effectively attenuated during waste water treatment so they are often discharged with treated sewage into receiving waters. Because of their high water solubility many pharmaceuticals concentrate in the hydrosphere, therefore their attenuation in the hydrosphere is important.

Photodegradation of NSAIDs[edit]

Non-steroidal anti-inflammatory drugs (NSAIDs) are a group of pharmaceuticals that are used widely for their analgesic (pain reducing), antipyretic (fever reducing) and anti-inflammation properties. NSAIDs include ibuprofen, ketoprofen, naproxen, aspirin, diclofenac, indomethacin, tolmetin, etc. In the United States, ibuprofen is taken by 17% of adults, aspirin by 17% of adults, and naproxen by 3.5% of adults per week (in 2002). In the year 2001, 30 million tablets of various NSAIDs were sold in United States. Most of these drugs are available over the counter.

Propionic acid derivatives[edit]

Propionic acid derivative NSAIDs like ibuprofen have been detected in surface waters of Germany, Finland and France and Canada. These compounds are known to undergo direct photolysis using several pathways, one of which results into cleavage of carbon- carbon bond to generate carboxy groups. These intermediates are further transformed by hydrogen abstraction, dimerization, and reaction with oxygen to generate number of photodegradation products. They may also react with the scavenger hydroxyl radicals to form hydroxyl compounds. NSAIDs, especially those that have amine and chlorine groups, upon photolysis generate carbazoles with quinone groups, aldehydes, and carboxylic acids.

Indole and indene acetic acid derivatives[edit]

One of the most commonly known NSAID of this class is indomethacin. The average concentration of this compound in waste water is 0.47 µg/l. This compound also undergoes direct photolysis to yield up to eight photo-products. The main route for its photo-chemical degradation is decarboxylation. This is then followed by oxygenation to yield alcohol and aldehyde. Photolysis can also result into hydrolysis of CO-N bond and oxidative cleavage of C-C bond.

Applications[edit]

Torkan Packaging in W.A. was the first company to introduce Photo-degradable plastic shopping bags in Australia in the 1980s.

Six-pack soda can rings have been photo-degradable LDPE plastic since the 1990s.

References[edit]

Citations[edit]

Sources[edit]

  • Castell, JV; Gomez-L, MJ; Miranda, MA; Morera, IM (2008), "Photolytic degradation of Ibuprofen. Toxicity of the isolated photoproducts on fibroblasts and erythrocytes", Photochemistry and Photobiology 46 (6): 991–96, doi:10.1111/j.1751-1097.1987.tb04882.x 
  • Salgado, R;Pereira, VJ; Carvalho, G; Soeiro, R; Gaffney, V; Almeida, C; Vale Cardoso, V; Ferreira, E; Benoliel, MJ; Ternes, TA; Oehmen, A; Reis, MAM; Noronha, JP (2013), "Photodegradation kinetics and transformation products of ketoprofen, diclofenac and atenolol in pure water and treated wastewater", Journal of Hazardous Materials (244-245): 516–52, doi:10.1016/j.jhazmat.2012.10.039 
  • Temussi, F; CErmola, F; DellaGreca, M; Iesce, MR; Passananto, M; Previtera, L; Zarrelli, A (2011), "Determination of photostability and photodegradation products of indomethacin in aqueous media", Journal of Pharmaceutical and Biomedical Analysis 56: 678–83, doi:10.1016/j.jpba.2011.07.005