- Radiation chemistry
Radiation chemistry is a subdivision of
nuclear chemistrywhich is the study of the chemical effects of radiation on matter; this is very different from radiochemistryas no radioactivity needs to be present in the material which is being chemically changed by the radiation. An example is the conversion of water into hydrogengas and hydrogen peroxide.
Reduction of organics by solvated electrons
A recent area of work has been the destruction of toxic organic compounds by irradiation; [Zhao C "et al" (2007) "Radiation Physics and Chemistry", 76:37-45] after irradiation, "
dioxins" (polychlorodibenzo-"p"-dioxins) are dechloroinated in the same way as PCBs can be converted to biphenylan inorganic chloride. This is because the solvated electrons react with the organic compound to form a radical anion, which decomposes by the loss of a chlorideanion. If a deoxygenated mixture of PCBs in isopropanolor mineral oilis irradiated with gamma rays, then the PCBs will be dechlorinated to form inorganic chlorideand biphenyl. The reaction works best in isopropanol if potassium hydroxide(" caustic potash") is added. The base deprotonates the hydroxydimethylmethyl radical to be converted into acetone and a solvated electron, as the result the G value (yield for a given energy due to radiation deposited in the system) of chloride can be increased because the radiation now starts a chain reaction, each solvated electron formed by the action of the gamma rays can now convert more than one PCB molecule. [Ajit Singh and Walter Kremers, "Radiation Physics and Chemistry", 2002, 65(4-5), 467-472] [Bruce J. Mincher, Richard R. Brey, René G. Rodriguez, Scott Pristupa and Aaron Ruhter, "Radiation Physics and Chemistry", 2002, 65(4-5), 461-465] If oxygen, acetone, nitrous oxide, sulfur hexafluorideor nitrobenzene[A. G. Bedekar, Z. Czerwik and J. Kroh, "Pulse radiolysis of ethylene glycol and 1,3-propanediol glasses—II. Kinetics of trapped electron decay", 1990, 36, 739-742] is present in the mixture, then the reaction rate is reduced. This work has been done recently in the USA, often with used nuclear fuelas the radiation source. [ [http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=10116942 Energy Citations Database (ECD) - - Document #10116942 ] ] [ [http://www.patentstorm.us/patents/6132561.html Process for the solvent extraction for the radiolysis and dehalogenation of halogenated organic compounds in soils, sludges, sediments and slurries - US Patent 6132561 ] ]
In addition to the work on the destruction of aryl chlorides it has been shown that
aliphaticchlorine and brominecompounds such as perchloroethylene, [V. Múka, *, R. Silber, M. Pospíil, V. Kliský and B. Bartoníek, "Radiation Physics and Chemistry", 1999, 55(1), 93-97] Freon(1,1,2-trichloro-1,2,2-trifluoroethane) and halon-2402 (1,2-dibromo-1,1,2,2-tetrafluoroethane) can be dehalogenated by the action of radiation on alkaline isopropanol solutions. Again a chain reaction has been reported. [Seiko Nakagawa and Toshinari Shimokawa, "Radiation Physics and Chemistry", 2002, 63(2), 151-156]
In addition to the work on the reduction of organic compounds by irradiation, some work on the radiation induced oxidation of organic compounds has been reported. For instance the use of radiogenic hydrogen peroxide (formed by irradiation) to remove sulfur from
coalhas been reported. In this study it was found that the addition of manganesedioxide to the coal increased the rate of sulfur removal. [P. S. M. Tripathi, K. K. Mishra, R. R. P. Roy and D. N. Tewari, "γ-Radiolytic desulphurisation of some high-sulphur Indian coals catalytically accelerated by MnO2", "Fuel Processing Technology", 2001, 70, 77-96] The degradation of nitrobenzeneunder both reducing and oxidising conditions in water has been reported. [Shao-Hong Feng, Shu-Juan Zhang, Han-Qing Yu, and Qian-Rong Li, "Radiation-induced Degradation of Nitrobenzene in Aqueous Solutions", "Chemistry Letters", 2003, 32(8), 718]
Reduction of metal compounds
In addition to the reduction of organic compounds by the solvated electrons it has been reported that upon irradiation a
pertechnetatesolution, at pH 4.1 is converted to a colloidof technetium dioxide. Irradiation of a solution at pH 1.8 soluble Tc(IV) complexes are formed. Irradiation of a solution at 2.7 forms a mixture of the colloid and the soluble Tc(IV) compounds. [T. Sekine, H. Narushima, T. Suzuki, T. Takayama, H. Kudo, M. Lin and Y. Katsumura, "Colloids and Surfaces A: Physicochemical and Engineering Aspects", 2004, "'249(1-3), 105-109] Gamma irradiation has been used in the synthesis of nanoparticlesof goldon iron oxide (Fe2O3). [http://www.chemistry.or.jp/gakujutu/chem-lett/cl-cont/GRA_03Aug/03080690PG.pdf] [Satoshi Seino, Takuya Kinoshita, Yohei Otome, Kenji Okitsu, Takashi Nakagawa, and Takao A. Yamamoto, "Magnetic Composite Nanoparticle of Au/γ-Fe2O3 Synthesized by Gamma-Ray Irradiation", "Chemistry Letters", 2003, 32(8), 690]
It has been shown that the irradiation of aqueous solutions of
leadcompounds leads to the formation of elemental lead, when an inorganic solid such as bentoniteand sodium formate are present then the lead is removed from the aqueous solution. [M. Pospίšil, V. Čuba, V. Múčka and B. Drtinová, "Radiation removal of lead from aqueous solutions- effects of various sorbants and nitrous oxide", "Radiation Physics and Chemistry", 2006, 75, 403-407]
Another key area uses radiation chemistry to modify polymers. Using radiation, it is possible to convert
monomers to polymers, to crosslink polymers, and to break polymer chains. [ [http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=7313004 Energy Citations Database (ECD) - - Document #7313004 ] ] [ [http://mitr.p.lodz.pl/biomat/raport/3_5_radiation_hydrogels.html IAEA report - Radiation Formation of Hydrogels for Biomedical Applications; the use of radiation technique - Mechanism of the radiation-induced crosslinking of polymers in aqueous solution ] ] Both man-made and natural polymers (such as carbohydrates [ [http://www-pub.iaea.org/MTCD/publications/PDF/te_1422_web.pdf IAEA-TECDOC-1422 ] ] ) can be processed in this way.
Both the harmful effects of radiation upon biological systems (induction of
cancerand acute radiation injuries) and the useful effects of radiotherapy involve the radiation chemistry of water. The vast majority of biological molecules are present in an aqueous medium; when water is exposed to radiation, the water absorbs energy, and as a result forms chemically reactive species that can interact with dissolved substances ( solutes). Water is ionized to form a solvated electronand H2O+, the H2O+ cation can react with water to form a hydrated proton (H3O+) and a hydroxyl radical (HO.). Furthermore, the solvated electron can recombine with the H2O+ cation to form an excited state of the water, this excited state then decomposes to species such as hydroxyl radicals (HO.), hydrogen atoms (H.) and oxygen atoms (O.). Finally, the solvated electron can react with solutes such as solvated protons or oxygen molecules to form respectively hydrogen atoms and dioxygen radical anions. The fact that oxygen changes the radiation chemistry might be one reason why oxygenated tissues are more sensitive to irradiation than the deoxygenated tissue at the centre of a tumor. The free radicals, such as the hydroxyl radical, chemically modify biomolecules such as DNA, leading to damage such as breaks in the DNA strands. Some substances can protect again radiation-induced damage by reacting with the reactive species generated by the irradiation of the water.
It is important to note that the reactive species generated by the radiation can take part in "following reactions", this is similar to the idea of the non-electrochemical reactions which follow the electrochemical event which is observed in
cyclic voltammetrywhen a non-reversible event occurs. For example the SF5 radical formed by the reaction of solvated electrons and SF6 undergo further reactions which lead to the formation of hydrogen fluorideand sulfuric acid. [K.-D. Asmus and J.H. Fendler, "The reaction of sulfur hexafluoride with solvated electrons", "The Journal of Physical Chemistry", 1968, 72, 4285-4289]
In water the dimerisation reaction of hydroxyl radicals can form
hydrogen peroxide, in saline systems the reaction of the hydroxyl radicals with chlorideanions form hypochloriteanions.
It has been suggested that the action of radiation upon underground
wateris responsible for the formation of hydrogen which was converted by bacteria into methane. [http://deepbio.princeton.edu/samp/papers/LinetalGCA69-893.pdf] . [LI-HUNG LIN, GREG F. SLATER, BARBARA SHERWOOD LOLLAR, GEORGES LACRAMPE-COULOUME and T. C. ONSTOTT, "Geochimica et Cosmochimica Acta", 2005, 69, 893-903.] A series of papers on the subject of bacteria living under the surface of the earth which are fed by the hydrogen generated by the radiolysis of water can be read on line. [http://wetlands.ifas.ufl.edu/sickman/SOS%206932/Ocean%20vent%20papers.pdf]
Industrial processing equipment
To process materials, either a gamma source or an electron beam can be used. The international type IV ("wet storage") irradiator is a common design (the JS6300 and JS6500 gamma sterilizers (made by 'Nordion International' [http://www.mds.nordion.com/] , which used to trade as 'Atomic Energy of Canada Ltd') are typical. [Features of the design are discussed in the
International Atomic Energy Agencyreport on a human erroraccident in such an irradiation plant [http://www-pub.iaea.org/MTCD/publications/PDF/Pub847_web.pdf] ] In these irradiation plants, the source is stored in a deep well filled with water when not in use. When the source is required, it is moved by a steel wire to the irradiation room where the products which are to be treated are present; these objects are placed inside boxes which are moved through the room by an automatic mechanism. By moving the boxes from one point to another, the contents are given a uniform dose. After treatment, the product is moved by the automatic mechanism out of the room. The irradiation room has very thick concrete walls (about 3 m thick) to prevent gamma rays from escaping. The source consists of 60Co rods sealed within two layers of stainless steel, the rods are combined with inert dummy rods to form a rack with a total activity of about 12.6PBq (340kCi).
While it is possible to do some types of research using an irradiator much like that used for gamma sterilization, it is common in some areas of science to use a "time resolved" experiment where a material is subjected to a pulse of radiation (normally
electrons from a LINAC. After the pulse of radiation, the concentration of different substances within the material are measured by emission spectroscopyor Absorption spectroscopy, hence the rates of reactions can be determined. This allows the relative abilities of substances to react with the reactive species generated by the action of radiation on the solvent (commonly water) to be measured. This experiment is known as pulse radiolysis[ [http://www.dur.ac.uk/oem.group/Research/radiolysis/Radiolysispage.htm pulse radiolysis ] ] which is closely related to Flash photolysis.
In the latter experiment the sample is excited by a pulse of light to examine the decay of the excited states by
spectroscopy[http://www.chem.uic.edu/chem343/Flash-Photolysis.pdf] ; sometimes the formation of new compounds can be investigated. [George Porter, Nobel lecture, 11 December 1967] [http://nobelprize.org/nobel_prizes/chemistry/laureates/1967/porter-lecture.pdf] Flash photolysis experiments have led to a better understanding of the effects of halogen-containing compounds upon the ozone layer. [http://www.bfrl.nist.gov/866/HOTWC/HOTWC2006/pubs/R0000232.pdf] chemosensor
The SAW chemosensor [ [http://www.jneurosci.org/cgi/reprint/22/13/5291?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&searchid=1&FIRSTINDEX=0&minscore=5000&resourcetype=HWCIT Abnormal Chemosensory Jump 6 Is a Positive Transcriptional Regulator of the Cholinergic Gene Locus in Drosophila Olfactory Neurons - Lee and Salvaterra 22 (13): 5291 - Journal of Neuroscience ] ] is nonionic and nonspecific. It directly measures the total mass of each chemical compound as it exits the gas
chromatographycolumn and condenses on the crystal surface, thus causing a change in the fundamental acoustic frequency of the crystal. Odor concentration is directly measured with this integrating type of detector. Column flux is obtained from a microprocessor that continuously calculates the derivative of the SAWfrequency.
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