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Sat Feb 10, 2018, 07:48 PM

Radiolytic degradation of CFCs.

In one of the rare successes of an environmental treaty, the CFC's, chlorofluorocarbons, were banned internationally by the 1987 protocol because of their potential for depleting ozone.

However these species, which are also greenhouse gases, are long lived in the atmosphere, and many remain in old refrigerators, air conditioners and even spray cans. Thus they are still present in the atmosphere.

They degrade by termination of chain reactions, induced in the upper atmosphere, primarily by interaction with UV, X-ray, and gamma radiation associated with outer space and the high ionosphere.

High energy radiation is of course, available in the lower atmosphere either by the use of accelerators - from a radiochemical perspective the most often utilized energy consuming tool in this regard is the electron beam - or from the decay of radioactive materials, most commonly in the lab either Co-60 made by bombarding natural cobalt with neutrons in a nuclear reactor, or by use of the fission product cesium-137, which is available on a macroscopic scale.

Most often when I look for literature on this subject, I am mostly concerned with halogenated species in water, for example the famous PCB's which continue to pollute the Hudson River, but while going through such literature today, I came across a study on the degradation of CFC's.

The paper is here: Chain reaction on de-halogenation of 1,2-dibromotetrafluoroethane and 1,1,2-trichlorotrifluoroethane induced by irradiation in alcohols (Nakagawa, Radiation Physics and Chemistry Volume 108, March 2015, Pages 29-32

It's a brief but interesting paper.

Some brief excerpts:

Chloro-fluoro aliphatic carbons (CFC) and bromo-fluoro aliphatic carbons (Halon) are considered environmental pollutants. Some techniques have been studied to decompose halogenated carbons. Irradiation with gamma-rays or high-energy electrons in alkaline 2-propanol solution seems to be one of the effective methods for degradation of chlorinated carbons, not only CFCs but also polychlorinated aromatic carbons, such as polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated biphenyls (PCB), polychlorinated phenols (PCP), polychlorinated benzenes (PCBz), and so on. There are many reports on the degradation of chlorinated carbons, and the reaction mechanisms have been well studied (Mucka et al., 1997; Schmid et al., 1997; Schmelling et al., 1998; Yamamoto and Tagawa, 1999; Hirota et al., 2000; Nakagawa and Shimokawa, 2002). On the contrary, few studies on the decomposition of brominated carbons exist. Though also investigated the degradation of 1,2-dibromotetrafluoroethane (Halon2402) in alkaline 2-propanol solution, they could not observe the product Halon2401, produced from the substitution of one of the bromine atoms in Halon2402 with a hydrogen atom. It is suspected that the mechanism for decomposition of Halon2402 in 2-propanol solution is entirely the same as that of chlorinated carbons.

In this report, the degradation of Halon2402 induced by irradiation in 2-propanol and methanol was studied in detail with careful de-oxygenation, and the charge transfer from an alcohol radical to Halon2402 was found to be the trigger of the chain reaction in pure alcohol

In this paper both Co-60 and Cs-137 were utilized.

It was found that in both methanol and isopropanol (rubbing alcohol) both bromo and chloro fluorocarbons were dechlorinated and or debrominated. However methanol was less efficient and detectable amounts of the probably inert (but still a greenhouse gas) tetrafluoroethylene was formed from the halon 2402 gave measurable yields (by Gas Chromatography) tetrafluoroethylene aka 1,1,2,2-tetrafluoroethene.

The halides formed were measured by old fashioned chemistry, a Mohr's titration, and so it is not clear the extent to which defluorination took place, since Mohr's titrations do not really distinguish between halides, if I recall their chemistry well.

Defluorination is a tougher not than dechlorination or debromination, but not actually impossible, at least with gamma radiation, particularly where gaseous or liquid water is available.

These reactions were conducted in alcoholic solvents in the absence of catalysts like titanium dioxide.

Thus as a practical matter, they are only applicable to the destruction of isolated CFC's.

Of course, the same chemistry as takes place in the upper atmosphere would also work at ground level. Ozone is essential in the upper atmosphere, but at ground level it is a serious air pollutant with huge health consequences. Irradiation of air, therefore, containing CFC's - as all air now does - would therefore destroy ozone. Further, owing to the Maxwell-Boltzmann distribution, CFC's, being relatively heavy molecules in comparison to air - which has a density very close to that of pure nitrogen - is far more concentrated at ground level than in the upper atmosphere.

The storage of radioactive materials in highly polluted cities would therefore mitigate these health issues significantly. However, realistically, there is too much fear and ignorance surrounding nuclear materials for that wise decision ever to happen, but well, the supposition that ignorance is not good for health is not a new concept and is well established and regrettably, widely observed.

I wish you a pleasant Sunday.

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