Formation and Degradation of Halogenated Organic Acids. Radiation versus Photocatalytically Induced Processes

Abstract

Radiation chemistry and photocatalysis are introduced as complementary methods for the study of the radical- and redox-induced degradation of organic substrates. Particular focus is devoted on the reductive and oxidative formation and destruction of halogenated organic acids, as intermediates in the degradation of halo- genated hydrocarbons, in dilute aqueous solutions. The specific aim of this comparative essay is to point out the fundamental similarities between a radiation chemical and photocatalytical approach but, at the same time, to demonstrate that and why significant differences may, nevertheless, be observed. The examples presented and discussed are concerned with (i) the oxidatively and reductively induced degradation of 1,1,1-trichloroethane to organic acids, (ii) corresponding experiments on the degradation of trifluoro-, trichloro- and tribromoacetic acid, (iii) peculiarities of the degradation processes in the absence of oxygen, and (iv) the importance of the cross­termination reaction between halogenated peroxyl radicals and superoxide, O

On the Role of Superoxide in the Radical-Induced Degradation of Halogenated Organic Compounds. Evidence for Cross-Termination between O2•- and Halogenated Peroxyl Radicals

“…Two further possible products, namely, CO and stable hydroperoxides, were also checked for, but neither was detectable under any experimental conditions in our present model system. This was done since (i) CO is a product formed in appreciable amounts from the chlorine and bromine analogues of • OOCF 2 CO 2 - , and (ii) hydroperoxides are actually postulated in our mechanism (see below) although only as short-lived molecular intermediates. The claim that no stable hydroperoxides (and/or peroxides) are formed is based on the fact that no I - → I 2 oxidation was observed anymore after treating the irradiated sample with catalase (160 U/mL), an enzyme which selectively destroys H 2 O 2 .…”

“…This statement also holds in view of a recent observation of ours that hydrolysis of oxalyl dibromide in diluted aqueous solutions does, in fact, not lead to oxalate but to stoichiometric amounts of bromide, carbon monoxide, and carbon dioxide (eq 24). 20 Since in the first stage of this hydrolysis process, Br(O)CCO 2 -, an analogue of F(O)CCO 2 -, is likely to be formed, we have checked for the possible formation of CO in our CDFA system. However, not even traces of CO could be detected.…”