Abiotic degradation/peroxidation

All three chloroacetic acids (chloroacetic acid [MCA], dichloroacetic acid [DCA], and trichloroacetic acid [TCA]) are naturally occurring (7), with TCA being identified in the environment most frequently (reviews (278, 405 108)). However, these chlorinated acetic acids also have anthropogenic sources. The major source of natural TCA appears to be the enzymatic (chloroperoxidase) or abiotic degradation of humic and fulvic acids, which ultimately leads to chloroform and TCA. Early studies (409) and subsequent work confirm both a biogenic and an abiotic pathway. Model experiments with soil humic and fulvic acids, chloroperoxidase, chloride, and hydrogen peroxide show the formation of TCA, chloroform, and other chlorinated compounds (317, 410-412). Other studies reveal an abiotic source of TCA (412, 413). 

Sun, Y. and J.J. Pignatello (1993). Activation of hydrogen peroxide by iron(III) chelates for abiotic degradation of herbicides and insecticides in water. J. Agile. Food Chem., 41 308-312. 

A good deal is now known about the kinetics of abiotic peroxidation and stabilisation of carbon-chain polymers and it is possible in principle to extrapolate to the time for ultimate oxidation from laboratory experiments. As already indicated, the key determinant of the time to bioassimilation is the antioxidant and if this is chosen to optimize the service life, bioassimilation can also be achieved as in the case of wood, straw, twigs, etc. It seems that straw is a particularly appropriate model for the biodegradation of the polyolefins since, like the polyolefins, it fully bioassimilated in biologically active soil over a period of about ten years. The most important conclusion from recent work is that nature does not depend on just one degradation mechanism. Abiotically initiated peroxidation is just as important, at least initially as biooxidation. 

Most of the studies on PLA degradation have concentrated on abiotic hydrolysis [35-37]. The effects of, e.g., residual monomer and other impurities, molecular weight and copolymerization on hydrolysis rate and properties have been studied [3,37-42]. Impurities, residual monomer [43,44], and peroxide modification [45] all increase the hydrolysis rate, while copolymerization can either increase (GA-copolymers) or decrease (CL, DXO-copolymers) the hydrolysis rate. Degradation of PLA and its copolymers in clinical applications ranging from absorbable sutures to drug delivery systems and artificial ligaments has also been widely studied [46-48]. 

Bioassimilation of polymers is primarily controlled by the rate of abiotic peroxidation or hydrolysis. Information is available in the published literature which could form the basis of biodegradation tests but has so far not been incorporated in standard test methods. However, in the long-term it is important that science-based test methods should be developed to permit users of degradable materials in agriculture, shipping and packaging to take advantage of these new materials. 

Nocardia and P. aeruginosa were shown to break the cw-PI chain by an oxidative mechanism since aldehyde groups were found to accumulate during microbial degradation. This is always the first product formed during the abiotic peroxidation of cw-PI and the evidence suggests that the bacteria initiate a radical-chain peroxidation. This will be discussed further in the context of polyolefin biodegradation. 

The hydroxyl radical is one of the most reactive free radicals known in chemistry. It is 10 more reactive than hydroperoxyf and it extracts a hydrogen atom at every encoimter with a hydrocarbon. It is thus also one of the most potent initiators of peroxidation known. It is not surprising then that the subsequent polymer degradation reactions are dominated by abiotic peroxidation chemistry. 

Chiellini et al. [58] extracted thermally peroxidised polyethylene with acetone and measured the rate of mineralization of the solvent free extracts in forest soil. This is compared with cellulose and a number of low molar mass control hydrocarbons in Fig. 2. Surprisingly, the peroxidation products were converted to carbon dioxide and water more rapidly than cellulose. The extracted polyethylene degraded at a similar rate to the pure hydrocarbons and it is evident from this work that the rate controlling process in the overall sequence of degradation reactions is the initial peroxidation of the polymer. It has been demonstrated [19] that the exposure of peroxidised PE to an abiotic water-leaching environment did not remove the peroxidation products from the polymer, whereas bioassimilation began immediately (see Fig. 2)... 

It was seen in Chapter 3 that hydrocarbon polymers, of which natural rubber is a naturally occurring example, degrade both abiotically and biotically by a peroxidation chain mechanism. This process is accelerated by the introduction of photosensitive... 

It is easier to achieve a carbon mass balance by temporally separating the peroxidation process from the biodegradation process. As discussed in Chapter 3, several workers have successfully applied this technique to degradable rubbers and polyolefins. CO2 formation begins abiotically during thermal (and photo-) oxidation and continues during the bioassimilation of the polymer. In the case of rubbers it has been found possible to correlate mass-loss with the mass of the protein produced by the polymer in soil. 

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