Photo-oxidation abiotic degradation

Dioxane is a cyclic ether compound that is miscible with water in all proportions and is also moderately volatile. It is resistant to hydrolysis and microbial degradation, but may undergo photolysis at water and soil surfaces. An estimate of the half-life for abiotic degradation in water with addition of ozone was 60 h. The half-life for photo-oxidation in air was 3.4 h. 1,4-Dioxane has a low adsorption potential and a high mobility/leaching potential in soil/water systems. No bioaccumulation of this chemical is expected. 

The absorption of solar radiation by abiotic sea water constituents initiates a cascade of reactions leading to the photo-oxidative degradation of organic matter and the concomitant production (or consumption) of a variety of trace gases and LMW... 

Whether biotic or abiotic, degradation is essentially lydiolytic or (photo/thermo)-oxidative (other modes exist such as acid attacks by Thiobacillus). For example, for the hydrolysis of poly(lactic add) (PLA), the reaction is veiy slow at low temperatures (20-25°C), but very fast at temperatures around 60°C (Figure 14.2). We can also mention the abiotic degradation of poly(ethylene) by photo- and thermo-oxidation (Figure 14.3) which results in a drastic decrease in molar masses, a loss of mechanical properties of the material, an increase in the carbonyl index up to 1713 cm and the progressive appearance of degradation molecules, such as ketones, alcohols, acids and so on. 

Figure 14.3. Example of the abiotic degradation ofpoly(ethylene) by photo- and thermo-oxidation (according to Jacques Lemaire CNEP)...

Abiotic hydrolysis, photo-oxidation and physical disintegration of polymers may enhance the biodegradation rate of polymers by increasing their surface contact area for microbial colonisation or by reducing the molecular weight [113]. In the case of lactic acid homo- and heteropolymers, abiotic hydrolysis is the most important reaction for initiating the environmental degradation... 

In general, abiotic degradation of plasties in the enviromnent results in a partial degradation which is due to photo-oxidation, hydrolysis, oxidation and photolysis. This will result in the formation of fragments whieh may remain in the environment for an indefinite period or may be further degraded biologically. 

Several of the more common commodity polymers like the polyolefins are susceptible to photo-oxidation. For a polymer like polyethylene, photo-oxidation leads to increasing amounts of carbonyl compounds. In-chain ketone groups act as sensitisers by UV light absorption. Through the well-known Norrish type I and II degradations radicals, end-vinyl and ketone groups are formed. Other products often observed in photo-oxidised low-density polyethylene (LDPE) are esters [5]. Scheme 1 shows one mechanism for abiotic ester formation. By Norrish type I cleavage the radical formed can react with an alkoxyl radical on the polyethylene (PE) chain. 

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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