Biodegradation End Products

2 End of Life Scenarios of Biodegradable Polymers 3.2.1 Biodegradation End Products 

Hereafter, organic and inorganic carbon exchanges are briefly described in order to give a quick overview of the principles that are involved and the role of biodegradation. 

Plants are able to synthesise organic molecules starting from inorganic substances. This is characteristic of autotrophic organisms which are able to uptake and assimilate inorganic compounds and convert inorganic elements into organic molecules. 

For example. Equation 3.1 shows the synthesis of glucose (C6Hi20c), a core molecule in plant metabolism, starting from atmospheric COiand soil H2O  

Similarly, the biodegradation process affects more complex organic molecules such as natural polymers (starch, cellulose and so on) and some man-made polymers. This is indeed the case of biodegradable polymers used for the production of plastic articles that have been designed with the aim of being biodegraded in the soil or in composting plants. 

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Green chemistry also calls for design for biodegradable end products, principally, by employing chemicals from renewable sources, and dictates the use of real-time, on-line analysis for better process control. 

Complex organics and toxics are chemically oxidized to end products of CO2 and H2O or to iatermediate products which are nontoxic and biodegradable. 

Most uses of chloroisocyanurates are regulated by the EPA under EIERA. Cyanuric acid (or cyanurate) is ultimately the end product of use of chloroisocyanurates in bleaching, sanitizing, and disinfection appHcations. Since the N-chloro derivatives are biocidal, biodegradation studies have centered on the residual CA. Biodegradation occurs (128). 

Laboratory studies have provided inconsistent results regarding the degree to which MTBE is biodegraded anaerobically to end products and the extent to which other oxygenates are biodegraded under anaerobic conditions. 

Environmental fate Chemicals released in the environment are suscephble to several degradahon pathways, including chemical (i.e., hydrolysis, oxidation, reduction, dealkylahon, dealkoxylation, decarboxylahon, methylation, isomerization, and conjugation), photolysis or photooxidahon and biodegradation. Compounds transformed by one or more of these processes may result in the formation of more toxic or less toxic substances. In addihon, the transformed product(s) will behave differently from the parent compound due to changes in their physicochemical properties. Many researchers focus their attention on transformahon rates rather than the transformahon products. Consequently, only limited data exist on the transitional and resultant end products. Where available, compounds that are transformed into identified products as weh as environmental fate rate constants and/or half-lives are listed. 

When looking at the life cycle of biodegradable plastics, two aspects are of particular importance the end-of-life options and the use of renewable resources in the material production (the major part of the currently available biodegradable plastic products are made of blends of fossil-based polymers and polymers derived from biomass). 

The BAT system operates based on principles of aerobic cometabolism. In cometabohsm, enzymes that the microbes produce in the process of consuming one particular compound (e.g., phenol) have the collateral effect of transforming another compound that normally resists biodegradation (e.g., chlorinated ethenes, especially lesser chlorinated ethenes such as dichloroethene or vinyl chloride). The BAT system operates under these principles by sorbing the chlorinated compounds from a vapor stream onto powdered activated carbon (PAC) where they are cometabolically transformed into a combination of end products, including new biomass, carbon dioxide, inorganic salts, and various acids. 

Bioremediation aims at innocuous end-products. In aerobic systems, CP mineralization is frequently achieved. Anaerobic processes often produce less-chlorinated end-products, which are environmentally less harmful and easier to degrade by aerobic microorganisms than the starting compounds. The pathways of CP biodegradation are well characterized. 

It is often not possible to state at what point in a metabolic loop biosynthesis has been completed and biodegradation begins. An end product X that serves one need of a cell may be a precursor to another cell component Y which is then degraded to complete the loop. The reactions that convert X to Y can be regarded as either biosynthetic (for Y) or catabolic (for X). 

A particular mention goes to Mater-Bi, produced by Novamont, who have revolutionised starch-based biomaterials for two decades. The commercial success of this biodegradable and biocompostable plastic relies on two main factors the scale economy that allows the reduction of costs, and the diversity of formulations to develop different end products (plastic bags, tableware, toys, etc.). More than 210 references in Chemical Abstracts are available on this (registered) keyword, and the number of patents related to different formulations and developments is also impressive. Mater-Bi can be essentially described as a blend of starch with a small amount of other biodegradable polymers and additives. The actual compositions are still known only by a very few people. 

One of the first applications of biodegradable materials is based on the cooked, extruded, and expanded starch known from the food and chemical sectors (Fig. 14.23). Starch is cooked with water in the extruder and chemically modified as necessary or mixed with plasticizers, then expanded to a starch foam and dried. The extrudate is ground so that the functional properties thus created can be used in the food/chemicals sector. The foamed, cut, and dried extrudate is the end product for loose-fill packaging applications. The degree of expansion is a measure of the foam texture. It increases strongly with product temperature at the die, helped by a higher specific mechanical energy input. However, both measures increase the water-solubility of the product. 

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