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Biodegradation is the chemical breakdown of materials by a physiological environment. The term is often used in relation to ecology, waste management and environmental remediation (bioremediation). Organic material can be degraded aerobically with oxygen, or anaerobically, without oxygen. A term related to biodegradation is biomineralisation, in which organic matter is converted into minerals. Biosurfactant, an extracellular surfactant secreted by microorganisms, enhances the biodegradation process. To make it more simple biodegradation means an object which disappears in its own time.

Biodegradable matter is generally organic material such as plant and animal matter and other substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms. Some microorganisms have the astonishing, naturally occurring, microbial catabolic diversity to degrade, transform or accumulate a huge range of compounds including hydrocarbons (e.g. oil), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pharmaceutical substances, radionuclides and metals. Major methodological breakthroughs in microbial biodegradation have enabled detailed genomic, metagenomic, proteomic, bioinformatic and other high-throughput analyses of environmentally relevant microorganisms providing unprecedented insights into key biodegradative pathways and the ability of microorganisms to adapt to changing environmental conditions.[1]

Contents

Methods of measuring biodegradation

Biodegradation can be measured in a number of ways. The activity of arobic microbes can be measured by the amount of oxygen they consume or the amount of carbon dioxide they produce. It can be measured by anaerobic microbes and the amount of methane or alloy that they may be able to produce.

Plastics

Biodegradable plastics. There are two main types of biodegradable plastics in the market: hydro-biodegradable plastics (HBP) and oxo-biodegradable plastics (OBP). Both will first undergo chemical degradation by hydrolysis and oxidation respectively. This results in their physical disintegration and a drastic reduction in their molecular weight. These smaller, lower molecular weight fragments are then amenable to biodegradation.

OBPs are made by adding a small proportion of compounds of specific transition metals (iron, manganese, cobalt and nickel are commonly used) into the normal production of polyolefin (PE & PP) and polystyrene. The additives act as catalysts* in speeding up the normal oxidative degradation, with the overall process increased by up to several orders of magnitude (factors of 10). (*It should be noted that catalysts of many kinds are widespread in Nature; others are used very commonly by industry. By definition, it takes only a small amount of catalyst to do what is required and the catalyst is not consumed in the reaction.) The products of the catalyzed oxidative degradation of the polyolefins are precisely the same as for conventional polyolefins because, other than a small amount of additive present, the plastics are conventional polyolefins. Many commercially useful hydrocarbons (e.g., cooking oils, polyolefins, many other plastics) contain small amounts of additives called antioxidants that prevent oxidative degradation during storage and use. Antioxidants function by ‘deactivating’ the free radicals that cause degradation. Lifetime (shelf life + use life) is controlled by antioxidant level and the rate of degradation after disposal is controlled by the amount and nature of the catalyst.

Since there are no existing corresponding standards that can be used directly in reference to plastics that enter the environment in other ways other than compost - i.e. as terrestrial or marine litter or in landfills, OBP technology is often attacked by the HBP industry as unable to live up to the standards (which are actually the standards for composting). It has to be understood that composting and biodegradation are not identical. OBP can however be tested according to ASTM D6954, and (as from 1.1.2010) UAE 5009:2009.


HBPs tend to degrade and biodegrade somewhat more quickly than OBP, but they have to be collected and put into an industrial composting unit. The end result is the same - both are converted to carbon dioxide (CO2), water (H2O) and biomass. OBP are generally less expensive, possess better physical properties and can be made with current plastics processing equipment. HBP emits methane in anaerobic conditions, but OBP does not.

Polyesters play a predominant role in hydro-biodegradable plastics due to their potentially dydrolysable ester bonds. HBP can be made from agricultural resources such as corn, wheat, sugar cane, or fossil (petroleum-based)resources , or blend of thee two. Some of the commonly-used polymers include PHA (polyhydroxyalkanoates), PHBV (polyhydroxybutyrate-valerate), PLA (polylactic acid), PCL (polycaprolactone), PVA (polyvinyl alcohol), PET (polyethylene terephthalate) etc. It would be misleading to call these "renewable" because the agricultural production process burns significant amounts of hydrocarbons and emits significant amounts of CO2. OBPs (like normal plastics)are made from a by-product of oil or natural gas, which would be produced whether or not the by-product were used to make plastic.

HBP technology claims to be biodegradable by meeting the ASTM D6400-04 and EN 13432 Standards. However, these two commonly quoted standards are related to the performance of plastics in a commercially managed compost environment. They are not biodegradation standards. Both were developed for hydro-biodegradable polymers where the mechanism including biodegradation is based on reaction with water and state that in order for a production to be compostable, the following criteria need to be met:

  1. Disintegration, the ability to fragment into non-distinguishable pieces after screening and safely support bio-assimilation and microbial growth;
  2. Inherent biodegradation, conversion of carbon to carbon dioxide to the level of 60% and 90% over a period of 180 days for ASTM D6400-04 and EN 13432 respectively; There is therefore little or no carbon left for the benefit of the soil, but the CO2 emitted to atmosphere contributes to climate-change.
  3. Safety, that there is no evidence of any eco-toxicity in finished compost and soils can support plant growth; and
  4. Toxicity, that heavy metal concentrations are less than 50% regulated values in soil amendments

See also

References

  1. ^ Diaz E (editor). (2008). Microbial Biodegradation: Genomics and Molecular Biology (1st ed.). Caister Academic Press. ISBN 978-1-904455-17-2. http://www.horizonpress.com/biod. 

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