Biodegradable additives: Difference between revisions

Biodegradable additives are additives that enhance the biodegradation of polymers by allowing microorganisms to utilize the carbon within the polymer chain as a source of energy. Biodegradable additives attract microorganisms to the polymer through quorum sensing after biofilm creation on the plastic product. Additives are generally in masterbatch formation that use carrier resins such as polyethylene (PE), polypropylene (PP), polystyrene (PS) or polyethylene terephthalate (PET).

Most common synthetic plastics are not biodegradable, and both chemical and physical properties of plastics play important roles in the process of plastic degradation. The addition of biodegradable additives can influence the mechanism of plastic degradation by changing the chemical and physical properties of plastics to increase the rate of degradation.^[1] Biodegradable additives can convert the plastic degradation process to one of biodegradation. Instead of being degraded simply by environmental factors, such as sunlight (photo-oxidation) or heat (thermal degradation), biodegradable additives allow polymers to be degraded by microorganisms and bacteria through direct or indirect attack.

While some plastic additives merely affect the surface of plastics (ex. colorants), effective biodegradable additives must change the interior of the plastics and their chemical properties, as well.^[2] Good biodegradable additives expedite the rate of degradation by reducing the strength of certain properties of the polymers and increasing their attractiveness to microorganisms.

Mechanism of biodegradation[edit]

In general, the process of microbial plastic biodegradation results in a considerable decrease in polymer molecular weight, causing the plastic to lose its structural integrity. There are several different ways in which microorganisms can carry out the process of plastic degradation, and the mechanism differs slightly depending on the environmental conditions.

Direct Action[edit]

Some microorganisms can directly consume plastic fragments and use the carbon as a nutritional source. For example, Brevibacillus borstelensis, Rhodococcus rubber, Pseudomonas chlororaphis, and Comamonas acidovorans TB-35 have all been shown experimentally to use direct action to consume polyethylene.^[3] For other less commonly used plastics, researchers have only found one strain of microbe capable of directly degrading a specific plastic. More research is currently being done to discover other microbial strains that can effectively biodegrade plastics.

Polymer molecular weight plays a significant role in whether microorganisms can use direction action to break down plastics because it is quite difficult for microorganisms to directly degrade high molecular weight polymers. Functional groups on the polymer also determine whether a polymer will be directly degraded, with large substituents being more difficult to degrade.^[4]

Indirect Action[edit]

Microbes involved in the breakdown of fossil-based plastics typically use an indirect mechanism in which microbial enzymes break down the plastic. Through indirect action, the metabolic products of the microorganism affect the properties of the plastic, resulting in degradation.^[3]

Enzyme-based microbial biodegradation can occur under two conditions: aerobic and anaerobic. Plastics are typically made up of hydrophobic polymers, so the first step of biodegradation under both conditions involves the breakdown of the polymer by the enzyme into smaller constituents such as oligomers, dimers, and monomers.^[6] The breaking down the plastic into smaller molecules is known as hydrolysis or oxidation, and this process increases the hydrophilicity of the polymer.^[4] Hydrolysis or oxidation is the most important step in the mechanism since it initiates the entire process of plastic biodegradation.^[5] Once hydrolysis or oxidation occurs, the microorganisms can act directly on the lower molecular weight products and utilize the carbon in these fragments as a source of energy.^{[citation needed]}

Common enzymes involved in microbial plastic biodegradation include lipase, proteinase K, pronase, and hydrogenase, among others.^[3] The efficacy of these enzymes depends on the type of plastic being degraded. Furthermore, the products of microbial biodegradation will differ depending on the environmental conditions.

Aerobic Conditions[edit]

Under aerobic conditions, the microorganisms will use oxygen as an electron acceptor. The resulting products are carbon dioxide (CO₂) and water (H₂O).^[5] Examples of aerobic conditions for microbial biodegradation include landfills and sediments.^[4]

Anaerobic[edit]

Under anaerobic conditions, the lack of oxygen requires that the bacteria use a different source for an electron acceptor. Common electron acceptors used by anaerobic bacteria are sulfate, iron, nitrate, manganese…