Maya Ellison
Biodegradable plastics are designed to be broken down by microorganisms into natural substances like water, carbon dioxide, and biomass. This process, known as microbial degradation, typically occurs within a few months to years, depending on the material's chemical composition and environmental factors such as temperature and moisture. While biodegradable plastics offer an eco-friendly alternative to traditional plastics, their effectiveness depends on proper disposal methods and the availability of industrial composting facilities. Additionally, the production and use of biodegradable plastics must be carefully managed to avoid contaminating recycling streams and ensure they fulfil their environmental promise.
| Characteristics | Values |
|---|---|
| Breakdown Process | Microbial Degradation |
| Number of Steps | 3 |
| First Step | Colonization of the plastic surface by microorganisms |
| Second Step | Hydrolysis |
| Third Step | Mineralization |
| Breakdown Products | Water, Carbon Dioxide, Inorganic Compounds, Biomass |
| Breakdown Time | Weeks to Months |
| Compostable | Yes |
| Composting Requirements | Optimal Conditions, Industrial Composting Facilities |
| Eco-Friendly | Yes |
| Eco-Friendly Conditions | Proper Disposal, Well-Managed Waste System |
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What You'll Learn
Microorganisms
Biodegradable plastics are broken down by microorganisms through a process called biological degradation. This process involves the conversion of large polymeric molecules into simpler ones by the action of biologically active enzymes produced by specific microorganisms. The enzymes produced by microorganisms play a crucial role in breaking down the complex structures of plastics into smaller molecules that can be easily assimilated by the microbes. This process can occur under environmental conditions, known as in vitro biodegradation, or within the guts of insects and other organisms, known as in vivo biodegradation.
The microbial community comprises a diverse range of microorganisms that work together to enhance the efficiency of plastic degradation. This community includes bacteria, fungi, and microalgae, each contributing to the breakdown of plastics in its own way. For example, bacteria and fungi have been found to be more efficient at degrading certain plastics compared to microalgae. Additionally, the pretreatment of plastics, such as through pyrolysis or temperature optimization, can make them more susceptible to microbial metabolism and enzymatic action.
The ability of microorganisms to degrade plastics is influenced by various factors, including the structure of the plastic polymers, ambient temperature, and the availability of certain enzymes. For example, plastics with long-chain molecules and high molecular weight, such as traditional petroleum-based plastics, are more challenging for microbes to break down. On the other hand, bio-based biodegradable plastics, which are derived from biological sources, are more susceptible to microbial degradation. Starch-based polymers, for instance, are commonly used in bio-based biodegradable plastics due to their availability, low cost, and biodegradability.
The diversity of microbial metabolisms is truly remarkable, with microorganisms from different taxa, including Firmicutes, Proteobacteria, Ascomycetes, and Basidiomycetes, all contributing to the degradation of bioplastics. These microbes are distributed across various ecosystems, including terrestrial and marine soil, compost facilities, and insect guts. The presence of these microorganisms in different environments ensures the breakdown of biodegradable plastics even after they have been disposed of.
The process of microbial plastic degradation holds great potential for reducing plastic pollution and its associated ecological and human health risks. By understanding the interaction between microbes and polymers, researchers can develop more efficient and eco-friendly strategies for producing biodegradable plastics. Additionally, the use of bioplastics, which can be broken down by microorganisms, is a step towards mitigating the environmental impact of traditional plastics. Overall, the role of microorganisms in breaking down biodegradable plastics is crucial for promoting ecological health and sustainability.
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Composting
Biodegradable plastic is defined by its ability to break down completely into substances found in nature within a reasonable timeframe, usually weeks to months. This timeframe is an important component when the term "biodegradable" is used for marketing purposes. However, it is important to note that not all biodegradable plastics are compostable. Compostable plastic refers specifically to biodegradation into soil-conditioning material (compost) under certain conditions.
To be labelled commercially "compostable", plastic must be able to be broken down by biological treatment at a commercial or industrial composting facility. These facilities use heat and humidity to facilitate the breakdown process. Industrial composting at high temperatures takes less time, with some modified polyesters breaking down faster at higher temperatures.
Some biodegradable plastics are designed for composting in a backyard compost bin, such as those made from spirulina. These plastics are made from powdered blue-green cyanobacteria cells and are designed to degrade on a similar timescale to a banana peel. Spirulina-based plastics also have the added benefit of being fire-resistant, instantly self-extinguishing when exposed to fire.
It is important to check the labels on plastic products to determine whether they are suited for home composting or require commercial composting facilities. Properly disposing of biodegradable and compostable plastics is essential to ensure they break down as intended and do not contribute to plastic pollution.
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Industrial composting
To be labelled as "compostable," plastics must be able to break down through biological treatment at a commercial or industrial composting facility. This process typically takes place within six months and must leave no toxic residue that could harm plant growth.
Biodegradable plastics are made from bio-based sources such as seaweed, sugar beets, or other plants, rather than fossil fuels. When properly managed, these materials can offer environmental benefits. However, it is important to note that not all compostable plastics are biodegradable, and vice versa. Compostable plastics are specifically designed and tested for processing in industrial composting facilities, whereas biodegradable plastics can break down in soil or water.
The industrial composting process for biodegradable plastics involves the following key steps:
- Collection: Biodegradable plastics are collected separately from other waste streams to ensure they are directed to the appropriate treatment facilities.
- Pre-treatment: The plastics may undergo pre-treatment processes such as sorting, shredding, or grinding to increase the surface area and facilitate biodegradation.
- Composting: The biodegradable plastics are then subjected to controlled conditions in the industrial composting facility. This includes maintaining specific temperatures, humidity levels, and providing an environment conducive to microbial activity.
- Degradation: Microorganisms, such as bacteria and fungi, break down the plastic polymers into smaller molecules. This process can take weeks to months, depending on the material and environmental conditions.
- Verification: Regular testing and verification are conducted to ensure that the plastics are effectively degraded and that no toxic residues remain.
- End Products: The final products of the biodegradation process are carbon dioxide, water, inorganic compounds, and biomass, which can be used as compost for soil enhancement.
While industrial composting of biodegradable plastics offers a promising solution to the plastic waste crisis, it is important to note that the infrastructure for such composting facilities is currently limited. Additionally, further research is needed to optimise the composting conditions and ensure complete biodegradation without any environmental or health risks.
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Temperature and moisture
The ideal temperature range to favour the growth of mesophilic microorganisms is between 15 and 28°C. Biodegradation tests are often carried out at a constant temperature within this range. The biodegradation rate is influenced by temperature, and a higher temperature will increase the rate of biodegradation.
The presence of moisture is also important. Moisture, in the form of water, is one of the end products of biodegradable plastic breakdown. However, moisture is also necessary at the beginning of the process, as it is required for microorganisms to adhere to the surface of the plastic and begin the breakdown process.
The rate of biodegradation is influenced by other environmental factors, such as nutrients, pH, and gas exchange. The bioactivity of the location, for example, whether the plastic is in a landfill, marine environment, or backyard, will influence the rate of biodegradation.
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Chemical composition
Biodegradable plastics are those that can be broken down into organic components in months or years, as opposed to the decades or centuries taken by traditional plastics. The rate of degradation depends on the chemical composition and material characteristics of the plastic, which dictate the type of microbes that can consume them.
The chemical composition of biodegradable plastics varies, but they are typically made from biologically-based polymers sourced from non-fossil materials, such as plant-based materials. These polymers can be broken down and digested by microbes in aerobic environments, where methane is not a byproduct. The mechanism for biodegradation varies between aerobic and anaerobic processes and the environment in which it takes place, such as a landfill, marine environment, or backyard compost.
The chemical composition of biodegradable plastics can be altered to generate a wide variety of properties, allowing for customization to fit specific applications. For example, lignin-based polymer composites are bio-renewable natural aromatic polymers with biodegradable properties. Lignin is a byproduct of polysaccharide extraction from plant material and is more environmentally friendly than other alternatives due to its low weight and neutral CO2 release during biodegradation.
Additionally, some biodegradable plastics are made from polyhydroxyalkanoates (PHAs), which are produced by bacteria. These plastics will biodegrade back into methane and can be naturally digested by marine microorganisms if they reach the ocean. Polyhydroxyalkanoate (PHA) was first observed in bacteria in 1888 by Martinus Beijerinck, and later chemically identified by French microbiologist Maurice Lemoigne in 1926.
The biodegradation process can be facilitated by hydrolysis, a two-step process where an enzyme first binds to the ingested polymer to catalyze hydrolytic cleavage. The polymer is then broken down into molecules with lower molecular weights that are mineralized to carbon dioxide, water, and biomass. This process can be influenced by factors such as temperature and moisture levels, which can impact the bioactivity of microorganisms.
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Frequently asked questions
The primary purpose of biodegradable plastics is to replace traditional plastics that persist in landfills and harm the environment.
Biodegradable plastics break down through microbial degradation, which involves three steps: colonization of the plastic surface, hydrolysis, and mineralization. First, microorganisms populate the exposed plastics. Next, the bacteria secrete enzymes that bind to the carbon source or polymer substrates and then split the hydrocarbon bonds. The process results in the production of H2O and CO2.
Biodegradable plastics are often made from bio-based sources such as seaweed, sugar beets, or other plants, instead of fossil fuels.
Biodegradable plastics require the right environmental conditions to break down effectively. These include temperature, moisture, and oxygen levels. For example, compostable plastics require higher temperatures, pressure, and specific chemical ratios, which can only be achieved in industrial composting plants.
No, not all biodegradable plastics break down effectively. Many biodegradable plastics are designed to degrade in industrial composting systems, which require well-managed waste systems. If these plastics are discarded into conventional waste streams such as landfills, rivers, or oceans, they may not break down and can contribute to plastic pollution.
