Sofia Martinez
Plastic pollution is a pressing global issue, with plastic waste accumulating in the environment at an alarming rate. While the understanding of plastic degradation in the environment is limited, it is known that plastics can persist for centuries or longer without complete degradation. The degradation rate varies depending on the type of plastic and environmental factors such as temperature, sunlight exposure, and the presence of metal ions in the water. Some plastics, like poly(ethylene terephthalate) (PET), are highly resistant to biodegradation, leading to environmental concerns. However, recent studies have shown that even PET degrades more rapidly than previously thought in certain conditions. The mechanisms of plastic degradation include physical changes, such as cracking and embrittlement, and chemical changes, such as bond cleavage and oxidation. Microbial degradation also plays a significant role in reducing the negative impact of plastics, and the use of bacterial strains and enzymes shows potential for efficient plastic degradation. While the complete degradation of plastics is challenging, understanding the factors influencing degradation rates is crucial for mitigating the environmental impact of plastic pollution.
| Characteristics | Values |
|---|---|
| Environmental degradation mechanisms | Physical (cracking, embrittlement, flaking) and chemical (bond cleavage, oxidation) |
| Plastic degradation factors | Type of plastic, environmental conditions, extrapolation method |
| Plastic waste generation rate | 400 Mt year-1 |
| Plastic waste recycling rate | 10% |
| Plastic waste incineration rate | 14% |
| Biodegradation of plastics | Microbial degradation, bacterial degradation, UV-light, oxygen, water, pollutants, photo-oxidation, mechanical degradation, thermo-oxidative degradation, hydrolytic degradation, biodegradation by microorganisms |
| Plastic degradation time estimates | Media estimates: 10-20 years or 500-1000 years; Marine environment: 58 years (bottles) to 1200 years (pipes); Ocean water: 50% degradation in 4.5 years and 100% degradation in 72 years |
| Plastic types | Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC) |
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What You'll Learn
Microbial degradation
Plastic waste is a persistent pollutant that can remain in the natural environment for hundreds of years or longer. The accumulation of plastics in the environment is a growing concern, and their resistance to degradation poses a serious threat to the ecosystem and human life.
The literature suggests that microbial-mediated degradation is a more suitable and eco-friendly approach compared to other methods. Microbial enzymatic degradation relies on the ability of bacterial enzymes to break down plastics efficiently. However, the knowledge of environmental plastic degradation is still limited, and further exploration of bacterial interactions with plastics is necessary to identify key biodegradable microorganisms.
While microbial degradation has shown promising results, it is important to note that plastics with intrinsic properties such as durability and resistance to degradation can hinder the process. These plastics, often made of recalcitrant polymers, may not provide a suitable substrate for microbial attachment and enzymatic reaction. As a result, complete degradation becomes challenging, and only partial degradation into small monomers may occur.
In natural environments, microbes have been observed to decompose biodegradable plastics on land, in rivers, and along the seashore. However, the ability of deep-sea microbes to degrade plastics is still uncertain. Studies have reported the degradation of biodegradable polyesters, polyhydroxyalkanoates, and polysaccharide esters at various deep-sea locations, but the mechanisms remain to be fully understood.
Overall, microbial degradation holds potential for mitigating the environmental impact of plastics. By understanding the interaction between microbes and plastics, we can develop more efficient biodegradation methods and reduce the burden of plastic waste on the environment.
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Environmental factors
Physical and Chemical Factors
Plastics can undergo physical or chemical degradation in the environment. Physical degradation refers to changes in the bulk structure of plastics, such as cracking, embrittlement, and flaking. On the other hand, chemical degradation involves changes at the molecular level, including bond cleavage and oxidation of long polymer chains, resulting in the formation of new, shorter-chain molecules. The specific surface degradation rate (SSDR) is a metric used to quantify the degradation rate of plastics, taking into account different natural environments. For example, the SSDR for high-density polyethylene (HDPE) in the marine environment ranges from 0 to approximately 11 μm year–1, leading to estimated half-lives from 58 years to 1200 years.
UV-Light, Oxygen, Water, and Pollutants
The combination of UV-light, oxygen, water, and pollutants contributes to polymer weathering, a significant factor in plastic degradation. UV-light, along with oxygen, leads to photo-oxidation, which is the most influential factor in the weathering process. This causes chain breaking in plastics, resulting in embrittlement and eventual fragmentation into microplastics. The presence of water can also impact the degradation rate, as seen in the faster degradation of PET in ocean water due to the catalytic effect of metal ions.
Microbial and Bacterial Degradation
Microbial and bacterial degradation play a crucial role in reducing the negative impact of plastics on the environment. The interaction between plastics and microbial communities is essential for biodegradation. Bacterial enzymes can break down plastics, although this process may require longer periods and more enzymatic activities for certain polymers. Additionally, the presence of specific bacterial strains can enhance the degradation process, improving human health and safety.
Temperature and Size
Temperature and the size of plastic objects influence their degradation rate. Higher temperatures can accelerate degradation, particularly in aggressive environments. The size of the plastic object also matters; larger pieces of plastic will take longer to degrade, similar to other materials like metal or glass. However, it is important to note that the degradation of plastics is not solely due to their size but also their composition and other environmental factors.
Natural Environment and Landfills
Plastics exposed to the natural environment can undergo gradual degradation. However, when plastics are disposed of in landfills, degradation becomes challenging due to the lack of oxygen and other necessary factors. Landfills contribute to the accumulation of plastic waste, leading to widespread environmental contamination.
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Different degradation rates
The degradation rate of plastics depends on several factors, including the type of plastic, the size and shape of the plastic item, and the environmental conditions.
The rate at which plastics degrade varies significantly. Biodegradation can take decades, while some industrial processes can completely decompose a polymer in hours. For example, the degradation rate of Polyethylene (PE) or Polypropylene (PP) plastic is similar to that of a leaf. On the other hand, some plastics like Polyethylene Terephthalate (PET) can take much longer to degrade. The degradation rate of PET can be accelerated by using enzymes like LC-cutinase, which has been found to increase the degradation rate by 230-970 times.
The environment in which the plastic degrades also plays a crucial role. For instance, plastic in landfills can persist for more than 20 years due to the anaerobic conditions, which limit degradation. In contrast, plastic exposed to sunlight and other environmental factors can degrade much faster. Polyethylene grocery bags, for instance, have been found to disintegrate in less than a year when left outdoors.
The size and shape of the plastic item also affect the degradation rate. As plastic items break down into smaller pieces, their combined surface area increases, which can accelerate the degradation process.
Additionally, the presence of certain bacteria and enzymes can influence the degradation rate. Thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria in extreme environments have the potential to degrade synthetic plastics. These bacteria produce plastic-degrading enzymes that can break down polymers into smaller molecules.
It is worth noting that the term "degradation" can refer to different processes, including depolymerization, chemical modification, alteration of physical properties, mass loss, or complete mineralization to CO2 and H2O. The definition used can also impact the perceived degradation rate.
While the exact degradation rates vary, it is clear that plastics can degrade much faster than the public perception of hundreds or even thousands of years. However, the degradation process can result in the release of harmful chemicals and microplastics into the environment, which can have negative ecological consequences.
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Plastic composition
Plastic is a synthetic or semisynthetic material composed primarily of polymers. They are organic polymers of high molecular weight, which are mixed with other substances. Plastics are composed of various elements such as carbon, hydrogen, oxygen, nitrogen, sulphur, and chlorine. They can also be produced from silicon atoms (known as silicone) along with carbon.
Plastics can be divided into two distinct categories based on their chemical composition. The first category is made up of polymers having only aliphatic (linear) carbon atoms in their backbone chains. Polypropylene is an example of this category. The second category is made up of heterochain polymers. These compounds contain atoms such as oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. Polycarbonate is an example of a heterochain polymer.
The world's first fully synthetic plastic was Bakelite, invented in 1907. Synthetic plastics are derived from crude oil, natural gas, or coal. Bio-based plastics, on the other hand, come from renewable products such as carbohydrates, starch, vegetable fats and oils, bacteria, and other biological substances. The vast majority of plastic in use today is synthetic due to the ease of manufacturing methods involved in processing crude oil. However, the growing demand for limited oil reserves is driving the need for newer plastics from renewable resources.
Plastics are classified into two types based on their reversibility of chemical processes: thermoplastics and thermosets. Thermoplastics do not undergo chemical change in their composition when heated and can be molded repeatedly. Examples include polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). Thermosets, on the other hand, can only melt and take shape once. After they solidify, they stay solid and retain their shape permanently. If reheated, thermosets decompose instead of melting. Examples of thermosets include epoxy resin and polyimide.
The formation and accumulation of plastics in the environment have caused serious ecological damage worldwide. Plastic waste is persistent and can remain in the natural environment for hundreds of years or longer. The environmental degradation mechanisms for plastics can be classified as physical or chemical. Physical degradation refers to changes in the bulk structure, such as cracking, embrittlement, and flaking. Chemical degradation refers to changes at the molecular level, such as bond cleavage or oxidation of long polymer chains to create new molecules.
There are four mechanisms by which plastics degrade in the environment: photodegradation, thermo-oxidative degradation, hydrolytic degradation, and biodegradation by microorganisms. Biodegradation by microorganisms is of great significance in reducing the negative impact of plastics. Certain bacterial strains can efficiently degrade plastics, improving human health and safety.
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Biodegradable additives
Plastic waste is a significant contributor to environmental pollution and ecological damage worldwide. While some reports claim that plastics do not degrade at all, others provide estimates ranging from 10-20 years to 500-1000 years for plastic bags, and over 70 to 450 years for plastic bottles. The variability in these estimates arises from differences in environmental conditions, such as the marine or terrestrial environment, and the specific type of plastic.
The persistence of plastic waste in the environment is due to its inherent resistance to degradation. Plastics are designed to have high stability and durability, which makes them useful but also challenging to dispose of responsibly. However, there are four primary mechanisms through which plastics can degrade:
- Photodegradation: Degradation caused by sunlight or UV radiation.
- Thermo-oxidative degradation: Degradation caused by heat.
- Hydrolytic degradation.
- Biodegradation by microorganisms: This process involves the use of microbial enzymes to break down plastics.
- Attract microorganisms to the polymer through quorum sensing after biofilm creation on the plastic product.
- Reduce the strength of certain properties of the polymers, making them more accessible to microbial attack.
- Increase the surface area of plastic products, enabling a greater number of microbes to attach and consume the plastic.
- Convert the plastic degradation process from one driven by environmental factors like sunlight and heat to one of biodegradation by microorganisms.
Starch, for example, is a biodegradable additive that blends with certain synthetic plastics like polyvinyl alcohol (PVA) and polyester, making them completely biodegradable. The presence of starch allows microorganisms to directly consume the plastic, as the material becomes more hydrophilic. Additionally, bioaugmentation, or the addition of microbial strains to plastics, can enhance biodegradability. For instance, the bacteria Geobacillus thermoleovorans can successfully attach to and increase the rate of microbial degradation of poly(lactic acid) (PLA).
Overall, biodegradable additives offer a potential solution to the problem of plastic waste by accelerating the degradation process and making it more environmentally friendly. Companies like BioSphere are developing custom biodegradability solutions in pellet, liquid, and powder formats, compatible with a wide range of plastic products and production processes.
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Frequently asked questions
Plastics in the environment are degraded by a combination of UV-light, oxygen, water, and pollutants, known as polymer weathering. This weathering causes chain-breaking, which leads to embrittlement and the eventual breaking apart of the plastic. As the plastic fragments get smaller, their combined surface area increases, facilitating the leaching of additives into the environment.
There are four types of plastic degradation: photodegradation, thermo-oxidative degradation, hydrolytic degradation, and biodegradation by microorganisms. Photodegradation is the most common, leading to thermo-oxidative degradation.
The rate of degradation varies. Biodegradation can take decades, while industrial processes can decompose polymers in hours. The exact rate of degradation depends on factors such as temperature, size, and amount of sunlight exposure.
Polypropylene garden chairs may become brittle and snap, while polycarbonate car headlamp covers can turn yellow and foggy over time. Polyethylene grocery bags were found to disintegrate in less than a year when left outdoors. PET bottles in the sea showed severe degradation after 15 years, with characteristic chemical bonds almost gone.
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