Sofia Martinez
Plastic is a broad term for different polymers with high molecular weight, which can be degraded by various processes. Traditional plastics are derived from petroleum, but bioplastics made from renewable sources are increasingly being used as substitutes. Biodegradable plastics are those that can be broken down by microorganisms and enzymes, and they offer many advantages, such as increased soil fertility and reduced waste management costs. To assess the biodegradability of plastics, laboratory tests are conducted to simulate degradation in different environments, such as soil, compost, and aquatic systems. These tests consider the chemical and physical properties of the plastics, as well as the characteristics of the microorganisms involved in the degradation process. While there are standard methods for testing, it is challenging to replicate the dynamic conditions of natural or industrial environments in a laboratory setting.
| Characteristics | Values |
|---|---|
| Biodegradability test | Laboratory test methods, spectroscopy, chromatography, iodine test, disintegration test |
| Biodegradability factors | Microorganisms, enzymes, temperature, moisture content, oxygen, nutrients, etc. |
| Biodegradability environment | Soil, sea, compost, activated sludge, etc. |
| Biodegradability duration | 12 weeks (European standard) |
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What You'll Learn
- Home testing: Cut plastic into squares, bury in compost for 12 weeks, and examine
- Laboratory testing: Test in varied environments, like soil, compost, and aquatic systems
- Chemical testing: Use spectroscopy or chromatography to analyse chemical composition
- Biodegradation factors: Assess the influence of surface conditions, like surface area and hydrophobic properties
- Microorganisms: Evaluate the types of microorganisms and their growth conditions
Home testing: Cut plastic into squares, bury in compost for 12 weeks, and examine
To check the biodegradability of plastics at home, you will need to perform a test over a 12-week period. This test will involve cutting the plastic into squares, burying them in compost, and examining them after the 12-week period to see if they have decomposed.
First, cut the plastic you want to test into three or four 4-inch (10 cm) squares. Make sure the plastic you are testing says "biodegradable" or "compostable" on it, as traditional plastics will not biodegrade. Plastics made from cornstarch or plant pulp are usually biodegradable.
Next, prepare your compost bin. Cut pieces of yarn or twine that are about twice the height of your compost bin. You will need one piece of yarn for each test square. Tie each piece of yarn to a square by cutting a small slit or hole in the square and threading the yarn through, making a knot at one end.
Bury the test squares in your compost bin and wait for 12 weeks. The European standard for biodegradable material is 12 weeks, so if your plastic hasn't broken down by then, it is not considered biodegradable.
After 12 weeks, carefully dig up the test squares and examine them. Plastic that is starting to break down will get holes, become cracked, change colour, and reduce in size. If the plastic looks the same as when you buried it, it has not biodegraded.
It is important to note that this home test is not a definitive way to determine biodegradability. More advanced techniques, such as spectroscopy or chromatography, can be used to confirm the chemical composition and biodegradability of a plastic material.
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Laboratory testing: Test in varied environments, like soil, compost, and aquatic systems
Laboratory testing is a critical aspect of assessing the biodegradability of plastics in varied environments, including soil, compost, and aquatic systems. Here's an overview of the key considerations and methodologies employed in laboratory testing:
Soil Environment:
Soil is a natural environment where biodegradation studies are commonly conducted. The complexity and expense of these tests are well-known, and they involve assessing the degradation of the plastic material over time. ASTM D6400 and ISO 17088 are recognized as comprehensive standards for soil biodegradation testing, although they may require additional characterizations specific to certain plastics.
Compost Environment:
Compost, a biologically active environment with high microbial diversity, enhances the biodegradation process. Laboratory testing in compost environments involves simulating composting conditions, including temperature profiles, to mirror natural self-heating processes. The measurement of microbial carbon dioxide production and oxygen consumption are critical indicators of biodegradation. However, due to the heterogeneous nature of compost, assessing complete biodegradation can be challenging.
Aquatic Systems:
Aquatic tests are essential for understanding the biodegradation of plastics in marine environments. These tests may involve simulating pelagic conditions or specific zones, such as the benthic zone with sunlight (photic zone or sublittoral zone). Aquatic tests often employ bioreactors containing seawater or a seawater/sediment interface to assess the biodegradation of plastics over time. The density of plastics also plays a role in aquatic systems, with biodegradable plastics typically having a density higher than 1, causing them to sink.
Standardization and Validation:
It is important to note that the field of biopolymer biodegradation assessment is still evolving, and there is a need for standardized research methods and policies. The current lack of uniformity in testing approaches can lead to discrepancies in biodegradability rates for the same biopolymer. Therefore, ongoing efforts aim to establish consistent data and labeling systems by adapting guidelines to test biodegradability in varied environments.
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Chemical testing: Use spectroscopy or chromatography to analyse chemical composition
The chemical composition of plastic plays a crucial role in product safety and recyclability. It is essential to identify hazardous materials in post-consumer plastics to increase effective recycling efforts.
Spectroscopy and chromatography are two detailed analytical techniques that can be used to confirm the chemical composition of plastic materials.
Spectroscopy, specifically IR spectroscopy, can be used to detect the presence of starch in plastic. A small sample of the plastic is dissolved in a solvent, such as water or alcohol, and then mixed with an iodine solution. If the plastic is starch-based, the iodine solution will turn blue or violet, indicating the presence of starch. Spectroscopy can also be used to identify hazardous materials in plastics, ensuring human safety, and aiding in recycling efforts. With advancements in spectrophotometric technology, we can now quickly and accurately identify the chemical composition of plastics and differentiate specific materials.
Chromatography, specifically liquid chromatography, can be used in parallel with nanofractionation-bioactivity detection and mass spectrometry to screen for estrogenic compounds in consumer-electronics plastics. This technique helps address the emerging concern of chemical safety in consumer products.
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Biodegradation factors: Assess the influence of surface conditions, like surface area and hydrophobic properties
The surface conditions of plastics, including their surface area, hydrophilic, and hydrophobic properties, play a significant role in influencing the biodegradation mechanism.
Surface Area
The surface area of a plastic material is an important factor in the biodegradation process. Increasing the surface area of a polymer can enhance its susceptibility to biodegradation. For instance, when PCL (poly-epsilon-caprolactone) is blended with starch, the surface area of PCL increases, making it more prone to biodegradation.
Hydrophobic Properties
Hydrophobicity, or the lack of affinity for water, is another critical aspect of plastic biodegradation. Hydrophobic polymers, such as PS (polystyrene), tend to have high molecular weights and are often non-biodegradable. PS, for example, is a synthetic hydrophobic polymer that is recyclable but not biodegradable. While some reports indicate a degree of biodegradation when exposed to specific microorganisms, the overall biodegradation level is typically very low.
The hydrophobic nature of certain plastics can hinder their interaction with water-soluble microorganisms and enzymes, which are often crucial for initiating biodegradation. Therefore, the hydrophobic properties of plastics can significantly influence their biodegradability.
In summary, both the surface area and hydrophobic characteristics of plastics are essential factors that can either promote or hinder the biodegradation process. Understanding these surface conditions is crucial for assessing the environmental impact and potential benefits of biodegradable plastics.
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Microorganisms: Evaluate the types of microorganisms and their growth conditions
Microorganisms, including bacteria, fungi, and algae, play a crucial role in the biodegradation of plastics. These microorganisms possess unique characteristics and abilities that enable them to break down and utilise plastic polymers as their carbon source. The evaluation of the biodegradability of plastics should consider the types of microorganisms involved, their growth conditions, and the enzymes they produce.
Bacteria are a diverse group of microorganisms that can be further classified into different types, such as thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria. These bacterial classifications are based on their optimal growth conditions and adaptations to specific environments. For example, thermophilic bacteria thrive in high-temperature environments, while psychrophilic bacteria are adapted to cold environments.
Fungi are another essential type of microorganism in plastic biodegradation. They play a significant role, as demonstrated by a study where approximately 30% of the fungi isolated from plastic litter in a lake were found to degrade Impranil®, a plastic dispersant. Fungi exhibit characteristics such as small size, fast absorption, strong adaptability, and easy variation, contributing to their effectiveness in breaking down plastics.
Algae, including microalgae, have also been explored for their potential in plastic biodegradation. While they represent a smaller proportion of the microorganisms involved, their ability to survive in harsh environments and produce specific enzymes contributes to their significance in the process.
The growth conditions for these microorganisms are critical factors in the biodegradation process. Temperature is particularly influential, with most studies focusing on temperatures above 20°C. However, cold-adapted microorganisms, such as those found in alpine and Arctic soils, have the capacity to produce enzymes that are active at lower temperatures (0–30°C). Other growth conditions, such as pH, moisture content, oxygen levels, and nutrient availability, also play a role in creating optimal conditions for microbial growth and plastic biodegradation.
In addition to the types of microorganisms and their growth conditions, the enzymes they produce are key players in the biodegradation process. Enzymes like PET hydrolase and PCL-cutinase have been studied for their ability to degrade specific polymers. By understanding the interaction between these enzymes and plastics, we can enhance their catalytic efficacy and develop more efficient biodegradation processes.
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Frequently asked questions
If you have a product that says “biodegradable” or “compostable” on it, you can make some compost and place your plastic in it. Cut the plastic into three 4-inch squares, ensuring they are all roughly the same size. Tie a piece of yarn to each square and bury them in your compost bin, ensuring the yarn is hanging over the side. Dig up your test squares after 12 weeks and examine them. If the plastic has started to biodegrade, it will have holes, cracks, changes in colour, and a reduction in size.
Bioplastics are usually made from renewable sources such as cornstarch or plant pulp. Traditional plastics, made from petroleum-based polymers, are not biodegradable. If a product is biodegradable, it will say so on the packaging.
There are a variety of methods used by scientists to test the biodegradability of plastics. Some of these include spectroscopy, chromatography, and thermogravimetric analysis. Scientists also test biodegradability in different environments, such as soil, compost, and aquatic systems.
