Sofia Martinez
Plastic waste is a pressing global issue, with an estimated generation rate of 400 Mt per year. As plastic waste accumulates in the environment, it is essential to understand its degradation to develop effective solutions. The degradation of plastics can be assessed through various methods, including laboratory tests, field studies, and advanced analytical techniques. These methods involve measuring changes in mass, mechanical properties, chemical composition, and biological activity. Laboratory tests simulate different environmental conditions, such as composting and marine degradation, to evaluate the disintegration and biodegradation of plastics. Field studies focus on the fate of plastics in specific habitats, like the shoreline and seawater, by monitoring oxygen consumption and mechanical degradation. Advanced analytical approaches, including thermal gravimetric analysis and differential scanning calorimetry, provide insights into the long-term degradation of microplastics and the impact of weathering. Understanding plastic degradation is crucial for managing plastic waste, developing biodegradable alternatives, and mitigating environmental risks associated with plastic pollution.
| Characteristics | Values |
|---|---|
| Specific Surface Degradation Rate (SSDR) | Varies across different natural environments, with values ranging from 0 to 11 μm year–1 for HDPE in the marine environment |
| Half-life | Estimated using mean SSDR, ranging from 58 years for bottles to 1200 years for pipes |
| Mechanical properties | Tensile strength at break decreased to -66% in 2 years when exposed to seawater |
| Biodegradation | Complete biodegradation may not result in 100% conversion to CO2 due to microorganism growth and multiplication; values generally between 60-100% |
| Disintegration | Visual disappearance of plastic items in the tidal zone |
| Oxygen consumption | Monitored to determine biodegradation of plastic items lying on sediment |
| Prolonged exposure to seawater | Simulated pelagic domain to test decay of mechanical properties |
| Composting | ISO 20200 (2004) test method used to determine disintegration of plastic products during composting by simulating the process in a laboratory reactor |
| ISO 14855 (2005) test method | Measures biodegradation under composting conditions by monitoring the evolution of CO2 |
| Differential scanning calorimetry (DSC) | Used to gauge the extent of plastic degradation due to marine weathering by measuring deviations in heat capacity and phase transitions |
| Thermal gravimetric analysis (TGA) | Provides accurate measurements of changes in thermal properties of microplastics during degradation |
| Particle size measurements | Used to approximate the rate and duration of degradation for certain plastic polymers under natural marine conditions |
| Radioactive labelling | Inclusion of small amounts of radioactive carbon allows unequivocal measurements of biodegradation using highly sensitive scintillation counters |
| Cavity ring-down spectrometry (CRDS) | Used to confirm biodegradation by measuring 13CO2 resulting from plastic mineralisation |
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What You'll Learn
Measuring changes in mass
The mass loss method can be used to study the biodegradation of plastic in composting plants. The plastic product under analysis is put into a reactor and mixed with synthetic organic waste. After three months, the level of disintegration is determined by sieving the obtained compost. If the product has degraded, its particles will pass through the sieve with the compost and will not be distinguishable.
Another method to measure mass loss involves putting a plastic sample inside bioreactors containing mature compost and monitoring the evolution of CO2. This method can determine the extent of biodegradation, as the CO2 is proof of biodegradation. However, the conversion to CO2 will not be 100% because microorganisms grow and multiply, assimilating some of the plastic.
The rate of mass loss can also be influenced by the type of plastic. For example, the SSDR values for LDPE decomposing on land vary by a factor of 50. The highest reported accelerated SSDR for LDPE is 83 μm year-1, while the lowest is 3.7 μm year-1. This variation may be due to differences in crystallinity, as blending can increase the volume fraction of amorphous regions, which show higher degradation rates.
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Testing in compost
Testing the degradation of plastic in compost is a complex process that involves various factors and methods. One of the most widely recognised methods is the ISO 14855 (2005) test method, which measures the ultimate aerobic biodegradability of plastic materials under controlled composting conditions. This method involves placing a plastic sample inside bioreactors containing mature compost and monitoring the evolution of carbon dioxide (CO2) as proof of biodegradation. The presence of CO2 indicates that the plastic is undergoing aerobic biodegradation, which is the final outcome of a complete degradation process. The data obtained from this method can provide valuable insights into the biodegradability of plastics in composting conditions.
Another recognised test method is the ISO 20200 (2004) test, which focuses on simulating a composting process in a laboratory setting. This involves placing the plastic product under analysis into a reactor and mixing it with synthetic organic waste. After a specified period, typically three months, the level of disintegration is determined by sieving the obtained compost. If the plastic has successfully degraded, its particles will pass through the sieve along with the compost and will not be visually distinguishable. However, it is important to note that this method only measures physical degradation and does not provide information on the continued biodegradation of the fragments.
In addition to these standardised methods, there are various other approaches to testing plastic degradation in compost. One study, conducted by T. Kijchavengkul et al. in 2006, utilised an automatic laboratory-scale respirometric system to measure the biodegradability of plastic polymers in olive-mill waste compost. This research provided insights into the biodegradation rates and conditions that influence the process.
When testing plastic degradation in compost, it is essential to consider the specific conditions and treatments applied to the compost. For example, temperature and moisture levels play a significant role in the degradation process. Elevated temperatures have been found to increase the rate of degradation, while moisture can impact the process as well. Additionally, the presence of microorganisms, such as mycelial microorganisms, can significantly contribute to the biodegradation of plastics in compost.
Overall, testing plastic degradation in compost involves a combination of standardised methods, laboratory simulations, and the consideration of various environmental factors. By employing these techniques, researchers can gain a deeper understanding of how plastics degrade in composting conditions and develop strategies to mitigate plastic waste issues, promoting the efficient use of renewable resources.
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Monitoring oxygen consumption
One method for monitoring oxygen consumption involves the use of transparent glass reactors, which enable the correlation between oxygen consumption and the physical status of samples through visual inspections. This method was used in a study that tested the degradation of Mater-Bi carrier bags and LDPE carrier bags. The Mater-Bi bags showed substantial disintegration after 9 months, while the LDPE bags showed no visible degradation. The blank reactors (with only sediment and seawater) showed an average total oxygen consumption of 120 mg/L. The Mater-Bi bags showed a much higher oxygen consumption, ranging from 215 to 376 mg/L.
Another study simulated the conditions experienced by plastic items buried in a wet sandy matrix. This test was designed as a qualitative disintegration test based on visual assessment. The Mater-Bi bags showed total degradation, with no residual film visible, while the control samples of polyethylene remained intact. To determine the ultimate biodegradability, the test should be performed within a closed reactor and oxygen consumption measured.
The percentage of biodegradation can be calculated by dividing the theoretical oxygen demand (ThOD) by the measured oxygen consumption. ThOD is determined by the elemental composition of the material being tested, following ISO14851.
Overall, monitoring oxygen consumption is an important technique for understanding the degradation of plastics in marine environments, particularly for plastics that end up on the sea floor.
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Assessing crystallinity
One of the key methods for assessing crystallinity in plastics is through differential scanning calorimetry (DSC). This technique involves heating a sample of the plastic at a controlled rate while measuring the amount of heat absorbed or released. The presence of crystalline regions within the plastic will result in distinct peaks on a DSC curve, indicating melting or crystallisation events. By analysing these peaks, the degree of crystallinity can be quantified.
Another technique used to evaluate crystallinity is wide-angle X-ray scattering (WAXS). This method involves directing an X-ray beam at a plastic sample and measuring the scattering pattern produced. The regularity and arrangement of polymer chains in crystalline regions cause distinct diffraction peaks in the scattering pattern, which can be analysed to determine the degree and structure of crystallinity.
Additionally, advanced techniques such as solid-state nuclear magnetic resonance (NMR) spectroscopy and transmission electron microscopy (TEM) can provide valuable insights into crystallinity. NMR spectroscopy can distinguish between crystalline and amorphous regions by detecting differences in molecular mobility, while TEM allows direct visualisation of crystalline structures within plastics.
It is important to recognise that crystallinity plays a significant role in the degradation behaviour of plastics. In general, crystalline regions within a plastic are more resistant to degradation compared to amorphous regions. This is because the tightly packed, ordered structure of crystalline domains hinders the penetration of degrading agents, such as water, oxygen, or microorganisms. Consequently, the degradation of plastics often initiates in the amorphous regions, where polymer chains are more accessible and susceptible to chemical or biological attack.
Furthermore, the presence of crystallinity can influence the mechanism of degradation. For example, in the case of oxidative degradation, crystalline regions may exhibit different oxidation pathways compared to amorphous regions. This is due to variations in polymer chain mobility and accessibility, which can lead to distinct oxidation products and rates of degradation.
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Using radioactive carbon
Radioactive carbon, specifically Carbon-14, is a useful tool for measuring the degradation of plastics. Carbon-14 is produced in the upper atmosphere when carbon dioxide is bombarded by solar radiation, and this carbon variant can be used to date formerly living organisms. By measuring the proportion of Carbon-14 in a plastic sample, scientists can estimate how long it has been since the plastic was created. This is because, once a plant, animal, or fungus dies, it ceases to exchange carbon with the environment, and the proportion of Carbon-14 decreases over time.
This method can be applied to plastics by conducting experiments over shorter periods and then extrapolating the results. For example, if 2% of a plastic mass disappears over the course of a year, it can be predicted that it will take approximately 50 years for 100% of the plastic to degrade. This method is particularly useful for plastics that do not biodegrade, such as polyethylene bags, which are not recognised as food by microorganisms and do not produce carbon dioxide in respirometry tests.
However, it is important to note that this method assumes a fairly stable rate of degradation, which may not always be the case for plastics. The degradation of plastics can be influenced by various factors, including the shape of the plastic piece, the presence of additives, and the specific environment in which the plastic is located. For example, plastic buried in the sediment or floating in the ocean may experience different rates of degradation due to variations in sunlight exposure, temperature, and other factors.
To address these complexities, researchers have developed test methods that simulate different marine environments, such as the pelagic zone, tidal zones, and the water-sediment interface. By suspending plastic specimens in synthetic sea salt solutions or burying them in sand kept wet with seawater, scientists can measure carbon dioxide evolution or mass loss to assess biological degradation. These tests provide a more comprehensive understanding of plastic degradation rates in diverse marine habitats.
Overall, using radioactive carbon, specifically Carbon-14, is a valuable technique for measuring the degradation of plastics, especially when combined with other analytical methods to account for the various factors influencing degradation rates.
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Frequently asked questions
There are several methods to measure the degradation of plastic in a marine environment. One method is to measure the disintegration of plastic products by simulating a composting process in a reactor in a laboratory. Another method is to measure the oxygen consumption (biodegradation) of plastic items lying on the sediment. A third method is to measure the decay of mechanical properties of plastic after prolonged exposure to seawater.
The ISO 20200 (2004) test method can be used to measure the disintegration of plastic products during composting. In this method, the plastic product is mixed with synthetic organic waste in a reactor. After 3 months, the level of disintegration is determined by sieving the compost. If the plastic has degraded, its particles will pass through the sieve along with the compost and will not be distinguishable.
Biodegradation of plastic can be measured by monitoring the evolution of CO2 using the ISO 14855 (2005) test method. The plastic sample is placed inside bioreactors containing mature compost, and the CO2 produced during biodegradation is monitored. Other methods include using radioactive carbon in polymeric materials, cavity ring-down spectrometry (CRDS), and gas chromatography (GC-MS/FID).
