Degradation of Plastics: Dream or Reality?

The word plastic comes from the Greek word “plastikos”, which means ‘able to be molded into different shapes’. Plastics are made up of linking of monomers together by chemical bonds. Polythene comprises of 64% of total plastic, which is a linear hydrocarbon polymers consisting of long chains of the ethylene monomers. General formula of polyethylene is CnH2n, where ‘n’ is the number of carbon atoms. The plastics we use today are made from inorganic and organic raw materials, such as carbon, silicon, hydrogen, nitrogen, oxygen and chloride. The basic materials used for making plastics are extracted from oil, coal and natural gas. Plastics include polythene, propylene, polystyrene, polyurethane, nylon etc. Polyethylene either LDPE (low density polyethylene) or HDPE (high density polyethylene) is a thermoplastic polymer made by monomers of ethylene, used mostly as thin films and packaging sheets.

From last three decades uncontrolled use of the plastics for packaging (e.g. fast food), transportation, industry and agriculture in rural as well as urban areas, has elevated serious issue of plastic waste disposal and its pollution. Light-weight, inertness, durability, strength and low cost are the main advantages of plastic while it has disadvantages such as, it is recalcitrant to biodegradation and difficult to degrade naturally.

The global use of plastic is growing at a rate of 12% per year and around 0.15 billion tones of synthetic polymers are produced worldwide every year. Accumulation rate of plastic waste in the environment is 25 million tons/year and is consequently considered a serious environmental danger. Plastic are estimated to be 20% municipal solid waste (MSW) in United States and Germany, 7.5% of MSW in Western Europe and 25% in Australia. In Turkey, 11 million tons of plastic were disposed per year. In the year 1999-2000, India imported more than 120,000 tons of plastic. Annually, India generates 5.6 million metric tonnes of plastic waste with Delhi accounting for a shocking 689.5 metric tonnes per day. According to Central Pollution Control Board (CPCB) of India, total plastic waste which is collected and recycled in the country is likely to be 9,205 tonnes per day (approximately 60% of total plastic waste) and 6,137 tonnes remain uncollected and littered. Major offender in generating such waste are four metros with Delhi contributing 689.5 tonnes a day, followed by Chennai (429.4 tonnes), Kolkata (425.7 tonnes) and Mumbai (408.3 tonnes). The figures only serve to confirm the common areas of masses of plastic in industrial, residential and slum areas of Indian cities and towns.

PLASTIC RECYCLING & DEGRADATION:

Recycling of plastics too is a major concern because hardly 50% of the total plastics produced are being recycled. The rate of production is tremendous compared to its degradability. The pollution caused by recycling is high, but that could be compensated by reuse of such materials. Plastics these days are being reused by varying their applications. Bottles, cans, containers can be used for storage. Drums, tubes, sheets etc are all being reused in many ways. Reusing plastics has lots of creativity involved. There are many artists who have developed new recycling techniques which can be replicated by anyone. Schools and colleges could teach recycling of plastics as an art, which would further create awareness in the community. Below is a list of plastic recycling symbols:

Source: http://pubs.sciepub.com/jaem/5/1/2/

There are quite a few physical and chemical ways polymers and plastics can be degraded. These are completely non biological. They are illustrated below:

  1. Thermo-oxidative Processes: Here, the polymer is heated to extremely high temperatures and undergoes oxidation in the primary chain of the polymer. Basically, here, the backbone of the polymer gets chemically altered.

  2. Mechanical Processes: Here, the polymer is subject mechanical stress or change which in turn causes the polymer properties to change.

  3. Ultrasonic Methods: When the polymer is put through an ultrasonic environment, the polymer chain may vibrate at the frequency of the environment and can split and dislodge.

  4. Hydrolytic Environments: When the plastic is kept in moist or acidic conditions, chain scission can occur through hydrolysis of the main chain. This hydrolysis occurs in polymers with functional groups that are sensitive to the effects of water.

  5. Other Chemical Methods: Here, corrosive chemicals, gases or liquids are used to decompose the polymer. Ozone, atmospheric pollutants, and acids like nitric, sulfuric, and hydrochloric will attack and degrade most polymers through chain scission and oxidation. This is quite similar to oxidative or hydrolytic processes.

  6. Photo-oxidative Methods: The absorption of radiation by polymers, or their impurities, due to exposure to sunlight or high energy radiation can result in the breaking of chemical bonds in polymers resulting in photo-degradation. This is even more possible if the polymers have some semiconducting properties in the visible or solar light region.

Other methods are landfills, incineration and disposal in oceans, which are having deadly consequences on life on earth. These methods are not feasible for long term duration.

Among all these methods, the most significant routes for plastic decomposition are the thermo-oxidative, biological and the photo-oxidative routes. Biological processes will always be a very important method of plastic decomposition because of their efficiency, ability to attach specific polymer sites and reduced environmental concerns!

BIOLOGICAL PROCESS OF PLASTIC DEGRADATION:

The safest method which can be implied at large scale is biological process. Biological processes don’t harm the environment and the byproducts could be of great applications. Below is an overview of the biological approach:

Microorganisms such as bacteria, fungi and actinomyectes are involved in the degradation of both natural and synthetic plastics. Plastics are usually biodegraded aerobically in nature, anaerobically in sediments and landfills and partly aerobically in compost and soil. Carbon dioxide and water are produced during aerobic biodegradation, while anaerobic biodegradation produces carbon dioxide, water and methane.

Biodegradation of polymers involves following steps:

  • Attachment of the microorganism to the surface of the polymer.
  • Growth of the microorganism, using the polymer as a carbon source.
  • Ultimate degradation of the polymer.

Microorganisms are able attach to a polymer’s surface, as long as the latter is hydrophilic. Once the organism is attached to the surface, it is able to grow using the polymer as its carbon source. In the primary degradation stage, the extracellular enzymes secreted by the organism cause the main chain to cleave, leading to the formation of low-molecular weight fragments, like oligomers, dimers or monomers. These low molecular weight compounds are further used by the microbes as carbon and energy sources. Small oligomers may also diffuse into the organism and get assimilated in its internal environment.

List of different microorganisms reported to degrade different types of plastics:

STORY OF A PLASTIC DEGRADING WORM:

Federica Bertocchini – amateur beekeeper, as well as science researcher – made a curious discovery. The larvae of wax moths (Galleria mellonella) are not only known for their use as fishing bait, but also for the problems they cause to beekeepers (the creatures live on beeswax). Bertocchini, having placed the larvae into a plastic bag to prevent them from swarming her hives, found that the larvae were able to chew their way out. “I went back to the room where I had left the worms and I found that they were everywhere, the bag was full of holes,” Bertocchini said.

This discovery prompted Bertocchini and other scientists at Cambridge University to begin examining the eating patterns of waxworms. Upon placing the larvae upon polyethylene plastic, they discovered that each worm could create almost two holes per hour. To put this into context – in under a month, 100 worms could degrade (on average) 5.5 grams of plastic!

To prove that the worms weren’t simply releasing smaller pieces of plastic (due to the mechanical movement of the masticatory system), Bertocchini and colleagues smeared blended larvae onto the plastic and then… waited. Surprisingly, they found that even the waxworms liquid could create holes in the plastic. This outcome convinced the team that it was either an enzyme (in their saliva, or their gut) or bacteria in the larvae that were capable of degrading plastic.

this discovery might just turn the tables on the ways in which the battle against plastic waste is being fought

Spectroscopy analysis – measuring the light absorbed, emitted or dispersed by a material – was used to confirm the degradation of plastic following contact with homogenate larvae. The scientists concluded that waxworms could breakdown plastic using the same enzymes that they use to eat beeswax; by attacking the chemical bonds within polyethylene, the worms transform it into an “un-bonded” material named ethylene glycol (a chemical regularly used within antifreeze). A very high-resolution type of skimming probe microscopy only provided more evidence supporting the idea that the larvae are capable of modifying the unity of polyethylene.

Commenting on these findings, Paolo Bombelli (a biochemist at Cambridge University) said, “…if a single enzyme is responsible for this chemical process, its reproduction on a large scale using biotechnological methods should be achievable.” Bombelli seems to believe that the results of this research are promising, and have the potential to lead to a solution for the problem of plastic waste.

From here, the next step is finding which enzyme specifically is involved in the degeneration. And…after that? The range of possibilities is wide. The compound could be inserted into either bacteria or phytoplankton, in order to reduce the amount of plastic waste in uninhabited regions. Furthermore, the findings could potentially be tailored to an industrial scale (considering the larvae’s ability to degrade even the hardest of plastics). Finally, the enzymes might be spattered directly onto locations such as landfill sites.

This discovery might just turn the tables on the ways in which the battle against plastic waste is being fought. I am hopeful that it provides the possibility of moving away from the endless accumulation of plastic in landfills and seas, and towards the biodegradation of plastics. For the time being, however – let’s carry on trying to reuse more, and produce less.

REFERENCES:

WEBLIOGRAPHY:

  1. http://www.businessinsider.com/plastic-eating-worms-reduce-pollution-moth-larvae-2017-5?IR=T

  2. https://www.dogonews.com/2017/5/25/can-these-plastic-eating-wax-worms-help-reduce-our-trash

  3. https://thepanoptic.co.uk/2017/05/28/waxworms-hungry-weapon-targeting-plastic-waste/

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