Recycling Plastic Waste

Perjantai 14.10.2022 klo 15.16 - Nikk Nikinmaa

One of the biggest problems with recycling plastic waste has been that a mixture of different plastics cannot be used as a feedstock for generating new plastic products. This means that either the consumers have to be quite careful and knowledgeable in sorting plastics or recycling stations need to carry out expensive sorting of plastics for further use. In a recent article in Science (October 13, 2022; DOI: 10.1126/science.abo4626) Sullivan et al. present a method that can be used for a mixture of plastics to generate starting material for commercial use. The method is comprised of two steps: first oxidizing the plastic mixture with metal-based catalysis (Co and Mn) to intermediates that can be digested by engineered bacteria to produce a feedstock for new commercial products. By varying the way that the bacteria are engineered, different starting products can be generated.

To my eyes, this method is quite safe and practical. It avoids the inherent problem with plastic-eating bacteria. I remember the book Mutant 59 by Kid Pedler and Gerry Davis (from 1971) where the plastic waste was planned to be eaten by bacterial mutation, which was very effective in consuming plastics, favoring them over any other foodstuff. The mutant naturally got accidentally free and started devouring everything made of plastics. Planes started crashing down, because all the plastics covering electric leads were eaten up and motors short-circuited. Most things in our houses became non-functional or half-disappeared etc., naturally, if you look around: most things are at least partly made of plastics. Since in the method described by Sullivan et al. the engineered bacteria eat only the oxidized end product of catalysis, they can only live in the recycling plant. By combining this method with effective collection of plastic waste, much of the plastics problem can be removed.

However, even if we get smokers to put their cigarette butts to waste bins, there will be one significant source of small plastic particles. Almost half of the microplastics in the environment is tire wear particles. They are impossible to collect. Thus, even if the recycling of all other plastics were more than 90% successful, this source would remain. It can only be reduced by drastically reducing road traffic. In this regard, electric cars are not a solution, the only solution could be using rail traffic and public transport more generally.  

Plastics in the Environment

Sunnuntai 14.4.2019 klo 14.51 - Mikko Nikinmaa

When one thinks about durable, light, easy-to-use and mouldable material, plastics certainly come to mind. It is hard to image that the plastic age is only 50-60 years old. One can hardly image life without plastic containers, plastic-insulated wires, plastic parts in household appliances and cars, and artificial fibres in clothing. This plastics era has produced and is producing so much of the useful materials that the world is choking to them. A very useful, detailed review about plastics, their use and environmental problems generated is written by CJ Rhodes in Scientific Progress 101: 207-260 (https://journals.sagepub.com/toc/scia/101/3).

In a way, lumping many different materials under one common name, plastics is wrong, because the foam used in insulation, fibres of clothing, and plastic packaging of food are very different. However, two things are common: First, most of the plastics are oil-based – more than 90 % of plastics are made of oil. In addition to oil-based products, plastic-like materials can be tree- or other plant material-based. However, these materials are exactly as problematic as oil-based ones except for not being fossil fuel-based. Second, the materials degrade slowly. The average life length of plastics is tens to hundreds of years. This means that virtually all plastics ever produced can still be present. The persistence is the major reason for the environmental problems generated by plastics.

Of the different plastics only less than ten percent are recycled, a little more than ten percent are burned and 80 % are currently ending up in the environment or in garbage dumps. This distribution of the fate of used plastics is the second major reason for the environmental problems. The most visible plastic pollution is that of the oceans, especially the Pacific Garbage Gyre in North Pacific, but virtually all major seas and beaches have significant amounts of trash. The waste problem is most pronounced in Asia, as all of the countries with most environmental plastic waste are there: China, Indonesia, Philippines, Vietnam and Sri Lanka. These countries contribute more than 55 % of world’s total waste. Because recycling is so limited, factories, which would use recycled plastics in their production, have an eternal material supply problem.

About 40 % of all the plastics is used in packaging. This has also the shortest half-life of use, less than a year. Thus, packaging is by far the most extreme case of single-use attitude. The second most important plastics user is building and construction industry with about 20 % proportion. However, the material is turned over in 35 years, decreasing the yearly amount of waste. A much bigger problem is textile industry, as the proportion is 20 %, and the turnover time is less than 5 years. Much of the rest is used in machines and electronics, and the material is turned over in 5-20 years.

The plastic waste can be macro-, micro- or nanosized. The macro-sized, visible material slowly degrades, but can disturb animal life up to hundreds of years. The materials are especially problematic, when they cause strangulation of animals or block their intestine thus inhibiting normal digestion. In addition, animals may feed on plastic materials, which naturally cannot be digested. Also, plastic trash affects the visibility of water. Micro-sized plastic is characterized as material of less than 5 mm but more than 100 nm in any dimension. Next to nothing is known about its appearance in soils, but an increasing number of studies has demonstrated its presence in water. It is notable that much more than 95 % of micro-sized plastic is removed in wastewater treatment plants. Much of the microplastics is ingested by animals. The lack of digestibility is a problem, and can cause starvation, when the alimentary canal is filled with material that cannot be digested. However, very little is actually known about if and how microplastic particles affect organisms. One of the possibilities is that it is not the particles themselves, but dissolved toxic chemicals initially adsorbed on microplastic particles that cause problems. The role of microplastics would in this case be increasing the surface area whereby all the toxic chemicals adsorbed to plastics can diffuse or otherwise be absorbed into the cells. However, the actual mechanisms by which microplastics affect animal functions are poorly known. In fact, one reviewer recently pointed out that the ingestion and egestion of microplastics have been extensively studied, but next to nothing is known about their toxic effects if any. The same is true for nanoplastics, materials with at the most one dimension 100 nm. Their cellular uptake and effects at environmental level are virtually unknown. This is a general problem in nanotoxicology: one has repeatedly shown that nanomaterials can be toxic at high concentrations, but their effects at environmental or predicted environmental level are unknown. One possibility is that they cause inflammation in any tissue they come in contact with.

Since the most important proportion of plastic waste is packaging, and single-use products largely associated with food and drink, it is were nice to know that the amount of trash can easily be decreased immensely by reducing overpackaging and by recycling. Effective plastic collection would be cheap and could be implemented anywhere in the world. Virtually all plastic material used in packaging needs to be recyclable and most of the plastics can minimally be burned. This would be equivalent to burning the same amount of oil. One can say that the major way of combatting plastic waste is to increase the effectiveness of plastic collection. At the moment, out of all trash found in the environment, more than 3/4 is plastics. Finally, replacing plastics by less “eternal” material where possible is needed. This, as reducing packaging, would be something that the production side needs to do. However, the consumers can affect, what is produced, by choosing the products so that overpackaging does not sell any more. The consumers can, on the other hand, do much on the recycling side.