Environment and energy

“Spent all these days in my laboratory and found many interesting things,” wrote Belgian-American chemist Leon Baekeland in his journal in June 1907. For four days he had been experimenting with condensation reactions between phenol and formaldehyde impregnated in wood blocks. “Have applied for a patent for a substance which I shall call Bakalite.” It was the first plastic made from synthetic components and the beginning of a materials revolution.

There’s no denying that plastics are amazing materials. They are made from polymers – long, stringy molecules composed mostly of a carbon backbone with a cornucopia of different functional atoms and groups branching off, from simple halogen atoms to aromatic rings and oxygen-containing ester linkages. Plastics can be hard or bendy. They are easily melted and reformed. And they are among the cheapest and most enduring materials on the planet. But that’s the problem. Plastics have spawned a consumer revolution in disposable goods that persist for decades, even centuries. And while public concern about plastic pollution is now strong, turning those sentiments into positive action can be tricky.

One person seeking solutions is Sally Beken, a chemist from Innovate UK. She heads the UK Circular Plastics Network, which aims to cut waste by bringing users of plastic together. For her and many others working on plastic waste, the problem is not the plastics themselves, but our poor husbandry of them. The good news is that technology is constantly making it easier to retrieve, reuse and recycle plastic, with developments in physics playing an important role. But despite the progress, the biggest challenge may be yet to come.

Sorting out the problem

To minimize the carbon footprint of a plastic product, you would ideally reuse it multiple times. But the additional plastic needed to make something robust enough so that it can be reused isn’t always balanced by the amount of reuse it gets. A Tupperware takeaway container, for example, would need to be used 200 times to leave a lower environmental footprint than its expanded polystyrene counterpart, even when the latter cannot be recycled. Sometimes it’s not even possible to reuse a plastic product – for example, plastic tubs can crack, rendering them useless. That’s why many people who are looking for ways to handle plastics more sustainably are instead trying to develop more efficient circular economies for these materials rather than simply trying to make each product last longer.

What makes recycling plastics more complicated than, say, cardboard, is the sheer proliferation of different types, all of which need distinct treatments. “For food packaging, we don’t need all these different types, we could manage with just three,” says Beken. And if we did just use fewer types, the volumes of each would increase, making it more economic to recycle.

But if we can’t limit the number of types of plastic, why not find smarter ways to sort them? In 2017 more than two million tonnes of plastic were used as packaging in the UK alone, and with many recycling stations still requiring people to separate the plastics by eye and by hand, there’s clearly got to be a better way.

One company leading the way is the French recycling business Paprec, which currently has 210 sites handling around 12 million tonnes of waste each year. Although it employs operators to sort some of the plastic by colour, and manually remove rivets, screws and the like from industrial plastic waste, a lot of the sorting is now automated. However, even the sorting approach depends on the type of plastic. In the case of polyvinyl chloride (PVC) and polyethylene terephthalate (PET), sorting is fully automated and based on the plastic’s optical properties. Cameras analyse the spectra of the waste before air nozzles blow off different types to redirect them, either for different treatments or ultimately landfill. Such optical sorting is the dominant automated approach in a lot of recycling units, but the technique often struggles with dark plastic. That’s because it traditionally uses near-infrared radiation, which is not reflected enough from darker materials to distinguish between different types.

While equipment is now commercially available over a broader spectrum of frequencies so that dark plastics can be sorted, additional plastic properties can be exploited instead. Paprec, for example, also uses “flotation sorting” which differentiates materials according to whether they float or sink. The technique neatly combines sorting with the rinsing stage of the recycling process, but the problem is the materials need to have density differences of at least 0.2 g cm^–3. Unfortunately, for polyethylene (PE, typically used in packaging and plastic bags) and polypropylene (PP, used in packaging and labelling) this is not the case. Another densimetric approach is to suck the lighter plastics off from a vibrating inclined plate – plastics too heavy to suck up are then redirected elsewhere. This requires a density difference of at least 0.3 g cm^–3, so is also not able to separate PE and PP. In fact, these two polymers are particularly problematic because they are often mixed within materials, making it even harder to isolate them.

Another option is “triboelectric sorting”. Using friction to charge a plastic’s surface, you can use charged electrodes to attract and reject different types of plastic depending on whether they become positively or negatively charged. Triboelectric sorting is effective on quite a few plastics including PET drinks bottles, engineering thermoplastics, plastics from electronic and cable wastes, PVC window profiles and even plastic production waste. Unfortunately, the waste must be dry – and multiple stages of plastic recycling are wet or involve washing.

“One bin to rule them all”

Clever and ingenious though these different sorting approaches may be, mixed plastic waste still requires a host of sorting treatments to separate it all. And since not all recycling depots have all the available techniques, the reality is a lot of potentially recyclable types of plastic end up in landfill. As an alternative, researchers in the Wolfson Centre for Materials Processing at Brunel University London in the UK devised a fluorescent tagging system that not only allows all plastics to be optically sorted, but can also draw distinctions based on use, separating containers used for food from those for pesticides for example.

Known as PRISM (plastic packaging recycling using intelligent separation technologies for materials), the technique involves writing a code onto the plastic using a dye containing phosphors – light-emitting molecules like those used in strip lighting. The phosphors emit ultraviolet (UV) light, so the code is visible only to detectors operating at that frequency. All you need to do is attach a UV detector to the optical sorters and attract buy-in from plastic manufacturers to tag their products. The first full-scale trial of PRISM was demonstrated by TOMRA – a Norwegian firm working on recycling innovations – which in 2017 claimed it could collect 98% of labelled plastics with 95% accuracy.

The fluorescent-tagging approach has many fans, but what do you base the tagging system on? Sorting has traditionally been centred on distinguishing plastics based on the polymer’s carbon backbone, or the monomer unit that is repeated to make up the polymer chain. But thanks to the ever-changing versatility of many plastics, this approach has shortcomings. Milk bottles and laundry detergent bottles, for example, are often both made from high-density polyethylene (HDPE), but the former readily soften when filled with hot water while the latter are more robust. A plastic’s properties are also affected by the type and amount of additives, by the molecular weight (how many repeating units on average make up each molecule for the plastic), by the percentage of recycled content, and by the source of that recycled content. As a result, a laundry bottle made from milk-bottle plastic will not have the flexural strength expected of it.

“The problems around plastics are not solved by a single discipline,” says Michael Shaver, a polymer scientist at the University of Manchester, who heads a UK Research and Innovation (UKRI) project called RE3 – Rethinking Resources and Recycling. Along with 25 stakeholders from across the supply chain, RE3 includes a project that Shaver runs called “One bin to rule them all”, which is seeking better recycling infrastructures so that all plastic waste can be sorted, recycled and valued even if it is disposed of in a single bin. For Shaver, what we need is a system or marker that sorts based on real value – the properties and potential uses of the retrieved plastic – not the attributes assumed from the chemistry of the backbone.

Shaping up

The simplest way to recycle plastic is to mechanically extrude and recast the product without meddling significantly with the chemistry of the polymers involved. The plastic is ground into small pieces, formed into pellets and poured like cereal into a turning “screw” that transports, melts and pressurizes the plastic so that it can be cast in a liquid form around moulds where it re-solidifies into the shape of the desired product. In Europe 99% of all recycled plastic is processed in this way.

Drinks bottles have been one success story in mechanical recycling, with most manufacturers now using the same PET material, making these much easier to sort and recycle. In fact, voluntary pledges from packaging producers alongside content targets set in the EU Single Use Plastics Directive have resulted in increased demand for recycled PET. “You can’t get enough of it,” says Beken, who points out that the UK has been importing recycled PET from Belgium because there isn’t the capacity to meet demand in the UK.

But even with PET recycling, there are disparities. Bottles are readily retrieved and can be extruded into high-quality recycled products, whereas there are fewer separate reprocessing lines for the plastic trays used for meat or ready meals, where the cost per tonne obtained is higher. In addition, plastic trays are often lined with different materials. When they are extruded, materials of the same type start to congregate – “phase separate” from other types – so that the plastic is no longer homogenous, leading to lower-quality recycled products. These problems mean that potentially recyclable materials could end up in landfill.

“Mechanical recycling is about consistency,” says Shaver, who believes that current efforts towards circular plastic economies are hampered by a total lack of standardization in the quality of recycling. “Legislation is the big thing.”

Enlisting the chemists

One active area of research is working out the impact of repeated mechanical recycling on a plastic’s properties. Inevitably, when a plastic is extruded and reshaped, there will be some wear and tear – such as reductions in polymer lengths, or the introduction of impurities – that will limit how many times the same polymers can be recycled. Shaver’s group is, for example, extruding plastics to try and understand the chemical processes at play during mechanical recycling so that they can enlist chemistry to combat the degradation taking place.

Chemistry also offers alternative methods for recycling, albeit with a larger loop between the waste product and its reincarnation. Strategies include breaking down a polymer into its monomer units, which can then be subject to some of the original polymerization reactions to produce a high-quality product once again. Alternatively, the polymer could be broken into oligomers – shorter chains that contain multiple monomer units – therefore preserving some of the original work done to produce the plastic. Such chemical approaches could be vital in the textile industry, where fibres integrate plastics with other materials that mechanical processes can’t easily separate.

Chemical processing could also help recycle “microplastics” – microscopic scraps chipped off larger plastic objects – but the main challenge is retrieving them in the first place. Most efforts to capture microplastics have focused on preventing these substances from entering the environment where they can pollute waterways and ultimately the food chain. While macroplastics are not absorbed by the body, and simply pass through the digestive system, we do not entirely understand the impact of ingested microplastics. Shaver points out, for example, that some common plastics, such as PET, have oxygen-containing ester groups in the polymer chain. “These functional groups are common in biological systems, and thus have the potential for more significant ecotoxicological impact,” he points out.

The problem is we’re all creating microplastics even if we don’t mean to. Microplastic fibres, for example, are released whenever you wash your clothes, seeping through domestic sewage treatment plants and remaining in the sludge that is often spread on agricultural land. “None of the current domestic or utilities infrastructure was designed to deal with such tiny particles,” says Adam Root, founder of a new UK firm called Matter, which is developing commercial and domestic microplastic harvesting systems. Products to capture microplastics – such as bags to put clothes in during washing – are already available on the market, but few people know about them or use them. To combat the problem from another angle, Root has developed an external regenerative filter that can be retro-fitted to existing washing machines, and an internal unit for new models. Both will be launched later this year.

Ideally, microplastics would be separated from all waste sewage before they spread into the environment. But traditional optical sorting is difficult for microplastics in sediments because background signals and surface degradation muddy the spectra collected. In 2017 researchers at the University of East Anglia in Norwich, UK, reported a fluorescent tagging technique for microplastics in which they stained the plastics with a compound called “Nile Red” to help identify them in sediments. Spectral shifts in the fluorescence of Nile Red due to the polarity of a plastic’s surroundings could also help identify how hydrophobic the material is and therefore distinguish certain types of plastic. This kind of staining might also be applicable to nanoplastics – even tinier bits of plastic – that Shaver points out are an emerging concern. But there is still a long way to go before anyone has the infrastructure to capture micro- and nanoplastics at a scale where their value can be retrieved in recycling.

PETase – a biological solution

In 2016, while picking through 250 pieces of debris from a PET recycling plant in Sakai prefecture, Japan, a team of scientists reported seeing a sediment sample harbouring a consortium of micro-organisms that appeared to be feeding on the PET.

Further analysis led the researchers – Kenji Miya-moto ​at Keio University, Kohei Oda at Kyoto Institute of Technology, and collaborators – to home in on a specific strain of bacteria that is key to the process. Named by the researchers as Ideonella sakaiensis, the bacteria releases two enzymes – PETase, which can hydrolyse the PET, and MHETase, which hydrolyses the reaction intermediate, mono(2-hydroxyethyl) terephthalic acid. The result: the environmentally benign biproducts terephthalic acid and ethylene glycol (Science 351 1196).

The discovery galvanized the scientific community, with several groups across the world racing to understand and potentially improve the enzymes’ activity. Among those keen to develop the work of Miyamoto and Oda was an international team of researchers, led by H Lee Woodcock at the University of South Florida in the US, John McGeehan at the University of Portsmouth in the UK, and Gregg Beckham at the National Renewable Energy Laboratory in Colorado, US. In their efforts to establish the structure of PETase, Beckham and his team performed X-ray crystallography at the UK’s Diamond Light Source, which produces X-rays that are intense and bright enough to compensate for the fact that PETase crystals are hard to form. The team was able to identify the 3D structure of PETase and even engineer it to improve its activity. As Beckham explains, the molecular structure gives insights into the potential mechanism by which PETase evolved from an enzyme that most likely works on a natural substrate – such as plant-cell-wall polymers cutin or suberin – to one that can degrade man-made PET.

It might seem impressive that in just the five decades since PET waste first began to amass around the world, bacteria have evolved to make it a source of carbon nutrition. However, Beckham suggests that what the discovery and analysis of the Ideonella sakaiensis and PETase really illustrate is how much further the enzyme can be optimized for the task. “This is exciting for the scientific community, though, because it means that we can harness tools like directed evolution [for which Frances Arnold shared the Nobel Prize for Chemistry in 2018] to make even better variants of this and enzymes like it for industrial use.”

However, others remain sceptical of the positive role something like PETase can play. Setting aside scare stories of enzymes running havoc and reducing all plastic to compost overnight, Shaver questions the focus on PET as a polymer, which is already easily recycled. Mechanical recycling to recover the product or even chemical recycling to recover the oligomers are tighter circular systems that would seem more efficient than taking the plastic back to the monomer, he argues. Shaver highlights the potential interest there might be in finding an enzyme that works on polymers with no oxygen on the backbone. Such an enzyme might tackle some of the most persistent types of plastic.

Beckham, meanwhile, points out that plastic bottles – the main success story of PET recycling – comprise just 30% of PET used. In fact, carpets and clothing are the main consumers of PET, but they are not readily recycled. Furthermore, mechanical recycling itself produces a fraction of small particles – “fines” – that are beyond the process’s reach.

“Understanding where and when biology can play a role in recycling of PET (among others) in terms of the economic and sustainability perspectives is important,” Beckham says. His group, for instance, is comparing chemical recycling technologies for PET and many other plastics (including mixed plastics) with biological solutions, which may reach beyond PET. In March 2020, researchers in Germany reported the discovery of a bacteria that could break down polyurethane, a plastic widely used in refrigerators, buildings, footwear and furniture that it is currently very expensive to recycle.

The role of the past in the future

While companies can be doing all the right things in switching to products that are more compatible with sustainable circular plastics economies, it’s down to consumers to play their role too. Beken highlights Gumdrop, a UK-based firm that collects used chewing gum and turns it into bins for collecting more of the gum. Most commercial chewing gum is based on a synthetic rubber called polyisobutylene mixed with food-grade plasticizers, and it takes hundreds of years for each piece of gum to degrade completely. The problem Gumdrop has faced, however, is that people throw cigarettes in the bins, which contaminates the gum and prevents it from being recycled. As Shaver puts it: “What matters is not whether any type of plastic is recyclable, biodegradable or compostable – but whether it will be recycled, biodegraded or composted.” And that’s perhaps the biggest problem facing plastic recycling – how to deal with our fickle human behaviour.