Covid plastics crisis

To fight Covid requires lots of plastics – in disposable face masks, gloves, and medical gowns and lab test kits, for example. But how can we avoid adding to the already burgeoning plastics pollution problem? XiaoZhi Lim reports.

By the International Solid Waste Association’s estimate, single-use plastic consumption in the US jumped 250-300% since the Covid pandemic began. In China and Spain, medical waste grew 300% (Chem. Eng. J., doi: 10.1016/j.cej.2020.126683); meanwhile in tiny Singapore, from just eight weeks of lockdown, 1400t in additional plastic waste was created from home delivery and takeout.

Part of the increase in single-use plastic consumption is essential: personal protective equipment (PPE) such as disposable face masks, gloves and medical gowns, and laboratory equipment such as test kits are needed. But consumption of single-use plastic containers and individual packaging has also increased – a ‘double whammy’ in the rise of plastics, says Rolf Halden, an environmental engineer at Arizona State University, US.

Around the world, concerns over surface transmission of the coronavirus led governments to roll back policies for curbing single-use plastic (Environ. Sci. Technol., doi: 10.1021/acs.est.0c02178). Fees or bans on single-use plastic such as grocery shopping bags were postponed or suspended in several US states; some went as far as to prohibit shoppers from bringing their own reusable bags. Bans on single-use plastics have also been delayed in the UK, Canada, and South Australia, while a proposed tax on virgin plastics in Italy was delayed to 2022.

‘The pandemic has reversed positive change that was already induced towards reducing plastics,’ Halden said. ‘That’s regrettable and it’s a move in the wrong direction.’

Preventing plastic pollution while protecting against Covid will be difficult. Consumers are expected to continue wearing face masks, a significant portion of which are disposable three-layer surgical masks that are light and easily littered. Meanwhile, shifts to online shopping and home food delivery services creates demand for plastic packaging, much of which is not mechanically recyclable. And potentially contaminated plastic waste from medical settings will be difficult to process.

Finding solutions that do not lock us into a single-use mindset is key, says Shanar Tabrizi of non-profit Zero Waste Europe. While it may be simple to fall back on incineration or waste-to-energy systems to handle increased volumes of plastic waste, they are not sustainable in the long run, she said. ‘The first option should always be waste prevention or reuse.’ Meanwhile, products should be designed for recyclability, and infrastructure needs to be built for materials collection, sorting and recycling. These are ‘long-term investments which better retain resources within the loop.’

Safe for reuse

Reusing plastic items is one of the most effective solutions to cutting down on waste. In May 2020, researchers from University College London (UCL) estimated that if every person in the UK wore just one disposable mask/day for a year to fight Covid, that would create 124,000t of plastic waste (University College London Press, doi: 10.14324/111.444/000031.v1).

‘It’s a huge environmental burden because those are made of polypropylene,’ said Gang Sun, a materials scientist at the University of California Davis, US.

Over 95% of that plastic waste would be eliminated if people switched to reusable masks that could be washed, the UCL researchers found. Even if reusable masks were paired with single-use filters, it would still generate 60% less waste than simply relying on single-use masks.

To make face masks attractive for reuse, Sun and his colleagues developed a material that could disinfect itself when exposed to light (ACS Appl. Mater. Interfaces, doi: 10.1021/acsami.0c15540). Current PPEs are all designed to act only as a barrier, he explains, so their surfaces could still be a source of live pathogens and pose a contamination risk during removal.

For disinfection, Sun selected rose bengal, a molecule being investigated for use in photodynamic treatment of aggressive eye infections and skin cancer. Rose bengal kills cells by generating reactive oxygen species locally when a laser shines on it, Sun explains. The researchers first modified the fibres in plain cotton fabric with 2-diethylaminoethyl chloride, making them positively charged. The positive charges help to bind and retain negatively charged rose bengal ions. Within 60 minutes of exposure to sunlight, 99.9999% of E. coli bacteria and T7 bacteriophage were killed. Sun attributes the mask’s disinfection efficiency to the positive charges, which also help to attract microorganisms. The masks remained effective against bacteria after the equivalent of 10 hand washing cycles.

To cut the use of packaging plastics, reusable or refillable systems where consumers return used containers to be washed and reused would help. While these systems were already gaining momentum in cities like New York, San Francisco and Hong Kong, the pandemic has shifted consumer preferences towards single-use – partly because of advocation from the plastic manufacturers that reusables allow the virus to spread. ‘The plastic industry didn’t miss a beat to assert that single-use plastic was the safest option,’ says Cassia Patel of non-profit Oceanic Global.

In June 2020, over 125 scientists including epidemiologists, virologists, biologists, chemists and doctors endorsed the safety of reusable containers (greenpeace.org/usa/wp-content/uploads/2020/06/Health-Expert-Statement_125-experts.pdf ). While early studies showed that the coronavirus could survive on surfaces for a few days, Covid primarily spreads through aerosols rather than surface transmission, the researchers wrote. ‘Single-use plastic is not inherently safer than reusables and causes additional public health concerns once it is discarded.’

Slashing single-use plastic packaging is key to making a dent in plastic pollution. ‘When we think of the pandemic’s impact on plastic use, all our eyes go to medical supplies,’ Halden said. But the growth in packaging, particularly small, individual wrappers and disposable foodware that cannot be recycled, is far more worrying, he said. ‘In terms of scale, those impacts are much bigger.’

Biodegradables

Reusable models could cut down on single-use plastic, but Patel acknowledges they are difficult to implement everywhere, particularly in suburban and rural areas where residents are dispersed. Covid has made it harder to adopt reusable models as people move out of cities or work remotely instead of being together in a large office building, where collection bins are more easily located.

Biodegradable materials could help reduce the impact of single-use items – if facilities for industrial composting or anaerobic digestion are available, Patel says.

For example, polylactic acid is a well-established compostable polymer that could be made from plant feedstocks such as corn, cassava, or sugar beet. The polymer is well-suited for food packaging or disposable foodware applications, and already commands a market size over $600m in 2019. But it requires elevated temperatures for break down, such as those in industrial composting facilities.

Another class of biodegradable polymers, polyhydroxyalkanoates (PHA), do not require the high temperatures of an industrial composting facility for degradation. The polyesters can be broken down by naturally occurring bacteria even in the cold marine environment, says Molly Morse of Mango Materials, a San Francisco-based firm producing PHA from methane. PHA can also be made through fermentation of other feedstocks ranging from sugars, vegetable oils, or agricultural waste.

Since the pandemic began, Morse has seen a lot of interest for PHA. ‘In 2020, every company we had ever spoken to reached back out to check on our status in a narrow time frame,’ she said. Most inquiries were for packaging applications, in particular flexible films, Morse said. But she believes PHA could be well-suited for making PPE, especially if paired with anaerobic digestion facilities where their biodegradation could be controlled. Then, the methane formed from the degradation could be used to produce electricity, heat, fuel, or even new PHA, she said.

Chemical treatments

Incineration or waste-to-energy processes is a straightforward strategy to dispose of plastic waste and keep it out of the environment. Pyrolysis or gasification technologies to turn plastic back into synthetic crude oils is also possible.

But those approaches simply turn the plastic pollution problem into a climate problem, says Tabrizi. ‘Why release the carbon embedded in the plastic to the atmosphere by burning it as fuel, if this can be avoided through reuse or recycling?’

Susannah Scott, a chemical engineer at the University of California Santa Barbara, US, points out that energy recovery approaches recoup only a small fraction of the energy invested in plastic. High-temperature pyrolysis could depolymerise some polymer types such as polystyrene into their monomers, which can then be re-polymerised. But this would require a lot of energy for polyolefins, which contain only single carbon-carbon and carbon-hydrogen bonds. ‘It’s simply not worth it,’ she says.

Instead, Scott and her colleagues have been working on converting polyethylene, which makes up 36% of plastic waste by mass, into other possible useful materials. ‘One of them is to make carbon-based chemicals that we would otherwise make from fossil fuel anyway,’ Scott said. Turning waste plastics into specialty chemicals that command high prices could also be more economically viable than converting them into fuels of low value.

Scott’s team initially tried out another research group’s approach of mixing short hydrocarbon molecules with polyolefins to obtain medium-length alkanes. But the researchers realised that they were getting aromatic compounds instead. Since alkylaromatic compounds are used in many applications such as detergents, lubricants, and refrigeration fluids, the researchers decided to pursue the goal of producing alkylaromatic compounds from polyolefins.

At a relatively mild temperature of 280°C, Scott and her colleagues converted low-density and high-density polyethylene into liquid alkylaromatic compounds by heating with a platinum-based catalyst for 24 hours (Science, doi: 10.1126/science.abc5441). Scott believes the process should be generally suitable for waste polyolefins, including polypropylene from face masks. The researchers are currently working to improve the catalyst’s activity, selectivity, and stability, while also figuring out how to engineer the process to operate in a continuous, scalable fashion.

The pandemic has reversed positive change that was already induced towards reducing plastics. That’s regrettable and it’s a move in the wrong direction.
Rolf Halden Arizona State University, US

In the long run, new materials designed for chemical recycling in non-energy-intensive ways are needed, Tabrizi says. Stefan Mecking and his colleagues at the University of Konstanz in Germany developed one such possibility: polyethylene-like materials that contain built-in weak points (Nature, doi: 10.1038/s41586-020-03149-9). Using 1,18-octadecanedioic acid, the researchers first prepared 1,18-octadecane diol and 1,18-octadecane dimethylester, then polymerised them. The resulting polymers resemble polyethylene but with ester or carbonate ‘break’ points located at every 18th carbon atom.

The polymers can be melted and extruded, making them suitable for 3D printing. Heating in basic ethanol or methanol at 120°C converted 99% of the polymer back to 1,18-octadecane diol, which crystallised from solution. The researchers recovered the pure 1,18-octadecane diol monomers by filtration and a single recrystallisation. They also managed to recover the monomers when the polymer was mixed with pieces of a polypropylene beverage cup and a high-density polyethylene bottle cap, which remained unreacted.

Solutions may be in demand, but existing petroleum-based plastics are being manufactured at such large scales, it is very difficult for new materials to compete price-wise. Even Mango Materials, which is making PHA from low-cost methane, needs larger infrastructure and larger production scales to get prices down, Morse said. ‘There is just so much steel in the ground for petroleum-based plastics.’