Several years ago I went on a camping trip with our cub scouts. One of the other leaders loaned me a plastic chair. As I relaxed, suddenly it cracked and fell apart. It was made of some sort of plastic/poly, and when we looked at it we could tell that years in the sunshine had made it weak. It was obvious that the sunlight had broken down some chemical bond in the plastic. When I applied my weight to the chair, it just broke down. Down being the operative word! But why? Evidently the ultraviolet (UV) rays of the sun caused a photochemical effect within the polymer structure of the chair – and crack!
I got up and dusted myself off, and carried the broken chair to my car. I would take it home and dispose of it. Then I went on my way without thinking much beyond being able to put the broken chair at the curb for pickup to be recycled. However, others have taken this photochemical effect and enhanced it to help reduce the problem of plastic pollution.
Flash forward to a road trip I recently took out west. Often we would stop and buy tomatoes and cheese and english muffins and make a classic roadside sandwich for lunch. Other times we would get a pre-made sandwich to go from a convenience store when we stopped for supplies or for gasoline for the car. In a roadside rest stop I would sit at a picnic table and relax in the shade of a tree while enjoying my lunch. Ham and cheese with mustard is always a favorite of mine. But after I have peeled back the plastic film from the formed, plastic package, and after I have enjoyed the sandwich, what to do with the plastic sandwich box? It has bits of mustard and bread and cheese crumbs stuck to it. Our county’s recycling protocols ask that I do not recycle this with other “clean” plastic waste. My sandwich container is contaminated. The organics, and the chemicals in the mustard will cause problems down the recycling line as this waste plastic cannot be recycled because of the contaminants. The contaminants create weaknesses in the reformed plastics. So my sandwich container, along with other plastics contaminated with food and other organic and chemical residues is NONRECYCLABLE!
But there are clever people who work on the issue of recycling old plastic into new, and they have developed a process that addresses the issue of contaminated, nonrecyclable plastic. An article in Chemistry World reports on work that may help reduce the burden of nonrecyclable plastics on the waste streams, public spaces, and the world’s oceans.
The process being studied not only reduces the pollution burden but also aids in the recovery of the energy that was put into the making of the plastic in the first place.
Moritz Kuehnel and Erwin Reisneer of Swansea University and the University of Cambridge respectively and their colleagues have devised a process that uses a photocatalyst to degrade otherwise nonrecyclable plastics. Their testing included pure plastic product as well as plastic that was contaminated with various organic and chemical residues which would normally cause the plastic to not be recycled into new usable products. Their process uses cadmium sulfide (CdS) quantum dots as a photcatalyst for sunlight. These quantum dots are high quality nanoparticles which are generally fabricated to act as catalysts in chemical processes. There are four elements to the process; the plastic, the CdS quantum dots, water, and sunlight. From the process, called photoreforming, hydrogen is generated.
The hydrogen has many applications including, as we look down the road, as fuel for hydrogen fuel cell cars! However, there continues to be a debate on whether hydrogen fuel cell powered cars or electric vehicles are the next giant leap forward in zero carbon transportation. But the process reported in Chemistry World may help address the energy intensive generation of hydrogen from water. The chemistry in the reported process has been shown to work, and now the team as well as others is working to scale up the process so it is economical and beneficial as it has the potential to convert huge amount of contaminated and nonrecyclable plastic wastes into useful chemicals and fuels.
The Chemistry World article may be found at https://www.chemistryworld.com/news/sunlight-converts-plastic-waste-to-hydrogen-fuel/3009467.article