Sunlight can be harnessed to transform polystyrene waste and surplus sulfur into valuable organic materials. The team behind this innovative process believes it could provide a solution to the millions of tonnes of polystyrene waste produced annually, eliminating the requirement for energy-demanding techniques.
The chemical sector generates over 20 million tonnes of polystyrene each year for applications in food packaging, electronics, and insulation. However, less than 1% of this polystyrene is recycled yearly. Different methods – including electrolysis, oxidation, and pyrolysis – present opportunities for recycling this plastic, but these often involve significant energy consumption or lack efficiency, according to study leader Qing-An Chen from the Dalian Institute of Chemical Physics in China.
‘By merging surplus elemental sulfur with non-degradable polystyrene waste, and facilitating the transformation using clean solar energy, we tackle both plastic pollution and the effective use of sulfur resources,’ Chen clarifies. Elemental sulfur is a byproduct of crude oil refining, with millions of tonnes accumulating each year.
Exposing polystyrene and elemental sulfur to sunlight for just two minutes under normal conditions resulted in the formation of several organic products. This included a diphenylated thiophene, utilized in semiconductor applications, and 1,3,5-triphenylbenzene, a multifunctional organic building block that can cost as much as $400 (£300) per kg.
This technique successfully upcycled various post-consumer waste items, such as spoons, cups, food packaging, falcon tubes, and assay plates.
Mechanistic investigations disclosed that sunlight energy produces sulfur radicals, which can then extract hydrogen atoms from the polystyrene backbone. Hydrated sulfur radicals subsequently engage with the polymer product through a sequence of steps to yield the final compounds.
‘Selectivity and yield are among the most significant hurdles for this project,’ comments Andrew Dove, a polymer chemist at the University of Birmingham in the UK. ‘Separation is typically energy-consuming or not conducted very sustainably… so enhancing [the selectivity] to circumvent separation would be a crucial objective.’
A considerable portion of the final reaction mixture consists of partially degraded polystyrene – nearly 40% by weight – although the team discovered that this product could either be utilized to fully depolymerise polystyrene or function as a UV-blocking additive in polystyrene films.
Chen notes that the team is currently focused on scaling up the upcycling process, as well as gaining a deeper understanding of the mechanism behind the conversion. ‘We [also] intend to broaden this method to other prevalent plastics, such as polyethylene, polypropylene, and polyvinyl chloride,’ he concludes.