Two For One: Upcycling Plastic Waste Into Useful Material That Leverages Solar Energy

Scientists develop a facile and cost-effective strategy to repurpose polymeric foam in various sunlight-powered applications.

Humanity needs innovative ways to combat the ever-increasing plastic pollution and energy consumption problems. In a recent study, an international team of scientists showcased a novel method to upcycle plastic foam waste into a highly versatile, cost-effective, and durable material for solar energy harvesting. Their approach has applications in water purification, ethanol distillation, and oil spillage cleanup, thereby constituting a great addition to the waste-to-energy nexus.

To avoid jumping straight into a global environmental crisis, we need to change our ways and fully embrace the concepts of sustainability and a circular economy. Appropriate waste management, recycling, and upcycling are among the main pillars of such still-idealistic societies, in which non-biodegradable plastic waste would be fully recovered and given a new purpose. Another foundational aspect of a sustainable society would be the use of clean and renewable energy sources for most of its endeavors, including the processing of plastic waste. But what if we could hit these two metaphorical birds with one stone?

Researchers are paying increasing attention to the waste-to-energy nexus, which encompasses technologies that simultaneously achieve waste reduction and energy production. Pyrolysis is one of the most common approaches to generate fuels from plastic waste, including discarded polymeric foams. Unfortunately, pyrolysis and other conventional ways of managing plastic waste consume energy and require highly technical skills. Thus, it’s high time we develop alternative strategies to breathe new life into plastic waste.

In a recent study published in Small (https://onlinelibrary.wiley.com/doi/full/10.1002/smll.202102459), an international team of scientists led by Prof. Yong Sik Ok from Korea University and Prof. Rui Xiao from Southeast University developed an innovative and remarkably simple method to turn polymeric foam waste into solar energy harvesters. Their approach involves taking advantage of the porous yet mechanically resilient structure of polymeric foam and use it, in combination with thermal energy procured from sunlight, for a variety of sustainability-related applications.

However, polymeric foams are not good at heating up by simply absorbing unfocused sunlight. The method proposed by the team involves first submerging and stirring the foam in a pyrrole-containing solution for a few hours. In this study, they used polymelamine formaldehyde, a plastic foam commonly used in packaging and as a cleaning abrasive. After washing and drying, the foam ends up coated in pyrrole, an organic compound with much better absorption characteristics in the solar spectrum. The resulting black, sponge-like material is extremely lightweight, inexpensive, and durable.

One application of this pyrrole-coated melamine foam (PMF) is water purification. The idea is to partially submerge the PMF in a water-containing recipient covered by a larger dome-like structure. Because of the capillary effect, water rises through the complex porous structure of PMF, which in turn heats up by absorbing sunlight. As the PMF transfers this heat to water, it quickly generates large amounts of steam, which condenses on the inner walls of the dome and eventually trickles down as clean water. “Our experiments showed that PMF could achieve solar evaporation rates superior to most photothermal conversion materials using solar energy alone, without using fossil fuels or electricity,” highlights Prof. Ok.

In a similar manner, PMF can be used for ethanol distillation. In this case, one begins the process with a diluted mixture of ethanol and water. Because ethanol evaporates many times faster than water, the liquid collected at the base of the dome after condensation and trickling will have a higher concentration of ethanol than the initial solution. By collecting the liquid accumulated in the dome and repeating this process several times, one can end up with a highly concentrated ethanol solution; the researchers reported that distillation from 10 vol% ethanol to 85 vol% ethanol was possible within just five cycles.

Yet another application of PMF demonstrated by the team was the cleaning of floating oil spillages. The porous structure and mechanical properties of the material make it act like an oil sponge, which can be reused over a hundred times with minimal losses in absorption capacity.

The positive qualities of PMF make it an attractive alternative to plastic foam waste upcycling in remote areas or emergency situations, as Prof. Ok remarks, “Owing to its advantages of ultra-facile fabrication, mechanical robustness, and cost-effectiveness, we anticipate that PMF may be widely applicable to portable energy-efficient systems for ecofriendly applications, thereby alleviating the serious environmental concerns arising from increasing energy consumption and pollution.”

Let us hope more and more people follow in the steps of this research team and come up with novel ways to reach sustainable development goals.