Polyethylene (PE) has become the most produced plastic in the world today. From shopping bags to furniture panels, from toys to aircraft parts, polyethylene is found in almost every corner of life and industry. Besides PE, common plastics include polyethylene terephthalate (PET) and polystyrene (PS). PET is currently the main component of mineral water bottles, while PS is widely used in protective packaging and sponge filling. Excellent physical and chemical properties and low manufacturing costs make our lives inseparable from these plastic products. However, the degradation and recycling of plastic waste has not been well resolved, threatening the global environment and ecology.
At present, plastic recycling mainly includes physical methods of mechanical processing and chemical recycling methods. Mechanical processing will be limited by various additives, pollutants and other impurities in waste plastics, and the performance of recycled plastics will drop significantly. The chemical recycling method is to realize reuse by converting waste plastics into high-purity monomers or small molecular compounds, which is a recycling method with great development potential. However, the vast majority of waste plastics are a mixture of plastics of different materials. When recycling, they need to be sorted according to the chemical composition of the plastics, which is time-consuming and costly. The quality and value of recycled materials are often lower than the original plastics. In order to solve these problems, the researchers adopted a two-step method of chemical oxidation and biodegradation, which can degrade mixed waste plastics without classification, convert mixed plastic waste into valuable small molecule intermediates, and realize the efficient recycling of plastic waste.
The first step in the two-step process uses catalytic oxidation to break down the polymer chains into three types of organic acids. The researchers said that they were inspired by a series of studies on autooxidation and a series of subsequent studies on autooxidation. Plastics can undergo autooxidation reactions under the conditions of cobalt or manganese-based catalysts.
In order to avoid the corrosiveness of bromine, the researchers used N-hydroxyphthalimide (NHPI) instead of bromide as the initiator and cocatalyst of autoxidation. After determining the optimal autoxidation conditions for three common plastics PE, PS and PET, the researchers mixed the three plastics and catalyzed oxidation at high temperature (210 °C) for 5.5 hours to generate three types of acid products (benzoic acid, terephthalic acid and dicarboxylic acid) and the product yields observed for decomposition of the polymers alone.
In the second step of the two-step method, the researchers used the microbe Pseudomonas putida. Pseudomonas putida in nature usually uses acetate, benzoate, and C4 and C5 dicarboxylates as carbon sources, which are very close to the mixed organic acid products obtained from the first step of autoxidation. They further adjusted the gene expression of the strain through genetic engineering, and the obtained engineered strain could convert organic acid into β-ketoadipate or polyhydroxy fatty acid salt.
Subsequently, the researchers biotransformed the organic acid intermediates obtained in the first chemical reaction of the three plastics and their mixtures. Among them, the molar yield of β-ketoadipic acid can reach 75.5±8.5% when the oxidized small molecules of three mixed plastics are used as substrates.
The authors use oxygen and a catalyst to break down the plastic mixture into smaller, biofriendly compounds that can then be bioengineered into valuable chemicals—in other words, a technology that can be used directly without the need for recyclers. By sorting plastics by type, this technology solves the problem of plastic waste that is difficult to recycle.
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Reference
- Mixed plastics waste valorization through tandem chemical oxidation and biological funneling.
Kevin P. Sullivan, Allison Z. Werner, Kelsey J. Ramirez, Lucas D. Ellis, Jeremy R. Bussard, Brenna A. Black, David G. Brandner, Felicia Bratti, Bonnie L. Buss, Xueming Dong, Stefan J. Haugen, Morgan A. Ingraham, Mikhail O. Konev, William E. Michener, Joel Miscall, Isabel Pardo, Sean P. Woodworth, Adam M. Guss, Yuriy Román-Leshkov, Shannon S. Stahl, Gregg T. Beckham.
