Recent advances and challenges in the biotechnological upcycling of plastic wastes for constructing a circular bioeconomy

Description Integrating oxidation and bioconversion provides a solution to recycle mixed plastics Since the early 20th century, plastics have played an ever-increasing role in human society. There is polystyrene, a spongy material widely used in protective packaging, such as styrofoam. There is polyethylene, the most produced plastic today, whose different density variants have been used in everything from plastic bags to poolside furniture. There is also polyethylene terephthalate (PET), which is the main ingredient in many synthetic fibers and water bottles. The numerous advantages offered by these nondegradable plastics, such as versatile physical properties and low manufacturing costs, have yet to be meaningfully challenged by another class of materials in the marketplace. However, the accumulative environmental cost of plastic waste is increasingly becoming an irreversible global catastrophe. On page 207 of this issue, Sullivan et al. (1) report an expansion of the toolbox for recycling plastic mixtures into other useful chemicals through a process comprising chemical oxidation and biological conversion.

Recycling plastic using a hybrid process

Plastic production worldwide has doubled in the last two decades and is expected to reach a four-fold increase by 2050. The durability of plastic makes them a perfect material for many applications, but it is also a key limitation to their end-of-life management. The current plastic lifecycle is far from circular, with only 13% being collected for recycling and 9% being successfully recycled, indicating the failure of current recycling technology. The remaining plastic waste streams are thus incinerated, landfilled, or worse, mismanaged, leading to them leaking into the environment. To promote plastic circularity, keeping material in the loop is a priority and represents a more sustainable solution. This can be achieved through the reuse of plastic items, or by using plastic waste as a resource for new materials, instead of discarding them as waste. As the discovery of plastic-degrading/utilizing microorganisms and enzymes has been extensively reported recently, the possibility of developing biological plastic upcycling processes is opening up. An increasing amount of studies have investigated the use of plastic as a carbon source for biotechnological processes to produce high-value compounds such as bioplastics, biochemicals, and biosurfactants. In the current review, the advancements in fossil-based plastic bio- and thermochemical upcycling technologies are presented and critically discussed. In particular, we highlight the developed (bio)depolymerization coupled with bioconversion/fermentation processes to obtain industrially valuable products. This review is expected to contribute to the future development and scale-up of effective plastic bioupcycling processes that can act as a drive to increase waste removal from the environment and valorize post-consumer plastic streams, thus accelerating the implementation of a circular (plastic) economy.

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Critical Review on the Progress of Plastic Bioupcycling Technology as a Potential Solution for Sustainable Plastic Waste Management

Upcycling of poly(ethylene terephthalate) to produce high-value bio-products.

Polyhydroxyalkanoates (PHAs) have attracted increasing interest as sustainable plastics because of their biorenewability and biodegradability in the ambient environment. However, current semicrystalline PHAs face three long-standing challenges to broad commercial implementation and application: lack of melt processability, mechanical brittleness, and unrealized recyclability, the last of which is essential for achieving a circular plastics economy. Here we report a synthetic PHA platform that addresses the origin of thermal instability by eliminating α-hydrogens in the PHA repeat units and thus precluding facile cis-elimination during thermal degradation. This simple α,α-disubstitution in PHAs enhances the thermal stability so substantially that the PHAs become melt-processable. Synergistically, this structural modification also endows the PHAs with the mechanical toughness, intrinsic crystallinity, and closed-loop chemical recyclability. Description Methyl groups improve a plastic Polyhydroxyalkanoates are a promising class of plastics accessible chemically or microbially from relatively sustainable feedstocks. However, their performance properties lag behind commercial petrochemically derived alternatives. Zhou et al. report that replacing reactive carbonyl-adjacent hydrogens in the monomer with methyl groups produces a polyhydroxyalkanoate that is more robust in multiple respects (see the Perspective by Guillaume). The dimethyl polymer is both crystalline and ductile, structurally sound during melt processing, and subject to efficient degradation to recover the monomer on treatment with base. —JSY Replacing reactive hydrogens in the monomer improves the properties of a type of polyester in multiple respects.

Chemically circular, mechanically tough, and melt-processable polyhydroxyalkanoates

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida, to funnel these oxygenated compounds into a single exemplary chemical product, either β-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products. Description Funneling mixed waste with microbes Current plastic recycling methods require sorting by chemical composition, a method that is expensive and results in products that are of lower quality and value than the starting plastic. If plastic waste could instead be converted to valuable chemical intermediates, then economical use of mixed waste as a feedstock might be feasible. Sullivan et al. developed a two-stage oxidation and biological funneling approach that can break down and reform mixtures of common consumer plastics (see the Perspective by Yan). The end products can be adjusted by metabolic engineering of the microbes in the second step, which should enable tailored conversion into various platform or specialty chemicals. —MAF Autoxidation and biological funneling converts mixed plastics to a single product.

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Morgan A Ingraham

Papers

Mixed plastics waste valorization through tandem chemical oxidation and biological funneling