Two-stage chemical and bioconversion approach turns mixed plastic waste into valuable chemical products

A hybrid process combining chemical oxidation and bioconversion via bioengineered microbes provides a novel approach for recycling mixed plastic waste into a single, useful chemical product, researchers report. While current plastic recycling methods require challenging and expensive sorting of plastic types and yield end products of lower quality and value, Kevin Sullivan et al. show that this new two-stage approach is effective in converting mixtures of high-density polyethylene (HDPE), polystyrene (PS), and polyethylene terephthalate (PET) – the most abundant types of post-consumer plastics – into valuable specialty chemicals, including polyhydroxyalkanoates, a family of bioplastics suitable for various medical materials and applications. “Sullivan et al. highlight how hybrid chemical and biological processes can enable plastic recycling that is otherwise unachievable,” writes Ning Yan in a related Perspective. Plastic waste is becoming an irreparable global environmental crisis that our current plastic reclamation and recycling infrastructure simply cannot address. To remedy this, researchers have explored a number of chemical and biological processes to develop alternative recycling routes for converting mixed plastic waste into commercially valuable chemicals. However, the chemical diversity and complexity of commonly used plastics have proved to be a significant barrier to achieving this goal. Here, Sullivan et al. present a two-step oxidation and biological funneling process to convert mixtures of common consumer plastics into simple chemical end products. According to the report, mixed plastics were first broken down into multiple organic acid intermediates by metal ion-promoted oxidation. During the second step, these oxygenated compounds were digested by a genetically modified soil bacterium, Pseudomonas putida, and subsequently bioconverted into a single chemical product. While the authors demonstrate the approach by producing β-ketoadipate or polyhydroxyalkanoates, they note that chemical end products could be adjusted by metabolic engineering of the microbes used, which could enable tailored mixed plastic conversion into a variety of platform or specialty chemicals.