(385p) Fermentable Organic Acids Production from Mixed Plastic Waste in the Presence of Nitric Acid

Since the advent of plastics, human life without plastics has been unimaginable, and plastics are being consumed in various fields such as packaging, building construction, textiles, transportation, electronics and everyday life. Forty million tons of plastic waste was generated in the US in 2021, but the recycling rate was only ~5%, while ~85% was reclaimed in landfills and ~10% incinerated. With the concern about this unmanageable amount of plastic waste, plastic upcycling has been considered. Most accomplishments have come from polyethylene terephthalate (PET) because the polymer backbone thereof can be readily degraded with a high yield of monomers. On the other hand, although polyethylene (PE), polypropylene (PP), and polystyrene (PS) have occupied a substantial amount of plastic waste, their upcycling has been restricted because of their robust non-activated C-C linked polymer backbone. One promising approach is an oxidative deconstruction of the polymer backbone by a universal chemical process, resulting in a mixture of organic acids. This strategy can be successfully developed when the resulting mixture of organic acids is metabolized by microorganisms without purification of each organic acid, called biological funneling.

Here in, we developed a deconstruction of LDPE, HDPE, PP, and PET or mixtures thereof in the presence of nitric acid to provide fermentable organic acids. Briefly, 1 g of each plastic debris was placed in a PTFE-lined hydrothermal reactor and 60 mL of 2 M nitric acid was charged. For mixed plastic experiments, we placed 0.5 g of each LDPE, HDPE, PP, and PET, a total of 2 g of plastics. The oxidative deconstruction of plastics was conducted at 180 °C for a designated time. For PE and PP, a liquefaction of solid debris was completely achieved after 6 h, giving an aqueous mixture of acetic acid and C₄-₇ dicarboxylic acids, and acetic acid and lactic acid, respectively. The organic acid concentrations of the resulting solution were 10.9 g/L from HDPE, 9.6 g/L from LDPE, and 10.2 g/L from PP. For PET, the vanishment of solid debris was completely achieved after 8 h, giving solid crystal of terephthalic acid with a yield of 89.5 mol%. When all 4 different plastics were mixed together, the resulting solution after 8 h of deconstruction had a total of 20.0 g/L of acetic acid, lactic acid and C₄-₇ dicarboxylic acids, and a comparable yield of terephthalic acid was obtained. The aqueous organic acid solutions and terephthalic acid were “ready for feeding” to microorganisms after pH adjustment without further purification.