Zero-waste extraction of polyhydroxy(butyrate-co-valerate) (PHBV) from mixed cultures using natural deep eutectic solvents (NADES): Unlocking the roles of molecular interactions, polarity, and viscosity

The use of conventional plastics in our modern society presents many environmental problems, both due to the accumulation of waste in ecosystems and CO2 emissions. Polyhydroxyalkanoates (PHAs), such as the co-polymer polyhydroxy(butyrate-co-valerate) (PHBV), could serve as an alternative due to their biodegradability, biocompatibility and production from renewable raw materials. However, the downstream processing of PHBV often relies on organic solvents and harsh process conditions. This study presents a novel optimized downstream process for the extraction and purification of PHBV from mixed microbial cell (MMC) biomass, as well as recycling of the solvents used in the process. For this purpose, hydrophobic natural deep eutectic solvents (NADESs) based on the terpenes thymol and menthol were prepared, and their Hansen solubility parameters (HSPs) as well as physicochemical characteristics such as viscosity and relative polarity were analyzed, before optimization of the process was carried out. Using NADESs has several advantages over traditional solvents and extraction methods due to their low toxicity, biodegradability and tunability for different applications. After screening, the most effective NADES proved to be thymol and decanoic acid in a 1:1 M ratio, while methanol was used as an anti-solvent in a 1:3 volumetric ratio. The use of these solvents for the extraction of PHBV was then optimized and a final PHBV yield of ≈ 85 wt% and purity of ≈ 92 wt% was achieved. Furthermore, both the NADES and the methanol were recycled, closing the process loop. These results were promising, as yields were comparable to those of organic solvents. Moreover, analysis of HSPs and physicochemical characteristics could help to streamline the identification process of worthwhile NADESs solvents for use in extraction.