The application of plastic materials has brought many conveniences to human life. However, a significant increase in consumption of plastic products due to population growth has led to concerns about resulting environmental and resource issues. From 1950 to 2015, the cumulative generation of plastic waste reached 6.3 billion tons, only 9% of which was recycled (Ali et al., 2022). According to estimates, by 2050, the cumulative recycling of plastic waste will still not reach 30% of the total plastic waste generation (Geyer et al., 2017). The massive and continued production of petrochemical polymers accelerates the depletion of fossil-based resources. Traditional disposal approaches for plastic waste, such as incineration and landfilling, further exacerbate environmental pollution and resource loss. It is necessary to vigorously develop biopolymers like polyhydroxyalkanoates (PHAs) and polylactic acid to enhance the sustainability of the environment and resources by transforming a linear economy into a circular economy.

PHAs are linear polyesters synthesized by microorganisms as energy storage substances. As the first isolated and characterized homopolymer of PHAs, polyhydroxybutyrate (PHB) has higher crystallinity and lower oxygen permeability than other PHA types (McAdam et al., 2020; Zhang et al., 2022b). It has been considered a potential substitute for some petroleum-based polymers with similar physical properties, such as polypropylene (PP) and polyethylene (PE) (Zhang et al., 2022b). In addition, as there are ester bonds on the main chain and methyl functional groups on the side chain, PHB has better barrier permeability and thermoplasticity than petroleum-based polymers (Vasudevan and Natarajan, 2022). Excellent biocompatibility and biodegradability also make PHB attractive as an ideal candidate for biomedical materials. The shortcomings of PHB include higher brittleness and lower thermal stability. Blending with plasticizers has been considered the simplest way to improve the properties of PHB (Turco et al., 2021).

The universal production capacity of PHB is about 30,000 tons per year, less than 0.1% of that of PP (Mostafa et al., 2020). The market competitiveness of PHB is still low due to an 8-10-time higher production cost than petroleum-based polymers, especially the cost of substrates accounting for approximately 50-60% of the total cost of PHB production (Zhang et al., 2022b). The valorization of carbon-rich waste as substrates may be a cost-effective approach to increase the commercialization potential of PHB. However, compared with pure sugar substrates, the more complex components in carbon-rich waste may lead to the unstable performance of PHB-producing strains. Targeted selection of strains and fermentation strategies are essential to maintain efficient microbial PHB production from specific substrates (Wang et al., 2023b; Wang et al., 2021a). Developing efficient recovery strategies can further improve the yield and quality of PHB to enhance the feasibility of practical PHB applications. The comprehensive process improvement may reinforce the economic feasibility and scale-up operability of PHB production.

In addition to the production process, waste disposal approaches also affect the sustainability of PHB as a bioplastic. PHA-degrading bacteria and fungi may decompose various PHAs into CO2 and water in aqueous and soil environments. However, the accumulation of PHA waste in nature may still occur due to the long degradation period (Kang et al., 2022). In addition, as CO2 and water are low-value degradation products, biodegradation in the traditional sense has limitations in thoroughly solving the issues of carbon loss. Therefore, developing valorization routes for bioplastic wastes is needed to increase their circular value and sustainability potential. By cleaving the ester bond of PHB, extracellular PHB depolymerase can decompose PHB into 3-hydroxybutyric acid (3-HB) monomers specifically. Chemical treatments may degrade PHB into 3-HB and derivatives under specific reaction conditions. These small-molecule organic compounds may serve as platform chemicals for other industries or as substrates for PHB resynthesis (Mensah and Bruijnincx, 2022). This integrated conversion approach focusing on the combination of waste disposal and resource derivation can effectively improve carbon recycling to enhance the economic potential of PHB (Fig. 1). It can be considered an extension of the circular bioeconomy of PHB-based industries.

This review comprehensively discussed recent advances in microbial synthesis, recovery, and valorization of PHB to demonstrate the feasibility of improving PHB production efficiency and broadening the fields of PHB waste valorization. It may provide ideas for the design of PHB-based integrated circular manufacturing.