Bio-based plastics – a sustainable solution to plastic pollution

Introduction

Global annual plastic production is >410 × 10⁶ tonnes with an annual rate increase of 4%.¹ Thus, plastic constitutes the third highest waste source globally making it a huge human health and environmental concern.² Most of this plastic is non-biodegradable. Polyethylene terephthalate (PET) is the dominant material (production of 70%) in the global fibres market, whereas polyethylene (PE; 36%), polypropylene (PP; 21%) and polyvinylchloride (PVC; 12%) are the main non-fibre plastics produced.³ The largest global plastic volumes are used in the following commercial sectors: packaging (35%), construction (16%), textiles (14.5%), consumer goods (10.3%), automotive (10.1%), electronics (6.2%) and agricultural industries (3.4%). Packaging is the main source of waste, with 141 × 10⁶ tonnes produced in 2015 alone, of which 96.6% was unrecycled.³ A significant portion of non-biodegradable plastic end up in the environment. It is estimated that up to 80% of plastics found in oceans comes from the land⁴ and ~2 × 10⁶ tonnes of plastic debris leach into rivers annually³ in both developing and developed nations. Moreover, wealthier nations ship their waste to developing countries with inferior regulations and waste management infrastructure.⁵ Malaysia has been the world’s largest importer of plastic waste² and also has one of the largest plastic production industries globally; however, along with other developing nations in South-east Asia, Malaysia lacks appropriate waste management systems.⁶ Up to 85% of the plastic waste ends up in landfill and the marine environment.⁷ This waste is harmful because microplastics (<1–5 mm) are generated and consumed by marine organisms resulting in bioaccumulation and biomagnification. Today, microplastics has been reported to be detected in air, tap water, fish and salt.⁸ Microplastics can also absorb and carry contaminants (e.g. hydrophobic organic chemicals).⁹ In addition, plastic can leach oestrogenic-like chemicals (e.g. bisphenol A) when exposed to sunlight or certain temperatures that may result in metabolic disorders in mammals such as obesity and type-2 diabetes.¹⁰

Plastic pollution is also very costly; the impact on tourism, fishing and shipping industries is estimated to be US$1.3 × 10⁹ year^–1 in the Asia–Pacific region alone.¹¹ The Malaysian government has established a roadmap to zero single-use plastic 2018–30¹² joining other nations with similar measures such as New Zealand, Thailand, India and the European Union.

Bio-based plastics and the circular economy

Not all bio-based plastics are environmentally degradable; however, bio-polybutylene succinate (bioPBS), polylactic acid (PLA), polyglycolic acid (PGA) and polyhydroxyalkanoates (PHAs) are, with low carbon, biodegradable and compostable (industrial composting) options available (Table 1).

Global plastic production is dominated by fossil-based and durable plastics, PP being the largest produced (Table 1). Such plastics are largely responsible for the global plastic and microplastic problem. Research has been conducted on their degradation by microorganisms.¹⁴ A recent review has listed bacteria, fungi, engineered microorganisms, microbial consortia, algae and animals capable of degrading conventional plastics.¹⁵ However, such an approach is likely to represent just one alternative among several to the microplastic problem considering the increasing volume of conventional plastics produced every year (>410 × 10⁶ tonnes).

Ideally, bio-based plastics should be made from renewable or recycled sources to comply for a circular economy and be part of a sustainable plastic life cycle (where waste becomes raw material). In the current linear economy, only 9% are recycled, 12% incinerated, with the remainder sent to landfill.³ A circular economy approach has been proposed to be able to reduce greenhouse gas emissions by ~62%.¹⁶ Some of the renewable sources include lignocellulosic biomass such as wheat straw and sugarcane bagasse,¹⁷ which are cheap, although they do require a pretreatment to obtain the fermentable cellulose and hemicellulose sugars.¹⁸

Bio-based plastic production

Global bio-based plastic production has more than doubled since 2010.¹⁹ Close to half of the 2.18 × 10⁶ tonnes of bioplastics manufactured in 2023 was for packaging, the most prevalent type of plastic pollution. The uptake of bioplastics (~1%) used in Australia is currently limited by high production costs, with most bioplastics in Australia imported from Thailand and Brazil. However, the commercial market of PHAs has been estimated to reach annual volumes of 100,000 tonnes in future years.³ Bio-based and degradable polyesters are PLA, bioPBS and PHAs (Fig. 1); all have the advantage of being able to be cleaved by enzymatic activity or hydrolysis.

Fig. 1.

Chemical structures of polylactic acid (PLA), polybutylene succinate (PBS) and polyhydroxyalkanoate (PHA).

PHAs can be produced by a range of different bacteria (Fig. 2a) including Bacillus safensis,²⁰ Cupriavidus necator,²¹ and archaeal strains Haloferax mediterranei²² and Halogeometricum borinquense.²³

Fig. 2.

(a) Fluorescent Nile red staining of cells accumulating polyhydroxylalkanoic acids (PHAs) on agar plates. (b) Transmission Electron Microscopy image showing intracellular PHA production (white granules) by one of RMIT University’s isolates from rotted sugar cane bagasse.

PHA is stored intracellularly, representing up to 80% of their cell volume (Fig. 2b). PHA can be synthesised from biomass compounds, e.g. sugar cane bagasse or liquefied plastic wastes²⁴ (Fig. 1). The most common monomers consist of the rigid poly-3-hydroxybutyrate (P3HB) and the softer and more flexible poly-(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), making them good substitutes for PE and PP.²⁵ The key enzymes and steps involved in PHA production are 3-ketothiolase: acetyl coenzyme A (CoA) to acetoacetyl CoA and PHA synthases that polymerise hydroxyacyl-CoA monomers to form polymeric PHA chains. Research is currently aimed at enhancing PHA yields by increasing the cell size (Fig. 3) along with improving stress tolerance²⁶ through the application of CRISPR-Cas9 approaches. Further, more environmentally friendly processes for cell extraction are also being developed.^26,27

Fig. 3.

Key enzymes in the production of PHAs and schematic of CRISPR-Cas9 gene editing.

Recycling of plastics

To be part of a complete circular economy, bioplastics should undergo biological recycling through aerobic composting or anaerobic degradation at their end of life (EOL), whereas fossil-based plastics should be upcycled by chemical or biological depolymerisation or by undergoing thermolysis for those with functional (PET) and non-functional backbones (PE and PP) respectively (Fig. 4).

Fig. 4.

A circular plastic economy. Plastic waste becomes raw material to be recycled. Composting or incineration of captured methane from anaerobic digestion is a net-zero addition to the carbon cycle since it is captured by photosynthesis to create new biomass.

Policy

The plastic waste pollution is one of current three most pressing global environmental crises. Solving plastic waste problem cannot be achieved without evidence-based government policy intervention. The circular economy of plastic is being prioritised by governments and international bodies. The United Nations has made it a priority since 2018. The Ellen McArthur Foundation promotes science-based policies for a circular economy.³⁵ The European Union aims to recycle 50% of plastic packaging by 2030 and certain items of single-use plastic have been banned. The adoption of extended producer responsibility (EPR) schemes has helped local governments deal with packaging materials, but the scope needs to include other plastic industries. Household waste management systems should be implemented globally as has been initiated in some European countries and Japan.³⁶ China has plans to increase their bioplastics production, specifically PLA and ban all non-recyclables whereas Japan, Malaysia, Singapore and South Korea have created bioplastic subsidies.³⁷ Recent discussion of introduction of recycled content in plastic products is encouraging and adoption of government policy shift will certainly enhance the recycling of plastic and reduce the problem of plastic pollution in the environment.

Conclusions

Bio-based plastics have the potential to create a circular economy and gene editing is very promising not only to increase bioplastic polymerisation (especially for PHAs), but also for biological depolymerisation during recycling. Moreover, public education on bioplastics needs to increase and labels need to be standardised for local and global scales to prevent confusion and avoid ‘greenwashing’. Deposit-refund systems for plastic bottles should be implemented globally and mimic the successful EU cases. Policies such as taxation of non-bio-based plastics should increase the demand for bioplastics and promote a more circular economy.