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. 2024 Apr 3;17(4):e14458. doi: 10.1111/1751-7915.14458

Can bioplastics always offer a truly sustainable alternative to fossil‐based plastics?

Lara Serrano‐Aguirre 1,2, M Auxiliadora Prieto 1,2,✉
PMCID: PMC10990045  PMID: 38568795

Abstract

Bioplastics, comprised of bio‐based and/or biodegradable polymers, have the potential to play a crucial role in the transition towards a sustainable circular economy. The use of biodegradable polymers not only leads to reduced greenhouse gas emissions but also might address the problem of plastic waste persisting in the environment, especially when removal is challenging. Nevertheless, biodegradable plastics should not be considered as substitutes for proper waste management practices, given that their biodegradability strongly depends on environmental conditions. Among the challenges hindering the sustainable implementation of bioplastics in the market, the development of effective downstream recycling routes is imperative, given the increasing production volumes of these materials. Here, we discuss about the most advisable end‐of‐life scenarios for bioplastics. Various recycling strategies, including mechanical, chemical or biological (both enzymatic and microbial) approaches, should be considered. Employing enzymes as biocatalysts emerges as a more selective and environmentally friendly alternative to chemical recycling, allowing the production of new bioplastics and added value and high‐quality products. Other pending concerns for industrial implementation of bioplastics include misinformation among end users, the lack of a standardised bioplastic labelling, unclear life cycle assessment guidelines and the need for higher financial investments. Although further research and development efforts are essential to foster the sustainable and widespread application of bioplastics, significant strides have already been made in this direction.

End‐of‐life (EoL) scenarios of bioplastics from a circular plastic economy perspective.

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Global plastics usage has reached 460 million tonnes (Mt) per year, with over two‐thirds consisting of short‐lived products, projecting to nearly triple by 2060. Similarly, plastic waste is expected to increase from 353 Mt in 2019 to 1014 Mt in 2060, with a projected accumulation of 493 Mt in aquatic environments if no action is taken (OECD, 2022). The carbon footprint of plastics could be responsible for 19% of the allowed global greenhouse gas emissions in order to limit global warming to 1.5°C by 2040 (Pew Charitable Trusts and SYSTEMIQ, 2020). Aside from its environmental cost, plastic pollution exerts a significant impact on human health, society and economies. The current linear plastic economy is estimated to cost between USD 300 and 600 billion annually (UNEP, 2023). Moreover, the balance between the benefits of plastics and their social costs is particularly skewed in the world's poorest nations within the framework of a universally recognised human right to a clean, healthy and sustainable environment (UN General Assembly Resolution 76/300 of 28 July 2022).

In March 2022, the United Nations Environment Assembly, part of the United Nations Environment Programme, reached a compromise to address plastic pollution and establish an international legally binding agreement by 2024. This resolution highlighted that plastic pollution is a global environmental issue, especially in the marine environment, and underscored the urgent need for international cooperation to tackle the problem and promote sustainable alternatives and technologies to achieve the transition to a circular plastic economy (UNEA, 2022). As a result, the global demand for new plastic alternatives with lower environmental impact has significantly increased.

Undoubtedly, conventional plastics offer several advantages that make them valuable in various industries and applications, including packaging, food preservation, electronic and electrical devices, the construction industry and biomedical devices, among others. These materials are typically lightweight, inert, durable and highly resistant to corrosion, moisture and chemicals, with excellent insulation properties. Furthermore, they are versatile in terms of design and are often cost‐effective to manufacture. Unlike conventional plastics, which are synthetic and predominantly derived from fossil fuels, bioplastics can be natural polymers (biopolymers) or synthetic polymers derived from renewable feedstock, resulting in a lower carbon footprint. The term “bioplastic” is controversial, given that it comprises materials that can be either bio‐based (also known as “drop‐in polymers”, e.g. bio‐polyethylene [bioPE], bio‐polyethylene terephthalate [bioPET]), compostable/biodegradable but not bio‐based (e.g. polybutylene adipate‐co‐terephthalate [PBAT], polycaprolactone [PCL]) or a combination of both properties (e.g. polyhydroxyalkanoates [PHAs], polylactic acid [PLA], polybutylene succinate [PBS], starch‐based polymers) (Rosenboom et al., 2022). According to European Bioplastics (https://www.european‐bioplastics.org/), the most widespread and promising types are PLA, PHA and starch blends. PLA is a bio‐based and industrial compostable polyester whose monomers, L and/or D‐lactic acid, are produced via microbial fermentation and further chemically polymerised to yield PLA. Conversely, PHAs are intracellular biopolymers produced by many bacteria under nutrient imbalance, such as nitrogen or phosphorous limitation, and are accumulated as granules coated by proteins involved in their biosynthesis and metabolism, such as polymerases, depolymerases and phasins (Prieto et al., 2016). Global bioplastics production capacity is set to significantly increase from approximately 2.18 Mt in 2023, with 52% comprising biodegradable polymers, to approximately 7.43 Mt in 2028 (62% biodegradable). Currently, bioplastic production is mainly centred in Asia (41.4%) and Europe (26.5%) (https://www.european‐bioplastics.org/).

Biodegradable plastics are a clear alternative to conventional plastics for certain applications in which composting is recommended (high organic matter contamination) or applications in which adequate waste collection and treatment is not feasible (e.g. mulch films). In this context, biodegradation should be differentiated from composting. According to a report from the Science Advice for Policy by European Academies (SAPEA, https://sapea.info/topic/biodegradability‐of‐plastics/), “plastic biodegradation” is the microbial conversion of all its organic constituents to carbon dioxide, new microbial biomass and mineral salts under oxic conditions or conversion to carbon dioxide, methane, new microbial biomass and mineral salts under anoxic conditions. Conversely, “composting” is defined by the European Environment Agency (EEA, https://www.eea.europa.eu/) as the controlled biological decomposition of organic material in the presence of air to form a humus‐like material. Controlled methods of composting include mechanical mixing and aerating, ventilating the materials by dropping them through a vertical series of aerated chambers or placing the compost in piles out in the open air and mixing it or turning it periodically.

The million‐dollar question is how to properly approach implementing bioplastics in everyday life, from cradle to grave. Beyond that, what are the key points for implementing a circular bioplastics‐based system?

CONTROLLED END‐OF‐LIFE SCENARIOS FOR BIOPLASTICS: THE KEY CHALLENGE FOR A CIRCULAR ECONOMY

The advantages of deploying renewable solutions for bio‐based plastic production processes are indisputable. However, possible bioplastics End‐of‐Life (EoL) scenarios and their impact on human health and the environment are still subject to discussion. During the late 1990s, it was generally assumed that biodegradable plastics could solve the problem of plastic accumulation in the environment, assuming that in the most obvious EoL settings (e.g. the open environment and landfills) their disintegration would not negatively affect ecosystems.

It is currently accepted that biodegradable plastics can confer benefits in relation to conventional plastics in applications where it is challenging to remove or collect a particular product or its fragments from the environment after use or where it is difficult to separate plastic from organic material that is destined for a composting waste stream or wastewater treatment. Cases of unwanted littering due to uncontrolled abrasion may also be considered.

Plastic biodegradability in the open environment should be considered as a systems property, given that it depends not only on intrinsic material properties (e.g. chemical structure, molecular weight, crystallinity, glass transition temperature) but also on environmental conditions such as (i) abiotic factors that can affect material integrity (e.g. pH, temperature) and (ii) biotic environmental conditions of the site where the material might end up, which could favour or hinder the action of enzymes secreted by microorganisms in that ecosystem (Sander et al., 2024). It is essential to consider the toxic effects of the massive release of hydrolysed biopolymer components on the target ecosystem or that of other components that are part of the formulation of the final material, added to confer the properties required for the applications for which they were manufactured. This situation needs to be studied in a holistic context.

Once bioplastics end up in the open environment (e.g. soil, estuaries, oceans), their structure and biodegradability can be affected by their surrounding conditions (e.g. sunlight, oxygen, pH, temperature, moisture), causing the breakdown of polymer chains. However, these processes do not imply complete mineralisation, and the influence of abiotic and biotic factors might result, similarly to their petrochemical counterparts, in the formation of microplastics and/or nanoplastics before their complete mineralisation. Although micro‐ and nanoplastics from biodegradable polymers blends are supposed to be temporary in the open environment, they should not be overlooked, given that they can easily migrate, and their complete biodegradation strongly depends on environmental conditions (Cucina et al., 2021) (Figure 1). In addition, micro(bio)plastics can behave as vectors, interacting with other contaminants, such as metals and persistent organic pollutants, and transporting them to other locations (Miri et al., 2022). Micro(bio)plastics release might also disrupt soil and aquatic biota by changing the physicochemical properties of ecosystems (Ali, Abdelkarim, et al., 2023; Ali, Ali, et al., 2023). Therefore, further research and a deeper understanding of the environmental impact of microplastics from bioplastics are required. The fact that microplastics are made of biodegradable polymers is not sufficient to assume no ecotoxicity.

FIGURE 1.

FIGURE 1

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Degradability of biopolymers in various environments according to the standardised testing protocols. Taken and modified from Narancic et al. (2018) and www.renewable‐carbon.eu/graphics.

The environmental effect of additives included in biodegradable plastics is another field not sufficiently explored. The compounding process of bioplastic blends requires dispersive and distributive mixing procedures performed at moderate shear rates and low product temperatures to ensure the lowest molecular weight degradation and thereby ensure the desired properties. During this process, and similar to fossil‐based plastics, bioplastics are commonly blended with other polymers and additives such as plasticisers, antioxidants, slip additives and stabilisers to obtain the desired material properties. There is a need for the establishment of ad hoc ecotoxicity protocols with the final goal of developing environmentally safe components to be included in bioplastic formulations (Wang et al., 2023; Zimmermann et al., 2020). Various natural additives, such as carbonates, silicates and polysaccharidic compounds, have been studied from a sustainability perspective to modulate the properties of bioplastic blends (Oluwasina et al., 2021).

EoL scenarios for bioplastics need to be considered in parallel with their synthesis and production processes development for safe and sustainable implementation in the market. Life cycle assessment (LCA) analysis is essential for evaluating the potential environmental and economic impact of bioplastic products, including material processing, distribution and recycling of waste or final disposal, before their widespread application (Ali, Abdelkarim, et al., 2023; Ali, Ali, et al., 2023; Hottle et al., 2013). LCAs must be in accordance with general international standards, such as ISO 14040/14044:2006, and specific standards for bioplastics, such as EN 16760. However, many parameters are still not well defined, and developing systematic guidelines becomes essential to achieve bioplastic environmental sustainability (Van Roijen & Miller, 2022; Walker & Rothman, 2020).

BIOPLASTIC RECYCLING: A PENDING TOPIC TO BE ADDRESSED

In light of the current situation of environmental plastic pollution and in terms of the circular economy, it is evident that intrinsic biodegradability is not the only factor to be considered as an efficient solution; reduction, reuse and recycling strategies are necessary to maintain (bio)plastics and their derivatives in the circular value change (Figure 2).

FIGURE 2.

FIGURE 2

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End‐of‐life (EoL) scenarios of bioplastics from a circular plastic economy perspective. Taken and modified from https://www.european‐bioplastics.org/.

When reuse is no longer possible, bioplastic recycling is the best EoL scenario (Rosenboom et al., 2022). However, suitable sorting and additive removal strategies for bioplastics are still needed. Currently, only drop‐in polymers can be integrated into the existing recycling streams; the remaining bioplastics are treated as contaminants of conventional plastics (see the comment by Pereyra‐Camacho & Pardo, 2024, in this issue). When bioplastics are mixed with fossil‐based plastics, various established sorting technologies (e.g. gravity and triboelectric‐based sorting) could be suitable. Although bioplastics such as PLA or PCL can be separated from polyolefins by gravity‐based sorting, this process is slow; its efficiency depends on density differences between polymers and requires a mechanical pre‐treatment of the plastics before sorting (Taneepanichskul et al., 2022). However, near‐infrared spectroscopy has recently been employed for accurately identifying and characterising mixed polymer blends without any material pre‐treatment (Gundupalli et al., 2017). Employing optical sensors, specific reflected spectra of each polymer are compared to a database, and plastic types are separated into different streams. Spectral‐based sorting has been successfully proven for differentiating PLA from other plastics such as PET (Fredi & Dorigato, 2021). This technique is highly automatic, and it involves low environmental impact, as well as digital watermarking and tracer‐based sorting; however, its implementation could require high economic cost (Taneepanichskul et al., 2022). Altogether, these sorting technologies can be used for improving the sorting effectiveness of mixed recycling collections, including bioplastics.

Once polymers have been properly sorted, recycling can be addressed with a mechanical, chemical or biological (enzymatic or microbial) approach. Currently, mechanical recycling is considered the most advisable EoL scenario for bioplastics, given that it has low environmental and economic impact. Although this approach could be the most feasible possibility, there are many drawbacks to be addressed related to the low quality of remoulded products due to the inferior thermomechanical properties of bioplastics compared with that of conventional plastics (Lamberti et al., 2020). Chemical recycling, mainly achieved by solvolysis or thermolysis, is based on plastic depolymerisation into monomers and repolymerisation into new materials with desired properties or production of valuable chemicals; it is considered an optimal strategy for conventional plastic recycling (Hong & Chen, 2017). However, this alternative might not be optimal for biodegradable plastics, given that there is the possibility of applying enzymatic catalysts for polymer hydrolysis.

Enzymatic hydrolysis of bioplastics can be an effective strategy for recycling, while being less energy consuming, more selective and less harmful to the environment (Lee & Liew, 2021). This approach requires lower temperatures, and product mixtures are typically less complex than those obtained by chemical recycling (Merchan et al., 2022). Enzymatic recycling is especially interesting for bioplastics based on aliphatic polyesters, such as PHAs, PBS and PLA. Many extracellular plastic‐degrading enzymes from bacteria and fungi have previously been identified, not only from isolated microorganisms but also from metagenomic DNA analysis (Satti & Shah, 2020; Zhu et al., 2022) (Table 1). Polymer‐enriched environments, such as landfills and compost, can be employed for the detection of plastic‐degrading enzymes, due to the great microbial abundance and diversity in these habitats (Chow et al., 2023). These enzymes are mainly hydrolases (e.g. carboxylesterases, depolymerases, lipases and cutinases), which break the ester bond of polymer chains, between carboxylic acid and alcohol groups from monomers. Due to their broad substrate specificity, these enzymes can frequently hydrolyse different polyester types, and their activity also depends on the physical properties of the material. Generally, polymers with high molecular weight, melting temperature and crystallinity are less susceptible to enzymatic hydrolysis (Urbanek et al., 2020). Implementing enzymatic recycling strategies requires protein engineering strategies for improving catalytic efficiency or enhancing thermostability (Zhu et al., 2022). The latter is a key property when plastics have high glass transition temperatures (T g), above which polymer flexibility and enzyme accessibility are increased.

TABLE 1.

Examples of bioplastic‐degrading enzymes. Taken and modified from (Urbanek et al., 2020).

Microorganism Substrate specificity Accession number Reference
scl‐PHA depolymerases
Alcaligenes faecalis AE122 P(3HB) AAB40611.1 Asano and Watanabe (2001), Kita et al. (1995)
Bacillus megaterium N‐18‐25‐9 P(3HB) BAF35850.1 Takaku et al. (2006)
Leptothrix sp. HS P(3HB) BAA92354.1 Takeda et al. (1998, 2000)
Comamonas acidovorans YM1609 P(3HP), P(4HB), P(3HB), PEA, PES BAA19791.1 Kasuya et al. (1997, 1999)
Diaphorobacter sp. PCA039 P(3HB‐co‐3 HV) ACI48814.2 Zhang et al. (2010)
Marinobacter sp. NK‐1 P(3HB), P(3HP), P(4HB) BAC15574.1 Kasuya et al. (2000)
Penicillium funiculosum IFO6345 P(3HB), P(3HB‐co‐3 HV) BAG32152.1 Miyazaki et al. (2000)
Pseudomonas stutzeri YM1006 P(3HP), P(3HB), PES, P(4HB), PEA BAA32541.1 Ohura et al. (1999)
Schlegelella sp. KB1a P(3HB) AAT09963.1 Romen et al. (2004)
Streptomyces ascomycinicus DSM 40822 P(3HB), P(3HB‐co‐3 HV) AAF86381.1 García‐Hidalgo et al. (2013)
Streptomyces exfoliatus DSM 41693 P(3HB), P(3HB‐co‐3 HV) AAB02914.1 García‐Hidalgo et al. (2012), Klingbeil et al. (1996)
Thermus thermophilus HB8 P(3HB) BAD70022.1 Papaneophytou et al. (2009)
mcl‐PHA depolymerases
Bdellovibrio bacteriovorus HD100 P(3HO‐co‐3HHx) CAE81078.1 Martínez et al. (2012)
Pseudomonas alcaligenes M4–7 P(3HO), P(3HN), P(HPV) AAQ72538.1 Kim et al. (2005)
Pseudomonas sp. GK13 P(3HO), P(3HD‐co‐3HO) AAA64538.1 Schirmer et al. (1993), Schirmer and Jendrossek (1994)
Streptomyces exfoliatus DSM 41693 P(3HO‐co‐3HHx), PHACOS WP_024761024.1 Martínez et al. (2015)
Streptomyces roseolus SL3 P(3HO), PCL, PLA AFQ93688.1 Gangoiti et al. (2012)
Streptomyces venezuelae SO1 P(3HP), P(3HB), P(3HB‐co‐3 HV), P(3HO), PCL AFQ93689.1 Santos et al. (2013)
Cutinases and esterases
Amycolatopsis sp. K104–1 PLA BAD02196.1 Matsuda et al. (2005), Nakamura et al. (2001)
Aspergillus oryzae PBS, PBSA, PCL PLA BAM28634.1 Liu et al. (2009), Maeda et al. (2005)
Cryptococcus flavus GB‐1 PBS, PBSA, PCL, PBAT, PDLA, PLLA BAT32793.1 Watanabe et al. (2015)
Fusarium sp. FS1301 PBS, PCL AAB05922.1 Mao et al. (2015)
Pseudozyma antarctica JCM 10317 PBS, PBSA, PCL, PLLA BAN66731.1 Shinozaki et al. (2013)
Bacillus pumilus PBAT, PBSA, PBS, PCL, PES BAV72205.1 Muroi et al. (2017)
Saccharomonospora viridis AHK190 (Cut190_ S226P_R228S) PBSA, PBS, PCL, PBTA, P(3HB), PDLA, PLLA, PET BAO42836.1 Kawai et al. (2014)
Thermobifida alba AHK119 PET, PBAT, PBSA, PBS, PCL, PDLA, PLL BAK48590.1 Hu et al. (2010)
Thermobifida fusca DSM 43793 PBAT, PET, PTT CAH17554.1 Eberl et al. (2008), Kleeberg et al. (2005), Müller et al. (2005)
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Bioplastics produced directly by microorganisms such as PHAs are susceptible to hydrolysation by specific depolymerases that can be classified as scl‐PHA (short chain length, from 3 to 5 carbon atoms) or mcl‐PHA (medium chain length, from 6 to 14 carbon atoms) depolymerases, according to their substrate specificity in terms of the monomer side chain (Choi et al., 2020; Prieto et al., 2016). In addition, these enzymes can be exo‐ or endo‐type hydrolases if their enzymatic reaction products are only monomers or oligomers, respectively (Urbanek et al., 2020). Interestingly, the reaction products are building blocks ((R)‐3‐hydroxy fatty acid monomers) that can be used to produce antibiotics, vitamins or pheromones (Guzik, 2021; Radivojevic et al., 2016). Lipases can also efficiently hydrolyse aliphatic polymers, commonly with the following substrate preference: PBSA > PCL > PBS ≥ PHB(V) = PLA (Urbanek et al., 2020). Likewise, cutinases, which are produced by phytopathogenic fungi to hydrolyse cutin, can also degrade some polyester‐type plastics, including aromatic polyesters such as (bio)PET (Kawai et al., 2014; Wei & Zimmermann, 2017). Unlike aromatic polyesters, aliphatic‐aromatic co‐polyesters, such as PBAT, can be enzymatically degraded more easily (Perz et al., 2016). However, further research on bioplastic hydrolytic enzymes is crucial for developing and implementing downstream recycling routes, promoting the sustainable large‐scale use of bioplastics (Xu et al., 2023).

As mentioned above, bioplastic waste treatment can involve not only enzymatic recycling but also composting and anaerobic digestion. Composting is defined as the controlled biological decomposition of organic material in the presence of air to form a humus‐like material. This process is performed in two consecutive steps: (1) the breakdown of the polymer into low‐molecular‐weight compounds and (2) the microbial uptake and metabolic utilisation. However, plastic biodegradation is highly dependent on biotic and abiotic environmental factors, in addition to material properties, and favourable conditions are not always met in home or industrial composting facilities (Lambert & Wagner, 2017). Anaerobic digestion allows the production of CH4 from plastic waste and the subsequent release of CO2 and H2O, recovering heat and energy for use (Stagner, 2016). Furthermore, this process can release volatile fatty acids that can be used as a carbon source for other bioproducts, such as microbial PHA production (Cerrone et al., 2014). These strategies are advisable when bioplastics are mixed with organic solid waste to avoid leakage into the environment, and they can be coupled (Cucina et al., 2021).

These bioplastic recycling strategies have been scientifically analysed and demonstrated at lab scales; however, further research is needed for their industrial implementation (Kumar et al., 2023). According to LCA analysis, mechanical recycling appears to cause less environmental impact than chemical recycling. However, although LCA data on enzymatic recycling processes are scarce, enzyme production costs do not appear to be a drawback (Singh et al., 2021).

Incineration and landfills are unacceptable EoL scenarios for bioplastics, being inappropriate from a circular and sustainable perspective.

BIOPLASTICS APPLICATIONS: OPPORTUNITIES AND CHALLENGES

Among the applications for bioplastics, packaging, mulching and disposable products are the most interesting markets, because they entail short‐ to medium‐lived products. Furthermore, bioplastics use is expected to increase in other sectors, such as the textile industry, agriculture, sport fishing, hunting, horticulture and electronics (https://www.european‐bioplastics.org/).

Most research efforts have focused on sustainable while profitable production systems; however, the great challenge is to implement proper waste management systems based on the circular economy concept, given that existing bioplastic alternatives to fossil‐based polymers tend to be costlier and are usually mismanaged. Moreover, misinformation about bioplastics among end users can lead to incorrect disposal practices, resulting in environmental leakage. Hence, there is an urgent need for global standardisation in bioplastic labelling to provide consumers clear information about chemical composition and recyclability options. Easily understandable labelling, together with deposit‐refund systems or other incentives, could encourage consumers to engage in proper bioplastic waste management. In economic terms, fees for fossil‐based plastics could enhance the cost competitiveness of bioplastics. Moreover, economic investments and financial incentives are essential for the design and implementation of improved production and recycling technologies (Rosenboom et al., 2022).

In conclusion, bioplastics have the potential to play a key role in the transition towards a sustainable circular economy that addresses the detrimental impact of conventional plastics on the environment. Following the waste hierarchy, it is crucial to prioritise the reduction, reuse and efficient recycling of plastic waste or, alternatively, its utilisation for energy recovery when recycling is not feasible. The use of biodegradable polymers not only implies a decrease in greenhouse gas emissions but also tackles the issue of plastic waste that ends up in the environment, particularly in applications where it is not possible to remove the product after its use. However, it must be emphasised that biodegradable plastics should not be considered as an alternative to proper waste management practices or as a complete solution to the prevailing plastic waste problem. Efficient bioplastic recycling strategies must be developed with their increasing production, to prevent their future accumulation in the environment.

AUTHOR CONTRIBUTIONS

Lara Serrano‐Aguirre: Conceptualization; visualization; writing – original draft; writing – review and editing. M. Auxiliadora Prieto: Conceptualization; resources; supervision; validation; visualization; writing – original draft; writing – review and editing.

CONFLICT OF INTEREST STATEMENT

Authors have no conflict of interest.

ACKNOWLEDGEMENTS

Authors want to acknowledge the financial support from the CSIC Interdisciplinary Thematic Platform (PTI) Sustainable Plastics towards a Circular Economy (PTI‐Susplast), the Spanish Ministry of Science, Innovation and Universities MCIU/AEI/10.13039/501100011033 under the research grant BIOCIR (PID2020‐112766RB‐C21) and REVOLUZION (PLEC2021‐008188/Next Generation EU/PRTR) and European Union's Horizon 2020 Research and Innovation Program, grant agreement no. Mix‐UP‐870294.

Serrano‐Aguirre, L. & Prieto, M.A. (2024) Can bioplastics always offer a truly sustainable alternative to fossil‐based plastics? Microbial Biotechnology, 17, e14458. Available from: 10.1111/1751-7915.14458

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