Biocatalytic recycling of polyethylene terephthalate plastic

Abstract

The global production of plastics made from non-renewable fossil feedstocks has grown more than 20-fold since 1964. While more than eight billion tons of plastics have been produced until today, only a small fraction is currently collected for recycling and large amounts of plastic waste are ending up in landfills and in the oceans. Pollution caused by accumulating plastic waste in the environment has become worldwide a serious problem. Synthetic polyesters such as polyethylene terephthalate (PET) have widespread use in food packaging materials, beverage bottles, coatings and fibres. Recently, it has been shown that post-consumer PET can be hydrolysed by microbial enzymes at mild reaction conditions in aqueous media. In a circular plastics economy, the resulting monomers can be recovered and re-used to manufacture PET products or other chemicals without depleting fossil feedstocks and damaging the environment. The enzymatic degradation of post-consumer plastics thereby represents an innovative, environmentally benign and sustainable alternative to conventional recycling processes. By the construction of powerful biocatalysts employing protein engineering techniques, a biocatalytic recycling of PET can be further developed towards industrial applications.

This article is part of a discussion meeting issue ‘Science to enable the circular economy’.

1. Introduction

More than eight billion tons of plastics have been produced to date, predominantly from fossil feedstocks [1] (figure 1). The use of plastics is increasing worldwide rapidly and it has been estimated that by 2050 a staggering amount of 33 billion tons could be produced if current consumption rates continue [2]. The major part of these plastic products is short-lived, disposable packaging materials becoming waste often after a single use. The huge amount of plastic waste accumulating in the environment is of increasing concern owing to its general recalcitrance and poor biodegradability with detrimental effects to terrestrial and marine ecosystems, and eventually to humans [3]. In particular, microplastic has been considered as potentially hazardous to human health owing to toxicity of the plastic particles and the release of chemical compounds when ingested or inhaled [4].

Figure 1.

Global production, use and fate of polymer resins, synthetic fibres and additives 1950–2015 (in million metric tons) [1]. (Online version in colour.)

Only a small fraction (about 9%) of the total plastic waste produced is recycled back to bottles or food packages while the majority is incinerated or ends up in landfills or in marine environments [5,6]. It is obvious that the amount of plastic waste that can be reused needs to be increased drastically to cope with this problem. An increasing awareness of this issue is reflected in the recent commitments of the plastic-producing industry and stake holders in favour of a circular plastics economy as well as in the recent introduction of legislative and regulatory measures, e.g. by the European Union (https://www.pcep.eu; https://www.ellenmacarthurfoundation.org/our-work/activities/new-plastics-economy; https://ec.europa.eu/info/research-and-innovation/research-area/environment/plastics-circular-economy_en).

Plastics such as polyethylene terephthalate (PET), polyurethane (PUR), polyethylene (PE), polypropylene (PP), polystyrene and polyvinyl chloride are almost exclusively manufactured using fossil-based feedstocks. Owing to the diversity in their chemical structures, these synthetic plastics show a very different susceptibility to degradation by microorganisms or their enzymes [7]. While polymers containing C–C bonds such as PP and PE are difficult to attack by enzymes, plastics containing ester bonds such as PET and polyester PUR can be considered as realistic targets for enzymatic degradation and recycling. While PET constitutes only a smaller part of the global solid plastic waste, it is the most suitable post-consumer plastic to be recycled in a biocatalytic process. This is on one hand owing to recent advances in the understanding of the mechanism of enzymatic PET degradation by microbial polyester hydrolases [8]. On the other hand, the very high recovery rate of PET beverage bottles owing to efficient deposit return systems, at least in several European countries, is providing access to a continuous feed of high purity plastic composed of only PET after cleaning and removal of labels and caps [9]. Following enzymatic hydrolysis, this facilitates the recovery of the monomers without tedious downstream processes for the production of new bottles. Considering that hundreds of millions of PET beverage bottles are produced per day worldwide, an efficient biocatalytic bottle-to-bottle recycling could be a step forward towards a circular plastics economy.

2. Enzymatic degradation of PET

PET is a synthetic polyester composed of terephthalic acid (TPA) and ethylene glycol (EG). It is used worldwide in large amounts for the production of textile fibres and packaging materials owing to its light weight, durability and chemical resistance. As a thermoplastic polymer, it becomes pliable at elevated temperatures and can be easily shaped by polymer processing techniques to produce parts such as containers or beverage bottles with excellent material properties. The PET polymer contains both amorphous and semi-crystalline microstructures depending on its processing and thermal treatment [10]. The extent of an enzymatic hydrolysis of PET has been shown to depend on the degree of its crystallinity since the highly ordered structure of the crystalline polymer regions cannot be attacked by the enzyme [11]. At the glass transition temperature of PET, the flexibility of the amorphous regions increases and becomes more susceptible to hydrolysis by polyester hydrolases [12]. An enzymatic degradation of PET is therefore most favourably performed at temperatures at or above its glass transition temperature of about 65°C requiring thermostable biocatalysts [13]. When amorphous PET is reacted with a polyester hydrolase, TPA and EG are obtained as the main products. Transient hydrolysis products formed such as bis-(2-hydroxyethyl) terephthalate (BHET) and mono-(2-hydroxyethyl) terephthalate (MHET) are further hydrolysed to TPA and EG depending on the reaction time and conditions (figure 2).

Figure 2.

Products obtained from amorphous PET films degraded by a polyester hydrolase. TPA, terephthalic acid; EG, ethylene glycol; MHET, mono-(2-hydroxyethyl) terephthalate; BHET, bis-(2-hydroxyethyl) terephthalate [7].

Thermostable polyester hydrolases such as lipases and cutinases from both fungal and bacterial origin have emerged as the most promising catalysts for the hydrolysis of PET to its monomeric building blocks TPA and EG [7,14]. Since the synthetic polymer PET is not the natural substrate of polyester hydrolases such as cutinases, which have evolved to cleave ester bonds in the plant polyester cutin, their activity against synthetic polymers is often too low for an application in a biocatalytic recycling process. Efforts to improve the activity and thermostability of polyester hydrolases by protein engineering and the structure elucidation of several of the enzymes have resulted in the construction of highly active biocatalysts and contributed to a better understanding of the mechanism of enzymatic PET degradation [7,14–16] (figure 3).

Figure 3.

Surface representation of a polyester hydrolase with a model compound composed of two repeating units of PET polymer (blue carbon atoms) docked within the substrate groove. Three independently docked conformations are shown in stick representation. The position of the active serine is marked on the surface in red [17]. The docking provides information on the relevant amino acids at the active site of the enzyme and the construction of superior variants. (Online version in colour.)

3. Biocatalytic recycling of post-consumer PET

Recycling is an essential component of effective waste management for plastic waste. Increasing the recycling capacity of post-consumer PET plastic by recovering its monomers as building blocks for the synthesis of new polymers or chemicals is considered an effective countermeasure to reduce the amounts of plastic waste thereby closing the loop in a circular plastics economy [18,19]. However, the widely employed mechanical recycling of PET consists of melting and filtration steps to produce recycled PET (rPET) of mostly non-food quality. This results in a limited bottle-to-bottle cycle with presently only a fraction of the rPET actually going back to new bottles. The shortening of the polymer chains resulting in quality loss, the accumulation of additives in rPET, the loss of PET material in the process (3–10%) and the high energy consumption are further disadvantages of this process. PET can also be depolymerized by various chemical methods to yield monomers that can be recycled to manufacture new PET or other chemicals. While these processes presently are gaining more consideration and improvements, they suffer from high energy costs and require very high capacity lines to become cost-efficient [20]. By contrast, in a biocatalytic PET recycling process, the post-consumer PET plastic is treated in an aqueous solution at moderate temperatures (60°C to 70°C) requiring a low energy input. The resulting monomers TPA and EG can be easily recovered by precipitation and distillation, respectively, and used for the synthesis of novel PET. The manufacture of other products from the hydrolysates such as aromatic polyester polyols to produce PUR could also be targeted [21,22]. The use of the enzymatic PET hydrolysates as substrates for microbial cell factories to produce value-added bioplastics such as polyhydroxy alkanoates has also been suggested [8].

The feasibility of biocatalytic recycling of PET has already been demonstrated. A dual enzyme reactor composed of a polyester hydrolase and a carboxyl esterase could be used to efficiently hydrolyse amorphous PET films [23]. The carboxyl esterase improved the degradation rate of PET by transforming the transient hydrolysis products MHET and BHET of the polyester hydrolase to TPA and EG. PET films could also be efficiently degraded by a polyester hydrolase in an enzyme reactor where the hydrolysis products were continuously removed using an ultrafiltration membrane [24] (figure 4). By optimization and scale-up of the enzyme reactor system, a 50% weight loss of post-consumer PET food package containers could be recently obtained with a single polyester hydrolase after 96 h of reaction at 70°C [25].

Figure 4.

Biocatalytic PET recycling with a polyester hydrolase in an ultrafiltration membrane reactor [24]. (Online version in colour.)

Since the crystalline regions of PET are not readily hydrolysed by the presently developed polyester hydrolases, mechanical or chemical pre-treatment methods will likely be a prerequisite for the complete degradation of semi-crystalline PET waste such as beverage bottles. High-temperature treatments to decrease the crystallinity of PET prior to a biocatalytic treatment have also been suggested [26]. However, introducing pre-treatment steps requiring the input of additional energy and chemicals would jeopardize the validity of a “green” biocatalytic recycling process likely turning it economically and ecologically unattractive. A low energy pre-treatment with the purpose to increase the surface area of the plastic in combination with more powerful engineered polyester hydrolases may be a more preferable route to develop an efficient biocatalytic recycling process for semi-crystalline PET waste [27–29].

Besides from converting single PET back to its monomeric constituents, a biocatalytic process may also be used advantageously for the recycling of mixed plastic wastes such as composite laminates or multi-layered plastic containers often employed in food packaging. In addition to PET, they contain other plastics, e.g. PP or PE. These compound materials provide additional useful functions for example as a gas barrier or the protection of food against light. However, they are difficult to recycle by conventional processes and usually need to be incinerated after they become waste. A treatment with polyester hydrolases could remove the PET and allow the recycling of the components. The removal of PET in mixed fabric waste to increase the recyclability of multi-component textiles could also become a further area of application for biocatalytic processes [30].

4. Conclusions

Owing to their particular properties and versatility, plastics have become an indispensable part of our daily life. Ignoring the necessity to recover and re-use plastics in an efficient way has created a considerable global problem. The conversion of plastic waste to recover its constituents and to recycle them to new products is a viable strategy to save valuable resources and to avoid contamination of the environment. Enzymes have already been shown to be versatile tools for the modification and degradation of PET. The biocatalytic recycling of PET as an emerging technological opportunity represents an environmentally benign alternative to conventional recycling with the aim to discard the single-use model of plastic in favour of a circular economy.

Data accessibility

This article has no additional data.

Competing interests

I have no competing interests.

Funding

This work was supported by grants from the German Federal Ministry of Education and Research (project 031A227E and project 031B0113H) and from the European Union's Horizon 2020 Research and Innovation Program under Agreement no. 633962.