Mechanochemical and mechanobiological recycling of postconsumer polyethylene terephthalate (PET) plastics under microwave irradiation: a comparative study

Exploring new solutions to improve the environmentally friendly degradation of fossil based postconsumer plastic waste is key in the development of effective techniques to increase the efficiency of plastics degradation while using mild, green depolymerization conditions. In this context, we introduce a novel, ultrafast mechanical pretreatment for postconsumer (PC) polyethylene terephthalate (PET) plastics that is based on a dissolution/reprecipitation approach under microwave (MW) irradiation. Fourier transform infra-red (FTIR) and Differential scanning colorimetry (DSC) analyses indicates a significant increase, up to 3.78 in the carbonyl index and a 2-fold decrease in crystallinity index of the pretreated PC PET sample when compared to the untreated one. Degradation efficiency of both untreated and pretreated PC PET was evaluated using enzymatic and MW assisted chemical degradation techniques. Results show that following MW assisted hydrolytic depolymerization, pretreated PC PET conversion rate of 95 % and terephthalic acid (TPA) monomer yield of 87.4 % were obtained and were significantly higher than that of untreated PC PET. While the proposed pretreatment approach did not show a significant improvement on the enzymatic degradation of PC PET, it did result in a 1.2-fold increase in the pretreated PC PET conversion rate, yielding solely TPA as a value-added monomer. This presents an advantage in the economic cost of the degradation process if applied on a larger scale.


Introduction
Packaging is an economic sector that consumes the most macromolecular synthesised materials such as plastics [1]. Polyethylene terephthalate (PET) of the most widely used plastics, in the format of synthetic fibres in the textile industry and as rigid and flexible packaging materials for the food industry due to its unique chemical and physical characteristic [2]. Nevertheless, great amounts of PET continue to accumulate in the ecosystem causing enormous challenges for the environment [3]. Hence, reducing PET plastic consumption and processing of fossil based polymeric materials are recommended to solve PET pollution and operation of a sustainable circular economy [4].
Recycling of PET plastic waste can take different routes through washing and re-melting, or by chemically and biologically depolymerizing it to lower molecular weight components. Chemical recycling of PET constitutes the chemical breaking down of the polymer chains into monomers which can be then repolymerized into virgin plastics or synthetic chemicals [5]. Nevertheless, chemical recycling, typically involves long durations of high pressures and temperatures, in addition toxic chemicals, making the process very harsh and energy-intensive [2]. As an emerging alternative, biocatalytic degradation of PET, has been proven to work under mild conditions without the harmful chemicals usually used in chemical recycling [2]. However, to achieve complete degradation of polymer, long-term incubation periods are considered obligatory even when using the most active PET hydrolases discovered to date [6]. Additionally, the increase in PET crystallinity by means of this physical aging results in the inhibition of PET enzymatic degradation which renders the process ineffective and requires some form of support to boost its action [7]. Such booster can be provided by simple and straight forward pretreatment techniques which in general make PET recycling more amenable for degradation.
As a continuation of our work on PET chemical and biocatalytic recycling [8][9][10][11], in this paper, pretreatment of postconsumer (PC) PET is delivered using a dissolution/ reprecipitation approach. During the mechanical pretreatment technique described here, PC PET is treated with a solvent/non-solvent system where the polymeric materials are dissolved and then recovered by reprecipitation [1]. The proposed dissolution/reprecipitation technique has several advantages which includes converting the plastic waste into an acceptable format that is compatible with conventional industrial processing equipment and allows the removal of additives and insoluble impurities from the PET polymeric material. Additionally, using this pretreatment process, PET structural and morphological changes are expected rather that the complete degradation of PET.
Bearing in mind that the degradation of PET, through chemical or enzymatic processes, depends on the mobility of the amorphous areas and the crystallinity index of the polymer [2,7]. Thus, the evaluation of the ability of the dissolution/reprecipitation pretreatment in creating amorphous structures on the surface of PET and shortening of the PET polymeric chains is important in establishing routes to accelerate its chemical and biocatalytic degradation.
In this study, we designed and applied a novel, ultrafast dissolution/reprecipitation method under microwave (MW) irradiation as a pretreatment for PC PET plastic waste. M-cresol/ethanol was selected as the solvent/nonsolvent system. M-cresol has been reported as a suitable solvent for PET polymer in addition to being an excellent MW absorber while ethanol is a widely known non-solvent for different polymeric materials [9,12,13]. Structural modifications of pretreated PET were assessed by FTIR and DSC. We then investigated the effect of the proposed pretreatment on PC PET degradation by chemical and biocatalytic depolymerization techniques, generating useful insights for comparative PET degradation. The chemical recycling process involved a MW assisted hydrolytic depolymerization of the pretreated PC PET using sodium carbonate (Na 2 CO 3 ) dissolved in ethylene glycol (EG) as depolymerizing agent. Simultaneously, LCC-ICCG enzyme [14] was used for the enzymatic degradation of pretreated PC PET and the exclusive release of TPA as value added monomer.

Experimental
Material PC PET flakes were provided by Shabra recycling (Ireland), m-cresol was obtained from Fisher scientific (UK), sodium bicarbonate, ethylene glycol, sodium carbonate, sodium dihydrogen phosphate and sodium phosphate dibasic were all purchased from Sigma Aldrich (UK). The solvents used were of reagent grade.

Mechanical pretreatment of PC PET flakes using dissolution/re-precipitation approach
The PC PET flakes (2 g), sodium bicarbonate (1 g) and m-cresol (10 mL) were added into a stoppered conical flask and allowed to mix on a stirrer for 5.0 min. The system was then transferred into a domestic oven microwave and exposed to 2.0 min of (350Watt) MW irradiation power at 190 °C temperature to allow good dissolution of PET according to our previous work [9]. The MW irradiated solution was then poured on hot into the non-solvent ethanol (25 °C) for re-precipitation of PC PET. The precipitate was washed with ethanol three times, filtrated and dried in an oven at 60 °C overnight. The pretreated PC PET flakes were obtained in the form of fluffy flakes and were stored in sealed container for FTIR and DSC characterization.

Chemical green depolymerization of untreated and pretreated PC PET flakes
The efficiency of the depolymerization of untreated and pretreated PC PET flakes under MW irradiation was evaluated following our previously published work with slight modification [8]. The tested PC PET samples (1 g) were stirred with sodium carbonate (Na 2 CO 3 ) of 10 % (w/v) concentration in 10 mL of ethylene glycol (EG). MW irradiation of 350 W power was used in the treatment of the samples for 1.5 min. Unreacted PC PET was precipitated, filtered out and dried overnight at 70 °C for further characterization. The soluble monomers present in the filtrate were identified and quantified using HPLC assay. PC PET depolymerization was determined via the following equation: The selectivity of TPA, MHET and BHET was quantified by peak area normalization method from the HPLC chromatograms and the yield of TPA was calculated using the following equation [15]: The TPA monomer was then precipitated by the addition of 2 mL of conc. HCl (34 %) to the cooled filtrate. The separated TPA was washed, dried in the oven and confirmed its identity by FTIR characterization.

Enzymatic degradation of untreated and pretreated PC PET flakes using LCC-ICCG as a biocatalyst
Both untreated and pretreated PC PET powder were enzymatically degraded to test the efficiency of the proposed dissolution/re-precipitation approach in improving the yield of PET biodegradation. Hydrolysis of the tested PC PET samples took place in 1 mL of 0.1 M phosphate buffer pH 7 under 1200 rpm agitation at 70 °C for 4 days. Reactions were initiated after incubating 10 mg of each material with 0.06 μg of LCC-ICCG [14] dialyzed in Tris-HCl buffer pH 7, while another 0.03 μg of the enzyme was supplemented every 24 h. In control reactions, Tris-HCl buffer pH 7 was added in equal amount. Each reaction was performed in duplicates. After 4 days, the reactions were terminated by adding 0.1 % (v/v) of 6 M HCl to each sample and centrifuged for 15 min at 20000 x g. The residual material was separated out, washed, and dried for further characterization. The supernatant was passed in 0.2 µm syringe filters for filtration and characterized by HPLC.
Details for enzyme expression, material characterization and instrumentation are provided in supplementary information as S1.

Mechanical pretreatment of PC PET flakes using dissolution/re-precipitation approach
The dissolution/re-precipitation pretreatment process resulted in significant morphological modifications in the PC PET flakes from the original sample as demonstrated in Fig. 1. The pretreated PC PET flakes showed a complete disappearance of the light blue colour found in the original sample and appearance of white irregular, easily cut flakes. Additionally, the pretreated PC PET showed a fluffy, spongy construction which could be attributed to the addition of sodium bicarbonate in the treating solvent acting as a blowing agent and producing that fluffy like structure. The crystallinity and carbonyl indices of the PC PET flakes were also affected by the pretreatment process. As illustrated in Figs. S1 and S2 and Table 1, the crystallinity index of the obtained PC PET flakes was decreased by 1.4-1.9 folds from the original sample. The carbonyl index, on the other hand, was increased from 3.08 in the untreated PC PET to 3.78 in the pretreated PC PET flakes. Thus, it can be indicated that the pretreatment process led to the production of an amorphous, hydrophilic PET that was ready to be depolymerized either by chemical or biological degradation processes. It is also worth mentioning that the DSC results of the obtained PC PET post the pretreatment process showed the appearance of an additional melting point temperature (T m ) peak at 177 °C beside the usual T m peak of PET at 246 °C. This could be attributed to the production of lower molecular weight PET oligomers exhibiting a melting endotherm peaking at temperature lesser than that of the original PC PET flakes [16].

Chemical green depolymerization of untreated and pretreated PC PET flakes
The MW assisted hydrolytic depolymerization of untreated and pretreated PC PET flakes was performed using sodium carbonate as depolymerizing agent in ethylene glycol. The effects of PET pretreatment on the rate of PC PET conversion, soluble monomers selectivity and the percentage yield of produced TPA are exemplified in Fig. 2. A 20% PC PET conversion rate was observed upon performing the hydrolytic depolymerization of untreated PC PET flakes. Such percentage was elevated to reach 95% when hydrolysing pretreated PC PET flakes. Such noticeable enhancement in the efficiency of PET depolymerization with the pretreated PC PET flakes at only 1.5 min MW irradiation time could be attributed to the modifications that took place in the properties of the PC PET flakes after applying the proposed mechanical pretreatment approach. As shown in Fig. 2, the TPA yield percentage increased from 18.72% for untreated PC PET sample to 87.38% for pretreated PC PET. Noticeably, the selectivity of MHET was very low and with almost similar values for both samples (6.29% for untreated PC PET flakes and 7.91% for pretreated PC PET flakes). Such results indicate that the pretreatment process did not have a significant effect on varying the selectivity of depolymerization products obtained except for TPA. Additionally, BHET was not observed among the produced monomers post the chemical recycling process which could be attributed to the complete depolymerization of BHET into TPA and very trace amounts of MHET under MW irradiation in 1.5 min. FTIR characterization was using to confirm the identity and purity of the produced TPA after the hydrolytic depolymerization in the MW. As shown in Fig. 3, the TPA precipitated from the untreated and pretreated chemically depolymerized PC PET samples had FTIR spectra that was quite similar to the TPA reported in literature [8]. The TPA characteristic carboxylic acid -OH group, the carboxylic C=O group and the ether C-O group stretching were observed at 3064 cm -1 , 1673 cm -1 and 1280 cm -1 , respectively for all analysed samples indicating the production of TPA in the MW assisted hydrolysis reaction.

Enzymatic degradation of untreated and pretreated PC PET flakes
After the pretreatment step, both untreated and pretreated PC PET were enzymatically depolymerized, as the crystallinity decrease could potentially improve the yield of degradation. According to Table 2, the PC PET conversion (%) of the untreated material was approximately 66%, whereas the pretreated material showed an increase of up to 1.2-fold in the corresponding percentage. In both cases, a similar amount of TPA was detected, which was the main hydrolysis product of the enzymatic degradation. However, it should be noted that just under 60 % of pretreated PC PET was Based on the amount of the detected products in the pretreated PC PET, it can be assumed that the enzyme mainly acts on the released MHET, which is further converted to TPA [17,18]. Nonetheless, the hydrolysis of MHET towards TPA can also be achieved at lower temperatures using enzymes with MHETase activity, thus, reducing the operational costs in large-scale application. This has been reported by Nikolaivits et al, where a feruloyl esterase was used to convert MHET to yield solely TPA which resulted in improved cost effectiveness of the proposed degradation process [11].
Overall, the self-hydrolysis of PET after mechanochemical pretreatment and the enzymatic hydrolysis of unpretreated PET are two approaches with comparable degradation yields. Consequently, the selection of the recycling method for PC PET should be based on the sustainability of each process, taking into consideration factors such as time, energy, and cost requirements.

Conclusion
We investigated the morphological and the structural changes as well as degradability of PC PET plastic waste after applying our novel, ultrafast dissolution/reprecipitation mechanical pretreatment approach. The approach involved treating PC PET with m-cresol/ethanol as solvent/ non-solvent system and heating under MW irradiation for 2.0 min. Macroscopic analysis revealed that the pretreated PC PET had a fluffy, spongy and white coloured appearance in contrast to the coloured flakes appearance of the untreated PET. DSC and FTIR spectra showed a decrease in the crystallinity index and an increase in the carbonyl index of the pretreated samples which indicated the rendering of PC PET to be more amenable for degradation. Considering the chemical recycling of pretreated PC PET by MW assisted hydrolytic depolymerization resulted in increased PET conversion rate and yield of produced monomers specially that of TPA. Within 1.5 min, 95.0 % PET conversion and 87.4 % yield of TPA was achieved. On the other hand, the enzymatic hydrolysis of the pretreated PET did not show a significant increase in conversion rate compared to the untreated material, as both samples achieved a PET conversion rate of approximately 69%. When considering the released products, TPA accounted for 96.5% in the case of the untreated material, which was nearly 10% higher compared to the mechanochemical approach. At the same time, the enzymatic treatment of the pretreated material resulted in TPA as the sole product, emphasizing the advantageous selectivity of the enzymatic hydrolysis process. The pretreatment process improved or maintained the PET conversion rate and yield of monomers in the pretreated PC PET compared to the untreated material in both enzymatic and chemical degradation techniques. These findings suggest the potential advantages of incorporating pretreatment and degradation methodologies in PC PET recycling programs.