Evaluation of the Degradation of Plastics PET and PE Type Under Composting Conditions Using Coffee Pulp as Substrate

The versatility of the mechanical and thermal properties of plastics have become an essential product in modern life around the world [1,2] not only due to their cheap manufacturing, stability, and durability but also to their extensive and wide variety of applications. Because of these advantages, plastic production has risen progressively since the 1950s, reaching 367 Mt in 2020, according to the report of Plastics Europe [3,4]. The most manufactured plastics are used in packaging as bottles and bags of short-life products, resulting in a huge consumption of single-use plastics, which are discarded easily [4,5]. The significant amount of plastic generated from these actions leads to the accumulation of millions of metric tonnes of plastic waste in the environment and landfills [2,6,7] causing devastating environmental pollution that affects ecosystems, wildlife, and human health in addition to waste management problems [2,4,5,8]. Among those, the most common types of plastics polluting and accumulating as solid waste in the environment are polyethylene terephthalate (PET), polypropylene (PP), polyethylene (LDPE-HDPE), vinyl chloride (PVC), polyurethane (PU) and polystyrene (PS). All of them are characterized by high durability and resistance to degradation in environmental conditions [5]. The serious problem of plastic pollution has generated a wakeup call for searching for effective and friendly alternatives for the treatment of plastics with the aim of reducing their impact on the environment, alleviating disposal problems and achieving their sustainability [4,9]. In recent years, a promising alternative for solid waste management by composting processes has emerged as an attractive option to the conventional recycling process (such as incineration), which generates toxic compounds released in the air and low heat efficiency due to its high moisture content [10]. The composting strategy can reduce the impact of plastic at the end of its useful life, mainly when it comes to plastic products that are most likely mismanaged or whose recycling is not feasible, such as those used for packaging (PET and PE). Furthermore, the decrease in the plastic’s environmental impact through this treatment process helps reduce the use of fossil resources and encourages their integration into natural cycles [4]. On the other hand, reducing the waste stream to landfills saves energy and allows to valorize some organic compounds with high content of nutrients and microorganisms from agricultural processes [11] as coffee pulp in the coffee processing sector, which represents about 44.0% of beans in a wet base. In Colombia, coffee production represents 85% of the country’s agricultural production. In coffee processing, only 9.5% of the total weight of the coffee bean is an exploited product for consumption in the preparation of beverages, and 90.5% are by-products of the pulping, which are considered waste that does not have well-defined disposal and management and has become contaminants in soils and water resources [12]. The coffee pulp production in Colombia is approximately 2.25 million tons per year [13]. In this way, the composting process arises as a potential alternative to take advantage of the microbiological value of the coffee pulp and get its valorization within the coffee production chain, as well as mitigate the pollution effects as a by-product by being an efficient and cost-effective biological process for treating waste [14,15].

Therefore, the goal of this work is to contribute to the knowledge of alternative strategies for solid waste management to reduce the serious environmental impact of plastics and take advantage of agricultural waste for composting processes. Therefore, this study was carried out to evaluate the degradation capacity of PET and PE plastics over time under composting conditions at the laboratoryscale using coffee pulp as substrate. The samples were subjected to physical (weight), structural, morphological, and thermal analysis. In addition, the bacterial community of the compost was examined by 16S rRNA gene clone analysis to identify the principal microorganisms that affect the composting processes and the degradation of plastics.

Experimental conditions

The degradability of PET and PE plastics was tested in the laboratory under aerobic composting conditions. PET samples were mainly disposable plastic bottles (soda and water drink bottles), and PE samples were packaged in small bags obtained from a solid waste collection point within the Universidad Católica de Manizales (UCM). The samples of both plastics were cleaned and kept dry. The PET samples were cut into sizes of 3.0cm x 3.0cm squares with an approximate weight of 0.3820g in each piece, whereas the PE samples were pieces of 9.0cm x 9.0cm with a weight of 0.4220g. Every piece of plastic sample was rigorously measured in such a way that weight only varied in the last significant digit (relative error of ±1×10-4g). These sample conditions were in accordance with ISO 20200:2015 [16].

The coffee pulp and soil used as a substrate for compost were obtained from a rural area (municipality of Neira) near Manizales in the central west of Colombia. The coffee pulp was pre-treated by drying at room temperature for 5 days and deposited into an Aluminum container to be dried in an oven at 60.0 °C for 13 hours to reduce the water content of the organic matter from the previous composting treatment.

Composting preparation

Figure 1:Image of one of the experimental reactors for PET samples within the compost.

The compost was prepared by mixing a 30% dry mass of coffee pulp and 35% soil with sawdust (20%), fresh and dry organic matter (7%) and Urea (1%). The compost was deposited in Polyvinyl Chloride (PVC) reactors (20.0cm x 30.0cm x 10.0cm) with a small hole (made home) for ventilating the samples. Each reactor was loaded with a total mass of 1Kg of compost, and each plastic system was conducted in quadruplicate (R1, R2, R3 and R4) to evaluate the effect of composting conditions on its degradation using coffee pulp as a substrate. On the compost, 18 pieces of PET and PE samples (labelled) were located and covered with it. An image of one of the reactors set up for the experiment is shown in Figure 1. The reactors of both plastic systems were kept in a controlled environment at an average temperature of 58.0 °C (±1.0 °C) in an oven. To investigate the degradation process of the plastic samples as a function of time, they were taken out of each of the reactors every five days, placed in a desiccator, and weighed after 48 hours at room temperature. This process was repeated over the entire experimental period (90 days). The reactors were also weighed every five days to determine water loss by evaporation. When weight was up to 50%, they were recovered with distilled water using a fine spray until they reached the weight at the beginning of the experiments.

Characterization of the samples

The weight variation of treated samples was calculated according to the following relationship:
W (mg)=mD0-mDT, where mD0 corresponds to the initial mass of the samples, and mDT denotes the measured mass of the dried samples after the composting treatment during a certain period of time, which was specified by T days. That is to say, mD5 denotes the weight of the sample maintained in compost conditions for 5 days. The results are expressed as an average weight determined for four replicates at the same treatment time.

ATR-FTIR spectra were collected on an FTIR spectrometer equipped with a PIKE Miracle single reflection ATR accessory. The spectra were measured with a resolution of 4cm^-1 in the range of 4000-400 cm^-1 (64 scans).

The evaluation of the thermal stability of the samples was carried out using a TQ-500 apparatus from TA Instruments under an air atmosphere. The samples were examined at 50 to 600 °C with a heating rate of 10K/min. The surface morphology of PET and PE samples before and after testing was observed by SEM-JSM 6301 model. The dried samples were imaged at high vacuum at 1.0kV and WD 6.0mm.

The microbiological analysis for 16S rDNA

The ADN extraction from the compost containing coffee pulp was performed using a NORGEN kit with a stool DNA isolation kit according to the manufacturer’s instructions. After the extraction process, DNA was quantified by the nanodrop spectrophotometer method and separately stored at -20 °C for subsequent sequencing according to the Illumina Mi Seq platform. The extracted DNA was used for experiments of microbial diversity (Metataxonomic) for bacteria with 16Sr RNAV3-V4 variable regions using oligonucleotides 341F (CCTACGGGNGGCWGCAG) and 805R (GACTACHVGGGTATCTAATCC). The sequencing was run on a MiSeq machine for 2 x 300 bp paired-end sequencing (Illumina). The diversity, abundance, and community structure of bacteria were studied with the last version of MOTHUR platform.

Statistical analysis

The weight variation on the processed samples under composting conditions during the treatment time (90 days) was analyzed with an experimental design of repeated measures using a Mauchly test, previous verification of the Huynh-Feld condition on the response of the variances. The sample size as experimental unit was four (reactors: R1, R2, R3 and R4) for each plastic system (PE and PET) with 18 samples over time. The data were divided in four series (S1…S4) with a set of five weight measures at different times; S1: D5, D10, D15, D20 and D25, S2: D25, D30, D40, D45 and D50, S3: D50, D55, D60, D65 and D70, S4: D70, D75, D80, D85 and D90. These data were analyzed independently with an intra-subject effect test to determine variances in the weight of the samples over time. Also, data were subjected to an intra-subject contrast test (polynomial contrast) to know the trend on the weight measures as a function of time, where time represents a quantitative factor, and plasticity acts as a qualitative factor. Finally, an inter-subject effect test was applied to the weight data to contrast the effect of the plastic type on the measures. All the data was analyzed with a significant P value < 0.05 using SPSS software.

Weight loss of the samples

The experiments to determine the weight loss of the treated PET and PE samples under composting conditions are summarized in Figure 2(a) & 2(b), respectively. The samples were recovered every five (5) days from the compost incubated at 58.0 ± 1.0 °C. The weight loss in the samples at each specific time was calculated with respect to the weight of the initial samples (D0). A statistically significant change in the weight loss for the treated PET samples at 25 days (D25) was observed. The results indicated a progressive gain of weight on the samples from D15 to D25. This fact could be related to the adhesion of the compost material to the surface of the samples, which is unlikely given a change in the physical parameters of the compost (temperature fluctuations or increased humidity). However, this effect was not noticed in the samples at longer treatment (incubation) times (D30 to D90), where no statistically significant changes in weight were observed.

Figure 2:Weight changes (ΔW) as a function of time of PET (a) and PE(b) samples in compost at 58°C. All data are the average weight of the samples ± standard error of the mean of 4 replicate measurements for each system.

In the case of treated PE samples, the statistical analysis did not reveal any effect on plastic degradation over treatment time. The results showed that the composting conditions with coffee pulp as substrate had no significant effect on the degradation of the plastics over the total time evaluated in this study (90 days). The degradation processes under composting conditions are influenced by factors that not only include the physicochemical characteristics of the plastics under study, such as their molecular structure, molecular weight, degree of crystallinity, and melting temperature, but also their own factors in terms of the variation of the soil, the organic and inorganic compositions of the compost employed, as well as the types of degrading microbiota and the incubation temperature [2].

Some previous studies on the polymer degradation of PET and PE under natural and composting conditions have not evidenced a significant degradation of the plastics [1,11,17,18]. The effect of biodegradable materials as PLA, PHB or TPS in the recycling process of PET was studied by Aldas et al. [18] Their results showed that these biodegradable polymers had no effect on the degradation of PET under the composting conditions used in that case. On the contrary, the presence of PET interferes with the degradation process of the biodegradable polymers under composting conditions, as observed by thermogravimetric analysis, since the degradation temperature of the blends decreased only a few degrees, and the samples did not show a significant weight loss after 30 days of treatment. On the other hand, a study of the biodegradable kinetics of PE under controlled composting conditions showed that the surface of PE and PE/starch remained unchanged after the treatment [11,19]. These observations showed that the study of weight loss alone is insufficient to determine the degradation degree of plastics. Thereby, the evaluation of the physicochemical and structural properties by using other characterization techniques such as FTIR, SEM or TGA enables the identification of specific behaviors that are only evidenced in detail through those techniques, and it will elucidate changes in the plastic samples subjected to composting treatment [20].

On the other hand, to understand how degradation processes of the plastic considered non-degradable occur under composting conditions, the determination of the parameters and conditions of the compost, nature, and environment where composting happens are relevant factors to getting an effective process of degradation or disintegration of the plastics without pre-treatment.

FT-IR characterization

A detailed FT-IR analysis of the treated samples allowed us to assess the effect of composting conditions as a function of time, checking the variation in the intensity and position of the bands assigned to the vibrations of the functional groups of PET and PE samples [21]. The FT-IR spectra for PET at different times of composting are shown in Figure 3(a). As can be seen, there are no appreciable changes in the spectra of treated samples of PET in comparison with the initial plastic material. So, the intense band at around 1714cm^-1 is associated with strong carbonyl bond stretching of the -C=O group [22,23] whereas the broad and intense band sat 1240cm^-1 and 1095cm^-1 are assigned to the typical stretching vibrations of ether C-O-C [22] and ester bonds [23,24], respectively. The C-C stretching aromatic bonds appeared at a wavenumber of 1409cm^-1 [25], while the band of low intensity at 1338cm^-1 is associated with the wagging of the ethylene units [22]. At lower wavenumbers, the FT-IR spectra showed an intense band at 1017cm^-1 that can be assigned to C-H aromatic ring in plane bending [24]. The band at 970cm^-1 has been assigned to -CC- and - C=C- bonds [26] or to -O-CH₂- stretching of ethylene glycol segments [27]. The bands at 871 and 846cm^-1 are attributed to outof- plane bending of the C-H aromatic ring [28] and rocking bending of the C-H bonds of –CH₂- [29], respectively, whereas the band at 723cm^-1 is assigned to the aromatic in-phase -C-H- out of plane bend [22,23,30].

Figure 3:FT-IR spectra of the treated plastic samples of PET (a) and PE (b) under composting conditions as a function of time.

The FT-IR spectra of treated PE samples shown in Figure 3(b) did not reveal significant changes in their bands as a function of the treatment time. The spectra present the following main transmittance bands at 2914 and 2848cm^-1 correspond to the C-H stretching vibration bonds, the bands at 1471 and 719cm^-1 are associated with the bending and rocking vibrations, of the -CH₂- bonds in the polymer, respectively [23-31]. The fact that PE samples treated under composting conditions showed a similar behavior to that observed in treated PET samples indicated that no significant chemical changes took place owing to the treatment.

Thermogravimetric analysis

The thermal stability and decomposition of the samples were evaluated by TG/DTA at different treatment times. The TG/DTA curves and the degradation temperatures for PET and PE samples during composting treatment over time are shown in Figure 4(a) & 4(b), respectively. The PET samples undergo two stages of degradation, the first one with a maximum around 418 °C, and a weight loss of 83.0% due to the thermal degradation of the PET backbone owing to the random scission of ester links in the main chain with the formation of different oligomers, and the second one at 547 °C with a weight loss of 17.0% [18,32-34]. All treated PET samples under composting conditions showed a similar trend in the thermogravimetric analysis. This indicates that thermal stability of the treated PET samples was not appreciably affected by the composting treatment. Similar results were also reported by Girija et al. [33] who investigated the thermal and mechanical properties of PET blends with various natural polymers like starch or cellulose derivative compounds, and they found that there were no significant changes in the decomposition temperature of PET in the blends.

Figure 4:Thermogravimetry curves (left side) and graphic representation of the variations in the decomposition temperatures as a function of time under composting conditions (right side) for PET (a) and PE (b). DT1, DT2, DT3 and DT4 denote the different stages observed in the thermal degradation of each polymer.

However, in the case of treated PE samples, the thermogravimetric analysis showed different behaviors for thermal degradation of this polymer as a function of time under composting conditions in contrast with those of PET. The thermal degradation mostly occurred in four stages on PE samples, as can be seen in Figure 4(b). For the initial sample (D0), the maximum degradation rate took place at 357 °C with a weight loss of 38.0%, followed by a second stage at 380°C with a weight loss of 25.0%, the third one at 449 °C with a weight loss of 28.0%, and a last one at 533 °C with a weight loss of 9%. For the treated PE sample at 20 days (D20), the main decomposition took place in the second stage at 341 °C with a weight loss of 37%. For this sample, the thermal degradation happened at lower temperatures than for the untreated PE sample (D0). As composting time increased, for example, in D60 and D90 samples, the higher thermal degradation occurred in the first stage as in D0, at 358 °C and 359 °C with a weight loss of 57% and 69%, respectively.

These differences between the thermal behavior of treated PET and PE samples could be due to the chemical structure of PE used for packaging (low-density polyethylene, LDPE). Its less compact structure with a lot of branches [35] gives place to random and distinct interactions of the plastic with the surrounding environment. As a consequence of these interactions, the polyethylene decomposes, resulting in a random distribution of volatile matter that contains not only different hydrocarbons but also organic residues and microorganisms from the compost.

This consideration agrees with the higher thermal stability against degradation observed for treated PET samples under composting conditions, since PET plastic has a higher degree of polymerization than PE plastic [36,37].

The effect of the composting process on PET and PE samples was monitored by SEM images and by digital photos to analyze and detect changes in the morphology as a function of the treatment time. The digital photos of the surface appearance of treated PET samples are shown in Figure 5 (insets). As can be seen, the samples lost stiffness as the treatment time increased. The plastic samples began to curve at 20 days (D20), and this change in appearance was more evident at 90 days (D90) of treatment under composting conditions. On the other hand, after 30 days of composting, the treated PET samples showed some brown areas on their surface. This result would be related to the adhesion of organic matter from the compost to the surface of the samples, which increased at long treatment times (D90). The SEM images of the surfaces showed a uniform and smooth aspect until sample D20 of composting. However, from D30 to D90 horizontal and vertical stripes were observed, as well as a change in the surface roughness of the plastic, which could probably also be a consequence of the adhesion of the compost to the surface of the samples, as has been mentioned above. Furthermore, this effect of composting observed by microscopy agrees with the fact that D30 showed an increase in its weight, according to Figure 2(a), which would highlight that the adhesion of the compost on the surface of the plastic could hide or reduce the response of the polymer to the degradation process. At the end of the treatment time, samples D80 and D90, a layer of compost can be observed on the surface. This observation well coincides with tonality changes and the appearance of brown spots on the surface of the plastic.

Figure 5:SEM micrographs and digital photos (inset) of the surface of treated PET samples at different time of the composting process.

Morphological and appearance studies for the samples

The appearance and morphologic changes in treated PE samples are shown in Figure 6. In this case, the effect of composting on the surface of the samples was exhibited earlier than for treated PET samples. The surface of the samples became rough after 10 days of treatment (D10), and random stripes and adhesion of compost on the surface took place at an early stage in the process. Other significant changes in the appearance of the PE samples were the changes in the tonality towards brown on most samples, like what happened in treated PET samples. From the above observations in appearance and surface morphology changes, it can be concluded that the treated PE and PET samples remained unchanged. They did not exhibit changes related to degradation after composting treatment, such as erosion, which would indicate that they were not attacked by the microorganisms under the composting conditions established in this study. These results are consistent with FT-IR and weight loss analysis, which evidenced that the degradation of PE and PET was negligible [11,38]. Furthermore, the adhesion observed by microscopy (SEM) was in good agreement with thermogravimetric analysis over the testing period. It is necessary to point out that the variation of the weight loss on these samples would be related to the amount of compost attached to the surface under composting conditions. The negligible weight loss of PE samples as a function of composting time, displayed in the Figure 2(b), the physical characteristics of this plastic and the difference on the surface area in contact with the compost, would cause this effect (compost adhesion) to appear in an early stage of the treatment in comparison with those of PET.

Figure 6:SEM micrographs and digital photos (inset) of the surface of treated PE samples at the end of the composting process.

Microbiological analysis of compost

The 16S rRNA gene clone analysis was used to identify the bacterial community in compost containing coffee pulp as substrate, which was used for the composting processes of PET and PE samples after 90 days. The classification analysis and relative abundance of bacteria at the phylum, family, and genus levels (taxonomic groups in the classification of organisms) are shown in Figure 7. As can be seen, Proteobacteria, Bacteriodota, Actinobacteriota, Firmicutes, Chloroflexi and Myxococcota were the six most dominant phyla in the compost samples. Between them, Proteobacteria and Bacteroidota were the most abundant phylums with around 55.0%, whereas Myxococcota only contributed 1.0% of the bacterial content in the compost.

Figure 7:Composition of bacterial communities of the compost based on coffee pulp substrate for the composting treatment of PET and PE plastics.

In terms of family, the analysis of the relative abundance revealed that Sphingobacteriaceae was the most abundant family, with 21.0% of the total bacterial 16S rRNA sequence, followed by Rhizobiaceae, Streptosporangiaceae and Flavobacteriaceae which contributed to 9.0% each to the bacterial population in the compost. Lastly, Planococcaceae and Bacillacea have the lowest proportion in the composition of bacterial communities. Sphingobacterium (~20.0%) and Nonomuraea (10.0%) comprised the main genera bacterial composition in the compost.

In a previous paper, it was reported that the Proteobacteria, Firmicutes and Actinobacteria are the most abundant phyla of bacterial communities found during composting processes [39] as in the findings of this research. However, it has been reported that species belonging to the genera, like Pseudomons, Ralstonia, Stenotrophomonas, Klebsiella, Acinetobact, Rodococcus, Staphylococcus, Streptococcus, Stretomyces and Bacillus, are able to degrade some types of PE [1,4,10,14,40]. However, they were not found in the compost medium employed in this study for PET and PE samples. Some of these bacterial communities were also found in the microbial structure sequencing during the spontaneous coffee-bean fermentation process in Colombia [41]. Therefore, the lack of this kind of bacterial community in the compost after the composting processes of PET and PE samples could be related to their negligible degradation and disintegration, considering that composting physical conditions like humidity and temperature play a crucial role not just for the degradation of polymer materials, influenced by the nature of each plastic, but also for the effective action of the microorganisms present in the compost [1,11,38,40,42]. On the other hand, it has been reported that the growth of microbial community controlled under certain laboratory conditions (constant abiotic conditions) of temperature, humidity, and aeration, can have a grave impact on the activity of the microorganism in the degradation processes of plastics [40]. Thereby, determining the nature of the environment where the process occurs will allow control and the best conditions to reach it, which guarantees more reproducible results.

The nature of PET and PE plastics as well as the composting conditions (microbial communities present) influenced the degradation process under the experimental conditions applied in this study. The weight variation tracking during 90 days of composting treatment did not evidence statistically significant changes that suggested degradation because of the composting process. However, the thermal and morphological characterization of the PE samples showed an important effect on the thermal stability and surface appearance due to a compost adhesion effect from the early stage of composting, in contrast with the results of the PET samples, which remained mostly unchanged.

On the other hand, the bacterial community of compost determined by 16S rRNA gene clone analysis did not have a significant effect on the degradation process of the PET and PE plastic samples.

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