The studies on plastic degradation are very important for the development of biodegradable plastics, and for reduction of pollution, since plastic waste can remain in the environment for decades or centuries. We have showed the degradation of oxo-biodegradable plastic bags and green polyethylene by Pleurotus ostreatus. This fungus can also produce mushrooms using these plastics. The plastic degradation was possibly by three reasons: (a) presence of pro-oxidant ions or plant polymer, (b) low specificity of the lignocellulolytic enzymes, and (c) the presence of endomycotic nitrogen-fixing microorganisms. In this chapter, the plastic bags’ degradation by abiotic and microbial process using the exposure to sunlight and the use of a white-rot fungus will described. The physical, chemical, and biological alterations of plastic were analyzed after each process of degradation. The degradation of plastic bags was more effective when the abiotic and biotic degradations were combined.
- green polyethylene
- Pleurotus sp.
- plastic bag
About 800 million metric tons (Mt) of plastics were produced worldwide in the last 67 years, and 79% of this production is accumulated in the environment . According to these authors, in 2050 is estimated an accumulation of about 12,000 Mt in landfills or in the natural environment that represent an annual accumulation of ~339 Mt. Therefore, the development of efficient degradation process is very important to avoid this annual accumulation.
The human population on the Earth in 2050 will be about 10 billion people . Thus, adequate disposal of plastics wastes is important for the maintenance of the natural resources to supply this population. Furthermore, the development of degradable plastics is necessary to prevent the accumulation of plastic waste in the ocean .
In Brazil, the National Solid Waste Policy  establishes the selective collection, separation of solid waste, recycling and the shared responsibility for the appropriate management of these wastes among manufacturers, distributors, consumers and the government. However, in these 7 years of law it has observed satisfactory results only in selective collection. In 2014, with the deadline for replacing dumps to landfills, new deadlines for 2021 are in discussion in the National Congress .
Our results of biotic and fungal degradation of oxo-biodegradable plastics and green polyethylene could contribute for development of a process of degradation of these residues using white rot fungi. These microorganisms can grow under adverse temperature, nutrient and moisture conditions that facilitate composting and fermentation processes.
The plastics polymers degradation is analyzed by alterations in mechanical, optical or electrical characteristics, cracking, fission, corrosion, discoloration, phase separation, chemical transformations and formation of new functional groups after degradation process .
Unlike of the petroleum-derived synthetic polymers, the biodegradable plastics polymers, when discarded in the environment, can be degraded by non-biological and biological processes . Exposure to ultraviolet light, thermal heating, and treatment with acidic or basic substances function as term initiators or photo-oxidation of polyethylene . After this oxidation fragments of polyethylene are degraded by action of microbial enzymes .
Oxo-biodegradable or d2W plastics are polymers that contain a pro-oxidant additive to accelerate photo or thermo-oxidation [8, 9]. So, these polymers when exposed to ultraviolet light or at high temperatures are cleaved in low molecular mass compounds that are assimilated by microorganisms . Several studies have shown the biodegradable plastics degradation, after exposed to ultraviolet light or heat, by bacteria and fungi [10, 11, 12, 13, 14, 15, 16].
The plastic bags of green polyethylene are produced using low-density polyethylene (LDPE) and green polymers obtain of sugarcane . We have showed the green polyethylene degradation by
The microbial enzymes, such as depolymerase, esterase and lignolytic ones, that cleave the polymers in small chain compounds, may be involved in the plastics degradation [6, 13, 18, 19]. Thus, white rot fungi have a great potential, because they are enzymes producers and have shown their ability for treatment of industrial waste [20, 21, 22].
The white rot,
Thus, in this chapter described the plastics bags degradation, by abiotic and microbial process, using the exposure to sunlight and
2. Methods and Results
The degradation of two plastic polymers used in the production of supermarket plastic bags was evaluated ( Figure 1 , I). The oxo-biodegradable and green polyethylene polymers were submitted the abiotic and biotic degradation ( Figure 1 , II). The oxo-biodegradable bags contain titanium oxide as pro-oxidant additive and low-density polyethylene .
The abiotic degradation of the plastic bags was the exposure to sunlight up to 120 days ( Figure 1 , III). This exposure was in the summer time in a green house. In this season, the sunlight is from 6:00 am to 5:00 pm.
For the biotic degradation ( Figure 1 , IV at VIII) the plastic polymers without ( Figure 1 , II) or with the exposure to sunlight ( Figure 1 , IV) was used
In each glass flask fours discs of agar (6–8 mm) containing the mycelium of
After inoculation the glass flask were incubated at 25°C for 30, 60, 90 and 120 days ( Figure 1 , VIII).
The alterations in plastic polymers ( Figure 1 , IX) after each time of incubation were performed ( Figure 1 , III, IV at VIII). These alterations were compared with analysis done before of the exposure to sunlight.
Physical alterations ( Figure 1 , IX a), such as wrinkles on the surface, formation of holes and cracks, crumbling, discoloration, were performed by digital photograph and scanning electron microscopy (SEM) with a magnification of 50,000 ( Figure 1 , IX a2). Mechanical properties, such as, energy at break and load at tensile strength were made in universal testing equipment (Instron model 3367) ( Figure 1 , IX a3).
Chemical changes ( Figure 1 , IX b) by Fourier transform infrared spectroscopy (FTIR) ( Figure 1 , IX b1) and SEM coupled with X-ray diffraction ( Figure 1 , IX b2) were determined. These alterations were the disappearance or formation of new functional groups in spectrum of FTIR with scanning of 500 at 4000 cm−1 wavenumbers and the decrease in-oxidant additive concentration by spectrum of X-ray diffraction.
The mycelial growth ( Figure 1 , IX c), the main agent of the biological alterations, was evaluated by dry mass ( Figure 1 , IX c1), respiratory activity ( Figure 1 , IX c2) determined without interruption for 120 days of incubation, electronic micrograph ( Figure 1 , IX c3), digital photography ( Figure 1 , IX c3) and lignocellulolytic enzymes activity ( Figure 1 , IX c5).
The capacity of
A total of 240 days were the time applied to degradation of the plastic bags, being 120 days of exposure to sunlight and 120 days of fungal incubation. According to the manufacturer, depending on environmental conditions, for example, the exposure to oxygen and outdoor element, oxo-biodegradable plastic bags decompose within a maximum period of 18 months after disposal [26, 27]. They also add that in only 121 days the biodegradability index of d2W plastics was 88.86% . Our time of abiotic degradation is the same those used for calculating the biodegradability index and corresponds to ¼ of the required time for the decomposition of these bags. However, after 4 months of exposure to sunlight we did not observe any fragmentation of the plastic bags, only the appearance of small cracks and the bleaching of the film were observed ( Figure 2 ). Da Luz et al. [14, 15] also showed changes in mechanical properties of oxo-biodegradable and green polyethylene after 120 days of exposure to sunlight. According to them, this time of exposure is insufficient for other physical or chemical changes, concluding that the mechanical properties alterations, such as the reduction of breaking energy and elasticity facilitated the fungal colonization of plastic waste. The chemical and physical changes in the low-density polyethylene (LDPE) was observed after pretreated of the LDPE sheets with low discharge plasma (O2, 3.0 × 10−2 mbar, 600 V) for 6 minutes [29, 30]. According to authors, this pretreated was important by plastics biodeterioration by
In a new experiment, we observed a fragmentation of oxo-biodegradable plastics bags after 21 months of exposure to sunlight ( Figure 3 ). The control samples were cut with a scissors ( Figure 3A ), but after that exposure to sunlight, it was no longer possible to cut the bags. These plastics were easily fragmented using the hand, resulting in a powder ( Figure 3B , C ). This result shows that there are needed more than 18 months of exposure to sunlight to completely degradation of the plastics. However, this result is promising, shows the ability of abiotic degradation of these bags and enables new testing using these bags with exposure to sunlight in a period equal to or greater than 18 months and inoculation of microorganisms to complete degradation of the remaining polymers. Degradation analysis with
The oxo-biodegradable polyethylene degradation, assessed by carbonyl index, was observed through exposure to sunlight, up to 90 days, in soil with of moisture and pH control . However, these authors concluded that the polyethylene films without pro-oxidant additive had greater structural and superficial modifications, than the films with the additive. Thus, action of the pro-oxidants by the effect of sunlight depends on e conditions and time of exposure to sunlight.
In the plastic bags the presence of titanium was identified, a component of the pro-oxidant additive ( Figure 4 ). This element presents a higher relative concentration than the other elements analyzed and it is uniformly distributed on the surface of the bags. This homogeneous distribution was also observed to manganese, iron and cobalt ( Figure 4 ). Furthermore, with the exception of titanium and cadmium, the other elements analyzed are important for fungal metabolism ( Figure 4 ). These micronutrients may be elicitors or enzyme cofactors. Thus, the presence of these elements may also have contributed to the
Mycelial growth of
The respiratory activity of
The loss of plastic dry mass was influenced by the time of exposure to sunlight and fungal incubation ( Figure 7 ). Fungal growth was lower in plastic polymers without exposure to sunlight than in others with different time of exposure to sunlight. This result shows that
In this study, we observed the formation of cracks and holes in oxo-biodegradable plastics and green polyethylene after fungal growth ( Figures 8 and 9 ). Comparing the Figure 2B and 8 it is observed that these changes in plastic polymers were caused by
In a simulation according to ASTM G160–03 of polyethylene films degradation with and without pro-oxidant additive through the exposure to sunlight on the soil, different genera or microbial groups,
The Figures 8 and 9 show the plastic degradation with 30 days of exposure to sunlight and 30 days of incubation with different scale enlargements. According to Da Luz et al. , the low specificity of the lignocellulolytic enzymes and presence of pro-oxidant ions and endomycotic nitrogen-fixing microorganisms were the main reasons for the biotic degradation of oxo-biodegradable plastics. Gómez-Méndez et al.,  observed activities of laccase, manganese peroxidase (MnP) and lignin peroxidase during
The laccase produced by the fungus
After 120 days exposure to sunlight, no changes in the FTIR spectrum of oxo-biodegradable plastics was observed. This result shows that pro-oxidant oxidation by sunlight was not sufficient for cleavage of the polymer chain or it there is no oxidation thereof. However, in a previous study, a reduction of the relative concentration of titanium on the surface of oxo-biodegradable plastics wastes after exposure to sunlight was observed . According to these authors, the oxidation of the pro-oxidant may have occurred initially by sunlight and then by co-metabolism with the extracellular fungal enzymes. The authors concluded that the presence of this pro-oxidant proved to be important to cause the breakage of this chain in fragments that were used as a source of carbon and energy by fungus.
In polyethylene green, which contain none pro-oxidant additive, no changes in the FTIR spectrum after exposure to sunlight was observed.
The formation of bands of the bonds oxygen-hydrogen and carbon-hydrogen at 3500–3000 cm−1 and carbon–oxygen and ether or peroxide at 1500–1000 cm−1 were the main changes in the FTIR spectra observed in plastic waste after
In studies on the plastics degradation for
The intensity of the degradation was higher in the green polyethylene than in the oxo-biodegradable polymers ( Figures 8 and 9 ). The green polyethylene degradation by fungus was possible due to the presence of sugarcane polymers in the composition of the bags, low specificity of the lignocellulolytic enzymes and presence of endomycotic nitrogen-fixing microorganisms. In addition, Da Luz et al.  was observed mineralization in green polyethylene with longer times of exposure to sunlight and fungal incubation.
Similar to Da Luz et al. , during the time of incubation we also observed the mushrooms formation in the plastic ( Figure 10 ). The conversion of plastic waste into fungal biomass and mushrooms would be a very important biotechnological innovation for plastic waste degradation that has been increased by millions of tons in recent years [1, 3, 16] and for environmental sustainability. However, the presence of toxic compounds and heavy metals, and also due to the low productivity and high costs are the main limitations to mushrooms production. Productivity in mushrooms can be increased by altering the composition of substrate, as for example, adding different proportions of agroindustrial residue and plastic.
The exposure to sunlight up to 120 days is insufficient to initiate degradation of oxo-biodegradable and green polyethylene plastic bags. However, this exposure is important for
The authors are grateful to the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Fundação Arthur Bernardes (FUNARBE) and Núcleo de Microscópia e Microanálise da UFV.
Conflict of interest
The authors declare no conflict of interest.