3. Disposal Methods of Poly(vinyl chloride) from the Environment
In the face of the increasing levels of environmental pollution from waste plastics, the development of highly efficient and environmentally friendly methods for their degradation is urgently needed. In 2000, the European Commission published a Green Paper for PVC waste
[77], which assessed various environmental and health aspects and the possibility of reducing its impact on the environment. It paid particular attention to measures leading to solutions to PVC waste management problems. For example, in the Vinyl2010 Voluntary Commitment, it was suggested to reduce organochlorine emissions through the sustainable use of additives and various controlled-cycle management strategies. Its successor, VinylPlus, set an annual recycling target of 900,000 tons by 2025 and at least 1,000,000 tons by 2030
[11][78].
It seems promising to carry out complete dechlorination processes before degradation. Owing to full dechlorination, PVC can be treated in the same way as common halogen-free plastics
[79]. Such methods include chemical modifications, the near-critical methanol process for PVC dechlorination and recovery of additives, and the near-critical process using an aqueous ammonia solution. Among these techniques, the well-known method is the chemical modification of PVC through the substitution of some chlorine atoms with various nucleophilic reagents. Another technique used to convert waste into energy with simple, fast reactions is hydrothermal treatment. In this technique, super- or sub-critical water is used as a solvent and reagent for the reaction of organic compounds
[80].
Moreover, despite the frequent use of certain chemicals in the hydrothermal dechlorination of PVC waste, their role in this process has not been fully understood. Analyses conducted by Zhao et al. with the use of Na
2CO
3, KOH, NaOH, NH
3·H
2O, CaO, and NaHCO
3 in water containing Ni
2+ showed that the alkalinity of the additives has a significant impact on the effectiveness of the dechlorination process. The most effective additive in these studies was Na
2CO
3 (concentration 0.025 M), with a maximum efficiency of 65.12%
[81]. The processes carried out using subcritical water-NaOH (CW-NaOH) and subcritical water-C
2H
5OH (CW- C
2H
5OH) proved that the main mechanism in the case of the dechlorination in CW-NaOH is the nucleophilic substitution of hydroxyl group in PVC, while in CW-C
2H
5OH—the nucleophilic substitution and direct dehydrochlorination were the equally significant processes
[82]. The key parameter of the dechlorination process is temperature. As the efficiency of this process also decreases with a decrease in temperature, the above-mentioned additives were used to improve the efficiency. Unfortunately, the incorporation of the additives not only increased the costs of the dechlorination process, but also generated secondary pollution
[83]. Temperature was also shown to be important in the removal of chlorine (Cl) from PVC in gas–liquid fluidized bed reactor studies where hot N
2 was used as the fluidizing gas to fluidize the polymer melt
[84].
Although poly(vinyl chloride) is a commercially important polymer, it is also one of the most sensitive to UV radiation. A study by Yang et al. showed that the rate of the photoaging of plastics is faster than other ageing processes; therefore, it is one of the most common methods of PVC degradation
[85]. The UVA radiation in deionized water, sea sand, and air was used to photodegrade plastics. The results showed that PVC effectively absorbs the UVA radiation in air, and this is where the ageing efficiency was the greatest. The ageing process included photoinitiation, chemical bond breaking, and oxygen oxidation
[72][85].
Under the influence of UV radiation (in the wavelength range of 253–310 nm) and in the presence of oxygen and moisture, PVC underwent very rapid processes of dehydro-chlorination and peroxidation to form polyenes. The irradiated material crumbled, lost its stretch, elasticity, and impact resistance, and the surface of the degraded polymer was significantly modified, i.e., loss of abrasion resistance, gloss, and interfacial free energy were observed
[86][87].
The use of the photodegradation process makes it easier to dispose plastics from the environment. In order to accelerate the photodegradation of plastics, semiconductor photocatalysts such as TiO
2, ZnO, Fe
2O
3, CdS, and ZnS were also used. For example, it was observed that the addition of ZnO to PVC increased the decomposition of the composite by 4.13% in the case of artificial UV radiation and by 9.7% in the case of solar radiation, respectively
[88]. A photodegradable composite film was prepared by doping poly(vinyl chloride) plastic with nano-graphite (Nano-G) and a TiO
2 photocatalyst. After exposure to the UV radiation (for 30 h), the weight loss rates of Nano-G/PVC, TiO
2/PVC, and Nano-G/TiO
2/PVC films were 7.68%, 8.94%, and 17.24%, respectively, while pure PVC decreased its weight by only 2.12%
[89].
PVC is less biodegradable than other plastics
[90][91]. Therefore, there have been many studies on the thermal decomposition and photodegradation of PVC, but there are a few reports in the literature on the biodegradation of poly(vinyl chloride) compared to other polymers
[92], and microorganisms capable of decomposing it, both in the aquatic environment
[93] and in the soil, are sought
[94].
3.1. Recycling and Utilization of PVC
PVC can be recycled using various material and energy recovery methods
[95].
Recycling techniques include:
mechanical methods—consisting in extruding and mixing the material with primary polymers,
chemical methods—changing the polymer structure of the material using chemical and thermal agents
[96].
The mechanical recycling is the most-recommended way to recycle PVC
[41]. The conventional mechanical recycling processes are based on the separation, shredding, and application of shredded material with an unchanged chemical composition to a processing equipment. In this technique, plastics are collected and sorted by hand and/or machines at recycling plants and then flaked in a high-speed mill and cleaned with a detergent and water. Finally, the dry flakes are melted and cast into pellets from which new products can be made
[97]. The limitation of the mechanical or secondary recycling is that it cannot be used in the case of the unmodified PVC waste of a known composition and origin
[95].
For economic and environmental reasons, the feedstock recycling of PVC is used, including waste that cannot be mechanically recycled. This relatively simple method of PVC recycling allows for energy recovery, which consists of the gasification of fuels or direct combustion in specialized thermal utilization plants. In the case of energy recovery, a fraction of PVC is mixed with other types of waste. The thermal process consists of two steps: dechlorination and the use of the remaining hydrocarbons. Through the thermal recycling of PVC waste, hydrogen chloride is recovered, and other recovered chemicals can find various applications, especially in the chlorine industry
[41]. Poly(vinyl chloride) (PVC) waste with a high chlorine content is the source of chlorine, providing hazardous chlorinated organic pollutants, which can be reused as chemicals, fuels, and feedstock
[98].
Some new mechanical recycling technologies are based on selective dissolution for the recycling of PVC in an economically feasible way. However, currently, only a small amount of PVC post-consumer waste is being recycled. Incineration, in conjunction with municipal waste disposal, is a simple option that allows for the partial recovery of energy and chemical substances when state-of-the-art technology is applied
[99].
One of the common chemical recycling techniques is pyrolysis, divided into hydrocracking, thermal cracking, and catalytic cracking
[21]. Although pyrolysis is an effective method for converting PVC waste into energy, it yields products containing significant amounts of chlorine
[100][101]. The release of harmful substances such as polychlorinated dibenzo-p-dioxins (dioxins) and polychlorinated dibenzofurans (furans) also occurs in processes such as incineration
[96]. During the thermal degradation of PVC, HCl is eliminated, leading to the formation of conjugated double bonds, and it, in turn, attacks other compounds with double bonds, leading to the production of organochlorine compounds
[102]. Even if PVC is landfilled instead of incinerated, during the process, it may release, among others, phthalates and heavy metals such as lead, cadmium, and tin
[79]. Therefore, these processes, including storage, pose a significant risk of releasing chlorinated organic compounds, microplastics, and pollutants into soils and waters
[103]. Due to the low efficiency of the recycling and the tendency to cause secondary pollution, the traditional methods of disposal of the plastic waste—incineration and landfilling—have been banned
[96]. Therefore, it has become important to develop techniques to reduce Cl migration.
3.2. Biodegradation of PVC Waste
The biodegradation of plastics found in the soil is a complex process. The efficiency of this process is influenced by the availability of substrates assimilable by microbial consortia, molecular weight, surface and morphological characteristics, as well as the structure of the polymers
[104]. The biodegradation includes the formation of microbial biofilms on plastic surfaces, followed by the enzymatic degradation of the polymer structure, which leads to the release of oligomers and monomers
[8]. The biochemical transformation of resistant polymers by microorganisms usually involves the transformation of complex compounds into simpler forms, leading to a reduction in the molecular weight, as well as the loss of the mechanical strength and surface properties of plastics. The biochemical degradation processes of PVC consists of five stages: colonization, biodeterioration, biofragmentation, assimilation, and mineralization. The first stage of the biodegradation mechanism is the colonization of the microorganisms on the plastic surface. It involves the adhesion of living microorganisms (bacteria and fungi) to the surface of plastics and their use for microbial growth and reproduction. During colonization, the microorganisms form biofilms, which causes damage to the polymer surface
[105]. The physical and chemical actions of the microorganisms lead to the biodeterioration and superficial degradation of many kinds of polymers, including PVC. They causes changes in their physical, mechanical, and chemical properties
[106].
The prolonged exposure to light, high temperatures, and chemicals in the atmosphere facilitates the biodeterioration process. The microorganisms penetrate the polymers and increase pores and cracks. On the other hand, some microbial species with chemolithotrophic potential promote oxidation and reduction reactions, and chemical biodeterioration
[105]. Biofragmentation is a lytic process that allows for the breakdown of polymers into monomers, dimers, or oligomers. The process involves a decrease in the molecular weight of polymers and the oxidation of the lower-weight molecules using specific enzymes (oxidoreductases and hydrolases), as well as free radicals
[107]. An enzymatic depolymerization of plastics released monomers that were transported into cells, where they underwent a series of enzymatic reactions leading to complete degradation and the formation of CH
4, CO
2, H
2O, and N
2 [108]. The mineralization stage can be aerobic or anaerobic, and it was catalyzed by several enzymes: cutinase, laccase, esterase, peroxidase and lipase in a study
[105]. PVC-MPs significantly inhibited the chemical oxygen demand (COD) removal efficiency of anaerobic granular sludge (AGS) by 13.2−35.5%, accompanied by 11.0−32.3% decreased formation of methane and 40.3−272.7% increased accumulation of short-chain fatty acids
[109].
Examples of the biodegradation of PVC with different kinds of microrganisms
[93][110], bacteria (
Pseudomonas, Mycobacterium,
Bacillus, and
Acinetobacter), and fungi (
Basidiomycotina,
Deuteromycota,
Ascomycota) were reported recently. Surface damage and a molecular-weight decrease were observed
[111][112][113][114]. Also, an extracellular lignin peroxidase of the fungus
Phanerochaete chrysosporium showed PVC-degrading activity
[115]. Alternatively, the biodegradation of PVC occurred with bacteria isolated from the larva’s gut microbiota (
Spodoptera frugiperda). Enzymatic assays (e.g., catalase-peroxidase, dehalogenases, enolase, aldehyde dehydrogenase, and oxygenase) caused the depolymerization of PVC
[116].
Enzyme specificity and temperature are of great importance in the degradation of plastics. Moreover, the use of several microbial consortia and several enzyme complexes allows for an increase in the biodegradation efficiency compared to a single enzyme or single microorganisms
[8]. It is known, however, that the biodegradation of PVC involves three main reactions, including the chain depolymerization, oxidation processes, and the mineralization of the resulting intermediates
[117]. An effective approach to the bioremediation of the environment from plastics is their initial thermal treatment. After the thermo-oxidative modifications of PVC, it was noticed that
Achromobacter denitrificans bacteria isolated from compost were able to eliminate 12.3% of the plastic, which was evidenced by its weight reduction
[118].
Of all higher organisms, only some insects are capable of degrading various plastics and converting them into monomeric compounds. In particular, insects in their larval stages have shown the ability to degrade plastics
[119]. Insects that metabolize plastic include yellow mealworms (
Tenebrio molitor), giant mealworms (
Zophobas atratus), and superworms (
Z. atratus). It is thought that this unique “plastic-eating” phenomenon may be related to the ability of some of these insects to degrade lignin
[120]. The insects have also been shown to ingest polymers, with the actual ingestion being led by the microorganisms inhabiting their guts
[105][120][121][122].
Some bacteria, e.g.,
Pseudomonas citronellolis, were capable of degrading PVC films. A 45-day incubation resulted in a fragmentation of the material and a decrease in its average molecular weight by 10%. The maximum weight loss during the further stages of the experiment was 19%
[123]. Almost 12% weight loss of PVC was also observed in an experiment conducted in anaerobic microcosms using enriched anaerobic consortia from marine samples (waste and water). In addition, this material showed lower thermal stability after 7 months of incubation
[93].
Changes in the mechanical properties of the material were also observed in the analyses carried out using isolates of marine bacteria of the genus
Vibrio, Altermonas and
Cobetia. The most effective microorganisms in the elimination of PVC turned out to be the
Altermonas BP-4.3 strain, in which case, after 60 days of incubation, a 1.76% loss in the weight of the poly(vinyl chloride) film was observed
[124].
Micrococcus luteus from areas heavily polluted with plastics was able to mineralize 8.87% of PVC. This level was achieved in cultures maintained for 70 days with mineral substrate
[125].
The importance of microorganisms in processes such as the depolymerization of PVC was also demonstrated using the example of microorganisms living in the intestines of
Tenebrio molitor larvae. For biodegradation tests, rigid PVC microplastic powders (MPs) were used (70–150 μm), with weight-, number-, and size-average molecular weights (M
w, M
n, and M
z) of 143,800, 82,200, and 244,900 g/mol, respectively, as the sole diet at 25 °C. The ingested PVC was broadly depolymerized, and the M
w, M
n, and M
z values decreased by 33.4%, 32.8%, and 36.4%, respectively. After 5 weeks of experiments involving the incorporation of poly(vinyl chloride) into the larvae diet, their survival rate with PVC as the only component of the diet was maintained at the level of up to 80%
[110]. The ability of
T. molitor to eliminate PVC was also confirmed by Bożek et al. after 21 days of the exposure of mealworm to poly(vinyl chloride) in the diet; a 3% loss in the mass of the material was observed
[121]. Two bacterial strains isolated from oil-contaminated soil (
Pseudomonas aeruginosa and
Achromobacter sp.) showed the ability to degrade PVC containing epoxidized vegetable oil (75% by weight), resulting in a change in the material’s surface topography and a decrease in its tensile strength during an incubation period of 180 days
[113]. Some microorganisms are capable of degrading PVC. However, the PVC materials used in these study were largely plastics containing plasticizers. It was found that some bacterial strains acted mainly on the PVC additives, and there was a low ability to degrade PVC without the additives
[123].
Apart from bacteria, microscopic filamentous fungi were also tested as organisms potentially capable of degrading PVC. Analyses carried out using
Chaetomium globosum (ATCC 16021) have shown, for example, that this fungus was able to adhere to the surface of PVC, which was the first stage of the degradation process
[111]. An exposure of PVC fragments containing plasticizers (dioctyl phthalate and dioctyl adipate) to the atmosphere for a period of 2 years showed that between the 25th and 40th weeks, the surface of the plastic was dominated by
Aureobasidium pullulans. After 80 weeks, the next microorganisms identified were, e.g.,
Rhodotorula aurantiaca and
Kluyveromyces spp. All tested strains of
A. pullulans grew in the presence of PVC, using it as a carbon source, degrading plasticizers, and producing an extracellular esterase and reducing the substrate weight during growth
[126].
Biodegradation potential, through adhesion to poly(vinyl chloride) by
Lentinus tigrinus PV2,
Aspergillus niger PV3, and
Aspergillus sydowii PV4, was also confirmed (Ali, 2014)
[127]. For fungi of the genus
Aspergillus (
A. Niger Sf1 and
A. glaucus Sf2), it was observed, among others, that there was 10% and 32% weight loss of PVC over 4 weeks of the experiment, respectively, while for
Bacillus licheniformis Sb1 and
Achromobacter xylosoxidans Sb2, with the same observation time, the values were 15% and 17%, respectively
[128].
Phanerochaete chrysosporium PV1 strain showed the potential for PVC film degradation, for which Fourier transform infrared spectroscopy and nuclear magnetic resonance analysis showed significant structural changes in the material. This was confirmed by peaks corresponding to alkenes appearing, decreases in peak intensity appearing in the case of C–H stretching, and a decrease in the weight of the analyzed PVC itself
[127]. The decrease in the PVC weight was also demonstrated during the experiment conducted for 12 weeks with strains isolated from the soil. The loss of 0.064 g/m
2 for
Mucor hiemalis, 0.300 g/m
2 for
Aspergillus versicolor, 0.341 g/m
2 for
Aspergillus niger, 0.619 g/m
2 for
Aspergillus flavus, 0.082 g/m
2 for
Penicillium sp., 0.240 g/ m
2 for
Chaetomium globosum, 0.330 g/m
2 for
Fusarium oxysporum, 0.240 g/m
2 for
Fusarium solani, 0.364 g/m
2 for
Phoma sp., and 0.145 g/m
2 for
Chrysonilia sitophila was observed, respectively. The ability of
Mucor sp. fungi to grow in the presence of poly(vinyl chloride) as the only source of carbon and energy was also demonstrated
[129].
With regard to the fact that additives added to polymers may increase their physical and chemical degradation, it has also been shown that the addition of a small amount of cellulose to PVC may cause changes in its properties and facilitate its microbiological degradation
[90][130][131].
Under aerobic conditions, vinyl chloride (VC) served as the sole source of carbon and energy for
Pseudomonas putida strain AJ and
Ochrobactrum strain TD, which were isolated from hazardous waste sites. Analyses conducted on the biodegradation of vinyl chloride, used as a monomer for PVC production, showed that alkene monooxygenase is responsible for its metabolism in AJ strains of
Pseudomonas putida and AD
Ochrobactrum bacteria [132]. The degradation of acetate-modified PVC (PVA) involved, among others, enzymes such as oxidases
[133]. The activity of PVA oxidase was correlated with PVA dehydrogenase. The β-diketone group was introduced into the PVA polymer molecule through the product of the reaction carried out by the dehydrogenase. This product, through an active site of serine hydrolase, initiated the oxidation reaction by PVA oxidase. This was followed by hydrolysis to form the monomer
[134]. While there is a lot of data on the enzymes involved in the degradation of modified poly(vinyl chloride), scientific reports on the mechanisms and enzymes involved in the degradation of PVC are virtually nonexistent
[133]. This is due to the high chemical stability and hydrophobicity of the C-C skeleton of PVC
[135]. Among the few data available, there is a mention that, in the case of genus
Cochliobolus, in the degradation of low molecular weight PVC, laccase is involved
[136].