Plastic Waste Management Strategies and Their Environmental Aspects: A Scientometric Analysis and Comprehensive Review

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Abstract

Plastic consumption increases with the growing population worldwide and results in increased quantities of plastic waste. There are various plastic waste management strategies; however, the present management progress is not sustainable, and plastic waste dumping in landfills is still the most commonly employed strategy. Being nonbiodegradable, plastic waste dumping in landfills creates several environmental and human health problems. Numerous research studies have been conducted recently to determine safe and ecologically beneficial methods of plastic waste handling. This article performed a bibliographic analysis of the available literature on plastic waste management using a computational approach. The highly used keywords, most frequently cited papers and authors, actively participating countries, and sources of publications were analyzed during the bibliographic analysis. In addition, the various plastic waste management strategies and their environmental benefits have been discussed. It has been concluded that among the six plastic waste management techniques (landfills, recycling, pyrolysis, liquefaction, road construction and tar, and concrete production), road construction and tar and concrete production are the two most effective strategies. This is due to significant benefits, such as ease of localization, decreased greenhouse gas emissions, and increased durability and sustainability of manufactured materials, structures, and roadways. Conversely, using landfills is the most undesirable strategy because of the associated environmental and human health concerns. Recycling has equal benefits and drawbacks. In comparison, pyrolysis and liquefaction are favorable due to the production of char and fuel, but high energy requirements limit their benefits. Hence, the use of plastic waste for construction applications is recommended.

Keywords: environmental pollution, plastic waste, plastic waste recycling, waste management, scientometric analysis

1. Introduction

Industrial growth results in a huge number of goods for human activities and a massive quantity of waste in the environment as a result of used items being discarded following human activities. These wastes include gaseous, liquid, and solid wastes [1,2,3]. Plastic waste (PW) is a typical industrial waste, and its disposal into landfills creates serious environmental concerns [4,5,6]. Plastic items have become an essential part of people’s everyday lives and are utilized in a variety of sectors, including construction, healthcare, electronics, agriculture, the automotive industry, and packaging [7]. Plastic demand continues to grow due to its numerous advantages, including resistance to erosion, durability, convenience, simplicity of production, and cheap cost [8,9]. Worldwide plastic output has increased at an annually compounded rate of 8.4% since 1950; plastic manufacture reached 0.36 billion tons in 2018 and is expected to surpass 0.50 billion tons in 2025 [10,11,12]. Around 60% of PW is not recycled and encounters the environment [13]. The primary plastics in demand include polyvinyl chloride (PVC), polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyurethane (PUR), and polystyrene (PS) [14]. Similarly, PW is derived from the aforementioned key plastic types. For instance, the packaging sector consumes the most plastics produced and is the primary source of PW due to the extensive usage of disposable items [12]. PW discarded by the packaging industry is mostly composed of PET, PS, PE, and PP [15]. PW degrades slowly and can persist in the environment for hundreds of years; it is thus referred to as nonbiodegradable waste. These industries contribute to the ever-growing quantity of global nonbiodegradable PW, resulting in a variety of environmental challenges [16].

Recycling is a promising option for lowering the demand for new raw materials and reducing waste in landfills [17,18,19]. Additionally, waste recycling benefits the environment by minimizing pollution caused by raw material extraction, conserving energy, and even providing domestic jobs [20]. Figure 1 shows the life cycle of plastic products from consumption to recycling/disposal. Numerous approaches have been developed and utilized in the management of nonbiodegradable PW to date. These technologies may be classified into two categories: traditional PW and advanced PW treatment. The conventional methods of PW disposal are incineration and landfilling, both of which are constrained by a specific bottleneck. Incinerating PW consumes substantial energy and produces harmful by-products [21]. CO₂, persistent organic compounds, particulate matter, acidic gases, and heavy metals are all extremely hazardous by-products that contribute to global warming and a variety of health issues, including lung function problems, respiratory symptoms, and an increased risk of cancer [22,23]. Landfilling has historically been used to handle PW, and even today, the majority of PW is disposed of in landfills [24,25]. Nonbiodegradable PW, on the other hand, decomposes slowly under landfill settings, requiring a growing quantity of land due to the rising volume of PW disposal. Additionally, the contact of PW with groundwater and harmful compounds soluble in water in landfills can result in the production of toxic leachate, which can cause the deterioration of the surrounding soil [26]. As a result, landfilling has been deemed an extremely adverse managing option and has been subject to a variety of restrictions [27]. To address the limitations of conventional methods, such as incineration and landfilling, a variety of innovative plastic management strategies have been established, including pyrolysis, liquefaction, and construction applications [28,29,30,31].

Plastic products’ life cycle [32].

As research on PW management expands due to the growing trend towards environmental protection, scholars confront information burdens that could hinder creative exploration and academic partnerships. It is therefore necessary to devise and implement a technique that enables scholars to obtain crucial information from the most dependable sources possible. A scientometric approach may aid in alleviating this weakness via a software application. The goal of this study is to undertake a scientometric analysis of bibliographic data published up to 2021 on PW management and a review of the different management strategies for PW that help reduce environmental pollution. A scientometric analysis can perform the quantitative evaluation of vast bibliometric data using a suitable software application. Conventional review studies are deficient in providing a complete and correct connection between disparate portions of the literature. Science mapping, co-occurrence, and co-citation are among the extremely difficult aspects of current exploration. The scientometric study includes the detection of the sources with the most articles, keyword concurrence, the leading authors in terms of papers and citations, and the regions actively participating in researching PW management. The Scopus database was used to extract bibliometric data for 6,101 relevant papers, which were then analyzed using VOSviewer software. Furthermore, the impact of various management strategies for PW on the environment was discussed. The aim was to analyze the benefits and drawbacks of each management technique from the literature and suggest the most desirable ones.

2. Scientometric Analysis Methods

This study employed scientometric analysis [33,34,35,36,37] along with a traditional review for the quantitative evaluation of the different aspects of the bibliographic data. Numerous articles have been published on the subject matter, and it is very important to pick a highly reliable search engine. Scopus and the Web of Science are two highly accurate search engines for that purpose [38,39]. The bibliographic data for the present study on PW management was collected from Scopus, as it is commonly recommended by scholars [38,40,41]. The search term “plastic waste management” returned 7756 articles in the Scopus database as of March 2022. To exclude unnecessary papers, various filter preferences were employed. The classifications “article”, “review”, “conference paper”, and “conference review” were chosen as the “document types”. The “source kind” was selected as “journal” and “conference proceeding”. The “publishing year” constraint was chosen to include papers published up through “2021”, and the selected “language” was “English”. Subsequent to the application of these conditions, 6,101 records were retained. The same procedure was also reported by various scholars [42,43,44,45].

Scientometric studies use science mapping, a method invented by researchers for interpreting bibliometric records for a number of reasons [46]. Data from Scopus was stored in the comma separated values (CSV) format for further assessment by employing compatible software. For further analysis, VOSviewer (version 1.6.17, Leiden University, Leiden, The Netherlands) was utilized to construct the scientific visualization and quantitative assessment of the literature. VOSviewer is an open-source and freely available visualization product that is broadly employed in a range of disciplines and well-reviewed by scholars [47,48,49,50,51]. Thus, the current study’s objectives were met by utilizing VOSviewer. In VOSviewer, the retrieved CSV files were imported, and further analysis was carried out while keeping data consistency and reliability. The sources of publications, the most frequently used keywords, the authors with the most citations, and countries’ participation were evaluated during the bibliographic evaluation. The several aspects, their associations, and co-occurrence were visualized through diagrams, while their statistical values were listed in tables. Figure 2 demonstrates the flowchart of the scientometric review.

Flowchart of the analysis procedure.

3. Results and Discussions

3.1. Subject Areas and Annual Publication Pattern

The Scopus analyzer was employed to perform such analysis as to determine the most relevant research areas. The three leading document-producing areas were found to be Environmental Science, Engineering, and Materials Science, as displayed in Figure 3 . For the searched terms “plastic waste management”, publications of the Environmental Science, Engineering, and Materials Science disciplines comprised around 38%, 10%, and 6% of the documents, respectively, accounting for a total of 54% contribution based on the document count. Furthermore, the type of documents was also analyzed for the searched keywords in the Scopus database, as depicted in Figure 4 . This analysis revealed that, for “plastic waste management”, journal articles, conference papers, journal reviews, and conference reviews comprised nearly 75%, 14%, 10%, and 1% of the total documents, respectively. Figure 5 illustrates the annual trend of publications in the present study area from 1965 to 2021, as the first document on the subject research area was found to have been published in 1965. Steady growth in the publication count was noticed, with an average of around 19 articles annually up to 2000. This annual average of publications rose to about 129 from 2001 to 2015. The number of publications rose substantially in the last six years (2016–2021), with an average of around 592 publications per year.

The subject areas of the articles.

Various types of documents published in related fields of study.

Annual publication trend for articles.

3.2. Publication Sources

This investigation was carried out on the retrieved bibliometric data with the VOSviewer. The “analysis type” was selected as “bibliographic coupling”, and the “analysis unit” was kept as “sources”. A threshold of at least 50 papers per source was established, and 17 of the 1555 sources satisfied these constraints. The publication sources (journals) that published a minimum of 71 papers, including data on PW recycling from up to 2021, are displayed in Table 1 , together with their number of citations received in that time period. Waste Management; Resources, Conservation and Recycling, and Science of the Total Environment are the leading three sources/journals in terms of the number of papers, with 548, 270, and 227 papers published, respectively. Moreover, the aforementioned journals also received the most citations in the related research area with 25,171, 13,142, and 11,555 citations, respectively. Notably, this analysis would provide a basis for forthcoming scientometric studies in this subject area. In addition, past manual evaluations were unable to provide science mapping visualization maps. Figure 6 represents a visualization map of the journals that have published at least 71 documents. The box size is proportional to the journal’s article count in the current research; a bigger box size implies a larger role. As an example, Waste Management has larger box dimensions than the others, denoting that it is a highly significant journal in that area. Two clusters were generated, each represented by a different color (red and green) in the illustration. Clusters are constructed depending on the extent of research sources or the frequency with which they are cited together [52]. In this analysis, clusters are formed based on the frequency with which they are cited together. The VOSviewer constructed clusters of journals identified by varying colors based on co-citation patterns in the published articles. For example, the green cluster comprises eight journals that are repeatedly cited in similar articles. Moreover, closely-spaced frames (journals) have stronger connections than widely spread frames. As an example, Waste Management is more directly related to Resources, Conservation and Recycling than it is to the Journal of Environmental Management.

Science mapping of publication sources with a minimum of 50 articles in the relevant study area. Box size is proportional to the number of articles published in a particular journal, and color refers to the clustering of journals based on co-citation patterns in the published articles.

Table 1

Sources of publications with a minimum of 71 papers in the related field up through 2021.

3.3. Keywords

Keywords are crucial in research as they define and indicate the basic topic of the study domain [53]. For the analysis, the “analytical type” was selected as “co-occurrence” and the “analysis unit” as “all keywords”. The lowest number of occurrences for a keyword was kept 100. As a result of these limits, 221 of the 36,091 keywords satisfied the condition. The leading 20 keywords that were employed frequently in the published papers in the subject area are displayed in Table 2 . The top five most frequently occurring keywords on the subject topic include “waste management”, “plastic”, “recycling”, “plastics”, and “plastic waste”. “Plastics” and “plastic” are the same keywords and need to be considered as a single keyword. However, no such option in the VOSviewer is available to merge the “plastic” and “plastics” keywords. So, they were treated separately. We might add their numerical values manually (2290 + 1548 = 3838), but they will not be displayed in the figure. This analysis revealed that PW recycling has been under study most often for waste management and sustainable development. Figure 7 shows the visualization map of keywords in terms of their co-occurrences, their links, and the density connected with their rate of recurrence. The dimension of a keyword circle in Figure 7 a reflects its frequency, while its position implies its co-occurrence in articles. Furthermore, the image demonstrates that the specified keywords have greater circles than the others, signifying that they are vital keywords in the research of PW management. Clusters of keywords have been highlighted distinctively in the graph to reflect their co-occurrence across a variety of publications. Clustering by color is based on patterns of co-occurrence among multiple keywords in the published articles. Green, red, blue, and yellow suggest the presence of four clusters ( Figure 7 a). As seen in Figure 7 b, diverse colors correspond to different levels of keyword density. The colors are ordered by their density, with red having the maximum and blue having the minimum density. “Waste management”, “plastic”, “recycling”, and “plastics” have red areas, suggesting a greater density. This finding will assist future writers in picking keywords that will expedite the discovery of published data in a specific area.

Co-occurrence of keywords: (a) Scientific mapping: circle size is proportional to the number of co-occurrences of a particular keyword; different colors show distinct clusters (red: cluster 1, green: cluster 2, blue: cluster 3, and yellow: cluster 4), and clustering by color is based on patterns of co-occurrence among multiple keywords in the published articles; (b) Density mapping.

Table 2

The leading 20 most utilized keyword combinations in the related study area.

3.4. Authors

Citation numbers show how influential a researcher is in a given research domain [54]. In the VOSviewer, “co-authorship” was chosen as the “analysis kind”, and “authors” as the “analysis unit”. The fewest number of papers required for a writer was set at 10, and 60 of the 18,236 writers satisfied this criterion. Table 3 displays the leading writers in terms of publications and total citations in the research of PW management, as assessed by data obtained from the Scopus search engine. The average number of citations for each author was computed by dividing the total number of citations by the total number of papers. It was complicated to measure a scientist’s efficacy considering all aspects, such as the number of publications, total citations, and average number of citations. Conversely, the writer’s evaluation was defined by equating each aspect independently, i.e., total publications, total citations, and average citations. When it comes to overall papers, the leading three authors are Li, J. with 39; Wang, H. with 36; and Zhang Y. with 34 papers. When it comes to total citations, Thompson, R.C. takes the lead with 7055; Wilcox, C. is second with 5130; and Li, J. is third with 1964 total citations in the present study area. Furthermore, when comparing average numbers of citations, the following authors come out on top: Thompson, R.C. has about 588; Wilcox, C. has about 395; and Al-Salem, S.M. has about 140 average citations. The visualization of writers with at least 10 papers and the connection of the most notable authors is shown in Figure 8 . It was noticed that 41 of the 60 authors were connected based on citations. This analysis disclosed that several authors are interconnected based on citations in the research of PW management.

Science mapping of authors: (a) With a minimum of 10 publications; (b) Linked authors based on citations. Box size is proportional to the number of publications by a particular author, and color refers to the clustering of authors based on patterns of co-citation in the publications.

Table 3

Authors with at least 10 publications in the related study field up through 2021.

3.5. Documents

The number of citations a document receives indicates its influence within a specific field of research. Citation-dense papers are often regarded as pioneers in their respective fields of research. For this assessment, “analysis kind” was set as “bibliographic coupling” and “analysis unit” as “documents”. The limit for the lowest number of citations set to 100, and 384 of the 6101 documents met this criterion. The leading 10 articles based on citations in the field of PW management are given in Table 4 , along with their writers and citations details. Jambeck, J.R. [55] obtained 4313 citations for their publication “Plastic waste inputs from land into the ocean”. Geyer, R. [13] and Hidalgo-Ruz, V. [56] received 3675 and 2007 citations, respectively, for their particular works and were rated in the leading 3. However, only 10 articles received more than 1000 citations up through 2021. Additionally, Figure 9 demonstrates the writer’s mapping of documents, connections based on citations, and the density of the connected documents in the current study topic. The VOSviewer study determined that 286 of 384 documents were connected based on citations. Figure 9 a illustrates the scientific visualization of the contributors to the study of the topic under study whereas Figure 9 b represents the mapping of connected documents based on citations. Furthermore, the density mapping ( Figure 9 c) demonstrates the increased density concentration of the top articles.

Scientific visualization of documents: (a) Documents with a minimum of 100 citations; (b) Linked documents based on citations: circle size is proportional to the number of co-occurrences of a particular keyword, and different colors show distinct clusters formed based on the pattern of co-citations; (c) Density of linked documents.

Table 4

Leading 10 most cited articles up through 2021 in the related study area.

3.6. Countries

Different nations have provided more to the current research area than others have and intend to. The network mapping was established to enable readers to view locations dedicated to eco-friendly construction. The “analysis type” was selected as “bibliographic coupling”, and the “analysis unit” as “countries”. The lowest document limit for a nation was set 30, and 48 countries satisfied this condition. Table 5 shows the top 20 countries based on publications in the current research area. The United States, India, and China presented the most papers with 871, 581, and 551 documents, respectively. However, the United States has the highest total number of citations at 42,924, then the United Kingdom with 30,071, and China with 19,944 total citations. Figure 10 depicts the science mapping visualization as well as the density of nations connected by citations. The size of a box is comparative to the impact of a nation on the subject topic ( Figure 10 a). According to the density visualization, the nations with the highest involvement had a greater density ( Figure 10 b).

Science mapping of the top contributing countries: (a) Network visualization: box size is proportional to the number of publications by a particular country, and different colors show distinct clusters created based on the pattern of co-citations of countries in published articles; (b) Density visualization.

Table 5

Leading 20 countries based on published documents in the present research area through 2021.

4. Management Techniques for Plastic Waste

The management techniques for PW are broadly classified into six categories, namely landfills, recycling, pyrolysis, liquefaction, road construction and tar, and concrete production [63], as displayed in Figure 11 . These techniques are briefly discussed in the subsequent subsections.

Various management techniques for plastic waste.

4.1. Landfills

Landfilling is the most rudimentary technique of PW disposal. Landfills contain a great deal of garbage and have been linked to a number of issues. It is not a sustainable means of disposing of PW [64]. The process of landfilling PW from generation to disposal is shown in Figure 12 , and Figure 13 illustrates the issues connected with the disposal of PW based on a review of the literature. Disposing of PW in landfills may exacerbate land shortages and hinder the operations of waste management organizations [5,65,66]. Additionally, when PW encounters bodies of water, it contaminates them [67,68]. Hence, dumping PW creates concerns for human health and the environment [69]. Landfills have long been recognized as contaminating the soil [68,70]. Thus, landfilling PW must be avoided, and other management techniques should be followed as described in the following sections to help protect the environment.

Flowchart of plastic waste management from generation to landfill/shredding [63].

Disadvantages of plastic waste disposal in landfills.

4.2. Recycling

Generally, recycling is the procedure through which PW is re-extruded. PW is mostly recycled mechanically, which is one of the most cost-effective methods [71,72]. The first phase is shredding or cutting, which involves cutting PW using saws or shears into tiny fragments that are simpler to carry. In the contaminant-separation process, paper pieces, dirt, and smaller particles are removed from PW with the help of a cyclone separator. PW with varying densities is separated using a flotation method in order to manage plastic with varying densities. The next step is milling, which collects and mills the individual polymers. Without the pre-processing phases stated prior to milling, the plant’s efficiency is reduced. Following that, the milled PW is cleaned with water. Chemical washing is also useful for some types of material handling (most notably for removing adhesive from PW), where caustic soda and wetting agents are utilized. The materials are then collected and kept or transferred for further processing during the agglutination process. Extrusion of the plastic results in the formation of strands, which are subsequently pelletized to create a single-polymer plastic. The items are quenched by cooling them with room temperature water. Granulated plastic is then offered on the market as grocery bags, blinds, shutters, and other home items [63].

Recycling is based on the notion of remolding plastic material. It is difficult to turn the full amount of PW into a reusable product. This mass reduction throughout the recycling process is accounted for as plastic emission [73]. Another downside of recycling is the significant amount of energy consumed throughout the process [74]. In comparison to the initial product, the durability of these items is significantly reduced. However, the most prudent course of action regarding plastic is to drastically minimize its usage and reliance. Guidelines limiting the usage of recycled materials have been enacted. For example, used PET containers cannot be utilized to package drinks [75]. The manufacture of wood–plastic composites is one recycling process that has gained increasing interest [76]. This approach of PW management entails the creation of novel materials by mixing PW with woody waste biomass in various amounts. The primary advantages of this technology are the capacity to regulate the characteristics of the materials generated and the efficiency with which two distinct forms of waste may be removed in the same process.

Panels made using a mixture of macadamia shells and automotive PW have exhibited more strength and resistance to fire than panels made entirely of automotive PW [77]. Similarly, it was discovered that when PW was combined with straw flour, its mechanical characteristics greatly improved [78]. It was also reported that the field of PW and woody biomass composites is still in its initial stages and needs additional investigations before its long-term viability can be determined.

4.3. Pyrolysis

The pyrolysis of PW has been investigated as a method for converting home and industrial PW to fuel by subjecting it to severe process conditions, most notably elevated temperature [79]. It entails the degradation of polymeric plastic molecules with a high molecular weight into light gas and liquid hydrocarbons in the absence of oxygen to avoid the creation of oxygen-containing by-products, such as sulfur and carbon oxides, in a reactor devised to endure severe conditions [79]. There are two primary ways by which it is carried out, which are characterized by the presence or absence of a catalyst. Thermal pyrolysis comprises the application of high temperatures and pressure to PW, resulting in the destruction of the molecule by a mixture of scission, in which the carbon chain is cracked around the center, chain crosslinking, and cyclization of linear structures [80]. On the other hand, catalytic pyrolysis utilizes a catalyst to boost the efficiency of the degradation and decrease the energy needs. The outputs of PW pyrolysis vary according to the kind and amount of feedstock utilized and the reactor employed. These may be classified into three distinct classes, including gaseous and liquid hydrocarbons and char. The pyrolysis liquid products are essential and can be utilized as a direct fuel or after mixing with gasoline, motor oil, or diesel, provided they sustain necessary characteristics [81].

The most-often obtained liquid products are paraffin (octane, heptane, and butane), olefins, isoparaffins, propane, and aromatics [82]. Char is a carbonaceous, solid substance that is produced as a by-product of the manufacturing of liquid oil and natural gas. Increases in the proportion of char generated were found to be associated with increases in the pyrolysis temperature [83]. Pyrolysis produces gaseous products mostly composed of lighter hydrocarbons formed by successive successful cracking, as well as some volatile impurities from char. The yield of gaseous products from the pyrolysis of PW, including low-density polyethylene (LDPE) and polystyrene and is much less than that of liquid products, with an increase in gaseous yield being proportional to a rise in temperature [84]. In comparison, polyvinyl chloride and polyethylene terephthalate-based PW yielded substantial gases (>75% by weight) because of their different processes and low energy requirements [85] These gaseous products (methane, butane, ethane, and propane) can be used to generate electricity.

Temperature has the greatest effect on a pyrolysis reaction of PW, with the endothermic characteristic of pyrolysis resulting in enhanced rates of conversion [86], as well as considerations for extreme operating settings [87]. Additionally, high temperatures were shown to be favorable for cyclization reactions between the products [88] but detrimental to the yield of waxes [89]. Residence time also influences the products, with greater residence time being associated with an increase in the fraction of linear hydrocarbon products and the conversion rate [8]. Pyrolysis of PW can also be carried out in conjunction with biomass, with a range of synergistic effects on the type and yield of the products, including a decrease in the formation of tar and an increase in the liquid yield [82,90]. Microwave pyrolysis is another method that has gained popularity. It entails causing the breakdown of polymeric molecules using high-intensity microwaves that raise the molecules’ surface temperature [91]. The plastic material is turned into a fuel made up of a mixture of hydrocarbons during the pyrolysis of PW.

4.4. Liquefaction

Hydrothermal liquefaction has a prolonged history of being used to convert biomass, primarily of algal origin, to bio-oil. It entails the transformation of cellular material into valuable liquid fuel. The method has been adapted to absorb PW and is particularly appealing since it allows for the recovery of plastic for reuse alongside liquid gasoline [92]. Typically, PW is liquefied in the presence of a biomass source, a process known as co-liquefaction. In comparison to alternative waste to value technologies, liquefaction of biomass results in a more even distribution of components among the products. Higher carbon content in the products should naturally result in improved fuel performance [93]. It is worth noting that polymers may be liquefied in the absence of biomass although this is a less popular process than co-liquefaction. For any liquefaction method, catalyst choice is critical and has a significant impact on the process’s efficiency. In most situations, heterogeneous catalysts are used in the liquefaction process to reduce the likelihood of corrosion and to optimize the reaction’s interactions [94].

Depolymerization, disassociation, and recombination are the three processes in the operation of a hydrothermal liquefaction system. Depolymerization of the three major components found in biomass into smaller molecules such as proteins, tiny carbohydrates, and amino acids occurs when a solvent and/or catalyst are added, as well as in high temperature and high pressure conditions [95]. The smaller molecules dissociate further to produce components in their native state, which then recombine to form the product. Bio-oil is created when recombination is combined with a polymerization event. The creation of char and coke arises from further polymerization and degradation of the bio-oil. The lack of a polymerization step leads to the production of biogas [96]. The use of polymers in liquefaction tends to reduce the amount of coke produced. As a result, using plastics in liquefaction can be thought of as having two benefits: recycling PW and improving existing liquefaction products [97]. It has been shown that liquefaction requires a huge amount of water, which might be considered a drawback. Liquefaction is likewise a high-temperature process that consumes a lot of energy [98].

4.5. Road Construction and Tar

Tar is an organic compound with a variety of different structures and compositions. Tar is produced in significant amounts when PW is co-gasified or co-pyrolyzed with other compounds such as heavy metals [99,100]. The extremely condensable behavior of the organic components of tar makes its existence in gas-manufacturing facilities an onerous prospect, owing to its proclivity for slagging and inhibiting catalysts [99]. While tar has a detrimental influence on gas-manufacturing facilities, it is widely employed in other sectors and adds value. It is used to treat plant and human diseases and has a wide range of applications in the coatings industry [101]. The most prevalent use of liquid tar is in road construction [102]. In several states, especially developing ones, roads are constructed using a bitumen base and successive layers over it [103]. These layers get compressed over time, forming a robust and sturdy structure. These layers eventually erode and are replenished over time. The use of PW to coat bitumen is a technology that has seen a recent spike in popularity [104,105]. The benefits are twofold: non-biodegradable PW may be disposed of efficiently, and roads using PW have demonstrated greater performance than regular tar roads [106]. Nearly one ton of PW is used to pave one kilometer of road, resolving the complicated issue of garbage disposal and subsequent emissions [107]. It was discovered that using PW in the road construction process saves about $40,000 per kilometer of road [108]. India is the most vocal proponent of this technique, followed by the Netherlands and the United Kingdom. Other developing nations, such as Malaysia and Ghana, are also embracing the usage of PW in road construction [63].

The addition of PW results in a higher melting point for bitumen as well as greater flexibility. Roads with plastic in them have also been found to have a higher rainfall tolerance. Furthermore, the possibility of using a mixed plastic feed decreases the possible costs associated with PW segregation. Plastic roads have a higher level of ultraviolet resistance and have a longer lifespan. The stiffening effect of PW in bitumen is due to a gradual rise in the attraction forces among the bitumen and the PW over time. Furthermore, the application of oxidizing and linking agents improved the bonding forces even more [109]. In addition, the PW-modified road has been shown to carry heavier loads for lengthy periods of time [110].

4.6. Concrete Production

Concrete is widely used in the building sector and hence serves as a cornerstone of industrial advancement [111,112,113,114,115,116,117]. Recent concrete research has emphasized the use of various ingredients, with a focus on lighter materials, as alternatives to natural aggregates in concrete [118,119]. PW has the potential to be employed in the manufacture of concrete as an aggregate substitute [120,121,122,123]. Incorporating PW has a detrimental influence on concrete strength properties [124]. These recycled plastic aggregates, on the other hand, have the potential to enhance a variety of material characteristics and may be used in sound-insulating, thermal, and lightweight materials [25]. In structural concrete applications where lesser stresses are applied and durability is less crucial, a particular proportion of PW may be used [125]. Gu and Ozbakkaloglu [126] reported that PW is preferable for lightweight concrete manufacture. Additionally, because of its better functional performance, concrete comprising PW is suitable for thermal and soundproofing applications [127,128,129]. Due to the reduced conductivity of plastic and the higher porosity of composites containing PW as an aggregate, it was observed that composites manufactured with PW had a thermal conductivity five times less than normal composites [130]. Thermal conductivity reduced with increasing content due to the hydrophobicity of plastic, resulting in the creation of voids in the mixtures [131]. Numerous studies have revealed that PW, due to its porous nature, has the potential to be used as a sound-absorbing material in concrete [128].

The use of PW in concrete will decrease not just dependency on natural resources but also manufacturing costs. However, using PW in large quantities in concrete is not recommended due to the considerable loss of strength. Usually, a replacement ratio of 10–15% of PW can result in a material with acceptable mechanical properties [132,133,134,135]. The increased air content and lower bonding capacity of plastic aggregates in concrete are the major causes of decreased strength. As a result, it is recommended that more research be conducted on the long-term behavior of plastic aggregates in concrete and their impact on the environment and service life. Various researchers have explored further usage of PW in construction, including plastic bottles in concrete blocks [136], plastic bottle bricks [137], and plastic fibers in concrete [138]. The management strategies for PW other than landfilling and their applications in various products are shown in Figure 14 .