Materials challenges and opportunities to address growing micro/nanoplastics pollution: a review of thermochemical upcycling

Materials challenges and opportunities to address growing micro/ nanoplastics pollution: a review of thermochemical upcycling S. Parrilla-Lahoz a , S. Mahebadevan a , M. Kauta a , M.C. Zambrano b , J.J. Pawlak b , R.A. Venditti b , T.R. Reina a , c , M.S. Duyar a , * a Department of Chemical and Process Engineering, University of Surrey, Guildford, GU2 7XH, UK b Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Raleigh, NC 27695-8005, USA c Inorganic Chemistry Department &Material Science Institute, University of Seville-CSIC, Avda. Am erico Vespucio 49, Sevilla, 41092, Spain article info Article history: Received 14 May 2022 Received in revised form 1 July 2022 Accepted 10 July 2022 Available online 4 August 2022 Keywords: Plastics Recycling Thermochemical conversion Pyrolysis Gasification abstract Micro/nanoplastics have sparked attention in recent years due to their widespread presence in the environment. Currently, several waste valorization approaches are under development in order to upcycle micro/nanoplastics. Thermal conversion technologies such as pyrolysis, gasification, liquefaction, or hydrothermal carbonization can yield high-value solid products, oil, and gases from plastics waste. The common thermal conversion technologies investigated focus on maximizing the production of oil and gases (such as H 2 and CH 4 ) for use as fuel. Except for hydrogen, when these products are used to generate energy, the carbon emissions generated are comparable to those produced by traditional fossil fuels. Herein, we present a review of the current efforts to capture and convert plastic waste into valuable products with an emphasis on identifying the need to develop processes specifically for micro/nanoplastics while also preventing the release of CO 2 emissions. We identify the development of efficient catalytic materials as a critical research need for achieving economically viable thermochemical conversion of micro/nanoplastics. ©2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Plastics have become an increasingly present aspect of our daily lives. They offer valuable properties such as dexterity and diversity, while being lightweight and relatively inexpensive to produce [1]. Plastics are used in the manufacturing of everyday items such as clothing and cosmetics, and further play crucial roles in constructing transportation vehicles such as automobiles and airplanes. In medicine, plastic products are critical as antiseptic and disposable containers and instruments that provide a high degree of hygiene [2]. However, despite the value of plastics for today's consumers, the environmental impacts associated with plastics after-use have started to plague human society and jeopardize the ecosystem's balance [3]. Plastic manufacturing has seen incredible growth in the past century, from 2 million metric tons in 1950 to 348 million metric tons in 2017 [4]. Between 26 and 36% of the world's plastics are single-use, intended for immediate disposal [5]. This is particularly concerning when considering that most plastic items end up in the environment after use. Due to the toughness and durability of plastics, they stay there for centuries. Plastic degrades in around 500e1000 years [6]. Despite their usefulness in the medical sector, increasing plastic use has also been linked to adverse health effects, through plastic particle deposition in the human body [7]. 1.1. Plastic health/environmental consequences The long-term environmental and health consequences of plastics are currently not entirely understood, and current findings point toward several causes for concern. Environmental problems include the destruction of habitat for wildlife [8], hazard of ingestion [9], and plastic-facilitated transport of organisms to new ecosystems [10,11]. Human exposure to plastic pollution is shown to Abbreviations: EGD, European Green Deal; HDPE, high-density polyethylene; LDPE, low density polyethylene; PP, polypropylene; PS, polystyrene; PET, Polyethylene terephthalate; PVC, polyvinyl chloride; PE, Polyethylene; RY, rayon; AC, acrylic; AAFA, American Apparel and Footwear Association; HTC, hydrothermal carbonization; CNTs, carbon nanotubes; CVD, chemical vapor deposition; CCUS, carbon capture utilization and storage. *Corresponding author. E-mail address: [email protected] (M.S. Duyar). Contents lists available at ScienceDirect Materials Today Sustainability journal homepage: https://www.journals.elsevier.com/ materials-today-sustainability https://doi.org/10.1016/j.mtsust.2022.100200 2589-2347/©2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Materials Today Sustainability 20 (2022) 100200 affect the respiratory, circulatory, and lymphatic systems [12]. Accumulation can occur in the liver, kidney, and gut [12]. Plastic particle transport and deposition in the human body is proposed to cause endocrine disruption [13]. Carcinogenicity and endocrine disruption can occur when certain polymers and their associated additives are inhaled or ingested for an extended time [7]. Inhalation, ingestion, and cutaneous absorption of plastics and micro/ nanoplastics have been identified as the three basic exposure modes. Fig. 1 shows a schematic approach to health issues caused by plastic and microplastic exposure. Respiratory-related disorders have been epidemiologically associated with inhaled polymeric particles, including nasal cavity cancer, airway impaction, respiratory disease, and lung cancer [14,15]. Micro/nanoplastics ingested through food products may have neurological and psychological consequences, including diminished parental behavior, selfreported sexual dysfunction, and neurotoxic reaction [16]. Plastics have been shown to have reproductive consequences including breast cancer, prostate cancer, decreased sperm count, ovarian cancer, and overall impaired fetal development [17,18]. Moreover, consumption may result in metabolic disease, bladder cancer, large bowel cancer, diabetes, liver disease, and more [15,19]. Macro and micro/nanoplastics used on the skin are primarily associated with irritation. The consequences on these systems may be profound with chronic exposure; nevertheless, a lot remains unknown about the implications on the human population [7]. 1.2. Plastic pollution elimination strategies Plastics are mainly produced from fossil fuels, and the transition to a carbon-neutral economy will require substantial shifts in the plastics life cycle. The European Commission unveiled the European Green Deal (EGD) [20] in December 2019, aiming to make Europe the first climate-neutral continent by 2050. In addition, the EU created the EU Taxonomy Regulation [21], which establishes criteria for what economic activities qualify as ‘sustainable’. The regulations can contribute significantly to climate change mitigation while also contributing to other environmental objectives such as transitioning to a circular economy, waste prevention, and recycling. The EU Taxonomy Regulation EU) 2020/852 has developed a comprehensive list of waste-related activities that meet this criterion, notably excluding waste-to-energy incineration, and including waste prevention and recycling [22]. The United Kingdom follows the same strategy regarding zero-waste targets [23]. Recently, it has been estimated that 8 million tons (Mt) of plastic waste reach the ocean each year [24]. This amount of plastic annually entering the ocean is expected to triple by 2040. Therefore, achieving an environmentally friendly and efficient strategy to eliminate plastic pollution is necessary [4]. The Break The Plastic Wave Report [4] estimates that it is possible to reduce plastic release to the ocean by 80% with an appropriate system change. In a proposed scenario (Fig. 2), it is proposed to reduce the utilization of plastic by 30%, substitute plastics with other materials by 17%, and recycle 20% more through chemical conversion in order to become closer to a zero-waste system. While scaling up recycling is vital, it is not technically or economically feasible to completely eradicate plastic pollution by recycling all plastic items [4]. The value of mechanically recycled plastic is limited by the quality criteria for food-grade plastic. Most plastic can only be mechanically recycled two or three times before deteriorating in quality [25]. This is not a technological limiting factor in chemical conversion where plastic polymers are transformed/depolymerized to their fundamental molecular building blocks. It is estimated that 20% of plastic waste may be suitable for chemical recycling [4]. Plastic-to-plastic chemical conversion enables the reintroduction of waste into the petrochemical process to make plastic, lowering the demand for petrochemical raw materials extraction [26]. On the other hand, substitution of alternative and less environmentally harmful materials to plastic should be examined wherever feasible to lower the use of plastics. Substitutes are often more expensive than plastics, and their carbon footprints vary depending on the material/geography [27]. Designing materials for reuse, recycling, and biodegradation is another necessary action item. Even after adopting a proposed scenario as in Fig. 2, including the reduction, substitution, and recycling of plastics, it is estimated that approximately 30% of plastics will be disposed of and mismanaged. To reduce the amount of disposed of and mismanaged plastics, innovation and research are needed [4]. Numerous indigenous microorganisms, such as bacteria and fungus, present in wastewater sludge, soils, and landfills can biodegrade manufactured plastics. Microorganisms can utilize these monomers and oligomers as substrates to produce biomass and carbon dioxide under aerobic conditions [28], through the process of microbial biodegradation. However, the output gasses streams of aerobic and anaerobic metabolism lead to increased carbon dioxide emissions. Another drawback of simple biodegradation is the slow rate, which is not sufficient to handle the massive amount of plastic that has been generated and accumulated already and is continuing to be released to the environment [28]. The shift of the current scenario needs to be achieved with a collective effort to develop and innovate different waste management technologies simultaneously. Waste collection innovations can assist in resolving a variety of issues, mainly helping prevent leakage to the environment. New garbage aggregation models, improved communication with waste producers, and improved logistics for collectors can make waste collection more efficient. In addition, decentralized waste storage, processing, and treatment can empower residents while redirecting resources away from trash disposal and minimizing the possibility of mismanagement [4]. Once the waste has been collected, it needs to be processed to prevent the release of both plastic and any derived greenhouse gases into the atmosphere. This is where plastic ‘upcycling’can be a solution, which is the focus of this review. 1.3. Plastic recycling/upcycling Material properties greatly influence approaches to upcycling. In the case of plastics waste, the material properties are Fig. 1. Plastic and microplastic human health impacts [7]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 2 determined by the polymers and additives that are combined to form plastics. Plastics are generally classified into two types: thermoplastics and thermosets (Fig. 3)[29]. When heated, thermoplastics melt and flow; when cooled, they solidify. This heating and cooling process can be repeated numerous times, and thermoplastic materials are generally melted first to reprocess them. High-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) materials are widely utilized in the manufacture of a variety of consumer goods, including lids, carrying bags, and fast-food packaging. Polyethylene terephthalate (PET) is used to make bottles for carbonated beverages. Polyvinyl chloride (PVC) is utilized to manufacture flooring, footwear, and bottles. These are mainly single-use plastics [30]. Furthermore, unlike thermoplastics, thermoset materials cannot be remelted and will decompose rather than melt. This is because they are chemically bonded during the curing process. This results in a chemical structure that is highly dense, imparting stiffness, and brittleness. Additionally, they have a longer useful life (10 years or more) and are utilized in smaller amounts than thermoplastic materials [31]. Due to the different categories and types of plastics with completely different properties, separating them before any recycling process may be necessary. Furthermore, it has been shown that combined polymers are chemically incompatible with one another [31]. The required different temperatures to melt and process them are also key considerations for remelting them as a mix in the case of different types of thermoplastics. Otherwise, if thermoplastic is reprocessed and mixed with thermoset, the mixture will not be homogeneous, resulting in a portion of the materials (thermoset component) decomposing, creating losses in process efficiency and economics as the energy is expended toward decomposition rather than toward melting the mixture [31]. Most plastics disposed of and mismanaged eventually degrade under weathering and aging into micro/nanoplastics. Micro/nanoplastics are widely detected in marine and coastal environments and are defined as plastic debris with less than 5 mm in size. Micro/ nanoplastics have gained increased attention in recent years as they are present everywhere in alarmingly large quantities [32]. Currently, micro/nanoplastics are categorized according to their origin into primary and secondary micro/nanoplastics (Fig. 3). Due to their small size and large surface-to-volume ratio, micro/nanoplastics can also sorb and accumulate pollutants which they can then transfer to organisms, causing toxicity across the food chain [32]. Mechanical recycling is the most prevalent form of recycling for plastics. Initially, recovered plastics are crushed down to a size suitable for reprocessing/mechanical recycling. This has been a long-standing practice in the plastics industry's manufacturing facilities. This recycling process, termed primary recycling, makes economic sense since manufacturers eliminate their own waste and increase production yields [33]. Secondary recycling entails a reprocessor reclaiming used material from various sources. Material may come in several shapes and sizes, including bales, moldings, and enormous plastic lumps. To an acceptable feed supply size, size reduction, cleaning, sorting, and regranulation may also be required [34]. Chemical recycling is breaking down polymers into smaller molecules that may be readily separated from contaminants. This technique, also known as feedstock recycling or tertiary recycling, produces raw materials for petrochemical operations or feedstock that may be utilized to make monomers for new polymers or other petroleum products [33]. Finally, thermochemical processing, also known as quaternary recycling, is the conversion of solid wastes into conversion products with the discharge of heat energy. It may be used for two things: volume reduction and energy recovery [1,34,35]. It should be noted that all of the above forms of recycling are complementary to each other and potentially necessary to achieve a circular economy [4]. Quaternary recycling has been one approach for plastics pollution mitigation that has gathered attention in the scientific community. It is also called ‘upcycling’, where plastic waste is converted into different products of greater value. Thermochemical conversion technologies such as pyrolysis, gasification, liquefaction, or hydrothermal carbonization have been explored in this approach, to obtain valuable oil and gases from micro and macroplastics [1,35]. Although thermochemical conversion appears promising when dealing with mixed discarded plastic, it usually occurs at high temperatures (300e800  C). Catalysts may play an essential role in increasing thermochemical conversion energy efficiency, encouraging focused reactions, and enhancing product selectivity [36]. Furthermore, co-processing methods that Fig. 2. Plastic in the system change scenario proposes for the break the plastic wave report. Including micro/nanoplastics and macroplastics [4]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 3 include mixing of biomass with polymers, such as co-pyrolysis, co-liquefaction, and co-gasification, offer significant environmental and economic benefits [1]. In one application, researchers explored the properties of typical solid feedstock combinations, emphasizing elemental composition, proximate analysis, and heating value [37]. The various feedstocks’effective hydrogen to carbon ratios are analyzed to guarantee the quality of petrochemical equivalent products generated from co-pyrolysis of plastic wastes and biomass. HDPE, LDPE, and PP were found to have a positive synergetic impact on liquid yield in co-pyrolysis with solid biomass. In contrast, PET, PS, and PVC have a positive synergetic effect on solid waste or gas yield [38]. In another study, microplastic co-gasification with biomass was investigated to achieve a better-quality syngas production than when only biomass was used as a feedstock [37]. The previous research team carried out an Aspen simulation where higher H 2 content was obtained when microplastic fraction was higher than biomass, achieving a better quality syngas production [37]. Most thermal conversion technologies target the production of hydrocarbon oils and gases and sometimes H 2 [1,37,38]. When the resulting hydrocarbon fuels are combusted to generate energy, the carbon emissions are similar to traditional fossil fuels. Nevertheless, by using efficient and selective catalysts in the thermochemical conversion of polymers, the operating temperature requirement may be significantly reduced while increasing carbon conversion and process efficiency. Hence, with the suitable catalyst, thermal conversion technologies can be optimized in order to maximize H 2 production and minimize oil fraction. In addition, it is possible to target valuable solid products that ‘trap’the carbon rather than release it, such as through the production of carbon black or carbon nanotubes [40,41]. Herein, we provide examples of current initiatives to upcycle microplastic waste into energy carriers (such as H 2 ) and valuable solid carbon products. A thorough analysis of the different types of micro/nanoplastics, collection devices, viable products, and waste valorization techniques, including the advantages and disadvantages of the processes, are provided. While prior research has focused on the broader applications of pyrolysis, bulk-plastic upcycling, or non-thermal pathways for plastic conversion, the present review is focused on emerging thermal conversion methods for micro/nanoplastics such as pyrolysis, gasification, hydrothermal carbonization, and liquefaction, presenting a more detailed analysis of these techniques. There are present in the literature reviews for the interested reader, focusing on bulk plastic upcycling or non-thermal pathways for plastic recycling [42e44]. 2. Micro/nanoplastics 2.1. Micro/nanoplastics classification Micro/nanoplastics are defined as untreated waste with a diameter of less than 5 mm in size that have a significant synthetic plastic content. They have gained increasing attention due to prevalence in the environment and their possible detrimental effects on animals, humans, and ecosystems [45]. Microplastic contamination in the environment is classified by source as primary and secondary. Fig. 4 shows the different release mechanisms for micro/nanoplastics and common changes in their physicochemical structure after being discarded. Primary micro/nanoplastics are purposefully produced as microscopic particles and are directly released to the environment by sewage spills or home and industrial effluents [46]. Primary micro/nanoplastics can be composed of plastics pellets, nurdles, powders, and fibers used as personal care and cleaning products additives or industrial materials. These particles have rounded or amorphous shapes. For example, microbeads can come from facial scrubs; artificial microspheres are used in cosmetics and detergents and artificial resin pellets are used as raw materials for industrial purposes [47]. Fig. 4 shows the common processes and products that use them, resulting in primary micro/nanoplastics released into the environment. Secondary micro/nanoplastics arise from the degradation of larger plastic pieces. The disintegration of these polymers is driven by UV radiation, thermal aging, bio-film growth, and oxidation [48]. Degradation is a term that refers to a sequence of chemical events that result in the breakdown of the structures of plastic polymers. It is further classified into photodegradation, thermal degradation, biological degradation, and thermooxidative degradation. These micro/nanoplastics are produced Fig. 3. Macro plastics and micro/nanoplastics composition [39]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 4 mainly by breaking down plastic items extensively utilized in packaging, construction, agriculture, transportation, textiles, and household products [47]. Additional degradation of primary and secondary micro/nanoplastics alters their characteristics, affecting their physical and chemical properties. The changes in properties such as color, surface morphology, hydrophilicity, crystallinity, particle size, and density, directly affect environmental interactions. The degree of crystallinity of polymers directly affects their mechanical characteristics. Semi-crystalline polymers are durable and resistant to abrasion, while amorphous polymers are soft and flexible. Moreover, micro/nanoplastics having a low density will float in the water column, whereas those with a higher density than water would sink and deposit as sediments. Some studies demonstrated that micro/nanoplastics of small size (0.02e1 mm) and with lower density are more abundant than heavier and larger ones. However, the density and grade of crystallinity of micro/nanoplastics are properties that easily change with weathering and aging processes [32,49]. Microplastic polymers are typically composed of PE, PP, PS, PVC, and polyester (PET) and can sometimes contain acrylic [47]. The chemical structure and composition of some of these polymers are depicted in Fig. 5 [50]. The elemental composition of these polymers contains mainly carbon and hydrogen. Furthermore, it is crucial to pay special attention to one of the main sources of microplastic generation, textile fibers, schematically illustrated in Fig. 6. There are three main types of fibers: natural, obtained from renewable resources such as cotton (cellulosic) and wool (protein bases) that do not contribute to micro/nanoplastic pollution; synthetic, derived mostly from nonrenewable petrochemical resources such as polyester and nylon that contribute to micro/nano plastic pollution; and semisynthetic or regenerated, produced from dissolving cellulose from wood and other sources to spin filament cellulose fibers. These fibers have different names depending on the process and solvent used (rayon, lyocell, Tencel) [51]. In fact, 63% of textile fibers are sourced from petrochemicals and synthetic fibers, such as nylon, acrylic, polyester, and PP. The manufacturing and disposal of synthetic fibers generate significant amounts of carbon dioxide emissions [52]. Cotton dominates the remaining 37% of fibers produced. Microfibers from synthetic fabrics are released throughout clothes manufacturing, usage, and disposal stages of clothes [53]. Microplastics derived from laundering make up 35% of the total micro/nanoplastics emitted to the ocean from primary sources (Fig. 4). However, there are other textile micro/nanoplastics emissions to the air and soil that end up eventually in the ocean and are worth being considered (Fig. 6) [54]. In addition, it is necessary to add that micro/nanoplastics fibers are emitted to the air in the form of dust from clothing and emission of particles from clothing dryers. These microfibers will travel through the air into the ocean and the human respiratory systems [55,56]. From secondary sources, plastic items that exceed the 5-mm scale and enter the environment are known to deteriorate further, possibly producing micro/nanoplastics. This is true for fishing and aquaculture equipment and sanitary items [56]. Additionally, landfilling of synthetic clothes is another significant source of microfibers [53]. Fast fashion's disposable nature and throwaway culture have created a severe environmental, social, and economic crisis. According to the American Apparel and Footwear Association, more than 16 million tons of textile waste were created in the United States in 2015, with just 15% recycled, 19% burned for energy recovery, and the remainder (66%) thrown into landfills [56]. Synthetic polymer-based clothing is estimate to take 200 years to degrade in a landfill and first they will fragmentize into microplastics [56]. There are numerous reasons for recycling, reusing, and upcycling textiles and everyday clothing: (1) these wastes are valuable and reusable; (2) they contain materials that if recovered can conserve water and energy used to manufacture new clothes and textiles; (3) their recycling and reuse can reduce the carbon footprint and greenhouse gas emissions associated with the manufacture of new clothes; and (4) the waste would otherwise occupy a significant amount of space in landfills for a substantial amount of time [56]. Fig. 4. Properties changes of micro/nanoplastics after degradation [modified from Refs. [32,49]]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 5 2.2. Micro/nanoplastics collection devices For micro/nanoplastics upcycling, first, it is necessary to collect them. In the case of the primary micro/nanoplastics, it is essential to capture them directly from the sources avoiding their emission to the aquatic environment. To abate secondary micro/nanoplastics pollution, it is crucial to develop technologies that can capture these efficiently. Several collection devices have been developed to remove micro/nanoplastics from the aquatic environment [57]. There is currently no available solution to recapture particles released to the air and soil directly. It is predicted that nearly 80% of microplastic in the open sea comes from sewage effluent discharges, wastewater treatment plant (WWTP) effluents discharges, and urban runoff [58]. Studies on micro/nanoplastics in municipal WWTPs show that the overall efficiency of existing treatment methods to remove microplastic Fig. 5. Common micro/nanoplastics chemical formulation [50]. Fig. 6. Textile microfibers sources [information from Refs. [53,54,56]]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 6 from water is estimated to be above 90% [58]. However, with these methods, most microplastic particles end up in a sludge that is generally applied on agricultural land as fertilizer and can enter waterbodies via runoff. Despite the high effectiveness of the treatment systems in removing micro/nanoplastic particles, the amount of microplastic particles discharged is a serious problem due to the large volume of wastewater treated [58]. Washing machine effluents constitute a significant source of micro/nanoplastics released into the aquatic environments. Synthetic fibers from textiles and microbeads from detergents can be removed or captured at the household level via devices that can be used while operating washing machines. There are several available commercial collection devices (Cora Ball and the Lint LUV-R filter are two examples depicted in Fig. 7) for decreasing fiber release from clothes washing. The methods shown in Fig. 7 have been reported to have 26% (Fig. 7a) and 87% (Fig. 7b) capture efficiency respectively, suggesting that their widespread implementation can create an ample yet distributed supply of collected micro/nanoplastics waste [58]. Micro/nanoplastics removal is not a currently regulated aspect of water treatment [59]. However, France recently introduced new legislation aimed to curb this pollution, requiring that by 2025, all new washing machines have a filter to capture plastic microfibers that fall off garments while washing [60]. In addition, in California, entities that operate industrial, institutional, or commercial laundry facilities are required to adopt the use of the best available control technology to capture microfibers that are shed during washing, as identified by the state board (Bills AB-802/AB-129). Australia committed in the 2021 National Plastic Plan (NPP) to work with the textile and white goods sectors on an industry-led phase-in of microfiber filters on new residential and commercial washing machines by 1 July 2030. Researchers have demonstrated that the installation of filters in washing machines in 97 homes of a small town of Canada connected to the WWTP found a significant reduction in microfibers in the treated final effluent after filter installation [61]. Regarding secondary micro/nanoplastics and primary micro/ nanoplastics released to the aquatic environment, their collection and detection are highly complex. Flotation methods are currently used to extract micro/nanoplastics from sediments. Some examples are decanting, elutriation, aeration, centrifugation, froth flotation, air-induced overflow, separation funnel, and overflow [62]. Other collection methods reported are categorized depending on the removal characteristics, such as filtration and surface attachment methods (e.g. coagulation, flocculation, and sedimentation, electrocoagulation, adsorption, magnetization, and microorganism aggregation) [38]. Note that these techniques are mainly utilized presently to sample small quantities of micro/nanoplastics [38,62]. If widespread micro/nanoplastics collection is implemented in homes via washing machine collection devices, it will be necessary to prevent the release of these plastics back into the environment. There is no way of ensuring this, as the collected fibers or used filters can make their way to landfills. It is possible to use upcycling techniques to create valuable products out of the captured micro/ nanoplastics if the filters and/or micro/nanoplastics can be collected from individual households. It may also be possible to use small-scale upcycling processes. Bulk plastics upcycling techniques can also utilize the carbon in micro/nanoplastics, so that it is not oxidized to CO 2 and released as a greenhouse gas. 3. Plastics valorization techniques The main pathways investigated for plastics valorization are biochemical degradation of plastics and thermochemical processing [1]. The biochemical approach breaks the polymers into monomers and oligomers by enzymes but is typically only effective for cellulose and other natural polymer products. The thermochemical approach is based on converting the polymers into a mix of products consisting of gas, oil, and char/tar. This review paper seeks to identify opportunities in thermochemical plastics upcycling [1,28,63,64]. 3.1. Thermochemical conversion of micro/nanoplastics Most thermochemical waste conversion technologies under research use liquefaction, gasification, and/or pyrolysis. Some novel technologies are also being explored for plastic upcycling, such as hydrothermal carbonization, microwave-assisted conversion, plasma-assisted conversion, and photoreforming [63]. 3.1.1. Liquefaction Liquefaction is a thermochemical solid-to-liquid conversion method that converts waste organics to energy-dense oil at high pressures, often with catalysts and high-pressure H 2 [65]. The addition of H 2 increases the process conversion toward oil products [66]. Traditionally, liquefaction has been used to process biomass, achieving thermal decomposition at temperatures ranging from 250 to 450  C and pressures between 1 and 20 MPa [67]. Biomass liquefaction is assumed to commence with the heat rupture of linking bonds with the subsequent production of free radical species. Hydrogen addition can pevent the recombination of radicals [68]. This approach can be applied to plastic waste. Liquefaction may also be conducted under hydrothermal high-pressure conditions. Hence, several liquefaction approaches have been developed for various solid waste valorization applications, and some of these methods have been applied to decompose plastics waste [63]. Therefore, the biggest drawback of using the liquefaction process to obtain oils is the use of high-pressure H 2 supply as a co-reactant to boost the conversion of the feedstock and make the process efficient [65]. In addition, the use of a high-pressure H 2 source imposes some safety and cost-related limitations. Another concern for this technology is the use of oil products as fuels, which will contribute to increased anthropogenic CO 2 emissions as the plastics are derived from traditional fossil fuels [69]. Liquefaction can be performed over a wide range of temperatures and pressures depending on the feedstock used [70]. An alternative way to perform the process is by eliminating the highpressure H 2 source and adding a solvent that can work as a hydrogen donor for the system. Water is used as the aqueous reaction media and hydrogen source in hydrothermal liquefaction, known as hydrothermal carbonization, which is carried out at temperatures ranging from 280 to 370  C and pressures ranging from 10 to 25 MPa [71]. Typically, waste plastic is co-liquefied in the presence of a biomass source to produce oil rich in aliphatic hydrocarbons [72]. Compared to alternative waste to value technologies, liquefaction of biomass results in a more even distribution of components among the products. Greater carbon content in the feedstock should improve fuel performance [73]. For any liquefaction process, the catalyst selection is critical and significantly impacts the process efficiency. In most situations, heterogeneous catalysts are used in the liquefaction process [74]. In liquefaction, alkali catalysts such as sodium carbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide may increase oil yields while reducing char production [75]. The catalytic liquefaction of plastics using hydrogen as coreactant has been documented in some studies since 1996. One of the earlier research projects consisted of applying this method to a feedstock mixed with coal. The catalyst used included the protonated zeolite Socony mobil-5, ferrihydrite treated with acid, coprecipitated alumina-silica, and ternary ferrihydrite-Al 2 O 3 - S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 7 SiO 2 . The organic fraction obtained from the process was gasoline, kerosene, and heavy oil and the selectivity toward lighter hydrocarbons in the oil increased with the H 2 pressure [76,77]. Other related studies have been reported (Table 1), and a thorough and detailed literature review of this technique can be found elsewhere [1]. When plastics are liquefied, more viscous oil is generated than when lignocellulosic biomass and other biogenic wastes are pyrolyzed to create highly oxygenated oil. Additionally, the low oxygen content of oils generated through plastic liquefaction contributes to their heating value and fuel qualities. Because plastic-derived oils have a low moisture content, catalytic fuel upgrading procedures such as hydro processing, hydrotreating, and hydrodeoxygenation are often far less energy-intensive [74]. Pei and colleagues co-liquefied microalgae Spirulina in subcritical and supercritical ethanol with a synthetic polymer, HDPE. A mixture of microalgae and HDPE was liquefied together to make an oil rich in aliphatic hydrocarbons. Elevated pressure may also inhibit intermediate condensation, cyclization, and repolymerization [72]. High pressures used during liquefaction may prevent the production of the gas phase hence boosting the output of liquid oil [78]. Commercial application of this technology was established in 2000 by Sapporo Plastic Recycling (“SPR”) in Japan. SPR constructed a plastic liquefaction plant with the potential to recycle over 50 tons per day of mixed plastic waste. The advanced thermal method recovers light oil used as a chemical feedstock for the manufacturing of new polymers, a medium fuel oil similar to diesel, and heavy oil used to produce power for export to the grid from this waste stream [79]. In addition, Klean Industries designs and builds facilities that transform waste plastics into premium oils in Canada. Their technology employs a continuous liquefaction process that indirectly heats plastic waste and a catalytic reaction to produce hydrocarbons, which are subsequently condensed to produce standard diesel and a patented heating fuel mix. These oils are compatible with diesel engines and generators without modification. Each ton of waste plastic may be converted into about 950 gallons of premium diesel fuel at a Klean facility. Diverse polymers may be treated concurrently without sorting, and common impurities such as grit, paper, metal, food residue, and oils do not need to be removed prior to processing. The improved method can accommodate large loads of PVC, PET, and optimal polyolefinpolymers without causing reactor damage [79]. 3.1.2. Hydrothermal carbonization Hydrothermal carbonization (HTC) is a relatively recent approach to treating wet organic waste. HTC (also referred to as wet pyrolysis) is attracting attention also for the valorization of polymeric waste. The technology has a maturity of almost 110 years and is a way of mimicking the natural phenomena of mineralization in aqueous media, as in coal formation from natural biomass [85]. HTC products are gases (or incondensable vapors), liquids rich in absorbed inorganics and unreacted components from the reactants and solids rich in carbon (akin to coal) [85]. Modern applications of HTC have used various waste forms of biomass, municipal solid waste, plastics, and bulk textiles as the reactant, aiming for solid carbon, different gases (such as CO 2 , CO, CH 4 and C 2 H 4 ), and oil products [86]. Applications of the process have been successful in producing solids rich in carbon with microporous structures, and nanomaterial forms. Further inspection of the integral structure of the solids indicates the existence of functional groups of oxides, sulphides, halides, based on the selection of feedstock used [85]. Subcritical and supercritical water are two fluid states of water that are used in hydrothermal liquefaction and gasification processes [87]. Wet feedstocks can react in subcritical water at 180e260  C to form hydrochar, a highly carbonaceous substance, utilized for energy production and agriculture, as activated carbon adsorbent among other applications [88]. Subcritical water acts as a solvent and reagent in organic chemical reactions. Hydrolysis is the first stage in the hydrothermal treatment of organic matrices, continued by defunctionalization processes, including dehydration and decarboxylation, and eventually recondensation and aromatization [89]. Due to the high ionic strength of subcritical water, between 180 and 280  C, it works as both a reactant and a catalyst, promoting feedstock dissolution and recondensation into solid products throughout the HTC process. The solid products might be utilized as coal alternatives [89]. Research effort have demonstrated that when HTC process use PVC as feedstock, aliphatic and alicyclic hydrocarbons, benzene, naphthalene, diphenyl, phenanthrene, pyrene, and their alkyl derivatives, in addition to valuable platform chemicals such as acetic acid, furfural, lactic acid, propionic acid, phenolic compounds, hydroxymethylfurfural, levulinic acid, formic acid, and succinic acid may be found in hydrochar [90]. Water is an excellent solvent for hydrothermal conversion processes, due to its low cost, non-toxicity, and abundance [91,92]. Organics are hydrolyzed into low molecular weight molecules during HTC. Due to the deconstructed molecules' instability and reactivity, they repolymerize into high molecular weight compounds [1]. Fig. 7. (a) Lint LUV-R. (b) Cora ball [58]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 8 Other investigations have indicated a successful formation of hydrochar from non-catalytic HTC [93]. These authors conducted 3h experiments at temperatures ranging from 200 to 300  C using baby diapers as raw materials. The resulting gas accounted for less than 1% of the overall product mass, whereas the hydrochar and liquid phases accounted for about 10% and 90% of the total product mass, respectively. The process used water predominately as a solvent [93]. Further investigations have demonstrated that the quality of the solvent dictates the overall composition and grade of solid hydrochar produced [35]. However, the selection of the solvent is not limited to pure water. The use of seawater has been demonstrated as an effective method for producing value-added carbonaceous solids. In a study experimenting with using four types of plastics (polyethylene, PET, PP, and nylon), HTC was performed at low temperature (200e300  C) and a constant residence time of 3 h using sea water as solvent [35]. Additionally, for HTC conducted at different temperatures (200  C, 250  C and 300  C), the authors demonstrated that an increase in the process temperature led to a major increase in carbon content in the hydrochar, reducing at the same time its oxygen content [35]. Hydrothermal carbonization is typically performed in batch reactors with or without a catalyst. Fig. 8 shows the different HTC mechanisms that have been investigated for textile bulk waste processing. HTC of cotton and synthetic fiber in an autoclave reactor using deionized water as the solvent was investigated [86]. The specified reactor temperatures and residence times were 230 and 280  C, for 30, 60, and 90 min [35,86]. Traditional HTC is susceptible to heat losses, non-selective heating, and uncontrollable side reactions, resulting in low yields and long residence times. However, it has been proposed the adoption and development of microwave assisted HTC (MHTC), can improve selectivity and reaction kinetics while achieving homogenous heat distribution [94]. The main drawback of MHTC is the low energy efficiency and high cost of the technology [94]. Additionally, co-HTC using lignocellulosic biomass has been suggested as a method to valorize PVC-containing medical waste. The organic chlorine in PVC may be converted to inorganic chlorine (hydrochloric acid, HCl) in the HTC process by hydrolysis, defunctionalization, recondensation, and aromatization. After washing the solid goods with condensed water, the inorganic-Cl with a high-water solubility may be eliminated. Lignin addition considerably increases the dichlorination efficiency in the HTC process [95]. SEM micrographs of PVC-derived solid fuels before and after HTC can be appreciated in Fig. 9 [95]. HTC is a promising thermochemical process for the production of solid carbon-rich fuel (hydrochar) and other high-value-added products due to the low temperatures required, and suitability for processing wet feedstocks, which can also be leveraged for micro/ nanoplastics waste recovery in aquatic environments. While HTC can operate at lower reaction temperatures than combustion, pyrolysis, and gasification, it requires highly pressurized water, to enable the hydrolysis, aromatization, dehydration, recondensation, and decarboxylation processes resulting in the generation of highvalue-added products [35]. In comparison with traditional liquefaction, this method does not require high-pressure H 2 and uses water as a hydrogen donor for the reaction. On the other hand, there is an environmental impact which needs to be considered, if the resulting hydrochar is combusted to produce energy. 3.1.3. Gasification Gasification is a thermochemical process that transforms solid organic matter into synthesis gas. Synthesis gas, or syngas, is mainly composed of hydrogen and carbon monoxide, with some carbon dioxide [96]. Gasification could be carried out in the presence of air, steam, or aqueous media. While conventional Table 1 Plastic liquefaction experimental results. Year/Author Feed Co-reactant Pressure (MPa) Temperature ( o C) Catalyst Residence time (min) Products Ref Feng et al. (1996) Coal/PPE, PPE, PP, PP/ coal, PP/PPE, PP/PPE/ coal Waste oil/tetralin mixture 5.6e0.7 420e460 HZSM-5 zeolite catalyst, ferrihydrite treated with critic acid, Al 2 O 3 -SiO 2 , ternary ferrihydrite, Al 2 O 3 -SiO 2 20e60 min Gasoline, kerosene, heavy oil [77] Luo and Curtis (1996) LDPE, HDPE, PET, PS. Mixture with coal Tetralin/hexadecane or none 2.8e8.6 400e440 HZSM-5 zeolite catalyst, Al 2 O 3 , super nova-D, Octacat, Octacat-50G (fluid catalytic cracking catalyst) 30e120 min Benzene [80] Zmierczak et al. (1996) Plastics and rubbers (PS) NA 3.4e17.2 350e450 Solid superacid catalysts: ZrG 2 /SO 4 2 and Fe 2 O 3 /SO 4 2 15e120 min Benzene, ethylbenzene, toluene, alkylbenzene, naphthalenes, and other hydrocarbons [81] Shabtai et al. (1997) HDPE, PP, polybutadiene NA 3.4e13.8 350e450 Solid superacid catalysts: SO 4 2 /Fe 2 O 3 , SO 4 2 /ZrO 2 ,SO 4 2 /Al 2 O 3 , and a Ptmodified SO 4 2 /ZrO 2 30e180 Benzene, alkylbenzenes, tetralins, indanes and bicyclic arenes, cycloparaffins and olefins [82] Pei el al. (2012) Spirulina/HDPE Subcritical and supercritical ethanol 9.3e11.9 320e380 Non catalytic NA Bio-oil rich in aliphatic hydrocarbons [72] Wang et al. (2014) Lignite, wheat straw, and plastic waste Subcritical water 2.0e5.0 260e320 Non catalytic 30 min Benzene, asphaltene, preasphaltene [83] Wu et al. (2017) Microalgae Dunaliella tertiolecta/PP Subcritical and supercritical water NA 320e370 Non catalytic 0e40 min Hydrocarbons, carbonyl compounds, acid, nitrogen compounds. [84] S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 9 hydrogen is particularly valuable as we move to a more sustainable energy and transportation system [168]. While hydrogen generation from nuclear energy is not heavily advocated in European hydrogen programs, it may become a viable option in other parts of the globe, such as China and Russia. This is sometimes called ‘purple hydrogen’and is produced electrochemically using electricity generated from nuclear power stations [164]. In contrast to the previously mentioned conventional methods, the by-product of turquoise hydrogen via methane pyrolysis is solid carbon in the form of filamentous carbon or carbon nanotubes. The by-products can be used in subsequent production processes or may be more easily stored, resulting in a lower carbon footprint [169]. Pyrolysis is not yet commercially competitive with SRM in terms of hydrogen generation, but there are examples of large-scale commercial applications when the carbon product is sold as well [170,171]. The hydrogen generated by the technologies studied in the present review can be categorized as turquoise hydrogen if it is generated by thermal decomposition of plastics with most of the carbon ending up in the solid phase. If a low emission energy source can power the process, the carbon emissions related to hydrogen production can be further reduced. Low-carbon hydrogen generation technologies are essential for a decarbonized economy. Waste valorization techniques should also be developed according to a hydrogen economy vision. This requires more investigation, particularly in developing selective pathways to hydrogen, and presents opportunities for catalyst discovery. 6.2. High-value carbon Carbon nanomaterials such as CNTs, carbon nanofibers (CNFs) [172], carbon nanosheets (CNS) [173], cup-stacked carbon nanotubes (CS-CNT) [174], and hollow carbon spheres have been created from plastic waste (HCS) [175]. Since their discovery by Iijima, carbon nanotubes (CNTs) have attracted much interest because of their thermal stability, excellent thermal and electrical conductivity, great mechanical strength, high elasticity, excellent tensile strength, flexibility, and semiconducting properties. CNTs have found uses in the automobile sector, where they are employed as conductive polymers and for plastic reinforcing, as catalytic materials, etc. [176]. Furthermore, CNTs are used as catalyst or catalytic support in various essential scientific disciplines (e.g., energy production and storage, electronics, and medicine) [177]. Chemical vapor deposition of synthetic hydrocarbons is the most popular technique for producing CNTs. Single-step (pyrolysis/gasification), two-step (pyrolysisereforming or pyrolysisegasification), and multistep thermochemical processes have been suggested to create CNTs and H 2 from plastic waste [36]. Carbon nanotubes consist of carbon bonded materials, graphene, where carbon atoms are closely organized in an atomicscale honeycomb (hexagonal) pattern. CNTs are cylinders fabricated of rolled-up graphene sheets [178]. Carbon nanotubes could be synthesized from plastics by applying controlled carbonization process in an oxidizing atmosphere. During this process, the carbonization of plastic occurs in two steps. Initially, there is the degradation of plastics, which is followed by the carbonization of intermediary products with a catalyst. The most important mechanism for the high yield of CNTs is to promote the degradation of plastics to intermediary products such as hydrocarbons and aromatics that are efficient carbon sources. However, this is an emerging area of research with limited knowledge of the growth mechanisms, and yields of CNTs are typically below 10 wt% from carbonization [102]. Carbon nanotubes offer a potentially substantial source of revenue for upcycling schemes. The selling price depends on a few parameters, such as type, purity, and length of the nanotube. A wide range of nanotube sizes (diameter) can be sold; 20 m m, 30 m m, 40 nm, and >50 nm. Multiwalled carbon nanotubes are priced at above £60.00/g [179]. Single-walled carbon nanotubes are available in two forms: 30 m m with hydroxy functionalization at 90% purity and 20 m mat 95% purity. One gram of these forms of single-walled carbon nanotubes costs £500 [179]. Most established methods to synthesize CNTs employ a vacuum or hydrocarbon process gases at atmospheric pressure. The research work of Takeuchi et al. [180] outlines the various synthesis techniques and the types of CNTs that are produced. The most commonly used CNT synthesis techniques are chemical vapor Fig. 12. Distribution of the chemical compounds found in gases of HTC at a temperature of 200 C(HTC200), 250 C(HTC250) and 300 C(HTC300) [35]. S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 16 deposition, arc discharge, laser ablation, high-pressure carbon monoxide deprotonation, plasma torch, super-growth CVD, liquid electrolysis, and natural, incidental, and controlled flame synthesis [180]. These methods to produce CNTs are energy energy-intensive, non-environmentally friendly due to the gas emissions, and costly. Therefore, the prices of the resulting nanomaterials are very high [181]. Therefore, carbon nanomaterial (CNM) manufacturing methods that are more sustainable and cost-effective are of significant business interest. In this regard, cost-effective and inventive alternative carbon sources, such as plastic waste might be very appealing [136]. Chemical vapor deposition (CVD) is the dominant mode for the production of CNTs worldwide [41]. The catalytic vapor phase deposition of carbon was first discovered in 1952. However, the actual synthesis of CNTs was developed in 1993, and in 2007, researchers developed the process to grow CNTs that grew to an impressive 18 mm. The growth of carbon nanotubes on aluminasupported nickel catalyst via thermal CVD was studied by Dikonimos Makris in 2005 [182]. Commercially, alumina-supported nickel catalysts are used to reform methane, but there is a method that can use these catalysts to produce a large number of carbon nanotubes. Various parameters that encourage growth, such as time, temperature, pressure, and concentration, were tested by Th. Dikonimos Makris and co-workers, and these were optimized for maximum carbon yield [182]. Effect of growth time on carbon amount and growth rate formed at constant temperature (600  C), pressure (100 mbar), and H 2 /CH 4 ¼10/70 ratio were studied. The carbon yield is maximized as the growth time is extended. However, the growth rate reduces over time due to the limited catalyst distribution on the surface [182]. Another factor that affects the growth rate involves the nucleation time of the nanotubes, i.e., some of the CNTs nucleate at a fast rate and are still expanding in length, while other CNTs start to nucleate. According to Gong and co-authors, based on high-pressure carbonization, 80 (weight percentage) wt% CNT yield with diameters ranging between 20 and 60 nm could be achieved [102]. Continuous production of CNTs has been found in employing 2 stage pyrolysisegasification reactors loaded with nickel-based catalysts. This method produced hydrogen as a co-product and CNTs with diameters ranging from 10 to 20 nm. In contrast, the yield was far lower as the two-stage method only managed to yield CNTs of less than 20  C. Lastly, a pyrolysisecombustionecarbonization method was found to produce CNTs that were 30e100 nm in outer diameter and 30 m m long. Therefore, not only does the method determine the overall quality of the CNTs, other factors such as the type of catalyst, composition of feed, and type of feed determine the yield and growth of CNTs during production [102]. Several important factors contribute to the final morphology of the carbon produced in this process. Liu YF et al. (2003), have discovered that the ratio between toluene and benzene in the carbon source has an important influence on the carbon nanotube morphology. Straight carbon nanotubes are more prevalent (compared to curled nanotubes) when pure benzene is used as the carbon source and the number of curled nanotubes increases as the amount of toluene in the carbon source increases [183]. While mixed bulk plastics are a difficult and highly variable feedstock for conversion to high purity CNTs of a certain type, there are some wastes, such as synthetic textiles, that may provide a more consistent supply of hydrocarbons (through the thermochemical transformations outlined herein) to be able to synthesize higher value nanostructured carbons. It is worth mentioning that other types of carbon materials can also be produced with the technologies analyzed in this review. Graphite is made up of carbon layers (with covalent and metallic bonding inside each layer) and layers connected by a weak van der Waals interaction. Graphene layers are the carbon layers of graphite. Graphite is anisotropic, having excellent electrical and thermal conductivity inside the layers (owing to in plane metallic bonding) and low electrical and thermal conductivity perpendicular to the layers (due to the weak Van Der Waals forces between the layers) [184]. The market price of high purity graphite powder is around US $15.00e10.00/kg. Due to graphite utilization as lithium batteries component, this price has steadily increased in time with increasing demand [185]. Moreover, Carbon fibers have a crystallographic pattern that aligns carbon layers parallel to the fiber axis, even if the layers themselves are not flat. For lightweight constructions, carbon fibers have often utilized as a reinforcement in polymer-matrix composites [184]. Amorphous carbon has comparable bonding and structure to graphite, but without long-range order. The degree of crystallinity of amorphous carbon increases with heating (called degree of graphitization). Numerous carbons utilized in practice, such as carbon fibers, are not completely graphitic but have a broad range of graphitization degrees, depending on the temperature of heat treatment. Therefore, amorphous carbon can be utilized as an intermediate product to produce CNTs or graphite [184]. 7. Conclusion, challenges, and future work Each of the technologies that have been described herein has its advantages and drawbacks. However, plastics waste conversion also presents broader challenges that need to be overcome before wide-scale implementation. It is known that 8 million tons (Mt) of plastic waste reaches the ocean each year [24]. This amount of plastic is estimated to be triple by 2040 [4]. In addition, plastic has to be collected from the environment and processed. Hence, the scale-up of thermochemical conversion technologies is vital to ensure the circular economy of plastic and micro/nanoplastics. The main challenge is the high cost and time demand of the collection of plastic wastes once they have made their way into the environment. Furthermore, it is necessary to develop more thorough methods that allow the collection of micro/nanoplastics from the aquatic environment, such as those released in vast quantities from washing machines. Micro/nanoplastics collection methods (i.e., those applicable to washing machines) are not able to separate different kinds of plastics (i.e., nylon vs. polyester). They are likely to result in a variable composition waste, containing household contaminants. This can present a challenge for catalytic upcycling methods since micro/nanoplastics collected may contain some contaminants and non-polymer species that can poison and deactivate the catalyst. The reactor design and process optimization will become essential for managing the variability of feedstocks. There are opportunities to leverage the know-how developed for biomass valorization, especially in discovering durable catalysts and developing processes for handling wet solid feedstock. Micro/nanoplastics in water present a potentially highly variable reactant that includes some biomass (such as organic fibers from cotton, for example). The transport cost from the collection point to the processing plant has also to be considered as this is a distributed source of hydrocarbons. Among thermochemical upcycling techniques, gasification and pyrolysis are promising for large-scale applications. Liquefaction and HTC are efficient in producing oil products but present high costs due to the need for high pressures and the utilization of different solvents. Regarding gasification and pyrolysis, there are opportunities to target hydrogen and solid carbon product formation that result in sequestration of the carbon (rather than contributing to increasing GHG emissions) alongside low emission fuels (i.e., hydrogen) production. The use of a catalyst can enable S. Parrilla-Lahoz, S. Mahebadevan, M. Kauta et al. Materials Today Sustainability 20 (2022) 100200 17 greater selectivity toward desired products. Computational chemistry offers a broad toolkit for investigating the mechanistic aspects of catalytic processes and can produce practical solutions to allow rational catalyst design through first-principles approaches. Recent advances in automated systems for reaction path analysis present an opportunity for removing human bias from computational catalysis investigations, leading to potentially faster, more effectively guided experimental efforts [186]. The primary barrier to commercialization is the catalytic thermochemical conversion technologies' cost-competitiveness in a no incentive scenario where the products have to compete with fossil fuel-derived alternatives. Numerous thermochemical conversion technologies may be unable to compete with established petrochemical corporations without substantial subsidies for two reasons: (1) the generated oil has poor economic value, and (2) sorting the mixed feedstock is costly. It is necessary to minimize plastics consumption through a cultural change while also finding technologies to complete the loop by recycling and substituting to accomplish a long-term shift in the system. Our suggested solutions may be used to minimize leakage into the environment by producing high-value goods, although they alone will not be sufficient for sustainability. It is also essential to completely change the way we produce plastics for end uses that consider them a necessity. Designing biodegradable plastics and new pathways for circular use are crucial aspects of ensuring sustainability. Bioplastic innovation is one way to prevent harmful effects of waste leakage to the environment. When evaluating upcycling techniques for existing plastics waste, it is essential to consider the entire life cycle of products, particularly to target products that do not result in oxidation of the carbon contained in plastics to CO 2 . The plastics and climate change issues are interlinked due to the use of fossil-derived carbon in plastics. Thus, it is essential to focus on waste management and valorization technologies that do not simply shift the carbon to the gas phase, thus contributing to climate change. For micro/nanoplastics waste, upcycling to carbon nanomaterials and green energy vectors can be a suitable and sustainable solution since the product downstream use is not expected to contribute to increasing GHG emissions. Moreover, the currently high prices for some carbon nanomaterial products can incentivize the collection and utilization of plastic waste. Due to the global scale of the problem and the rapid pace at which we need progress, ambitious collaboration efforts (i.e., between basic scientific research, engineering development, industries, governments, and computational studies) are needed to accelerate research, develop, and optimize promising processes and create viable solutions for a circular economy. Author contribution S. Parrilla-Lahoz: Writing eoriginal draft, conceptualization, visualization, and investigation. Stephen Mahebadevan: Investigation, writing eoriginal draft. Mbinaye Kauta: Investigation, writing eoriginal draft. Marielis C. Zambrano: Investigation, writing ereview and editing. Joel J. Pawlak: Funding acquisition, supervision, writing ereview and editing. Richard A. Venditti: Funding acquisition, supervision, writing e review and editing. T.R. Reina: Funding acquisition, conceptualization, project administration, and supervision. Melis Duyar: Funding acquisition, supervision, conceptualization, visualization, project administration, and writing ereview and editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments/Financial support Financial support for this work was provided by the University Global Partnership Network (UGPN) Research Collaboration Fund (RCF) and the Chemical and Process Engineering Department at University of Surrey. References [1] S. Nanda, F. Berruti, Thermochemical conversion of plastic waste to fuels: a review, Environ. Chem. Lett. 19 (2021) 123e148, https://doi.org/10.1007/ s10311-020-01094-7. [2] F. Shahul Hamid, M.S. Bhatti, N. Anuar, N. Anuar, P. Mohan, A. 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