The sources of rubber waste can vary by region, generally the largest source is used or discarded tires that contain vulcanized rubber, typically making up to 50% of total rubber waste. Other major sources are rubber waste from automotive and conveyor belt manufacturing industries, as well as municipal solid wastes, such as clothing and shoe soles (Cummins, 2019). Based on these sources, waste rubber types range from natural rubber (NR) to synthetic rubbers such as styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), ethylene-propylene-diene monomer (EPDM), polyurethane (PU) and silicone rubber. All these rubber products and materials are a challenging source of waste and realizing a suitable and sustainable way to reuse waste rubber has been a global challenge for many years. Traditionally, waste rubber, i.e., end-of-life (EOL) tires, were dumped on landfills where they can form a hazardous ecosystem as potentially harmful substances used during the manufacture of tires, such as antioxidants and antiozonants [e.g., butylated hydroxyanisole, BHA and butylated hydroxytoluene, BHT (Armada et al., 2022) as well as 6PPD, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (Hu et al., 2022)], as well as heavy metals can leach into the groundwater. Additionally, these dumped tires can act as a breeding ground for mosquitos, forming a further threat for the environment and human health (Yee, 2008; González et al., 2020). Given the lack of suitable recycling options, and regulation, huge landfill sites appeared and developed over the past centuries. Currently, the EU scraps a total of 3.6 million tons of worn tires per year, with global numbers reaching one billion tons (Cummins, 2019). An additional environmental problem emerged, with tire stockpiles forming inextinguishable fire hazards that have excellent properties to sustain combustion, and once ignited, burning thousands and millions of tires, as for example happened regularly in the US in the 1980s and 90s and more recently in Seseña, Spain in 2016 and in the world's largest tire dump in Sulaibiya, Kuwait in 2020. Rubber tires though do not easily ignite since rubber does not conduct heat very well and ignition therefore requires prolonged periods at very high temperature, and indeed a number of these accidents were caused by either very poor management and/or malfunctions at tire collection and destruction sites, or by arsonists. These fires typically last for a long time, from several days to months, a fire involving 10 million tires in Heyope, Wales began in 1989 and still smoldered 15 years later as the tires were packed too densely for firefighters to extinguish. All these fires caused significant damage to local ecosystems, including contamination of residential drinking water wells and contaminating soils with carbon black, as well as releasing harmful greenhouse gases including carbon dioxide, carbon monoxide and sulfur oxides into the atmosphere (Downard et al., 2015; Singh et al., 2015). As a result of these increasing problems associated with EOL tires, the United Nations Environmental Programme (UNEP) recognizes that waste tires are becoming a burgeoning problem for national and local governments to ensure a sustainable management of solid waste (Jain, 2016). Until around 20 years ago, up to and partially more than 50% of EOL tires were either dumped illegally or disposed in landfill sites. Legislation started to act upon that, with e.g., stockpiling of whole tires in landfills being banned in the EU in 2003 and shredded tires in 2006 according to the EU landfill directive 1999/31/EC. However, because of the lack of suitable reuse options for waste rubber, these large landfill sites partially still remain. It is estimated that there are currently 4 billion such tires in landfills and stockpiles worldwide (Cummins, 2019).

One problem of reusing or recycling waste rubber, apart from its immense quantity, is that rubber is a thermosetting polymeric material that unlike thermoplastic materials, cannot be remolded by simply reheating it. The sulfur that is used to crosslink rubber polymers during vulcanization, creates very strong chemical bonds and products that do not easily decompose. Various methods have been suggested to break these bonds and allow to revulcanise rubber by adding fresh sulfur to create new rubber products. An overview of these technologies and their pathways to potential end products are illustrated in Figure 1. Most of these methods, however, do not only break the sulfur crosslinks but also the carbon bonds. This typically leads to low value rubber products, e.g., rubber mats for playgrounds or roofing materials, where the initial physical characteristics of the rubber get destroyed, at least partially. To allow the reuse of rubber for high value products, such as rubber tires, the sulfur crosslinks need to be removed selectively which is a very challenging process. Some of the potential methods and their value for a circular economy will be discussed here with a focus on biotechnological solutions.

The first step in rubber recycling and reuse is collection of rubber waste materials. In 2019, 95% of waste tires were collected and treated for material recycling and energy recovery in Europe, with the remaining 5% either being stocked and waiting for treatment or not being able to be tracked down (European Tyre and Rubber Manufacturers' Association, 2021). Until the 2010's, a substantial amount of these tires, estimated at up to 10% (ETRMA, 2015), was exported, primarily to China, where the tires were burned as an energy source. The introduction of the waste import ban in China in 2018 heavily restricted this disposal option and increased the pressure to find alternative disposal routes. This is currently gaining even more importance as Europe heads toward a waste export ban. Surely, in terms of a circular economy, rubber waste combustion, either via export to other countries outside the EU or within the EU, has never been an ideal solution. It helps to decrease amounts of rubber waste, however, can lead to a production of greenhouse gases such as carbon dioxide and sulfur oxides if not handled and managed correctly. Also, the calorific value of burning a tire is approximately only 25%−30% of the energy needed to produce a tire (Snyder, 1998), therefore not very energy efficient. Regardless, in Europe close to 1.5 million tires are being burned every year (European Tyre and Rubber Manufacturers' Association, 2021), primarily in cement production where shredded or whole tires are added to the slurry as it enters the cement kiln. As waste rubber tires form a cheap alternative to the usual heat sources, typically oil, gas or petroleum coke, they are a welcomed addition to the heat process by most cement manufacturers. A recent economic and environmental assessment of using waste tires as fuel in cement clinker production reported a cost reduction of around 25%, when a combination of 40% scrap tires and 60% petroleum coke was used instead of 100% coke (Castañón et al., 2021). Additionally, in the same study and using the same test conditions, a reduction of 17% NO_x, 28% SO₂ and up to 60% CO at the exit of the kiln was observed, showing that there is also potential environmental benefit when cement kilns are operated efficiently. There is another technological advantage of using waste tires as fuel in cement kilns, as it is not necessary to remove the metal reinforcements in the tires prior to incineration as they can partially replace the iron additions used as a flux in the composition of cement raw mix (Castañón et al., 2021). Further research is required, however, to fully determine all the potential environmental implications of using tires as fuel and under which conditions EU limits, e.g., 500 mg/Nm³ NO_x and 50 mg/Nm³ SO₂ according to Directive 2010/75/EU for cements kilns co-incinerating waste, can be met.

The economic viability of utilizing waste tires in cement kilns depends on a number of factors, such as the ability to obtain gate or tipping fees. Although alternative fuels, e.g., plastic waste, may lead to greater competition, the comparable high calorific value of tires make them an appealing option.

From a sustainability perspective, environmental benefits depend on the original heat source being used. Industries can transition from fossil fuel to waste tires for a share of their energy demand and generate less greenhouse gas or other polluting emissions. Alternative fuels, such as waste tires, can therefore be a more sustainable option to decrease air pollution.

Another traditional disposal route for used tires is material recovery, in particular crumbing and reusing the resulting tire crumb as infill in compounds for road, sports, and playground surfaces. In Europe, around every third EOL tire is currently being used for that recycling route (Cummins, 2019). Scrap tire rubber can be for example incorporated into asphalt paving mixes using two different methods, a wet and a dry process. In the wet process, crumb rubber typically acts as an asphalt cement modifier to improve the rheological properties of asphalt binder. while in the dry process, crumb rubber is used to modify bitumen or as a partial substitute for aggregates (Lim et al., 2021; Riekstins et al., 2021; Hussein et al., 2022; Li et al., 2022). Both technologies can play an important role for the circular economy by reducing resource consumption and by alleviating environmental pollution, however, their application is part of a long ongoing debate around the safety of crumbed rubber and the potential release of carcinogenic substances used during the tire manufacture, such as antioxidants and antiozonants as well as polycyclic aromatic hydrocarbons (PAHs), into the environment. Over the last decade numerous articles concluded that crumbed rubber infill poses no threat to human health (Simcox et al., 2011; Pavilonis et al., 2014; Oomen and De Groot, 2017; Schneider et al., 2020a,b,c), but as the concerns remain and as crumbed rubber is part of the microplastic classification, it is likely only a question of time for this application to be restricted or even fully banned in the EU. Indeed, after long discussions the European Commission and the EU member states decided to prohibit the use of granules and mulches as infill if they exceed 20 mg/kg PAHs to make playing on artificial sport pitches and playgrounds safer. Recent studies, however, showed over-standard content of PAHs in rubber crumb in a large number of samples (Armada et al., 2022; Grynkiewicz-Bylina et al., 2022). Furthermore, the European Chemical Agency (ECHA) also proposed wide-ranging restrictions on intentional uses of microplastics. Indeed, on 25th September 2023, the European Commission announced to adopt measures that restrict microplastics intentionally added to products under the EU chemical legislation REACH (European Commission, 2023). An international policy review on the current state of regulations for artificial turf and rubber crumb infill has also been published very recently by Zuccaro et al. (2022). If this market is being completely ruled out in the future, however, current recycling alternatives would basically be incineration or illegal landfill, or exports as described above. Therefore, additional recovery technologies for waste rubber have been developed over the last years, including its use in construction materials such as cementitious concrete, asphalt concrete and granular materials for earth structure. Various studies have shown advantageous properties when rubber crumb was mixed into the compounds, as recently reviewed by Mushunje et al. (2018), Mohajerani et al. (2020), and Singh et al. (2023).

As the granulation of used tires requires advanced treatment and processing steps, the financial investment for granulation and its applications can generally be higher than for other recycling methods. In addition, some markets for ground rubber applications, such as use in artificial sports fields, have already declined due to the ongoing debate about the possible release of carcinogenic substances during tire production. Recent measures by the European Commission, such as the restriction of intentionally added microplastics to products, will further hinder this application.

Granulation is considered material recycling and is therefore high up in the hierarchy of rubber waste management. However, the above-mentioned uncertainty about potential health and environmental hazards associated with certain applications of rubber granules will most likely further limit this recovery method in the future.

Beside energy and material recovery, a third recovery option for waste tires is oil recovery, also known as pyrolysis. Here, EOL tires are decomposed into oil, gas, steel, and char using high temperatures and pressure under anoxic conditions. Though this process traditionally only had minor importance with < 1% of waste tires being used to extract oil and gas (ETRMA, 2017), this technology has gained traction more recently with large chemical companies investing substantial resources to make this process more cost-effective and process oil and gas from EOL tires a lucrative alternative to usual oil and gas. Pyrolysis also generates valuable by-products such as carbon black that can be blended into various materials, e.g., plastic and ethylene-vinyl acetate (EVA) foam. The potential of waste tire pyrolysis has been reviewed in the past (Martínez et al., 2013; Williams, 2013) and also again more recently (Dick et al., 2020; Hoang et al., 2020; Campuzano et al., 2023).

All products of the pyrolysis process, including high-quality carbon black, oil, syngas as well as residual steel have various potential applications. Currently, however, the economic viability of pyrolysis is still quite low in relation to virgin or traditional materials. However, with increasing oil and gas prices and the increasing level of technical feasibility, pyrolysis might become more important in the near future. Indeed, particularly the development of tire-derived material carbon black is in the process of commercialization by a number of European companies.

Still, from an environmental point of view, pyrolysis can cause significant air pollution if environmental standards are not upheld, therefore high environmental standards with continuous monitoring of emissions are crucial while developing this technology further.

As an alternative to the recovery options described above, the lifetime of some tires can potentially be extended with a process called retreading. Here, the waste tire is stripped off its tread and replaced by a new one. Truck tires for examples are typically retreaded up to three times which can be very profitable as it only requires 30% of the energy and 25% of the raw materials needed in comparison to produce new tires (Sienkiewicz et al., 2012), therefore also significantly reducing the amount of waste tires pilling up in the environment (Dabić-Ostojić et al., 2014; Ounsaneha et al., 2020). Since life cycle studies in the literature for tires are rare, it is very difficult to fully assess the environmental benefits from retreading. One such attempt was made by Gutowski et al. (2011), they calculated a 68% energy saving for a retreaded heavy-duty truck tire (compared to the production of a new one), using life cycle inventory data for the materials and manufacturing phase only. However, they also pointed out that, if the use phase was also considered, depending on the increased rolling resistance of a retreaded tire as well as its reduced lifetime, energy savings reduce to 28% or even become unfavorable if the rolling resistance fraction is larger (depending on driving behavior), e.g., 33%, and expected lifetime is less.

Waste tires can also be repurposed and directly recycled retaining their original form in several civil engineering applications such as garden planters, playground equipment, outdoor furniture, road embankments, erosion barriers, artificial reefs and many others (Sienkiewicz et al., 2012). Most of these applications, however, are not widely practiced currently because of the potential negative impacts of leachates on the environment.

Though retreading is in line with the circular economy and to expand the lifetime of products, it is currently only an established practice for truck and aircraft tires but not for automobile tires because of economic profitability. The market for civil engineering applications of whole or shredded tires also remains small, however, these applications often offer economic advantages over traditional materials. Additionally, using waste tires in civil engineering can create products with a high added value due to the advantageous technical properties of rubber tires.

Despite some potential improvements in environmental impact compared to alternative construction materials, such as the use of stone, gravel and sand, the overall performance of such applications in construction can be considered lower than other recycling routes for used tires due to the materials they replace.

In terms of the waste hierarchy of the circular economy, if the reuse of materials is not possible, recycling is the next desirable option. For both routes, reuse and recycling, a material should be reused for its originally intended purpose. In the case of waste tires, recycling would mean the use of recycled tire rubber as a compounding ingredient for new tires. Multiple recycling technologies have emerged over the past years but many fall short for this desired recycling goal. As mentioned before, to recycle rubber crumb, the S–S and C–S crosslinks between the polymer chains of the rubber need to be broken in a process called desulfurization (see Figure 2). Most technologies developed to date achieve this to some extent but also break the carbon chain bonds, resulting in poor physical properties of the recycled rubber crumb and therefore limiting its potential applications and restricting it to low-value rubber products. This process is called reclamation and uses mechanical, thermal energy and/or chemicals to convert vulcanized scrap rubber into a reusable rubber material that can be mixed, processed and vulcanized again (Isayev, 2001). The following provides a short overview of the key reclamation methods developed to date and their current application potential.

Though some of the above presented technologies show desulfurization rates close to 100% and therefore claim to demonstrate complete devulcanisation, their value to the circular economy and the global rubber market and its applications remains limited because of the concurrent degradation of the rubber polymer and the resulting poor mechanical properties of the reclaimed rubber. For a general comparison of the pros and cons of the different techniques in rubber waste management and their main applications, see Table 1.

To end the waste rubber problem, the focus therefore needs to shift from recycling to the reuse of rubber waste as a valuable material for high-value products. As mentioned before, reusing waste rubber crumb for these applications requires to break the S–S and C–S crosslinks between the rubber polymer chains while keeping the C–C bonds intact (see Figure 2). This can only be achieved by “true” devulcanisation but not by reclamation. To date the biggest potential to achieve devulcanisation without degrading the rubber polymer has been demonstrated using biotechnological methods using specific microorganisms and desulfurization enzymes. Several key parameters are crucial for a successful and economically feasible devulcanisation.

These studies all show substantial promise that microbial desulfurization of waste tire crumb could be a sustainable approach to deal with the waste tire problem in future, particularly in accordance with shifting to a circular economy where materials are (re)used for their intended purpose and no waste is created. Here, microbial devulcanisation, the only true devulcanisation approach that allows to reuse the waste rubber materials for new high-value rubber products, clearly surpasses alternative recycling routes. For a successful commercialization, a few challenges still must be overcome, however. Our understanding of microbial devulcanisation, e.g., the key enzymes involved and potential feedback inhibition by sulfate released into culture media, need to be studied further to optimize the process. Also, factors that could impact desulfurization rates such as mesh size, GTR amount, required sterility of crumb and bacterial loading, as well as optimal process conditions (media composition, incubation length, temperature, etc.) need to be investigated and determined. For scaling-up biotechnological devulcanisation processes, additional challenges will occur. Given the hydrophobic nature of rubber crumb, mixing will be an important factor for any process using GTR. Efficient mixing will be key, not only to guarantee product homogeneity but also to supply sufficient oxygen levels for aerobic devulcanisation processes. Finally, research gaps in assessing environmental and economic impacts need to be closed for the various recycling options before a wider commercialization.

Overcoming these hurdles will require more in-depth research. Establishing a biotechnological approach to tackle the waste tire problem, however, seems to be within reach over the next few years. Their development should be a major focus now for waste rubber management and perhaps the only possible solution, at least in terms of a circular economy, to counteract the increasing pressure waste tires put on our environment.

DG: Writing – original draft.