Review
A review of potentially harmful chemicals in crumb rubber used in synthetic football pitches

https://doi.org/10.1016/j.jhazmat.2020.124998Get rights and content

Highlights

  • Crumb rubber (CR) from recycling end-of-life tires is used as synthetic turf infill.

  • Potentially hazardous chemicals were reviewed in CR, water leachates and nearby air.

  • 8-carcinogenic PAH levels from 1.91 to 24.67 ± 18.31 mg/kg surpass the legislated limit.

  • Zn was the prevalent metal, up to 15,494 mg/kg in CR and 34,170 μg/L in water leachates.

  • Other contaminants linked to tire making like VOCs, plasticizers or PCBs were found.

Abstract

Recycling end-of-life tires (ELTs) reduces waste and provides a low-cost source of energy and materials such as crumb rubber, used as infill in artificial turf football pitches. However, some concerns were raised and remain about its safety. The potentially toxic human exposure to chemicals such as polycyclic aromatic hydrocarbons (PAHs), metals and others (volatile organic compounds (VOCs), plasticizers, antioxidants and additives) existing in ELTs (and in the resulting crumb rubber) is being studied, with no definitive conclusions. The literature existing so far suggests the possibility of their release from synthetic turf infill into the environment as water leachates and to the air surrounding the pitches, but there is the need of further research, also to assess the contribution of other materials present in synthetic turf. The database available comprised crumb rubber infill studies from pitches in 6 countries (USA, Norway, Netherlands, Portugal, Italy, Spain) and revealed a myriad of hazardous chemicals, with benzo[a]pyrene (n.d.–4.31 ± 3.95 mg/kg) and zinc (n.d.–14150 ± 1344 mg/kg) often exceeding the established limits. A dependence on indoor/outdoor conditions and the age of the source material was evaluated, often showing significative differences. From this standpoint, this review is intended to add knowledge about the presence of contaminants in this recycled material, aiming to ensure the safety of end-users and the environment.

Introduction

The number of vehicles circulating in the road network is increasing as a consequence of societal and technological development. In fact, in the European Union (EU), the passenger car fleet grew 5.7% from 2015 (243 million units) to 2018 (257 million) (EAMA, 2018), contributing to a tire production of 4.9 kton according to the European Tyre & Rubber Manufacturers Association (ETRMA, 2017).

One of the critical problems arising from this evidence is the management of end-of-life tires (ELTs) (Zanetti et al., 2015), that after being retreaded or regrooved can no longer be used on vehicles. The bulk of ELTs come from car and truck tires, and the world’s greatest generator is China, with 10,260 kton in 2015, followed by the United States with 3581 kton and the EU with 3190 kton (an increase from 2883 kton in 2013) (WBCSD, 2008). In Europe, the main reuse routes of ELTs are rubber granulates and powder (41%) and cement kilns (37%) (WBCSD, 2018).

Manufactured by a process of rubber vulcanization which culminates in the generation of several reaction products, tires are generally constituted by 40–60% of rubber polymer (30% synthetic and 20% natural), 20–35% of reinforcing agents (e.g. carbon black and silica), 15–20% of aromatic extender oils and a smaller percentage of vulcanization additives, antioxidants and processing aids (e.g. steel cord, steel bead wire, polyester and nylon) (ECHA, 2017, Li et al., 2010). Truck tires usually have a higher natural rubber content then passenger car tires (WBCSD, 2008). Some of these components and chemicals may be lost during their lifetime, (and some others acquired from the contact to asphalt, for instance), but many are still present in ELTs. And so, tires can be a hazard to the environment as most of them are simply dumped (legally or illegally), generating long-lasting potentially toxic waste piles in landfills (Lee and Yoo, 2011, Llompart et al., 2013).

The most used process to address this issue has been tire incineration for energy recovery/production (McKee, 2015, Watterson, 2017), as fuel for cement kilns or asphalt concrete (Siddique and Naik, 2004). Pyrolysis is one of the most effective and sustainable ways for recycling, due to its eco-friendly process (Sagar et al., 2018). However, burning tires can be harmful since the fumes emitted incorporate many toxic chemicals such as benzene, metals (mainly lead), benzo[a]pyrene, butadiene and styrene. Moreover, dioxins and furans are also released due to the chlorine content in tires, highlighting the need of appropriate facilities able to mitigate these negative effects (Laboy-Nieves, 2014).

In the interest of a sustainable waste management to potentiate the added value of circular economy and a healthier environment, safer alternatives were sought. One of them aims the recycling of the main component of tires: rubber. But being a chemically cross-linked polymer, it cannot be molten, solubilized or reshaped without considerable degradation of its properties (Laresgoiti et al., 2004). A valid option is the reduction to granules of different sizes, commonly named “crumb rubber” (Jim, 2017). Crumb rubber is usually produced in dedicated factories, using two main processes: mechanic and cryogenic. In the first one, the tire is ground repeatedly at ambient temperature until the desired size (ca. 0.3 mm). During this process, steel and textile parts are removed with a separator and electromagnetic equipment, respectively (Sunthonpagasit and Duffey, 2004). On the other hand, the cryogenic method uses liquid nitrogen to freeze partially shredded tires until − 80 to − 100 °C before crushing them into smaller particles (in the order of 50 µm) in a hammer mill (Liang and Hao, 2000, Watterson, 2017). The material produced has an apparent advantage over the mechanic option: absence of thermal and chemically degradation, i.e., the molecular chains of the rubber polymers are not destroyed. Consequently, those granules have a highly elastic behavior (Gomes et al., 2010), and a finer and brittle crumb powder represents a highly value-added material (Wang et al., 2019). Recycled crumb rubber is used in different applications, of which 43% in synthetic turf (including infill). About 45% are employed in other sport surfaces (school sports areas, athletics tracks, tennis and basketball courts and recreation areas), geotechnical/asphalt applications, shock-absorbing pavements (for instance in children playgrounds), molded tuber goods, and bumpers (floor mats or liners for cars and trucks) (ECHA, 2017). The remaining portion (≈10%) appears in shock absorbing pads for rails and machinery, adhesives and sealing compounds, pipe insulations and linins, among other uses. This review focuses on the main application of crumb rubber: synthetic football pitches.

Current artificial turf surfaces are comprised of three main components: the backing, an infill system (commonly consisting of silica sand and cryogenic/mechanical rubber granulates) and turf fibers (Wannop et al., 2017). Crumb rubber is used as infill material and not as pile (grass substitute), mimicking the mineral soil substrate of natural turf and supporting the pile blades (Jim, 2017, Watterson, 2017). It can be used exclusively or in a mixture with sand or alternative infills (EPA, 2019), reducing soil hardness and shear strength, as well as the water content (Isayev, 2005). The first well-publicized use of crumb rubber as infill was AstroTurf, a synthetic turf installed on the Houston Astrodome in 1966. This first generation was basically a short pile carpet with a foam backing (Daines, 2008). With time, new concepts of synthetic turf fields were developed, for instance with a grave/stone base to promote the drainage and fiber blades of polyethylene attached to a support material over the base, composed by multi-layered polypropylene and urethane (EPA, 2019). Although natural turf is generally preferred for official and amateur sports competitions, synthetic turf brings advantages that make it a well-spread alternative. Consistency of the surface, faster installation, lower maintenance costs, reduced water, fertilizer and pesticide use, higher resistance to rain, snow or actually every climate and weather conditions are some of the features that favor synthetic turf, ultimately providing an extended number of playable hours (McKee, 2015, Simon, 2010). Of all the materials that may form the synthetic pitches, researchers have been focusing their attention on crumb rubber (as does this review), as it is deemed as the material that can potentially cause the most impact to the environment and human health due to the chemicals in its constitution (Perkins et al., 2019).

A football pitch needs between 20,000 and 40,000 tires to create enough crumb rubber infill and there is a need to refill the granules periodically due to the degradation caused by users or wash off from the pitch (Watterson, 2017, Zhang et al., 2008). It is estimated that 95% of the synthetic pitches use a recycled rubber infill (EPA, 2019). The Fédération Internationale de Football Association (FIFA), which issues certification of artificial pitches (under ISO 17025) for several levels of international competitions, reported 3437 certified pitches worldwide from 2006 to 2016 (UK, Holland and Turkey being the predominant countries), with an exponential growth from 2010 onwards (FIFA, 2017). But this only represents a small fraction of all pitches installed. ~The European Synthetic Turf Organization (ESTO) indicates that nowadays about 1200–1400 new football pitches are installed (new or as replacing old ones) each year in the EU. Additionally, in 2016, over 13,000 synthetic turf pitches and 47,000 mini pitches used for football practices existed in the EU and this number will increase to 21,000 and 72,000, respectively, which represents a growing business with considerable revenues (ECHA, 2017). But there are some issues open for discussion that needs a thorough evaluation. The remaining synthetic pitches (5%) use mainly infills of natural products such as cork or coir, a coconut-derived material (Watterson, 2017). The latter products are seemingly more environmentally friendly but, for instance, cork-based crumb can be 4 times more expensive per cubic meter. In fact, in 2017 according to the Finnish Football Association (FFA), the prices of the alternative infill materials compared to crumb rubber (EUR 500/ton, i.e., cost per field EUR 22500 – EUR 50000) were the following: EPDM-TPO-TPE-granules approximately EUR 2000/ton, i.e., per field EUR 90000 – EUR 200000; Granulated cork approximately EUR 1500/ton, i.e., per field EUR 67500 – EUR 150000 (ECHA, 2017).

In terms of composition, the materials that comprise tire rubber are known to include potentially harmful compounds such as polycyclic aromatic hydrocarbons (PAHs), metals, phthalates, volatile organic hydrocarbons (VOCs) and other semi-volatile organic hydrocarbons (SVOCs) (ECHA, 2017, Simon, 2010). Thus, it is likely that most of these compounds are present in crumb rubber (Li et al., 2010). The origin of PAHs - a large class of organic compounds composed of two or more fused aromatic rings and formed through the release or incomplete combustion of fossil fuels especially during the pyrolysis process - in crumb rubber is mainly due to the presence of a highly aromatic oil used as an additive in the production of tires (ECHA, 2017, Zhang et al., 2008). Their presence in crumb rubber can be variable and depends on the processes followed from the manufacturing to the recycling of the tires. Other parameters such as granulometry (Plesser and Lund, 2004), coatings (Gomes et al., 2010, Menichini et al., 2011) and the age of the pitch (Marsili et al., 2015, Ruffino et al., 2013, Zhang et al., 2008) were reported as potentially influencing the levels of PAHs in crumb from the infill.

Concerning metals, zinc appears as the most prevalent element. It derives from zinc oxide, a vulcanization aid used during rubber production (Iraola-Arregui et al., 2018). The presence of these chemicals in crumb rubber used in sports facilities can be a problem per se, but also because they can be transferred into the environment and organisms and ultimately have a negative effect in human health. One of the problems associated with synthetic turf football pitches is caused by rainwater and watering for maintenance purposes, which creates a leachate that may be an input of metals and hazardous organic compounds from the crumb rubber onto the receiving sewage waters, groundwater and/or natural surface waters. This can constitute a negative impact to drinking water and surface water resources via groundwater recharge (Connecticut Department of Environmental Protection, 2010, Simon, 2010). Moreover, Zn and PAHs were also detected in the leachate from tire rubbers compounds in lab-controlled conditions (San Miguel et al., 2002). The leaching of Zn from crumb rubber was found to be as much as 20 times higher than from agricultural treatment applications (Williams and Maguire, 2015). Lim and Walker (2009) indicated that the leaching potential depends on the rubber type, with truck tires exhibiting the highest leaching potential (Lim and Walker, 2009).

On the other hand, VOCs are more prone to volatilization and can also be released from the crumb rubber, whereas PAHs and other SVOCs have relatively lower vapor pressures and may therefore partition between the atmospheric gas phase and aerosol particles (Chu et al., 2010, Li et al., 2010, Pavilonis et al., 2014). Synthetic turf fields have the capacity to absorb heat, which increases the surface temperature to levels much higher than the surrounding air, hence facilitating the granules-air partition (Daines, 2008). As mentioned previously, regular watering of the synthetic turf is advised not only to maintain the properties of the material but also to prevent excessive temperature rise in the warmer seasons, that can go above 70 °C (Jim, 2017). This entails a potential environmental risk, as the inhalation of these chemicals could be hazardous to users (Simon, 2010). Other examples of potentially toxic compounds found in tires composition are nitrosamines (formed during the vulcanization process), benzothiazoles (accelerators in the vulcanization process) and secondary amines (antioxidants for the rubber) (van Rooij and Jongeneelen, 2010). One important perspective to address in terms of the pollutant load in water leachates from the pitches or the air above them is the possibility that a fraction of those chemicals of concern measured is released from other materials present in the synthetic turf (pile blades, non-rubber infill parts, plastics, bitumen or in some cases the soil where the pitch stands on) (Cheng et al., 2014). The literature in this topic is still limited, but it will also be included in this review.

To this day, there are already some scientific reports and studies that focus on the presence of unsafe compounds in crumb rubber used in synthetic football pitches, although the determination of some of them in the granules may be complicated due to the vulcanization process (ECHA, 2017). But the last reports available from entities closely linked with legislative and or/regulatory bodies (e.g., ECHA, 2017, EPA, 2019) state that there are still considerable gaps deserving a close attention and a thorough research. In fact, while the majority of attention has been paid to the presence of potentially hazardous chemicals in crumb rubber, with a controversy between those who advocate crumb rubber safety and those who raise concerns, little is known about the transfer of those compounds to the surrounding environment (ECHA, 2017, EPA, 2019).

This review intends to be a key instrument for future efforts, providing a complete overview of the concentrations of potentially harmful chemicals such as PAHs, metals, VOCs, SVOCs and other relevant additives and antioxidants found in crumb rubber used as infill of synthetic football pitches. The discussion also includes the levels in the main matrices to where these compounds can be transported after their release from the crumb (air and water leachates), as well as a summary of environmental and human effects of this hazardous material. This effort comprises a comprehensive and unprecedented analysis of all chemicals originating from the tire manufacturing and recycling processes, including their behavior in and around the target facilities: synthetic turf football grounds.

Section snippets

Methodology

This review was constructed with a judicious investigation of the scientific publications (including journal articles and technical reports) concerning studies of the levels of potentially hazardous chemicals in crumb rubber used in synthetic football fields. It was possible to collect documents on this subject published between 2004 and 2020. This literature search was done in the available databases: Scopus®, Elsevier®, ScienceDirect®, Springer® and Google® Scholar, using the keywords

Chemicals in crumb rubber/water leachates/volatiles from synthetic turf football pitches

In literature there is a general acknowledgment that most chemicals of concern present in the rubber employed in tires will end up in the crumb rubber (Re Depaolini et al., 2017, Diekmann et al., 2019). And depending on the use of this recycled material, the release of those compounds to the environment may occur in several forms (Perkins et al., 2019). Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 show the levels of chemicals such as PAHs, metals and other organic

Human health concerns

When so many compounds with potentially harmful effects are present in a matrix employed in public facilities, it is mandatory to assess the health risks that may be associated with its use. In synthetic fields the people more likely to be in contact with crumb rubber include children and adults playing football and workers in charge of their installation or maintenance (ECHA, 2017).

Regarding regulatory aspects, there is a debate if the definition of a specific concentration limit for rubber

Conclusions

This review provided crucial and comprehensive information about the levels of potentially harmful chemicals such as polycyclic aromatic hydrocarbons (PAHs) metals, plasticizers and other additives and antioxidants commonly present in crumb rubber (from ELTs) acting as infill of synthetic football pitches. From the studies in literature, it was proved that these chemicals may also be released from the crumb rubber, namely: (i) volatilizing into the air and (ii) leaching into water in contact

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

This work was also financially supported by: (i) Project UID/EQU/00511/2019 - Laboratory for Process Engineering, Environment, Biotechnology and Energy – LEPABE funded by National funds through FCT/MCTES (PIDDAC); (ii) Project 01-0145-FEDER-028101 - SAFEGOAL - Safer Synthetic Turf Pitches with Infill of Rubber Crumb from Recycled Tires, funded by FEDER funds through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) and by national funds (PIDDAC) through FCT/MCTES.

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