Application of material from used car tyres in geotechnics—an environmental impact analysis

Introduction

Globally, the rubber and car industries generates hundreds of millions of used tyres each year. That mass is foreseen to increase steadily, at a rate in line with the increase in the number of motor vehicles. In the EU as a whole, 2017 saw around 3.4 million tons of used tyres of passenger cars generated, while figures for Poland include 275,000 tonnes (along with 200,000 tonnes of truck tyres) (Central Statistical Office, 2017). Similar data on the production of passenger car tyres in Poland are available from the European tyre and Rubber Manufacturers Association—ETRMA. The conclusion (ETRMA, 2017) would thus be that 79% of the tyres produced in Poland are recycled or recovered, with 42% becoming recycled material (granulation tyres) or retread, along with 29% going for energy recovery (burning in cement kilns) or energy recycling (pyrolysis). The fact that the remaining 21% of spent tyres go unused must represent great potential for the tyres to gain use in other parts of the economy, e.g., in civil engineering. Rubber material from tyre-recycling (in fine or granulated form) does gain successful use in road construction, where the application is to modify the composition of asphalt (Liu, Cai & Liu, 2018). The bituminous surfaces arising from this activity are demonstrably more durable and more flexible, of lesser luminance, and of greater resistance to abrasion. They also achieve greater roughness, while serving to reduce the level of noise arising as tyres still in use on vehicles make contact.

In addition, rubber also gains use in the manufacture of clothing, car mats, mats for farm animals, wipers, playground surfaces, and so on (Duda et al., 2016). However, at present there are no examples in Poland of such pro-ecological solutions as direct tyre-recycling processes, whereby waste material processed to only a limited extent generates products of new and desired features. In countries such as the USA, France, Japan, the UK and Spain, as well as the Scandinavian countries, rubber aggregate in the form of shreds and chips is most often used as filler in earth and engineering constructions (embankments, retaining structures, backfill of abutments or drainage infrastructure). Similar applications appear where the combined form of the tyre bale is made use of. The first key advantage of such limited either fragmentation or compression of tyres is the way in which the recycling process then only takes between one-tenth or one-sixteenth as much energy as does the fragmentation of granular tyres, or tyre pyrolysis (Winter, Watts & Johnson, 2006).

The high level of energy consumption inherent in far-reaching recycling of used tyres reflects the complex construction, wherein rubber is augmented by both textile and steel cord. This leaves used car tyres as extremely durable waste not naturally inclined to decompose and resistant to the action of water, various chemicals and extreme temperatures alike. This in turn means a major long-term threat associated with the possibility of fire taking hold at dumps where large numbers of tyres are present, as a result of vandalism (arson), the self-ignition of loose tyres or the role of nature (as with a lightning strike). Table 1 reports on selected cases of fire involving tyres that were being stored, in which the level of environmental hazard is very high indeed, for tyre fires release harmful chemicals capable of polluting air, water and soil.

Currently-applied methods of disposal of tyres do not suffice to guarantee environmental safety, hence the prohibition on the disposal of tyres at conventional landfills. In Poland, provisions of the Act of 11 May 2001 (Journal of Laws, 2001) in force in this regard require that tyre manufacturers recover 75% of the tonnage of manufactured tyres coming off the market, with at least 15% of that taking the form of recycling. As of now, the management of used car tyres in Poland entails material recycling (high energy granulation) and energy recovery (burning tyres in cement kilns). It will be clear from this that used tyres are among types of waste capable of exerting the most-severe environmental impact.

A background to that is the presence in tyres of dozens of different synthetic chemicals intended to confer the right properties, affect the flexibility, strength and durability of the product, and facilitate its processing. Table 2 shows the typical composition of the rubber from which tyres are made. The features determining the performance of a tyre, i.e., its resistance to mechanical damage and road conditions (e.g., as regards water and temperature), are also responsible for the difficulties associated with its redevelopment after use (PAS: 107, 2012; Duda et al., 2016).

More specifically, a significant part of the environmental problem posed by rubber is down to additives, which include vulcanising agents, vulcanisation accelerators, vulcanisation accelerator activators, fillers that confer specific mechanical properties or reduce costs of production, softeners that facilitate processing and improve elasticity at low temperatures, anti-aging substances and substances protecting against fatigue. Rainfall provides for the leaching of these substances out of tyres, which thus constitute a significant source of pollutants exerting a negative impact on the environment. According to Wagner et al. (2018), tires may contain additional substances in amount of 5 to 10 mass %. Preservatives (halogenated cycloalkanes), antioxidants (amines, phenols), desiccants (calcium oxides), plasticizers (aromatic and aliphatic esters), processing aids (mineral oils, peptizers) can be leached from the matrix under favorable conditions.

Polycyclic aromatic hydrocarbons (PAHs) are among the substances washed out of tyres both new and used. Literature data suggested that, in circumstances of a basic pH, the concentrations of polycyclic aromatic hydrocarbons leached from rubber chips may exceed limit values set for drinking water. PAHs can derive from soot, special fillers or residual oil. New car tyres differ from used ones in generating slightly higher concentrations of eluted PAHs (1.2 ppb in new tyres and 0.6 ppb in old ones) (Miller & Chadik, 1993; Water Opcert School, 2020; United States Environmental Protection Agency, 2020). In addition, research by Capolupo et al. (2020) conducted on the elution of various components from car tires in the form of microplastics (grain diameter <1 mm) into freshwater and marine leachates confirm the presence of such substances as benzothiazole, 2(3H)-benzothiazolone and phthalimide. Benzothiazole was found in the highest concentrations in the freshwater and marine leachates (2,313 and 1,460 g/L, respectively). In turn, Hennebert et al. (2014) confirms in its research the possibility of leaching phthalic acid esters (PAE) and polycyclic aromatic hydrocarbons from car tires.

Another group of substances potentially leaching from tyres includes organic compounds containing nitrogen and sulphur in their structure, serving as vulcanising agents, antioxidants and/or gases. Volatile organic compounds are used at the stages during which tyres are manufactured, in order to ensure adequate flexibility and viscosity. However, most VOCs are removed from the tyre as vulcanisation progresses, such that only about 8% of the original total ultimately remains in a tyre (Miller & Chadik, 1993). According to Miller & Chadik (1993) it is possible for there to be leaching from shreds of rubber chemical compounds, such as the aromatics (e.g., ketones) contained in gasoline, as well as carboxylic acids and aniline. According to Gasteiger (2010), during analysis of the leaching of components from car tires into water, aniline was the main identified leaching agent and occurred at levels almost an order of magnitude higher than other identified components. Leaching aniline from tires is also confirmed by Stier, Horgan & Bonos (2020). Hennebert et al. (2014) indicated that the concentration of aniline can reach 5.73 mg/kg in eluates from the shredded tires.

Leachates are also found to contain several other volatile substances, including benzene and methylbenzene. However, no benzene was found in the field leaching test, though the presence of methylbenzene at low concentrations was confirmed. In addition, trimethylbenzene and ethyltoluene were found to be at considerable concentrations. As mentioned above, these processes occur as tyres are in operation on vehicle wheels. However, the appearance of these pollutants in rainwater does not result in high concentrations, on account of the dilution effect.

Literature data (Grefe, 1989; Edil & Bosscher, 1992; Miller & Chadik, 1993; Humphrey & Katz, 1995; Kim, 1995; Selbes, 2009; Turner & Rice, 2010; Hennebert et al., 2014; Selbes et al., 2015; Redondo-Hasselerharm et al., 2018; Liu et al., 2020; Mohajerani et al., 2020) also confirm the presence of elements such as selenium, arsenic, calcium, iron, zinc and chromium in tyres; while silver and mercury were not found. Grefe (1989) reports that zinc, barium, iron and manganese were present in laboratory-modelled processes of leaching. Chromium has also been detected at low concentrations, while those of iron and manganese have both been found to exceed permissible values for drinking water (Water Opcert School, 2020; United States Environmental Protection Agency, 2020).

Kim (1995) presents the results of leaching from rubber chips, with samples taken and tested (filtered or unfiltered) 790 and 830 days on from the onset of the washing process in columns consisting of a mixture of rubber chips and soil or else soil itself. Analyses were performed for such metals as zinc, barium, arsenic, lead and chromium, but concentrations in samples tested were not seen to exceed levels permissible for drinking water. The highest concentration of lead was recorded with unfiltered leaching carried out on a column consisting of a mixture of rubber chips and soil.

Miller & Chadik (1993) also analysed the effect of pH on the quality and level of leaching of chemical compounds from tyres, finding no correlation between pH value and concentrations of leached metal ions. According to the authors, this may reflect the adsorption of metal ions by rubber chips. Other studies (Liu, Mead & Stacer, 1998; Mohajerani et al., 2020) have shown that metals are leached from rubber materials at the highest concentrations where prevailing conditions are acidic. On the other hand, PAHs and petroleum hydrocarbons are leached from rubber materials in highest concentrations where pH conditions are basic. The Environment Agency Regulatory Position Statement 085 indicates that tire bales should not be used below the water level in Source 1 or 2 Conservation Zones or in very acidic environments (peat bogs etc.) where the soil pH is 5 or less, in order to reduce risk of leaching. However, the latest scientific reports include Mohajerani et al. (2020) still indicate the need for further comprehensive research on the leaching of toxic heavy metals and other substances from used rubber products. Indicates the need to test leachate at various pH values and the ratio of liquids to solids.

At present tyre bales are being successfully used in mainly civil engineering applications, in landfill construction where innovation is less constrained than in other sectors of the construction industry, and in unpaved roads (Winter, Watts & Johnson, 2006; Simm, Winter & Waite, 2008). It is rubber material from tyres recycled in the form of bales that is best suited to civil-engineering applications, especially in transport infrastructure, geotechnics and hydrotechnics.

The Faculty of Civil and Environmental Engineering and Architecture of Rzeszow University of Technology has been joined by executive and design companies within the framework of the ReUse research project aimed at developing and implementing an innovative, cheap and environmentally-friendly building material recycled from waste in the form of tyre bales. Engineering constructions filled with tyre bales are exposed to aggressive rainwater, potentially causing leaching of organic and/or inorganic compounds from the tyres generally considered hazardous to the environment and people.

Intact tyres are less prone to leaching than shredded tyre rubber. The leaching research indicates that the available leaching surface on the tyre material is an important leaching factor, especially for zinc and PAH. Decreasing the available surface by using larger fractions of tyre shreds is favourable in an environmental point of view (Edeskär, 2006; Selbes et al., 2015). Large size tire shreds can be also an economical alternative compared to the small size tire shreds in the construction of the tire shred embankment (Khan & Shalaby, 2002). In addidion, Birkholz, Belton & Guidotti (2003) showed that fresh shredded tyre produced a moderate toxic threat to aquatic species, if run-off was not diluted. However, the danger receded as the material aged. However, according to Gualtieri et al. (2005) the growth of tire particles used in the sample does not correlate directly with Zn leaching, because particle aggregation may occur, thus limiting the surface area of exposed granular material. Laboratory tests of heavy metal leaching from new and used tires were also conducted by Fenner & Clarke (2003). Used tires have been exposed to the aquatic environment for thirteen years. The results show that the factor limiting their use may be the release of cadmium and leaching of vulcanizing chemicals. It has been found that tires can be used sustainably in a river or coastal environment, provided that local conditions at the construction site are carefully considered. The purpose of the research carried out and described in this article has been to determine whether tyre bales cause pollution of rainwater, and then groundwater and soil; as well as what level of leaching of undesirable substances takes place.

Materials & Methods

Tyre bales

The pressing and packing of tyres has represented a response to the obligation that the input of tyre-related components be reduced, also given the increasing risk that the tyres present at dumps might self-ignite. Tyre bales considerable potential for use in construction, particularly where their low density and ease of handling give them an advantage. Tyre bales used in the experiment comprised 135 whole waste passenger-vehicle tyres compressed into a lightweight block with the weight of around 1,000 kg and the density of circa 500 kg/m3. Each bale contained only one type and size of tyres. The bales were produced according to PAS 108 (PAS: 108, 2007) in a tyre baler capable of compressing whole tyres to reach the density not lower than 420 kg/m3. The bales measured approximately 1.30 ×2.05 ×0.75 m and were secured by six galvanized steel tie wires running around the length and depth of the bale (Fig. 1). Apart from tyre bale dimensions -slightly modified to facilitate transport - the remaining PAS 108 (PAS: 108, 2007) production requirements, i.e., compressing force, density (mass/volume ratio) and number and tension of steel tie wires, were fulfilled.

The impact of tyre bales on water quality

The environmental testing of tyre bales proceeded in stages. Stage I saw the tyres from tyre bales washed with tap water (WW) (Fig. 2), with both the collected leachate (M) and inputting water made the subject of physico-chemical analyses. The used tyres involved were obtained from a tyre store and not cleaned preliminarily in any way. In Stage II of the testing, washed tyre bales were placed in two separate pools (Fig. 3), i.e., one filled with tap water at pH 8.71 and a second acidified to pH 3.87. The goal was for the first pool to mirror conditions in which rain and surface/ground water is present, while the second simulated situations of acid rain.

The tyre bales were left immersed for 120 days, with sampling on days 0, 21, 68, 99 and 120. Based on the reviewed literature and previous experience in site contamination, factors that may affect the rate of leaching and/or the concentration of tyre leachate compounds in soil, surface water and groundwater also include contact time with water. The longer the tyres are in contact with water, the greater the risk of groundwater contamination. In our research, the experiment lasted up to 120 days due to the durability of the tires. Also, the half-life times of plastics can reach several dozen days, for example in marine water—60 days, in soil -120 days, in fresh or estuarine water—40 days, in marine sediment—180 days, in fresh or estuarine sediment—120 days (European Commission, REACH Online, 2007; Muniyasamy et al., 2019). The purpose of this experiment was to make sure that the effect of prolonged contact of tires with water would not be leaching harmful components from the tires. Due to the fact that no significant changes in water composition were observed after 120 days, it was decided not to continue the experiment any longer. All the experiments were done in duplicated, with an observed deviation of less than 5%. Additional effluent, formed in the sludge-well located at the bottom of the test setup (Fig. 4)—as intended for mechanical tests of backfill pressure distribution—was also analysed (Duda & Siwowski, 2020a; Duda & Siwowski, 2020b). Samples there were taken after the first flushing of tyres, and on the last day of the trial.

The scope of the physico-chemical analyses of generated waters and wastewaters reflected known technological parameters of tyres, information in the literature, and indicators listed in the EU’s Water Framework Directive. Results were in particular set against provisions in the Polish law implementing the Directive, as well as Directive 2010/75/EU of the European Parliament and of the Council of November 24, 2010 on industrial emissions (integrated pollution prevention and control), as recast.

Where the Polish domestic law was concerned, reference was made to “Regulation 1”, i.e., the Regulation of the Minister of the Maritime Economy and Inland Navigation of 12 July 2019 on substances particularly harmful to the aquatic environment and conditions to be met when introducing sewage into waters or into the ground, as well as when discharging rainwater or snowmelt into waters or for water equipment (the Official Journal of Laws of 2019, item 1311); as well as “Regulation 2”, i.e., the Regulation of the Minister of the Maritime Economy and Inland Navigation of 29 August 2019 on the requirements to be met by surface waters used to supply the population with water intended for human consumption (the Official Journal of Laws of 2019, item 1747)

The comparison of the results obtained with the standards arising from “Regulation 1” is of decisive importance where any further treatment of effluent arising is concerned, while the comparison with standards laid down in “Regulation 2” represents auxiliary information in this regard. The scope and characteristics of methodologies involved in the physico-chemical analysis are as summarised in Table 3.

Table 3:

The scope of research and research methodologies used.

Σ16PAHs: naphthalene (Na), acenaphthylene (Acy), acenaphthene (Ace), fluoren (Flu), phenanthrene (Fen), anthracene (An), fluoranthene (Fl), pyrene (Pir), benzo [a] anthracene (BaA), chrysene (Ch), benzo [b] fluoranthene (BbF), benzo [k] fluoranthene (BkF), benzo [a] pyrene (BaP), indeno [1,2,3-cd] pyrene (IP), dibenzo [ah] anthracene (DBA), benzo [ghi] perylene (BghiP).

Parameter	Unit	Method	Method details
Reaction	pH	In situ meter	pH-meter, MultiLine P4
Temperature	°C	In situ meter	HQ30D Digital single channel multimeter
Dissolved Oxygen	mg L−1	In situ meter	HQ30D Digital single channel multimeter
Conductivity in 20° C	µS cm−1	In situ meter	Conductivity meter, Elmetron IP 67
Total suspension	mg L−1	PN-EN-872:2007	Samples volume: 100 mL
Solutes	mg L−1	PN-78/C-04541	Samples volume: 100 mL
Total Organic Carbon	mg L−1	PN-EN 1484:1999	Samples volume: 20 mL TOC/N analyzer
Total Nitrogen	mg L−1	PN-EN ISO 11905-1:2001	Samples volume: 20 mL TOC/N analyzer
NH4+, NO3−, Cl−, PO43−, SO43−, Na+, K+, Mg2+, Ca2+,	mg L−1	PN-EN ISO 14911:2002	Samples volume: 100 mL Ion chromatography analysis
Dibutyl phthalate, Bis(2-ethylhexyl) phtalate	mg L−1	PN-EN ISO 18856	Samples volume: 100 mL SPE extraction GC/MS chromatography analysis
Σ16PAH’s	mg L−1	PN-ISO 18287	Samples volume: 100 mL SPE extraction GC/MS chromatography analysis
Cd, Zn, Pb, Cu	mg L−1	PN-EN ISO 11885	Samples volume: 100 mL Mineralization in MARS 6 microwave mineralizer ICP-OES analysis

DOI: 10.7717/peerj.9546/table-3

Results

Discussion

In this study, heavily worn tires that could already be free of mobile forms of pollution were used. The results obtained indicate the environmental safety of their use. No evidence has been obtained that the use of compressed tyre bales increases the concentration of substances in the tires. It has not been found that this material increases the occurrence of metals, chlorides and sulfates, which have an impact on secondary (aesthetic) drinking water standards. Particularly important is the lack of exceedances for zinc, which is used in the production of tires in large quantities (Rhodes, Ren & Mays, 2012). This confirms the findings of other researchers who do not obstruct leaching of this metal from waste tyre bales (Simm, Wallis & Collins, 2004). Hylands & Shulman (2003) summarise the results of laboratory and field studies to determine the level of leachates from tyres. The result indicates that for all regulated metals and organics the results for post-consumer tyres are well below regulatory levels. Substances which could potentially leach from post-consumer tyre materials are already present in groundwater in developed area. Studies suggest that leachate levels for the majority of contaminants fall below the allowable regulatory limits and will have negligible impact on the general quality of water in close proximity to tyres. However, the National Water Research Institute in Canada conducted tests to determine the toxicity of new car tires and their impact on aquatic animals. Research reports published by the Institute show that both old and new tires are toxic to the analyzed rainbow trout. According to the authors of the study, the toxicity of tires is not related to the substances collected during their use or storage, but is only the result of production processes. A higher level of toxicity has been reported for used car tires than for unused new tires (Day et al., 1993). However, research conducted by Redondo-Hasselerharm et al. (2018) indicate that the car tire, including the chemicals associated with this material, did not adversely affect the four freshwater benthic invertebrates. According to the authors, the car tire is a low risk for freshwater benthic invertebrates. However, the potential long-term effects caused by the slow release and gradual environmental increase of bioavailable zinc and other substances caused by aging of the rubber particles are not expressed in these experiments and still require further research.

Conclusions

1. The washing of tyre bales and collection and disposal of effluent generated will suffice to protect the natural environment where the resulting product is embedded against contamination.

2. The effluent from tyre-washing is enriched only slightly at the time of contact, to the extent that visual checks on tyres used to form a bale (with non-selection of the most-soiled tyres) may render washing prior to installation unnecessary.

3. There is no evidence that tyre bales embedded in the ground represent a real or potential source of pollution, or pose a threat that water and soil quality in the vicinity will deteriorate.

4. The quality of sedimentation-well sludge is also satisfactory, to the extent that no treatment as per the regulations for industrial effluent are necessitated.

5. The conditions of the simulation may reasonably be deemed extreme, given immersion and thus constant contact between tyres and water for a period of 120 days. In real conditions, tyre bales will be in contact with water for a few hours, after which various changes associated with self-cleaning, concentration and a density gradient will ensue, to the extent any negative impact on the environment is further curbed.