Effect of vacuum UV and UV-C treatment on degradation and ecotoxicity of tire wear microrubber leachates

The largest fraction of microplastic pollution in many countries originates from car tire abrasion resulting in tire wear microrubber particles (TWMP) (Boucher & Friot 2017; Kole et al. 2017). TWMP are released due to the heat created while driving and the shear forces between the road and the tire. The shear forces result in relatively large particles, while the heat generated on the tire's surface releases smaller particles (Kole et al. 2017). TWMP vary in size from less than 2 μm to coarse particles of 500–1,000 μm with average sizes often dominated by smaller particles of less than 100 μm (Grigoratos & Martini 2014; Wagner et al. 2018; Gieré & Dietze 2023). The specific TWMP size depends on factors, such as driving speed, driving style, vehicle characteristics, meteorological parameters, tire composition and age and road surface characteristics (Grigoratos & Martini 2014; Kole et al. 2017). About 1.3 million tonnes of tire wear are released into the environment each year in Europe (Wagner et al. 2018).

Aquatic ecosystems are one of the primary sinks for TWMP and about half of the total microplastic pollution in many aquatic ecosystems stems from tire erosion (Boucher & Friot 2017; Baensch-Baltruschat et al. 2020). This large contribution from TWMP to environmental microplastic pollution is not expected to decrease in the near future (Sommer et al. 2018). In addition, vehicle tires that are no longer suited for use (end-of-life tires) are occasionally collected and reused to serve different purposes. Such recycled products can also represent significant sources of TWMP pollution. Unfortunately, tire rubber contains a range of potentially toxic substances that can leach into the environment and contaminate soils and surface waters even though the rubber particles themselves are trapped or otherwise immobilized (Wik et al. 2009; Wagner et al. 2018; Baensch-Baltruschat et al. 2020; Capulupo et al. 2020; Halle et al. 2020; Lu et al. 2021; Gieré & Dietze 2023). TWMP toxicants include metals, such as zinc, copper, aluminum, lead, polycyclic aromatic hydrocarbons (PAHs) and additives, such as phthalates, benzothiazoles (BTZ) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) (Wagner et al. 2018; Gieré & Dietze 2023). It has therefore been suggested that of all possible environmental plastic contaminations, leachates from car tire rubber likely have the greatest toxicological effect on aquatic organisms (Capolupo et al. 2020).

Natural degradation of TWMP is slow and can take hundreds or even thousands of years, and some constituents are considered persistent in the environment. Interestingly, UV-based advanced oxidation processes (AOPs) have the potential for the treatment of water containing toxic and persistent substances (Zoschke et al. 2014; Duca et al. 2017; Lin et al. 2020). The underlying principle of UV-based AOPs is the in situ generation of radicals such as hydroxyl radicals (•OH) that will react with organic or inorganic constituents in the contaminated water. Vacuum UV (VUV) treatment will generate such radicals including •OH without the need for additions of catalysts or radical precursors (Zoschke et al. 2014). In addition, combined VUV-UVC treatment has the potential for both direct and indirect photolysis of many environmental pollutants. The indirect photolysis is carried out by photochemically produced reactive intermediates generated mainly by VUV (e.g., 185 nm) whereas the direct photolysis is mediated mainly by the UVC radiation (e.g., 254 nm). The latter occurs when the target molecule absorbs light that promotes excitation leading to electron transfer and subsequent degradation. However, it is not known how AOPs such as VUV-UVC affect the ecotoxicity of TWMP leachates, and there is a need to establish basic knowledge regarding changes in toxicity to aquatic organisms before large-scale applications are considered.

This study examined the effects of short-wave UV irradiation on the degradation and ecotoxicity of substances in TWMP leachates. Samples were exposed to different UV regimes including UVC (254 nm) and combined VUV (185 nm) and UVC (VUV-UVC). The kinetics of compound removal was determined in relation to treatment time and UV type and UV fluence (dose). The potential ecotoxicity of TWMP constituents was examined before and after UV treatment using aquatic test organisms to assess treatment efficiency with respect to bioactive compounds and the occurrence of toxic transformation products.

TWMP were obtained from the tire recycling company Genan (Viborg, Denmark) and consisted of the product GENAN 120 Mesh. This product consists of cryomilled recycled tire rubber with nominal particle sizes <125 μm. GENAN 120 is micronized rubber produced from off-road vehicle tires, passenger car tires and truck tires. Genan obtained the size fraction <125 μm by sieving the particles through an Air Jet sieve 200 LS (Hosokawa Alpine). The specific density of the TWMP was 1.16 g/cm³. The TWMP were suspended in tap water and the particle size distribution and concentrations of TWMP were determined using a Multisizer™ 4e Coulter Counter (Beckmann-Coulter, USA). Particles were sized and counted using a combination of apertures (50, 100 and 2,000 μm). The tap water used for creating TWMP suspensions consisted of municipal tap water obtained from Aalborg Municipality (Denmark). The municipality produces tap water from groundwater (12–16 dH) abstracted directly from chalk aquifers without water treatment or disinfection (no chlorination). The nonvolatile organic carbon (NVOC) concentration and the chemical oxygen demand (COD) were 1.3 and 0.4 mg/L, respectively. The pH and turbidity of the tap water were 7.6 and <0.05 FTU, respectively.

This study focused on TWMP leachates rather than rubber particles because less is known about the effects of UV on leachates despite being abundant and somewhat mobile in the environment. TWMP leachates were established by suspending 1 g/L cryomilled tire rubber in tap water in 1 L Bluecap bottles followed by incubating on a magnetic stirrer. The solutions were subsequently transferred to Greiner centrifuge tubes (Sarstedt) and centrifuged at 3,000 g in a ScanSpeed 1236R centrifuge (LaboGene). The supernatants were then filtered through Advantec GF-75 glass fiber filters with a nominal pore size of 0.3 μm (Advantec, Japan) to create TWMP leachates.

Experiments for testing the effects of VUV-UVC treatment on large volumes of TWMP leachates were conducted in a continuous-flow UV photoreactor as described previously (Del Puerto et al. 2022). The 4 L UV photoreactor (Figure 1) was operated with cooling at 10 °C and a flow of 2 L/min. The UV photoreactor was equipped with a UVC sensor and a low-pressure high-output amalgam VUV Hg lamp which simultaneously emitted VUV (185 nm) and UVC (254 nm) at a radiation flux of 14 and 56 W, respectively (UltraTherm 200 W LPHO TOC UV, Ultraaqua A/S, Denmark). UV fluence (J/cm²) was calculated on the basis of the measured UV irradiance during the treatments (25–34 mW/cm²) and the treatment times. The treatment times were varied between 0 and 32 min.

PAHs, BTZ, 6PPD and phthalates in leachates and spiked samples were extracted and quantified partly as described by Polyakova et al. (2013). Na₂SO₄ (0.2 g) was added to 2 mL water samples and then extracted with 2 mL toluene-hexane (1:6) followed by centrifugation to separate phases (Polyakova et al. 2013). Naphthalene [CAS 91-20-3] and phenanthrene [CAS 85-01-8] were added as internal standards, and the PAH extraction efficiency varied between 83.4 and 101.2%. The toluene-hexane extract was analyzed using an Agilent 7890A Gas Chromatograph equipped with flame ionization detection, a Combi PAL autosampler and a DB5 column (30 m × 0.32 mm; 0.25 μm film thickness). The initial column temperature was 150 °C followed by ramping at 15 C/min to 260 °C, and then 6 min at 260 °C. Helium was used as carrier gas. The analysis of PAHs in TWMP leachates focused on phenanthrene, pyrene, fluoranthene and benzo(a)pyrene as model compounds because they are relatively abundant in tire rubber and/or have carcinogenic properties. The analysis of spiked samples before and after UV irradiation included BTZ [CAS 95-16-9], dimethyl phthalate (DMP) [CAS 13-11-3] and diethyl phthalate (DEP) [CAS 84-66-2]. These organics were spiked into water samples at 10 and 100 mg/L without the use of carrier solvents.

The content of metals and other elements in TWMP leachates was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using an Optima 8,000 system (Perkin Elmer) and inductively coupled plasma mass spectrometry (ICP-MS) using an SCIEX ELAN DRC-e system (PerkinElmer). The following 25 elements were considered: Al, Sb, As, Ba, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Ag, Se, Ti, Zn, P, Sr, V, Tl and Ca. Zinc (Zn) was the dominant metal in TWMP leachates and routine measurements of Zn ions were carried out using a Supelco Spectroquant^® Photometric zinc test (Merck). Zn complexed with a 1-(2-pyridylazo)-2-naphthol derivative was quantified at 565 nm using a Genesys 20 spectrophotometer (Merck).

The ecotoxicity of TWMP leachates and constituents was determined before and after UV treatment. Toxicity assays were conducted using the unicellular green microalga Raphidocelis subcapitata, the zooplankton organism Daphnia magna and the luminescent bacterium Aliivibrio fischeri. The bacterium Escherichia coli was assayed in a genotoxicity assay. The genotoxicity test consisted of the SOS Chromotest that determines DNA mutations in a genetically engineered E. coli strain (Environmental Bio-detection Products, Inc., Canada). A growth inhibition test with R. subcapitata was conducted in 96-well microplates in accordance with ISO 8692 2012. Plates were incubated for 72 h at 22 °C on a shaker at 70 rpm with continuous illumination (8,000 lx). Algal growth was measured as absorbance at 450 nm using a Thermo Multiskan Plate Reader (Thermo Scientific). Toxicity test with D. magna was conducted in accordance with ISO 6341 except that the test organisms were incubated individually in 24-well microplates (Nunc). The toxicological endpoint for D. magna was inhibition of mobility determined by inspection of the animals after 24 and 48 h (ISO 6341 2012). Toxicity test with A. fischeri was conducted in accordance with ISO 11348-1. A. fischeri was incubated in white 96-well plates (CulturPlate, Perkin Elmer) and changes in bioluminescence were quantified after 30 min using a Victor X2 Multilabel Plate Reader (Perkin Elmer).

It should be noted that the concentrations of pyrene, fluoranthene and benzo(a)pyrene measured in the leachates shown in Figure 3 were above the aqueous solubility of these PAHs. The TWMP leachates were created by incubating tire microrubber particles in water followed by filtration through filters with a nominal pore size of 0.3 μm. Hence, it is likely that the TWMP filtrate contained nano-size tire rubber particles (<0.3 μm) that contributed to the elevated PAH concentrations after extraction. Tire rubber particles are well-known sources of PAHs (Wagner et al. 2018; Gieré & Dietze 2023), and nanoparticles are good carriers of hydrophobic PAHs (Mahgoub 2019).

Zinc (Zn) was the dominant metal in the investigated TWMP leachates (0.5–1.6 mg/L). Elements such as magnesium (Mg), strontium (Sr), calcium (Ca), potassium (K) and sodium (Na) were also abundant in TWMP leachates (0.29–1.47 mg/L). This is in line with studies reporting zinc to be a dominant element in tire rubber with a mean mass fraction of 11.2 g/kg which corresponds to about 1% of the tire mass (O'Loughlin et al. 2023). Zinc and zinc oxide (ZnO) are added to tire rubber as a vulcanization accelerator to convert soft rubber into a stable product that can carry the weight of vehicles. The concentration of detectable zinc ions was affected by UVC and VUV-UVC treatment (Figure 4(d)). Quantification of zinc was based on ICP-MS analysis and a photometric method where Zn ions were complexed with 1-(2-pyridylazo)-2-naphthol. The ICP-MS analysis indicated that zinc was present in all samples and increased after UV treatment which may have been due to accelerated weathering of nano-size tire rubber particles in the leachates. UV weathering of tire rubber particles can mobilize metals and increase zinc concentrations in leachates (Simon et al. 2021). In contrast, the spectrophotometric 1-(2-pyridylazo)-2-naphthol method suggested that the concentration of free zinc ions decreased during the VUV-UVC treatment. Zinc was obviously not degraded, but a fraction appeared to be photochemically converted to a form that was less detectable by the traditional 1-(2-pyridylazo)-2-naphthol method. Undissolved or complex-bound zinc will not be detected by the 1-(2-pyridylazo)-2-naphthol method unless a pre-digestion is performed. This could suggest that the VUV-UVC treatment facilitated the complexation of some of the zinc ions.

The apparent genotoxicity of TWMP leachates was also attenuated by VUV-UVC treatment as indicated by an SOS test detecting DNA damage in a genetically engineered E. coli strain. Untreated TWMP leachates showed a weak genotoxic response whereas VUV-UVC treated TWMP leachates did not show any detectable genotoxic responses (data not shown).

A considerable amount of TWMP and their leachates eventually end up in stormwater, wastewater and surface waters. TWMP leachates contain a mixture of organic and inorganic constituents that are potentially toxic to aquatic life. This study investigated the effects of UVC and combined VUV-UVC treatment of TWMP leachates on compound removal and ecotoxicity. VUV-UVC treatment mitigated dissolved zinc and degraded common organic constituents in TWMP leachates. The potential ecotoxicity of leachates decreased after VUV-UVC treatment which was likely due to the formation of less toxic or less bioavailable transformation products. The study indicates the potential of VUV-UVC treatment as an advanced oxidation method to mitigate toxic compounds in TWMP leachates.

The authors wish to thank Helle Blendstrup, Sofie Albrekt Hansen and Timo Kirwa for laboratory assistance, and Linda Birkebæk Madsen and Dorte Spangsmark for ICP analyses of TWMP leachates. We also thank ULTRAAQUA A/S for providing a collimated VUV-UVC beam system, a continuous-flow VUV-UVC photoreactor and a VUV Power meter.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.