Tire wear microrubber particles (TWMP) are a major source of environmental contamination. Degradation of TWMP is slow and leachates contain toxic constituents including heavy metals, polycyclic aromatic hydrocarbons (PAHs) and organic additives. Few studies have addressed methods to mitigate the potential ecotoxicity of TWMP leachates. This study investigated the effects of UV-C (254 nm) and combined UV-C and vacuum UV (185 nm) treatment (VUV-UV-C) of TWMP leachates on degradation and ecotoxicity. VUV-UV-C treatment mitigated dissolved zinc and degraded the TWMP constituents fluoranthene, pyrene and benzo(a)pyrene by up to 90%, and the additives benzothiazole and phthalates by up to 70%. The potential ecotoxicity and genotoxicity of TWMP constituents were examined before and after UV treatment in bioassays with Escherichia coli, the luminescent bacterium Aliivibrio fischeri, the microalga Raphidocelis subcapitata and the crustacean Daphia magna. VUV-UV-C treatment decreased the potential ecotoxicity up to five-fold as indicated by changes in median effective concentrations (EC50). This was likely due to the formation of less toxic and less bioavailable transformation products. The VUV-UV-C treatment did not require the addition of oxidants or catalysts, and the study indicated a potential of VUV-UV-C as an advanced oxidation process to mitigate toxic compounds in TWMP leachates from urban or industrial sources.

  • Leachates from tire wear microrubber particles (TWMP) exhibited a substantial potential for ecotoxicity.

  • VUV-UVC treatment removed toxic compounds from TWMP leachates.

  • In vivo bioassays showed significantly lower ecotoxicity in VUV-UVC-treated leachates.

  • VUV-UVC is an effective advanced oxidation technology that does not require the addition of catalysts or oxidant precursors.

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.

Tire wear microrubber particles

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.

VUV and UVC irradiation

Combined VUV and UVC irradiation of aqueous solutions of TWMP leachates and TWMP constituents were carried out using a Collimated UV Beam Tube system (ULTRAAQUA A/S, Denmark) equipped with a 4 W low-pressure high-output amalgam VUV Hg lamp (NIQ 40/18, Heraeus, Germany) (Figure 1). The UV lamp simultaneously emitted VUV (185 nm) and UVC (254 nm) with fluence rates at the collimated beam tube window of 10 and 100 μW/cm2, respectively. The VUV-UVC Collimated beam system was continuously flushed with N2 (99.9999%) to limit quenching by O2 (O3 formation). The Collimated beam system was temperature controlled at 18 °C using an HTS 15 heat transfer station (Huber). Water samples with TWMP leachates in 25 mL glass petri dishes were placed in direct contact with the Collimated Beam Tube window (no gas phase) to avoid quenching and samples were stirred with a magnetic stirrer during UV irradiation. UV doses (fluence in J/cm2) were calculated based on measured UVC and VUV irradiance, the water depth and volume in the petri dish, the UV absorption coefficient and the reflection factor (Bolton et al. 2015). VUV irradiance was measured using a Hamamatzu H9535-185 VUV Power meter, and UVC irradiance was measured using a ZED Reference UV Radiometer (TinyMeter). To compare the effect of combined VUV-UVC irradiation with the effect of UVC irradiation alone (no VUV), monochromatic UVC irradiation of aqueous solutions of TWMP leachates was carried out using an 8 W UVP 3UV lamp (Analytic Jena) equipped with a monochromatic UVC tube (254 nm). Samples with TWMP leachates in 25 mL glass petri dishes were placed on a magnetic stirrer 12 cm below the UVC lamp. The UVC irradiance was measured using an Extech SDL470 UVC meter and was 120 μW/cm2 at 12 cm distance.
Figure 1

Collimated beam tube for VUV-UVC irradiation of TWMP leachates (a), and VUV-UVC flow reactor for treatment of large samples of TWMP leachates (b).

Figure 1

Collimated beam tube for VUV-UVC irradiation of TWMP leachates (a), and VUV-UVC flow reactor for treatment of large samples of TWMP leachates (b).

Close modal

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 and organic additives in TWMP leachates

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.

Metals in TWMP leachates

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).

Ecotoxicity of VUV and UVC-treated TWMP constituents

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).

Calculations and statistics

The toxicity response of the test organisms was expressed as inhibition (I) relative to control samples: I = 1 − (Ri/Rc), where Ri and Rc are responses measured for inhibited and control samples, respectively. Concentration–response curves were fitted using a four-parameter inhibition model using iterative nonlinear regression:
formula
(1)
where I is the relative inhibition, X is the concentration of TWMP in mg/L, EC50 is the median effective concentration in mg/L, m1, m2 and m3 are model parameters (KalaidaGraph 5.0, 2023). The EC50 values from the different treatments were compared using the Wilcoxon rank sum test (Mann–Whitney U test) with a significance level of p < 0.05.

Tire rubber microrubber particles

The TWMP used in the current study had a density of 1.16 g/cm3 and varied in size from <1 to 130 μm and was dominated in numbers by particles with sizes <100 μm, and in weight by particles >10 μm (Figure 2). This is in the same range as typical TWMP from tire abrasion found in the environment (Gieré & Dietze 2023). About 6 million tons of such tire wear material is released into the environment each year (Gieré & Dietze 2023).
Figure 2

Particle size (a) and weight (b) distribution for TWMP used in this study.

Figure 2

Particle size (a) and weight (b) distribution for TWMP used in this study.

Close modal

Effect of UV treatment on the degradation of TWMP constituents

PAHs are examples of well-known toxicants present in tire wear. PAHs in TWMP leachates were degraded by UVC and VUV-UVC treatment (Figure 3). The concentration of pyrene, fluoranthene and benzo(a)pyrene decreased with increasing UV doses (Figure 3). About 57–79% of these PAHs were degraded by UVC treatment at 4.5 J/cm2 (Figure 3(a)) whereas a combined VUV-UVC dose of 4.5 J/cm2 degraded 86–90% (Figure 3(b)). The combined VUV-UVC dose of 4.5 J/cm2 corresponded to approximately 0.9 J/cm2 VUV and 3.6 J/cm2 UVC. The degradation results for UVC are in the same range as reported previously for the removal of PAHs from wastewater (Rosinska 2021) whereas published results for noncatalytic PAH degradation by VUV are scarce. Interestingly, the initial pseudo-first-order rate constant for the removal of combined pyrene, fluoranthene and benzo(a)pyrene was about two times greater for VUV-UVC treatment compared to UVC treatment (Figure 3(d)). Pseudo-first-order reaction kinetics can be applied as a convenient model to facilitate the comparison of otherwise complex UV reactions in some batch and flow UV reactors (e.g., Shi et al. 2021). In the present study, the observed PAH degradation during the VUV-UVC treatment was likely due to a combination of direct UV photolysis mediated by photon absorption and then indirect photolysis mediated by reactive species produced by the VUV.
Figure 3

Degradation of pyrene, fluoranthene and benzo(a)pyrene in TWMP leachates after UVC treatment (a), after VUV-UVC treatment (b), comparison of UVC and VUV-UVC degradation of the sum of pyrene, fluoranthene and benzo(a)pyrene (‘PAHs’) (c) and pseudo-first-order rate constants for UVC and VUV-UVC degradation of the three PAHs (d).

Figure 3

Degradation of pyrene, fluoranthene and benzo(a)pyrene in TWMP leachates after UVC treatment (a), after VUV-UVC treatment (b), comparison of UVC and VUV-UVC degradation of the sum of pyrene, fluoranthene and benzo(a)pyrene (‘PAHs’) (c) and pseudo-first-order rate constants for UVC and VUV-UVC degradation of the three PAHs (d).

Close modal

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).

Organic additives in tire rubber leachates include phthalates and aromatic heterocyclic sulfur-containing compounds. Such compounds were also removed by VUV-UVC (Figure 4). In these experiments, known concentrations of DMP, DEP and BTZ were spiked into water samples (10 mg/L) and treated by the VUV-UVC flow system shown in Figure 1. The combined VUV-UVC treatment degraded about 34–70% of DMP, DEP and BTZ within 32 min corresponding to a UV dose of 16 J/cm2 (Figure 4(b)). The pseudo-first-order rate constant for DMP and BTZ were somewhat comparable resulting in half-lives of 16–23 min. BTZ is added to tire rubber to accelerate vulcanization and enhance abrasion resistance and occurs in concentrations of more than 10 g/kg in TWMP. BTZ is also used in other industrial products than tire rubber and is therefore common in municipal wastewater and in surface waters (Lai et al. 2023). Conventional wastewater treatment plants often have incomplete removal of BTZ (e.g., 22–78%) but new water/wastewater AOP has a potential for more efficient BTZ removal (Yang et al. 2021; Lai et al. 2023). This study expands these observations by showing the degradation of BTZ by a VUV-UVC process without the addition of catalysts or oxidant precursors.
Figure 4

Degradation of diethyl phthalate (DEP), dimethyl phthalate (DMP) and benzothiazole (BTZ) after VUV-UVC treatment as a function of irradiation time (a) and UV fluence (b). Pseudo-first-order rate constants for VUV-UVC degradation of DEP, DMP and BTZ (c). Changes in zinc concentrations measured by ICP-MS and photometry after VUV-UVC treatment (d).

Figure 4

Degradation of diethyl phthalate (DEP), dimethyl phthalate (DMP) and benzothiazole (BTZ) after VUV-UVC treatment as a function of irradiation time (a) and UV fluence (b). Pseudo-first-order rate constants for VUV-UVC degradation of DEP, DMP and BTZ (c). Changes in zinc concentrations measured by ICP-MS and photometry after VUV-UVC treatment (d).

Close modal

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.

Effect of UV treatment on toxicity of TWMP leachates

The ability of UV treatment to mitigate the ecotoxicity of TWMP leachates was assessed in bioassays with R. subcapitata, A. fischeri and D. magna (Figure 5). VUV-UVC treatment of TWMP decreased the toxicity to the freshwater microalga R. subcapitata (Figure 5(b)). UVC treatment alone showed a minor effect (Figure 5(a)). The traditional test organism A. fischeri was not inhibited by TWMP leachates and the concentration–response curve was therefore not fitted using Equation (1) (Figure 5(c)). The greatest concentration of TWMP leachates did not affect the luminescence or stimulate the metabolic activity of A. fischeri which may be partly due to the short 30 min incubation time used in this standard test (ISO 11348–1 2007). Hence, this suggests that acute tests with A. fischeri are not applicable for assessing the ecotoxicity of TWMP leachates. In contrast, the survival of the freshwater crustacean D. magna was negatively affected by TWMP leachates, but VUV-UVC treatment decreased the toxicity (Figure 5(d)). The relative mortality was somewhat comparable in raw (unfiltered) and filtered (0 J/cm2) leachate and the toxicity in untreated samples increased with increasing exposure time (24 vs 48 h). However, treatment with 2.75 and 4.5 J/cm2 VUV-UVC significantly (p < 0.05) decreased the mortality for D. magna exposed for 48 h to the TWMP leachate (Figure 5(d)). The relative mortality after 48 h was only 10% in samples treated with 4.5 J/cm2 VUV-UVC which is comparable to the natural mortality in water samples without TWMP leachate.
Figure 5

Changes in toxicity of TWMP leachates to the green microalga R. subcapitata after 4.5 J/cm2 UVC treatment (a), and after 4.5 J/cm2 combined VUV-UVC treatment (b). Changes in toxicity of TWMP leachates to the luminescent bacterium A. fischeri after 4.5 J/cm2 combined VUV-UVC treatment (c). The concentrations on the X-axes in (a–c) correspond to the amount of TWMP that was used to create the leachates. (d) The relative mortality of D. magna after 24 and 48 h exposure to TWMP leachates before and after VUV-UVC treatment (n = 24). A letter has been assigned depending on whether mortalities are significantly different from each other (P < 0.05).

Figure 5

Changes in toxicity of TWMP leachates to the green microalga R. subcapitata after 4.5 J/cm2 UVC treatment (a), and after 4.5 J/cm2 combined VUV-UVC treatment (b). Changes in toxicity of TWMP leachates to the luminescent bacterium A. fischeri after 4.5 J/cm2 combined VUV-UVC treatment (c). The concentrations on the X-axes in (a–c) correspond to the amount of TWMP that was used to create the leachates. (d) The relative mortality of D. magna after 24 and 48 h exposure to TWMP leachates before and after VUV-UVC treatment (n = 24). A letter has been assigned depending on whether mortalities are significantly different from each other (P < 0.05).

Close modal

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).

In experiments with the freshwater microalga R. subcapitata, the median effective concentration (EC50) for TWMP leachates was 1.67 ± 0.290 mg/L [±standard error; n = 6] (Figure 6). The traditional classification of hazardous substances considers compounds with EC50 < 1 mg/L as ‘very toxic’ for aquatic life and substances with 1 mg/L < EC50 < 10 mg/L as ‘toxic’ for aquatic life (United Nations 2021). Interestingly, the EC50 value increased with increasing UV dose which suggests a decreasing toxicity (Figure 6). The toxicity decreased two- and five-fold after 4.5 J/cm2 UVC and 4.5 J/cm2 VUV-UVC treatment, respectively. This could be explained by the decreased concentrations of PAHs and organic additives in the TWMP leachates after VUV-UVC treatment (Figures 3 and 4). An attenuated concentration of bioavailable zinc (Figure 4(d)) and degradation of other TWMP constituents may also have been contributing factors. Regardless, the results suggested a potential for attenuating the ecotoxicity of TWMP leachates by VUV-UVC treatment and to a lesser extent UVC treatment (Figure 6). The efficiency of AOPs such as VUV-UVC in large-scale applications will depend on a range of factors such as the presence of other organic or inorganic constituents in the water (Duca et al. 2017). For example, the presence of elevated levels of organic matter as indicated by high COD levels can be a challenge for AOP processes because reactants are mainly consumed by other compounds than the target molecules (Andreozzi et al. 1999). Hence, it has been suggested that the best results will be obtained if COD levels are ≤5,000 mg/L which may be achievable in some urban and industrial water systems (Andreozzi et al. 1999). The proposed VUV-UVC treatment could then have a potential for the treatment of water and wastewater from areas such as rubber industries (e.g., rubber manufactory plants, rubber recycling facilities), effluents from stormwater retention ponds and for polishing municipal wastewater effluents (tertiary treatment). VUV/UVC could potentially also be used to treat concentrated wastes from some of the facilities mentioned earlier.
Figure 6

Changes in toxicity of TWMP leachates to the green microalga R. subcapitata shown as median effective concentration (EC50) before (gray box) and after UVC treatment (a) and after combined VUV-UVC treatment (b). The horizontal line within each box represents the median values for six independent experiments and the upper and lower limits of a box represent the distance between the upper and lower quartiles (interquartile distance). Individual data points are shown as bullets (n = 6).

Figure 6

Changes in toxicity of TWMP leachates to the green microalga R. subcapitata shown as median effective concentration (EC50) before (gray box) and after UVC treatment (a) and after combined VUV-UVC treatment (b). The horizontal line within each box represents the median values for six independent experiments and the upper and lower limits of a box represent the distance between the upper and lower quartiles (interquartile distance). Individual data points are shown as bullets (n = 6).

Close modal

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.

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