Microplastic particles from roads and traffic : occurrence and concentrations in the environment

Abstract

The impact of roads and traffic on the environment have been studied for several decades, and negative impact on both the terrestrial and aquatic environment from pollutants have been demonstrated. Road pollution is typically related to high concentrations of particles, such as mineral particles (quartz, feldspar) and micro- and nanoparticles from the abrasion of tires and road surfaces. In recent years, the research interest for tire and road wear particles have increased substantially due to the increased research interest in micro- and nanoparticle pollution. As tires and some types of road surfaces contain synthetic rubbers, these particles are also included in the plastic pollution terminology as tire wear particles (TWP), tire wear particles combined with mineral particles (TRWP) and road wear particles with polymer-modified bitumen (RWPPMB). Road pollution is also typically high in particle-associated pollutants, such as metals (zinc (Zn), copper (Cu), cadmium (Cd), nickel (Ni), lead (Pb)) and organic micropollutants such as polycyclic aromatic hydrocarbons compounds (PAC), organophosphates compounds (OPC), benzothiazoles, hexa(methoxymethyl)melamine (HMMM) and N-1,3-dimethylbutyl-N 0-phenyl-p-phenylenediamine-quinone (6-PPD-quinone). Especially the acute toxic effect of 6-PPD-quinone and benzothiazoles in the environment have been linked to the release of tire wear particles. Roads and traffic are estimated as the largest source of microplastic particles (MP) from land to the marine environment, and TWP are estimated to be the main microplastic source, with abrasion particles from road markings (RMP) and RWPPMB as the second and third source. Road transport is an essential part of modern society and predictions estimate that the number of vehicles will almost double over the next 30 years. It is therefore crucial for the environment on the planet that road-associated microplastic particles (RAMP) are assessed and mitigated. To reliably assess the levels of RAMP and ensure correct and efficient mitigation measures, there is an urgent need for more environmental data. However, comparisons between current available data on RAMP are hampered by the lack of standardized methods for both sampling and analysis. There are currently several initiatives in the research community and on governmental level to harmonize the assessments of MP, such as Horizon2020-project EUROqCHARM (https://www.euroqcharm.eu/en). There are also efforts made to unify the analytical methods for TWP/TRWP specifically, such as the European TRWP Platform (https://www.csreurope.org/trwp). At the national level, several countries have implemented action plans against plastic pollution. In the National Transport Plan (NTP, 2022-2033), the Norwegian government have incorporated the need to improve the knowledge of microplastic release from roads and traffic and how to reduce the negative impact on the environment. The presented thesis aimed to contribute knowledge on the sources of MPs from roads and traffic, on the occurrence and concentrations of microplastic in different environmental compartments and to assess possible remedial actions for road-associated microplastic particles. To investigate potential new sources of RAMP, the microplastic concentrations in road de-icing salt from both sea salt and rock salt sources were assessed, and the annual release of microplastic particles from road de-icing salt in Scandinavia was estimated (Paper I). The results demonstrated that MPs are present in road de-icing salt in Scandinavia, however the contribution was negligible compared to the other three sources of RAMP previously identified. The annual release of MPs from road de-icing salt was estimated to contribute to less than 0.003% of the total estimated microplastic release, compared to TWP (90%), RM (9%) and RWPPMB (0.5%). However, the results support the need to identify and assess all sources of RAMP in order to evaluate the realistic levels of microplastic pollution and to reduce the negative impact on the environment. Although the work of this thesis focused on the high road salt consumption in Scandinavia, high salt consumption is also observed in several other countries, such as Germany, the United Kingdom, Ireland, the US, Canada and China. As different road de-icing salts are used in different countries, future research should assess the microplastic levels in the salts used locally to realistically address the annual release of microplastic particles from this source. One of the challenges with describing the environmental impact of MPs, including TWP and RWPPMB, is knowing what the relevant environmental concentrations are. Thus, reliable and comparable quantification methods must be developed so that these levels can be assessed across different studies in time and space, and between different environmental compartments. Current available literature presents several different analytical methods for analysing RAMP, mostly focused on TWP. For single-particle analysis, methods such as Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy, Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX) and Micro-X-ray Fluorescence (µXRF) have been utilized. For mass concentration analysis, Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Liquid-Chromatography Mass Spectrometry (LC-MS/MS), Thermal Desorption Gas Chromatography Mass Spectroscopy (TED-GC/MS) and Pyrolysis Gas Chromatography Mass Spectroscopy (PYR-GC/MS) are the most used methods in current literature. For mass-based analysis, the challenge is finding suitable marker compounds that are reliable for both reference material and environmental samples, stable in different types of matrices and accessible for a high throughput of samples in order to establish environmental concentration levels across different types of samples. For PYR-GC/MS, the International Organization for Standardization (ISO) has published two technical specifications for quantifying TWP/TRWP in soil/sediment and air samples. However, these methods are currently not adjusted for the presence of synthetic rubbers in the road surface wear layer (RWPPMB), such as styrene butadiene styrene rubbers (SBS) or scrap tires, which as applied in many countries for roads with high traffic volume. As SBS is currently the only rubber added to RWPPMB on state and county roads as well as some municipality roads of Norway, the current thesis aimed to improved quantification methods for TWP/TRWP and RWPPMB. The improved method (Paper II) proposed in this thesis utilizes multiple pyrolysis markers for the quantification of styrene butadiene rubber (SBR) and butadiene rubber (BR) from tires and SBS from the road surface in environmental samples. The suggested markers were benzene (mz 78), α-methylstyrene (mz 117), ethylstyrene (mz 118) and butadiene trimer (mz 91). The proposed markers substantially lowered the standard deviation of the results to 40% s.d. compared to 62% (4-VCH), 77% (SB dimer) and 85% (SBB trimer) for the single marker compounds proposed in previous studies. The multiple pyrolysis markers also demonstrated good recoveries in complex road matrices (88–104%), which further validated the strength of the method. The proposed method also included an improved calculation step from the measured rubber concentration to the mass of TWP and RWPPMB. This step included the use of local emission factors, traffic data and locally relevant reference tires. The calculations were performed with Monte Carlo simulation. The use of Monte Carlo simulation also enabled the uncertainties related to these calculations to be reported. Incorporating assessments of the uncertainty is important, as the rubber concentration in commercial tires are highly variable. The average percentage of SBR+BR rubbers in personal and heavy vehicles tires reported in the current thesis were 31% (of total tire tread) and 33%, respectively. However, for personal vehicles and heavy vehicles, the variation between different tire types and brands were large. These results differ substantially from previous studies were 40-50% SBR+BR have been assumed for all personal vehicle tires and 50% natural rubber (NR) have been proposed for truck tires. Thus, the use of locally relevant reference tires will improve the quantification of TWP in environmental samples. As TWP in the environment are exposed to other road particles on the road surface, tire wear particles are often reported as agglomerate particles mixed with mineral particles from the road, defined as tire and road wear particles (TRWP). Based on a limited number of studies, previous quantification methods assume that all TRWP particles contain TWP and minerals in a 1:1 ratio. In the present thesis, we propose an improved method for calculating TRWP based on the concentration of TWP and the current data available on mineral content for TRWP. The calculations for TRWP are also performed with Monte Carlo simulation. Even though the proposed method is hampered by the limited knowledge on mineral content of TRWP, it demonstrates the possibility to optimize quantification methods for locally relevant data, such as different road surfaces, different driving patterns or other variables, as future publications contribute with improved data. The improved quantification methods for TWP, RWPMB and TRWP were further used to analyse the concentration levels in roadside snow (Paper III) and in different compartments of a road tunnel (Paper IV). The TWP concentrations in roadside snow (76.0–14 500 mg/L meltwater; 222–109 000 mg/m2 mass loads) far exceeded concentration levels reported for snow and road runoff in previous studies, as well as the concentrations reported for tunnel wash water (TWW) in the present thesis (untreated: 14.5-47.8 mg/L; treated: 6.78-29.4 mg/L). As concentrations of RWPMB had not been assessed in previous studies, only comparison between the roadside snow (14.8–9550 mg/L; 50.0–28 800 mg/m2), the tunnel road surface (0.578-258 mg/m2) and the TWW (untreated: 11.5-38.1 mg/L; treated: 5.40-23.4 mg/L), in which the roadside snow has substantially higher concentrations compared to the tunnel samples. This demonstrates the potential for snow piles to accumulate RAMP and potentially pose a higher acute release risk to the environment compared to road runoff and tunnel wash water. Compared to the mass of total particles in the snow (TSS), the percentage of TWP and RWPMB combined were 5.7% (meltwater) and 5.2% (mass load). For the road tunnel, the concentrations of TWP, RWPPMB and TRWP were assessed for the road surface, the gully-pots and the TWW, including TWW after sedimentation treatment. The concentration on the road surface were significantly higher in the side bank area (TWP: 2650 ± 1120, RWPPMB : 2110 ± 892; TRWP: 3840 ± 1620 mg/m2) and the outlet area (TWP: 1520 ± 2210, RWPPMB : 1210 ± 1760; TRWP: 2200 ± 3200 mg/m2) compared to the other surface areas, suggesting that these are important areas for accumulation. The mass percentage of TWP, RWPPMB and TRWP were higher in the bank area (3.8%, 3.0% and 5.5%) and the outlet (6.4%, 5.1% and 9.2%) compared to the average mass percentage. For gully pots (GP), the highest concentration of TWP, RWPPMB and TRWP were reported from the inlet GP (TWP: 24.7 ± 26.9 mg/g, RWPPMB: 17.3 ± 48.8 mg/g, TRWP: 35.8 ± 38.9 mg/g). The mass percentage of TWP (5.4%), RWPPMB (4.3%) and TRWP (7.8%) were also higher at the inlet GP compared to the other gully pots. For the tunnel wash water, the mass percentage of TWP, RWPPMB and TRWP did not change substantially from the untreated TWW (TWP 2.1%, RWPPMB 1.7% and TRWP 3.0%) to the treated TWW (TWP 2.5%, RWPPMB 2.0% and TRWP 3.6%), although there was a small increase in the percentage for the treated water. The concentrations of TWP, RWPPMB and TRWP were 38.3 ± 10.5, 26.8 ±7.33 and 55.3 ±15.2 mg/L in the untreated TWW and 14.3 ± 6.84, 9.99 ±4.78 mg/L and 20.7 ±9.88 mg/L in the treated TWW, respectively. The current treatment of TWW for this tunnel (sedimentation) retained 63% of the RAMP and 69% of the TSS, indicating a lower retention efficiency for microplastic particles compared to the total particle load. The study on roadside snow also explored the different variables potentially explaining the variation of RAMP concentrations along roads. The road types (peri-urban highway, urban highway and urban city roads) were the most important variable explaining the variation, however, the main traffic variable was speed limit. This is contradictory to previous road studies, were Annual Average Daily Traffic (AADT) has been reported as the main explanatory variable. Other statistically significant explanatory variables were distance from the road and the combination of speed and AADT. The reported concentrations of TWP, RWPPMB and TRWP in roadside snow and the road tunnel both validates the improved analytical method proposed in this thesis and contributes new data on the environmental concentrations of RAMP. More data on environmental concentrations are needed in order to assess and evaluate the levels of microplastics from roads and traffic, from different types of roads, such as highways, urban roads and country-side areas, and for different traffic variables such as speed, AADT, inclination and road maintenance. It is also necessary to evaluate the efficiency of different types of measures taken to mitigate negative environmental impacts from road pollution, including MP. To be able to evaluate and assess the efficiency of different types of mitigation measures and types of water treatments used for road and tunnel runoff, it is important to increase the number of studies across different countries, climates, road types and driving patterns, as well as using comparable methods for sampling and analysis. In short, the presented thesis provides a validated analytical method for mass quantification of microplastic particles from tire and road wear in different environmental matrices, new knowledge on the concentration levels of RAMP in different environmental compartments including the retention efficiency of TWW treatment and new knowledge on potential new sources of MPs from roads and traffic. This thesis answers to the needs defined by Norwegian government (NTP, 2022-2033), as well as providing new and improved knowledge for the research community.

Miljøpåvirkningen av vei og trafikk har blitt studert i flere tiår, og det er påvist at veiforurensing kan ha negativ påvirkning på både det terrestriske og akvatiske miljøet. Veiforurensning er typisk relatert til høye konsentrasjoner av partikler, som mineralpartikler (kvarts, feltspat) og mikro- og nanopartikler fra slitasje av bildekk og veidekker. De siste årene har forskningsinteressen for dekk- og veislitasjepartikler økt betydelig på grunn av den økte interessen for mikro- og nanoplast. Siden bildekk og enkelte typer veidekker inneholder syntetisk gummi, er disse slitepartiklene også inkludert i plastforurensningsterminologien som dekkslitasjepartikler (TWP), dekk og veislitasjepartikler (TRWP) og veislitasjepartikler med polymermodifisert bitumen (RWPPMB). Veiforurensning inneholder også typisk høye konsentrasjoner av partikkelbundet forurensning, som for eksempel metaller (sink (Zn), kobber (Cu), kadmium (Cd), nikkel (Ni), bly (Pb)) og organiske miljøgifter, som polysykliske aromatiske hydrokarbon-forbindelser (PAC), organofosfat-forbindelser (OPC), benzotiazoler, heksa(metoksymetyl)melamin (HMMM) og N-1,3-dimetylbutyl-N 0-fenyl-p-fenylendiamin-kinon (6-PPD-kinon). Særlig de akutte toksiske effektene av 6-PPD-kinon og benzotiazoler i miljøet har vært knyttet til utslipp av dekkslitasjepartikler. Vei og trafikk er estimert som den største kilden til mikroplastpartikler (MP) fra land til havmiljø, og TWP/TRWP er estimert til å være den viktigste mikroplastkilden, med slitasjepartikler fra veimerking (RM) og veislitasjepartikler med polymermodifisert bitumen (RWPPMB) som de nest største og tredje største kildene.