Aging of tire and road wear particles in terrestrial and freshwater environments - A review on processes, testing, analysis and impact.

The environmental fate of tire and road wear particles (TRWPs) receives increasing attention due to the per capita emission volumes of 0.2-5.5 kg/(cap year) and recent reports on the environmental hazard of TRWP constituents. It is expected that aging impacts TRWPs fate in the environment but detailed knowledge is quite limited, yet. Making use of information on tire aging, the available knowledge on environmental aging processes such as thermooxidation, photooxidation, ozonolysis, shear stress, biodegradation and leaching is reviewed here. Experimental techniques to simulate aging are addressed as are analytical techniques to determine aging induced changes of TRWPs, covering physical and chemical properties. The suitability of various tire wear test materials is discussed. Findings and methods from tire aging can be partially applied to study aging of TRWPs in the environment. There is a complex interplay between aging processes in the environment that needs to be considered in future aging studies. In addition to existing basic qualitative understanding of the aging processes, quantitative understanding of TRWP aging is largely lacking. Aging in the environment needs to consider the TRWPs as well as chemicals released. Next steps for filling the gaps in knowledge on aging of TRWPs in the environment are elaborated.

Organic Markers of Tire and Road Wear Particles in Sediments and Soils: Transformation Products of Major Antiozonants as Promising Candidates.

Tire and road wear particles (TRWPs) are one of the main sources of particulate traffic emissions, but measured data on TRWP contents in the environment are scarce. This study aims at identifying organic compounds suitable as quantitative markers for TRWPs by a tiered multistep selection process involving nontarget screening and subsequent identification by liquid-chromatography high-resolution mass spectrometry. Starting from several thousands of signals recorded in the extract of tire particles, the rigorous selection process considered source specificity, tendency of leaching, analytical sensitivity and precision, and stability during aging. It led to three transformation products of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) as the most suitable marker candidates: N-formyl-6-PPD, hydroxylated N-1,3-dimethylbutyl-N-phenyl quinone diimine, and 6-PPD-quinone. A linear response in standard addition experiments with tire particles and the correlation with TRWP contents in a diverse set of environmental samples imply that these compounds are promising candidates as markers for the quantification of TRWPs. Organic markers for TRWP contents in the environment would allow TRWP quantification with the traditional tandem MS (LC-MS/MS) equipment of an organic trace analytical laboratory and, thus, allow easy generation of data on TRWP occurrence in sediments and soils and other environmental matrices.

Comprehensive characterization of tire and road wear particles in highway tunnel road dust by use of size and density fractionation.

Tire and road wear particles (TRWPs) are a major component of non-exhaust traffic emissions, but knowledge about their physico-chemical properties is limited. Road dust of a highway tunnel was fractionated by size and density, and fractions were analyzed for TRWPs, metals, seven tire tread indicator chemicals (benzothiazoles, 6-PPD and DPG) and effects in in-vitro bioassays. TRWP content in tunnel dust was very high (11-12%). The peak of the TRWP mass distribution was in the size fraction 20-50 µm, with 31-36% of the total TRWP mass and a content of up to 260 mg/g. The mass of organic tire constituents peaked in the smallest analyzed size fractions (<20 µm) with 35-55% of their total mass. They also peaked in the density fraction 1.3-1.7 g/cm³, indicating a lower TRWP density and a higher contribution of TP to TRWP (approx. 75%) than expected. Video-based shape analysis and SEM showed elongated particles, likely TRWPs, to be present in those size and density fractions ascribed to TRWPs by chemical analysis. But also irregular heteroagglomerates could be found. Solvent extracts of size and density fractions induced effects in bioassays indicative of the activation of the arylhydrocarbon receptor (AhR-CALUX) and the adaptive response to oxidative stress (AREc32). Similar comprehensive characterization of road dust from other sites may be needed to decide on whether TRWPs occurring in high concentrations in tunnel dust are suited as representative test materials for analytical purposes and TRWP fate studies.

The diverse metal composition of plastic items and its implications.

Plastic items from urban, freshwater and marine environments as well as from household items and electric supplies were analyzed for their metals and metalloids arsenic, barium, bismuth, cadmium, cobalt, chromium, copper, manganese, nickel, iron, lead, antimony, tin and zinc. Total metal contents ranged from 3 µg/kg (5th percentile) up to 7 g/kg (95th percentile). The median content of most metals was below 1 mg/kg and did not exceed legal limits. Iron and zinc were the metals with the highest contents, with medians of approximately 50 mg/kg. Multivariate statistics (k-means clustering and principal component analysis) did not reveal a polymer specific metal composition except for samples of tire tread rubber that was obtained from passenger car tires. Investigation on the potential origin of the metals in plastics revealed that pigments were the most likely source. In comparison to natural and anthropogenic materials in rivers, oceans and air, the metal content of plastic items was within the same order of magnitude, except for antimony and zinc contents. Literature data on the adsorption capacities of plastics suggested that the inherent content of barium, iron, antimony and zinc was dominating the total content in the studied samples. Compared to suspended sediments in rivers, the metal flux into marine environment transported with plastic items was found to be negligible due to the three orders of magnitude lower masses. The different properties, however, may consequently lead to the transport of plastics and their constituents into pristine and remote environments which natural particulate matter may not reach.

Characterization of tire and road wear particles from road runoff indicates highly dynamic particle properties.

Tire and road wear particles (TRWPs) are heteroagglomerates of tire rubber and other particles deposited on the road surface and one of the main contributors to non-exhaust emissions of automobile traffic. In this study, samples from road environments were analyzed for their TRWP contents and concentrations of eight organic tire constituents. TRWP concentrations were determined by quantifying Zn in the density fraction <1.9 g/cm³ and by thermal extraction desorption-gas chromatography-mass spectrometry (TED-GC/MS) and the concentrations ranged from 3.7 to 480 mg TRWP/g. Strong and statistically significant correlations with TRWPs were found for 2-hydroxybenzothiazole and 2-aminobenzothiazole, indicating that these substances may be suitable markers of TRWPs. The mass distribution of TRWPs in road dust suggests that the main mass fraction formed on roads consists of coarse particles (>100 µm). Data for a sedimentation basin indicate that the fine fraction (<50 µm) is preferentially transported by road runoff into receiving waters. The size distribution and density data of TRWP gathered by three different quantitation approaches also suggest that aging of TRWPs leads to changes in their particle density. An improved understanding of the dynamics of TRWP properties is essential to assess the distribution and dissipation of this contaminant of emerging concern in the environment.

Source-related smart suspect screening in the aqueous environment: search for tire-derived persistent and mobile trace organic contaminants in surface waters.

A variant of suspect screening by liquid chromatography-high-resolution mass spectrometry (LC-HRMS) is proposed in this study: Samples of a potential source of contamination and of an environmental sample close to this source are first analyzed in a non-targeted manner to select source-related suspects and to identify them. The suspect list compiled from such an exercise is then applied to LC-HRMS data of environmental samples to ascribe and to identify persistent and mobile contaminants in the water cycle that may originate from the source under study. This approach was applied to tire crumb rubber (source) and road dust (close to source); by comparison of the two data sets, 88% of the features detected in tire leachate could be excluded. Of the 48 suspects remaining, a total of 41 could be tentatively identified as either related to hexamethoxymethyl melamine or cyclic amines, benzothiazoles, or glycols. Subsequently, environmental samples were searched for these suspects: 85% were determined in an urban creek after a combined sewer overflow and 67% in the influent of a municipal wastewater treatment plant (WWTP). These exceptionally high rates of positive findings prove that this source-related smart suspect screening effectively directs the effort of selecting and identifying unknown contaminants to those related to the source of interest. The WWTP effluent and the urban creek during dry weather also showed the presence of numerous contaminants that may stem from tire and road wear particles (TRWP) in road runoff. Contribution from other sources, however, cannot be ruled out. Graphical abstract.

Relationship between Discharge and River Plastic Concentrations in a Rural and an Urban Catchment.

Rivers play a major role in the transport of plastic debris from inland sources such as urban areas into the marine environment. The present study examined plastic particle concentrations and loads (>500 µm) upstream and downstream of an urban subcatchment over 15 months and investigated the relationship between river water discharge (Q) and plastic concentration (C). The plastic particle concentration increases by 0.8 g/1000 m3 or 79 n/1000m3 from the rural to the urban subcatchment. In the rural subcatchment, C does not increase with increasing Q (p = 0.57), whereas a positive relationship between C and Q exists downstream of the urban catchment (p = 0.00003). Combined sewer overflows likely contribute additional plastic loads during high flow conditions. Based on the C-Q relationship, we estimate the total plastic export in 2016 from the entire catchment to be 3.0 × 106 n/year or 2.6 × 104 n/(km2 year) and 15 n/(cap year). Because of the positive C-Q relationship, 90% of the plastic load is transported during 20% of the time. The analysis of time-resolved plastic concentration data in rivers provides a data-driven tool to better estimate plastic loads and to better understand the catchment controls of plastic in rivers.

Tire and road wear particles in road environment - Quantification and assessment of particle dynamics by Zn determination after density separation.

In this study, a method for the determination of tire and road wear particle (TRWP) contents in particulate samples from road environment was developed. Zn was identified as the most suitable elemental marker for TRWP, due to its high concentration in tire tread and the possibility of separation from other Zn sources. The mean concentration of 21 tire samples was 8.7 ± 2.0 mg Zn/g. Before quantification in samples from road environment, TRWP were separated from the particulate matrix by density separation. Method development was conducted using shredded tread particles (TP) as a surrogate for TRWP. Recovery of TP from spiked sediment was 95 ± 17% in a concentration range of 2 - 200 mg TP/g. TP determination was not affected by other Zn containing solids or spiked Zn-salts. By adjusting the density of the separation solution to 1.9 g/cm³, more than 90% of total TRWP were separated from the sample matrix. TRWP concentrations in particulate matter collected in two road runoff treatment systems ranged from 0.38 to 150 mg TRWP/g. Differences in quantified TRWP contents of the two systems indicate changes in particle dynamics due to ageing and aggregation processes. The developed method allows TRWP determination in road runoff and in environments that are influenced by road traffic. The validated separation procedure can also be applied for TRWP characterization in future studies.

Tire wear particles in the aquatic environment - A review on generation, analysis, occurrence, fate and effects.

Tire wear particles (TWP), generated from tire material during use on roads have gained increasing attention as part of organic particulate contaminants, such as microplastic, in aquatic environments. The available information on properties and generation of TWP, analytical techniques to determine TWP, emissions, occurrence and behavior and ecotoxicological effects of TWP are reviewed with a focus on surface water as a potential receptor. TWP emissions are traffic related and contribute 5-30% to non-exhaust emissions from traffic. The mass of TWP generated is estimated at 1,327,000 t/a for the European Union, 1,120,000 t/a for the United States and 133,000 t/a for Germany. For Germany, this is equivalent to four times the amount of pesticides used. The mass of TWP ultimately entering the aquatic environment strongly depends on the extent of collection and treatment of road runoff, which is highly variable. For the German highways it is estimated that up to 11,000 t/a of TWP reach surface waters. Data on TWP concentrations in the environment, including surface waters are fragmentary, which is also due to the lack of suitable analytical methods for their determination. Information on TWP properties such as density and size distribution are missing; this hampers assessing the fate of TWP in the aquatic environment. Effects in the aquatic environment may stem from TWP itself or from compounds released from TWP. It is concluded that reliable knowledge on transport mechanism to surface waters, concentrations in surface waters and sediments, effects of aging, environmental half-lives of TWP as well as effects on aquatic organisms are missing. These aspects need to be addressed to allow for the assessment of risk of TWP in an aquatic environment.

Micropollutants in drinking water from source to tap - Method development and application of a multiresidue screening method.

A multi-residue screening method for simultaneous measurement of a wide range of micropollutants in drinking water (DW) resources was developed. The method was applied in a field study in central Sweden on water from source to tap, including samples of surface water (upstream and downstream of a wastewater treatment plant, WWTP), intake water before and after a DW treatment plant (DWTP, pilot and full-scale), treated DW leaving the plant and tap water at end users. Low detection limits (low ng L-1 levels) were achieved by using large sample volumes (5 L) combined with ultra performance liquid chromatography high resolution mass spectrometry (UPLC-HRMS). In total, 134 different micropollutants were analyzed, including pesticides, pharmaceuticals and personal care products (PPCPs), drug-related compounds, food additives, and perfluoroalkyl substances (PFASs). Of these 134 micropollutants, 41 were detected in at least one sample, with individual concentrations ranging from sub ng L-1 levels to ~80 ng L-1. Two solid phase extraction (SPE) cartridges (Oasis HLB and Bond-Elut ENV) were shown to be complementary in the field study, with three compounds detected exclusively using HLB. The total concentration in treated drinking water (56-57 ng L-1) was at a similar level as upstream from the WWTP (79-90 ng L-1). The composition of micropollutants changed along the water path, to a higher fraction of food additives and PFASs. Median treatment efficiency in the full-scale DWTP was close to 0%, but with high variability for individual compounds. In contrast, median treatment efficiency in the pilot-scale DWTP was ~90% when using nanofiltration followed by a freshly installed granulated active carbon (GAC) filter.