Researchers at the University of Vienna (Austria) have demonstrated that tire wear particle (TWP)-derived compounds are readily taken up by lettuce with measured maximum leaf concentrations between ∼0.75 for N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine (6PPD) and 20 μg g^–1 for hexamethoxymethyl melamine (HMMM).

In an open-access paper in the ACS journal Environmental Science & Technology, they said that although these compounds were metabolized in the plant, they identified several transformation products, most of which proved to be more stable in the lettuce leaves than the parent compounds.

Furthermore, continuous leaching from TWP led to a resupply and replenishment of the metabolized compounds in the lettuce leaves. The stability of metabolized TWP-derived compounds with largely unknown toxicities is particularly concerning and is an important new aspect for the impact assessment of TWP in the environment, they concluded.

The extent of continuous tire wear particle (TWP) emissions into the environment remains poorly quantified, but world-wide emissions are estimated to amount to 5.9 million tons per year. TWP are expected to be introduced to farmland soils via several pathways, including atmospheric deposition and road runoff. Additionally, high retention of TWP in wastewater treatment plants (WWTP), with an estimated amount of >93% of TWP retained, implicates that WWTP sludge is an important source of TWP to farmlands when biosolids are applied as fertilizers. It was estimated that in Germany, between 1400 and 2800 tons of TWP per year are deposited on agricultural land through the application of biosolids.

TWP can introduce a large suite of organic compounds to farmland soils with unknown effects on biota. Aside from rubber and fillers, tires contain additives that are, to date, indispensable for their functionality. These include vulcanization accelerators, activators, plasticizers, processing aids, and antioxidants.

The vulcanization accelerators 1,3-diphenylguanidine (DPG) and benzothiazole (BTZ) have been detected at concentrations in the μg L^–1 range in rivers. Although these concentrations are close to the compounds’ predicted no effect concentrations (PNEC), it was shown that at higher concentrations, they are toxic to fish embryos. Similar concentrations have been reported for hexamethoxymethyl melamine (HMMM), a cross-linking agent with over 30 known transformation products. TWP have been implicated as the predominant source of these compounds in rivers. Not only the parent compounds but also their transformation products, many of which are still unknown, may exert harmful effects on biota. For example, the anti-ozonant N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine (6PPD) is transformed into a significantly more toxic quinone transformation product (6PPD-q), which is responsible for mortalities of coho salmon through the introduction of street runoff into surface waters, where it has been detected in the concentration range of μg L^–1.

—Castan et al.

The researchers noted that if regulatory thresholds are only defined according to the concentrations of original TWP-derived compounds, they will underestimate the sum of parent compounds and transformation products with so far largely unknown toxicities.

Resources

• Stephanie Castan, Anya Sherman, Ruoting Peng, Michael T. Zumstein, Wolfgang Wanek, Thorsten Hüffer, and Thilo Hofmann (2022) “Uptake, Metabolism, and Accumulation of Tire Wear Particle-Derived Compounds in Lettuce” Environmental Science & Technology doi: 10.1021/acs.est.2c05660