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Tire Wear Particles as a Source of Organic Contaminants in the Agroenvironment: Release of Tire Wear-Derived Compounds and their Plant Uptake
Summary
Tiny bits of worn-off tire rubber in soil release chemicals that food crops like lettuce and alfalfa can absorb through their roots and store in their leaves. One chemical in particular, called DPG, built up at especially high levels in lettuce, and since tire particles keep breaking down slowly over time, they may act as a long-term, ongoing source of this chemical entering our food. While researchers are still studying what these tire chemicals do to human health, this study shows a clear pathway from road dust to the food on our plates.
Tire wear particles (TWPs) are introduced into agricultural soils, raising concern about the release of tire wear-derived compounds (TWCs) to the environment and their uptake by plants. This study examined the release of six TWCs from TWPs and their subsequent uptake by alfalfa and lettuce plants grown in soil containing 0.1 and 1% TWPs. The release of TWCs followed a two-stage kinetic pattern; an initial fast release of surface-bound compounds followed by a prolonged, diffusion-controlled release of compounds entrapped within the rubber matrix. 1,3-Diphenylguanidine (DPG) and 2-aminobenzothiazole (A-BTZ) exhibited the highest concentrations (113 and 10 ng/g, respectively) in the alfalfa shoots. However, their concentrations declined with each of the five successive harvests. Hexamethoxymethyl melamine (HMMM) and N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) were detected only in the first harvest of alfalfa shoots, while benzothiazole (BTZ) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) were not detected. Transformation products (TPs) of the studied TWCs were detected in the alfalfa shoots, suggesting either uptake and translocation of the parent compounds followed by in-plant metabolism, and/or direct uptake of the TPs from the soil solution. Similar to alfalfa, in lettuce leaves, DPG exhibited the highest concentration (395 ng/g), followed by 6PPD (4.1 ng/g), 6PPD-quinone (0.9 ng/g), HMMM (0.6 ng/g), and A-BTZ (0.4 ng/g). BTZ was also not detected. A comparison of our data for lettuce with literature data indicated that TWCs introduced via TWPs exhibit reduced bioavailability compared to those introduced with irrigation water, except for DPG, which exhibited elevated concentrations when introduced via TWPs. This suggests that TWPs are acting as a long-term source for bioavailable DPG.