Uptake of tire-derived compounds in lettuce under realistic growing conditions

Tire wear particles represent a major fraction of global microplastic pollution, potentially entering agricultural ecosystems through biosolid application, treated wastewater irrigation, or atmospheric deposition. These particles contain high concentrations of organic additives and associated transformation products (tire-derived compounds), posing ecological and human health risks once released into the environment as some of these compounds exhibit high toxicity to fish and have been detected in human blood and urine.

We investigated the uptake by edible crops under realistic conditions of five tire-derived compounds: Benzothiazole (BTH), Diphenylguanidine (DPG), Hexamethylmethoxymelamine (HMMM), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its transformation product 6PPD-quinone. In greenhouses, we grew lettuce (Lactuca sativa) in three well characterized soils varying in their physical-chemical properties collected from the Israeli Negev region. During the growing season, irrigation water was spiked every two days with tire-derived compounds at 500 μg/L. As a control, pots without plants were spiked with tire-derived compounds. After two months the plants were harvested and partitioned into three sections: inner leaves, outer leaves and roots. Soil samples were collected from the top soil (0-10 cm) and the bottom soil (10 cm - bottom of pot). Soil pore water samples were also collected at various time points. Tire-derived compound concentrations were quantified in all samples with HPLC-MS/MS. 

 

The bioavailable concentrations of DPG, HMMM and BTH in soil pore water decreased dramatically within 4 h of spiking, likely due to soil sorption, transport and plant uptake. At harvest, DPG, HMMM, 6PPD and 6PPDq were detected in the top soil, while in the bottom soil DPG, 6PPD and 6PPDq were detected only sporadically and at trace concentrations, suggesting limited vertical mobility in the soils. Compared to the no-plant controls, in pots with plants DPG and 6PPD had lower concentrations in top soil, indicating depletion by the plants. In the roots, tire-derived compounds were detected at concentrations in the order of 6PPDq > DPG > HMMM. 6PPD and BTH were not present at quantifiable concentrations. 6PPD, 6PPDq, DPG and HMMM were detected in the lettuce leaves at concentrations following the order of HMMM (max 62.1 ng/g dw) > 6PPDq (max 53.8 ng/g dw)  > DPG (max 42.3 ng/g dw)  >> 6PPD (max 2.1 ng/g dw). BTH was sporadically detected in leaves but was not quantifiable. Concentrations in the outer leaves were generally higher than in the inner leaves. The lowest leaf concentrations were found in plants grown in the soil with the highest clay content, likely due to a higher sorption and reduced bioavailability of the compounds in this soil.

Our findings show that tire-derived compounds can be taken up by edible crops under realistic growing conditions. Although the spiked concentrations were likely higher than environmental concentrations, our results indicate that plant uptake is a potential pathway for tire-derived compounds to enter the human food chain.