Uptake of tire-wear derived compounds by lettuce grown in three soils

Tire-wear derived compounds have recently been detected in commercially grown leafy vegetables, raising concern about their uptake and accumulation in crops under realistic agricultural conditions. Lettuce (Lactuca sativa L.) cultivated in three agricultural soils, which varied in sand content (25-82 %), clay content (4-27 %), cation exchange capacity (11 meq/100 g-21 meq/100 g), and organic matter content (1.0-2.6 %), were regularly irrigated with water containing 1 mg/L hexamethoxymethylmelamine (HMMM), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), 1,3-diphenylguanidine (DPG), 6PPD-quinone, and benzothiazole (BTZ). The first four tire-wear derived compounds accumulated in lettuce leaves at maximum concentrations of 66 ng/g, 3.7 ng/g, 33 ng/g and 94 ng/g respectively. Accumulation of HMMM and 6PPD in lettuce was limited by the formation of non-extractable residues in all soils (up to 93 % for HMMM and 79 % for 6PPD), while higher bioaccessibility of DPG and 6PPD-quinone (maximum 4 % and 23 % non-extractable residue formation, respectively) led to greater accumulation in plants. In the Nir Oz soil, with the highest sand (82 %) and lowest clay (4 %) content, tire-wear derived compounds accumulated at higher concentrations in the plants. Four transformation products of HMMM, five of DPG, five of 6PPD, and two of 6PPD-quinone were detected in soils, irrespective of soil type or the presence of a plant. Five transformation products of HMMM, one of DPG, six of 6PPD, and one of 6PPD-quinone were also detected in plants, irrespective of the soil type in which plants were grown. This study comprehensively tracks the fate of model tire-wear derived compounds in the soil-plant continuum, including non-extractable residue formation, bioaccumulation, and transformation. The results indicate that these compounds can accumulate in edible plants under agriculturally realistic conditions, with important implications for human exposure.