In-vitro digestion of Tire and Road Wear Particles: Bioavailability of metals and Polycyclic Aromatic Hydrocarbons

The potential impact of Tire and Road Wear Particles (TRWP) on aquatic organisms has recently gained attention since the occurrence of TRWP in the aquatic environment has been observed in surface water and sediments of numerous regions. Moreover, the ingestion of TRWP by aquatic species, including fish, has recently been demonstrated. However, data regarding the bioavailability and toxicity of contaminants associated with these particles are still lacking. This study aimed (i) to characterize the solubilization potential of metals and PAHs from tire particles and TRWP in Simulated Gastric Fluids (SFG) and Simulated Intestinal Fluids (SIF) designed to mimic Oncorhynchus mykiss (Rainbow Trout) gut conditions and (ii) to assess the impact of influencing factors such as co-ingestion of natural food organic matter on the compounds solubilization. Our results showed that solubilization of all metals and PAHs were enhanced by up to 10-fold in simulated gut fluids compared to solubilization in the control (phosphate buffer saline solution). Zn was highly solubilized after an in-vitro digestion of 24-h with a final concentration in SIF of 3,2 µg.L-1 corresponding to 23 % of the total Zn concentration in TRWP. Five PAHs (Fluoranthene, Phenanthrene, Pyrene, Benzo (e) Pyrene and Benzo (g,h,i) perylene) were quantified in SIF after an incubation time of 24-h and the concentration of individual PAHs in SIF ranged from 42 to 3092 ng.L-1 which represented 0,4 – 1,5 % of their total concentration measured in tire particles. Metals were highly solubilized by the acidic SGF (pH = 2) but only a weak solubilization was observed in neutral SIF (pH = 7,4). Contrastingly, PAHs were not solubilized in SGF and highly solubilized in SIF which was proved to contain high quantities of proteins and surfactants forming micelles. This emphasise the importance of pH in the solubilization of metals and of bile surfactants and organic matter in the solubilization of PAHs from TRWP and brings new insights on the bioavailability of metals and organic pollutants from TRWP. Overall, our results demonstrate the importance of accounting for the ingestion of TRWP as an important route of exposure to metals and PAHs for aquatic organisms. As the solubilization of potential toxic compounds was greatly enhanced by the gut fluids and leads to bioavailability of these compounds, further studies investigating the uptake by the epithelial cells and transfer throughout the whole organism will be performed.