Wastewater inflows can promote Cd and Zn mobilization from tire and road wear particles in rivers

Tire and road wear particles (TRWP) are particulate emissions of rising concerns. Due to their potential adverse effects on organisms and the rising emission levels the European Union has included TRWP in the new Euro 7 standard, however, regulatory limits have not been established. TRWP are emitted into all compartments, with up to 20 % reaching the aquatic environment. Here they may act as a carrier for other pollutants such as antibiotics, plasticizers or heavy metals. The binding of heavy metals like Cd, Zn to TRWP is relatively weak. Consequently, changes in environmental conditions, including increased salinity or decrease in pH can reduce adsorption or induce desorption of previously adsorbed Cd and Zn. In this study we investigate the influence of anthropogenic dissolved organic matter (DOM) on the adsorption behavior of heavy metals to TRWP. A case study using water samples from the River Hrazdan (Armenia) revealed that untreated wastewater promotes the mobilization of Zn, whereas water samples associated with algal blooms does not have a comparable effect. Further controlled laboratory experiments with artificially defined proportions of treated wastewater demonstrated that Cd and Zn adsorption is hindered when the proportion of treated wastewater exceeds 25-50 % of the experimental solution. This was linked to the presence of humic-rich DOM and potential complexation mechanisms. Cr, Ni, As and Pb were not affected under these conditions. These findings are relevant for regions where summer droughts or climate change may result in river systems increasingly dominated by (treated) wastewater inflows.