Potential deterioration of chemical water quality due to trace metal adsorption onto tire and road wear particles - Environmentally representative experiments.

Tire wear particles are an increasing issue in particle emissions to the environment. Germany-wide approximately 100,000 t tire wear particles are emitted every year into the environment which are estimated to be one third of the microplastic emissions. Up to 20% are estimated to reach inland surface waters. Their behavior in the aquatic environment is understudied. Tire wear particles have an overly hydrophobic surface that is capable of adsorbing substances like trace elements. In this study we investigated the adsorption and desorption of trace metals onto and from the particle surface of tire-related samples in water samples of the Freiberger Mulde, a river with naturally elevated concentration of trace elements. The priority trace metals Cr, Ni, Zn, Cd and Pb show a significant adsorption onto the particle surface of tire-related samples. Tire wear particles themselves revealed adsorption of mainly Ni, Cd and Pb. Regarding the German classification for suspended matter in freshwaters, an endangering of the chemical water quality is expected due to the adsorption process and not due to the particles themselves. Upcoming electromobility is expected to increase the Zn (increased tire abrasion) and decrease the Cu amount (reduced brake abrasion) released to freshwaters.

Detailed insights in adsorption process of heavy metals on tire wear particles.

Road traffic induced tire wear particles (TWP) attracted widespread attention due to their potential environmental impact. Here, the adsorption process of heavy metals like Pb2+ and Cd2+ on tire wear particles produced by filing (TWP-f) is studied to elucidate the underlying kinetics and thermodynamics. This work includes voltammetric experiments to investigate the concentration and temperature dependency of the adsorption. The adsorption kinetics in buffer solution spiked with heavy metals follows a pseudo-second-order rate equation involving rate-controlling boundary layer adsorption and a side-by-side intraparticle diffusion process. Meanwhile, the adsorption tendencies under the studied conditions for TWP-f were Pb2+ > Cd2+. The equilibrium adsorption data were modulated by the Langmuir, Freundlich, and Dubinin-Radushkevich (DR) isotherms. Both the Freundlich and DR isotherms were found to be feasible for describing the adsorption on TWP-f. The adsorption energy obtained from the DR isotherm is 1.6 kJ mol-1 for Pb2+ and 2 kJ mol-1 for Cd2+, indicating physisorption as the dominating force. According to the Freundlich isotherm, multilayer adsorption is proposed. The thermodynamic parameters show that the adsorption of Pb2+ and Cd2+ is endergonic. Due to small Gibbs enthalpy values near the thermodynamic equilibrium, the adsorption process is mainly dependent on the ambient conditions. So, close-to-nature experiments were conducted to verify the received results. Therefore, tire and road wear particles including road sediments (TRWP+RS) were added to prefiltered freshwater samples of the river Freiberger Mulde (having naturally elevated trace element concentrations). The adsorption kinetics were investigated by ICP-MS/MS emphasizing the pseudo-second-order rate equation. Moreover, it is suggested that the tire wear particles in the TRWP+RS sample are majorly responsible for the adsorption of at least Cd2+.