Mobility of non-exhaust brake wear dust from road traffic in saturated and unsaturated porous media mimicking subsurface environments

Road traffic is a major source of atmospheric pollution, especially in urban areas, contributing significantly to particulate matter (PM) emissions. While electric vehicles (EVs) help reduce exhaust emissions, they do not substantially address non-exhaust emissions (NEEs), such as brake wear dust (BWD), which remains a significant source of PM, particularly in urban environments. This study investigates at a preliminary level the environmental fate of BWD, studying at the laboratory scale its mobility and behaviour in unsaturated and saturated porous media, which simulate subsoil and aquifer conditions. BWD, produced through friction between brake pads and rotors during deceleration, can settle on road surfaces, posing risks to soil and water quality through runoff and infiltration. Laboratory tests were conducted here to highlight BWD transport mechanisms in porous media. BWD mobility is influenced by ionic strength and flow velocity, with higher rates promoting particle transport, and higher ionic strengths inhibiting it. The study also highlights the importance of soil composition, with sandy soils allowing for greater BWD mobility than clay-rich soils. The findings evidence the need for continued research to better understand the environmental risks posed by BWD, particularly in unsaturated soils. Additionally, the potential of BWD to act as a carrier for other pollutants requires further investigation.