Speciation and mobility of metals from vehicular non-exhaust particulate matter

Non-exhaust vehicle particulate matter emissions (NEVPM) are gaining prominence in air quality policies due to reduced exhaust emissions and the high metal load from tyre and brake wear. Once deposited, NEVPM components can be mobilised by rainwater, contributing to urban water contamination. This study examines the chemical composition of tyres, brake pads (BPs), and brake dust. Brake pads were thermally treated to simulate braking temperatures, and both treated BPs and brake dust underwent leaching tests to assess metal release and speciation. Tyre formulations showed consistent carbon content and inorganic additives, whereas BPs comprised complex mineral phases with variable Al, Fe, Ti, Cu, and trace metals. Thermal treatment induced mineralogical transformations, including oxidation and formation of metal oxides and sulphates, altering metal mobility and bioavailability. Leaching tests revealed toxic metal releases, particularly Zn, Cu, Pb, Mn, Cr(VI), and As, often exceeding regulatory thresholds. Cr(VI) formation increased post-treatment, underscoring health risks. Brake dust predominantly contained fine PM enriched with Fe oxides, Cu, Zn, Ba, Sb, and other trace elements, highlighting brake wear as a major urban pollution source. These findings emphasise tyre and brake debris as significant metal pollution sources, underscoring the need for stricter regulatory controls and safer material formulations.