Surface topography, mineralogical survey, and Pb isotopic compositions of vehicle wheel weights: Major heavy metal contaminants of roadways

Wheel weights used for balancing tires are among the top five anthropogenic lead (Pb) sources contaminating the surface environment in modern society. In the present study we characterized size, shape, and stages of degradation of wheel weights from the U.S.A. to understand abrasion, dissolution, and transfer of metallic Pb. In a previous study of wheel weights [Ayuso and Foley 2020, Jour. Geochem Explor.], we identified partially abraded and chemically corroded textures with coatings of reconstituted Pb particles, including litharge (PbO) and/or plattnerite (PbO₂), in a matrix of clays, feldspars, silica, and iron-oxides. New examples of finely comminuted Pb-rich particles (by Field Emission-Scanning Electron Microscopy) include platy sheaths, rods, and tabular crystals up to about 30 µm in length, slender needles less than ~0.5 µm in width, and submicrometer-sized flakes, chips, and masses of indeterminate form.  Abraded Pb oxide particles are mostly <1 µm-size consisting of acicular needles and hair-like crystals; the size and particle shapes can have significant adverse health impacts if ingested, inhaled, or imbedded in the human body. Pb isotope analysis (by Thermal Ionization Mass Spectrometry and High Resolution Inductively Coupled Plasma Mass Spectrometry) provides a way to identify the main lead exposure pathways. Lead isotopic ratios for wheel weights (bulk and acid-leachates) in plots of ²⁰⁶Pb/²⁰⁷Pb (~1.130-1.230) vs. ²⁰⁸Pb/²⁰⁷Pb (~2.410-2.470) are analytically indistinguishable. The ²⁰⁸Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁷Pb data implicate a lead source from Mississippi Valley-type deposits (U.S.A.) and a less radiogenic endmember which may link to an international lead source, such as ore deposits of China. Degradation of wheel weight particles will influence the Pb isotopic composition of local surface and groundwater regardless of whether the contribution is from solid particles derived from grain erosion or from dissolved forms of Pb. Geochemical modelling supports the argument that pH is a primary factor in lead corrosion; however, the range of sizes (<63 µm) and morphologies (needles, hairs) identify an additional health concern associated with intake of weathered products of Pb components found in water and soils. The legacy of past and current uses of Pb metal in manufactured products will continue to be of global concern because these industrial products remain readily available through both primary and secondary global markets.