Surface topography, mineralogy, and Pb isotope survey of wheel weights and solder: Source of metal contaminants of roadways and water systems

Abstract

Wheel weights and solders are among the most important anthropogenic lead sources in modern society. Characterization of size, shape, and stages of degradation of wheel weights and solders by scanning electron microscopy provides fundamental knowledge toward better understanding of processes related to abrasion, dissolution, and transfer of metallic lead to the surface environment. Our studies of wheel weights identified partially abraded and chemically corroded textures, with coatings of reconstituted Pb particles, including litharge (PbO) and/or plattnerite (PbO2), in a matrix of soil components, mainly clays, feldspars, silica, and iron-oxides. Pb oxide particles abraded from wheel weights have a major component of <1 μm-size particles and consist of acicular needles and hair-like crystals are of concern because the size and particle shapes are known to have significant adverse health impacts if ingested, inhaled, or imbedded in the human body.Pb isotope analysis (by TIMS and HR-ICPMS) provides a way to identify the main lead exposure pathways if wide differences in isotopic ratios exist among possible local sources. Lead isotopic ratios for wheel weights from the U.S. plot along narrow arrays in plots of 206Pb/207Pb (~1.130–1.230) vs. 208Pb/207Pb (~2.410–2.470). The apparent trend in the 208Pb/207Pb and 206Pb/207Pb plot implicates a lead source from central U.S. from Mississippi Valley-type deposits, and a less radiogenic endmember which may link to an international lead source, such as from deposits of China. Wheel weights and corresponding leachates are analytically indistinguishable. 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 Pb. The legacy of past and current uses of lead metal in manufactured products will continue to be of global concern for the foreseeable future because Pb-rich products, such as wheel weights and solders, remain commercially available through both primary and secondary global markets.