Weathering of leaded paint and chromate primer: Transformation of engineered into incidental nanoparticles

Nanoparticles (NPs) containing Pb are concerning due to the exceptionally hazardous nature of the element and its widespread use. This study characterizes the weathering of Pb-bearing paint chips (PCs) into Pb-bearing NPs in the surficial soils of Germantown, Philadelphia, USA. The PCs are composed of nano- to micrometer-size engineered particles (EPs) of massicot (PbO, Pb-pigment), Zn-chromates (ZnCrO₄, primer) and white paint components (rutile, TiO₂, barite, BaSO₄, wurtzite, ZnS). These EPs transform into incidental nanoparticles (INPs) of litharge and massicot (both PbO), crocoite (PbCrO₄), and phoenicochroite (Pb₂CrO₅), Zn-(hydr)oxides, Cr-(hydr)oxides and anatase/brookite (TiO₂) INPs. With increasing degree of weathering, the altered PCs become depleted in Zn and Pb relative to layers composed of massicot and Zn-chromates with the latter two elements being released as INPs or solutes into the bulk soil. Chemical and mineralogical characterization of colloidal fractions leached from collected soil samples indicate that pyromorphite (Pb₅(PO₄)₃OH) is the only observed Pb-bearing phase, suggesting that the observed Pb-bearing INPs in the PCs are either not mobile or transform into the former phase. The results of this study suggest that the transformation of larger Pb-, Cr- and Zn-bearing EPs into INP's increase the reactivity, bioaccessibility and their health risks.