ENVIRONMENTAL ASPECTS OF LEAD CHROMATE

Lead chromate is a synthetic mineral pigment widely used in yellow traffic paint. Although lead chromate is reported to have a low solubility in water of 1.34 x 10^-7 mol/L at 25°C, potential exists to mobilize lead and hexavalent chromium over time and during road treatment applications. Primary concerns with lead in traffic paint are the potential to enter aquifers and impact drinking water supplies along with ecological risks to wildlife. Scanning Electron Microscopy (SEM) of lead chromate from aged traffic paint in the environment exhibits textures suggesting that some dissolution or degradation occurs over time. To determine the effect of road treatment on lead chromate, a synthetically manufactured powder and a natural mineral sample of crocoite were mixed with 6 different solutions that contained varying concentrations of either sodium chloride or calcium chloride. Unmodified PbCrO₄ powder with a mass of 0.5 ± 0.01 g was placed into polystyrene vials with 35 mL solutions of NaCl and CaCl₂ at concentrations of 0.05, 0.01 and 0.25 M. Suspension mixtures were kept with lids loose to allow atmospheric CO₂ to equilibrate with solution waters to mimic road treatment conditions. The solutions were allowed to react for approximately one month. Pb and Cr concentrations were measured on an Agilent 720ES axial-viewing Inductively coupled plasma-optical emission spectroscopy using the Pb 220.2 nm and Cr 267.7 nm wavelengths with Rh 233.4 nm used as the internal standard. CaCl₂ solutions produced higher concentrations of Pb, 81 to 199 ppm, than NaCl solutions which produced Pb concentrations of 7 to 147 ppm. Cr, however, was not detected in solutions. SEM images of reacted solids from the 0.25 M NaCl experiments do not show demonstrable dissolution features but occasionally in limited samples show some fading in contrast, which may indicate selective lead removal. SEM images of reacted solids from the 0.25 M CaCl₂ show more irregular crystals and also exhibit a gradational contrast, suggesting selective lead removal. Batch experiments clearly show that dissolution of PbCrO₄ is promoted by NaCl and CaCl₂ solutions with concentrations between 0.05 and 0.25 M. Results indicate traffic paint derived lead chromate is an environmental concern, the impact of which may not be fully recognized.