The solubility of Pb in coastal marine rainwaters: pH-dependent relationships

Data on the particulate ↔ dissolved speciation of total Pb (ΣPb) in 175 rainwater samples from a variety of coastal marine environments have been used to construct a composite ‘pH–% Pb solubility’ plot which covers the pH range found in natural rainwaters. The % ΣPb solubility values of the samples display large variations, ranging from >90 to <10%. The ‘pH–% ΣPb solubility’ relationship is constrained by a relatively sharp classical pH-edge, extending from pH 5.0 at the high solubility end to pH 5.8 at the low solubility end. At pH values greater than that of this ‘limiting’ pH-edge, the % ΣPb solubility is < 10% at any solution pH. The composite plot therefore confirms that pH-mediated adsorption/desorption reactions play a dominant role in controlling the solubility of the ΣPb in precipitation. However, the plot shows a wide range of ΣPb solubilities at pH values to the more acidic side of the ‘limiting’ pH-edge. Lead in the dissolved state has the most significant effect on aquatic systems, and the major environmental consequence of the variation in ΣPb solubilities at pH values less than that of the ‘limiting’ pH-edge is that even in urban regions, where the scavenged aerosol has relatively high concentrations of Pb in a potentially soluble form, the actual % ΣPb solubility in rainwater can have large variations (< 10 to > 90% of the ΣPb) over a narrow pH range.