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

Data on the particulate <----> dissolved speciation of total Pb (Sigma Pb) 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 % Sigma Pb solubility values of the samples display large variations, ranging from > 90 to < 10%. The 'p H-% Sigma Pb solubility' relationship is constrained by a relatively sharp classical pH-edge, extending from pH similar to 5.0 at the high solubility end to pH similar to 5.8 at the low solubility end. At pH values greater th an that of this 'limiting' pH-edge, the % Sigma Pb solubility is < similar to 10% at any solution pH. The composite plot therefore confirms that pH-me diated adsorption/desorption reactions play a dominant role in controlling the solubility of the Sigma Pb in precipitation. However, the plot shows a wide range of Sigma Pb solubilities at pH values to the more acidic side of the 'limiting' pH-edge. Lead in the dissolved state has the most significa nt effect on aquatic systems, and the major environmental consequence of th e variation in Sigma Pb solubilities at pH values less than that of the 'li miting' 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 % Sigma Pb solubility in rainwater can have large variations (< sim ilar to 10 to > similar to 90% of the Sigma Pb) over a narrow pH range. (C) 2000 Elsevier Science Ltd. All rights reserved.