Influence of UV-B irradiation in lead speciation from an estuarine sample

The influence of UV-B irradiation in estuarine water from the salt marsh of Corroios, Tagus river, was studied in the laboratory in terms of lead speciation. The filtered water was exposed to UV-B irradiation with a intensity of 540 mu W/cm(2). The effect of UV-B (280-320 nm) was studied from the titration with Pb2+, followed by anodic stripping voltammetry in differential-pulse mode, after different periods of irradiation time, from 1 h up to 10 days. It was found that, before UV-B irradiation, Pb2+ speciation can be interpreted assuming two different types of ligands: one macromolecular, with an heterogeneous behaviour represented by two main groups, while the other is a small ligand, homogeneous in terms of Pb2+ complexation. Stability constant values indicate that the heterogeneous ligand presents a humic-like behaviour, with minor sites related to phenolic, amino or sulphur type groups, and major sites related to carboxylic type groups. Small homogeneous ligands, probably released by bioorganisms, also present stability constants of carboxylic type groups. From the results it can be observed that UV-B irradiation decreases complexing strength of the medium as well as its heterogeneity, both effects becoming evident after 1 h of irradiation. Even so, for the concentration range studied (C-Pb less than or equal to 2 x 10(-6) M) more than 80% of Pb-II still remains complexed to the weaker group of the macromolecular ligand. Since UV-B doses are similar to maximum spring solar intensity, the results suggest both chemical and ecological implications. (C) 2000 Elsevier Science Ltd. All rights reserved.