Characterization of heavy metal pollution in the sediments of the lagoon of Venice by sequential selective extractions

In this study heavy metal pollution in surface sediments of the lagoon of Venice (Italy) has been investigated. A new procedure of sequential selective extractions has been developed to determine the partitioning of heavy metals among the different components of sediments; it provides metal concentrations in five fractions: 1) exchangeable, 2) bound to carbonates, 3) bound to Fe-Mn oxides/hydroxides, 4) bound to organic matter and sulphides, 5) bound to resistant authigenic phases and 6) residual. Microwave heating has been used as an alternative to conventional heating techniques, allowing to shorten considerably the time required for extraction; the perfect control of experimental conditions, such as temperature and pressure, improved greatly the reproducibility of the various extraction procedures. A particular attention has been posed to the evaluation of efficiency and selectivity. to this purpose, the composition of the sediment and of the residue of each extraction has been determined by means of x-ray diffraction, mossbauer and ir spectroscopy and scanning electron microscopy; the comparison of the results was used to identify the geochemical phases affected by each extractant.The analytical procedure has been applied to sediment samples collected in various areas of the lagoon of Venice, characterized by different morphological, hydrodinamical and physicochemical conditions. The use of sequential extractions can provide information on the amounts of heavy metals that can be potentially released to the overlying water column under particular environmental conditions, becoming thus bioavailable. This approach is useful to assess the risk posed to the lagoon ecosystem by metal contaminated sediments and permits to estimate anthropogenic heavy metal content