Heavy metal speciation in the sludge of the Venice canals: evaluation of selectivity and efficiency of extraction procedures

The city of Venice is crossed by a complex network of canals, where high amounts of organic matter and pollutants are discharged through different sources, such as urban waste water, sewage, boat traffic and others. Suspended particles settle on the bottom of the canals, producing an organic-rich and greatly reduced sludge, which constitutes a sink for pollutants such as heavy metals. The sediments of the canals of the city presents high levels of metal contamination, comparable to those measured in the most polluted areas of the lagoon of Venice, such as the industrial zone. Sediment disturbance by boat traffic and by tidal currents can cause the release of metals to the overlying water column; moreover, these pollutants can be released in consequence of dredging and disposal operations. Thus, besides the determination of total metal concentrations, an evaluation of the fraction of sediment-bound metals that can be potentially released is of fundamental importance. To this purpose, different procedures are commonly used; they are based either on single-step extractions, or on sequential selective extractions that allow to evaluate metal speciation. In this study a composite sediment sample from the Venice canals was used to develop suitable extraction procedures and to evaluate their efficiency and selectivity. Microwave heating was used as an alternative to conventional heating techniques, allowing to shorten considerably the time required for extraction. Optimization of microwave digestion procedures was carried on by investigating the use of different conditions and by comparing the results with those obtained by conventional extraction techniques. An attempt was made to verify extraction efficiency and selectivity; to this purpose, X-ray diffraction, Mössbauer spectroscopy, FTIR spectroscopy and scanning electron microscopy were used to determine the compositions of both the sediment and the residues of extractions. They were then compared, in order to identify which geochemical phases have been solubilized by each extraction procedure