Heavy metals in surface sediments of the lagoon of Venice: evaluation of selectivity and efficiency of extraction procedures

In this study new procedures for the extraction of heavy metals from sediments were developed. Microwave heating was used as an alternative to conventional digestion techniques, allowing to shorten considerably the time required for extraction and to improve the reproducibility of the various procedures. The study concerned both single step acid extractions, used to evaluate the anthropogenic and total content of metals, and sequential selective extractions, used to determine the partitioning of heavy metals among the different components of sediments. 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. Attention has been focused on the evaluation of efficiency and selectivity of the newly developed procedures. To this purpose, X-ray diffraction, Mössbauer spectroscopy, FTIR spectroscopy and scanning electron microscopy were used to determine the composition both of the sediment and of the residues of extractions; the results were then compared, in order to identify which geochemical phases have been solubilized by each extraction procedure. The results pointed out the main factors that are likely to affect efficiency and selectivity. In particular, the extraction used for determining the anthropogenic fraction of metals yielded an incomplete solubilization of authigenic sulphides and a partial attack of residual phases. Sequential extractions showed some indetermination due to non efficient solubilization of carbonates. Moreover, the effect of the presence of organic matter coatings on the surface of sediment particles was underlined