Extraction of Anthropogenic Heavy Metals from Reduced Sediments: Indeterminations due to Authigenic Sulphides and Clay Minerals

A mineralogical and micro-chemical study was performed by X-ray diffraction and SEM/EDS techniques in a reduced sediment sample from the Venice Lagoon to investigate selectivity and significance of two widely used acid extractions (cold IN HCl and hot 8N HNO3) for the evaluation of anthropogenic metal concentrations. Hydrochloric acid determines an incomplete and variable solubilization of authigenic iron sulphides, yielding a partial extraction of the associated metals (Hg, Mn, Zn and less Ni and Cu). Nitric acid is effective in solubilizing all sulphides, however, it leads to an overestimate of the anthropogenic metal content since it partially dissolves the lattice of some clay-minerals, particularly chlorites. When these acid extractions are used to asses metal pollution in aquatic ecosystems, the possible over or under-estimate of the anthropogenic concentration must be taken into account, considering the mineralogical composition of the sediment.