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The accumulation of acid-volatile sulfides and its relation with other trace metal-binding sediment constituents may be important mechanisms to determine the suitability of anaerobic sediments to trap trace metals. An investigation into the relationships among acid-volatile sulfides, total organic carbon, and reactive (hydrogen chloride-soluble) iron, manganese, and trace metals of environmental concern (cadmium, copper, nickel, lead, and zinc) was conduced in anaerobic sediments from a disturbed mangrove ecosystem in Sepetiba Bay (SE Brazil). The two main metal contaminants in the study area (cadmium and zinc) presented levels above regional backgrounds (showing enrichment factors up to 8.5 and 9.1, respectively) and a significant correlation, suggesting a contamination source similarity. Positive correlations among total organic carbon, acid-volatile sulfides, and iron and negative correlations of manganese with total organic carbon and acid-volatile sulfides occurred, probably because of (1) organic matter fuelling of acid-volatile sulfides production and depletion of manganese levels due to manganese oxide consumption by microbial respiration and (2) manganese oxide depletion in reaction with acid-volatile sulfides. The elevated acid-volatile sulfides concentrations and inventories observed, exceeding those of metals of environmental concern, indicate the sediment suitability to trap these metals as metal sulfides. While comparatively low manganese concentrations are probably unable to influence acid-volatile sulfides distribution, results evidenced that acid-volatile sulfides may affect manganese distribution negatively. To our knowledge, this negative metal-sulfur relationship was demonstrated for the first time in sediments from a tropical coastal ecosystem, which may help elucidate the behavior of manganese and manganeseassociated elements in anaerobic mangrove sediments.

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Sediment re-suspension experiments have been conducted to predict contaminants release from sediments to the water column, during dredging operations. In this context, polluted, anoxic estuarine sediments from Rio de Janeiro, SE Brazil, were suspended in oxygenated estuarine water, in laboratory experiments intended to simulate their dispersion by flood flow or dredging operations, in order to measure any release into solution of heavy metals originally present as sulphides that might suffer oxidation. Oxidation of sulphides to sulphate acidified the waters but only after at least 5 h of suspension. Furthermore, the oxidation of acid volatile sulphide (AVS) to sulphate was more rapid and only proceded to completion within 5 days, when large quantities of sulphide forming metals other than Fe were not present. In sediment heavily polluted with zinc, oxidation of AVS was slower and incomplete, resulting in soluble release of a much smaller fraction of the Zn present in the sediment and a maximum dissolved zinc concentration that was much lower than that resulting from less contaminated sediment. The maximum percentages of sulphide-bound metals appearing in solution at any time during re-suspension were low, less than 46% in all cases and typically less than 10%. These maxima were manifested only after acidification by sulphate formation. Appreciable metal dissolution would not occur in an estuary if dilution and dispersion separated the sediment from acid generated or if dredged material settled before acidification occurred.