Relation of Reactive Sulfides with Organic Carbon, Iron, and Manganese in Anaerobic Mangrove Sediments: Implications for Sediment Suitability to Trap Trace Metals

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 manganese-associated elements in anaerobic mangrove sediments.