Sulfide and iron control on mercury speciation in anoxic estuarine sediment slurries

In order to understand the role of sulfate and Fe(III) reduction processes in the net production of monomethylmercury (MMHg), we amended anoxic sediment slurries collected from the Venice Lagoon, Italy, with inorganic Hg and either potential electron acceptors or metabolic byproducts of sulfate and Fe(III) reduction processes, gradually changing their concentrations. Addition of sulfide (final concentration: 0.2–6.3 mM) resulted in an exponential decrease in the sulfate reduction rate and MMHg concentration with increasing concentrations of sulfide. Based on this result, we argue that the concentration of dissolved sulfide is a critical factor controlling the sulfate reduction rate, and in turn, the net MMHg production at steady state. Addition of either Fe(II) (added concentration: 0–6.1 mM) or Fe(III) (added concentration: 0–3.5 mM) resulted in similar trends in the MMHg concentration, an increase with low levels of Fe additions and a subsequent decrease with high levels of Fe additions. The limited availability of dissolved Hg, associated with sulfide removal by precipitation of FeS, appears to inhibit the net MMHg production in high levels of Fe additions. There was a noticeable reduction in the net MMHg production in Fe(III)-amended slurries as compared to Fe(II)-amended ones, which could be caused by a decrease in the sulfate reduction rate. This agrees with the results of Hg methylation assays using the enrichment cultures of anaerobic bacteria: whereas the enrichment cultures of sulfate reducers showed significant production of MMHg (4.6% of amended Hg), those of Fe(III), Mn(IV), and nitrate reducers showed no production of MMHg. It appears that enhanced Fe(III)-reduction activities suppress the formation of MMHg in high sulfate estuarine sediments.     

Relocation effects of dredged marine sediments on mercury geochemistry: Venice lagoon,Italy

Understanding the biogeochemical process of Hg is critical in the overall evaluation of the ecological impacts resulting from the reuse of Hg-contaminated dredged sediment. Sediment banks (V1 and V2) were constructed with freshly dredged sediments from a navigational channel in Venice Lagoon, Italy, with the goal of clarifying potential differences in the biogeochemistry of Hg between the reused dredged sediments and those from surrounding sites (SS1 and S2). Toward this purpose, Hg and monomethylmercury (MMHg) concentrations, and Hg methylation rates (MMRs) in the surface 2.5 cm sediments were monitored, along with ammonium, iron, sulfate and sulfide concentrations in the pore waters of banks and surrounding sites from November 2005 to February 2007. Pore water analyses indicate that the bank sediments are characterized by lower levels of sulfate and iron, and by higher levels of ammonium and sulfide compared to the surrounding sediments. With respect to Hg speciation, the fractions of MMHg in total Hg (%MMHg/Hg) and the MMRs were significantly lower in the bank V1 compared to those in the reference site SS1, whereas the %MMHg/Hg and the MMRs were similar between V2 and S2. A negative correlation is found between the logarithm of the particle-water partition coefficient of Hg and the MMR, indicating that the reduced MMRs in V1 are caused by the limited concentrations of dissolved Hg. Organic matter appears to play a key role in the control of MMR via the control of Hg solubility.     

Evidence for enhanced bioavailability of trace elements in the marine ecosystem of Deception Island, a volcano in Antarctica

This study assessed whether trace elements present at Deception Island, an active submarine volcano in the Antarctic Peninsula, show enhanced biological availability to the local marine community. Using a weak acid extraction method to dissolve organic material and leach associated but not constitutive trace elements of sediments, fifteen elements were measured from seafloor sediment, seawater particulates, and tissues of benthic (bivalves, brittlestars, sea urchins) and pelagic (demersal and pelagic fishes, krill) organisms collected in the flooded caldera. The highest element concentrations were associated with seafloor sediment, the lowest with seawater particulates and organism tissues. In the case of Ag and Se, concentrations were highest in organism tissue, indicating contamination through the food chain and biomagnification of those elements. The elements Al, Fe, Mn, Sr, Ti, and to a lesser extent Zn, were the most concentrated of the trace elements for all sample types. This indicates that the whole ecosystem of Deception Island is contaminated with trace elements from local geothermal activity, which is also reflected in the pattern of element contamination in organisms. Accordingly, element concentrations were higher in organisms collected at Deception Island compared to those from the neighboring non-active volcanic King George Island, suggesting that volcanic activity enhances bioavailability of trace elements to marine organisms. Trace element concentrations were highest in digestive tissue of organisms, suggesting that elements at Deception Island are incorporated into the marine food web mainly through a dietary route.     

Macrobenthic community response to copper in Shelter Island Yacht Basin, San Diego Bay, California

We examined Cu contamination effects on macrobenthic communities and Cu concentration in invertebrates within Shelter Island Yacht Basin, San Diego Bay, California. Results indicate that at some sites, Cu in sediment has exceeded a threshold for “self defense” mechanisms and highlight the potential negative impacts on benthic faunal communities where Cu accumulates and persists in sediments. At sites with elevated Cu levels in sediment, macrobenthic communities were not only less diverse but also their total biomass and body size (individual biomass) were reduced compared to sites with lower Cu. Cu concentration in tissue varied between species and within the same species, reflecting differing abilities to “regulate” their body load. The spatial complexity of Cu effects in a small marina such as SIYB emphasizes that sediment-quality criteria based solely on laboratory experiments should be used with caution, as they do not necessarily reflect the condition at the community and ecosystem levels.     

Distribution, Speciation, and Extractability of Cadmium in the Sedimentary Phosphorite of Hahotoé-Kpogamé (Southern Togo)

The geochemistry and extractability of Cd in the phosphorite deposits of Hahotoé-Kpogamé (southern Togo) have been studied using various methodologies such as Cd distribution in profiles, grain-size dependence analysis of Cd content, Cd localization using scanning electron microscopy (SEM), sequential acid extraction, and a leaching experiment with artificial seawater. Results demonstrate that in the phosphorite deposits of Hahotoe-Kpogame, Cd is enriched by a factor of 157 compared to shale and by a factor of 3 compared to average world phosphorite composition. The main carrier of Cd appears to be apatite. This is evidenced by significant high positive correlations between the Cd content and P₂O₅ (in bulk sedimentr = 0.7 and in the 1–0.2 mm fraction r = 0.9). The grain-size dependence of Cd contents (concentrations decreasing with decreasing grain size) and SEM analysis supports these conclusions. Secondary Cd carriers include calcite, goethite, and various bone fragments. Sequential extraction tests with acetic acid and EDTA show an extraction rate reaching up to 40%. Leaching experiments with artificial seawater show evidence of Cd release in seawater. This leads to the conclusion that the processing of phosphorite by wet sieving using seawater and dumping of phosphorite tailings into the coastal waters of Togo can be a major source of marine pollution with Cd.