Seasonal cycling of sulfur and iron in porewaters of a Delaware salt marsh

An extensive pore water data set has been gathered in the Great Marsh, Delaware over various seasons, salinities, and tides. The data all point to a complimentary redox cycle for sulfur and iron which operates seasonally and tidally. Surface oxidizing conditions prevail in summer, with more reducing conditions at depth during the winter. During the spring tides which flood the marsh, pyrite oxidation occurs releasing excess dissolved iron (II) and sulfate to the porewaters, and precipitating authigenic solid iron phases. The redox conditions in the porewaters of the upper zone during the summer is poised between mildly oxidizing and mildly reducing conditions as shown by pE calculations. This redox environment and intermediate iron-sulfur redox species may be important for the stimulation of plant growth (photosynthesis) and sustenance of a viable microbial community (heterotrophy and chemoautropy)

Redox reactions and weak buffering capacity lead to acidification in the Chesapeake Bay

The combined effects of anthropogenic and biological CO2 inputs may lead to more rapid acidification in coastal waters compared to the open ocean. It is less clear, however, how redox reactions would contribute to acidification. Here we report estuarine acidification dynamics based on oxygen, hydrogen sulfide (H2S), pH, dissolved inorganic carbon and total alkalinity data from the Chesapeake Bay, where anthropogenic nutrient inputs have led to eutrophication, hypoxia and anoxia, and low pH. We show that a pH minimum occurs in mid-depths where acids are generated as a result of H2S oxidation in waters mixed upward from the anoxic depths. Our analyses also suggest a large synergistic effect from river-ocean mixing, global and local atmospheric CO2 uptake, and CO2 and acid production from respiration and other redox reactions. Together they lead to a poor acid buffering capacity, severe acidification and increased carbonate mineral dissolution in the USA\u27s largest estuary

Metal–organic complexation in the marine environment

We discuss the voltammetric methods that are used to assess metal–organic complexation in seawater. These consist of titration methods using anodic stripping voltammetry (ASV) and cathodic stripping voltammetry competitive ligand experiments (CSV-CLE). These approaches and a kinetic approach using CSV-CLE give similar information on the amount of excess ligand to metal in a sample and the conditional metal ligand stability constant for the excess ligand bound to the metal. CSV-CLE data using different ligands to measure Fe(III) organic complexes are similar. All these methods give conditional stability constants for which the side reaction coefficient for the metal can be corrected but not that for the ligand. Another approach, pseudovoltammetry, provides information on the actual metal–ligand complex(es) in a sample by doing ASV experiments where the deposition potential is varied more negatively in order to destroy the metal–ligand complex. This latter approach gives concentration information on each actual ligand bound to the metal as well as the thermodynamic stability constant of each complex in solution when compared to known metal–ligand complexes. In this case the side reaction coefficients for the metal and ligand are corrected. Thus, this method may not give identical information to the titration methods because the excess ligand in the sample may not be identical to some of the actual ligands binding the metal in the sample

Evidence for Aqueous Clusters as Intermediates During Zinc Sulfide Formation

Using zinc sulfide as an example, we demonstrate a plausible stepwise process for the formation of minerals from low temperature aqueous solutions. The process occurs with the formation of soluble complexes that aggregate into soluble rings and clusters. The final moiety in solution has a structure similar to the moiety in the first formed solid, which is a restatement of the Ostwald step rule. Titrations of aqueous Zn(II) with bisulfide indicate that sulfide clusters form at concentrations of 20 μM (or less) of metal and bisulfide. Precipitation does not occur according to voltammetric measurements using a mercury electrode and UV-VIS (ultra-violet to visible) spectroscopic data. UV-VIS data and filtration experiments indicate that the material passes through 0.1 μm Nuclepore and 1000 dalton filters. The complexes form rapidly (kf \u3e 108Ms−1), are kinetically inert to dissociation and thermodynamically strong. Although a neutral complex of 1:1 (ZnS) empirical stoichiometry initially forms, an anionic complex with an empirical 2 Zn:3 S stoichiometry results with continued addition of sulfide. Gel electrophoresis confirms the existence of a cluster that is negatively charged with a molecular mass between 350 and 750 daltons. On the basis of known mineral and thiol complex structures for these systems, a tetrameric cluster unit of Zn4S6(H2O)44− is likely. Molecular mechanic calculations show that this cluster is structurally analogous to ZnS minerals (particularly sphalerite) and is a viable precursor to mineral formation and a product of mineral dissolution. The formation of Zn4S6(H2O)44− can occur from condensation of Zn3S3(H2O)6 rings, which are neutral molecular clusters. The Zn atoms on one Zn3S3(H2O)6 ring combine with the S atoms on another Zn3S3(H2O)6, to lead to higher order clusters with loss of water. The Zn4S64− species form by the cross-linking of two neutral Zn3S3 rings by added sulfide; thus a Zn–S–Zn bridge forms across the rings with subsequent rearrangement and condensation to Zn4S64−; this combination results in a sphalerite-like cluster. If the rings condense without additional sulfide, a wurtzite-like structure could form. All condensations result in sulfide displacement of water from Zn to form Zn–S bonds. Water loss is an example of an entropy-driven process, which leads to a more favorable thermodynamic process. These clusters would be resistant to oxidation by O2. Voltammetric experiments indicate neutral and anionic clusters for Zn and agree with ion chromatographic data from the sulfidic waters of the Black Sea

On the Existence of Free and Metal Complexed Sulfide in the Arabian Sea and its OMZ

Free hydrogen sulfide was not detected in the oxygen minimum zone (OMZ) of the Arabian Sea during legs D1 (September 1992) and D3 (October–November 1992) of the Netherlands Indian Ocean Programme (NIOP). However, sulfide complexed to metals was detected by cathodic stripping square wave voltammetry at 2 nM or less throughout the water column. A slight increase in sulfide was measured in the OMZ relative to the surface waters and may be related to sulfur release from organic matter during decomposition. Sulfide complexes are of two general types at low concentrations of metal and sulfide. First, metals such as Mn, Fe, Co and Ni form complexes with bisulfide ion (HS−) that are kinetically labile to dissociation and are reactive. Second, metals such as Cu and Zn form multinuclear complexes with sulfide (S2−) that are kinetically inert to dissociation; thus, they are less reactive than free (bi)sulfide and the labile metal bisulfide complexes. Zinc and copper sulfide complexes are important in allowing hydrogen sulfide to persist in seawater which contains measurable oxygen

Determination of the Stability Constants for Metal Ion Complexes Using the Voltammetric Oxidation Wave of the Anion and the DeFord and Hume Formalism

The voltammetric oxidation peaks for anions and other ligands that react at the Hg electrode can be used for the determination of stability constants of metal–ligand complexes using the DeFord and Hume formalism. Metal–ligand complexes with known stability constants that span the range of log β=2.9–5.9 were determined using the oxidation wave of the ligand as metal was varied. For the larger stability constants, it was possible to titrate concentrations of metal to ligand that are smaller than the ligand concentration and obtain reliable data (βjCxj approaches 1 in this case) indicating that an excess of titrant is not required to obtain reliable β values. Data reduction with different versions of the Solver tool (included with Microsoft Excel) is discussed and blind use of these programs without knowledge of the physical chemistry of the system is discouraged. The use of Solver gives β values that are in agreement with previous work that used (non)linear regression to evaluate each Fn(X) versus [X] function

Evidence for Aqueous Clusters as Intermediates During Metal Sulfide Mineral Formation

Extended abstract for presentation at V.M. Goldschmidt Conference in Toulouse, France

Quantifying Elemental Sulfur (S\u3csup\u3e0\u3c/sup\u3e), Bisulfide (HS\u3csup\u3e-\u3c/sup\u3e), and Polysulfides (S\u3csub\u3ex\u3c/sub\u3e\u3csup\u3e2-\u3c/sup\u3e) Using a Voltammetric Method

Free polysulfides (S42−), elemental sulfur (S0) and bisulfide (HS−) can be separated at discrete peak potentials using either cyclic voltammetry (CV) or linear sweep voltammetry (LS) with extremely fast scan rates. A scan rate of 1000 mV/s was found to be optimal for peak separation. Differences in the nucleophilic nature of Sx2− versus HS− and the electrochemical irreversibility of the S0 in Sx2− allowed for peak separation. For 10 μM solutions at a scan rate of 1000 mV/s, peak potentials were found to occur at −0.66 V for HS− and −0.69 V for S0. S42− was found to give two discrete peaks at −0.70 and −0.81 V, representing a two step mechanism. In mixed laboratory standards, all the three sulfur species were uniquely identified. Flectrochemical analyses revealed similar peak separation patterns under different environmental conditions, ranging from estuarine pore waters to hydrothermal vents. In estuarine sediments, the sulfur speciation was found to change throughout a core profile, with S0 dominant in the top layers (0–6 cm), Sx2−dominant in the transition zone (6–7 cm), and HS− dominant in the deeper sediment (\u3e7 cm). In hydrothermal vent waters, different regions were observed to have different sulfur speciation. In diffuse flow regions away from chimneys containing trace O2 concentrations, polysulfides, elemental sulfur and bisulfide were discretely identified