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Aquatic Geochemistry (v.5, #1)

Potentiometric Determination of Silver Thiolate Formation Constants Using a Ag2S Electrode by Nicholas W. H. Adams; James R. Kramer (pp. 1-11).

Formation constants for silver thiolates were obtained by titration of the ligand in a constant temperature, ionic strength and pH medium and measuring the potential change at a Ag2S electrode. A non-linear equation was derived from which the first and second silver formation constants, β1 and β2′, and the sulfide group acid dissociation constant, Ka′, were determined. An overall estimate of the uncertainty in the derived parameters was obtained using a Monte Carlo approach. The procedure was compared to a previous work on AgHS°. Log β1′, log β2′ and - log Ka′ results were obtained for cysteine (11.9 ± 0.5, 15.2 ± 0.4, 7.8 ± 0.1), glutathione (12.3 ± 0.3, 14.3 ± 0.8, 8.8 ± 0.3) and 3-mercaptopropanoic acid (12.0± 0.4, 14.0 ± 0.4, 10.5 ± 0.3) at 20 °C and 0.01 m ionic strength.

Keywords: Silver sulfide; thiols; formation constants

Climatic and Anthropogenic Variations in the Sulfide Distribution in the Black Sea by S. K. Konovalov; V. N. Eremeev; A. M. Suvorov; A. Kh. Khaliulin; E. A. Godin (pp. 13-27).

Information on the depth of the sulfide onset in the Black Sea has been analyzed for the period 1910–1995. Correlation between the depth of the sulfide onset and the density structure of the layer of the main pycnocline is significant for the entire period covered. Correlation coefficients (R) between the depth of the sulfide onset and the position of isopycnal surfaces in the layer of the main pycnocline vary, on average, 0.71–0.88. The average value of density (σt) at the depth of sulfide onset, defined as a sulfide concentration equal to 3 μM, is close to 16.17. Oscillations in the average depth of the sulfide onset range over 24 m and indicate that a steady-state trend does not occur. The period of these oscillations may occur over a century timescale. The dataset for the period 1960–1995 has been used to analyze temporal variations in concentrations of sulfide inside the anoxic zone of the Black Sea. The results of isopycnal analysis demonstrate that possible temporal variations in the average position of sulfide onset versus density (σt) scale do not exceed 0.15, which is close to the limit of uncertainties for data obtained before 1988. In contrast, a prominent increase in sulfide concentration, as well as nutrient levels, within the anoxic zone is shown and is related to anthropogenic impact.

Electrochemical Evidence for Pentasulfide Complexes with Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+ by Steven J. Chadwell; David Rickard; George W. Luther III (pp. 29-57).

A series of stable pentasulfide complexes of the common base metals, Mn, Fe, Co, Ni, Cu and Zn exist in aqueous solutions at ambient temperatures. Pure sodium pentasulfide was prepared and reacted with the divalent cations of Mn, Fe, Co, Ni, Cu and Zn in aqueous solution at ambient temperature. The S52- complexes were found to exist as determined by voltammetric methods.Pentasulfide complexes with compositions assigned as [M(η1-S5)] and [M2(μ- S5)]2+ occur for Mn, Fe, Co and Ni where only one terminal S atom in the S52- binds to one metal (η1 = mono-dentate ligand or M-S-S-S-S-S, μ = ligand bridging two metal centers or M-S-S-S-S-S-M). Conditional stability constants are similar for all four metals with log β1 between 5.3 and 5.7 and log β2 between 11.0 and 11.6. The constants for these pentasulfide complexes are similar to the tetrasulfide complexes and are approximately 0.4–0.8 log units higher than for comparable bisulfide complexes [M(SH)]+ as expected based on the higher nucleophilicity of S52- compared to HS-. Voltammetric results indicate that these are labile complexes.As with the bisulfide and tetrasulfide complexes, Zn(II) and Cu(II) are chemically distinct from the other metals. Zn(II) reacts with pentasulfide to form a stable monomeric pentasulfide chelate, [Zn(η1-S5)] with log β = 8.7. Cu(II) reacts with pentasulfide to form a complex with the probable stoichiometry [Cu(S5)]2 with log β estimated to be 20.2. As with the other four metals, these complexes are comparable with the tetrasulfide complexes. Discrete voltammetric peaks are observed for these complexes and indicate they are electrochemically inert to dissociation. Reactions of Zn(II) and Cu(II) also lead to significant breakup of the polysulfide.The relative strength of the complexes is Cu > Zn > Mn, Fe, Co, Ni. Cu displaces Zn from [Zn(η1- S5)] and both Cu and Zn displace Mn, Fe, Co and Ni from their pentasulfide complexes.

Keywords: Metals; pentasulfide; metal-pentasulfide complexes; stability constants; voltammetry

Framvaren and the Black Sea – Similarities and Differences by David W. Dyrssen (pp. 59-73).

The following determinations in the Norwegian fjord Framvaren and the Black Sea have been compared: carbon-14, carbon-13, alkalinity, total dissolved inorganic carbon, sulfide, tritium (HTO), trace metals, silica, ammonium and phosphate. The historical development of the two anoxic basins is quite different. The carbon-14 age of the total inorganic dissolved carbonate in the deep water is 2000 years in the Black Sea, but only 1600 in Framvaren. The fresh water supply and composition are different. The rivers entering the Black Sea have a high alkalinity, but the river input and runoff to Framvaren has a very low alkalinity. The alkalinity, carbonate and sulfide concentrations in the anoxic waters below the chemoclines are much higher in Framvaren. This is mainly an effect of the different surface to volume ratios. The difference in carbon-13 (-8‰ for the Black Sea deep water, -19‰ in the Framvaren bottom water) is mainly due to the smaller imprint of the decomposition of organic matter on the Black Sea deep water.The concentration of trace metals in the particulate form are about the same in the deep water. About 76% of the molybdate in seawater is lost in the sulfidic water of Framvaren, and about 82–96% of the molybdate carried into the Black Sea by the Bosporus undercurrent is lost in the deep water. The relation between silica, ammonium and phosphate can be understood if part of the ammonium is being removed by denitrification, a process that most likely has been going on for thousands of years.

Keywords: Framvaren fjord; Black Sea; carbon isotopes; tritium; alkalinity; total carbonate; sulfide; molybdenum; denitrification

Sulfides in Sandy Sediments: New Insights on the Reactions Responsible for Sedimentary Pyrite Formation by John W. Morse (pp. 75-85).

The formation of sedimentary iron sulfides was studied in sandy sediments of the Laguna Madre, TX, in order to better understand how this process operates in sediments where reactive iron is likely to be limiting for sulfide mineral formation. These sediments usually had reactive iron and total reduced sulfide concentrations one to two orders of magnitude less than in typical shallow water terrigenous muds, but organic-C concentrations typical of fine-grained sediments due to the extensive presence of seagrass beds. This resulted in moderate (0–150 μm) dissolved H2S concentrations with maximum concentrations in the upper (3–:5 cm) root zone. Based on citrate dithionite extractable-Fe the degree of sulfidization was usually 100% or greater. Acid volatile sulfides (AVS) typically comprised roughly 60% of total reduced sulfur and the proportion of AVS generally increased instead of decreasing with depth. The unusual proportion of TRS as AVS and persistence of AVS are attributed to exceptionally slow pyrite formation kinetics. The probable reasons for these slow reaction kinetics are the high (>7.8) pH of the sediments, which favors the slow polysulfide pathway for pyrite formation, high (typically about 2–4 mm) dissolved organic carbon concentrations that inhibit growth of pyrite and the low concentration of reactants which greatly increases the average transport distances necessary for diffusion controlled reactions.

Keywords: Sulfides; pyrite; anoxic sediments; iron

Selective Extraction Chemistry of Toxic Metal Sulfides from Sediments by D. Craig Cooper; John W. Morse (pp. 87-97).

Experimental evidence suggests that formation of metal sulfides in anoxic sediments limits the bioavailability of several toxic elements. Our ability to quantify the processes by which these metal sulfides form is dependent upon our ability to determine the speciation of solid phase metals in sediments. Our work indicates that an entire suite of Cu-Fe and Ni-Fe sulfide minerals can form upon the exposure of mackinawite to aqueous Cu and Ni. Furthermore, we have demonstrated that the solubility of pure metal sulfide minerals and their iron-metal derivatives in HCl directly correlates with the observed trends in the ''Degree of Trace Metal Pyritization'' in natural sediments. Current extraction schemes cannot distinguish discrete trace metal sulfides from trace metals associated with pyrite.

Sulphur Enrichment in Organic Matter of Eastern Mediterranean Sapropels: A Study of Sulphur Isotope Partitioning by Hilde F. Passier; Michael E. Böttcher; Gert J. De Lange (pp. 99-118).

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmMS) in eastern Mediterranean sediments containing three sapropels of different ages and with different organic carbon contents (sapropel S1 in core UM26, formed from 5–9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum organic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Sulphur isotopic compositions (δ34S) of the organic matter ranged from -29.5 to +15.8‰ and the atomic S/C ratio was 0.005 to 0.038. The organic sulphur in the sediments is a mixture of sulphur derived from (1) incorporation of 34S-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur with an isotopic signature close to seawater sulphate. The calculated biosynthetic fraction of organic sulphur in non-sapropelic sediments ranges from 68–87%. The biosynthetic fraction of the organic sulphur of the sapropels (60–22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and thus an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.

Keywords: Eastern Mediterranean; sapropel; organic sulphur; sulphur isotopes; C-irmMS; pyrite

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Aquatic Geochemistry (v.5, #1)

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Aquatic Geochemistry (v.5, #1)