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Aquatic Geochemistry (v.13, #3)
Chemistry of Neutral and Alkaline Waters with Low Al3+ Activity Against Hydroxyaluminosilicate HASB Solubility. The Evidence from Ground and Surface Waters of the Sudetes Mts. (SW Poland) by Dariusz Dobrzyński (pp. 197-210).
A wide set of aqueous chemistry data (574 water analyses) from natural environments has been used to testify and validate of the solubility of synthetic hydroxyaluminosilicate (HASB), Al2Si2O5(OH)4. The ground and surface waters represent regolith and/or fissure aquifers in various (magmatic, sedimentary and metamorphic) bedrocks in the Sudetes Mts. (SW Poland). The solubility of HASB in natural waters was calculated using the method proposed by Schneider et al. (Polyhedron 23:3185–3191, 2004). Results confirm usefulness and validity of this method. The HASB solubility obtained from the field data (logKsp = −44.7 ± 0.58) is lower than it was estimated (logKsp = −40.6 ± 0.15) experimentally (Schneider et al. Polyhedron 23:3185–3191, 2004). In the waters studied the equilibrium with HASB is maintained at pH above 6.7 and at [Al3+] ≤ 10−10. Silicon activity (log[H4SiO4]) ranges between −4.2 and −3.4. Due to the calculation method used, the Ksp mentioned above cannot be considered as a classical solubility constant. However, it can be used in the interpretation of aluminium solubility in natural waters. The HASB has solubility lower than amorphous Al(OH)3, and higher than proto-imogolite. From water samples that are in equilibrium with respect to HASB, the solubility product described by the reaction, $$ { ext{Al}}_{{ ext{2}}} { ext{Si}}_{{ ext{2}}} { ext{O}}_{{ ext{5}}} { ext{(OH)}}_{{ ext{4}}} ;{ ext{ + }};{ ext{6H}}^{{ ext{ + }}} ; leftrightarrow ;{ ext{2Al}}^{{{ ext{3+}}}} ;{ ext{ + }};{ ext{2H}}_{{ ext{4}}} { ext{SiO}}_{{ ext{4}}} ;{ ext{ + }};{ ext{H}}_{{ ext{2}}} { ext{O}} $$ is calculated to be logKsp = 14.0 (±0.7) at 7°C.
Keywords: Hydroxyaluminosilicate; Water chemistry; Natural water; The Sudetes Mts.; Poland
Geochemistry of Flooded Underground Mine Workings Influenced by Bacterial Sulfate Reduction by Amber J. Roesler; Christopher H. Gammons; Gregory K. Druschel; Harry Oduro; Simon R. Poulson (pp. 211-235).
Unlike the majority of the water in the flooded mine complex of Butte Montana, which includes the highly acidic Berkeley pit lake, groundwater in the flooded West Camp underground mine workings has a circum-neutral pH and contains at least 8 μM aqueous sulfide. This article examines the geochemistry and stable isotope composition of this unusual H2S-rich mine water, and also discusses problems related to the colorimetric analysis of sulfide in waters that contain FeS(aq) cluster compounds. The West Camp mine pool is maintained at a constant elevation by continuous pumping, with discharge water that contains elevated Mn (90 μM), Fe (16 μM), and As (1.3 μM) but otherwise low metal concentrations. Dissolved inorganic carbon in the mine water is in chemical and isotopic equilibrium with rhodochrosite in the mineralized veins. The mine water is under-saturated with mackinawite and amorphous FeS, but is supersaturated with Cu- and Zn-sulfides. However, voltammetry studies show that much of the dissolved sulfide and ferrous iron are present as FeS(aq) cluster molecules: as a result, the free $$ {hbox{H}}_{hbox{2}} {hbox{S}},{hbox{ + }},{hbox{HS}}^{hbox{ - }} $$ concentration of the West Camp water is poorly constrained. Concentrations of dissolved sulfide determined by colorimetry were lower than gravimetric assays obtained by AgNO3 addition, implying that the FeS(aq) clusters are not completely extracted by the Methylene Blue reagent. In contrast, the clusters are quantitatively extracted as Ag2S after addition of AgNO3. Isotopic analysis of co-existing aqueous sulfide and sulfate confirms that the sulfide was produced by sulfate-reducing bacteria (SRB). The H2S-rich mine water is not confined to the immediate vicinity of the extraction well, but is also present in flooded mine shafts up to 3 km away, and in samples bailed from mine shafts at depths up to 300 m below static water level. This illustrates that SRB are well established throughout the southwestern portion of the extensive (>15 km3) Butte flooded mine complex.
Keywords: Mine water; Hydrogen sulfide; Iron sulfide clusters; Bacterial sulfate reduction; Geochemistry; Arsenic; Rhodochrosite
Dissolution of Carbonate Sediments Under Rising pCO2 and Ocean Acidification: Observations from Devil’s Hole, Bermuda by Andreas J. Andersson; Nicholas R. Bates; Fred T. Mackenzie (pp. 237-264).
Rising atmospheric pCO2 and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO2 levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO2. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO3 m−2 h−1. On a yearly basis, this range corresponds to 175–701 g CaCO3 m−2 year−1; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO3 m−2 year−1. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO2 arising from burning of fossil fuels.
Keywords: Climate change; CO2 ; Ocean acidification; Carbonate minerals; CaCO3 dissolution; Mg-calcite; Coral reef; Calcification
Fred T. Mackenzie and Abraham Lerman, Carbon in the Geobiosphere-Earth’s Outer Shell; Topics in Geobiology Volume 25
by John W. Morse (pp. 265-266).