The heat capacity of a natural monticellite and phase equilibria in the system CaO-MgO-SiO2-CO2

The heat capacity of a natural monticellite (Ca_1.00Mg_.09Fe_.91Mn_.01Si_0.99O_3.99) measured between 9.6 and 343 K using intermittent-heating, adiabatic calorimetry yields C_p⁰(298) and S₂₉₈⁰ of 123.64 ± 0.18 and 109.44 ± 0.16 J · mol⁻¹K⁻¹ respectively. Extrapolation of this entropy value to end-member monticellite results in an S⁰₂₉₈ = 108.1 ± 0.2 J · mol⁻¹K⁻¹. High-temperature heat-capacity data were measured between 340–1000 K with a differential scanning calorimeter. The high-temperature data were combined with the 290–350 K adiabatic values, extrapolated to 1700 K, and integrated to yield the following entropy equation for end-member monticellite (298–1700 K): S_T⁰(J · mol⁻¹K⁻¹) = S₂₉₈⁰ + 164.79 In T + 15.337 · 10⁻³T + 22.791 · 10⁵T⁻² − 968.94. Phase equilibria in the CaO-MgO-SiO₂ system were calculated from 973 to 1673 K and 0 to 12 kbar with these new data combined with existing data for akermanite (Ak), diopside (Di), forsterite (Fo), merwinite (Me) and wollastonite (Wo). The location of the calculated reactions involving the phases Mo and Fo is affected by their mutual solid solution. A best fit of the thermodynamically generated curves to all experiments is made when the S⁰₂₉₈ of Me is 250.2 J · mol⁻¹ K⁻¹ less than the measured value of 253.2 J · mol⁻¹ K⁻¹.A best fit to the reversals for the solid-solid and decarbonation reactions in the CaO-MgO-SiO₂-CO₂ system was obtained with the ΔG⁰₂₉₈ (kJ · mole⁻¹) for the phases Ak(−3667), Di(−3025), Fo(−2051), Me(−4317) and Mo(−2133). The two invariant points − Wo and −Fo for the solid-solid reactions are located at 1008 ± 5 K and 6.3 ± 0.1 kbar, and 1361 ± 10 K and 10.2 ± 0.2 kbar respectively. The location of the thermodynamically generated curves is in excellent agreement with most experimental data on decarbonation equilibria involving these phases.     

Evaluation of the origin and fate of nitrate in the Abbotsford Aquifer using the isotopes of15N and18O in NO3−

Extensive NO₃⁻ contamination of groundwater in the Abbotsford aquifer to levels above drinking water limits is a major problem in the Fraser Lowlands of southwestern British Columbia, Canada. Nitrate concentrations in the aquifer ranged from 0 to 151 mg/l NO₃⁻, with a median concentration of 46 mg/l NO₃⁻. Of 117 wells sampled, 54% had NO₃⁻ concentrations exceeding the drinking water limit of 45 mg/1. Approximately 80% of the study area had groundwater NO₃⁻ concentrations exceeding 40 mg/1 NO₃⁻. Potential NO₃⁻ source materials were poultry manure N and synthetic NH₄ based fertilizers. Theδ¹⁵N of solid poultry manure samples ranged between + 7.9 and + 8.6‰ (AIR). Four brands of synthetic fertilizers commonly used hadδ¹⁵N values between −1.5 and −0.6‰. Ammonia volatilization caused theδ¹⁵N of groundwater NO₃⁻ produced from poultry manure N to range between +8 and +16‰. Theδ¹⁸O values of groundwater NO₃⁻, by contrast, mostly ranged between +2 and +5‰ (SMOW). This narrow range ofδ¹⁸O values fell within the expected range of NO₃⁻ produced by nitrification of reduced N forms such as poultry manure N and NH₄ fertilizers, and had a similar range ofδ¹⁸O values as NO₃⁻ in the upper part of the unsaturated zone below raspberry fields and beneath former manure piles. Theδ¹⁵N-NO₃⁻ andδ¹⁸O-NO₃⁻ data confirmed that NO₃⁻ in the aquifer was predominantly derived from poultry manure and to a lesser extent from synthetic fertilizers. Theδ¹⁸O-NO₃⁻ data further suggested the nitrification process occurred mainly in the summer months, with the soil NO₃⁻ produced subsequently flushed into the aquifer during fall recharge. Theδ¹⁵N-NO₃⁻andδ¹⁸O-NO₃⁻ data conclusively indicated that no significant bacterial denitrification is taking place in the Abbotsford aquifer.     

The ion-pair constant and other thermodynamic properties of HCl up to 350°C

Solubilities of silver chloride in aqueous hydrochloric acid solutions have been determined from 100 up to 350°C. From these measurements, the ionisation constant of HC1 has been evaluated up to 225°C. Evidence is presented to show that a protonated silver species, HAgCl₂⁰, exists at 275°C and above. Available experimental data up to 200°C have been firted to Pitzer's equation to generate an algorithm to calculate stoichiometric activity and osmotic coefficients of HCl up to 350°C and concentrations up to at least 3.0 m. Using the present results and those of Wrightet al. (1961), Pearsonet al. (1963) and Lukashowet al. (1976), the dissociation constant (K_d) of HCl as a function of temperature is described by the equation log₁₀K = 2136.898 + 1.020349T−4.5045 × 10⁻⁴T²−50396.40/T−901.770 10g₁₀T (Tin °K) which is valid in the range 25–350°C. Calculated enthalpy (ΔH⁰), entropy (ΔS⁰) and heat capacity change (ΔC_p⁰) functions for HCl dissociation have been rationalized in terms of changing solute and solvent characteristics as temperature is raised.     

Current status of models with hot and cold dark matter

An analysis of a five-parameter family of cosmological models in a spatially flat Friedmann Universe with a zero Λ term is presented. The five parameters are (1) σ₈, the dispersion of the mass fluctuations in a sphere with radius 8h ^?1 Mpc, where h=H ₀/100 km s^?1 Mpc^?1 and H ₀ is the Hubble constant; (2) n, the slope of the density-perturbation spectrum; (3) Ω_v, the normalized energy density of hot dark matter; (4) Ω_b, the baryon density; and (5) h, the normalized Hubble constant. The density of cold dark matter is determined from the condition Ω _cdm >1?Ω_v?Ω _b . Analysis of the models is based on comparison of computational results with observational data for: (1) the number density and mass function of galaxy clusters (a so-called Press-Schechter formalism) and (2) the cosmic microwave background anisotropy. The first method enabled us to determine the value σ₈=0.52±0.01 with high accuracy. Using the resulting normalization of the density-perturbation spectrum, we calculated a model for the anisotropy of the cosmic microwave background radiation on large scales (l?10, where l is the harmonic number) and the required contribution of cosmological gravitational waves, characterized by the parameter T/S. The restrictions on T/S become weaker as Ω_v increases. Nevertheless, even when Ω_v≤0.4, models with h+n≥1.5 require a considerable contribution from gravitational waves: T/S?0.3. On the other hand, in models with Ω_v≤0.4 and a scale-invariant density-perturbation spectrum (n=1), we find T/S ?10(h?0.47). The minimization of T/S is possible only for the family of models with red spectra (n<1) and small h (<0.6). The value of Ω_v is determined most accurately by the data onΔT/T near the first acoustic peak (l?200). By imposing a general restriction on the amplitude of gravitational waves T/S∈[0, 3] and taking into account the available observational data on the amplitude of the acoustic peak of Sakharov oscillations, ranges of possible values n and Ω_v are derived. If the baryon number is constrained by nucleosynthesis data, the models under consideration can have both moderately red and blue power spectra n∈[0.9, 1.2] with a rather high concentration of hot particles Ω_v∈[0.2,0.4]. The conditions that n<0.9 and/or Ω_v<0.2 decrease the relative amplitude of the acoustic peak by over 30% compared to its value in the standard cold-dark-matter (CDM) model normalized using COBE data.     

Mechanism of kaolinite dissolution at room temperature and pressure Part II: kinetic study

Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

Source parameters and scaling relations for small earthquakes in the Kachchh region of Gujarat,India

The scaling relationships for stress drop and corner frequency with respect to magnitude have been worked out using 159 accelerograms from 34 small earthquakes (M _w 3.3–4.9) in the Kachchh region of Gujarat. The 318 spectra of P and S waves have been analyzed for this purpose. The average ratio of P- to S-wave corner frequency is found to be 1.19 suggestive of higher corner frequency for P wave as compared to that for S wave. The seismic moments estimated from P waves, M ₀(P), range from 1.98 × 10¹⁴ N m to 1.60 × 10¹⁶ N m and those from S waves, M ₀(S), range from 1.02 × 10¹⁴ N m to 3.4 × 10¹⁶ N m with an average ratio, M ₀(P)/M ₀(S), of 1.11. The total seismic energy varies from 1.83 × 10¹⁰ J to 2.84 × 10¹³ J. The estimated stress drop values do not depend on earthquake size significantly and lie in the range 30–120 bars for most of the events. A linear regression analysis between the estimated seismic moment (M ₀) and corner frequency (f _c) gives the scaling relation M ₀ f _c ³  = 7.6 × 10¹⁶ N m/s³. The proposed scaling laws are found to be consistent with similar scaling relations obtained in other seismically active regions of the world. Such an investigation should prove useful in seismic hazard and risk-related studies of the region. The relations developed in this study may be useful for the seismic hazard studies in the region.     

Iodine diagenesis in non-pelagic deep-sea sediments

Measurements of sediment geochemistry and porewater speciation have been made using eight cores containing turbidite sections from the Madeira and Nares Abyssal Plains. The results have been used to evaluate how the diagenetic chemistry of iodine in these sediments compares with that in sediments undergoing steady-state diagenesis. The behaviour of iodine is related to the development of a redox front within the turbidite, between the organic-rich anoxic sediment and its oxic cap, and the downward migration of the front through the turbidite with time. In contrast to the steady-state case, sediment I contents and I/ C ratios increase downwards through the oxidised section reaching a maximum at the redox front (up to ~ 100 μ/g I; molar I/C~ 20 × 10⁻⁴) below which values drop dramatically (I/C ~ 5 × 10⁻⁴). A strong iodate enrichment (up to ~3 μmol kg⁻¹) is observed in the oxidised section of the sediment. At the front interconversion of I⁻ and IO⁻₃ species occur and below the front porewater IO⁻₃ is absent and I~ concentrations increase with depth (as in other cases of anoxic diagenesis) up to ~ 10 μmol kg⁻. In the oxidised section of the sediment the I enrichment has been supplied by upward transport of iodide with the increasing I content, with depth being accounted for by progressive diagenetic enrichment with time.     

Coda wave attenuation parallel and perpendicular to the Mexican Pacific coast

We calculated the quality factor, Q_c, at frequencies from 6 to 24 Hz using coda waves of 97 aftershocks of the Petatlan, Mexico, earthquake (March 14, 1979; M_S=7.6). The data were recorded parallel (between Acapulco and Playa Azul) and perpendicular (between Petatlan and Mexico City) to the coast. The results are the following: at 12 and 24 Hz there is no significant difference in the attenuation (Q_c⁻¹) along the two paths; at 6 Hz, Q_c⁻¹ has a large scatter in both directions. This observation indicates strong site effects at this frequency; average Q_c⁻¹ is slightly higher between Petatlan–Acapulco (toward SE) than between Petatlan–Playa Azul (toward NW); and at high frequencies, Q_c⁻¹ remains essentially constant perpendicular to the coast. These results show that the large seismic wave amplifications in Mexico City are caused by shallow site effects.     

Laboratory determination of thermal diffusion constants for N2/N2 in air at temperatures from −60 to 0°C for reconstruction of magnitudes of abrupt climate changes using the ice core fossil-air paleothermometer

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio ²⁹N₂/²⁸N₂ has stayed extremely constant in the atmosphere for ≥10⁶ years. However, prior to this work no data were available for thermal diffusion of ²⁹N₂/²⁸N₂ in air, but only in pure N₂. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators.Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Ω) for ²⁹N₂/²⁸N₂ in air is +(14.7 ± 0.5) × 10⁻³ per mil/°C when the average temperature is -30.0°C. The corresponding value for ²⁹N₂/²⁸N₂ in pure N₂ that we find is +(15.3 ± 0.4) × 10⁻³ per mil/°C at -30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/T_K − 1232/T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for ²⁹N₂/²⁸N₂ in air with temperature in the range of -60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of ²⁹N₂/²⁸N₂ in the firn air. The combined newly obtained data constitute a calibration of the fossil-air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores.     

Standard Gibbs free energy of formation for Cu2O,NiO, CoO,and FexO: High resolution electrochemical measurements using zirconia solid electrolytes from 900–1400 K

Galvanic cells with oxygen-specific solid electrolytes made of calcia-stabilized zirconia have been used to make equilibrium measurements of the standard Gibbs free energy of formation, Δ_fG⁰_m,(T), for copper (I) oxide (Cu₂O), nickel (II) oxide (NiO), cobalt (II) oxide (CoO), and wüstite (Fe_xO) over the temperature range from 900–1400 K. The measured values of Δ_fG⁰_m at 1300 K are −73950, −123555, −142150, and −179459 J · mol⁻¹ for Cu₂O, NiO, CoO, and Fe_0.947O, respectively. The precision of these measurements is ± 30–60 J · mol⁻¹, and their absolute accuracy is estimated to be ± 100–200 J·mol⁻¹. Using values of –76.557, −94.895, −79.551, and −71.291 J · K⁻¹ · mol⁻¹ for the entropies of formation, Δ_fS_m⁰, (298.15 K), the calculated enthalpies of formation, Δ_fH_m⁰, (298.15 K), are −170508, −240110, −237390, and −266458 J · mol⁻¹ for Cu₂O, NiO, CoO, and Fe_0.947O, respectively. These values of Δ_fS_m⁰ (298.15 K) and Δ_fH_m⁰ (298.15 K) are in good agreement with the best available calorimetric measurements.     

N-nitrodimethlyamine in natural and drinking water of high cancer incidence regions of Guangdong,China

The Guangdong province of China contains the most clearly described high-incidence of hepatocellular carcinoma (HCC) and nasopharyngeal carcinoma (NPC) areas in the world. The geographical heterogeneity of cancer incidence in the region suggests that many carcinogenic risk factors might be present in the regional geochemical environment. This paper presents the concentrations of a wide range of known carcinogens in two high cancer incidence areas in Guangdong and compared them to a low cancer incidence area in the same province. N-Nitrosamines, NO₃⁻, NO₂⁻, and ammonium were detected in groundwater, surface water, and drinking-water. The concentrations of the 7 trace metal and metalloid elements As, Cd, Ni, Cu, Pb, Zn, and Hg were determined in surface soil samples and all water samples. The results show that, compared with the guidelines or limit values for drinking-water quality in the world, the high cancer incidence areas have hazardous high levels of N-nitrodimethlyamine (NDMA) in all kinds of water. Oppositely, the low cancer incidence area has a safe low level of NDMA in water bodies. The levels of NO₃⁻, NO₂⁻, and ammonium in water have the same character, although they have different expression between the two high-risk areas. The distribution of the 7 tested trace elements in surface soil has no significant correlation with cancer incidence. On the other hand, high concentrations of carcinogenic N-Nitrosamines in drinking-water and natural water bodies were identified for the first time in the high NPC and HCC incidence area.     

The solubility of BaCO3(cr) (witherite) in CO2-H2O solutions between 0 and 90°C,evaluation of the association constants of BaHCO3+(aq) and BaCO30(aq) between 5 and 80°C,and a preliminary evaluation of the thermodynamic properties of Ba2+(aq)

One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO₃(cr) = Ba²⁺(aq) + CO₃²⁻(aq) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by logK = 607.642 + 0.121098T − 20011.25/T − 236.4948 logT where T is in degrees Kelvin. The logK of BaCO₃(cr), the Gibbs energy, the enthalpy and entropy of the reaction at 298.15 K are −8.562, 48.87 kJ · mol⁻¹, 2.94 kJ · mol⁻¹ and −154.0 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO₃⁺(aq) and BaCO₃⁰(aq) Three different methods were used to evaluate the association constant of BaHCO₃⁺(aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba²⁺(aq) + HCO₃⁻(aq) = BaHCO₃⁺(aq) is given by logK_BaHCO3⁺ = −3.0938 + 0.013669T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15°K are 0.982, −5.606 kJ · mol⁻¹, 23.26 kJ · mol⁻¹ and 96.8 J · mol⁻¹ · K⁻¹, respectively. The temperature dependence of the equilibrium constant for the reaction Ba²⁺(aq) + CO²⁻₃(aq) = BaCO₀³(aq) is given by logK_BaCO3⁰ = 0.113 + 0.008721T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15° K are 2.71, −15.49 kJ · mol⁻¹, 14.84 kJ · mol⁻¹ and 101.7 J· mol⁻¹ · K⁻¹.The above model leads to reliable calculations of the aqueous speciation and solubility of witherite in the system BaCO₃-CO₂-H₂O from 0 to more than 90°C. Literature data on witherite solubility were re-evaluated and compared with the results of this study.Problems in the thennodynamic selections of Ba compounds are considered. Newer data require the revision of Δ_fH° and Δ_fG° of Ba²⁺(aq) to −532.5 and −555.36 kJ · mol⁻¹, respectively, for agreement with solubility data.     

Celestite (SrSO4(s)) solubility in water,seawater and NaCl solution

Celestite solubility measurements have been conducted in pure water at temperatures from 10 to 90°C. Equilibrium was achieved with respect to a crystalline solid phase from both undersaturated and supersaturated solutions. The measurements show that the solubility undergoes a maximum near 20°C. LogK values for the solubility reaction are adequately described by the following expression over the temperature range 283.15 to 363.15 K: −logK= −35.3106+0.00422837T+318312/T²+14.99586 logT.The following thennodynamic values for the dissolution reaction of SrSO_4(s), at 25°C have been derived: ΔG_R⁰ = 37852 ± 30 Jmol⁻¹ΔH_R⁰ = −1668±920Jmol⁻¹ΔS_R⁰= −132.6±3.2JK^−1mol−1Celestite solubility measurements were also determined in NaCl solutions up to 5 m concentration and from 10 to 40°C. These data are in good agreement with the work of StrÜbel (1966), who reports solubility measurements to temperatures of 100°C.The application of the Pitzer relations and the solubility constants determined in this study to calculate celestite solubility in NaCl solutions yields excellent agreement between predicted values and experimental measurements over the entire range of temperature and NaCl concentration conditions. For the limited number of solubility measurements in seawater-type solutions and mixed-salt brines, the agreement using the Pitzer relations is within three percent of the measured solubility.     

Effect of confining pressure on the diffusion of HTO, 36Cl− and 125I− in a layered argillaceous rock (Opalinus Clay): diffusion perpendicular to the fabric

The through- and out-diffusion of HTO, ³⁶Cl⁻ and ¹²⁵I⁻ in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, D_e, ranged between (5.6±0.4)×10⁻¹² and (6.7±0.4)×10⁻¹² m² s⁻¹ for HTO, (7.1±0.5)×10⁻¹³ and (9.1±0.6)×10⁻¹³ m² s⁻¹ for ³⁶Cl⁻ and (4.5±0.3)×10⁻¹³ and (6.6±0.4)×10⁻¹³ m² s⁻¹ for ¹²⁵I⁻. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for ³⁶Cl⁻ and 0.080 for ¹²⁵I⁻. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike ³⁶Cl⁻, ¹²⁵I⁻ sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, K_d, for ¹²⁵I⁻ is of the order of 1–2×10⁻⁵ m³ kg⁻¹. The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archie's Law with exponent m=2.5.     

Structure and stability of the Fe(II)–Fe(III) green rust “fougerite” mineral and its potential for reducing pollutants in soil solutions

Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [Fe^II_(1−x) Fe^III_x (OH)₂]^x+·[(x/n) Aⁿ⁻·(m/n) H₂O]^x−, where x is the ratio Fe^III/Fe_tot, and reflects the structure in which brucite-like layers alternate with interlayers of anions Aⁿ⁻ and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl⁻) and the hydroxysulphate GR2(SO₄²⁻) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl⁻) the structure of the fougerite GR mineral, [Fe^II_(1−x) Fe^III_x (OH)₂]^x+·[x OH⁻·(1−x) H₂O]^x-  Fe(OH)_(2+x)·(1−x) H₂O, is proposed displaying interlayers made of OH⁻ ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH⁻) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the E_h-pH diagram of Fe(OH)_(2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH)_(2+x)]=(1−x) μ°[Fe(OH)₂]+x μ°[Fe(OH)₃]+RT [(1−x) ln (1−x)+x ln x]+A₀ x (1−x), where μ°[Fe(OH)₂]=−492.5 kJ mol⁻¹, μ°[Fe(OH)₃]=−641 kJ mol⁻¹ and A₀=−243.9 kJ mol⁻¹ at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe₃(OH)₈, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH)₃ is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and E_h-pH diagrams of Fe(OH)_(2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO₃⁻, Se(VI) or Cr(VI) are fully illustrated.     

A plug flow-through reactor for studying biogeochemical reactions in undisturbed aquatic sediments

A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores. The experimental approach was designed to preserve the structural, geochemical and microbiological integrity of the sediment sections and, hence, retrieve reaction parameters that apply to in-situ conditions.Inert tracer breakthrough experiments were performed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46–83%) and mean grain size (10⁻⁴−10⁻² cm). Longitudinal dispersion coefficients were determined from the breakthrough curves of Br⁻. The curves were also used to check for early breakthrough or trailing, that is, features indicative of non-ideal flow conditions. Sediment plugs that exhibited these features were eliminated from further experiments.Dimensionless equilibrium adsorption coefficients (K) of NH₄⁺, were calculated from measured retardation times between the breakthrough of NH₄⁺ and Br⁻. The values of K at 5°C vary between 0.3 and 2.3, with the highest value obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The values for the marine and estuarine sediments agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7m) and temperature (range 5–25°C) in an estuarine sediment confirm that the main sorption mechanism for NH₄⁺ is ion exchange.The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO₄²⁻ reduction in a salt-marsh sediment. Recirculation homogenizes solute concentrations within the reactor, hence facilitating the derivation of reaction rate expressions that depend on solution composition. The rate of microbial S0₄⁻ reduction was found to be nearly independent of the dissolved SO₄²⁻ concentration in the range of 2.2−1 mM. Fitting of the experimental rates to a Monod relationship resulted in a maximum estimate of the half-saturation concentration, K_s, of 240 μM. This value is comparable to those reported for a pure culture of SO₄²⁻-reducing bacteria, but is significantly smaller than the only other K_s value reported in the literature for SO₄²⁻ utilization in a natural marine sediment.     

The solubility of gold and silver in the system AuAgSO2H2O at 25°C and 1 atm.

During supergene alteration of auriferous carbonate ore, the weathering fluids formed are likely to be alkaline and therefore unsuitable as a medium for gold transport as a chloride complex. Secondary gold remobilization in such deposits can often be attributed instead to gold complexing by sulphur-bearing ligands. Gold and silver solubility in the systems AuSO₂H₂O and AgSO₂H₂O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S₂O₃)₂³⁻ (at φO₂ > 10⁻⁶⁰) and Au(HS)⁻₂ (atφO₂ < 10⁻⁶⁰), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S₂O₃)³⁻₂ in moderately oxidizing, and bisulphide complexes, AgHS⁰ and Ag(HS)⁻₂ in reducing, alkaline media. Silver solubility in highly oxidized, neutral or acid solutions is increased by formation of AgS₂O₃⁻, Ag⁺ and AgSO₄⁻ complexes.Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na₂S₂O₃ do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na₂S₂O₃, for example, is increased both by the presence of silver-thiosulphate complexes and alloyed silver. It is possible that such behaviour is due to the formation a mixed metal complex of the type (Au, Ag)(S₂O₃)₂³⁻.The nature and mineral association of secondary gold in the oxidized zone of carbonate ore at Wau. in Papua New Guinea, is consistent with prior remobilization as a thiosulphate complex. Here the secondary gold is coarsely crystalline, alloyed with 50–75 at% Ag and enriched at the watertable and with manganese dioxide in the oxidized zone.     

Biogeochemical cycling in an organic-rich coastal marine basin. 6. Temporal and spatial variations in sulfate reduction rates

Rates of sulfate reduction were measured over a 3 year period in the anoxic nearshore sediments of Cape Lookout Bight, North Carolina, using both a tube incubation method and a ³⁵S-sulfate direct injection technique. The methods yielded similar depth-integrated rates over the upper 30 cm ranging from less than 10 mol SO⁼₄ · m⁻² · y⁻¹ in winter to greater than 50 mol SO⁼₄ · m⁻² · y⁻¹ in summer. There were also seasonal changes in the Arrhenius activation energies for the sulfate reduction rates indicating that the assumption that E_a is constant with temperature is not always valid. The time averaged annual turnover rate for all three years was 20.4 (±11.4) mol SO⁼₄ · m⁻² · y⁻¹. Surface rates ranged seasonally from less than 0.01 to over 3 mM SO⁼₄ · d⁻¹ between winter and summer, respectively. A subsurface rate maximum was observed to develop during the summer months which accounted for 28 percent of the annual depth integrated sulfate reduction rate. Subsurface rate maxima are the result of changes in the chemistry (substrate type and/or concentration) and the microbiology in the sediments. The possibility of the subsurface maximum being an artifact of the ³⁵S method is also discussed. However, the sulfate reduction rates compare well with previous measurements of the carbon sediment-water plus burial fluxes and with a depth integrated CO₂ production rate modelled from a ΣCO₂ concentration profile from the same site.