The dissolution of biotite and chlorite at 25°C in the near-neutral pH region
ABSTRACT We studied the dissolution of biotite and chlorite in laboratory systems with flow-through and batch reactors. The initial dissolution of biotite in the near-neutral pH region, under N2(g) atmosphere is highly non-stoichiometric. A slow linear release of iron during a period of weeks indicates a surface-chemical-reaction-controlled mechanism of release for iron. The release of potassium is much faster than that of iron. A parabolic dependence of accumulated release with time suggests a diffusion-controlled mechanism of potassium release. The rates of magnesium, aluminium and silicon release are between those for potassium and iron and approach that of iron with time. There is no significant influence of (bi)carbonate or pH on biotite dissolution rate or stoichiometry in the pH region 7 < pH < 8.5. The release rates of elements from chlorite are close to stoichiometric and similar to the iron release rate from biotite. In closed batch reactors at near-basic pH the composition of test solutions in contact with biotite is apparently controlled by gibbsite (Al), kaolinite (Si) and Fe(III)-(hydr)oxide. We estimated a turn-over time (101−102 yr) for molecular oxygen and a time scale (10 months) to develop characteristic Fe(II) concentrations for a granitic groundwater.
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ABSTRACT: The solubilization of arsenic (As) from an ore material (native Arsenic [As, trig.] with Lollingite [FeAs2, rh.]) was characterized in leaching tests lasting for ≤ 99 days. The experiments were performed with materials of different particle sizes (≤ 2 mm), in different waters and under test conditions relevant to As mobilization at near surface contaminated sites. The impact of dolomite [CaMg(CO3)2], metallic iron (Fe0), and pyrite (FeS2) on As release was accessed. Two different types of batch experiments were conducted with a constant amount of the base material and different types of water (deionised, mineral, spring, and tap water). For comparison parallel experiments were conducted with 0.1M EDTA, 0.1M Na2CO3 and 0.1M H2SO4. The results indicated no significant effect of carbonate addition on As solubilization. Fe0 and FeS2 addition essentially slowed the initial As solubilization. H2SO4 was the sole leaching agent significantly influencing As solubilization from the base material. The general trend assuming that “the smaller the particle size the quicker the As release” was not strictly verified because in samples of smaller particle sizes (d < 0.063) As was partly oxidized to more stable species.Engineering in Life Sciences 10/2013; 8(6):622–630. · 1.63 Impact Factor
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ABSTRACT: The standard thermodynamic properties (DGf , DHf , S°, and V°), at 298.15 K, 1 bar, of 16 vermiculites and their heat-capacity coefficients were computed in this research following the approach of Wolery and Jove-Colon (2007). In this way, data consistent with those of other sheet silicates contained in the ther- modynamic database data0.ymp.R5 of the EQ3/6 software package were obtained. Although the uncertainty of these data is too high to investigate exchange reactions involving vermi- culites, they can be profitably used to predict the conditions of vermiculite formation during weathering. The shallow groundwaters interacting with granitoid and gneissic rocks and overlying soils, from an area of the Sila Massif (Calabria Region, Italy), were taken into account for an example of application. Results of speciation–saturation calculations for these waters show that: (i) in general, production of vermicu- lites hosting Mg2+ and Ca2+ ions in the interlayer sites is favoured with respect to generation of vermicu- lites whose interlayer sites are occupied by Na+ and K+ ions; (ii) the possibly forming solid phases (all metastable with respect to Mg–Fe–vermiculite) are kaolinite, Mg–Al–vermiculite, and Mg–Mg–Fe–ver- miculite, in order of increasing pH values. In detail, kaolinite/Mg–Al–vermiculite coexistence occurs at pH close to 6.7, whereas Mg–Al–vermiculite/Mg–Mg–Fe–vermiculite coexistence occurs at pH close to 7.3.Applied Geochemistry 05/2013; 35:264-278. · 1.71 Impact Factor
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ABSTRACT: Chlorite dissolution rates were measured in a series of batch reactor experiments testing the effect of pCO2, pH, chloride and bicarbonate concentrations and temperature. Chlorite is an important diagenetic mineral in sedimentary basins, often found cementing mineral grains and filling pore space in formations that may serve as reservoirs for storing carbon dioxide. Conflicting reports of whether chlorite acts as a barrier to reservoir rock reactivity or leads to enhanced porosity due to dissolution, after the injection of supercritical CO2 into a reservoir, makes studying the reactivity of chlorite in contact with CO2 saturated waters pertinent. Measured dissolution rates were initially rapid and decreased over time as the saturation state of solution relative to chlorite increased. Temperature had the strongest effect on dissolution rate, with an apparent activation energy of 16 ± 0.5 kJ mol−1 and rate constant of log k0 = −9.56 ± 0.07 mol m−2 s−1 assuming a rate law of the form: rate = k0exp(−EA/RT). The apparent activation energy is lower than previously accepted values, but is consistent with a study of chlorite dissolution using flow through techniques (Smith et al., 2013). Mineral dissolution rates are typically proton enhanced, but the lack of a significant pH effect or pCO2 effect on chlorite dissolution rate in this study suggests that the use of NaHCO3 to buffer the pH of CO2 saturated solutions led to an inhibition of mineral dissolution in competition with the expected pH effect. This is supported by the observed dissolution rate increasing dramatically (half a log unit) with the use of an organic acid buffer (KHpthalate) under CO2 free conditions. The effect of chloride (NaCl ∼5 to 50 g/L) was found not to affect the dissolution rate of chlorite. Various empirical rate laws are proposed and fit to the data and lead to the development of a surface complex model describing proton promoted dissolution and bicarbonate inhibition of chlorite dissolution rates. The model can be applied to predict the rate of chlorite dissolution under elevated pCO2 conditions relevant to the storage of CO2 in reservoirs from 50 to 275 °C in contact with fluids ranging in pH from 3.4 to 5.4.Geochimica et Cosmochimica Acta 01/2014; 125:225–240. · 3.88 Impact Factor