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1.
Dissolution and precipitation rates of brucite (Mg(OH)2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10−4 to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10−6 to 5·10−4 M). Brucite surface charge and isoelectric point (pHIEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pHIEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m2) and lack of dependence on ionic strength predicts the dominance of >MgOH2+ species at pH < 8 and their progressive replacement by >MgOH° and >MgO− as pH increases to 10-12. Rates are proportional to the square of >MgOH2+ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH2+ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:
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2.
The dissolution of siderite (FeCO3) and rhodochrosite (MnCO3) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10−8.65 mol m−2 s−1 for 5.5 < pH < 12 and increases as [H+]0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10−10 mol m−2 s−1 for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10−7.5 mol m−2 s−1, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O2 or [Fe3+(OH)4]− species and surficial >FeII hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10−4.93[H+] + 10−8.45 mol m−2 s−1. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10−8.45 to 10−9.0 mol m−2 s−1. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.  相似文献   

3.
The influence of Al(OH)4− on the dissolution rate of quartz at pH 10-13 and 59-89 °C was determined using batch experiments. Al(OH)4− at concentrations below gibbsite solubility depressed the dissolution rate by as much as 85%, and this effect was greater at lower pH and higher Al(OH)4− concentration. Dissolution rates increased with increasing temperature; however, the percent decrease in rate due to the presence of Al(OH)4− was invariant with temperature for a given H+ activity and Al(OH)4− concentration. These data, along with what is known about Al-Si interactions at high pH, are consistent with Al(OH)4− and Na+ co-adsorbing on silanol sites and passivating the surrounding quartz surface. The observed pH dependence, and lack of temperature dependence, of inferred Al(OH)4− sorption also supports the assumption that the acid-base behavior of the surface silanol groups has only a small temperature dependence in this range. A Langmuir-type adsorption model was used to express the degree of rate depression for a given in situ pH and Al(OH)4− concentration. Incorporation of the rate data in the absence of aluminate into models that assume a first-order dependence of the rate on the fraction of deprotonated silanol sites was unsuccessful. However, the data are consistent with the hypothesis proposed in the literature that two dissolution mechanisms may be operative in alkaline solutions: nucleophilic attack of water on siloxane bonds catalyzed by the presence of a deprotonated silanol group and OH− attack catalyzed by the presence of a neutral silanol group. The data support the dominance of the second mechanism at higher pH and temperature.  相似文献   

4.
《Geochimica et cosmochimica acta》1999,63(13-14):2009-2018
Oxygen and hydrogen isotope fractionation factors between brucite and water were experimentally determined by chemical synthesis techniques at low temperatures of 15° to 120°C. MgCl2, Mg3N2, and MgO were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by x-ray diffraction (XRD) for crystal structure and by scanning electron microscope (SEM) for morphology. It is observed that oxygen isotope fractionations between brucite and water are temperature dependent regardless of variations in aging time, the chemical composition, and pH value of solutions. Brucites derived from three different starting materials yielded consistent fractionations with water at the same temperatures. These suggest that oxygen isotope equilibrium has been achieved between the synthesized brucite and water, resulting in the fractionation equation of 103lnα=1.56×106/T2−14.1. When the present results for the brucite–water system are compared with those for systems of gibbsite–water and goethite–water, it suggests the following sequence of 18O-enrichment in the M−OH bonds of hydroxides: Al3+ − OH > Fe3+ − OH > Mg2+ − OH.Hydrogen isotope fractionations between brucite and water obtained by the different synthesis methods have also achieved equilibrium, resulting in the fractionation equation of 103lnα=−4.88×106/T2−22.5. Because of the pressure effect on hydrogen isotope fractionations between minerals and water, the present calibrations at atmospheric pressure are systematically lower than fractionations extrapolated from hydrothermal exchange experiments at high temperatures of 510° to 100°C and high pressures of 1060 to 1000 bar. Comparison of the present results with existing calibrations involving other low-temperature minerals suggests the following sequence of D-enrichment in hydroxyl-bearing minerals: Al3+ − OH > Mg2+ − OH > Fe3+ − OH.  相似文献   

5.
Lead- and Pb-As-jarosites are minerals common to acidic, sulphate-rich environments, including weathering zones of sulphide ore deposits and acid rock or acid mine drainage (ARD/AMD) sites, and often form on or near galena. The structures of these jarosites are based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, comprised of slightly distorted FeO6 octahedra and SO42− (-AsO43− in Pb-As-jarosites) tetrahedra. To better understand the dissolution mechanisms and products of the break down of Pb- and Pb-As-jarosite, preliminary batch dissolution experiments were conducted on synthetic Pb- and Pb-As-jarosite at pH 2 and 20 °C, to mimic environments affected by ARD/AMD, and at pH 8 and 20 °C, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2). All four dissolutions are incongruent. Dissolution of Pb-jarosite at pH 2 yields aqueous Pb, Fe and SO42−. The pH 8 Pb-jarosite dissolution yields aqueous Pb, SO42− and poorly crystalline Fe(OH)3, which does not appear to resorb Pb or SO42−, possibly due to the low solution pH (3.44-3.54) at the end of the experiment. The pH 2 and 8 dissolutions of Pb-As-jarosite result in the formation of secondary compounds (poorly crystalline PbSO4 for pH 2 dissolution; poorly crystalline PbSO4 and Fe(OH)3 for pH 8 dissolution), which may act as dissolution inhibitors after 250 to 300 h of dissolution. In the pH 2 dissolution, aqueous Fe, SO42− and AsO43− also form, and in the pH 8 dissolution, Fe(OH)3 precipitates then subsequently resorbs aqueous AsO43−. The dissolutions probably proceed by preferred dissolution of the A- and T-sites, which contain Pb, and SO42− and AsO43−, respectively, rather than Fe, which is sterically remote, within the T-O-T Pb- and Pb-As-jarosite structures. These data provide the foundation necessary for further, more detailed investigations into the dissolution of Pb- and Pb-As-jarosites.  相似文献   

6.
Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO3H regulates dissolution for pH < 7 while the surface concentration of >MnOH2+ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ1/2(kco + kca)[>CO3H] + χ1/2(kmo + kma)[>MnOH2+ ], where R is the dissolution rate (mol m−2 s−1), 2χ1/2 is the fraction of surface sites located at steps, [>CO3H] and [>MnOH2+ ] are surface concentrations (mol m−2), and kco, kca, kmo, and kma are the respective dissolution rate coefficients (s−1) for the >CO3H and the >MnOH2+ surface species on obtuse and acute steps. We find kco = 2.7 s−1, kca = 2.1 × 10−1 s−1, kmo = 4.1 × 10−2 s−1, kma = 3.7 × 10−2 s−1, and χ1/2 = 0.015 ± 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.  相似文献   

7.
The dissolution of well crystallized gibbsite far at from equilibrium was studied in batch and mixed flow through reactors. The dissolution experiments were carried out between pH 2 and 6 in the presence of 10 mmol L−1 citrate, at pH 2 and 3 in the presence of 10 mmol L−1 chloride, nitrate, and sulfate, and at pH 2 and 3 in the presence of 1.5 mmol L−1 silica at 20°C. The dissolution rate of gibbsite, RAl (mol m−2 s−1), increases in the order of chloride ≈ nitrate < silica < sulfate ≈ citrate. In presence of silica, sulphate, and citrate dissolution is catalysed by the formation of aluminium complexes at the gibbsite surface (pH 2 and 3). From pH 2 to 3 no effect of RAl on hydrogen activity is predicted as singly coordinated surface sites at the edges of the platy gibbsite crystals, [≡AlOH2+0.5] ≈ [≡AlOH], are almost saturated with protons. However at pH >3 dissolution is slowed by a decrease of [≡AlOH2+0.5].Gibbsite dissolution rates measured in closed and open systems were identical within the experimental and analytical uncertainty. This observation indicates that gibbsite dissolution is a surface controlled process. If dissolution of gibbsite occurs close to equilibrium RAl values may be predicted by an approximately linear function of ΔGr.  相似文献   

8.
The dissolution of chlorite with intermediate Fe-content was studied macroscopically via mixed flow experiments as well as microscopically via atomic force microscopy (AFM). BET surface area normalized steady state dissolution rates at 25 °C for pH 2 to 5 vary between 10−12 and 10−13 mol/m2.s. The order of the dissolution reaction with respect to protons was calculated to be about 0.29. For pH 2 to 4, chlorite was found to dissolve non-stoichiometrically, with a preferred release of the octahedrally coordinated cations. The additional release of octahedrally coordinated cations may be due to the transformation of chlorite to interstratified chlorite/vermiculite from the grain edges inward.In-situ atomic force microscopy performed on the basal surfaces of a chlorite sample, which has been preconditioned at pH 2 for several months, indicated a defect controlled dissolution mechanism. Molecular steps with height differences which correspond to the different subunits of chlorite, e.g. TOT sheet and brucite like layer, originated at surface defects such or compositional inhomogenities or cracks, which may be due to the deformation history of the chlorite sample. In contrast to other sheet silicates, at pH 2 nanoscale etch pits occur on the chlorite basal surfaces within flat terraces terminated by a TOT-sheet as well as within the brucite like layer. The chlorite basal surface dissolves layer by layer, because most of the surface defects are only expressed through single TOT or brucite-like layers. The defect controlled dissolution mechanism favours dissolution of molecular steps on the basal surfaces compared to dissolution of the grain edges. At pH 2 the dissolution of the chlorite basal surface is dominated by the retreat of 14 Å steps, representing one chlorite unit cell.The macroscopic and microscopic chlorite dissolution rates can be linked via the reactive surface area as identified by AFM. The reactive surface area with respect to dissolution consists of only 0.2% of the BET-surface area. A dissolution rate of 2.5 × 10−9 mol/m2s was calculated from macroscopic and microscopic dissolution experiments at pH 2, when normalized to the reactive surface area.  相似文献   

9.
《Geochimica et cosmochimica acta》1999,63(19-20):3261-3275
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.  相似文献   

10.
《Geochimica et cosmochimica acta》1999,63(19-20):3133-3143
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11.
Far from equilibrium enstatite dissolution rates both open to atmospheric CO2 and CO2 purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO2 conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO.Steady-state dissolution rates of enstatite increase above pH 10 when CO2 was purged by performing the experiments in a N2 atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO32− above pH 10.33. It is argued that CO32− forms a >Mg2-CO3 complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si-O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO2-purged enstatite dissolution in moles per cm2 per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature.  相似文献   

12.
The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV-Vis spectrophotometry to identify and measure the thermodynamic properties of Au(III) aqueous complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solutions in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, we derived log dissociation constants for the following mixed and end-member halide and hydroxide complexes: [AuCl3Br]−, [AuCl2Br2]−, [AuBr3Cl]− and [AuBr4]−; [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]− and [Au(OH)4]−; and [AuBr3(OH)]−, [AuBr2(OH)2]− and [AuBr(OH)3]−. These are the first reported results for the mixed chloride-bromide complexes. Increasing temperature to 80 °C resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl(4−n)(OH)n]− series of complexes (n = 0-4), there is an excellent agreement between our spectrophotometric results and previous electrochemical results of Chateau et al. [Chateau et al. (1966)]. In other experiments, the iodide ion (I−) was found to be unstable in the presence of Au(III), oxidizing rapidly to I2(g) and causing Au to precipitate. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atmospheric), acidic (pH < 5) conditions. Native gold solubility under atmospheric oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alkaline conditions, increasing chloride, especially at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. The results of our study increase the understanding of gold aqueous geochemistry, with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.  相似文献   

13.
The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxide minerals. The results suggest the following sequence of 18O-enrichment in the common hydroxides: limonite > gibbsite > goethite > brucite > diaspore. The hydroxides are significantly enriched in 18O relative to the corresponding oxides. The sequence of 18O-enrichment in the hydroxides and oxides of trivalent cations is as follows: M(OH)3 > MO(OH) > M2O3. There are also considerable fractionations within the polymorphos of Al(OH)3. The internally consistent fractionation factors for hydroxide–water systems are obtained for the temperature range of 0 to 1200 °C, which are comparable with the data derived from synthesis experiments and natural samples at surficial temperatures. Temperature dependence of oxygen isotope fractionations between goethite, gibbsite, boehmite and diaspore and water are significant enough for the purpose of geothermometry. Thus the hydroxide–water pairs hold great promise of serving as reliable paleothermometers in surficial geological environments. Received: 22 January 1997 / Revised, accepted: 2 June 1997  相似文献   

14.
In light of recent work on the reactivity of specific sites on large (hydr)oxo-molecules and the evolution of surface topography during dissolution, we examined the ability to extract molecular-scale reaction pathways from macroscopic dissolution and surface charge measurements of powdered minerals using an approach that involved regression of multiple datasets and statistical graphical analysis of model fits. The test case (far-from-equilibrium quartz dissolution from 25 to 300 °C, pH 1-12, in solutions with [Na+] ? 0.5 M) avoids the objections to this goal raised in these recent studies. The strategy was used to assess several mechanistic rate laws, and was more powerful in distinguishing between models than the statistical approaches employed previously. The best-fit model included three mechanisms—two involving hydrolysis of Si centers by H2O next to neutral (>Si-OH0) and deprotonated (>Si-O−) silanol groups, and one involving hydrolysis of Si centers by OH−. The model rate law is
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15.
Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3−, NH4+, Fe2+, and Mn2+ and SO42− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, NO3−, Fe(OH)3 and SO42−, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm−2 yr−1, of which 77% were due to O2, 17% to NO3− and 3% to Fe(OH)3 and 3% to SO42−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d−1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m−2 yr−1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m−2 yr−1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.  相似文献   

16.
Sorption of Cm(III) and Eu(III) at trace concentrations onto Ca-montmorillonite (SWy-1) and Na-illite (Illite du Puy) has been studied under anaerobic conditions by batch sorption experiments and time-resolved laser fluorescence spectroscopy (TRLFS). Comparison of the results from spectroscopic and batch sorption experiments with Cm and Eu indicates the existence of outer-sphere complexes at pH <4 in the experiments with Na-illite (0.25 g/L solid; 2.5 × 10−7 mol/L Cm; 0.1 mol/L NaClO4). In the case of Ca-montmorillonite, (0.25 g/L solid, 2.5 × 10−7 mol/L Cm or 10−6 mol/L Eu, 0.066 mol/L Ca(ClO4)2), Cm/Eu outer-sphere complexes do not form at significant levels due to the Ca2+ competition for the clay mineral cation-exchange sites. TRLFS spectra indicate the formation of inner-sphere surface complexes at pH >5 for both clay minerals. Five H2O/OH− molecules remain in the first metal ion coordination sphere of the sorbed Eu/Cm. Measured fluorescence lifetimes of sorbed Eu/Cm and peak deconvolution of Cm-spectra are consistent with the formation of surface complexes of the form ≡S-O-Eu/Cm(OH)x(2−x)(H2O)5−x. At pH ≥ 12 Cm becomes incorporated into a surface precipitate at the Ca-montmorillonite surface presumably composed of Ca(OH)2 or calcium silicate hydrate. A dramatic shift of the fluorescence emission band by more than 20 nm and a clear increase in the fluorescence lifetime suggests the almost complete displacement of coordinated H2O and OH−. The pH dependent Eu sorption data obtained in batch experiments are consistent with spectroscopic data on Eu and Cm within experimental uncertainties thus demonstrating the validity of Eu as a homologue for trivalent actinides. Parameterization of a two-site protolysis nonelectrostatic surface complexation and cation exchange model using the batch sorption data and spectroscopic results is discussed in Part 2 of this work.  相似文献   

17.
Biotite dissolution under conditions of high pH and high aluminum, sodium, and nitrate concentrations analogous to those found in tank wastes at the Hanford Site was investigated using continuously stirred flow-through reactors at 22 to 25 °C. Experiments were designed to simulate tank leaks into the Hanford vadose zone where Fe(II) from biotite is the dominant reducing agent available to immobilize certain contaminants. Both non-steady-state and steady-state dissolution kinetics were quantified; interest in non-steady-state kinetics derives from the inherently transitory nature of tank leaks. Biotite was conditioned in pH 8 solutions to simulate the alkaline environment of the Hanford sediment, and then reacted in pH 10-14 solutions, some including 0.055 M Al(NO3)3 and/or 2 M or 6 M NaNO3. Initial dissolution transients (intervals of rapid release rates that decay to slower steady-state rates) showed fast preferential release of K followed by near-stoichiometric release of Si, Al, and Mg, and slower release of Fe. Each increase in pH resulted in a second transient with the greatest amounts of Si, Al, and K released at pH 14, followed by pHs 13, 12, 11, and 10. Fe release also was highest at pH 14, but unchanging at pHs 10-13 within experimental error. Transient releases at high pH are attributed to dissolution of amphoteric secondary phases such as ferrihydrite that are inferred from saturation calculations and solid analyses to form during the conditioning interval. Transient release of Si was inhibited by the presence of 0.055 M Al(NO3)3; the effects of Al(NO3)3 and NaNO3 on the release rates of Al, Fe, Mg, and K were variable and generally outweighed by the effect of pH. Quasi-steady-state release rates were slowest at pH 11-12 (10−12.2 mol biotite m−2 s−1 for Si) and increased in either direction in pH away from this minimum (to 10−11.5 at pHs 8 and 14 for Si). Fe release rates at high pH were sufficient to account for observed Cr(VI) reduction at Hanford. The net release rates of the major framework cations, from which the biotite dissolution rate is inferred, may reflect the precipitation of secondary phases or the alteration of biotite to vermiculite. The most extensive solid-phase alterations were observed in Na-enriched solutions.  相似文献   

18.
The dissolution behavior of two biogenic Mg-calcites, the echinoid, Tripneustes (12 mol% MgCO3), and the red alga, Neogoniolithon (18 mol% MgCO3 plus brucite), was studied using free-drift methods in distilled water and phosphate-spiked solutions at 25°C and PCO2 = 1 atm. Small concentrations of phosphate strongly inhibit dissolution rates. Inhibition increases with increased phosphate levels and with approach toward saturation. Near saturation, dissolution rates are reduced by 103 by the presence of 0.6 μmol adsorbed-P/m2. The magnitude of phosphate inhibition is similar to that observed for low-Mg calcite, and like calcite, the mechanism of inhibition is probably by adsorption of P at surface kink sites. Phosphate appears to inhibit removal of Mg and Ca equally during Mg-calcite dissolution. Rates of brucite dissolution are unaffected by phosphate.Mg-calcites containing >8.5 mol% MgCO3 should be thermodynamically unstable relative to aragonite in most natural waters. Recent work, however, shows that in contrast to its effect on the behavior of Mg-calcites. phosphate does not inhibit aragonite dissolution. The presence of phosphate might thus reverse the relative stability of these two minerals during diagenesis of shallow marine carbonate sediments.  相似文献   

19.
《Applied Geochemistry》2002,17(4):399-408
The sorption of U(VI) onto the surface of olivine has been experimentally investigated at 25 °C under an air atmosphere as a function of pH, solid surface to volume ratio and total U concentration. Sorption has been observed to decrease as the extent of carbonate complexation of U(VI) in solution increases, which is attributed to the competition between aqueous and solid ligands for the coordination of U. The experimental results have been interpreted by means of two different approaches: (1)a semi-empirical model, exemplified by the application of a Langmuir isotherm and (2) a non-electrostatic thermodynamic surface complexation model which includes the formation of the surface species: >SO–UO2+ and >SO–UO2(OH). The following stability constants for these species have been determined from the thermodynamic analysis: K(>SO–UO2+)=289±71 and K(>SO–UO2(OH))=(3.4±0.4)×10−6. The comparison of the sorption of U onto olivine with granites of different origin indicate that the use of this mineral as additive to the backfill of deep high level nuclear waste repositories could retard the migration of U from the repository to the geosphere.  相似文献   

20.
Forward dissolution rates of Na-Montmorillonite (Wyoming) SWy-2 smectite (Ca0.06Na0.56)[Al3.08Fe(III)0.38Mg0.54] [Si7.93 Al0.07]O20(OH)4 were measured at 25 °C in a mixed-flow reactor equipped with interior dialysis compartment (6-8 kDa membrane) as a function of pH (1-12), dissolved carbonate (0.5-10 mM), phosphate (10−5 to 0.03 M), and nine organic ligands (acetate, oxalate, citrate, EDTA, alginate, glucuronic acid, 3,4-dihydroxybenzoic acid, gluconate, and glucosamine) in the concentration range from 10−5 to 0.03 M. In organic-free solutions, the Si-based rates decrease with increasing pH at 1 ? pH ? 8 with a slope close to −0.2. At 9 ? pH ? 12, the Si-based rates increase with a slope of ∼0.3. In contrast, non-stoichiometric Mg release weakly depends on pH at 1 ? pH ? 12 and decreases with increasing pH. The empirical expression describing Si-release rates [R, mol/cm2/s] obtained in the present study at 25 °C, I = 0.01 M is given by
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