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1.
Biogenic carbonates spawned from microbial activities are common occurrences in soils. Here, we investigate the carbonate biomineralization mediated by the bacterium Bacillus megaterium, a dominant strain separated from a loess profile in China. Upon completing bacterial cultivation, the ensuring products are centrifuged, and the resultant supernatant and the concentrated bacterial sludge as well as the un-separated culture are added separately into a Ca-CO3 containing solution for crystallization experiments. Results of XRD and SEM analysis indicate that calcite is the dominant mineral phase formed when the bacteria are present. When the supernatant alone is used, however, a significant portion of vaterite is also precipitated. Experimental results further reveal that the bacteria have a strong tendency to colonize the center area of the calcite {1 0  4} faces. Observed crystal morphology suggests that the bacterial colony may promote the growth normal to each individual {1 0  4} face of calcite when the cell concentration is high, but may retard it or even cause dissolution of the immediate substrate surfaces when the concentration is low. SEM images taken at earlier stages of the crystallization experiments demonstrate the nucleation of calcite on the bacterial cell walls but do not show obvious morphological changes on the nanometer- to submicron-sized nuclei. δ13C measurements unveil that the crystals grown in the presence of bacteria are further enriched in the heavy carbon isotope, implying that the bacterial metabolism may not be the carbon sources for the mineralization. Based upon these findings, we propose a mechanism for the B. megaterium mediated calcite mineralization and conclude that the whole process involves epi- and inter-cellular growth in the local microenvironments whose conditions may be controlled by cell sequestration and proton pumping during bacterial respiration.  相似文献   

2.
Seeded calcite growth experiments were conducted at fixed pH (10.2) and two degrees of supersaturation (Ω = 5, 16), while varying the Ca2+ to solution ratio over several orders of magnitude. The calcite growth rate and the incorporation of Sr in the growing crystals strongly depended on the solution stoichiometry. At a constant degree of supersaturation, the growth rate was highest when the solution concentration ratio, r = [Ca2+]/[], equaled one, and decreased symmetrically with increasing or decreasing values of r. This behavior is consistent with the kink growth rate theory for non-Kossel crystals, assuming that the frequency factors for attachment to kink sites are the same for the cation and anion. Measured Sr partition coefficients, DSr, ranged from 0.02 to 0.12, and correlated positively with the calcite growth rate.  相似文献   

3.
Previous works have opened the debate on the importance of dawsonite in CO2 sequestration by mineral trapping, although there is a lack of systematic studies assessing its long-term stability. With an open eye to this application, we have systematically investigated the stability of synthetic dawsonites (MAlCO3(OH)2, M = , Na+, and K+) in a variety of aqueous media (, Cl−, , , and OH−) at ambient pressure and 323 K. The three compositions dissolved under strong acid (pH 2) and basic (pH 14) conditions. In water (pH 7), dawsonites were not stable, evolving into boehmite (NH4-form) and bayerite (Na and K-forms). The most interesting transformations occurred upon contacting the synthesized dawsonites with ammonium, potassium, and sodium carbonate solutions. NH4-dawsonite was converted into Na-dawsonite and K-dawsonite in Na2CO3 and K2CO3 (pH 12), respectively. Na-dawsonite transformed into NH4-dawsonite in (NH4)2CO3 (pH 10). The NH4-dawsonite and Na-dawsonite can be successively inter-converted in the appropriate carbonate solution. These transformations are thought to follow a dissolution-precipitation mechanism. In the pH range of 10-12, K-dawsonite was the most stable among the three synthesized samples. Besides structural changes, the treatment of dawsonites in carbonate solutions caused remarkable morphology and porosity alterations of the product samples. Our study provides an improved understanding of the chemistry of dawsonite-type compounds. In particular, the narrow pH range at which they are stable poses serious constraints in their potential use for geochemical fixation of CO2.  相似文献   

4.
Ammoniojarosite [(NH4,H3O)Fe3(OH)6(SO4)2], a poorly soluble basic ferric sulfate, was produced by microbiological oxidation of ferrous sulfate at pH 2.0-3.0 over a range of concentrations (5.4-805 mM) and temperatures (22-65 °C). Ammoniojarosites were also produced by chemical (abiotic) procedures in parallel thermal (36-95 °C) experiments. At 36 °C, schwertmannite [ideally Fe8O8(OH)6(SO4)] was the only solid product formed at <10 mM concentrations. Between 11.5 and 85.4 mM , a mixed product of ammoniojarosite and schwertmannite precipitated, as identified by X-ray diffraction. In excess of 165 mM , ammoniojarosite was the only solid phase produced. An increase in the incubation temperature using thermoacidophiles at 45 and 65 °C accelerated the formation of ammoniojarosite in culture solutions containing 165 mM . Both the biogenic and chemical ammoniojarosites were yellow (2Y-4Y in Munsell hue), low surface area (<1 m2/g), well crystalline materials with average co and ao unit cell parameters of 17.467 ± 0.048 Å and 7.330 ± 0.006 Å, respectively. Strong positive correlations were observed between unit cell axial ratios (co/ao) and increasing synthesis temperature in both biotic and abiotic systems. All samples were N deficient compared to stoichiometric ammoniojarosite, and both chemical and X-ray data indicated partial replacement of by H3O+ to form solid solutions with 0.14-0.24 mole H3O+ per formula unit. The morphology of the biogenic jarosites included aggregated discs, pseudo-cubic crystals and botryoidal particles, whereas the chemical specimens prepared at 36-95 °C were composed of irregular crystals with angular edges. Morphological information may thus be useful to evaluate environmental parameters and mode of formation. The data may also have application in predicting phase boundary conditions for Fe(III) precipitation in biogeochemical processes and treatment systems involving acid sulfate waters.  相似文献   

5.
The inhibition of heterogeneous calcite precipitation by orthophosphate was investigated under four different solution compositions using a pH-stat system. The system composition was designed to maintain a constant degree of supersaturation with respect to calcite, but with different carbonate/calcium ratios and pH values during precipitation. Inhibition in the presence of orthophosphate was found to be more effective at lower carbonate/calcium ratios and lower pH values. With the assumption that the calcite precipitation rate is proportional to the surface concentration of active crystal-growth sites, the reduction in the rate of calcite precipitation by phosphate can be explained by a Langmuir adsorption model using a conditional equilibrium constant and total phosphate concentration. Through a detailed analysis of chemical speciation in the solution phase and calcite surface speciation using chemical equilibrium computer modeling, the “conditional” equilibrium constants obtained at different solution compositions were found to converge to a single “non-conditional” value if only was considered in the adsorption reaction. This suggests that is the responsible species for inhibition of calcite precipitation because it adsorbs to the surface and blocks the active crystal-growth sites. The standard enthalpy change (ΔH0) and standard entropy change (TΔS0) of the adsorption reaction, determined by experiments performed from 15 to 45 °C, were 58.5 and 98.3 kJ/mol, respectively. The high positive values of the standard enthalpy change and the standard entropy change suggest that the adsorption reaction is an endothermic reaction, chemisorptive in nature, and driven by the entropy change, most likely resulting from the dehydration process that accompanies the adsorption of onto the calcite surface.  相似文献   

6.
Partitioning of strontium during spontaneous calcite formation was experimentally studied using an advanced CO2-diffusion technique. Results at different precipitation rates and T = 5, 25, and 40 °C show that at constant temperature Sr incorporation into calcite is controlled by the precipitation rate (R in μmol/m2/h) according to the individual expressions
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7.
Dissolved inorganic nitrogen, largely in reduced form (), has been documented in thermal waters throughout Yellowstone National Park, with concentrations ranging from a few micromolar along the Firehole River to millimolar concentrations at Washburn Hot Springs. Indirect evidence from rock nitrogen analyses and previous work on organic compounds associated with Washburn Hot Springs and the Mirror Plateau indicate multiple sources for thermal water NH4(T), including Mesozoic marine sedimentary rocks, Eocene lacustrine deposits, and glacial deposits. A positive correlation between NH4(T) concentration and δ18O of thermal water indicates that boiling is an important mechanism for increasing concentrations of NH4(T) and other solutes in some areas. The isotopic composition of dissolved NH4(T) is highly variable (δ15N = −6‰ to +30‰) and is positively correlated with pH values. In comparison to likely δ15N values of nitrogen source materials (+1‰ to +7‰), high δ15N values in hot springs with pH >5 are attributed to isotope fractionation associated with loss by volatilization. NH4(T) in springs with low pH typically is relatively unfractionated, except for some acid springs with negative δ15N values that are attributed to condensation. NH4(T) concentration and isotopic variations were evident spatially (between springs) and temporally (in individual springs). These variations are likely to be reflected in biomass and sediments associated with the hot springs and outflows. Elevated NH4(T) concentrations can persist for 10s to 1000s of meters in surface waters draining hot spring areas before being completely assimilated or oxidized.  相似文献   

8.
Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO2 in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO2 into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), siderite (FeCO3), as well as the far less common mineral, dawsonite (NaAlCO3(OH)2). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO2 sequestration.In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: , from under- and oversaturated solutions at 50-200 °C in basic media at 1.0 mol · kg−1 NaCl. The solubility products (Qs) obtained were extrapolated to infinite dilution to obtain the solubility constants (. Combining the fit of these values and fixing  at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: , and . Subsequently, we were able to derive values for the Gibbs energy of formation (, enthalpy of formation ( and entropy ( of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals.  相似文献   

9.
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10.
The influence on olivine/melt transition metal (Mn, Co, Ni) partitioning of substitution in the tetrahedral network of silicate melt structure has been examined at ambient pressure in the 1450-1550 °C temperature range. Experiments were conducted in the systems NaAlSiO4-Mg2SiO4- SiO2 and CaAl2Si2O8-Mg2SiO4-SiO2 with about 1 wt% each of MnO, CoO, and NiO added. These compositions were used to evaluate how, in silicate melts, substitution and ionization potential of charge-balancing cations affect activity-composition relations in silicate melts and mineral/melt partitioning.The exchange equilibrium coefficient, , is a positive and linear function of melt Al/(Al + Si) at constant degree of melt polymerization, NBO/T. The is negatively correlated with the ionic radius, r, of the M-cation and also with the ionization potential (Z/r2, Z = electrical charge) of the cation that serves to charge-balance Al3+ in tetrahedral coordination in the melts. The activity coefficient ratio, (γM/γMg)melt, is therefore similarly correlated.These melt composition relationships are governed by the distribution of Al3+ among coexisting Q-species in the peralkaline (depolymerized) melts coexisting with olivine. This distribution controls Q-speciation abundance, which, in turn, controls (γM/γMg)melt and . The relations between melt structure and olivine/melt partitioning behavior lead to the suggestion that in natural magmatic systems mineral/melt partition coefficients are more dependent on melt composition and, therefore, melt structure the more alkali-rich and the more felsic the melt. Moreover, mineral/melt partition coefficients are more sensitive to melt composition the more highly charged or the smaller the ionic radius of the cation of interest.  相似文献   

11.
Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: , , , FeCl3(H2O)3, FeCl3(H2O)2, , FeCl5H2O2−, , ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation.Relative to , all models show a nearly linear decrease in 56Fe/54Fe as the number of Cl− ions per Fe3+ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl− at 20 °C. At 20 °C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for , 8.04-9.12‰ for , 7.61-8.73‰ for , 7.14-8.25‰ for , and 3.09-4.41‰ for . The fractionation between and ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe3+/Fe2+ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models.Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe-X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for , ,, respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl3(H2O)2 vs. FeCl3(H2O)3 and 5.25‰ vs. 4.94‰ for vs. FeCl5H2O2− with the B3LYP/6-31G(d) model). As more Fe-Cl bonds substitute for Fe-OH2 bonds (with a resulting decrease in β), the lengths of the Fe-Cl bonds and the Fe-O bonds increase.Preliminary modeling of shows an Fe3+/Fe2+ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H2O molecules to models of improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8-1.0‰. An additional hydration sphere around increases 1000 ln β by only 0.1‰.Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record.  相似文献   

12.
The enthalpy of mixing of the calcite-rhodochrosite (Ca,Mn)CO3 solid solution was determined at 25 °C from calorimetric measurements of the enthalpy of precipitation of solids with different compositions. A detailed study of the broadening of powder X-ray diffraction peaks shows that most of the precipitates are compositionally homogeneous. All the experimental enthalpy of mixing (ΔHm) values are positive and fit reasonably well (R2 = 0.86) to a Guggenheim function of three terms:
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13.
Microbiological reduction of a biogenic sulfated green rust , was examined using a sulfate reducing bacterium (Desulfovibrio alaskensis). Experiments investigated whether could serve as a sulfate source for D. alaskensis anaerobic respiration by analyzing mineral transformation. Batch experiments were conducted using lactate as the electron donor and biogenic as the electron acceptor, at circumneutral pH in unbuffered medium. transformation was monitored with time by X-ray diffraction (XRD), Transmission Mössbauer Spectroscopy (TMS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The reduction of sulfate anions and the formation of iron sulfur mineral were clearly identified by XPS analyses. TMS showed the formation of additional mineral as green rust (GR) and vivianite. XRD analyses discriminated the type of the newly formed GR as GR1. The formed GR1 was as indicated by DRIFTS analysis. Thus, the results presented in this study indicate that D. alaskensis cells were able to use as an electron acceptor. , vivianite and an iron sulfur compound were formed as a result of reduction by D. alaskensis. Hence, in environments where geochemical conditions promote biogenic formation, this mineral could stimulate the anaerobic respiration of sulfate reducing bacteria.  相似文献   

14.
We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite (Ilm), armalcolite (Arm) and anhydrous lunar silicate melt, to constrain geochemical models of the formation of titanium-rich melts in the Moon. Experiments were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3 GPa and 1300-1400 °C using a synthetic Ti-enriched Apollo ‘black glass’ composition in the CaO-FeO-MgO-Al2O3-TiO2-SiO2 system. Ilmenite-melt and armalcolite-melt partition coefficients (D) show highly incompatible values for the rare earth elements (REE) with the light REE more incompatible compared to the heavy REE ( 0.0020 ± 0.0010 to 0.069 ± 0.010 for ilmenite; 0.0048 ± 0.0023 to 0.041 ± 0.008 for armalcolite). D values for the high field strength elements vary from highly incompatible for Th, U and to a lesser extent W (for ilmenite: 0.0013 ± 0.0008, 0.0035 ± 0.0015 and 0.039 ± 0.005, and for armalcolite 0.008 ± 0.003, 0.0048 ± 0.0022 and 0.062 ± 0.03), to mildly incompatible for Nb, Ta, Zr, and Hf (e.g. 0.28 ± 0.05 and : 0.76 ± 0.07). Both minerals fractionate the high field strength elements with DTa/DNb and DHf/DZr between 1.3 and 1.6 for ilmenite and 1.3 and 1.4 for armalcolite. Armalcolite is slightly more efficient at fractionating Hf from W during lunar magma ocean crystallisation, with DHf/DW = 12-13 compared to 6.7-7.5 for ilmenite. The transition metals vary from mildly incompatible to compatible, with the highest compatibilities for Cr in ilmenite (D ∼ 7.5) and V in armalcolite (D ∼ 8.1). D values show no clear variation with pressure in the small range covered.Crystal lattice strain modelling of D values for di-, tri- and tetravalent trace elements shows that in ilmenite, divalent elements prefer to substitute for Fe while armalcolite data suggest REE replacing Mg. Tetravalent cations appear to preferentially substitute for Ti in both minerals, with the exception of Th and U that likely substitute for the larger Fe or Mg cations. Crystal lattice strain modelling is also used to identify and correct for very small (∼0.3 wt.%) melt contamination of trace element concentration determinations in crystals.Our results are used to model the Lu-Hf-Ti concentrations of lunar high-Ti mare basalts. The combination of their subchondritic Lu/Hf ratios and high TiO2 contents requires preferential dissolution of ilmenite or armalcolite from late-stage, lunar magma ocean cumulates into low-Ti partial melts of deeper pyroxene-rich cumulates.  相似文献   

15.
Copper partitioning in a melt-vapor-brine-magnetite-pyrrhotite assemblage   总被引：4，自引：0，他引：4  
The effect of sulfur on the partitioning of Cu in a melt-vapor-brine ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 140 MPa, fO2 = nickel-nickel oxide (NNO), logfS2=-3.0 (i.e., on the magnetite-pyrrhotite curve at NNO), logfH2S=-1.3 and logfSO2=-1. All experiments were vapor + brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl2 in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2σ) for Cu between melt-vapor, melt-brine and vapor-brine are , , and , respectively, in the S-free system. The partition coefficients (±2σ) for Cu between melt-vapor, melt-brine and vapor-brine are , , and , respectively, in the S-bearing system. Apparent equilibrium constants (±1σ) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of are 34 ± 21 and 128 ± 29 in the S-free and S-bearing runs, respectively. The values of are 33 ± 22 and60 ± 5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases.  相似文献   

16.
The solubility of gold was measured in KCl solutions (0.001-0.1 m) at near-neutral to weakly acidic pH in the presence of the K-feldspar-muscovite-quartz, andalusite-muscovite-quartz, and pyrite-pyrrhotite-magnetite buffers at temperatures 350 to 500°C and pressures 0.5 and 1 kbar. These mineral buffers were used to simultaneously constrain pH, f(S2), and f(H2). The experiments were performed using a CORETEST flexible Ti-cell rocking hydrothermal reactor enabling solution sampling at experimental conditions. Measured log m(Au) (mol/kg H2O) ranges from −7.5 at weakly acid pH to −5.9 in near-neutral solutions, and increases slightly with temperature. Gold solubility in weakly basic and near-neutral solutions decreases with decreasing pH at all temperatures, which implies that Au(HS)2 is the dominant Au species in solution. In more acidic solutions, solubility is independent of pH. Comparison of the experimentally measured solubilities with literature values for Au hydrolysis constants demonstrates that at 350°C dominates Au aqueous speciation at the weakly acidic pH and f(S2)/f(H2) conditions imposed by the pyrite-pyrrhotite-magnetite buffer. In contrast, at temperatures >400°C becomes less important and predominates in weakly acid solutions. Solubility data collected in this study were used to calculate the following equilibrium reaction constants:
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17.
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18.
Comparative concentrations of carbonate and hydroxide complexes in natural solutions can be expressed in terms of reactions with bicarbonate that have no explicit pH dependence (). Stability constants for this reaction with n = 1 were determined using conventional formation constant data expressed in terms of hydroxide and carbonate. Available data indicate that stability constants appropriate to seawater at 25 °C expressed in the form are on the order of 104.2 for a wide range of cations (Mz+) with z = +1, +2 and +3. Φ1 is sufficiently large that species appear to substantially dominate MOHz−1 species in seawater. Evaluations of comparative stepwise carbonate and hydroxide stability constant behavior leading to the formation of n = 2 and n = 3 complexes suggest that carbonate complexes generally dominate hydroxide complexes in seawater, even for cations whose inorganic speciation schemes in seawater are currently presumed to be strongly dominated by hydrolyzed forms (). Calculated stability constants, and , indicate that the importance of carbonate complexation is sufficiently large that carbonate and hydroxide complexes would be generally comparable even if calculated Φ2 and Φ3 values are overestimated by two or more orders of magnitude. Inclusion of mixed ligand species in carbonate-hydroxide speciation models allows cation complexation intensities (MT/[Mz+]) to be expressed in the following form:
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19.
High concentrations of naturally occurring radium pose environmental and health concerns in natural and industrial systems. The adsorption of Ra2+ in saline water is limited compared to its adsorption in fresh water, but the process of co-precipitation may be effective in decreasing its concentration. However, despite its importance, Ra co-precipitation has rarely been studied in high ionic strength environments such as those in evaporitic systems.The fate of Ra in the reject brine of a desalination plant was studied via evaporation batch experiments at ionic strengths (I) ranging from 0.7 to 7.0 mol kg−1. Precipitation sequences revealed that Ra co-precipitated with barite, even though the latter was a trace mineral compared to the precipitated gypsum. The concentration-based effective partition coefficient, , for the co-precipitation reaction was 1.04 ± 0.01. This value of is significantly lower than the value for relatively diluted solutions (1.8 ± 0.1). This low value of is mainly the result of a kinetic effect but is also slightly affected by the ionic strength.Both effects are quantitatively examined in the present paper. It is suggested that a kinetic effect influences the nucleation of (Ra,Ba)SO4, reducing the value of the partition coefficient. This kinetic effect is caused by the favorable nucleation of a more soluble phase (i.e., a phase with a higher BaSO4 fraction). An additional decrease in the partition coefficient results from the ionic strength effect. Considering the activity of Ra2+ and Ba2+ in the solution (rather than their concentration) makes it possible to determine the activity-based partition coefficient (), which accounts for the ionic strength effect. was calculated empirically from the experiments and theoretically via a kinetic model. The two derived values are consistent with one another and indicate the combined effect of ionic strength and precipitation kinetics.Finally, the common assumption that γRa2+/γBa2+=1 was re-examined using a numerical model to predict the experimental results. As the ionic strength increases, this assumption becomes less appropriate for predicting the change in as calculated in the experiments. Understanding the co-precipitation of Ra in such systems is crucial for risk assessments in which both Ra concentration and ionic strength are relatively high.  相似文献   

20.
The adsorption of phosphate onto calcite was studied in a series of batch experiments. To avoid the precipitation of phosphate-containing minerals the experiments were conducted using a short reaction time (3 h) and low concentrations of phosphate (?50 μM). Sorption of phosphate on calcite was studied in 11 different calcite-equilibrated solutions that varied in pH, PCO2, ionic strength and activity of Ca2+, and . Our results show strong sorption of phosphate onto calcite. The kinetics of phosphate sorption onto calcite are fast; adsorption is complete within 2-3 h while desorption is complete in less than 0.5 h. The reversibility of the sorption process indicates that phosphate is not incorporated into the calcite crystal lattice under our experimental conditions. Precipitation of phosphate-containing phases does not seem to take place in systems with ?50 μM total phosphate, in spite of a high degree of super-saturation with respect to hydroxyapatite (SIHAP ? 7.83). The amount of phosphate adsorbed varied with the solution composition, in particular, adsorption increases as the activity decreases (at constant pH) and as pH increases (at constant activity). The primary effect of ionic strength on phosphate sorption onto calcite is its influence on the activity of the different aqueous phosphate species. The experimental results were modeled satisfactorily using the constant capacitance model with >CaPO4Ca0 and either >CaHPO4Ca+ or > as the adsorbed surface species. Generally the model captures the variation in phosphate adsorption onto calcite as a function of solution composition, though it was necessary to include two types of sorption sites (strong and weak) in the model to reproduce the convex shape of the sorption isotherms.  相似文献   

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