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1.
Zhaosheng Qian Hui Feng Wenjing Yang Yingjie Wang 《Geochimica et cosmochimica acta》2010,74(4):1230-1032
In this paper, the structure of the Al30O8(OH)56(H2O)2618+(Al30) polyoxocation in aqueous solution is investigated, including an exploration of its water-exchange reaction using a supramolecular model. Thirty-one solvent water molecules were explicitly included in the supramolecular model to approximate the influence of the solvent. The calculated results indicated that both the gas-phase and the supramolecular models could correctly reproduce the structure of the Al30 polyoxocation, but the supramolecular model described the structure more accurately. Using the supramolecular model, we calculated the 27Al NMR chemical shifts of various aluminum atoms using HF and GIAO methods, and they compared well to the chemical shifts determined experimentally. The water-exchange reaction of the Al30 polyoxocation could not be simulated with the gas phase model because of a proton-transfer reaction that is induced by the highly positive charge of the Al30 polyoxocation. However, the inclusion of an explicit second solvation sphere lowered the acidity of the coordinated water molecules and allowed simulation of the water exchange reaction. 相似文献
2.
Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution
Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al2O8Al28(OH)56(H2O)2618+(aq) (Al30) are reported at pH≈4.7 and 32-40°C. The Al30 molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al30 molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ3-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ2-OH and μ3-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of 17O-enriched water, growth of the 17O-NMR peak near 37 ppm, which is assigned to μ2-OH and μ3-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the 17O-NMR spectra, assigned to bound water molecules (η-OH2), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH2 sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH2 sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH2 sites on the Al30. 相似文献
3.
The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)12(aq) molecule and aqueous solutions
Alasdair P. LeeBrian L. Phillips William H. Casey 《Geochimica et cosmochimica acta》2002,66(4):577-587
We report rates of oxygen exchange with bulk solution for an aqueous complex, IVGeO4Al12(OH)24(OH2)128+(aq) (GeAl12), that is similar in structure to both the IVAlO4Al12(OH)24(OH2)127+(aq) (Al13) and IVGaO4Al12(OH)24(OH2)127+(aq) (GaAl12) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl12 molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al13 and GaAl12. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using 17O-NMR.Apparent rate parameters for exchange of the bound water molecules (η-OH2) are kex298 = 200 (±100) s−1, ΔH‡ = 46 (±8) kJ · mol−1, and ΔS‡ = −46 (±24) J · mol−1 K−1 and are similar to those we measured previously for the GaAl12 and Al13 complexes. In contrast to the Al13 and GaAl12 molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl12 species.The two topologically distinct μ2-OH sites in the GeAl12 molecule exchange at greatly differing rates. The more labile set of μ2-OH sites in the GeAl12 molecule exchange at a rate that is faster than can be measured by the 17O-NMR isotopic-equilibration technique. The second set of μ2-OH sites have rate parameters of kex298 = 6.6 (±0.2) · 10−4 s−1, ΔH‡ = 82 (±2) kJ · mol−1, and ΔS‡ = −29 (±7) J · mol−1 · K−1, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ2-OH sites in the Al13 and GaAl12 molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ2-OH site with bulk solution for the GeAl12 molecule, which contrasts with Al13 and GaAl12 molecules. 相似文献
4.
Although, the kinetic reactivity of a mineral surface is determined, in part, by the rates of exchange of surface-bound oxygens and protons with bulk solution, there are no elementary rate data for minerals. However, such kinetic measurements can be made on dissolved polynuclear clusters, and here we report lifetimes for protons bound to three oxygen sites on the AlO4Al12(OH)24(H2O)127+ (Al13) molecule, which is a model for aluminum-hydroxide solids in water. Proton lifetimes were measured using 1H NMR at pH ∼ 5 in both aqueous and mixed solvents. The 1H NMR peak for protons on bound waters (η-H2O) lies near 8 ppm in a 2.5:1 mixture of H2O/acetone-d6 and broadens over the temperature range −20 to −5 °C. Extrapolated to 298 K, the lifetime of a proton on a η-H2O is τ298 ∼ 0.0002 s, which is surprisingly close to the lifetime of an oxygen in the η-H2O (∼0.0009 s), but in the same general range as lifetimes for protons on fully protonated monomer ions of trivalent metals (e.g., Al(H2O)63+). The lifetime is reduced somewhat by acid addition, indicating that there is a contribution from the partly deprotonated Al13 molecule in addition to the fully protonated Al13 at self-buffered pH conditions. Proton lifetimes on the two distinct sets of hydroxyls bridging two Al(III) (μ2-OH) differ substantially and are much shorter than the lifetime of an oxygen at these sites. The average lifetimes for hydroxyl protons were measured in a 2:1 mixture of H2O/dmso-d6 over the temperature range 3.7-95.2 °C. The lifetime of a hydrogen on one of the μ2-OH was also measured in D2O. The τ298 values are ∼0.013 and ∼0.2 s in the H2O/dmso-d6 solution and the τ298 value for the μ2-OH detectable in D2O is τ298 ∼ 0.013 s. The 1H NMR peak for the more reactive μ2-OH broadens slightly with acid addition, indicating a contribution from an exchange pathway that involves a proton or hydronium ion. These data indicate that surface protons on minerals will equilibrate with near-surface waters on the diffusional time scale. 相似文献
5.
Activation volumes for exchange of oxygen between bulk aqueous solution and sites in the GaO4Al12(OH)24(H2O)127+(aq) (GaAl12) complex were measured by variable-pressure 17O NMR techniques. Near 322 K, rates of exchange for the less labile set of bridging hydroxyls in the GaAl12 decrease by a factor of about two with increasing pressure from 0.1 to 350 MPa. These data indicate a substantially positive activation volume of ΔV‡ = +7 ± 1 cm3/mol, which is the first activation volume measured for a bridging hydroxyl in a polynuclear complex. This result suggests significant bond-lengthening in the activation step. Electrostriction effects should be small because exchange occurs via a pH-independent path under the experimental conditions. The second, more labile set of bridging hydroxyls exchange too rapidly for the variable-pressure techniques employed here. The exchange of bound-water molecules on the GaAl12 was observed at P = 350 MPa using the 17O-NMR line-broadening technique. Comparison with previous measurements at 0.1 MPa indicates decreasing line width from 0.1 to 350 MPa for temperatures at which exchange dominates, yielding an activation volume of ΔV‡ = +3(± 1) cm3/mol. This activation volume is smaller than the value for the Al(H2O)63+ complex, suggesting that water exchange on the larger GaAl12 complex has less dissociative character although the average charge density is lower. 相似文献
6.
E. Davesne K. Dideriksen B.C. Christiansen K.B. Ayala-Luis H.C.B. Hansen 《Geochimica et cosmochimica acta》2010,74(22):6451-6467
In a recent study, sulphate-bearing green rust (GRSO4) was shown to incorporate Na+ in its structure (NaFeII6FeIII3(OH)18(SO4)2(s); GRNa,SO4). The compound was synthesised by aerial oxidation of Fe(OH)2(s) in the presence of NaOH. This paper reports on its free energy of formation .Freshly synthesised GRNa,SO4 was titrated with 0.5 M H2SO4 in an inert atmosphere at 25 °C, producing dissolved Fe2+ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions:
7.
Pyromorphite Pb5(PO4)3Cl and mimetite Pb5(AsO4)3Cl are isostructural minerals with apatite. Due to their high environmental stability, they have gained considerable attention as metals sequestration agents in water treatment and contaminated soil remediation. Pyromorphite and mimetite can form a continuous solid solution series in near-Earth surface environments. Precipitation of the end members and intermediate members of the series is likely to occur in the areas where the cost-effective in situ immobilization reclamation method, based on phosphate amendments, is applied. In contrast to the widely studied thermodynamic parameters of pyromorphite and mimetite, knowledge of the thermodynamics of their solid solutions is sparse. To supplement the data, a number of compounds from the pyromorphite-mimetite series were synthesized at room temperature using a method to simulate the conditions in the near-Earth surface environments. Afterwards, batch dissolution and dissolution-recrystallization experiments of seven synthesized precipitates were conducted at 25 °C, pH = 2 and in a 0.05 M KNO3 background electrolyte. The experiments were carried out for a period of 6 (dissolution) and 14 (dissolution-recrystallization) months. A plateau in the [Pb] evolution patterns was used to determine equilibrium. All seven dissolutions were congruent, and the ionic activity products (IAP) of the minerals from the pyromorphite-mimetite solid solution series were calculated based on the dissolution reaction: . The IAPs for pyromorphite and mimetite exhibit a significant difference in values over three orders of magnitude between approximately 10−79 for pyromorphite and approximately 10−76 for mimetite. The series appeared to be ideal, and Lippmann and Roozboom diagrams were used for better understanding of its thermodynamics. The results indicated a strong tendency of pyromorphite to partition into the solid phase in the series, which explains some of the naturally observed phenomena. The improvement of the lattice stability of the mimetite due to isostructural phosphate substitutions in anionic sites was observed. The thermodynamic data reported in this study supplement existing databases used in geochemical modeling. 相似文献
8.
Wenjing Yang 《Geochimica et cosmochimica acta》2010,74(4):1220-1032
The formation mechanism of Al30O8(OH)56(H2O)2618+ (Al30) has been investigated by the density functional theory based on the supermolecule model and kinetic analysis on the 27Al nuclear magnetic resonance (NMR) experimental results in monitoring Al30 synthesis process. The theoretical chemistry calculations on the four possible schemes show that δ-Na-Al13 is the reasonable intermediate followed by the substitution of Na with Al to form δ-Al14, and Na+ plays an important role in stabilizing the intermediate (δ-Na-Al13) in the transformation. The kinetic analysis on the 27Al NMR experimental data indicates that ε-Al13 decomposes and isomerizes in the formation of Al30, while Al monomers facilitate the decomposition of ε-Al13 and so the isomerization of ε-isomers to δ-isomers effectively. The favorable formation mechanism of Al30 includes three steps: (1) ε-Al13 decomposes and rearranges into the isomer δ-Al13; (2) Na+ reacts with δ-Al13 to stabilize the intermediate δ-Na-Al13, followed by Al monomers replacing Na to form δ-Al14; (3) δ-Al14 reacts with the Al monomers in the solution to finally form Al30. Both Al monomers and Na+ are important in the transformation. Al monomers are the basic building units and helpful to the isomerization while Na+ can well stabilize the isomer δ-Al13 to yield intermediate δ-Na-Al13. The results also show that other isomers of ε-Al13 (β-Al13 and α-Al13) form in the formation of Al30, and their calculated 27Al NMR tetrahedral resonance shifts are consistent with the experimental 27Al NMR tetrahedral signals in the preparation process of Al30. 相似文献
9.
Jörgen Rosenqvist 《Geochimica et cosmochimica acta》2004,68(17):3547-3555
Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ2-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of 18O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in 18O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H218O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates 18O into the least-reactive sites, which are presumably μ2-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH2 sites exchange within fractions of a second and should be isotopically normal using this procedure.When suspended in isotopically normal electrolyte solutions, we find that the rates of release of 18O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ2-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ2-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution. 相似文献
10.
Fluoride replacement of oxygens in the GaO4Al12(OH)24(H2O)127+(aq) molecule [GaAl12] was studied via 19F nuclear magnetic resonance (NMR) at 4 < pH < 5 and 278 K in order to elucidate similar reactions at the surfaces of clays. Peaks are identified in the 19F-NMR spectra that correspond to both terminal and bridging fluorides on the GaAl12 molecule, with relative peak positions similar to those previously identified in fluoridated aluminum (hydr)oxide mineral surfaces (Nordin, J. P., Sullivan, D. J., Phillips, B. L., and Casey, W. H. [1999], “Mechanisms for fluoride-promoted dissolution of bayerite [β-Al(OH)3(s)] and boehmite [γ-AlOOH(s)]-19F-NMR spectroscopy and aqueous surface chemistry,” Geochim. Cosmochim. Acta63, 3513-3524). Fluoride substitutes for oxygen at three different sites in the GaAl12 molecule, but at dramatically different rates.The kinetics of fluoride substitution follow a rate law that includes parallel and reversible transfer of fluoride from nonbridging sites to the two bridging sites. The essential features of the rate law are as follows: (1) fluoride replaces bound water molecules (η-OH2) within minutes at 278 K at rates that are quantitatively similar to fluoride uptake by Al(H2O)63+(aq) to form AlF2+(aq) at similar conditions; (2) fluoride substitutes onto the two topologically distinct μ2-OH sites at different rates, as was previously observed for oxygen exchange, but here, the reaction is complete in hours to days at 278 K. Most importantly, rates of fluoride substitution onto μ2-OH sites are 102 times more rapid than the corresponding rates of oxygen exchange with bulk waters, indicating that fluoride considerably labilizes the molecule, as is also observed at the surfaces of minerals. The largest cause of this labilization is the reduced molecular charge on the GaAl12 upon replacement of bound waters by fluoride, which for mineral surfaces corresponds to a reduction in surface charge density. 相似文献
11.
The solubility of crystalline Mg(OH)2(cr) was determined by measuring the equilibrium H+ concentration in water, 0.01-2.7 m MgCl2, 0.1-5.6 m NaCl, and in mixtures of 0.5 and 5.0 m NaCl containing 0.01-0.05 m MgCl2. In MgCl2 solutions above 2 molal, magnesium hydroxide converted into hydrated magnesium oxychloride. The solid-liquid equilibrium of Mg2(OH)3Cl·4H2O(cr) was studied in 2.1-5.2 m MgCl2. Using known ion interaction Pitzer coefficients for the system Mg-Na-H-OH-Cl-H2O (25°C), the following equilibrium constants at I = 0 are calculated:
12.
Richard E. Zeebe 《Geochimica et cosmochimica acta》2005,69(11):2753-2766
The stable boron isotope ratio (11B/10B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)3, and borate ion, B(OH)4− (, hereafter α(B3-B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same α(B3−B4) = 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for α(B3-B4) varies between ∼1.020 and ∼1.050 at 300 K. However, several arguments suggest that α(B3-B4) ? 1.030. Measured isotopic shifts in various 10B-, 2D-, and 18O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in α(B3-B4) do not bias pH reconstructions provided that organism-specific calibrations are used. 相似文献
13.
I. V. Pekov N. V. Chukanov I. M. Kulikova D. I. Belakovsky 《Geology of Ore Deposits》2007,49(7):530-536
Phosphoinnelite, an analogue of innelite with P > S, has been found in a peralkaline pegmatite vein crosscutting calcite carbonatite
at the phlogopite deposit, Kovdor pluton, Kola Peninsula. Cancrinite (partly replaced with thomsonite-Ca), orthoclase, aegirine-augite,
pectolite, magnesioarfvedsonite, golyshevite, and fluorapatite are associated minerals. Phosphoinnelite occurs as lath-shaped
crystals up to 0.2 × 1 × 6 mm in size, which are combined typically in bunch-, sheaf-, and rosettelike segregations. The color
is yellow-brown, with vitreous luster on crystal faces and greasy luster on broken surfaces. The mineral is transparent. The
streak is pale yellowish. Phosphoinnelite is brittle, with perfect cleavage parallel to the {010} and good cleavage parallel
to the {100}; the fracture is stepped. The Mohs hardness is 4.5 to 5. Density is 3.82 g/cm3 (meas.) and 3.92 g/cm3 (calc.). Phosphoinnelite is biaxial (+), α = 1.730, β = 1.745, and γ = 1.764, 2V (meas.) is close to 90°. Optical orientation
is Z^c ∼ 5°. Chemical composition determined by electron microprobe is as follows (wt %): 6.06 Na2O, 0.04 K2O, 0.15 CaO, 0.99 SrO, 41.60 BaO, 0.64 MgO, 1.07 MnO, 1.55 Fe2O3, 0.27 Al2O3, 17.83 SiO2, 16.88 TiO2, 0.74 Nb2O5, 5.93 P2O5, 5.29 SO3, 0.14 F, −O=F2 = −0.06, total is 99.12. The empirical formula calculated on the basis of (Si,Al)4O14 is (Ba3.59Sr0.13K0.01)Σ3.73(Na2.59Mg0.21Ca0.04)Σ3.04(Ti2.80Fe
0.26
3+
Nb0.07)Σ3.13[(Si3.93Al0.07)Σ4O14(P1.11S0.87)Σ1.98O7.96](O2.975F0.10)Σ3.075. The simplified formula is Ba4Na3Ti3Si4O14(PO4,SO4)2(O,F)3. The mineral is triclinic, space group P
or P1. The unit cell dimensions are a = 5.38, b = 7.10, c = 14.76 ?; α = 99.00°, β = 94.94°, γ = 90.14°; and V = 555 ?3, Z = 1. The strongest lines of the X-ray powder pattern [d, ? in (I)(hkl)] are: 14.5(100)(001), 3.455(40)(103), 3.382(35)(0
2), 2.921(35)(005), 2.810(40)(1
4), 2.683(90)(200,
01), 2.133(80)(
2), 2.059(40)(204, 1
3, 221), 1.772(30)(0
1, 1
7, 2
2, 2
3). The infrared spectrum is demonstrated. An admixture of P substituting S has been detected in the innelite samples from
the Inagli pluton (South Yakutia, Russia). An innelite-phosphoinnelite series with a variable S/P ratio has been discovered.
The type material of phosphoinnelite has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences,
Moscow.
Original Russian Text ? I.V. Pekov, N.V. Chukanov, I.M. Kulikova, D.I. Belakovsky, 2006, published in Zapiski Rossiiskogo
Mineralogicheskogo Obshchestva, 2006, No. 3, pp. 52–60.
Considered and recommended by the Commission on New Minerals and Mineral Names, Russian Mineralogical Society, May 9, 2005.
Approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, July 4, 2005 (proposal
2005-022). 相似文献
14.
The rates of Sb(III) oxidation by O2 and H2O2 were determined in homogeneous aqueous solutions. Above pH 10, the oxidation reaction of Sb(III) with O2 was first order with respect to the Sb(III) concentration and inversely proportional to the H+ concentrations at a constant O2 content of 0.22 × 10−3 M. Pseudo-first-order rate coefficients, kobs, ranged from 3.5 × 10−8 s−1 to 2.5 × 10−6 s−1 at pH values between 10.9 and 12.9. The relationship between kobs and pH was:
15.
16.
The heat capacities of synthetic pyrope (Mg3Al2Si2O12), grossular (Ca3Al2Si3O12) and a solid solution pyrope60grossular40 (Mg1.8Ca1.2Al2Si3O12) have been measured by adiabatic calorimetry in the temperature range 10–350 K. The samples were crystallized from glasses in a conventional piston-cylinder apparatus.The molar thermophysical properties at 298.15 K (J mol?1 K?1) are:
Cop | So298?So0 | Ho298?Ho0/T | |
Pyrope | 325.31 | 266.27 | 47852 |
Grossular | 333.17 | 260.12 | 47660 |
Py60Gr40 | 328.32 | 268.32 | 47990 |