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1.
Arsenic(III) adsorption reactions are thought to play a critical role in the mobility of arsenic in the environment. It is the nature of the As(III) surface species that must be known on a wide variety of minerals and over a range of pH, ionic strength and surface coverage in order to be able to predict adsorption behavior. EXAFS and XANES spectroscopic studies have identified bidentate, binuclear inner-sphere surface species and/or an outer-sphere species, but only a few oxides have been examined. These results need to be integrated with a predictive surface complexation model in order to ascertain the environmental conditions under which the different surface species may be important on a wide range of solids. In the present study, the surface species information from XAFS and XANES studies has been built into a recent extension of the triple-layer model (ETLM) for the formation of inner-sphere complexes of anions that takes into account the electrostatics of water dipole desorption during ligand exchange reactions. The ETLM has been applied to regress surface titration, proton coadsorption, and As(III) adsorption data over extensive ranges of pH, ionic strength, electrolyte type and surface coverage for magnetite, goethite, gibbsite, amorphous hydrous alumina, hydrous ferric oxide (HFO), ferrihydrite, and amorphous iron oxide. Two principal reactions forming inner- and outer-sphere As(III) surface species,
2.
Sulfate and selenate adsorption on iron oxides are important reactions in natural systems under a very wide range of pH values, ionic strengths, and electrolyte compositions. Under such conditions, spectroscopic and theoretical calculations have demonstrated the potential importance of a variety of surface species. Understanding the variations in the surface speciation of these oxyanions is fundamental to prediction of their partitioning between minerals and aqueous solutions. In the present study, published experimental spectroscopic and theoretical molecular evidence of the identity of sulfate/selenate surface species are integrated with a surface complexation model consistent with a wide variety of experimental adsorption, surface titration, and proton coadsorption data to define the surface speciation of sulfate and selenate on iron oxides under a wide range of conditions. The analysis was carried out with the extended triple layer model (ETLM) taking into account the electrostatics of water dipole desorption during ligand exchange reactions. On seven out of eight goethites studied, sulfate and selenate surface reactions can be represented by the formation of a monodentate-mononuclear inner-sphere and a bidentate-binuclear outer-sphere (or H-bonded) species according to
3.
The formation constants of neodymium complexes in sulfate solutions have been determined spectrophotometrically at temperatures of 30-250 °C and a pressure of 100 bars. The dominant species in the solution are NdSO4+ and Nd(SO4)2−, with the latter complex being more important at higher temperature. Equilibrium constants were calculated for the following reactions:
4.
J.A. Tossell 《Geochimica et cosmochimica acta》2006,70(20):5089-5103
Boric acid, B(OH)3, forms complexes in aqueous solution with a number of bidentate O-containing ligands, HL−, where H2L is C2O4H2 (oxalic acid), C3O4H4 (malonic acid), C2H6O2 (ethylene glycol), C6H6O2 (catechol), C10H8O2 (dioxynaphthalene) and C2O3H4 (glycolic acid). McElligott and Byrne [McElligott, S., Byrne, R.H., 1998. Interaction of and in seawater: Formation of . Aquat. Geochem.3, 345-356.] have also found B(OH)3 to form an aqueous complex with . Recently Lemarchand et al. [Lemarchand, E., Schott, J., Gaillardeet, J., 2005. Boron isotopic fractionation related to boron sorption on humic acid and the structure of surface complexes formed. Geochim. Cosmochim. Acta69, 3519-3533] have studied the formation of surface complexes of B(OH)3 on humic acid, determining 11B NMR shifts and fitted values of formation constants, and 11B, 10B isotope fractionations for a number of surface complexation models. Their work helps to clarify both the nature of the interaction of boric acid with the functional groups in humic acid and the nature of some of these coordinating sites on the humic acid. The determination of isotope fractionations may be seen as a form of vibrational spectroscopy, using the fractionating element as a local probe of the vibrational spectrum. We have calculated quantum mechanically the structures, stabilities, vibrational spectra, 11B NMR spectra and 11B,10B isotope fractionations of a number of complexes B(OH)2L− formed by reactions of the type:
5.
Wojciech Piasecki 《Geochimica et cosmochimica acta》2008,72(16):3964-3979
The distribution of yttrium and the rare earth elements (YREE) between natural waters and oxide mineral surfaces depends on adsorption reactions, which in turn depend on the specific way in which YREE are coordinated to mineral surfaces. Recent X-ray studies have established that Y3+ is adsorbed to the rutile (1 1 0) surface as a distinctive tetranuclear species. However, the hydrolysis state of the adsorbed cation is not known from experiment. Previous surface complexation models of YREE adsorption have suggested two to four cation hydrolysis states coexisting on oxide surfaces. In the present study, we investigate the applicability of the X-ray results to rare earth elements and to several oxides in addition to rutile using the extended triple-layer surface complexation model. The reaction producing a hydrolyzed tetranuclear surface species
6.
The nature of adsorbed arsenate species for a wide range of minerals and environmental conditions is fundamental to prediction of the migration and long-term fate of arsenate in natural environments. Spectroscopic experiments and theoretical calculations have demonstrated the potential importance of a variety of arsenate surface species on several iron and aluminum oxides. However, integration of the results of these studies with surface complexation models and extrapolation over wide ranges of conditions and for many oxides remains a challenge. In the present study, in situ X-ray and infrared spectroscopic and theoretical molecular evidence of arsenate (and the analogous phosphate) surface speciation are integrated with an extended triple layer model (ETLM) of surface complexation, which takes into account the electrostatic work associated with the ions and the water dipoles involved in inner-sphere surface complexation by the ligand exchange mechanism.Three reactions forming inner-sphere arsenate surface species
7.
Robert H. Byrne 《Geochimica et cosmochimica acta》2010,74(15):4312-5647
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:
8.
The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 °C over a range of pH (4.0-7.1) and carbonate concentrations . Distribution coefficients, defined as , where [MSi]T is the total concentration of sorbed YREE, MT is the total YREE concentration in solution, and [Si] is the concentration of amorphous ferric hydroxide, initially increased in magnitude with increasing carbonate concentration, and then decreased. The initial increase of is due to sorption of YREE carbonate complexes , in addition to sorption of free YREE ions (M3+). The subsequent decrease of , which is more extensive for the heavy REEs, is due to the increasing intensity of YREE solution complexation by carbonate ions. The competition for YREEs between solution complexation and surface complexation was modeled via the equation:
9.
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:
10.
Caroline L. Peacock 《Geochimica et cosmochimica acta》2005,69(15):3733-3745
We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)n−6 and binuclear (Cu2O6Hn)n−8 inner-sphere complexes on variable-charge ≡AlOH sites and as Cu2+ on ion exchangeable ≡X--H+ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)6 polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)2Cu(OH)20, tridentate (≡Al3O(OH)2)Cu2(OH)30 and ≡X--Cu2+ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions
11.
Regina A. Easley 《Geochimica et cosmochimica acta》2011,75(19):5638-5647
Lead speciation in many aqueous geochemical systems is dominated by carbonate complexation. However, direct observations of Pb2+ complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium over a range of ionic strength. Through spectrophotometric observations of formation at 25 °C in NaHCO3-NaClO4 solutions, formation constants of the form were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation:
12.
The stability of yttrium-acetate (Y-Ac) complexes in aqueous solution was determined potentiometrically at temperatures 25-175 °C (at Ps) and pressures 1-1000 bar (at 25 and 75 °C). Measurements were performed using glass H+-selective electrodes in potentiometric cells with a liquid junction. The species YAc2+ and were found to dominate yttrium aqueous speciation in experimental solutions at 25-100 °C (log [Ac−] < −1.5, pH < 5.2), whereas at 125, 150 and 175 °C introduction of into the Y-Ac speciation model was necessary. The overall stability constants βn were determined for the reaction
13.
Steady-state magnesite dissolution rates were measured in mixed-flow reactors at 150 and 200 °C and 4.6 < pH < 8.4, as a function of ionic strength (0.001 M ? I ? 1 M), total dissolved carbonate concentration (10−4 M < ΣCO2 < 0.1 M), and distance from equilibrium. Rates were found to increase with increasing ionic strength, but decrease with increasing temperature from 150 to 200 °C, pH, and aqueous CO32− activity. Measured rates were interpreted using the surface complexation model developed by Pokrovsky et al. (1999a) in conjunction with transition state theory (Eyring, 1935). Within this formalism, magnesite dissolution rates are found to be consistent with
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16.
Guillaume Desbois 《Geochimica et cosmochimica acta》2007,71(21):5233-5243
Hydrogen-deuterium exchange in tourmaline single crystals of elbaite composition from Nepal has been studied at 1 atm and at temperatures between 973 and 1073 K. H/D ratios were determined after each annealing experiment using micro FTIR-spectroscopy. Diffusion coefficients (10−16-10−15 m2 s−1) were determined by fitting the data using a 3D numerical simulation. The rate of diffusion is two to three times faster along the c direction than along directions parallel to the basal plane. The diffusion laws are, respectively:
17.
Arsenic, iron and sulfur co-diagenesis in lake sediments 总被引：3，自引：0，他引：3
Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (ΣS−II), total zero-valent sulfur (ΣS0), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, 210Pb and 137Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As (). The model defines depth intervals where As is either released to (positive ) or removed from (negative ) the porewaters.At two of the sites, whose bottom water were oxygenated at sampling time, a production zone ( = 12 × 10−18 mol cm−3 s−1-71 × 10−18 mol cm−3 s−1) is inferred a few cm below the sediment-water interface, coincident with sharp porewater As and Fe peaks that indicate an intense coupled recycling of As and Fe. This process is confirmed by solid As and Fe maxima just below the sediment surface. In these two lakes a zone of As consumption ( = −5 × 10−18 mol cm−3 s−1 to −53 × 10−18 mol cm−3 s−1), attributed to the slow adsorption of As to authigenic Fe oxyhydroxides, occurs just above the production zone. A second-order rate constant of 0.12 ± 0.03 cm3 mol−1 s−1 is estimated for this adsorption reaction.Such features in the porewater and solid profiles were absent from the two other lakes that develop a seasonally anoxic hypolimnion. Thermodynamic calculations indicate that the porewaters of the four lakes, when sulfidic (i.e., ΣS−II ? 0.1 μM), were undersaturated with respect to all known solid As sulfides; the calculation also predicts the presence of AsV oxythioanions in the sulfidic waters, as suggested by a recent study. In the sulfidic waters, the removal of As ( = −1 × 10−18 mol cm−3 s−1 to −23 × 10−18 mol cm−3 s−1) consistently occurred when saturation, with respect to FeS(s), was reached and when AsV oxythioanions were predicted to be significant components of total dissolved As. This finding has potential implications for As transport in other anoxic waters and should be tested in a wider variety of natural environments. 相似文献
18.
Nikolay N. Akinfiev 《Geochimica et cosmochimica acta》2009,73(6):1597-1608
Knowledge of the solubility of quartz over a broad spectrum of aqueous fluid compositions and T-P conditions is essential to our understanding of water-rock interaction in the Earth’s crust. We propose an equation to compute the molality of aqueous silica, mSiO2(aq), mol·(kg H2O)−1, in equilibrium with quartz and water-salt-CO2 fluids, as follows:
19.
H. Gailhanou J.C. van Miltenburg J. Olives E.C. Gaucher 《Geochimica et cosmochimica acta》2007,71(22):5463-5473
The heat capacities of the anhydrous international reference clay minerals, smectite MX-80, illite IMt-2 and mixed-layer illite-smectite ISCz-1, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 6 to 520 K (at 1 bar). The samples were chemically purified and Na-saturated. Dehydrated clay fractions <2 μm were studied. The structural formulae of the corresponding clay minerals, obtained after subtracting the remaining impurities, are K0.026Na0.435Ca0.010(Si3.612Al0.388) (Al1.593Mg0.228Ti0.011)O10(OH)2 for smectite MX-80, K0.762Na0.044(Si3.387Al0.613) (Al1.427Mg0.241O10(OH)2 for illite IMt-2 and K0.530Na0.135(Si3.565Al0.435)(Al1.709Mg0.218Ti0.005)O10(OH)2for mixed-layer ISCz-1. From the heat capacity values, we determined the molar entropies, standard entropies of formation and heat contents of these minerals. The following values were obtained at 298.15 K and 1 bar:
(J mol−1 K−1) S0 (J mol−1 K−1) Smectite MX-80 326.13 ± 0.10 280.56 ± 0.16 Illite IMt-2 328.21 ± 0.10 295.05 ± 0.17 Mixed-layer ISCz-1 320.79 ± 0.10 281.62 ± 0.15 - Full-size table
20.
Sorption of phosphate onto calcite; results from batch experiments and surface complexation modeling 总被引：3，自引：0，他引：3
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