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1.
Owen W Duckworth 《Geochimica et cosmochimica acta》2003,67(10):1787-1801
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
2.
Melchor González-Davila 《Geochimica et cosmochimica acta》2005,69(1):83-93
The oxidation of Fe(II) with H2O2 at nanomolar levels in seawater have been studied using an UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5 to 8.2), H2O2 (7.2 × 10−8 M to 5.2 × 10−7 M), HCO3− (2.05 mM to 4.05 mM) and Fe(II) (5 nM to 500 nM) as a function of temperature (3 to 35 °C) on the oxidation of Fe(II) are presented. The oxidation rate is linearly related to the pH with a slope of 0.89 ± 0.01 independent of the concentration of HCO3−. A kinetic model for the reaction has been developed to consider the interactions of Fe(II) with the major ions in seawater. The model has been used to examine the effect of pH, concentrations of Fe(II), H2O2 and HCO3− as a function of temperature. FeOH+ is the most important contributing species to the overall rate of oxidation from pH 6 to pH 8. At a pH higher than 8, the Fe(OH)2 and Fe(CO3)22− species contribute over 20% to the rates. Model results show that when the concentration of O2 is two orders of magnitude higher than the concentration of H2O2, the oxidation with O2 also needs to be considered. The rate constants for the five most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3, Fe(CO3)22−) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO3− and the conditions found in fresh waters. 相似文献
3.
The oxidation of Mn(II) by O2 to Mn(III) or Mn(IV) is thermodynamically favored under the pH and pO2 conditions present in most near surface waters, but the kinetics of this reaction are extremely slow. This work investigated whether reactive oxygen species, produced through illumination of humic substances, could oxidize Mn at an environmentally relavent rate. The simulated sunlight illumination of a solution containing 200 μM Mn(II) and 5 mg/L Aldrich humic acid buffered at pH 8.1 produced ∼19 μM of oxidized Mn (MnOx where x is between one and two) after 45 minutes. The major oxidants reponsible for this reaction appear to be photoproduced superoxide radical anion, O2−, and singlet molecular oxygen, 1O2. The dependencies of MnOx formation on Mn(II), humic acid, and H+ concentration were characterized. A kinetic model based largely on published rate constants was established and fit to the experimental data. As expected, analysis of the model indicates that the key reaction rate controlling MnOx production is the rate of decomposition of a MnO2+ complex formed from the reaction of Mn(II) with O2−. This rate is strongly dependent on the Mn(II) complexing ligands in solution. The MnOx production in the seawater sample taken from Bodega Bay, USA and spiked with 200 μM Mn(II) was well reproduced by the model. Extrapolations from the model imply that Mn photo-oxidation should be a significant reaction in typical surface seawaters. Calculated rates, 5.8 to 55 pM h−1, are comparable to reported rates of biological Mn oxidation, 0.07 to 89 pM h−1. Four fresh water samples that were spiked with 200 μM Mn(II) also showed significant MnOx production. Based on these results, it appears that Mn photo-oxidation could constitute a significant, and apparently unrecognized geochemical pathway in natural waters. 相似文献
4.
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:
5.
This paper presents the results of extensive field trials measuring rates of Fe(II) oxidation at a number of Fe-bearing mine drainage discharges in the UK. Batch experiments were carried out with samples taken at regular intervals and Fe(II) concentration determined spectrophotometrically using 2′2-bipyridyl as the complexing agent. Initial concentrations for Fe(II) were 5.65-76.5 mg/L. Temperature, pH and dissolved O2 (DO) were logged every 10 s, with pH at the start of the experiments in the range 5.64-6.95 and alkalinity ranging from 73 to 741 mg/L CaCO3 equivalent. A numerical model based on a fourth order Runge-Kutta method was developed to calculate values for k1, the rate constant for homogeneous oxidation, from the experimental data. The measured values of pH, temperature, [Fe(II)] and DO were input into the model with resulting values for k1 found to be in the range 2.7 × 1014-2.7 × 1016 M−2 atm−1 min−1. These values for k1 are 1-3 orders of magnitude higher than previously reported for laboratory studies at a similar pH. Comparison of the observed Fe(II) oxidation rates to data published by other authors show a good correlation with heterogenous oxidation rates and may indicate the importance of autocatalysis in these systems. These higher than expected rates of Fe oxidation could have a significant impact on the design of treatment schemes for the remediation of mine drainage and other Fe-bearing ground waters in the future. 相似文献
6.
The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH2PO4+, FeHPO4 and FePO4− have been determined for the first time. FePO4− was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M−1 s−1. FeH2PO4+ and FeHPO4 were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10−2 M−1 s−1 and (1.2 ± 0.8) × 10−1 M−1 s−1, respectively. 相似文献
7.
The kinetics of iodide (I−) and molecular iodine (I2) oxidation by the manganese oxide mineral birnessite (δ-MnO2) was investigated over the pH range 4.5-6.25. I− oxidation to iodate proceeded as a two-step reaction through an I2 intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I− from solution was first order with respect to I− concentration, pH, and birnessite concentration, such that −d[I−]/dt = k[I−][H+][MnO2], where k, the third order rate constant, is equal to 1.08 ± 0.06 × 107 M−2 h−1. The data are consistent with the formation of an inner sphere I− surface complex as the first step of the reaction, and the adsorption of I− exhibited significant pH dependence. Both I2, and to a lesser extent, sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I2 and , as well as the biophilic nature of I2, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds. 相似文献
8.
Passive treatment systems for mine drainage use no energy other than gravity, but they require greater area than active treatment systems. Researchers are considering “hybrid” systems that have passive and active components for increased efficiency, especially where space limitations render passive-only technology ineffective. Flow-through reactor field experiments were conducted at two large net-alkaline anthracite mine discharges in central Pennsylvania. Assuming an Fe removal rate of 20 g m−2 day−1 and Fe loading from field data, 3.6 × 103 and 3.0 × 104 m2 oxidation ponds would be required for the passive treatment of Site 21 and Packer 5 discharges, respectively. However, only a small area is available at each site. This paper demonstrates aeration to drive off CO2, increase pH, and increase Fe(II) oxidation rates, enabling treatment within a small area compared to passive treatment methods, and introduces a geochemical model to accurately predict these rates as well as semi-passive treatment system sizing parameters. Both net-alkaline discharges were suboxic with a pH of ≈5.7, Fe(II) concentration of ≈16 mg L−1, and low Mn and Al concentrations. Flow rates were ≈4000 L min−1 at Site 21 and 15,000 L min−1 at Packer 5. Three-h aeration experiments with flow rates scaled to a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases from 12 to 22 °C, dissolved O2 increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases from 16 to <0.05 mg L−1. A 17,000-L pilot-scale reactor at Site 21 produced similar results although aeration was not as complete as in the smaller reactor. Two non-aerated experiments at Site 21 with 13 and 25-h run times resulted in pH changes of ?0.2 and Fe(II) concentration decreases of less than 3 mg L−1. 相似文献
9.
Microbial Mn(II) oxidation kinetics in response to oxygen concentration were assessed in suboxic zone water at six sites throughout the Black Sea. Mn(II) oxidation rates increased asymptotically with increasing oxygen concentration, consistent with Michaelis-Menten enzyme kinetics. The environmental half-saturation constant, KE, of Mn(II) removal (oxidation) varied from 0.30 to 10.5 μM dissolved oxygen while the maximal environmental rate, VE−max, ranged from 4 to 50 nM h−1. These parameters varied spatially and temporally, consistent with a diverse population of enzymes catalyzing Mn oxide production in the Black Sea. Coastally-influenced sites produced lower KE and higher VE−max constants relative to the Western and Eastern Gyre sites. In the Bosporus Region, the Mn(II) residence time calculated using our KE and VE−max values with 0.1 μM oxygen was 4 days, 25-fold less than previous estimates. Our results (i) indicate that rapid Mn(II) oxidation to solid phase Mn oxides in the Black Sea’s suboxic zone is stimulated by oxygen concentrations well below the 3-5 μM concentration reliably detected by current oceanographic methods, (ii) suggest the existence of multiple, diverse Mn(II)-oxidizing enzymes, (iii) are consistent with shorter residence times than previously calculated for Mn(II) in the suboxic zone and (iv) cast further doubt on the existence of proposed reactions coupling solid phase Mn oxide production to electron acceptors other than oxygen. 相似文献
10.
Permanganate (MnO4−) has widely been used as an effective oxidant for drinking water treatment systems, as well as for in situ treatment of groundwater impacted by various organic contaminants. The reaction stoichiometry of As(III) oxidation by permanganate has been assumed to be 1.5, based on the formation of solid product, which is putatively considered to be MnO2(s). This study determined the stoichiometric ratio (SR) of the oxidation reaction with varying doses of As(III) (3-300 μM) and MnO4− (0.5 or 300 μM) under circumneutral pH conditions (pH 4.5-7.5). We also characterized the solid product that was recovered ∼1 min after the oxidation of 2.16 mM As(III) by 0.97 mM MnO4− at pH 6.9 and examined the feasibility of secondary heterogeneous As(III) oxidation by the solid product. When permanganate was in excess of As(III), the SR of As(III) to Mn(VII) was 2.07 ± 0.07, regardless of the solution pH; however, it increased to 2.49 ± 0.09 when As(III) was in excess. The solid product was analogous to vernadite, a poorly crystalline manganese oxide based on XRD analysis. The average valence of structural Mn in the solid product corresponded to +III according to the splitting interval of the Mn3s peaks (5.5 eV), determined using X-ray photoelectron spectroscopy (XPS). The relative proportions of the structural Mn(IV):Mn(III):Mn(II) were quantified as 19:62:19 by fitting the Mn2p3/2 spectrum of the solid with the five multiplet binding energy spectra for each Mn valence. Additionally, the O1s spectrum of the solid was comparable to that of Mn-oxide but not of Mn-hydroxide. These results suggest that the solid product resembled a poorly crystalline hydrous Mn-oxide such as (MnII0.19MnIII0.62MnIV0.19)2O3·nH2O, in which Mn(II) and Mn(IV) were presumably produced from the disproportionation of aqueous phase Mn(III). Thermodynamic calculations also show that the formation of Mn(III) oxide is more favorable than that of Mn(IV) oxide from As(III) oxidation by permanganate under circumneutral pH conditions. Arsenic(III), when it remained in the solution after all of the permanganate was consumed, was effectively oxidized by the solid product. This secondary heterogeneous As(III) oxidation consisted of three steps: sorption to and oxidation on the solid surface and desorption of As(V) into solution, with the first step being the rate-limiting process as observed in As(III) oxidation by various Mn (oxyhydr)oxides reported elsewhere. We also discussed a potential reaction pathway of the permanganate oxidation of As(III). 相似文献
11.
The oxygen isotope fractionation factor of dissolved oxygen gas has been measured during inorganic reduction by aqueous FeSO4 at 10−54 °C under neutral (pH 7) and acidic (pH 2) conditions, with Fe(II) concentrations ranging up to 0.67 mol L−1, in order to better understand the geochemical behavior of oxygen in ferrous iron-rich groundwater and acidic mine pit lakes. The rate of oxygen reduction increased with increasing temperature and increasing Fe(II) concentration, with the pseudo-first-order rate constant k ranging from 2.3 to 82.9 × 10−6 s−1 under neutral conditions and 2.1 to 37.4 × 10−7 s−1 under acidic conditions. The activation energy of oxygen reduction was 30.9 ± 6.6 kJ mol−1 and 49.7 ± 13.0 kJ mol−1 under neutral and acidic conditions, respectively. Oxygen isotope enrichment factors (ε) become smaller with increasing temperature, increasing ferrous iron concentration, and increasing reaction rate under acidic conditions, with ε values ranging from −4.5‰ to −11.6‰. Under neutral conditions, ε does not show any systematic trends vs. temperature or ferrous iron concentration, with ε values ranging from −7.3 to −10.3‰. Characterization of the oxygen isotope fractionation factor associated with O2 reduction by Fe(II) will have application to elucidating the process or processes responsible for oxygen consumption in environments such as groundwater and acidic mine pit lakes, where a number of possible processes (e.g. biological respiration, reduction by reduced species) may have taken place. 相似文献
12.
Claudia L. Caldeira Virginia S.T. Ciminelli Kwadwo Osseo-Asare 《Geochimica et cosmochimica acta》2010,74(6):1777-1789
The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO3−, FeCO30, Fe(CO3)(OH)− and FeCO32−) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O2, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O2 is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface. 相似文献
13.
Jinghao ZhangLeonard W Lion Yarrow M Nelson Michael L ShulerWilliam C Ghiorse 《Geochimica et cosmochimica acta》2002,66(5):773-781
The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O2, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O2 and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria. 相似文献
14.
《Applied Geochemistry》2004,19(4):611-622
Subsurface aeration is the in situ oxidation of Fe from groundwater that is used to make drinking water potable. When subsurface aeration is applied to an anaerobic groundwater system with pH>7, Fe(II) is oxidised heterogeneously. The heterogeneous oxidation of Fe(II) can result in the in situ formation of Fe colloids. To study this, the effect of substances commonly found in groundwater (e.g. PO4, Mn, silicate and fulvic acid) on the heterogeneous oxidation process was measured. The heterogeneous oxidation of Fe(II) becomes retarded when PO4, Mn, silicate or fulvic acid is present in the groundwater in addition to Fe(II). Phosphate and fulvic acid retarded the oxidation process most. The heterogeneous oxidation was described using a model with a homogeneous (k1) and an autocatalytic oxidation rate constant (k′2). From the modelling it followed that the homogeneous oxidation rate constant was not affected or even slightly elevated whereas the autocatalytic oxidation rate constant decreased remarkably by the addition of PO4, Mn, silicate or fulvic acid. From speciation calculations it followed that the decreased availability of the Fe(II) species can only explain a small part of the retarded autocatalytic oxidation process. Therefore exploratory calculations were performed to gain insight into whether the adsorption of PO4 or fulvic acid could explain the retarded autocatalytic oxidation. These calculations showed that the adsorption of fulvic acid could explain the retarded autocatalytic oxidation process. In contrast the adsorption of PO4 only partly explained the retarded autocatalytic oxidation process. In terms of colloid formation this study shows that the heterogeneous oxidation of Fe(II) in presence of PO4, Mn, silicate or fulvic acid leads to the formation of Fe colloids. 相似文献
15.
E.C Todd 《Geochimica et cosmochimica acta》2003,67(5):881-893
The nature of the surface oxidation phase on pyrite, FeS2, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur LIII edge, and iron LII,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS2 can be detected in the X-ray absorption spectra. In a 1.667 × 10−3 mol/L Fe3+ solution with ferric iron present as FeCl3 in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH. 相似文献
16.
Owen W Duckworth 《Geochimica et cosmochimica acta》2004,68(3):607-621
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
17.
Ma Catalina Alfaro-De la TorreAndr Tessier 《Geochimica et cosmochimica acta》2002,66(20):3549-3562
A dated core from the profoundal zone in a pristine oligotrophic acidic lake was analyzed for Cd as well as for Al, Ca, Fe, Mg, Mn, Pb, Ti and total carbon and nitrogen. Overlying water and porewater samples were also obtained on six occasions at the same site, and yielded vertical profiles of pH and dissolved Cd, Ca, Fe, Mg, Mn, sulfide, SO4−2, organic and inorganic carbon concentrations. These extensive porewater and sediment geochemical data were used, together with information on infaunal benthos, to decipher the sedimentary record of Cd contamination. Depth variation of sediment Ca concentrations indicate that the lake suffered from progressive acidification starting about 1950. The present-day accumulation rate of Cd (JaccCd = 5.4 ± 0.4 × 10−11 mol cm−2 yr−1) in the sediments is the sum of the flux of Cd deposited with settling particles (JSCd = 3.3 ± 0.2 × 10−11 mol cm−2 yr−1) and the fluxes of dissolved Cd across the sediment-water interface due to molecular diffusion (JDCd = 1.8 ± 0.3 × 10−11 mol cm−2 yr−1), bioturbation (JBCd = 1.1 ± 0.2 × 10−14 mol cm−2 yr−1) and bioirrigation (JICd = 0.27 ± 0.05 × 10−11 mol cm−2 yr−1). Biological mixing of the sediments was negligible. The shape of the vertical profile of total Cd concentration with depth in the sediment appears to be determined more by its input history than by post-depositional mobilization and redistribution in the sediment column. 相似文献
18.
Maurizio Pettine Francesca Gennari Frank J. Millero 《Geochimica et cosmochimica acta》2008,72(23):5692-5707
The oxidation of Cr(III) has been studied in NaCl solutions in the presence of two siderophore models, acetohydroxamic acid (Aha) and benzohydroxamic acid (Bha), the natural siderophore Desferal (DFOB) and the synthetic aminocarboxilate (ethylenedinitrilo)-tetra-acetic acid (EDTA) as a function of pH (8-9), ionic strength (0.01-2 M) and temperature (10-50 °C), at different Cr(III)-organic compound ratios. The addition of Aha and Bha caused the rates to increase at low ligand/Cr(III) ratios and decrease at high ratios. The variation of the pseudo first order rate constant (k1) as a function of the ligand concentration has been attributed to the formation of three Cr(III)-organo species (1:1, 1:2, 1:3), which can form in the presence of monohydroxamic acids. A kinetic model has been developed that gives a value of 600 (min−1) for the pseudo first order rate constant k1CrAha2+ and values approaching zero for and k1CrAha3. These kinetic results demonstrate that these monohydroxamic acids are able to bind with Cr(III) under experimental conditions that may occur in natural waters and can increase the oxidation rates of Cr(III) with H2O2 by a factor of 3.5 at an Aha/Cr(III) ratio of about 50-100.The monohydroxamic acids also affect the rates on aged products of Cr(III), suggesting that these ligands are able to affect the oxidation rates by releasing reactive Cr(III). DFOB and EDTA do not have a great effect on the oxidation of Cr(III) with H2O2. This is thought to be due to the much longer times they need to form complexes with Cr(III) compared to Aha and Bha. The rates for the formation of DFOB and EDTA complexes with Cr(III) are not competitive with the rates of the formation of aged Cr(III). After allowing Cr(III) and DFOB to react for 5 days to form the complex, reaction rates of Cr(III) with H2O2 appear to be lowered probably because of steric hindrance of the chelated Cr(III). 相似文献
19.
Polythionates (SxO62−) are important in redox transformations involving many sulfur compounds. Here we investigate the oxidation kinetics and mechanisms of trithionate and tetrathionate oxidation between pH 0.4 and pH 2 in the presence of Fe3+ and/or oxygen. In these solutions, Fe3+ plus oxygen oxidizes tetrathionate and trithionate at least an order of magnitude faster than oxygen alone. Kinetic measurements, coupled with density functional calculations, suggest that the rate-limiting step for tetrathionate oxidation involves Fe3+ attachment, followed by electron density shifts that result in formation of a sulfite radical and S3O30 derivatives. The overall reaction orders for trithionate and tetrathionate are fractional due to rearrangement reactions and side reactions between reactants and intermediate products. The pseudo-first order rate coefficients for tetrathionate range from 10−11 s−1 at 25°C to 10−8 s−1 at 70°C, compared to 2 × 10−7 s−1 at 35 °C for trithionate. The apparent activation energy (EA) for tetrathionate oxidation at pH 1.5 is 104.5 ± 4.13 kJ/mol. A rate law at pH 1.5 and 70°C between 0.5 and 5 millimolar [Fe3+] is of the form:
20.
We have performed holographic interferometry measurements of the dissolution of the (0 1 0) plane of a cleaved gypsum single crystal in pure water. These experiments have provided the value of the dissolution rate constant k of gypsum in water and the value of the interdiffusion coefficient D of its aqueous species in water. D is 1.0 × 10−9 m2 s−1, a value close to the theoretical value generally used in dissolution studies. k is 4 × 10−5 mol m−2 s−1. It directly characterizes the microscopic transfer rate at the solid-liquid interface, and is not an averaged value deduced from quantities measured far from the surface as in macroscopic dissolution experiments. It is found to be two times lower than the value obtained from macroscopic experiments. 相似文献