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1.
Lead- and Pb-As-jarosites are minerals common to acidic, sulphate-rich environments, including weathering zones of sulphide ore deposits and acid rock or acid mine drainage (ARD/AMD) sites, and often form on or near galena. The structures of these jarosites are based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, comprised of slightly distorted FeO6 octahedra and SO42− (-AsO43− in Pb-As-jarosites) tetrahedra. To better understand the dissolution mechanisms and products of the break down of Pb- and Pb-As-jarosite, preliminary batch dissolution experiments were conducted on synthetic Pb- and Pb-As-jarosite at pH 2 and 20 °C, to mimic environments affected by ARD/AMD, and at pH 8 and 20 °C, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2). All four dissolutions are incongruent. Dissolution of Pb-jarosite at pH 2 yields aqueous Pb, Fe and SO42−. The pH 8 Pb-jarosite dissolution yields aqueous Pb, SO42− and poorly crystalline Fe(OH)3, which does not appear to resorb Pb or SO42−, possibly due to the low solution pH (3.44-3.54) at the end of the experiment. The pH 2 and 8 dissolutions of Pb-As-jarosite result in the formation of secondary compounds (poorly crystalline PbSO4 for pH 2 dissolution; poorly crystalline PbSO4 and Fe(OH)3 for pH 8 dissolution), which may act as dissolution inhibitors after 250 to 300 h of dissolution. In the pH 2 dissolution, aqueous Fe, SO42− and AsO43− also form, and in the pH 8 dissolution, Fe(OH)3 precipitates then subsequently resorbs aqueous AsO43−. The dissolutions probably proceed by preferred dissolution of the A- and T-sites, which contain Pb, and SO42− and AsO43−, respectively, rather than Fe, which is sterically remote, within the T-O-T Pb- and Pb-As-jarosite structures. These data provide the foundation necessary for further, more detailed investigations into the dissolution of Pb- and Pb-As-jarosites. 相似文献
2.
Göril Möschner Barbara Lothenbach Andrea Ulrich Ruben Kretzschmar 《Geochimica et cosmochimica acta》2008,72(1):1-18
The solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca4[Fe(OH)6]2(SO4) · 6H2O) and Fe-monocarbonate (Ca4[Fe(OH)6]2(CO3) · 6H2O) are formed. The solubilities of these hydrates at 25 °C are: 相似文献
3.
4.
Yavapaiite, KFe(SO4)2, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO4)2, ΔH°f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol−1.K−1 and ΔG°f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe2O3 + H2O has been calculated, at pH2O = 1 atm, the phase boundary lies at 219 ± 2°C. 相似文献
5.
根据X射线衍射(XRD)分析发现: A Fe3(SO4)2(OH)6(A=K+、H3O+)系列铁钒的XRD数据十分相近,难以用XRD区别,需通过能谱(EDS)辅助分析,才能区分此类铁矾。另外,此类铁矾的003和107面网间距d随K+含量增大而增大,且呈一元三次方程的关系;而033和220面网间距d随K+含量增大而减小,呈一元二次方程的关系。对该现象从铁矾晶体结构方面进行解释:K+、H3O+离子位于较大空隙中,且沿着Z轴方向排列,当K+、H3O+离子之间相互替换时,会导致该铁矾晶体结构在Z轴方向有较明显的变化。 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.
Iván A. Reyes Ister Mireles Francisco Patiño Thangarasu Pandiyan Mizraim U. Flores Elia G. Palacios Emmanuel J. Gutiérrez Martín Reyes 《Geochemical transactions》2016,17(1):3
Background
The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact.Results
This work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe3[(SO4)2 ? X(CrO4)X](OH)6). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H3O+/OH?). It was found that the substitution of CrO4 2?in Y-site and the substitution of H3O+ in M-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K+, SO4 2? and CrO4 2? from the KFe3[(SO4)2 ? X(CrO4)X](OH)6 structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)3 in alkaline medium dissolutions. The dissolution of KFe3[(SO4)2 ? X(CrO4)X](OH)6 exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order n, activation energy E A and dissolution rate constants for each kinetic stages of induction and progressive conversion.Conclusions
The kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K+, SO4 2? and CrO4 2? from the KFe3[(SO4)2 ? X(CrO4)X](OH)6 structure, although CrO4 2? is quickly adsorbed by Fe(OH)3 solid residues. The precipitation of phases such as KFe3[(SO4)2 ? X(CrO4)X](OH)6, and the absorption of Cr(VI) after dissolution can play an important role as retention mechanisms of Cr(VI) in nature.8.
Seal or cap-rock integrity is a safety issue during geological carbon dioxide capture and storage (CCS). Industrial impurities such as SO2, O2, and NOx, may be present in CO2 streams from coal combustion sources. SO2 and O2 have been shown recently to influence rock reactivity when dissolved in formation water. Buoyant water-saturated supercritical CO2 fluid may also come into contact with the base of cap-rock after CO2 injection. Supercritical fluid-rock reactions have the potential to result in corrosion of reactive minerals in rock, with impurity gases additionally present there is the potential for enhanced reactivity but also favourable mineral precipitation.The first observation of mineral dissolution and precipitation on phyllosilicates and CO2 storage cap-rock (siliciclastic reservoir) core during water-saturated supercritical CO2 reactions with industrial impurities SO2 and O2 at simulated reservoir conditions is presented. Phyllosilicates (biotite, phlogopite and muscovite) were reacted in contact with a water-saturated supercritical CO2 containing SO2, or SO2 and O2, and were also immersed in the gas-saturated bulk water. Secondary precipitated sulfate minerals were formed on mineral surfaces concentrated at sheet edges. SO2 dissolution and oxidation resulted in solution pH decreasing to 0.74 through sulfuric acid formation. Phyllosilicate dissolution released elements to solution with ∼50% Fe mobilized. Geochemical modelling was in good agreement with experimental water chemistry. New minerals nontronite (smectite), hematite, jarosite and goethite were saturated in models. A cap-rock core siltstone sample from the Surat Basin, Australia, was also reacted in water-saturated supercritical CO2 containing SO2 or in pure supercritical CO2. In the presence of SO2, siderite and ankerite were corroded, and Fe-chlorite altered by the leaching of mainly Fe and Al. Corrosion of micas in the cap-rock was however not observed as the pH was buffered by carbonate dissolution. Ca-sulfate, and Fe-bearing precipitates were observed post SO2-CO2 reaction, mainly centered on surface cracks and an illite rich illite-smectite precipitate quantified. Water saturated impure supercritical CO2 was observed to have reactivity to rock-forming biotite, muscovite and phlogopite mineral separates. In the cap-rock core however carbonates and chlorite were the main reacting minerals showing the importance of assessing actual whole core. 相似文献
9.
Barry R. Bickmore Kathryn L. Nagy A. Riley Brinkerhoff 《Geochimica et cosmochimica acta》2006,70(2):290-305
The influence of Al(OH)4− on the dissolution rate of quartz at pH 10-13 and 59-89 °C was determined using batch experiments. Al(OH)4− at concentrations below gibbsite solubility depressed the dissolution rate by as much as 85%, and this effect was greater at lower pH and higher Al(OH)4− concentration. Dissolution rates increased with increasing temperature; however, the percent decrease in rate due to the presence of Al(OH)4− was invariant with temperature for a given H+ activity and Al(OH)4− concentration. These data, along with what is known about Al-Si interactions at high pH, are consistent with Al(OH)4− and Na+ co-adsorbing on silanol sites and passivating the surrounding quartz surface. The observed pH dependence, and lack of temperature dependence, of inferred Al(OH)4− sorption also supports the assumption that the acid-base behavior of the surface silanol groups has only a small temperature dependence in this range. A Langmuir-type adsorption model was used to express the degree of rate depression for a given in situ pH and Al(OH)4− concentration. Incorporation of the rate data in the absence of aluminate into models that assume a first-order dependence of the rate on the fraction of deprotonated silanol sites was unsuccessful. However, the data are consistent with the hypothesis proposed in the literature that two dissolution mechanisms may be operative in alkaline solutions: nucleophilic attack of water on siloxane bonds catalyzed by the presence of a deprotonated silanol group and OH− attack catalyzed by the presence of a neutral silanol group. The data support the dominance of the second mechanism at higher pH and temperature. 相似文献
10.
Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O2 conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen (PO2), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O10(OH,F)2 for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO2 conditions were achieved in a newly developed glove box (PO2 ? 6 × 10−4 atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO2; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO2 at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe3+ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO2. The results infer that the Fe2+ and Fe3+ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering. 相似文献
11.
Pyrite (FeS2) oxidation: A sub-micron synchrotron investigation of the initial steps 总被引：1，自引：0，他引：1
Anand P. Chandra 《Geochimica et cosmochimica acta》2011,75(20):6239-6254
Pyrite is an environmentally significant mineral being the major contributor to acid rock drainage. Synchrotron based SPEM (scanning photoelectron microscopy) and micro-XPS (X-ray photoelectron spectroscopy) have been used to characterise fresh and oxidised pyrite (FeS2) with a view to understanding the initial oxidation steps that take place during natural weathering processes. Localised regions of the pyrite surface containing Fe species of reduced coordination have been found to play a critical role. Such sites not only initiate the oxidation process but also facilitate the formation of highly reactive hydroxyl radical species, which then lead the S oxidation process.Four different S species are found to be present on fresh fractured pyrite surfaces: S22−(bulk) (4-fold coordination), S22−(surface) (3-fold coordination), S2− and S0/Sn2− (metal deficient sulfide and polysulfide respectively). These species were found to be heterogeneously distributed on the fractured pyrite surface. Both O2 and H2O gases are needed for effective oxidation of the pyrite surface. The process is initiated when O2 dissociatively and H2O molecularly adsorb onto the surface Fe sites where high dangling bond densities exist. H2O may then dissociate to produce OH radicals. The adsorption of these species leads to the formation of Fe-oxy species prior to the formation of sulfoxy species. Evidence suggests that Fe-O bonds form prior to Fe-OH bonds. S oxidation occurs through interactions of OH radicals formed at the Fe sites, with formation of SO42− occurring via S2O32−/SO32− intermediates. The pyrite oxidation process is electrochemical in nature and was found to occur in patches, where site specific adsorption of O2 and H2O has occurred. Fe and S oxidation was found to occur within the same area of oxidation probably in atomic scale proximity. Furthermore, the O in SO42− arises largely from H2O; however, depending on the surface history, SO42− formed early in the oxidation process may also contain O from O2. 相似文献
12.
Natural and constructed clay liners are routinely used to contain waste and wastewater. The impact of acidic solutions on the geochemistry and mineralogy of clays has been widely investigated in relation to acid mine drainage systems at pH > 1.0. The impact of H2SO4 leachate characterized by pH < 1.0 and potentially negative pH values on the geochemistry and mineralogy of clays is, however, not clear. Thus, laboratory batch experiments were conducted on three natural clay samples with different mass ratios of smectite, illite and kaolinite to investigate the impact of H2SO4 on the geochemistry and mineralogy of aluminosilicates from pH 5.0 to −3.0. Batch testing was conducted at seven pH treatments (5.0, 3.0, 1.0, 0.0, −1.0, −2.0 and −3.0) using standardized H2SO4 solutions for four exposure periods (14, 90, 180, and 365 d). Aqueous geochemical and XRD analyses showed: increased dissolution of aluminosilicates with decreasing pH and increasing exposure period, that smectite was more susceptible to dissolution than illite and kaolinite, precipitation of an amorphous silica phase occurred at pH ? 0.0, and anhydrite precipitated in Ca-rich clays at pH ? −1.0. In addition, global dissolution rates were calculated for the clays and showed good agreement to literature smectite, illite and kaolinite dissolution rates, which suggests global dissolution rates for complex clays could be determined from monomineralic studies. A stepwise conceptual model of the impact of H2SO4 on aluminosilicate geochemistry and mineralogy between pH 5.0 and −3.0 is proposed. 相似文献
13.
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
14.
CO2-water-basalt interaction. Numerical simulation of low temperature CO2 sequestration into basalts
The interaction between CO2-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water-rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO2 concentration and reaction progress (ξ). The calculations were carried out at 25-90 °C and pCO2 to 30 bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300 °C in order to simulate mineral saturations relevant to basalt alteration in CO2-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO2-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO2 enriched waters (pH <6.5), SiO2 and simple Al-Si minerals, Ca-Mg-Fe smectites and Ca-Mg-Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO2 mineralization, the pH increased to >8 resulting in Ca-Mg-Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO2 and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca-Fe smectites) and carbonates at low pH, and zeolites and clays (Mg-Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study. 相似文献
15.
The solubility of KFe(CrO4)2·2H2O, a precipitate recently identified in a Cr(VI)-contaminated soil, was studied in dissolution and precipitation experiments. Ten dissolution experiments were conducted at 4–75°C and initial pH values between 0.8 and 1.2 using synthetic KFe(CrO4)2·2H2O. Four precipitation experiments were conducted at 25°C with final pH values between 0.16 and 1.39. The log KSP for the reaction
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16.
Dr. F. Scordari 《Mineralogy and Petrology》1981,28(1):17-29
Summary The mineral fibroferrite has the chemical formula Fe(OH)SO4·xH2O; the value forx has not been definitely settled, but as a rule it is found to be near five. Several symmetries are given in the literature.A sample from Saint Felix de Paillères, France, proved to be rhombohedral with space group R3; lattice constants for the hexagonal cell area=24.176,c=7.656 Å. As calculated from the experimental density (=1.95 g·cm–3)Z=18 for this cell. Intensities were collected on an automated X-ray diffractometer from a thin fiber extended along [00.1]. The structure was determined by Patterson and Fourier methods. Least squares refinement with 818 observed reflections resulted inR=0.076.The structure contains hydroxo-bridged {Fe(OH)(H2O)2SO4} spiral chains built of [Fe(OH)2(H2O)2O2] octahedra and SO4 tetrahedra. Hydrogen bonds provide connections between these chains. The spiral chains are a stereoisomer variant of the hydroxo-bridged linear chains of Fe(OH)SO4, butlerite and parabutlerite. A comparison of these compounds is givenm to understand the relationship between the structure and their water content.
With 2 Figures
Paper presented at the Fifth European Crystallography Meeting, Copenhagen, Denmark 1979. 相似文献
Fibroferrit: Ein Mineral mit einer {Fe(OH)(H2O)2SO4} Spiralkette und seine Beziehung zu Fe(OH)SO4, Butlerit und Parabutlerit
Zusammenfassung Das Mineral Fibroferrit hat die chemische Formel Fe(OH)SO4·xH2O; der Wert furx scheint nicht endgültig geklärt zu sein, liegt aber meist nahe 5. Verschiedene Symmetrien werden in der Literatur angegeben.Eine Probe von Saint Felix de Paillères, Frankreich, erwies sich als rhomboedrisch mit der Raumgruppe R3; die Gitterkonstanten der hexagonalen Zelle sinda=24,176,c=7,656 Å. Die experimentelle Bestimmung der Dichte (=1,95 g·cm–3) führt für diese Zelle zuZ=18. Von einer nach [00.1] gestreckten dünnen Faser wurden die Intensitäten auf einem automatischen Röntgendiffraktometer gesammelt. Die Struktur wurde mit Patterson-und Fouriersynthesen gelöst. Eine Verfeinerung nach der Methode der kleinsten Quadrate führte für 818 beobachtete Reflexe aufR=0,076.Die Struktur enthält durch Hydroxil-Gruppen verknüpfte {Fe(OH)(H2O)2SO4}-Spiralketten, die aus [Fe(OH)2(H2O)2O2]-Oktaedern und SO4-Tetraedern aufgebaut sind. Die Spiralketten von Fibroferrit sind eine stereoisomere Variante der annähernd linearen Fe–O–S-Ketten von Fe(OH)SO4, Butlerit und Parabutlerit. Diese Verbindungen werden mit Fibroferrit verglichen, um Beziehungen zwischen Struktur und Wassergehalt zu verstehen.
With 2 Figures
Paper presented at the Fifth European Crystallography Meeting, Copenhagen, Denmark 1979. 相似文献
17.
Pyrite plays the central role in the environmental issue of acid rock drainage. Natural weathering of pyrite results in the release of sulphuric acid which can lead to further leaching of heavy and toxic metals from other associated minerals. Understanding how pyrite reacts in aqueous solution is critical to understanding the natural weathering processes undergone by this mineral. To this end an investigation of the effect of solution redox potential (Eh) and various anions on the rate of pyrite leaching under carefully controlled conditions has been undertaken.Leaching of pyrite has been shown to proceed significantly faster at solution Eh of 900 mV (SHE) than at 700 mV, at pH 1, for the leach media of HCl, H2SO4 and HClO4. The predominant effect of Eh suggests electrochemical control of pyrite leaching with similar mechanism(s) at Eh of 700 and 900 mV albeit with different kinetics. Leach rates at 700 mV were found to decrease according to HClO4 > HCl > H2SO4 while at 900 mV the leach rate order was HCl > HClO4 > H2SO4. Solution Fe3+ activity is found to continually increase during all leaches; however, this is not accompanied by an increase in leach rate.Synchrotron based photoemission electron microscopy (PEEM) measurements showed a localised distribution of adsorbed and oxidised surface species highlighting that pyrite oxidation and leaching is a highly site specific process mediated by adsorption of oxidants onto specific surface sites. It appears that rates may be controlled, in part, by the propensity of acidic anions to bind to the surface, which varies according to , thus reducing the reactive or effective surface area. However, anions may also be involved in specific reactions with surface leach products. Stoichiometric dissolution data (Fe/S ratio), XPS and XRD data indicate that the highest leach rates (in HCl media at 900 mV Eh) correlate with relatively lower surface S abundance. Furthermore, there are indications that solution Cl− assists oxidation especially at higher Eh through the prevention of surface S0 buildup at reactive surface sites. 相似文献
18.
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
19.
Geochemical reactions and dynamics during titration of a contaminated groundwater with high uranium, aluminum, and calcium 总被引：1，自引：0，他引：1
This study investigated possible geochemical reactions during titration of a contaminated groundwater with a low pH but high concentrations of aluminum, calcium, magnesium, manganese, and trace contaminant metals/radionuclides such as uranium, technetium, nickel, and cobalt. Both Na-carbonate and hydroxide were used as titrants, and a geochemical equilibrium reaction path model was employed to predict aqueous species and mineral precipitation during titration. Although the model appeared to be adequate to describe the concentration profiles of some metal cations, solution pH, and mineral precipitates, it failed to describe the concentrations of U during titration and its precipitation. Most U (as uranyl, UO22+) as well as Tc (as pertechnetate, TcO4−) were found to be sorbed and coprecipitated with amorphous Al and Fe oxyhydroxides at pH below ∼5.5, but slow desorption or dissolution of U and Tc occurred at higher pH values when Na2CO3 was used as the titrant. In general, the precipitation of major cationic species followed the order of Fe(OH)3 and/or FeCo0.1(OH)3.2, Al4(OH)10SO4, MnCO3, CaCO3, conversion of Al4(OH)10SO4 to Al(OH)3,am, Mn(OH)2, Mg(OH)2, MgCO3, and Ca(OH)2. The formation of mixed or double hydroxide phases of Ni and Co with Al and Fe oxyhydroxides was thought to be responsible for the removal of Ni and Co in solution. Results of this study indicate that, although the hydrolysis and precipitation of a single cation are known, complex reactions such as sorption/desorption, coprecipitation of mixed mineral phases, and their dissolution could occur simultaneously. These processes as well as the kinetic constraints must be considered in the design of the remediation strategies and modeling to better predict the activities of various metal species and solid precipitates during pre- and post-groundwater treatment practices. 相似文献
20.
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