Selective leaching of antimony and arsenic from mechanically activated tetrahedrite,jamesonite and enargite

In this study, the changes in surface area, morphology and leachability of antimony and arsenic from tetrahedrite, jamesonite and enargite mechanically activated by a high-energy planetary mill were investigated. It appears that the leaching of antimony from tetrahedrite and jamesonite and arsenic from enargite in alkaline solution of sodium sulphide is temperature-sensitive reaction. The temperature dependencies of all reactions were investigated in the interval 313–363 K. Resulting experimental activation energies were E_a = 111–182 kJ mol^− 1 for mechanically activated minerals. The values of E_a are characteristic for processes controlled by surface chemical reactions.     

Magnetic properties and neutron diffraction study of two manganese sulfosalts: monoclinic MnSb<Subscript>2</Subscript>S<Subscript>4</Subscript> and benavidesite (MnPb<Subscript>4</Subscript>Sb<Subscript>6</Subscript>S<Subscript>14</Subscript>)

Mn²⁺Sb₂S₄, a monoclinic dimorph of clerite, and benavidesite (Mn²⁺Pb₄Sb₆S₁₄) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe²⁺Sb₂S₄) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb₂S₄ shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb₂S₄ interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.     

Magnesium self-diffusion in orthoenstatite

Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic directions in natural orthoenstatite (En₈₈Fs₁₂). Experiments were conducted at 1 atm in CO-CO₂ gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of ²⁵Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched ²⁵MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling. The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E _a , and frequency factors, D _o, where D = D _o exp (−E _a /RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E _a  = 360 ± 52 kJ/mole and D _o = 1.10 × 10⁻⁴ m²/s; b-axis, E _a  = 339 ± 77 kJ/mole and D _o = 6.93 × 10⁻⁶ m²/s and c-axis, E _a  = 265 ± 66 kJ/mole and D _o = 4.34 × 10⁻⁹ m²/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/10⁶ years) for the metal portions. Received: 22 January 1997 / Accepted: 2 October 1997     

Kinetics and mechanisms of the leaching of low Fe sphalerite

The surface speciation and leaching kinetics of 38- to 75-μm sphalerite (0.45 wt.% Fe) particles reacted in O₂ purged perchloric acid (at pH 1.0) at 25, 40, 60, and 85 °C over a leach period of 144 h were investigated. In all cases, an initial rapid leach rate is observed followed by a slower leach rate. These two leach regimes can each be adequately modeled using straight-line interpolation, and thus two activation energies (E_a) have been derived. E_a for the fast and slow Zn dissolution rates were 33 ± 4 kJ mol⁻¹ and 34 ± 4 kJ mol⁻¹ respectively, suggesting the same rate-determining step.     

Calculation of the energetics for the oxidation of Sb(III) sulfides by elemental S and polysulfides in aqueous solution

Recent experimental studies have reported the existence of two new Sb sulfide species, Sb₂S₅²⁻ and Sb₂S₆²⁻, in alkaline sulfidic solutions in equilibrium with stibnite, Sb₂S₃, and orthorhombic S. These species contain Sb(V), which has also recently been identified in similar solutions using EXAFS by other researchers. This represents a significant change from the consensus a decade ago that sulfidic solutions of Sb contained only Sb(III) species. I have calculated from first principles of quantum mechanics the energetics for the oxidation of the Sb(III) sulfide dimer Sb₂S₄²⁻ to the mixed Sb(III,V) dimer Sb₂S₅²⁻ and then to the all Sb(V) dimer, Sb₂S₆²⁻. Gas-phase reaction energies have been evaluated using polarized valence double zeta effective core potential basis sets and Moller-Plesset second order treatments of electron correlation. All translational, rotational and vibrational contributions to the gas-phase reaction free energy have been calculated. Hydration energies have been obtained using the COSMO version of the self-consistent reaction field polarizable continuum method. Negative free energy changes are calculated for the oxidation of the dianion of the III,III dimer to the III,V dimer by both small polysulfides, like S₄H⁻, and elemental S, modeled as S₈. For the further oxidation of the III,V dimer to the V,V dimer the reaction free energies are calculated to be close to zero. The partially protonated Sb III,III dimer monoanion HSb₂S₄⁻ can also be oxidized, but the reaction is not so favorable as for the dianion. Comparison of the calculated aqueous deprotonation energies of H₂Sb₂S₄, H₂Sb₂S₅ and H₂Sb₂S₆ and their dianions with values calculated for various oxyacids indicates that the III,V and V,V dimers will have pK_a2 values <5, so that their dianions will be the dominant species in alkaline solutions. These results are thus consistent with the recent identification of Sb₂S₅²⁻ and Sb₂S₆²⁻ species. I have also calculated the Raman spectra of Sb₂S₅²⁻ and Sb₂S₆²⁻ to assist in their identification. The calculated vibrational frequencies of the III,V and V,V dimers are characteristically higher than those of the III,III dimer I previously studied. The III,V dimer may contribute shoulders to the Raman spectrum.     

Electron paramagnetic resonance spectroscopy of Fe<Superscript>3+</Superscript> ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe³⁺ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E ₁′. However, the two broad signals do not correspond to any known Fe³⁺ centers in the quartz lattice, but are most likely attributable to Fe³⁺ clusters on surfaces. The absolute numbers of spins of the Fe³⁺ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E _a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe³⁺ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe³⁺ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.     

Reversible hydration in synthetic mixite, BiCu6(OH)6(AsO4)3·nH2O (n≤3): hydration kinetics and crystal chemistry

 The presence of zeolitic water, with a reversible hydration behaviour, was determined by structural and kinetic studies on synthetic mixite BiCu₆(OH)₆(AsO₄)₃·nH₂O (n≤3). X-ray diffraction and infrared-spectroscopic investigations were performed on single crystals. Isothermal thermogravimetric experiments were carried out to determine the reaction kinetics of the de- and rehydration processes. The single-crystal structure refinement of a fully hydrated crystal yielded five partially occupied Ow positions (Ow=oxygen atom of a H₂O molecule) within the tube-like channels of the hexagonal [BiCu₆(OH)₆(AsO₄)₃] framework. For the partially dehydrated form, with n≈1, at least two of these sites were found to be occupied significantly. In addition, the structural investigations allowed two different intra-framework hydrogen bonds to be distinguished that are independent of the extra-framework water distribution and are responsible for the stability of the self-supporting framework. The kinetic analysis of the rate data in the 298–343K temperature range shows that the dehydration behaviour obeys a diffusion-controlled reaction mechanism with an empirical activation energy of E _a ^dehyd=54±4 kJ mol^–1. A two-stage process controls rehydration of which the individual steps were attributed to an initial surface-controlled (E _a ^hyd-I=6±1 kJ mol^–1) and subsequent diffusion-controlled reaction mechanism (E _a ^hyd-II=12±1 kJ mol^–1). The estimated hydration enthalpy of 42±5 kJ mol^–1 supports the distribution model of molecular water within the channels based on a purely hydrogen-bonded network. Received June 26, 1996 / Revised, accepted November 11, 1996     

Cu and Fe chalcopyrite leach activation energies and the effect of added Fe

The leaching kinetics of chalcopyrite (CuFeS₂) concentrate in sulfuric acid leach media with and without the initial addition of Fe³⁺ under carefully controlled solution conditions (E_h 750 mV SHE, pH 1) at various temperatures from 55 to 85 °C were measured. Kinetic analyses by (i) apparent rate (not surface area normalised), and rate dependence using (ii) a shrinking core model and (iii) a shrinking core model in conjunction with Fe³⁺ activity, were performed to estimate the activation energies (E_a) for Cu and Fe dissolution.The E_a values determined for Cu and Fe leaching in the absence of added Fe³⁺ are within experimental error, 80 ± 10 kJ mol⁻¹ and 84 ± 10 kJ mol⁻¹, respectively (type iii analyses E_a are quoted unless stated otherwise), and are indicative of a chemical reaction controlled process. On addition of Fe³⁺ the initial Cu leach rate (up to 10 h) was increased and Cu was released to solution preferentially over Fe, with the E_a value of 21 ± 5 kJ mol⁻¹ (type ii analysis) suggestive of a transport controlled rate determining process. However, the rate of leaching rapidly decreased until it was consistently slower than for the equivalent leaches where Fe³⁺ was not added. The resulting E_a value for this leach regime of 83 ± 10 kJ mol⁻¹ is within experimental error of that determined in the absence of added Fe³⁺. In contrast to Cu release, Fe release to solution was consistent with a chemical reaction controlled leach rate throughout. The Fe release E_a of 76 ± 10 kJ mol⁻¹ is also within experimental error of that determined in the absence of added Fe³⁺. Where type (ii) and (iii) analyses were both successfully carried out (in all cases except for Cu leaching with added Fe³⁺, <10 h) the E_a derived are within experimental error. However, the type (iii) analyses of the leaches in the presence of added Fe³⁺ (>10 h), as compared to in the absence of added Fe³⁺, returned a considerably smaller pre-exponential factors for both Cu and Fe leach analyses commensurate with the considerably slower leach rate, suggestive of a more applicable kinetic analysis.XPS examination of leached chalcopyrite showed that the surface concentration of polysulfide and sulfate was significantly increased when Fe³⁺ was added to the leach liquor. Complementary SEM analysis revealed the surface features of chalcopyrite, most likely due to the nature of the polysulfide formed, are subtly different with greater surface roughness upon leaching in the absence of added Fe³⁺ as compared to a continuous smooth surface layer formed in the presence of added Fe³⁺. These observations suggest that the effect of Fe³⁺ addition on the rate of leaching is not due to the change in the chemical reaction controlled mechanism but due to a change in the available surface area for reaction.     

The role of Fe2+ and Fe3+ in synthetic Fe-substituted tetrahedrite

Summary Tetrahedrites with the composition between Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ were synthesized at 457 °C and 500 °C from the elements and carefully studied by Mössbauer spectroscopy of⁵⁷Fe. Between Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ iron is predominantly ferric. Between Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ iron is predominantly ferrous and occupies the tetrahedral M1-sites.

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Zusammenfassung Die Rolle von Fe²⁺ und Fe³⁺ in synthetischen Tetraedriten mit Fe-Substitution Tetraedrite mit einer Zusammensetzung zwischen Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ wurden bei 457 °C und 500 °C aus den Elementen synthetisiert und sorgfdltig mit Mössbauer-Spektroskopie von⁵⁷Fe untersucht. Zwischen Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ ist Eisen überwiegend dreiwertig. Zwischen Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₁Fe₂Sb₄S₁₃ ist Eisen überwiegend zweiwertig und besetzt die tetraedrisch koordinierten M1-Plätze.

     

Viscoelastic behaviour of mica-based glass-ceramic aggregate

The present study deals with the small strain torsion deformation of MACOR glass-ceramic samples at high temperatures (450–850 °C) and over a range of low frequencies (20 Hz–5 mHz). The samples of MACOR ceramic consist of 55 vol% randomly oriented, sheet-like fluorophlogopite mica crystals (∼100–20 μm in planar size, 1–2 μm in thickness) and 45 vol% of isotropic alumino-borosilicate glass matrix. Measurements of the complex shear modulus show that the sample does not possess the relaxed shear viscosity even at temperatures above the glass transition temperature of the glass matrix. The maximum of the imaginary component G ^′′() of the shear modulus is ∼0.15 of the unrelaxed value G _∞, the relaxation strength Δ≈0.9. The activation energy of the peak of G ^′′() is ∼245 kJ mol⁻¹. Using this value of E _a , the data obtained at various frequencies and temperatures have been reduced to a master curve using the dimensionless variable ωτ, where ∼₀ exp(−E _a /RT). The internal friction Q⁻¹(ωτ) is ∝1/()^0.35−0.4 in the low-temperature high-frequency range (1); passes through a maximum at ∼1 and trends asymptotically to a value Q⁻¹∼0.25–0.30 at ≪1. The behaviour of Q ⁻¹(ωτ) differs from that of a Caputo body by the presence of the resolved peak which may be attributed to the slow mechanical relaxation of mica crystals due to rotation as well as flexing and bending modes of crystal deformation. Received: 26 June 1998 / Revised, accepted: 13 January 1999     

Hydroxyl in mantle olivine xenocrysts from the Udachnaya kimberlite pipe

The incorporation of hydrogen in mantle olivine xenocrysts from the Udachnaya kimberlite pipe was investigated by Fourier-transform infrared spectroscopy and secondary ion mass spectrometry (SIMS). IR spectra were collected in the OH stretching region on oriented single crystals using a conventional IR source at ambient conditions and in situ at temperatures down to −180°C as well as with IR synchrotron radiation. The IR spectra of the samples are complex containing more than 20 strongly polarized OH bands in the range 3,730–3,330 cm⁻¹. Bands at high energies (3,730–3,670 cm⁻¹) were assigned to inclusions of serpentine, talc and the 10 Å phase. All other bands are believed to be intrinsic to olivine. The corresponding point defects are (a) associated with vacant Si sites (3,607 cm⁻¹ E || a, 3,597 E || a, 3,571 cm⁻¹ E || c, 3,567 E || c, and 3,556 E || b), and (b) with vacant M1 sites (most of the bands polarized parallel to a). From the pleochroic behavior and position of the OH bands associated with the vacant M1 sites, we propose two types of hydrogen—one bonded to O1 and another to O2, so that both OH vectors are strongly aligned parallel to a. The O2–H groups may be responsible for the OH bands at higher wavenumbers than those for the O1–H groups. The multiplicity of the corresponding OH bands in the spectra can be explained by different chemical environments and by slightly different distortions of the M1 sites in these high-pressure olivines. Four samples were investigated by SIMS. The calculated integral molar absorption coefficient using the IR and SIMS results of 37,500±5,000 L mol H₂O cm⁻² is within the uncertainties slightly higher than the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003) (28,450±1,830 L mol H₂O cm⁻²). The reason for the difference is the different distributions of the absorption intensity of the spectra of both studies (mean wavenumber 3,548 vs. 3,570 cm⁻¹). Olivine samples with a mean wavenumber of about 3,548 cm⁻¹ should be quantified with the absorption coefficient as determined in this study; those containing more bands at higher wavenumber (mean wavenumber 3,570 cm⁻¹) should be quantified using the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003).

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)−diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W _CaTs−Di ^V = −0.29 ± 0.11 cm³ mol⁻¹ and W _Di−CaTs ^V = −1.14 ± 0.14 cm³ mol⁻¹. Heat capacities were determined between 5 and 923 K by heat-pulse at 5−302 K and differential-scanning calorimetry at 143−923 K. The precision of the low and high temperature C _p data is better than ±1%. Polynomials of the form C _p = a + bT ^−1/2 + cT ⁻² + dT ⁻³ were fitted to the C _p data in the temperature range between 250 and 925 K. Thermal entropy values [S ₂₉₈ − S ₀] and [S ₉₀₀ −S ₀] as well as enthalpies [H ₂₉₈ − H ₀] and [H ₉₀₀ − H ₀] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.     

Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction

Thermal behaviour and kinetics of dehydration of gypsum in air have been investigated using in situ real-time laboratory parallel-beam X-ray powder diffraction data evaluated by the Rietveld method. Thermal expansion has been analysed from 298 to 373 K. The high-temperature limits for the cell edges and for the cell volume, calculated using the Einstein equation, are 4.29 × 10⁻⁶, 4.94 × 10⁻⁵, 2.97 × 10⁻⁵, and 8.21 × 10⁻⁵. Thermal expansion of gypsum is strongly anisotropic being larger along the b axis mainly due to the weakening of hydrogen bond. Dehydration of gypsum has been investigated in isothermal conditions within the 348–403 K range with a temperature increase of 5 K. Dehydration proceeds through the CaSO₄·2H₂O → CaSO₄·0.5H₂O → γ-CaSO₄ steps. Experimental data have been fitted with the Avrami equation to calculate the empirical activation energy of the process. No change in transformation mechanism has been observed within the analysed temperature range and the corresponding E _a is 109(12) kJ/mol.     

Thermodynamics of hydration of Na- and K-clinoptilolite to 300 °C

The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and P _H2O<30 bars. The water content of clinoptilolite can be predicted as a function of water activity and temperature with the equation: a _H2O = [exp[[−ΔH _h ^∘/nRT] + [ΔS _h ^∘/nR] − 1/nRT· [W₁ X _h + W₂ X _{h 2}]− ln(X _a/X _h)]]⁻¹ where T is degrees in Kelvin, ΔH _h ^∘ is the standard molal enthalpy of hydration, ΔS _h ^∘ is the entropy of hydration, X _h and X _a are, respectively, the mole fractions of the hydrous and anhydrous components of the solid solution, W ₁ and W ₂ are interaction parameters, n is the maximum number of moles of H₂O per formula unit (based on 12 oxygens), and R is the gas constant. This equation can be used to locate clinoptilolite-H₂O isohydrons in a _H2O-T space below the liquid-vapor equilibrium curve of water. The standard molal Gibbs free energy of hydration is −47.62 ± 5.52 kJ/mol H₂O and −5.40 ± 2.71 kJ/mol H₂O for the Na- and K-clinoptilolite, respectively. These standard-state thermodynamic properties of clinoptilolite hydration are in good agreement with previous data at low H₂O pressures. The experiments indicate that clinoptilolite progressively dehydrates with increasing temperature at pressures along the liquid-vapor equilibrium curve. Kinetic data above 150 °C show that clinoptilolite dehydration and hydration reactions are fast and reversible and that steady-state hydration states are attained in minutes. Received: 19 June 1998 / Revision, accepted 14 December 1998     

Remediation of heavy metal-contaminated sediments by solid-bed bioleaching

 Weisse Elster River sediment from the Leipzig Lowlands region (Saxony, Germany) is anthropogenically polluted by heavy metals. Sediment dredged from a trap to the south of Leipzig was characterized in detail. When freshly dredged sediment contacts air, the material turns acidic because of oxidation processes, the heavy metals become soluble and the sediment poses an environmental risk. We are therefore developing a sediment-treatment process based on heavy metal removal by bioleaching. Leaching experiments were carried out in suspension and in the solid bed. The heavy metals were solubilized to nearly the same extent by H₂SO₄ dosage (pure chemical extraction) and addition of elemental sulphur (microbial oxidation of S⁰ to H₂SO₄). With increasing dosage of the leaching agent, Zn, Cd, Ni, Cu and Cr were more and more solubilized, whereas Pb was only dissolved in small amounts. The addition of 2% S⁰ is considered an optimum dosage. When 5% S⁰ was added to the sediment, the pH dropped to 1.76 and large amounts of undesirable compounds such as Ca, Al and Fe were solubilized. The higher the temperature, the faster the metals were solubilized in both suspension and the solid bed. The temperature optimum for activating the indigenous S⁰-oxidizing microbes of the sediment lies between 30 and 40  °C. Conditioning of freshly dredged sediment with plants makes it suitable for solid-bed leaching; the kinetics of heavy metal solubilization from sediment conditioned for 6 months with Phragmites australis was the same as from long-term stored sediment. Received: 26 November 1999 · Accepted: 5 May 2000     

In situ high-temperature powder X-ray diffraction study on the spinel solid solutions (Mg1?xMnx)Cr2O4

Using a conventional high-T furnace, the solid solutions between magnesiochromite and manganochromite, (Mg_1−x Mn _x )Cr₂O₄ with x = 0.00, 0.19, 0.44, 0.61, 0.77 and 1.00, were synthesized at 1,473 K for 48 h in open air. The ambient powder X-ray diffraction data suggest that the V–x relationship of the spinels does not show significant deviation from the Vegard’s law. In situ high-T powder X-ray diffraction measurements were taken up to 1,273 K at ambient pressure. For the investigated temperature range, the unit-cell parameters of the spinels increase smoothly with temperature increment, indicating no sign of cation redistribution between the tetrahedral and octahedral sites. The V–T data were fitted with a polynomial expression for the volumetric thermal expansion coefficient (a_T = a₀ + a₁ T + a₂ T ^{- 2} \alpha_{T} = a_{0} + a_{1} T + a_{2} T^{ - 2} ), which yielded insignificant a ₂ values. The effect of the composition on a ₀ is adequately described by the equation a ₀ = [17.7(8) − 2.4(1) × x] 10⁻⁶ K⁻¹, whereas that on a ₁ by the equation a ₁ = [8.6(9) + 2.1(11) × x] 10⁻⁹ K⁻².     

Thermodynamics of arsenates,selenites, and sulfates in the oxidation zone of sulfide ores: IV. Eh-pH diagrams of the Me-Se-H<Subscript>2</Subscript>O systems (Me = Co,Ni, Fe,Cu, Zn,Pb) at 25°C

The behavior of selenium at the Earth’s surface and nearby at low temperatures and pressures is controlled by variations of the redox potential and the acidity of solutions. These parameters determine the migration of selenium and its precipitation as various solid phases. Understanding the mechanism of selenium’s behavior under surface conditions, which is important for solving environmental problems, is an urgent task of contemporary mineralogy and geochemistry. The activities of components in natural waters beyond the zones of natural (oxidation zones) and man-made contamination with selenium (a _ΣSe = 10⁻⁹, a _ΣFe = 10⁻⁵, a _ΣCu = 10⁻⁷, a _ΣZn = 5 × 10⁻⁷, a _ΣCo = 10⁻⁸, a _ΣNi = 6 × 10⁻⁸, a _ΣPb = 10⁻⁸) and in waters formed in the oxidation zone (a _ΣSe = 10⁻⁵–10⁻⁴, a _ΣFe = 10⁻², a _ΣCu = 10⁻², a _ΣZn = 5 × 10⁻², a _ΣCo = 10⁻³, a _ΣNi = 10⁻², a _ΣPb = 10⁻⁴) have been estimated. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 7.0) software package. The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution, 624 solid phases, and 10 gases. The Eh-pH diagrams of the Me-Se-H₂O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) were plotted for the average contents of these elements in underground water and for their contents in oxidation zones of sulfide deposits. The formation of Co, Ni, Fe, Cu, Zn, and Pb selenites and selenates at the surface is discussed.     

Thermodynamics of arsenates,selenites, and sulfates in the oxidation zone of sulfide ores: Part III: Eh-pH diagrams of the Me-As-H<Subscript>2</Subscript>O systems (Me = Co,Ni, Fe,Cu, Zn,Pb) at 25°C

The behavior of arsenic at the Earth’s surface and nearby at low temperatures and pressures depends mainly on the redox potential and the acidity-alkalinity of the crystallization media. These parameters determine the migration of arsenic compounds and their precipitation as various solid phases. Understanding the mechanism of arsenic’s behavior under surface conditions, which is important for solving environmental problems, is an urgent task of contemporary mineralogy and geochemistry. The activities of the components in natural waters beyond the zones of natural (oxidation zones) and man-made contamination with arsenic (a _ΣAs = 3 × 10⁻⁸, a _ΣFe = 10⁻⁵, a _ΣCu = 10⁻⁷, a _ΣZn = 5 × 10⁻⁷, a _ΣCo = 10⁻⁸, a _ΣNi = 6 × 10⁻⁸, a _ΣPb = 10⁻⁸) and in waters formed in the oxidation zone (a _ΣSe = 10⁻³, a _ΣFe = 10⁻², a _ΣCu = 10⁻², a _ΣZn = 5 × 10⁻², a _ΣCo = 10⁻³, a _ΣNi = 10⁻², a _ΣPb = 10⁻⁴) have been estimated. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 7.0) software package. The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution, 624 solid phases, and 10 gases. The Eh-pH diagrams of the Me-As-H₂O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) were plotted for the average contents of these elements in the underground water and for their higher contents in the oxidation zones of sulfide deposits. The formation of Co, Ni, Fe, Cu, Zn, and Pb arsenates at the surface is discussed.     

Hydrogen peroxide leaching of hydrolyzed titania residue prepared from mechanically activated Panzhihua ilmenite leached by hydrochloric acid

The hydrolyzed titania residue with mainly precipitated hydrated TiO₂ is prepared from mechanically activated Panzhihua ilmenite leached by hydrochloric acid. We use H₂O₂ as a coordination agent to leach Ti from the hydrolyzed titania residue. The effects of H₂O₂ to-hydrolyzed titania residue mass ratio, leaching pH value, leaching temperature, leaching time, and H₂O₂ concentration on leaching of hydrolyzed titania residue by H₂O₂ are investigated. The recovery of Ti is above 98% under the optimal conditions. XRD shows that the peroxo-titania powder prepared by boiling the aqueous titanium peroxide solution is crystalline anatase phase. A well-crystallized anatase TiO₂ and rutile TiO₂ can be obtained through calcining at the temperature of 800 and 1000 °C, respectively. EDS shows that the impurities including silicon are enriched in the leaching slag. Little Si is detected as the only impurity in the synthetic TiO₂ due to the weak alkaline condition of leaching.