Wide Range Of Solution Compositions Research Articles

Thorium oxide dissolution in HNO3-HF mixture: kinetics and mechanism

Abstract This paper is an attempt to find out thorium oxide dissolution mechanism in HNO3-HF mixture. In a previous paper, several parameters effects on thorium oxide dissolution have been described, with specific focus on hydrofluoric acid effect, which can lead to an increase of the dissolution rate if present in small amount, but precipitates as ThF4 at higher content. Based on this previous study, experimental data were fitted using several dissolution models in order to find out the best one. Finally, a revisited model based on literature and considering the ThF4 formation was proposed. It describes the main steps of dissolution and is able to fit the experimental data for a wide range of solution compositions. This point is crucial since it allows considering an extrapolation of the established model to not-yet-studied conditions.

Kinetics of Ferric Ion Reduction on Chalcopyrite and its Influence on Leaching up to 150 °C

The reduction kinetics of ferric ion on chalcopyrite in the quaternary H2SO4-Fe2(SO4)3-FeSO4-H2O system were systematically studied by cathodic potentiodynamic polarization (0.5mV/s) through a wide range of solution compositions and temperatures (25–150°C). Results reveal that all of the pseudo-polarization plots exhibit reproducible well-defined linear Tafel regions and thus the temperature dependence of various kinetic parameters, such as the exchange current density, transfer coefficient and rate constant, has been determined. The exchange current densities of the Fe3+/Fe2+ couple obtained by extrapolating the cathodic linear Tafel line to the reversible potential are on the order of 10−7-10−5 A/cm2 in the range of 25–150°C, substantially less than that on platinum due to the formation of a passive film. In general, at the same nominal Fe3+/Fe2+ ratio, increasing the temperature from 25 to 150°C was observed to give rise to an increase in i0, especially when a larger amount of free ferrous existed in the solution. Based on the obtained exchange current densities, the rate constants for ferric ion reduction were calculated, and further analysis indicates that the rate constants can be well described by the Arrhenius equation. Experimental evidence clearly shows an anomalous independent variation of Tafel slope with temperature and a corresponding unexpected increased linear variation of transfer coefficient with respect to temperature (rather than being constant throughout the investigated temperature range). Analysis by mixed potential theory indicates that in the acidic iron sulfate leaching system the control scenario highly depends on both the solution composition and temperature. The importance of the cathodic ferric ion reduction reaction on the overall leaching process is progressively increased when increasing the nominal Fe3+/Fe2+ ratio and temperature. The leaching reaction is under anodic control when the nominal Fe3+/Fe2+ ratio is around 1:1, whereas at higher nominal Fe3+/Fe2+ ratios and temperatures the reaction is under mixed control, by both anodic oxidation and cathodic reduction reactions.

Speciation of the H2SO4–Fe2(SO4)3–FeSO4–H2O system and development of an expression to predict the redox potential of the Fe3 +/Fe2 + couple up to 150 °C

A thermodynamic model is developed and shown to reliably simulate the speciation of the quaternary H2SO4–Fe2(SO4)3–FeSO4–H2O system through a wide range of solution compositions and temperatures (25°C–150°C). The main species involved (including H+, Fe2+, Fe3+, SO42−, HSO4−, FeHSO4+, FeSO4°, FeHSO42+, Fe(SO4)2−, FeSO4+, Fe2O3 and H2O) were investigated, and their thermodynamic data were collected and critically assessed to evaluate their activity coefficients and equilibrium constants. Results show that the species distribution depends highly on the initial total amount of iron, acidity, nominal Fe3+/Fe2+ ratio and temperature. Modeling results also reveal that most of the Fe(III) is distributed as complexes or precipitates and the free Fe3+ accounts for only a minor percentage, whereas a large amount of Fe(II) exists in the form of free Fe2+, with the rest dissolved as complexes. Moreover, based on the model results, the temperature dependence of the real Fe3+/Fe2+ ratios and their activity coefficient ratios was subsequently obtained and employed to systematically study the redox potential of the Fe3+/Fe2+ couple. The redox potentials predicted by the model increase with the nominal Fe3+/Fe2+ ratio and temperature when the nominal Fe3+/Fe2+ ratios are 10:1, 100:1 and 1000:1, with its values in the range of 736–956mV (all potentials in the present study are quoted with respect to the Standard Hydrogen Electrode at 25°C) from 25°C to 150°C in the studied acidic iron sulfate solutions. At a nominal Fe3+/Fe2+ ratio of 1:1, the predicted potentials are relatively stable with respect to temperature, typically between 671mV and 679mV, because the real Fe3+/Fe2+ ratio in solution is much lower for such a condition. The speciation model explains the change of redox potential with temperature for all nominal Fe3+/Fe2+ ratios. An expression is also developed to calculate the redox potential and is supported by experimental results from the literature. It appears that the redox potential can be easily and accurately determined only based on the variables of temperature and nominal Fe3+/Fe2+ ratio, independent of the nominal concentrations of H2SO4 and Fe3+. Moreover, this expression is probably applicable to more general cases at or around room temperature. The proposed model was validated by reliable prediction of measured redox potential from 25°C to 150°C. Comparison of previously published results of ferric solubility, together with an analysis of the calculated pH and ionic strength, provides additional justification for the model developed herein. These findings contribute to the research on aqueous speciation of acid iron sulfate solutions, the reduction kinetics of the Fe3+/Fe2+ couple, as well as the mechanistic analysis in industrial leaching processes of chalcopyrite and other sulfide minerals.

Fluorescence and absorbance spectroscopy of the uranyl ion in nitric acid for process monitoring applications

Near real time process monitoring of the ura- nium content in an aqueous fuel recycling plant is a desired component of an advanced safeguards suite; Ultraviolet- Visible spectroscopy and Time Resolved Laser induced Fluorescence Spectroscopy can contribute to this technol- ogy gap. This work presents the observation of the spec- troscopic parameters (molar absorptivities, fluorescent response) of the uranyl ion across the range of conditions expected in reprocessing chemistry. From this data, a monitor using the ratio of the absorbance of the uranyl ion at 403 and 426 nm has been developed. This technique can determine the nitrate solution concentration and can be coupled with a condition appropriate molar absorptivity to determine the uranyl concentration. This method provides a reliable technique for online, real time process monitoring of the uranyl and nitrate concentration under a wide range of solution compositions.

Effect of complexation of poly(acrylic acid) with Cu2+ ions on the size of copper nanoparticles prepared via reduction in aqueous solutions

Sols of zero-valence copper are prepared via the chemical reduction of Cu(II) ions by hydrazine borane in aqueous solutions of high-molecular-mass poly(acrylic acid), which forms stable complexes with copper ions at 20°C in a wide pH range. The study of the composition of coordination centers, the ligand surrounding of metal ions, and the character of distribution of copper ions over poly(acrylic acid) coils in a wide range of solution compositions and pH values shows that the size of copper nanoparticles in the sols can be controlled by varying the ratio between ligand groups (carboxylate anions in poly(acrylic acid)) and copper ions in the reaction system during the synthesis of sols. This effect can be accomplished either by variation in the initial composition of solution or change in pH (the degree of ionization of the initial poly(acrylic acid) in the presence of copper ions).

Complexation in the Cu(II)–LiCl–H 2O system at temperatures to 423 K by UV-Visible spectroscopy

Thermodynamic data for the solubility and formation of copper(I)and copper(II) species are required to model innovative designs of the electrolysis cells used in the thermochemical copper-chloride hydrogen production process. Cumulative formation constants of Cu 2+(aq) complexes with Cl −(aq) were determined by Principal Component Analysis of UV-spectra, obtained over a very wide range of solution compositions and temperature, coupled with an appropriate model for activity coefficients of the solution species. Shortcomings in the existing database and future work to extend these studies to higher temperatures and much higher copper concentrations are discussed.

Brackish water reverse osmosis (BWRO) operation in feed flow reversal mode using an ex situ scale observation detector (EXSOD)

The feasibility of operating brackish water reverse osmosis (BWRO) desalination in a feed flow reversal (FFR) mode so as to mitigate mineral scaling was evaluated with feed solutions containing calcium, sodium, sulfate and chloride ions. The premise of the FFR approach is to enable dissolution of surface mineral crystals, that form first at the BWRO plant tail elements, by periodic exposure to the undersaturated feed solution. Automatic triggering of FFR was accomplished by integrating an Ex-situ scale observation detector (EXSOD) with the BWRO plant controller. The EXSOD system consisted of a small high pressure plate-and-frame RO cell, with its membrane surface digitally imaged in real-time, to monitor the onset and development of membrane mineral scaling. During normal feed flow (NFF) operation, the EXSOD monitored a concentrate side-stream from the BWRO plant tail element, signaling the BWRO plant controller to initiate FFR when the specified scaling threshold was reached. Once the plant feed flow was reversed, the EXSOD feed stream was switched from the BWRO plant concentrate to the plant's permeate in order to dissolve mineral crystals formed on the EXSOD membrane during NFF operation. Successful operation of automated FFR, whereby scale formation was prevented, required adjustment of the EXSOD operating conditions such that the saturation index in the EXSOD RO cell was at a level that would not result in scale detection significantly before or after scaling would occur in the RO plant. Although the onset of mineral scaling was detected at the level of appearance of the first observable mineral crystal, the results suggest that it would be more practical to trigger FFR based on a threshold that considers both the mineral crystal number density and fractional surface scale coverage. Current work is continuing to demonstrate the broad applicability of FFR mode of RO mineral scale-free operation (with reduced or no antiscalant addition) over a wide range of solution compositions and operating conditions.

Structure and composition of binary monolayers self-assembled from sodium 2-mercaptoetanosulfonate and mercaptoundecanol mixed solutions on silver and gold supports

Voltammetric reductive desorption, scanning tunneling microscopy (STM) and surface-enhanced Raman scattering (SERS) were used to determine the composition and structure of mixed, two-component monolayers of sodium 2-mercaptoetanosulfonate (MES) and mercaptoundecanol (MUL) on silver and gold supports. Monolayers were prepared by self assembling from ethanolic solutions of varying composition. Preferred adsorption of MUL was found in the electrochemical experiments on Au(111). The presence of two well-separated reductive desorption peaks in the voltammograms of the mixed monolayers on Au(111) indicated the existence of MES-rich and MUL-rich phases in a wide range of solution compositions. STM imaging confirmed formation of a few-nanometer-wide thiol domains for xMES greater than 0.5 in the solution used for SAMs preparation. On the contrary, SERS experiments pointed at dominant adsorption of MES on rough Ag and Au substrates. Nearly exclusive adsorption of the MES for xMES greater than 0.5 was observed on the rough Ag surface.

Mixed monolayers of bovine serum albumin with a nonionic surfactant (Tween 80)

Mixed monolayers of bovine serum albumin (BSA) with a nonionic surfactant (Tween 80) are obtained by spreading solutions containing both components over the surface of a subphase (water with pH 6) over a wide range of solution compositions. According to compression-expansion isotherms, the mixed monolayers are of the condensed type when the BSA concentrations in the solution are far higher than or equal to the surfactant concentration. Such monolayers mostly consist of BSA-Tween 80 (1: 1) complexes. In contrast, a BSA monolayer is of the expanded type. When Tween 80 in the solution prevails over Tween 80, the monolayers become unstable. The results of this work pertain to the monitoring of the properties of protein-surfactant mixtures and design of Langmuir-Blodgett (LB) films.

The structure and rheology of mixed micellar solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide

The paper presents the results of viscosity measurements by capillary and rotational viscometry for aqueous solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide and data on the hydrodynamic radii of aggregates obtained from dynamic light scattering measurements. A wide range of solution compositions predominantly containing dodecyldimethylamine oxide was studied at 308.15 K; the total content of surfactants was from 1 to 15 wt %, pH ∼ 8. Solution viscosities were found to change by several orders of magnitude when the composition of the system was varied; the dependence of viscosity on the ratio of surfactants passes through maximum and inflections. The dependences of viscosity and hydrodynamic radii on the ratio between two surfactants were similar in character. The conclusion was drawn that various nanostructural rearrangements occurred in the solutions studied and synergistic effects were observed.

Experimental evaluation of the isotopic exchange equilibrium 10B(OH) 3+ 11B(OH) 4−= 11B(OH) 3+ 10B(OH) 4− in aqueous solution

The precision of spectrophotometric measurements of indicator absorbance ratios is sufficient to allow evaluation of small isotopically induced differences in the dissociation constant of boric acid ( K B). The quotient of 11 K B and 10 K B, obtained using isotopically ⩾99% pure borate/boric acid buffers, provides an equilibrium constant for the reaction 10B(OH) 3+ 11B(OH) 4 −⇔ 11B(OH) 3+ 10B(OH) 4 − which heretofore had not been experimentally determined. Previous theoretical and semi-empirical evaluations of this equilibrium, which is important for assessments of the paleo-pH of seawater and the paleo- pCO 2 of the atmosphere, have yielded constants, 11–10 K B= 10 K B/ 11 K B, that have ranged between 1.0194 and approximately 1.033. The experimentally determined value 11–10 K B=1.028 5±0.001 6 (mean±95% confidence interval) obtained at 25 °C and 0.63 molal (mol kg −1 H 2O) ionic strength is in much better agreement with recent theoretical assessments of 11–10 K B that have ranged between 1.026 and 1.033, than the much-cited original estimate (1.0194) of Kakihana et al. (1977) [Fundamental studies on the ion-exchange separation of boron isotopes. Bulletin of Chemical Society of Japan 50, 158–163]. Since the activity quotient for the fractionation reaction is almost equal to unity, it is expected that the 11–10 K B value obtained in this study will be applicable over a wide range of solution compositions and ionic strengths.

TEM-EDX Observations of the Microstructure of Electrodeposited Ni-Sn Alloys

The microstructure of Ni-Sn alloys electrodeposited at 50–5000 A/m 2 from 65 � C chloride solutions containing 5–40 metal% of Sn were analyzed by TEM-EDX. Ni-Sn alloy deposition showed a characteristic behavior in that Ni and Sn were codeposited at an atomic ratio of 1:1 under a wide range of solution compositions and current densities. Ni and Sn EDX mapping images of a cross section of Ni-50 at% Sn alloy deposited at over 1000 A/m 2 showed a lamellar pattern parallel to the interface between substrate and deposit, indicating the alternate deposition of Ni-Sn alloys with different compositions. The Ni-Sn alloys were identified by EDX line analysis as Ni3Sn4 and Ni3Sn2. It was concluded that Ni-50 at%Sn alloys deposited over 1000 A/m 2 were not composed of the previously reported metastable-phase NiSn single alloy, but instead consisted of both Ni3Sn4 and Ni3Sn2 alloys in a thermodynamically stable phase. [doi:10.2320/matertrans.47.1550]

A new force field for atomistic simulations of aqueous tertiary butanol solutions

We present a new tert-butanol force field parametrized to reproduce the mixture thermodynamics of tert-butanol/water over a wide range of solution compositions at room temperature and atmospheric pressure. The experimental Kirkwood-Buff integrals, which quantify preferential solvation of solution components by the same species or by the other components, were used as target values to be reproduced. Water was modeled using the simple point charge model. In the range of alcohol mole fractions between 0.02 and 0.98, our optimized model satisfactorily reproduces alcohol-alcohol, water-water, and alcohol-water aggregation behavior. As a consequence, the solution activity derivatives are reproduced as well. A comparison has been made with solution activities obtained by free energy calculations (i.e., thermodynamic integration). It clearly shows that the Kirkwood-Buff based approach performs superior in predicting solution activities of liquid mixtures. The new tert-butanol model has been used to examine the solution structure and hydrophobic interactions in aqueous tert-butanol at the various mixture compositions. A comparison is made with structural data obtained by neutron diffraction.

Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control

Dissolution and precipitation rates of brucite (Mg(OH) 2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10 −4 to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10 −6 to 5·10 −4 M). Brucite surface charge and isoelectric point (pH IEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pH IEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m 2) and lack of dependence on ionic strength predicts the dominance of >MgOH 2 + species at pH < 8 and their progressive replacement by >MgOH° and >MgO − as pH increases to 10–12. Rates are proportional to the square of >MgOH 2 + surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH 2 + concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity: R=k Mg + · {>MgOH 2 +} 2 · (1−Ω 2) where k Mg + is the dissolution rate constant, {> i} is surface species concentration (mol/m 2), and Ω is the solution saturation index with respect to brucite. Measurements of nonsteady state brucite dissolution rates, in response to cycling the pH from 12 to 2 (pH-jump experiments), indicate the important role of surface hydroxylation — that leads to the formation of Mg oxo or –hydroxo complexes — in the formation of dissolution-active sites. Replacement of water molecules by these oxygen donor complexes in the Mg coordination sphere has a labilizing effect on the dynamics of the remaining water molecules and thus increases reaction rates.

The anomalous codeposition of tungsten in the presence of nickel

Deposition of tungsten-rich alloys from plating baths that do not contain ammonia is reported. Alloys with nominal composition of NiW and even NiW 2 have been formed for the first time by electroplating. It was shown in this paper, and in our previous work [Electrochem. Solid-State Lett. 3 (2000) 543; Proc. Advanced Metalization Conf. (AMC 2000) p. 337; Langmuir 17 (2001) 8270; J. Electrochem. Soc. 149 (2002) C100; J. Appl. Surf. Sci. 200 (2002) 1], that alloys of tungsten and nickel having any composition between a few a/o and 67 a/o of tungsten can be electroplated, by suitable choice of the experimental conditions. The most important factors among these are the concentrations of citrate, which acts as the complexing agent, the ratio of concentrations of nickel and tungstate ions and the pH of the solution. New indirect evidence for the existence of a mixed metal complex of the type [(Ni)(WO 4)(Cit)(H)] 2− was obtained by: (a) the pH dependence of the composition of the alloy and (b) by the linear dependence of the partial current density for deposition of tungsten on the calculated concentration of this complex, over a wide range of solution compositions. Removing ammonia from the plating bath allows the deposition of tungsten-rich alloys, but this is achieved at the cost of a substantial reduction in Faradaic efficiency.

Raman study of aluminum speciation in simulated alkaline nuclear waste.

The chemistry of concentrated sodium aluminate solutions stored in many of the large, underground storage tanks containing high-level waste (HLW) at the Hanford and Savannah River Nuclear Reservations is an area of recent research interest. Not only is the presence of aluminate in solution important for continued safe storage of these wastes, the nature of both solid and solution aluminum oxyhydroxides is important for waste pretreatment. Moreover, for many tanks that have leaked high aluminum waste in the past, little is known about the speciation of Al in the soil. In this study, Raman spectroscopy has been used to investigate the speciation of the aqueous species in the Al2O3-Na2O-H2O system over a wide range of solution compositions and hydration. A ternary phase diagram has been used to correlate the observed changes in the spectra with the composition of the solution and with dimerization of aluminate that occurs at elevated aluminate concentrations (>1.5 M). Dimerization is evidenced by growth of new Al-O stretching bands at 535 and 695 cm(-1) at the expense of the aluminate monomer band at 620 cm(-1). The spectrum of water was strongly influenced by the high concentrations of Na+ and OH- (>17 M). Upon increasing the concentration of NaOH in solution, the delta-(H-O-H) bending band of water (v2 mode) increased in frequency to 1663 cm(-1), indicating that the water contained in the concentrated caustic solution was more strongly hydrogen bonded at the higher base content. In addition, the sharp, well-resolved band at 3610 cm(-1), assigned to the v(O-H) of free OH-, increased in intensity with increasing NaOH. Analysis of the v(O-H) bands in the 3800-2600 cm(-1) region supported the overall increase in hydrogen bonding as evidenced by the increase in relative intensity of a strongly hydrated water band at 3118 cm(-1). Taking into consideration the activity of water, the molar concentrations of the monomeric and dimeric aluminate species were estimated using the relative intensities of the Al-O stretching bands from the Raman spectra. A constant apparent log Kdimer value was obtained at aluminate concentrations >1.5 M with a value of 0.97+/-0.04 at approximately 25 degrees C. This study represents the first spectral-based estimation of a thermodynamic equilibrium constant for the Al2O3-Na2O-H2O system.

Dolomite surface speciation and reactivity in aquatic systems

The surface charge of dolomite (CaMg(CO3)2) was measured as a function of pH (6.5–11.5), pCO2 (10−3.5, 0.01, and 0.96 atm) and ionic strength (0.01, 0.1, and 0.5 M NaCl) using potentiometric titrations in a limited residence time reactor. Dolomite zeta potential (ζ) was determined using streaming potential and electrophoresis techniques at pH 2 to 12 in solutions having ionic strengths from 0.001 to 0.1 M NaCl as a function of aqueous Ca2+, Mg2+, and CO32− concentrations. The point of zero charge (PZC) and isoelectric point (IEP) of dolomite are the same (pH ∼8 at pCO2 ∼10−3.5 atm) and very close to those of calcite and magnesite. On the basis of these results, a surface complexation model (SCM) is proposed that postulates the presence of three distinct primary hydration sites: >CO3H°, >CaOH°, and >MgOH°. The intrinsic stability constants of dolomite surface reactions were determined by fitting the pH dependence of the surface charge and taking into account the isoelectric points and ζ-potential values for a wide range of solution compositions. In most natural aquatic environments, dolomite surface speciation can be modeled using the following species: >CO3−, >CO3Me+, >MeOH2+, >MeHCO3o, and >MeCO3−, where Me = Ca, Mg. The speciation model presented in this study allows description of metal and ligand adsorption onto dolomite surface and provides new insights on the mechanisms that control dolomite dissolution/crystallization in aqueous solutions. In particular, it is shown that dolomite dissolution is controlled by the protonation of >CO3H° surface complexes at pH < 6 and by hydrolysis of >MeOH2+ groups at higher pH.

Processes at the magnesium-bearing carbonates/solution interface. I. a surface speciation model for magnesite

The surface charge of magnesite (MgCO 3) was measured at 25°C as a function of pH (4.6 to 11) and ionic strength (0.01, 0.1 and 0.5 M NaCl) under pCO 2 from 10 −3.5 to 0.96 atm. The acid-base titrations were performed in a limited residence time reactor following the approach developed by Charlet et al. (1990) for rhodochrosite and siderite. Magnesite zeta potential (ζ) was determined as a function of pH (1.5 to 12), ionic strength (0.001 to 0.1 M) and Mg 2+ and HCO 3 − concentrations. Based on these results and spectoscopic data on the calcite and dolomite/water inerfaces, a surface complexation model is presented which postulates the formation of the primary hydration species >CO 3H° and >MgOH°. Within this scheme, magnesite surface speciation is governed by the following species: >CO 3H°, >CO 3 −, >CO 3Mg +, >MgOH°, >MgO −, >MgOH 2 +, >MgHCO ° 3, and >MgCO 3 −. The intrinsic stability constants of these species were determined by fitting the pH dependent surface charge over a wide range of solution composition. For the conditions investigated in this study, the measured ζ-potential values of magnesite are in good agreement with the sign of the surface potential and its dependence on solution composition calculated using the surface complexation model generated in this study.

An Experimental Study of Quartz Precipitation and Dissolution Rates at 200°C

The understanding and quantification of quartz-water interactions is essential for modelling mass transfer in crustal fluids. Quartz dissolution kinetics at hydrothermal temperatures have been the subject of many investigations. Recently, Berger et al. (1994) used a mixed flow reactor to produce data over a wide range of solution compositions, which enabled an improved understanding of the quartz dissolution mechanism. Much less work has been performed on quartz precipitation, and in particular the effect on the rates of changing solution composition. Rimstidt and Barnes (1980) and Bird and Boon (1985) used a closed system technique to generate rate constants from their data, but to date no direct quartz precipitation rate measurements have been performed, In this study direct measurements of quartz dissolution/precipitation rates were made as a function of solution saturation state at the near equilibrium conditions typical of natural processes. Experiments were performed at 200~ using a titanium mixed flow reactor system described by Berger et al. (1994). These reactor systems are ideally suited to investigate the rates of water/mineral interactions at both near and far from equilibrium conditions (Gautier et al., 1994). Mixed flow reactors afford numerous advantages over closed system reactors for kinetic studies including 1) allowing direct measurement of steady-state rates, and 2) permitting rate measurements at specific fluid compositions by either changing the inlet solution composition or the fluid flow rate. The same natural quartz crystals were used in both dissolution and precipitation experiments. The crystals were ground and sieved to the 50-125 micron size fraction. Fine particles were removed by ultrasonic cleaning in deionized water. To remove remaining fine particles and the disturbed surface layer produced by grinding, the quartz grains were cleaned in a mixed flow reactor for five days at 250~ with deionized water. The B.E.T. specific surface area measured after the cleaning procedure was 0.098 _+ 0.005 m2/g. The aqueous silica-rich inlet solutions used for precipitation experiments were prepared by concentrating deionized water originally equilibrated with silicic acid at 90~ When starting a precipitation run, the reactor was filled with deionized water and heated to 200~ Silica-rich solution was then pumped through the reactor. This procedure avoided any potential precipitation of non-quartz silica phases on the quartz grains that could occur from supersaturated solutions at temperatures less than 200~ To assess the possible effect of silica precipitation on the reactor walls and tubing, a blank test was made by pumping a silica-rich solution through the reactor containing no quartz sample. Only a slight decrease of the outlet silica concentration, less than one percent of the inlet concentration, was observed. Dissolution rates were measured at 200~ in atmosphere equilibrated deionized water (pH of 5,65 at 25~ Steady-state outlet silica concentrations ranged frolI1 4.13 x 10 -5 to 2.70 x 10 -3 m. Each of the dissolution data points shown in Fig. 1 corresponds to an individual run conducted at a specific flow rate. Quartz solubility was measured at 200~ in a 5 x 10 -4 m NaOH solution using the same solid as used for the kinetic study but of a smaller size fraction (40-60 microns). The measured solubility of 4.55 x 10 3 m of Si corresponds to a calculated quartz dissociation constant at 200~ of 4.27 x 10 -3. Precipitation experiments were performed using inlet fluids containing from 145 to 253 mg/kg of Si and at flow rates ranging from 0.01 to 0.18 ml/min. Steady-state outlet concentrations ranged from 141 to 198 mg/kg of Si. The B.E.T. specific surface area measured after the last experiment was 0.0745 _+ 0.005 m2/g, which is slightly lower than the original measurement. Dissolution rates exhibit a linear dependance on the saturation state consistent with rate laws derived from the Transition State Theory and the principle of detailed balancing (Lasaga, 1981) given by:

MgO by Injection CVD

Epitaxial YBa 2Cu 3O 7 layers with 45° in-plane orientation have been grown by Injection CVD on MgO substrates polished off-axis to within 1.4–1.9° of the [100] direction. This new single-source CVD process is based on computer-controlled injection of precise microdoses of a metal–organic precursor solution into a CVD reactor. A wide range of solution compositions was tested to investigate compositional effects on phase purity, surface morphology, texturing and superconducting properties of the prepared films. The highest quality films with pure 45° texture had a smooth surface, zero resistance T c ( R=0) of 88–89 K, and critical current density J c (77 K) above 10 6 A/cm 2.