Papers by George Demopoulos
Crystal Growth & Design, 2016
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Metallurgical Transactions B, 1985
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Nano Energy, 2015
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Industrial & Engineering Chemistry Research, 2015
ABSTRACT In this paper the air sparging-oxidation kinetics of hydrolyzed iron(II) sulfate slurrie... more ABSTRACT In this paper the air sparging-oxidation kinetics of hydrolyzed iron(II) sulfate slurries in connection to acidic effluent treatment by neutralization for the removal of contaminants like arsenic(V) were investigated. It was shown in the absence of As(V) and forced air sparging that Fe(II) sulfate of initial concentration 75 mmol/L hydrolyzed completely between pH 7.5 and 8.5 at 22 °C. Subsequent oxidation by forced air sparging of the ferrous hydroxide slurry was found to proceed via a series of transformations starting from ferrous hydroxide to green rust, to lepidocrocite or magnetite depending on the pH and rate of oxidation, and finally to goethite. The oxidation kinetics at pH 8 or higher were governed by oxygen mass transfer while at pH 6 they were chemically controlled with autocatalytic behavior arising from the effect of in situ formed ferric oxyhydroxide. In the presence of As(V), both Fe(II) and As(V) precipitated from solution starting at pH 4 with the latter ultimately dropping below 1 mg/L past pH 6.5 via the apparent formation of ferrous arsenate compound. Subsequent oxidation by air sparging of the Fe(II)-As(V) slurry at constant pH 8 led to destabilization of the arsenate-carrying phase resulting in partial release of As(V). The bulk control of As(V) in the latter case appears to switch from ferrous arsenate to arsenate adsorption on in situ formed iron(III) oxyhydroxide.
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Industrial & Engineering Chemistry Research, 2015
ABSTRACT In this paper the air sparging-oxidation kinetics of hydrolyzed iron(II) sulfate slurrie... more ABSTRACT In this paper the air sparging-oxidation kinetics of hydrolyzed iron(II) sulfate slurries in connection to acidic effluent treatment by neutralization for the removal of contaminants like arsenic(V) were investigated. It was shown in the absence of As(V) and forced air sparging that Fe(II) sulfate of initial concentration 75 mmol/L hydrolyzed completely between pH 7.5 and 8.5 at 22 °C. Subsequent oxidation by forced air sparging of the ferrous hydroxide slurry was found to proceed via a series of transformations starting from ferrous hydroxide to green rust, to lepidocrocite or magnetite depending on the pH and rate of oxidation, and finally to goethite. The oxidation kinetics at pH 8 or higher were governed by oxygen mass transfer while at pH 6 they were chemically controlled with autocatalytic behavior arising from the effect of in situ formed ferric oxyhydroxide. In the presence of As(V), both Fe(II) and As(V) precipitated from solution starting at pH 4 with the latter ultimately dropping below 1 mg/L past pH 6.5 via the apparent formation of ferrous arsenate compound. Subsequent oxidation by air sparging of the Fe(II)-As(V) slurry at constant pH 8 led to destabilization of the arsenate-carrying phase resulting in partial release of As(V). The bulk control of As(V) in the latter case appears to switch from ferrous arsenate to arsenate adsorption on in situ formed iron(III) oxyhydroxide.
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The Journal of Physical Chemistry C, Mar 6, 2011
In this study, the electronic properties of N719 adsorbed onto anatase were comparably investigat... more In this study, the electronic properties of N719 adsorbed onto anatase were comparably investigated by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) techniques. Sensitized TiO2 films made from two different ...
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Journal of Chemical and Engineering Data, 2005
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Journal of Chemical and Engineering Data, 2005
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J Chem Eng Data, 2008
ABSTRACT This work reports the stability and solubility of nesquehonite in several salts (NaCl, N... more ABSTRACT This work reports the stability and solubility of nesquehonite in several salts (NaCl, NH4Cl, MgCl2, and KCl) over the temperature range of (15 to 35) °C. The needle-like nesquehonite used in this work was prepared by the reaction of analytical pure MgCl2 with Na2CO3. The concentration investigated for all salts was up to 4 mol·dm−3 NaCl, 3.5 mol·dm−3 NH4Cl, 4 mol·dm−3 MgCl2, and 1.0 mol·dm−3 KCl at ambient temperature. The solubility of nesquehonite in pure water was found to decrease with temperature within the temperature range in which nesquehonite is the stable phase. In NaCl solutions, the solubility of nesquehonite initially increases to a maximum value and then decreases gradually with an increase of the common salt concentration. It was further found that the addition of MgCl2, NH4Cl, or KCl causes the solubility of nesquehonite to increase due apparently to complexation. XRD and SEM examination of the equilibrated solids showed that nesquehonite is stable in pure water up to 50 °C, but its stability region becomes smaller in concentrated brines.
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Hydrometallurgy, 2009
Controlling silica level in Bayer liquor is critical in order to prevent scaling or alumina quali... more Controlling silica level in Bayer liquor is critical in order to prevent scaling or alumina quality issues. The conventional method of removing silicates from aluminate solutions requires the introduction of calcium oxide or calcium hydroxide. In this work, Friedel's salt (FS: 3CaO·A12O3·CaCl2·10H2O) is proved for the first time to remove up to 95% silica from sodium aluminate solution. FS is a mineral anion exchanger belonging to the layered double hydroxides (LDHs) which was prepared by adding calcium chloride to sodium aluminate over the temperature range of 50–90 °C. It was characterized by XRD, SEM, Particle Size Analyzer and TG-DSC. FS prepared at 50 °C has a relatively high desilication capacity, better than calcium oxide.Experimental parameters affecting the desilication process, such as temperature, sodium aluminate liquor composition, initial silica concentration (4–10 g/L) and FS dosage were investigated in detail and a comparison of desilication between FS and CaO was carried out. The desilication products (DSP) were mainly calcium aluminium silicates, identified by XRD to be Chabazite and Wadalite and the final chloride concentration in the sodium aluminate solution after anion exchange with FS was ∼ 0.015 g/L. The rate of desilication by FS was first order with a rate constant 2.582 × 106 min− 1. The apparent activation energy was estimated to be 57.7 kJ mol− 1 over the temperature range of 80–110 °C.
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Langmuir, 2010
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Journal of Crystal Growth, Mar 1, 2008
In this study, homogeneous (unseeded) precipitation of nesquehonite (MgCO 3·3H 2O) by the reactio... more In this study, homogeneous (unseeded) precipitation of nesquehonite (MgCO 3·3H 2O) by the reaction of MgCl 2 with (NH 4) 2CO 3 in supersaturated solutions was investigated. Factors that influence the precipitation of MgCO 3·3H 2O, such as reaction temperature, initial concentration, stirring speed, titration speed, equilibration time, have been studied. SEM images and particle size distribution show that the temperature, initial concentration and titration speed have significant effect on nesquehonite's crystal morphology and particle size. In addition, stirring speed and equilibration time also have some influence on its properties. X-ray powder diffraction (XRD) results show that the obtained crystals compositions are greatly affected by the reaction temperature. With the morphological transformation, their corresponding composition also change from MgCO 3· xH 2O to Mg 5(CO 3) 4(OH) 2·4H 2O in the interval of 288-333 K. With the optimization of operating conditions, the crystals can grow up to a length of about 40 mum and a width of 5 mum, indicating good filtration properties. High-purity nesquehonite obtained in this study was calcined to produce highly pure MgO at 1073 K as shown by XRD results.
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Hydrometallurgy, 2015
ABSTRACT The coprecipitation of arsenic(V) with ferric iron was studied through the use of contin... more ABSTRACT The coprecipitation of arsenic(V) with ferric iron was studied through the use of continuous circuit coprecipitation experiments that involved lime neutralization of acidic sulfate solutions (Fe/As molar ratio of 4) to pH 8. The influence of coprecipitation circuit design on arsenic removal was evaluated through one, two and three-stage experiments as well as the use of solids recycling in a two-stage circuit. The two-stage (operating respectively at pH 4 and 8) continuous circuit configuration produced the lowest residual arsenic concentration and the lowest specific surface area coprecipitate. Two-stage continuous experiments were also used to examine the influence of nickel and aluminum, co-ions that are common in industrial solutions. Nickel was not observed to significantly influence the residual dissolved arsenic under the conditions tested. Aluminum was found to be a suitable equimolar substitute for a portion of the ferric iron. X ray diffraction and Raman spectroscopic data indicated that the coprecipitates consisted of a mixture of gypsum, poorly crystalline ferric arsenate and (arsenic-bearing) ferrihydrite. Calculations based on the pH of point of zero charge (pHpzc) suggested that the content of ferric arsenate ranged from 24% to 57% and was influenced by the coprecipitation circuit design. The highest ferric arsenate content was observed with the two-stage coprecipitation circuit which also yielded the lowest residual arsenic concentration. The results indicated that subtle chemical differences induced in the coprecipitates by the process could be of significant influence to the geochemical stability of arsenic.
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Solvent Extraction and Ion Exchange
ABSTRACT
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Solvent Extraction and Ion Exchange
ABSTRACT
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Solvent Extraction and Ion Exchange
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Applied Surface Science, 2015
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Journal of Hazardous Materials, 2015
Gypsum precipitates as a major secondary mineral during the iron-arsenate coprecipitation process... more Gypsum precipitates as a major secondary mineral during the iron-arsenate coprecipitation process for the removal of arsenic from hydrometallurgical effluents. However, its role in the fixation of arsenic is still unknown. This work investigated the incorporation of arsenic into gypsum quantitatively during the crystallization process at various pHs and the initial arsenic concentrations. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray absorption near edge spectroscopy (XANES) and scanning electron microscopy (SEM) were employed to characterize the coprecipitated solids. The results showed that arsenate was measurably removed from solution during gypsum crystallization and the removal increased with increasing pH. At lower pH where the system was undersaturated with respect to calcium arsenate, arsenate ions were incorporated into gypsum structure, whereas at higher pH, calcium arsenate was formed and constituted the major arsenate bearing species in the precipitated solids. The findings may have important implications for arsenic speciation and stability of the hydrometallurgical solid wastes.
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Papers by George Demopoulos