Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.     

Co2 conversion and oxalate stability on alkali promoted metal surfaces: Sodium modified Al(100)

CO₂ conversion on alkali promoted metals in aprotic systems has been followed with surface sensitive spectroscopies. New results on sodium modified aluminum(100) are presented and compared with previous studies on magnesium [1], aluminum [2], and bulk alkali metals [3]. Electron energy loss spectra reveal two different states of CO₂ adsorption at 100 K and monolayer sodium coverage. Vibrational bands at 650 cm^–1 and 2325 cm^–1 correspond to weakly bound molecular CO₂ and a multitude of bands between 2300 cm^–1 and 460 cm^–1 to oxalate ions with low, possibly unidentate, coordination. Gentle annealing increases the coordination as apparent by vibrational shifts. This corresponds to oxalate to carbonate conversion, a process which is completed around room temperature. CO desorption was detected at 285 K and Auger measurements reveal a 13 C/O stoichiometry after high temperature annealing. We observe no release of CO₂ above 110 K but an additional weak state of CO desorption around 470 K. High temperature annealing causes decomposition of all intermediates and leaves the aluminum surface covered with an irreducible carbide and oxide overlayer. We suggest that CO₂ reduction and dimerization to C₂O₄ ^–2 is a common path to yield carbon deposition on all alkali promoted surfaces in hydrogen deficient systems. In contrast, oxalate decomposition is related to the specific chemistry of each substrate.     

Influence of inorganic and organic additives on the composition of the precipitate of synthetic hydroxylapatite and calcium oxalate monohydrate

Synthetic hydroxylapatites are prepared with additives, such as Mg²⁺, CO ₃ ^2? , and C₂O ₄ ^2? . An increase in the concentration of magnesium leads to the formation of struvite. In the Ca(NO₃)₂-(NH₄)HPO₄-Na₂CO₃-NH₄OH-H₂O system, an excess of carbonate ions leads to the formation of calcite. When the synthesis is performed using oxalate ions as additives, calcium oxalate does not form the inherent phase. Calcium oxalate monohydrate is synthesized with additives, such as CO ₃ ^2? , HPO ₄ ^2? , and SO ₄ ^2? ions and urea, glycine, and glutamic acid. X-ray powder diffraction analysis has revealed that the composition of the CaC₂O₄ · H₂O precipitate remains unchanged under these conditions and in the presence of the aforementioned additives.     

Electrodissolution de la millérite en milieu chlorhydrique

Résumé L'électrodissolution deMS synthétisé a été étudiée en milieu d'acide chlorhydrique à 25° C. L'influence de la concentration des ions d'hydrogène (pH 0.46–2.68), de nickel (0–1 M) et de chlore (1–4 M) a été examinée. Les études potentiocinétiques et potentiostatiques ainsi que les diagrammesE-pH de NiS-H₂O ont clarifié la cinétique électrochimique et le mécanisme de dissolution deNiS. La surface attaquée et les produits de corrosion ont été examiné par rayons-X, microscopie électronique à balayage, dispersion des rayons-X et absorption atomique.

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The electrodissolution of syntheticMS has been studied in hydrochloric acid at 25° C. The influence of hydrogen ions (pH 0.46–2.68), nickel ions (0–1 M) and chloride ions (1–4 M) has been examined. Potentiokinetic and potentiostatic studies and alsoE-pH diagrams of NiS-H₂O clarified the electrochemical kinetics and mechanism of the dissolution ofNiS. The attacked surface and dissolution products were examined by X-rays, scanning electron microscopy, X-ray dispersion and atomic absorption.

     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

Fluxes of CO2 and CH4 in soil profiles of a mountainous watershed of Nepal as influenced by land use, temperature, moisture and substrate addition

Effects of land use, moisture, temperature and substrate on production of CO₂ and consumption of CH₄ were measured in a series of laboratory incubation experiments on bulk soil samples from 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths under four dominant land uses [forest, grazing land, irrigated rice on level terraces (Khet), and upland maize–millet on sloping terraces (Bari)] of Mardi watershed (area=144 km²), Nepal. In addition, baseline physical and chemical properties of these soils were measured. The production of CO₂-C day^–1 per unit soil organic carbon (SOC) content in topsoil was lowest in grazing land, indicating a possibility of higher C sequestration with this land use than with other land uses. There was a decreasing trend of CO₂ emission with soil depth in all land uses, as was also the case with the SOC content. The CO₂ emission was increased by 90, 58, 27 and 23% for Bari, Khet, grazing and forest soil, respectively, with increase in moisture level from 40 to 60% (w/w). The CO₂ release from forest soil went up from 15 to 50 g CO₂ g^–1 dry soil with increase in temperature from 5 to 15 °C and it further increased to 67 g CO₂ g^–1 at 20 °C with estimated Q ₁₀ values of 3.4 and 1.8, respectively. Significantly higher amounts of CO₂ were emitted from all the land use types upon addition of glucose to the soil, indicating high potential of microbial activity. Consumption of CH₄ was more rapid in the soil from 10 to 20 cm depth for all the land use types. There was a 89% increase in CH₄ consumption in forest soil with increase in moisture level from 40 to 60%, while it was decreased by 38, 73, and 40% for Khet, Bari and grazing soil, respectively. Addition of (NH₄)₂SO₄ inhibited CH₄ oxidation in soils of all land uses, indicating a negative effect of N fertiliser input on CH₄ uptake in soil.     

Electrochemical insertion of sodium into graphite in molten sodium fluoride at 1025 °C

The electrochemical insertion of sodium into graphite was studied in molten sodium fluoride at 1025 °C. The results obtained evidenced two mechanisms for sodium insertion into graphite: sodium intercalation between the graphite layers and sodium sorption into the porosity of the material. Subsequent internal rearrangement of inserted sodium occurred, via transference from the pores towards the intercalation sites. In addition, the intercalation compound was found to undergo a fast decomposition process (k = 2.55 × 10⁻⁹ mol s⁻¹). X-ray diffraction analysis was used to confirm the formation of a high stage compound (Na_0.1C₈), the composition of which was consistent with compositions observed in the case of chemical vapor and electrochemical insertion of sodium, during experiments in the sodium perchlorate-ethylene cabonate electrolyte.     

Investigation into the phase formation in the ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> system

Nanopowders of solid solutions with different compositions are prepared in the zirconia-enriched region of the ZrO₂-CeO₂ system. The crystallization of these powders and the formation of the monoclinic, cubic, and tetragonal solid solutions of the composition (Zr_{1 – x} Ce_x)O₂ are investigated. It is found that the unit cell parameters of the solid solutions increase as the cerium content increases. This confirms the fact that cerium ions [r(Ce⁴⁺) = 1.11 ] substitute for zirconium ions [r(Zr⁴⁺) = 0.98 ] in these solid solutions. The average size of crystallites of the solid solutions under investigation increases from 5 to 60 nm in the temperature range 500–1200°C.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Panova, Glushkova, Nefedova.     

Electrolytic oxidation of cuprocyanide electroplating waste waters under different pH conditions

The main purpose of this work was to investigate the electrolytic oxidation of cuprocyanide solution with various total cyanide to copper molar ratios ranging from 2.8 to 20 and under different pH conditions. In strong alkaline solution (pH12), cuprocyanide ions Cu(CN) _n ^/(n–1)– , wheren=2, 3 or 4, are directly electroxidized, and copper oxide precipitates on the anode. Cyanate ions, as well as nitrogen gas, were detected as the products and 0.30–0.43 g mol of total cyanide was destroyed per Faraday. For less alkaline solutions (pH<12), cuprocyanide ions first dissociated to free cyanide ions and then electroxidized. At a pH of 10.5–11.7, cyanate ion and brown azulmin polymer were produced in the anolyte. In the neutral solution (pH=7.0–8.6), carbonate and ammonium ions and azulmin were formed and 0.52–0.56 g mol of total cyanide was destroyed per Faraday. In weak acidic solution (pH=5.2–6.8), oxalate and ammonium ions and white oxamide were produced and 1.01–1.18 g mol of total cyanide were destroyed per Faraday.Nomenclature C _CN molar concentration of total cyanide (kmol m^–3) - C _Cu molar concentration of total copper (kmol m^–3) - C _d equivalent concentration of cyanide destroyed due to the formation of cupric oxide (kg m^–3) - C _f concentration of cyanide destroyed by dissociation of complex ion to free cyanide ion and then electroxidized (kg m^–3) - C _i initial concentration of total cyanide (kg m^–3) - C _t change of total cyanide concentration during electrolysis (kg m^–3) - F Faraday constant (96 487 C mol^–1) - K ₁,K ₂,K ₃ formation constant of dicyanocuprate, tricyanocuprate and tetracyanocuprate ions - R molar ratio of total cyanide concentration to total copper concentration (i.e.C _CN/C _Cu) - W weight of precipitates on electrodes or in anolyte (kg) - angle of incidence     

Development of a model for the anodic behavior of T60 titanium in chlorinated and oxygenated aqueous media. Application to the specific conditions of hydrothermal oxidation (1 MPa<pressure<30 MPa, 20 °C<temperature<400 °C)

This work evaluates the anodic electrochemical behavior of titanium metal in hydrothermal oxidation conditions (up to 400 °C and 28 MPa) in chlorinated media in order to estimate the supercritical water oxidation reactors reliability for the treatment of less than 10% organic-waste waters. The titanium room temperature dissolution mechanism in chlorinated acidic medium (pH<0) is not fundamentally modified by oxygen. Deduced from the ‘current-potential’ and ‘valence-potential’ curves, it is based on four crucial elementary steps leading to two branches: a so-called active branch corresponding to a trivalent dissolution (its effect is inversely proportional to the pH), and a passive branch (TiO₂ oxide formation with a very limited tetravalent dissolution). In hydrothermal oxidation (pH>1), only the second branch is effective. The titanium protection is directly related to the oxide stability in high pH systems. The mechanism model is expressed in terms of ‘current-potential’ laws, which provide kinetic parameters using optimization calculations. The different elementary steps reaction rates were estimated as well as the evolution of the reaction intermediates coverage ratios with the potential. The quantification of each elementary step was performed to understand and/or orient the materials behavior according to different factors (pH, chloride ions contents, potentials…).     

Electron paramagnetic resonance of color centers in nanoporous glasses impregnated with copper β-diketonatewith the use of supercritical carbon dioxide

The spectral properties of nanoporous glasses containing Cu²⁺ ions, which are introduced into glass pores in the form of organometallic compound of copper -diketonate dissolved in supercritical CO₂, are investigated for the first time. The analysis of the EPR and optical absorption spectra demonstrates that Cu²⁺ ions are located in octahedral sites with symmetry D_4h. It is revealed that the intensity of the EPR spectrum decreases by one order of magnitude after annealing of the glass at a temperature of 200°C, which correlates with the temperature of decomposition of copper -diketonate molecules (250°C) with the formation of CuO or metallic copper. This process is accompanied by a change in the spectrum shape and the spin-Hamiltonian parameters, even though these parameters also correspond to the octahedral environment with symmetry D^4hIt is assumed that the remaining Cu²⁺ ions form new complexes with fragments of decomposed molecules. According to the optical data, the inference is made that high-temperature annealing leads to the formation of copper metal nanoparticles in the glass.Original Russian Text Copyright © 2004 by Fizika i Khimiya Stekla, Bagratashvili, Bogomolova, Jachkin, Krasilnikova, Rybaltovskii, Tsypina, Chutko.     

Validation of a population balance model for olivine dissolution

The dissolution of silicate minerals, among them olivine, in water enables its subsequent reaction with carbon dioxide to form magnesium carbonate, a process called aqueous mineral carbonation. A general model for the dissolution of olivine, based on a population balance approach, has been developed. For this purpose, the dissolution rate of olivine has been measured as a function of varying particle size and pH at . Three separate particle populations in three different size ranges were used: a sub 90, a 90-180, and a 180- size fraction. The pH was varied between 2 and 4.75 using HCl. Experiments were carried out in a flow-through reactor under a nitrogen atmosphere of 20 bar. The dissolution extent varied from 12% up to complete dissolution, depending on experimental conditions. Particle size distributions of the different size fractions were measured with a Coulter Multisizer^®. Using the assumption of a surface controlled reaction, the solution to the population balance equation was coupled with a reactor model. Data were fitted to the model to obtain a shape modulated dissolution rate, , combining the volume shape factor k_v with the dissolution rate D, which has the dimensions of a velocity. Including earlier published dissolution experiments an overall correlation for the dissolution rate was regressed. Using the general model, the limitations of the simplified model employed in an earlier publication are illustrated.     

Interaction between BaCO3 and OPC/BFS composite cements at 20 °C and 60 °C

A BaCO₃ slurry, containing radioactive ¹⁴C, is produced during the reprocessing of spent nuclear fuel. This slurry is encapsulated in a Portland-blastfurnace slag composite cement. The effect of BaCO₃ on the hydration of OPC and Portland-blastfurnace slag cements has been studied in this work. Samples containing a simulant BaCO₃ slurry were cured for up to 720 days at 20 and 60 °C and analysed by XRD, SEM(EDX) and ICC. BaCO₃ reacted with OPC to precipitate BaSO₄ from a reaction between soluble sulfate and BaCO₃. Calcium monocarboaluminate subsequently formed from the carbonate released. The monocarboaluminate precipitated as crystals in voids formed during hydration. At 60 °C in OPC, it was not identified by XRD, suggesting the phase is unstable in this system around this temperature. In the Portland-blastfurnace slag cements containing BaCO₃, less monocarboaluminate and BaSO₄ were formed, but the hydration of BFS was promoted and monocarboaluminate was stable up to 60 °C.     

Effect of carbon dioxide on the selectivities obtained during the partial oxidation of methane and ethane over Li+/MgO catalysts

AtT 650 °C carbon dioxide either formed during reaction or added to the system increases the selectivity for the desired hydrocarbon products during the oxidative coupling of methane and the oxidative dehydrogenation of ethane reaction over Li⁺/MgO catalysts. Similarly, CO₂ inhibits secondary reactions of CH₃-radicals with the surface of the Li⁺/MgO. The improved selectivities are attributed to the poisoning effect that CO₂ has on the secondary reactions of alkyl radicals with the surface.     

Acid‐Resistant Coatings on Marble

Inorganic coatings are being developed to protect marble monuments and sculpture from weathering. In this work, the acid resistance of hydroxyapatite (HAP), calcium oxalate, and calcium tartrate coatings on Carrara marble were compared. To quantify the rate of attack on calcite, the pH of the solution was measured. This approach was validated by confirming that the rate of dissolution of untreated calcite inferred from the change in pH agrees with data in the literature. Calcium tartrate coatings were incomplete, and the mineral is so soluble that it offered no significant protection. Calcium oxalate forms coherent coatings, so it serves as a sacrificial coating in spite of having solubility comparable to that of calcite. HAP was deposited from aqueous solutions of 1M diammonium hydrogen phosphate (DAP), with or without millimolar additions of CaCl₂ (which improved coverage) and (NH₄)₂CO₃ (which resulted in cracking). The best HAP coatings remained porous; nevertheless, they were comparable to oxalate coatings in the short term and superior in the long term, reducing the dissolution rate by about 40%.     

Stability and reactivity of thaumasite at different pH levels

Thaumasite (CaCO₃·CaSO₄·CaSiO₃·15H₂O) has been reported to form at low temperatures (below 15 °C) during sulfate attack. Reactions between calcium silicate hydrate (C-S-H) and Ca⁺², CO₃⁻², SO₄⁻², CO₂ and water, or between ettringite and C-S-H, CO₃⁻² and/or CO₂ and water, result generally in the formation of thaumasite. In some instances, thaumasite may be affected by the presence of other chemicals in the surrounding environment (i.e., phosphates and ammonia in agricultural soil). There are insufficient data regarding the stability of thaumasite at different pH levels in the presence of other chemical ions. Understanding this issue might help in the detection of the thaumasite form of sulfate attack, and, therefore, in one's choice of the appropriate protection technique. This work reports the reactivity of thaumasite with phosphate, carbonate and bicarbonate ions at different pH levels ranging from 6.00 to 12.00, as well as the stability of thaumasite at high pH levels (greater than 12.00). Thaumasite was found to react with these ions at pH levels at and below 12.00; however, thaumasite was stable with minimal reactivity at pH levels greater than 12.00.     

Temperature dependence, 0 to 40 °C, of the mineralogy of Portland cement paste in the presence of calcium carbonate

Thermodynamic calculations disclose that significant changes of the AFm and AFt phases and amount of Ca(OH)₂ occur between 0 and 40 °C; the changes are affected by added calcite. Hydrogarnet, C₃AH₆, is destabilised at low carbonate contents and/or low temperatures < 8 °C and is unlikely to form in calcite-saturated Portland cement compositions cured at < 40 °C. The AFm phase actually consists of several structurally-related compositions which form incomplete solid solutions. The AFt phase is close to its ideal stoichiometry at 25 °C but at low temperatures, < 20 °C, extensive solid solutions occur with CO₃-ettringite. A nomenclature scheme is proposed and AFm-AFt phase relations are presented in isothermal sections at 5, 25 and 40 °C. The AFt and AFm phase relations are depicted in terms of competition between OH, CO₃ and SO₄ for anion sites. Diagrams are presented showing how changing temperatures affect the volume of the solid phases with implications for space filling by the paste. Specimen calculations are related to regimes likely to occur in commercial cements and suggestions are made for testing thermal impacts on cement properties by defining four regimes. It is concluded that calculation provides a rapid and effective tool for exploring the response of cement systems to changing composition and temperature and to optimise cement performance.     

Impedance spectroscopy of the Ti(IV)/Ti(III) redox couple in the molten LiCl-KCl eutectic melt at 470°C

The electrochemical oxidation of Ti³⁺ ions in LiCl-KCl eutectic melt at 470°C occurs in two different ways according to the current densities. In our experimental conditions (concentration of Ti³⁺ ions 0.11 mol kg^–1) for potentials lower than –0.48 V (vs chlorine electrode) and current densities lower than 10 mA cm^–2 soluble Ti⁴⁺ ions are formed, whereas, for higher potentials solid K₂ TiCl₆ precipitates at the electrode resulting in a partial passivation of the electrode. Both reactions have been studied by means of impedance measurements which have shown the reversibility of the electrochemical reaction (k ₀=0.25 cm s^–1; =0.3) and the porosity of the K₂ TiCl₆ layer electrochemically formed. The thickness of this layer has been estimated from the amount of electricity involved in the reaction and from the resistance and the capacitance characterising the deposit, measured at high frequencies. This comparison shows that the deposit is composed of a very thin (0.1 m) barrier layer covered by a very thick (70 m) porous external layer.Paper presented at the EUCHEM Conference on Molten Salts 1988, St Andrews, Scotland, 6 July 1988.     

Long service life IrO2/Ta2O5 electrodes for electroflotation

Ti/IrO₂-Ta₂O₅ electrodes prepared by thermal decomposition of the respective chlorides were successfully employed as oxygen evolving electrodes for electroflotation of waste water contaminated with dispersed peptides and oils. Service lives and rates of dissolution of the Ti/IrO₂-Ta₂O₅ electrodes were measured by means of accelerated life tests, e.g. electrolysis in 0.5M H₂SO₄ at 25°C and j = 2 A cm^–2. The steady-state rate of dissolution of the IrO₂ active layer was reached after 600–700 h (0.095 g Ir h^–1 cm^–2) which is 200–300 times lower than the initial dissolution rate. The steady-state rate of dissolution of iridium was found to be proportional to the applied current density (in the range 0.5–3 A cm^–2 ). The oxygen overpotential increased slightly during electrolysis (59–82 mV for j = 0.1 A cm^–2 ) and the increase was higher for the lower content of iridium in an active surface layer. The service life of Ti/IrO₂ (65 mol%)-Ta₂O₅ (35 mol%) in industrial conditions of electrochemical devices was estimated to be greater than five years.List of symbols a constant in Tafel equation (mV) - b slope in Tafel equation (mV dec^–1) - E potential (V) - f mole fraction of iridium in the active layer - j current density (A cm^–2) - l number of layers - m _Ir content of iridium in the active layer (mg cm^–2) - r dissolution rate of the IrO₂ active layer (g Ir h^–1 cm^–2) - T _c calcination temperature (°C) - _{O 2} oxygen overpotential (mV) - _{O 2} difference in oxygen overpotential (mV) - _A service life in accelerated service life tests (h) - _S service life in accelerated service life tests related to 0.1 mg Ir cm^–2 (h) - _p polarization time in accelerated service life tests (h)     

Concentration effects of hydrochloric acid on the anodic behaviour of lead sulphide

The anodic dissolution of lead sulphide is studied at various chloride concentrations and at different pH values. At 25° C it is found that in hydrochloric acid the dissolution rate reaches a maximum around 3.0 mol dm^–3. It has also been observed that at concentrations between 0.7 and 1.2 mol dm^–3, a crystalline sulphur deposit formed during the dissolution process leads to an independent peak on theI-E curve whereas at higher concentrations it merges with the PbCl₂ peak formation. pH has no significant effect on the dissolution rate. The results of a systematic study on the kinetics of the dissolution process as a function of concentration, temperature and pH are discussed.