Oxidation of arsenopyrite (FeAsS) in acid Part II: Stoichiometry and reaction scheme

The electrochemical oxidation of arsenopyrite (FeAsS) in 0.01 iv1 chloride solution at pH 2 has been investigated and the effect of electrode potential, temperature and arsenopyrite mineral composition on the reaction stoichiometry studied. Iron, arsenic and sulfur products were formed in the ratio 1 : 1 : 1, for all conditions for arsenic deficient and stoichiometric arsenopyrite. Product speciation was dependent on temperature and potential, but not on arsenopyrite composition. At 25°C, Fe(ii), Fe(iii), As(iii), As(v), S, SO ₄ ^2– and S(x) (which could be a polythionate such as tetrathionate or pentathionate) were formed and 9e^– produced per mol of arsenopyrite oxidized. At 75°C, practically no S(x) was formed and 7.5 e^– produced per mol of arsenopyrite oxidized. A qualitative reaction scheme, based on the decomposition of thiosulfate to polythionate in the presence of As(iii), is outlined.     

Electrosynthesis of a manganese(iii) aminopolycarboxylate complex in alkaline media

Mn(iii)CyDTA [(trans-cyclohexane-1,2-diamine-N,N,N,N-tetraacetato) manganate(iii)] was generated electrochemically from Mn(ii)CyDTA (6–54 mm) in 0.1 m NaHCO₃ at pH 9.0 and 10.5, respectively, 25 °C, for two CyDTA/Mn molar ratios (1/1 and 2/1). A divided batch electrochemical reactor was employed with anode current densities from 2.6 to 102 A m^–2. Separate cyclic voltammetry experiments of Mn(iii)CyDTA in alkaline media showed a prepeak behaviour, indicating the adsorption of Mn(ii) species. The visible anodic deposit, formed during the electrosynthesis of Mn(iii)CyDTA at pH 10.5 and 1/1 CyDTA/Mn molar ratio on stainless steel and PbO₂/Pb, reduces the current efficiency for Mn(iii). For a Mn(ii) concentration of 18 mm and at 13 A m^–2, the graphite and platinized titanium anodes gave a current efficiency for Mn(iii) of 78% and 66%, respectively, without a visible deposit. A 2/1 CyDTA/Mn molar ratio, avoided a visible anodic deposit formation, but gave lower current efficiencies for Mn(iii) than in the case of a 1/1 ligand to metal ratio. The electrosynthesis of Mn(iii)CyDTA is recommended for routine preparation of the complex and is also suitable for in situ electrochemically mediated oxidations in alkaline media (up to pH 11).     

Elution Behavior of Metal Ions with Mixed Glycine-Nitric Acid Eluents in Dowex 50W-X8 Column: Separation of Th(IV), Ce(IV), Bi(III), Fe(III), and Al(III)

Abstract

Distribution coefficients (K) determined by the batch technique in acidic glycine media using Dowex 50W-X8 cation exchanger (H⁺?form, 100–200 mesh size) revealed that this medium can effectively be employed to separate a number of tetravalent and trivalent metal ions from bivalent metal ions. In fixed glycine (0.40 M) and varying concentration of nitric acid (0.10 to 1.0 M), a number of mixtures containing two or three metal ions were resolved on columns using about 8 g of exchanger. In 0.40 M glycine-1.0 M HNO₃ medium, Th(IV)/Ce(IV) were separated from Al(III)/Fe(III)/Bi(III)/Co(II)/Ni(II)/Cu(II)/Zn(II)/Cd(II)/Hg(II)/Pb(II)/Ag(I) and also Al(III)/Bi(III) from a number of divalent metal ions. In 0.40 M glycine-0.50 M HNO₃ medium, the resolution of following ternary mixtures were also achieved: Th(IV)/Ce(IV)-Al(III)/Bi(III)-Fe(III)/Co(II)/Ni(II)/Cu(II)/Zn(II)/Cd(II)/Hg(II)/Pb(II)/Ag(I). Th(IV)/Al(III)/Fe(III)/Bi(III) were also separated from other divalent metal ions in 1.60 M glycine-0.50 M HNO₃ medium. The values of K, elution characteristics of metal ions, elution curves, and the results of the resolution of a number of mixtures of metal ions along with standard deviations are reported.     

Selective electrodeposition of metals from simulated waste solutions

Most industrial waste streams contain a number of different heavy metals. The feasibility of selective electrodeposition of cadmium, cobalt and nickel as their pure metals (99%) from binary mixtures has been investigated. Single cation linear sweep voltamograms and nucleation potential measurements have been made and used to predict the optimum potentials and conditions for selective electro-deposition from the mixed-cation solutions. The anions used in this work are SO₄ ^2–, Cl^– and BF₄ . The optimum potential for removal by electrodeposition of Cd from a solution containing Cd(ii) and Co(ii) or Ni(ii) without codepositing Co or Ni has been evaluated from a simple diagram constructed from nucleation potential measurements. It is shown that the [Cd(ii)] remaining in the solution after exhaustive electrolysis at a fixed potential is determined by the Nernst equation, and not by the electrolysis time provided that a critical minimum time has been exceeded.     

Studies on promising cell performance with H2SO4 as the catholyte for electrogeneration of Ag2+ from Ag+ in HNO3 anolyte in mediated electrochemical oxidation process

Electrochemical performance of a divided cell with electrogeneration of Ag²⁺ from Ag⁺ in 6 M HNO₃ anolyte has been studied with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte. This work arose because in mediated electrochemical oxidation (MEO) processes with Ag(II)/Ag(I) redox mediator, HNO₃ is generally used as catholyte, which, however, produces NO _x gases in the cathode compartment. The performance of the cell with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte has been compared in terms of (i) the acid concentration in the cathode compartment, (ii) the Ag⁺ to Ag²⁺ conversion efficiency in the anolyte, (iii) the migration of Ag⁺ from anolyte to catholyte across the membrane separator, and (iv) the cell voltage. Studies with various concentrations of H₂SO₄ catholyte have been carried-out, and the cathode surfaces have been analyzed by SEM and EDXA; similarly, the precipitated material collected in the cathode compartment at higher H₂SO₄ concentrations has been analyzed by XRD to understand the underlying processes. The various beneficial effects in using H₂SO₄ as catholyte have been presented. A simple cathode surface renewal method relatively free from Ag deposit has been suggested.     

Physicochemical and electrocatalytic properties of LaNiO3 prepared by a low-temperature route for anode application in alkaline water electrolysis

LaNi_1–x Fe_xO₃ (x = 0, 0.25, 0.5) has been synthesized by the hydroxide solid solution precursor method for electrochemical characterization as oxygen anode in strongly alkaline medium. Studies were made at the oxide film, which was obtained by the oxide-slurry painting technique. The cyclic voltammetric study showed the formation of a diffusion-controlled quasireversible redox couple, Ni(ii)/Ni(iii), (E ⁰ - 430 ± 10 mV) at the oxide surface in 1 m KOH. The reaction was observed to follow approximately first-order kinetics in OH^– concentration. Values of the Tafel slope ranged between 59 and 86 mV decade^–1 with all the oxide film electrodes. The electrocatalytic activity was found to be greatest with the Ni/LaNi_0.75Fe_0.25O₃ electrode. A comparison was made between the electrocatalytic activities of LaNiO₃ prepared by the hydroxide solid solution precursor and by the hydroxide coprecipitation technique.     

Role of dissolution of Mn(iii) species in discharge and recharge of chemically-modified MnO2 battery cathode materials

Various experimental techniques, including cyclic-voltammetry, constant current discharge and recharge, with a detecting electrode for determination of soluble Mn(iii) intermediate species, were used to study the reaction mechanism involved in the reduction of a chemical modified form of MnO₂ that is multiply rechargeable. The results show how dissolution of Mn(iii) species plays an important role in the overall path of the reduction reaction and also in the rechargeability of the material. Mainly an heterogeneous reduction mechanism is involved; substantial loss of capacity can arise if diffusion of the soluble Mn(iii) species into the bulk electrolyte solution is allowed to take place. In practical cells, this effect must be minimized using cells with starved electrolyte, i.e. with only a small free volume-fraction of KOH electrolyte in the MnO₂-carbon electrode structure. Soluble Mn(iii) species were detected in in situ experiments by means of (a) a spectro-electrochemical procedure, (b) a collector electrode system at which dissolved Mn(iii) was reoxidized to MnO₂, and (c) a rotating ring-disc electrode.     

A sustainable mediated electrochemical process for the abatement of NOx from simulated flue gas by using Ag(I)/Ag(II) redox mediators

Electrochemical removal of NO and NO₂ by using Ag(I)/Ag(II) redox mediator system in nitric acid medium by two-stage scrubbing process was investigated. Experiments were carried out for the complete removal of NO and NO₂ from the stimulated flue gas at room temperature and atmospheric pressure. The process parameters like current density, Ag(I) concentration, HNO₃ concentration, initial concentration of NO, Ag(I) concentration and temperature were studied and optimized. A removal efficiency of >99% was achieved using this sustainable redox process. Ag(II)/Ag(I) can be regenerated and reused for the scrubbing of waste gases continuously and there is no other gases emission during scrubbing.     

A comparison of the electrocatalytic activities of ordered mesoporous carbons treated with either HNO3 or NaOH

Ordered mesoporous carbon (OMC) was treated with HNO₃ or NaOH. The two treated OMCs have many oxygen-containing functional groups. Those treated with HNO₃ have more acidic surface groups than those treated with NaOH. Nicotinamide adenine dinucleotide (NADH) and H₂O₂ were selected as marker molecules for the comparison of the electrocatalytic property of the OMCs. A comparison between the cyclic voltammograms shows that the oxidation peak potential of NADH is 0.614 V at a bare glassy carbon electrode (GCE), 0.205 V at OMC/GCE, 0.223 V at NaOH-treated OMC/GCE, and 0.0 V at HNO₃-treated OMC/GCE (vs. Ag/AgCl). The results indicate that the HNO₃-treated OMC/GCE exhibits the highest electrocatalytic activity for NADH oxidation. Thus, acidic groups rather than other oxygen-containing functional groups, play a very important role in the catalytic activity of OMC.     

Preparation and characterization of thin films of LaNiO3 for anode application in alkaline water electrolysis

LaNiO₃ electrodes were prepared, in the form of thin films on platinum by the methods of spray pyrolysis and sequential coating of mixed metal nitrate solutions followed by thermal decomposition. The films were adherent and of p-type semiconducting. Cyclic voltammetric studies indicated the formation of a quasireversible surface redox couple, Ni(iii)/Ni(ii), on these films before the onset of oxygen evolution in 1 m KOH. The anodic Tafel slopes were 40 and 65 mV decade^–1, on the sprayed LaNiO₃ film and on the film obtained by a layer method, respectively. The reaction order with respect to OH^– was found to be 2.2 on the sprayed oxide film and 1.2 on the layer film. The sprayed oxide film was found to be electrocatalytically more active. It is suggested that the oxygen evolution reaction proceeds on both the film electrodes via the formation of the physisorbed H₂O₂ as an intermediate in the rate determining step.     

Electrolytic oxidation of Ce(III) in nitric acid and sulfuric acid media using a flow type cell

The electrochemical oxidation behavior of Ce(III) in nitric acid and sulfuric acid media with various concentrations and compositions were investigated by cyclic voltammetry (CV) and potentiostatic electrolysis. In nitric acid media, the peak potential separation for the redox reactions of the Ce(III)/Ce(IV) couple shifted to base side with the increasing concentration of nitric acid from 1 to 6 M. The kinetics of the Ce(III)/Ce(IV) couple is rapid in high concentration nitric acid. The formal potential is independent of both proton and nitrate concentrations while the standard rate constant increases with added protons but is independent of nitrate concentration. Constant-potential electrolysis also shows that a high proton concentration is electrochemically favorable for the electron transfer of the Ce(III)/Ce(IV) couple and for a high Ce(IV) yield in nitric acid media. The current efficiency was ca. 75% for the oxidation process of Ce(III) at 298 K. A Ce(IV) yield of ca. 80% was achieved for the electrolysis of 100 mM Ce(III) in 6 M HNO₃ solution. In sulfuric acid media, the peak potential separation for the redox reactions of the Ce(III)/Ce(IV) couple shifted to noble side with rising concentration of sulfuric acid from 0.1 to 2 M and then shifted to base side with further increase in the concentration. A Ce(IV) yield of ca. 95% was achieved for the potentiostatic electrolysis of 100 mM Ce(III) in 3 M H₂SO₄ solution.     

Destruction of aniline by mediated electrochemical oxidation with Ce(IV) and Co(III) as mediators

Mediated electrochemical oxidation has been employed to test the feasibility of treating soluble organic wastes. We report Ce(IV)- and Co(III)-mediated electrochemical oxidation of aniline at various electrodes in acidic media as an example of organic waste. Aniline was oxidized by an electrogenerated electron transfer mediator, Ce⁴⁺ or Co³⁺, in the anolyte and carbon dioxide was produced as a final oxidation product. Carbon dioxide was collected by bubbling through a barium hydroxide solution. When a powerful oxidizing agent, Ce(IV) or Co(III), was used as an electron shuttling mediator, parameters affecting the coulombic efficiency for aniline oxidation were the standard oxidation potentials of the mediators, their concentrations and the reaction temperature. Intermediate species produced during the oxidation of aniline were identified by cyclic voltammetric and absorption spectroscopic measurements.     

Autoxidation of thiols with cobalt(II)phthalocyanine-tetra-sodium sulfonate attached to poly(vinylamine), 6. Immobilized catalysts by crosslinking of poly(vinylamine)

Poly(vinylamine), being both carrier and promotor of the thiol oxidation catalyst cobalt(II)phthalocyanine-tetra-sodium sulfonate (CoPc(NaSO₃)₄), was crosslinked with α,α′-dichloro-p-xylene, thus yielding porous hydrophilic networks. The effects of experimental parameters such as stirring speed, particle size, degree of crosslinking, distribution of catalytic sites (CoPc(NaSO₃)₄) in the catalyst particles, pH, temperature, and thiol concentration were investigated. Reaction rates observed for the immobilized catalyst system appeared to be 4–30 times lower as compared with the water soluble polymeric catalyst system but still higher than those of the polymer free CoPc(NaSO₃)₄ catalyst. At a stirring speed around 3 · 10³ rpm not the mass transfer from the bulk to the catalyst particles but intra-particle diffusion limits the reaction rate. Accordingly, an uncrosslinked polymeric catalyst anchored to silica, with catalytic sites situated close to the particle surface, exhibited comparatively high activity, i.e. only four times lower than the soluble polymeric catalyst. In addition, the heterogeneous catalyst systems showed resemblance in kinetic behaviour with the soluble polymeric thiol oxidation catalyst.     

Homogenous and Heterogeneous Catalytic Activity of Azo-Linked Schiff Base Complexes of Mn(II), Cu(II) and Co(II)

Abstract  

Azo linked Schiff-base[L] complexes of Mn(II)(1), Cu(II)(2) and Co(II)(3) obtained by template method, in the reaction of 4-(benzeneazo) salicylaldehyde with 1,2-propanediamine in the present of metal acetate, respectively. Complexes are used as catalyst for oxidation of cyclohexene with tert-butylhydroperoxide (TBHP); oxidation of cyclohexene catalyzed by these complexes gave 2-cyclohexene-1-one and 2-cyclohexene-1-ol as major products. Conversion of cyclohexene achieved was 95–100% with (1), (2) and (3), with selectivity of 57, 92 and 100% for 2-cyclohexene-1-one, respectively. The encapsulated Cu(II) complex (Cu–NaY) catalyzes the oxidation of cyclohexene using TBHP as oxidant in good yield. (Cu–NaY) under optimized reaction condition gave three reaction products. A maximum of 100% conversion of cyclohexene has been achieved where selectivity of 2-cyclohexene-1-one was 83%.     

Strong effects of counterions on the electrochemistry of poly(N‐methylaniline) thin films

Poly(N‐methylaniline) thin films show different cyclic voltammetric behaviour when cycled in HClO₄, HBF₄, HCl or HNO₃. While in the first two acids the film shows profiles peak potentials similar to those of polyaniline, profiles in HCl and HNO₃ show higher peak potentials for oxidation and a different shape. The free energy for the process, calculated from the oxidation peak potential, shows a linear correlation with the free energy of hydration of the anions present in the test solution. An energy cycle for the oxidation process is proposed to explain the results. The reaction mechanism assumes that anions have to lose their hydration shell to form the polymer salt during electrochemical oxidation of the films. Electropolymerization also depends on the anion present in the solution. While this is possible with low hydration energy anions (ClO₄^?, BF₄^?), it is difficult or impossible when the electrolyte solution contains other anions (Cl^?, NO₃^?) where a higher oxidation potential of the preformed polymer is observed. © 2002 Society of Chemical Industry     

Synthesis and Properties of Fully Triphenylamine-based Polyamides Bearing 3,5-bis(Trifluoromethyl) and/or 3,5-dimethyl Substituents on the Pendent Phenyl Units

Four new fully triphenylamine-based polyamides coded as polyamide (CF₃,CF₃), polyamide (CF₃,CH₃), polyamide (CH₃,CF₃), and polyamide (CH₃,CH₃) were synthesized by the phosphorylation polyamidation reaction from various combinations of 3,5-bis(trifluoromethyl)-4′,4″-dicarboxytriphenylamine, 3,5-dimethyl-4′,4″-dicarboxytriphenylamine, 3,5-bis(trifluoromethyl)-4′,4″-diaminotriphenylamine, and 3,5-dimethyl-4′,4″-diaminotriphenylamine. All the polyamides were amorphous and readily soluble in many common organic solvents and could be solution-cast into transparent, flexible, and strong films with good mechanical properties. They had useful levels of thermal stability associated with high glass-transition temperatures of 268–287°C and 10?wt%-loss temperatures in excess of 500°C. Cyclic voltammograms of the film of polyamide (CH₃,CH₃) on the indium-tin oxide-coated glass substrates exhibited two oxidation redox couples with E_1/2 around 0.82 and 1.29?V vs. Ag/AgCl in tetrabutylammonium perchlorate/acetonitrile solution, accompanied by a color change from pale yellow neutral state to dark green oxidized state. The CF₃-substituted polyamides displayed a higher oxidation potential and less electrochemical stability as compared to the CH₃-substituted analogues.     

Gel-Liquid Extraction and Separation of U(VI), Th(IV), Ce(lll), and Co(ll)

Abstract

The gel-liquid extraction of U(VI), Th(IV), Ce(III), and Co(II) has been investigated in the 0.01 to 2 M HNO₃ range using a gel prepared by swelling styrene divinylbenzene with di-(2-ethylhexyl)phosphoric acid. Obtained results indicate that all of the tested cations can be extracted and that the extraction coefficients increase in the order Ce(III) < Co(II) < Th(IV) < U(VI) and generally decrease with acidity. Under suitable conditions, separation of Th(IV), Ce(III), or Co(II) from U(VI) or of Th(IV) from Ce(III) can be achieved. Kinetic studies indicate that the extraction process is controlled by a progressive shell sorption mechanism.     

Linoleic acid metabolism in the red algaLithothamnion corallioides: Biosynthesis of 11(R)-hydroxy-9(Z),12(Z)-octadecadienoic acid

Incubation of [1-¹⁴C]linoleic acid with an enzyme preparation obtained from the red algaLithothamnion corallioides Crouan resulted in the formation of 11-hydroxy-9(Z),12(Z)-octadecadienoic acid as well as smaller amounts of 9-hydroxy-10(E),12(Z)-octadecadienoic acid, 13-hydroxy-9(Z),11(E)-octadecadienoic acid and 11-keto-9(Z),12(Z)-octadecadienoic acid. Steric analysis showed that the 11-hydroxyoctadecadienoic acid had the (R) configuration. The 9- and 13-hydroxyoctadecadienoic acids were not optically pure, but were due to mixtures of 75% (R) and 25% (S) enantiomers (9-hydroxyoctadecadienoate), and 24% (R) and 76% (S) enantiomers (13-hydroxy-octadecadienoate). 11-Hydroxyoctadecadienoic acid was unstable at acidic pH. In acidified water, equal parts of 9(R,S)-hydroxy-10(E),12(Z)-octadecadienoate and 13(R,S)-hydroxy-9(Z),11(E)-octadecadienoate, plus smaller amounts of the corresponding (E),(E) isomers were produced. In aprotic solvents, acid treatment resulted in dehydration and in the formation of equal amounts of 8,10,12- and 9,11,13-octadecatrienoates. The enzymatic conversion of linoleic acid into the hydroxyoctadecadienoic acids and the ketooctadecadienoic acid was oxygen-dependent; however, inhibitor experiments indicated that neither lipoxygenase nor cytochrome P-450 were involved in the conversion. This conclusion was supported by experiments with¹⁸O₂ and H₂ ¹⁸O, which demonstrated that the hydroxyl oxygen of the hydroxy-octadecadienoic acids and the keto oxygen of the 11-ketooctadecadienoic acid were derived from water and not from molecular oxygen. The term “oxylipin” was introduced recently (ref. 1) as an encompassing term for oxygenated compounds which are formed from fatty acids by reaction(s) involving at least one step of mono- or dixoygenase-catalyzed oxygenation.     

Photoanodic response of iron oxide electrode in aqueous solution and its application to Pb removal under visible light irradiation

The iron oxide electrode was prepared from thermal oxidation of iron at 600 °C for 3 h in air atmosphere. This electrode with the structure of Fe₃O₄ and α-Fe₂O₃ showed the response of photoanodic current to the light with wavelength shorter than 600 nm. The band gap energy of this electrode was 1.99 eV. The onset potential of distinct steady photocurrent and also the flatband potential were 0.80 and 0.09 V vs. Ag/AgCl, respectively, in 0.1 M HNO₃ aqueous solution. The cell consisting of the iron oxide photoanode in HNO₃-Pb(NO₃)₂ and the graphite cathode in H₂SO₄-Ce(SO₄)₂ caused the PbO₂ deposit on the surface of the former electrode due to visible light irradiation without application of voltage. By holding the potential of this electrode at more positive value than 0.90 V, the photoanodic removal rate of Pb²⁺ in HNO₃-Pb(NO₃)₂ solution was higher than that observed when Ce⁴⁺ was used as electron acceptor.     

DISTRIBUTION BEHAVIOUR OF U(VI), Th(IV) AND Pa(V) FROM NITRIC ACID MEDIUM USING LINEAR AND BRANCHED CHAIN EXTRACTANTS

ABSTRACT

The extraction behaviour of 1M solutions of tri-2-ethylhexyl phosphate (TEHP), di-2-ethyl hexyl isobutyramide (D2EHIBA), tri-n-butyl phosphate (TBP) and di-n-hexyl hexanamide (DHHA) in n-dodecane towards U(VI), Th(IV) and Pa(V) in the presence of 220 g/L of Th from nitric acid medium has been studied. The limiting organic concentrations (LOC) of thorium (g/L) for 1 M TBP and 1 M DHHA are evaluated as 31, 20 ( at 1 M HNO₃) and 25,13 (at 4 M HNO₃) respectively. The distribution ratio (D) values of U(VI), Th(IV) and Pa(V) in the presence of thorium (220 g/L) at. 1 M HNO₃ suggest that branching in the alky group of amides suppresses the extraction considerably. In view of the selective extraction of U over Th by 5 % TBP in THOREX process at 4 M HNO₃, distribution behaviour is also studied employing a lower concenfration (0·18 M) of extractant for comparison purpose, Separation factor (S. F.) values for U(VI) over Th(IV) under different experimental conditions consistently varied in the order: D2EHIBA > DHHA > TEHP > TBP. The quantitative extraction of ²³³U from a synthetic mixture containing ²³³U (10^?5 M). ²³³Pa (10^?11 M) and thorium (220 g/L) at 1 M HNO₃ using 1 M solution of D2EHIBA in n-dodecane is achieved in three stages, Stripping and reusability studies of D2EHIBA have also been carried out.