Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)_(aq) + Cr(VI)_(aq) ; 3Fe(III)_(aq) + Cr(III)_(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)_(aq) is supplied by dissolving FeSO₄ · 7(H₂O)_(s) into the wastewater, while in the electrochemical process Fe(II)_(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)_(aq) and Fe(III)_(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)_2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H₂O–e^– and Fe(III)–Fe(II)–H₂O–e^– both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.     

Decomposition of nitrous oxide over the catalysts derived from hydrotalcite-like compounds

Catalytic decomposition of nitrous oxide into nitrogen and oxygen has been carried out on ‘in situ' thermally calcined hydrotalcites of general formula M(II)–M(III)-CHT where M(II)=Mg, Co, Ni, Cu or Zn and M(III)=Al, Fe or Cr having different M(II)/M(III) atomic ratios. The reaction was performed in a recirculatory static reactor at 50 Torr (133.3 Pa) initial pressure of N₂O in the temperature range 423–773 K. Among the catalysts screened, Ni and Co containing catalysts with Al as the trivalent cation showed good activity (even at 423 K) wherein 50% and 100% conversion was achieved at 463 K and 523 K, respectively. Results on effect of composition for Co–Al system indicated that the activity increased with increase in the active metal ion concentration (Co²⁺), with closer dependence on the surface concentration (as determined by XPS). The observed activity pattern is explained on the basis of redox property and electronic effects. These materials were further evaluated under flow conditions (at Air Products and Chemicals, USA) simulating the industrial process streams (10% N₂O, 2% H₂O, 2% O₂ and balance N₂ and GHSV=18,500). Among the non-precious metal ions investigated, cobalt-based catalysts offered comparable activity similar to metal-exchanged zeolites for removing N₂O from combustion and Nylon-6,6 processes.     

Kinetics of Sn electrodeposition from Sn(II)–citrate solutions

The influence of solution chemistry on the electrodeposition of Sn from Sn(II)–citrate solutions is studied. The distribution of various Sn(II)–citrate complexes and citrate ligands is calculated and the results presented as speciation diagrams. At a SnCl₂·H₂O concentration of 0.22 mol/L and citrate concentration from 0.30 mol/L to 0.66 mol/L, SnH₃L⁺ (where L represents the tetravalent citrate ligand) is the main species at pH below about 1.2 and SnHL⁻ is the main species at pH above about 4. Polarization studies and reduction potential calculations show that the Sn(II)–citrate complexes have similar reduction potentials at a given solution composition and pH. However, the Sn(II)–citrate complexes become more difficult to reduce with higher total citrate concentration and higher solution pH. Nevertheless, SnHL⁻ which forms at higher pH is a favored Sn(II)–citrate complex for Sn electrodeposition due to better plated film morphology, likely as a result of its slower electroplating kinetics. Precipitates are formed from the Sn(II)–citrate solutions after adding hydrochloric acid (to lower the pH). Compositional and structural analyses indicate that the precipitates may have the formula Sn₂L.     

An unusual dichromium(II,II) compound bearing di-2-pyridyl ketone oximate ligands and prepared by the ligand-assisted reduction of a trichromium(III,III,III) complex in air

The reaction of the oxide-centred triangular, trichromium(III,III,III) complex [Cr₃O(O₂CCMe₃)₆(H₂O)₃](O₂CCMe₃) with di-2-pyridyl ketone oxime, (py)₂CNOH, in MeCN under aerobic and refluxing conditions yields the pivalate-free, dichromium(II,II) complex [Cr₂{(py)₂CNO}₄] · 2H₂O (1 · 2H₂O). The dinuclear complex can also be prepared by the reaction of [Cr(CO)₆] with (py)₂CNOH in refluxing MeCN/H₂O in air. The two high-spin Cr^II atoms are doubly bridged by two 2.1110 oximate ligands, while a chelating 1.0110 (py)₂CNO⁻ ion completes distorted trigonal bipyramidal coordination at each metal centre. The dimers are stabilized by intramolecular stacking interactions between the terminal (py)₂CNO⁻ ligands, and the structural effects of these interactions are discussed.     

Spectroscopic and Thermal Studies of Mn(II), Fe(III), Cr(III) and Zn(II) Complexes Derived from the Ligand Resulted by the Reaction Between 4-Acetyl Pyridine and Thiosemicarbazide

Transition metal complexes of Mn(II), Fe(III), Cr(III) and Zn(II) metal ions with a general formulas [Mn(L)₂(Cl)₂]·4H₂O (I), [Fe(L)₂(Cl)₂]·Cl·6H₂O (II), [Cr(L)₂(Cl)₂]·Cl·6H₂O (III) and [Zn(L)₂(Cl)₂]·2H₂O (IV) where L = 4-acetylpyridine thiosemicarbazone, have been synthesized and interpreted using CHN elemental analysis, magnetic susceptibility measurements, molar conductance, thermal analysis and spectroscopic techniques; i.e., infrared, electronic UV/vis, ¹H-NMR and mass. The manganese(II), ferric(III), chromium(III) and zinc(II) complexes have octahedral geometry. The molar conductance measurements reveal that the Mn(II) and Zn(II) chelates are non-electrolytes but Fe(III) and Cr(III) have an electrolytic behavior. The IR spectra show that the 4-acetylpyridine thiosemicarbazone free ligand is coordinated to the metal(II) chlorides as a neutral bidentate ligand through both of the lone pair of electrons of the C=N azomethine group and C=S group. X-ray powder diffraction gives an impression that the resulting complexes are amorphous and different from the start materials. The thermogravimetric studies indicate that uncoordinated water molecules are lost in the first and second decomposition steps. The activation thermodynamic parameters E*, ΔH*, ΔS* and ΔG* are estimated from the differential thermogravimetric analysis (DTG) curves using Horowitz–Metzger (HM) and Coats–Redfern (CR) methods. The ligand and its complexes have been screened for antibacterial and antifungal activities against two bacteria; i.e., Escherichia coli (Gram −ve) and Bacillus subtilis (Gram +ve) and two fungi, i.e., tricoderma and penicillium activities).     

Electrodeposited amorphous iron(III) oxides as anodes for photoelectrolysis of water

Deposition of amorphous iron(III)-oxide films on a conducting glass substrate was achieved via a cathodic bias in a 0.1 M hydrated ammonium iron(II) sulfate ((NH₄)₂Fe(SO₄)₂·6H₂O) solution at −1.6 V versus Ag/AgCl. Analysis by X-ray absorption near edge structure confirmed the iron(III) feature of the amorphous films. The deposited films exhibited n-type semiconducting characteristics by showing photoresponses under an anodic bias. The Mott–Schottky method and cyclic voltammetry were employed to characterize the semiconducting properties of the deposited films, which included the band gap (2.2 eV), the potentials of the conduction and valence band edges and flat band (−0.6, +1.6 and −0.58 V versus Ag/AgCl at pH 7, respectively), and the donor density (1 × 10²²/cm³). The deposited iron(III)-oxide films were suitable to serve as an anode for water splitting under illumination.     

A hemidirected 1-D coordination polymer [Pb2(H2O)2(HBTC)2] · 3H2O extended in holodirected 3-D by weak Pb–O interactions

A new complex, [Pb₂(H₂O)₂(HBTC)₂] · 3H₂O (H₃BTC = 1,3,5-Benzenetricarboxylic acid) (1), has been synthesized under hydrothermal condition. The single-crystal analysis shows that 1 consists of 1-D double-chains with Pb(II) six-coordinated by three H₃BTC and one H₂O molecule with the Pb–O bond distances in the range of 2.56–2.76 Å. When the Pb–O bonding limit extends from 2.76 to 2.90 Å, the potential weak bonds of Pb–O can be found and the coordination number of Pb will increase from six to nine. As a result, the coordination geometry of Pb(II) transforms from hemidirected to holodirected and an infinite 3-D framework is obtained by the connection of the double-chains. The IR spectrum and the TGA–DTA curve of 1 are also reported in this paper.     

Highly selective solvent extraction of Zn(II) and Cr(III) with trioctylmethylammonium chloride ionic liquid

This study investigates the recovery of Zn(II) and Cr(III) from aqueous solutions based on solvent extraction with trioctylmethylammonium chloride [TOMA⁺][Cl-], commercialy named Aliquat 336. Single metal solutions and binary mixtures of both metals were considered. The effect of relevant operating conditions such as pH, contact time, initial concentration, O/A phase volumetric ratio, and temperature were evaluated. Additionally, loading capacity and stripping studies were performed. Results showed that [TOMA⁺][Cl^?] is an effective extracting agent for Zn(II), reaching maximum removal capacity at pH 1.8 and demonstrating fast extraction kinetics. Extraction efficiencies above 99% were achieved at 0.5, 0.75, and 1.00 O/A volumetric phase ratios for 0.1 g/L initial Zn(II) concentration. At 1 g/L and 10 g/L concentration, for the same O/A ratios, approximately 88% of the initial Zn(II) was extracted. In contrast, it was found that negligible amounts of Cr(III) were transferred to the [TOMA⁺][Cl^?] phase at the 1-5 pH range. Selectivity studies showed that Zn(II) removal is boosted in the presence of Cr(III), although no Cr(III) is extracted. [TOMA⁺][Cl^?] exhibited a high Zn(II) storage capacity, since after 25 loading cycles with 1 g/L, the loading capacity reached approximately 13.5 g/L, and after five loading cycles with 5 g/L, the capacity reached 19.4 g/L. Stripping tests revealed that NaOH is an efficient agent for the removal of Zn(II) from the ionic liquids, reaching 98.5% removal after two cycles, whereas HNO₃ is not a suitable agent, reaching less than 40% removal after three cycles. [TOMA⁺][Cl^?] revealed high potential for separating Zn(II) from Cr(III).     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Electroplating of cadmium from acidic bromide baths

Earlier work on the electroplating of cadmium from acidic bromide solution containing 0.3M CdBr₂·4H₂O, 0.1M HC1, 0.4M H₃BO₃ and 2.0M KBr (Bath I) has been reviewed and extended to an examination of the influence of the organic additives 5g gelatin 1^–1 and 2.5g melamine 1^–1 (Bath II). The effects of the plating current density, plating time and pH on the cathodic polarization and the current efficiency of cadmium electroplating from Bath II, as well as on the morphology and the microhardness of the as-plated cadmium electrodeposits are discussed. It was observed that the additivecontaining Bath II yields more coherent, brighter and harder cadmium plates than the additive-free Bath I. The optimum operating conditions for obtaining satisfactory plates from Bath II at 25° C are:i=0.6–1.6 A dm^–2;t=10–15 min and pH 3.6–1.9.     

Electrosynthesis of cobalt (III) ions in concentrated H3PO4 medium

The electrosynthesis of cobalt(III) ions from cobalt(II) ions has been studied in 29, 57 and 85 wt % H₃PO₄ solutions. It has been shown that the conversion of Co(II) to Co(III) is limited by the chemical reaction between Co(III) ions and water. A kinetic study demonstrated that this reaction becomes more rapid as the Co(III) ion concentration increases. In order to find the best conditions for the electrolysis, the effect of some experimental parameters on the current yield and the chemical efficiency has been examined. A comparison between gold and platinum anodes has also been made. The following conditions were found to be the best: anode material, gold; initial Co(II) ion concentration, 5×10^–3 M; solvent, 85 wt % H₃PO₄ solution; current density, 1 mA cm^–2; temperature, 20–30° C. Under these conditions the maximum value of current yield, and chemical efficiency are 66% and 48% respectively.

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Résumé L'electrosynthèse des ions cobalt(III) à partir des ions cobalt(II) a été étudiée dans les solutions H₃PO₄ 29, 57, et 85% en masse. II a été montré que la conversion de Co(II) en Co(III) est limitée par la réaction chimique entre les ions Co(III) et l'eau. Une étude cinétique a montré que cette réaction devient plus rapide au fur et à mesure que la concentration de l'ion Co(III) augmente. Dans le but de trouver les conditions favorables pour l'electrolyse, l'effet de certains paramètres expérimentaux, sur les rendements électrique et chimique, a été examiné. Les résultats obtenus avec une anode en or et une anode en platine ont été comparés. Les conditions d'électrolyse suivantes ont été retenues: le matériau de l'anode, Au; concentration initiale de l'ion Co(II), 5×10^–3 M; solvant, solution H₃PO₄ 85% en masse; densité de courant, 1 mA cm^–2; température, 20–30° C Dans ces conditions, les valeurs des rendements faradique et chimique maximum sont respectivement de 66% et 48%.

     

Terminal ligand effect on the structure variation of Copper(II) complexes

Two copper(II)–poly(pyrazolyl)methane complexes [Cu₂(bpm)₄(ta)](ClO₄) ₂ · 2H₂O 1 and {[Cu₂(tpm)₂(ta)₂] · H₂O}_n 2 (bpm = bis(3,5-dimethylpyrazolyl)methane, tpm = tris(pyrazolyl)methane, H₂ta = terephthalic acid) were synthesized and characterized by elemental analysis, FT-IR and UV–vis spectra. 1 is a binuclear complex, while 2 exhibits as a 1D zigzag coordination polymer.     

New Mononuclear Cu(II) Complexes and 1D Chains with 4-Amino-4H-1,2,4-triazole

The crystal structures of two mononuclear Cu(II) NH₂trz complexes [Cu(NH₂trz)₄(H₂O)](AsF₆)₂ (I) and [Cu(NH₂trz)₄(H₂O)](PF₆)₂ (II) as well as two coordination polymers [Cu(μ₂-NH₂trz)₂Cl]Cl·H₂O (III) and [Cu(μ₂-NH₂trz)₂Cl] (SiF₆)_0.5·1.5H₂O (IV) are presented. Cationic 1D chains with bridging bis-monodentate μ₂-coordinated NH₂trz and bridging μ₂-coordinated chloride ligands are present in III and IV. In these coordination polymers, the Cu(II) ions are strongly antiferromagnetically coupled with J = −128.4 cm⁻¹ for III and J = −143 cm⁻¹ for IV (H = −J∑S_iS_i+1), due to the nature of the bridges between spin centers. Inter-chain interactions present in the crystal structures were taken into consideration, as well as g factors, which were determined experimentally, for the quantitative modeling of their magnetic properties.     

Formation, Characterization and Electrical Conductivity of Polycarbosilazane-Cu(II), -Ni(II) and -Cr(III) Chloride Metallopolymers

A series of transition metal-polycarbosilazane complexes have been prepared by the reaction of poly(N,N-bis(dimethylsilyl)ethylenediamine), [–Si(CH₃)₂NHCH₂CH₂NH–] _n , with Cu(II), Ni(II), and Cr(III) chloride. The resulting complexes were characterized by infrared (FT-IR) and UV-visible spectroscopy, magnetic susceptibility measurements, thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). The average chain-chain spacing in these materials were estimated from XRD data and found to be 6.88, 7.91, 7.09, and 6.33 Å in metal-free, Cu(II)-, Ni(II)-, and Cr(III)-containing polycarbosilazanes, respectively. DC electrical conductivity measurements showed that all these metal-polycarbosilazane complexes exhibit semiconductor behavior while the metal-free matrix is an insulator.     

Solvothermal synthesis of 1D and 2D cobalt(II) and nickel(II) coordination polymers with 2,5-dihydroxy-p-benzenediacetic acid

1D cobalt(II) and nickel(II) coordination polymers {[Co(dba)(H₂O)₄] · H₂O}_n (1) and {[Ni(dba)(H₂O)₄] · H₂O}_n (2) (H₂dba = 2,5-dihydroxy-p-benzenediacetic acid) were synthesized under low temperature solvothermal condition. When 4,4′-bipyridine (bpy) was introduced to the synthetic systems of 1 and 2, respectively, two novel 2D coordination polymers {[Co(dba)(bpy)] · 0.5H₂O}_n (3) and [Ni(dba)(bpy)(H₂O)₂]_n (4) with different structures were obtained. All of the compounds were characterized by elemental analysis, FT-IR, UV–Vis spectra and single crystal X-ray diffraction.     

Hexanuclear 4d–4f complexes [Ln2Ag4(ina)8(H2O)10][NO3]2 · 4H2O (Ln = Sm, Eu, Dy): Syntheses, structures and photoluminescent properties

Hexanuclear 4d–4f heterometallic complexes, [Ln₂Ag₄(ina)₈(H₂O)₁₀][NO₃] ₂ · 4H₂O [Ln = Sm (1), Eu (2), Dy (3) and Hina = isonicotinic acid], have been synthesized by the hydrothermal reaction of lanthanide oxides, Ag^I, and isonicotinic acid at a suitable temperature. Single-crystal X-ray diffraction studies indicate that these 4d–4f complexes consist of extended 1D zigzag chains structure built upon [Sm₂Ag₄(ina)₈(H₂O)₁₀] subunits connected by Ag–Ag interactions. Furthermore, the photoluminescent properties of the complex 2 were studied.     

2-Hydroxy-4-Benzamidothiosemicarbazide as a Chelating Reagent. Part I-Studies on its Cobalt(II) and Cobalt(III) Complexes

Three distinct cobalt complexes of 2-hydroxy-4-benzamidothiosemicarbazide, [Co(C₈H₉N₄O₂S) (H₂O)₃] Cl (I), [Co(C₃H₉N₄O₂S) (OH) (H₂ O)₂] (II) and Na [Co(C₈H₉N₄ O₂ S) (OH)₃] H₂ O (III) have been prepared at pH 3.0, 9.0 and 12.5, respectively. Magnetic measurements show subnormal values of the magnetic moment for high-spin octahedral cobaltous complexes. A possible correlation has been sought in nephelauxetic ratio, calculated from electronic spectral data. The values of 10 Dq, L.F.S.E. and Racah's interelectronic repulsion parameters have also been evaluated. The complex (III) is diamagnetic in nature.     

Study of Ti(III) solutions in various molten alkali chlorides. I. Chemical and electrochemical investigation

The effect of alkali metal cations in molten chloride baths on the behaviour of Ti(III) in these melts has been studied. With caesium chloride the percentage of total dissolved titanium present as Ti(II) after 72 hours at 700° C is 0.1%, whereas in lithium chloride it is 2%. In the presence of excess titanium metal the proportion of Ti(II) is always much higher. In LiCl it depends strongly on the area of titanium exposed and may reach 85%. In CsCl, on the other hand, this factor has no effect since a fine dispersion of powdery titanium appears at the very beginning of the reaction. The amount of Ti(II) remains close to 50%. In all the baths studied, the electrochemical reduction of Ti(III) at 700° C occurs in two steps: Ti(III) + e Ti(II)E _1/2 = – 1.6V versus Cl^–/Cl₂ Ti(II) + 2e Ti(O)E _1/2=–2.1V versus Cl^–/Cl₂. These results are different from those previously obtained in CsCl-LiCl (40–60 mol%) at 400° C, where reduction is a one-step process.     

Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part I: Emulsion structure, electrode kinetics and batch electrolysis

The mediated electrosynthesis of H₂O₂ in acidic solutions (pH 0.9–3.0) was investigated in a three-phase, aqueous/organic/gaseous system using 2-ethyl-9,10-anthraquinone (EtAQ) as mediator (redox catalyst). The main hydrogen peroxide producing route is the in situ mediating cycle: EtAQ electroreduction–homogeneous oxidation of anthrahydroquinone (EtAQH₂). The organic phase was composed of tributylphosphate solvent (TBP) with 0.2 M tetrabutylammonium perchlorate (TBAP) supporting electrolyte, 0.06 M tricaprylmethylammonium chloride (A336) surface active agent, and 0.1–0.2 M EtAQ mediator. Part I of this two part work deals with the physico-chemical characteristics of the emulsion electrolyte (e.g., ionic conductivity, emulsion type, H₂O₂ partition between the aqueous and organic phases), and kinetic aspects (both electrode and homogenous) of the mediation cycle. Furthermore, batch electrosynthesis experiments are presented employing reticulated vitreous carbon cathodes (specific surface area 1800 m² m^–3) operated at superficial current densities of 500–800 A m^–2. During 10 h batch electrolysis involving the emulsion mediated system with O₂ purge at 0.1 MPa pressure, H₂O₂ concentrations in the range 0.53–0.61 M were obtained in 0.1 M H₂SO₄ (pH 0.9) and 2 M Na₂SO₄(acidified to pH3). The corresponding apparent current efficiencies were from 46 to 68%. Part II of the present work describes investigations using flow-by fixed-bed electrochemical cells with co-current upward three-phase flow.     

Solely μ1,1-azido-bridged heterometallic compound

Solely end-on azide-bridged bimetallic Cu(II)–Mn(III) tetranuclear compound [Mn(salophen)(H₂O)]₂[Cu₂(N₃)₆]·4H₂O shows strong ferromagnetic coupling, giving rise to a ground spin state of S = 5.