Degradation of 4,6-dinitro-o-cresol from water by anodic oxidation with a boron-doped diamond electrode

Anodic oxidation of 4,6-dinitro-o-cresol (DNOC) has been studied in a cell of 100 ml with a boron-doped diamond anode and a graphite cathode, both of 3-cm² area. Solutions containing up to approximately 240 mg l⁻¹ of compound in the pH range 2.0-12.0 have been treated at 100, 300 and 450 mA between 15 and 50 °C. Total mineralization is always achieved due to the great amount of hydroxyl radical (OH) produced as oxidant on the anode surface. Total organic carbon is more rapidly removed in acid medium, being the optimum pH 3.0. The degradation rate increases when temperature, current and DNOC concentration increase. However, at 100 mA depollution becomes more effective from 71 mg l⁻¹ of initial pollutant. A pseudo first-order kinetics for DNOC decay is always found by reversed-phase chromatography, with a rate constant practically independent of pH, as expected if the same electroactive species is oxidized in all media. Ion-exclusion chromatography allowed the detection of oxalic acid as the ultimate carboxylic acid. The mineralization process leads to the complete release of NO₃⁻ ions from the destruction of nitroderivative intermediates. These products are oxidized simultaneously with accumulated oxalic acid up to the end of electrolyses. Comparative treatment of the same solutions with a Pt anode yields a quite poor depollution because of the generation of much lower amounts of reactive OH on its surface.     

Electrochemical incineration of indigo textile dye in filter-press-type FM01-LC electrochemical cell using BDD electrodes

This work shows results obtained in the incineration of 1 mM indigo textile dye (536 ppm COD) in 0.05 M NaCl aqueous media (which resembles a denim laundry industrial wastewater). Microelectrolysis and macroelectrolysis studies indicated that oxidation of indigo dye was carried out via hydroxyl radicals (OH) formed by water oxidation on the BDD surface, instead of active chlorine as usually occurs by using DSA. Electrolyses in a FM01-LC reactor was performed at Reynolds between 1600 < Re < 18300, and a fixed current density of 5.3 and 15 mA cm⁻². The experimental set-up achieved 100% efficiency in color removal, indigo mineralization and current efficiency. Estimated energy consumption, at Re = 12892 and J = 5.3 mA cm⁻², was 9 kWh m⁻³. Experimental data revealed that hydrodynamic conditions do not influence either the indigo degradation rate or the current efficiencies; therefore indigo degradation must involve a complex mechanism.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Inactivation of Escherichia coli in Na2SO4 electrolyte using boron-doped diamond anode

Electrochemical disinfection in chloride-free electrolyte has attracted more and more attention due to advantages of no production of disinfection byproducts (DBPs), and boron-doped diamond (BDD) anode with several unique properties has shown great potential in this field. In this study, inactivation of Escherichia coli (E. coli) was investigated in Na₂SO₄ electrolyte using BDD anode. Firstly, disinfection tests were carried on at different current density. The inactivation rate of E. coli and also the concentration of hydroxyl radical (OH) increased with the current density, which indicated the major role of OH in the disinfection process. At 20 mA cm⁻² the energy consumption was the lowest to reach an equal inactivation. Moreover, it was found that inactivation rate of E. coli rose with the increasing Na₂SO₄ concentration and they were inactivated more faster in Na₂SO₄ than in NaH₂PO₄ or NaNO₃ electrolyte even in the presence of OH scavenger, which could be attributed to the oxidants produced in the electrolysis of SO₄²⁻, such as peroxodisulfate (S₂O₈²⁻). And the role of S₂O₈²⁻ was proved in the disinfection experiments. These results demonstrated that, besides hydroxyl radical and its consecutive products, oxidants produced in SO₄²⁻ electrolysis at BDD anode played a role in electrochemical disinfection in Na₂SO₄ electrolyte.     

An electrochemical study of H2O2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H₂O₂ concentration range of 10⁻⁴ to 10⁻² mol dm⁻³. The Tafel slopes for H₂O₂ reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec⁻¹, respectively, independent of pH and the concentration of H₂O₂. Both the reduction and oxidation of H₂O₂ on γ-FeOOH have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH⁻. For both cases the electroactive species is the molecular H₂O₂, not its base form, HO₂⁻. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HOOH⁻ and HO for the reduction, and HO₂H⁺, HO₂, and O₂⁻ for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH⁻ and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the γ-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H₂O₂ decomposition.     

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH₃CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D⁻). Dissociative electron transfer between RX and D⁻ yields an intermediate radical (R), which further reacts with D⁻ either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on ED/D−°, allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λR/R⁻. The standard reduction potentials obtained for the radicals CH₂CN, CH₂CO₂Et and CH(CH₃)CO₂Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λR/R⁻ (from 122 to 164 kJ mol⁻¹) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R.     

Structure–activity studies of dodecatungstophosphoric acid impregnated bentonite clay catalyst in hydroxyalkylation of p-cresol

Bentonite clay impregnated with dodecatungstophosphoric acid (20% DTP/BNT) showed an excellent activity, selectivity and stability [95% product yield with 94% selectivity to 2, 2′-methylenebis (4-methylphenol), DAM] for the hydroxyalkylation of p-cresol with formaldehyde at 353 K and for a mole ratio of 5. Ammonia-TPD results showed that an increase in total concentration of acid sites from 4.9 of parent bentonite to 11.6 micromoles per surface area NH₃ (μmolS^− 1 NH₃) of 20% DTP/BNT was due to a strong interaction of protons of bulk DTP with surface hydroxyl groups of BNT as evidenced by ³¹P NMR studies.     

Spectroelectrochemical study of perchloroethylene reduction at copper electrodes in neutral aqueous medium

The potential-dependent chemical reaction of perchloroethylene (PCE) on copper in neutral noncomplexing aqueous media is explored by means of surface-enhanced Raman spectroscopy (SERS), linear sweep voltammetry and preparative electrolysis at controlled potential. Voltammetric peaks associated with copper oxide reduction in Na₂SO₄ solution in the presence and the absence of Cl⁻ are correlated with simultaneously acquired SER spectra. Perchloroethylene undergoes a dechlorination process at potentials at E ≤ −0.3 V vs. Ag/AgCl/KCl (3 M), as shown by the emergence of an intense CuCl stretching band at 290 cm⁻¹ and a CH stretching band together with the presence of Cl⁻ in the catholyte. In the potential region between 0 and −0.9 V vs. Ag/AgCl/KCl (3 M) a broad band assigned to CC structures is observed in the triple-bond region (∼1900 cm⁻¹, FWHM = 180 cm⁻¹). In addition, dichloroethylene (DCE) is detected (but not trichloroethylene (TCE)) in this potential region during preparative electrolysis. At potentials lower than −1 V vs. Ag/AgCl/KCl (3 M) carbon residues are the main product, detected on the copper surface by SERS (and confirmed by XPS), whereas in solution higher levels of dichloroethylene and trichloroethylene are detected with a DCE/TCE ratio below 1.     

Direct synthesis of H2O2 acid solutions on carbon cathode prepared from activated carbon and vapor-growing-carbon-fiber by a H2/O2 fuel cell

Direct synthesis of H₂O₂ acid solutions was studied using a gas-diffusion cathode prepared from activated carbon (AC), vapor-growing-carbon-fiber (VGCF) and poly-tetra-fluoro-ethylene (PTFE) powders, with a new H₂/O₂ fuel cell reactor. O₂ reduction to H₂O₂ was remarkably enhanced at the three-phase boundary (O₂(g)-electrode(s)-acid(l)) at the [AC + VGCF] cathode. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk acid solutions were essential for H₂O₂ accumulation. Synergy of AC and VGCF was observed for the H₂O₂ formation. RRDE and cyclic voltammetry studies indicated that the surface of AC functioned as the active phase for O₂ reduction to HO₂, and VGCF functioned as an electron conductor and a promoter to convert HO₂ to H₂O₂. A maximum H₂O₂ concentration of 353 mM (1.2 wt%) was accomplished under short-circuit conditions (current density 12.7 mA cm⁻², current efficiency 40.1%, geometric area of cathode 1.3 cm², reaction time 6 h).     

In situ ESR spectroscopic evidence of the spin-trapped superoxide radical, O2, electrochemically generated in DMSO at room temperature

Superoxide radical (O₂⁻) was both electrochemically generated and detected at room temperature. In situ ESR spectroelectrochemistry with spin trapping was used for the radical detection. That is, the O₂⁻ radical was obtained in a DMSO solution under cyclic voltammetry conditions as soon as the potential of the dioxygen electroreduction was reached. This radical reacted then with a 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin-trap reagent present in solution to form the DMPO-OOH adduct. The hyperfine coupling constants determined for the adduct were a_N = 1.285 mT, and in accord to those reported in literature.     

Diffusivity of anion vacancies in WO3 passive films

The WO₃ films were grown in 0.1 M HClO₄ aqueous solution, at different formation potentials (E_f) in the range of 2.0-7.0 V versus sce, on W electrode. The anion diffusion coefficient (DO) of WO₃ films was calculated from EIS spectra, following the surface charge approach (at high-field limit approximation), the Point Defect Model and the Mott-Shottky analysis. Among the parameters necessary to evaluate DO, the half-jump distance (a) is very relevant, given that a small variation in a has a great impact in the calculation of DO. In this work, it is proposed the half-jump distance (a) should be evaluated from spectroscopic data (available in the literature). The value of a (∼1.9 Å) is taken from lattice constants of a-WO₃ (amorphous-WO₃), with different values of N (coordination number), and the lattice constants of m-WO₃ (monoclinic-WO₃). The calculated value of DO was ∼3 × 10⁻¹⁷ cm²/s.     

A new method for characterizing the growth and properties of polyaniline and poly(aniline-co-o-aminophenol) films with the combination of EQCM and in situ FTIR spectroelectrochemisty

A new method of in situ piezoelectric Fourier transform infrared (FTIR) spectroelectrochemisty, i.e., the combination of in situ FTIR and electrochemical quartz crystal microbalance (EQCM), was developed to study the electropolymerization of aniline and aniline-co-o-aminophenol, to investigate the properties of the polymers in 0.2 M HClO₄. The piezoelectric electrochemical studies showed that the copolymerization process was changed in the presence of o-aminophenol and the copolymer exhibited different electrochemical behaviors from polyaniline and poly-o-aminophenol. The effects of the molar ratio of o-aminophenol on the copolymerization speed and the scan rate or pH values on the electroactivity of the copolymer were also investigated. The results suggested that the copolymer formed in the case of F = 0.1 had good stability and electroactivity than polyaniline at different pH values. The results obtained by the way of in situ piezoelectric FTIR spectroelectrochemisty indicated that the copolymerization process and the properties of the copolymer were different from that of polyaniline. The polymerization mechanisms and the structure of the two polymers were also different from each other. The copolymer formed through head-to-tail coupling of the two monomers via NH groups was a new polymer rather than a mixture of polyaniline and poly-o-aminophenol.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.     

Influence of metal coordination on conductivity behavior in poly(butadiene-acrylonitrile)-CoCl2 system

The metal complex formation and the electrical properties of amorphous solid polymer electrolytes, based on poly(butadiene-acrylonitrile) copolymer (PBAN) and CoCl₂, have been studied over the homogeneity region of the system limited by the CoCl₂ concentration of 1.89 mol kg⁻¹. It has been found that ionic conductivity is carried out by the unipolar anion transfer at lower CoCl₂ concentrations (up to 0.10 mol kg⁻¹). As the CoCl₂ concentration increases, electronic conductivity appears in addition to ionic conductivity, and the former becomes dominant, starting from 0.38 mol kg⁻¹. It has been shown that the nature of charge carriers is determined by the composition of metal complexes formed by CoCl₂ and the macromolecular solvent PBAN. At lower concentrations, the [Co₂L₂Cl₄]⁰ dimers are the predominant species (L being macromolecule side groups CN), and their dissociation is followed by the formation of mobile Cl⁻ anions and immobile binuclear [Co₂Cl₃]⁺ complexes. As CoCl₂ concentration increases, polynuclear [Co_nL₂Cl_2n]⁰ (n > 2) complexes appear (L being CN and CC groups of PBAN). Specific features of chemical bonds in π-complexes of transition metals result in the appearance of electronic charge carriers. The abrupt increase in conductivity observed at the highest CoCl₂ concentration is connected with the formation of a percolation network of polynuclear [Co_nL₂Cl_2n]⁰ complexes.     

Ionic conductivity, infrared and Raman spectroscopic studies of 1-methyl-3-propylimidazolium iodide ionic liquid with added iodine

Polyiodides (I_x⁻, x = 3 and 5) and 2I⁻…I₂ adducts were established from the Raman spectra study of 1-methyl-3-propylimidazolium iodide (MPIm⁺I_x⁻; 1 ≤ x ≤ 5) ionic liquids containing various amounts of iodine (0 mol ≤ I₂ ≤ 2 mol). The existence of I₃⁻ and 2I⁻…I₂ was established for 1 ≤ x ≤ 2.5, symmetric I₃⁻ ions for x = 3, while linear and discrete I₅⁻ was substantiated for 3 ≤ x ≤ 5. The presence of polyiodide species in MPIm⁺I_x⁻ (1 ≤ x ≤ 5) was correlated with an enhanced ionic conductivity, attributed to the established relay-type Grotthus mechanism. Two-step conductivity increase was also reflected in decrease of the hydrogen bond interactions between the CH ring groups and polyiodides. While in the concentration range 1 ≤ x ≤ 3 (triiodides and tetraiodides) IR bands changed only slightly in intensity, in the concentration range x > 3 the CH stretching bands (3040-3170 cm⁻¹) split and the new band at 1585 cm⁻¹ appeared in the IR spectra beside the already existing Im⁺ ring stretching mode at 1566 cm⁻¹.     

Structure of chemically prepared poly-(para-phenylenediamine) investigated by spectroscopic techniques

The structure of chemically prepared poly-p-phenylenediamine (PpPD) was investigated by Resonance Raman (RR), FTIR, UV-VIS-NIR, X-ray photoelectron (XPS), X-ray Absorption at Nitrogen K edge (N K XANES), and Electron paramagnetic Resonance (EPR) spectroscopies. XPS, EPR and N K XANES data reveal that polymeric structure is formed mainly by radical cations and dication nitrogens. It excludes the possibility that PpPD chains have azo or phenazinic nitrogens, as commonly is supposed in the literature. The RR spectrum of PpPD shows two characteristic bands at 1527 cm⁻¹ and 1590 cm⁻¹ that were assigned to νCN and νCC of dication units, respectively, similar to polyaniline in pernigraniline base form. The presence of radical cations was confirmed by Raman data owing to the presence of bands at 1325/1370 cm⁻¹, characteristic of νC-N of polaronic segments. Thus, all results indicate that PpPD has a doped PANI-like structure, with semi-quinoid and quinoid rings, and has no phenazinic rings, as observed for poly-o-phenylenediamine.     

Di-ureasil xerogels containing lithium bis(trifluoromethanesulfonyl)imide for application in solid-state electrochromic devices

In this study we report the characterization of a prototype solid-state electrochromic device based on poly(ethylene oxide) (PEO)/siloxane hybrid networks doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The polymer networks prepared, designated as di-ureasils and represented as d-U(2000), were produced by a sol-gel procedure and are composed of a siliceous framework to which both ends of polyether chains containing about 40 CH₂CH₂O units are covalently bonded through urea linkages. Samples with compositions of 200 ≥ n ≥ 0.5 (where n is the molar ratio of CH₂CH₂O to Li⁺) were characterized by thermal analysis, complex impedance measurements and cyclic voltammetry at a gold microelectrode. Electrolyte samples were obtained as self-supporting, transparent, amorphous films and at room temperature the highest conductivity was observed with the d-U(2000)₃₅LiTFSI composition (3.2 × 10⁻⁵ Ω⁻¹ cm⁻¹). We report the results of preliminary evaluation of these polymer electrolytes as multi-functional components in prototype electrochromic displays. Device performance parameters such as coloration efficiency, optical contrast and image stability were also evaluated. The electrolytes with n > 8 presented an optical density above 0.56 and display assemblies exhibited good open-circuit memory and stable electrochromic performances.     

Voltammetric detection of the interactions between RNO2 and electron acceptors in aqueous medium at highly boron doped diamond electrode (HBDDE)

This paper describes the electrochemical behavior of the nitrofurazone (NFZ), in predominantly aqueous medium, in the absence and presence of glutathione (reduced form) (GSH), l-cysteine (Cys) and O₂ using a highly boron doped diamond electrode (HBDDE). In presence of [Thiol] ≥ 3.7 × 10⁻² mol L⁻¹ NFZ is directly reduced to RNO-Thiol adducts in an electrochemical process involving two electrons and two protons. On the other side, O₂ acts as a RNO₂^•− scavenger and the velocity constant for the reaction, kO₂, is 60 L mol⁻¹ s⁻¹. The process is catalytic and can be used to the analytical determination of NFZ in the range of 9.9 × 10⁻⁷ ≤ [NFZ] ≤ 1.1 × 10⁻⁵ mol L⁻¹ at pH 8.0, with sensitivity of 2.2 × 10⁶ μA mol⁻¹ cm⁻² and detection limit of 3.4 × 10⁻⁷ mol L⁻¹. The analytical parameters were similar to those obtained at pH 4.0 using the direct reduction of NFZ to the respective amine derivative in a process involving six electrons and six protons. The characterization of NFZ global reduction process in aqueous medium and at relative low scan rate, 100 mV s⁻¹, was only possible due the intrinsic superficial characteristics of the HBDDE, which stabilize the RNO₂⁻ free radical, allowing to work in a large potential window, without losing the RNO₂⁻ oxidation signal.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.