Portugaliae Electrochimica Acta

Special Issue on the IX Iberic Meeting of Electrochemistry

Electrochemical Advanced Oxidation Processes (EAOPs) for Environmental Applications

Conventional processes for water treatment are inefficient for the remediation of wastewaters containing toxic and biorecalcitrant organic pollutants. A large number of advanced oxidation processes (AOPs) have been successfully applied to degrade pollutants present in waters. These methods are based on the generation of a very powerful oxidizing agent such as hydroxyl radical (·OH) in solution, able to destroy organics up to their mineralization. In recent years new AOPs based on the electrochemical technology are being developed. Electrochemical advanced oxidation processes (EAOPs) are environmentally friendly emerging methods for the decontamination of wastewaters contaminated with toxic and persistent herbicides, pesticides, chlorophenols, nitrophenols, polychlorinated biphenyls, pharmaceuticals, etc. This paper reports the fundamentals, main characteristics and recent developments of EAOPs such as anodic oxidation and electro-Fenton alone and coupled with other physicochemical processes. These techniques utilize electrolytic systems such as three-electrode divided and two-electrode undivided cells with different cathodes as working electrodes (carbon-felt or O2-diffusion cathode) and auxiliary electrodes (Pt, PbO2, boron-doped diamond (BDD) or iron anode). The effect of several experimental parameters that largely influence the degradation rate of organic pollutants is discussed. Chromatographic analyses and total organic carbon (TOC) and chemical oxygen demand (COD) measurements show a quick disappearance of initial pollutants and their aromatic and aliphatic reaction products in all cases. The great capacity of oxidation and/or mineralization of all these EAOPs to decontaminate acidic aqueous solutions of common herbicides and pesticides is described.

Mineralization of Drugs in Aqueous Medium by Advanced Oxidation Processes

At present chemical (AOPs) and electrochemical (EAOPs) advanced oxidation processes with ability to destroy organic pollutants in waters are being developed. These methods are based on the production of hydroxyl radical (OH) as oxidant. In AOPs OH radical can be obtained from Fentons reaction between Fe2+ and H2O2 added to the medium, photoreduction of Fe3+ species or reaction between ozone and Fe2+. In EAOPs this radical is formed from water oxidation on the surface of a high O2-overvoltage anode and/or Fentons reaction between Fe2+ added to the medium and H2O2 electrogenerated at the cathode by the two-electron reduction of oxygen. The present work reports the mineralization of several aromatic drugs such as paracetamol, ibuprofen, clofibric acid and salicylic acid by AOPs based on ozonation catalyzed by Fe2+, Cu2+ and/or UVA light and EAOPs like anodic oxidation, electro-Fenton and photoelectro-Fenton, which are environmentally friendly electrochemical methods. For the latter processes, the drug decay with time and the evolution of aromatic intermediates and generated carboxylic acids are studied. Anodic oxidation is only effective when a boron-doped diamond (BDD) anode is used, whereas the photoelectro-Fenton process with Fe2+, Cu2+ and UVA light is the most potent method to completely destroy the drugs. The combined use of catalysts Fe2+, Cu2+ and UVA light in catalyzed ozonation also leads to overall decontamination of drug solutions.

Actualmente se están desarrollando métodos químicos (AOPs) y electroquímicos (EAOPs) de oxidación avanzada que son capaces de destruir contaminantes orgánicos en aguas. Estas técnicas están basadas en la producción del radical hidroxilo (OH) como oxidante. En los AOPs el radical OH se puede obtener a partir de la reacción de Fenton entre el Fe2+ y el H2O2 adicionados al medio, de la fotorreducción de especies de Fe3+ o bien de la reacción entre el ozono y el Fe2+. En los EAOPs dicho radical se forma a partir de la oxidación del agua en la superficie de un ánodo de elevado sobrepotencial de O2 y/o de la reacción de Fenton entre el Fe2+ añadido al medio y el H2O2 electrogenerado en el cátodo por la reducción bielectrónica del O2. El presente trabajo describe la mineralización de algunos fármacos aromáticos como el paracetamol, el ibuprofeno, el ácido clofíbrico y el ácido salicílico usando AOPs basados en la ozonización catalizada por Fe2+, Cu2+ y/o luz UVA y EAOPs como la oxidación anódica, el electro-Fenton y el fotoelectro-Fenton, que son métodos electroquímicos compatibles con el medio ambiente. Para los últimos procesos, se ha estudiado la desaparición de los fármacos con el tiempo y la evolución de los intermedios aromáticos y de los ácidos carboxílicos generados. La oxidación anódica sólo es efectiva cuando se utiliza un ánodo de diamante dopado con boro (BDD), mientras que el proceso fotoelectro-Fenton con Fe2+, Cu2+ y luz UVA es el método más potente para destruir totalmente los fármacos. El uso conjunto de los catalizadores Fe2+, Cu2+ y luz UVA en la ozonización catalizada también conduce a la descontaminación total de las disoluciones de fármacos.

Electrostatic Effects in Biosorption: The Role of the Electrochemistry

Biosorption is the passive sequestration of pollutants by non-metabolizing non-living biomass. The technique emerged in 1980s as a possible alternative method for wastewater treatments in an environmentally friendly manner. In this paper it is shown that a basic research in biosorption processes is closely related to the field of Electrochemistry according to the conceptual division of this area in Ionics and Electrodics.

Electroactive Properties and Electrochemical Stability of Poly(3,4-ethylenedioxythiophene) and Poly(N-methylpyrrole) Multi-layered Films Generated by Anodic Oxidation

Multi-layered films of poly(3,4-ethylenedioxythiophene), and poly(N-methylpyrrole), have been prepared using a layer-by-layer electrodeposition technique. The electrochemical properties and the conductivity of the films formed by 3, 5 and 7 layers have been compared with their homopolymers and with the copolymers prepared from 3,4-ethylenedioxythiophene and N-methylpyrrole with different concentration ratios. The electroactivity and stability of the multi-layered systems were higher than those of PEDOT homopolymer and copolymers. Furthermore, the electrochemical properties improved when the number of layers increased. In addition, the electrical conductivity of the multi-layered films is higher than those of poly(N-methylpyrrole) and copolymers, and comparable to the PEDOT homopolymers. These results indicate that multi-layered systems show better performance than their homopolymers and/or copolymers derivatives for some applications like anticorrosive coatings or sensor materials.

Preparación y Caracterización de un Electrodo Compósito de PVC/TTF-TCNQ: Aplicaciones en un Sistema de Inyección de Flujo

It was prepared a new polyvinyl chloride (PVC)/tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) composite electrode which showed good physical and electrochemical characteristics. Different PVC/TTF-TCNQ-graphite proportions were tested and the relation 1/10 (p/p) PVC/TTF-TCNQ provided the best results in terms of sensitivity, coefficients of variation and mechanical resistance. This electrode has interesting applications as a working electrode. So, employment is remarkable in a flow injection system and had been satisfactory checked to detect ascorbic acid. Also, the electrode exhibits selective voltammetric behaviour in aqueous solution that affords its use as an ion detector. The proposed electrode showed good kinetics property with a low background current and a relatively reproducible signal. Moreover, the electrode can be prepared in a simple way and its surface easily recovered.

Se ha preparado un electrodo compósito de cloruro de polivinilo (PVC)/tetratiofulvaleno-tetracianoquinodimetano (TTF-TCNQ) que presenta buenas características físicas y electroanalíticas. El estudio con distintas proporciones de PVC/TTF-TCNQ-grafito ha mostrado que la relación 1/10 (p/p) PVC/TTF-TCNQ es la que mejores resultados ofrece en términos de sensibilidad, coeficientes de variación y de resistencia mecánica. Este electrodo presenta interesantes aplicaciones como electrodo de trabajo. Así, destaca su empleo en sistemas de inyección de flujo, habiéndose aplicado satisfactoriamente a la determinación de ácido ascórbico. También ha mostrado su utilidad como sensor voltamperométrico de iones. El electrodo propuesto presenta buenas propiedades cinéticas, con una baja corriente de fondo y una señal relativamente reproducible. Además, puede ser preparado de una manera simple, y su superficie es fácilmente regenarable.

Modification of the Pet-Membrane/Solution Interface: Effect on Electrical Parameters

Electrochemical characterization of a symmetric track-etched polyethylenthereftalate (PET) membrane in contact with KCl solutions at different concentrations was carried out by measuring impedance spectroscopy (IS) and membrane potential (MP), which allow the estimation of membrane electrical resistance and ion transport numbers, respectively. IS measurements permit us the characterization of membranes in "working conditions" (in contact with electrolyte solutions) and the determination of electrical parameters for the membrane and the membrane/solution interface (resistance, capacitance and Warburg impedance) by analyzing the impedance plots and using equivalent circuits as models. The non-reproducibility of membrane potential values for two series of measurements and the asymmetry of the impedance curves show the modification of the membrane/aqueous solution interface. This point was confirmed by the time evolution of the membrane system electrical resistance and the increase of nitrogen content obtained from X-ray photoelectron spectroscopy (XPS) analysis for dry and PET samples maintained in water for different periods of time, and the results indicate an increase in nitrogen content, which is attributed to bacterial presence on the membrane surfaces. These results seem to indicate modification of the PET-membrane/solution interface due to fouling and the possibility of its determination by electrical measurements.

Electrochemical Oxidation Mechanism of Photochromic Switches: Electrodimerisation, Ring Closure or Ring Opening?

Simple photochromic dithienylethylenes with either perfluoro or perhydro cyclopentene ring, and a variety of substituents have been prepared and their electrochemical behavior explored by cyclic voltammetry. All present two electron irreversible oxidation waves in their open form, but the radical cation of the open isomers can follow three different reaction pathways: dimerisation, ring closure, or ring reopening. Whereas the chloro derivative follows a dimerisation mechanism (EC2E mechanism), the phenylthio substituted compound displays an efficient oxidative ring closure (ECE or DISP1 mechanism). Interesting electrochromic behavior is associated with this compound, a redox process occurring in the range 0.5-1.5V is observed by monitoring the absorption species changes (colored species) in function of the applied potential. Furthermore, electrochromic properties are also found in the corresponding ring closed isomers. Depending on the substituents on the thiophene ring and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring closed isomers via EC mechanism. These observations should be taken into account for the potential design of three-state conjugated systems and photoelectrical molecular switching.

Study of Molecules with Multiple Redox Centers Using Differential Staircase Voltammetry at Spherical Electrodes and Microelectrodes

The study of the reversible reduction/oxidation molecules containing multiple interacting or noninteracting redox centers has been carried out using the multipotential step technique Differential Staircase Voltammetry (DSCV). Methods to obtain the formal potentials and other characteristic parameters of the process are proposed for any interaction degree between the centers, when using spherical electrodes of any size (from planar to ultramicrospherical electrodes) and, in particular, the advantages of microelectrodes are discussed. The theoretical predictions have been tested with two experimental systems: ethyl viologen in acetonitrile and pyrazine in aqueous acid media, finding an excellent agreement between the theory and experiments. Therefore, the formal potentials for these systems have been evaluated using DSCV.

Cathodic Behaviour of Cysteine at a Platinum Electrode

The electroreduction behaviour of cysteine was investigated using cyclic, square wave and differencial pulse voltammetric techniques at a platinum working electrode. The reduction of cysteine occurs at a potential of -0.36 V independent of pH. It is a reversible process, controlled mainly by diffusion and in the mechanism of reduction 1 electron per molecule is involved. Using the voltammetric techniques: Cyclic Voltammetry, Square Wave Voltammetry and Differencial Pulse Voltammetry, different parameters (pH, frequency, step potential, pulse amplitude, scan rate) were optimized in order to develop an electrochemical procedure for determination of cysteine in pharmaceutical products. The repeatability, reproducibility, precision and accuracy of the methods were studied. No electroactive interferences from the excipient were found in the pharmaceutical compounds.

Comportamiento Frente a la Corrosión de Aleaciones Metálicas Amorfas de Base Fe/Co en Soluciones Salinas: Nuevos Materiales para Biosensores de Efecto GMI

The objective of this investigation has been the study of corrosion resistance of Fe2.5Co64.5Cr3Si15B15, Fe3Co67Cr3Si15B12 and Fe5Co70Si15B10 amorphous metallic alloys obtained by the melt spinning technique, used as based materials to create a new type of giant magnetoimpedance (GMI) biosensor. The corrosion behaviour has been studied in phosphate buffered saline (PBS) solutions at pH 7.3 and 37.5 ºC. The electrochemical characterization of alloys has been made by means of DC techniques, obtaining the corrosion potential, pitting and protection potentials, as well as the perfect and imperfect passive regions of alloys. In this work, the experimental results obtained are discussed in order to study their corrosion behaviour in artificial biological solutions and thus determine their possible use as GMI-biosensor prototype materials.

En este trabajo se ha estudiado la resistencia a la corrosión de aleaciones metálicas amorfas obtenidas por solidificación ultrarrápida de composición Fe2.5Co64.5Cr3Si15B15, Fe3Co67Cr3Si15B12 y Fe5Co70Si15B10, utilizadas como materiales base para la creación de biosensores basados en el efecto de magnetoimpedancia gigante, efecto GMI. El comportamiento frente a la corrosión se ha estudiado en soluciones salinas tamponadas de fosfato a pH 7.3 y 37.5 ºC. La caracterización electroquímica de las aleaciones se ha realizado mediante técnicas de corriente continua, obteniéndose los potenciales de corrosión, picadura y protección, así como las regiones de pasividad perfecta e imperfecta de las aleaciones. En el trabajo se discuten los resultados experimentales obtenidos, con el fin de predecir su comportamiento frente a la corrosión en medios biológicos artificiales, y determinar su posible utilización como materiales prototipo para biosensores de efecto GMI.

Electrochemical Response of a Vitreous Carbon Electrode Modified by a Thiomacrocyclic Compound Film

A Langmuir-Schaefer (LS) film of a thiomacrocyclic (ThM) compound was deposited on the surface of a glassy carbon electrode (GCE) rod, from a subphase containing Cu(II) ions. The voltammetric response of this modified GCE when the ThM was bonded to Cu2+, showed that the LS film moved the oxidation peaks of copper to more positive values. On the other hand, a LS film of the ThM compound was deposited on the surface of a GCE rod from a subphase of pure water. When the voltammetric response of the GCE-ThM electrode was studied in a Cu2+-SO4(2-) solution, it was found that the modified electrode increases its sensitivity respect to Cu2+ at low bulk Cu2+ concentrations in solution, and a surface-complexation reaction is proposed to explain the effect of the LS film on the GCE surface.

The Anodic Oxidation of Aluminium: Fabrication and Characterization

A nano-porous anodized aluminium oxide layer was synthesized using two step anodization process, in which the dents of aluminium formed in the first anodization step (one day) worked as the initial sites of the pore growth in the second anodization step (one hour). The influence of the voltage on the structure of aluminium oxide has been studied. For different anodization temperatures, it has been found that higher temperatures decrease the pore size, although lower temperatures yielded higher symmetry. The electrolyte used has high influence on the ordered pore domain size. Sulfuric acid generated very small pore sizes, phosphoric acid produced a very big pore size and the largest domain size was observed for samples anodized in oxalic acid.

Se han obtenido óxidos de aluminio con dominios bien definidos en el rango de las 2 µm mediante dos etapas de anodizado, una primera que produce la texturización de la superficie del aluminio tras un largo periodo de anodización y una segunda a menor tiempo. Se ha estudiado la influencia del potencial en el proceso de anodizado encontrando que dependiendo del electrolito empleado existe un potencial óptimo para la generación de estructuras bien ordenadas. Se ha comprobado como la temperatura del electrolito es fundamental para obtener dichas estructuras, observándose como al incrementar la temperatura disminuye el tamaño de poro, aunque las estructuras más ordenadas se obtienen a temperaturas bajas. Otro factor que influye en el orden de los óxidos formados es el electrolito, comprobando que el empleo de ácido sulfúrico produce tamaños de poro muy pequeños, mientras que las estructuras crecidas en ácido fosfórico presentan tamaño de poro grande. El ácido oxálico muestra tamaños de poro intermedio entre los dos electrolitos citados.

Temperature Behavior of the Electrical Conductivity of Emim-Based Ionic Liquids in Liquid and Solid States

In this paper we present experimental measurements of the temperature dependence of the electrical conductivity, s, in four ionic liquid compounds (ILs) in both the liquid and solid states and at atmospheric pressure. The chemicals measured are composed by the 1-ethyl-3-methyl-imidazolium (EMIM+) cation, which has been combined with four different anions: Cl-, Br-, BF4- and ethyl sulfate (ES-). In the liquid state, the temperature dependence of s, for the four ILs follows the Vogel-Tamman-Fulcher (VTF) equation with high precision. Around the transition between the solid and liquid states, the electrical conductivity of the three ILs with lighter anions presents an hysteresis loop, that can be explained as the apparition of supercooled liquid. In contrast, the EMIM-ES presents a smooth transition, without any jump in the s, value or any hysteresis loop (probably because its melting point is reported to be below the minimum temperature measured by us). Finally, the jump in s, is not related with the glass transition because its temperature value is well below the minimum temperature measured, and at that glass transition temperature the s, value is below the resolution of our conductivity meter (2 nS/cm).

Electrochemical Evaluations of Glutamate at a Gold Electrode

The voltammetric behaviour of glutamate at a gold electrode was studied by means of cyclic voltammetry. From 2.6 to 11.5 pH values, glutamate originated a single cathodic peak. Potential of the peak, Ep, was -0.8 V, and it was independent from pH. The reduction of glutamate was found irreversible and mainly controlled by diffusion. An analytical approach for analysis of glutamate by means of square wave voltammetry was developed. Plots of current versus concentration presented a linear behaviour from 2.5×10-4 to 2.7×10-3 M. The detection limit was 6.3×10-5 M. Interference from compounds co-existing with glutamate in food was negligible, and in favour of an application of the proposed method to the analysis of real samples. Determination of glutamate in pure solutions resulted in acceptable deviation from the stated concentration. Relative errors ranged -2.5 to +1.6 %.