Portugaliae
Electrochimica Acta

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.

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 Fenton’s 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 Fenton’s 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.

Biosorption is the passive sequestration of pollutants by non-metabolizing non-living biomass. The technique emerged in 1980’s 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.

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.

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.

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.

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.

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.

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.

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.

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+-SO42- 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.

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.

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).

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 %.