Annalisa Vacca

Electrochemical treatment of wastewater containing phenolic compounds: oxidation at boron-doped diamond electrodes

This work investigates the performance of BDD electrodes during oxidation of aqueous solutions of phenol. The main reaction intermediates are identified, the effect of operating conditions on the faradic yield of the process, and the degree of mineralization achievable under different experimental conditions are evaluated. Due to the crucial role of mass transfer in the process, an impinging jet cell is used for the experiments. The results indicate that if a minimum value of current density is imposed, suitable initial conditions can be set at which the removal of the reactant is always under mass transfer control and the process is carried out at a faradic yield of about unity, up to the near-complete disappearance of total organic load. High current density and high mass transfer coefficient must be used in order to carry out the process with high space-time yield. The performance of BDD is compared to that obtained at Ti/RuO2 anodes.

Electrochemical treatment of phenolic waters in presence of chloride with boron-doped diamond (BDD) anodes: Experimental study and mathematical model

This work deals with an experimental and numerical study on the electrochemical treatment of waters containing phenolic compounds with boron-doped diamond (BDD) anodes. Anodic oxidation of m-cresol, as a model of phenolic compound, was investigated by galvanostatic electrolyses. The electrolyses were carried out under different experimental conditions by using an impinging-jet flow cell inserted in a hydraulic circuit in a closed loop. On the basis of the experimental results a mathematical model was implemented to simulate the effect of the chemistry of organic compounds and solution on the process, in particular the effect of chlorides on the kinetics of m-cresol oxidation. The effect of hydrodynamics of the cell on the mass transfer towards the electrode surface was also considered. The model was validated through comparison with experimental data: the results showed that the proposed model well interpreted the complex effect on removal efficiency of such operative parameters as current density, hydrodynamic of the reactor and chemistry of the solution. The model predictions were utilised to obtain quantitative information on the reaction mechanism, as well as to predict the performance of the process under different operative conditions, by calculating some relevant figures of merit.

Competitive Sorption of Heavy Metal Ions by Soils

An experimental study is presented on heavy metal pollution of soils. In particular, Pb2+, Zn2+, Cd2+ are considered as most representing the pollution of a Sardinian (Italy) area, where important mining activities connected to the extraction of blende and galena have been carried out in the past. Results of batch experiments involving adsorption from mono-component metal solutions are examined showing that at the fixed working pH of 6.5, a fraction of metal was always retained by ion exchange; surface precipitation was assumed to be responsible for the remaining fraction of metal retained. Competitive adsorption is also investigated by performing multicomponent sorption isotherms. The aim of the work was to evaluate the exchangeable fraction of metal retained in the soil, when it is contaminated by multicomponent heavy metal solutions. A mathematical model is presented which, using only parameters from binary adsorption data, is able to predict the fraction of metal retained by ion exchange from multicom...

Preparation and characterisation of transparent and flexible PEDOT:PSS/PANI electrodes by ink-jet printing and electropolymerisation

Novel flexible PEDOT:PSS/PANI bilayered thin-film electrodes were successfully prepared by ink-jet printing of PEDOT:PSS on a PEN substrate and subsequent electrochemical polymerisation of PANI. Scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to systematically investigate the structure, morphology and electrochemical behaviour of the electrodes. The PEDOT:PSS/PANI bilayer nanofilms have been tested as pH sensors. As a comparison, either PEDOT:PSS or PANI electrodes have been tested for pH measurements. The results show that the sensitivity of the PEDOT:PSS/PANI bilayer film is higher than that of PEDOT:PSS and PANI electrodes, indicating a synergistic effect of the two polymers. A reversible near-Nerstian behaviour of the open circuit voltage with pH was found, with a slope of 58.9 mV pH−1 unit in a wide range of pH values from 2 to 14.

Electrochemical treatment of water containing Microcystis aeruginosa in a fixed bed reactor with three-dimensional conductive diamond anodes

An electrochemical treatment was investigated to remove Microcystis aeruginosa from water. A fixed bed reactor in flow was tested, which was equipped with electrodes constituted by stacks of grids electrically connected in parallel, with the electric field parallel to the fluid flow. Conductive diamond were used as anodes, platinised Ti as cathode. Electrolyses were performed in continuous and in batch recirculated mode with flow rates corresponding to Re from 10 to 160, current densities in the range 10-60Am(-2) and Cl(-) concentrations up to 600gm(-3). The absorbance of chlorophyll-a pigment and the concentration of products and by-products of electrolysis were measured. In continuous experiments without algae in the inlet stream, total oxidants concentrations as equivalent Cl2, of about 0.7gCl2m(-3) were measured; the maximum values were obtained at Re=10 and i=25Am(-2), with values strongly dependent on the concentration of Cl(-). The highest algae inactivation was obtained under the operative conditions of maximum generation of oxidants; in the presence of microalgae the oxidants concentrations were generally below the detection limit. Results indicated that most of the bulk oxidants electrogenerated is constituted by active chlorine. The prevailing mechanism of M. aeruginosa inactivation is the disinfection by bulk oxidants. The experimental data were quantitatively interpreted through a simple plug flow model, in which the axial dispersion accounts for the non-ideal flow behaviour of the system; the model was successfully used to simulate the performances of the reactor in the single-stack configuration used for the experiments and in multi-stack configurations.

Analysis of photocurrent and capacitance of TiO2 nanotube–polyaniline hybrid composites synthesized through electroreduction of an aryldiazonium salt

TiO2 nanotube–polyaniline hybrid composites were synthesized by a multi-step electrochemical procedure as follows: (1) electrochemical oxidation of Ti foil to obtain TiO2 ordered nanotubular electrodes, (2) functionalization of TiO2 by electroreduction of 4-nitrobenzendiazonium salt, (3) electrochemical reduction of the nitro group to amine, (4) galvanostatic electropolymerization of aniline onto the surface of the aminophenyl-modified TiO2 electrode. Samples were also prepared by direct electropolymerization of PANI on TiO2 nanostructures without the deposition of 4-nitrobenzendiazonium as under-layer. Different times for the electropolymerization step were applied. The composite materials were morphologically and electrochemically characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance analysis. The improvement of the most relevant properties of the PANI–TiO2 composites for their application as photocatalysts and as supercapacitors was checked.

Photo-electrochemical behavior at different wavelengths of electrochemically obtained TiO2 nanotubes

The results of an experimental study on the photo electrochemical behavior of nanotubular TiO2 structures are presented in this work. TiO2 samples were prepared by electrochemical anodization of Ti foils and submitted to thermal annealing. The influence of the current transient during the anodization, and of the annealing temperature on the photochemical response of the samples at different wavelengths was studied. Different behavior of the samples was observed, which may be attributed to the distributions of defects and to their different sensitivity to the temperature. The analysis of the performance of the samples in absence or in the presence of glycerol, used as hole scavenger, provided more information on the photo-catalytic properties of these structures.

TiO2–WO3 nanostructured systems for photoelectrochemical applications

TiO2/WO3 mixed metal oxide is used in the present work as an anode for the photoelectrosplitting of water. TiO2 is used as the support under-layer in the form of either a compact or nanotubular structure. The over-layer of WO3 is obtained via cathodic electrodeposition, by means of pulse potential technique (PPT). The performances of the samples in neutral supporting electrolyte are compared when the samples are irradiated with light at two wavelengths, 365 nm and 400 nm. The effect of the WO3 loading on the performance is also investigated, as well as the charge transfer mechanism. The results from runs carried out in solutions containing glycerol are used to study the possible role of the WO3 over-layer in the whole working mechanism of the combined structure.

Case Studies in the Electrochemical Treatment of Wastewater Containing Organic Pollutants Using BDD

A critical review is presented in this chapter on the possible applications of boron-doped diamond (BDD) as anode material to perform oxidation of organic compounds in aqueous solution. The oxidation of model substances is studied as well as that of the main classes of pollutants, such as phenols, dyes, pesticides and drugs, surfactants, which make some problems of degradation with the traditional wastewater treatments. The presented results indicate that organic compounds refractory to other oxidation techniques are successfully oxidized at BDD, even if the reaction mechanism is differently dependent on the organic compound and the electrolyte composition. Economic considerations reveal that electrochemical oxidations at BDD are less expensive than other advanced oxidation processes, indicating that in the near future this technology can become a competitive treatment for the removal of refractory compounds from wastewater.

Electrochemical removal of microcystis aeruginosa in a fixed bed reactor

The removal of M. aeruginosa from natural waters has been investigated by using a fixed bed cell with 3dimensional electrodes in continuous mode. Steady state values of chlorophyll-a pigment, contained in the microalgae, were obtained, as well as concentrations of oxidizing species in the outlet stream of the system. In particular, attention was paid to two key parameters, flow rate and current density, to determine their influence on the removal of microalgae and on the concentration. Experiments were performed in a plastic cylindrical single pass cell, filled with glass spheres, connected to a hydraulic circuit. Grids of Titanium, arranged in stacks in-series, were employed as electrodes packing. The grids of the cathode packing were coated with platinum, while the grids of the anode packing were coated with Ir/Ru mixed oxides. The effect of the operative parameters on the possible mechanism of removal, namely electric field and electro-generated oxidants was investigated. The results show that the removal of M. aeruguinosa may be related to the synergistic effect of electrogenerated oxidants produced from the chloride ions and electric field, although the main mechanism is the killing by long life oxidants electro-generated at the anode surface.