Electrochemical incineration of Oxalic Acid - Reactivity and engineering parameters

Mass transfer measurements were carried out to test a disk-shaped parallel-plate electrochemical cell, based on a new design. The impinging-jet-cell concept, confined between parallel plates, was adapted to a configuration with one central inlet and several peripheral exit sections, leading to more effective hydrodynamics within the cell. Measurements of mass transfer coefficient were performed using the limiting diffusion current technique based on ferro-cyanide ion oxidation, and overall mass transfer coefficients were correlated to Reynolds numbers ranging from 30 to 200. A comparison with literature on similar devices showed higher mass transfer coefficients can be obtained in the cell described in the present work. From the mass transfer standpoint, this type of cell could be a valuable tool in electrochemical wastewater treatment applications. The electrochemical oxidation of oxalic acid was tested at different anode materials (Pb/PbO2, boron-doped diamond, Ti/Pt and Ti/IrO2–Ta2O5), showing that the new cell design enables limitations usually encountered with conventional batch cells to be overcome. However, the nature of the anode material remains an important parameter for the elimination of organic substrates

Electrochemical incineration of organics. Combined role of mediation and electrode material

In recent years, the perspectives of application of electrochemistry for environmental pollution abatement have been thoroughly investigated. The feasibility of electrochemical incineration of organic substrates in wastewater, in particular, has drawn much attention since pioneering papers by Stucki and Comninellis in early 90’s. In these works, the influence of the nature of the electrode material on the faradaic efficiency of the anodic mineralization of organics has been considered in detail, showing that optimal conditions for the process in question are met at high-oxygen-overpotential anodes, like SnO2 and PbO2. Basing on these evidences, Comninellis has proposed a general model, which satisfactorily accounts for the different results described in the literature. More recent results, obtained at conductive diamond electrodes, characterized by very high oxygen overpotential, also fit in the model predictions quite well. Model organic substrates, like glucose, aldonic and aldaric acids, show a complex reactivity toward the anodic mineralization. In fact, in these cases, besides the central role of adsorbed hydroxyl radicals, also the mode of adsorption of the organic species has to be taken into account. Simpler molecules, like oxalic acid, also support the view that co-electrosorption of hydroxyl radicals and organic species decides of the rate of the anodic mineralization, volcano-plot-approaches of the type applied long since for ethylene electrochemical oxidation being possibly a good interpretative tool. These considerations afford further evidence to the importance of the nature of the electrode material in electrochemical incineration. Taking into consideration the role of mediators, like Cl-, of particular interest for its common presence in many different types of wastewater, interpretations may be attempted, bringing again the role of the electrode material in a prominent position. Chloride-mediated incineration can be properly accounted for by assumptions on co-adsorbed hydroxy- and chloro-radicals at the electrode surface and on the effect of this on the rate of oxygen evolution reaction

Kinetic mechanism of the electroxidation of oxalic acid at different electrode materials

Oxalic acid (OA) represents one of the proposed metabolites of the anodic oxidation of more complex organic molecules; in spite of its simple structure, its mineralization is strongly dependent on the nature of the electrode material at which the process is carried out. Sargysian and Vasil’ev [1] pointed out such dependence, investigating the kinetic behavior of OA at different metal electrodes (Rh, Pd, Os, Ir, Pt and Au), at a DSA-type anode (RuO2-TiO2) and at glassy carbon. Their conclusions, also in agreement with our recent results on the topic [2], highlighted the important role played by the anion adsorption step, claiming that OA is oxidized with increasing difficulty at electrode materials having higher oxygen affinity. In the present work, the analysis of the dependence of the OA electroxidation on the nature of the electrode material has been extended to highly conductive, boron-doped diamond (BDD) electrodes, with either oxygen and fluorine at their surfaces. At variance with glassy carbon, BDD-based electrodes should permit elucidating the aspects of the OA kinetic behavior at carbonaceous materials, without exhibiting the mechanical and chemical limitations of graphite and GC. Moreover, the different hydrophobic/hydrophilic character of the BDD and F-BDD electrode surfaces, which necessarily reflect in the structure of the electrode/electrolyte double-layer, will be also taken into consideration

Electrochemical incineration in the presence of halides

The influence of different halides on electrochemical incineration of oxalic acid was studied. Experiments were initially carried out at a Pt electrode in alkaline media, in which case, in the absence of mediating effects, a slow electroxidation was observed. However, with halides, the rate of the electrochemical incineration was increased significantly, following the order F- > Br- > Cl-, and the rate of the mediated process was practically independent of applied current density. An explanation has been attempted for the role of halides, in relation with the oxygen evolution reaction, concomitant with the electrochemical incineration

Electrosynthesis of dehydrocholic acid from cholic acid

The electrochemical oxidation of cholic acid has been investigated in the recent past, the target product, dehydrocholic acid, being of great interest, in consideration of its medicinal activity (dissolution of gallstones in mammals). Our previous study took into consideration both the direct oxidation and the chlorine-mediated process; high chemical yields were obtained at Ti-supported lead dioxide and graphite electrodes, but only in the presence of chlorides in solution. On the contrary, very low faradaic yields have unfortunately been obtained at less ecologically harmful materials (like platinum); moreover, the mediated electro-chemical oxidation did not occur in aquo-organic media. The present communication reports on cholic acid oxidation, carried out at boron-doped diamond, platinum and iridium dioxide-based electrodes; the electrochemical synthesis has been obtained in a micro-pilot plant and the chemical yield has been determined by a quantitative analysis based on HP-TLC and OPLC methods

Electrokinetic extraction of surfactants and heavy metals from sewage sludge

Waste management represents a quite serious problem involving aspects of remediation technologies and potential re-utilization in different fields of human activities. Of course, wastes generated in industrial activities deserve more attention because of the nature and amount of xenobiotic components, often difficult to be eliminated. However, also ordinary wastes of urban origin are drawing more and more attention, depending on the concentration of noxious substances like surfactants and some heavy metal, which may eventually require expensive disposal. In the present paper, a research has been carried out on the application of electrokinetic treatments for the abatement of the above xenobiotic components from sewage sludge generated in urban wastewater treatment plants. Experiments were carried out on a laboratory scale, in a 250mm×50mm×100mmcell, using 250–300 g of sludge for each test and current densities between 2.4 and 5.7mAcm−2. As a general result, quite significant abatements of heavy-metal ions and surfactants were achieved, with relatively low energy consumption

Electrokinetic extraction of surfactants and heavy metals from a municipal wastewater sludge

The disposal of sewage sludge from domestic wastewater treatment plants is a growing problem worldwide. The European Community has developed the draft of "Working document on sludge" to promote the (re)use of sewage sludge. In Italy, sewage sludge could be disposed on agricultural lands but, in last years, this use has been limited by Italian legislation to avoid the contamination by and the accumulation of both heavy metals and organic compounds in the soils. Analogous regulations have decreased the maximum attainable limit for these pollutants. In order to increase the recycling of sludge on agricultural lands, thus avoiding their accumulation, we need a powerful treatment as electroremcdiation. The present work reports on a laboratory investigation on the use of an electrokinetic method for the removal of anionic surfactants like linear alkylbenzenesulphonate (LAS) and heavy metals from wastewater sludge

Ossidazione di acido ossalico a diversi materiali elettrodici

L’ossidazione anodica di sostanze organiche, anche di natura complessa, porta in genere alla formazione di metaboliti più semplici, prima della decomposizione completa ad anidride carbonica ed acqua, e tra essi l’acido ossalico (OA) è probabilmente l’intermedio più ricorrente. Nonostante abbia struttura semplice, la sua completa mineralizzazione risulta essere fortemente dipendente dalla natura del materiale elettrodico al quale il processo elettrochimico ha luogo. Sargisyan e Vasil’ev [Elektrokhimiya, 18 (1982) 845] furono i primi a sottolineare tale dipendenza, investigando il comportamento cinetico a diversi metalli (Rh, Pd, Os, Ir, Pt e Au), come pure su anodi dimensionalmente stabili (RuO2-TiO2) e su carbone vetroso. Dal loro lavoro risulta chiaramente il ruolo giocato dallo stadio di adsorbimento dell’anione organico, come pure la crescente difficoltà ad ottenere il processo ossidativo su materiali aventi un’elevata affinità per l’ossigeno. Più recentemente, tali conclusioni sono risultate confermate da dati da noi ottenuti sull’ossidazione di OA ad elevati potenziali anodici [Electrochimica Acta, 49 (2004) 4027]. Allo scopo di arricchire il quadro, il presente lavoro è stato basato su misure eseguite a diversi materiali elettrodici, comprendenti platino, ossidi misti, carbone vetroso e diamante conduttore, modificato in superficie dalla presenza di ossigeno e fluoro