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Kinetics of the electrochemical oxidation of organic compounds at BDD anodes: modelling of surface reactions
No full textThis paper presents the results of a numerical study of the kinetics of electrochemical oxidation of different organic substances at boron doped diamond (BDD). It is well established that oxidation of organics at BDD anodes takes place in the potential region of oxygen evolution, through reaction steps in which OH radicals are involved: these radicals can react with organic compounds to give more oxidised substances, or with water to give oxygen. Because of the high reactivity of OH radicals these reactions are confined to a thin film adjacent to the electrode surface. A mathematical model was implemented, which accounts for chemical and electrochemical reactions, as well as for the transport phenomena involved in the process: the parameters of the model were derived from experimental data. The model allowed calculation of the trend with time of reactant concentration, reaction intermediates and oxidisable agents: their space profiles in the reactor were also obtained. The numerical predictions of the model were compared with experimental results obtained from galvanostatic electrolyses of different organic compounds: (a) 2,4,6-trihydroxy-1,3,5 triazine also known as cyanuric acid (CA), which is well known to be refractory to oxidation with OH; (b) atrazine (ATR) the most oxidisable precursor of CA, and (c) phenol which is considered as an OH scavenger because of its high reactivity. The agreement between experimental and model predicted data was good in all the examined conditions
Effetto del trasferimento di materia sulla cinetica di ossidazione elettrochimica su anodi di diamante drogato al boro per il trattamento di acque di scarico
No full textCo-disposal of industrial wastes to obtain an inert material for environmental reclamation
No full textThe present work deals with the treatment of red mud from Eurallumina Bayer plant with gypsum, in order to obtain an inert which might be disposed without risk. The proposed process has particular concern in the local contest in which the Bayer plant is located. In fact, some power plants or fluoride production plants, that are present nearby the Eurallumina plant, might be economic sources of gypsum. Moreover, the inert material resulting from the proposed process should be disposed in the old mine sites, also present in the same area, and than re-vegetated, thus effectively contributing to an environmental reclamation of this territory. The present work compares the characteristics of effluents obtained by leaching with water, beds of either red mud coming from the thickening filters of the Bayer plant (RMF) or from the holding ponds (RMP), or their mixtures with gypsum. The results obtained up to now show that the process is able to lower pH at values near 9, which is stable to further leaching runs. Also the physical characteristics of the resulting material are enhanced, in terms of both particle size distribution and hydraulic permeability. The addition of 10% sandy soil is sufficient to give permeability in the order of 10(-4) cm/s, which is typical of sandy soil
Electrochemical degradation of diuron and dichloroaniline at BDD electrode
No full textThe degradation of diuron and 3,4-dichloroaniline (DCA) has been studied at boron-doped diamond (BDD) anode. A three electrode impinging jet cell was used to improve the mass transfer of the reactant to the electrode surface. In the whole range of experimental conditions examined, the reactants were completely mineralised and passivation of the electrode surface was not observed. HPLC and ionic chromatography analyses revealed that the reaction involves the oxidative ring-cleavage and the formation of stoichiometric amounts of chloride and ammonium ions. Depending on the specific experimental conditions, short-chained organic acids were also identified as intermediate products. Values of global current efficiencies greater than 20% were obtained even operating with low reactant concentration (0.17 mm for diuron and 2 mM for dichloroaniline) provided that a suitable ratio between applied current and mass transfer rate was guaranteed. (C) 2003 Elsevier Ltd. All rights reserved
Competitive sorption of heavy metal ions by soils
No full textAn 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 multicomponent solution
Behavior of cobalt oxide electrodes during oxidative processes in alkaline medium
No full textThe behaviour Of Co3O4 electrodes was investigated in alkaline solution during oxidative runs at different anodic potentials up to the value of oxygen evolution reaction (OER). Voltammetric results indicate the presence of several solid-state surface redox transitions (SSSRT) which lead to the formation of Co(IV) containing species which are the specific catalytic phases for OER. Electrochemical impedance measurements were done in different ranges of potential and frequency in order to better characterise the SSSRT and the capacitive response of the electrode/electrolyte interface. (c) 2007 Elsevier Ltd. All rights reserved
Product and by-product formation in electrolysis of dilute chloride solutions
No full textThe results of an experimental study on electrochemical disinfection of water are presented. Attention was paid to the behaviour of chlorine compounds during electrolysis of water containing chlorides, with particular regard to the selectivity of the process towards the production of oxidising agents. Two reactor configurations were tested: a stirred tank cell and a filter press cell inserted in a hydraulic circuit. Both cells were equipped with boron doped diamond (BDD) anodes. Experiments were performed in batch and continuous mode. The effect of such operating parameters, current density, stirring rate or recirculating flow rate, on the behaviour of the process was investigated. The results at BDD anodes show that low current densities and perfect mixing of the system should be adopted in order to obtain high values of the concentration of oxidising agents avoiding the formation of such undesired by-products as chlorite, chlorate and perchlorate ions. Runs were also performed in which BDD was substituted by a commercial (Ti/RuO2) DSA anode and the results obtained with the two materials are compared
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