Electrokinetic remediation of soils polluted by heavy metals (mercury in particular)

The electrokinetic approach, for the extraction of contaminants from a soil matrix, requires the application of electric fields of suitable intensity, through saturated portions of the soil. When aimed at the removal of species present in metallic form (like mercury) or as poorly conductive minerals (like some sulfides), pollutants require to be preliminary dissolved, an action that can be facilitated by adding appropriate chemicals. In this paper, we show that the presence of an electric field is decisive, possibly because the pollutant-containing particles were able to act as polarizable species (and thus as improper electrodes). At first, tests were performed on small amounts of soil (about 200 g, in plastic bench cells); then, the process was scaled up, testing approximately 400 kg of soil. A 60% of total mercury was removed, in less than 3 months, by adopting specific expedients, in terms of hydraulic control as well as of plant design

Procedimento per depurare una matrice terrosa contaminata da metalli pesanti

La presente invenzione concerne un procedimento per depurare una matrice terrosa contaminata da metalli pesanti comprendente almeno: (a) predisporre almeno un pozzetto catodico all’interno di detta matrice terrosa, detto pozzetto comprendendo almeno un catodo immerso in una soluzione elettrolitica (soluzione catodica) inclusa all’interno di un contenitore catodico permeabile ai liquidi e alle specie ioniche; (b) predisporre almeno un pozzetto anodico all’interno di detta matrice terrosa, detto pozzetto comprendendo almeno un anodo immerso in una soluzione elettrolitica (soluzione anodica) inclusa all’interno di un contenitore anodico permeabile ai liquidi e alle specie ioniche; (c) immettere acqua, preferibilmente acqua comprendente un agente complessante (soluzione complessante), in detta matrice terrosa così da impregnare detta matrice terrosa sino a raggiungere un grado di saturazione pari ad almeno il 70% e formare complessi ionici metallici con detti metalli contaminanti; (d) applicare una differenza di potenziale a detti anodo e catodo così da far migrare detti complessi ionici metallici; (e) recuperare detti metalli contaminanti da una o più fra dette soluzione anodica, soluzione catodica e acqua di impregnazione di detta matrice terrosa

Studio della reazione di sviluppo di cloro quale metodo diagnostico per la caratterizzazione di elettrodi ad ossido

La reazione anodica di sviluppo di cloro (ChlER, Chlorine Evolution Reaction) è uno degli argomenti più investigati nel campo dell’elettrocatalisi, grazie al suo impatto immediato sulla tecnologia elettrochimica. L’industria del cloro-soda si avvale dell’utilizzo di elettrodi cosiddetti DSA (Anodi Dimensionalmente Stabili), costituiti da un metallo valvola di base, avente una sua geometria e che funge da conduttore di corrente, sul quale viene depositato un film a base di ossidi metallici aventi proprietà catalitiche nei confronti delle reazioni desiderate. Uno studio dei più importanti ossidi metallici utilizzati come coatings nei DSA (quali, ad esempio, RuO2 e miscele RuO2-TiO2) e dei meccanismi cinetici proposti per la ChlER su tali materiali (tra i quali si annoverano i meccanismi di Volmer-Tafel, Volmer-Heyrovsky, Kristhalik ed Eremburg) è riportato nella “Review Article” di Trasatti [1]. Le conoscenze già acquisite sulla cinetica della ChlER consentono di utilizzare la reazione come processo-modello per la caratterizzazione di nuovi materiali, permettendo altresì un confronto di prestazioni tra materiali differenti. Argomento della presente comunicazione sono le proprietà elettrochimiche di miscele IrO2-TiO2, preparate mediante differenti tecniche di sintesi (sol-gel, e sputtering reattivo) e a diversa temperatura. Su tali materiali si è sviluppato uno studio dettagliato della cinetica della reazione di sviluppo di cloro, i cui parametri cinetici sono stati acquisiti attraverso un’elaborazione inizialmente proposta da Conway [2, 3], e successivamente estesa dal nostro gruppo di ricerca [4]. [1] S. Trasatti, Electrochim. Acta 1987, 32, 369 [2] B.E. Conway, D.M. Novak, J. Chem. Soc., Faraday Trans. 1 1979, 75, 2454 [3] B.E. Conway, D.M. Novak, J. Electroanal. Chem. 1979, 99, 133 [4] S. Ferro, A. De Battisti, J. Phys. Chem. B 2002, 106, 224

On the electrolysis of dilute chloride solutions: Influence of the electrode material on Faradaic efficiency for active chlorine, chlorate and perchlorate

In the present work, the electrolytic process of diluted aqueous chloride solutions was investigated at Ti/RuO2.2SnO2 and Ti/Pt electrodes, at different values of current density, temperature and electrolysis time. The time evolution of chlorine-related species (i.e. active chlorine (dissolved Cl2, HClO, OCl-), chlorite, chlorine dioxide, chlorate and perchlorate) was investigated in order to establish whether their formation and consumption was related to either chemical or electrochemical path of reactions. The estimated faradic efficiencies demonstrated the better catalytic activity of the Ti/RuO2.2SnO2 electrode towards the chlorine evolution reaction with respect to the Ti/Pt anode, and the key-role played by the temperature, which reflects the different activation energies of the two competing electrochemical reactions, i.e. chloride and water oxidations. The concentration trends of chlorate and perchlorate indicated that the electrochemical route was responsible for their presence in the bulk solution, instead of a chemical path. The low concentration levels assessed for chlorites throughout the tests did not suggest the preponderance of the chemical over the electrochemical depletion process; however, further potentiodynamic tests suggested their high reactivity towards both anodic and cathodic surfaces, thus suggesting that the electrochemical path of depletion could prevail over the chemical. Conversely, due to a solution pH unfavourable to the stability of chlorine dioxide, its low concentration level was associated to a chemical depletion route

Influence of the nature of the electrode material and process variables on the kinetics of the chlorine evolution reaction. (I) The case of IrO2-based electrocatalysts

Kinetic studies on the chlorine evolution reaction (ChlER) on oxide-based materials have been the subject of a number of papers since the seventies, following the introduction of DSAs (Dimensionally Stable Anodes) in chlor-alkali plants. On the basis of experimental data, different pathways have been proposed for the reaction over the years. Actually, specific experimental conditions and different approaches in sample preparation may lead to conflicting explanations. In the present paper, the ChlER kinetics has been studied at four electrode materials based on iridium and titanium oxides (with a 1:2 molar ratio). Electrodes were synthetized at two temperatures (350 and 450 °C) and by two different preparation methods: physical vapor deposition (rf-magnetron sputtering) and a conventional sol-gel technique, using special precursors developed in our laboratory. Both methodologies guarantee a high level of reproducibility. As also observed by other authors, experimental data have shown a lack of linearity in Tafel plots, high b slopes and reaction orders with respect to chloride ≤ 1, which have been justified on the basis of a Volmer-Heyrovsky pathway, by considering a model proposed by Tilak and Conway in 1992. This approach highlighted the role of the adsorbed intermediates, also at low overpotentials, for all electrode materials. To analyze further the kinetics, Langmuir and Frumkin models for intermediates adsorption were considered. Values for the lateral interaction parameter g were estimated, which ranged between 1 and 10, in all cases. Concerning the effect of pH, its influence on the ChlER rate seems to be related only with electrode surface modifications, without any involvement of protons in the rate determining step of the process. A slight inhibiting effect was assessed, by increasing the protons concentration. Eventually, impedance spectroscopy analysis did not appear sensitive to intermediate adsorption, plausibly because of the low variation of the coverage within the Tafel region; a poorly resolved contribution related to porosity was found in the case of samples prepared at 350 °C

Charge-storage process in IrO2-SnO2 mixed-oxide electrodes. Role of coating composition, solution pH and Temperature

The effects of both pH and temperature on the kinetics of redox processes that involve the charging/discharging of electrocatalytic sites, at dimensionally stable anodes, have been investigated. At first, electrodes based on Ir and Sn oxides, having different molar compositions, were studied by X-ray diffraction in order to characterize the system under investigation, and with the four-probe method to evaluate how the film resistivity varies with the oxide composition. Then, cyclic voltammetric experiments were carried out at different temperatures to estimate the apparent activation energy (Wa) of the charging/discharging process of active sites. The Wa values obtained were rather low (about 0.5 kcal mol-1) and thus realistically not attributable to any chemical reaction. By excluding a role for the various physicochemical parameters that could influence the process, we suggest that the “double injection/double ejection” mechanism is related to a Grotthuss-type movement of protons, which takes its advantage from an electrode-electrolyte interface extending for a few monolayers within the oxide coating. The interface thus represents a relatively rigid and organized structure in which protons can move without any hindrance

On the oxygen evolution reaction at IrO2-SnO2 mixed-oxide electrodes

Taking advantage of an innovative sol-gel preparation, Ti-supported coatings based on iridium and tin oxides have been exploited for carrying out a thorough investigation of the oxygen evolution reaction. The research has been performed on devices having an Ir concentration between 20 and 90 mol% of noble-metal, at different temperatures (in the range from 2 to 60 °C) and at four different [H+] concentrations(between 1.0 and 10−3 mol l−1). Approximately constant activation energies have been obtained, with values close to 12.4 kcal mol−1; moreover, the electrode composition proved not to affect the reaction pathway in a significant manner. At all devices, the OER appears to take place with one and the same mechanism, as suggested by the relative constancy of Tafel slopes and reaction orders with respect to H+. The mechanistic path has been confirmed also on the basis of the reaction order with respect to the active site. The effective catalytic activity of the mixed-oxide coating is not influenced by the noble-metal content

ELECTROKINETIC REMEDIATION OF SOILS CONTAMINATED BY MERCURY

The electrokinetic approach, for the extraction of metal ions from dispersed solid matrixes, has received extensive description in the scientific literature of the last twenty years. The application of electric fields of suitable intensity, through saturated portions of soils, can determine the displacement of charged species. In principle, this effect has two components: one is the electromigration, the other is the component due to the electroosmotic drag of the spatial ionic charge facing the charged surface of soil particles. In the case of electromigration, the electric field has essentially the role of “driving force” for the movement of ionic species through the water impregnating the dispersed solids and the soil in particular. The effect remains even if the solid matrix is finely divided. The application of this technology, when aimed to the removal of mercury present in metallic form, requires its prior dissolution, which can be facilitated by adding appropriate chemicals. Designing an intervention requires a preliminary speciation of the contaminant in the soil, as well as the execution of electrochemical tests at a laboratory scale. The analysis of soil with speciation of the pollutant was carried out following the approach proposed by Boszke et al. “Mercury mobility and bioavailability in soil from contaminated area”, Environmental Geology (2008) 55: 1075-1087. The soil analysis was repeated after the electrochemical laboratory test, for an assessment of the amount of removed Hg. In addition, results obtained with this method were compared with those acquired using the alternative EPA 3200 procedure for speciation. The electrochemical laboratory test was performed on approximately 400 kg of soil, and for a period of about three months. After treatment, the soil analysis showed a significant reduction of total Hg (approximately 60% of the total). Basing on these results, an intervention in the field is being planned: estimation of time required to achieve the objective of remediation has been based on the removal of mobile and mobilizable forms of Hg