Conductive-diamond Anodes Research Articles

Electrochemical treatment of soil-washing effluent with boron-doped diamond electrodes: A review

In recent years, electrochemical technologies have been widely used to remove contaminants at lab-scale and semi-pilot scale. Boron-doped diamond (BDD) electrodes have been considered as efficient materials for the abatement of persistent organic pollutants owing to their outstanding properties, such as rapid rates of electron-transfer for soluble redox systems, wide electrochemical potential window for water discharge reactions in aqueous and non-aqueous electrolytes, and high stability. Similar to other applications of electrochemical technology, wastes display medium to high ionic conductivity. Therefore, one of the applications highlighted for the electrolysis with these new electrodes is the treatment of soil-washing fluids, because in the polluted streams, washing of polluted soils provides a suitable conductivity to the effluent. In this context, this review summarizes the application of conductive diamond anodes for the electrochemical treatment of soil-washing effluents contaminated with different persistent organic pollutant such as pesticides, hydrocarbons, dyes, and organochlorine compounds, in single anodic oxidation processes and in other more complex processes such as electro-Fenton, photoelectrolysis, or sonoelectrolysis. Finally, the challenges and future research directions of electrochemical technology are discussed and outlined at pilot and prototype scale.

Applicability of electrochemical oxidation using diamond anodes to the treatment of a sulfonylurea herbicide

In this work, the electrolysis of chlorsulfuron (ClSF, a sulfonylurea herbicide) with conductive-diamond anodes is studied. The influences of the nature of the supporting electrolyte (0.05moldm−3 NaCl and Na2SO4), the applied current density (30 and 100mAcm−2) and the initial herbicide concentration (20,50 ad 100mgdm−3) have been evaluated. Total mineralization of the herbicide was attained regardless of the supporting electrolyte medium used, although electrolyte clearly influenced on the efficiency and rate of the oxidation process. Likewise, the process was found to be more efficient at low current densities. Results point out the importance of mediated electrochemical processes in the removal of ClSF. This influence was confirmed by additional tests in which the electrolysis was combined with UV light or ultrasound irradiation: the use of UV irradiation results in a less efficient process, while US improves importantly the rate of the mineralization as compared to the single electrolytic process. The strong oxidation conditions of electrolysis leads to the formation of reaction intermediates (2-amino-4-methoxy-6-methyl-1,3,5-Triazine, 2-chlorobenzenesulfonamida and oxalic acid) in agreement with the mechanisms proposed in the literature for other AOPs.

Mechanism and kinetics of electrochemical degradation of uric acid using conductive-diamond anodes

ABSTRACTUric acid (UA) is one of the principal effluents of urine wastewaters, widely used in agriculture as fertilizer, which is potentially dangerous and biorefractory. Hence, the degradation of UA (2,6,8-trihydroxy purine) in aqueous solution of pH 3.0 has been studied by conductive-diamond electrochemical oxidation. Hydroxyl radicals formed from water oxidation at the surface of boron-doped diamond anodes were the main oxidizing agents. Effects of current density and supporting electrolyte on the degradation rate and process efficiency are assessed. Results show that the increase of current density from 20 to 60 mA cm–2 leads to a decrease in the efficiency of the electrochemical process. In addition, the best degradation occurred in the presence of NaCl as conductive electrolyte. Interestingly, an almost total mineralization of 50 ppm UA was obtained when anodic oxidation was performed at low current densities (20 mA cm–2) and in the presence of NaCl. This result confirmed that the electrolysis using diamond anodes is a very interesting technology for the treatment of UA. The identification of UA transformation products was performed by high-performance liquid chromatography (HPLC). HPLC analysis of treated solutions revealed that oxalic acid and urea were the two intermediates found. Oxalic acid was the most persistent product. Based on detected intermediates and bibliographic research, a mechanism of UA mineralization by anodic oxidation has been proposed. Ionic chromatography analysis confirmed the release of and ions during UA mineralization.

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.

CONDUCTIVE-DIAMOND ELECTROCHEMICAL OXIDATION OF A PHARMACEUTICAL EFFLUENT WITH HIGH CHEMICAL OXYGEN DEMAND (COD). KINETICS AND OPTIMIZATION OF THE PROCESS BY RESPONSE SURFACE METHODOLOGY (RSM)

Conductive-diamond electrochemical oxidation processes are promising technologies for treating biorefractory industrial wastes with organic loads below 20000 mg L-1. In this work, electrochemical oxidation of a real industrial pharmaceutical wastewater with conductive-diamond anodes has been studied. The electrolyses were carried out in discontinuous operation mode under galvanostatic conditions, using a bench-scale plant equipped with a single compartment electrochemical flow cell. For optimizing the process and studying the interaction between the operating conditions, different experiments were performed by modifying the current density (from 25 to 180 mA cm-2) and recirculation flow rate (from 105 to 565 ml min-1) with residence times between 0 and 570 minutes. The corresponding contribution of these two operative parameters on COD removal and its evolution versus residence time was studied. A time of 98 minutes was obtained in order to evaluate the highest influence of operative parameters. For this time, the current density was found to have a considerable positive effect, while the flow rate proved to be a statistically insignificant variable. ANOVA test reported significance for three of the five involved variables and Response Surface Methodology technique was used to optimize COD removal.

Irradiation-assisted electrochemical processes for the removal of persistent organic pollutants from wastewater

This work focused on the removal of persistent organic pollutants (POPs) from wastewater using irradiation-assisted electrochemical technologies, i.e. sonoelectrolysis, photoelectrolysis and sono-photoelectrolysis. Single-irradiation processes (sonolysis and photolysis) and electrochemical oxidation using conductive diamond anodes (current density of 30 mA cm−2) were also evaluated for comparison. Three POPs with different molecular structures (sulfamethoxazole, metoprolol and caffeine, initial concentration of 100 mg dm−3) were studied to evaluate the robustness of the selected technologies and the oxidation mechanisms involved in each case. Results show that the single application of the irradiation technologies led to the removal of only a small amount of POPs and no mineralisation, with the nature of the pollutant showing a marked effect; the opposite was observed for the single application of conductive diamond electrochemical oxidation (CDEO), which is a highly robust and efficient technology for the degradation of all types of POPs. Sonoelectrolysis, photoelectrolysis and sono-photoelectrolysis processes (ultrasound conditions: 200 W, ultraviolet conditions: 254 nm, 4 W) may show synergistic, antagonistic or nil effects with respect to a single electrochemical oxidation event, depending on the nature of the treated molecule. The differences observed may be related to the different chemical nature of the organic species studied, indicating an important role of mediated oxidation processes, which may be enhanced with ultrasounds and ultraviolet radiation techniques.

Treatment of synthetic urine by electrochemical oxidation using conductive-diamond anodes.

In this work, the electrochemical oxidation of synthetic urine by anodic oxidation using boron-doped diamond as anode and stainless steel as cathode was investigated. Results show that complete depletion of chemical oxygen demand (COD) and total organic carbon (TOC) can be attained regardless of the current density applied in the range 20-100 mA cm(-2). Oxalic and oxamic acids, and, in lower concentrations, creatol and guanidine were identified as the main intermediates. Chloride ions play a very important role as mediators and contribute not only to obtain a high efficiency in the removal of the organics but also to obtain an efficient removal of nitrogen by the transformation of the various raw nitrogen species into gaseous nitrogen through chloramine formation. The main drawback of the technology is the formation of chlorates and perchlorates as final chlorine products. The increase of current density from 20 to 60 mA cm(-2) led to an increase in the rate of COD and TOC removals although the process becomes less efficient in terms of energy consumption (removals of COD and TOC after applying 18 Ah dm(-3) were 93.94 and 94.94 %, respectively, at 20 mA cm(-2) and 89.17 and 86.72 %, respectively, at 60 mA cm(-2)). The most efficient conditions are low current densities and high temperature reaching total mineralization at an applied charge as low as 20 kAh m(-3). This result confirmed that the electrolysis using diamond anodes is a very interesting technology for the treatment of urine.

Electrochemical production of peroxocarbonate at room temperature using conductive diamond anodes

This work describes a method for synthesizing sodium peroxocarbonate by electoxidation of carbonate on conductive-diamond thin film electrodes in a sodium carbonate solution at room temperature. The effect of several parameters including: electrolyte concentration, current intensity, addition of Na2SiO3 as a stabilizing agent, and increments in temperature for sodium peroxocarbonate formation has been studied in an electrochemical H cell of two compartments separated by a cation-exchange membrane. The oxidant formation has been confirmed by cyclic voltammetry and in situ reduction during water electrolysis. The optimal experimental conditions are: the current density of 34.3 mA cm−2, an electrolyte concentration of 1 M, and the addition of 4 g/L of sodium metasilicate in the two compartments of the electrochemical cell.

High efficiencies in the electrochemical oxidation of an anthraquinonic dye with conductive-diamond anodes

Oxidation of anthraquinonic dye Acid Blue 62 by electrolysis with conductive-diamond electrodes is studied in this work. COD, TOC, and color have been selected to monitor the degradation of the molecule as a function of several operating inputs (current density, pH, temperature, and NaCl concentration). Results show that the electrochemical oxidation of this model of large molecules follows a first order kinetics in all the conditions assessed, and it does not depend on the pH and temperature. The occurrence of chloride ions in wastewaters increases the rate of color and COD removal as a consequence of the mediated oxidation promoted by the chlorinated oxidizing species. However, chloride occurrence does not have an influence on the mineralization rate. First-order kinetic-constants for color depletion (attack to chromophores groups), oxidation (COD removal), and mineralization (TOC removal) were found to depend on the current density and to increase significantly with its value. A single model was proposed to explain these changes in terms of the mediated oxidation processes. Rate of mineralization remained very close to that expected for a purely mass transfer-controlled process. This was explained assuming that mediated oxidation does not have a significant influence on the mineralization in spite it has some effect on intermediate oxidation stages. The efficiency of the oxidation was found to depend mainly on the concentration of COD being negligible the effect of the other inputs assessed except for the occurrence of chloride ions. Opposite, the efficiency of mineralization depends on concentration of TOC and current density and it did not depend on the chloride occurrence. This observation was found to have an important influence on the power required to remove a given percentage of the initial TOC or COD. To decrease COD efficiently, the occurrence of chloride in the solution is very important, while to remove TOC efficiently, it is more important to work at low current densities and chloride effect is negligible. Energy consumption could be decreased by folds using the proper conditions.

Sonoelectrolysis of Wastewaters Polluted with Dimethyl Phthalate

In this work, the removal of dimethyl phthalate (DMP) by electrolyses and sonoelectrolyses with conductive-diamond anodes has been studied and compared. Both treatments have been shown to be very effective, attaining complete mineralization in the 1.6–160.0 mg dm–³ range of initial concentrations of DMP. The current density strongly influences the rate and efficiencies of the processes, because of an increase in the pseudo-steady-state concentration of oxidants and the importance of processes occurring near the electrode surface. Ultrasound irradiation produces small differences in the depletion of DMP but significantly improves the chemical oxygen demand depletion rate, mineralization rate, and efficiency. Monomethyl phthalate, phthalic acid, and 4-hydroxyphthalic acid are the main aromatic intermediates, and maleic acid, tartaric acid, and especially oxalic acid are the most significant nonaromatic intermediates found. Concentrations obtained are very small in most conditions studied. In the presence of chloride ions, chlorophenols are found.

Use of conductive-diamond electrochemical-oxidation for the disinfection of several actual treated wastewaters

This work assesses the disinfection for reuse of treated urban wastewaters using electrochemical-oxidation with conductive-diamond anodes, without adding any external chemical reagent. Three actual effluents of municipal wastewater treatment plants (sample collected from the secondary clarifiers) have been treated using this technology, in every case, a complete removal of Escherichia coli have been achieved. In addition, it has been found that working within the range of current densities proposed in this work (1.3–13.0Am−2), this technology does not form any hazardous by-products (neither perchlorates nor halo-compounds), even at trace level. Combined effects of chloramines and hypochlorite seem to be the main mechanisms to explain disinfection. The primary disinfectant produced in every electrolyses depends on the current density, and on the concentration of both: chlorides and non-oxidized nitrogen, present in the raw treated wastewater. Power consumption as low as 0.2kWhm−3 is enough to assure the complete disinfection of a particular treated wastewater. The lower the applied current density, the lower the energy required to get the disinfection of the treated wastewater.

Effect of the cathode material on the removal of nitrates by electrolysis in non-chloride media

In this work, the effect of the cathode material (conductive diamond, stainless steel, silicon carbide, graphite or lead) and the current density (150–1400Am−2) on the removal of nitrates from aqueous solutions is studied by electrolysis in non-divided electrochemical cells equipped with conductive diamond anodes, using sodium sulphate as the electrolyte. The results show that the cathode material very strongly influences both the process performance and the product distribution. The main products obtained are gaseous nitrogen (NO, N2O and NO2) and ammonium ions. Nitrate removal follows first order kinetics, which indicates that the electrolysis process is controlled by mass transfer. Furthermore, the stainless steel and graphite cathodes show a great selectivity towards the production of ammonium ions, whereas the silicon carbide cathode leads to the highest formation of gaseous nitrogen, which production is promoted at low current densities.

Influence of the supporting electrolyte on the electrolyses of dyes with conductive-diamond anodes

In this work, electrolyses of solutions containing one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) synthetic dyes were carried out using conductive-diamond anodes. From the obtained results, clearly the oxidation was largely influenced by the supporting electrolyte used: (i) in the presence of chloride ions, the primary mechanism is mediated electrooxidation in solution with formation of a significant amount of intermediates; (ii) in the presence of sulfate ions, the dyes are directly mineralized by hydroxyl radicals. In every condition, the synthetic dyes were completely removed from the solutions. The kinetics of the electrooxidation process was mass-transport controlled, although the proper treatment of such dye solutions was assured with energy consumptions below 60kWhm−3.

Electro-oxidation of As(III) with dimensionally-stable and conductive-diamond anodes

In this work, arsenic oxidation by an electrochemical process was studied in a batch bench-scale electrolysis plant equipped with mono- and bi-compartment cells, dimensionally-stable anodes (DSAs) and conductive-diamond anodes (CDAs). The results demonstrate that the electrolysis is an adequate technology to transform As(III) into As(V) species, which is an important pre-treatment stage for removing of arsenic from water using technologies such as coagulation or electro-coagulation. The process requires large current densities in non-divided cells to obtain a good As(V)/As(III) ratio, but it can be more efficiently performed at low current densities in cells divided by cationic membranes. The presence of chlorides or sulphates can significantly affect the results due to the formation of powerful oxidants that contribute to the net oxidation process.

Electrochemical Synthesis of Peroxyacetic Acid Using Conductive Diamond Electrodes

In this work, the synthesis of peroxyacetic acid and peroxoacetate salts by electrolysis with conductive-diamond anodes is described, using three different raw materials: ethanol, acetaldehyde, and sodium acetate. Results show that peroxyacetic acid is produced at significant concentrations during the electrolyses of the three raw materials, although sodium acetate achieves the highest efficiency and, at the same time, the smallest carbon mineralization. Acetaldehyde behaves as a good raw material during a first stage but, afterward, it catalyzes the decomposition of peroxoacetate to the acetate anion. Moderate current densities and slightly alkaline pH seems to promote the synthesis of peroxyacetic acid and to restrain raw matter mineralization.

Use of low current densities in electrolyses with conductive-diamond electrochemical — Oxidation to disinfect treated wastewaters for reuse

In the search for new disinfection technologies for reused water, in this work the effect of current density on the disinfection capability, during the electrolyses with conductive diamond anodes of the effluent of a municipal wastewater treatment plant was evaluated. It was found that this electrolytic technology can disinfect completely for current densities ranging from 1.3 to 130 A m − 2 , but the disinfection rate and chlorine speciation are significantly affected by particular current density applied. In order to avoid the occurrence of non desired species, the lower value of this range is proposed. In those conditions, complete disinfection can be obtained and hypochlorite and chloramines are the main species, not being produced chlorate, perchlorate or organo-chlorinates species.

Improvements in the Electrochemical Production of Ferrates with Conductive Diamond Anodes Using Goethite as Raw Material and Ultrasound

The improvement in the electrochemical production of ferrates with conductive diamond anodes using goethite as raw material and ultrasounds has been studied. The conductive-diamond electrolysis of suspensions of goethite leads to a higher ferrate concentration than that of Fe(OH)3 suspensions and of iron-powder bed. With this raw material, the ferrate concentration increases continuously with a constant production rate, after a first more efficient stage in which results are similar to the other two raw materials. In addition, the ferrate production rate is significantly improved in the sonoelectrochemical system, suggesting that the amount of soluble iron ready to be oxidized to ferrates increases with the ultrasound effect. The initial iron concentration only seems to have an influence on the first stage of the process. On the other hand, the concentration of ferrate strongly depends on the hydroxide anion concentration: the higher is the hydroxide concentration, the higher the ferrate concentration is.

Synthesis of novel oxidants by electrochemical technology

In this work it has been studied the electrochemical production of different oxidants with conductive-diamond anodes. This technology was found to allow the production of stable oxidants that can not be easily synthesized using other electrodes or by other more usual techniques. Thus, it has been found that monoperoxophosphoric acid, and peroxodiphosphate, peroxodisulphate and ferrate salts can be easily produced by this technology, when the right operation conditions are used. In contrast, the production of pure hypochlorite is not favoured, because this technology leads to the formation of a mixture of hypochlorite with other oxoanions of chlorine in a higher oxidation state, particularly chlorates and perchlorates, which seems to be the final product in these electrolyses. The production of perbromates salts has also been pointed, although in this case only preliminary results are shown. In every case, it has been observed two marked trends in the process efficiency as function of the current density applied. This fact can be explained in terms of the contribution of hydroxyl radicals in the oxidation mechanisms that occur on diamond surfaces. Results obtained in this work also allow explaining the influence of the electrolyte salt on the efficiency of the electrochemical oxidation of wastewaters.

Advanced oxidation processes for the treatment of wastes polluted with azoic dyes

In this work, synthetic wastes polluted with azoic dyes have been treated with conductive-diamond electrochemical oxidation, Fenton oxidation and ozonation. Eriochrome black T (EBT), methyl orange (MO) and congo red (CR) were selected as model pollutants. Although the three technologies are classified as advanced oxidation processes and, consequently hydroxyl radicals are assumed to be involved in their oxidation mechanism, the results obtained in this work show important differences. The three technologies are able to successfully decolourise the wastes during the first stages, which means that the breakage of the azoic group is one of the first steps in the oxidation mechanisms. However, the efficiencies and the mineralization percentages obtained depend strongly on the oxidation technique and on the concentration of pollutant. For diluted wastes, the electrochemical oxidation is least efficient because the primary oxidation mechanisms that occur in conductive-diamond anodes are greatly affected by mass transport limitations. For highly-loaded wastes, in the Fenton process an important fraction of refractory carbon remains at the final stages of the treatment and, consequently, the process is the less efficient. The efficiency of ozonation and electrochemical oxidation are very similar (especially during the first stages), although the energy consumption required by the electrochemical process to remove at fixed percentage of COD or TOC is significantly smaller than that of ozonation.

Electrochemical treatment of the effluent of a fine chemical manufacturing plant

In this work, the electrochemical oxidation of an actual industrial wastewater with conductive-diamond anodes has been studied. The wastewater is the effluent of a fine chemicals plant. This effluent consists of an aqueous solution of solvents (ketones and alcohols) with a high concentration of aromatic compounds coming from the raw materials, intermediates and products of the different processes of the plant and its COD is around 6000 mg dm −3. The electrolyses were carried out in a discontinuous operation mode under galvanostatic conditions, using a bench-scale plant equipped with a single compartment electrochemical flow cell. The conductive-diamond electrochemical oxidation (CDEO) allowed achieving the complete mineralization of the waste with high current efficiencies. These efficiencies seem to strongly depend on the concentration, pH and temperature but not on the current density (in the range studied). This confirms that besides the hydroxyl radicals mediated oxidation, CDEO combines other important oxidation processes such as the direct electrooxidation on the diamond surface and the oxidation mediated by other electrochemically formed compounds generated on this electrode. Other two advanced oxidation processes (ozonation and Fenton oxidation) have been also studied in this work for comparison purposes. Both technologies were able to treat the wastes, but they obtained very different results in terms of efficiency and mineralization. The efficiency of ozonation and electrochemical oxidation were very similar (especially during the first stages), although the energy consumption required by the electrochemical process to remove at fixed percentage of COD or TOC was significantly smaller than that of ozonation. The possible accumulation of carboxylic acid as final products excludes the use of Fenton oxidation as a sole treatment technology.