Study of the Electrocoagulation Process Using Aluminum and Iron

Aug 21, 2007 - Iron and aluminum have been widely used as electrode materials in electrocoagulation systems according to the literature, because they ...
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Ind. Eng. Chem. Res. 2007, 46, 6189-6195

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Study of the Electrocoagulation Process Using Aluminum and Iron Electrodes Pablo Can˜ izares, Carlos Jime´ nez, Fabiola Martı´nez, Cristina Sa´ ez, and Manuel A. Rodrigo* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, UniVersidad de Castilla La Mancha, Campus UniVersitario s/n, 13005 Ciudad Real, Spain

In this work, the electrocoagulation process using aluminum and iron electrodes has been used to treat synthetic wastewaters polluted with three different types of pollutant models: kaolin suspensions, dye solutions, and oil-in-water emulsions. It was obtained that both electrodes can achieve high efficiencies (above 80%) in the treatment of the three wastes. However, there are strong differences in the electrochemical coagulation or breakup mechanism that can be explained in terms of the speciation of the dissolved metals and especially in terms of the significant concentrations of monomeric and polymeric ionic species that appear in the treatment with aluminum electrodes. In every case, sweep coagulation explains the coagulation of kaolin suspension with both aluminum and iron electrodes. However, in the case of aluminum, the neutralization charge mechanism should also be considered for low reagent doses. The coagulation of EBT (Eriochrome Black T) solutions and the breakup of O/W emulsions (oil-in-water emulsions) have been explained by the binding of the pollutants to metal hydroxide precipitates. This binding is promoted for aluminum electrodes because of the adsorption of cationic reagent species on the surface of the aluminum hydroxide. Introduction Conventional chemical coagulation consists of the direct dosing of a coagulant solution to the wastewater in order to reduce the electrical repulsion forces that inhibit the aggregation of particles. In the chemical coagulation process, the addition of hydrolyzing metal salts (of Fe3+ or Al3+) as coagulant reagents is typical. The electrochemical coagulation method, on the contrary, consists of the in situ generation of coagulants by electrolytic oxidation of an appropriate anode material (e.g., iron or aluminum).1,2 Recent research in wastewater treatment using electrochemically assisted coagulation has shown that this could be a competitive technology with the conventional coagulation process.3-11 Iron and aluminum have been widely used as electrode materials in electrocoagulation systems according to the literature, because they are cheap and have been demonstrated to be very effective on the electrocoagulation process. Depending on the use, one of them is preferred. Thus, for applications that are not continuous in time, aluminum electrodes are the best choice,5 because iron can be oxidized easily and corrosion problems on the electrodes are reported when the cell is not connected. In this context, the use of iron as the electrode material has another additional problem because of the color of Fe(III) salts. In this work, the electrocoagulation process using aluminum and iron electrodes has been used to treat synthetic wastewaters polluted with three different types of pollutant models: • Kaolin, as a model of colloidal pollutant, since clays behave as hydrophobic colloids in water. These compounds consist of flat sheets of alternating layers of silicon oxides and aluminum oxides, held together by ionic attraction for cations sandwiched between the sheets. In water solutions, aluminum (+3) or silicon (+4) can be replaced with sodium (+1), potassium (+1), or ammonium (+1) ions, resulting in a negative charge of the particle. This charge is responsible for the electric repulsion of kaolin particles and, thus, for the stability of the colloidal suspension. The aggregation of colloids can be accomplished * To whom correspondence should be addressed. Tel.: +34 902204100. Fax: +34 926 29 53 18. E-mail: [email protected].

either by destabilization or by the enmeshment of the colloidal particles in a growing hydroxide precipitate formed in the waste (sweep coagulation). From the theoretical point of view, two are considered to be the main processes involved in the destabilization of colloidal suspensions:12,13 the decrease in the thickness of the diffuse part of the electrical double layer, caused by an increase in ionic strength, and the neutralization of the particle charge by the specific adsorption of counterions. However, it is unlikely that a sufficient increase in ionic strength would be a practical coagulation method.12 Hence, counterion adsorption is considered to be the primary mechanism in most coagulation processes. This process can be promoted by the addition of monomeric or polymeric ionic reagents. In the later case, the aggregation of particles can also be caused by polymer bridging12 (adsorption of different functional groups of a polymer onto different colloids). • Dyes as a model of soluble organic pollutants. There are many soluble organic compounds that can coagulate, ranging from simple molecules like phenol14 to more complex structures like dyes. The functional groups contained in the molecule and, particularly, their interaction with the coagulant reagents can explain this process for every particular case. However, in the literature, a special attention is paid to dyes, because of the huge amount discharged by different types of industries.15 This group consists of colored substances with a complex chemical structure (many functional groups) and a high molecular weight. These compounds are also highly soluble in water and are persistent once discharged into a natural environment. Thus, their removal from industrial effluents is also subject of the major importance from the environmental point of view. The removal of dissolved organic matter by coagulation is widely reported in the literature.16,17 Thus, two are supposed to be the primary mechanisms for the removal of dissolved organic matter by coagulation:12,13,18 the binding of the metal species to the anionic sites of the organic molecules, neutralizing their charge and resulting in reduced solubility, and the adsorption of organic substances on amorphous metal hydroxide precipitates. • Oil-in-water emulsions as a model of emulsified effluents. An emulsion consists of a dispersion of one immiscible liquid into another, through the use of a chemical reagent that reduces

10.1021/ie070059f CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007

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the interfacial tension between the two liquids to achieve stability. Commonly, this chemical reagent consists of an amphiphile molecule that contains both hydrophilic and hydrophobic groups. In the case of oil-in-water (O/W) emulsions, the reagent is frequently a mineral oil. As a result of the adsorption of this reagent, the oil droplets have a net charge on their surfaces, which causes repulsion forces between them. These repulsion forces between droplets explain the stability of the emulsions. As a consequence of the small size of droplets in the dispersion, the macroscopic appearance of the emulsion is that of a homogeneous liquid, although this mixture is really a heterogeneous system.19 The destabilization methods involve the addition of coagulant agents to the wastewater, which promote the breakup of the emulsion due to the reduction of the superficial charge of the droplets, causing the coalescence of the oil droplets, and the subsequent separation of the aqueous and oily phases by means of conventional settling or dissolvedair flotation. According to the literature,19 the main destabilization mechanism is the attachment of adsorbing macromolecules to more than one droplet at a time (bridging flocculation). Destabilization by nonadsorbing polymers can also be promoted through the mechanism of depletion flocculation. The first mechanism normally involves electrically charged species as reagent, as these species can combine (by attractive electrical forces) with the opposite electrically charged active sites that are present on the surface of the droplet. In this work, the aim is to understand the differences obtained in the electrocoagulation of these three models of wastewaters as a function of the electrode material. To do that, batch experiments have been carried out under the same operation conditions using iron and aluminum as electrode materials. Results have been interpreted in terms of the species formed during the metal salt hydrolysis. In addition, a study of the dissolution of the aluminum and iron electrodes is also carried out to clarify the role of the chemical dissolution on the global electrodissolution process. As is known, and according to the literature,20-22 the amounts of dissolved aluminum or iron are not always in agreement with Faraday’s law. This work will help to understand this process. Experimental Section Experimental Devices. The electrocoagulation experiments have been carried out in a bench-scale plant with a singlecompartment electrochemical flow cell that can work in batchoperation mode (Figure 1). Aluminum or iron electrodes were used as the anode and cathode. Both electrodes were square in shape (100 mm side), each with a geometric area of 100 cm2 and with an electrode gap of 9 mm. The electrical current was applied using a dc power supply FA-376 Promax. The current flowing through the cell was measured with a 2000 digital multimeter Keithley. The wastewater was stored in a 5000 mL glass tank stirred by an overhead stainless steel rod stirrer Heidolph RZR 2041 and thermostatized by means of a water bath, which allowed maintaining the temperature at the desired set point. The wastewater was circulated through the electrolytic cell by means of a peristaltic pump. Aluminum and Iron Speciation. The characterization of the hydrolyzed aluminum or iron species generated has been carried out by the ferron method.23-29 This method consists of the timed spectroscopy monitoring of metal (aluminum or iron) ferron (8hydroxy-7-iodo-5-quinolinesulfonic acid) reaction, to form a complex of probable composition25 Me(ferron)3, which has a maximum absorbance of 364 nm. Monomeric species react almost instantaneously with ferron, whereas polymeric species

Figure 1. Layout of the bench-scale plant. Detail of the electrochemical flow cell.

have a much slower reaction rate with this compound. The particles of precipitate practically do not react with ferron. Therefore, this method allows distinguishing among monomeric, polymeric, or precipitate species. The analytical measurement has been carried out by filtering the samples using micropore membranes of 0.45 µm to remove the particles of precipitate. Once the sample is filtered, an aliquot is added to the volume of saturated ferron solution freshly prepared (as ferron is not stable27) so that ferron is in excess, at pH 5 in an acetate buffered solution. Immediately, the absorbance of the sample is monitored with time, until a constant value is obtained, which is indicative of the end of the reaction. By plotting the logarithm of the unreacted metal (aluminum or iron) vs time, the ratio of hydrolyzing-metal species that react quickly and slowly with ferron (that is, monomeric and polymeric species) can be estimated. Extrapolation of the linear parts of the curve to zero time yields information on the amount of metal that is bound in complexes of different degrees of polymerization.23-25 The measurement of total and soluble metal (filtered with 0.45 µm) reports the ratio of soluble and precipitate metal. Experimental Procedure. Electrochemical coagulation experiments were carried out under galvanostatic conditions. Prior to every experiment, the electrodes were treated by rinsing with a solution of 1.30 M HCl in order to reject any effect due to the different prehistory of the electrodes. For the electrochemical coagulation experiments, the pollutant solution was pumped from the feed tank to the cell and then it was recirculated to the feed tank. Samples were taken at the outlet of the cell. After a time of settling (20 min in the case of kaolin and 1 h in the case of EBT and O/W emulsions), different measurements were made depending on the kind of synthetic wastewater studied: • In the case of kaolin, turbidity (using a 115 Velp Scientifica turbidimeter) and pH (using an inoLab WTW pH meter) were measured in the clarified liquid. • In the case of EBT, samples were first filtered and then the absorbance spectrum (using a UV-visible spectrophotomer Shimadzu UV-1603) and the pH (using an inoLab WTW pH meter) were measured for the filtered liquid. To estimate the

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Figure 2. Variation of aluminum and iron concentration electrogenerated in the electrochemical processes with the electrical charge passed in batch mode operation: [, aluminum; 0, iron; s, values predicted using Faraday’s Law; temperature, 25 °C; initial pH, 4.8; flowrate, 50 dm3 h-1; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3.

removal of EBT, the absorbance at 550 nm (corrected by the pH value) was measured (550 nm is the wavelength that shows the maximum absorbance). • In the case of O/W emulsions, chemical oxygen demand (COD; using a Hach DR2000 analyzer) and pH (using an inoLab WTW pH meter) were measured in the aqueous phase. To determine the amounts of aluminum and iron electrogenerated by the applied current at the different operation conditions, as well as the hydrolysis species formed for both metals, several experiments were performed. In these assays, the electrolyte only consists of NaCl, as well as NaOH or HCl added for any subsequent pH adjustment (without any pollutant). After the experiments, samples were monitored by the ferron method, and aluminum or iron concentration, zeta potential, and pH were then measured off-line. The aluminum and iron concentrations were determined by dilution (50:50 v/v) of samples with HNO3 4 N and measured using an inductively coupled plasma Liberty Sequential Varian according to a standard method30 (plasma emission spectroscopy). Zeta potential was measured using a Zetasizer Nano ZS (Malvern, U.K.). To study the chemical dissolution of the electrodes, some experiments were carried out in a batch operation using stirred beakers. These beakers were initially filled with a solution of chloride, at different pHs, and a piece of aluminum or iron was placed inside. Samples were taken from the beakers, and the pH and the aluminum or iron were measured according to the methods previously described. The metal dissolution rates were calculated after fitting the experimental data obtained in these essays to the mass balance equations of the batch reactor. The initial and final weights of the metal pieces were also used to confirm the results. Results and Discussion Electrodissolution of the Aluminum and Iron Electrodes. Figure 2 shows the variation of the aluminum and iron concentrations electrogenerated in the electrochemical processes with the electrical charge passed, compared to the expected values if the process was purely electrochemical (assuming the only anodic process is the metal oxidation). As can be seen, the electrodissolved metal concentration increases linearly with the electrical charge for both aluminum and iron, and the experimental values are greater than the values calculated if the process is considered to be purely electrochemical (according to Faraday’s Law). The differences observed between the experimental results and those expected if the process was purely electrochemical have been explained3,20,29 in terms of the chemical dissolution of the electrode surfaces.

Figure 3. Aluminum and iron dissolution rates as a function of the pH; supporting media, 3000 mg dm-3 of NaCl. The onset corresponds to a shorter range of pH for the electrochemical dissolution process using aluminum as electrode material: [, aluminum; 0, iron.

To confirm this, Figure 3 shows the aluminum and iron dissolution rates versus pH in a batch reaction system when a sheet of 4 cm2 area (and 0.8 mm and 1.5 mm thickness, respectively) is placed into a solution that contains sodium chloride. One can observe the presence of a minimum in the case of aluminum at close to neutral pH and that the dissolution rate is several orders of magnitude higher at alkaline pHs. On the contrary, in the case of iron, no minimum appears, and the dissolution rate increases significantly for acidic pHs. In every case, hydrogen bubbles evolving from the sheet surface were clearly observed, especially in the conditions in which the dissolution rates are higher. This observation is indicative that reaction 1 is taking place for aluminum sheets and reactions 2 and 3 are taking place in the case of iron sheets.

2Al + 6H2O f 2Al3+ + 3H2 + 6OH-

(1)

Fe + 2H2O f Fe2+ + H2 + 2OH-

(2)

4Fe + 10H2O + O2 f 4Fe3+ + 4H2 + 12OH-

(3)

The use of the same material to be applied as the anodes and the cathodes is an important matter in electrocoagulation processes, as it allows reversing the polarity of the cell, and hence, this helps to minimize problems that arise from the deposition of carbonate layers on the cathodic surface. In this context, it is important to note that the chemical dissolution of the electrode surfaces is promoted at alkaline pHs (for the case of aluminum) and at acidic pHs (for the case of iron). As is known, in each electrochemical cell, there is a pH profile between anode and cathode.3,20 On the anode, the water oxidation process generates a high concentration of protons, resulting in a lower pH. On the cathode, the water reduction process results in the formation of hydroxyl ions and, hence, in a higher pH. This means that, in the case of iron-based cells, on the anode surface is going to be promoted the chemical dissolution, while in the case of aluminum-based cells, the formation of coagulant on the cathode surface by chemical dissolution is a subject of major importance. This was clearly modeled for the case of aluminum in a previous work20 of our group. According to this model, a pH close to 2.1 in the nearness of the iron surface can explain the chemical dissolution of iron, while a pH near 11.1 on the proximities of the cathode surface in the aluminum-dosage process is able to explain the superfaradaic observation. Electrocoagulation of a Wastewater Polluted with Kaolin. Figure 4 shows the influence of the electrodic material on the

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Figure 5. Aluminum and iron speciation (expressed as aluminum and iron concentration) in a batch operation mode electrocoagulation assay: -×-, monomeric species; -9-, polymeric species; -[-, precipitate; current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 4.5; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3.

Figure 4. Removal of turbidity, pH, and zeta potential for an electrocoagulation assay of a wastewater polluted with kaolin in batch-mode operation using as electrode material: [, aluminum; 0, iron; current density, 0.5 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 4.5; kaolin concentration, 1000 mg dm-3; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3. (c) s, isoelectric point.

results obtained in the electrocoagulation of kaolin suspensions. Great differences are observed in the turbidity changes with the dose of coagulant reagents. In the case of aluminum, two zones of high efficiency can be clearly discerned: one for low aluminum doses (range 0.05-0.15 mmol dm-3) and another for high aluminum concentration (range > 0.6 mmol dm-3). Both zones are clearly split by a minimum in the turbidity removal (∼0.35 mmol dm-3). On the contrary, the coagulation with iron does not exhibit this minimum and only one highefficiency coagulation zone is observed. The pH increases in both cases up to a value of 7. In the case of iron, the change is very abrupt, while in the case of aluminum, there is a softer increase. The major differences are observed for the zeta potential. As can be seen in the case of aluminum electrocoagulation, a zeta potential of zero is achieved close to the concentration of aluminum in which the maximum of turbidity removal is obtained. In the case of iron electrocoagulation, the zeta potential is always negative except for the very high dosage in which it is close to zero. These observations have to be explained in terms of the aluminum or iron species formed in the reaction system. In this context, it is important to take into account that the aqueous chemistry of aluminum and iron is especially complex, as it involves not only monomeric ionic species and the formation of precipitates but also oligomeric and polymeric species.

Because of this large number of species, the quantification of the hydrolysis rate constants, and even of the particular speciation, is a complicated task, and there are significant differences between the values reported in the literature.12,13,27,31-34 For this reason, in this work a simple but commonly used method has been used to quantify the metal speciation. Thus, Figure 5 shows the concentrations of the iron and aluminum species measured by the ferron procedure in both coagulation essays. It can be observed that, in both cases, precipitates are the primary species. In the electrocoagulation with aluminum, a significant concentration of monomeric ions are observed, while in the case of iron, the concentrations are completely negligible. Taking into account this speciation and also the values of the zeta potential, it can be suggested that sweep coagulation is the primary mechanism in the coagulation with iron. In the coagulation with aluminum, two unconnected mechanisms operate: for low concentration, the primary mechanism is charge neutralization by monomeric cationic aluminum species, while for high doses, sweep coagulation with amorphous aluminum hydroxide explains the results. Electrocoagulation of a Wastewater Polluted with EBT. Figures 6 and 7 show the results obtained in the electrocoagulation of wastewaters polluted with EBT, using aluminum or iron as electrode materials. A similar behavior is obtained for both electrodic materials, although the doses required to meet the same efficiencies are lower in the case of aluminum and, in addition, the removal efficiencies are slightly higher for this material (very close to 100%). Measured pHs and zeta potentials increase during the treatment, and the values of the zeta potential are always negative. Significant concentrations of monomeric and polymeric ions are measured during the aluminum dosage, while the iron dosage does not exhibit the formation of these species. Taking into account these observations, the binding of EBT molecules to the surface of small iron or aluminum precipitated particles can be suggested as the primary mechanism. These precipitate particles can adsorb both the EBT molecules and

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Figure 6. Removal of EBT, pH, and zeta potential for an electrocoagulation assay of a wastewater polluted with EBT in batch-operation mode using as electrode material: [, aluminum; 0, iron; current density, 1.4 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 4; EBT concentration, 100 mg dm-3; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3. (c) s, isoelectric point.

Figure 7. Aluminum and iron speciation (expressed as aluminum and iron concentration) in a batch operation mode electrocoagulation assay: -×-, monomeric species; -9-, polymeric species; -[-, precipitate; current density, 1.4 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 4; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3.

the coagulant ionic species. In the later case, this results in positive charge surfaces that can enhance the adsorption of EBT

Figure 8. Removal of COD, pH, and zeta potential for an electrocoagulation assay of an O/W emulsion in batch operation mode using as electrode material: [, aluminum; 0, iron; current density, 2.2 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 8.5; oil concentration, 3000 mg dm-3; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3. (c) s, isoelectric point.

by electrostatic attraction between the negative functional groups of EBT and the positive adsorption sites of the precipitate. In the case of aluminum, the concentration of these ionic species is very important, and this explains the high efficiency and also the lower dose of coagulant needed. On the contrary, in the case of iron, the prevalence of the precipitate over the ionic species reduces the binding of the EBT molecules, resulting in a lower efficiency process. In this context, it is important to note that, although no soluble polymeric or monomeric iron species are found (according to the ferron procedure), it does not exclude that they could be directly adsorbed on the surface of the amorphous iron hydroxide. Electrocoagulation of an O/W Emulsion. Figure 8 shows the main parameters involved in the electrochemical breakup of an O/W emulsion using iron or aluminum electrodes (the oily phase of the emulsion is composed by a common lubricant oil (REPSOL ELITE TDI 15W40 provided by REPSOL-YPF, Spain) and a soluble oil (SOL 1000 provided by Molydal, France)). Likewise, Figure 9 shows the coagulant species in both treatments determined according to the ferron procedure. The influence of the dose of reagent on the treatment efficiencies looks like that observed in the case of EBT electrocoagulation, although, in this case, the higher efficiencies are obtained when iron is used as electrode material. The pH is maintained almost constant during the treatments. On the

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concentrations of monomeric and even (in some cases) polymeric ionic species. • Sweep coagulation can explain the coagulation of kaolin suspension with both aluminum and iron electrodes. However, in the case of aluminum, the neutralization charge mechanism should also be considered for low reagent doses. • The binding of EBT to metal hydroxide precipitates can explain the coagulation of this dye. This binding is promoted for aluminum electrodes because of the adsorption of cationic reagent species on the surface of the aluminum hydroxide. The same mechanisms explain the breakup of O/W emulsions. Acknowledgment This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through Project CTM2004-03817/TECNO. Literature Cited

Figure 9. (a) Aluminum speciation in a batch-operation mode electrocoagulation assay; (b) iron speciation in a batch-operation mode electrocoagulation assay: -×-, monomeric species; -9-, polymeric species; -[-, precipitate; current density, 2.2 mA cm-2; temperature, 25 °C; flowrate, 50 dm3 h-1; initial pH, 8.5; supporting media, 3000 mg dm-3 of NaCl; volume, 1.5 dm-3.

contrary, the zeta potential increases and there is a charge reversal in the case of aluminum but not in the case of iron. Precipitates are the primary species in both cases and only significant concentrations of monomeric ionic species are found for the treatment with aluminum. In this case, the results can be explained according to the same mechanism proposed for the EBT coagulation. Oil droplets can be absorbed onto the amorphous metal hydroxide particles formed during the treatment. The proximity of several oil droplets can make them join (coalescence) and form a bigger one. Because the surfaces of these oil droplets are negatively charged, the presence of adsorbed cationic groups on the surface of the metal hydroxide can promote the efficiency of the breakup process for lower doses of reagent. This explains the lower doses of aluminum required, as in this case a significant concentration of monomeric species are detected in the system. On the contrary, the higher efficiencies obtained for a high concentration of iron should be explained in terms of the neutrality of the precipitate surface. This neutrality avoids the strong interaction between the precipitate and the oil drop and then promotes the separation of the coalesced oil drop from the precipitate. In the case of aluminum, the strong electrostatic interaction can favor that very small particles of precipitate with an oil drop remain after the coalescence of the phase, and then they are measured as COD in the aqueous phase. Conclusions From this work, the following conclusions can be drawn: • Iron and aluminum electrodes can be successfully used as coagulants in the treatment of kaolin suspensions and EBT solutions. They can also be used effectively as reagents in the breakup of oil/water emulsions. • There are strong differences in the electrochemical coagulation (or breakup) with iron and aluminum electrodes. They can be explained in terms of the speciation of the dissolved metals. Amorphous hydroxides are the primary species in both cases. However, aluminum electrodissolution also yields significant

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ReceiVed for reView January 10, 2007 ReVised manuscript receiVed June 8, 2007 Accepted July 12, 2007 IE070059F