The electrodissolution of aluminum electrodes in aqueous solutions containing sulfate or chloride ions is studied in this work. The results obtained are important in order to obtain a better understanding of the electrocoagulation process, as the electrodissolution of the anode surface is its first step. It has been determined that both chemical and electrochemical dissolution play an important role in the aluminum generation. The chemical dissolution of aluminum is strongly influenced by the pH. Alkaline pHs increase the dissolution rate by orders of magnitude. Within the experimental conditions used, the supporting media does not seem to influence greatly the chemical dissolution process. The electrochemical dissolution process depends mainly on the specific electrical charge passed. Salinity does not significantly affect the electrodissolution rate. Good fittings between experimental and modeled data are obtained by modeling the system with a simple model based on two assumptions:  a highly segregated flow pattern and the calculation of aluminium species and pH from a pseudoequilibrium approach.

Dyes are common pollutants in a large variety of industrial wastewaters, and the treatment of these wastes by coagulation has been extensively studied in the literature. This work is focused on the comparison of the efficiencies of the chemical and the electrochemical coagulation processes with hydrolyzing aluminum salts, and it tries to determine the similarities or differences that exist between the two coagulation processes. To do this, Eriochrome Black T solutions were used as a model of dye-polluted wastewater, and experiments of both coagulation technologies were planned to meet the same operation conditions. The pH, the aluminum concentration, the type of electrolyte, and the mode of dosing of aluminum were found to influence the process. Moreover, the speciation of aluminum was found to be the key parameter to explain the results, in terms of the mechanisms previously proposed in the literature for dissolved organic matter coagulation.

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.

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The aim of this work is to study the differences between the hydrolyzing aluminum species formed in an aqueous solution when the aluminum is added by aluminum salt solution dosing and when it is supplied by electrodissolution. The dosing of aluminum is the first step in the coagulation processes, and it marks the more important differences between the coagulation and the electrocoagulation processes. It has been found that the speciation of aluminum in an aqueous solution does not depend directly on the dosing technology, but on the total concentration of aluminum and pH. This latter parameter changes in different ways for the solution dosing and the electrochemical dosing technologies, and this can be the main difference between both technologies: the pH value increases during the electrochemical process and decreases during the solution dosing process. In continuously operated processes, and feeding the solution dosing and the electrochemical dosing processes with solutions at different pHs (with the aim to obtain the same pH at the steady state), the results obtained in the speciation were nearly the same. More significant differences have been obtained in the comparison of the dosing processes for the discontinuous operation mode as it is impossible to maintain both the aluminum concentration and the pH at the same value. In the acidic range of pHs, the predominant species are the monomeric cationic hydroxoaluminum species. Increases in the pH lead to the coexistence of these monomeric species with increasing amounts of polymeric cations and precipitates. Under pHs close to neutrality, the predominant species are the aluminum hydroxide precipitates, and increases in the pH lead to the dissolution of the precipitates to form monomeric anionic hydroxoaluminum, which is the predominant species at alkaline pHs. The formation of precipitates is promoted in solutions containing sulfates. The ζ potential has been found to give important information and to depend mainly on the pH: pHs below 8 lead to positive values of the ζ potential, while higher values of pH cause negative ζ potentials. This behavior has been explained in terms of the formation of particles of aluminum hydroxide precipitate and of the adsorption of ionic species on their surface.

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The electrocoagulation of a synthetic wastewater has been studied in this work. The electrochemical process was carried out in a continuous single-flow electrochemical cell equipped with aluminum electrodes. Kaolin suspensions were used as a model of wastes polluted with colloids, as clays behave as hydrophobic colloids in water. The results obtained were useful to clarify the mechanisms that are involved in the electrocoagulation of this kind of waste and also to study the influence of the different operation conditions in the process. It has been found that the more important variables in the process were the aluminum concentration generated in the system and the pH. The concentration of aluminum generated in the electrochemical cell was always over the expected value (superfaradaic efficiencies) due to the important contribution of the chemical dissolution of the electrodes. This chemical dissolution of the electrodes depended strongly on the pH. A larger concentration of aluminum in the waste did not result in greater process efficiencies. For acidic pHs, a small concentration of aluminum achieved good coagulation efficiencies (80% removal of turbidity), while for alkaline pHs neither high nor low concentrations of aluminum yielded good coagulation results. For pHs close to neutral, a large concentration of aluminum was required to achieve good results. Two primary coagulation mechanisms can explain the experimental behavior of the system: at acid pH the neutralization of the superficial charge of the clays and at neutral pH (and also at high concentrations of aluminum) the enmeshment of the kaolin particles into a sweep floc.