Comparison of the aluminum speciation in chemical and electrochemical dosing processes

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 zeta potential has been found to give important information and to depend mainly on the pH: pHs below 8 lead to positive values of the zeta potential, while higher values of pH cause negative zeta 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.