Electrochemical dosing of iron and aluminum in continuous processes: A key step to explain electro-coagulation processes

The aim of this work is to study the main differences between aluminum and iron production during electrochemical coagulation processes, attending not only to the coagulant dosage, but also to the characteristics of the coagulants produced. To do this, the influence of both pH and current density have been studied. It has been observed that there is a super-faradaic production of aluminum because of a promoted corrosion at the alkaline pHs produced on the cathode surface. This efficiency decreases for high values of current density because of cathodic protection processes. On the contrary, for iron electrodes, it does not exist a super-faradaic dissolution within the range of current densities studied but simply a decrease in the efficiency for alkaline pH values and high current densities, because of the formation of Fe3+ instead of Fe2+ during the electro-dissolution process. With respect to speciation, it has been observed the formation of monomeric species at acidic pHs and precipitate species for pHs above 5. In addition, for aluminum, monomeric species are also the primary species for pHs above 9 and some polymeric species are formed at pHs between 4 and 6. The superficial charge of both metal precipitates is very similar. For pHs below 9, both aluminum and iron precipitates have a positive charge due to the adsorption of monomeric soluble hydroxometals positively charged or protons on the precipitate surface. For pHs above 9 the aluminum precipitate is negatively charged due to the adsorption of soluble hydroxometal species negatively charged or hydroxyl ions on the precipitate surface. On the contrary, iron precipitate does not have a superficial charge for alkaline pH, because there is almost no formation of soluble negatively-charged species.

► Analysis of the aluminum and iron production during electrochemical coagulation processes.
► The metal dissolution process in an electrochemical cell is due to both electrochemical and corrosion processes.
► Aluminum dissolved in the cell is higher than the expected values predicted by Faraday’s Law.
► Current density and pH strongly influence the generation of aluminum and iron inside the electrochemical cell.
► The main species produced for both metals are amorphous hydroxides.