Electrolytic mechanism with Al and Fe electrodes
The electrolytic process involves the generation of coagulants in situ by electrolytic oxidation of the sacrificial electrode material. Aluminum or iron is usually used as electrodes and their cations are generated by the dissolution of sacrificial anodes upon the application of direct current. The metal ions generated are hydrolyzed in the electrochemical cell to produce metal hydroxide ions according to the reactions (1)–(7) and the solubility of the metal hydroxide complexes formed depends on pH and ionic strength. Insoluble flocs are generated in a pH range between 6.0 and 7.0 as seen from the solubility diagrams of aluminum hydroxide at various pH values (Bensadok et al. 2008). The Al plates are also finding applications in wastewater treatment either alone or in combination with Fe plates due to the high coagulation efficiency of Al3+ (Chen 2004). Mollah et al. (2001) had reported that the electrolytic dissolution of the Al anode produces the cationic monomeric species such as Al3+ and Al(OH)2+ under acidic conditions. At appropriate pH values, they are transformed initially into Al(OH)3 and finally polymerized to Al n (OH)3n according to the following reactions: