What Anodes are Used for Electrocoagulation?

What Anodes are Used for Electrocoagulation?

Fe anode and Al anode are typically used for electro-coagulation processes, as their anodic dissolution products, Fe(OH)3 and Al(OH)3, can adsorb the suspended solids in the wastewater. The in situ generation of the coagulant has the advantages of simple equipment and less handling of chemicals. Moreover, since these coagulants have low solubility, so they can be easily removed from the treated water.

Fe → Fe2+ + 2e-  
Al → Al3+ + 2e- 

The dissolution reaction of Fe and Al as show above, and the generated Fe2+ ion can be further oxidized anodically or by aeration, to trivalent Fe3+. In these electrochemical reactions, current efficiency can be higher than 100%, especially in a solution containing chloride. Upon entering the solution, these metallic ions can be hydrolized to form corresponding hydroxides: Fe(OH)3 and Al(OH)3. The solubility products of these hydroxides are around, indicating their solubility is very low in aqueous solution over a range of pH. Furthermore these hydroxides can form extensive structures, so they can readily adsorb other insoluble particles and precipitate. Numerous studies and projects have been conducted to apply this technology to wastewater like industrial, farming and municipal wastewater. Besides, the metallic ions can form insoluble compounds or complex structures with ligand ions in the water.  

Wastewater treatment using electrochemical dissolution of iron anode and aluminum anode has the advantage of employing simple equipment. High removal efficiency has been achieved, and the process is capable of reducing many species to sub-ppm levels. The drawbacks of sacrificial anodes are also obvious. Above all, the process cost is still high, including both the material cost and energy cost. The operation requires a suitable pH range. Under acidic conditions, coagulation may not occur. Even if the acidity is suitable, the coagulants need to be removed from the anode surface, or they are likely to slow down the reactions. Also, the aluminum ion is known to have a negative biological effect and is therefore regulated. The concern of residual Al in the water may hinder its application in drinking water. Lastly, the target contaminates are immobilized but not destroyed, so it needs to be used in combination with a sludge treatment process.