Factors affecting the Faradaic efficiency of Fe(0) electrocoagulation

Abstract Electrocoagulation (EC) using Fe(0) electrodes is a low cost water treatment technology that relies on efficient production of Fe(II) from the electrolytic dissolution of Fe(0) electrodes (i.e. a high Faradaic efficiency). However, the (electro)chemical factors that favor Fe(0) oxidation rather than O 2 evolution during Fe(0) EC have not been identified. In this study, we combined electrochemical methods, electron microscopy and Fe measurements to systematically examine the interdependent effects of current density ( i ), anodic interface potential ( E A ) and solution chemistry on the Faradaic efficiency. We found that Fe(0) oxidation was favored (Faradaic efficiency >0.85) in chloride and bromide solutions at all i , whereas carbonate, phosphate, citrate, and nitrate solutions lead to Faradaic efficiencies 2 evolution) only depended on i in the sulfate and formate solutions. Experiments in binary-anion solutions revealed that molar ratios of [HCO 3 − ]/[Cl − ] near 100 and [NO 3 − ]/[Cl − ] near 20 separated the electrochemical domains of Fe(0) oxidation and O 2 evolution in the EC system. These molar ratios were supported by experiments in synthetic groundwater solutions. We also found that the E A vs i curves for solutions with poor Faradaic efficiency overlapped but were situated 2–4 V vs Ag/AgCl higher than those of solutions with high Faradaic efficiency. Therefore, the position of the E A vs i curve, rather than the E A alone, can be used to determine unambiguously the reaction occurring on the Fe(0) anode during EC treatment.