The impact of long-term operation on the Faradaic efficiency of Fe(0)-electrocoagulation

Abstract

Arsenic contamination of drinking water remains the largest chemical threat to millions of people, especially in rural Southeast Asia. One unconventional water treatment method that can be applied in remote communities is Iron-Electrocoagulation (Fe-EC), which is based on the electrochemical production of Fe-oxides in the contaminated water that bind arsenic. Although Fe-EC has been shown to effectively remove arsenic in extended field trials, the efficiency of field systems is several times lower than in laboratory studies and electrode surface layers are observed. We hypothesize that the Faradaic efficiency (FE) of field system is low, so that the generated Fe dose is lower than predicted by Faraday’s law of electrolysis. However, no long-term systematic studies have been performed in the laboratory to evaluate the impact of long-term operation on the FE. We performed laboratory experiments investigating the long-term FE over a range of conditions for 15 – 35 runs. We tested different electrolytes, Fe(0) anode purity, and varied operation parameters like charge dosage rate and polarity alternation. Our results show that the FE does decline continuously during repeated operation under typical field conditions, resulting in a lower Fe dose than expected. In addition, we find that a high FE can be maintained in electrolytes free from oxyanions or by applying charge dosage rates ≥ 15 C/L/min. The results also suggest that a low FE is related to the formation of macroscopic surface layers on the electrodes. Based on these results, we discuss potential strategies to maintain the efficiency of Fe-EC field systems under realistic conditions and evaluate the implications for arsenic removal.