Ohmic Losses Dominated Electrode Fouling during Long-Term Aluminum Electrocoagulation of Hypersaline and Divalent Cation-Rich Oilfield-Produced Water.

Electrode behavior was elucidated during long-term galvanostatic electrocoagulation (aluminum anode and aluminum cathode) of a hypersaline oilfield produced water rich in divalent cations. Electrode potentials progressively increased (i.e., fouling) for most operational conditions due to surface accumulation of calcite and brucite. The interfacial resistance resulting from partial insulation by electrodeposited salts was quantified by using electrochemical impedance spectroscopy. The potential drop associated with this resistance correlated strongly and positively with the increased overpotential required to maintain the galvanostatic operation and was statistically indistinguishable from the calculated ohmic drop, confirming that electrode fouling could be fully attributed to ohmic effects. This also ruled out the occurrence of electrochemical side reactions at elevated potentials, despite their thermodynamic feasibility (note that H 2(g) evolution is a non-Faradaic chemical reaction). We evaluated polarity reversal (PR) as a fouling mitigation strategy to restore electrode performance over a 4-fold variation in current density and a 100-fold variation in PR interval. The PR interval did not significantly influence performance, and fouling was effectively mitigated only at the highest applied current density (200 mA·cm-2). Results indicated the existence of a threshold current density and associated hydrogen bubble generation rate necessary to effectively control electrode fouling under the experimental conditions investigated. Foulant deposition also hindered the migration of electrodissolved aluminum ions away from the anode, facilitating their supersaturation, nucleation, precipitation, and entrapment, thereby decreasing the apparent Faradaic efficiency of coagulant dosing.