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One of the main challenges for the implementation of electrocoagulation (EC) in water treatment are fouling and passivation of the electrodes, especially for applications with high contaminant concentrations. For the first time, we investigated in this study the process of fouling mitigation by polarity reversal during the EC treatment of boiler blowdown water from oil-sands produced water, characterized by high silica concentrations (0.5–4 g L−1). This effluent is typically obtained from an evaporative desalination process in oil production industries. Potentiodynamic characterisation was used to study the impact of passivation on the anode dissolution. Although a charge loading of 4,800 C L−1 was found to remove about 98% of silica from a 1 L batch of 4 g L−1 Si solution, fouling reduced the performance significantly to about 40% in consecutive cycles of direct current EC (DC-EC) treatment. Periodic polarity reversal (PR) was found to reduce the amount of electrode fouling. Decreasing the polarity period from 60 to 10 s led to the formation of a soft powdery fouling layer that was easily removed from the electrodes. In contrast, with DC operation, a hard scale deposit was observed. The presence of organics in the field samples did not significantly affect the Si removal, and organics with high levels of oxygen and sulfate groups were preferentially removed. Detailed electrochemical and economic investigations suggest that the process operating at 85 °C achieve 95% silica removal (from an initial concentration of 481 mg L−1) with an electrical energy requirement of 0.52 kWh m−3, based on a charge loading of 1,200 C L−1, an inter-electrode gap of 1.8 cm and a current density of 16 mA cm−2.

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Electrocoagulation (EC) is a cost-effective and reliable technology to treat water and wastewater and has the ability to remove many types of contaminants. Studies have shown that EC operated with aluminum or iron electrodes exhibits higher treatment efficiencies than traditional chemical coagulation with aluminum sulfate or ferric chloride salts [1], [2]. EC involves the in-situ generation of metal hydroxide coagulant by the electrochemical dissolution of sacrificial metal anodes using a direct current, combined with generation of hydroxide ions at the cathode. However, material precipitation on the electrodes associated with long term operation is a major problem hindering the scale up of EC [3]. The growth of electrode surface layers increases passivation, which reduces the treatment efficiency and increases operating costs [4]. Polarity reversal during electrocoagulation, i.e. intermittently changing the direction of the current, is a method that can remove passivation layers on the electrodes formed during direct current operation [5]. The main goal of this study was to investigate the effect of polarity reversal on the reaction and electrode fouling mechanisms as well as the performance of electrocoagulation for the treatment of SAGD produced water. Total organic carbon and silicon removal efficiencies were measured to evaluate treatment performance. Laser scanning confocal microscopy was used to monitor the pH distribution close to electrodes as well as the formation of solid products in an electrocoagulation cell. Customized polycarbonate bench scale reactors were used to study the relationship between coagulant production, polarity reversal frequency, solution composition, and flowrate. The cycle time of the polarity reversals was varied from 5 to 600 s, and the Reynolds numbers was varied between 30 to 200. The Faradaic efficiencies for the coagulant dissolution at different operating conditions were determined by digesting the solid products followed by elemental analysis. The evolution of pH during polarity reversal revealed that at higher frequencies or higher flow rates, the thickness of the interfacial pH boundary layer was lower. The quantification of pH was used to study the effect of pH on passivation layer stability. It was found that at higher frequencies, Faradaic efficiencies were lower for EC with iron electrodes, whereas increased efficiencies were observed for aluminum electrodes due to increased susceptibility to non-Faradaic corrosion. EC using aluminum electrodes (Al-EC), employing polarity reversal at all frequencies led to a reduction in cell voltage and therefore a reduction in the required energy for treatment. References [1] M. Eyvaz, M. Kirlaroglu, T. S. Aktas, and E. Yuksel, “The effects of alternating current electrocoagulation on dye removal from aqueous solutions,” Chem. Eng. J., vol. 153, no. 1–3, pp. 16–22, 2009. [2] P. K. Holt, G. W. Barton, M. Wark, and C. A. Mitchell, “A quantitative comparison between chemical dosing and electrocoagulation,” Colloids Surfaces A Physicochem. Eng. Asp., vol. 211, no. 2–3, pp. 233–248, 2002. [3] S. Garcia-Segura, M. M. S. G. Eiband, J. V. de Melo, and C. A. Martínez-Huitle, “Electrocoagulation and advanced electrocoagulation processes: A general review about the fundamentals, emerging applications and its association with other technologies,” Journal of Electroanalytical Chemistry, vol. 801. pp. 267–299, 2017. [4] C. M. van Genuchten, S. R. S. Bandaru, E. Surorova, S. E. Amrose, A. J. Gadgil, and J. Peña, “Formation of macroscopic surface layers on Fe(0) electrocoagulation electrodes during an extended field trial of arsenic treatment,” Chemosphere, vol. 153, pp. 270–279, 2016. [5] M. Eyvaz, “Treatment of brewery wastewater with electrocoagulation: Improving the process performance by using alternating pulse current,” Int. J. Electrochem. Sci., vol. 11, no. 6, pp. 4988–5008, 2016.