Electrocoagulation is a cost effective separation technology for removal of organic and inorganic contaminants from water. However, the mechanism of contaminant removal and operational challenges such as electrode fouling have limited widespread application. In this study we  examine the influence of the electrolde boundary layer on the contaminant removal and electrode fouling processes, with silica removal from produced water as the target application. Bench scale testing has demonstrated the effectiveness of electrocoagulation for the removal of silica from the produced water with minimal electrical energy consumption. However, electrode fouling remains an important chanllenge. To study the electrode boundary layer, we developed a method using pH sensitive fluorescent dyes and laser scanning confocal microscopy (LSCM) to visualise the pH distribution close to the electrodes. Using ratiometric analysis the pH distribuion can be mapped in space and time. Acidic conditions are observed close to the anode, due to hydrolysis reactions as metal ions are dissolved into soluition, as well as proton generation associated with oxygen evolution. The pH at the electrode surface can be 2 or more pH units lower than the bulk.  At the cathode, strongly alkaline conditions can be observed, as hydroxide ions are generated from the hydrogen evolution reaction.  These pH boundary layers influence the coagulation processes through hydrolysis reactions in the anode boundary layer, and thus play an important role in the separation process. Furthermore, the rate of deposition of precipitates at the electrode surface will also be a function of the local pH at each electrode. This study thus provides new insights into contaminant removal and fouling processes in electrocoagulation.  We have also investigated the use of polarity reversal to control electrode fouling.Polarity reversal leads to rapid pH swings at the electrode surface, which enables reduction in the fouling but also infuences the coagulatio processes. The particle size distribution of precipitated coagulant changes depending on the frequency of the reversal. At high frequency there is less electrode fouling, but the particle size formed is smaller, so separation of the solids becomes more challenging.

Removal of silica from oil-sands produced water by electrocoagulation

256th National Meeting (2018)

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