Operando Visualization of Produced Water Treatment by Electrocoagulation

Abstract

Electrocoagulation (EC) involves the in-situ generation of metal hydroxide coagulant by the dissolution of the sacrificial metal anode and formation of hydroxyl ions at the cathode. Previous studies have shown that pH is a critical factor that directly affects the EC performance as the metal dissolution, the ionic speciation and equilibrium species, and metal hydroxide solubility are governed by the pH. In the present study, a combination of pH-sensitive fluorescent dyes, with different pKa, together with laser scanning confocal microscopy (LSCM), was employed to visualize the interfacial pH changes of EC in-operando. Quantification of interfacial pH in a wide pH range from ~1.5 to 8.5 with a high spatiotemporal resolution was achieved. The sucessful application of the method in EC provided new insights into the reaction mechanisms such as metal dissolution and gas evolution. The dynamics of the pH boundary layer within a few hundred microns of the electrode surface was monitored, and the effect of current density and flow rate was investigated. Furthermore, coupling the method for in-operando monitoring of interfacial pH with reflectance microscopy provided valuable information on the impact of operating conditions such as current type (DC or PR), current density, electrode material (iron or aluminum), the buffer capacity of the solution, and PR intervals (5 s to 60 s) on the interfacial pH, the morphology of electrode surface and formation of sludge. The interfacial pH in PR-EC oscillates in response to the changes in the direction of current flow, and is a function of the PR interval, and the buffer capacity of the solution. For both electrode materials, shear caused by hydrogen evolution contributes to removing the fouling layer off the electrode surface. Applying a current density of 16 mA cm?2 and 60 s PR intervals was found to reduce the amount of fouling for iron and aluminum electrodes while maintaining or improving the performance of the EC process and the lifetime of electrodes.Finally, the impact of the operating conditions on the sludge properties was investigated. The network strength of the sludge formed during EC treatment was studied using a rheometer. The sludge strength network is an essential factor for the separation steps after electrocoagulation. A weak network strength can lead to poor solid-liquid separation in settling tanks or blockage of flow channels in the filtration process. The network strength of the EC sludge formed with iron electrodes was found to be higher than that obtained with aluminum electrodes. When PR with 60 s intervals was used, a 70% increase in the sludge network strength was achieved with aluminum electrodes, while a similar network strength to DC was observed with iron electrodes. Although PR-EC was effective for removing the fouling layer from the electrode surface and enhancing the mechanical properties of the sludge, the frequency of PR should be chosen carefully based on the electrode material and the solution chemistry, in particular the type of contaminants and the buffer capacity. The present study provides valuable insights into implementing in-operando techniques to probe the dynamics of reaction involved in the electrocoagulation process and can readily be extended to other electrochemical systems.