An important challenge for continuous treatment by electrocoagulation (EC) is the passivation and fouling of electrodes which increases during the longer operation (Ingelsson et al., 2020). The application of polarity reversal (PR) and treatment performance of EC was investigated using continuous flow reactor for simultaneous removal of silica and hardness (calcium and magnesium) from produced water. Polarity reversal times (PRT) from 30 s to 10 min were studied at a fixed charge loading of 2000 C L-1 (i.e., the amount of charge passed per unit volume of produced water treated) using Fe and Al electrodes. Periodic PR was found to reduce the fouling and de-passivate the electrodes by changing the surface chemistry at the electrode-electrolyte interface (Ingelsson et al., 2020; Yasri et al., 2022). During 90 minutes of continuous treatment the chronopotentiometric data indicated that a PRT of 10 min was more effective in reducing the cell voltage for both Fe and Al electrodes [Fig. 1 (a & b)]. With a longer PRT of 10 min, there is more time for the acid boundary layer to form on the anode, which mitigates metal precipitation at the anode. In contrast the higher cell voltage that persists in direct current EC (DC-EC) could be due to the precipitation of Ca and Mg minerals in the alkaline solution on the cathode surface, leading to cathode passivation (Chow et al., 2021). After the first polarity reversal, both electrodes have been anodic, removing passivation layers from the surface and reducing the cell voltage. On reversing the polarity, the cathode becomes the anode, and the acidic pH on the formed at the electrode interface will facilitate the dissolution of Ca and Mg precipitates on the electrode surface, reducing the cell voltage significantly. For Fe-EC, the contaminant removal performance increased with PRT, and the highest removal was observed with DC-EC (Fig. 1c). The increasing removal performance with PRT for Fe-EC is consistent with the increase in faradaic efficiency observed of 55%, 68%, 85%, and 99.8% with 30 sec, 2 min and 10 min PRT, and DC respectively. The lower faradaic efficiencies for Fe-EC with short PRTs are likely due to redox reactions of iron species at the electrode surface (Chow et al. 2021).
The contaminant removal at a charge loading of 2000 C L–1 using DC-EC with Al electrodes was similar to that for PR-EC with PRTs of 2 to 10 min (Fig. 1d). Slightly lower Si and hardness removal was observed for Al-EC at the shortest PRT of 30 s. For Al-EC, the faradaic efficiency was also observed to increase with PRT, with values of 150%, 220% and 287% obtained at 30 sec, 2 min and 10 min PRT respectively. The super-faradaic (>100%) efficiencies can be explained by dissolution of the Al from the cathode under the local alkaline conditions that develop on the electrode surface. With a short PRT, there is less time for the alkaline pH to develop at the surface and hence the faradaic efficiency was reduced.
References:
Chow, H., Ingelsson, M., Roberts, E.P.L., Pham, A.L.T., 2021. How does periodic polarity reversal affect the faradaic efficiency and electrode fouling during iron electrocoagulation? Water Res. 203. https://doi.org/10.1016/j.watres.2021.117497
Ingelsson, M., Yasri, N., Roberts, E.P.L., 2020. Electrode passivation, faradaic efficiency, and performance enhancement strategies in electrocoagulation—a review. Water Res. 187, 116433. https://doi.org/10.1016/j.watres.2020.116433
Yasri, N.G., Ingelsson, M., Nightingale, M., Jaggi, A., Dejak, M., Kryst, K., Oldenburg, T.B.P., Roberts, E.P.L., 2022. Investigation of electrode passivation during electrocoagulation treatment with aluminum electrodes for high silica content produced water. Water Sci. Technol. 85, 925–942. https://doi.org/10.2166/wst.2022.012
Fig. 1. Impact of PR-EC on cell voltage with reversal time of 10 min (a) with Fe-EC and (b) with Al-EC and comparison of the performance of DC-EC and PR-EC for polarity reversal times of 30 sec, 2 min and 10 min of (c) Fe-EC and (d) Al-EC at flowrate 60 mL min-1 corresponds to (Re) = 116, charge loading 2000 C L⁻1 and current density 8 mA cm-2.