Enhanced oil droplet aggregation and demulsification by increasing electric field in electrocoagulation

doi:10.1016/j.chemosphere.2021.131123

Chemosphere

Volume 283, November 2021, 131123

https://doi.org/10.1016/j.chemosphere.2021.131123 Get rights and content

Highlights

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A higher electric field can promote the demulsification of oil droplets.
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Tandem EC is easier to form larger flocs under higher current density.
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Interaction of electric field and electrolyte concentration can improve removal rate.

Abstract

Electrocoagulation (EC) is an efficient technology for removing oil-in-water (O/W) emulsions. However, the role of the electric field in EC for demulsification remains unclear and an obstacle for improving reactor design and operation. Herein, demulsification and oil removal performance by EC under different electric field conditions were investigated. Increasing the EC electric field intensity was beneficial for oil removal, and tandem EC had a higher electric field intensity than parallel EC under the same current density. When the current density was 0.67 mA cm⁻², the chemical oxygen demand (COD) removal rates of tandem EC and parallel EC were 1 136.47 and 745.99 g COD kWh⁻¹, respectively. Oil droplets were polarized by the electric field, and then aligned and aggregated parallel to the direction of the electric field. Increasing electric field intensity accelerated the aggregation of oil droplets, as verified by physical fluid simulation. Furthermore, results showed a higher Al³⁺ dosage and larger electric field intensity in EC with increasing current density, which was conducive to oil droplet demulsification. These findings provide insight into and a theoretical basis for improving oil removal by EC processes.

Introduction

Efficient treatment and reuse of oily wastewater is of great significance to the sustainable development of the petrochemical industry and the ecological health of oil production areas (Jiang et al., 2019). Compared with chemical flocculation, EC exhibits higher oil removal efficiency as well as chemical free and reduced sludge production (Drouiche et al., 2009), and is thus an environmentally friendly and sustainable water purification process (Brillas and Martínez-Huitle, 2015; Arturi et al., 2019; Padmaja et al., 2020).Some researchers provided to treat real oily saline wastewater and oilfield produced water released from drilling oil sites by the use of EC treatment process(Ammar and Akbar, 2018), and verified that EC not only had a better removal rate for oily wastewater, but also for metal ions, fluoride, SDS, ammonia and turbidity in oily wastewater had a certain removal effect(Gobbi et al., 2018; Changmai et al., 2019; Liu et al., 2019; Taslimi Taleghani et al., 2019). In EC, iron (Fe)/aluminum (Al) ions are generated from sacrificial anode flocculate colloidal contaminants, while hydrogen evolution on cathodes can produce air floatation to remove suspended pollutants (Garcia-Segura et al., 2017). EC has been utilized in practical treatment of oily wastewater from petroleum refineries and drilling oil sites, and shows high efficiency in oil, chemical oxygen demand (COD), and particle removal (Al-Qodah et al., 2020). However, the effects of the electric field on demulsification remain poorly understood, thus hindering the optimization of EC reactor design and operation parameters.

Although EC is better than chemical coagulation in oil removal, the reason why is not clear. The electric field in EC may promote oil droplet demulsification (Ren and Kang, 2019; Chen et al., 2020; Hu et al., 2021), whereby oil droplets move along the electric field direction driven by the electric field force, then gather and demulsify (Ren and Kang, 2019). Ichikawa et al. (Ichikawa, 2007) studied the demulsification of dense O/W emulsions in a low-voltage electric field and found that it took place simultaneously over the entire space between two plate electrodes. However, the influence of the electric field in demulsification by EC has been largely ignored in oily wastewater treatment. In EC design, electrode arrangement can lead to different electric field forms and intensities (Hu et al., 2016). A tandem electrode configuration has a higher electric field intensity than parallel electrode configuration, and thus may have a higher demulsification effect as oil droplet emulsions break up faster under higher electric field intensities (Cotillas et al., 2020). Further, flocculation and flotation in EC can change the surface potential of oil droplets, and thus electric field, flocculation, and flotation may have synergistic effects on demulsification(Valero et al., 2011).

In this study, the role of the electric field in EC was examined for O/W emulsion demulsification. Oil removal rates of tandem and parallel electrode configurations were compared to highlight the significance of the electric field. The morphology and surface potential properties of Al-oil flocs in EC were investigated under different electric field intensities and current densities. The aggregation of oil-water droplets under the electric field was observed by electron microscopy and analyzed by theoretical stimulation. This study should provide a theoretical and practical basis for the optimization of EC reactor design and operation.

Section snippets

Chemicals and materials

All reagents used were of analytical grade. Sodium dodecyl sulfate (SDS, ≥98.0%) was employed as the model anionic surfactant. N-hexadecane was used as the main oil and was purchased from Aladdin (Shanghai, China). Sodium chloride, hydrochloric acid, sodium hydroxide, and methylene chloride were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was obtained using a Millipore system (Billerica, MA, USA).

The simulated O/W emulsion was prepared with

Significance of electric field in EC demulsification

The oil removal performance of EC under tandem and parallel electrode configurations was explored to study the response of demulsification to electric field intensity. The two electrode configuration forms ensured different electric field intensities but the same current density in EC. As shown in Fig. 2a and b, COD and turbidity removal under tandem and parallel EC after 30 min reached 77.9% and 98.0% and 66.9% and 89.1%, respectively. Thus, tandem EC showed 11.0% and 8.9% higher removal

Conclusions

The aggregation and demulsification of oil droplets using the electric field were investigated. When the current density was 0.67 mA cm⁻², COD removal by tandem EC with a higher electrode field intensity was higher than that of the parallel EC (11.0%). This was because the higher electric field intensity in EC was beneficial for COD removal of the O/W emulsion and promoted the demulsification of oil droplets. The pollutant content in the scum of the tandem EC was 12.2% higher than that of the

Credit author statement

Chengzhi Hu: Conceptualization, Methodology, Editing. Jingqiu Sun: Visualization, Investigation, Software, design. Kun Wu: Supervision, Validation. Saiguo Yang: Experiment, Writing – original draft, Writing- Reviewing and Editing, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No. 51978646), Young Scientists Fund of the National Natural Science Foundation of China (No. 52000174), and Chinese Academy of Sciences (QYZDY-SSW-DQC004) for funding.

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Enhanced oil droplet aggregation and demulsification by increasing electric field in electrocoagulation

Highlights

Abstract

Introduction

Section snippets

Chemicals and materials

Significance of electric field in EC demulsification

Conclusions

Credit author statement

Declaration of competing interest

Acknowledgements

Sci. Total Environ.

Chin. J. Chem. Eng.

Desalination

Appl. Catal. B Environ.

J. Hazard Mater.

Separ. Purif. Technol.

Chemosphere

J. Hazard Mater.

J. Electroanal. Chem.

J. Environ. Manag.

Chin. J. Chem. Eng.

Colloids Surf., A-Physicochem.Eng.Asp.

Separ. Purif. Technol.

Colloids Surf., A-Physicochem. Eng. Asp.

Separ. Purif. Technol.

Desalination

J. Hazard Mater.

Water Res.

Renew. Energy

Dyes Pigments

Separ. Purif. Technol.

Carbon