Fabrication of conductive ceramic membranes for electrically assisted fouling control during membrane filtration for wastewater treatment
Graphical abstract
Introduction
Membrane technology has been increasingly used in wastewater treatment and reuse, due to its many advantages, such as its maintaining stable and excellent effluent quality, ease of operation, and small required footprint (Padaki et al., 2015). Inorganic membranes, such as ceramic membranes (CMs), have advantages over organic membranes, namely their better physical and chemical stability, higher mechanical strength, greater durability under chemical and high-temperature conditions, and longer service life (Zhao et al., 2019). The rapid decrease in the price of CMs has led to their increased use in recent years. However, membrane fouling remains the biggest challenge to membrane applications, including those of CMs. Numerous efforts have been made to develop new methods and strategies for the effective prevention of membrane fouling during the filtration process. For example, Wu et al. (2006) dosed inorganic coagulants into a submerged membrane bioreactor to mitigate a fouling problem. Wang et al. (2013) used hydraulic flushing to minimize the fouling of ultrafiltration membranes. In addition to modified operations, forward flushing, backward flushing, air flushing, and frequent chemical cleaning, electrical technologies involving electrostatic forces or electrochemical reactions have also been used for membrane fouling control (Xu et al., 2021).
Electrically facilitated membrane fouling control is an innovative technique for fouling mitigation during membrane applications (Trellu et al., 2018). Most potential foulants, i.e., pollutants (e.g., particles, microbial cells, organic macromolecules), in water and wastewater are negatively charged (Leite et al., 2019). Thus, by negatively charging a membrane using electricity, foulant materials that approach the charged membrane will be repelled, resulting in effective fouling minimization. However, the application of this method to CMs (e.g., alumina (Al2O3) and titania (TiO2) membranes) is difficult, because of their non-conductive nature. Previous studies have used attached conductive metals (e.g., stainless steel or titanium (Ti) mesh) to CMs to form conductive membrane modules (Fu et al., 2019). However, such modules are rather complex and their power efficiency is low, due to power lost from the metal surface (Yuan et al., 2019). Conductive membranes may also be made directly from conductive materials, such as Ti4O7, a sub-stoichiometric Ti oxide species (Hua et al., 2020; Zaky and Chaplin, 2013). However, such membranes are too expensive to be used for wastewater treatment. Thus, more cost-effective methods need to be developed to fabricate conductive CMs.
Coating a conductive layer onto a membrane surface is a more practical and affordable way to make conductive CMs. Carbon materials, such as graphene, carbon nanotubes (CNTs), and carbon black, have been used to form conductive coating layers (Anis et al., 2021; Chen et al., 2020). However, there are several problems with this method. Specifically, it is difficult to coat carbon materials onto ceramic surfaces; a graphene layer greatly reduces the permeability of a coated membrane; and a coating of carbon black may not afford sufficient electrical conductivity (Pan et al., 2019; Yang et al., 2018). CNTs have a high conductivity and porosity, which can be more suitable for use as a conductive coating material. Zhang et al. (2011) attempted to coat CNTs onto a ceramic membrane by pyrolysis, to generate a CNT-coated membrane for organic wastewater treatment. However, it is difficult to add CNTs to a ceramic surface to create a coating of sufficient bond strength and stability (Ji et al., 2016). There is thus a need for an innovative and more effective coating technique to make conductive CMs, and hence enable electrically assisted membrane fouling control.
In this study, CNTs were used as the coating material on a flat-sheet CM to produce conductive membranes. Instead of directly coating CNTs onto the ceramic surface, polydopamine (PDA) was first coated on the surface of a CM support to create an intermediate adhesive layer, prior to the conductive CNT coating. An DC electric current was applied to the flat-sheet conductive CM (FSCCM) to create an electric field for membrane fouling control during the filtration process. The performance and effectiveness of this electrically assisted method of membrane fouling control were evaluated for the treatment of synthetic wastewaters, such as particle suspensions and oily wastewater. The key factors were investigated and the components of the fouling resistance to filtration were analyzed, thereby revealing the mechanism of fouling mitigation. The energy consumption of electrically assisted fouling control during a high-rate CM filtration process was also estimated.
Section snippets
Chemicals and materials
The CM support was purchased from Sinotsing Environment Technology (Shenzhen, China). It was a flat-sheet support base for coating the filtration layer to make ceramic membranes. The CM support was made of α-Al2O3 with a thickness of 1.0 mm, mean pore size of 1.0 μm, and porosity of 32%, and was operated in outside-in filtration mode from both sides. Multiwall carbon nanotubes (CNTs) with a diameter of 10–30 nm and length of 10–30 μm were purchased from Hongdachang Technology (Shenzhen, China).
Morphology of the coating layer of the ceramic membranes
The surface morphologies of the membrane support and FSCCM were investigated by SEM (Fig. 2). Fig. 2(a)–(c) show the cross-sections of the CM support and FSCCM product under different magnifications. The CNT coating layer with a thickness of ~2.5 μm can be clearly observable on the surface of the CM support. Fig. 2(d)–(f) show the surface details of the CM support and coated membrane under different magnifications. The CNTs were extensively crosslinked and uniformly deposited on the membrane
Conclusions
A novel conductive CM was fabricated for electrically assisted membrane fouling control during wastewater filtration. A flat sheet CM was first coated with dopamine to form an intermediate adhesive layer, and this was then with oxidized CNTs, followed by calcination in an argon atmosphere to form a conductive CNT-coated CM. The CNT-coated CM, an FSCCM, exhibited excellent electric conductivity and stability. It enabled electrically enhanced membrane filtration of synthetic wastewater containing
Credit author statement
Pu Li: Investigation, Conceptualization, Methodology, Writing- Original draft preparation. Chao Yang: Methodology. Feiyun Sun: Writing- Reviewing and Editing. Xiao-yan Li: Supervision, Writing- Reviewing and Editing.
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.
Acknowledgment
This research was financially supported by Grants 17210219 and T21-711/16R from the Research Grants Council of the Hong Kong Government, Project 51978369 from the National Natural Science Foundation of China, and Projects JCYJ20180508152004176 and KCXFZ202002011008448 from the Municipal Science and Technology Innovation Council of the Shenzhen Government, China.
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