Electrically conductive hydrophobic membrane cathode for membrane distillation with super anti-oil-fouling capability: Performance and mechanism

Highlights

• Conductive membrane cathode consisting of CNT on PVDF substrate is proposed.

• Electrostatic repulsion between oil droplet and membrane is firstly studied in MD.

• Negligible flux decline was found for hexadecane emulsion of 2000 ppm.

• Sliding of oil droplet along the membrane cathode is found.

• Foulant-membrane interacting dynamics is modulated.

Abstract

This study mainly focused on enhancing oil-membrane electrostatic repulsion towards anti-oil-fouling via electrically conductive hydrophobic membrane in electricity-assisted membrane distillation (MD). Carbon nanotubes (CNTs) were coated on commercial membranes to fabricate membrane cathode. For concentrated hexadecane-in-saline water emulsions, the modified membrane exhibited significantly less flux decline, < 5% in dealing with extreme high-concentration oil emulsion (2000 ppm) at cell potential of 3.0 V. The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation with in-situ DI rinse. The anti-fouling mechanism was systematically discussed regarding the hydrophilicity of the membrane interface, charge repulsion between the oil and membrane cathode as well as slippery property at the liquid-gas-solid triple-phase interface. A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode. Thermodynamics analysis suggested enhanced foulant-membrane electrostatic repulsion leads to a significant energy barrier which favored anti-fouling performance. Interestingly, the sliding dynamics of oil droplet along the interface of membrane cathode was found, tuned by the herein weak cell potential, which could also contribute to the fouling mitigation. Our results insight that the electrically conductive membrane cathode could modulate the foulant-membrane interaction which plays an important role in mitigating fouling/wetting occurred at the triple-phase interface.

Introduction

Oily wastewater is one typical industrial effluent particularly during oil and gas exploitation, where produced water, consisting of various organic and inorganic substances, has been largely generated and become a rapidly growing concern threatening the local ecological environment [1], [2], [3]. Towards the stringent discharge standard of this oily wastewater, highly efficient oil removal (particularly for droplets of diameter < 10 μm) from stable emulsified oil wastewater and recovery of clean water remains as one of the key challenges [1], [4]. Compared to conventional approaches like skimming, flotation, de-emulsification, electrical-driven processes and hydrocyclone techniques which are un-efficient and energy-extensive, membrane-based processes are prospective in dealing with this complex oily emulsion [5], [6], [7]. Specially, various pressure-driven membrane processes (from microfiltration to reverse osmosis) have been extensively investigated for oil emulsion treatment, but the complex compositions like the organics or salinity may restrict their further applications and cause fouling concerns [8], [9].

Recently, membrane distillation (MD), a thermally-driven membrane process, has been proposed as one prospective candidate for desalinating oily wastewater [10], [11], [12], thanks to its unique merits including satisfactory separation/purification, tolerance to salt content and adoptability to renewable energy [13], [14], [15]. Specifically, MD is capable to use low-grade heat to achieve extremely high retention (>99.99%) for less or non-volatile solute molecules in the feed solution, like inorganic salt, oil emulsion. Ideally, the porous membrane of high hydrophobicity allows only the penetration of volatile components (e.g., water vapor). But once liquid penetration occurs, a typical outcome of membrane fouling/wetting in MD, severe or complete loss of membrane selectivity can be expected, and this is especially true in dealing with a feed of low-surface-tension solutes like oil emulsion [16], [17], [18]. Researchers have extensive work on tuning membrane surface wettability against oil induced fouling/wetting issue, proposing tailor-made superhydrophobic [19], [20], [21], [22], [23], Janus (i.e., interface of asymmetric wettability) [10], [24], omniphobic (i.e., repel both water and oil) [25], [26], [27], [28], [29] membranes. However, oil emulsion exhibits attractive hydrophobic-hydrophobic interaction with MD membrane, which weaken not only the effectiveness of current (super-) hydrophobic membrane surfaces [23] but also the oil-repelling hydration layer in a Janus membrane [10].

Charge interaction between a solute particle and membrane surface is also an important player in membrane fouling issues yet less considered in MD studies, particularly compared with those in ultrafiltration or reverse osmosis where solute-membrane electrostatic repulsion has been reported influential in mitigating fouling issue [30], [31], [32], [33], [34]. Indeed, the introduction of an electrical field upon conductive membrane to modulate fouling has been well explored, with the involved electrokinetic mechanisms not only limited to electrostatic interaction but also others such as electrophoresis [35], [36]. However, such modifications of membrane-oil interaction under electro-assistance are rarely considered in MD [37], [38].

In MD, the (anti-) fouling phenomenon actually happens at a liquid-air-solid triple-phase interface involving the feed solution, trapped air stagnate in or near membrane pore and the membrane matrix, which could be considered more complex than that in pressure-driven membrane process (only liquid-solid phase involved) [19]. Thus, the dynamic interaction between membrane cathode and oil emulsion at such a different interface could be different and worth in-depth investigating. Recent studies have shown that the in-situ construction of the CNT layer improved fouling mitigation towards humic acid [23], [39] or bacteria in the MD process [37], with the anti-oil-fouling few reported. Moreover, the exact anti-fouling mechanism via the electrically conductive membrane in the MD process needs to be further clarified, in terms of what kind of electrokinetic behavior is responsible (electrostatic repulsion or electrophoresis) for fouling mitigation under the electrical field, and how the solute-membrane interaction could be modulated in the pair of oil emulsion and hydrophobic membrane.

This study aimed to unravel the potential role of electrostatic repulsion between oil droplet and membrane interface in the MD process. An electrically conductive composite membrane made of a hydrophilic layer of CNT coated on commercial PVDF hydrophobic membrane was performed in an electricity-assisted direct contact membrane distillation (e-DCMD), operated at a very low cell potential to rule out of the contribution of bubbling due to water electrolysis or electrophoresis. The outcome of this work would help understand the anti-oil-fouling mechanism of electrically conductive hydrophobic membrane and in developing MD systems.