Superhydrophilic polyvinylidene fluoride membrane with hierarchical surface structures fabricated via nanoimprint and nanoparticle grafting

Highlights

• Hierarchical membrane surface was prepared via nanoimprint and nanoparticle coating.

• Nanoimprint lithography resulted in a wave-like micro-scaled surface pattern.

• Nanoparticle grafting resulted in a secondary nano-scaled superhydrophilic layer.

• An enhanced rejection capability was achieved without losing water permeability.

• Hydration layer and turbulent flow gave rise to improved antifouling capability.

Abstract

The sustainable development of membrane separation technology is facing the severe challenge of membrane fouling problem. This paper proposed a novel strategy that combined nanoimprint lithography and nanoparticle grafting technology to fabricate antifouling polyvinylidene fluoride membranes. The nanoimprint process brought about a wave-like micro-scaled surface pattern with alternate concave and convex structures. The subsequent grafting process with amine-terminated nanoparticles further decorated the pattern into a hierarchically structured surface, which allowed a rapid (<1 s) complete wetting (i.e., contact angle = 0°) by water thus superhydrophilic. Besides, the obtained membrane (designated as imprinted_G) achieved an enhanced capability of solute rejection while maintaining a water permeability of ~8.4 × 10⁻⁷ m s⁻¹ kPa⁻¹ comparable with that of the pristine membrane (~8.6 × 10⁻⁷ m s⁻¹ kPa⁻¹). Furthermore, the superhydrophilic and hierarchical surface potentially led to a local hydration layer and turbulent flow, which resisted the adhesion and deposition of foulants, thereby endowing the membrane with dramatically enhanced antifouling capability. The imprinted_G membrane achieved the highest water production yield (~36% higher than the pristine membrane) with stable flux recovery rates in the filtration experiments with practical membrane bioreactor mixed liquor (containing activated sludge). Furthermore, the imprinted_G membrane exhibited a conspicuously improved capability in resisting biofouling of Bacillus thuringiensisd and Escherichia coli, demonstrating a promising prospect for practical applications.

Introduction

The increasingly severe water crisis is promoting the rapid development and innovation of water purification and wastewater reclamation technologies [[1], [2], [3]]. Intensive attention has been focused on the membrane-based water treatment technology due to its high efficiency, satisfying effluent quality, small footprint, and so forth [[4], [5], [6]]. Ultrafiltration and microfiltration, as the most representative low-pressure membrane separation technology, has been extensively used in various areas, such as wastewater reclamation, water purification, and desalination pretreatment [2,[7], [8], [9]]. However, the membrane fouling problem, which leads to reduced water yield and increased operational cost, has always been a daunting impediment challenging the further sustainable progress of the membrane technology [10,11]. In essence, membrane fouling results from the interaction between foulants and membrane surface including chemical bonding, deposition, adsorption, accumulation, and so on [12]. Therefore, the occurrence and evolution of membrane fouling is closely related with the physicochemical properties of the membrane (e.g., hydrophilicity, charge property, surface roughness, pore structures, etc.) [10,13].

Accordingly, membrane fouling can be efficiently harnessed by tuning the surface chemistry and physical topography of the membrane [4,14,15]. Consequently, the preparation of superhydrophilic membranes by maximizing surface hydrophilicity has been proved to be an effective method to control membrane fouling in recent years [[16], [17], [18], [19]]. The superhydrophilic surface potentially cultivates a local hydration layer which hampers the adhesion and deposition of foulants on the membrane surface [20]. Among the various modification methods including blending, surface coating, surface grafting, and so forth [[21], [22], [23]], the nanomaterial-based surface grafting method is widely acknowledged as the most effective strategy for the preparation of superhydrophilic membranes [10,11,23,24]. Especially for the ultrafiltration membranes, which possess lower porosity and relatively flatter surface than the more porous microfiltration membranes, it is more difficult to achieve superhydrophilicity on ultrafiltration membranes through conventional modification methods [19,25].

Another fouling control strategy is to fabricate surface-patterned membranes [[26], [27], [28], [29], [30], [31]] through different methods, such as template-based micro-molding [26], 3D printing [32], and direct nanoimprinting [33]. The resultant well-ordered membrane morphologies were found to bring about enhanced hydrodynamic disturbance in local surface, enlarged filtration area, and improved antifouling capability [30,[34], [35], [36], [37], [38]]. The relevant research usually focused on the influence of the surface morphology change on filtration flow patterns, which commonly advised that the altered local flow profile in the vicinity of the surface pattern (especially on the apex regions) contributed to the resistance of foulant deposition thus reduced membrane fouling [28,34,35]. But such physical topography change could also have profound effects on other physicochemical characteristics of the membrane, such as the wetting behavior of liquid–membrane interface, selective separation (e.g., oil–water separation) capability, and so on [25,39,40]. There is a rich possibility to further strengthen the membrane separation performance through combining the surface pattern and nanomaterial grafting technologies. However, such studies are rarely reported. Besides, most of the studies on fabrication or modification of novel antifouling membranes have adopted synthetic foulant solutions (prepared with model foulants such as sodium alginate, bovine serum albumin, and/or humic acid, etc.) or supernatant separated from practical wastewater (e.g., prefiltered mixed liquor with suspended solids removed) to conduct fouling filtration experiments. But this may lead to differences between the tested results and the actual situation [41]. As far as possible, it is necessary to use the experimental conditions consistent with the real situation to characterize the antifouling performance of the membranes.

In view of the above, the present study proposes a novel hierarchically patterned superhydrophilic membrane fabricated via a combined protocol of nanoimprint lithography and nanoparticle functionalization. Both the membranes and nanomaterials were systematically characterized in terms of morphology, chemistry, and hydrophilicity. A series of filtration experiments were performed to evaluated the permeability, selectivity, and antifouling capability (using a practical sludge containing mixed liquor) of the membranes. Biofouling experiments were also performed to evaluate the anti-biofouling performance of the membranes.