A novel ZnO nanoparticle blended polyvinylidene fluoride membrane for anti-irreversible fouling

Abstract

Irreversible membrane fouling is harmful for long-term operation of filtration. In this study, a novel anti-irreversible fouling polyvinylidene fluoride (PVDF) membrane was successfully fabricated using the wet phase separation methods. Nano-ZnO, with different dosages ranging from 6.7% to 26.7% (percentage of PVDF weight), was blended as an additive into the membrane matrix for the modification of the internal surfaces of membrane pores. A series of tests, such as filtration experiments, contact angle measurements, scanning electron microscope (SEM)/energy dispersive X-ray spectrometer (EDS) analyses and mechanical tests, were performed to characterize the modified membranes. The multi-cycle filtration experiments showed that the modified PVDF membranes demonstrated significant anti-irreversible fouling property. All the modified membranes achieved almost 100% water flux recovery after physical cleaning, whereas the raw membrane only reached 78% recovery. This promotion might be related to the increase of membrane hydrophilicity. The implantation of nano-ZnO into membrane inner surface (i.e., pore wall), as indicated by SEM/EDS tests, might be responsible for the enhancement of anti-irreversible fouling property. The water permeability of the modified membrane almost doubled by adding 6.7% nano-ZnO which was determined as the optimum dosage (within the dosage range in this study) for PVDF membrane modification. Additionally, the mechanical strength was found reinforced for modified membranes, which should also benefit the filtration application.

Introduction

Membrane filtration is one of the most promising technologies for application in water and wastewater treatment systems. However, it has been widely accepted that membrane fouling is one of the most stubborn problems hindering the practical application of membrane technologies [1], [2], [3], [4], [5], [6], [7], [8]. A great variety of researches have been carried out for a better understanding of the complex membrane fouling mechanisms [4], [9], [10], [11] and the fouling control strategies to maintain the predominant filterability of membranes [2], [12], [13], [14], [15]. In the microfiltration and ultrafiltration processes, membrane fouling includes reversible fouling and irreversible fouling [16]. The fouling that can be removed by physical cleaning methods, mainly refers to hydraulic cleaning methods (e.g. cross-flushing), is generally defined as reversible fouling [6], [16], [17]. Whereas, the fouling caused by adsorption and entrapment of dissolved pollutant into the membrane pores, which can scarcely be removed by physical cleaning methods and is thought to be harmful for the long-term operation of the filtration techniques [18], [19], is here defined as irreversible fouling or physically irreversible fouling. In general, the irreversible fouling can only be removed by chemical cleaning [20]. However, the chemical cleaning should be limited to a minimum frequency because repeated chemical cleaning may shorten the membrane lifespan [16]. Therefore, it is of great significance that the irreversible membrane fouling be efficiently controlled.

Membrane modification is an effective way to tune the surface characteristics, which closely correlates with anti-fouling properties. Hydrophilicity is one of the desirable surface properties of membrane which can mitigate membrane fouling. Unfortunately, most polymeric materials used for membrane fabrication is only weakly hydrophilic [21]. One example is polyvinylidene fluoride (PVDF) which possesses excellent mechanical and thermal strength, and is widely used in membrane bioreactors. Therefore, extensive efforts have been devoted to increasing the hydrophilicity of membranes via physical or chemical methods [22], [23], [24]. The incorporation of inorganic materials into the organic polymer matrix with blend strategy has attracted great interests due to their completely hydrophilic characteristic. Recent advances in nanotechnology have greatly expanded the ideas of membrane modification by introducing the nanoparticles to fabricate various kinds of nanoparticles-based-membranes [24]. Beneficial effects of certain types of nanoparticles on membrane modification have been reported, such as the amelioration of surface hydrophilicity and enhancement of antifouling property [25], [26], [27], [28], [29], [30]. But most of the attentions were focused on membrane surface modification. The surface property of internal pores, which is directly related with irreversible fouling, is barely concerned. Furthermore, the mainly used nano-materials in previous studies, such as TiO₂ and Al₂O₃ nanoparticles, are relatively expensive for practical applications in future.

Zinc oxide, with the completely hydrophilicity, is one of the most common raw materials in industry and suitable to be used to improve the hydrophilicity of the membrane. Furthermore, nano-sized zinc oxide (nano-ZnO) possesses not only the antibacterial nature [31] but also the valuable ultravioresistant property [32], [33], which might potentially benefit the antifouling performance and extend the service life and application field of membranes. Moreover, nano-ZnO is much cheaper than TiO₂ and Al₂O₃ nanoparticles (∼1/4 price according to the Chinese market quotes). However, the related application of nano-ZnO for membrane modification has not been reported.

In this research, a novel anti-irreversible fouling PVDF membrane, with nano-ZnO blended as an additive, was successfully fabricated using the non-solvent induced phase separation (NIPS) method [34]. The modification of internal surface of membrane pores was primarily concerned. Different dosage of nano-ZnO ranging from 6.7% to 26.7% (percentage of PVDF weight) was adopted for membrane modification. The filterability and anti-irreversible fouling property of the resultant membranes were evaluated through testing the water permeability, flux recovery and long-term filtration performance. A series of experiments, such as water contact angle (CA) measurements, scanning electron microscope (SEM)/energy dispersive X-ray spectrometer (EDS) analyses and mechanical tests, were carried out for membrane characterization. Through all of the experiments, an optimum dosage of nano-ZnO was proposed, and the mechanism of membrane filterability improvement caused by blending nano-ZnO was investigated.