1. Introduction
In the last few decades, membrane separation technologies have been viewed as one of the most prominent strategies through which to address water quality and scarcity issues in various fields, such as water desalination, ultrapure water production and wastewater treatment [
1]. Membrane fouling, arguably a major hindrance to membrane-based systems application, deteriorates membrane performance in terms of selectivity and productivity, shortens membrane life span and increases energy consumption [
2]. Targeting this thorny problem, membrane modification, focusing on surface and/or bulk hydrophilicity improvement, has been acknowledged as a prominent approach for fouling mitigation, based on the consensus that hydrophilicity favors the amelioration of membrane fouling [
3]. Modification strategies, such as surface coating [
4], surface grafting [
5] and the blending approach [
6], have been practiced intensively. Among these, blending modification easily enables the incorporation of hydrophilic polymeric materials and/or nanoparticles into the membrane surface and bulk, which offers a window of opportunity for membranes to be modified via the synergy effect between polymers and hydrophilic compatible additives [
7]. Organic materials and inorganic nanomaterials have frequently been blended to imbue membranes with desirable functional properties. Sandu et al. prepared microfiltration and ultrafiltration membranes by mixing acrylonitrile-vinyl acetate copolymers with poly (vinyl alcohol) [
8]. Căprărescu et al. prepared a biopolymeric membrane by blending cellulose acetate with chitosan (CHI)-silver material to remove metallic ions [
9]. Metal/metal oxide nanoparticles have received much attention in polymeric membrane modification in recent years [
10]. Hanshim et al. modified a PVDF hollow-fiber membrane by using SiO
2 particles as additives [
11]. Căprărescu et al. synthesized a composite polyvinyl alcohol membrane with excellent proton conductivity by blending SiO
2 nanoparticles [
12]. Liang et al. modified the anti-irreversible-fouling performance of a polyvinylidene fluoride (PVDF) membrane by blending ZnO [
13].
Among the inorganic metal oxide nanomaterials, TiO
2 nanoparticles have attracted considerable attention for preparing composite membranes for years owing to their advantages of nontoxicity, low cost, superhydrophilicity and satisfactory chemical stability, as well as their antifouling and antibacterial properties [
14]. In particular, the high water affinity of TiO
2 nanoparticles, arising from the rich hydrogen bonding between water and surface hydroxyl groups, benefits membrane hydrophilicity modification to a large extent [
15]. Nonetheless, the high surface energy of TiO
2 nanoparticles inevitably gives rise to aggregation from their initial size (typically, around 20 nm) to several hundreds of nanometers, which is detrimental to their dispersion, exertion of hydrophilicity and self-cleaning properties in the membrane. To alleviate the negative influence of nanoparticle aggregation on membrane hydrophilicity modification, a growing body of literature has developed around complementary approaches for minimizing particle-to-particle interactions in the preparation of nanocomposite membranes. Zeng et al. prepared halloysite (HNTs)-loaded TiO
2 and investigated the impact of TiO
2-HNTs on the hydrophilicity property and antifouling performance of TiO
2-HNTs/PVDF composite membranes [
16]. Razmjou et al. used aminopropyl triethoxysilane (APTES) as a silane coupling agent to ease the aggregation of TiO
2 nanoparticles, which improved the antifouling properties of a PES membrane [
17]. Ma et al. modified the surface of TiO
2 with -SO
3H groups using sulfonated poly (phthalazinone ether sulfone ketone) (SPPESK) as a catalyst and, subsequently, prepared TiO
2-SPEEK-PES nanocomposite membranes [
18].
The modification of PVDF membranes adopting TiO
2 nanoparticles has become a frequently discussed topic. The use of TiO
2 was also proven to efficiently enhance hydrophilicity, water permeability and anti-fouling performance in our previous study [
19]. Furthermore, extensive research has assessed how the incorporation of chemically functionalized/modified TiO
2 nanoparticles affects the morphology, structure and performance of composite membranes. Nevertheless, little emphasis has been placed on the influence of the addition of TiO
2 nanoparticles via the blending modification method without introducing additional chemical coating and changing the composition of the casting solution. The inner mechanism of TiO
2 nanoparticles’ dispersion status during the blending process has not yet been clearly revealed. In this research, TiO
2 nanoparticles were utilized as hydrophilic additives during a facile blending modification of a PVDF membrane, aiming at the application of this approach in a membrane bioreactor (MBR) in municipal sewage treatment. The dispersion of the TiO
2 nanoparticles in casting solutions was implemented without changing the recipes simply by altering the manner of addition or sequence of TiO
2 nanoparticles. The kinetics processes in the manifold casting solutions using an identical recipe were evaluated by multiple light scattering spectroscopy (MLiSSP). The influence on the membrane structure, properties and antifouling performance was examined systematically, aiming at preparing a PVDF/TiO
2 composite membrane with excellent comprehensive properties conveniently. PEG was used beforehand in the casting solution preparation process to serve as the dispersant of TiO
2 nanoparticles by virtue of its steric hindrance effects [
20]. PEG can assist the pore-forming of composite membranes during the immersing phase inversion process; this is attributed to its capacity to form the hydration layer via hydrogen bonds that are relatively easy to break and reform [
21]. The morphologies of the PVDF/TiO
2 composite membranes fabricated by diverse manners of addition of nanoparticles were characterized by scanning electron microscope (SEM), while the cross-sectional elemental compositions of the membranes were determined by energy-dispersive X-ray (EDX). The membranes’ pore size distribution and mechanical properties were also estimated. The physicochemical properties of the membranes were explored by determining the contact angle, zeta potential, porosity and pure water permeability. The functional groups on the membrane surfaces were examined by attenuated total reflectance-Fourier transform infrared (ATR-FTIR). The membranes’ antifouling performance was simulated by extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory and evaluated by batch filtration experiment. The stabilities of the casting solutions were also surveyed by multiple light scattering equipment (Turbiscan).