Mitigation of HA, BSA and oil/water emulsion fouling of PVDF Ultrafiltration Membranes by SiO₂-g-PEGMA nanoparticles

Highlights

• Synthesized SiO₂-g-PEGMA nanoparticles were used as hydrophilic additive.

• Novel hybrid PVDF membranes were successfully fabricated by Phase inversion method.

• Modified membranes exhibited higher hydrophilicity and antifouling property than plain membrane.

• Ultrafiltration performances of the membranes have been investigated using HA, BSA and o/w emulsion.

Abstract

In this work antifouling poly(vinylidene fluoride) (PVDF) ultrafiltration membranes were synthesized with direct blending of Poly(ethylene glycol) methyl ether methacrylate (PEGMA)-grafted SiO₂ (SiO₂-g-PEGMA) nanoparticles (NPs) in the casting solution by phase inversion method. Chemical structure of the SiO₂-g-PEGMA NPs was analyzed by Fourier transform infrared (FTIR) spectroscopy. Effect of different wt.% of SiO₂-g-PEGMA NPs on the structure and performance of prepared membranes were examined by Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy, Field emission scanning electron microscope (FESEM), Liquid–liquid displacement porosimetry (LLDP), pure water flux (PWF), hydraulic permeability, solute rejection study, water contact angle, water uptake, porosity and bovine serum albumin (BSA) adsorption. Modified membranes have higher solute rejection than plain PVDF membrane. Also, modified membrane with 0.5 wt% of SiO₂-g-PEGMA NPs showed enhanced porosity, pore density, pore area, PWF and permeability. Water contact angle and amount of adsorbed BSA for the modified membrane with 1 wt.% of SiO₂-g-PEGMA NPs were 50.7° and 0.05 mg/cm² compared to 68.7° and 0.17 mg/cm² respectively, for the plain membrane. Furthermore, antifouling property and rejection performance of modified PVDF membranes were investigated by humic acid (HA), BSA and oil-in-water (o/w) emulsion ultrafiltration experiment. It was observed from ultrafiltration study that the irreversible fouling and total fouling were decreased and achieved high flux recovery ratio for modified membrane.

Introduction

A large amount of oily wastewater is produced from various process industries such as refineries, metallurgical, textile, leather and steel processing plants. The separation of oily wastewater particularly oil-water emulsion is a serious problem. Membrane technology has been considered a prominence technology amongst worldwide for oily wastewater treatment due to high separation efficiency, small footprint, simple process and low operational cost. Among all polymers, PVDF is a promising candidate to prepare polymeric membrane for oily wastewater separation due to its good chemical property, superior thermal stability and excellent mechanical strength [1,2]. However, hydrophobic nature and low surface energy of PVDF membranes makes it susceptible to fouled by natural organic matter (NOM), protein and oily wastewater, results sharp decline in flux and permeability [3]. Fouling can be overcome by hydrophilic modification of PVDF membranes. Nowadays, preparation of antifouling membrane is an intense research area in separation and purification technology [4]. Hydrophilic modification of the membranes can be achieved by surface coating, surface grafting and blending of additives. Surface coating with hydrophilic layer provides the temporary hydrophilicity due to instability and could be washed out during filtration and cleaning process because of weak physical interaction between membrane surface and coated layer. While, surface grafting is effective method to modify the membrane with grafted chains through covalent bonding interaction onto the membrane surface. It can be achieved by ultraviolet irradiation, electron beam surface induced grafting polymerization, plasma treatment and living/controlled radical polymerisation. Hydrophilic polymers, amphiphilic copolymers and inorganic NPs [5] such as polyethylene glycol (PEG) [6], Polyvinylpyrrolidone (PVP) [7], poly(methylmethacrylate) (PMMA) [8], poly(methyl methacrylate-poly(ethylene glycol) methyl ether methacrylate) P(MMA-r-PEGMA) [9], polyvinylideneflouride-grafted-poly(ethylene glycol) methyl ether methacrylate (PVDF-g-PMAA), polyvinylideneflouride-grafted-poly(oxyethylene methacrylate) (PVDF-g-POEM) [10], polyvinylideneflouride-grafted-poly acrylic acid (PVDF-g-PAAc) [11], TiO₂, ZrO_2, Al₂O₃ [5,12], SiO₂ [13], TiSiO₄ [14], Fe₃O₄ [15], Ag [16], ZnO [17], and carbon nanotubes (CNTs) [18] are used as additive for direct blending in the casting solution. Among these methods, blending of additives is preferable and simple because in other methods one additional step is required to modify the membranes and its applications are limited to modify structure of membrane.

Zhao et al. synthesized PVDF membrane by blending of hyperbranched polyglycerol (HPG). They found that addition of HPG as modifier increases the porosity, surface pore size, hydrophilicity and PWF, and state that HPG act as pore forming agent and also contribute to raise the hydrophilicity of membrane [19]. Song et al. prepared PVDF membrane by blending of TiO₂ NPs and PEG in the dope solution. They found that addition of PEG-TiO₂ increases the HA removal and simultaneously reduces the flux due to small size of pores [20]. Li et al. fabricated styrene-alt-maleic anhydride (SMA)/PVDF blend membrane and immersed in TiO₂ NPs solution for seven days and investigated that the TiO₂ self-assembly membranes indicates the improved antifouling property and permeability as compare to SMA/PVDF blend membrane [21]. Moslehyani et al. modified PVDF membrane by blending multi-walled carbon nanotube (MWCNT). They had observed more than 99% rejection of TiO₂ pollutant for oxidized MWCNT nanocomposite membranes [22].

Although NPs are excellent hydrophilic additives; however agglomeration is one of the major drawbacks in the blending method due to small size and high surface energy of NPs. Agglomeration induces unstable conditions to casting solution which results non uniform membrane due to uneven distribution of NPs. This behaviour affects the structure and performance of membrane, and also reduces the fouling resistant ability of hydrophilic NPs additives [23]. Uniform dispersion of NPs can be achieved by sonication or strong mechanical stirring; however it is restricted to high concentration of NPs. Various methods such as surface modification of NPs, formation of NPs by sol-gel process and addition of third component to enhance the interaction between NPs had been used by various researchers to minimize the agglomeration [13,24].

Recently, functionalised NPs have become ideal candidates to incorporate into polymeric material for hydrophilic modification of membrane. Madaeni et al. synthesized PVDF membrane with functionalized TiO₂ NPs by acrylic acid monomer and they observed better fouling resistance of modified membranes [25]. Shen et al. prepared antifouling hybrid PVDF membrane with poly(N-acryloylmorpholine)-grafted ZrO₂ (ZrO₂-g-PACMO) NPs and they observed that modified membranes showed lower adsorption of protein than unmodified PVDF membrane [26]. Munirasu et al. established a new method for fabrication of asymmetric organic-inorganic nanoporous polymeric membrane by functionalisation of SiO₂ NPs with PMMA. They proposed that the new SiO₂-g-PMMA hybrid asymmetric nanoporous structure and synthesis process can be applicable in membrane fabrication for specific separation purpose [27]. Park et al. modified the SiO₂ NPs grafted with poly(oxyethylene methacrylate) (POEM) and ionic poly (styrene sulfonic acid) (PSSA). They discussed that the modified NPs showed better dispersion in alcohol than unmodified NPs [28].

In reviewing the previous work it is clear that several researchers reported the modification of membrane with unmodified and modified NPs but to the best of our knowledge hydrophilic modification of PVDF membrane with functionalized SiO₂ NPs by PEGMA chains is still lacking. Therefore, in this work, PEGMA was successfully grafted onto SiO₂ NPs and then novel hydrophilic PVDF membranes (hybrid) were synthesized by direct blending of SiO₂-g-PEGMA NPs in the casting solution via Phase inversion method. The synthesized membranes were characterized by FTIR-ATR, FESEM, LLDP, water CA, water uptake, porosity, protein adsorption study, PWF and hydraulic permeability. In ultrafiltration study, HA, BSA and o/w emulsion were used to represent NOM, protein and industrial pollutant, respectively as model foulants. Finally, flux recovery ratio and selection parameter were investigated to compare the performance membranes.