Novel membrane surface modification to enhance anti-oil fouling property for membrane distillation application
Introduction
Membrane distillation (MD) is an emerging separation technology that has potential to be used for seawater desalination. In a MD process, feed and permeate streams flow at the both sides of a hydrophobic microporous membrane, and water vapor transports from the feed to permeate side because of the vapor pressure gradient across the membrane. The advantages of applying MD for desalination include high purity of water product, no external hydraulic pressure required and applicability for highly concentrated salt solutions. However, there are several challenges to be addressed for MD industrial applications, such as membrane pore wetting by water molecules, relatively low mass flux as well as flux decay over time. Many efforts have been made to fabricate MD membranes with better performance, design more reasonable membrane modules to facilitate heat and mass transfers and optimize the MD system to achieve higher energy efficiency [1], [2], [3], [4]. Among these efforts, fabricating an appropriate membrane is the most important task as the membrane is the key component of the system.
A basic requirement for membrane distillation is that the membrane material should be intrinsically hydrophobic to prevent pore wetting. The popular hydrophobic materials include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) and polypropylene (PP), etc. However, currently most of the membranes used are microfiltration (MF) membranes, which are not specially designed for the MD process. The relatively large pores of MF membranes may render the problem of membrane wetting in the MD application.
Membrane surface modification can be applied to reduce maximum membrane pore size, narrow down its distribution or/and further enhance membrane hydrophobicity [5]. However, membrane fouling is a big concern for a highly hydrophobic membrane surface if it is in contact with an aqueous media containing hydrophobic species, as hydrophobic species present high affinity to the hydrophobic membrane surface, resulting in the blockage of the membrane pores and membrane wetting [6]. For the feed containing hydrophobic species such as oils, hydrophilic modification, which can not only reduce membrane pore size, but also protect the membrane pore mouth from fouling, seems to be more appropriate.
Generally speaking, there are two major strategies for hydrophilic surface modification: surface coating and surface grafting. Peng et al. fabricated composite membranes for MD use by coating a layer of polyvinyl alcohol/polyethylene glycol (PVA/PEG) onto the hydrophobic PVDF substrate [7]. Mansouri et al. also coated PVA and polyhydroxyethylmethacrylate (PHEMA) on the PP and PVDF membranes for osmotic distillation [8]. Nevertheless, the common limitation in surface coating is the instability of the coating layer on the membrane surface due to their relatively weak interaction. In contrast, surface grafting, which can be achieved by high energy plasma and ultraviolet (UV) irradiations or by chemical treatment [9], [10], [11], [12], [13], can produce covalent bonding between the membrane surface and the grafted molecules, leading to a stable modified layer on the membrane surface. However, most of reported plasma treatments were to make the membrane surface more hydrophobic to avoid pore wetting [5], [12], [14], [15].
Many attempts have been made to fabricate TiO2/polymer membranes in order to impart anti-fouling and anti-bacterial properties to the membrane [16], [17]. TiO2 particles were either entrapped in the membrane matrix by adding TiO2 particles into the casting solution or directly deposited onto the membrane surface [18]. Although a high TiO2 loading in the casting solution can be achieved, the anti-fouling effect was found to be significantly lower than that resulted from the direct TiO2 surface deposition [19]. However, for the surface deposition method, the immobilization of TiO2 particles was normally carried out by the filtration of an aqueous suspension or simple membrane dip-coating [16], [20], [21], [22], [23]. As a result, the bonding between the TiO2 particles and the targeted membrane is rather skeptical because no covalent force exists to maintain the stability. There are limited reports on incorporating TiO2 on the membrane surface via strong covalent bonding, especially for chemically stable PVDF membrane [24].
In addition, inadequate reports are available on membrane fouling for MD process in literature. Some relevant studies focused on the influence of operation conditions on oil fouling with little attention on the interaction of oil droplets and the membrane surface [8], [25]. Recently, the classical extended Derjaguin, Landau, Verwey and Overbeek (XDLVO) theory was used to study the organic deposition on pressure-driven membrane processes including reverse osmosis (RO) and ultrafiltration (UF) [26], [27]. A better understanding of the fouling mechanisms was expected by the quantitative assessment of physicochemical interactions at molecular level, together with the investigation of the effect of operation conditions.
The current study aims to explore a novel modification approach to obtain a highly hydrophilic membrane surface, and to investigate the impacts of membrane surface properties and operation conditions on the oil fouling during MD process for three different membranes, including one commercial PVDF membrane and two in-house modified membranes. The two membranes were modified by PEG grafting and TiO2 depositing onto the membrane surface. The interaction between mineral oil and membrane surface was quantitatively analyzed based on the interfacial free energy using the XDLVO theory. In addition, since the membrane surface charge plays an important role in fouling behavior and the discharge of coastal petrochemical industry effluents probably alters the pH of the near shore seawater, MD tests were performed at varied pH values of the feed solution to approach the practical desalination condition. Meanwhile, the influence of hydrodynamic conditions was also investigated in order to identify the optimum operating parameters for desalinating seawater with oil. It is expected that this study can provide a deep insight on the oil-fouling mechanism and explore an effective way to develop novel anti-wetting and anti-fouling membranes for the MD application.
Section snippets
Concept of surface tension
The wettability of a solid surface can be expressed by contact angle, , which is governed by Young's equation:where , and refer to the surface tensions of solid in contact with air, solid in contact with liquid, and liquid in contact with air, respectively. The surface tension of a solid surface can be written as following when the Good–Van Oss model is applied [28]:Where , and are the dispersive component (indicating Lifshitz–Van Der Waals
Materials and chemicals
All solutions and reagents were prepared with analytical grade chemicals. Deionized (DI) water was from Millipore Water Purification System. Polyvinylidene fluoride (PVDF) membrane (GVHP), with an average pore size of 0.22 µm and a thickness of 125 µm was purchased from Millipore. Sodium hydroxide (Sigma-Aldrich) was used as the alkali solution for the post treatment. Polyethylene glycol (PEG Mw∼1000) used for plasma grafting was purchased from Sigma-Aldrich. Titanium tetraisopropoxide (TTIP)
Confirmation of the surface modification mechanism
The chemical structures of commercial PVDF membrane, PVDF-PT membrane and PVDF-P membranes were examined by ATR-FTIR. Fig. 3 provides the spectroscopy results in the wave number range of interest (650–4000 cm−1). As can be seen from Fig. 3(a), the vibration bands at 1178 and 1275 cm−1 correspond to the asymmetrical and symmetrical stretching of CF2, respectively, and the vibration of CH2 groups appeared at the frequency of 1402 cm−1.
The change in the functional groups of PVDF-P membrane caused by
Conclusions
A novel and effective way to modify hydrophobic PVDF flat sheet membrane to obtain hydrophilic surface property via plasma induced grafting of PEG and subsequent TiO2 particles deposition onto the membrane surface has been reported. A series of characterizations were conducted for the virgin and modified membranes. Membrane distillation tests were also performed to investigate the influences of membrane hydrophobicity, membrane surface structure, pH value of the feed solution and hydrodynamic
Acknowledgments
This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB (EWI RFP #0901-IRIS-02-03). We are also grateful to the Singapore Economic Development Board for funding to Singapore Membrane Technology Centre.
References (41)
- et al.
Membrane distillation
J. Membr. Sci.
(1997) - et al.
A framework for better understanding membrane distillation separation process
J. Membr. Sci.
(2006) - et al.
Energy efficiency evaluation and economic analyses of direct contact membrane distillation system using Aspen Plus
Desalination
(2011) - et al.
Performance improvement of PVDF hollow fiber-based membrane distillation process
J. Membr. Sci.
(2011) - et al.
Progress in the production and modification of PVDF membranes
J. Membr. Sci.
(2011) - et al.
Desalination by membrane distillation adopting a hydrophilic membrane
Desalination
(2005) - et al.
Osmotic distillation of oily feeds
J. Membr. Sci.
(1999) - et al.
Surface grafting control of PEGylated poly(vinylidene fluoride) antifouling membrane via surface-initiated radical graft copolymerization
J. Membr. Sci.
(2009) - et al.
Antifouling microfiltration membranes prepared from acrylic acid or methacrylic acid grafted poly(vinylidene fluoride) powder synthesized via pre-irradiation induced graft polymerization
J. Membr. Sci.
(2010) - et al.
Preparation and characterization of modified nano-porous PVDF membrane with high antifouling property using UV photo-grafting
Appl. Surf. Sci.
(2009)
Plasma-induced immobilization of poly(ethylene glycol) onto poly(vinylidene fluoride) microporous membrane
J. Membr. Sci.
Surface modification of polyvinylidene fluoride-co-hexafluoropropylene (PVDF–HFP) hollow fiber membrane for membrane gas absorption
J. Membr. Sci.
CF 4 plasma surface modification of asymmetric hydrophilic polyethersulfone membranes for direct contact membrane distillation
J. Membr. Sci.
Preparation and characterization of fouling-resistant TiO2 self-assembled nanocomposite membranes
J. Membr. Sci.
Structural and performance properties of UV-assisted TiO2 deposited nano-composite PVDF/SPES membranes
Desalination
Studies on various reactor configurations for coupling photocatalysis and membrane processes in water purification
J. Membr. Sci.
Study on a photocatalytic membrane reactor for water purification
Catal. Today
Studies on various reactor configurations for coupling photocatalysis and membrane processes in water purification
J. Membr. Sci.
Heterogeneous photocatalytic degradation of pharmaceuticals in water by using polycrystalline TiO2 and a nanofiltration membrane reactor
Catal. Today
A new approach to improve antifouling property of PVDF membrane using in situ polymerization of PAA functionalized TiO2 nanoparticles
J. Membr. Sci.
Cited by (217)
Investigating mechanism of enhanced anti-fouling performance of MXene modified Janus MD membrane by XDLVO theory combined with surface elemental integration method
2024, Process Safety and Environmental ProtectionInterface engineered Ag-r-GO-CuFe<inf>2</inf>O<inf>4</inf>-Fe<inf>3</inf>O<inf>4</inf> heterojunction an efficient photocatalyst for water treatment and toxicity study in Trifolium plants
2024, Journal of Industrial and Engineering ChemistryA comprehensive review on state-of-the-art antifouling super(wetting and anti-wetting) membranes for oily wastewater treatment
2024, Advances in Colloid and Interface Science