Effects of superhydrophobic SiO₂ nanoparticles on the performance of PVDF flat sheet membranes for vacuum membrane distillation

Highlights

• PVDF flat sheet membranes were prepared by phase inversion for VMD.

• Superhydrophobic SiO₂ nanoparticles effect on membrane performance is investigated.

• Superhydrophobic SiO₂ nanoparticles increased the surface pore size.

• The superhydrophobic SiO₂ nanoparticles reduced the porosity of the membrane.

• The SiO₂ nanoparticles blended PVDF membranes showed enhanced VMD flux.

Abstract

Polyvinylidene fluoride (PVDF)/SiO₂ flat sheet composite membranes were prepared for vacuum membrane distillation (VMD) by the phase inversion immersion precipitation process. The effect of blending superhydrophobic SiO₂ nanoparticles into the PVDF dope solution was studied. The concentration of the nanoparticles in the dope solution was varied at different wt.% (1, 2, 4, 6, 7, 8 and 10 wt.%). The prepared membranes were characterized by scanning electron microscopy, water contact angle, porosity, liquid entry pressure of water, Fourier transformed infrared spectroscopy, and VMD at feed temperature of 27 °C. The nanoparticles enhanced the membrane performance through a reduction in the sponge-like layer thickness and an increase in surface pore size, leading to increased vapour flux with a maximum at 7 wt.%. The salt rejection was greater than 99.98% when a 35 g/L NaCl solution was used as feed. At this concentration, the smallest thickness of the sponge-like layer and largest macro-voids were also achieved. Beyond 7 wt.%, the sponge-like layer became predominant and the flux was reduced. With a vapour flux increase of up to 4 times (from 0.7 to 2.9 kg/m² h) when compared to the neat membrane, this nanocomposite membrane could be of great potential in the desalination process through VMD.

Introduction

The demand for drinkable water is showing an increasing trend due to the rise in global population. The total amount of water on earth is believed to be constant that goes through a re-cycling process. Though 70% of the earth is covered by water, only 2.5% is fresh water, the rest being saline and ocean based [1]. With the increasing effects of global temperature rise (climate changes), surface water is suffering from constant evaporation creating more shortage of potable water and making the saline water more saline. The situation does not seem to be improving along with difficult challenges associated with controlling global warming and pollution [1]. Since there has not been a cost effective means to circumvent the crisis, global health, economic growth and even social welfare are at risk.

It is therefore imperative that other means of fresh water is required to meet the rising population demand such as desalination of sea water to potable water. Several conventional processes like ultrafiltration (UF), microfiltration (MF), reverse osmosis (RO) and multistage vacuum evaporation have been employed for water treatment but an emerging separation technique is needed [2]. Membrane distillation (MD) is proving to be more efficient than other processes because of: (i) Lower operating pressure than RO, (ii) Close to 100% salt rejection, (iii) Lower energy consumption than multistage evaporation, and (iv) Lower operating temperature. MD has been used for different applications such as in the food industry, environmental protection and pharmaceuticals. In MD the driving force is the difference in vapour pressure across the micro-porous membrane created by the temperature gradient across the membrane [2].

Following extensive research on MD [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], different configurations have been analysed: vacuum membrane distillation (VMD), air gap membrane distillation (AGMD), direct contact membrane distillation (DCMD) and sweep gas membrane distillation (SGMD) [6], [9]. Irrespective of which configuration is being applied, a membrane which meets specific characteristics is always employed to carry out the separation process. In all MD configurations, one of the membrane characteristics is hydrophobicity which determines the wettability of the membrane surface. Hydrophobic membranes do not get wet easily, hence allowing only water vapour and not liquid to pass through the pores at pressures lower than the liquid entry pressure (LEP_w). At pressures greater than the LEP_w, the micro-pores become wet and part of the feed solution seeps onto the permeate side, thus allowing for no separation [3], [4], [8]. Researchers have used several polymers in preparing hydrophobic membranes because these polymers have presented low surface energies, high chemical resistance, thermal stability, and good mechanical strengths. These polymers include; polyvinylidine fluoride (PVDF), polypropylene (PP), polyethylene (PE) and polytetrafluoroethylene (PTFE) amongst which PVDF is the most widely used [27], [28]. Several techniques of membrane preparation have also been investigated in preparing membrane distillation membranes. These include; thermally induced phase separation (TIPS), immersion precipitations, vapour induced precipitation (VIP), and air casting of polymer solution, all of which involve some kind of solvent for polymer dissolution. The most widely used solvents for PVDF membrane preparation are dimethysulfoxide (DMSO), dimethyformamide (DMF), and dimethylacetylamide (DMAc) which are capable of completely dissolving the polymer at moderate temperatures [6], [9], [27], [28].

In the last decade of research in MD, scientists have placed special focus on the hydrophobicity of the developed membranes because an efficient MD process must result in reliable flux measurements and consistent conductivity readings of the resulting permeate. As such researchers have developed several means of achieving hydrophobic (water contact angle greater than 90°) and even superhydrophobic (contact angle greater than 130°) membrane surfaces [9]. Kuo et al. [16] obtained novel composite membranes by using alcohol as the coagulation agent through the phase invasion precipitation method. The membranes where reported to have contact angles greater than 130°. Razmjou et al. [24] fabricated hydrophobic membranes by coating the PVDF surface with TiO₂ which resulted in membranes with contact angles greater than 130°.

Recent research works have also included techniques to enhance the porosity, flux, and mechanical strength of the membranes. A blend of high and low molecular weight PVDF was used by Chen et al. [5], [6] to prepare flat sheet membranes of high porosity and improved LEP_w. Fontananova et al. [12] prepared membranes from PVDF copolymer (PVDF-co-hexafluoropropylene) and PVDF homopolymer. The copolymer membranes posed greater resistance to mass transport than the homopolymer due to a bi-layer formed at the top and bottom surfaces of the membrane. High porosity membranes where made by Khayet and Matsuura [14] using water as a pore forming agent. Wang and Chung [29] produced a mixed matrix hollow fibre membrane with high flux and porosity by adding hydrophobic clay particles into the dope. Other researchers have used a blend of polymers for the dope solution preparation. Yang et al. [32] fabricated membranes on support materials which were shown to enhance the flux by up to 15 fold when compared to the unsupported membrane.

The present research was carried out to fabricate hydrophobic PVDF/nanoparticles flat sheet composite membranes using superhydrophobic SiO₂ nanoparticles as additives. The hydrophobic porous membranes where prepared by the phase inversion precipitation method using deionised water as the nonsolvent additive and DMAc as the solvent. The membrane properties were investigated at different concentrations of the nanoparticles. They were characterized by scanning electron microscopy (SEM), VMD, porosity, liquid entry pressure of water (LEP_w), and water contact angle. The rejection of the membranes was also tested for desalination by using synthetic salt water.