PVDF membranes containing reduced graphene oxide: Effect of degree of reduction on membrane distillation performance
Graphical abstract
Introduction
Today, seawater desalination is mainly achieved by one of the following three technologies: multistage flash distillation (MSF), multi-effect distillation (MED), or reverse osmosis (RO). However, significant amounts of thermal energy in MSF and MED, and electrical energy in RO are required to carry out the separation on a large scale and typically rely on non-renewable energy sources. In addition, the capital investment and maintenance costs for these technologies can be very high. Consequently, alternative technologies that are capable of performing effectively but in a more sustainable way are currently being investigated. Membrane distillation (MD) seems to be a suitable option. In this process, only water vapor is able to permeate through a non-wetted porous hydrophobic membrane as a consequence of the difference in water vapor pressure between the two sides of the membrane. The pressure difference results from the thermal gradient between the hot feed and the cold permeate stream [1,2]. MD requires lower feed temperatures than those in thermal-based processes, and offers a variety of advantages when compared to conventional pressure-driven membrane separation processes; it can achieve higher rejection of ions at much lower operating pressures than for instance RO and it shows greater simplicity and higher resistance to fouling [[2], [3], [4], [5]].
Recently, MD has begun to be implemented at pilot plant scale, as described in the work by Duong et al., where a pilot-scale spiral-wound air gap MD system was successfully used to treat coal steam gas (CSG) RO brine with 80% water recovery [6]. The work by Woldemariam et al. also reports the use of MD at pilot scale, where the purification of effluent from a municipal wastewater treatment plant was carried out in an air gap membrane distillation (AGMD) system [7]. Yet, this technology has still not been fully implemented at the industrial level, with the lack of specially-tailored low-cost membranes being one of the major reasons for this [8]. Although commercial hydrophobic microfiltration (MF) membranes can be utilized for MD systems, they can still suffer from wetting issues and sub-optimal flux performance [4,8]. Thus, robust membranes specific for MD processes with high hydrophobicity, high porosity, narrow pore size distribution and good chemical stability need to be developed [3].
Polyvinylidenefluoride (PVDF) is a semicrystalline polymer characterized by having good chemical resistance and thermal stability, high mechanical strength and excellent aging resistance, which are very important requirements for practical applications [9]. Moreover, PVDF is soluble in some common solvents as dimethylacetamide (DMAc), N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Consequently, PVDF flat sheet and hollow fibre membranes can be produced at scale from solution by the widely used non-solvent induced phase separation (NIPS) process [[8], [9], [10], [11]]. Attempts have been carried out using PVDF as a base polymer due to its relatively high hydrophobicity, low thermal conductivity, and good processability. For instance, Tomaszewska used PVDF in combination with LiCl to prepare membranes for direct contact membrane distillation (DCMD) with 98% of salt rejection and a permeate flux of 9.7 LMH (L m−2 h−1) [12]. Furthermore, advances in nanotechnology have been used in water desalination technologies to improve salt rejection and flux [13]. Prince et al., prepared electrospun nanofiber membranes comprising PVDF blended with clay nanocomposites, which were tested for DCMD and gave a flux of 5.7 LMH and a rejection of 99.5% [14]. TiO2 was used by Meng et al. as a surface coating for PVDF membranes, leading to improvements in salt rejection and membrane stability [15]. In addition, Roy et al. immobilized functionalized carbon nanotubes (CNTs) on porous polypropylene supports, achieving higher fluxes in DCMD [16].
A considerable amount of literature has been published on using graphene oxide (GO) nanosheets in MD [[17], [18], [19]] and ultrafiltration (UF) applications [20,21]. GO can be prepared via several cost-effective mass production methods using inexpensive graphite as raw material with high yields [22]. Leaper et al. reported an 86% flux enhancement of neat PVDF membrane by adding 3 (aminopropyl)triethoxysilane (APTS)-functionalized GO [19]. Zahirifar et al. fabricated and tested octadecylamine functionalized graphene oxide/PVDF dual-layer membranes for desalination via AGMD, showing a decrease in water flux but boosted salt rejection as compared to the unmodified PVDF membrane [18]. Bahdra et al., immobilized GO on the surface of polytetrafluoroethylene (PTFE) membranes for desalination via DCMD, significantly improving the overall permeate flux with complete salt rejection [17].
GO can be partly reduced to graphene-like sheets (i.e., reduced graphene oxide, rGO) by removing the oxygen-containing groups, thus, recovering the conjugated structure in a large extent [23]. Although the electronic properties of rGO are far from those shown for pristine graphene, other features such as mechanical strength, high ratio area/volume and thermal/chemical stability are still presents in rGO nanosheets. The relatively high hydrophobicity of rGO compared to GO makes it a promising material for improving the hydrophobic membranes in MD systems.
In the present study, PVDF flat-sheet membranes incorporated with reduced graphene oxide (rGO) have been employed for water purification of aqueous solutions containing 35,000 mg L−1 of sodium chloride via AGMD. To the best of the authors' knowledge, this is the first time that rGO has been used for the preparation of MD membranes, and the first time that the amount of oxygen-containing functionalities has been linked to the morphology, hydrophobicity and desalination performance of MD membranes. GO and four different rGO fillers with increasing degrees of reduction have been prepared, characterized and incorporated into PVDF. The concentration of graphene-based nanoadditives in the membranes has been optimized for membranes prepared with the most reduced rGO sample, and that optimum concentration has been used in the preparation of membranes containing GO and the other three rGO fillers with lower reduction degrees.
Section snippets
Materials
For the preparation of graphene oxide (GO) graphite powder (Nature Graphit GmbH, Germany) was used. Potassium permanganate (KMnO4, 99%), hydrogen peroxide (H2O2, 30%), sulphuric acid (H2SO4, 98%), sodium hydroxide (NaOH), nitric acid (HNO3, 68%) and hydrochloric acid (HCl, 37%) were purchased from Sigma Aldrich, UK and were all used as received. For the preparation of membranes, PVDF powder with a molecular weight of 534,000 g mol−1 and N,N-dimethylformamide (DMF) were purchased from
Characterization of fillers
Raman spectroscopy is an effective way to identify the structural fingerprint of carbon materials (such as GO and rGO) since Raman scattering can observe vibrational modes these systems. In Fig. 2, the Raman spectra of the prepared GO and rGO with different degrees of reduction are shown. The characteristic G and D peaks are present for all the samples. It is well-known that the G peak at ~1600 cm−1 corresponds to the sp2 hybrid structure, which reflects the symmetry and crystallinity of carbon
Conclusions
Returning to the hypothesis posed at the beginning of this study, it is now possible to state that the addition of reduced GO, prepared using environmentally friendly ascorbic acid, into PVDF mixed matrix membranes significantly enhances the membrane distillation performance. This enhancement can be explained by differences in morphology (as evidenced by SEM, AFM, and pore-size data analyses) as well as water vapor interactions with the rGO nanosheets. The degree of GO reduction, as defined by
Acknowledgements
The authors would like to acknowledge EPSRC for funding this work (grant number EP/K016946/1). A. Abdel-Karim would like to thank the Ministry of Higher Education of Egypt and the Newton-Mosharafa Fund.
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