Reduced wrinkling in GO membrane by grafting basal-plane groups for improved gas and liquid separations
Graphical abstract
Introduction
Membrane technology is considered as one of the most promising technologies for industrial separation applications, such as desalination [1], [2], [3], water purification [4], [5], [6], [7] and gas purification [8], [9], [10], [11]. The increasing demands of energy and clean water nowadays push for fast development of new membranes with advanced functionalities. Graphene oxide (GO) membranes have attracted great interests in recent years due to the intrinsic physicochemical properties of GO such as excellent thermal stability, chemical inertness and tunable surface functionality. With rich oxygenated groups present at both edge and basal plane, GO nanosheets can be conveniently modified for different purposes. Especially, its two-dimensional (2-D) sheet structure provides a great opportunity to fabricate membranes via stacking technologies [12], [13], [14], [15]. GO membranes have been demonstrated excellent separation properties in both gas and liquid separation applications [16], [17], [18], [19]. However, there are still a few key challenges need to be addressed before GO membranes can be practically used including but not limited to weak mechanical strength at nanoscale thickness, low membrane integrity due to swelling and limited approaches to tune d-spacing [20], [21], [22].
Porous substrates are often used to support GO membrane and extend their application in wider range of operation conditions [23], [24], [25]. The d-spacing, interlayer distance of stacking GO nanosheets, is one of the most important factors that directly related to the separation performance of GO membrane [26], [27], [28]. The laminar structure formed by stacking GO nanosheets offers zigzag paths for molecule transportation. The size of path is determined by d-spacing. Thus, it is extremely important to control d-spacing when the size exclusion mechanism dominates in separation process. It has been reported that d-spacing of dried GO membrane and graphene membrane is ~ 0.80 and 0.34 nm, respectively. The steric hindrance exerted by oxygen functional groups on GO sheets is the main reason of larger d-spacing of GO membrane [27]. Meanwhile, the presence of such hydrophilic groups could further enlarge d-spacing to 6–7 nm in aqueous environment [27], which is almost ten-fold larger than the size of hydrated ions [29], [30]. Swelling not only weakens the separation performance, but also reduces mechanical strength of GO membranes. Thus, restricting GO swelling is another critical task to improve separation performance and maintain long term stability in liquid separation applications. Crosslinking is a widely adopted strategy to address swelling issue by bridging GO nanosheets via covalent bonds [31], [32], [33]. Besides crosslinking chemistry, the membrane fabrication method also has huge influence on membrane microstructure and thus separation performance. A recent study reveals that pressure-driven filtration can form more compacted laminar structure than the membrane prepared from vacuum filtration [34].
GO surface property is another important factor that dominates the membrane property. Two major streams of research were developed in recent years, one is GO reduction [13], [35], [36] and the other is GO modification with charged functional groups [37], [38]. GO reduction eliminates surface functional groups and reduces steric hindrance, leading to a more compact laminar structure and smaller d-spacing. However, the hydrophilic property is often sacrificed after reduction, which makes the dispersion in water very challenging as well as the membrane fabrication with good patterning. It is generally believed that, besides d-spacing, surface charge also plays an important role in separation by the well-known “Gibbs-Donnan” effect. For example, by grafting positively charged polyethyleneimine on GO membrane surface, improved cation rejection (Mg2+, Pb2+, Ni2+, Cd2+, and Zn2+) was observed [38].
During membrane fabrication, wrinkling phenomenon of GO sheets has been found and confirmed by both experimental and simulation studies. It is widely accepted that wrinkles are often formed during GO membrane fabrication processes, such as drop-casting, spin-coating, spraying, and filtration [39], [40], [41]. The soft nature, small thickness and large 2-D dimension make it very easy to fold and form wrinkles. Once the folding occurred at early stage of membrane formation, the folding-induced curvature will be amplified as membrane thickness increases. As a result, larger wrinkles will be formed on top of the GO membrane [40]. In a recent simulation study, Kim et al. found that edge-edge connection and plane-plane connection of GO sheets has dramatic influence on the folding of GO sheets [42]. Edge-edge connection is more likely to form wrinkled structure and thus greatly influence the packing of GO sheets. By reducing the wrinkles, improved Na2SO4 rejection rate from 21.3% to 85.8% was observed [40].
In this work, epoxide ring-opening reaction and subsequent modification with OA were proceeded to enrich the functional groups on the basal plane of GO sheets. By facilitating the plane-plane connection of GO sheets with activated in-plane groups, better packing (reduced wrinkling) of GO membrane would be expected and thus superior separation performance. GO membranes were fabricated by pressure-assisted filtration method on porous polymer support. Membrane thickness was simply controlled by the amount of filtration solution. Crosslinking was introduced in the membrane to improve the mechanical strength as well as reduce swelling in aqueous media. The GO modification, crosslinking reaction, microstructure and wrinkle structure of GO membrane were systematically characterized. The prepared membranes were tested in various separation applications including H2/CO2 separation, organic dye separation from water, and desalination.
Section snippets
Materials
Graphite power (SP-1) was purchased from Bay Carbon Inc, USA. Hydrochloric acid (HCl) was purchased from EMD Millipore Corporation. Potassium persulfate (K2S2O8, ≥ 99.0%), methylene blue (MB, C16H18ClN3S·3H2O) were purchased from Fisher Scientific. Potassium permanganate (KMnO4, ≥ 99.0%), phosphorus pentoxide (P2O5, ≥ 98.0%), sulfuric acid (H2SO4, 95.0–98.0%), hydrogen peroxide (H2O2, 30 wt% in H2O), ethylenediamine (EDA, ≥ 99.0%), dopamine hydrochloride, magnesium chloride (MgCl2, anhydrous,
Results and discussion
The thickness of membrane not only affects its microstructure, it is also one of the important parameters that influences separation. Thin membrane is preferred to facilitate large flux, but it generally results in lower selectivity. In this work, membranes with three different thicknesses were prepared on top of porous PES support with a thickness of about 100, 200 and 400 nm, respectively, Fig. 4. Obviously, the increase of membrane thickness followed the same trend as increasing the amount
Conclusion
To sum up, this work provided a novel modification method to fabricate OAGO membranes with excellent permselectivity in gas separation, dye removal, and desalination applications. The OA modification introduced active functional groups on the basal plane of GO nanosheets, and subsequent crosslinking with EDA improves membrane stability in liquid environment. After modification and crosslinking, strong plane-plane connection was constructed with tightly ordered laminar structure. The OAGO/EDA-2
Acknowledgement
The Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund (#55570-DNI10) and NSF (CBET-1603264). We also thank the National Energy Solutions Institute – Smart Energy Source (NESI-SES) and the Technology and Business Development Program (TBDP) (P2017-0033) for partial funding of this work.
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