Controlled fabrication and optimisation of crosslinked graphene oxide membranes for enhanced water purification performance

Graphene Oxide (GO) is an equitable next generation membrane material and significant graphene alternative owing to its large-scale production scalability specifically and physicochemical characteristics. The use of GO as a water purification and desalination membrane was first demonstrated by Nair et al in 2012. As such, its feasibility as a nanofiltration separation membrane material is still in its primary usage hence the need for optimisations, modifications and understanding of its permeation mechanisms. Major limitations in the use of GO as a separation membrane material include the widening of the membrane interlayer spacing (pore-gap) during operation and poor membrane stability. This doctoral research in consequence looked into the use of different crosslinkers to enhance the performance and stability of GO membranes through both inter and intralayer crosslinking. Successively, p-Phenylenediamine, 1,3,5–triazine – 2,4,6 triamine (melamine) and polyethyleneimine were systematically introduced onto the GO nanosheets via the dip-assisted layer-by-layer method to fabricate crosslinked GO membranes. Principally, the feasibility of the use of the aforementioned crosslinkers in interconnecting GO nanosheets and fabricating thin-films/membranes via the layer by layer assembly method was explored. The nature of interaction between GO and the crosslinkers was analysed and subsequently crosslinked thin films were fabricated to demonstrate the control of key characteristics like thickness. Respective characterizations were undertaken, proving successful thin-film assembly. Following thin film fabrication, the thesis goes on to look into the nanofiltration performance of respective membranes assembled on poly (acrylonitrile) and polycarbonate substrates. The impact of crosslinking in enhancing performance was apparent. Successively, the impact of the physicochemical properties of GO, specifically its lateral size and surface chemistry onto the nanofiltration performance of the crosslinked membranes was studied. Ultimately, GO and crosslinker concentration alteration on membrane nanofiltration performance was analysed. These were optimisation stages where the aim was to determine the optimum lateral-size of GO nanosheets and concentration effects in membrane stability and performance

Systematic covalent crosslinking of graphene oxide membranes using 1,3,5 triazine 2,4,6 triamine for enhanced structural intactness and improved nanofiltration performance

From its physicochemical characteristics, graphene oxide (GO) is a promising versatile next generation membrane material. Its unique characteristics like ultrafast permeation and hydrophilicity makes it a favourable separation membrane nanomaterial in water purification. However, a fundamental problem in the use of GO in nanofiltration is decreased performance overtime due to the pore size widening phenomenon. This paper explored the use of an amine group containing compound, 1,3,5 triazine, 2,4,6 triamine (melamine) to covalently interlink the GO nanosheets to counteract this swelling phenomenon. Prior to membrane fabrication, covalent interactions between GO and the crosslinker, melamine were successfully confirmed through thermogravimetric analysis (TGA), x-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD) and Fourier Transform Infra-Red (FTIR) spectroscopy characterisations. Following these characterisations, crosslinked membranes were successfully fabricated and enhanced nanofiltration performance was confirmed. Resultantly, the surface morphology of the membranes was recorded via Scanning Electron Microscopy (SEM) characterisations while a lab-scale nanofiltration device was constructed for flux and rejection analysis. Evidently, performance improvement with covalent crosslinking was imminent as an up to a 100% rejection of methylene blue was achieved for the crosslinked membranes. Structural integrity of GO membranes has indeed been improved through crosslinking

Interfacial crosslinked controlled thickness graphene oxide thin-films through dip-assisted layer-by-layer assembly means

The augmentation of research in graphene based thin films has been of great interest to various current industrial stakeholders. This is mainly due to the wide scope of films applications, ranging from nanoelectronics to separation membranes. Therefore, establishing a relation between graphene based thin film key characteristics and the fabrication operating conditions is of high significance. This study entails the successful fabrication of controlled-thickness crosslinked graphene oxide (GO) thin films on inexpensive silicon-based glass slide substrates. The method of film fabrication used is the dip-assisted layer-by-layer assembly, which has an added advantage of step-control of thin film thickness, good film uniformity and continuity. The thickness was primarily tuned through the use different sized crosslinkers; a covalent based sub-nanometer sized p-phenylenediamine and an electrostatic based polyethyleneimine on an interchangeable assembly with GO. Pre film fabrication, Fourier Transform Infra-Red and X-Ray Photoelectron Spectroscopy characterizations were carried out to determine the nature of interactions between GO and the crosslinkers. Post film fabrication, scanning electron microscopy, water contact angle measurements and profilometry analysis were undertaken for film continuity, hydrophilicity and thickness measurements respectively. A strong linear trend between film thickness of the differently crosslinked films and the number of bi-layers was established

Influence of graphene oxide's characteristics on the fabrication and performance of crosslinked nanofiltration membranes

Graphene oxide (GO) is emerging as an excellent next generation material for water purification membranes. Its ability to be fabricated cost-effectively in large quantities and featured characteristics, such as hydrophilicity, makes it an equitable graphene alternative in respective nanometric applications, including nanofiltration. In this study, the influence of key properties of the GO sheets, such as lateral size, surface chemistry and colloid stability, on the successful fabrication and subsequent water purification performance of crosslinked nanofiltration membranes is analysed. GO water suspensions with nanosheets of different lateral sizes and distribution of oxygenated functional groups were prepared by controlling the sonication time (from 0 to 180 min) starting from commercial GO. The variation of the physicochemical characteristics of the resulting GO sheets was comprehensively studied by means of atomic force microscopy, UV–Vis absorption spectroscopy, zeta potential measurements and X-Ray photoelectron spectroscopy. The morphology of the subsequently fabricated membranes was hereafter examined via scanning electron microscopy, while their nanofiltration performance was investigated against methylene blue solution. The influence of GO's physicochemical characteristics on membrane performance was apparent, with the average rejection values ranging from 59.8% to 98.4% at a changing lateral size and surface chemistry

Enhanced covalent p-phenylenediamine crosslinked graphene oxide membranes: towards superior contaminant removal from wastewaters and improved membrane reusability

The increasing depletion of freshwater necessitates the re-use and purification of wastewaters. Among the existing separation membrane materials, graphene oxide (GO) is a promising candidate, owing to its tunable physicochemical properties. However, the widening of GO membranes pore gap in aqueous environments is a major limitation. Crosslinking agents can be incorporated to alleviate this problem. This study describes a comparative analysis of uncrosslinked and p-Phenylenediamine (PPD) crosslinked GO membranes’ water purification performance. Dip-coating and dip-assisted layer-by-layer methods were used to fabricate the uncrosslinked and crosslinked membranes respectively. The covalent interaction between GO and PPD was confirmed by Fourier Transform Infra-Red and X-ray Photoelectron Spectroscopy. The excellent membrane topographical continuity and intactness was assessed by means of Scanning Electron Microscopy, while water contact angle measurements were undertaken to evaluate and confirm membrane hydrophilicity. The improvement impact of the crosslinker was manifested on the enhancement of the stability and performance of the membranes during nanofiltration tests of aqueous solutions of methylene blue in a homemade nanofiltration cell operated at 1 bar

Crosslinked graphene oxide membranes: Enhancing membrane material conservation and optimisation

Background: Graphene Oxide (GO) has recently shown great promise in water purification as a potential substitute to conventional membrane materials. However, GO membranes face some challenges associated to their swelling due to the accumulation of water molecules in their oxidized regions. The use of crosslinkers has been proven as an effective way to improve GO membranes stability and performance. Nevertheless, optimisations to efficiently use materials and resources are a necessity. These include the determination of the influence of GO and crosslinker amounts on membrane structure, operation, and efficiency. Methods: Consequently, in this study crosslinked membranes with different GO and p-phenylenediamine (crosslinker) concentrations were fabricated to establish relationships between the quantity of the selected materials and membranes performance. FESEM was undertaken to investigate the structural quality together with thickness measurements. The performance of the membranes was evaluated via a pressure assisted nanofiltration cell using aqueous methylene blue (MB) as feed solution. Significant findings: A notable enhancement in MB separation from around 75 to 98% was observed at an increasing GO and crosslinker concentrations. The permeation flux decreased correspondingly owing to tortuosity lengthening at higher GO concentrations. Based on performance rates and XPS characterizations the optimum crosslinker and GO concentrations were deduced