Highly Hydrophobic Electrospun Reduced Graphene Oxide/Poly(vinylidene fluoride-co-hexafluoropropylene) Membranes for Use in Membrane Distillation

Cyrene-Enabled Green Electrospinning of Nanofibrous Graphene-Based Membranes for Water Desalination via Membrane Distillation

High-performance and sustainable membranes for water desalination applications are crucial to address the growing global demand for clean water. Concurrently, electrospinning has emerged as a versatile manufacturing method for fabricating nanofibrous membranes for membrane distillation. However, widespread adoption of electrospinning for processing water-insoluble polymers, such as fluoropolymers, is hindered by the reliance on hazardous organic solvents during production. Moreover, restrictions on industrial solvents are tightening as environmental regulations demand greener alternatives. This critical challenge is addressed here by demonstrating, for the first time, the fabrication of nanofibrous electrospun membranes of PVDF-HFP, poly(vinylidene fluoride)-co-hexafluoropropylene using a renewable, environment- and user-friendly solvent system containing Cyrene (dihydrolevoglucosenone), dimethyl sulfoxide, and dimethyl carbonate. The same solvent system was further used to produce nanocomposite graphene oxide (GO) and graphene nanoplatelet (GNP)-containing nanofibrous electrospun membranes. When tested for water desalination via membrane distillation, these membranes either outperformed or matched the performance of those produced with hazardous organic solvents, achieving salt rejection rates of >99.84% and long-term stability. The economic viability of the green solvent system was further validated through Monte Carlo simulations. This work demonstrates the potential to move fluoropolymer electrospinning from dimethylformamide-based systems to greener alternatives, enabling the consistent production of high-quality nanofibrous membranes. These findings pave the way for more sustainable manufacturing practices in membrane technology, specifically for water desalination via membrane distillation.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas-liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas-liquid interface), thus the proceeding of the wetting layer.

Advantages, limitations, and future suggestions in studying graphene-based desalination membranes

The potential of novel 2D carbon materials such as nanoporous single-layer graphene and multilayer graphene oxide membranes is based on their possible advantages such as high water permeability, high selectivity capable of rejecting monovalent ions, with high salt rejection, reduced fouling, and high chemical and physical stability. Here we review how the field has advanced in the study of their performances in various desalination approaches such as reverse osmosis, forward osmosis, nanofiltration, membrane distillation, and solar water purification. The research on making high-performance graphene membranes which started with reverse osmosis applications is seemingly evolving towards other directions.

Pore wetting in membrane distillation treatment of municipal wastewater desalination brine and its mitigation by foam fractionation

Reverse Osmosis (RO) desalination is an important step of wastewater reuse as it can remove salts and trace contaminants. However, RO also generates high salinity brines that need to be dealt with. Membrane distillation (MD), a process largely unaffected by salinity, provides a way to treat desalination brines up to high water recovery and has been proposed as a solution for RO brine management. However, pore wetting of membranes in MD is one of the major hurdles that prevents its implementation in wastewater treatment systems, as amphiphilic organic compounds present in wastewater can lead to pore wetting and loss of selectivity over time. The objective of this study was to identify a pre-treatment strategy to prevent wetting in MD treatment of municipal wastewater RO brines. We compared three pre-treatments with different separation or removal mechanisms: foam fractionation, advanced oxidation, and ultrafiltration. We evaluated membrane wetting by measuring the change in conductivity in the distillate and identified the most effective pre-treatment to prevent wetting in MD. The results show that wetting is prevented by pre-treating the brine with foam fractionation. The effectiveness of foam fractionation as a wetting control strategy was confirmed for a high wetting propensity synthetic water using sodium dodecyl sulfate as a model wetting compound. Finally, the effect of the pre-treatments on the desalination brine was evaluated to understand the nature of the compounds removed by each treatment. The results of this study will help implement MD as a treatment process for desalination brines in municipal wastewater reuse systems.

3D Printing of UV-Curable Polyurethane Incorporated with Surface-Grafted Nanocellulose

The recognition of nanocellulose has been prominent in recent years as prospect materials, yet the ineffectiveness of nanocellulose to disperse in an organic solvent has restricted its utilization, especially as a reinforcement in polymer nanocomposite. In this study, cellulose has been isolated and defibrillated as cellulose nanofibrils (CNF) from oil palm empty fruit bunch (EFB) fibers. Subsequently, to enhance its compatibility with UV-curable polyurethane (PU)-based resin, the surface hydrophilicity of CNF has been tailored with polyethylene glycol (PEG), as well as reduced graphene oxide (rGO). The dispersibility of reinforced modified CNF in UV-curable PU was examined through the transmittance interruption of resin, chemical, and mechanical properties of the composite printed using the stereolithographic technique. Evidently, the enhanced compatibility of modified CNF and UV-curable PU was shown to improve the tensile strength and hardness of the composites by 37% and 129%, respectively.

Fouling and wetting in the membrane distillation driven wastewater reclamation process - A review

Fouling and wetting of membranes are significant concerns that can impede the widespread application of the membrane distillation (MD) process during high-salinity wastewater reclamation. Fouling, caused by the accumulation of undesirable materials on the membrane surface and pores, causes a decrease in permeate flux. Membrane wetting, the direct permeation of the feed solution through the membrane pores, results in reduced contaminant rejection and overall process failure. Lately, the application of MD for water recovery from various types of wastewaters has gained increased attention among researchers. In this review, we discuss fouling and wetting phenomena observed during the MD process, along with the effects of various mitigation strategies. In addition, we examine the interactions between contaminants and different types of MD membranes and the influence of different operating conditions on the occurrence of fouling and wetting. We also report on previously investigated feed pre-treatment options before MD, application of integrated MD processes, the performance of fabricated/modified MD membranes, and strategies for MD membrane maintenance during water reclamation. Energy consumption and economic aspects of MD for wastewater recovery is also discussed. Throughout the review, we engage in dialogues highlighting research needs for furthering the development of MD: the incorporation of MD in the overall wastewater treatment and recovery scheme (including selection of appropriate membrane material, suitable pre-treatment or integrated processes, and membrane maintenance strategies) and the application of MD in long-term pilot-scale studies using real wastewater.