Beneficial CNT Intermediate Layer for Membrane Fluorination toward Robust Superhydrophobicity and Wetting Resistance in Membrane Distillation

Bio-Inspired Functional Materials for Environmental Applications

With the global population expected to reach 9.7 billion by 2050, there is an urgent need for advanced materials that can address existing and developing environmental issues. Many current synthesis processes are environmentally unfriendly and often lack control over size, shape, and phase of resulting materials. Based on knowledge from biological synthesis and assembly processes, as well as their resulting functions (e.g., photosynthesis, self-healing, anti-fouling, etc.), researchers are now beginning to leverage these biological blueprints to advance bio-inspired pathways for functional materials for water treatment, air purification and sensing. The result has been the development of novel materials that demonstrate enhanced performance and address sustainability. Here, an overview of the progress and potential of bio-inspired methods toward functional materials for environmental applications is provided. The challenges and opportunities for this rapidly expanding field and aim to provide a valuable resource for researchers and engineers interested in developing sustainable and efficient processes and technologies is discussed.

Graphene Oxide/Polyethyleneimine-Modified Cation Exchange Membrane for Efficient Selective Recovery of Ammonia Nitrogen from Wastewater

Competition for the migration of interfering cations limits the scale-up and implementation of the Donnan dialysis process for the recovery of ammonia nitrogen (NH4+-N) from wastewater in practice. Highly efficient selective permeation of NH4+ through a cation exchange membrane (CEM) is expected to be modulated via tuning the surface charge and structure of CEM. In this work, a novel CEM was designed to form a graphene oxide (GO)-polyethyleneimine (PEI) cross-linked layer by introducing self-assembling layers of GO and PEI on the surface of a commercial CEM, which rationally regulates the surface charge and structure of the membrane. The resulting positively charged membrane surface exhibits stronger repulsion for divalent cations compared to monovalent cations according to Coulomb's law, while, simultaneously, GO forms π-metal cation conjugates between metal cations (e.g., Mg2+ and Ca2+), thus limiting metal cation transport across the membrane. During the DD process, higher NH4+ concentrations resulted in a longer time to reach Donnan equilibrium and higher NH4+ flux, while increased Mg2+ concentrations resulted in lower NH4+ flux (from 0.414 to 0.213 mol·m-2·h-1). Using the synergistic effect of electrostatic interaction and non-covalent cross-linking, the designed membrane, referred to as GO-PEI (20) and prepared by a 20 min impregnation in the GO-PEI mixture, exhibited an NH4+ transport rate of 0.429 mol·m-2·h-1 and a Mg2+ transport rate of 0.003 mol·m-2·h-1 in single-salt solution tests and an NH4+/Mg2+ selectivity of 15.46, outperforming those of the unmodified and PEI membranes (1.30 and 5.74, respectively). In mixed salt solution tests, the GO-PEI (20) membrane showed a selectivity of 15.46 (~1.36, the unmodified membrane) for NH4+/Mg2+ and a good structural stability after 72 h of continuous operation. Therefore, this facile surface charge modulation approach provides a promising avenue for achieving efficient NH4+-selective separation by modified CEMs.

Membrane Design Criteria and Practical Viability of Pressure-Driven Distillation

Pressure-driven distillation (PD) is a novel desalination technology based on hydraulic pressure driving force and vapor transport across a hydrophobic porous membrane. In theory, PD offers near-perfect rejection for nonvolatile solutes, chlorine resistance, and the ability to decouple water and solute transport. Despite its advantages, pore wetting and the development of a reverse transmembrane temperature difference are potential critical concerns in PD, with the former compromising the salt rejection and the latter reducing or even eliminating the driving force for vapor transport. We herein present an analysis to evaluate the practical viability and membrane design principles of PD with a focus on the dependence of flux and salt rejection (SR) on membrane properties. By modeling the mass transfer in a PD process under different conditions, we arrive at two important conclusions. First, a practically detrimental reverse transmembrane temperature difference does not develop in PD under all relevant circumstances and is thus not a practical concern. Second, for a PD process to achieve an acceptable SR, the membrane pores should be at the nanometer scale with a highly uniform pore size distribution. This analysis demonstrates the practical viability of PD and provides the principles for designing robust and high-performance PD membranes.

Membrane Distillation Hybrid Peroxydisulfate Activation toward Mitigating the Membrane Wetting by Sodium Dodecyl Sulfate

The fouling/wetting of hydrophobic membrane caused by organic substances with low-surface energy substantially limits the development of the membrane distillation (MD) process. The sulfate radical (SO4 ·−)-based advanced oxidation process (AOP) has been a promising technology to degrade organics in wastewater treatment, and peroxydisulfate (PDS) could be efficiently activated by heat. Thus, a hybrid process of MD-AOP via PDS activated by a hot feed was hypothesized to mitigate membrane fouling/wetting. Experiments dealing with sodium dodecyl sulfate (SDS) containing a salty solution via two commercial membranes (PVDF and PTFE) were performed, and varying membrane wetting extents in the coupling process were discussed at different PDS concentrations and feed temperatures. Our results demonstrated permeate flux decline and a rise in conductivity due to membrane wetting by SDS, which was efficiently alleviated in the hybrid process rather than the standalone MD process. Moreover, such a mitigation was enhanced by a higher PDS concentration up to 5 mM and higher feed temperature. In addition, qualitative characterization on membrane coupons wetted by SDS was successfully performed using electrochemical impedance spectroscopy (EIS). The EIS results implied both types of hydrophobic membranes were protected from losing their hydrophobicity in the presence of PDS activation, agreeing with our initial hypothesis. This work could provide insight into future fouling/wetting control strategies for hydrophobic membranes and facilitate the development of an MD process.

Non-Fluorinated Flexible Superhydrophobic Surface with Excellent Mechanical Durability and Self-Cleaning Performance

Although plenty of superhydrophobic surfaces have been developed owing to their tremendous potential applications, it is still a great challenge for the superhydrophobic surfaces to possess environmental friendliness, biocompatibility, and mechanical durability simultaneously. Herein, a non-fluorinated flexible superhydrophobic surface was designed by constructing a film-substrate system with labyrinth-like wrinkles combining an intrinsically hydrophobic Zn film and a polydimethylsiloxane (PDMS) substrate. Excellent superhydrophobicity with a contact angle up to 168.5° and a slide angle as low as 0° has been achieved on the Zn/PDMS surface, which is attributed to the micro-/nano-textured structures of the labyrinth-like wrinkles, providing sufficient air pockets to form a stable Cassie-Baxter state. Furthermore, the Zn/PDMS surface retains excellent superhydrophobicity under stretching, bending, and twisting mechanical deformation up to 500 cycles due to the stability of the micro-/nano-textured structures of the labyrinth-like wrinkles protected by the fantastic self-healing ability of the micro-cracks. Additionally, the Zn/PDMS superhydrophobic surface possesses an outstanding self-cleaning performance for various contaminants. The present work provides a valuable routine to design non-fluorinated flexible superhydrophobic surfaces with superb mechanical durability and self-cleaning property as promising functional layers for flexible electronics, wearable devices, biomedical engineering, and so forth.

Preparation of ECTFE Porous Membrane for Dehumidification of Gaseous Streams through Membrane Condenser

Due to the good hydrophobicity and chemical resistance of poly(ethylene trifluoroethylene) (ECTFE), it has been an attractive potential material for microfiltration, membrane distillation and more. However, few porous hydrophobic ECTFE membranes were prepared by thermally induced phase separation (TIPS) for membrane condenser applications. In this work, the diluent, di-n-octyl phthalate (DnOP), was selected to prepare the dope solutions. The calculated Hassen solubility parameter indicated that ECTFE has good compatibility with DnOP. The corresponding thermodynamic phase diagram was established, and it has been mutually verified with the bi-continuous structure observed in the SEM images. At 30 wt% ECTFE, the surface contact angle and liquid entry pressure reach their maximum values of 139.5° and 0.71 MPa, respectively. In addition, some other basic membrane properties, such as pore size, porosity, and mechanical properties, were determined. Finally, the prepared ECTFE membranes were tested using a homemade membrane condenser setup. When the polymer content is 30 wt%, the corresponding results are better; the water recovery and condensed water yield is 17.6% and 1.86 kg m⁻² h⁻¹, respectively.

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.