Influence of microporous membrane properties on the desalination performance in direct contact membrane distillation

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Preparation and characterization of polysulfone-polyurethane membranes for recovery of simulated wastewater from industrial textile processes

Techniques using membranes for the treatment of wastewaters usually promote higher quality of treated water when compared to other processes. Among them, pervaporation has advantages in terms of selectivity in addition to low working pressure, which can prevent clogging problems. Polysulfone and polyurethane have complementary characteristics and are interesting in the context of membranes for industrial applications. In this sense, the aim of this work was to prepare and characterize polysulfone/polyurethane-based membranes and tested them with a simulated wastewater containing the reactive black dye and sodium chloride by pervaporation. In their manufacture, thermal treatment (at 60°C) and photo-radiation treatment (using ultraviolet light) were also applied. The characterizations were performed using different analytical tools. In general, it was possible to verify that all membranes have a dense layer. The thermal analysis allowed to define that the indicated working temperature is below 50°C. With respect to the simulated wastewater treatment, all membranes reached 100% selectivity. Concerning the saline solution, the mean selectivity was around 98.5%. Moreover, the permeate flow values were within the range presented by commercial membranes ranging from 1.6 to 2.4 kg m^-2 h^-1. Although for the photoirradiated membranes the photo-graft reaction has occurred, among all membranes, the blend without any treatment stood out from the others, presenting the highest permeate flow of the simulated wastewater. Finally, the results reveal that these membranes are capable of recovering wastewater from textile processes, in addition to having the potential to remove salts from water through the pervaporation process.Highlights Polysulfone/polyurethane-based membranes were not yet evaluated for wastewater recovery.Modifications in the membrane characteristics promoted variations in the permeate flow.Changes in physical-chemical properties of membrane as a result of a photoinitiation reaction.Removal efficiency achieved was 100% for reactive black dye and 98.5% for sodium chloride.A new way of performing pervaporation on the recovery of aqueous solutions.

Efficient Estimation of Permeate Flux of Asymmetric Ceramic Membranes for Vacuum Membrane Distillation

Ceramic membranes have the advantages of high mechanical strength and thermal stability and are promising candidates for membrane distillation. Ceramic membranes are generally designed to have a multilayer structure with different pore sizes to create a high liquid entry pressure and obtain a high permeability. However, these structural characteristics pose significant difficulties in predicting permeate flux in a ceramic membrane contactor for vacuum membrane distillation (VMD). Here, a modeling approach was developed to simulate the VMD process and verified by comparing the simulated results with the experimental data. Furthermore, correlations are proposed to simplify the calculations of permeate flux for VMD using asymmetric ceramic membranes by assuming those multilayers to be an effectively quasi-symmetric layer and by introducing a correction factor. The simulation results indicated that this simplified correlation was effective and enabled a quick estimation of the effect of membrane parameters on permeate flux.

Porous Hydrophobic-Hydrophilic Composite Hollow Fiber and Flat Membranes Prepared by Plasma Polymerization for Direct Contact Membrane Distillation

High water vapor flux at low brine temperatures without surface fouling is needed in membrane distillation-based desalination. Brine crossflow over surface-modified hydrophobic hollow fiber membranes (HFMs) yielded fouling-free operation with supersaturated solutions of scaling salts and their precipitates. Surface modification involved an ultrathin porous polyfluorosiloxane or polysiloxane coating deposited on the outside of porous polypropylene (PP) HFMs by plasma polymerization. The outside of hydrophilic MicroPES HFMs of polyethersulfone was also coated by an ultrathin coating of porous plasma-polymerized polyfluorosiloxane or polysiloxane rendering the surface hydrophobic. Direct contact membrane distillation-based desalination performances of these HFMs were determined and compared with porous PP-based HFMs. Salt concentrations of 1, 10, and 20 wt% were used. Leak rates were determined at low pressures. Surface and cross-sections of two kinds of coated HFMs were investigated by scanning electron microscopy. The HFMs based on water-wetted MicroPES substrate offered a very thin gas gap in the hydrophobic surface coating yielding a high flux of 26.4-27.6 kg/m2-h with 1 wt% feed brine at 70 °C. The fluxes of HFMs on porous PP substrates having a long vapor diffusion path were significantly lower. Coated HFM performances have been compared with flat hydrophilic membranes of polyvinylidene fluoride having a similar plasma-polymerized hydrophobic polyfluorosiloxane coating.

Large-pore-size membranes tuned by chemically vapor deposited nanocoatings for rapid and controlled desalination

Though membranes with pore size larger than 1 μm are much desired to increase the permeate flux of membrane distillation (MD), the vulnerability of large-pore-size membranes to pore wetting results in the penetration of saline water and consequent failure of MD operation. We report modification of large-pore-size membranes by chemically vapor deposited nanocoatings to achieve both high salt rejection and high permeate flux. The chemical vapor modification not only led to enhanced surface hydrophobicity and increased liquid entry pressure in membranes, but also significantly improved membrane wetting resistance at high temperature. Membranes with 1.0 and 2.0 μm pore size were successfully used for MD desalination with salt rejection higher than 99.99% achieved. Enlarging the pore size from 0.2 μm to 2.0 μm contributed to 48-73% enhancement in the permeate flux of the modified membranes. The modified large-pore-size membranes maintained the high permeate flux at elevated saline concentration and extended the operation time.

Interplay of the Factors Affecting Water Flux and Salt Rejection in Membrane Distillation: A State-of-the-Art Critical Review

High water flux and elevated rejection of salts and contaminants are two primary goals for membrane distillation (MD). It is imperative to study the factors affecting water flux and solute transport in MD, the fundamental mechanisms, and practical applications to improve system performance. In this review, we analyzed in-depth the effects of membrane characteristics (e.g., membrane pore size and distribution, porosity, tortuosity, membrane thickness, hydrophobicity, and liquid entry pressure), feed solution composition (e.g., salts, non-volatile and volatile organics, surfactants such as non-ionic and ionic types, trace organic compounds, natural organic matter, and viscosity), and operating conditions (e.g., temperature, flow velocity, and membrane degradation during long-term operation). Intrinsic interactions between the feed solution and the membrane due to hydrophobic interaction and/or electro-interaction (electro-repulsion and adsorption on membrane surface) were also discussed. The interplay among the factors was developed to qualitatively predict water flux and salt rejection considering feed solution, membrane properties, and operating conditions. This review provides a structured understanding of the intrinsic mechanisms of the factors affecting mass transport, heat transfer, and salt rejection in MD and the intra-relationship between these factors from a systematic perspective.

Performance Comparison between Polyvinylidene Fluoride and Polytetrafluoroethylene Hollow Fiber Membranes for Direct Contact Membrane Distillation

Increasing water demand coupled with projected climate change puts the Southwestern United States at the highest risk of water sustainability by 2050. Membrane distillation offers a unique opportunity to utilize the substantial, but largely untapped geothermal brackish groundwater for desalination to lessen the stress. Two types of hydrophobic, microporous hollow fiber membranes (HFMs), including polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), were evaluated for their effectiveness in direct contact membrane distillation (DCMD). Water flux and salt rejection were measured as a function of module packing density and length in lab-scale systems. The PVDF HFMs generally exhibited higher water flux than the PTFE HFMs possibly due to thinner membrane wall and higher porosity. As the packing density or module length increased, water flux declined. The water production rate per module, however, increased due to the larger membrane surface area. A pilot-scale DCMD system was deployed to the 2nd largest geothermally-heated greenhouse in the United States for field testing over a duration of about 22 days. The results demonstrated the robustness of the DCMD system in the face of environmental fluctuation at the facility.

Membranes with Intrinsic Micro-Porosity: Structure, Solubility, and Applications

Microporous polymer membranes have been widely studied because of their excellent separation performance. Among them, polymers of intrinsic micro-porosity (PIMs) have been regarded as a potential next-generation membrane material for their ultra-permeable characteristics and their solution-processing ability. Therefore, many reviews have been reported on gas separation and monomers for the preparation of PIMs. This review aims to provide an overview of the structure-solubility property. Different structures such as non-network and network macromolecular structure made of different monomers have been reviewed. Then their solubility with different structures and different separation applications such as nanofiltration, pervaporation, and gas/vapor separation are summarized. Lastly, we also provide our perspectives on the challenges and future directions of the microporous polymer membrane for the structure-property relationship, anti-physical aging, and more.