Making waves: Magneto-responsive membranes with special and switchable wettability: new opportunities for membrane distillation.

Membrane distillation (MD) has promising potential in the water purification and wastewater treatment industries; however, fouling and wetting are the main obstacles to its commercialization, and higher fluxes and energy efficiencies are essential. Magneto-responsive membranes (MagMem) with integrated magnetic nanoparticles (MNPs) enable in situ fouling mitigation and switchable separation by nano-mixing or nano-heating, triggered by external magnetic fields, in a range of membrane processes, but not yet been demonstrated in MD. This perspective discussed the potential paths of MagMem utilization in MD based on the research status and dilemmas of MD. It can be envisioned that MagMem will lead to a paradigm shift in MD, especially by in situ fouling/wetting mitigation and enhancing energy efficiency via in-place actuation and localized heating by MNPs. Moreover, remotely controllable pore tuning and specific or switchable wettability can also be anticipated. Overall, MagMem provides attractive opportunities for advanced robust and efficient MD.

Distinct impacts of natural organic matter and colloidal particles on gypsum crystallization.

Gypsum scaling via crystallization is a major obstacle limiting the applications of membrane-based technologies and heat exchangers in engineered systems. Herein, we perform the first comparative investigation on the impacts of natural organic matter (Suwannee River humic acid, SRHA) and colloidal particles on the gypsum crystallization process in terms of induction time and crystal morphology. Results show that the presence of SRHA significantly increases the induction time of gypsum crystallization. Specifically, at a solution saturation index of 4.92, the induction time increases 6.5-fold in the presence of 6 mg/L SRHA, compared to the case without SRHA. SRHA also alters the morphology of the formed calcium sulfate crystals, resulting in a polygon-like shape, differing from the characteristic needle-like shape of gypsum in the absence of additives. These changes in crystal morphology are attributed to the adsorption of SRHA on the gypsum crystal surface, blocking the active sites for gypsum growth. In contrast, in the presence of colloidal particles, the observed induction time of gypsum crystallization either decreases or increases, depending on the competitive interplay between the enhancement effect in the nucleation step and the inhibition effect in the subsequent crystal growth step. Furthermore, the formed gypsum crystals in the presence of colloidal particles exhibit a needle-like morphology similar to the crystals formed in the absence of any additives. Our study provides fundamental understanding of gypsum crystallization in feedwaters containing natural organic matter and colloidal particles, highlighting the importance of feedwater composition in gypsum scaling.

One-pot synthesis of magnetic Prussian blue for the highly selective removal of thallium(I) from wastewater: Mechanism and implications.

Thallium (Tl) often enters the environment via mineral exploitation and utilization. The main restriction of Tl removal is the interference of high concentrations of coexisting ions in wastewater, therefore, enhancing the selectivity for Tl is essential to its treatment. Magnetic Prussian blue particles (Fe3O4@PB), an ion-sieving material with an open structure, were synthesized through a "one-pot" method at room temperature for the highly selective removal of Tl+. The removal percentage of Tl+ was over 92% even when the concentration of coexisting ions (e.g. Zn2+, Cd2+, Cu2+, and Pb2+) were 10,000 times higher than the initial concentration of Tl+. The maximal experimental removal capacity was 528 mg Tl/g Fe3O4@PB, and the removal percentage remained steady at pH 3-10. The high selectivity of Fe3O4@PB for Tl+ is attributed to the fact that hydrated Tl+ has a smaller hydrated diameter and a lower hydration free energy than other coexisting ions, while the rapid adsorption kinetics of Tl+ results from the negative surface charge and the network of nanocapillaries of the Fe3O4@PB. Overall, a new low-cost material that is easy to synthesize and has superior Tl+ removal capacity with extremely high selectivity for Tl+ was obtained for effective magnetic removal of thallium from wastewater.

Negative Pressure Membrane Distillation for Excellent Gypsum Scaling Resistance and Flux Enhancement.

Membrane distillation (MD) has potential to become a competitive technology for managing hypersaline brine but not until the critical challenge of mineral scaling is addressed. The state-of-the-art approach for mitigating mineral scaling in MD involves the use of superhydrophobic membranes that are difficult to fabricate and are commercially unavailable. This study explores a novel operational strategy, namely, negative pressure direct contact membrane distillation (NP-DCMD) that can minimize mineral scaling with commercially available hydrophobic membranes and at the same time enhance the water vapor flux substantially. By applying a negative gauge pressure on the feed stream, NP-DCMD achieved prolonged resistance to CaSO4 scaling and a dramatic vapor flux enhancement up to 62%. The exceptional scaling resistance is attributable to the formation of a concave liquid-gas under a negative pressure that changes the position of the water-air interface to hinder interfacial nucleation and crystal growth. The substantial flux enhancement is caused by the reduced molecular diffusion resistance within the pores and the enhanced heat transfer kinetics across the boundary layer in NP-DCMD. Achieving substantial performance improvement in both the scaling resistance and vapor flux with commercial membranes, NP-DCMD is a significant innovation with vast potential for practical adoption due to its simplicity and effectiveness.

Module-scale analysis of low-salt-rejection reverse osmosis: Design guidelines and system performance.

Low-salt-rejection reverse osmosis (LSRRO) is a novel reverse osmosis (RO)-based technology that can highly concentrate brines using moderate operating pressures. In this study, we investigate the performance of LSRRO membrane modules and systems using module-scale analysis. Specifically, we correlate the observed salt rejection of an LSRRO module with the water and salt permeabilities of the RO membrane. We then elaborate the impact of membrane properties and operating conditions on the performance of a 2-stage LSRRO, providing design guidelines for LSRRO systems. We further compare the performance of 2-stage and 3-stage LSRRO systems, showing that an LSRRO system with more stages is not always favored due to a larger energy consumption. The performance of a 3-stage LSRRO in treating different feed solutions for minimal/zero liquid discharge (MLD/ZLD) applications is then evaluated. Based on our results, when treating feed waters with a relatively low salinity (e.g., 0.1 M or ∼5,800 mg L-1 NaCl), the 3-stage LSRRO can achieve a concentrated brine that can be directly sent to the thermal brine crystallizers (i.e., brine concentration > 4 M or ∼240,000 mg L-1 NaCl), and the corresponding specific energy consumption (SEC) is only ∼3 kWh m-3. When treating feed waters with a relatively high salinity (e.g., 0.6 M or ∼35,000 mg L-1 NaCl), the brine from the 3-stage LSRRO can be ∼80 % more concentrated compared to that from conventional RO, while the corresponding SEC does not exceed 6 kWh m-3. Our results demonstrate that LSRRO can substantially advance minimal/zero liquid discharge (MLD/ZLD) applications because it can significantly minimize the use of thermal brine concentrators. We conclude with a discussion on the practicability of LSRRO and highlight future research needs.

Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance.

Although membrane distillation (MD) has been identified as a promising technology to treat hypersaline wastewaters, its practical applications face two prominent challenges: membrane wetting and fouling. Herein, we report a facile and scalable approach for fabricating a Janus MD membrane comprising a dense polyvinyl alcohol (PVA) surface layer and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate. By testing the Janus membrane in direct contact MD experiments using feeds containing a sodium dodecyl sulfate (SDS) surfactant or/and mineral oil, we demonstrated that the dense Janus membrane can simultaneously resist wetting and fouling. This method represents the simplest approach to date for fabricating MD membranes with simultaneous wetting and fouling resistance. Importantly, we also unveil the mechanism of wetting resistance by measuring the breakthrough pressure and surfactant permeation (through the PVA layer) and found that wetting resistance imparted by a dense hydrophilic layer is attributable to capillary pressure. This new insight will potentially change the paradigm of fabricating wetting-resistant membranes and enable robust applications of MD and other membrane contactor processes facing challenges of pore wetting or/and membrane fouling.

Comparison of Energy Consumption of Osmotically Assisted Reverse Osmosis and Low-Salt-Rejection Reverse Osmosis for Brine Management.

Minimum and zero liquid discharge (MLD/ZLD) are emerging brine management strategies that attract heightened attention. Although conventional reverse osmosis (RO) can improve the energy efficiency of MLD/ZLD processes, its application is limited by the maximum hydraulic pressure (ΔPmax) that can be applied in current membrane modules. To overcome such limitation, novel RO-based technologies, including osmotically assisted RO (OARO) and low-salt-rejection RO (LSRRO), have been proposed. Herein, we utilize process modeling to systematically compare the energy consumption of OARO and LSRRO for MLD/ZLD applications. Our modeling results show that the specific energy consumption (SEC) of LSRRO is lower (by up to ∼30%) than that of OARO for concentrating moderately saline feed waters (<∼35,000 mg/L TDS) to meet MLD/ZLD goals, whereas the SEC of OARO is lower (by up to ∼40%) than that of LSSRO for concentrating higher salinity feed waters (>∼70,000 mg/L TDS). However, by implementing more stages and/or an elevated ΔPmax, LSRRO has the potential to outperform OARO energetically for treating high-salinity feed waters. Notably, the SEC of both OARO and LSRRO could be 50% lower than that of mechanical vapor compressor, the commonly used brine concentrator in MLD/ZLD applications. We conclude with a discussion on the practicability of OARO and LSRRO based on membrane module availability and capital cost, suggesting that LSRRO could potentially be more feasible than OARO.

Nanopore-Based Power Generation from Salinity Gradient: Why It Is Not Viable.

In recent years, the development of nanopore-based membranes has revitalized the prospect of harvesting salinity gradient (blue) energy. In this study, we systematically analyze the energetic performance of nanopore-based power generation (NPG) at various process scales, beginning with a single nanopore, followed by a multipore membrane coupon, and ending with a full-scale system. We confirm the high power densities attainable by a single nanopore and demonstrate that, at the coupon scale and above, concentration polarization severely hinders the power density of NPG, revealing the common, yet significant, error in linearly extrapolating single-pore performance to multipore membranes. Through our consideration of concentration polarization, we also importantly show that the development of materials with exceptional nanopore properties provides limited enhancement of practical process performance. For a full-scale NPG membrane module, we find an inherent tradeoff between power density and thermodynamic energy efficiency, whereby achieving a high power density sacrifices the energy efficiency. Furthermore, we derive a simple expression for the theoretical maximum energy efficiency of NPG, showing it is solely related to the membrane selectivity (i.e., S2/2). Through this relation, it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e., S = 1), reinforcing our optimistic full-scale simulations which result in a (practical) maximum energy efficiency of 42%. Finally, we assess the net extractable energy of a full-scale NPG system which mixes river water and seawater by including the energy losses from pretreatment and pumping, revealing that the NPG process-both in its current state of development and in the case of highly optimistic performance with minimized external energy losses-is not viable for power generation.

Intrapore energy barriers govern ion transport and selectivity of desalination membranes.

State-of-the-art desalination membranes exhibit high water-salt selectivity, but their ability to discriminate between ions is limited. Elucidating the fundamental mechanisms underlying ion transport and selectivity in subnanometer pores is therefore imperative for the development of ion-selective membranes. Here, we compare the overall energy barrier for salt transport and energy barriers for individual ion transport, showing that cations and anions traverse the membrane pore in an independent manner. Supported by density functional theory simulations, we demonstrate that electrostatic interactions between permeating counterion and fixed charges on the membrane substantially hinder intrapore diffusion. Furthermore, using quartz crystal microbalance, we break down the contributions of partitioning at the pore mouth and intrapore diffusion to the overall energy barrier for salt transport. Overall, our results indicate that intrapore diffusion governs salt transport through subnanometer pores due to ion-pore wall interactions, providing the scientific base for the design of membranes with high ion-ion selectivity.

Minimal and zero liquid discharge with reverse osmosis using low-salt-rejection membranes.

Minimal and zero liquid discharge (MLD/ZLD) are wastewater management strategies that are attracting heightened attention worldwide. While conventional reverse osmosis (RO) has been proposed as a promising technology in desalination and MLD/ZLD processes, its application is limited by the maximum hydraulic pressures that current RO membranes and modules can withstand. In this study, we develop low-salt-rejection RO (LSRRO), a novel staged RO process, that employs low-salt-rejection membranes to desalinate or concentrate highly saline feed streams, requiring only moderate hydraulic pressures. Based on process modeling, we demonstrate that LSRRO can overcome the hydraulic pressure limitations of conventional RO, achieving hypersaline brine salinities (>4.0 M NaCl or 234 g L-1 NaCl) that are required for MLD/ZLD applications, without using excessively high hydraulic pressures (≤70 bar). In addition, we show that the energy efficiency of LSSRO is substantially higher than traditional thermally-driven phase-change-based technologies, such as mechanical vapor compressor (MVC). For example, to concentrate a saline feed stream from 0.1 to 1.0 M NaCl, the specific energy consumption (SEC) of 4-stage LSRRO ranges from 2.4 to 8.0 kWh of electrical energy per m3 of feedwater treated, around four times less than that of MVC, which requires 20-25 kWhe m-3. Furthermore, compared to osmotically mediated RO technologies that require bilateral countercurrent stages to treat hypersaline brines, LSRRO is eminently more practical as it can be readily implemented by using 'loose' RO or nanofiltration membranes in conventional RO. Our study highlights LSRRO's potential for energy efficient brine concentration using moderate hydraulic pressures, which would drastically improve the energetic and economic performance of MLD/ZLD processes.

Pathways and challenges for efficient solar-thermal desalination.

Solar-thermal desalination (STD) is a potentially low-cost, sustainable approach for providing high-quality fresh water in the absence of water and energy infrastructures. Despite recent efforts to advance STD by improving heat-absorbing materials and system designs, the best strategies for maximizing STD performance remain uncertain. To address this problem, we identify three major steps in distillation-based STD: (i) light-to-heat energy conversion, (ii) thermal vapor generation, and (iii) conversion of vapor to water via condensation. Using specific water productivity as a quantitative metric for energy efficiency, we show that efficient recovery of the latent heat of condensation is critical for STD performance enhancement, because solar vapor generation has already been pushed toward its performance limit. We also demonstrate that STD cannot compete with photovoltaic reverse osmosis desalination in energy efficiency. We conclude by emphasizing the importance of factors other than energy efficiency, including cost, ease of maintenance, and applicability to hypersaline waters.

Nanoparticle-templated nanofiltration membranes for ultrahigh performance desalination.

Nanofiltration (NF) membranes with ultrahigh permeance and high rejection are highly beneficial for efficient desalination and wastewater treatment. Improving water permeance while maintaining the high rejection of state-of-the-art thin film composite (TFC) NF membranes remains a great challenge. Herein, we report the fabrication of a TFC NF membrane with a crumpled polyamide (PA) layer via interfacial polymerization on a single-walled carbon nanotubes/polyether sulfone composite support loaded with nanoparticles as a sacrificial templating material, using metal-organic framework nanoparticles (ZIF-8) as an example. The nanoparticles, which can be removed by water dissolution after interfacial polymerization, facilitate the formation of a rough PA active layer with crumpled nanostructure. The NF membrane obtained thereby exhibits high permeance up to 53.5 l m-2h-1 bar-1 with a rejection above 95% for Na2SO4, yielding an overall desalination performance superior to state-of-the-art NF membranes reported so far. Our work provides a simple avenue to fabricate advanced PA NF membranes with outstanding performance.

Novel Janus Membrane for Membrane Distillation with Simultaneous Fouling and Wetting Resistance.

A novel Janus membrane integrating an omniphobic substrate and an in-air hydrophilic, underwater superoleophobic skin layer was developed to enable membrane distillation (MD) to desalinate hypersaline brine with both hydrophobic foulants and amphiphilic wetting agents. Engineered to overcome the limitations of existing MD membranes, the Janus membrane has been shown to exhibit novel wetting properties unobserved in any existing membrane, including hydrophobic membranes, omniphobic membranes, and hydrophobic membranes with a hydrophilic surface coating. Being simultaneously resistant to both membrane fouling and wetting, a Janus membrane can sustain stable MD performance even with challenging feed waters and can thus potentially transform MD to be a viable technology for desalinating hypersaline wastewater with complex compositions using low-grade-thermal energy.

Membrane fouling and wetting in membrane distillation and their mitigation by novel membranes with special wettability.

Membrane distillation (MD) has been identified as a promising technology to desalinate the hypersaline wastewaters from fracking and other industries. However, conventional hydrophobic MD membranes are highly susceptible to fouling and/or wetting by the hydrophobic and/or amphiphilic constituents in these wastewaters of complex compositions. This study systematically investigates the impact of the surface wetting properties on the membrane wetting and/or fouling behaviors in MD. Specifically, we compare the wetting and fouling resistance of three types of membranes of different wetting properties, including hydrophobic and omniphobic membranes as well as composite membranes with a hydrophobic substrate and a superhydrophilic top surface. We challenged the MD membranes with hypersaline feed solutions that contained a relatively high concentration of crude oil with and without added synthetic surfactants, Triton X-100. We found that the composite membranes with superhydrophilic top surface were robustly resistant to oil fouling in the absence of Triton X-100, but were subject to pore wetting in the presence of Triton X-100. On the other hand, the omniphobic membranes were easily fouled by oil-in-water emulsion without Triton X-100, but successfully sustained stable MD performance with Triton X-100 stabilized oil-in-water emulsion as the feed solution. In contrast, the conventional hydrophobic membranes failed readily regardless whether Triton X-100 was present, although via different mechanisms. These findings are corroborated by contact angle measures as well as oil-probe force spectroscopy. This study provides a holistic picture regarding how a hydrophobic membrane fails in MD and how we can leverage membranes with special wettability to prevent membrane failure in MD operations.

Composite Membrane with Underwater-Oleophobic Surface for Anti-Oil-Fouling Membrane Distillation.

In this study, we fabricated a composite membrane for membrane distillation (MD) by modifying a commercial hydrophobic polyvinylidene fluoride (PVDF) membrane with a nanocomposite coating comprising silica nanoparticles, chitosan hydrogel and fluoro-polymer. The composite membrane exhibits asymmetric wettability, with the modified surface being in-air hydrophilic and underwater oleophobic, and the unmodified surface remaining hydrophobic. By comparing the performance of the composite membrane and the pristine PVDF membrane in direct contact MD experiments using a saline emulsion with 1000 ppm crude oil (in water), we showed that the fabricated composite membrane was significantly more resistant to oil fouling compared to the pristine hydrophobic PVDF membrane. Force spectroscopy was conducted for the interaction between an oil droplet and the membrane surface using a force tensiometer. The difference between the composite membrane and the pristine PVDF membrane in their interaction with an oil droplet served to explain the difference in the fouling propensities between these two membranes observed in MD experiments. The results from this study suggest that underwater oleophobic coating can effectively mitigate oil fouling in MD operations, and that the fabricated composite membrane with asymmetric wettability can enable MD to desalinate hypersaline wastewater with high concentrations of hydrophobic contaminants.

Environmental Applications of Interfacial Materials with Special Wettability.

Interfacial materials with special wettability have become a burgeoning research area in materials science in the past decade. The unique surface properties of materials and interfaces generated by biomimetic approaches can be leveraged to develop effective solutions to challenging environmental problems. This critical review presents the concept, mechanisms, and fabrication techniques of interfacial materials with special wettability, and assesses the environmental applications of these materials for oil-water separation, membrane-based water purification and desalination, biofouling control, high performance vapor condensation, and atmospheric water collection. We also highlight the most promising properties of interfacial materials with special wettability that enable innovative environmental applications and discuss the practical challenges for large-scale implementation of these novel materials.

Synthesis of Ketones through Microwave Irradiation Promoted Metal-Free Alkylation of Aldehydes by Activation of C(sp(3))-H Bond.

In this paper, a novel methodology for the synthesis of ketones via microwave irradiation promoted direct alkylation of aldehydes by activation of the inert C(sp(3))-H bond has been developed. Notably, the reactions were accomplished under metal-free conditions and used commercially available aldehydes and cycloalkanes as substrates without prefunctionalization. By using this novel method, an alternative synthetic approach toward the key intermediates for the preparation of the pharmaceutically valuable oxaspiroketone derivatives was successfully established.