The growing concern over anthropogenic climate change and increasing water demands has exacerbated the issue of water scarcity worldwide. As a result, the need for innovative water treatment technologies that can effectively address this challenge has become more urgent.
Nontraditional water sources, including wastewater, seawater, and brackish water, are being considered as viable alternatives, but their safe utilization requires efficient water treatment systems capable of removing almost all dissolved constituents from contaminated water. In recent times, membrane technologies, particularly reverse osmosis (RO), have emerged as leading contenders for water reuse and desalination due to their numerous advantages, such as high energy efficiency, ease of operation, and compact design.
However, despite their widespread adoption, RO systems have faced long-standing performance limitations mainly related to the materials used in the membranes. Current polymeric salt-rejecting membranes face a trade-off where high permeability comes at the expense of decreased water-salt selectivity.
Additionally, these membranes often fail to remove harmful contaminants, as certain low-molecular-weight neutral species can pass through them. Examples of poorly rejected contaminants include boron, urea, N-nitrosodimethylamine (NDMA), and 1,4-dioxane. Moreover, the polymeric materials employed in RO membranes are vulnerable to chemical oxidation, which reduces their longevity and prevents their effective cleaning with common disinfectants like chlorine and ozone.
Given these limitations, there is a compelling need to explore alternative separation processes for advanced water treatment. Distillation-based technologies, which rely on gas-liquid phase changes, have been utilized for centuries and possess distinct advantages over RO.
The phase change inherent in distillation enables the removal of all low-volatility species from water, including those that are poorly rejected by RO. Furthermore, distillation systems can tolerate feedwaters containing harsh oxidants, solvents, and other chemicals.
Pressure-Driven Distillation: An Innovative Solution
The concept of pressure-driven distillation presents a novel approach to water purification, where applied pressure is harnessed to drive vapor flow through a unique air-trapping membrane. In this system, pressure induces evaporation on the feed side of the membrane, gas-phase diffusion through the pore, and condensation on the permeate side of the membrane.
The development of such a technology holds great promise, as it could potentially combine the energy efficiency and small footprint of RO with the complete removal of nonvolatile species and the ability to handle harsh feedwaters.
To realize this vision, researchers have focused on the synthesis of nanoporous membranes with sub-200-nm-thick air layers to investigate transport and desalination performance. The results of proof-of-concept experiments have been highly promising, demonstrating near-complete rejection (exceeding 99%) of nonvolatile solutes such as sodium chloride, boron, and urea. Membranes with a 27.1-nm pore diameter have exhibited successful operation at pressures up to 48.3 bar. Furthermore, by reducing the air layer thickness to 119.0 nm, the technology has shown normalized permeabilities of up to 8.9 kg m−2 hour−1 bar−1 without compromising salt rejection. Additionally, the air-trapping membranes have exhibited remarkable resistance to sustained exposure to high concentrations of chlorine and ozone.
Fig. 1. Design of ultrathin air-trapping membranes for pressure-driven vapor transport.
(A) Schematic diagram of pressure-driven water vapor transport through a nanoporous membrane with an ultrathin air gap. (B) Schematic of the fabrication process: A porous alumina membrane is modified with a hydrophilic masking layer, sputtered with controlled metal deposition into pores, selectively coated with a hydrophobic layer on the metal surface, and treated to remove the residual masking layer. (C) Water contact angle on the top and bottom surfaces of the membrane. SEM of the (D) top surface and (E) cross section of the upper surface of the membrane.
Potential Advantages and Applications
Pressure-driven distillation has several advantages that make it a viable alternative to traditional RO systems. By employing air-trapping membranes, this technology can achieve superior permeability, selectivity, and oxidation resistance compared to conventional salt-rejecting membranes that rely on polyamide or cellulose acetate chemistry.
As a result, the high selectivity of air-trapping membranes toward nonvolatile solutes may eliminate the need for costly secondary treatment processes, particularly for removing boron, which can harm agriculture at even relatively low concentrations. In the context of water reuse applications, this technology may significantly enhance the safety of product water by efficiently removing contaminants like NDMA, urea, and other micropollutants, potentially reducing the reliance on downstream processes.
Moreover, pressure-driven distillation offers exciting possibilities for ultrapure water production and water recycling in space applications, where efficiency and resource management are critical considerations. The versatility of this technology suggests that it can be adapted to various membrane materials and structures, providing opportunities for tailored and chemically robust water treatment systems.
The incorporation of hydrophobic alumina membranes, for instance, has demonstrated resistance to chemical oxidants like chlorine and ozone, which can reduce harmful disinfection byproducts and inhibit fouling.
Challenges and Future Prospects
While pressure-driven distillation shows great promise, several challenges remain to be addressed before widespread implementation can be achieved. For instance, efforts are needed to comprehensively identify and mitigate issues related to wetting, fouling, and scale-up. Air-trapping membranes are vulnerable to wetting from low-surface tension liquids and foulants, necessitating further investigation into fouling behavior. Additionally, fabricating large-scale hydrophobic porous membranes and optimizing their structure for high applied pressure without compromising permeability is a critical area of research.
In terms of theoretical understanding, there are important knowledge gaps related to interfacial and evaporation phenomena underlying the pressure-driven distillation process. Continued research is needed to establish a comprehensive theoretical framework for transport in air-filled nanopores, including the role of interfacial resistances, nanoconfinement, cluster evaporation, and other related behaviors.
The development of pressure-driven distillation technology has the potential to revolutionize advanced water treatment by overcoming the limitations of traditional RO systems. The innovative approach of using applied pressure to drive vapor flow through air-trapping membranes opens up exciting possibilities for efficient water purification with enhanced selectivity, permeability, and resistance to chemical oxidation.
As the research in this area continues to progress, the technology’s scalability and adaptability to various membrane materials and structures will likely pave the way for its successful implementation in diverse applications, ranging from large-scale desalination projects to ultrapure water production in space missions.
With continued efforts to address existing challenges and expand our understanding of the underlying processes, pressure-driven distillation holds the potential to significantly alleviate water scarcity and improve the availability of safe, clean water resources worldwide.
reference link : https://www.science.org/doi/10.1126/sciadv.adg6638