Membrane technologies using reverse
osmosis (RO) and nanofiltration
(NF) have been widely implemented in water purification and desalination
processes. Separation between species at the molecular level is achievable
in RO and NF membranes due to a complex and poorly understood combination
of transport mechanisms that have attracted the attention of researchers
within and beyond the membrane community for many years. Minimizing
existing knowledge gaps in transport through these membranes can improve
the sustainability of current water-treatment processes and expand
the use of RO and NF membranes to other applications that require
high selectivity between species. Since its establishment in 1949,
and with growing popularity in recent years, Eyring’s transition-state
theory (TST) for transmembrane permeation has been applied in numerous
studies to mechanistically explore molecular transport in membranes
including RO and NF. In this review, we critically assess TST applied
to transmembrane permeation in salt-rejecting membranes, focusing
on mechanistic insights into transport under confinement that can
be gained from this framework and the key limitations associated with
the method. We first demonstrate and discuss the limited ability of
the commonly used solution-diffusion model to mechanistically explain
transport and selectivity trends observed in RO and NF membranes.
Next, we review important milestones in the development of TST, introduce
its underlying principles and equations, and establish the connection
to transmembrane permeation with a focus on molecular-level enthalpic
and entropic barriers that govern water and solute transport under
confinement. We then critically review the application of TST to explore
transport in RO and NF membranes, analyzing trends in measured enthalpic
and entropic barriers and synthesizing new data to highlight important
phenomena associated with the temperature-dependent measurement of
the activation parameters. We also discuss major limitations of the
experimental application of TST and propose specific solutions to
minimize the uncertainties surrounding the current approach. We conclude
with identifying future research needs to enhance the implementation
and maximize the benefit of TST application to transmembrane permeation.
Membrane technologies that enable the efficient purification of impaired water sources are needed to address growing water scarcity. However, state-of-the-art engineered membranes are constrained by a universal, deleterious trade-off where membranes with high water permeability lack selectivity. Current membranes also poorly remove low–molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. We report a water desalination technology that uses applied pressure to drive vapor transport through membranes with an entrapped air layer. Since separation occurs due to a gas-liquid phase change, near-complete rejection of dissolved solutes including sodium chloride, boron, urea, and
N
-nitrosodimethylamine is observed. Membranes fabricated with sub-200-nm-thick air layers showed water permeabilities that exceed those of commercial membranes without sacrificing salt rejection. We also find the air-trapping membranes tolerate exposure to chlorine and ozone oxidants. The results advance our understanding of evaporation behavior and facilitate high-throughput ultraselective separations.
Desalination technologies using salt-rejecting membranes are a highly efficient tool to provide fresh water and augment existing water supplies. In recent years, numerous studies have worked to advance a variety of membrane processes with different membrane types and driving forces, but direct quantitative comparisons of these different technologies have led to confusing and contradictory conclusions in the literature. In this Review, we critically assess different membrane-based desalination technologies and provide a universal framework for comparing various driving forces and membrane types. To accomplish this, we first quantify the thermodynamic driving forces resulting from pressure, concentration, and temperature gradients. We then examine the resistances experienced by water molecules as they traverse liquid- and air-filled membranes. Last, we quantify water fluxes in each process for differing desalination scenarios. We conclude by synthesizing results from the literature and our quantitative analyses to compare desalination processes, identifying specific scenarios where each process has fundamental advantages.
Pressure-driven distillation is a separation process
in which hydraulic
pressure is used to drive water vapor transport across an air-trapping
porous hydrophobic membrane. Current development of pressure-driven
distillation is limited by a lack of robust, large-area membranes.
Here, we report desalination using pressure-driven vapor transport
through scalable polymeric polytetrafluorethylene membranes. The membranes
showed pressure-driven water flow with near-complete rejection of
sodium chloride (greater than 99%) under hydraulic pressures of up
to 10.3 bar. Membrane structure, surface chemistry, and desalination
performance were found to be unaffected by doses of sodium hypochlorite
up to 3000 ppm h. Flux decline due to biofouling from Pseudomonas
aeruginosa bacterium was effectively mitigated using chlorine.
Membranes also exhibited high temperature resilience with operation
up to 60 °C. Overall, this work demonstrates the use of large-area
polymeric materials in pressure-driven distillation and highlights
key advantages in chlorine and heat tolerance.
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