Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications

Abstract

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance (Science2018360518521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

Demystifying the Role of Surfactant in Tailoring Polyamide Morphology for Enhanced Reverse Osmosis Performance: Mechanistic Insights and Environmental Implications

“…This trend is consistent with previous studies. ,, The increased permeance can be attributed to the enlarged nanovoids from TFC-0.2 to TFC-2 (see the TEM graphs in Figure ), which can be resulted from more generated nanobubbles by their higher MPD storage . Previous studies have shown that nanovoids are effective in improving the water transport pathways in TFC membranes due to their “gutter effect”. ,− On the other hand, the declined permeance for TFC-8 can be explained by its thicker polyamide film (see the TEM graphs in Figure ), which may offset the benefit of its larger nanovoids . The addition of TW generally improved the water permeance compared to TFC membranes (Figure a), thanks to the more prominent nanovoids of TFC–TW membranes (Figure ).…”

“…We further found that TW addition could reduce membrane fouling propensity-the TFC–TW membrane experienced lower flux decline and less accumulation of HA on its surface compared to the TFC membrane (Figure a,b). The reduced fouling tendency can be attributed to the more prominent nanovoids in TFC–TW, which could potentially regulate the water transport and flux distribution above its polyamide film. ,, Interestingly, TFC shows much sharper flux decline than TFC–TW at the initial stage of fouling test, which can be ascribed to more defects in its polyamide layer. Since defects can result in hot spots of high-localized flux, foulants would preferentially deposit at these regions at the initial stage to accelerate membrane fouling. ,, It is worthwhile to note that the current study only investigated fouling tendency over relatively short filtration duration (96 h).…”

“…The reduced fouling tendency can be attributed to the more prominent nanovoids in TFC−TW, which could potentially regulate the water transport and flux distribution above its polyamide film. 28,31,47 Interestingly, TFC shows much sharper flux decline than TFC−TW at the initial stage of fouling test, which can be ascribed to more defects in its polyamide layer. Since defects can result in hot spots of high-localized flux, foulants would preferentially deposit at these regions at the initial stage to accelerate membrane fouling.…”

“…This peculiar phenomenon prompts us to further investigate the exact mechanism(s) on how surfactants regulate membrane performance. Indeed, the membrane performance has been widely reported to be dependent on the nanovoids-containing roughness feature of its polyamide layer. ,,,− Specifically, more prominent nanovoids could typically lead to higher membrane permeance, thanks to the increased filtration area − and gutter effect. ,− Presumably, surfactants may benefit the formation of nanovoids during the IP reaction to promote membrane performance.…”

Polyamide Thin-Film Composite Janus Membranes Avoiding Direct Contact between Feed Liquid and Hydrophobic Pores for Excellent Wetting Resistance in Membrane Distillation

“…For the pristine PVDF membrane, the effective filtration area (i.e., the pore area on the membrane surface) is smaller than the total membrane surface area. For the TFC Janus membrane, however, the effective filtration area could be the total surface area of the polyamide layer and thus might be higher than the membrane surface area considering the crumpled morphologies of the polyamide layer. , This enhanced filtration area might explain the enhanced vapor flux. Other mechanisms, such as confined evaporation in the polyamide layer, might also contribute to the enhanced vapor flux.…”