Deciphering the Role of Amine Concentration on Polyamide Formation toward Enhanced RO Performance

Abstract

Polyamide surface morphology and its underneath nanosized voids have crucial influence on the separation performance of thin film composite (TFC) polyamide reverse osmosis membranes. Although there have been numerous studies reporting the impact of amine monomer concentration on polyamide formation and membrane performance, the observations and interpretations in the existing literature remain controversial. In this study, we performed interfacial polymerization (IP) of polyamide films over a wide range of m-phenylenediamine (MPD) concentration (0.05−8.0 w/w %). For the first time, we demonstrate that the water permeance of the resultant TFC membranes is governed by the competing effects of (1) promoted polyamide film growth for forming thicker polyamide films and (2) improved nanofoaming effect that results in more extensive nanovoids at higher MPD concentrations. To dissect these competing mechanisms, we further adopted a free-interface IP strategy to suppress the nanofoaming effect. The corresponding polyamide nanofilms had negligible nanovoids and monotonously increased film thickness, leading to decreased water permeance at high MPD concentrations. In contrast, the conventional TFC membranes exhibited optimal water permeance at the intermediate MPD concentration of 2.0 w/w %, which results from the trade-off between improved nanovoid formation (which promotes higher permeance) and increased film growth (which limits permeance). On the other hand, the better film growth at greater MPD concentration was generally beneficial for achieving better membrane rejection. The current study unveils the fundamental chemistry−morphology−performance relationship of TFC polyamide membranes and provides important implications on their synthesis and environmental applications.

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

“…The conceptual model (c) further highlights the benefits of (1) an altered transport path ( A real , the dark blue line) with the increased substrate porosity, (2) an increased surface area ( A rough , the superimposed red line), and (3) both together with the reduced thickness of the crumpled polyamide layer ( A rough and thin , the superimposed yellow line). Figure (a) was adapted from ref with permission. Copyright 2022 American Chemical Society.…”

“…The crumpled polyamide morphologies, equivalent to the effect of the increasing substrate porosity of a flat polyamide rejection layer, could significantly improve membrane water permeance thanks to the greatly shortened water transport pathways, which approach the ideal water permeance (light blue line). Alternatively, the effect of the nanovoids within the crumpled polyamide rejection layer could be interpreted through their self-gutter effect on shortening the transport path to approach the ideal water permeance. , It is also interesting to note that, as an added advantage, the altered water transport pathways tend to result in more uniform flux distribution, which is beneficial to reducing fouling tendency by decreasing the accumulation of foulants in the localized hot spot zone , (see the section “Crumpled Polyamide Film and Local Flux”).…”

“…Alternatively, the effect of the nanovoids within the crumpled polyamide rejection layer could be interpreted through their self-gutter effect on shortening the transport path to approach the ideal water permeance. 9,28 It is also interesting to note that, as an added advantage, the altered water transport pathways tend to result in more uniform flux distribution, which is beneficial to reducing fouling tendency by decreasing the accumulation of foulants in the localized hot spot zone 151,156 (see the section "Crumpled Polyamide Film and Local Flux"). We further benchmark the theoretical water permeance of the rough NF membrane with an ideal rejection layer (without the effect of substrate, superimposed in red color in Figure 4c), which could even successfully exceed the ideal water permeance of the smooth NF membrane because of the additional benefit of increased filtration area.…”

“…With numerous in-depth studies on the polyamide selective layer formed by the interfacial polymerization reaction, the morphology of the polyamide layer is found to have a significant influence on the permeance of TFC membranes. For example, in RO membranes formed by m -phenylenediamine (MPD) and TMC (Figure a), the “ridge-and-valley” morphology of the formed polyamide layer greatly improves the permeance of an RO membrane by increasing the effective filtration area − and creating voids in the polyamide layer, ,− whose self-guttering effect further improves membrane permeance. ,− Unlike RO membranes, commercially available NF membranes formed by PIP and TMC are relatively smooth (Figure a). Inspired by the highly efficient water transport in TFC RO membranes, , one may wonder if NF membranes with crumpled polyamide layers may effectively enhance water permeance over their smooth counterparts to overcome the permeance–selectivity trade-off.…”

Unveiling the Growth of Polyamide Nanofilms at Water/Organic Free Interfaces: Toward Enhanced Water/Salt Selectivity

“…The difficulty in resolving the growth mechanism and defect formation in conventional IP is at least partially caused by the ridge-and-valley surface roughness features of typical polyamide RO membranes. , To avoid such interferences, we adopt a novel free interface fabrication strategy to form smooth polyamide nanofilms by suppressing the nanofoaming-induced surface roughness. ,,,− The series of smooth polyamide films formed over predetermined time steps enable us to unravel the evolution of their physiochemical properties (e.g., thickness, density, crosslinking degree, and defects) as well as correlate these properties to membrane separation performance. These fundamental insights into the growth dynamics of polyamide rejection films and their structure–performance correlation provide important guidance to the future development of high-performance RO membranes.…”