Planting Anion Channels in a Negatively Charged Polyamide Layer for Highly Selective Nanofiltration Separation

Abstract

A nanofiltration (NF) membrane with high salt permeation and high retention of small organics is appealing for the treatment of high-salinity organic wastewater. However, the conventional negatively charged NF membranes commonly show high retention of divalent anions (e.g., SO 4 2−), and the reported positively charged NF membranes normally suffer super low selectivity for small organics/Na 2 SO 4 and high fouling potential. In this work, we propose a novel "etching−swelling−planting" strategy assisted by interfacial polymerization and mussel-inspired catecholamine chemistry to prepare a mixcharged NF membrane. By X-ray photoelectron spectroscopy depth profiling and pore size distribution analysis, it was found that such a strategy could not only deepen the positive charge distribution but also narrow the pore size. Molecular dynamics confirm that the planted polyethyleneimine chains play an important role to relay SO 4 2− ions to facilitate their transport across the membrane, thus reversing the retention of Na 2 SO 4 and glucose (43 vs 71%). Meanwhile, due to the high surface hydrophilicity and smoothness as well as the preservation of abundant negatively charged groups (−OH and −COOH) inside the separation layer, the obtained membrane exhibited excellent antifouling performance, even for the coking wastewater. This study advances the importance of vertical charge distribution of NF membranes in separation selectivity and antifouling performance.

Nanofiltration Membranes with Salt-Responsive Ion Valves for Enhanced Separation Performance in Brackish Water Treatment: A Battle against the Limitation of Salt Concentration

“…As shown in Figure g, both the PA and PA-ZNs membranes exhibited the CO stretching vibration of amide I observed at 1625 cm –1 . The peaks located at 1440 and 3440 cm –1 were attributed to the stretching vibration of O–H, indicating the presence of –COOH groups . These results verified the formation of the polyamide layer of PA and PA-ZNs membranes.…”

“…24 The peaks located at 1440 and 3440 cm −1 were attributed to the stretching vibration of O−H, indicating the presence of −COOH groups. 13 These results verified the formation of the polyamide layer of PA and PA-ZNs membranes. In addition, the zwitterionic nanospheres displayed characteristic peaks at 964 and 1040 cm −1 corresponding to −SO 3 − and −N + (CH 3 ) 3 , respectively.…”

“…NF membranes with high pore size homogeneity can be achieved by controlling the diffusion of reaction monomer and incorporating nanoparticles . Surface coating or grafting, incorporation of nanomaterials, and adoption of new monomers have been reported to regulate membrane electrical characteristics, including membrane charge and dielectric constant. – However, these membrane construction strategies are based on the artificial control of membrane properties, which may not effectively cope with complex solution environments because the properties of fabricated membranes are fixed or can fluctuate only within a small range. A paradigm shift in which membrane properties are regulated by environmental factors is necessary.…”

The role of ion‐membrane interactions in fast and selective mono/multivalent ion separation with hierarchical nanochannels

“…The ATR‐FTIR spectra confirmed decreases in the characteristic peaks of the C=O (1715 cm −1 ), –CH 2 (1455 and 1375 cm −1 ), and –SO 3 (1150 and 1030 cm −1 ) groups from the PAM substrates, 52 while the new peak for C–N (1496 cm −1 ) was a result of out‐of‐plane bending vibrations from the amplified amide bands (Figure S4). This suggested the successful fabrication of the (PEI/PSS)‐GA polyelectrolyte‐based membrane.…”

The Adverse Effect of Concentration Polarization on Ion–Ion Selectivity in Nanofiltration

“…Due to their more open pore structure compared to reverse osmosis (RO) membranes, nanofiltration (NF) membranes have been used in various applications that require selective separation between solutes, such as water softening, groundwater purification, and concentration of products in certain industries. − More recently, with a growing demand for more challenging separations that require precise selectivity (e.g., lithium recovery from salt-lake brines), NF membranes have become a leading candidate to pursue such selectivity. − Relying on steric hindrance, Donnan (charge) exclusion, and other molecular mechanisms, − NF membranes of different materials and fabrication approaches were proposed to create selectivity toward different solutes. ,, Although success has been abundantly reported, − the selectivity of most NF membranes is relatively limited, unspecific, and, for comparison, a few orders of magnitude lower than the selectivity of ion-selective biological channels. − …”