Researchers led by Wan Yinhua, PhD, from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences say they have developed a novel antifouling nanofiltration membrane for various types of industrial liquid separation. The membrane applies new knowledge about the role of pore size distribution in filtration, according to the scientists, who published their study (“Enhancing the Antifouling Ability of a Polyamide Nanofiltration Membrane by Narrowing the Pore Size Distribution via One-Step Multiple Interfacial Polymerization”) in ACS Applied Materials & Interfaces.

Nanofiltration membranes have received a lot of attention in the field of water purification and biomanufacturing due to their ability to accurately separate targeted solutes from other components. However, the application of nanofiltration membranes in industry suffers from membrane fouling that causes a significant decline in separation performance.

For example, for the most prevalent polyamide thin-film composite membrane prepared by interfacial polymerization (IP), the intrinsic heterogeneous mass transfer of the IP process results in wide pore size distribution and causes uneven permeation flux distribution on the membrane during filtration, thus weakening the antifouling ability of the nanofiltration membrane.

Moreover, commonly used nanofiltration membranes have abundant hydrophobic sites (i.e., benzene rings) in their polyamide chains. These sites are prone to adsorb hydrophobic foulants.

Goal to enhance antifouling membrane performance

The researchers attempted to enhance the antifouling performance of a polyamide nanofiltration membrane by narrowing its pore size distribution via a one-step multiple IP process.

“Application of nanofiltration membranes in industries still has to contend with membrane fouling that causes a significant loss of separation performance. Herein, an innovative approach to design antifouling membranes with a narrowed pore size distribution by IP assisted by silane coupling agents is reported,” the investigators wrote.

“An aqueous solution of piperazine anhydrous (PIP) and γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560) is employed to perform IP with an organic solution of trimesoyl chloride and tetraethyl orthosilicate (TEOS) on a porous support. In accordance with the results of molecular dynamics and dissipative particle dynamics simulations, the reactive additive KH560 accelerates the diffusion rate of PIP to enrich at the reaction boundary.

Enhancing the antifouling ability of a polyamide nanofiltration membrane by narrowing pore size distribution via one-step multiple interfacial polymerization. [LIU Lulu]
“Moreover, the hydrolysis/condensation of KH560 and TEOS at the aqueous/organic interface forms an interpenetrating network with the polyamide network, which regulates the separation layer structure. The characterization results indicate that the polyamide–silica membrane has a denser, thicker, and uniform separation layer. The mean pore size of the polyamide–silica membrane and the traditional polyamide membrane is 0.62 and 0.74 nm, respectively, and these correspond to the geometric standard deviation (namely, pore size distribution) of 1.39 and 1.97, respectively.

“It is proved that the narrower pore size distribution endows the polyamide–silica membrane with stronger antifouling performance (flux decay ratio decreases from 18.4 to 3.8%). Such a membrane also has impressive long-term antifouling stability during cane molasses decolorization at a high temperature (50 °C),” the authors wrote.

The reactive additive KH560 accelerates the diffusion rate of PIP so it becomes enriched at the reaction boundary. Moreover, the hydrolysis/condensation of KH560 and TEOS at the aqueous/organic interface forms an interpenetrating network with the polyamide network, thus regulating the separation layer structure.

“This work not only provides a novel one-step multiple IP strategy to prepare antifouling nanofiltration membranes, but also emphasizes the importance of pore size distribution in fouling control for various industrial liquid separations,” said Luo Jianquan, PhD, of IPE, corresponding author of the study. “Such a nanofiltration membrane promises to improve the robustness of thin-film composite nanofiltration membranes in industrial liquid separation.”