Nanofillers dissolution as a crucial challenge for the performance stability of thin-film nanocomposite desalination membranes

Abstract

With the aim to improve the intrinsic separation performance, integration of nanofillers toward polyamide thin-film nanocomposite (TFN) membranes is a very active research area. Mesoporous silica nanoparticles (MSN), among other types of nanomaterials, are often used for that purpose. However, one of the distinguishing features of MSN is their dissolution in basic aqueous solutions. To date, no investigation in the literature has considered to study the influence of MSN nanofillers dissolution on the separation performance of the respective TFN polyamide membranes. This paper investigates how hydrophobic functionalization of MSN can improve the stability of MSN in aqueous solutions and enhance the stability of the respective TFN membranes over a prolonged filtration time at different pH values. The results showed that TFN membranes containing the octadecyltrichlorosilane-functionalized MSN had only ≈ 6% decline in salt rejection compared to ≈ 35% decline for the membrane containing unfunctionalized nanofillers accompanied by increasing water permeability after 240 h filtration time (120 h at pH 5, then 120 h at pH 9). Furthermore, the durability of the barrier layer after a prolonged use for desalination was also investigated via filtration of solutions of dextran with different molecular weight and assessing the dextran rejection for the native and used membranes. From the results of all parallel analyses of nanofiller and TFN membrane structure it is concluded that OTS functionalization of MSN cannot only enhance the TFN membrane separation performance, but also reduce the dissolution tendency of MSN. This was attributed to the formation of a protective organic layer leading to significantly enhanced long-time stability of the respective TFN membranes.