Improving the biofouling resistance of polyamide thin-film composite membrane via grafting polyacrylamide brush on the surface by in-situ atomic transfer radical polymerization

Abstract

Membrane biofouling represents a major challenge ever-present in the practical application of polyamide (PA) thin-film composite (TFC) reverse osmosis (RO) membranes. This work focused on grafting polyacrylamide (PAAm) onto PA-TFC via an in-situ atomic transfer radical polymerization (ATRP) method to improve the biofouling resistance of PA-TFC. The ATRP initiators, isobutyryl bromide (iPB), were introduced into PA matrix using a bifunctional small molecule, 2-bromoisobutyryl bromide (BIBB), added in organic hexane solutions to participate in interfacial polymerization through the condensation reaction with m-phenylenediamine. The successful introduction of iPB was confirmed by membrane surface characterizations, and the iPB density in PA layer was tuned by varying the BIBB concentrations in hexane solutions based on the variation in water permeance and salt rejections of iPB-loaded PA-TFC. The ATRP for the gradual growth of PAAm brush was thus in-situ achieved on the PA-TFC surface from the pre-incorporated iPB groups and the length of PAAm chains was facilely controlled via tuning ATRP polymerization time. The PAAm grafted PA-TFC exhibited a minimum decrement of water permeance and essentially retained salt rejections relative to the un-grafted ones. More importantly, the grafted membranes presented synergistically enhanced anti-adhesion and bacteriostatic performance towards either Gram-negative Escherichia coli (E. coli) or Gram-positive Bacillus subtilis (B. subtilis). Also, the grafted membranes presented a lower final flux decline ratio (FDRf) and higher final flux recovery ratio (FRRf) in relation with the un-grafted membranes in the dynamic bovine serum albumin (BSA) fouling test. Particularly, the membrane grafted with longer PAAm chains from extended grafting time showed better anti-biofouling performance: the sterilization ratio towards E. coli and B. subtilis achieved up to 98.8% and 99.0%, respectively, and FDRf of just 3.7% and FRRf of 97.6% after 3 × 8 h BSA fouling test for the membrane subjected to 2 h ATRP grafting. The PAAm grafting induced greater water wettability of membrane surface and the flexible chain structure of PAAm were proposed to account for the enhanced anti-adhesion and bacteriostasis towards bacteria, which collaboratively led to the improved biofouling resistance.