Impact of end groups of small molecules grafted to reverse osmosis membranes on their separation and antifouling performance

In this study, surface hydroxylation and methoxylation were employed to fabricate polyamide (PA) reverse osmosis (RO) membranes with enhanced permeability and antifouling performance. Diethanolamine (DEA) and bis(2-methoxyethyl) amine (BMEA) were grafted onto PA of RO membranes, and the impact of their end groups of hydroxyls and methoxyls on PAs were investigated. Benefiting from the increased surface hydrophilicity (particularly for the DEA-grafted RO) and the tailored cross-linking structure of PA layers, both DEA- and BMEA-grafted membranes showed high water permeance (1.58-fold than the pristine RO membrane) with highly competitive NaCl rejection of 99 %. The DEA-grafted membrane exhibited the enhanced antifouling performance against both positively and negatively charged foulants, while the BMEA-grafted membrane exhibited exceptional performance against small cationic foulants. Quantum chemistry simulations reveal that hydroxyl groups grafted on PA possess a relatively low binding energy with both positively and negatively charged small foulants, whereas methoxyl groups grafted on PA exhibited an exceptionally low binding energy with positively charged small foulants, which rendered them less likely to be adsorbed onto the corresponding grafted PA and to form a loose cake layer. Our study provides new insights into design of small molecules with different end groups used for the simple and effective modification of antifouling RO membranes.