Surface modification of FO membrane for improving ammoniacal nitrogen (NH4+-N) rejection: Investigating the factors influencing NH4+-N rejection

Forward osmosis (FO) has attracted attention for wastewater treatment owing to its energy-saving characteristics; however, the low rejection of ammoniacal nitrogen (NH4+-N) for normal polyamide thin-film composite (TFC) FO membranes limits their practical applications for NH4+-N recovery or concentration from wastewater. Herein, ethylenediamine (EDA)- and 2-aminoethanol (AEA)-grafted TFC FO membranes were prepared to improve the NH4+-N rejection. EDA and AEA have similar molecular structures; however, their end groups differ. EDA has two primary amino groups, whereas AEA has one amino group and one hydroxy group. Both EDA- and AEA-grafted FO membranes significantly improved the NH4+-N rejection, and a further increase in NH4+-N rejection was noted (up to 97.5%) with increasing grafting times. These membranes showed high rejection, even at a feed solution with higher pH of 7.6 (89.7% and 83.0% for EDA- and AEA-grafted membranes, respectively). The carboxylic group density of the pristine and grafted membranes was measured to investigate the factors influencing the improvement in NH4+-N rejection. A significant correlation was found between the carboxylic group density and NH4+-N rejection, while the correlation between the surface zeta potential and NH4+-N rejection was unclear. This means not only positive charge chemical grafting can improve the NH4+-N rejection, but also the neutral chemical grafting can improve the NH4+-N rejection. It also indicates that reducing membrane surface carboxylic group density could alleviate cation exchange during FO process. This study clarified the factors influencing NH4+-N rejection and provided a novel idea for designing a FO membrane with high NH4+-N selectivity suitable for ammoniacal nitrogen concentration from wastewater. [Display omitted] •High NH4+-N rejection TFC FO membrane was developed by grating EDA or AEA.•Membrane surface -COO- density was controlled via multiple times catalytic reaction.•A significant correlation between NH4+-N rejection and surface carboxylic group density was revealed.•Surface carboxylic group density is the decisive factor for NH4+-N rejection.