Insights into simultaneous ammonia-selective and anti-fouling mechanism over forward osmosis membrane for resource recovery from domestic wastewater

The inferior selectivity towards ammonia nitrogen (NH₄⁺-N) and severe fouling of the forward osmosis (FO) membrane during the pre-concentration of domestic wastewater (DW) greatly hinder the practical realization of FO-based processes in DW reduction and resource utilization. Herein, surface amine functionalization strategy was, for the first time, adopted to explore the feasibility of fabricating FO membrane with simultaneous ammonia-selective and anti-fouling capacities. Polyethylenimine (PEI), owing to its abundant positively charged primary (-NH₃⁺) and tertiary (-NR₃⁺) amines, was introduced as a protective barrier to impose synergistic interactions of diffusion resistance and electrostatic repulsion to the external NH₄⁺. Benefiting from this, the PEI-grafted membrane exhibited significantly enhanced NH₄⁺-N selectivity with rejection rate of 95.88% over that of 70.36% for the pristine membrane. Besides, the incremental surface hydrophilicity and smoothness as well as the nearly neutral surface potential endowed the PEI-grafted membrane with robust anti-fouling capacity in the concentration of raw DW. Meanwhile, deriving from the reinforced NH₄⁺-N selectivity and the alleviated membrane fouling, the grafted membrane with moderate PEI load possessed a superior NH₄⁺-N removal efficiency of 86.11%. The maximum concentration experiment of DW indicated that the simultaneous ammonia-selective and anti-fouling capacities of the PEI-grafted membrane greatly leaned upon the inhibitions of both the PEI layer incapacitation and the NH₄⁺-N cake-enhanced polarization. The presented strategy on surface amine functionalization enlightened the fabrication of ammonia-selective and anti-fouling FO membrane and significantly facilitated the development of FO-based technologies in resource recovery from DW.