Development and performance characterization of a polyamide nanofiltration membrane modified with covalently bonded aramide dendrimers

A first generation of amine terminated aramide dendrimers (G1-NH ₂) was covalently attached to the polyamide (PA) active layer of a commercially available nanofiltration (NF) membrane. Amide bonds between G1-NH₂ and PA free carboxylic groups were formed by activation of the carboxylic groups with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) or 2-chloro-1-methylpyridinium iodide (CMPI), followed by aminolysis. Dendrimer attachment was assessed by indirectly measuring the concentration of carboxylic groups and amine groups before and after membrane modification with RBS using yttrium and tungstate ions (Y³⁺ and WO₄^2-) as ion probes. RBS analyses showed a decrease in the concentration of carboxylic groups and an increase in amine groups on the membrane active layer, consistent with dendrimers attaching covalently to the active layer. Permeation experiments with Rhodamine WT (R-WT) revealed that the water and solutes permeability decreased after modification with dendrimer G1-NH₂. Water permeability of G1-NH₂ modified membrane decreased by 16-19% using EDC combined with sulfo-N-hydroxysuccinimide (s-NHS), and by 17-33% using CMPI. The permeability of the electrolyte BaCl₂ decreased by 54% after G1-NH₂ modification using EDC/s-NHS and only by 20% using CMPI, the latter consistent with a weaker Donnan exclusion effect. The permeability of the larger solute R-WT decreased by 82% in modified G1-NH₂ membranes when using EDC/s-NHS, and 64% for cross-linking reagent CMPI. Thus, the use of EDC/s-NHS was more favorable because it resulted in higher gains in solute rejection with lower losses in water permeability.