Enhancing the performance of nanofiltration membranes by modifying the active layer with aramide dendrimers

The fully aromatic polyamide active layer of a commercial nanofiltration membrane was modified with three generations (G1, G2, and G3) of aramide dendrimers, all with oligoethylene glycol chains on their peripheries. Permeation experiments revealed that the rejection of Rhodamine WT, used as a surrogate for organic contaminants, improved 1-2 orders of magnitude for membranes modified with G2 and G3 dendrimers at loadings of 0.7-3.5 μg/cm² (dendrimer layer thicknesses of ∼1-6 nm) compared to the performance of unmodified membranes. In contrast, the corresponding water permeability of dendrimer-modified membranes decreased by only ∼30%. Although an enhancement in the rejection of H₃AsO₃, NaCl, and BaCl₂ was also observed for dendritic membranes, the effect was less pronounced than that for rhodamine WT. Characterization of membranes modified with 3.5 μg/cm² dendrimers G2 and G3 by Rutherford backscattering spectrometry with the aid of heavy ion probes (Ag⁺ and Ba²⁺) revealed that accessibility of the larger Ba²⁺ probe to carboxylate groups on the active layer decreased for the membranes modified with dendrimers.