Aggregation-induced overestimation of membrane selectivity in dye/salt separation: Mechanistic insights and rational design

Selective separation of dyes and inorganic salts is central to dyeing effluent treatment and resource recovery. However, the rejection mechanisms of low-molecular-weight dyes and the reliability of membrane performance evaluation under realistic conditions remain insufficiently understood. Here, the separation behaviors of representative dyes, Methyl Orange (MO) and Congo Red, were systematically investigated using a dense nanofiltration (NF) membrane and a tight ultrafiltration (UF) membrane, combining experiments and molecular simulations. Results reveal that dye aggregation critically governs apparent membrane selectivity. At high concentrations, dyes form J- or H-type aggregates through π–π stacking and electrostatic interactions, increasing their hydrodynamic size and markedly enhancing the rejection of otherwise permeable small dyes by the UF membrane. Disruption of aggregation by ethanol addition, elevated ionic strength, or reduced pH diminishes size-sieving and electrostatic effects, leading to substantial rejection decline, particularly for the UF membrane. In addition, electrostatic adsorption of cationic dyes and pore blockage induced by anionic dyes further affect flux stability. When treating real dyeing effluent, the UF membrane exhibits significantly reduced rejection of low-concentration, weakly aggregated dyes, whereas the NF membrane maintains dye rejection but suffers from excessive salt rejection. Guided by these insights, a dense NF membrane with a heterogeneous charge architecture was fabricated via interfacial polymerization. The membrane, with a molecular weight cut-off of 235 Da, integrates a weakly negative surface and internally distributed positive microdomains, enabling high MO rejection with relatively low Na 2 SO 4 rejection. This work demonstrates that dye aggregation can induce overestimation of membrane selectivity and highlights the necessity of evaluating intrinsic rejection capability toward small-molecule dyes under realistic conditions.