Interplay between silicon quantum dot surface functionality and diisocyanate chemistry in polyurea thin-film nanocomposite nanofiltration membranes for dye/salt separation

Dye/salt separation requires nanofiltration membranes that retain organic dyes while allowing dissolved salts to pass. This study examined how silicon quantum dot (SiQD) surface functionality and organic diisocyanate chemistry affect polyurea thin-film nanocomposite membranes prepared by interfacial polymerization. Three diamine-functionalized SiQDs were combined with three organic diisocyanates to compare their effects on membrane properties and separation performance. Among the investigated combinations, the membrane prepared with m -phenylenediamine (MPDA)-functionalized SiQDs and 1,3-bis(isocyanatomethyl)cyclohexane (BIMC) showed the best overall dye/salt separation performance. Under the determined fabrication conditions, the MPDASiQDs–BIMC/polysulfone membrane had a pure-water flux of 39.11 ± 4.47 LMH and rejected 98.24 ± 0.46% Rose Bengal, 97.93 ± 0.68% Methyl Blue, and 96.11 ± 0.91% Congo Red, while Na₂SO₄ and NaCl rejections were 27.92 ± 2.37% and 8.69 ± 1.83%, respectively. For the Rose Bengal/NaCl feed, the membrane rejected 97.84% of Rose Bengal and 5.83% of NaCl. For the Rose Bengal/Na₂SO₄ feed, it rejected 97.70% of Rose Bengal and 17.84% of Na₂SO₄. Rose Bengal rejection also remained above 96% across feed concentrations of 10–600 mg L −1 , although flux decreased at higher concentrations. During continuous filtration for approximately 168 h, dye rejection remained above 90%, while flux gradually decreased to approximately 80% of its initial value. These results show that SiQD surface functionality and diisocyanate chemistry can be varied to obtain polyurea–SiQD membranes that retain dyes while allowing salts to pass.