Selective separation of dye and salt by PES/SPSf tight ultrafiltration membrane: Roles of size sieving and charge effect

Polyethersulphone (PES)/sulfonated polysulphone (SPSf) tight ultrafiltration (TUF) membrane was tailored for dye/salt selective separation via non-solvent induced phase separation (NIPS) method using adipic acid (AA) as a pore forming agent. Sucrose and PEG (200–1000 Da) as neutral model molecules and eight kinds of charged dye model molecules (269.3–960.8 Da) were employed to explore the roles of size sieving and charge effect during the membrane separation process. Results showed that the membrane obtained from the casting solution with the mass ratio of PES/SPSf = 84/16 and 11 wt% AA possessed a fully sponge-like structure with an average pore size of 1.8 nm and the molecular weight cut off (MWCO) of 7250 Da. The pure water permeance of the membrane was up to 72.2 L/m2 h bar and displayed a reasonable retention for the neutral molecules including PEG 1000 (46.0%), PEG 400 (19.7%) and sucrose (17.2%). Simultaneously, the membrane exhibited a higher rejection for dyes with the similar molecular weight (Mw) such as methyl orange (MO) (69.4%), acid blue 25 (AB25) (92.6%) and Evans blue (EB) (100%). The high rejection for these dyes was mainly attributed to the synergy between the size sieving and charge effect. Furthermore, the rejection of membrane for methyl red (MR), MO and AB25 was 50.1%, 69.4% and 92.6%, respectively. Molecular dynamics (MD) simulations revealed that the hydration radius of AB25 (1.04 nm) was bigger than that of MR (0.94 nm) and MO (0.99 nm), leading to higher rejection illustrating that size sieving played an important role in the rejection of dyes (Mw less than 696.7 Da) with the same charge. Specifically, the contribution rate of steric hindrance and charge effect to the dye rejection gradually decayed as an increase on the molecular weight of dyes (over 416.0 Da). In addition, the rejection of MR, MO, indigo carmine (IC) and acid fuchsin (AF) decreased with an increase in the ionic strength of feed solutions due to the shielding effect of space charge. In summary, our work provides a profound understanding of the membrane selective separation mechanism for dye wastewater treatment and reuse.