Salting-Out Effect-Enhanced Organics Adsorption Diminishes the Chloride/Sulfate Separation Selectivity of Nanofiltration Membranes For High Salinity Wastewater

Nanofiltration (NF), exploiting its tailorable pore size and surface charge properties, emerges as a core technology for salt separation in zero liquid discharge (ZLD). However, the coexisting organics in high-salinity wastewater raise concerns regarding their potential adverse impacts on NF membrane salt separation performance. Herein, we investigated this long-overlooked phenomenon and its underlying mechanisms by focusing on Cl^-/SO₄^2- separation in an actual reverse-osmosis concentrate treatment. The results demonstrated that the coexisting organics compromised the Cl^-/SO₄^2- separation factor from 100-150 to 20-40. Static adsorption experiments revealed that organic adsorption is governed by electrostatic interactions in low-salinity solutions but dominated by nonelectrostatic interactions due to the enhanced charge-shielding and salting-out effects in high-salinity solutions. Consequently, organic adsorption not solely is limited to positively charged organics but also involves neutral and negatively charged organics, with the adsorption amount increasing with the hydrophobicity of the organic compounds and the membrane. Performance characterization revealed that organic adsorption induced the attenuation of the membrane surface negative charge density and increased the molecular weight cutoff, thereby weakening the steric, Donnan, and dielectric exclusion. This study yields critical insights into strategies for enhancing salt separation performance, optimizing NF process design, and elevating the economic viability of ZLD systems in industrial practice.