Effect of post-treatment on permeance and fouling of nanofiltration membrane.

The effects of two post-treatments with CaCl₂ and NaOH on nanofiltration (NF) membranes were investigated according to the physicochemical properties and antifouling performance. The membrane post-treated with NaOH (NF-OH^-) exhibited enhanced hydrophilicity and greater negative surface charge, accompanied by collapsed nodular structures. Compared with the pristine NF membrane, NF-OH^- demonstrated a 50% increase in permeance, but showed compromised separation performance, with SO₄^2- rejection of 97.3% and Cl^-/SO₄^2- separation factor of 46. In contrast, the CaCl₂-treated membrane (NF-Ca²⁺) displayed improved hydrophilicity with reduced negative charge and denser nodular morphology. While NF-Ca²⁺ achieved 25% higher permeance than the pristine membrane, it maintained comparable SO₄^2- rejection and Cl^-/SO₄^2- separation factor. Fouling experiments using biological treatment effluent of leachate revealed that NF-OH^- showed greater fouling propensity and lower cleanability than the other two membranes. This is likely due to the stronger negative charge and smoother surface morphology of the NF-OH^- membrane, which promoted foulant adhesion while limiting surface turbulence. Conversely, the NF-Ca²⁺ membrane demonstrated superior antifouling characteristics and higher permeance recovery, attributable to its reduced negative surface charge and denser nodular morphology. These findings underscore the critical influence of surface charge and nanostructure on the filtration and antifouling behavior of NF membrane.