Characterizing dissolved organic matter fouling of nanofiltration membranes and evaluating effects of naproxen retention

Nanofiltration (NF) membrane fouling caused by two types of natural dissolved organic matter (DOM), namely Qingcaosha reservoir (QCS) DOM and Taihu Lake (TH) DOM, on naproxen (NPX) retention, were assessed. The results indicated that NPX retention could be interpreted with respect to the DOM molecular weights (MW) and the fouling characteristics. Organics with medium- and low-MW, e.g. less than 10kDa, significantly affected NPX retention and resulted in membrane fouling. DOMs with medium MW (1.5-10kDa) mainly had strong hydrophobic characteristics, and adhered well to the NF membrane surface to form a fouling layer, which altered the hydrophobicity and negative charge of the membrane surface. The hydrophobicity and electronegativity of the adhered DOM fouling layer was responsible for the decreased membrane flux and the change in NPX retention. TH DOM contained more hydrophobic organics with MW 1.5-10kDa than QCS DOM, and the cohesion free energy and adhesion free energy between the membrane and foulants were more negative for TH DOM, which caused more severe membrane fouling. The surface free energy between membranes and foulants is a useful parameter for membrane fouling analysis, and a mechanism of influencing NPX retention in the presence of DOM was proposed. The increased NPX removal by QCS DOM-fouled membrane was mainly attributed to electrostatic repulsion, whereas the decreased NPX rejection by TH DOM-fouled membrane was mainly attributed to cake-enhanced concentration polarization.