Influence of dominant salts on the removal of trace micropollutants by hollow fiber nanofiltration membranes

Nanofiltration (NF) is seen as a promising advanced treatment technology to deal with the increasing concen-tration of micropollutants (MPs) in water sources globally. To further improve the successful implementation of NF, an increased understanding of membrane transport mechanisms is important. One key aspect among these mechanisms concerns the influence of solution composition on the overall filtration performance. Although several studies report the influence of feed solution composition on the removal of micropollutants by NF membranes, the underlying mechanisms are often not fully understood. In this study, the impact of dominant salts (NaCl, Na2SO4, MgCl2) on the removal of trace MPs by commercial hollow fiber NF membranes (dNF40, dNF80) was investigated. Common conditions where salt concentrations greatly exceed MP concentrations were applied to assess their potential influence at environmentally relevant concentrations. Experimental observations reveal that a dominant salt alters MP transport behaviour substantially. The impact of a dominant salt on MP transport via electrostatic coupling increases for MPs of higher mobility inside the membrane. Overall, higher mobility, i.e. lower removal, of MPs through the dNF80 membranes was observed. Correspondingly, the strongest impact was observed for the dNF80 membrane, where the removal of positively charged atenolol increased from about 60% to >90% in the presence of Na2SO4. A theoretical transport model (DSPM&DE) was used to assist the interpretation of experimental observations further. Model predictions reveal the relevance of two effects: the influence of a dominant salt on the charge-based membrane properties and the electrostatic coupling in the form of the arising membrane potential.