Influence of inorganic salt on retention of ibuprofen by nanofiltration

The retention of ibuprofen by a nanofiltration membrane (Desal 5 DK) was studied, considering the influence of the different co-existing salt (namely, NaCl, Na2SO4, CaCl2) with varying concentration (ionic strength from 10 to 100 mM). The result shows that the Desal 5 DK cannot reject the ibuprofen satisfactorily (highest retention rate approximated to be ca.40%) in absence of salt, which agrees with the strong solute-membrane attraction. On the other hand, the presence of salt differentiates the membrane performance. An enhanced permeability of ibuprofen is only found when Na2SO4 is present, giving lowest retention rate compared with other cases (without salt and with chloride-induced salts). NaCl improves the ibuprofen retention in a moderate extent. Donnan exclusions of the anions and membrane charge screening may be involved for these two mixtures. CaCl2 gives the highest retention of ibuprofen mainly due to the specific interaction of calcium and carboxyl group, and the lowest adsorption of ibuprofen on membrane as well as acceptable permeate flux. Further study is needed to better understand the influence of salt on such emerging pollutants retention by membrane.