Effects of coexisted substances on the separation characteristics and mechanisms of perfluorooctanoic acid by polyamide composite nanofiltration membrane

Worldwide widely distributed perfluorooctanoic acid generally coexists with Ca2+/Mg2+ and natural organics in natural water bodies. However, there is no systematic study reported on the retention behavior of perfluorooctanoic acid in the presence of Ca2+/Mg2+ and typical dissolved organics during nanofiltration. Herein, the influences of Ca2+/Mg2+ and/or humic acid/sodium alginate on perfluorooctanoic acid removal by nanofiltration were investigated, respectively. The dominant rejection mechanism of perfluorooctanoic acid was further revealed by combining the surface morphology and charge characteristics of the virgin and fouled membranes. Result showed that, under 0.6 MPa, perfluorooctanoic acid rejection obviously increased when the concentration of Ca2+/Mg2+ increased from 0 to 2 mM in feedwater. This improvement was attributed to the complexing and bridging of Ca2+/Mg2+ with perfluorooctanoic acid which could increase the size exclusion effect. When humic acid/sodium alginate coexisted with perfluorooctanoic acid, perfluorooctanoic acid rejection increased distinctly because the adsorption of humic acid/sodium alginate on nanofiltration membrane surface enhanced the sieving and electrostatic repulsion effects of nanofiltration membrane to perfluorooctanoic acid. Additionally, when Ca2+/Mg2+ and humic acid/sodium alginate coexisted with perfluorooctanoic acid in feedwater, perfluorooctanoic acid rejection was the highest in all feed conditions because Ca2+/Mg2+ could neutralizing charge, compressing double electric layer, and complexating and bridging with humic acid/sodium alginate as well as nanofiltration membrane. At this time, the foulant cake layer acted as an extra rejection layer, and sieving effect was the main rejection mechanism of perfluorooctanoic acid by nanofiltration membrane.