Adsorption of perfluorooctane sulfonate on carbon nanotubes: influence of pH and competitive ions

Perfluorooctane sulfonate (PFOS) is one of the persistent organic pollutants that has aroused global concern. Carbon nanotubes (CNTs) have drawn much research attention due to their unique properties. This study was aimed to investigate the adsorption behaviour of PFOS with relation to CNTs and clarify their interactions. Electrostatic repulsion suppressed the sorption of PFOS to multi-walled carbon nanotubes (MWCNTs), resulting in low sorption at high pH. With the increase of anion concentration, the removal of PFOS decreased by 20 to 30% because ionic strength changed the aggregation of the MWCNTs. Unlike inorganic anions, cationic metals including Cu(II) and Pb(II) had a significant impact on the sorption of PFOS onto the MWCNTs. At low concentrations of Cu(II) and Pb(II), PFOS removal decreased due to the suppressive effect of ionic strength, while the removal of PFOS increased rapidly with increasing metal concentrations, especially for functionalized MWCNTs. The enhanced adsorption of PFOS at high Cu(II) and Pb(II) concentrations was attributed to the adsorbed cationic metals which might further adsorb PFOS.