Coadsorption of copper and perfluorooctane sulfonate onto multi-walled carbon nanotubes

► Sorption and desorption of PFOS and Cu(II) on MWCNTs in single- and bi-solute systems were studied. ► Sorption capacities of PFOS and Cu(II) in bi-solute system were higher than those in single-solute system. ► Desorption behaviors of PFOS and Cu(II) in bi-solute system were different from those in single-solute system. ► Cu formed inner-sphere complexes without PFOS, while out-sphere complexes with PFOS. Multi-walled carbon nanotubes (MWCNTs) are superadsorbents for perfluorooctane sulfonate (PFOS) and heavy metals, which coexist in wastewater. The interaction between PFOS and Cu(II) with regard to their adsorption and desorption on MWCNTs was evaluated in this study. The adsorption of PFOS on MWCNTs enhanced with an increase of Cu(II). Desorption of PFOS showed no hysteresis without Cu(II), while pronounced hysteresis with Cu(II). Increases of the adsorption and the occurrence of desorption hysteresis of PFOS in the presence of Cu(II) may due to the PFOS adsorption via a Cu(II) bridge. The adsorption of Cu(II) increased with an increase of PFOS. The desorption hysteresis factors in the presence of PFOS were lower than those in the absence of PFOS. Increases of the adsorption and decreases of desorption hysteresis factors may be attributed to the Cu(II) adsorption via a PFOS bridge. Results obtained by X-ray absorption spectroscopy verified the formation of inner-sphere complexes between Cu(II) and functional groups of MWCNTs without PFOS, while out-sphere complexes with large amount of PFOS. The effect of Cu(II) and PFOS adsorption on the zeta potential of MWCNTs was also studied. The results showed that the coexistence of Cu(II) and PFOS greatly affects their sorption and desorption behaviors on MWCNTs thereby their fate and transport in wastewater.