Surface-bound humic acid increased Pb2+ sorption on carbon nanotubes

► Humic acid (HA) was coated on commercially available carbon nanotubes (CNTs). ► Surface-bound HA introduced oxygen-containing functional groups and negative charges. ► Sorption capacities of the HA-CNT complexes increased with increasing HA content. ► Pb2+ could be electrostatically attracted into diffuse layer of the HA-CNTs. ► The loaded Pb2+ could be inner-spherically complexed with -COOH group on the HA-CNTs.