title: Sorption of aqueous Zn[II] and Cd[II] by multiwall carbon nanotubes: the relative roles of oxygen-containing functional groups and graphenic carbon.
abstract: Exposure of multiwalled carbon nanotubes (MWCNTs) to oxidizing acids and other oxidants introduces oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl groups onto the surface. This research evaluated how changes in oxygen concentration and distribution of oxygen-containing functional groups influenced the sorption of aqueous zinc and cadmium on MWCNTs. Sorption results with natural char, activated carbon, and a suite of MWCNTs (of varying surface oxygen content) were obtained. Results confirmed that surface oxygen enhances the sorption of both Zn[II] and Cd[II] from aqueous solution. Although Zn[II] sorbed more strongly than Cd[II] for all materials studied, surface oxidation had more effect on the sorption of Cd[II] than of Zn[II]. Additional sorption experiments with Zn[II] and 16 MWCNTs of varying surface oxidation level and functional group distribution revealed the relative contributions of different types of surface sites to sorption. Sorption isotherms were fit using a two-site Langmuir adsorption model that incorporated the independent characterization of functional group distribution. Results showed that carboxyl-carbon sites were over 20 times more energetic for zinc sorption than unoxidized carbon (graphenic-carbon) sites, though both site types are important contributors to sorption.
authors.affiliation: Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Hyun-Hee
authors.initials: HH
authors.lastname: Cho
authors.forename: Kevin
authors.initials: K
authors.lastname: Wepasnick
authors.forename: Billy A
authors.initials: BA
authors.lastname: Smith
authors.forename: Fazlullah K
authors.initials: FK
authors.lastname: Bangash
authors.forename: D Howard
authors.initials: DH
authors.lastname: Fairbrother
authors.forename: William P
authors.initials: WP
authors.lastname: Ball