Sorption of Aqueous Zn[II] and Cd[II] by Multiwall Carbon Nanotubes: The Relative Roles of Oxygen-Containing Functional Groups and Graphenic Carbon

• Published in: Langmuir 2010-01, Vol.26 (2), p.967-981
• Main Authors: Cho, Hyun-Hee, Wepasnick, Kevin, Smith, Billy A, Bangash, Fazlullah K, Fairbrother, D. Howard, Ball, William P
• Format: Article
• Language: English
• Subjects: Adsorbents; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Interfaces: Adsorption, Reactions, Films, Forces; Surface physical chemistry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la902440u

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.