Surfactant removal with multiwalled carbon nanotubes.

The ability of multiwalled carbon nanotubes (MWCNTs) to remove a non-ionic surfactant, Triton X-100 (TX100), an anionic surfactant, sodium dodecylbenzenesulonate (SDBS), and a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB) from the aqueous phase was investigated. Untreated, OH-, and COOH-functionalized MWCNTs with different outer diameters and chemical composition were examined and compared. As both the concentrations of surfactants and MWCNTs initially added may affect removal efficiency of surfactants, a relationship between the initial concentration ratio of surfactants and MWCNTs (Rc ) and the removal efficiency (E) was established. The results showed that for a given Rc (e.g., 0.8), removal efficiency of the tested surfactants by a specific MWCNT (e.g., the untreated one with outer diameter <8 nm) decreased in the following order: TX100 (52.3%) > SDBS (26.2%) > CTAB (3.8%). TX100 was more readily removed by MWCNTs than SDBS and CTAB, due to its longer aliphatic chain compared to SDBS and CTAB thus higher hydrophobicity, and stronger π-π interactions with the aromatic structure of the surfaces of graphite sheets relative to CTAB. Based upon the established relationship between Rc and E of surfactants by MWCNTs, the maximum removal efficiency and the most appropriate Rc of TX100 and SDBS by two MWCNTs (UT8 and OH8) were derived. It was interesting to notice that, except for the case to remove TX100 using UT8, even though a large quantity of UT8 or OH8 was added to the TX100 or SDBS removal systems, they cannot be completely removed, with the maximum removal efficiency in the range of 55.88-87.17%. This mostly resulted from strong aggregation of MWCNTs thus reducing their readily accessible surface area and porosity for sorption.