Sorption behavior of carbon nanotubes: Changes induced by functionalization, sonication and natural organic matter

The effect of functionalization on the sorption behavior of carbon nanotubes (CNTs) remains poorly understood, especially when combined with other factors affecting dispersion. The sorption behavior of a series of functionalized CNTs towards pyrene has therefore been systematically evaluated over a wide range of concentrations and dispersion scenarios. When studied as purchased (in the absence of humic acids and sonication treatment), sorption isotherms showed that OH-, COOH- and NH₂-CNTs exhibited significantly different sorption affinity for pyrene. Sonication greatly increased both the sorption affinity and the maximum capacity of all types of functionalized CNTs, to an extent that overwhelmed the differences initially observed (increase of up to 1.5 orders of magnitude lead to log K_d values close to 9L/kg). Results demonstrate that a significant proportion of the CNT surface was unavailable to pyrene prior to sonication. The presence of humic acids enhanced dispersion but decreased sorption, especially when combined with sonication. Sorption affinity, however, remained very high in all cases (log K_d>7.5L/kg), suggesting that CNTs can act as strong sorbents under a wide range of conditions.