Adsorption and desorption of thallium(I) on multiwalled carbon nanotubes

Increasing applications of multiwalled carbon nanotubes (MWCNTs) and thallium (Tl) would raise their exposure risks to aquatic environment. MWCNTs might affect the environmental behavior of Tl(I) by adsorption, and it is critical to understand the interactions between them. The adsorption kinetics, isotherms, thermodynamics, and desorption of Tl(I) by three MWCNTs were studied. In addition, the influences of solution pH, ionic strength on adsorption were also evaluated. The adsorption of Tl(I) on MWCNTs started at very low pH values and increased abruptly, then maintained high level with increasing pH values at pH > 6.5. MWCNTs with more hydrophilic groups and negative surface charges had the greatest Tl(I) absorption capacity. The existence of K+ significantly inhibited Tl(I) adsorption on purified-MWCNTs. Tl(I) adsorption on MWCNTs was spontaneous occurred by ion exchange with H+/Na+/K+ at the exchange sites and forming outer-sphere surface complex. Moreover, Tl(I) could be easily desorbed from MWCNTs and the lower pH benefited the desorption.

► Cation exchange and outer-sphere complexes predominated the adsorption processes.
► Adsorption of Tl(I) on MWCNTs were spontaneous chemisorption.
► CNTs with more negative surface charges had the greatest Tl(I) absorption ability.
► Tl+ was harder to replace K+ than to Na+ in adsorption on MWCNTs.
► Lower pH did benefit the desorption of Tl(I) from MWCNTs.