Effect of carbon nanotubes on Cd(II) adsorption by sediments

• Effect of CNTs on Cd(II) adsorption by sediments was investigated.
• The pseudo-second-order rate constant decreased with rising CNTs/sediments ratios.
• Qmax of Cd(II) increased and KL decreased with increasing CNTs/sediments ratios.
• Cd(II) adsorption is a spontaneous, endothermic, and physical reaction.

With increasing production and application, carbon nanotubes (CNTs) will enter into the environment in significant masses in upcoming years. CNTs have great potential to disturb the bioavailability and toxicity of other contaminants because of their high and strong adsorption capacity. This study investigated the influence of CNTs on the Cd(II) adsorption behavior by sediment–CNTs mixture at different CNTs/sediments ratios. The results showed that the adsorption kinetic data were well described by the pseudo-second-order model, and the pseudo-second-order rate constant (k2) decreased from 0.122 to 0.070 g/(mg min) with increasing CNTs/sediments ratios from 0% to 10%. Because of the different functional groups in CNTs (carboxyl groups, pKa = 2.81) and sediments (humic acid, pKa = 5.83), the removal efficiencies of Cd(II) by CNTs increased at pH 3.0, while the removal efficiencies of Cd(II) by sediments increased significantly at pH 5.7. The adsorption capacity of Cd(II) increased by the CNTs polluted sediments with ascending CNTs/sediments ratios. Cd(II) adsorption by sediments–CNTs mixture is a spontaneous (ΔG○ < 0), endothermic, and physical reaction (ΔH○ < 41 kJ/mol). Moreover, the spontaneity of the Cd(II) adsorption by sediment–CNTs mixture decreased, as indicated by less negative ΔG○ values, with increasing CNTs/sediments ratios. This is among the first study to investigate the effect of CNTs on metal adsorption by sediments. The results indicate CNTs released into sediments would change the adsorption behavior of Cd(II) by sediments and then alter the potential risk of Cd(II) and CNTs in sediments.