Investigation of thermodynamics of interaction between humic acids and CNTs with various oxygen contents by isothermal titration calorimetry

The interaction between carbon nanotubes (CNTs) with various oxygen contents and humic acids (HA) was studied through batch adsorption experiments and the isothermal titration calorimetry (ITC) technique. The effects of CNTs surface chemistry and solution ionic strength were evaluated. Adsorption results show that the binding strength between CNTs and HA and equilibrium adsorption capacity of HA increase with the decrease in CNTs oxygen content decreases and the increase in ionic strength, implying that hydrophobic interaction could be one of the main interaction forces between CNTs and HA. ITC results indicate that interaction between CNTs and HA is exothermic and that a stable CNT-HA complex could be formed. CNTs solutions become more ordered after binding with HA. The negative enthalpy and entropy changes obtained from the ITC analysis imply the existence of hydrogen bonds between CNTs and HA. This could provide a better understanding of interactions between humic substances and CNTs, and thus the migration and fate of CNTs in natural environments.