Study on Kinetics, Thermodynamics and Mechanism for Carbon Nanotubes Adsorbing Humic Acid

Abstract

The dissolved organic carbon in natural water was mainly from soluble humic acid(HA) which had much harms in industrial and drinkable water. As a new adsorbent, the multi-walled carbon nanotubes (MCNTs) were used to remove HA at different pH value, different MCNTs dosages and different HA initial concentration, a series of experiments including batch adsorption, adsorption isotherm were carried. The adsorption kinetics data was fitted with pseudo-second-order rate model, their linear correlations was above 0.99, and equilibrium adsorption capacity obtained from fitting curve was 27mg/g, consistent with the experimental results (25.5mg/g). The adsorption isotherm test was carried at 25°C−50°C, the equilibrium adsorption capacity was 29.7mg/g by Langmuir model, which was similar to the experiment results. The free energy (ΔG), enthalpy(ΔH) and entropy (ΔS) were obtanined with Clapeyron-Clausius and Gibbs-Helmholtz equation in the adsorption isotherm, which all were negative, indicated that HA adsorbed by MCNT was an exothermic process of entropy reduction. MCNT’s outer surfaces, layer interspace, inner cavity and aggregated pores became efficient adsorption space for HA. The π−π interactions between HA and MCNT played an important role in the adsorption process.