Interactions Between Carbon Nanotubes And Humic Acid

Due to the development of science and technology, nanomaterials (such as carbon nanotubes, CNTs) have been used more and more widely and frequently around the world. However, the exposure, migration and transformation of nanomaterials might have great influence on normal metabolism of human and other living cultures as well as the safety of ecology. It has attracted more and more attention about the migration and transformation of nanomaterials in the environment in order to understand their fate and ecotoxicity in the environment. Humic substances are the main polymeric organic matters that exists abundantly in soil and natural water. It consists large aromatic skeletons and a great amount of active functional groups, such as carboxyl, hydroxyl, keto, aldehyde etc. They have high absorption and complexation capability to organic pollutants and heavy metals. When CNTs enter into natural water bodies or sewage treatment systems, they might be adsorbed to these organic matters immediately. When these nanomaterials are covered with humic substances, their surface charge, chemical activity and spatial resistance will change dramatically, and thus will affect their migration and transformation in water bodies and their environmental toxicity. The surface characteristics of CNTs and the solution conditions of water will have a great influence on the interaction between CNTs and humic substances. Based on advanced analytical technology of thermodynamics (e.g., isothermal titration calorimetry), this work proposed a thennodynamic analytic method for the interaction between CNTs and humic acids. The effects of surface oxygen contents of CNTs, solution ionic strength and pH on the interaction also have been investigated, and the interaction mechanisms have been revealed. This work would provide a theoretical basis and technical supports on the migration and transformation of CNTs in the aqueous environment. The main contents and results are as follows:1. The effects of CNT surface oxygen content and the ionic strength on adsorption kinetics and isotherms of humic acids (HA) on CNTs were investigated. The results reveal that the HA adsorption capability increased with increasing ionic strength and decreasing oxygen content, which might be due the hydrophobic interaction between the CNTs and HA. However, there is no significant change for absorption kinetics of HA onto CNTs with various oxygen contents at various ionic strengths. Adsorption isotherms could be fitted using Langmuir model and Freundlich model, and both model fitted the experimental results well. The equilibrium adsorption capacity of CNTs and the interaction intensity increased with increasing the solution ionic strengths and decreasing the oxygen content of CNTs. The maximum adsorption capacity of CNTs calculated from Langmuir model decreased with increasing the CNT oxygen content and the solution ionic strength.2. The isothermal titration calorimetry (ITC) technique was used to determine the thermodynamic parameters of the interaction between CNTs and HA. The typical binding capacity of CNTs towards HA was calculated to be32.48mg TOC/g (5.41×10-7mol/g), while the binding constant was1.49x106L/mol. The changes of the binding enthalpy and the Gibbs free energy were calculated to be-4420kJ/mol and-35.22kJ/mol, respectively, which implied that the binding reaction was exothermal and thermodynamically favorable. A stable CNTs-HA complexes could be formed. The entropy change of the interaction was-14.04kJ/mol/K, implying that the degree of disorder of the system reduced after CNT combined with HA. The enthalpy and entropy changes depended on the surface oxygen content of CNTs, and they were the highest when the CNTs had the least surface oxygen content. However, the number of binding sites was the largest when CNTs had the lowest surface oxygen content. Furthermore, the solution conditions also influenced the interaction. The binding constant increased with increasing ionic strength, while the binding sites decreased with increasing ionic strength.