title: Bound and unbound humic acids perform different roles in the aggregation and deposition of multi-walled carbon nanotubes.
abstract: Natural organic matter influences the carbon nanotube transport in aqueous environments. The role of bound humic acid (HA) on carbon nanotubes and unbound HA in bulk solution in the aggregation and deposition of carboxylated multi-walled carbon nanotubes (C-MWNTs) was examined in NaCl and CaCl2electrolyte solution. Time-resolved dynamic light scattering and quartz crystal microbalance with dissipation monitoring were employed to investigate the C-MWNT aggregation and deposition kinetics, respectively. The critical coagulation concentration (CCC) of C-MWNTs is 30mM in NaCl and 3mM in CaCl2. The bound HA results in CCCs of 32mM in NaCl and 2.9mM in CaCl2. However, the existing unbound HA causes much slower aggregation in both NaCl and CaCl2electrolytes and results in CCCs of 86mM in NaCl and 5.8mM in CaCl2. The HA adsorption experiment confirms the additional adsorption of unbound HA in the presence of cations, which can increase the steric effect between C-MWNTs. The more negative charge of C-MWNTs in the presence of unbound HA also stabilizes the suspension. In contrast, the bound HA on C-MWNTs has a more remarkable effect on the deposition rate on the SiO2surface than the unbound HA. Bound HA changes the C-MWNT surface functional groups, leading to differences in the interaction between C-MWNTs and the SiO2surface. Hence, the C-MWNTs dispersed by their covalently bonded oxygen-containing groups on the carbon framework and dispersed by the bound HA show nearly the same aggregation rates but quite different deposition rates. The additional unbound HA adsorption does not change the surface functional groups or the changing trend of the CNT deposition rate. Distinguishing the role of bound and unbound HA in the aggregation and deposition of carbon nanomaterials is important to predict their transport in various natural waters.
authors.affiliation: Environment Research Institute, Shandong University, Jinan 250100, China.
authors.forename: Xuezhi
authors.initials: X
authors.lastname: Yang
authors.affiliation: Environment Research Institute, Shandong University, Jinan 250100, China.
authors.forename: Qi
authors.initials: Q
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Jiangsu 210093, China.
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Qu
authors.affiliation: Environment Research Institute, Shandong University, Jinan 250100, China. Electronic address: jiangw@sdu.edu.cn.
authors.forename: Wei
authors.initials: W
authors.lastname: Jiang
keywords: Aggregation; Carbon nanotubes; Deposition; Humic acid; Silica surface