Bound and unbound humic acids perform different roles in the aggregation and deposition of multi-walled carbon nanotubes

•Unbound HA mitigates CNT aggregation due to the additional adsorption with cations.•Bound HA changes the CNT surface groups and their interaction with SiO2.•Bound HA affects the deposition on SiO2 more remarkably than unbound HA.

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 CaCl2 electrolyte 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 30 mM in NaCl and 3 mM in CaCl2. The bound HA results in CCCs of 32 mM in NaCl and 2.9 mM in CaCl2. However, the existing unbound HA causes much slower aggregation in both NaCl and CaCl2 electrolytes and results in CCCs of 86 mM in NaCl and 5.8 mM 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 SiO2 surface than the unbound HA. Bound HA changes the C-MWNT surface functional groups, leading to differences in the interaction between C-MWNTs and the SiO2 surface. 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.

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