Removal of humic acid from aqueous solution by a carbon nanotubes/ reduced graphene oxide composite hydrogel

In this study, three-dimensional (3D) porous network carbon nanotubes/reduced graphene oxide composite hydrogels (CNT-rGH) were synthesized through a facile hydrothermal method for the removal of macromolecular humic acid (HA). Brunauer-Emmett-Teller analyses indicated that incorporation of carboxylated multi-walled carbon nanotubes (COOH-MWCNT) renders the hydrogels with increased surface area, larger mean pore diameter, and higher intensity of mesoporous and macroporous structure. Such improvement was shown to facilitate adsorption site exposure on hydrogels, which enhanced the HA adsorption capacity. Incorporation of 50% proportion of COOH-MWCNT in the CNT-rGH (CNT50-rGH) exhibits optimal adsorption performance for HA. In addition, the initial aqueous pH value and the ionic strength were systematically studied to evaluate their impacts on HA adsorption by the CNT50-rGH. Adsorption kinetics data show that HA adsorption onto CNT50-rGH is in agreement well with pseudo-second-order kinetic model, indicating the principle and mechanism of chemisorption. The Langmuir model is the most relative adsorption isotherm to depict the HA adsorption, and the maximum HA adsorption capacity of the CNT50-rGH is determined at 270.27 mg/g. Through the XDLVO theory and controlled experiments reveals that hydrogen bonding interaction is the main adsorption mechanism of CNT50-rGH for HA adsorption.