Covalent organic framework@cellulose nanofibrils@carboxymethyl cellulose composite hydrogel beads for the removal of nickel ions from aqueous solutions

A novel adsorbent, covalent organic framework (COF)@cellulose nanofibrils (CNF) @carboxymethyl cellulose (CMC) composite hydrogel bead (C-CCHB), was prepared for Ni2+ removal from aqueous solutions. The imine COF provides the main active sites for adsorption. CNF is used as the carrier for grafting COF to improve the dispersion of COF and the reinforcing material. CMC is used as the hydrogel matrix to facilitate easy recycling. The equilibrium adsorption capacity (qe) of the C-CCHB is vastly influenced by the mass ratio of COF@CNF/CMC. The optimized mass ratio of COF@CNF/CMC is 0.050/1 (g/g). The maximum adsorption is reached at pH=6. The qe of Ni2+ increases with a rise in initial concentration of the solution. Based on Langmuir isotherms, the maximum adsorption capacity of C-CCHB is 289.50 mg/g. The adsorption of Ni2+ complies with the pseudo-second-order kinetic model. The C-CCHBs still maintain preferable adsorption properties after the seventh cycle of adsorption–desorption. The Ni2+ adsorption of the C-CCHB is mainly attributed to the chelating effect and electrostatic interaction.