Mechanically stable core-shell cellulose nanofibril/sodium alginate hydrogel beads with superior cu(II) removal capacity

Nanocellulose hydrogels are promising sustainable biosorbents for removing heavy metal ions for wastewater treatment. However, the nanocellulose hydrogels reported thus far typically suffer from inferior adsorption performance and/or poor mechanical stability, thus limiting their industrial applications. Achieving the goals of mechanical stability and high removal capability remains a crucial technical challenge, which may be addressed, as presented in this study, by developing novel core-shell carboxymethylated cellulose nanofibril (CMCNF)/sodium alginate (SA) hydrogel beads (CAbs). By immobilizing CMCNFs (shell) on the surface of the SA hydrogel bead (core) via electrostatic attractions and hydrogen bonding, a mechanically stable hydrogel bead with a core-shell configuration was constructed, which shows a Cu(II) removal capacity of up to 221 mg/g that exceeds that of CMCNFs and most other nanocellulose structural adsorbents. Furthermore, both the formation principle of the core-shell structure and the Cu(II) removal mechanism were explored in detail. Finally, we demonstrated a potential application of core-shell CAbs to treat drinking water with a low concentration of Cu(II) using a homemade column adsorption device. This work brings sustainable nanocellulose adsorbents a step closer to industrial applications for Cu(II) wastewater treatment.