HCNCs for water treatment: additives, aerogels, and hydrogel beads. (A) Adding ENCC to a copper solution results in their aggregation and precipitation on the time scale of seconds, providing a facile method to remediate polluted aqueous media. Supernatant may contain star-like ENCC–Cu(ii) aggregates if ENCC charge is not fully neutralized. The precipitate contains raft-like ENCC–Cu(ii) aggregates. (B) Copper removal capacity of ENCC versus equilibrium copper concentration (C_e), showing a maximum removal capacity ∼185 mg g⁻¹. (C) Schematic of reusable, anionic, highly porous aerogel adsorbents prepared from the Schiff based reaction of BNCC with CMCT. (D) Freundlich isotherm fit to the experimental data for the adsorption of a cationic dye (methylene blue) by BNCC–CMCT aerogels in solutions with no salt or 0.1 M of NaCl at standard temperature and pressure (STP). (E) Schematic of ENCCs directly used or incorporated into alginate hydrogel beads for dye removal. (F) Dye uptake and removal percentage versus the initial dye concentration, showing a maximum dye uptake ∼1400 mg g⁻¹ for ENCC. (Panels A and B) Adapted from ref. 76 with permission from American Chemical Society, Copyright 2015. (Panels C and D) Adapted from ref. 80 with permission from American Chemical Society, Copyright 2016. (Panels E and F) Adapted from ref. 85 with permission from American Chemical Society, Copyright 2019.