Ferrous ion-induced cellulose nanocrystals/alginate bio-based hydrogel for high efficiency tetracycline removal

The complex synthesis process, high cost, toxicity and difficult separation of traditional adsorbents have limited their application for the removal of emerging organic contaminants (EOCs) from the aquatic environment. A completely bio-based alginate reinforced with cellulose nanocrystals hydrogel via Fe2+ cross-linking (CNC/Alg-Fe2+), was successfully fabricated as an effective adsorbent under unique low temperature heat treatment (100 °C) for tetracycline (TC) removal. The metal ions (Fe3+, Fe2+ and Ca2+) enabled bindings with hydroxyl, sulfate half ester and carboxyl groups coexisted on CNC/Alg surface via chelation, strong hydrogen bonds and electrostatic interactions. Among composite materials with varied cation cross-linkers, hydrogel cross-linked with Fe2+ showed the maximum adsorption capacity of TC (741.66 mg·g−1), circa 1.9 and 6.9 times higher than that of hydrogel cross-linked with Fe3+ (394.16 mg·g−1) and Ca2+ (108.14 mg·g−1) respectively at 298 K, due to enhanced chelation between Fe2+ and TC. The efficient adsorption of TC was attributed to surface complexation, cation bridging, n-π electronic donator-acceptor (n-π EDA) interactions, hydrogen bonds and electrostatic interactions synergistically. Besides, adsorption capacity of CNC/Alg-Fe2+ retained 84.6% after 5 regeneration cycles indicating high reusability. This study provides a simple but effective bio-based hydrogel absorbent with stable structure, ultrahigh adsorption capacity, excellent reusability and easy-separation for the purification of TC-containing biomedical wastewater.