Fabrication of all-starch-based hydrogels as eco-friendly water-absorbent resin: Structure and swelling behaviors

Water-absorbent resin has gained wide applications due to the capability in absorbing and retaining substantial amounts of water, while it's a challenge to fabricate a full biobased water-absorbent resin with excellent biodegradability and eco-friendliness. In this study, starch was sulfonated (SS) and crosslinked with epichlorohydrin to fabricate all-starch-based hydrogels (SSH) as water-absorbent resin with advantages of intrinsic biodegradability and low cost. The results confirmed that the hydrogen atoms of -OH groups in starch chains were partially replaced by -SO₃^- and the substitution degree (DS) of SS reached 0.008-0.344. By controlling DS and gelation process of SS, the swelling ratio (Q_e) of SSH was improved in distilled water, reaching 244.47 g/g for samples prepared using SS with medium DS (SS_MDSH). SS_MDSH showed relatively loose network structure with low cross-linking density and large pore size. Meanwhile, -SO₃^- groups on SS_MDSH chains facilitated strong ion-dipole interactions with water molecules, resulting in an increase in content of non-freezing bound water within hydrogels and thus improvement in water absorption capacity. Besides, SSH showed desired fertilizer absorption performance and complete biodegradability in α-amylase solution, which made it to be a promising candidate in agricultural fields as eco-friendly water-absorbent resin.