Fabrication of starch-based high-performance adsorptive hydrogels using a novel effective pretreatment and adsorption for cationic methylene blue dye: Behavior and mechanism

Herein, NaOH solution of a finite concentration was used to pretreat starch, which serves a novel effective pretreatment. A starch-based high-performance adsorptive hydrogel (STAH) was successfully prepared by grafting polyacrylic acid (PAA) onto starch and then crosslinking with N,N'-methylene-bisacrylamide (MBA). This adsorbent was employed to adsorb typical organic cationic dye contaminants (methylene blue (MB)) from high concentration effluents. The effect of the pretreatment time of starch on adsorption capacity of STAH was investigated in detail based on starch molecular weight, solid state C-13 nuclear magnetic resonance spectra and grafting parameters. The results suggested that the optimal adsorbent (STAH20) presented the highest grafting parameters (50.06% reaction ratio of C6 in starch, 90.79% grafting efficiency, and 248.49% grafting ratio of PAA). STAH(20) exhibited maximum adsorption capacity (2967.66 mg/g) in optimal conditions and excellent reusability after 4 rounds of recycling. The adsorption process of STAH(20) for MB was consistent with the Langmuir model, obeyed the pseudo-first order model with intraparticle diffusion, and was endothermic and spontaneous. Using this pretreatment, the semi-crystalline structure of starch was destroyed, and numerous hydroxyl groups were gradually exposed with time, which is conducive to the graft reactions. The number of -COOH/ -COO- groups increased, which significantly enhances the hydrogen-bonding interaction and electrostatic interaction between STAH and MB and improves the adsorption capacity. Our study provides a method for low-energy, inexpensive, effective and simple starch pretreatment for the fabrication of STAH that exhibits significant potential for application in the removal of organic cationic dye contaminants.