Synthesis and characterization of zwitterionic hydrogels with Ag@g-C3N4 for adsorption-photocatalytic removal of methylene blue and methyl orange dyes

Most hydrogels for dye treatment only have the adsorption ability for anionic or cationic dyes, while a few zwitterionic hydrogels that can adsorb both anionic and cationic dyes need a complicated regeneration process when they are reused. To prepare reusable zwitterionic adsorbent hydrogels, in this paper, a zwitterionic composite hydrogel with Ag@g-C₃N₄ was prepared, which not only can adsorb both anionic and cationic dyes but also has photocatalytic degradation ability. The scanning electron microscopy images show that the optimum hydrogel has a large number of micro pores for dye adsorption. The adsorption capacity of the hydrogel achieves the highest in mixed dye concentration of 50 mg·L^-1 with pH value of 7–9. Benefiting from the synergistic effect of adsorption and photocatalytic degradation, the removal efficiency of methyl orange and methylene blue in mixed dyes was up to 93.86% and 91.80%, respectively. The photocatalytic activity trapping experiments showed that superoxide radicals as well as hydroxyl radicals played a major role in photocatalytic degradation. After 5 consecutive photocatalytic degradations, the removal ratio of the hydrogel for dyes was still above 80%, indicating good photocatalytic degradation and reusability properties. This research suggested the zwitterionic composite hydrogel has a promising prospect in removing dyes from wastewater treatment.