Rapid Fabrication of Starch-Humic Acid Composite Hydrogel via an Internal Mixer for Dye Adsorption

To develop a rapid, scalable, and eco-friendly hydrogel for wastewater decontamination, an industrial internal mixer was employed to fabricate a green hydrogel adsorbent based on starch and humic acid. Starch-grafted polyacrylamide hydrogels incorporated with humic acid were rapidly synthesized within 5 min using an internal mixer as the reactor. Starch was gelatinized in situ, followed by graft polymerization with acrylamide via free-radical polymerization and cross-linking with N,N'-methylenebisacrylamide in the same reactor. Humic acid was introduced as a natural modifier to boost the dye adsorption performance of the starch-based hydrogel. The adsorption capacity for methylene blue was evaluated under different humic acid dosages, and the maximum adsorption capacity occurred at a humic acid dosage of 20 g. Additional batch experiments revealed that the adsorption capacity increased with increasing the solution pH (from 22.6 to 54.0 mg g^-1 over pH 4-12) and the initial dye concentration (from 14.6 to 34.1 mg g^-1 over 40-200 mg L^-1), while it decreased with increasing the adsorbent dosage (from 35.9 to 10.6 mg g^-1 over 0.1-0.9 g). Distinguished from conventional laboratory stirred reactors that require 2-3 h for hydrogel synthesis, the internal mixer achieves a one-pot synthesis within 5 min, showing outstanding potential for industrial large-scale production. This work provides a time-efficient, industrially compatible strategy to prepare eco-friendly starch-humic acid hydrogels, which show promising potential as sustainable adsorbents for dye-contaminated wastewater treatment.