Abstract:

Objective A large amount of industrial wastewater is generated in the dyeing process. The dye molecules not only cause the water coloration to deepen sharply, damaging the ecological landscape of the water body, but also hinder the photosynthesis of phytoplankton in the water body, thereby triggering the problem of water eutrophication, which seriously threatens the balance of the aquatic ecosystem. Therefore, it is of significance for developing efficient, environmentally friendly, and low-cost wastewater treatment technologies for the dyeing industry. Specifically, modified starch-based flocculants were prepared using cassava starch as the raw material and then combined with polyaluminum chloride (PAC) for the treatment of simulated dyeing wastewater.

Method A new type of modified starch-based flocculant was prepared using cassava starch, which is widely available, inexpensive and biodegradable, during which introducing two monomers, acrylamide (AM) and sodium acrylate (AANa), were introduced into the starch molecular chain through graft copolymerization reaction. Through single-factor experiments, the dosage of initiator, the molar ratio of AM to AANa, the reaction temperature and reaction time were optimized to ensure the optimal structure and performance of the flocculant. Analytical characterization techniques such as Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ¹³C-NMR were utilized to characterize the molecular structure, chemical composition,and microscopic morphology of the modified flocculant, and to verify the effectiveness of the graft copolymerization reaction. Three commonly used disperse dyes, namely dispersed red FB, dispersed blue E-4R and dispersed yellow RGFL, were selected and mixed in a certain proportion to prepare simulated printing and dyeing wastewater. Through static flocculation experiments, with turbidity removal rate and color removal rate as the core evaluation indicators, the influences of flocculant dosage, wastewater pH value and reaction temperature on the flocculation effect were investigated, and the synergistic effect of PAC and the modified starch-based flocculant was explored.

Results When the initiator mass fraction was 0.6%, the molar ratio of AM to AANa was 1∶0.75, the reaction temperature was 70℃ and the reaction time was 3 h, the flocculation performance of the prepared flocculant was the best. When PAC and the modified flocculant were used in combination, the turbidity and color removal rates of disperse dyes in the simulated dyeing wastewater reached over 90% at the pH value of 3-7 and at 20-40℃.When the flocculant dosage was 120 mg/L, the pH value was 6 and the temperature was 20℃, the treatment effect of the simulated dyeing wastewater was the best, with a turbidity removal rate of 99.28% and a color removal rate of 98.34%.

Conclusion The anionic starch-based modified flocculant prepared from cassava starch by graft copolymerization with AM and AANa exhibits excellent flocculation performance under optimized synthesis conditions. Its combined use with PAC can efficiently remove turbidity and color from simulated dyeing wastewater within a broad range of pH and temperature, and achieves the optimal treatment effect under the condition of flocculant dosage 120 mg/L, pH value 6 and temperature 20 ℃, which provides an effective approach for solving the pollution problems caused by dyeing wastewater.

Key words: cassava starch, acrylamide, sodium acrylate, disperse dye, flocculation performance, flocculant, printing and dyeing wastewater, wastewater treatment