ZnO/Cassava Starch-Based Hydrogel Composite for Effective Treatment of Dye-Contaminated Wastewater

Abstract

Industrial expansion has increased the discharge of contaminated wastewater. Wastewater can be treated by adsorption with petroleum-based hydrogels but the materials are not biodegradable and therefore cause secondary toxic waste. In this work, hydrogel composites are prepared based on non-biodegradable polyacrylamide and biodegradable materials of cassava starch (CS) and poly(vinyl alcohol). The effect of CS content on the porous structure is studied. The highest water absorption capacity of 74 g g−1 is obtained from a hydrogel 30 wt% of CS. Within 4 h, the hydrogel effectively adsorbs the cationic dyes methylene blue (MB) and crystal violet, and the anionic dyes congo red and reactive orange. The maximum adsorption capacity toward MB is 993 mg g−1. Experimental data indicate a monolayer adsorption via chemisorption. Silica-coated ZnO photocatalyst particles are synthesized via a sol–gel method and coated on the outer surface of the hydrogel. Under sunlight, the hydrogel composite degrades almost 90% of adsorbed dye. The hydrogel composite is capable of effective photodegradation for at least three cycles under artificial UV irradiation and four cycles under sunlight, but adsorption capacity remains higher than 80% at the eighth cycle. The hydrogel composite also shows antibacterial activities, indicating an additional beneficial property for industrial wastewater treatment.