Double-function ZnO/starch biodegradable hydrogel composite for methylene blue adsorption and photocatalytic...

An eco-friendly material for the removal of dyes from wastewater was developed. Biodegradable polymers (BP), cassava starch and poly(vinyl alcohol), were used to replace polyacrylamide. The hydrogel containing 50 wt% of BP (BP50) could absorb 34 times its dry weight of water. The hydrogel could adsorb Zn²⁺ and ZnO photocatalyst particles could be formed via a simple precipitation method. The incorporation of ZnO did not affect the adsorption efficiency of the ZnO/BP50 hydrogel composite towards methylene blue (MB). At initial concentrations (C_o) below 4500 mg/g, the hydrogel composite removed ∼99 % of MB from solution in 3 h. The highest adsorption capacity of 1170 mg/g was obtained when C_o was 6000 mg/g and at a dose of 0.10 g/20 mL. The hydrogel composite degraded 95 %-98 % of adsorbed MB at rates of 0.19 h^-1 and 1.77 h^-1 under UV irradiation and sunlight, respectively, with exposure times of 16 h for UV but only 2 h for sunlight. The material remained effective for at least 10 cycles of photodegradation under sunlight and removed 86 % of MB in solution on the 10^th cycle. The composite also showed antibacterial activities and biodegradability in soil. These results indicated this material would not generate after-process toxic waste.