Photocatalytic decolorization of textile effluent over ZnO nanoparticles immobilized on eucalyptus bark biochar: Parametric optimization, kinetic and economic analyses

Heterogeneous photocatalysis via combination of semiconductor-based material and light is considered one of the most promising advanced oxidation processes for degradation of non-biodegradable contaminants of drinking water and industrial effluents into harmless species. This work delves into the preparation and photocatalytic evaluation of ZnO nanoparticles doped with eucalyptus bark biochar (ZnO@EB) developed via sol-gel-hydrothermal method. Varying amounts (10–50 wt %) of ZnO nanoparticles were incorporated into the eucalyptus biochar (EB) framework, followed by hydrothermal treatment at 110 °C for 24 h, and 1.5 g of ZnO immobilized on 3.5 g of EB (30%ZnO@EB) exhibited excellent activity for photocatalytic degradation of dye in textile industry effluent. The photocatalytic decolorization of textile effluent under solar light irradiation using the 30%ZnO@EB composite was optimized. Influence of operational parameters on the decolorization efficiency of textile effluent was evaluated by the Box-Behnken design. Optimization results showed that the maximum decolorization efficiency of 94.8 $\pm$ 1.09% was achieved at the optimum conditions of 2.99 g/L photocatalyst dosage, 3.04 effluent pH and 101.7 min irradiation time. The pseudo-first-order Langmuir-Hinshelwood model with apparent rate constants of 0.029$\pm$ 0.44, 0.027$\pm$ 0.71 and 0.023$\pm$ 0.08 min⁻¹ (at effluent pH of 3, 7 and 11, respectively) excellently predicted the photocatalytic degradation kinetic. Additionally, the spent 30%ZnO@EB composite was easily separated from the treated solution and reused up to ten times for decolorization process without significant activity loss.