Combined photocatalytic and biological process for textile wastewater treatments.

Azo dyes, common in textile wastewater, have high photolytic and chemical stabilities, which make them difficult to be removed using conventional treatments. This study aims to evaluate a combined process using heterogeneous photocatalysis, with ZnO/UV or TiO2 /UV (0.6 g catalyst L-1 solution/2-hr UV radiation), and a biological process for textile wastewater treatment. After the proposed treatments, the color and organic matter removals from synthetic wastewater (SW) and industrial wastewater (IW) were evaluated. For SW, the coupled photocatalytic (ZnO/UV or TiO2 /UV)-biological system promoted a high extent of color removal (98%) and total organic carbon (TOC) reduction (>80%). Promising results were obtained with IW using combined photocatalytic (TiO2 /UV)-biological treatments, reaching 97% and 63% of color and TOC removal, respectively. This process, coupling heterogeneous photocatalysis and a bioprocess, has proved to be a good alternative for the treatment of textile wastewater, not only for color removal but also for dye mineralization purposes. PRACTITIONER POINTS: A combined process using heterogeneous photocatalysis (ZnO/UV or TiO2 /UV) and biological process was evaluated for synthetic (SW) and industrial (IW) textile wastewaters treatment. For SW, coupled process promoted high extent of colour and organic matter removals. For IW, promising results were obtained with TiO2 /UV-biological treatment (97% of colour and 63% of organic matter removals).

Decolorization and mineralization of reactive dyes by a photocatalytic process using ZnO and UV radiation.

Reactive dyes are one of the major pollutants in textile wastewater and a concern because they are not easily degraded by conventional wastewater treatments. Heterogeneous photocatalysis has been considered an effective option for treating wastewater containing those dyes. This research work assesses the photocatalytic degradation of reactive dyes using UV irradiation and pure or impregnated ZnO. In addition to photocatalysis, separate photolysis and adsorption experiments were conducted but showed low efficiency. The dye degradation was monitored by UV-Vis spectroscopy and mineralization was determined by total organic carbon (TOC) analyses. Total color removal was achieved after 30 min of irradiation using pure ZnO. The Black 5 dye photocatalytic decolorization reaction followed first-order kinetics, while Yellow 145, Red 4 and Blue 21 dyes followed zero-order kinetics. TOC removals in the range of 70-80% were achieved after 240 min of individual photocatalytic treatment with ZnO. The performance of each photocatalyst was also compared when the four dyes were mixed together and the order of efficiency in the mineralization process was as follows: Fe/ZnO > ZnO > Co/ZnO. This result was explained by the crystal field theory.