Heterogeneous photocatalytic treatment of simulated dyehouse effluents using novel TiO2-photocatalysts

Heterogeneous photocatalytic treatment of two different synthetic dyehouse effluents called RI and RII, that were simulating textile wastewater from an integrated plant and a dyehouse mill and containing various reactive dyestuffs and associated auxiliary chemicals, was investigated by using different TiO2 types and reaction conditions, The effect of initial pH and effluent dilution ratio on treatment efficiency was examined and photocatalytic activities of novel TiO2 materials were compared in terms of ultraviolet-visible absorption kinetics, TOC removal, and Adsorbable Organic Halogens (AOX) formation, respectively. For comparative purposes, % photonic efficiencies zeta (%) based on the photocatalytic mineralization rates were calculated for the investigated reaction conditions. The sensitivity of the treatment efficiency to reaction pH was rather pH-independent in the pH range 4-9 for both recipe mixtures and reaction rates were drastically inhibited at higher pH specially for RII effluent due to its high carbonate content. Increasing the strength of the dyehouse effluents had negative influence upon the photocatalytic reaction rates. It was also established that for RI, TOC kinetics could be fitted very well to the empirical Langmuir-Hinshelwood kinetic model (K-TOC=29 l mg(-1); k(TOC)=0.246 mg (min l)(-1)) for the Degussa P25-mediated photocatalytic treatment, whereas this model could not be applied to RII. The highest photonic efficiency was obtained with Sachtleben Mikroanatas IF9308/18 TiO2 powder for recipe mixture RI at pH=10.5 and for RII at pH=4 being 4.9 and 4.6%, respectively. Best obtained overall TOC removals were around 30% for RI at pH=7.0 (P25) and 37% for RU at pH=4.0 (Millenium PC 500 TiO2 powder) after 1 h treatment. No clear correlation existed between adsorptive properties of the investigated TiO2 semiconductors and their corresponding reaction rates. Considerable amounts of AOX was formed particularly at pH=4 after the photocatalytic oxidation as a consequence of the high chloride content in the synthetic wastewater. (C) 2000 Elsevier Science B.V. All rights reserved.