Photocatalytic degradation and chlorination of azo dye in saline wastewater: kinetics and AOX formation

Chloride ion as one of the major salt components in dyestuff wastewaters can greatly affect the homogenous OHOH radical-based advanced oxidation processes (AOPs), but little information is available now for heterogeneous AOPs like UV/TiO₂ under similar conditions. Here the effects of chloride on kinetics and reaction intermediates of dye degradation in heterogeneous photocatalysis were examined. The dye degradation efficiency in UV/TiO₂ process was investigated as a function of a wide range of salinity and pH. The chloride ion was found to have a dual effect on both the dye decoloration and mineralization in UV/TiO₂ system due to different mechanisms involved. Higher Cl⁻ concentration would inhibit dye degradation, especially in acidic medium. AOX (halogenated organic compounds adsorbable on activated carbon) generated increased with the increasing content of chloride ion present in the solution. Several toxic chlorinated byproducts were firstly identified during the dye degradation in UV/TiO₂ process using GC–MS method and possible decomposition pathways were proposed. These findings may have significant technical implications for optimizing the photochemical technologies in salt-rich wastewater treatment.