Sonophotolytic degradation of azo dye reactive black 5 in an ultrasound/UV/ferric system and the roles of different organic ligands

The sonophotolytic advance oxidation system (US/UV/Fe3+) could achieve synergistic degradation of reactive black 5 (RB5), as compared to UV/Fe3+ and US/Fe3+ systems. A synergy factor of 2.5 based on the pseudo-first-order degradation rate constant (kobs) was found, along with enhancements in organic detoxification and mineralization. The presence of organic ligands could affect the US/UV/Fe3+ system differently. Oxalate, citrate, tartrate and succinate could enhance the RB5 degradation, while NTA and EDTA exhibited strong inhibitions. The influence of these ligands on kobs(RB5) in the US/UV/Fe(III)-ligand systems followed the sequence of oxalate > tartrate > succinate > citrate > without ligand > NTA > EDTA, while they could be degraded simultaneously with the kobs(ligand) order of oxalate > citrate > tartrate > succinate > NTA > EDTA. Monitoring of iron species and the generated H2O2 and
• OH revealed that the ligands in the US/UV/Fe(III)-ligand system could play different mechanistic roles: (1) promoting H2O2 production, (2) accelerating Fenton reaction, and (3) competing with RB5 for reacting with
• OH. Among the ligands, oxalate exhibited the most significant enhancement of RB5 oxidation in the sonophotolytic system, and the process was pH-dependent. An initial reaction lag in RB5 degradation was observed when Fe2+ was used in lieu of Fe3+ as the catalyst in the sonophotolytic system.

► Synergy in RB5 degradation was achieved in the sonophotolytic US/UV/Fe3+ system.
► Organic ligands affected differently the US/UV/Fe3+ system.
► Polycarboxylic ligands enhanced RB5 degradation, while amino ligands showed strong inhibitions.
► Oxalate significantly enhanced RB5 degradation in the US/UV/Fe3+ system.
► The US/UV/Fe(III)-oxalate system was pH-dependent.