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/Fe(3+)) could achieve synergistic degradation of reactive black 5 (RB5), as compared to UV/Fe(3+) and US/Fe(3+) systems. A synergy factor of 2.5 based on the pseudo-first-order degradation rate constant (k(obs)) was found, along with enhancements in organic detoxification and mineralization. The presence of organic ligands could affect the US/UV/Fe(3+) 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 k(obs)(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 k(obs)(ligand) order of oxalate > citrate > tartrate > succinate > NTA > EDTA. Monitoring of iron species and the generated H(2)O(2) and •OH revealed that the ligands in the US/UV/Fe(III)-ligand system could play different mechanistic roles: (1) promoting H(2)O(2) 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 Fe(2+) was used in lieu of Fe(3+) as the catalyst in the sonophotolytic system.