Effective degradation of primary color direct azo dyes using Fe0 aggregates-activated persulfate process.

The present study examined the oxidation power of a Fe0 aggregates/persulfate (PS/Fe0) system for the degradation of the wastewater containing mixed primary direct dyes (i.e., Sirius® Gelb S-2G, Sirius® Red F3B, and Sirius® Turkis GL01). Results indicated that decolorization efficiency was determined by operating parameters of the PS/Fe0 system and the structural complexity of dye molecules. System efficiency increased with increasing persulfate and Fe0 dosages. Faster decolorization was observed in experiments conducted at pH < 10. The process obeyed a first-order kinetics. Slow heterogeneous reactions were observed at high initial pH (>10.5) and low PS concentration (<2 × 10-3 M). Inhibitory effect occurred in systems containing salts Na2SO4, NaCl, Na2CO3, and Na2HPO4 at 1 × 10-2 M. The effect was suppressed when reaction temperature was raised to 55 °C. Heat enhanced not only decolorization efficiency, but also COD removal. Complete decolorization of a mixed dye containing ADMI (the American Dye Manufacture Institute) 15105 was achieved within10 min in the PS/Fe0/55 °C system with an initial pH of 6.0 and dosages of 5 × 10-3 M Na2S2O8 and 0.5 g/L Fe0. Low molecular weight intermediates including organic acids were identified. Due to a relatively low activation energy (4.68 kcaL/mol), the PS/Fe0 system exhibited higher efficiency at higher temperature. This study demonstrated that Fe0-activated PS is a promising process for the treatment of textile wastewaters containing mixed azo direct dyes.