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.