Effect of Fenton's treatment on the biodegradability of chromium-complex azo dyes

Pretreatment of an acid dyebath effluent bearing a new generation chromium complex azo dyestuff (C_o = 350 mg/L) with Fenton's reagent was investigated. Preliminary optimisation (baseline) experiments were conducted to determine the Fe²⁺, H₂O'2 concentrations and pH required to the highest possible COD and colour removals. Kinetic studies were carried out at varying temperatures (20°C < T < 70°C) to establish a relationship between COD abatement and H₂O₂ consumption. The activation energy found for catalytic H₂O₂ decomposition (E_a = 9.8 kJ/mol) appeared to be significantly less than that of fermentative (E_a = 23 kJ/mol) and of thermal (E_a = 76 kJ/mol) H₂O₂ decomposition, implying that H₂O₂ decomposition during the Fenton's reaction occurs more spontaneously. The experimental studies indicated that approximately 30% COD and complete colour removal could be achieved under optimised Fenton pretreatment conditions (Fe²⁺ = 2 mM; H₂O₂ = 30 mM; pH = 3; at T = 60 °C). Long-term activated sludge experiments revealed that although the raw and pretreated acid dyebath effluent contained practically the same amount of "readily biodegradable" COD (inert COD fraction ≤ 10%), biodegradation of the chemically pretreated acid dye effluent proceeded appreciably faster than that of the untreated acid dyebath effluent.