Pretreatment of an acid dyebath effluent bearing a new generation chromium complex azo dyestuff (Co=350 mg/L) with Fenton's reagent was investigated. Preliminary optimisation (baseline) experiments were conducted to determine the Fe2 + , H2O2 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 (Ea=9.8 kJ/mol) appeared to be significantly less than that of fermentative (Ea=23 kJ/mol) and of thermal (Ea=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.

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