Revealing Inaccuracies in Using Chemical Oxygen Demand to Evaluate Azo Dye Mineralization: A Case for Control Experiments.

The degradation of azo dyes using a wide range of techniques including photocatalysts, electrochemical methods, and microbes has been employed toward resolving azo dye pollution. The efficiency of the systems designed for azo dye degradation is often evaluated using chemical oxygen demand (COD). COD values of a series of azo dyes were determined by following the Standard Methods for the Examination of Water and Wastewater Chemical Oxygen Demand Method 5220D (SM5220D) using five water-soluble azo dyes: chrysoidine G, methyl orange, basacryl red, Bismarck brown Y, and acid violet 3. A control experiment was designed, referred to as the thermal digestion control COD method, in which heating in concentrated sulfuric acid was first performed without the addition of the oxidizing agent used in COD experiments, potassium dichromate. After thermal digestion was completed, the oxidizing agent, potassium dichromate, was added to determine the COD value of the thermally digested products. Comparison of the COD values obtained from thermal digestion control experiments to SM5220D COD showed inconsistent values between these two methods, suggesting that there are multiple degradation pathways during the digestion process that can result in artificially low COD values. Such artificially low COD values would incorrectly suggest that a method of degradation mineralized more dyes than it actually did. These results indicate that caution should be used when reporting COD values to determine the efficiency of azo dye degradation systems. The authors recommend that the control experiment developed here be performed first to inform researchers if the COD is appropriate to use with the chosen azo dyes. Only in this way can newly developed degradation systems be more accurately assessed.