Treatment and toxicity evaluation of methylene blue using electrochemical oxidation, fly ash adsorption and combined electrochemical oxidation-fly ash adsorption

Treatment of a basic dye, methylene blue, by electrochemical oxidation, fly ash adsorption, and combined electrochemical oxidation-fly ash adsorption was compared. Methylene blue at 100mgL^-1 was used in this study. The toxicity was also monitored by the Vibrio fischeri light inhibition test.When electrochemical oxidation was used, 99% color and 84% COD were removed from the methylene blue solution in 20min at a current density of 428Am^-2, NaCl of 1000mgL^-1, and pH₀ of 7. However, the decolorized solution showed high toxicity (100% light inhibition).For fly ash adsorption, a high dose of fly ash (>20,000mgL^-1) was needed to remove methylene blue, and the Freundlich isotherm described the adsorption behavior well.In the combined electrochemical oxidation-fly ash adsorption treatment, the addition of 4000mgL^-1 fly ash effectively reduced intermediate toxicity and decreased the COD of the electrochemical oxidation-treated methylene blue solution. The results indicated that the combined process effectively removed color, COD, and intermediate toxicity of the methylene blue solution.