Enhancement of the biodegradability of a mixture of dyes (methylene blue and basic yellow 28) using the electrochemical process on a glassy carbon electrode

The coupling of an electrochemical process with a biological treatment for the degradation of methylene blue (MB) and basic yellow 28 (BY28) considered separately or in mixture on a glassy carbon electrode was examined in this study. It was shown that color removal efficiency and mineralization yield of MB, BY28, and their mixture increased with the working potential and decreased with the initial dye concentration. The optimal conditions were found to be E=2.4V/SCE, [MB](0)=50mg L-1, [BY28](0)=50mg L-1, pH=2, T=25 degrees C, and =600rpm, which led to 100% color removal after 120 and 240min of reaction time for BY28 and MB, respectively. Under these optimal conditions, the mineralization yield of BY28, MB, and their mixture (50mg L-1 of each dye) was close to 59, 57, and 54% within 360min of reaction time, respectively. The biological oxygen demand (BOD5)/chemical oxygen demand (COD) ratio increased substantially after 360min of pre-treatment from 0.04 to 0.27 for the dyes mixture. Microbial degradation was therefore performed for the pre-treated mixture solution and the results showed significant mineralization yield leading to an overall dissolved organic carbon decrease of 78% for the coupled process. It was therefore shown the presence of residual refractory compounds at the end of the culture which was illustrated by the decrease of the BOD5/COD ratio (0.045) obtained for the final solution. However, biodegradability was improved after a recycling of the solution in the electrochemical oxidation pre-treatment during 180min leading to a BOD5/COD ratio of 0.73.