Electro-reductive deiodination of iohexol catalyzed by vitamin B12 and biodegradability investigation

[Display omitted] •Electroreduction of iohexol led to its total deiodination.•Vitamin B12 catalyzes the electroreduction reaction.•Vitamin B12 leads to a faster reaction and improved current efficiency.•Total dehalogenation of iohexol was not sufficient to improve its biodegradability.•Stable oxidized by-products were formed during the biological treatment of iohexol. Owing to their high consumption for X-ray diagnose and to their rapid excretion from organism, iodinated X-ray contrast media (ICM) are often found in source waters. It is a serious public health concern due to their potential to generate highly toxic disinfection by-products. Reductive deiodination of ICM is considered as a promising approach to degrade them and allows the possible recovery of iodide ions. Electrocatalytic reduction of iohexol, an ionic ICM, was performed in the presence of vitamin B12 as catalyst. Vitamin B12 was chosen owing to its catalytic activity toward dehalogenation processes and to its positive effect on biodegradation of pollutants. The conditions for electrocatalysis were optimized to favor the electrocatalytic process over the direct reduction, leading to the enhancement of the reaction kinetic and the current efficiency. A total deiodination of iohexol was achieved after 1 h of electrolysis. A biological treatment with activated sludge underlined a low decrease of the dissolved organic carbon even after electroreduction. However, the concentration of iohexol decreased and a significant biotransformation of iohexol occurred, slightly improved by the presence of vitamin B12. The identification of the main by-products after the biotransformation of iohexol highlighted the oxidation of the primary and secondary alcohols of the alkyl chains.