Mineralization of 2,4-D by advanced electrochemical oxidation processes

The mineralization process for 2,4-dichlorophenoxyacetic acid (2,4-D) at pH ca. 3 has been studied by advanced electrochemical oxidation processes (AEOPs), such as electro-Fenton and photoelectro-Fenton processes, in which a Pt anode and a carbon–polytetrafluoroethylene O2-fed cathode, for in situ production of H2O2 are used. A solution of 230 ppm 2,4-D with a low salt content can be completely mineralized by the photoelectro-Fenton process at low current, whereas the electro-Fenton process leads to ca. 90% of mineralization. In both methods, 2,4-D is quickly destroyed at the same rate. The high degradation power of these AEOPs is due to the large production of oxidizing hydroxyl radicals by reaction between electrogenerated H2O2 and Fe2+ added to the solution. The higher mineralization rate found for photoelectro-Fenton is accounted for by the fast photolytic decomposition of some intermediates by UV light. Classical anodic oxidation with a graphite cathode and anodic oxidation in the presence of electrogenerated H2O2 are much less efficient methods to degrade 2,4-D and its oxidation products. 2,4-Dichlorophenol, 4,6-dichlororesorcinol, chlorohydroquinone and chlorobenzoquinone have been identified as intermediates by GC–MS and their evolution for each process has been followed by reverse-phase chromatography. Chloride ion is released from these chloroderivatives and accumulates in the medium. Short-chain acids, as glycolic, glyoxylic, maleic, fumaric and oxalic, have been detected by ion-exclusion chromatography. A general reaction pathway involving all these intermediates is proposed.