Photocatalytic degradation of concentrated solutions of 2,4-dichlorophenol using low energy light: Identification of intermediates

This study reports on Fenton and photo-assisted Fenton induced oxidation of 2,4-dichlorophenol (from here on 2,4-DCP). The pre-treatment of concentrated aqueous solutions of 2,4-DCP (453mgl−1) up to bio-compatibility is shown to be possible using low energy 36W black fluorescent mercury light. The degradation process of 2,4-DCP was observed to occur within minutes. Catalytic amounts of Fe3+-ion (10mg) were only necessary to obtain acceptable degradation kinetics. Degradation of the 2,4-DCP was not observed in the dark. The total organic carbon (TOC), the adsorbable organic halogen compounds (AOX) and the amount of free chloride (Cl−) were followed in time during 2,4-DCP oxidation under light irradiation. The rate of disappearance of 2,4-DCP and that of Cl−-ion formation were not similar. This was ascribed to the formation of aliphatic and aromatic chlorinated long-lived intermediates. The latter intermediates were identified by liquid–liquid extraction, solid–liquid extraction and coupled GC/MS analysis. Based on the experimental results obtained, a mechanism for the radical intervention is suggested for the abatement of 2,4-DCP. A relatively short pre-treatment time (54min) was necessary to reduce by 97% the toxicity (ecotoxicity) of a concentrated solution of 2,4-DCP under low energy light irradiation in the photo-reactor.