Electro-Fenton and photocatalytic oxidation of phenyl-urea herbicides: An insight by liquid chromatography–electrospray ionization tandem mass spectrometry

The degradation pathways exhibited by three phenyl-urea herbicides: isoproturon (ISO), chlortoluron (CHLT) and chloroxuron (CHLOX), during photocatalytic (on supported TiO2 under intense solar radiation) and electro-Fenton (EF) treatment were investigated by HPLC coupled to electrospray ionization single and tandem mass spectrometry (HPLC–ESI-MS and MS/MS). In particular, the dependence of degradation efficiency on the initial concentration ratio between substrate and Fe(III) ion was assessed for the EF treatment and a 1:1 ratio was found to be optimal. A comparison between photocatalytic and EF degradation, in terms of structures, number and evolution on a similar time scale (up to 5 h) of by-products, was then performed. Similar reactivities were found in the two cases, hydroxylation (single and multiple, with H, alkyl groups or Cl substitution, depending on the herbicide) and demethylation on the dimethylamino moiety (eventually followed by hydroxylation) being the most relevant processes in by-products generation. The scale of EF degradation efficiency for the three herbicides was almost identical to the photocatalytic one (ISO > CHLT ≈ CHLOX), yet electro-Fenton proved to be a more efficient process, generally leading to a faster further degradation of by-products.