Photochemical versus coupled photochemical-biological flow system for the treatment of two biorecalcitrant herbicides: metobromuron and isoproturon

Physicochemical pre-treatment for the destruction of two non-biodegradable herbicides (metobromuron and isoproturon) were conducted by different advan ced oxidation processes, i.e. the systems UV/TiO2, UV/TiO2/H2O2, Fe3+/H2O2, and UV/Fe3+/H2O2. The degradation rates were always higher for the photo-F enton reactions when compared with the heterogeneous photocatalytic systems . Optimal concentrations of Fe3+ and H2O2 for the abatement of both herbici des in the photo-Fenton system were found. During the phototreatment, exper iments were made to obtain information concerning the evolution of: (a) org anic carbon and initial compound concentration, (b) the oxidation state, (c ) ions, (d) the toxicity, (e) the biodegradability, and (f) the chemical na ture of the intermediates. These analysis show that the solution resulting from the photodegradation of metobromuron is not appropriate for a biologic al treatment, probably due to the formation and the accumulation in the bul k of bromide intermediates, which are highly bio-recalcitrant. In contrast, the solution resulting from the phototreatment of isoproturon is biologica lly compatible and its complete mineralization can be performed by biologic al means. A combined photo-Fenton and biological flow reactor for the degra dation of isoproturon was operated in continuous mode at laboratory scale. In this coupled system, 100% of the initial concentration of isoproturon an d 95% of TOC were removed. Some field experiments under direct sunlight wer e carried out at the Plataforma Solar de Almeria in Spain. The obtained res ults demonstrated that the solar catalytic treatment is effective for the p urification of water contaminated by the herbicides under study. (C) 2000 E lsevier Science B.V. All rights reserved.