Photooxidation of foramsulfuron: Effects of char substances

The photodegradation of the herbicide foramsulfuron (FRS) in simulated sunlight was studied in water and in the presence of photocatalysts and chars. The photodegradation in distilled water was rather slow and the metabolite 2-amino-4,6-dimethoxypyrimidine was the only by-product detected. In the presence of TiO2 and ZnO, a very fast photodegradation occurred affording a complete mineralization of the herbicide. The presence of a biochar (BC), derived from the pyrolysis of woodlands, in irradiated FRS solution caused a slight slowdown of the FRS photodegradation rate. Instead, the addition of a liquid hydrochar (HC), obtained by the hydrothermal carbonization of maize, halved the half-life time of the herbicide. The addition of BC to FRS solutions containing TiO2 or ZnO did not affect the herbicide degradation rate. On the other hand, the addition of HC, while not influencing significantly the FRS degradation catalysed by ZnO, slowed very much the TiO2-promoted degradation. The different trends of FRS photodegradation in the presence of BC or HC have been interpreted on the basis of the physico-chemical characteristics of the two chars. The addition of 2-propanol or EDTA, which are selective scavengers of radicals and holes respectively, to catalysed systems increased FRS half-life time. The addition of 2-propanol reduced the FRS photodegradation rate to a higher extent than EDTA, thus emphasizing the role of radicals in the photodecomposition mediated by the two semiconductors.