Solar-assisted photodegradation of pesticides over pellet-shaped TiO2-kaolin catalytic macrocomposites at semi-pilot-plant scale: Elucidation of photo-mechanisms and water matrix effect

The behavior of novel pellet-shaped bulky titania-kaolin photocatalysts is herein evaluated for its application in the solar-assisted photocatalytic treatment of a bio-recalcitrant pesticide mix (imidacloprid, pyrimethanil and methomyl) in solar lab-scale and semi-pilot scale raceway pond photoreactors. The photocatalyst consists of 1 cm long titania-kaolin macro-composites that combine some advantages of both nanoparticulate and supported catalysts without being either. This opening the opportunity to extend the use of this photocatalyst to complex wastewater effluents. In addition, their stability and reusability potential was also evaluated, and reactive species leading to the solar-degradation of each pesticide were also identified using radical scavengers. Total removal of pesticides (2 mg L-1 each) with an optimal pellets dosage of 34.8 g L-1 was addressed, whether in ultrapure water (180-270 min of solar irradiation) or in two basic pH Municipal Wastewater Treatment Plant (MWWTP) effluents (<= 300 min of solar irradiation) where methomyl was always the most recalcitrant pesticide. Lower Total Organic Carbon (TOC) removal (approximate to 53-56%) was also found under the effect of these complex MWWTP effluents than in ultrapure water (>60%). Those results are very promising when comparing to the almost negligible TOC removal achieved with the Titania powder under basic MWWTP effluents due to the strong effect of particle aggregation. Very good photocatalyst stability and durability was shown along three consecutive cycles after applying a low-cost recovery protocol consisting of several washing and drying cycles without addressing a significant photoefficiency loss (TOC approximate to 60-56%). The application of scavengers revealed that hydroxyl radical generated from photoinduced holes was the dominant species in the degradation of pyrimethanil and methomyl whereas reactive oxygen species formed from conduction band electrons were mainly involved in the photo-oxidation of imidacloprid.

Automated summary

The behavior of novel pellet-shaped bulky titania-kaolin photocatalysts in the solar-assisted photocatalytic treatment of a bio-recalcitrant pesticide mix was evaluated. The results showed promising removal of pesticides and good stability and durability of the photocatalyst. The identification of reactive species and degradation mechanisms of different pesticides were also investigated.