Application of Solar Photocatalysis and Solar Photo-Fenton Processes for the Removal of Some Critical Charged Pollutants: Mineralization Trends and Formation of Reaction Intermediates

The present study investigated the efficiency of the titanium dioxide (TiO₂)-assisted solar photocatalytic degradation (SPCD) process and the solar photo-Fenton degradation (SPFD) process for the removal of some critical charged aqueous-phase pollutants under solar radiation conditions in Dhahran. The findings revealed that phenol and its reaction intermediate compounds can be successfully removed via the SPCD process at acidic pH values via a one sun-type re-circulating plug flow-type reactor. A similar trend was noted for the solar photo-Fenton process, which also indicated high phenol degradation at pH of 4 and 6; however, at pH 10, the overall total organic carbon removal was low because of increased Fe-species precipitation and reduced ·OH radical formation. The SPCD process showed greater ammonia removal efficiency than the SPFD process. Based on the successful application of the SPCD process, we further investigated the degradation of other critical charged cationic and anionic pollutants using the SPCD process. Tetramethylammonium (TMA) SPCD-based degradation resulted in near-complete TMA degradation within a reaction time of 4 h. Furthermore, the intermediates produced by TMA solar-energized degradation showed a stepwise de-methylation of TMA into trimethylammonium, dimethylammonium, and methylammonium, followed by mineralization to NH4+/NH3 and NO3-. In general, the results of the present work confirmed successful application of the SPCD process for the removal of several important pollutants.