The role of sunlight in improving the performance of Fe3+/S(IV) process under natural reoxygenation: Optimized oxygen utilization and enhanced reactive species generation

The efficiency of Fe3+/S(IV) process is always limited by the easy depletion of oxygen, waste of reagents and strict acidic condition. Herein, the influence of solar energy on Fe3+/S(IV) process under natural reoxygenation was investigated to degrade a typical emerging contaminant, bisphenol A (BPA), mainly focusing on the improvement of O-2 utilization and reactive species variation. Both simulated sunlight (SSL) and real sunlight (SL) could enhance the regeneration of Fe2+, thereby largely reducing the iron demand and increasing the utilization rate of oxygen and sulfite. The effective pH range could also be broadened to neutral by SSL. Moreover, both SO4 circle- and circle OH deriving from SO3 circle- were assumed as the main contributors to BPA degradation. High-valence Fe (IV) contributing a minor role to BPA removal, was for the first time found in Fe3+/S(IV) and SSL/Fe3+/S(IV) processes. Additionally, SSL could not only promote the generation of circle OH, SO4 circle- and Fe(IV), but also increase the ratio of [circle OH] (ss) to [SO4 circle-](ss). Besides, the steady-state concentrations of circle OH ([circle OH](ss)) and SO4 circle- ([SO4 circle-](ss)) were determined to be 1.30 x 10(-13) M and 6.44 x 10 13 M, respectively, in SSL/Fe3+/sulfite process, increasing by 17 and 7 times than that in Fe3+/S(IV) process. Finally, both the potential mechanisms in SSL/Fe3+/S(IV) process and BPA degradation pathways were proposed. This study may provide new insights into the mechanisms involved in SL/Fe3+/S(IV) process and a promising alternative for efficient removal of ECs in water.