Photochemical behavior of ferrihydrite-oxalate system: Interfacial reaction mechanism and charge transfer process

Heterogeneous photochemical reactions associated with natural iron (hydr)oxides and oxalic acid have attracted a great deal of scientific attention in the application of organic pollutants degradation. However, the reaction mechanism is still unclear due to the complicated iron cycles and reactive oxygen species (ROS) generation. In this study, the in situ attenuated total reflectance-Fourier transform infrared spectroscopy was implemented to investigate the adsorption process and photochemical behavior of oxalic acid on the surface of ferrihydrite. A comprehensive reaction mechanism from the perspective of charge transfer process, including homogeneous-heterogeneous iron cycling and ROS generation, was illustrated in detail. We found that oxalic acid was first adsorbed on the surface of ferrihydrite with a mononuclear bidentate binding geometry. Interestingly, this mononuclear bidentate complex on the surface of ferrihydrite was stable under visible light irradiation. Subsequently, the whole complex departed from ferrihydrite surface through non-reduction dissolution with the form of Fe(C2O4)(+). In the solution, the Fe(C2O4)(+) complexes would quickly convert to Fe(C2O4)(2)(-) complexes. Under visible light irradiation, the electrons generated from the photolysis of Fe(C2O4)(2)(-) complex reacted with O-2 to form O-2(center dot-)/center dot OOH. Meanwhile, Fe(III) was reduced to Fe(II). Finally, the produced O-2(center dot-)/center dot OOH could react with Fe(II) through a one-step way to generate center dot OH, which possessed higher center dot OH formation efficiency than that through the two-step way of H2O2 as the intermediates. This study helps us with understanding of in-situ photochemical reaction mechanism of ferrihydrite-oxalic acid system, and also provides guidance to effectively utilize widespread iron (hydr)oxides and organic acids in natural environment to develop engineered systems for water treatment. (C) 2019 Elsevier Ltd. All rights reserved.