Environmental photochemistry of iron complexes and their involvement in environmental chemical processes

Iron is one of the most abundant metals in the continental crust, while most of dissolved iron is complexed with organic ligands. The irradiated iron complexes in the environment undergo direct photolysis and secondary (photo)chemical reactions, generating Fe(II), organic radicals and some reactive oxygen species (ROS). Environmental photochemistry of iron complexes can greatly affect ROS dynamics, organics degradation and redox cycling of other elements. Therefore, it is becoming a hot topic in the international field of environmental sciences research. This review firstly summarizes three types of iron complexes including inorganic Fe complexes, simple organic Fe complexes and macrocyclic organic Fe complexes, and photoreduction mechanisms of iron complexes. Secondly, the potential oxidants of Fe(II) in acidic aquatic environment are introduced. The oxidation kinetics, possible reaction mechanism and influencing factors (such as dissolved oxygen concentration, pH, ionic strength, temperature and natural organic matters concentrations) of (photo)chemical oxidation of Fe(II) are elucidated. This review also highlights recent findings in the study of environmental processes involving iron photochemistry. Finally, the future prospects in this field are discussed based on the current status.