Photochemical Generation of Cl Radicals from Iron Chloride/Sodium Chloride Salt Pans

Recent research has identified a new photocatalytic mechanism that generates significant amounts of chlorine over the North Atlantic, involving iron from mineral dust, chloride from sea spray, and sunlight. While it is known that charge transfer absorption in iron chlorides (FeCl_<i>n</i>^3-<i>n</i>) reduces Fe³⁺ to Fe²⁺ and oxidizes Cl^- to neutral chlorine (Cl⁰), important questions remain regarding the chemical state of iron before and after the reaction and the mechanism of chlorine release. Brines and solid precipitates form from iron, sodium, and chloride in atmospheric particles, and iron is distributed among hydrated Fe(II) and Fe(III) chlorides, oxides, and hydroxides, depending on the concentration, pH, and humidity. In this study, we investigate chlorine production from solids prepared by evaporating aqueous solutions of iron and sodium chlorides and then subjecting the resulting solids (salt pans) to UV radiation. We characterize the chemical transformations of iron before and after photoreduction by using X-ray diffraction (XRD). The key findings are as follows: (1) Iron-free sodium chloride pans did not produce detectable chlorine. (2) A direct correlation was observed between UV irradiation and chlorine production from iron/sodium chloride salt pans (ISP). (3) The efficiency of chlorine release rate depends inversely on ambient humidity. (4) Photolysis alters the crystalline structure of iron(III) chlorides. (5) Water is released during the photoreduction of Fe(III) to Fe(II) chlorides. (6) Prior to photolysis, recrystallization from brine produces smaller FeCl₃·6H₂O crystals that rapidly decompose upon irradiation. These findings are consistent with the overall reaction: [FeCl₃·6H₂O + <i>h</i>ν → FeCl₂·2H₂O + 4H₂O + Cl].