Photolytic degradation of molecular iodine adsorbed on model SiO2 particles

Abstract

A molecular derivatization method followed by gas chromatographic separation coupled with mass spectrometric detection was used to study photolytic degradation of I2 adsorbed on solid SiÜ2 particles. This heterogeneous photodegradation of I2 is studied at ambient temperature in synthetic air to better understand I2 atmospheric dispersion and environmental fate. The obtained laboratory results show a considerably enhanced atmospheric lifetime of molecular iodine adsorbed on solid media. The heterogeneous atmospheric residence time (x) of I2 is calculated to be x»187 minutes, i.e., x»3 hours. The obtained heterogeneous lifetime of I2 is shown to be considerably longer than its destruction by its principal atmospheric sink, namely, photolysis. The observed enhanced atmospheric lifetime of I2 on heterogeneous media will likely have direct consequences on the atmospheric transport of I2 that influences the toxicity or the oxidative capacity of the atmosphere.

Photodegradation of Molecular Iodine on SiO2 Particles: Influence of Temperature and Relative Humidity

“…This represents a Brobdingnagian number of potential surfaces for gas-phase I2-particle heterogeneous reactivity, a hypothesis that has been supported by early laboratory experiments [19][20][21][22] that show that gas phase I2 readily adsorbs on solid atmospheric particles. Further, we showed in a recent study that the photodegradation rate of molecular iodine on solid media was very different (much lower) from the photodegradation rate in the homogeneous phase [23]. This difference in reactivity will likely have a direct impact on the long-range gas-phase I2 atmospheric transport and reactive fate [24,25].…”

“…A schematic representation of the experimental system used in this work is published elsewhere [23]. A 500 cm 3 Pyrex bulb attached to a modified rotary evaporator (Laborota 4000 efficient, Heidolph) was used to study the heterogeneous photodegradation of molecular iodine adsorbed on solid SiO2 particles in the presence of solar radiation in the ultraviolet and visible region of the solar spectrum [23]. Under typical experimental conditions, about 500 mg of dried SiO2 particles coated with molecular iodine was placed within the Pyrex bulb.…”

“…To simulate the complex physical and chemical nature of atmospheric aerosol content, AEROSIL R812 silica particles were used [23] to study the influence of the temperature and relative humidity on the heterogeneous photo-degradation of I2. These model particles were chosen because it has been estimated that as much as 20 to 30% of the atmospheric particles' content includes SiO2.…”

“…Molecular iodine was coated on the SiO2 particles surface using a liquid/solid adsorption method as reported in our earlier work [23]. The particles coating was carried out in the absence of UV visible radiation to minimize any photo-degradation of molecular iodine.…”

“…The apparatus and the experimental protocol used to study the photo-degradation of molecular iodine on solid media has been documented before [23]. Therefore, only details relevant to this work are given below.…”