Photolysis Kinetics of Toluene, Ethylbenzene, and Xylenes at Ice Surfaces

Abstract

Benzene, toluene, ethylbenzene, and xylenes (BTEX) are important organic pollutants. These compounds do not undergo direct photolysis in natural waters because their absorbance spectra do not overlap with solar radiation at the Earth's surface. Recent research has suggested that benzene is able to undergo direct photolysis when present at ice surfaces. However, the photolysis of toluene, ethylbenzene, and xylenes (TEX) at ice surfaces has not been investigated. Using fluorescence spectroscopy, photolysis rate constants were measured for TEX in water, in ice cubes, and in ice granules which reflect reactivity at ice surfaces. No photolysis was observed in water or ice cubes. Photolysis was observed in ice granules; rate constants were (4.5 ± 0.5) × 10(-4) s(-1) (toluene), (5.4 ± 0.3) × 10(-4) s(-1) (ethylbenzene), and (3.8 ± 1.2) × 10(-4) s(-1) (xylenes). Photolysis of TEX molecules appears to be enabled by a red shift in the absorbance spectra at ice surfaces, although photosensitization may also occur. The results suggest that direct photolysis could be an important removal pathway for TEX in snow-covered environments.

Effects of Chromophoric Dissolved Organic Matter on Anthracene Photolysis Kinetics in Aqueous Solution and Ice

“…5,7,9,10,14,16,36−39 Rate constants of reactions occurring in ice granules largely reflect reactions at ice surfaces due to the high surface-area-to-volume ratios, whereas rate constants acquired from experiments performed with ice cubes largely reflect reactivity in liquid water, since reagents are primarily confined to liquid inclusions within the bulk ice. 5,7,10,13,15 Several potential reasons for faster anthracene photolysis at ice surfaces compared to in aqueous solution have been investigated. Many factors, including temperature differences, solid versus liquid environments, an altered wavelength dependence, larger anthracene concentrations, changes in pH, changes in polarity, and presence of hydroxyl radicals have been reported not to be responsible for the observed differences.…”

Bathochromic Shift in the UV–Visible Absorption Spectra of Phenols at Ice Surfaces: Insights from First-Principles Calculations

“…These characteristics make snow and ice unique environments for environmentally relevant chemical reactions such as the photodegradation of pollutants, which may be transformed into more volatile molecules that can then be released into the atmosphere. , While extremely important, to date, only a few direct photochemical reactions have been studied on ice and snow. Some works show an increase in the rate of photodegradation at the air–ice interface compared to that in solution (e.g., for naphthalene, toluene, ethylbenzene, and xylene), , while others show that photodegradation proceeds at similar rates at the air–ice interface and in solution (e.g., for nitrate, nitrite, hydrogen peroxide, anthracene, pyrene, and fluoranthene). ,− A possible reason for the observed photodegradation enhancement for molecules solvated at the air–ice interface is that their molar absorptivities are shifted to lower energy (bathochromic shift), where there is a higher sunlight photon flux. This is a hypothesis that has been previously examined for a variety of chemicals. ,− Since the summer polar actinic flux at the surface of the Earth increases by a factor of approximately 1 million between 290 and 310 nm, compounds that absorb light below this range of wavelengths would normally not be readily photolyzed by sunlight in solution, but could undergo degradation in snow if there is a bathochromic shift in their absorbance toward wavelengths with higher photon flux.…”

“…4,5 While extremely important, to date, only a few direct photochemical reactions have been studied on ice and snow. Some works show an increase in the rate of photodegradation at the air−ice interface compared to that in solution (e.g., for naphthalene, toluene, ethylbenzene, and xylene), 6,7 while others show that photodegradation proceeds at similar rates at the air−ice interface and in solution (e.g., for nitrate, nitrite, hydrogen peroxide, anthracene, pyrene, and fluoranthene). 8,11−13 A possible reason for the observed photodegradation enhancement for molecules solvated at the air−ice interface is that their molar absorptivities are shifted to lower energy (bathochromic shift), where there is a higher sunlight photon flux.…”

Physical Characterization of Frozen Saltwater Solutions Using Raman Microscopy

“…Many physical and chemical processes occur very differently at liquid and frozen water surfaces. For example, a number of aromatic pollutants have been reported to photolyze more rapidly at ice surfaces than at liquid water surfaces, and some aromatic pollutants that do not photolyze in aqueous solution can photolyze at ice surfaces. − Heterogeneous reaction kinetics have also been reported to differ at frozen and liquid water surfaces: ozonation of several aromatic species has been reported to be much faster at ice surfaces than at liquid water surfaces, while hydroxyl radicals (OH), which react rapidly with aromatic species at liquid water surfaces, have been reported to be unreactive toward a range of aromatic species at ice surfaces. ,− If solutes cause the surface to be coated in a liquid “brine”, then these physical and chemical processes will occur as though the surface is a liquid rather than a solid and can be modeled as such. There is some evidence that this might be the case: the photolysis rate constant of the aromatic dye harmine was larger at frozen freshwater surfaces than in aqueous solution, but no enhancement compared to in aqueous solution was observed at the surface of frozen NaCl solutions when initial NaCl concentrations exceeded 0.2 M. These results were interpreted as indicating that the reaction environment experienced by harmine transitioned from that of an ice surface to a liquid brine as the NaCl concentration increased …”