Degradation of organic pollutants in/on snow and ice by singlet molecular oxygen (<sup>1</sup>O*2) and an organic triplet excited state

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Snowpacks contain a variety of chemicals, including organic pollutants such as toxic polycyclic aromatic hydrocarbons (PAHs). While PAHs undergo photodegradation in snow and ice, the rates of these reactions remain in debate. Some studies report that photochemical reactions in snow proceed at rates similar to those expected in a supercooled aqueous solution, but other studies report faster reaction rates, particularly at the air−ice interface (i.e., the quasi-liquid layer, or QLL). In addition, one study reported a surprising nonlinear dependence on photon flux. Here we examine the photodegradation of two common PAHs, anthracene and pyrene, in/on ice and in solution. For a given PAH, rate constants are similar in aqueous solution, in internal liquid-like regions of ice, and at the air−ice interface. In addition, we find the expected linear relationship between reaction rate constant and photon flux. Our results indicate that rate constants for the photochemical loss of PAHs in, and on, snow and ice are very similar to those in aqueous solution, with no enhancement at the air−ice interface.

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Photodegradation Rate Constants for Anthracene and Pyrene Are Similar in/on Ice and in Aqueous Solution

Hullar

Magadia

Anastasio

2018

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“…These results suggest that benoxacor served as a photosensitizer for metolachlor photodegradation on quartz and, to a lesser extent, in water, but not on kaolinite. The absence of sensitized reaction on kaolinite may be attributed to the greater distance between metolachlor and benoxacor as a result of the larger specific surface area provided by kaolinite particles. , The stronger sensitizing effect of benoxacor on quartz than in water was similar to that observed for phenol, furfuryl alcohol, tryptophan, and bisphenol A in ice, where sensitized reactions by excited triplet state 3,4-dimethoxybenzaldehyde and 1 O 2 were more than 50 times faster than those in water. , Two factors may have contributed to the stronger sensitizing effect on quartz: First, the half-life of reactive species such as •OH and 1 O 2 are typically longer in air than in water. , Second, metolachlor and benoxacor molecules were ostensibly in closer proximity on quartz, facilitating the interaction between metolachlor and the reactive species generated by benoxacor.…”

Section: Results and Discussionmentioning

confidence: 74%

“…45,46 The stronger sensitizing effect of benoxacor on quartz than in water was similar to that observed for phenol, furfuryl alcohol, tryptophan, and bisphenol A in ice, where sensitized reactions by excited triplet state 3,4-dimethoxybenzaldehyde and 1 O 2 were more than 50 times faster than those in water. 47,48 Two factors may have contributed to the stronger sensitizing effect on quartz: First, the half-life of reactive species such as •OH and 1 O 2 are typically longer in air than in water. 49,50 Second, metolachlor and benoxacor molecules were ostensibly in closer proximity on quartz, facilitating the interaction between metolachlor and the reactive species generated by benoxacor.…”

Section: Environmental Science and Technologymentioning

confidence: 99%

Sunlight Photolysis of Safener Benoxacor and Herbicide Metolachlor as Mixtures on Simulated Soil Surfaces

Caywood

Sivey

et al. 2019

Benoxacor is a safener paired with the high-use herbicide S-metolachlor. Commercial formulations containing both compounds are sprayed onto soil pre-emergence to enhance yields of corn. In this study, we evaluated the sunlight photolysis of metolachlor and benoxacor, individually and as mixtures, in three different reaction environments: in water and on two soil-simulating surfaces (quartz and kaolinite). When irradiated individually, benoxacor degraded at least 19 times faster than metolachlor in each reaction environment, consistent with its higher molar absorptivity within the solar spectrum than metolachlor. When metolachlor and benoxacor were irradiated as mixtures, benoxacor promoted metolachlor degradation on quartz and, to a lesser extent, in water, but not on kaolinite. On quartz, at a benoxacor/metolachlor molar ratio of 0.1:1, metolachlor degraded 1.8 times faster than in the absence of benoxacor; as the benoxacor/metolachlor ratio increased, metolachlor degradation rate also increased. The photolysis rate of benoxacor depended on its initial surface concentration and was promoted by metolachlor. Benoxacor photoproducts were capable of absorbing sunlight and serving as photosensitizers for metolachlor degradation. These results illustrate how a safener can influence the photochemistry of its coformulated herbicide and suggest that such mixture effects should be considered when evaluating the environmental fate of agrochemicals.

“…The environmentally relevant chemical transformations taking place in frozen solutions have recently attracted attention among environmental researchers, − and the studies on them are well-justified considering the fact that the majority of freshwater on Earth exists in the frozen state. The solutes in frozen solutions are highly concentrated in the ice grain boundary region, which contains a small fraction of liquid water between the freezing point and the eutectic point of water, and the liquid-like grain boundary layer may have reaction conditions (e.g., solute concentration, pH, and ionic strength) that are very different from those of aqueous solution. ,,, As a result, many kinds of homogeneous and heterogeneous chemical reactions occurring in frozen solutions often exhibit highly accelerated kinetics.…”

Section: Introductionmentioning

confidence: 99%

Ligand-Specific Dissolution of Iron Oxides in Frozen Solutions

Menacherry

Kim

Lee

et al. 2018

The freezing-enhanced dissolution of iron oxides by various ligands has been recently proposed as a new mechanism that may influence the supply of bioavailable iron in frozen environments. The ligand-induced dissolution of iron oxides is sensitively affected by the kind and concentration of ligands, pH, and kind of iron oxides. While most ligands are thought to be freeze-concentrated in the ice grain boundary region along with iron oxides to enhance the iron dissolution, this study found that some ligands, such as ascorbic acid, suppress the iron dissolution in frozen solution relative to that in aqueous solution. Such ligands are proposed to be preferentially incorporated in the ice lattice bulk and not freeze-concentrated in the liquid-like grain boundary. The experimental analysis estimated that the ionized forms of ligands (e.g., iodide ions) are hardly present in the ice bulk region (<3%) and enhance the iron dissolution in frozen solution (relative to that in aqueous solution), whereas some neutral ligands (e.g., undissociated ascorbic acid) are significantly trapped in the ice bulk (>50%) and suppress the iron dissolution compared to the aqueous counterpart. The present results reveal that the ligand-induced dissolution of iron oxide in frozen solution is not always enhanced relative to aqueous solution but depends upon the kind of ligand and experimental conditions.