An Investigation of the Major Transformations and Loss Mechanisms of Mercury and Selenium in the Surface Ocean

The importance of methylmercury (CH3Hg) photochemical degradation, an understudied process in marine ecosystems, was investigated in variety of coastal and oceanic waters from the northeastern U.S. as well as the Atlantic, Pacific and Arctic Oceans. Degradation rate constants ranged from 0.87 to 1.67 day-1,but did not correlate with the environmental parameters measured. Further experiments investigating the reaction mechanism observed little effect of nitrate, chloride, and bromide ions. CH3Hg loss per year due to photodegradation was modeled across latitudes from the Equator to the Arctic using water column integrated rates determined for coastal wetlands, estuaries and the open ocean. A global photochemical demethylation rate of 25.3 Mmol yr-1 was calculated, representing an important flux in the biogeochemical cycle of CH3Hg. Air-sea exchange of elemental mercury (Hg0), another important process in the biogeochemical cycle of Hg, was investigated on the U.S. GEOTRACES cruise in the Arctic Ocean in 2015. High resolution measurements of Hg0 in surface waters and the atmosphere were used to calculate evasional fluxes, and Hg concentrations determined in aerosols and precipitation were used to estimate atmospheric deposition. Overall, concentrations of dissolved Hg0 were near saturation in ice-free waters (32 ± 30 fM), but were highly enriched under contiguous ice (101 ± 98 fM, up to 544 fM). Predicted peaks in Hg0 evasion, although blocked by the sea ice barrier, were as high as 270 pmol m-2 h-1. From these estimates we can better predict the effect of a changing climate on Hg dynamics in the Arctic. The photochemical cycling of selenium (Se), an essential micronutrient, was also studied in marine waters. Inorganic Se(IV) and Se(VI) were found in nutrient-type distributions in samples collected during the Metzyme cruise on the equatorial Pacific Ocean in 2011. Photochemically mediated redox transformation pathways studied could not explain the enhanced concentrations of Se(IV) observed, indicating that a biological process is likely involved. Photodegradation was an important sink of dimethyl selenide ((CH3)2Se) in a variety of natural waters, with reaction rate constants ranging from 18.1 to 47.0 day-1. The global loss of (CH3)2Se due to photodegradation was estimated at 28.0 Gmol yr-1

Reactive Oxygen Species and Chromophoric Dissolved Organic Matter Drive the Aquatic Photochemical Pathways and Photoproducts of 6PPD-quinone under Simulated High-Latitude Conditions

The photochemical degradation pathways of 6PPD-quinone (6PPDQ, 6PPD-Q), a toxic transformation product of the tire antiozonant 6PPD, were determined under simulated sunlight conditions typical of high-latitude surface waters. Direct photochemical degradation resulted in 6PPDQ half-lives ranging from 17.5 h at 20 °C to no observable degradation over 48 h at 4 °C. Sensitization of excited triplet-state pathways using Cs+ and Ar purging demonstrated that 6PPDQ does not decompose significantly from a triplet state relative to a singlet state. However, assessment of processes involving reactive oxygen species (ROS) quenchers and sensitizers indicated that singlet oxygen and hydroxyl radical do significantly contribute to the degradation of 6PPDQ. Investigation of these processes in natural lake waters indicated no difference in attenuation rates for direct photochemical processes at 20 °C. This suggests that direct photochemical degradation will dominate in warm waters, while indirect photochemical pathways will dominate in cold waters, involving ROS mediated by chromophoric dissolved organic matter (CDOM). Overall, the aquatic photodegradation rate of 6PPDQ will be strongly influenced by the compounding effects of environmental factors such as light screening and temperature on both direct and indirect photochemical processes. Transformation products were identified via UHPLC-Orbitrap mass spectrometry, revealing four major processes: (1) oxidation and cleavage of the quinone ring in the presence of ROS, (2) dealkylation, (3) rearrangement, and (4) deamination. These data indicate that 6PPDQ can photodegrade in cool, sunlit waters under the appropriate conditions: t1/2 = 17.4 h tono observable decrease (direct); t1/2 = 5.2-11.2 h (indirect, CDOM)

Methods for reactive oxygen species (ROS) detection in aqueous environments

This review summarizes direct and indirect analytical methods for the detection and quantification of the reactive oxygen species (ROS): 1O2, O2·−/HOO·, H2O2, HO·, and CO3·− in aqueous solution. Each section briefly describes the chemical properties of a specific ROS followed by a table (organized alphabetically by detection method, i.e., absorbance, chemiluminescence, etc.) summarizing the nature of the observable (associated analytical signal) for each method, limit of detection, application notes, and reaction of the probe molecule with the particular ROS