Charles M. Sharpless

Lifetimes of Triplet Dissolved Natural Organic Matter (DOM) and the Effect of NaBH4 Reduction on Singlet Oxygen Quantum Yields: Implications for DOM Photophysics

The natural lifetimes of triplet dissolved organic matter ((3)DOM) were determined by an O(2) saturation kinetics study of singlet oxygen quantum yields (Φ(1O2)) in buffered D(2)O. At least two distinct (3)DOM pools are present, and the observed lifetime range (∼20 to 80 μs) leads to a dependence of Φ(1O2) on O(2) concentrations between 29 and 290 μM. Thus, steady-state (1)O(2) concentrations will depend on [O(2)] in natural waters. The lifetimes are essentially identical for DOM samples of different origins and do not vary with excitation wavelength. However, Φ(1O2) varies greatly between samples and decreases with excitation wavelength. These data strongly suggest that (3)DOM quantum yields decrease with excitation wavelength, which gives rise to the Φ(1O2) variation. Borohydride reduction of several samples in both D(2)O and H(2)O lowers the absorbance and (1)O(2) production rates, but it does not alter Φ(1O2). This is consistent with a model in which (1)O(2) sensitizing chromophores are borohydride reducible groups in DOM, such as aromatic ketones. Interpreted in the framework of a charge transfer (CT) model for DOM optical properties, the collective data suggest a model in which electron acceptor moieties are important (1)O(2) sensitizers and where CT interactions of these moieties disrupt their ability to produce (1)O(2).

Nitrate photosensitized degradation of atrazine during UV water treatment

Abstract.Increasingly, ultraviolet radiation (UV) is being used to treat chemical contaminants in waste and drinking water. To understand the significance of indirect photolysis during such processes, the effect of nitrate on UV treatment of atrazine (ATZ) was investigated at pH 7 in phosphate buffer and solutions containing Suwannee River natural organic matter (NOM). With a medium-pressure mercury lamp, environmentally relevant levels of nitrate (0.7–7 mg-N L–1) lead to initial rates of ATZ removal that are enhanced compared to those without nitrate, and comparison of the product distributions obtained with nitrate and with hydrogen peroxide indicates that this can be attributed to hydroxyl radical (•OH) production from nitrate absorption below 250 nm. Consistent with this, the observed rates decrease as the reaction progresses, presumably due to •OH scavenging by photochemically generated nitrite. In solutions containing NOM, this effect is not observed, and the addition of nitrate leads to lower rates of ATZ removal, albeit higher than predicted on the basis of light-screening by nitrate alone. In order to model this effect, the •OH quantum yield from nitrate photolysis below 250 nm was determined by two methods, competition kinetics between ATZ and 1-octanol and formaldehyde production by reaction of •OH with methanol, and it was found to lie in the range 0.09 to 0.14. This is the first report of the quantum yield at these wavelengths, and this information allows the photolysis rates to be modeled as a function of nitrate concentration and water quality.

Triplet photochemistry of effluent and natural organic matter in whole water and isolates from effluent-receiving rivers.

Effluent organic matter (EfOM), contained in treated municipal wastewater, differs in composition from naturally occurring dissolved organic matter (DOM). The presence of EfOM may thus alter the photochemical production of reactive intermediates in rivers that receive measurable contributions of treated municipal wastewater. Quantum yield coefficients for excited triplet-state OM (3OM*) and apparent quantum yields for singlet oxygen (1O2) were measured for both whole water samples and OM isolated by solid phase extraction from whole water samples collected upstream and downstream of municipal wastewater treatment plant discharges in three rivers receiving differing effluent contributions: Hockanum R., CT (22% (v/v) effluent flow), E. Fork Little Miami R., OH (11%), and Pomperaug R., CT (6%). While only small differences in production of these reactive intermediates were observed between upstream and downstream whole water samples collected from the same river, yields of 3OM* and 1O2 varied by 30-50% between the rivers. Apparent quantum yields of 1O2 followed similar trends to those of 3OM*, consistent with 3OM* as a precursor to 1O2 formation. Higher 3OM* reactivity was observed for whole water samples than for OM isolates of the same water, suggesting differential recoveries of photoreactive moieties by solid phase extraction. 3OM* and 1O2 yields increased with increasing E2/E3 ratio (A254 nm divided by A365 nm) and decreased with increasing electron donating capacities of the samples, thus exhibiting trends also observed for reference humic and fulvic acid isolates. Mixing experiments with EfOM and DOM isolates showed evidence of quenching of triplet DOM by EfOM when measured yields were compared to theoretical yields. Together, the results suggest that effluent contributions of up to 25% (v/v) to river systems have a negligible influence on photochemical production of 3OM* and 1O2 apparently because of quenching of triplet DOM by EfOM. Furthermore, the results highlight the importance of whole water studies for quantifying in situ photoreactivity, particularly for 3OM*.

Impact of hydrogen peroxide on nitrite formation during UV disinfection

One concern with UV disinfection of water is the production of nitrite when polychromatic UV sources are utilized. Based on previous work, it was hypothesized that a small addition of hydrogen peroxide (H(2)O(2)) may be useful in controlling nitrite during UV disinfection. However, it was found that H(2)O(2) addition (5 or 10mg/L) during polychromatic UV irradiation of drinking water at doses used for disinfection significantly increases the levels of nitrite produced relative to solutions without H(2)O(2). Enhancement rates ranged from approximately 15% to 40% depending upon pH and H(2)O(2) concentration; the relative increase in the NO(2)(-) yield was greater at pH 6.5 than at pH 8.3. The observed effects are tentatively ascribed to a combination of enhanced superoxide production and increased hydroxyl radical scavenging when H(2)O(2) is added. These results indicate that H(2)O(2) cannot be used to control nitrite production during UV disinfection and that enhanced nitrite formation will occur if H(2)O(2) is added during UV water treatment to achieve advanced oxidation of contaminants.

Production of photo-oxidants by dissolved organic matter during UV water treatment.

Dissolved organic matter (DOM) irradiated by sunlight generates photo-oxidants that can accelerate organic contaminant degradation in surface waters. However, the significance of this process to contaminant removal during engineered UV water treatment has not been demonstrated, partly due to a lack of suitable methods in the deep UV range. This work expands methods previously established to detect (1)O2, HO•, H2O2, and DOM triplet states ((3)DOM*) at solar wavelengths to irradiation at 254 nm, typical of UV water treatment. For transient intermediates, the methods include a photostable probe combined with selective scavengers. Quantum yields for (1)O2, (3)DOM* and H2O2 were in the same range as for solar-driven reactions but were an order of magnitude higher for HO•, which other experiments indicate is due to H2O2 reduction. With the quantum yields, the degradation of metoxuron was successfully predicted in a DOM solution irradiated at 254 nm. Further modeling showed that the contribution of DOM sensitization to organic contaminant removal during UV treatment should be significant only at high UV fluence, characteristic of advanced oxidation processes. Of the reactive species studied, (3)DOM* is predicted to have the greatest general influence on UV degradation of contaminants.

Partial Photochemical Oxidation Was a Dominant Fate of Deepwater Horizon Surface Oil

Following the Deepwater Horizon (DWH) blowout in 2010, oil floated on the Gulf of Mexico for over 100 days. In the aftermath of the blowout, substantial accumulation of partially oxidized surface oil was reported, but the pathways that formed these oxidized residues are poorly constrained. Here we provide five quantitative lines of evidence demonstrating that oxidation by sunlight largely accounts for the partially oxidized surface oil. First, residence time on the sunlit sea surface, where photochemical reactions occur, was the strongest predictor of partial oxidation. Second, two-thirds of the partial oxidation from 2010 to 2016 occurred in less than 10 days on the sunlit sea surface, prior to coastal deposition. Third, multiple diagnostic biodegradation indices, including octadecane to phytane, suggest that partial oxidation of oil on the sunlit sea surface was largely driven by an abiotic process. Fourth, in the laboratory, the dominant photochemical oxidation pathway of DWH oil was partial oxidation to oxygenated residues rather than complete oxidation to CO2. Fifth, estimates of partial photo-oxidation calculated with photochemical rate modeling overlap with observed oxidation. We suggest that photo-oxidation of surface oil has fundamental implications for the response approach, damage assessment, and ecosystem restoration in the aftermath of an oil spill, and that oil fate models for the DWH spill should be modified to accurately reflect the role of sunlight.