Evaluating the photochemical reactivity of disinfection byproducts formed during seawater desalination processes.

Reverse osmosis (RO) is widely used for seawater desalination but pre-chlorination of intake water produces halogenated disinfection byproducts (DBPs). The fate and environmental impacts associated with the discharge of DBP-containing RO brine wastewater are unknown. Therefore, to evaluate if photochemistry plays a role in DBP degradation in seawater, we collected samples at a desalination plant, which were desalted and concentrated using two-inline solid phase extraction (SPE) techniques combining reverse-phase polymeric (PPL) and weak anion exchange (WAX) resins. Both filtered water samples and SPE samples (extracts reconstituted in open ocean seawater) were exposed to simulated sunlight in a custom-built flow-through system. Optical property analysis during irradiation experiments did not provide distinguishing features between intake water and RO reject water (brine). Extractable organic bromine (organoBr) concentrations were low in intake water samples (7.8 µg Br L-1) and did not change significantly due to irradiation. OrganoBr concentrations in laboratory-chlorinated raw water were much higher (135 µg Br L-1) and on average decreased by 42 % after 24 h irradiation. However, while organoBr concentrations were highest in RO reject water (473 µg Br L-1), changes in organoBr concentrations in PPL SPE samples after 24 h irradiation were variable, ranging from a 1-46 % loss. Furthermore, most bromine-containing molecular ions identified by high resolution mass spectrometry that were present in RO reject water before irradiation were also found after both 24 h and 50 h exposures. Although only one RO reject water sample was tested in this study, results highlight that hundreds of yet to be identified brominated DBPs in RO reject water could be resistant to photodegradation or phototransform into existing DBPs in the environment. Future work examining the biolability of DBPs in RO reject water, as well as the interplay between photochemical and biological DBP cycling, is warranted.

Direct Evidence of a Light-Dependent Sink of Superoxide within Chromophoric Dissolved Organic Matter.

Superoxide (O2• -) is produced photochemically in natural waters by chromophoric dissolved organic matter (CDOM) via the reaction of molecular oxygen with photoproduced one-electron reductants (OERs) within CDOM. In the absence of other sinks (metals or organic radicals), O2• - is believed to undergo primarily dismutation to produce hydrogen peroxide (H2O2). However, past studies have implicated the presence of an additional light-dependent sink of O2• - that does not lead to H2O2 production. Here, we provide direct evidence of this sink through O2• - injection experiments. During irradiations, spikes of O2• - are consumed to a greater extent (∼85-30% loss) and are lost much faster (up to ∼0.09 s-1) than spikes introduced post-irradiation (∼50-0% loss and ∼0.03 s-1 rate constant). The magnitude of the loss during irradiation and the rate constant are wavelength-dependent. Analysis of the H2O2 concentration post-spike indicates that this light-dependent sink does not produce H2O2 at low spike concentrations. This work further demonstrates that simply assuming that the O2• - production is twice the H2O2 production is not accurate, as previously believed.

Biodegradation of Terrigenous Organic Matter in a Stratified Large-Volume Water Column: Implications of the Removal of Terrigenous Organic Matter in the Coastal Ocean.

Large amounts of terrigenous organic matter (TOM) are delivered to the ocean every year. However, removal processes of TOM in the ocean are still poorly constrained. Here, we report results from a 339-day dark incubation experiment with a unique system holding a vertically stratified freshwater-seawater column. The quality and quantity of dissolved organic matter (DOM), RNA-based size-fraction microbial communities, and environmental factors were high-frequency-monitored. Microbial processes impacted TOM composition, including an increased DOM photobleaching rate with incubation time. The mixed layer had changed the bacterial community structure, diversity, and higher oxygen consumption rate. A two-end member modeling analysis suggested that estimated nutrient concentrations and prokaryotic abundance were lower, and total dissolved organic carbon was higher than that of the measured values. These results imply that DOM biodegradation was stimulated during freshwater-seawater mixing. In the bottom layer, fluorescent DOM components increased with the incubation time and were significantly positively related to highly unsaturated, oxygenated, and presumably aromatic compound molecular formulas. These results suggest that surfaced-derived TOM sinking leads to increased DOM transformation and likely results in carbon storage in the bottom water. Overall, these results suggest that microbial transforming TOM plays more important biogeochemical roles in estuaries and coastal oceans than what we know before.

Effects of Added Humic Substances and Nutrients on Photochemical Degradation of Dissolved Organic Matter in a Mesocosm Amendment Experiment in the Gulf of Finland, Baltic Sea.

Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large-scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The in situ DOM optical properties became similar among the control, humic- and humic+nutrients-amended mesocosm samples toward the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions did not significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.

Photoproduction Rates of One-Electron Reductants by Chromophoric Dissolved Organic Matter via Fluorescence Spectroscopy: Comparison with Superoxide and Hydrogen Peroxide Rates.

One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to determine the photoproduction rates of OER from a diverse set of CDOM samples. 3AP reacts with OER to produce the hydroxylamine, which is then derivatized with fluorescamine and quantified spectrofluorometrically. Although less sensitive than traditional methods for measuring RO2•-, measuring RH provides a simpler and faster method of estimating RO2•- and is amenable to continuous measurement via flow injection analysis. Production rates of OER (RH), superoxide (RO2•-), and hydrogen peroxide (RH2O2) have a similar wavelength dependence, indicating a common origin. If all the OER react with molecular oxygen to produce superoxide, then the simplest mechanism predicts that RH/RH2O2 and RO2•-/RH2O2 should be equal to 2. However, our measurements reveal RH/RH2O2 values as high as 16 (5.7-16), consistent with prior results, and RO2•-/RH2O2 values as high as 8 (5.4-8.2). These results indicate that a substantial fraction of superoxide (65-88%) is not undergoing dismutation. A reasonable oxidative sink for superoxide is reaction with photoproduced phenoxy radicals within CDOM.

Combined fluorescent measurements, parallel factor analysis and GC-mass spectrometry in evaluating the photodegradation of PAHS in freshwater systems.

To better understand the transformation and photochemical fate of PAHs in aquatic environments, a custom-designed closed-circuit recirculation photodegradation system, combined with inline semi-continuous fluorescence and absorbance measurements, as well as modelling of excitation-emission (EEM) measurements with parallel factor analysis (PARAFAC), and GC-MS analysis, were combined to create a robust tool for holistically assessing the photodegradation of individual PAHs, their mixtures and photoproduct formation. Selected compounds included in the US EPA priority list, representing 2- to 6-ring compounds, were monitored individually and in mixtures, during 24 h photodegradation experiments. Experiments were conducted in solutions simulating ideal (ultrapure water) and environmentally relevant conditions (1.00 mg L-1 Suwannee River Natural Organic Matter (SRNOM)). The fluorescence, primary PARAFAC components and quantification data obtained by GC-MS, indicated that the decline in parent molecule concentration occurred rapidly within 200 min. The degradation rates of parent PAHs increased with aromaticity (6-ring ≫ 2-ring PAHs) and followed pseudo-first order degradation kinetics. The presence and transformation of degradation products, were captured by PARAFAC. NOM influenced the diversity of photoproducts. From the GC-MS results, photoproducts were only detected in Ant, BAnt and the PAH mixture solutions, but optical property analyses indicated that diverse changes occurred with all PAHs. Spectrometric and chromatographic data demonstrated that parent PAHs and photoproducts co-existed at various stages, which is significant for freshwater systems contaminated with these compounds if photoproducts have higher-toxic potential. These results may be used to model the hazard-potential associated with PAHs present in freshwater systems and understanding the mechanisms that govern their environmental fate.

Simulated Sunlight Selectively Modifies Maillard Reaction Products in a Wide Array of Chemical Reactions.

The photochemical transformation of Maillard reaction products (MRPs) under simulated sunlight into mostly unexplored photoproducts is reported herein. Non-enzymatic glycation of amino acids leads to a heterogeneous class of intermediates with extreme chemical diversity, which is of particular relevance in processed and stored food products as well as in diabetic and age-related protein damage. Here, three amino acids (lysine, arginine, and histidine) were reacted with ribose at 100 °C in water for ten hours. Exposing these model systems to simulated sunlight led to a fast decay of MRPs. The photodegradation of MRPs and the formation of new compounds have been studied by fluorescence spectroscopy and nontargeted (ultra)high-resolution mass spectrometry. Photoreactions showed strong selectivity towards the degradation of electron-rich aromatic heterocycles, such as pyrroles and pyrimidines. The data show that oxidative cleavage mechanisms dominate the formation of photoproducts. The photochemical transformations differed fundamentally from "traditional" thermal Maillard reactions and indicated a high amino acid specificity.

The chemodiversity of algal dissolved organic matter from lysed Microcystis aeruginosa cells and its ability to form disinfection by-products during chlorination.

Algal-derived dissolved organic matter (ADOM) originating from lysed Microcystis aeruginosa cells was investigated as precursor material to form disinfection by-products upon disinfection with free chlorine. Non-targeted ultrahigh resolution 12 T negative mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed high molecular diversity in solid-phase extracted and ionizable components of Microcystis aeruginosa ADOM. The toxin microcystin LR was effectively degraded by free chlorine, which was expected. However, we found a high diversity of disinfection by-products associated with the addition of free chlorine to the water-soluble and solid-phase extractable fraction of ADOM and of double-bond moieties in abundant and known unsaturated fatty acids. Aromatic DOM precursors were absent from known metabolites of Microcystis aeruginosa and no evidence for aromatic disinfection by-products (DBPs) was found, despite N-containing compounds. A large diversification of N-containing molecular formulas was observed after chlorination, which seems indicative for the breakdown and oxidation of larger peptides. Additionally, a diverse group of N-compounds with presumed chloramine functional groups was observed. This study highlights the importance to evaluate ADOM and its ability to form different DBPs when compared to allochthonous or terrestrially-derived DOM.

Blending remote sensing data products to estimate photochemical production of hydrogen peroxide and superoxide in the surface ocean.

Hydrogen peroxide (H2O2) and its precursor, superoxide (O2(-)), are well-studied photochemical products that are pivotal in regulating redox transformations of trace metals and organic matter in the surface ocean. In attempts to understand the magnitude of both H2O2 and O2(-) photoproduction on a global scale, we implemented a model to calculate photochemical fluxes of these products from remotely sensed ocean color and modeled solar irradiances. We generated monthly climatologies for open ocean H2O2 photoproduction rates using an average apparent quantum yield (AQY) spectrum determined from laboratory irradiations of oligotrophic water collected in the Gulf of Alaska. Because the formation of H2O2 depends on secondary thermal reactions involving O2(-), we also implemented a temperature correction for the H2O2 AQY using remotely sensed sea surface temperature and an Arrhenius relationship for H2O2 photoproduction. Daily photoproduction rates of H2O2 ranged from <1 to over 100 nM per day, amounting to ∼30 µM per year in highly productive regions. When production rates were calculated without the temperature correction, maximum daily rates were underestimated by 15-25%, highlighting the importance of including the temperature modification for H2O2 in these models. By making assumptions about the relationship between H2O2 and O2(-) photoproduction rates and O2(-) decay kinetics, we present a method for calculating midday O2(-) steady-state concentrations ([O2(-)]ss) in the open ocean. Estimated [O2(-)]ss ranged from 0.1-5 nM assuming biomolecular dismutation was the only sink for O2(-), but were reduced to 0.1-290 pM when catalytic pathways were included. While the approach presented here provides the first global scale estimates of marine [O2(-)]ss from remote sensing, the potential of this model to quantify O2(-) photoproduction rates and [O2(-)]ss will not be fully realized until the mechanisms controlling O2(-) photoproduction and decay are better understood.