Bulk Dissolved Organic Matter Research Articles

Fate of dissolved organic matter substructure in a full-scale wastewater treatment plant by using size exclusion chromatography multi-excitation-emission matrix analysis

Dissolved organic matter (DOM) in wastewater treatment plant (WWTP) effluent threats aquatic environment and challenges engineered aquatic systems. In order to predict its environmental risk and guide the selection of suitable strategy and technology, understanding the removal efficiency and the fate of DOM along the treatment is helpful. Herein, to qualitatively and quantitatively estimate the composition of DOM in a full-scale WWTP in northern China, a combined approach of chromatography and spectroscopy was employed, and the contribution of size-dependent substructure to the global feature of DOM was discussed simultaneously. The results suggest that the WWTP could remove the free proteins of low molecular size by physical treatment with removal ratios of 46.4% and 41.0% for tyrosine and tryptophan, respectively, whereas humic-like and humic-protein complex accumulate in the effluent of treatment with a ratio of 40.6%. Biotreatment is of importance for changing the proportion of different types of DOM, especially for anaerobic tank and the anoxic tank, which results in the increase of humification. Size exclusive chromatography (SEC) coupled with multi emission maps identifies six DOM substructures. Two protein-like DOM were removed by the WWTP, except the composition with the molecular weight ranging from 0.75 to 0.25 kDa, whereas humic-like DOM with whole size range accumulated to varying extent. The standard deviation analysis states that the structure variation of bulk DOM in the WWTP is primarily contributed by the decrease of small-sized proteins and the increase of humic-like DOM. This study presents insight into the fate and behavior of DOM components during the full-scale WWTP treatment and provides guidelines for altering DOM-treating technologies. Besides, innovative strategies suggest using DOM as a resource based on the critical review of previous research.

Distinct Brain Networks are Involved in Reach Directed at Moving and Static Targets

Dissolved organic matter (DOM) and carbon nanotubes are introduced into aquatic environments. Thus, it is important to elucidate whether their interaction affects DOM amount and composition. In this study, the composition of DOM, before and after interactions with single-walled carbon nanotubes (SWCNTs), was measured and the adsorption affinity of the individual structural fractions of DOM to SWCNTs was investigated. Adsorption of DOM to SWCNTs was dominated by the hydrophobic acid fraction, resulting in relative enhancement of the hydrophilic character of non-adsorbed DOM. The preferential adsorption of the HoA fraction was concentration-dependent, increasing with increasing concentration. Adsorption affinities of bulk DOM calculated as the normalized sum of affinities of the individual structural fractions were similar to the measured affinities, suggesting that the structural fractions of DOM act as independent adsorbates. The altered DOM composition may affect the nature and reactivity of DOM in aquatic environments polluted with carbon nanotubes.

Connecting the Age and Reactivity of Organic Carbon to Watershed Geology and Land Use in Tributaries of the Hudson River

AbstractWe characterized the dissolved organic matter (DOM) under baseflow conditions from a set of rivers in the Mohawk and Hudson River watersheds. The rivers in this study drain a range of bedrock geologies and land cover. We identify how those factors influence riverine DOM reactivity and the source, age, and composition of the biolabile DOM. We performed laboratory incubation experiments to characterize each river's reactive and non‐reactive DOM pools. Measurements of dissolved organic carbon concentration, radiocarbon, Ultraviolet‐visible spectroscopy absorbance, and Fourier‐transform ion cyclotron resonance mass spectrometry (FTICR‐MS) analysis were performed at each incubation start and end, allowing us to determine the quantity, age, and composition of the reactive and nonreactive DOM pools. We find that lithology controls bulk DOM ages, with watersheds underlain by shale/limestone having the most aged DOM and crystalline/metasedimentary watersheds having the youngest DOM. We observe that for a given lithology, bulk DOM age increases with the proportion of agricultural land in the watershed–suggesting agricultural practices mobilize aged DOM. FTICR‐MS analysis reveals that both lithology and land cover influence DOM composition. Shale/limestone watersheds showed DOM compositions distinct from other watershed lithologies, and the percentage of nitrogen‐containing DOM correlated with agricultural influence. In two of the studied rivers we find that the biolabile DOM fraction is older than the bulk DOM (upwards of 7 kyr) revealing that aged DOM may be preferentially consumed in these rivers. Our findings provide insight into how riverine carbon cycles may respond to watershed disturbances that influence DOM inputs to rivers.

UV-based advanced oxidation of dissolved organic matter in reverse osmosis concentrate from a potable water reuse facility: A Parallel-Factor (PARAFAC) analysis approach

Disposal of reverse osmosis concentrate (ROC) from advanced water purification facilities is a challenge associated with the implementation of reverse osmosis-based treatment of municipal wastewater effluent for potable reuse. In particular, the dissolved organic matter (DOM) present in ROC diminishes the quality of the receiving water upon environmental disposal and affects the toxicity, fate, and transport of organic contaminants. This study investigates UV-based advanced oxidation processes (UV-AOPs) for treating DOM in ROC using a Parallel Factor Analysis (PARAFAC) approach. DOM composition and degradation were tested in UV-only and three UV-AOPs using hydrogen peroxide (H2O2), free chlorine (Cl2), and persulfate (S2O82−). The four-component PARAFAC model consisted of two terrestrial humic-like components (CUVH and CVisH), a wastewater/nutrient tracer component (CNuTr), and a protein-like (tyrosine-like) component (CPrTy). Based on the observed loss in the maximum fluorescence intensity of the components, DOM degradation was determined to be dependent on UV fluence, oxidant dose, and dilution factor of the ROC (i.e., bulk DOM concentration). CVisH was most the photolabile component in the UV-only system, followed by CNuTr, CPrTy, and CUVH, respectively. Furthermore, UV-H2O2 and UV-S2O82− displayed faster overall reaction kinetics compared to UV-Cl2. The degradation trends suggested that CNuTr and CPrTy consisted of chemical moieties that were susceptible to reactive oxygen species (HO•) but not reactive chlorine species; whereas, CVisH was sensitive to all reactive species generated in the three UV-AOPs. Compared to other components, CPrTy was recalcitrant in all treatment scenarios tested. Calculations using chemical probe-based analysis also confirmed these trends in the reactivity of DOM components. The outcomes of this study form a foundation for characterizing ROC reactivity in UV-AOP treatment technologies, to ultimately improve the sustainability of water reuse systems.

Relationships between dissolved black carbon and dissolved organic matter in streams

Black carbon (BC) is a pyrolyzed product derived from incomplete combustion. A major fraction of BC produced by landscape fires is initially deposited onto onsite soils. Atmospheric deposition of soot is known to be an important source of soil BC, especially in watersheds that are not affected by landscape fires. The transport of the dissolved fraction of oxidized BC in soil, defined as dissolved black carbon (DBC), to streams is considered one of the important loss pathways of BC in soil, but the mechanism is not well documented. We measured the quantity and quality of DBC, determined by a benzenepolycarboxylic acid method, and the quantitative and qualitative parameters of bulk dissolved organic matter (DOM) in streams in Hokkaido, northern Japan, whose catchments were not affected by landscape fire for at least 110 years. DBC with relatively low polycondensed signatures occurred in the streams, irrespective of differences in watershed characteristics and seasons, suggesting that atmospheric deposition of soot into the catchment is probably a major source of stream DBC. The DBC concentration was linearly related to the dissolved organic carbon (DOC) concentration, irrespective of the differences in watershed characteristics and seasons. Furthermore, the polycondensation degree of DBC was observed to correlate with the qualitative parameters of bulk DOM. Such quantitative and qualitative relationships between DBC and bulk DOM imply that the transfer mechanism from soils to streams of soot-derived polycondensed DBC is linked with that of higher plant-derived, high-molecular-weight aromatic DOM.

Effect of Decreasing Biological Lability on Dissolved Organic Matter Dynamics in Streams

AbstractRespiration of dissolved organic matter (DOM) in streams contributes to the global CO2 efflux, yet this efflux has not been linked to specific DOM sources and their respective uptake rates. Further, removal of DOM inferred from longitudinal concentration gradients in river networks has been insufficient to account for observed CO2 outgassing. We hypothesize that understanding in‐stream dynamics of DOM, which is a heterogeneous mixture spanning a wide range of biological labilities, requires considering that DOM lability decreases during downstream transport. To test this hypothesis, we paired seasonal bioreactor measurements of DOM biological lability with whole‐stream tracer data from White Clay Creek, Pennsylvania, USA, and used a particle‐tracking model to predict in‐stream DOM dynamics. The model simulates continuous inputs of DOM and uses storage time in the stream bioactive regions plus kinetic parameters from bioreactors to assess differential uptake of DOM fractions (i.e., fractionation) in the stream. We compared predictions for in‐stream dynamics of bulk DOM concentration (quantified as dissolved organic carbon) and fluorescent DOM components. Our model‐data synthesis approach demonstrates that more labile fractions of DOM in stream water preferentially originate and are consumed within short travel distances, causing spiraling metrics to change with downstream distance. Our model can account for local sources of rapidly cycled labile DOM, providing a basis for improved interpretation of DOM dynamics in streams that can reconcile apparent discrepancies between respiratory outgassing of CO2 and longitudinal DOM concentration gradients within river networks.

Metal binding by dissolved organic matter in hypersaline water: A size fractionation study using different isolation methods

The influence of dissolved organic matter (DOM) on mineral extraction from salt lake brines depend on DOM quality. This study contributes to our knowledge of DOM’s metal binding behavior in hypersaline environments by characterization of DOM from lakes in the Qaidam Basin, i.e., Qarhan Lake (LQDOM), Da Qaidam (DQDOM) and West Ginair Salt Lake (WGDOM). The DOM was fractionated based on solid phase extraction (SPE) and ultrafiltration (UF), and the spectral and metal binding behavior of these fractions were studied by absorption spectroscopy, Pb(II) titration techniques and fluorescence parallel factor (PARAFAC) analysis. The results showed that bulk DOM generally contained more dissolved organic carbon (DOC), lower specific UV absorbance (SUVA254), higher fluorescence and biological indices, comparable humification index, and lower condition stability constants compared to the other nature waters. Compared with UF, SPE-derived DOM exhibited higher DOC recovery and aromaticity and lower carbohydrate yield. It appeared that the SPE procedure used affects the spectral composition of bulk DOM to a larger extent than UF. Source and molecular weight (MW)-dependent differences in abundance and quality of brine DOM was indicated by higher SUVA254 in high MW DOM, for LQDOM and DQDOM, and humic-like fluorophores were mainly in high MW-DOM in each lake. Moreover, the high MW humic-like component exhibited higher metal binding potential than the bulk and low MW counterparts for LQDOM and DQDOM, while the inverse was observed for WGDOM. This study revealed the effects of isolation techniques on interpretation of DOM characteristics, and meanwhile highlighted the importance of origin- and MW-dependent DOM in manipulating the behavior, fate, and bioavailability of heavy metals in salt lake brine.

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects.

Permafrost degradation is delivering bioavailable dissolved organic matter (DOM) and inorganic nutrients to surface water networks. While these permafrost subsidies represent a small portion of total fluvial DOM and nutrient fluxes, they could influence food webs and net ecosystem carbon balance via priming or nutrient effects that destabilize background DOM. We investigated how addition of biolabile carbon (acetate) and inorganic nutrients (nitrogen and phosphorus) affected DOM decomposition with 28‐day incubations. We incubated late‐summer stream water from 23 locations nested in seven northern or high‐altitude regions in Asia, Europe, and North America. DOM loss ranged from 3% to 52%, showing a variety of longitudinal patterns within stream networks. DOM optical properties varied widely, but DOM showed compositional similarity based on Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) analysis. Addition of acetate and nutrients decreased bulk DOM mineralization (i.e., negative priming), with more negative effects on biodegradable DOM but neutral or positive effects on stable DOM. Unexpectedly, acetate and nutrients triggered breakdown of colored DOM (CDOM), with median decreases of 1.6% in the control and 22% in the amended treatment. Additionally, the uptake of added acetate was strongly limited by nutrient availability across sites. These findings suggest that biolabile DOM and nutrients released from degrading permafrost may decrease background DOM mineralization but alter stoichiometry and light conditions in receiving waterbodies. We conclude that priming and nutrient effects are coupled in northern aquatic ecosystems and that quantifying two‐way interactions between DOM properties and environmental conditions could resolve conflicting observations about the drivers of DOM in permafrost zone waterways.

Evaluation of the biodegradability of fluorescent dissolved organic matter via biological filtration

AbstractFluorescence spectroscopy to evaluate the removal of dissolved organic matter (DOM) via biofiltration under varying operational and water quality conditions was investigated in this study. Three feedwaters (lake, river, and wastewater effluent) were examined at three temperatures (6, 22, and 28 °C) and three empty bed contact times (EBCTs; 5, 15, and 30 min) to simulate seasonal and operational variations experienced by utilities. Parallel factor (PARAFAC) analysis was used to characterize the fluorescing DOM and to determine the component removal. Three components were found in the lake and river feedwaters, and four components were found in the wastewater effluent. In natural source waters, protein‐like components were removed most effectively via biofiltration. In the wastewater effluent, a humic‐like component common in nutrient‐rich waters was removed most effectively, and the protein‐like component was removed second most effectively. Increased EBCT and temperature increased the removal efficiency of bulk DOM and the protein‐like components.

The nonconservative distribution pattern of organic matter in the Rajang, a tropical river with peatland in its estuary

Abstract. Southeast Asian peatland-draining rivers have attracted much attention due to their high dissolved organic carbon (DOC) yield and high CO2 emissions under anthropogenic influences. In August 2016, we carried out a field investigation of the Rajang River and its estuary, a tropical system located in Sarawak, Malaysia. The Rajang has peatland in its estuary, while the river basin is covered by tropical rainforest. DOC-δ13C in the Rajang ranged from −28.7 ‰ to −20.1 ‰, with a U-shaped trend from river to estuary. For particulate organic carbon (POC), δ13C ranged between −29.4 ‰ and −31.1 ‰ in the river, and there was a clear increasing trend towards more enriched δ13C values with higher salinity. In the estuary, there was a linear conservative dilution pattern for dissolved organic matter composition (as quantified by D- and L-amino acid enantiomers) plotted against DOC-δ13C, whereas when plotted against salinity, dissolved D- and L-amino acid enantiomer values were higher than the theoretical dilution value. Together, these data indicate that the addition of DOC to the estuary (by peatland) not only increased the DOC concentration but also altered its composition, by adding more biodegraded, 13C-depleted organic matter into the bulk dissolved organic matter. Alteration of organic matter composition (addition of a more degraded subpart) was also apparent for the particulate phase, but patterns were less clear. The Rajang was characterized by DOC to DON (dissolved organic nitrogen) ratios of 50 in the river section, with loss of DON in the estuary increasing the ratio to 140, suggesting an unbalanced export of organic carbon and nitrogen. Where affected by anthropogenic activities, further assessment of organic carbon to nitrogen ratios is needed.

Molecular weight-dependent heterogeneities in photochemical formation of hydroxyl radical from dissolved organic matters with different sources

Dissolved organic matter (DOM) is ubiquitous in aquatic ecosystem and characterized by a wide range of molecular weight (MW) distribution. In this study, a total of nine bulk DOM samples, including five International Humic Substances Society (IHSS) standards and four naturally collected samples, were fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with MW-dependent heterogeneities in photochemical formation of hydroxyl radical (HO) was investigated. The formation rate of HO (RHO) for the bulk samples were 4.60–7.27 × 10−12 M/s/mg-C/L for IHSS standards and 4.63–7.66 × 10−12 M/s/mg-C/L for naturally collected samples. Regardless of sample types, the LMW fraction was found to exhibit generally higher RHO values than the HMW counterparts. For IHSS standards, the RHO decreased from 4.68–8.46 × 10−12 M/s/mg-C/L for LMW fraction to 3.67–6.66 × 10−12 M/s/mg-C/L for HMW fraction, and for naturally collected samples, the value of RHO decreased from 5.21–12.04 × 10−12 M/s/mg-C/L for LMW fraction to 3.25–6.49 × 10−12 M/s/mg-C/L for HMW counterpart. A positive correlation between the net RHO and the normalized intensities of fluorescent peak [Em/Ex: (400–500)/(230–250) nm] was found, showing that the HO formation was strongly coupled to the abundance of humic-like substances. The results indicate that aquatic DOM is an important pool for HO formation, and characterization of MW distribution rather than average MW is thus required to explain the DOM-induced formation potential of HO.

The role of fluorescent dissolved organic matter on mercury photoreduction rates: A case study of three temperate lakes

The formation of dissolved gaseous mercury (DGM) in lake water is mainly attributed to photochemical processes mediated by dissolved organic matter (DOM). In this study, we explored how different DOM components influence the Hg(II) photoreduction rate constant (kr) in lake surface water. For this purpose, the kr and the fluorescence properties of DOM were obtained from three Korean lakes with different trophic states. Three major fluorophores were identified by excitation-emission matrix fluorescence spectroscopy combined with a parallel factor analysis: plant-derived terrigenous humic-like DOM (C1), autochthonous DOM (C2), and soil fulvic-like DOM (C3). The principal component analysis (PCA) loading matrix demonstrated that kr increases when bulk DOM has high flavin-like and soil-derived fulvic-like fractions. The results of a Pearson’s correlation agreed with the outcome of the PCA analysis: kr showed a strong positive correlation with the soil fulvic-like DOM component (r = 0.92) and the redox index (r = 0.92). This was further confirmed by a partial least squares-regression model that predicted kr (r = 0.99) using multiple DOM components. Overall, our results suggest that kr can be modeled using fluorescence intensities of diverse DOM components, which in turn has the potential to be incorporated into Hg biogeochemical models to better predict the variability of Hg redox rates across lake systems.

Chemical Alterations of Dissolved Organic Matter by Permanganate Oxidation.

Dissolved organic matter (DOM) is ubiquitous in raw drinking water and can efficiently scavenge oxidants, such as permanganate. Here, changes to DOM induced by permanganate oxidation under typical drinking water treatment conditions (6 μM, 1 h) to bulk DOM properties, DOM functional groups, and DOM chemical formulae were examined for two DOM isolate types (terrestrial and microbial). Permanganate oxidation did not mineralize DOM, rather changes were compositional in nature. Optical properties suggest that permanganate oxidation decreased DOM aromaticity (decreased SUVA-254), decreased DOM electron-donating capacity, and decreased DOM average molecular weight (increased E2/E3 ratios). Fourier-transform-infrared spectroscopy second derivative analyses revealed that permanganate does not oxidize DOM alkene groups, suggesting permanganate access to functional groups may be important. Four ionization techniques were used with ultrahigh-resolution mass spectrometry: negative and positive ion mode electrospray ionization and negative and positive ion mode laser/desorption ionization. The results from all four techniques were combined to understand changes in DOM chemical formulae. It was concluded that nitrogen-containing aromatic compounds and alkylbenzenes were oxidized by permanganate to form nitrogen-containing aliphatic compounds and benzoic acid-containing compounds. This work highlights how multiple ionization techniques coupled with UHR-MS can enable a more detailed characterization of DOM.

Dissolved Metal(loid) Concentrations and Their Relations with Chromophoric and Fluorescent Dissolved Organic Matter in an Urban River in Shenzhen, South China

Urbanization is often accompanied by aquatic metal(loid) pollution, which is regulated by dissolved organic matter (DOM). However, the relationships between dissolved metal(loid) concentration and the bulk, chromophoric, and fluorescent DOM in black and odorous urban rivers are still poorly understood. Here, we investigated the dissolved metal(loid) concentrations of Zn, Cu, Cr, As, Pb, and Cd and their correlations with DOM-related parameters in water samples from a polluted urbanized watershed in Shenzhen, China. The results showed that the Zn and Cu concentrations in the mainstream and tributary exceeded the national standards, and the wastewater treatment plant (WWTP) was an important source, as indicated by the abrupt concentration increases downstream of the WWTP. The dissolved metal(loid) concentrations were not always significantly correlated with the dissolved organic carbon (DOC) concentration or the ultraviolet absorbance at 254 nm (UV254); however, they were more likely to be correlated with the maximum fluorescence intensity (Fmax) of protein-like fluorescent DOM components. A strong correlation between the Cu/DOC ratio and specific UV254 (SUVA254) previously reported did not exist in the present study. Instead, the Cu/DOC ratio was positively correlated with the Fmax/DOC ratios for protein-like fluorescent DOM components. Our study highlights that protein-like fluorescent DOM may be more important than humic-like fluorescence DOM and chromophoric DOM in terms of interacting with dissolved metal(loid)s in black and odorous urban rivers.

The importance of aromaticity to describe the interactions of organic matter with carbonaceous materials depends on molecular weight and sorbent geometry.

Dissolved organic matter (DOM) is ubiquitous in aquatic environments where it interacts with a variety of particles including carbonaceous materials (CMs). The complexity of both DOM and the CMs makes DOM-CM interactions difficult to predict. In this study we have identified the preferential sorption of specific DOM fractions as being dependent on their aromaticity and molecular weight, as well as on the surface properties of the CMs. This was achieved by conducting sorption batch experiments with three types of DOM (humic acid, Suwannee River natural organic matter, and a compost extract) and three types of CMs (graphite, carbon nanotubes, and biochar) with different geometries and surface complexities. The non-adsorbed DOM fraction was analyzed by size exclusion chromatography and preferentially sorbed molecular weight fractions were analyzed by UV/vis and fluorescence spectroscopy. All three sorbent types were found to preferentially sorb aromatic DOM fractions, but DOM fractionation depended on the particular combination of sorbent and sorbate characteristics. Single-walled carbon nanotubes only sorbed the smaller molecular weight fractions (<1 kDa). The sorption of smaller DOM fractions was not accompanied by a preference for less aromatic compounds, contrary to what was suggested in previous studies. While graphite preferentially sorbed the most aromatic DOM fraction (1-3 kDa), the structural heterogeneity of biochar resulted in reduced selectivity, sorbing all DOM > 1 kDa. The results explain the lack of correlation found in previous studies between the amount of aromatic carbon in a bulk DOM and its sorption coefficient. DOM sorption by CMs was generally controlled by DOM aromaticity but complex sorbent surfaces with high porosity, curvatures and functional groups strongly reduced the importance of aromaticity.

Systematic Variation in Marine Dissolved Organic Matter Stoichiometry and Remineralization Ratios as a Function of Lability

Remineralization of organic matter by heterotrophic organisms regulates the biological sequestration of carbon, thereby mediating atmospheric CO2. While surface nutrient supply impacts the elemental ratios of primary production, stoichiometric control by remineralization remains unclear. Here we develop a mechanistic description of remineralization and its stoichiometry in a marine microbial ecosystem model. The model simulates the observed elemental plasticity of phytoplankton and the relatively constant, lower C:N of heterotrophic biomass. In addition, the model captures the observed decreases in the C:N of more labile dissolved organic matter (DOM) and the C:N of its remineralization with depth, which are driven by a switch in the dominant source of DOM from phytoplankton to heterotrophic biomass. Only a model version with targeted remineralization of N‐rich components is able to simulate the observed profiles of preferential remineralization of N relative to C and the elevated C:N of bulk DOM. The model suggests that more labile substrates are associated with C‐limited heterotrophic growth and not with preferential remineralization, while less labile substrates are associated with growth limited by processing rates and with preferential remineralization. The resulting patterns of variable remineralization stoichiometry mediate the extent to which a proportional increase in carbon production resulting from changes in phytoplankton stoichiometry can increase the efficiency of the biological pump. Results emphasize the importance of understanding the physiology of both phytoplankton and heterotrophs for anticipating changes in biologically driven ocean carbon storage.

The Role of Dissolved Organic Matter Composition in Determining Photochemical Reactivity at the Molecular Level.

Dissolved organic matter (DOM) composition influences its ability to form photochemically produced reactive intermediates (PPRI). While relationships have been established between bulk DOM properties and triplet DOM (3DOM) and singlet oxygen (1O2) quantum yields, contradictory evidence exists for hydroxyl radical (•OH) and hydroxylating species. Furthermore, little is known about these relationships at the molecular level. We evaluated DOM composition and photochemical reactivity of water samples from a wastewater treatment plant and the St. Louis River in Minnesota and Wisconsin, U.S.A. Bulk characterization using ultraviolet-visible spectroscopy demonstrates that color and apparent size of DOM decrease downstream, while molecular composition analysis using Fourier-transform ion cyclotron resonance mass spectrometry reveals that saturation and chemodiversity is highest near Lake Superior. 3DOM quantum yield coefficients and 1O2 quantum yields increase downstream and correlate strongly with saturated formulas. Similar results are observed for carbon-normalized photodegradation rate constants of atorvastatin, carbamazepine, and venlafaxine, which react primarily with 3DOM and 1O2. In contrast, •OH quantum yields are lowest downstream and correlate with less saturated, more oxygenated DOM, suggesting that 3DOM is not its major precursor. Mixed relationships are observed for DEET, which reacts with multiple PPRI. Molecular-level compositional data reveal insights into the differing formation pathways of individual PPRI, but information about specific contaminants is needed to predict their photochemical fate.

Origin, distributions, and environmental significance of ubiquitous humic-like fluorophores in Antarctic lakes and streams

This study characterized dissolved organic matter (DOM) obtained from 47 lakes and 2 streams on ice-free areas at Lützow-Holm Bay and Amundsen Bay in East Antarctica (n = 74), where few biogeochemical studies have been historically conducted. Samples were analyzed for basic water chemistry and by resin fractionation, UV–vis spectroscopy, and excitation emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC). Salinity of the samples ranged very broadly from fresh to hypersaline as a result of evaporative concentration. There was a clear positive correlation between log-salinity and the spectral slopes of DOM (S275–295), an indicator of photodegradation. Thus, we interpreted the correlation as a progression of photodegradation by prolonged water retention time. Of the identified seven PARAFAC components, three ubiquitous humic-like components decreased as photodegradation progressed, while a photorefractory UVC humic-like component increased its relative abundance. A non-humic component, traditionally defined as Peak N, did not show a trend depending on photodegradation, and its level was high in nutrient-rich lakes, presumably due to high in-situ production. We found robust correlations between the relative abundance of the ubiquitous humic-like components and that of the Peak N component in the bulk DOM irrespective of water types or ice-free areas. We proposed there were common processes that generated the ubiquitous humic-like components from the Peak N component in the Lützow-Holm Bay and Amundsen Bay lakes and streams, such as bacterial processing of primary production-derived DOM and photochemical transformation of microbial DOM.

Photochemical formation of carbonate radical and its reaction with dissolved organic matters

The carbonate radical (CO3•−) is a strong oxidative radical that is generated via the reactions of HCO3−/CO32− with hydroxyl radical (HO•) or triplet states of dissolved organic matter (3DOM∗) in sunlit surface water. The bimolecular reaction rate constants of CO3•− with various DOM isolates (kCO3•−,DOM) were calculated as 15–239 (mg of C/L)−1 s−1 and were correlate to the bulk DOM properties, such as the content of phenolic moieties, the specific UV absorbance (SUVA), the E2/E3 value, and the fluorescence index (FI). The spectroscopic E2/E3 values was found to strongly correlated (R2 = 0.93) with kCO3•−,DOM, and an empirical equation was established. Our results also demonstrate that CO3•− is involved in the photobleaching of dissolved organic matter (DOM) and in particular reacts with electron-donor moieties, leading to faster decay rates at long wavelengths of UV–vis absorption. Furthermore, a model was developed to calculate the steady-state concentrations of CO3•− during DOM photobleaching. These results allow us to estimate the reactivity of DOM with CO3•− and to evaluate the role of CO3•− in sunlit surface water. It will also allow a better assessment of the concentration and utilization of CO3•− during the application of advanced oxidation processes.

Elucidating the Hidden Nonconservative Behavior of DOM in Large River‐Dominated Estuarine and Coastal Environments

AbstractUnderstanding the mixing behavior and biogeochemical cycling of heterogeneous dissolved organic matter (DOM) at the land‐ocean interface has been a major challenge, especially in large river‐dominated estuarine and coastal environments with multiple and complex end‐members of riverwater and seawater. Significant correlations and thus an “apparent” conservative behavior are generally observed for the bulk DOM when plotting concentration‐salinity relationships, but the relatively small deviations from the apparent conservative behavior are difficult to be quantified. In July 2016, the flood in the Changjiang River basin caused the Changjiang Diluted Water extended widely to the western East China Sea, making it a natural laboratory to examine the river‐sea mixing behavior of riverine DOM and other optical properties. Here a new biogeochemical approach was established to elucidate their hidden nonconservative behavior in the study area. Comparisons in the bulk dissolved organic carbon and the three chromophoric DOM properties among 13 pairs of same‐salinity patches (with similar salinities but quite different distances from the river mouth) revealed that the East China Sea shelf was a significant sink for all the three chromophoric DOM properties. The Changjiang Diluted Water surface branches transporting southward tended to serve as a sink for the bulk dissolved organic carbon while the ones transporting northward were less a sink or even a source. The different removal extents of various biogeochemical constituents can also be calculated from the same‐salinity patch comparisons. Our results obtained by the new approach provide detailed magnitudes of sink or source for different DOM properties and their in situ removal/production extents, giving new insights into biogeochemical processes in large river‐dominated estuarine and coastal environments.