Dissolved Organic Matter Isolates Research Articles

Influences of molecular composition in dissolved organic matter on its reactivity with chlorine radicals (Cl• and Cl2•−)

Dissolved organic matter (DOM) is a major scavenger of chlorine radicals (e.g., Cl• and Cl2•−) in water. However, there is limited literature addressing the impact of DOM molecular composition on its reactivity towards chlorine radicals. This work focused on discovering the relationships between DOM molecular composition and its reactivity towards Cl• and Cl2•−. The molecular composition of 23 DOM isolates from different sources was characterized using high-resolution mass spectrometry, and their rate constants with Cl• (kDOM-Cl•) and Cl2•− (kDOM-Cl2•−) were determined using laser flash photolysis and competition kinetics. Orthogonal partial least squares (OPLS) analysis model was then used to identify the most correlated and effective predictors of kDOM-Cl• and kDOM-Cl2•−. The kDOM-Cl• positively correlated to higher unsaturation and aromaticity while negatively correlated to the proportion of several N- and S-containing formulas. The kDOM-Cl2•− values showed positive correlation with unsaturation and aromaticity while showing negative correlation with the proportion of CHOPwa formulas. These observations suggest that while unsaturation and aromaticity serve as useful indicators of reactivity with chlorine radicals, non-oxygen heteroatom-containing formulas within the DOM isolates play a pivotal role in governing their interactions with chlorine radicals.

Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes.

Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. Under sunlight irradiation, DOM undergoes complete photooxidation to produce carbon dioxide (CO2) and partial photooxidation that alters the molecular composition of DOM. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to partial and complete photooxidation in surface waters is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate DOM photooxidation using two DOM isolates and DOM from 16 lakes that vary in trophic status and size. High ratios of oxygen consumption to dissolved inorganic carbon (DIC) production demonstrate that all samples undergo extensive partial photooxidation. At the molecular level, more oxidized, aromatic DOM formulas are associated with oxygen consumption and DIC production. Bulk level measurements indicate that DOM becomes less aromatic and lower in apparent molecular weight following partial photooxidation, and there is molecular level evidence of oxygen addition and loss of CO2 in all samples. However, formulas most susceptible to photooxidation vary depending on the initial DOM composition. Collectively, this work provides insights into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

Effect of salinity on molecular characterization of dissolved organic matter using ESI FT-ICR MS

Solid-phase extraction (SPE) coupled with negative-ion Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for molecular characterization of dissolved organic matter (DOM). However, little attention has paid to test whether the salinity of the sample and the presence of chloride ions in water samples affect the molecular composition of DOM extracted by SPE (SPE-DOM). In this study, one natural organic matter standard and several natural waters were selected to investigate how salinity affects the molecular composition of SPE-DOM and the selectivity of chloride ion adducts formation with respect to the molecular structure of SPE-DOM in negative ion ESI FT-ICR MS analysis. The results show that the molecular composition of SPE-DOM varied in a sample made by different salinity; and the variation pattern of DOM composition was different among different water samples under the treatment of consistent salinity gradients. The chloride ions can't be completely removed from cartridges in conventional SPE, thus leading organic compounds in SPE-DOM to form [M+Cl]‾ adducts when performing ESI FT-ICR MS analysis. In addition, the molecules with high H/C and low O/C ratios were likely to form [M+Cl]‾ ions. The relative abundance of [M+Cl]‾ ions could increase with the increase of salinity. These results are instructive to guide the pretreatment and molecular characterization of DOM in water samples with different salinity. Overall, we proposed a modification to the SPE to minimum reduce the formation of chloride ion adducts during the isolation of DOM.

Assessment of salinity and heat impacts on natural organic matter composition in bentonite for used nuclear fuel storage

Bentonite clay (MX-80, Wyoming, USA) is a proposed sealing material for the safe and long-term management of Canada's used nuclear fuel in a deep geological repository (DGR). Due to the physiochemical properties of bentonite, this buffer layer can limit radionuclide mobilization, and minimize air and water circulation, and hence suppress microbial activity near the used fuel container. However, natural organic matter (NOM) in mined Wyoming-type bentonite (MX-80) may serve as microbial substrates and cause further corrosion to the used fuel container. In the proposed DGR, bentonite will be exposed to high salinity due to saline groundwater and heat released from radioactive decay of radionuclide inventory of the used nuclear fuel. There is limited knowledge about how NOM quantity and quality in MX-80 may change after being exposed to the anticipated levels of salinity and heat in a DGR, especially at the molecular-level. To explore this, organic carbon contents and NOM composition (both soluble- and solid-phase) in MX-80 after exposure to different saline (NaCl) solutions (1 M, 2 M, 3 M, 4 M and 5 M) with and without heat exposure (90 °C) were analyzed. Dissolved organic carbon (DOC) solubility decreased with increasing salinity but these differences were not statistically significant when compared to the control (P < 0.05). DOC solubilization was not significantly altered with heat (P < 0.05). Dissolved organic matter (DOM) composition was analyzed with solution-state 1H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectroscopy. The NMR results indicated that DOM samples with varied salinities and heat exposure had similar chemical signatures. The results from UV–Vis analysis revealed a small decrease in less unsaturated components in DOM with NaCl solution compared to deionized water. The chemistry of solid-phase NOM that remained in MX-80 after DOM isolation was similar and suggests an insensitivity to either heat and/or salinity. The solubilized DOC contributes less than 1% to the total organic carbon in MX-80, thus, the anticipated level of salinity and heat in a DGR is unlikely to enhance NOM solubilization and change NOM chemistry in the bentonite markedly. As the bentonite in this study was only exposed to the heat and salinity in a short-term experiment, further studies examining NOM composition and reactivity under long-term saline and heat conditions are recommended.

Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes.

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq-), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq- reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq-), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC-1 s-1. kDOM,eaq- measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol-1 and that kDOM,eaq- could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq- exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq- probe revealed that continued eaq- exposure abates DOM chromophores and eaq- scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq- scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Evaluating the Microheterogeneous Distribution of Photochemically Generated Singlet Oxygen Using Furfuryl Amine.

Singlet oxygen (1O2) is an important reactive species in natural waters produced during photolysis of dissolved organic matter (DOM). Prior studies have demonstrated that 1O2 exhibits a microheterogeneous distribution, with [1O2] in the interior of DOM macromolecules ∼30 to 1000-fold greater than in bulk solution. The [1O2] profile for DOM-containing solutions has been determined mainly by the use of hydrophobic probes, which are not commercially available. In this study, we employed a dual-probe method combining the widely used hydrophilic 1O2 probe furfuryl alcohol (FFA) and its structural analogue furfuryl amine (FFAm). FFAm exists mainly as a cation at pH <9 and was therefore hypothesized to have an enhanced local concentration in the near-DOM phase, whereas FFA will be distributed homogeneously. The probe pair was used to quantify apparent [1O2] in DOM samples from different isolation procedures (humic acid, fulvic acid, reverse osmosis) and diverse origins (aquatic and terrestrial) as a function of pH and ionic strength, and all samples studied exhibited enhanced reactivity of FFAm relative to FFA, especially at pH 7 and 8. To quantify the spatial distribution of [1O2], we combined electrostatic models with Latch and McNeill's three-phase distribution model. Modeling results for Suwannee River humic acid (SRHA) yield a surface [1O2] of ∼60 pM, which is ∼96-fold higher than the aqueous-phase [1O2] measured with FFA. This value is in agreement with prior reports that determined 1-3 orders of magnitude higher [1O2] in the DOM phase compared to bulk solution. Overall, this work expands the knowledge base of DOM microheterogeneous photochemistry by showing that diverse DOM isolates exhibit this phenomenon. In addition, the dual-probe approach and electrostatic modeling offer a new way to gain mechanistic insight into the spatial distribution of 1O2 and potentially other photochemically produced reactive intermediates.

Decoupling Optical Response and Photochemical Formation of Singlet Oxygen in Size Isolated Fractions of Ozonated Dissolved Organic Matter.

The complex effects of ozonation on the photophysical and size-based properties of dissolved organic matter (DOM) were investigated using two DOM isolates, Suwannee River Fulvic Acid (SRFA) and Pony Lake Fulvic Acid (PLFA). A size exclusion chromatography system paired with absorbance, fluorescence, and total organic carbon detection was used to determine the fluorescence quantum yield (Φf) as a function of the apparent molecular weight (AMW). Size-based fractions of each isolate were collected and irradiated to measure singlet oxygen (1O2) quantum yield (Φ1O2). Φf decreased with ozonation in low AMW fractions, while increasing in high AMW fractions. Φ1O2 increased with ozone dose in low AMW fractions from ∼2 to ∼7% and ∼3 to ∼11% for PLFA and SRFA, respectively, indicating that these are the most photoreactive fractions of DOM. Decreases in Φf and concomitant increases in Φ1O2 in low AMW fractions indicated that chemical transformations occurred, likely including the conversion of phenols to quinones, particularly in SRFA. Results further suggest that the photoactive and fluorescent fractions of DOM are likely independent pools of chromophores from different AMW fractions. In PLFA, a linear response in Φ1O2, specific UV absorbance at wavelength 254 nm (SUVA254), and Φf with ozonation indicated the equal distribution of ozone-reactive moieties.

Evaluation of sequential dosing of potassium permanganate for microcystin‐LR removal

AbstractPermanganate can be scavenged by dissolved organic matter (DOM), which decreases microcystin (MC) removal efficiency, and permanganate attack on cyanobacterial cells can damage cells, resulting in the release of additional MCs. Here, the impact of sequential permanganate doses on MC‐LR removal in the presence of two DOM isolates (terrestrial‐ and algal‐derived) was examined. Although sequential permanganate dosing reduced the competition for permanganate from each DOM isolate, the overall MC removal efficiency was not as high when multiple smaller doses of permanganate were applied compared to one single dose. Potassium permanganate was not observed to lyse Microcystis cells up to 10 mg L−1. Sequential permanganate dosing did not alter the extent of lysis observed for a model bacterium. Mathematical simulations of permanganate oxidation in the presence of DOM suggested that sequential dosing would be most beneficial for enhancing MC‐LR removal when a small portion of highly reactive DOM exists.

Delineating the Effects of Molecular and Colloidal Interactions of Dissolved Organic Matter on Titania Photocatalysis.

In the face of significant challenges to practical applications of photocatalysis for water treatment, recent reports revealed a potential route to overcome a problem posed by dissolved organic matter (DOM). These studies showed that inhibition of photocatalytic processes by DOM is driven largely by competition for active surface sites on TiO2 or other catalysts, and controlling the type of DOM present in solution could significantly mitigate DOM fouling. Whether or not control of solution parameters could achieve the same preventative action is not known. Here, a series of DOM isolates, including humic acid (HA) and transphilic (TPI), hydrophobic (HPO), or colloidal fractions of organic matter from a membrane bioreactor mixed liquor supernatant, were tested for inhibitory activity under a range of pH values (3, 5, 7, and 9) and ionic compositions (NaCl, CaCl2, and Al2(SO4)3 with ionic strengths (IS) ranging from 0 to 3 M). The resulting TiO2-DOM agglomerates were monitored for size and ζ-potential. Inhibitory profiles were generated using para-chlorobenzoic acid (pCBA) as probe with varying concentrations of inhibitory DOM for each solution condition to discern the extent of surface-phase quenching of radicals. Manipulation of pH clearly impacted inhibition, and the effect varied by DOM type; for example, interference occurred at all pHs for HA, at neutral or basic pHs for TPI, and only at pH 7 for HPO. Particle sizes did not correlate with inhibitory action of DOM. Increases in ionic strength induced growth of TiO2 and TiO2-DOM agglomerates, but again, particle sizes did not correlate to inhibition by DOM. The changes to IS, regardless of ion type, were not affected by the presence of TPI or HPO. Since particle stability did not correlate directly with photocatalytic activity, we suggest that surface-based quenching reactions arise from site-specific adsorption rather than generalized particle destabilization and aggregation.

Size exclusion chromatography and stable carbon isotopes reveal the limitations of solid phase extraction with PPL to capture autochthonous DOM production

The study of the elemental and carbon isotopic composition of dissolved organic matter (DOM) is of great interest in deciphering the origin and transformations of organic matter in estuarine and coastal environments. Styrene-divinylbenzene copolymer (PPL) sorbent is currently the most commonly used material for the isolation of DOM from environmental samples. It is notably used for the development of molecular formula-based indices used to study DOM reactivity. However, any extraction procedure (including with PPL) will fractionate the DOM. If this fractionation is not well constrained it can lead to biased interpretations of the biogeochemical processes affecting DOM. In this work we investigate the fractionation effects of the PPL sorbent on the size class distribution of DOM and the carbon isotopic composition of the PPL retentate. The use of size exclusion chromatography, that does not require a pre-concentration step, allows a precise study of the fractionation of DOM (including aromaticity) by the PPL resin. Extractions performed on two types of humic substances dissolved in artificial seawater, using the PPL resin, showed high extraction yield (> 85%) and the stable isotopic carbon composition (δ13C) of these compounds was successfully recovered. These results indicate that salinity is not a parameter affecting extraction yield on PPL sorbent. For a hydrophilic compound (atropine) the extraction efficiency was low (33%) and δ13C signature was underestimated. Size exclusion chromatography measurements, in samples collected along a salinity gradient, demonstrate that the PPL sorbent strongly fractionates DOM. Although the DOM size class distributions in freshwaters and marine waters were initially different, their retentates were marked by similar size class distributions. This work demonstrates that PPL resin captures DOM compounds with less nitrogenous content and it seems to have a lower affinity for aromatic compounds of marine origin than of terrigenous origin. The study of DOM distribution in a macro-tidal estuary demonstrated the limitations of PPL resin extraction in capturing an internal N-rich DOM production event at the time of sampling. Futhermore, the isotopic composition of the PPL resin retentate appears to depend on the extraction efficiency of the more hydrophobic compounds which changed along the salinity gradient. This study recommends careful interpretations of data that only rely on PPL extractions, particularly for works tracking the origin of DOM in estuaries and comparing DOM composition across ocean biogeochemical domains.

Reactivity of Bromine Radical with Dissolved Organic Matter Moieties and Monochloramine: Effect on Bromate Formation during Ozonation.

Bromine radical (Br•) has been hypothesized to be a key intermediate of bromate formation during ozonation. Once formed, Br• further reacts with ozone to eventually form bromate. However, this reaction competes with the reaction of Br• with dissolved organic matter (DOM), of which reactivity and reaction mechanisms are less studied to date. To fill this gap, this study determined the second-order rate constant (k) of the reactions of selected organic model compounds, a DOM isolate, and monochloramine (NH2Cl) with Br• using γ-radiolysis. The kBr• of all model compounds were high (kBr• > 108 M-1 s-1) and well correlated with quantum-chemically computed free energies of activation, indicating a selectivity of Br• toward electron-rich compounds, governed by electron transfer. The reaction of phenol (a representative DOM moiety) with Br• yielded p-benzoquinone as a major product with a yield of 59% per consumed phenol, suggesting an electron transfer mechanism. Finally, the potential of NH2Cl to quench Br• was tested based on the fast reaction (kBr•,NH2Cl = 4.4 × 109 M-1 s-1, this study), resulting in reduced bromate formation of up to 77% during ozonation of bromide-containing lake water. Overall, our study demonstrated that Br• quenching by NH2Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1-2 mgC/L).

Fluorescence Quenching of Humic Substances and Natural Organic Matter by Nitroxide Free Radicals.

Fluorescence spectroscopy is one of the most frequently used techniques for studying dissolved organic matter (DOM) in natural and engineered systems. However, the spatial distribution and fluorophores, including local and interacting states, within DOM's larger structure remains poorly understood. In this study, we used two nitroxide fluorescence quenchers to evaluate the chemical and spatial heterogeneity of DOM fluorophores. Several results from quenching experiments with cationic 4-amino-TEMPO (tempamine), including downward-curving Stern-Volmer plots and spectral dependent quenching, show that multiple emitting species contribute to the observed emission even at a single excitation wavelength. Furthermore, for DOM isolates of diverse geographic origins (soil vs aquatic) and isolation procedures (reverse osmosis vs humic substances), the maximum extent of quenching occurs on the red edge of the emission spectra. For soil humic substance isolates, the spectral dependent quenching was significant enough to affect a blue shift in the average emission wavelength. The same soil humic substance isolates whose emission spectra were blue shifted by tempamine quenching were also blue shifted by decreasing solution pH and decreasing solvent polarity, which suggests a role for anionic fluorophores (e.g., hydroxybenzoic acids) in long wavelength fluorescence. Finally, curvature in Stern-Volmer plots indicate that between 10 and 50% of emitting species detected by steady-state fluorescence are inaccessible to quenching by tempamine, suggesting that this fraction of fluorophores may be inaccessible to water solvent. Results from this study provide an assessment of the spatial distribution of fluorophores within DOM and help to reconcile prior studies on the role of solvent polarity and pH on DOM fluorescence.

Static and dynamic sorption of DOM on Bond Elute PPL and Bondesil PPL sorbents: physical-chemical characteristics

ABSTRACT The objective of this work was to study sorption of dissolved organic matter (DOM) onto Bondesil PPL and Bond Elut PPL sorbents with particular attention to impact of sorption loading on molecular composition of the DOM isolated with a use of solid phase extraction (SPE). It was shown that the batch sorption isotherms were best fitted with the Langmuir model, the sorption constants were 8.6·103 L·kgOC −1 and 10.7·103 L·kgOC −1 for Bond Elut PPL and Bondesil PPL, respectively. Breakthrough curves were satisfactorily fitted by a logistic equation; the maximum sorbent loading was 13% (wt. dissolved organic carbon) for both sorbents. The relationship between loading values and optical descriptors of the DOM at the column exit showed preferential sorption of low molecular weight aromatic DOM components in the range of low loading values (up to 4%), the similar trends were observed for size exclusion chromatography (SEC) measurements. Much lesser impact of loading onto molecular composition was observed within the range from 4 to 8% dissolved organic carbon (DOC). Given high recovery values (70–85%) in this loading range, it can be recommended as preferential for conducting DOM isolation from natural waters using SPE PPL-modified sorbents.

Molecular-level composition of dissolved organic matter in distinct trophic states in Chinese lakes: Implications for eutrophic lake management and the global carbon cycle

Dissolved organic matter (DOM) is an abundant and mobile part of the aquatic environment and plays important roles in aquatic biogeochemical cycles and the global carbon cycle. Recently, eutrophication has become an important environmental issue in global lakes, but how eutrophication drives changes in the molecular composition of DOM along trophic gradients remains poorly understood. We thus characterized 67 DOM isolates from 11 lakes along a trophic gradient in China by using a combined approach including absorption spectroscopy, excitation-emission matrix fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our results indicated that dissolved organic carbon and absorption coefficients at 350 nm increased with increasing trophic status index. The ultraviolet absorbance at 254 nm and fluorescence intensity of all fluorescent components were higher in eutrophic lakes than in oligotrophic lakes. DOM in high trophic state lakes tended to be dominated by higher molecular weight, unsaturation degree, greater abundance of S-containing compounds, and condensed or polycyclic aromatic compounds than oligotrophic lakes. Additionally, autochthonous DOM characterized by more aliphatic compounds increased with the increasing trophic state. We concluded that nutrient input along with allochthonous DOM favors the lake eutrophication and subsequently increases the release and accumulation of autochthonous DOM. Consequently, eutrophication modifies the structure of the organic matter into more complex materials with increased input of allochthonous DOM and increased release of autochthonous DOM, which could accelerate global carbon cycle processes. Our results here have potential to contribute significantly to future studies of DOM dynamics in eutrophic lakes.

Comparative Studies on Sorption Recovery and Molecular Selectivity of Bondesil PPL versus Bond Elut PPL Sorbents with Regard to Fulvic Acids

Large scale isolation—in gram quantities—of dissolved organic matter (DOM) from natural waters is necessary for detailed investigation of its role in chemical and microbial processes driving carbon cycling under conditions of global climate change. The best candidate for a use in these large-scale experiments is a bulk sorbent Bondesil PPL, which has the same modification as the widely used Bond Elut PPL sorbent. There have been no studies so far reported on interchangeability of these sorbents with regard to DOM isolation. This work was devoted to comparative studies on sorption efficiency and molecular selectivity of these two sorbents—Bond Elut PPL and Bondesil PPL with regard to DOM components. Fulvic acids (FA) from peat water leachate were used as a model DOM. Laboratory solid phase extraction (SPE) setup was used for monitoring sorption recovery and extraction yield. It included three parallel experiments on pre-packed Bond Elut PPL cartridges (500 mg/3 mL) and three self-packed Bondesil PPL cartridges (500 mg/3 mL). Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) and 13C/1H nuclear magnetic resonance (NMR) spectroscopy were used for determination of molecular and structural group compositions of the FA isolates obtained with a use of two different sorbents. The results of this study allowed a conclusion on interchangeability of the two sorbents used in this study for the purposes of DOM isolation from natural waters. This conclusion was backed up by similarity of sorption behavior of the peat FA components on both sorbents and by high similarity of molecular compositions and carbon distribution among the main structural groups.

Role of Antioxidant Moieties in the Quenching of a Purine Radical by Dissolved Organic Matter.

Dissolved organic matter (DOM) has been known to inhibit the degradation of trace organic contaminants (TrOCs) in advanced oxidation processes but quantitative understanding is lacking. Adenine (ADN) was selected as a model TrOC due to the wide occurrence of purine groups in TrOCs and the well-documented transient spectra of its intermediate radicals. ADN degradation in the presence of DOM during UV/peroxydisulfate treatment was quantified using steady-state photochemical experiments, time-resolved spectroscopy, and kinetic modeling. The inhibitory effects of DOM were found to include competing for photons, scavenging SO4•- and HO•, and also converting intermediate ADN radicals (ADN(-H)•) back into ADN. Half of the ADN(-H)• were reduced back to ADN in the presence of about 0.2 mgC L-1 of DOM. The quenching rate constants of ADN(-H)• by the 10 tested DOM isolates were in the range of (0.39-1.18) × 107 MC-1 s-1. They showed a positive linear relationship with the total antioxidant capacity of DOM. The laser flash photolysis results of the low-molecular-weight analogues of redox-active moieties further supported the dominant role of antioxidant moieties in DOM in the quenching of ADN(-H)•. The diverse roles of DOM should be considered in predicting the abatement of TrOCs in advanced oxidation processes.

Effect of Solution pH on the Dual Role of Dissolved Organic Matter in Sensitized Pollutant Photooxidation.

Dissolved organic matter (DOM) has a dual role in indirect phototransformations of aquatic contaminants by acting both as a photosensitizer and an inhibitor. Herein, the pH dependence of the inhibitory effect of DOM and the underlying mechanisms were studied in more than 400 kinetic irradiation experiments over the pH range of 6–11. Experiments employed various combinations of one of three DOM isolates, one of two model photosensitizers, the model antioxidant phenol, and one of nine target compounds (TCs), comprising several aromatic amines, in particular anilines and sulfonamides, and 4-cyanophenol. Using model photosensitizers without antioxidants, the phototransformation of most TCs increased with increasing pH, even for TCs for which pH did not affect speciation. This trend was attributed to pH-dependent formation yields of TC-derived radicals and their re-formation to the parent TC. Analogous trends were observed with DOM as a photosensitizer. Comparison of model and DOM photosensitizer data sets showed increasing inhibitory effects of DOM on TC phototransformation kinetics with increasing pH. In systems with anilines as a TC and phenol as a model antioxidant, pH trends of the inhibitory effect could be rationalized based on the reduction potential difference (ΔEred) of phenoxyl/phenol and anilinyl/aniline couples. Our results indicate that the light-induced transformation of aromatic amines in the aquatic environment is governed by the pH-dependent inhibitory effects of antioxidant phenolic moieties of DOM and pH-dependent processes related to the formation of amine oxidation intermediates.

Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone

The application of oxidants for disinfection or micropollutant abatement during drinking water and wastewater treatment is accompanied by oxidation of matrix components such as dissolved organic matter (DOM). To improve predictions of the efficiency of oxidation processes and the formation of oxidation products, methods to determine concentrations of oxidant-reactive phenolic, olefinic or amine-type DOM moieties are critical.Here, a novel selective oxidative titration approach is presented, which is based on reaction kinetics of oxidation reactions towards certain DOM moieties. Phenolic moieties were determined by oxidative titration with ClO2 and O3 for five DOM isolates and two secondary wastewater effluent samples. The determined concentrations of phenolic moieties correlated with the electron-donating capacity (EDC) and the formation of inorganic ClO2-byproducts (HOCl, ClO2−, ClO3−). ClO2-byproduct yields from phenol and DOM isolates and changes due to the application of molecular tagging for phenols revealed a better understanding of oxidant-reactive structures within DOM.Overall, oxidative titrations with ClO2 and O3 provide a novel and promising tool to quantify oxidant-reactive moieties in complex mixtures such as DOM and can be expanded to other matrices or oxidants.

Binding strength of mercury (II) to different dissolved organic matter: The roles of DOM properties and sources

Dissolved organic matter (DOM) influences the environmental fate and toxic effects of trace metals such as mercury (Hg). However, because of limits in DOM analytical techniques and lack of sample diversity in past studies, it remains unclear whether the binding strength of DOM complexed with Hg(II) is related to the DOM properties. In this study, different DOM isolates (n = 26) from various sources were used to determine the conditional stability constant (logK) of DOM-Hg complexes using the equilibrium dialysis ligand exchange (EDLE) method. UV–Vis and fluorescence spectrometry were used to evaluate the correlation between logK values and DOM properties, such as chromophoric moieties, aromaticity, and molecular weight. Results demonstrated that the DOM from different sources presented an extensive range of binding strengths to Hg(II), because of their heterogeneous properties. Moreover, DOM chromophores, including aromaticity and molecular weight, are critical indicators of the DOM-Hg affinity in ambient-relevant circumstances. Significantly, higher terrestrial DOM led to greater DOM-Hg affinity. Additionally, this study supports that UV–Vis and fluorescence spectroscopy can be used to estimate DOM composition and its binding strength with Hg(II). Furthermore, the observed relationship between logK and DOM properties provided a possible pathway of explanation for the spatial co-variations between Hg(II) concentrations and DOM characters observed in previous field investigations.

Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride.

The reaction of dissolved organic matter (DOM) with sodium borohydride has been used to understand the geographic origin of DOM and investigate the photophysical model underlying DOM's optical properties. However, the physicochemical properties of DOM (e.g., molecular size and charge) that influence the kinetics and ultimate reducibility of DOM by borohydride remain poorly characterized. Herein, we studied the kinetics of DOM-borohydride reactions by recording absorbance and fluorescence spectra at a high temporal frequency (every ∼10 min for 24 h) for a diverse set of DOM isolates of aquatic and soil origin. The reducibility of DOM by sodium borohydride (as judged by relative removal of initial absorbance) varied appreciably among the DOM samples studied, with soil humic substances being less reducible than aquatic humic substances and natural organic matter. While statistically significant correlations were found between the reducibility of DOM and descriptors of molecular size, these descriptors were not able to differentiate the reducibility of soil versus aquatic DOM isolates that had similar bulk properties. Thus, it appears that the extent of absorbance removal by borohydride is largely driven by the origin of the humic substance isolate (aquatic vs soil) instead of molecular size or charge. Borohydride reduction resulted in increased fluorescence emission across UV and visible excitation wavelengths. However, the enhanced emission at visible excitation decreased over a time period of hours to days, suggesting that reduction of an important subset of DOM chromophores is reversible. This reversibility in fluorescence emission is consistent with the small role of quinones in the absorbance of DOM but suggests a more important role for quinone-containing charge-transfer contacts in the fluorescence of DOM, particularly at visible excitation wavelengths.