Triplet-State Photochemistry of Dissolved Organic Matter: Triplet-State Energy Distribution and Surface Electric Charge Conditions

Influence of dissolved organic matter on carbonyl sulfide and carbon disulfide formation from cysteine during sunlight photolysis

Carbonyl sulfide (COS) and carbon disulfide (CS2) are important atmospheric gases that are formed from organic sulfur precursors present in natural waters when exposed to sunlight. However, it remains unclear how specific water constituents, such as dissolved organic matter (DOM), affect COS and CS2 formation. To better understand the role of DOM, irradiation experiments were conducted in O2-free synthetic waters containing four different DOM isolates, acquired from freshwater to open ocean sources, and the sulfur-based amino acid, cysteine (CYS). CYS is a known natural precursor of COS and CS2. Results indicated that COS formation did not vary strongly with DOM type, although small impacts were observed on the kinetic patterns. COS formation also increased with increasing CYS concentration but decreased with increasing DOM concentration. Quenching experiments indicated that ˙OH was not involved in the rate-limiting step of COS formation, whereas excited triplet states of DOM (3CDOM*) were plausibly involved, although the quenching agents used to remove 3CDOM* may have reacted with the CYS-derived intermediates as well. CS2 was not formed under any of the experimental conditions. Overall, DOM-containing synthetic waters had a limited to no effect towards forming COS and CS2, especially when compared to the higher concentrations formed in sunlit natural waters, as examined previously. The reasons behind this limited effect need to be explored further but may be due to the additional water quality constituents present in these natural waters. The findings of this study imply that multiple variables beyond DOM govern COS and CS2 photoproduction when moving from freshwaters to open ocean waters.

Photochemical Characterization of Surface Waters from Lakes in the Adirondack Region of New York

The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φ_app,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH), for surface waters collected from 16 representative Adirondack lakes. Φ_app,³DOM* and Φ_app,¹O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φ_app,^•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S_290-400) as the most effective predictor of Φ_app,RI among measured water chemistry parameters and bulk DOM properties. Φ_app,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.

Distinct effects of copper on the degradation of β-lactam antibiotics in fulvic acid solutions during light and dark cycle

This study revealed the dual roles of Cu(II) on the β-lactam antibiotics degradation in Suwannee River fulvic acid (SRFA) solution during day and night cycle. Amoxicillin (AMX) and ampicillin (AMP) were selected as the representative β-lactam antibiotics. Cu(II) played a key role in the dark degradation of AMX and AMP via catalytic hydrolysis and oxidation. However, Cu(II) mainly exhibited an inhibitory effect on SRFA-involved photochemical degradation of AMX and AMP. In the presence of 500 nM of Cu(II), the degradation rate of AMX and AMP in the light condition were around 5 times higher than that in the dark condition, suggesting the photodegradation of β-lactam antibiotics was much more pronounced than catalyzed hydrolysis and oxidation. The triplet excited state of SRFA (³SRFA∗) primarily contributed to AMX and AMP photodegradation. Hydroxyl radicals (^•OH) and singlet oxygen (¹O₂) exhibited limit impacts. The redox cycle of Cu(II)/Cu(I) restricted the electron transfer pathway of ³SRFA∗ with AMX and AMP. During the day and night cycles for 48 h, Cu(II) served as a stronger inhibitor rather than a promotor. These findings highlight the interactions between Cu(II) and SRFA are distinct under day and night conditions, which could further affect the fate of β-lactam antibiotics in natural environments.

UV365 induced elimination of contaminants of emerging concern in the presence of residual nitrite: Roles of reactive nitrogen species

The presence of nitrite (NO₂^-) is inevitable with concentrations of several mg L^-1 in some typical water bodies. In this study, UV at wavelength of 365 nm was investigated to degrade contaminants of emerging concern (CECs) in the presence of NO₂^- at environmentally relevant concentrations (0.1-5.0 mg L^-1). Six selected CECs with different structures were efficiently removed because of the generation of reactive nitrogen species (RNS) and hydroxyl radical (HO^•) from photolysis of NO₂^-. Contributions of UV₃₆₅ photolysis, RNS, and HO^• to CEC degradation in UV₃₆₅/NO₂^- system were calculated, and RNS were found to be the predominant species that are responsible for CEC degradation. The second major contributor is HO^• for the degradation of selected CECs except for the case of sulfadiazine. Impacts of water matrix components (including dissolved oxygen, solution pH, and natural organic matter) on CEC degradation in UV₃₆₅/NO₂^- system were evaluated. Furthermore, evolution profiles of CECs and NO₂^- in UV₃₆₅/NO₂^- system were tracked when actual water samples were used as background, and a simultaneous removal of CECs and NO₂^- was observed. Transformation products of bisphenol A and carbamazepine were proposed according to the results of HPLC/MS and quantum chemistry calculations. Nitration induced by RNS and hydroxylation induced by HO^• are main reactions occurred during CEC degradation in UV₃₆₅/NO₂^- system. Overall, UV₃₆₅ is a potential technology to remove CECs and NO₂^- in aquatic environment when residual NO₂^- is present. Our present study also provides possibility for the application of sunlight to remediate water co-polluted by CECs and NO₂^-.

Insights on photochemical activities of organic components and minerals in dissolved state biochar in the degradation of atorvastatin in aqueous solution

This study systematically investigated the photocatalytic activity of dissolved state biochar (DSB) with different pyrolysis temperature to the degradation of atorvastatin (ATV), a medicine widely used to combat hyperlipidemia. It was found that the photocatalytic efficiency of DSB increased with the decrease of pyrolysis temperature, that is, DSB300 (DSB with 300 °C of pyrolysis temperature) had the greatest photocatalytic activity in same condition, which was attributed to the dual role of DSB300 as heterogeneous photocatalyst and photosensitizer. The mineral components were responsible for the heterogeneous photocatalytic activity of DSB300. Organic carbon components could synergistically enhance the heterogeneous photocatalytic activity by enhancement of electron-hole separation, and contribute to the formation of singlet oxygen (¹O₂) and triplet-excited state (³DSB*) as well. The identification of intermediate products and X-ray photoelectron spectroscopy (XPS) analysis of irradiated DSB300/ATV revealed that cross-coupling reaction between ATV and DSB existed in the photodegradation process of ATV. The detailed photodegradation pathways of ATV were proposed, which was triggered by oxygen insertion of pyrrole ring and hydroxyl addition. Meanwhile, the modification of DSB300 under irradiation was evidently attenuated with ATV as shown by multiple characterizations, which helped to keep the stability of DSB300 in photochemical reaction process.

Triplet Photochemistry of Dissolved Black Carbon and Its Effects on the Photochemical Formation of Reactive Oxygen Species

Dissolved black carbon (DBC) is an important component of dissolved organic matter pool; however, its photochemical properties are not fully understood. In this study, we determined the excited triplet-state quantum yields of DBC (³DBC*) and ¹O₂ quantum yields (Φ_¹O2) of six diverse DBCs using sorbic alcohol, 2,4,6-trimethylphenol (TMP), and furfuryl alcohol and compared the results with quantum yields of reference natural organic matters (NOMs). The average Φ_¹O2 of six DBCs (4.2 ± 1.5%) was greater than that of terrestrial NOM (2.4 ± 0.3%) and comparable to autochthonous NOM (5.3 ± 0.2%). Using TMP as a probe for oxidizing triplets, DBC presented significantly higher apparent quantum yield coefficients for degrading TMP (f_TMP) than the reference NOM, reflecting that the f_TMP values of low-energy ³DBC* were approximately 12-fold greater than those of low-energy ³NOM*. The differences in the f_TMP and Φ_¹O2 trends among the DBCs indicated that the ³DBC* responsible for these reactions may be from different sources. In addition, DBC was much more effective than NOM, on a carbon-normalized basis, during photodegradation of pharmaceutically active compounds. This result confirms that the presence of DBC can accelerate the photodegradation of contaminants that are susceptible to one-electron oxidation by triplets.

Kinetic Consideration of Photochemical Formation and Decay of Superoxide Radical in Dissolved Organic Matter Solutions

The photochemical formation and decay rates of superoxide radical ions (O₂^•-) in irradiated dissolved organic matter (DOM) solutions were directly determined by the chemiluminescent method. Under irradiation, uncatalyzed and catalyzed O₂^•- dismutation account for ∼25% of the total O₂^•- degradation in air-saturated DOM solutions. Light-induced O₂^•- loss, which does not produce H₂O₂, was observed. Both the O₂^•- photochemical formation and light-induced loss rates are positively correlated with the electron-donating capacities of the DOM, suggesting that phenolic moieties play a dual role in the photochemical behavior of O₂^•-. In air-saturated conditions, the O₂^•- quantum yields of 12 DOM solutions varied in a narrow range, from 1.8 to 3.3‰, and the average was (2.4 ± 0.5)‰. The quantum yield of O₂^•- nonlinearly increased with increasing dissolved oxygen concentration. Therefore, the quantum yield of one-electron reducing intermediates, the precursor of O₂^•-, was calculated as (5.0 ± 0.4)‰. High-energy triplets (³DOM*, E_T > 200 kJ mol^-1) and ¹O₂ quenching experiments indicate that ³DOM* and ¹O₂ play minor roles in O₂^•- production. These results are useful for predicting the photochemical formation and decay of O₂^•- in sunlit surface waters.

Dissolved Organic Matter Singlet Oxygen Quantum Yields: Evaluation Using Time-Resolved Singlet Oxygen Phosphorescence

Singlet oxygen (¹O₂) generation quantum yields from chromophoric dissolved organic matter (CDOM) have been reported for many samples over the past 4 decades. Yet even for standardized isolates such as those from the International Humic Substance Society (IHSS), wide-ranging values exist in the literature. In this manuscript, time-resolved ¹O₂ phosphorescence was used to determine the ¹O₂ quantum yields (Φ_Δ) of a variety of dissolved organic matter (DOM) isolates and natural waters. In general, the ¹O₂ quantum yield values in this study are in the middle, although below the median of the range of past reported values (e.g., for Suwannee River Natural Organic Matter IHSS isolate: 1.8% vs 0.23-2.89%). Notably, hydrophobic neutral fractions of DOM isolates were found to possess the highest ¹O₂ quantum yields, an interesting result given that these fractions are not retained in typical humic and fulvic acid isolation procedures that use XAD resins. The excitation wavelength dependence of ¹O₂ generation from CDOM was also examined, and an approximate linear decrease with longer excitation wavelength was observed. This work advances the understanding of CDOM photoprocesses, especially in relation to wavelength-dependent ¹O₂ production, which is valuable for assessing real-world environmental behavior.

Overview of the Phototransformation of Wastewater Effluents by High-Resolution Mass Spectrometry

Photochemical transformation driven by sunlight is one of the most important natural processes for organic contaminant attenuation. In the current study, statistical analysis-assisted high-resolution mass spectrometry was employed to investigate the phototransformation of nontarget features in wastewater effluents under various radical quenching/enhancing conditions. A total of 9694 nontarget features were extracted from the effluents, including photoresistant features, photolabile features, and transformation products. 65% of the wastewater effluent features were photoresistant, and the photolabile features could be classified into five groups: direct photolysis group (group I), HO^•-originated species-dominated group (group II), ³OM*-dominated group (group III), photochemically produced reactive intermediates combination-dominated group (group IV), and non-first-order degradation group (group V). The direct photolyzed features were observed to degrade significantly faster than the indirect photolyzed features. Moreover, group II dominated by HO^•-originated species contributed 34% to the photolabile features. The reaction types that occurred in the phototransformation process were analyzed by linkage analysis. The results suggested that oxygen addition and dealkyl group reactions were the most common reaction types identified in the phototransformation process. Overall, high-resolution mass spectrometry coupled with statistical analysis was applied here to understand the photochemical behavior of the unknown features in wastewater effluents.

Assessing the contribution of hydroxylation species in the photochemical transformation of primidone (pharmaceutical)

Pharmaceutical and personal care products (PPCPs) are a group of emerging contaminants that have frequently been detected in aqueous environments. Phototransformation driven by solar irradiation is one of the most important natural processes for the elimination of PPCPs. In this study, primidone (PMD) was chosen as a model "photorefractory" compound. A series of experiments were conducted to assess if reactive intermediates (RIs), such as hydroxyl radical (HO), singlet oxygen (¹O₂), and triplet states of dissolved organic matter (³DOM^⁎), inhibited or enhanced the photochemical transformation of PMD under simulated solar irradiation. The results indicate that HO plays a key role in the photodegradation of PMD and that dissolved oxygen can affect the degradation rate of PMD by promoting HO formation. Our results demonstrated that PMD can not only react with free HO (HO_-free) but also react with lower-energy hydroxylation agents (HO_-like). The contributions of HO_-free and HO_-like to PMD degradation in various dissolved organic matter (DOM) solutions were estimated by a methane-quenching experiment. The results indicated that the HO_-like species were important in the photodegradation of "photorefractory" compounds. The bimolecular reaction rate constant of the reaction of free HO with PMD was measured as (5.21 ± 0.02) × 10⁹ M^-1 s^-1 by using electron pulse radiolysis. Furthermore, PMD was used as a probe to estimate the steady-state concentration of HO_-free in various DOM solutions. Using the multivariate statistical strategies of orthogonal projection to latent structures discriminant analysis (OPLS-DA) and hierarchical clustering, 28 photochemical transformation products (TPs) of PMD were successfully identified from the DOM matrix.

Effects of photochemical and microbiological changes in terrestrial dissolved organic matter on its chemical characteristics and phytotoxicity towards cyanobacteria

Previous studies have shown that under laboratory conditions, dissolved organic matter (DOM) leached from plants can be differentially more phytotoxic to cyanobacteria, compared to green algae. This study examined how DOM source and transformation processes (microbial and photochemical) affect its chemical composition and phytotoxicity towards a cultured species of cyanobacteria (Raphidiopsis raciborskii) using a factorial experimental design. To complement cyanobacterial bioassays, the chemical composition and associated changes in DOM were determined using spectroscopic (nuclear magnetic resonance (NMR) and absorbance) and elemental analyses. Sunlight exposed DOM from leaves of the terrestrial plants, Casuarina cunninghamiana and Eucalyptus tereticornis had the most phytotoxic effect compared to DOM not exposed to sunlight. This phytotoxic DOM was characterised by relatively low nitrogen content, containing highly coloured and relatively high molecular mass constituents. Both mixed effect model and PCA approaches to predict inhibition of photosynthetic yield indicated phytotoxicity could be predicted (P < 0.001) based upon the following parameters: C: N ratio; gilvin, and lignin-derived phenol content of DOM. Parallel proton-detected 1D and 2D NMR techniques showed that glucose anomers were the major constituents of fresh leachate. With ageing, glucose anomers disappeared and products of microbial transformation appeared, but there was no indication of the appearance of additional phytotoxic compounds. This suggests that reactive oxygen species may be responsible, at least partially, for DOM phytotoxicity. This study provides important new information highlighting the characteristics of DOM that link with phytotoxic effects.

Oxidation of Microcystic-LR via the solar/chlorine process: Radical mechanism, pathways and toxicity assessment

Microcystin-LR (MC-LR) is the most widely distributed and harmful variant toxins released by cyanobacteria, which poses potential threaten to people and aquatic animals when entering natural water. In our research, solar/chlorine process was comprehensively investigated to degrade and detoxify MC-LR. Under the chlorine concentration of 1.0 mg L^-1, MC-LR (1.0 μM) was decreased by 96.7%, 26%, and 9% by solar/chlorine process, chlorination, and solar irradiation respectively. Quenching experiments confirmed that reactive chlorine species (RCS) and hydroxyl radical (HO) were the predominant reactive species in solar/chlorine process at neutral condition, and ozone was generated because of the participation of triplet-state oxygen (O(³P)). The respective contributions of each reactive species were calculated with the order as: RCS, HO, ozone, and solar irradiation. The presence of HCO₃^- and natural organic matter in water inhibited the degradation efficiency of MC-LR. Moreover, the transformation products of MC-LR generated during the solar/chlorine process were identified and a possible pathway was proposed. The hepatotoxicity of MC-LR and its transformation products was compared using protein phosphatase 2A. Our experimental results revealed that the concentration and hepatotoxicity of MC-LR both significantly decreased, and most products were not hepatoxic. Overall, the solar/chlorine process is a promising alternative technology to degrade MC-LR during eutrophication.

Photodegradation of 2-(2-hydroxy-5-methylphenyl)benzotriazole (UV-P) in coastal seawaters: Important role of DOM

Benzotriazole UV stabilizers (BT-UVs) have attracted concerns due to their ubiquitous occurrence in the aquatic environment, and their bioaccumulative and toxic properties. However, little is known about their aquatic environmental degradation behavior. In this study, photodegradation of a representative of BT-UVs, 2-(2-hydroxy-5-methylphenyl)benzotriazole (UV-P), was investigated under simulated sunlight irradiation. Results show that UV-P photodegrades slower under neutral conditions (neutral form) than under acidic or alkaline conditions (cationic and anionic forms). Indirect photodegradation is a dominant elimination pathway of UV-P in coastal seawaters. Dissolved organic matter (DOM) from seawaters accelerate the photodegradation rates mainly through excited triplet DOM (³DOM^⁎), and the roles of singlet oxygen and hydroxyl radical are negligible in the matrixes. DOM from seawaters impacted by mariculture exhibits higher steady-state concentration of ³DOM^⁎ ([³DOM^⁎]) relative to those from pristine seawaters, leading to higher photosensitizing effects on the photodegradation. Halide ions inhibit the DOM-sensitized photodegradation of UV-P by decreasing [³DOM^⁎]. Photodegradation half-lives of UV-P are estimated to range from 24.38 to 49.66 hr in field water bodies of the Yellow River estuary. These results are of importance for assessing environmental fate and risk UV-P in coastal water bodies.

Photocatalytic degradation of cephalexin by ZnO nanowires under simulated sunlight: Kinetics, influencing factors, and mechanisms

Increasing concentrations of anthropogenic antibiotics and their metabolites in aqueous environments has caused growing concerns over the proliferation of antibiotic resistance and potential adverse impacts to agro-environmental quality and human health. Photocatalysis using novel engineered nanomaterials such as ZnO nanowires may be promising for removing antibiotics from waters. However, much remains to be learned about efficiency and mechanism for photocatalytic degradation of antibiotics by ZnO nanowires. This study systematically investigated photodegradation of cephalexin using ZnO nanowires under simulated sunlight. The degradation efficiency of cephalexin was substantially increased in the presence of ZnO nanowires especially at circumneutral and alkaline condition (solution pH of 7.2-9.2). The photodegradation followed the first-order kinetics with degradation rate constants (k) ranging between 1.19 × 10^-1 and 2.52 × 10^-1 min^-1 at 20-80 mg L^-1 ZnO nanowires. Radical trapping experiments demonstrated that hydroxyl radicals (OH) and superoxide radicals (O₂^-) predominantly contributed to the removal of cephalexin. With the addition of HCO₃^- (1-5 mM) or Suwannee River natural organic matter (SRNOM, 2-10 mg L^-1), the k values were substantially decreased by a factor of 1.8-70 to 1.69 × 10^-3-6.67 × 10^-2 min^-1, probably due to screening effect of HCO₃^- or SRNOM sorbed on ZnO nanowires and scavenging of free radicals by free HCO₃^- or SRNOM in solution. Combining product identification by mass spectrometry and molecular computation, cephalexin photodegradation pathways were identified, including hydroxylation, demethylation, decarboxylation, and dealkylation. Overall, the novel ZnO nanowires have the potential to be used for removing antibiotics from contaminated waters.

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.

Sorbic Acid as a Triplet Probe: Reactivity of Oxidizing Triplets in Dissolved Organic Matter by Direct Observation of Aromatic Amine Oxidation

Sorbic acid (2,4-hexadienoic acid; HDA) isomerization is frequently used to probe triplet-state dissolved organic matter (³CDOM*) reactivity, but there remain open questions about the reaction kinetics of ³CDOM* with HDA due to the difficulties of directly measuring ³CDOM* quenching rate constants. Using our recently developed approach based on observing the radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) formed through oxidation of TMPD by ³CDOM*, we studied ³CDOM* quenching kinetics with HDA monitored via transient absorption spectroscopy. A competition kinetics-based approach utilizing formation yields of TMPD^•+ was developed, validated with model sensitizers, and used to determine bimolecular rate constants between ³CDOM* oxidants and HDA for diverse DOM isolates and natural waters samples, yielding values in the range of (2.4-7.7) × 10⁸ M^-1 s^-1. The unquenchable fraction of TMPD-oxidizing triplets showed that, on average, 41% of ³CDOM* oxidants cannot be quenched by HDA. Conversely, cycloheptatriene quenched nearly all TMPD^•+-forming triplets in CDOM, suggesting that most ³CDOM* oxidants possess energies greater than 150 kJ mol^-1. Comparing results with our companion study, we found slight, but noticeable differences in the ³CDOM* quenching rate constants by HDA and unquenchable triplet fractions determined by oxidation of TMPD and energy transfer to O₂ (¹O₂ formation) methods.

Sorbic Acid as a Triplet Probe: Triplet Energy and Reactivity with Triplet-State Dissolved Organic Matter via 1O2 Phosphorescence

Sorbic acid (2,4-hexadienoic acid; HDA) is commonly used as a probe and quencher for triplet-excited chromophoric dissolved organic matter (³CDOM*), an important transient species in natural waters, yet much remains unknown about its reactivity with ³CDOM* and its triplet energy. To better understand the quenching behavior of HDA, we measured HDA quenching rate constants for various humic substance isolates and whole waters with singlet oxygen (¹O₂) phosphorescence and determined the triplet energy of HDA. Low-temperature phosphorescence measurements determined the triplet energy of HDA to be 217 kJ mol^-1, whereas a complementary method based on triplet quenching kinetics found a triplet energy of 184 ± 7 kJ mol^-1. Time-resolved ¹O₂ phosphorescence measurements yielded different HDA quenching rate constants depending on the fitting method. Using an approach that considered the reactivity of the entire triplet pool produced values of (∼1-10) × 10⁸ M^-1 s^-1, while an approach that considered only the reactivity of the high-energy triplets output higher rate constants ((∼7-30) × 10⁸ M^-1 s^-1). In addition, the model based on high-energy triplet reactivity found that ∼30-60% of ³CDOM* is not quenched by HDA. Findings from this study provide a more comprehensive view on the use of HDA as a probe for ³CDOM*.

Natural Photosensitizers in Constructed Unit Process Wetlands: Photochemical Characterization and Inactivation of Pathogen Indicator Organisms

Dissolved organic matter (DOM) is a natural photosensitizer that contributes to the inactivation of microbial pathogens. In constructed treatment wetlands with open water areas DOM can promote sunlight disinfection of wastewater effluent, but a better understanding of DOM spectroscopic and photochemical properties and how they are impacted by different unit process wetlands is needed to inform design. The goals of this study were: (1) to investigate whether DOM isolates realistically represent the photochemistry of the source DOM in its original water and (2) to observe how changes of DOM along a treatment wetland affect its photochemistry, including pathogen inactivation. A pilot scale unit process wetland was studied that consisted of three different cells (open water, cattail, and bulrush) fed by secondary wastewater effluent. DOM was isolated using solid-phase extraction (SPE), photochemically characterized, and compared to the original water samples and standard DOMs. For MS2 coliphage, a virus indicator, the most efficient photosensitizer was the wastewater DOM isolated from the influent of the wetland, while for the bacterial indicator Enterococcus faecalis, inactivation results were comparable across wetland isolates. SPE resulted in isolation of 47% to 59% of whole water DOM and enriched for colored DOM. Singlet oxygen precursors were efficiently isolated, while some excited triplet state precursors remained in the extraction discharge. DOM processing indicators such as SUVA₂₅₄, SUVA₂₈₀, and spectral slopes including E₂/ E₃ ratios were reflected in the isolates. Photoinactivation of MS2 was significantly lower in both the reconstituted water samples and isolates compared to the original water sample, possibly due to disturbance of the trans-molecular integrity of DOM molecules by SPE that affects distance between MS2 and DOM sites with locally higher singlet oxygen production. For E. faecalis, results were similar in original water samples and isolates. Higher sorption of DOM to E. faecalis was roughly correlated with higher photoinactivation rates. To enhance sunlight disinfection in unit process wetlands, there is no advantage to placing open water cells after vegetated cells, as passage through the vegetated cells led to increased light absorption and lower singlet oxygen and triplet-state quantum yields and steady state concentrations.