Unraveling Multiple Pathways of Electron Donation from Phenolic Moieties in Natural Organic Matter

Electrochemistry and Molecular Compositions Reflect Electron Shuttling of Dissolved Organic Matter in High Arsenic Groundwater

Little is known about the electron shuttle ability of dissolved organic matter (DOM) and its effects on arsenic (As) mobilization, which makes the underlying mechanism of groundwater As enrichment elusive. In this study, both the electrochemical properties and molecular compositions of DOM in high As groundwater were quantified in the Hetao Basin, China. We found that, along the flow path, the average electron-transferring capacity (ETC) of DOM, including the capacities of electron-accepting and electron-donating, continuously increased from 2.85 to 3.59 mmole-/gC along with As concentrations. The increasing ETC reflected an increase in electron shuttle ability of DOM. Furthermore, the increasing electron shuttle ability was mainly attributed to the recalcitrant compounds in DOM, especially CHOS and CHONS formulas in highly unsaturated structures with high oxygen (HUSHO) and CHO and CHON formulas in aromatic structures (AS). The significantly positive correlation between As concentration and ETC indicated that recalcitrant DOM promoted groundwater As enrichment through electron shuttling for inducing the reductive dissolution of As-containing Fe(III) oxide minerals, which was further supported by our culture experiments showing that goethite was more reduced [133 μM Fe(II)] in the presence of DOM with a higher ETC (3.35 mmole-/gC) as electron shuttling than that [65.2 μM Fe(II)] with a relatively lower ETC (2.41 mmole-/gC). Our study highlights that recalcitrant DOM compounds with unsaturated and AS have high electron shuttle ability, promoting As enrichment in groundwater.

Microscopic mechanism of organic carbon sequestration and redox properties influenced by iron (Oxyhydr)oxides

Iron and organic carbon (OC) biogeochemical cycling is highly correlated, and dissolved organic matter (DOM), a highly reactive component of soil and water environments, is the main OC source. However, the micro-mechanism of the molecular fractionation of DOM, the spatial OC distribution on iron (oxyhydr)oxides, and how these factors further affect their redox properties remain to be fully understood. Therefore, this study investigated the DOM adsorption properties of iron (oxyhydr)oxides with different crystallinities at the molecular level through the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and transmission electron microscopy/electron energy loss spectroscopy (TEM-EELS) analyses of the liquid-solid phases. Owing to the limited number of adsorption sites, OC sequestration on goethite and hematite surfaces generally followed an "onion" model, in the order of preference of aromatic, aliphatic, and carboxylic acid-rich compounds. Combined with dielectric electrochemical tests and charge differential density calculations, the results revealed that the complexation effect produced by iron ions increased the electron-accepting capacity (EAC) of the DOM remaining in the aqueous solution. In contrast, molecular selective adsorption and oxidative polymerization significantly enhanced the EAC of DOM adsorbed on the surface fraction of iron (oxyhydr)oxides. These findings help elucidate the mechanism of OC sequestration by iron (oxyhydr)oxides. The increased EAC may affect various biogeochemical processes, such as methane production and microbial Fe(III) reduction, facilitating the prediction of OC cycling in natural environments.

Removal of Sulfonamide Antibiotics in Peracetic Acid-Mediated Natural Polyphenol Systems via an Overlooked Polymerization Pathway: Role of ortho-Quinones

Natural polyphenols can be oxidized into reactive quinones, which might play a key role in the removal of specific contaminants in natural polyphenol-related advanced oxidation processes (AOPs). In this study, peracetic acid (PAA) was employed in combination with natural protocatechuic acid (PCA) to remove sulfonamide antibiotics (SAs) from water. More than 95% removal of sulfamethoxazole (SMX) and other SAs was observed in the PCA/PAA system, and neutral pH conditions (5.0-8.0) were more conducive to the removal of SMX. The PCA/PAA system exhibited a great anti-interference ability against complex water matrices. ortho-Quinone, generated from the oxidation of PCA by PAA, played a dominant role in the SMX removal. Electrons tended to transfer from SMX to the generated ortho-quinones and form covalent bonds, resulting in the production of less toxic oligomers via the overlooked polymerization pathway. A reduction in the toxicity of the SMX solution was found following treatment with the PCA/PAA system. More interestingly, several polyphenols structurally related to PCA could also facilitate SMX removal using PAA as the oxidant. Overall, this study proposes a novel strategy for developing reactive quinones dominated AOPs with robust anti-interference performance, as well as enhances the understanding of contaminant removal via an overlooked polymerization pathway in natural polyphenol-related AOPs.

Electron exchange capacity of dissolved natural organic matter: further method development and interpretation using square wave voltammetry in dimethyl sulfoxide

Most measurements of the electron exchange capacity (EEC) of natural organic matter (NOM) have been done in water using mediated chronoamperometry (MCA), which gives precise results that are believed to be representative of the samples' current redox condition, but the broader significance of these EECs is less clear. In a recent study, we described a novel but complementary electrochemical approach to quantify EECs of 10 pyrogenic dissolved organic matter (pyDOM) and 6 standard/reference natural organic matter (NOM) materials without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Comparison of the results obtained by MCA and SWV showed that SWV in DMSO gave larger EECs than MCA, by several-fold for NOM and 1-2 orders of magnitude for pyDOM. In this study, we describe an improved protocol for calibration of the SWV/DMSO method, which largely eliminates the difference in EECs from SWV and MCA for the standard/reference NOM samples. The results show that values obtained via the SWV method depend on the specific redox standards used for calibration (i.e., calibrant model compounds), with slopes that span 1.5 orders of magnitude due to variations in current response factors. For pyDOM, the higher values of EEC obtained by SWV were further verified and rationalized. Like the calibrant model compounds, it is proposed that the relatively large EECs for some pyDOM samples from high-temperature chars reflect a combination of hydrodynamic influences in our electrochemical cell, primarily related to electrode surface area to volume ratio and pyDOM size. A detailed explanation of the calibration method, choice of working electrode, DOM sorption effects, and cosolvent effects are discussed. The results obtained with this method suggest that the capacity of NOM for donating, accepting, and storing elections is an operationally defined property, the significance of which will depend on application, e.g., to carbon, metal, or nutrient cycling, pollutant attenuation, etc.

Low-molecular weight organic acids can enhance the microbial reduction of iron oxide nanoparticles and pollutants by improving electrons transfer

The combined application of dissimilatory iron-reducing bacteria (DIRB) and Fe(III) nanoparticles has garnered widespread interest in the contaminants transformation and removal. The efficiency of this composite system relies on the extracellular electron transfer (EET) process between DIRB and Fe(III) nanoparticles. While modifications to Fe(III) nanoparticles have demonstrated improvements in EET, enhancing DIRB activity also shows potential for further EET enhancement, meriting further investigation. In this study, we demonstrated that the addition of low-molecular organic acids (LMWOAs) (oxalate, pyruvate, malate, citrate, or fumarate) can improve the reduction of Fe2O3 nanoparticles by Geobacter sulfurreducens PCA through three pathways: increasing intracellular electron production, enhancing the reductive activity of extracellular metabolites, and improving the electron-donating capacity of extracellular polymeric substances. The maximum reduction of Fe2O3 nanoparticles reached up to 72 %. Our results further showed that LMWOAs significantly boosted the removal rate and ratio of Cr(VI) and hexachlorobenzene (HCB) by accelerating the EET process. Following the introduction of LMWOAs, the maximum reduction ratio of Cr(VI) reached 98 ± 0.05 % within 24 h, while the degradation efficiency of HCB reached 92 ± 0.06 % within 9 h. Overall, our study provided a precise mechanism of the role of LMWOAs on the EET process and a new strategy for reductive bioremediation of pollutants.

Molecular Insights into the Synergistic Inhibition of Microplastics-Derived Dissolved Organic Matter and Anions on the Transformation of Ferrihydrite

Ferrihydrite (Fh), as a ubiquitous iron (oxyhydr)oxide, plays an essential role in nutrient cycling and pollutant transformation due to its high surface area and diversified reaction sites. In the natural environment, Fh transformation could be easily influenced by coexisting components (particularly dissolved organic matter (DOM) and anions). As a new and important carbon source, microplastic-derived DOM (MP-DOM) directly or indirectly affects the morphology and fate of Fh, but limited knowledge exists about the combined effect of MP-DOM and anions on Fh transformation. Herein, this study elucidates the joint effects of polystyrene DOM (PS-DOM) and anions (such as Cl-, SO42-, and PO43-) on Fh transformation. Single anions (especially PO43-) were shown to inhibit the transformation of Fh to hematite (Hm) by hindering the dissolution and recrystallization of Fe(III). However, the inhibitory effect was strongly enhanced when PS-DOM and anions coexisted, which is attributed to their synergetic effects on inhibiting dissolution/recrystallization by occupying more active sites and hindering electron transfer. Furthermore, Fh transformation was predominantly controlled by PS-DOM, especially those containing high-unsaturation, high-oxidation-state, and O-rich phenolic compounds. These findings provide a new perspective on the significance of considering the joint effects of DOM and anions in evaluating the transformation of iron minerals.

Heating-Induced Redox Property Dynamics of Peat Soil Dissolved Organic Matter in a Simulated Peat Fire: Electron Exchange Capacity and Molecular Characteristics

Peatlands store one-third of the world's soil organic carbon. Globally increased fires altered peat soil organic matter chemistry, yet the redox property and molecular dynamics of peat-dissolved organic matter (PDOM) during fires remain poorly characterized, limiting our understanding of postfire biogeochemical processes. Clarifying these dynamic changes is essential for effective peatland fire management. This study demonstrates temperature-dependent dynamic changes in the electron exchange capacity (EEC) of PDOM by simulating peat soil burning, significantly affecting microbial iron reduction. At low fire temperatures (200-250 °C), the EEC remains constant by releasing more phenolic moieties to enhance the electron-donating capacity (EDC). Higher temperatures (500 °C) diminish 90% of the EEC by consuming phenolic-quinone moieties. Pyrolytic PDOM (pyPDOM) contributes to 40% of the EEC of peat soil, with this contribution declining at higher temperatures. Phenolic-quinone moieties remain the primary redox-active moieties in pyPDOM. Fourier transform ion cyclotron resonance mass spectrometry analysis shows that postfire EDC depends more on phenolic types than abundance, with monophenol-like molecules (C < 12) being more significant than polyphenol-like (C ≥ 12). Quinone moieties in pyPDOM are associated with high-oxygen condensed aromatics, and their depletion reduces the electron-accepting capacity, weakening its electron shuttle effect in microbial iron reduction. Our findings enhance the understanding of the changes in PDOM redox properties during fires.

Tracking the Changes of DOM Composition, Transformation, and Cycling Mechanism Triggered by the Priming Effect: Insights from Incubation Experiments

The priming effect (PE) is recognized as an important mechanism influencing organic matter transformation in aquatic systems. The land-ocean aquatic continuum (LOAC) has received large quantities of dissolved organic matter (DOM) from various sources, which is an ideal interface for PE research. Here, we investigated the PE process by utilizing such a coastal environment to explore the turnover of DOM in the LOAC system. Suwannee River natural organic matter was selected as the background, and various external environmental samples were introduced to track the changes of organic carbon. The PE process together with the variations of DOM sources, compositions, and structures was characterized. Generally, river and estuary environments exhibited a positive PE, while the offshore zone showed a negative effect. Additionally, nutrients, salinity, and DOM composition all contributed to the PE. After the incubation, the feature of carbon sources transferred from terrestrial to autochthonous. The carbonyl and alcohol functional groups significantly decomposed, while the methyl and methylene groups increased and heteroatoms further accelerated the PE process. The data also shows that special parameters and molecular markers can be utilized to track the carbon response to the PE. This research indicates that the change of carbon flux and the imbalance of its budget in aquatic systems could be partially explained from the perspective of the PE.

Transformation mechanism, kinetics and ecotoxicity of kaempferol and quercetin in the gaseous and aqueous phases: A theoretical combined experimental study

The transformation and risk assessment of flavonoids triggered by free radicals deserve extensive attention. In this work, the degradation mechanisms, kinetics, and ecotoxicity of kaempferol and quercetin mediated by ∙OH, ∙OCH3, ∙OOH, and 1O2 in gaseous and aqueous environments were investigated using cell experiments and quantum chemical calculations. Three radical scavenging mechanisms, including hydrogen atom transfer (HAT), radical adduct formation (RAF) and single electron transfer (SET) were discussed. The results show that RAF and HAT are the main reaction mechanisms for the neutral kaempferol/quercetin, and SET mechanism is important for the anionic kaempferol/quercetin. The overall rate coefficient of kaempferol and quercetin with ∙OH were calculated at 273-323 K, and the aqueous rate coefficients are calculated by considering the rates of neutral and monoanionic forms multiplied with the molar fractions of each form. The values are 2.81 × 1010 and 8.63 × 1010 M-1 s-1 in the aqueous environment, and 2.31 × 10-10 and 1.18 × 10-10 cm3 molecule-1 s-1 in the gaseous environment at 298 K. Fluorescence probe and flow cytometry results show that kaempferol and quercetin can be efficiently degraded by free radicals, and quercetin has a better effect, which is consistent with the theoretical results in the aqueous environment. The transformation mechanism of Q-OH-P7a with ∙OH, O2 and NO was studied, and the stable product is Q-P1. Toxicology results show that most of the subsequent products of quercetin do not bioaccumulate and can be biodegraded, but most products still have toxic properties or harmful properties and show positive mutagenicity. This study provides new guidance for flavonoid degradation behavior and environmental risks.

Reduction of hexavalent chromium by compost-derived dissolved organic matter

Compost-derived dissolved organic matter (DOMC) is a heterogeneous assemblage of different redox-active organic molecules. We hypothesize that DOMC can interact with Cr(VI) and reduce it to Cr(III), thereby influencing the dynamics of Cr(VI) in soil and aquatic environments. Here, DOMC, along with soil humic substances isolated from red soil and black soil, were fractionated into humic acid fractions (i.e., HAC, HAB, and HAR) and fulvic acid fractions (i.e., FAC, FAB, and FAR), respectively. The reduction and interaction between Cr(VI) and the six organic matter fractions were investigated. The results showed that the total Cr(VI) reduction capacity (TRC) of the six organic matter fractions was 26.77-49.34 μM Cr(VI) per mg OM. The TRC of HA fractions was 35.54-49.34 μM Cr(VI) per mg OM, which exceeded that of FA fractions (26.77-31.29 μM Cr(VI) per mg OM). DOMC had a HA/FA ratio of 0.64, which was higher than that of black soil humic substance (0.59) and red soil humic substance (0.20). The sum of the TRC of DOMC was 35.57 μM Cr(VI) per mg OM, which was larger than that of black soil humic substance (32.87 μM Cr(VI) per mg OM) and red soil humic substance (33.01 μM Cr(VI) per mg OM). The TRC was positively correlated with TOC, TN, phenol C, alkyl C, and aromatic C contents and negatively correlated with E2/E3, O-alkyl C, and carboxyl C contents. The reduction of Cr(VI) at pH 6 was negligible, whereas 32-67% Cr(VI) was reduced at pH 2. The Cr(VI) reduction capacities (RC2, RC2, and RC6) at pH 2-6 were positively correlated (R2 > 0.71) with phenol C. Spectral analysis showed that there was no obvious complexation between Cr(VI) and the six organic matter fractions at pH 6, and thus the reduction of Cr(VI) was negligible, but solution pH could affect the accessibility of organic molecules to Cr(VI) and thus influence Cr(VI) reduction.

Biogenic amorphous FeOOH activated additional intracellular electron flow pathways for accelerating reductive dechlorination of tetrachloroethylene

Dissimilatory iron-reducing bacteria (DIRB) with extracellular electron transfer (EET) capabilities have shown significant potential for bioremediating halogenated hydrocarbon contaminated sites rich in iron and humic substances. However, the role and microbial molecular mechanisms of iron-humic acid (Fe-HA) complexes in the reductive dehalogenation process of DIRB remains inadequately elucidated. In this study, we developed a sustainable carbon cycling approach using Fe-HA complexes to modulate the electron flux from sawdust (SD), enabling almost complete reductive dechlorination by most DIRB (e.g., Shewanella oneidensis MR-1) that lack complex iron-sulfur molybdo enzymes. The SD-Fe-HA/MR-1 system achieved a 96.52% removal efficiency of tetrachloroethylene (PCE) at concentrations up to 250 μmol/L within 60 days. Material characterization revealed that DIRB facilitated the hydrolysis of macromolecular carbon sources by inducing the formation of amorphous ferrihydrite (FeOOH) in Fe-HA complexes. More importantly, the bioavailable FeOOH activated additional intracellular electron flow pathways, increasing the activity of potential dehalogenases. Transcriptome further highlight the innovative role of biogenic amorphous FeOOH in integrating intracellular redox metabolism with extracellular charge exchange to facilitate reductive dechlorination in DIRB. These findings provide novel insights into accelerating reductive dechlorination in-situ contaminated sites lacking obligate dehalogenating bacteria.

Influences of wildfire on the soil dissolved organic matter characteristics and its electron-donating capacity

Global increases in the intensity and frequency of wildfires are driving major changes in soil organic matter (SOM) characteristics, including soil dissolved organic matter (DOM). As the most crucial component of SOM, soil DOM plays a pivotal role in the carbon cycle and regulates the environmental fate of contaminants through its versatile reactivities, including electron-donating capacity (EDC). However, it is still being determined how wildfire influences key characteristics of soil DOM and subsequent effects on EDC in forest soils. Thus, we conducted our study to fill this gap with the forest soils of Jinyun Mountain Nature Reserve of China, which experienced an unprecedented wildfire event in 2022. The results from optical characterization, high-performance size-exclusion chromatography (HPSEC), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) showed decreasing molecular weight but elevating nitrogen-containing molecular formulas of soil DOM in the burned soils. This could be attributed to the Maillard reaction and microbial re-colonies. Additionally, wildfires increased the condensed aromatics and lignin components in soil DOM. In the burned soils, we observed increasing EDC of soil DOM, which accounts for an increase in lignin-derived phenolic components. Overall, the findings of this study demonstrate that eco-disturbances, such as wildfires, induce alterations in the properties of DOM, leading to variations in its reactivity and potentially influencing the fate of environmental pollutants beyond carbon dynamics alone. Thus, incorporating the dynamic properties of soil DOM, particularly in the context of climate change, can enhance the assessment of risks associated with contaminants in soil and water, providing valuable insights.

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide

This study addresses the fundamental processes that drive the photomineralization of dissolved organic matter (DOM) to carbon dioxide (CO2), deconvoluting the role of direct and sensitized photolysis. Here, a suite of DOM isolates and model compounds were exposed to simulated sunlight in the presence of various physical and chemical quenchers to assess the magnitude, rate, and extent of direct and sensitized photomineralization to CO2. Results suggest that CO2 formation occurs in a biphasic kinetic system, with fast production occurring within the first 3 h, followed by slower production thereafter. Notably, phenol model chromophores were the highest CO2 formers and, when conjugated with carboxylic functional groups, exhibited a high efficiency for CO2 formation relative to absorbed light. Simple polycarboxylated aromatic compounds included in this study were shown to be resistant to photomineralization. Quencher results suggest that direct photolysis and excited triplet state sensitization may be largely responsible for CO2 photoproduction in DOM, while singlet oxygen and hydroxyl radical sensitization may play a limited role. After 3 h of irradiation, the CO2 formation rate significantly decreased, and the role of sensitized reactions in CO2 formation increased. Together, the results from this study advance the understanding of the fundamental reactions driving DOM photomineralization to CO2, which is an important part of the global carbon cycle.

Dissolved organic matter isolates obtained by solid phase extraction exhibit higher absorption and lower photo-reactivity: Effect of components

Notable differences in photo-physical and chemical properties were found between bulk water and solid phase extraction (SPE) isolates for dissolved organic matter (DOM). The moieties extracted using modified styrene divinylbenzene cartridges, which predominantly consist of conjugated aromatic molecules like humic acids, contribute mainly to light absorption but exhibit lower quantum yields of fluorescence and photo-produced reactive intermediates (PPRIs). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed lignin as the moieties displaying most significant variance in abundance. In Van Krevelen-Spearman plot, we observed molecules positively or negatively correlated with DOM's optical and photochemical properties (including SUVA254, steady-state concentrations of ·OH, 1O2 quantum yield, etc.) were confined to specific regions, which can be delineated using a threshold modified aromaticity index (AImod) of 0.3. Based on the relationships between optical properties and PPRI production, it is suggested that the energy gap between ground state and excited singlet state (△ES1→S0), governing the inner conversion rate, serves as a determinant for apparent quantum yield of PPRIs in DOM, with intra-molecular charge transfer (CT) interactions potentially playing a pivotal role. Regarding DOM's photoreactivity with pollutants, this study has revealed, for the first time, that protein/amino sugars/amino acids could act as antioxidant groups in addition to phenols on the photolysis of sulfadiazine. These findings provide valuable insights into DOM photochemistry and are expected to stimulate further research in this area.

Positive profile of natural small molecule organic matters on emerging antivirus pharmaceutical elimination in advance reduction process: A deep dive into the photosensitive mechanism of triplet excited state compounds

Natural small molecular organic matter (NSOM), ubiquitous in natural waters and distinct from humic acid or fulvic acid, is a special type of dissolved organic matter (DOM) which is characterized as strong photosensitivity and simple molecular structure. However, little study had been directed on the role of NSOM in eliminating emerging contaminants in advanced reduction process (ARP). This study took three small molecular isomeric organic acids (p-hydroxybenzoic acid, pHBA; salicylic acid, SA; m-hydroxybenzoic acid, mHBA) as the representative substances of NSOM to explore these mechanisms on promoting Ribavirin (RBV, an anti COVID-19 medicine) degradation in ultraviolet activated sulfite (UV/Sulfite) process. The results demonstrated that the observed degradation rate constant of RBV (kobs-RBV) was 7.56 × 10-6 s-1 in UV/Sulfite process, indicating that hydrated electron (eaq-) from UV/Sulfite process could not effectively degrade RBV, while it increased by 178 and 38 times when pHBA and SA were introduced into UV/Sulfite process respectively, suggesting that pHBA and SA strongly promoted RBV degradation while mHBA had no promotion on RBV abatement in UV/Sulfite process. Transient absorption spectra and reactive intermediates scavenging experiment indicated that the triplet excited state pHBA and SA (3pHBA* and 3SA*) contributed to the degradation of RBV through non-radical process. Notably, eaq- played the role of key initiator in transforming pHBA and SA into their triplet states. The difference of kobs-RBV in UV/Sulfite/pHBA and UV/Sulfite/SA process was attributed to different generation pathways of 3pHBA* and 3SA* (high molar absorptivity at the wavelength of 254 nm and photosensitive cycle, respectively) and their second order rate constants towards RBV (kRBV-3pHBA* = 8.60 × 108 M-1 s-1 and kRBV-3SA* = 6.81 × 107 M-1 s-1). mHBA could not degrade RBV for its lack of intramolecular hydrogen bond and low molar absorptivity at 254 nm to abundantly transform into its triplet state. kobs-RBV increased as pH increased from 5.0 to 11.0 in UV/Sulfite/SA process, due to the high yield of eaq- in alkaline condition which promoted the generation of 3SA* and the stable of the absorbance of SA at 254 nm. By contrast, kobs-RBV underwent a process of first increasing and then decreasing in UV/Sulfite/pHBA process as the increase of pH, and its highest value achieved in a neutral condition. This lied in the exposure of eaq- increased as the increase of pH which promoted the generation of 3pHBA*, while the molar absorptivity of pHBA at 254 nm decreased as the increase of pH in an alkaline condition which inhibited the yield of 3pHBA*. The RBV degradation pathways and products toxicity assessment indicated that UV/Sulfite/pHBA had better detoxification performance on RBV than UV/Sulfite/SA process. This study disclosed a novel mechanism of emerging contaminants abatement through non-radical process in NSOM mediated ARP, and provide a wide insight into positive profile of DOM in water treatment process, instead of only taking DOM as a quencher of reactive intermediates.

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

Despite the widespread use of photochemical and optical properties to characterize dissolved organic matter (DOM), a significant gap persists in our understanding of the relationship among these properties. This study infers the molecular basis for the optical and photochemical properties of DOM using a comprehensive framework and known structural moieties within DOM. Utilizing Suwannee River Fulvic Acid (SRFA) as a model DOM, carboxylated aromatics, phenols, and quinones were identified as dominant contributors to the absorbance spectra, and phenols, quinones, aldehydes, and ketones were identified as major contributors to radiative energy pathways. It was estimated that chromophores constitute ∼63% w/w of dissolved organic carbon in SRFA and ∼47% w/w of overall SRFA. Notably, estimations indicate the pool of fluorescent compounds and photosensitizing compounds in SRFA are likely distinct from each other at wavelengths below 400 nm. This perspective offers a practical tool to aid in the identification of probable chemical groups when interpreting optical and photochemical data and challenges the current "black box" thinking. Instead, DOM photochemical and optical properties can be closely estimated by assuming the DOM is composed of a mixture of individual compounds.