Contrasting Impacts of Photochemical and Microbial Processing on the Photoreactivity of Dissolved Organic Matter in an Adirondack Lake Watershed

Photochemically generated reactive oxygen species (ROS) play numerous key roles in earth's surface biogeochemical processes and pollutant dynamics. ROS production has historically been linked to the photosensitization of natural organic matter. Here, we report the photochemical ROS production from three naturally abundant iron minerals. All investigated iron minerals are photoactive toward sunlight irradiation, with photogenerated currents linearly correlated with incident light intensity. Hydroxyl radicals ( • OH) and hydrogen peroxide (H 2 O 2 ) are identified as the major ROS species, with apparent quantum yields ranging from 1.4 × 10 −8 to 3.9 × 10 −8 and 5.8 × 10 −8 to 2.5 × 10 −6 , respectively. Photochemical ROS production exhibits high wavelength dependence, for instance, the • OH quantum yield decreases with the increase of light wavelength from 375 to 425 nm, and above 425 nm it sharply decreases to zero. The temperature shows a positive impact on • OH production, with apparent activation energies ranging from 8.0 to 17.8 kJ/mol. Interestingly, natural iron minerals with impurities exhibit higher ROS production than their pure crystal counterparts. Compared with organic photosensitizers, iron minerals exhibit higher wavelength dependence, higher selectivity, lower efficiency, and long-term stability in photochemical ROS production. Our study highlights natural inorganic iron mineral photochemistry as a ubiquitous yet previously overlooked source of ROS.

Section: Resultsmentioning

confidence: 99%

Sunlight-Driven Production of Reactive Oxygen Species from Natural Iron Minerals: Quantum Yield and Wavelength Dependence

Zheng

Zhou

et al. 2022

“…Dissolved organic matter (DOM) is a complex mixture of organic molecules derived from terrestrial and microbial sources. , Environmental processing of DOM further changes its molecular composition and reactivity. , DOM participates in reactions that affect microbial metabolism, contaminant transformation, − and drinking water treatment processes. − In surface waters, DOM is the predominant light absorbing component and forms photochemically produced reactive intermediates (PPRI) as a result. , …”

Section: Introductionmentioning

confidence: 99%

Dissolved Organic Matter Photoreactivity Is Determined by Its Optical Properties, Redox Activity, and Molecular Composition

Berg

Wammer

Remucal

2023

Predicting the formation of photochemically produced reactive intermediates (PPRI) during the irradiation of dissolved organic matter (DOM) has remained challenging given the complex nature of this material and differences in PPRI formation mechanisms. We investigate the role of DOM composition in photoreactivity using 48 samples that span the range of DOM in freshwater systems and wastewater. We relate quantum yields for excited triplet-state organic matter (f TMP ), singlet oxygen (Φ 1 Od 2 ), and hydroxylating species (Φ • OH ) to DOM composition determined using spectroscopy, Fourier-transform ion cyclotron resonance mass spectrometry, and electron-donating capacity (EDC). f TMP and Φ 1 Od 2 follow similar trends and are correlated with bulk properties derived from UV−vis spectra and EDC. In contrast, no individual bulk property can be used to predict Φ • OH . At the molecular level, the subset of DOM that is positively correlated to both Φ • OH and EDC is distinct from DOM formulas related to Φ 1 Od 2 , demonstrating that • OH and 1 O 2 are formed from different DOM fractions. Multiple linear regressions are used to relate quantum yields of each PPRI to DOM composition parameters derived from multiple techniques, demonstrating that complementary methods are ideal for characterizing DOM because each technique only samples a subset of DOM.

“…As depicted in eqs –, the concentration of PPRIs will keep increasing over time until TMP is fully consumed, a trend also supported by the modeled concentration profiles over an extended reaction period (Figure S13). Therefore, Model-3 is expected to outperform the other two models, especially in cases where the studied DOM sample exhibits a high value of k 3 DOM*,TMP or a significant quantity of TMP was degraded. , It is also important to recognize that Model-2 could remain effective under conditions such as the [TMP] 0 range below 200 μM and short reaction times. In summary, Model-3 offers a robust and versatile approach to simulate TMP degradation under a wide range of conditions.…”

Section: Resultsmentioning

confidence: 99%

“…Generally, aquatic DOM is more aliphatic and less oxidized compared to the more structurally complex and higher aromatic nature of soil DOM, 39 and aquatic DOM often exhibits higher photochemical reactivity than soil DOM. 12,23,24,40 Ultrapure water (18.2 MΩ•cm) was obtained from a Millipore Milli-Q system and used in all experiments.…”

Section: Methodsmentioning

confidence: 99%

Reassessing the Quantum Yield and Reactivity of Triplet-State Dissolved Organic Matter via Global Kinetic Modeling

Du,

Tang,

Yang

et al. 2024

Measuring the quantum yield and reactivity of triplet-state dissolved organic matter ( 3 DOM*) is essential for assessing the impact of DOM on aquatic photochemical processes. However, current 3 DOM* quantification methods require multiple fitting steps and rely on steady-state approximations under stringent application criteria, which may introduce certain inaccuracies in the estimation of DOM photoreactivity parameters. Here, we developed a global kinetic model to simulate the reaction kinetics of the hv/DOM system using four DOM types and 2,4,6trimethylphenol as the probe for 3 DOM*. Analyses of residuals and the root-mean-square error validated the exceptional precision of the new model compared to conventional methods. 3 DOM* in the global kinetic model consistently displayed a lower quantum yield and higher reactivity than those in local regression models, indicating that the generation and reactivity of 3 DOM* have often been overestimated and underestimated, respectively. The global kinetic model derives parameters by simultaneously fitting probe degradation kinetics under different conditions and considers the temporally increasing concentrations of the involved reactive species. It minimizes error propagation and offers insights into the interactions of different species, thereby providing advantages in accuracy, robustness, and interpretability. This study significantly advances the understanding of 3 DOM* behavior and provides a valuable kinetic model for aquatic photochemistry research.