Transient spectroscopic characterization and theoretical modeling of fulvic acid radicals formed by UV-A radiation

Dissolved organic matter (DOM) has been shown to inhibit the oxidation of aromatic amines initiated by excited triplet states, an effect that was attributed to the reduction of oxidation intermediates back to their parent compounds. The present study focuses on the quantification of an analogous inhibitory effect of DOM on aqueous oxidations induced by the sulfate radical (SO4 · –). Second-order rate constants for the SO4 · –-induced transformation of selected anilines and sulfonamide antibiotics were determined by competition kinetics in the presence and absence of DOM from three different isolates at pH 8. In the presence of 1 mgC L–1 of DOM, a significant reduction in the rate constant was observed for most of the compounds compared to DOM-free solutions, but for two electron-rich anilines, increases in the rate constant were measured. For 4-cyanoaniline and sulfamethoxazole, the DOM concentration dependence of the rate constant consisted of a sharp decrease up to ∼1.0 mgC L–1 of DOM followed by a region of slight changes or even increases for higher DOM concentrations (up to 5 mgC L–1). This behavior was attributed to the occurrence of the aforementioned inhibitory effect and a counteracting accelerated transformation of the contaminants due to reactions with secondary radical oxidants resulting from DOM oxidation by SO4 · –. Both effects of inhibition and secondary oxidants should be considered when assessing the abatement of aromatic amines in SO4 · –-based advanced oxidation processes.

Section: Second-ordermentioning

confidence: 99%

Inhibitory Effect of Dissolved Organic Matter on the Transformation of Selected Anilines and Sulfonamide Antibiotics Induced by the Sulfate Radical

Canonica

Schönenberger

2019

“…For many organic molecules this is possible, but the diverse mixture of triplets within 3 CDOM* leads to unresolvable TA signals, resulting from not only triplets but also photoproduced radicals and hydrated electrons. 7 Because 3 CDOM* is the direct precursor to 1 O 2 , it is possible to indirectly obtain kinetic information about 3 CDOM* through direct observation of 1 O 2 , which is formed via energy transfer from 3 CDOM* to O 2 but with varying yields. 8 1 O 2 is an excellent indirect probe because O 2 exhibits near diffusion-limited quenching rate constants with most triplets (∼1−3 × 10 9 M −1 s −1 in water) 9 and possesses a low singlet−triplet energy gap (94 kJ mol −1 ), 8 thereby allowing the capture of most triplets in 3 CDOM* when using high O 2 concentrations.…”

Section: ■ Introductionmentioning

confidence: 99%

“…If k TMP was known and consistent across DOM samples, TMP could be used as a quantitative probe to determine 3 CDOM* steady-state concentrations ([ 3 CDOM*] SS ) and intersystem crossing quantum yields (Φ ISC ). , Typically, triplet reactivity is assessed using transient absorption (TA) spectroscopy, wherein the triplet excited-state intermediates are directly observed, allowing the bimolecular reaction rate constant to be determined. For many organic molecules this is possible, but the diverse mixture of triplets within 3 CDOM* leads to unresolvable TA signals, resulting from not only triplets but also photoproduced radicals and hydrated electrons …”

Section: Introductionmentioning

confidence: 99%

Singlet Oxygen Phosphorescence as a Probe for Triplet-State Dissolved Organic Matter Reactivity

Erickson

Moor

Werner

et al. 2018

106

Triplet-state chromophoric dissolved organic matter (CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about the reactivity of these intermediates. To better understand the kinetic behavior and reactivity of CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (O) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O quenches almost all triplets with near diffusion limited rate constants, resulting in the formation of O, which is kinetically linked to the precursors. A kinetic model relating O phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural-water samples (hereafter referred to as CDOM*) was developed and used to determine rate constants governingCDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. CDOM* was found to exhibit smaller O and TMP quenching rate constants, ∼9 × 10 and ∼8 × 10 M s, respectively, compared with model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.

“…Therefore, time-resolved measurements have the potential to provide more quantitative information about 3 CDOM*. Past studies have reported the observation of several transients after laser excitation of CDOM. − However, these transients are mostly related to radical species, such as phenoxyl radical, and hydrated electrons. Hydrated electrons are overproduced under laser irradiation compared to natural sunlit conditions due to a biphotonic process .…”

Section: Introductionmentioning

confidence: 99%

Triplet-State Dissolved Organic Matter Quantum Yields and Lifetimes from Direct Observation of Aromatic Amine Oxidation

Schmitt

Erickson

McNeill

2017

Excited triplet state chromophoric dissolved organic matter (CDOM*) is a short-lived mixture of excited-state species that plays important roles in aquatic photochemical processes. Unlike the study of the triplet states of well-defined molecules, which are amenable to transient absorbance spectroscopy, the study of CDOM* is hampered by it being a complex mixture and its low average intersystem crossing quantum yield (Φ). This study is an alternative approach to investigating CDOM* using transient absorption laser spectroscopy. The radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), formed through oxidation byCDOM*, was directly observable by transient absorption spectroscopy and was used to probe basic photophysical properties of CDOM*. Quenching and control experiments verified that TMPD was formed from CDOM* under anoxic conditions. Model triplet sensitizers with a wide range of excited triplet state reduction potentials and CDOM oxidized TMPD at near diffusion-controlled rates. This gives support to the idea that a large cross-section ofCDOM* moieties are able to oxidize TMPD and that the complex mixture of CDOM* can be simplified to a single signal. Using the TMPD transient, the natural triplet lifetime and Φ for different DOM isolates and natural waters were quantified; values ranged from 12 to 26 μs and 4.1-7.8%, respectively.