Abstract
We report a dual probe system based on 4'-substituted biphenyl-2-carboxylic acids (BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes are converted to the corresponding benzocoumarins (BZCs) at different rates depending on the mechanism of oxidation; thus, two probes used simultaneously can differentiate strong triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (•OH) present in dissolved organic matter (DOM). Comparison of the ratios of BZC-CH3 and BZC-CF3 product formation using model photooxidants such as NaNO2, a •OH precursor, and model triplet sensitizer lumichrome gave a range of 2 to 250. Application of these probes to DOM isolates and whole natural waters afforded intermediate ratios. Although the oxidation potential of BPAs (>ca. 1.80 V SHE) is significantly higher than the estimated average reduction potential of typical 3CDOM* samples, these results have demonstrated the presence of a small pool of oxidants in the selected DOM isolates and whole water samples that is capable of oxidizing aromatic carboxylates. As an analytical tool, this probe pair can be used between pH 4-6 without affecting the product formation ratio and may find applications in various systems involving complex mixtures of photochemically produced oxidants of differing natures.
Highlights
SHE) is significantly higher than the estimated average reduction potential of typical 3CDOM* samples, these results have demonstrated the presence of a small pool of oxidants in the selected Dissolved organic matter (DOM)
Dissolved organic matter (DOM) plays a fundamental role in the global carbon cycle, and its photochemical and biological transformation in terrestrial and aquatic systems is critical to understanding the exchange of carbon between the land, water, and atmosphere.[1−3] The degradation of DOM result in the formation of diverse organic molecules, altering its spectroscopic and chemical characteristics as well as affecting the bioavailability of this carbon in food webs.[4]
Hydroxyl radical was generated by irradiation of NaNO2 (200 μM) in Milli-Q water at 365 nm and quantified by formation of hydroxyterephthalic acid (hTPA) from terephthalic acid (TPA) (10 μM) in the presence of the biphenyl-2-carboxylic acids (BPAs) probes
Summary
Dissolved organic matter (DOM) plays a fundamental role in the global carbon cycle, and its photochemical and biological transformation in terrestrial and aquatic systems is critical to understanding the exchange of carbon between the land, water, and atmosphere.[1−3] The degradation of DOM result in the formation of diverse organic molecules, altering its spectroscopic and chemical characteristics as well as affecting the bioavailability of this carbon in food webs.[4]. TPA (10 μM) was used as an internal probe for monitoring hydroxyl radical concentrations in BPA reactivity experiments as described by Page et al.[35] Steady-state hydroxyl radical concentrations in NaNO2 photolyses ranged from 3.2 × 10−15 to 1.2 × 10−14 M in the presence of the BPA probes in Milli-Q water containing 0.2 to 2.5% (v/v) MeCN. For paired probe experiments involving model oxidants and DOM samples, BPA probe stock solutions were prepared in water with addition of stoichiometric NaOH. Paired probe experiments with DOM were MeCN-free and sampled at 5−10 min intervals; aliquots (50 μL) were diluted with 20 μL MeCN prior to HPLC analysis. Hydroxyl radical was generated by irradiation of NaNO2 (200 μM) in Milli-Q water at 365 nm and quantified by formation of hTPA from TPA (10 μM) in the presence of the BPA probes.
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