Figure 1. (a) Water splitting and (b) methanol splitting upon UV excitation, illustrated for Au@TiO 2 .
Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO 2 , but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH 3 ) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH 3 group, regardless of other structural features. We then confirmed that all ArOCH 3 -containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO 2 and that lignin photodegradation may represent a source of hydroxyl radicals.
Oceanic phytoplankton populations are actively monitored via satellite measurements coupled with local validation. Increasingly, chlorophyll fluorescence is used for such measurements, both from remote platforms and in-situ. A critical aspect of extracting concentrations from steady-state fluorescence intensity measurements is the ability to quantify the extent of fluorescence quenching in the particular environment of interest. Here, we report a quantitative study of the rate constants for chlorophyll fluorescence quenching by common seawater components: halides, sulphate, calcium, magnesium, and ferric ions, as well as the effect of pH on fluorescence intensity. In general, halide anions and Fe(III) are very effective quenchers of chlorophyll fluorescence, with iodide showing a near diffusion-limited rate constant. Although the most concentrated anion in seawater is chloride, the quenching constant obtained from artificial sea water (1.3 × 10 9 M −1 s −1 ) is higher than that for chloride alone (1.8 × 10 8 M −1 s −1 ), suggesting that simply considering NaCl is inappropriate to model real seawater. Negligible quenching was observed from Ca 2+ , SO 4 2− , or Mg 2+ , and there was no effect of changing pH between pH = 5 and pH = 10. Quenching by ferric ions has an apparent quenching constant of 8 × 10 12 M −1 s −1 , which suggests iron−chlorophyll complex formation.
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