Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups
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Date
2022-09-02
Publication Type
Journal Article
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Abstract
Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO2, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH3) 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 ArOCH3 group, regardless of other structural features. We then confirmed that all ArOCH3-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 CO2 and that lignin photodegradation may represent a source of hydroxyl radicals.
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published
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Journal / series
Volume
56 (18)
Pages / Article No.
13449 - 13460
Publisher
American Chemical Society
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Subject
dissolved organic matter photodegradation; carbon monoxide; reaction mechanism; lignin; methanol; hydroxyl radicals; carbon cycling; methoxy-substituted aromatics
Organisational unit
03850 - McNeill, Kristopher / McNeill, Kristopher
Notes
Funding
188565 - Reactivity-based characterization of photoexcited natural organic matter (SNF)