Cyanohydrin Equilibria Implicate Non-Aromatic Aldehydes in Photochemical Production of Oceanic Carbon Monoxide

Carbonyls have previously been dismissed as significant precursors for carbon monoxide (CO) photoproduction from natural chromophoric dissolved organic matter (CDOM). Here, we used hydrogen cyanide (HCN), which reacts with carbonyls to form photochemically inert cyanohydrins, as a probe to re-examin...

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Bibliographic Details
Published in:Environmental science & technology 2024-09, Vol.58 (36), p.16066-16075
Main Authors: Zafiriou, Oliver C., Xie, Huixiang, Kieber, David J., Wang, Wei, Song, Guisheng, Cohen, Natalie
Format: Article
Language:English
Subjects:
Aldehydes
Biogeochemical Cycling
Borohydrides
Carbon monoxide
Carbonyl compounds
Carbonyls
Chemical equilibrium
Dissolved organic matter
Euphotic zone
Hydrogen cyanide
Ketones
Phenylacetaldehyde
Photochemicals
Photochemistry
Photolysis
Photoproduction
Precursors
Seawater
Solvation
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Summary:Carbonyls have previously been dismissed as significant precursors for carbon monoxide (CO) photoproduction from natural chromophoric dissolved organic matter (CDOM). Here, we used hydrogen cyanide (HCN), which reacts with carbonyls to form photochemically inert cyanohydrins, as a probe to re-examine the role of carbonyls in CO photoproduction. Adding HCN to low-absorbance euphotic zone seawater decreased CO photoproduction. Modeling [HCN] (∼5 to 364 μM) vs the percent decrease in CO photoproduction (%CO↓) yielded carbonyl-cyanohydrin dissociation equilibrium constants, K D, and maximum %CO↓, %CO↓max values. Four Atlantic and Pacific seawater K Ds (66.7 ± 19.6 μM) overlap aqueous aliphatic but not aromatic aldehyde K Ds. Phenylacetaldehyde (PA) and other β,γ-unsaturated aldehydes are proposed as prototypical CO precursors. Direct photolysis of ∼10 nM PA can supply the measured daily production of HCN-sensitive CO at an open-ocean site near Bermuda. HCN’s %CO↓max was 31 ± 2.5% in North Atlantic seawater vs the 13 ± 2.5% inhibition of CO photoproduction by borohydride, a dilemma since only borohydride affects most ketones. Borohydride also decreased CDOM absorption much more than did HCN. This puzzle probably reflects differing steric and solvation requirements in HCN- and borohydride-CDOM reactions. This study demonstrates cyanophilic aldehydes to be a significant source of open-ocean CO and reveals new clues regarding CDOM photochemistry mechanisms.
ISSN:0013-936X
1520-5851
1520-5851
DOI:10.1021/acs.est.4c04637