Photochemical Production of Carbon Monoxide from Dissolved Organic Matter: Role of Lignin Methoxyarene Functional Groups

Abstract

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.

Kinetics and Mechanisms of Aqueous-Phase Reactions of Triplet-State Imidazole-2-carboxaldehyde and 3,4-Dimethoxybenzaldehyde with α,β-Unsaturated Carbonyl Compounds

“…Product 2 might undergo elimination reactions, resulting in the loss of either formaldehyde (HCHO) or acetaldehyde (CH 3 CHO), leading to product 3 (MW = 192 g mol –1 ) appearing as m / z = 162 ([P 3 − CO] + ) or 4 (MW = 206 g mol –1 ) appearing as m / z = 162 ([P 4 − CO] + ), respectively. Recent studies also showed that the aromatic methoxy group produced CO after UV irradiation with conversion efficiency up to 17.6% . Further experiments with MACR in the presence of DMB were performed, but no product peak appeared within 3 h of irradiation time (Figure S13 in the Supporting Information).…”

“…Recent studies also showed that the aromatic methoxy group produced CO after UV irradiation with conversion efficiency up to 17.6%. 97 Further experiments with MACR in the presence of DMB were performed, but no product peak appeared within 3 h of irradiation time (Figure S13 in the Supporting Information). It should be mentioned that the UPLC/ESI(+)-MS method used fails to measure formaldehyde or acetaldehyde.…”

Carbon Monoxide Cycling in the Eastern Indian Ocean

“…Recently, Ossola et al. (2022) demonstrated for the first time a precise mechanism for CO production by CDOM photolysis based on the study of Stubbins et al. (2008).…”

Tracking the Photomineralization Mechanism in Irradiated Lab-Generated and Field-Collected Brown Carbon Samples and Its Effect on Cloud Condensation Nuclei Abilities

“…CO and CO 2 are the key oxidation products supporting a photomineralization mechanism and a decrease in TOC with UVB irradiation. 5,71 These end products were quantified using a GC-FID setup, and samples analyzed were the second and third batch of (NH 4 ) 2 SO 4 -methylglyoxal, the December sample for firewood smoke and the Padua Friday sample. SRFA results are reported in Borduas-Dedekind et al 5 In (NH 4 ) 2 SO 4 -methylglyoxal solutions, CO 2 concentrations increased on average by 762 μM (Figure 5).…”