Photooxidation and Its Effects on the Carboxyl Content of Dissolved Organic Matter in Two Coastal Rivers in the Southeastern United States

Abstract

Photodecarboxylation (often stoichiometrically expressed as RCOOH + (1/2)O2 --> ROH + CO2) has long been postulated to be principally responsible for generating CO2 from photooxidation of dissolved organic matter (DOM). In this study, the quantitative relationships were investigated among O2 consumption, CO2 production, and variation of carboxyl content resulting from photooxidation of DOM in natural water samples obtained from the freshwater reaches of the Satilla River and Altamaha River in the southeastern United States. In terms of loss of dissolved organic carbon (DOC), loss of optical absorbance, and production of CO2, the rate of photooxidation of DOM was increased in the presence of Fe redox chemistry and with increasing O2 content. The ratio of photochemical O2 consumption to CO2 photoproduction ranged from approximately 0.8 to 2.5, depending on the O2 content, the extent of involvement of Fe, and probably the initial oxidation state of DOM as well. The absolute concentration of carboxyl groups ([-COOH]) on DOM only slightly decreased or increased over the course of irradiation, possibly depending on the stages of photooxidation, while the DOC-normalized carboxyl content substantially increased in the presence of Fe redox chemistry and sufficient O2. Both the initial [-COOH] and the apparent loss of this quantity over the course of irradiation was too small to account for the much larger production of CO2, suggesting that carboxyl groups were photochemically regenerated or that the major production pathway for CO2 did not involve photodecarboxylation. The results from this study can be chemically rationalized by a reaction scheme of (a) photodecarboxylation/ regeneration of carboxyl: CxHyOz(COOH)m + aO2 + (metals, hv) --> bCO2 + cH2O2 + Cx-bHy'Oz'(COOH)m-b(COOH)b or of (b) nondecarboxylation photooxidation: CxHyOz(COOH)m + aO2 + (metals, hv) --> bCO2 + cH2O2 + Cx-bHy'Oz'(COOH)m.

Depth-dependent molecular composition and photo-reactivity of dissolved organic matter in a boreal lake under winter and summer conditions

“…Such highly oxidized formulas produced after solar simulated irradiation have been previously shown and confirmed our observation (Gonsior et al, 2009). This indicates that these molecules were photochemical products that are labile and depleted by either microbial degradation or additional photochemical decarboxylation (Xie et al, 2004;Xu and Wan, 2000) and the release of carbon mono-and dioxide and other small carboxylic acids (Bertilsson and Tranvik, 1998). Microbial respiration, whereby DOC is oxidized and dissolved inorganic carbon is formed, seemed to be an effective pathway based on dark incubation of water collected in the same lake in a previous study and the loss of about 7 % of the initial DOC within 5 months at 15 • C (Bastviken et al, 2004).…”

Photomineralization and photomethanification of dissolved organic matter in Saguenay River surface water

“…S R has been used to characterize the source, molecular size, and photoprocessing of CDOM (Helms et al, 2008). Consistent with the results of previous studies (Gao and Zepp, 1998;Xie et al, 2004;Lou and Xie, 2006), irradiation led to photochemical O 2 consumption, absorbance bleaching, and acidification (i.e. decrease in pH).…”

Effect of Humic Substance Photodegradation on Bacterial Growth and Respiration in Lake Water

“…DOM photoproducts are more oxidized than dark-treated controls (50,66), and these more oxidized products appear to be incorporated less efficiently by bacteria (24,48), which should lead to a decrease in BGE. However, we observed a statistically significant increase in the BGE after irradiation in three out of four samples (U test, P Ͻ 0.05) ( Table 2).…”