Photooxidation and its effects on the carboxyl content of dissolved organic matter in two coastal rivers in the Southeastern United States

Photodecarboxylation (often stoichiometrically expressed as RCOOH + 1/2O(2) --> 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 O-2 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 O-2 content. The ratio of photochemical O-2 consumption to CO2 photoproduction ranged from similar to0.8 to 2.5, depending on the O-2 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 O-2. 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) + aO(2) + (metals, hv) --> bCO(2) + cH(2)O(2) + Cx-bHyOz'(COOH)(m-b)(COOH)(b) or of (b) nondecarboxylation photooxidation: CxHyOz(COOH)(m) + aO(2)+ (metals, hv) --> bCO(2) + cH(2)O(2) + Cx-bHyOz'(COOH)(m).