Photochemical production of dissolved inorganic carbon from suwannee river humic acid

The photochemical mineralization of dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) is a key process in carbon cycling. Using a Suntest CPS solar simulator, Suwannee River humic acid (SRHA) was photooxidated to examine the effects of O₂ levels, the wavelength of incident light, and the concentration of Fe on the photoproduction of DIC. Increasing the O₂ abundance enhanced photodegradation of SRHA. The rate of DIC photoproduction under air saturation in the first 24 h (4.40 μmol/(L h)) was increased by a factor of 1.56 under O₂ saturation, but fell by only 36% under N₂ saturation. To evaluate the relative importance of UV-B, UV-A, and visible radiation in the photodegradation, we examined the above process using Mylar-d films and UF-3 and UF-4 plexiglass filters. The results indicated that the UV-B, UV-A and visible wavelengths accounted for 31.8%, 32.6% and 25.6%, respectively, of DIC production with simulated sunlight irradiation. The above results also indicated that photoproduction of DIC could take place in natural water at depths greater than those that UV light can reach. When 20 μmol/L desferrioxamine mesylate (DFOM, a strong Fe complexing ligand) was added, the rate of DIC photoproduction fell to 55.6% that of the original SRHA samples with 5.46 μmol/L Fe.