This study demonstrates the strong impact of photochemical degradation of soil dissolved organic matter (DOM) on its metal complexing capacity. The role of light in the fate of organically-bound metals transported from soils to surface waters was studied in laboratory experiments. We studied four humic and one fulvic acid isolates from different soil horizons in the Bohemian Forest (Czech Republic). Different concentrations of aluminium (Al) and iron (Fe) salts were added to the solutions of organic acids (initial dissolved organic carbon (DOC) concentration 0.5 mmol L−1), and the samples were irradiated in a reactor equipped with 350 nm irradiation lamps for 0 to 120 min. Aliquots of irradiated samples and dark controls were analyzed for DOC, ionic and organically-bound Al and Fe (Ali, Fei, and Alo, Feo, respectively), pH, and UV-VIS spectra. The initial Feo concentrations in the samples (2.09 to 5.66 µmol L−1) decreased from 21 to 52% during irradiation, while the initial Alo concentrations (2.28 to 5.37 µmol L−1) decreased from 7 to 41%. The greatest decrease in the organically-bound metal concentrations occurred for the fulvic acid, and the smallest decrease occurred for the humic acid from the deepest soil horizon. The extrapolation of laboratory experiments to in situ conditions suggested that the DOM's ability to bind metals changes greatly within the first few hours after groundwater enters the stream. The rapid degradation of organically-bound Al and Fe can be an important process in first and second-order streams, and lake epilimnia.
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