Experimental photochemical release of organically bound aluminum and iron in three streams in Maine, USA

Abstract

Laboratory photochemical experiments with stream water were done to characterize the photodegradation of dissolved organic carbon (DOC) and photochemical release of organically bound metals. The samples were collected from Bear Brook Watershed, Hadlock Brook, and Mud Pond Stream in Maine, USA, during January and April 2006. Filtered samples were irradiated in a reactor equipped with 350 nm irradiation lamps. Aliquots of irradiated samples were analyzed for DOC, dissolved aluminum (Al(d)) and iron (Fe(d)), pH, and UV-Vis spectra. Organically bound metals (Fe(o) and Al(o)) were measured after passing the sample through a column filled with a strong cation exchange resin (Dowex HCR-W2). UV radiation resulted in a decrease in DOC concentration and structural changes in DOC composition. UV-Vis spectra showed a decrease in aromaticity and molecular weight of DOC during irradiation. The DOC ranged from 0.1 to 0.35 mmol L(-1) at the beginning of experiments and decreased 5% to 37% after irradiation. Oxidation and structural changes in DOC resulted in the release of organically bound metals. Initial Fe(o) concentrations ranged from 0.16 to 0.79 μmol L(-1) and decreased 56% to 81% during the irradiation. The concentration of Al(o) ranged from 1.0 to 3.85 μmol L(-1) and declined steadily throughout the irradiation, resulting in 8% to 76% decline. Degradation of a small percentage of organically bound Al and Fe occurs rapidly enough so as to be an important process in first- and second-order streams. Irradiation energy absorbed by samples during hours of laboratory experiments equates to days in stream environment. Degradation of more refractory complexes occurs on a time scale that requires longer residence times, such as in lakes. This study demonstrated a strong impact of photochemical degradation of DOC on its metal-complexing ability and capacity. The results also suggest different binding properties of Fe and Al in their organic complexes.