HR: 08:45h
AN: H51G-04    [Abstracts]
TI: Photo-oxidation of Dissolved Organic Matter in River Water and its Effect on Trace Element Speciation
AU: * Shiller, A M
EM: alan.shiller@usm.edu
AF: University of Southern Mississippi, Department of Marine Science 1020 Balch Blvd, Stennis Space Center, MS 39529 United States
AU: Duan, S
EM: sduan@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AU: van Erp, P
EM: peter.vanerp@usm.edu
AF: University of Southern Mississippi, Department of Marine Science 1020 Balch Blvd, Stennis Space Center, MS 39529 United States
AU: Bianchi, T
EM: tbianch@tulane.edu
AF: Tulane University, Department of Earth and Environmental Sciences, New Orleans, LA 70118 United States
AB: Photochemical effects on the chemistry of fresh surface waters are now well established. One interesting contrast in the studies of photochemical effects on dissolved organic carbon (DOC) versus trace metals is the difference in time scales generally considered. For DOC studies, timescales of days to weeks are most common in experiments whereas for trace metals, it is the diel cycle that tends to be studied. We conducted a three-week incubation in natural light (with dark controls) of filtered water from the lower Pearl River (Mississippi) examining both the changes in DOC and changes in physical-chemical speciation of a suite of trace metals. During the incubation, DOC decreased in the light by about 20% while UV light absorbance decreased by nearly 40%. This implies both the photo-oxidation of the DOC as well as a shift to a proportionately less aromatic nature for the DOC. For the trace elements, a variety of behaviors were observed. Some elements showed no change in speciation; for instance, the alkali and alkaline earth metals as well as some oxyanions (e.g., Mo) and some other elements (e.g., Mn). Other elements, however, did show significant changes in the light. Fe, for example, is a key trace element in this system with a colloidal concentration over 2 $\mu$M. There was a significant, continuous decrease in dissolved ($<$0.02 $\mu$m) Fe in the light samples during the experiment. This is best explained by release of organically-complexed Fe during photo-oxidation of the low-to-medium molecular weight fraction of the DOC followed by subsequent precipitation of the released Fe as additional colloidal iron(III) oxyhydroxide. A number of other elements (Ce, Cu, Cr, Pb, V, and U) also showed decreases in the dissolved ($<$0.02 $\mu$m) fraction with time, implying a release from low molecular weight complexes followed by sorption onto Fe colloids. All of these elements have been previously found to be associated with colloidal Fe in other systems. A number of these elements also showed decreases in their retention by an anion exchange column, likewise implying a decrease in the organically-complexed form. Our results suggest that as fluvial DOC becomes more photo-refractory upon exposure to sunlight, there will be a transfer of certain trace elements from dissolved, complexed forms to colloidal or particulate adsorbed forms. This should lead to fundamental differences in metal speciation, transport, and bioavailability between low-order streams with fresh allochthonous carbon inputs and floodplain rivers dominated by photo-refractory carbon.
DE: 1806 Chemistry of fresh water
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
DE: 1065 Trace elements (3670)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting