| Abstract
| - The photochemical reduction of Cr(VI) by iron and aquaticdissolved organic matter (DOM) was investigated. DOMsampled from a number of surface waters (a eutrophicwetland, a blackwater stream, and river water from a mix-use watershed) was used in this study. Moreover, afulvic acid from Lake Fryxell, Antarctica, was also used torepresent a DOM derived from a strictly autochthonoussource. Cr(VI) reduction to Cr(III) at pH 5.5 was observedfor all target DOMs used in this study, but rates variedwidely. In general, photoreduction rates increased withincreasing iron concentrations, but the type of DOM appearedto influence the kinetics to a larger degree. The rate ofreduction was significantly greater for DOM derived fromterrestrial systems than from predominantly autochthonousmaterials even if additional iron was added to the later. Apositive correlation was observed between rates of Cr(VI) photoreduction and properties of the isolated DOMsamples whereby faster reduction was observed for largermore aromatic substrates. On the basis of the fast ratesreported for the dark reduction of Cr(VI) to Cr(III) by Fe(II)−organic ligands, we hypothesize that the rate-limitingstep in these reactions is the photoreduction of Fe(III) to Fe(II) by a ligand-to-metal charge-transfer pathway afterabsorption of light by Fe(III)−DOM complexes or by reductionof Fe(III) by superoxide or other intermediates formedafter light absorption by DOM. Thus, the rate of Cr(VI)photoreduction to Cr(III) in natural sunlit waters is dependentupon both the amount of iron present and the nature ofthe dissolved organic matter substrate.
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