Iron and carbon are important constituents of natural colloids, which intimately links the fate of these two elements in riverine systems. Iron may strongly affect the binding of trace metals by organic matter, e.g. through competition for binding sites, which highlights the importance of a correct appreciation of the Fe speciation in surface waters. However, the chemistry of Fe and C in natural colloids is complex and depend on many factors including the pH, the Fe:C ratio, and the redox speciation of Fe [1-3]. Two areas with a contrasting Fe chemistry were studied: a lowland area with widespread seepage of iron-rich groundwater, and an upland peat area. Samples of ten oxic, well-mixed streams were subjected to cascade filtration using conventional filtration (1.2 µm, 0.45 µm, 0.1 µm) and cross-flow ultrafiltration (CFF; 5 kDa). The colloidal fraction, here operationally defined as between 0.45 µm and 5 kDa, was isolated by CFF and subsequently freeze-dried. The speciation of colloidal Fe was determined by EXAFS spectroscopy at the Fe K-edge (MAX-lab, Lund, Sweden). In the rivers draining upland peat, Fe and C were predominantly recovered in the fraction between 5 kDa and 0.1 µm. Conversely, in the rivers draining the lowland with extensive seepage of iron-rich groundwater, Fe was most abundant in the > 0.1 µm fraction, whereas C was predominantly present < 0.1 µm. The EXAFS data reveal that colloidal Fe speciation is different in both study areas. It exists as mononuclear Fe complexed by dissolved organic matter, as colloidal hydrous ferric oxides (likely stabilized by adsorbed organic matter), or as a mixture of these. The colloidal Fe concentrations show considerable seasonal variability. Overall, this study contributes to a better understanding of colloidal Fe speciation and of its interaction with organic C.
2013. 643-643 p.