The iron status in colloidal matter from the Rio Negro, Brasil

Colloidal and particulate natural organic matter was size-fractionated and concentrated from the Rio Negro river using tangential-flow filtration (TFF). Flow field-flow fractionation (F-FFF), with UV absorbance detection (UVA), was used to investigate the molecular weight distributions of the organic colloids. To further characterize the nature of the Rio Negro colloids, the size distributions of the iron concentrations were determined by direct on-line coupling of the F-FFF to an inductively coupled plasma emission spectrometer (ICP-AES). The size distributions obtained by both F-FFF-UVA and F-FFF-ICP-AES were considerably smaller than expected from the stated pore size of the TFF membranes. These results demonstrate that care must be taken in using TFF to classify the size distribution of organic colloids and associated elements present in rivers. The iron distribution is more closely correlated to the organic matter distribution in the colloidal fraction than in the particulate fraction, but it is shifted towards heavier molecular weights for both fractions. The combination of electron paramagnetic resonance spectroscopy (EPR) and transmission electron microscopy (TEM) reveals the presence of iron occurring as: specific complexes with organic functional groups, as Fe-oxide/oxihydroxide phases, or as structurally incorporated component in kaolinite. Particulate and colloidal fractions are differentiated from each other with respect to the iron forms, which is in qualitative agreement with the F-FFF-ICP-AES results