Metal distribution in different size fractions of natural organic matter

In this paper, size-exclusion chromatography (SEC) was used to determine the metal concentration in different size fractions of a bog lake water. Two methods were applied: (a) preparative SEC with off-line metal concentration analysis and (b) direct coupling of an analytical SEC system on-line with an inductively-coupled plasma mass spectrometer (ICP-MS). In the preparative SEC measurements, maximum concentrations were found for different metal ions in different size fractions of the natural organic matter (NOM). Normalization of metal concentrations to dissolved organic carbon concentration (DOC) yielded two maxima in the high and in the very low molecular-weight fractions. Whereas good recoveries were found for AI, Fe, and Ni, only 40 % were obtained for Pb. This indicates that Pb formed labile complexes with NOM, and hence could strongly interact with the column material. In the experiments with the analytical SEC-ICP-MS system, the same trends were observed, but with even lower recoveries than in the preparative system. Sample preconcentration and storage were also investigated with respect to decrease in metal concentration. During the ultrafiltration preconcentration step AI and Fe were removed only to a small extent, whereas about half of the initial Pb was lost. This indicates that AI and Fe were mainly bound to high molecular-weight fractions of NOM. In contrast to that, AI and in particular Fe were removed from solution more than proportionally with respect to DOC because of aggregation of the NOM during storage, whereas Pb and Ni were concentrated relative to the DOC.