Characterization of interactions between natural organic matter and metals by tangential-flow ultrafiltration and ICP OES

摘要

Molecular size fractionation of the natural organic matter (NOM) in water samples from the Serra de Itabaiana National Park (Brazil) was used to characterize the temporal behavior of metal ion complexation. A special five-stage tangential-flow ultrafiltration device was used to separate NOM in six size fractions (F1: 100; F2: 100-50; F3: 50-30; F4: 30-10; F5: 10-5; F6: < 5 kDa). The distribution patterns obtained for one in natura sample showed that metals (Al, Ba, Cr, Cu, Fe, Mn, Ni, Pb and Sr) were preferentially complexed by the F1 fraction, which contained higher concentration of dissolved organic carbon (DOC). The following decreasing order of carbon distribution in the different fractions was obtained: F1 F3 F2 = F4 F6 F5. Fractions F2 and F5 were the most humified, and F6 the least. New distributions of DOC and metals were observed after 30 days when high concentrations of metals were added to filtered samples. Highest DOC was still found in F1, with carbon contents decreasing in the following order: F1 F2 F3 F4 F5 F6. The studied metals presented similar and homogeneous distributions in all fractions. Values of log K were similar for samples in natura and 30 days after addition of metals (7.1 and 7.0, 6.6 and 6.9, and 7.2 and 7.0 for Al, Cu and Fe-NOM, respectively), indicating a new dynamic equilibrium in the metals distribution after 30 days. The species formed between NOM and spiked metals showed distribution patterns that changed as a function of complexation time, indicating a transformation of the NOM binding sites, suggesting that the collective effect of the fractions is more important than individual site properties, and confirming that the most humified fractions exert a large influence on the complexation of metals in aquatic environments.