Multi-method study on aquatic humic substances from the "Rio Negro" - Amazonas state/Brazil. Emphasis on molecular-size classification of their metalcontents

The aquatic humic substances investigated in this study were conventionally isolated from Rio Negro waters - Amazonas State/Brazil by means of the col lector XAD 8. Molecular spectroscopy investigations of these humic substanc es using H-1-NMR and C-13-NMR indicated the presence of high aromaticity wi th few substituents, small amounts of carboxylic carbon, and an aliphatic/a romatic ratio of 1:1. The relatively high W-VIS absorbance E-350/E-450 rati o suggested the presence of relatively large proportions of aliphatic struc tures. Moreover, using a five-stage tangential-flow ultrafiltration (UF) de vice, the molecular-size distribution of the isolated humic substances was characterized. The fractionation patterns showed that metal traces remainin g in humic substances after their XAD 8 isolation have different size distr ibutions. For instance, Mn, Cu, Cd and Ni quantified by ICP-AES were prefer ably found in the high molecular-weight fractions (10-100 kDa).