Multi-method Study on Aquatic Humic Substances from the "Rio Negro" - Amazonas State/Brazil. Emphasis on Molecular-Size Classification of their Metal Contents

Abstract

The aquatic humic substances investigated in this study were conventionally isolated from Rio Negro waters - Amazonas State/Brazil by means of the collector XAD 8. Molecular spectroscopy investigations of these humic substances using 1H-NMR and 13C-NMR indicated the presence of high aromaticity with few substituents, small amounts of carboxylic carbon, and an aliphatic/aromatic ratio of 1:1. The relatively high UV-VIS absorbance E350/E450 ratio suggested the presence of relatively large proportions of aliphatic structures. Moreover, using a five-stage tangential-flow ultrafiltration (UF) device, the molecular-size distribution of the isolated humic substances was characterized. The fractionation patterns showed that metal traces remaining in humic substances after their XAD 8 isolation have different size distributions. For instance, Mn, Cu, Cd and Ni quantified by ICP-AES were preferably found in the high molecular-weight fractions (10-100 kDa).