Humic and fulvic acid characterization in sediments from a contaminated tropical river

Abstract

Humic acids (HA) and fulvic acids (FA), isolated from sediments of a tropical river basin in Venezuela, have been studied in order to ascertain the influence of human activities as well as the influence of the type of vegetation. Differences and similarities of the tropical acids with temperate humic substances are established.Samples from non-contaminated and contaminated locations were collected in tropical rain-forest and savanna areas and characterized by IR, 1H NMR and 13C NMR spectroscopy. The organic carbon and nitrogen contents were determined in sediments, and in the HA and FA.The anthropogenic wastes have a strong effect on the chemical characteristics of humic substances. For example, polluted sediments contain a higher proportion of the residual fraction (humin) and a lower proportion of fulvic acids than the non-polluted sediments. The nitrogen content in polluted HA and FA is higher than in the non-polluted humic substances, and the organic carbon and nitrogen contents in polluted sediment are higher than in the non-polluted sediments.The IR and 1H NMR spectra of the non-contaminated HA and FA of forest and savanna areas indicate the presence of oxygenated functional groups and aliphatic structures, whereas the tropical rain-forest HA and FA show a slightly higher aromatic character. Contamination increases the proportion of oxygenated functional groups and the aliphatic character of HA and FA, in particular the 1H NMR HA methyl and methylene proton signals. Humic acid spectra of samples collected in areas with high domestic inputs show large-chain alkyl structure signals similar to that of detergents like alkylbenzenosulphonates. Signals in the aromatic regions were detected in FA 13C NMR spectra in samples of the same zone, and appear to be similar to those reported for the phthalic acids used in the preparation of alkyl resins such as plasticizer.The vegetation of the area plays a key role in the Corg and N content of fluvial sediments, with average values of both elements in areas of tropical rain forest 1.7 times higher than those in savanna and temperate forest areas. The climate seems to affect only the C content in the most reactive organic fraction, i.e. the FA.