Acid-base properties of aquatic humic substances isolated from unpolluted tropical black water rivers

Abstract

The acid-base properties of tropical aquatic humic substances isolated from 2 Venezuelan black water rivers were studied. A model with discrete pK values was applied to potentiometric titration data, since direct determination of pK and concentration values from titration curves is not possible. In the computational procedure simulated curves have been compared with the titration curves to improve the calculations for pK and concentration values. Both rivers (Yuruani and Aponwao) are located in a zone where the human influence is still negligible. The geomorphological characteristics of this remote area (Gran Sabana) are singular in relation to the rest of the world. The rivers have been monthly studied at sampling site, during the rainy season (May–September). Six discrete acidic groups represented well the acidic characteristics of aquatic humic substances under consideration. The average pK values were 2.74, 4.37, 5.67, 7.09, 8.73 and 10.30, and the average concentration values for each group were 2.50, 1.80, 0.94, 0.62, 0.66, and 1.31 mmol/g humic substance, respectively. Considering the factors “month” and “river”, significant quantitative, but not qualitative, differences were in general found. A comparison with data from temperate ecosystems showed that the three most acidic groups presented similar acid-base characteristics, wheras the three other groups are quite different for the tropical aquatic humic substances under study.