Chemical and biological studies on environmental humic acids

Abstract

Abstract Forty humic acid samples were extracted from various types of soils, including aquatic sediments. To estimate the degree of humification, absorbance coefficients for 1% humic acid solutions at 600 nm () and ΔlogK values were measured. Mean values for decreased in the order: Andisol (A)>Oxisol (O), Mollisol (M), Vertisol (V)>Histosol (H)>Entisol (E), sediment (s), Inceptisol (I). Mean values for ΔlogK increased in the order: A<M, V, O<s, I, E<H. Andisol and sediment humic acids showed the highest and lowest degrees of humification, respectively. Linear correlation was found to be significant between and ΔlogK. The elemental composition of the various types of humic acids was compared. Andisol humic acids contained a large amount of carbon and a small amount of hydrogen and nitrogen. Hydrogen and nitrogen contents of aquatic humic acids were higher than those of the terrestrial humic acids. Mean values for the H/C ratio decreased in the order: s>I, E, O, H>M, V>A. A linear correlation was found to be significant between the H/C and N/C ratios. The lower the H/C ratios, the higher the values and therefore also the degree of humification. In the graph of H/C versus O/C, sediment and Entisol humic acids were placed in the area near plankton and plants. Andisol humic acids fell to the right of the coal band (i.e., had higher O/C ratio). As the demethanation and dehydration reactions lead to the decrease of the H/C ratio, the aliphatic and proteinoid parts are split off from the humic molecules and dehydrative condensation takes place in the humification process. Each source of humic acid was analyzed for the content of carboxyl, low pK a carboxyl (COOHs), carbonyl, phenolic, and alcoholic hydroxyl groups. Andisol humic acids were rich in carboxyl and carbonyl groups and poor in alcoholic hydroxyl groups. Entisol humic acids had low carboxyl and carbonyl contents. It is noteworthy that sediment humic acids had relatively high carboxyl and carbonyl contents. Linear correlations were found to be significant between COOH and COOHs, and COOH and C=O. values showed significant positive and negative linear correlations with the carboxyl and alcoholic hydroxyl groups, respectively