Properties of soil humic substances in fractions obtained by sequential extraction with pyrophosphate solutions at different pHs - II. Elemental composition and UV-VIS spectra of humic acids

Humic acids (HAs) obtained from four different soils by sequential extraction with pyrophosphate solutions at different pH values (i.e. at pH 3, 5, 7, 9, 11, and 13), were examined by ultraviolet and visible (UV-VIS) spectroscopy and elemental analysis. UV-VIS spectra from 230 to 700 nm were measured, and absorbance coefficients of 1% humic acid solutions at 600 nm (E-600(1%)) and Delta log K were calculated. The shape of the spectra of humic acids depended on the soil types rather than on the pH values. The E-600(1%) values were higher in the neutral region, and lower in the alkaline region, for each pH value they decreased in the following order: buried Andosol > Andosol > Histosol greater than or equal to Cambisol. The Delta log K values for each pH value were higher in the following order: Histosol > Cambisol > buried Andosol greater than or equal to Andosol. Although the linear correlation was found to be significant between the E-600(1%)) and Delta log K values, the relation between these optical properties and pH values differed among the soil types. However, in the neutral region, the higher E-600(1%) values of humic acids from buried Andosol and Andosol and the lower Delta log K values of humic acids from Cambisol and Histosol suggest that the humification degree was higher in the neutral region. Data of elemental composition and atomic ratios were obtained for each humic acid. The H/C ratios in the four soils increased gradually with increasing pH values from 5 to 13, and those for each pH value were commonly higher in the following order: Cambisol > Histosol > Andosol > buried Andosol. The O/C and O/H ratios decreased with increasing pH values. The results of the H/C versus O/C diagram suggested that decarboxylated humic acids were extracted at the higher pH values. A highly significant correlation coefficient was obtained between the H/C and O/H ratios (r = -0.891***), Results showing that the decrease of the pH values corresponded to the order of the humic acids in the correlation curve in the H/C versus O/H diagram suggest that the humic acids with a higher content of unsaturated bonds and a higher degree of oxidation were extracted at the lower pH values within the range from 5 to,13.