Abstract

Humic and fulvic acids present in soils of different depth were extracted and their acidic functional groups and structural characteristics were analyzed and compared. The purpose of this study was to present a basic data needed to evaluate the effect of humic substances on depth distribution and migrational behaviour of radioactive elements deposited on soil. Acidic functional groups of the humic and fulvic acids were analyzed by pH titration method, and their proton exchange capacity (PEC, <TEX>$mq\;g^{-1}$</TEX>) and average <TEX>$pK_a$</TEX> values were obtained. Structural characteristics of the humic and fulvic acids were analyzed using their CPMAS <TEX>$^{13}C$</TEX> NMR spectra and elemental composition data. pH titration data showed that fulvic acids have higher acidic functional group contents ranging from 5.5 to <TEX>$7.0meq\;g^{-1}$</TEX> compared with that of humic acids (<TEX>$3.8{\sim}4.8meq\;g^{-1}$</TEX>). From depth profiles, it has been found that PEC values of humic acids in deeper soil (> 8 cm) were higher than those at the surface soils. Elemental compositions (H/C ratio) and spectral features (<TEX>$C_{arom}/C_{aliph}$</TEX> ratio) obtained from CPMAS <TEX>$^{13}C$</TEX> NMR spectra showed that the aromatic character in humic acids was a relatively higher than that of fulvic acids, while lower in carboxyl carbon content. The aromatic character and carboxyl carbon contents of humic acids tend to increase as soil depth increased, but those of fulvic acid showed little differences by the soil depth range.