FUNCTIONAL-GROUP CONTENT OF SOIL AND SEDIMENTARY HUMIC ACIDS DETERMINED BY CP/MAS C-13 NMR RELATED TO CONDITIONAL ZN2+ AND CD2+ FORMATION-CONSTANTS

Cross polarization/magic angle spinning C-13 NMR spectra, with varying dipolar dephasing times, were obtained for seven humic acids from a variety of terrestrial soil and aquatic sedimentary sources. Interpretation of the spectra revealed differences in functional group content, such as an enrichment in amino acid groups in the lignin-poor sedimentary samples relative to lignin-rich sedimentary and soil samples. Plots of the natural log of signal intensity above 100 ppm versus dipolar dephasing time, were used to measure the degree of aromatic substitution in each humic acid. The percentage of non-protonated aromatic carbon (relative to total aromatic carbon) ranged from 49% to 62% for the lignin-poor humic acids, while the values for lignin-rich samples were consistently higher and ranged from 64% to 86%, indicating a greater degree of aromatic substitution of the lignin-rich humic acids relative to the lignin-poor humic acids. In addition, conditional formation constants for each humic acid with Zn2+ and Cd2+ were evaluated by the use of Scatchard plots and the modified gel filtration technique. The sedimentary humic acids were found to have higher conditional formation constants and this may be due to the higher amino acid content of these samples. Within a humic acid category (lignin-rich or lignin-poor), the samples with greater degrees of aromatic substitution were characterized by higher conditional formation constants, which may be due to a chelation effect.