Study on C-Functional Groups of Soil Humus Fractions Affected by Phosphate Using C 1s Near-edge X-ray Absorption Fine Structure Spectroscopy

Synchrotron based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was applied to study the C-functional group chemistry of various soil humus fractions (including fulvic acid, humic acid and humin) and to explore the reaction mechanisms between phosphate and humus. The results indicated that the humus fractions contained similar functional groups but at different proportions. Humic acid and humin, with the increasing acid. insolubility, exhibited higher content of hydrophobic functional groups and higher aromaticity while lower carboxylic C when compared with fulvic acid. The presence of phosphate could favor the release of the hydrophilic carboxylic functional groups into soil solutions through substitution reactions as well as electrostatic effects, and thus exert significant effects on the functional group chemistry of soil humic substances. The results will be great helpful to study the speciation and composition of C-functional groups with NEXAFS spectroscopy and to understand the reaction mechanisms of phosphate in soil micro-interfaces.