Near edge X-ray absorption fine structure (NEXAFS) of model compounds for the humic acid/actinide ion interaction

To understand spectral features of humic acid (HA) C 1s-near edge X-ray absorption fine structure (NEXAFS) with and without metal ion complexation, a set of model compounds is investigated. Halogenated benzoic acids and anthranilic acid are examined to demonstrate the effect of electron withdrawing groups on the C 1s-NEXAFS spectra of complex organic acids, including HA. The peak positions for aromatic and carboxylic groups in these spectra are in agreement with common assignments. The spectral position of the peak for substituted aromatic carbon shifts with increasing electronegativity of the substituent to higher photon energies. Polyacrylic acid (PAA) and different PAA metal ion complexes are investigated as model substances for metal cation complexation by HA. Tb(III)-, Zr(IV)- and U(VI)-PAA exhibit general spectral signatures previously observed for the PAA/Eu(III)-PAA system. For these different metal cations, similar spectral changes and distinct variations in peak intensities are observed going from the uncomplexed to the metal-loaded macromolecules. These spectral changes are comparable to those for U(VI)- and Th(IV)-HA systems.