Soft X-ray spectromicroscopy of humic acid europium(III) complexation by comparison to model substances

To provide the experimental basis for a deeper understanding of humic acid (HA) C 1s-near edge X-ray absorption fine structure (NEXAFS) spectra with and without metal ion complexation, a set of model compounds is investigated. Phthalic acid and hydroxyl substituted benzoic acids (benzoic, 4-hydroxy benzoic, protocatechuic, and gallic acid) are examined in an attempt to describe the C 1s-NEXAFS spectra of more complex organic acids—including HA—based on the building block principle. While the peak assignments and observed oscillator strengths for aromatic and carboxylic groups in these spectra are in good agreement with common assumptions, the spectral intensity in the range of the phenolic band is not readily explained with a simple building block model. The spectral signature observed for HA and Eu(III)–HA complexes is compared to the one obtained for polyacrylic acid (PAA) and Eu(III)–PAA, chosen as model substances for the complexation. In both systems strikingly similar spectral changes going from the uncomplexed to the metal-loaded macromolecules are evident. Therefore, a distinct complexation effect on the C 1s-NEXAFS is postulated. Possible effects of radiation damage on the C 1s-NEXAFS are found to be negligible.