STXM∕C 1s-NEXAFS study of Eu(III) and Uranyl humic acid aggregates at different pH

Abstract

Humic acids (HA) are chemically heterogeneous and structurally ill‐defined biopolymers which are able to bind traces of actinides or lanthanides. Due to their dimensions in the colloidal size range they may affect transport of these elements in aquatic systems. Eu(III)‐ and UO22+‐HA aggregates have been investigated by Scanning Transmission X‐ray Microscopy (STXM) and C 1s‐NEXAFS under systematic variation of pH. In the Eu(III)‐ and UO22+‐HA systems aggregate morphologies at near neutral pH were similar to those observed in previous studies: optically dense zones (high absorption at the carbon K‐edge) are embedded in a matrix of less dense material. C 1s‐NEXAFS signatures observed in the different zones, i.e., the intensity of the characteristic complexation feature previously experimentally described and recently theoretically characterized, strongly depends on sample pH. In the alkaline regime (pH 9) with added carbonate, co‐precipitation of Eu(III)‐carbonate (or ternary carbonate/(oxo)hydroxide complexes) with the Eu(III)‐HA majority fraction is observed but Eu(III) binding to HA over carbonate in the dense zones seems to be favoured. The UO22+‐HA system exhibits in alkaline solution more compact morphologies combined with a strong metal ion complexation effect in the NEXAFS. Eu(III) and UO22+ polyacrylic acid (PAA) aggregates used as HA model systems show similar spectral trends; these aggregates exhibit highly branched morphologies without segregation into zones with different NEXAFS signatures. The chemical environment such as pH or the type of metal cation strongly influences both HA aggregate morphologies and NEXAFS spectral signatures. These can, in turn, be used as indicators of the strength of lanthanide or actinide ion bound HA interaction.