Colloids and Surfaces A: Physicochemical and Engineering Aspects 197 (2002) 245 – 256 Combined AFM and STXM in situ study of the influence of Eu(III) on the agglomeration of humic acid Markus Plaschke a, *, Jo ¨ rg Rothe a, *, Thorsten Scha ¨fer a , Melissa A. Denecke a , Kathy Dardenne a , Susanne Pompe b , Karl-Heinz Heise b a Institut fu ¨r Nukleare Entsorgung, Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karlsruhe, Germany b Forschungszentrum Rossendorf, Institut fu ¨r Radiochemie, Postfach 510119, D-01314 Dresden, Germany Received 19 March 2001; accepted 8 August 2001 Abstract Humic acid (HA) agglomerates formed in aqueous solutions in the presence of trivalent Eu cations were investigated in situ with a combination of atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM). The micromorphologies of both natural HA and a melanoidine based synthetic HA observed by AFM in electrolyte solution are in fair agreement with previous AFM studies on humic substances. STXM micrographs of Eu(III) induced HA agglomeration reveal zones of high and low optical density with markedly distinct C K-NEXAFS, indicative of different humic functionalities. Particulate agglomerates observed by AFM can be correlated to the dense zones, whereas fibrous structures in AFM images can be associated with the low density areas. The Eu cation distribution within the agglomerates cannot be unambiguously deduced from their C K-NEXAFS spectra. The near edge X-ray absorption fine structure spectra can be correlated to a segregation of different HA fractions, possibly due to the presence of humic species with different affinities for metal cation complexation. STXM micrographs of purified Aldrich HA exhibit the presence of other yet unidentified, carbon-rich particles, independent of the addition of Eu(III). Both AFM and STXM results for the synthetic melanoidine based HA demonstrate a homogeneous morphology and chemical structure. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Humic acid; Synthetic humic acid; Atomic force microscopy; Scanning transmission X-ray microscopy; C K-NEXAFS www.elsevier.com/locate/colsurfa 1. Introduction Long term isolation and immobilization of long-lived radionuclides, especially actinide ele- ments, is the major goal of nuclear waste reposito- ries in geological formations. To ensure this goal, the impact of colloid-borne actinide transport in the geosphere must be understood. Actinide ions can form either ‘eigen-colloids’, or adsorb onto aquatic colloids formed by organic materials such as humic substances (HS) or inorganic materials such as hydrous ferric oxides (HFO) or clays. These colloids can be mobile in groundwater aquifers and thus enhance radionuclide migration [1]. * Corresponding author. Tel.: +49-7247-82-4747; fax: + 49-7247-82-3927. E-mail addresses: plaschke@ine.fzk.de (M. Plaschke), rothe@ine.fzk.de (J. Rothe). 0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(01)00901-3