Markus Plaschke

In situ AFM study of sorbed humic acid colloids at different pH

Abstract Humic acid colloids adsorbed on the basal plane of cleaved muscovite are investigated under in situ conditions by non-contact mode atomic force microscopy (AFM) in liquid (also called fluid tapping-mode AFM). Structures are found to be of nanometer scale, consisting of flat particles (8–13 nm in diameter), aggregates of particles (20–100 nm), chain-like assemblies, networks and torus-like structures. In contrast to former investigations colloids are investigated in aquatic solution and structures are not influenced by sample preparation. Nanostructure, surface coverage and particle sizes are found to depend on solution pH. Humic colloids can be distinguished from surface roughness and background noise by image processing. Furthermore, an approach to quantify the surface coverage is discussed. Therefore, non-contact mode AFM in liquid is shown to be a powerful method to study the interaction of colloids at solid–liquid interfaces.

Combined AFM and STXM in situ study of the influence of Eu(III) on the agglomeration of humic acid

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.

Accelerator mass spectrometry of actinides in ground- and seawater: An innovative method allowing for the simultaneous analysis of U, Np, Pu, Am, and Cm isotopes below ppq levels

(236)U, (237)Np, and Pu isotopes and (243)Am were determined in ground- and seawater samples at levels below ppq (fg/g) with a maximum sample size of 250 g. Such high sensitivity was possible by using accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) with extreme selectivity and recently improved efficiency and a significantly simplified separation chemistry. The use of nonisotopic tracers was investigated in order to allow for the determination of (237)Np and (243)Am, for which isotopic tracers either are rarely available or suffer from various isobaric mass interferences. In the present study, actinides were concentrated from the sample matrix via iron hydroxide coprecipitation and measured sequentially without previous chemical separation from each other. The analytical method was validated by the analysis of the Reference Material IAEA 443 and was applied to groundwater samples from the Colloid Formation and Migration (CFM) project at the deep underground rock laboratory of the Grimsel Test Site (GTS) and to natural water samples affected solely by global fallout. While the precision of the presented analytical method is somewhat limited by the use of nonisotopic spikes, the sensitivity allows for the determination of ∼10(5) atoms in a sample. This provides, e.g., the capability to study the long-term release and retention of actinide tracers in field experiments as well as the transport of actinides in a variety of environmental systems by tracing contamination from global fallout.

Chemical status of U(VI) in cemented waste forms under saline conditions

Abstract Retention of U(VI) in cemented waste forms reacting with NaCl and MgCl2 brines is investigated in long-term leaching experiments on full scale monoliths. Solution compositions were monitored over a period of 17 to 18 years. After termination of the leaching experiments, chemical and mineralogical compositions of solid reaction products were studied intensively. XRD, TRLFS and XANES/EXAFS analyses indicate uranophane (Ca(UO2)2(SiO3OH)2·5H2O) to be the dominant uranium bearing phase in the corroded cement. Other possible uranium phases such as soddyite, meta-schoepite, and di-uranate phases could not be identified by combining the results of the various experimental techniques.

Scanning transmission X-ray microscopy as a speciation tool for natural organic molecules

Summary A molecular-scale understanding of the basic processes affecting stability and transport behavior of actinide cations, complexes or hydroxide (‘eigencolloid’) species is prerequisite to performance assessment of nuclear waste disposal in geological formations. Depending on their functional group chemistry and macromolecular structure, naturally occurring organic molecules (NOM) possess a high tendency towards actinide complexation reactions. However, the compositional and structural heterogeneity of NOM and mixed aggregates with inorganic phases makes speciation by spectromicroscopy techniques highly desirable. The applicability of Scanning Transmission X-ray Microscopy (STXM) as a speciation tool for the characterization of NOM is demonstrated for a multifunctional natural organic acid (chlorogenic acid), Eu(III)-loaded humic acid (HA) aggregates and Eu(III)-oxo/hydroxide/HA hetero-aggregates. It is shown that in situ probing of HA functional group chemistry down to a spatial resolution <100 nm (i.e., less than femto-liter sampled volumes) is feasible, at the same time revealing morphological details on NOM aggregates and NOM/mineral associations.

The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP

A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the individual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.

Understanding humic acid / Zr(IV) interaction. A spectromicroscopy approach.

Complexation of Zr(IV) by humic acid (HA) and polyacrylic acid (PAA) is investigated from the point of view of the organic ligand. STXM Spectromicroscopy and C 1s‐NEXAFS point to different interaction mechanisms between Zr(IV) cations and oxo/hydroxo colloids and PAA. Under conditions where the metal aquo ion is stable, strong complexes are formed. In contrast, unspecific surface coating is identified when PAA is contacted with Zr(IV) oxo/hydroxide colloids. HA exhibits similar C 1s‐NEXAFS features indicating a complexation reaction.

STXM and LSLM investigation of Eu(III) induced humic acid colloid aggregation

Humic acids (HA) are potentially important in binding traces of actinides or lanthanides, thus affecting their transport in aquatic systems. Eu(III) induced HA colloid aggregation has been investigated by a combination of Scanning Transmission X-ray Microscopy (STXM) and Laser Scanning Luminescence Microscopy (LSLM). Both methods reveal the same aggregate morphology – optically dense zones embedded in a matrix of less dense material observed by STXM correspond to areas with increased Eu(III) luminescence yield in the LSLM micrographs. From these comparative measurements we infer the enrichment of Eu(III) cations in the optically dense zones. These areas also exhibit a C 1s-NEXAFS signature strongly differing from the signal extracted from the less dense areas. Spectral filtering of Eu(III) luminescence lines corresponding to valence transitions affected by organic acid complexation indicates that Eu(III) cations are more strongly bound in the dense zones.

Multiactinide Analysis with Accelerator Mass Spectrometry for Ultratrace Determination in Small Samples: Application to an in Situ Radionuclide Tracer Test within the Colloid Formation and Migration Experiment at the Grimsel Test Site (Switzerland)

The multiactinide analysis with accelerator mass spectrometry (AMS) was applied to samples collected from the run 13-05 of the Colloid Formation and Migration (CFM) experiment at the Grimsel Test Site (GTS). In this in situ radionuclide tracer test, the environmental behavior of 233U, 237Np, 242Pu, and 243Am was investigated in a water conductive shear zone under conditions relevant for a nuclear waste repository in crystalline rock. The concentration of the actinides in the GTS groundwater was determined with AMS over 6 orders of magnitude from ∼15 pg/g down to ∼25 ag/g. Levels above 10 fg/g were investigated with both sector field inductively coupled plasma mass spectrometry (SF-ICPMS) and AMS. Agreement within a relative uncertainty of 50% was found for 237Np, 242Pu, and 243Am concentrations determined with the two analytical methods. With the extreme sensitivity of AMS, the long-term release and retention of the actinides was investigated over 8 months in the tailing of the breakthrough curve of run 13-05 as well as in samples collected up to 22 months after. Furthermore, the evidence of masses 241 and 244 u in the CFM samples most probably representing 241Am and 244Pu employed in a previous tracer test demonstrated the analytical capability of AMS for in situ studies lasting more than a decade.

Synchrotron-Based X-Ray Spectromicroscopy of Organic Nanoparticles Complexing Actinides

Humic acids (HA) have a high binding capacity toward traces of toxic metal cations, including actinides, and have a propensity for forming nano- to micron-sized colloids. They can therefore act as transport vehicles for actinides in aquatic systems, which can have detrimental consequences for the safety of nuclear waste disposal. Modern scanning transmission X-ray microscopy (STXM) at the carbon K(1s)-edge is an excellent tool for investigations of such and other processes. STXM studies, combined with other sophisticated techniques and quantum chemical calculations, provide the necessary spatial resolution in the sub-μm range to resolve characteristic aggregate morphologies and identify molecular processes involved in HA–actinide interaction. Systematic experimental and theoretical investigations of reference systems allow successful interpretation of C 1s near edge X-ray absorption fine structure (NEXAFS) extracted from STXM image stacks of HA–metal aggregates. The results show that different HA domains (of likely structurally supramolecular HA associations) exhibit different interactions with metal cations. Metal cations bound to HA are enriched in a minority fraction containing higher densities of complexing carboxylic sites. This fraction is likely to play a dominant role in HA colloid-mediated transport of actinides and other toxic trace metals in the hydrosphere.