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The environmental fate of metal ions is influenced by their interactions with natural organic and inorganic ligands, which modify the ions’ structure and charge and thus influence their interactions with mineral phases. We investigate the impact of ubiquitous sulfate on the retention of trivalent f-element cations (M(III) = Am, Eu, Y) by muscovite. We combine ex situ alpha spectrometry and in situ surface X-ray diffraction (i.e., crystal truncation rod and resonant anomalous X-ray reflectivity) to determine M(III) coverages and interfacial structures at the molecular level. M(III) cations adsorb as two distinct outer-sphere (OS) complexes (i.e., adsorbed and extended OS complexes) whose coverages vary with increasing sulfate concentration, [SO42-]. When [SO42-] ≤ 0.4 mM, coverages increase and exceed the amounts needed for surface charge compensation of muscovite by a factor of ~3. This overcompensation is likely controlled by ion-ion correlations at the mineral/water interface rather than adsorption of MSO4+, which has a lower thermodynamic stability in the solutions and weaker electrostatic attraction to the mica surface than M3+. For higher [SO42-], MSO4+ and M(SO4)2- dominate solution speciation, leading to a strong decrease of the M(III) coverage due to their lower sorption affinity and weaker ion-ion correlations compared to M3+.

1 Introduction Transport of contaminants, e.g. radionuclides, in the environment depends strongly on their interactions with mineral phases. In a repository for radioactive waste, crystalline rock (e.g. granite) as one potential host rock in Germany and many other countries, may affect the mobility of radionuclides. Main constituents of granite are feldspars. In spent nuclear fuel, trivalent actinides (Am, Cm, but also Pu) contribute strongly to the radiotoxicity. Therefore, this work studies the retention of Am and Cm, as well as their rare earths element analogues (Eu, La, Lu, Nd, Y) on K-feldspar. By combining batch sorption experiments and time-resolved laser-induced fluorescence spectroscopy (TRLFS), a generic surface complexation model (SCM) was obtained that is valid for all investigated M3+. Thermodynamic sorption data were obtained and an understanding of sorption mechanisms on the molecular level was achieved. 2 Results Batch sorption experiments were performed over a broad range of environmental conditions (pH 4 – 10, [M3+] = 52 nM – 10 µM, 3 – 50 g/L K-feldspar (dp < 21 µm; 63 – 200 µm))[1]. Sorption is weak for pH < 5, strongly increases between pH 5 – 7 and reaches complete uptake at higher pH. By deconvolution of Cm emission spectra, an inner-sphere complex and its first two hydrolysis forms were found to be responsible for retention in this pH range. For determination of the deprotonation constant pKa of K-feldspar, as one important input parameter of the model, column titration experiments were conducted. Batch sorption results of all studied M3+ were used to develop two alternative SCMs. The experimental sorption data were used to determine surface complexation parameters by coupling the parameter estimation code UCODE with PHREEQC (SCM-A). In a second approach, spectroscopic data were also considered (SCM-B). A generic approach was used to develop the geochemical models that satisfactorily describe all of the derived M3+/K feldspar sorption edges

The mobility of radionuclides in the environment, in particular in the context of a deep geological repository for radioactive waste, is heavily influenced by their interactions with charged mineral surfaces. This study investigates the retention potential of feldspars, a main component of granite as one potential host rock for a repository. The focus is on the sorption of trivalent actinides (Am, Cm) and their rare earth analogues (Eu, La, Lu, Nd, Y) as a main source of radiotoxicity in spent nuclear fuel. A multi-method approach was used, consisting of traditional batch sorption experiments over a broad range of experimental conditions to determine uptake. Generally, retention increases with increasing pH and reaches quantitative retention at near neutral conditions. Furthermore, a spectroscopic study of the sorption structure on the molecular level was conducted. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) using the actinide Cm as a luminescent probe, shows that four surface complexes are formed, an inner sphere sorption complex and its two hydrolysis forms, as well as a ternary feldspar/Cm/silicate complex at alkaline conditions (pH > 10). Based on the observed comprehensive batch sorption dataset a generic surface complexation model (SCM-A) was developed that describes sorption of trivalent actinides and their rare earth analogues as a function of a variety of geochemical parameters (pH, ionic strength, metal concentration, solid-liquid ratio,…). In a second step, the dataset for the model was further increased by taking the quantitative spectroscopic results into consideration (SCM-B). The developed SCMs deliver surface complexation parameters of the formed sorption species, which are included in thermodynamic databases. This data is essential for the subsequent calculation of distribution coefficients in modern approaches like the Smart KD-concept[1] as well as reactive transport modeling. Therefore, this study provides a contribution to a m

The mobility of heavy metal contaminants and radionuclides in the environment is directly controlled by their interactions with charged mineral surfaces, hence an assessment of their potential toxicity, e.g. in the context of nuclear waste disposal sites, requires understanding of sorption processes on the molecular level. Here, we investigate the sorption of a variety of rare earth elements (REE) and trivalent actinides (Am, Cm) on K-feldspar using batch sorption and column transport experiments, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and a surface complexation model. Initially, a reliable pKa for K-feldspar’s surface deprotonation reaction was determined as 2.5 ± 0.02, in excellent agreement with a measured pHIEP of 2.8. Batch sorption experiments over a broad range of experimental conditions in terms of mineral grain size, pH, [M3+], ionic radius, solid/liquid ratio, ionic strength, and equilibration procedures were carried out to quantify macroscopic immobilization. Similar pH-dependent uptake behavior was found for all investigated trivalent REE and actinides. In parallel, spectroscopic investigations provided insight into surface speciation. Cm TRLFS spectra indicate the formation of three inner-sphere sorption complexes with increasing hydrolysis. Additionally, a ternary K-feldspar/Cm/silicate complex was found for pH > 10, and batch and spectroscopic data at low pH (< 4) point to small amounts of outer sphere sorption complexes. Based on TRLFS data, batch sorption, and titration data, a generic geochemical sorption model was developed, that describes sorption edges for all investigated M3+/K-feldspar systems satisfactorily. The derived stability constants for the binary sorption complexes (logK1-4 = −3.6, −7.7, −11.5, and −17.4, respectively) are in good agreement with previous studies on similar systems, and could successfully be used to reproduce literature data. The stability constants obtained for the surface complexes were includ

Sorption is one of the main processes, which determine the retention of radionuclides (RN) in a repository for nuclear waste. In a multi-barrier system, the host rock poses the ultimate barrier retarding the release of RN into the environment. Feldspars (e.g. orthoclase) are one of the main constituents of crystalline rock (e.g. granite), which is considered one potential host rock type in many countries (e.g. Finland, Sweden, Germany). In this study, the sorption of trivalent actinides (Cm, Am) and their rare earth analogues (Lu, Y, Eu, Nd, La) onto orthoclase (K feldspar) is investigated. For reliable predictions concerning the migration of RN, a process understanding on the molecular level of such processes is necessary. To achieve this, batch sorption experiments are combined with TRLFS and SCM. Batch experiments were performed covering a broad range of experimental conditions (pH 4-11, oxic and anoxic conditions, [M3+] = 10-6-10-4 M, 3-50 g/L orthoclase (grain size: < 21 and 63-200 µm; SSA: 4.2 and 0.2 m2g-1)). Weak retardation below pH 5, followed by a strong increase between pH 5 and 7 and complete removal from solution at pH ≥ 8 was observed for all investigated metals. Cm- and Eu-TRLFS-measurements suggested the formation of an outer-sphere surface complex at lower (pH<5) and two different inner-sphere surface complexes at higher pH values (pH > 5 and pH > 7.5, respectively). Surface precipitation was observed for higher metal concentrations (10-4 M). As the investigated metals revealed a similar behavior over a broad range of conditions, a generic approach was used for the SCM to describe the system as a whole. Experimental data of different series with different metals were simultaneously fitted by coupling PHREEQC with UCODE using the same underlying speciation model. Resulting generic stability constants for the involved surface complexes will be presented. The identification of comparable processes and their unified description with one suitable model

The fundamental aspects of uranyl-spectroscopy (absorption, luminescence, IR), data interpretation and subsequent conclusions to interpret the U(VI)-speciation will be discussed. Furthermore, it will be explained how spectroscopy (NMR and IR) can be used to identify the binding properties of organic molecules, exemplarily explained for a polyhydroxy-carboxylic acid.

Cellulosic material is present in low and intermediate level wastes in considerable amounts. In the case of ground water ingressing into cementitious backfill material a hyperalkaline environment will be established. Since cellulose will be relatively fast degraded under alkaline conditions, there is a high risk that small organic, water soluble molecules will be formed. These molecules can act as complexing agents for radionuclides and thereby affecting their sorption behavior and solubility adversely. Therefore the focus of current investigations is on the interaction of uranium (VI) with Isosaccharinic acid as main degradation product as well as with acetate. The progress and results of these investigations as part of the MIND project (work package 1) will be presented.

Low and intermediate level waste contain cellulosic material in considerable amounts. In a repository nuclear waste is often surrounded by cementitious backfill material. If water ingresses a hyperalkaline environment will be established. Since cellulose will be relatively fast degraded under alkaline conditions, there is a high risk that small organic, water soluble molecules will be formed. These molecules can act as complexing agents for radionuclides and thereby affecting their sorption behavior and solubility adversely. Therefore the focus of current investigations is on the interaction of uranium (VI) with Isosaccharinic acid (ISA) as main degradation product of cellulose first in the acidic pH range. The progress and results of these studies within the MIND project (work package 1) will be presented.