Erich Wieland

The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals

A general rate law on the surface-controlled dissolution of oxides and silicates is discussed. Combining concepts of surface coordination chemistry with established models of lattice statistics and activated complex theory we propose a general rate law for the acid- and ligand-promoted dissolution of minerals: R = kxa,PjS where R is the proton- or ligand-promoted dissolution rate [moles m−2s−1], k stands for the appropriate rate constant [s−1], xa denotes the mole fraction of dissolution active sites [−], Pj represents the probability to find a specific site in the coordinative arrangement of the precursor complex [−] and S is the surface concentration of sites [mol m−2]. Surface complexes (surface chelates and metal proton complexes) are precursors in the rate-limiting detachment of a central metal ion from the surface into the solution. We develop a mechanistic model, which clarifies the pH-dependence of dissolution rates. First appropriate surface protonation isotherms are derived. The surface protonation equilibria of different minerals at constant ionic strength match a single Freundlich isotherm with a slope of ca. 0.2. This result explains the frequently reported fractional pH-dependence of dissolution rat`es. Based on the Bragg-Williams approximation a lattice statistical procedure is outlined which permits the calculation of the probability Pj of the precursor complex. The consistency of reported experimental dissolution rates and activation energy is tested and, as a consequence, different possibilities to correlate reaction rates (kinetic rate constants [s−1] are not reported) or apparent activation energies with thermodynamic data in the form of free linear energy relations are explored. We postulate that the site energy (Madelung energy) of the most stable lattice constituent (generally a metal cation at a surface site) or the ion formation energy of the solid—characteristic of the free energy needed to break the essential bonds in the lattice—are suitable free energy parameters to be correlated with the dissolution rates (log RH at pH 5).

Dissolution kinetics of kaolinite in acidic aqueous solutions at 25 C

Abstract The dissolution of kaolinite is interpreted in terms of the surface complexation model. Acid/ base properties of the terminal OH groups and ion exchange reactions occurring at the kaolinite surface have been investigated. A three-site model incorporating solid-solution equilibria at aluminol groups of the edge and gibbsite surfaces and at negatively charged XO groups of the siloxane surface account for the protonation of kaolinite platelets in acidic solutions. The dissolution kinetics of kaolinite at 25°C has been studied as a function of solution pH. The dissolution of kaolinite is nonstoichiometric in the pH range 2–6.5 with a preferential release of silicon. Stoichiometry of the dissolution reaction is achieved, however, in the presence of oxalate as Al-complexing ligand. The detachment of aluminium from the lattice structure of the kaolinite surface and its readsorption on distinct surface sites occur simultaneously during the dissolution process causing the experimentally observed nonstoichiometry. Although oxalate and salicylate form surface complexes only with Al centers, they promote the release of both Al and Si centers during the dissolution process. The proton-promoted dissolution of kaolinite occurs at the edge surface (pH R H (Si) reflects sequential protonation of terminal OH groups on both surfaces. The dissolution reaction can be interpreted as a coupled release of Al and Si with the detachment of the Al center from the surface lattice structure as the rate-limiting step. The aluminium:proton Stoichiometry of the activated complex is 1:3 at the gibbsite surface and 1:1 at the edge surface.

The Effects of Complex-Forming Ligands on the Dissolution of Oxides and Aluminosilicates

The composition of natural waters is controlled to a significant extent by processes at the solid (particle)/water interface, above all by the dissolution and precipitation of minerals.

Diffusion of tritiated water and 22Na+ through non-degraded hardened cement pastes

Diffusion experiments through hardened cement pastes (HCP) using tritiated water (HTO) and 22Na(+), considered to be conservative tracers, have been carried out in triplicates in a glove box under a controlled nitrogen atmosphere. Each experiment consisted of a through-diffusion test followed by an out-diffusion test. The experimental data were inversely modelled applying an automated Marquardt-Levenberg procedure. The analysis of the through-diffusion data allowed the extraction of values for the effective diffusion coefficients, D(e), and the rock capacity factor, alpha. Good agreement between measured and calculated tracer breakthrough curves was achieved using both a simple diffusion model without sorption and a diffusion/linear sorption model. The best-fit K(d)-values were found to be consistent with R(d)-values measured in previous batch-sorption experiments. The best-fit values from the through-diffusion tests were then used to predict the results of subsequent out-diffusion experiments. Good agreement between experimental data and predictions was achieved only for the case of linear sorption. Isotopic exchange can only partially account for both the amount of tracer taken up in the batch-sorption tests and the measured retardation in the diffusion experiments and, hence, additional mechanisms have to be invoked to explain the data.

Determination of uranium(VI) sorbed species in calcium silicate hydrate phases: a laser-induced luminescence spectroscopy and batch sorption study.

Batch sorption experiments and time-resolved luminescence spectroscopy investigations were carried out to study the U(VI) speciation in calcium silicate hydrates for varying chemical conditions representing both fresh and altered cementitious environments. U(VI) uptake was found to be fast and sorption distribution ratios (R(d) values) were very high indicating strong uptake by the C-S-H phases. In addition a strong dependence of pH and solid composition (Ca:Si mol ratio) was observed. U(VI) luminescence spectroscopy investigations showed that the U(VI) solid speciation continuously changed over a period up to 6 months in contrast to the fast sorption kinetics observed in the batch sorption studies. Decay profile analysis combined with factor analysis of series of spectra of U(VI)-C-S-H suspensions, recorded with increasing delay times, revealed the presence of four luminescent U(VI) species in C-S-H suspensions, in agreement with the batch sorption data. Along with the aqueous UO(2)(OH)(4)(2-) species and a Ca-uranate precipitate, two different sorbed species were identified which are either bound to silanol groups on the surface or incorporated in the interlayer of the C-S-H structure.

Iodine species uptake by cement and CSH studied by I K-edge X-ray absorption spectroscopy

Summary The uptake of iodine species (I−/IO3−) by HCP (hardened cement paste) and a CSH (calcium silicate hydrate) phase under highly alkaline conditions has been investigated using X-ray absorption spectroscopy (XAS). The study was performed at the I K-edge (33.169 keV) instead of the I L3-edge (4.557 keV) to avoid interference with Ca (K-edge=4.038 keV), a major element in HCP and CSH phases. The XANES (X-rays absorption near-edge structure) spectra revealed no changes in the formal oxidation state of iodide (I(-I)−) and iodate (I(V)O3−) upon uptake by HCP and CSH. The EXAFS (extended X-ray absorption fine structure) oscillations from I− treated HCP and CSH samples were found to be extremely weak, limiting interpretation of the EXAFS data. The IO3− EXAFS spectra showed that the IO3− entity consisting of three oxygen atoms with a characteristic I-O distance (∼1.78 Å) is maintained upon uptake by HCP and CSH. XANES further indicated that CSH is not the uptake-controlling phase in HCP.

Structural Insight into Iodide Uptake by AFm Phases

The ability of cement phases carrying positively charged surfaces to retard the mobility of (129)I, present as iodide (I(-)) in groundwater, was investigated in the context of safe disposal of radioactive waste. (125)I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfate-containing AFm phases indicated that calcium-monosulfate (AFm-SO(4)) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm-I(2)) and Friedels salt (AFm-Cl(2)) for I-Cl mixtures, whereas interstratifications of AFm-I(2) and hemicarboaluminate (AFm-OH-(CO(3))(0.5)) were observed for the I-CO(3) systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm-I(2) and AFm-SO(4) for the I-SO(4) mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I-CO(3) and in I-SO(4) samples compared to pure AFm-I(2). This is assumed to be due to the introduction of stacking faults in I-CO(3) samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I-SO(4) samples on the other hand. The formation of a solid solution between AFm-I(2) and AFm-SO(4), with a short-range mixing of iodide and sulfate, implies that AFm-SO(4) bears the potential to retard (129)I.

The effect of α-isosaccharinic acid on the stability of and Th(IV) uptake by hardened cement paste

Summary The effect of α-isosaccharinic acid (ISA) on Th(IV) uptake by hardened cement paste (HCP) has been investigated under alkaline conditions (pH 13.3). Prior to performing the uptake studies the stability of HCP was determined in the presence of ISA. It was observed that the formation of Ca-ISA complexes in solution enhances portlandite solubility. The fraction of portlandite dissolved from the HCP matrix depends on the solid to liquid (S/L) ratio of the system and the ISA concentration in solution. Th(IV) uptake by HCP was found to be reduced above an aqueous ISA concentration of about 0−4 M. Reduction of Th(IV) uptake can be modelled taking into account the formation of a Th:ISA:Ca=1:2:1 complex in solution. It is indicated that the formation of ternary Th(IV)-ISA complexes may be important in cement systems. The final interpretation of the data however fails due to the large uncertainties in the distribution ratios measured in the absence of ISA.

Spectroscopic investigations of Np(V/VI) redox speciation in hyperalkaline TMA-(OH, Cl) solutions

Abstract The redox chemistry of Np(V/VI) was investigated in ∼0.6 M tetramethylammonium hydroxide/chloride (TMA-(OH, Cl)) solutions with 9 ≤ −log [H+] ≤ 13.5. Redox conditions were defined by the absence or presence of ClO− as oxidizing agent (Na-salt, 5 × 10−3 M and 5 × 10−2 M). The high total Np concentration ([Np]tot ∼ 2 × 10−3 M) led to the precipitation of solid phases in some of the samples. The carbonate concentration (as impurity of TMA-OH) was 2–3 × 10−3 M. UV-vis/NIR spectra obtained from the supernatant in TMA-(OH, Cl) solutions and absence of ClO− showed clear Np(V) features, identified as NpO2+, NpO2CO3− and (NpO2)x(CO3)y(OH)zx−2y−z. No NIR features were observed within 800 nm ≤ λ ≤ 1300 nm for samples with ClO−. XANES edge energies and features of these samples confirmed the predominance of Np(V) in the absence of ClO− and Np(VI) in the presence of ClO−, by comparison to XANES reference spectra of Np(III/IV/V/VI) prepared within the present work by in-situ electrolysis. A similar Np redox distribution was observed for the solid phases based on XANES and EXAFS measurements. EXAFS spectra indicative of NpVO2OH(s) and NpVIO3· xH2O(s) were obtained for samples in absence and presence of ClO−, respectively. The formation of a Na-Np(VI) phase in 5 × 10−2 M ClO− and −log [H+] ∼ 12 was also indicated from the EXAFS, chemical analysis and SEM-EDS. These results indicate that Np(VI) aqueous species and solid compounds prevail far below the oxidation border of water in alkaline solutions and also far below the EH border calculated with the current NEA data selection [1]. These observations are further supported by correlations of literature thermodynamic data for actinides (U, Np, Pu and Am), which predict the formation of NpO2(OH)3− and NpO2(OH)42− aqueous species with stability constants (log *βº1,3 and log *βº1,4) similar to those available for U(VI).

Uptake of Np(IV) by C–S–H Phases and Cement Paste: An EXAFS Study

Nuclear waste disposal concepts developed worldwide foresee the use of cementitious materials for the immobilization of long-lived intermediate level waste (ILW). This waste form may contain significant amounts of neptunium-237, which is expected to be present as Np(IV) under the reducing conditions encountered after the closure of the repository. Predicting the release of Np(IV) from the cementitious near field of an ILW repository requires a sufficiently detailed understanding of its interaction with the main sorbing components of hardened cement paste (HCP). In this study, the uptake of Np(IV) by calcium silicate hydrates (C-S-H) and HCP has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS studies on Np(IV)-doped C-S-H and HCP samples reveal that Np(IV) is predominantly incorporated in the structure of C-S-H phases having different Ca:Si ratios. The two main species identified correspond to Np(IV) in C-S-H with a Ca:Si mol ratio of 1.65 as in fresh cement and with a Ca:Si mol ratio of 0.75 as in highly degraded cement. The local structure of Np(IV) changes with the Ca:Si mol ratio and does not depend on pH. Furthermore, Np(IV) shows the same coordination environment in C-S-H and HCP samples. This study shows that C-S-H phases are responsible for the Np(IV) uptake by cementitious materials and further that incorporation in the interlayer of the C-S-H structure is the dominant uptake mechanism.