Surface Complexation and Its Impact on Geochemical Kinetics

The weathering of rocks, the formation of soils, the alteration and dissolution of sediments are a consequence of surface reac-. tions. Furthermore, many redox processes such as the oxidation of V02+, Mn2+ and Fe2+, the ncn-biotic degradation of organic substances and photosensitized processes are catalyzed by surfaces. The electric double layer theory, despite its efficiency in quantifying certain phenomena of colloid stability, has limitations because it neglects chemical speciation at the surface and does not provide information on the chemical structure of the interfacial region. The surfaces of naturally occurring solids are characterized by functional groups, e. g., OH- groups on the surface of hydrous oxides at on organic surfaces. Specific adsorption of - or interaction with - H+, OH-, metal ion s and ligands occurs through coordination at the surface; inner-sphere surface complexes can be formed. The form of occurrence of the individual compounds (speciation) needs to be known in order to understand their reactivity; especially the geometry of the coordination shell of surface sites or of reactants at surfaces is a prerequisite for interpreting reaction rates occuring at the particle-water interface. Some case studies on the oxidation of Mn2+ and V02+ and on the dissolution of hydrous oxides and silicates are presented. In each case, the kinetics of the processes and how it is affected by solution variables such as H+ and ligands (such as oxalate and other di- ar hydroxy-carboxylates) are explained by simple mechanistic models that involve the coordination at the mineral-solution interface. Simple rate laws are derived illustrating the rates\u27 dependence on the concentration (activity) of surface species

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates

Wet chemistry experiments were carried out to investigate the effect of the redox state and aqueous speciation on the uptake of neptunium by titanium dioxide (TiO2) and by calcium silicate hydrates (C-S-H) under alkaline conditions. TiO2 was chosen as a reference sorbent to determine the surface complexation behaviour of neptunium under alkaline conditions. C-S-H phases are important constituents of cement and concrete. They may contribute significantly to radionuclide retention due to their high recrystallization rates making incorporation the dominating sorption mechanism for many radionuclides (e.g. the actinides) on these materials. The sorption of neptunium on both solids was found to depend strongly on the degree of hydrolysis. On TiO2 Rd values for Np(IV), Np(V) and Np(VI) are identical at pH = 10 and decrease with progressing hydrolysis in case of Np(V) and Np(VI). On C-S-H phases, Rd values for the three redox states are also identical at pH = 10. While the Rd values for Np(VI) sorption on C-S-H phases decrease with progressing hydrolysis, the Rd values for Np(IV) and Np(V) sorption are not affected by the pH. In addition to the effect of hydrolysis, the presence of Ca is found to promote Np(V) and Np(VI) sorption on TiO2 whereas on C-S-H phases, the present wet chemistry data do not give unambiguous evidence. Thus, the aqueous speciation appears to have a similar influence on the sorption of the actinides on both types of solids despite the different sorption mechanism. The similar Rd values for Np(IV,V,VI) sorption at pH = 10 can be explained qualitatively by invoking inter-ligand electrostatic repulsion between OH groups in the coordination sphere of Np(V) and Np(VI). This mechanism was proposed earlier in the literature for the prediction of actinide complexation constants with inorganic ligands. A limiting coordination number for each Np redox state, resulting from the inter-ligand electrostatic repulsion, allows the weaker sorption of the highest hydrolysed Np(V,VI) species to be explaine

A luminescence line-narrowing spectroscopic study of the uranium(VI) interaction with cementitious materials and titanium dioxide

Non-selective luminescence spectroscopy and luminescence line-narrowing spectroscopy were used to study the retention of UO22+ on titanium dioxide (TiO2), synthetic calcium silicate hydrate (C-S-H) phases and hardened cement paste (HCP). Non-selective luminescence spectra showed strong inhomogeneous line broadening resulting from a strongly disordered UO22+ bonding environment. This problem was largely overcome by using luminescence line-narrowing spectroscopy. This technique allowed unambiguous identification of three different types of UO22+ sorbed species on C-S-H phases and HCP. Comparison with spectra of UO22+ sorbed onto TiO2 further allowed these species to be assigned to a surface complex, an incorporated species and an uranate-like surface precipitate. This information provides the basis for mechanistic models describing the UO22+ sorption onto C-S-H phases and HCP and the assessment of the mobility of this radionuclide in a deep geological repository for low and intermediate level radioactive waste (L/ILW) as this kind of waste is often solidified with cement prior to storage.Swiss National Cooperative for the Disposal of Radioactive Wast

EXAFS investigation on U(VI) immobilization in hardened cement paste: influence of experimental conditions on speciation

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the coordination environment of U(VI) in cementitious materials. The EXAFS measurements were carried out on U(VI)-doped samples prepared under varying conditions, such as samples from sorption, hydration and diffusion experiments, and using different cementitious materials, such as crushed hydrated hardened cement paste (HCP) and calcium silicate hydrates (C-S-H). The samples had U(VI) loadings ranging from 1700μg/g to 45000μg/g. Applying principal component analysis (PCA) on 13 EXAFS spectra (each spectra corresponding to aminimum of five different scans) of the low loading samples, one single species is obtained indicating asimilar U(VI) coordination environment for both HCP and C-S-H samples. This result confirms that C-S-H phases control the uptake of U(VI) in the complex cement matrix. The coordination environment structure of this species is similar to aU(VI) surface complex or to U(VI) in uranyl silicate minerals (two axial O atoms at 1.82±0.02 Å; four equatorial O atoms at 2.25±0.01 Å; one Si atom at 3.10±0.03 Å). At high U(VI) loading, PCA revealed asecond U(VI) species, with acoordination environment similar to that of U(VI) in calcium uranate (two axial O atoms at 1.94±0.04 Å; five equatorial O atoms at 2.26±0.01 Å; four Ca atoms at 3.69±0.05 Å and five U atoms at 3.85±0.04 Å). This study suggest that, at low U(VI) loading, U(VI) is bound to C-S-H phases in HCP while at high U(VI) loading, the immobilization of U(VI) in cementitious materials is mainly controlled by the precipitation of acalcium uranate-type phas

Speciation of iron(II/III) at the iron-cement interface: a review.

Steel is used as reinforcement in construction materials and it is also an important component of cement-stabilized waste materials to be disposed of in deep geological repositories for radioactive waste. Steel corrosion releases dissolved Fe(II/III) species that can form corrosion products on the steel surface or interact with cementitious materials at the iron-cement interface. The thermodynamically stable Fe species in the given conditions may diffuse further into the adjacent, porous cement matrix and react with individual cement phases. Thus, the retention of Fe(II/III) by the hydrate assemblage of cement paste is an important process affecting the diffusive transport of the aqueous species into the cementitious materials. The diffusion of aqueous Fe(II/III) species from the steel surface into the adjacent cementitious material coupled with the kinetically controlled formation of iron corrosion products, such as by Fe(II) oxidation, decisively determines the extension of the corrosion front. This review summarises the state-of-the art knowledge on the interaction of ferrous and ferric iron with cement phases based on a literature survey and provides new insights and proper perspectives for future study on interaction systems of iron and cement

Thermodynamics of Np(IV) complexes with gluconic acid under alkaline conditions: sorption studies

The complexation of Np(IV) with gluconic acid (GLU) under alkaline conditions was investigated in the absence of Ca by carrying out aseries of sorption experiments. The decrease of Np(IV) sorption on the sorbing material at increasing concentrations of GLU was interpreted as the formation of Np(IV)-GLU aqueous complexes. The modelling of experimental data according to the Schubert method [1] confirmed the formation of acomplex with aNp : GLU ratio 1 : 1. The stoichiometry of the complex Np(OH)4GLU− was proposed based on the experimental observation that no proton exchange occurred during the course of the complexation reaction and that Np(OH)4(aq) was the predominant hydrolysis product in the absence of GLU. Alog *β1,4,10 = −2.92±0.30 for the formation reaction Np4+ + 4H2O + GLU−⇔Np(OH)4GLU− + 4H+ was calculated based on the conditional stability constants determined from sorption experiments and using the Np(IV) thermodynamic data selected in the NEA reviews [2]. Linear free energy relationships (LFER) confirmed that the stoichiometry and stability of the Np(IV)-GLU complex characterized in this work are consistent with data available for Th(IV)-, U(IV)- and Pu(IV)-GLU complexe

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

The redox chemistry of Np(V/VI) was investigated in ∼0.6M 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, 5×10−3M and 5×10−2M). The high total Np concentration ([Np]tot ∼ 2×10−3M) led to the precipitation of solid phases in some of the samples. The carbonate concentration (as impurity of TMA-OH) was 2-3×10−3M. 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 800nm ≤ λ ≤ 1300nm 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. Asimilar 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 aNa-Np(VI) phase in 5×10−2M 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

Modification of the SSG/LRR-omega RSM for adverse pressure gradients using turbulent boundary layer experiments at high Re

A modification of the SSG/LRR-omega model for turbulent boundary layers in adverse pressure gradient is presented. The modification is based on a new wall law for the mean velocity at adverse pressure gradient. The wall law is found from two new joint DLR/UniBw experiments and from the analysis of a data base from the literature. The mean velocity profile in the inner layer is found to consist of a log-law region, which is thinner than its zero pressure gradient counterpart, and a half-power law region above the log law. An empirical correlation for the wall-distance of the transition from the log-law to the half-power law is presented. Then a modification of the omega-equation to account for a half-power law behaviour of the mean velocity is described. The modified SSG/LRR-omega model is then applied to the two joint DLR/UniBw experiments. The modification leads to a reduction of the mean velocity in the inner part of the boundary layer and makes the model more susceptible for flow separation, which is in good agreement with the experimental data