Radionuclide Interaction with Clays in Dilute and Heavily Compacted Systems: A Critical Review

Given the unique properties of clays (i.e., low permeability and high ion sorption/exchange capacity), clays or clay formations have been proposed either as an engineered material or as a geologic medium for nuclear waste isolation and disposal. A credible evaluation of such disposal systems relies on the ability to predict the behavior of these materials under a wide range of thermal–hydrological–mechanical–chemical (THMc) conditions. Current model couplings between THM and chemical processes are simplistic and limited in scope. This review focuses on the uptake of radionuclides onto clay materials as controlled by mineral composition, structure, and texture (e.g., pore size distribution), and emphasizes the connections between sorption chemistry and mechanical compaction. Variable uptake behavior of an array of elements has been observed on various clays as a function of increasing compaction due to changes in pore size and structure, hydration energy, and overlapping electric double layers. The causes for this variability are divided between “internal” (based on the fundamental structure and composition of the clay minerals) and “external” (caused by a force external to the clay). New techniques need to be developed to exploit known variations in clay mineralogy to separate internal from external effects

Natural Indices for the Chemical Hardness/Softness of Metal Cations and Ligands

Quantitative understanding of reactivity and stability for a chemical species is fundamental to chemistry. The concept has undergone many changes and additions throughout the history of chemistry, stemming from the ideas such as Lewis acids and bases. For a given complexing ligand (Lewis base) and a group of isovalent metal cations (Lewis acids), the stability constants of metal–ligand (ML) complexes can simply correlate to the known properties of metal ions [ionic radii (<i>r</i>_M^<i>n</i>+), Gibbs free energy of formation (Δ<i>G</i>°_{f,M^<i>n</i>+}), and solvation energy (Δ<i>G</i>°_{s,M^<i>n</i>+})] by 2.303<i>RT</i> log <i>K</i>_ML = (α*_MLΔ<i>G</i>°_{f,M^<i>n</i>+} – β*_ML<i>r</i>_M^<i>n</i>+ + γ*_MLΔ<i>G</i>°_{s,M^<i>n</i>+} – δ*_ML), where the coefficients (α*_ML, β*_ML, γ*_ML, and intercept δ*_ML) are determined by fitting the equation to the existing experimental data. Coefficients β*_ML and γ*_ML have the same sign and are in a linear relationship through the origin. Gibbs free energies of formation of cations (Δ<i>G</i>°_{f,M^<i>n</i>+}) are found to be natural indices for the softness or hardness of metal cations, with positive values corresponding to soft acids and negative values to hard acids. The coefficient α*_ML is an index for the softness or hardness of a complexing ligand. Proton (H⁺) with the softness index of zero is a unique acid that has strong interactions with both soft and hard bases. The stability energy resulting from the acid–base interactions is determined by the term α*_MLΔ<i>G</i>°_{f,M^<i>n</i>+}; a positive product of α*_ML and Δ<i>G</i>°_{f,M^<i>n</i>+} indicates that the acid–base interaction between the metal cation and the complexing ligand stabilizes the complex. The terms β*_ML<i>r</i>_M^<i>n</i>+ and γ*_MLΔ<i>G</i>°_{s,M^<i>n</i>+}, which are related to ionic radii of metal cations, represent the steric and solvation effects of the cations. The new softness indices proposed here will help to understand the interactions of ligands (Lewis bases) with metal cations (Lewis acids) and provide guidelines for engineering materials with desired chemical reactivity and selectivity. The new correlation can also enhance our ability for predicting the speciation, mobility, and toxicity of heavy metals in the earth environments and biological systems

TiO₂ Photocatalytic Cyclization Reactions for the Syntheses of Aryltetralones

This work focuses on a new strategy to overcome the overoxidation in heterogeneous TiO₂ photocatalysis and to realize high-efficiency photosynthesis. We demonstrate that TiO₂ photocatalysis can integrate C–C and CO formation in a tandem manner to achieve efficient oxidative cyclization for the syntheses of aryltetralones. This protocol does not need any additive besides the inexpensive and/or recyclable TiO₂, O₂, and (solar) light. High yields with excellent diastereoselectivities are obtained for a wide scope of electron-rich substrates. Our findings demonstrate that in contrast to the conventional overoxidation, as long as the radical cations possess sufficient reactivity toward nucleophilic addition, single-electron transfer processes in TiO₂ photocatalysis can be developed into a powerful tool to construct C–C bonds and even strained carbon rings

Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water

For sol–gel synthesis of titanium oxide, the titanium­(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium­(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO₄ and TiCl₄ without using organic ligands. Our results indicate that titanium­(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium­(IV) in water and, meanwhile, are new molecular materials for photocatalysis

Alkali Halide Cubic Cluster Anions ([Cs₈X₂₇]^19–, X = Cl, Br) Isolated from Water

Herein we report the syntheses and the X-ray structure of [Cs₈X₂₇]^19‑ (X = Cl, Br) clusters, the first binary cluster anions isolated in bulk crystal structures. They were obtained by electrostatic capture and face-directed recognition of the prenucleation [Cs_<i>m</i>Cl_<i>n</i>]^{(<i>n</i>−<i>m</i>)–} clusters from water solutions, using [M₄(OH)₈(OH₂)₁₆]⁸⁺ (M = Zr^IV or Hf^IV) as the counter cations. These compounds have been thoroughly characterized with a variety of techniques including vibrational spectroscopy and superionic conductivity analysis. This work not only provides structural models for a better understanding of the nucleation of binary materials but also shows that magic number binary clusters adopting a cubic lattice structure do form, in agreement with the time-honored theoretical and spectroscopic predictions

Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water

For sol–gel synthesis of titanium oxide, the titanium­(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium­(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO₄ and TiCl₄ without using organic ligands. Our results indicate that titanium­(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium­(IV) in water and, meanwhile, are new molecular materials for photocatalysis

Role of the Alkali-Metal Cation Size in the Self-Assembly of Polyoxometalate-Monolayer Shells on Gold Nanoparticles

Polyoxometalate (POM)-monolayer stability constants, <i>K</i>, for three POM anions vary with the cation size, in the same order as that for increasing ion-pair formation with α-SiW₁₁O₃₉^8– (<b>1</b>) in the early nucleation phase of monolayer self-assembly: Li⁺ < Na⁺ < K⁺ < Cs⁺. Cryo-TEM images demonstrating the use of the cation size to rationally control monolayer formation provide definitive evidence that the POM monolayers are electrostatically stabilized (ionic) shells, analogous in that respect to the monolayer walls of “hollow” POM-macroanion vesicles

Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water

For sol–gel synthesis of titanium oxide, the titanium­(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium­(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO₄ and TiCl₄ without using organic ligands. Our results indicate that titanium­(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium­(IV) in water and, meanwhile, are new molecular materials for photocatalysis

Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water

For sol–gel synthesis of titanium oxide, the titanium­(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium­(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO₄ and TiCl₄ without using organic ligands. Our results indicate that titanium­(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium­(IV) in water and, meanwhile, are new molecular materials for photocatalysis

Iodine/Visible Light Photocatalysis for Activation of Alkynes for Electrophilic Cyclization Reactions

Photocatalytic organic synthesis needs photocatalysts to initiate the reactions and to control the reaction paths. Available photocatalytic systems rely on electron transfer or energy transfer between the photoexcited catalysts and the substrates. We explore a concept based on the photopromoted catalyst coupling to the substrate and the phototriggered catalyst regeneration by elimination from the catalytic cycle. A catalytic amount of elementary I₂ is applied as both a visible light photocatalyst and a π Lewis acid, enabling the direct activation of alkyne CC bonds for electrophilic cyclization reactions, one of the most important reactions of alkynes. Visible light is crucial for both the iodocyclization of the propargyl amide and the deiodination of the intermediate. Singlet oxygen is found to play a key role in the regeneration of I₂. This system shows good functional group compatibility for the generation of substituted oxazole aldehydes and indole aldehydes. Hence, this study provides a readily accessible alternative catalytic system for the construction of heterocycle aldehyde derivatives by sunlight photocatalysis