Binding Of Strontium Research Articles

Decorating Channel Walls in Metal-Organic Frameworks with Crown Ethers for Efficient and Selective Separation of Radioactive Strontium(II).

Nuclear accidents and the improper disposal of nuclear wastes have led to serious environmental radioactive pollutions. The rational design of adsorbents for the highly efficient separation of strontium(II) is essential in treating nuclear waste and recovering radioactive strontium resources. Metal-organic frameworks (MOFs) are potential materials for the separation of aqueous metal ions owing to their designable structure and tunable functionality. Herein, a novel 3D MOF material MOF-18Cr6, in which 1D channels are formed using 18-crown-6-ether-containing ligands as channel walls, is fabricated for strontium(II) separation. In contrast to traditional MOFs designed by grafting functional groups in the framework pores, MOF-18Cr6 possesses regular 18-crown-6-ether cavities on the channel walls, which not only can transport and intake strontium(II) via the channels, but also prevent blockage of the channels after the binding of strontium(II). Consequently, the functional sites are fully utilized to achieve a high strontium(II) removal rate of 99.73 % in simulated nuclear wastewater. This study fabricates a highly promising adsorbent for the separation of aqueous radioactive strontium(II), and more importantly, can provide a new strategy for the rational design of high-performance MOF adsorbents for separating target substances from complex aqueous environments.

Decorating Channel Walls in Metal–Organic Frameworks with Crown Ethers for Efficient and Selective Separation of Radioactive Strontium(II)

AbstractNuclear accidents and the improper disposal of nuclear wastes have led to serious environmental radioactive pollutions. The rational design of adsorbents for the highly efficient separation of strontium(II) is essential in treating nuclear waste and recovering radioactive strontium resources. Metal–organic frameworks (MOFs) are potential materials for the separation of aqueous metal ions owing to their designable structure and tunable functionality. Herein, a novel 3D MOF material MOF‐18Cr6, in which 1D channels are formed using 18‐crown‐6‐ether‐containing ligands as channel walls, is fabricated for strontium(II) separation. In contrast to traditional MOFs designed by grafting functional groups in the framework pores, MOF‐18Cr6 possesses regular 18‐crown‐6‐ether cavities on the channel walls, which not only can transport and intake strontium(II) via the channels, but also prevent blockage of the channels after the binding of strontium(II). Consequently, the functional sites are fully utilized to achieve a high strontium(II) removal rate of 99.73 % in simulated nuclear wastewater. This study fabricates a highly promising adsorbent for the separation of aqueous radioactive strontium(II), and more importantly, can provide a new strategy for the rational design of high‐performance MOF adsorbents for separating target substances from complex aqueous environments.

Strontium increasing calcium accessibility from calcium citrate

Strontium chloride added to aqueous suspensions of metastable calcium citrate tetrahydrate increased calcium ion activity measured electrochemically without transition of metastable tetrahydrate to stable calcium citrate hexahydrate as shown by DSC. Calcium activity increase was explained by lower solubility of strontium citrate pentahydrate formed (8.9 × 10−4 M at 25 °C) increasing with temperature compared to calcium citrate tetrahydrate (1.6 × 10−3 M) decreasing with temperature. Strontium binding to citrate was found endothermic, ΔH0 = 45 kJ∙mol−1 at 25 °C, while calcium binding shows variation from ΔH0 = 94 kJ∙mol−1 at 10 °C becoming exothermic above physiological temperature with ΔH0 = −9 kJ∙mol−1 at 45 °C as determined from temperature and concentration variation in electric conductivity. These differences in solution thermodynamics and pH effect on complex formation between calcium and strontium citrate are discussed in relation to biomineralization.

Radium and strontium binding by a modified bauxite refinery residue – isotope exchange studies of pH-dependence, reversibility and ageing

The pH-dependence and reversibility of radium (Ra) and strontium (Sr) binding to a modified bauxite refinery residue (MBRR), and the effects of sample ageing, were studied in laboratory uptake/leaching experiments. Natural 226 Ra and stable strontium (Sr nat ) were placed in contact with the MBRR for an 8-day loading period (equilibrium pH c . 8.5). Following the addition of exchange isotopes 228 Ra and 85 Sr, the samples were then leached in pH-dependent experiments, where the pH was decreased incrementally from 9 to 3 over 7 h. A further suite of samples was aged at 4°, 23° and 65°C for 6 months after the initial addition of 226 Ra and Sr nat and then studied in a similar set of pH-dependent exchange experiments. The relative concentrations of the Ra and Sr isotope pairs ( 226 Ra/ 228 Ra and Sr nat / 85 Sr) provided insights into the adsorption strength, incorporation, reversibility and ageing effects. The Sr nat data showed that the amount of bound Sr released from aged MBRR samples as the pH decreases is substantially lower than the unaged sample, showing that Sr has been incorporated in less accessible phases during the ageing period. It appears that the uptake of 228 Ra by the higher-temperature aged samples is somewhat stronger than the unaged samples; however, Ra isotope exchange plots for aged and unaged samples are similar. Consequently, the stronger binding of 228 Ra by the higher-temperature aged samples is probably driven by translocation to kinetically less-favourable surface sites rather than a significant incorporation of Ra within the mineral solids during ageing.

Water-Mediated Differential Binding of Strontium and Cesium Cations in Fulvic Acid.

The migration of potentially harmful radionuclides, such as cesium ((137)Cs) and strontium ((90)Sr), in soil is governed by the chemical and biological reactivity of soil components. Soil organic matter (SOM) that can be modeled through fulvic acid (FA) is known to alter the mobility of radionuclide cations, Cs(+) and Sr(2+). Shedding light on the possible interaction mechanisms at the atomic level of these two ions with FA is thus vital to explain their transport behavior and for the design of new ligands for the efficient extraction of radionuclides. Here we have performed molecular dynamics, metadynamics simulations, and density-functional-theory-based calculations to understand the binding mechanism of Sr(2+) and Cs(+) cations with FA. Our studies predict that interaction of Cs(+) to FA is very weak as compared with Sr(2+). While the water-FA interaction is largely responsible for the weak binding of Cs(+) to FA, leading to the outer sphere complexation of the ion with FA, the interaction between Sr(2+) and FA is stronger and thus can surpass the existing secondary nonbonding interaction between coordinated waters and FA, leading to inner sphere complexation of the ion with FA. We also find that entropy plays a dominant role for Cs(+) binding to FA, whereas Sr(2+) binding is an enthalpy-driven process. Our predicted results are found to be in excellent agreement with the available experimental data on complexation of Cs(+) and Sr(2+) with SOM.

Synthesis of new periodic mesoporous organosilica (PMO) incorporated with macrocyclic host for strontium binding

A new kind of periodic mesoporous organosilica (PMO) material containing macrocyclic host was designed and synthesized in this work. The macrocyclic host molecule cis-dicyclohexano-18-crown-6, well-known as an excellent ionophore for strontium, was carefully modified and incorporated into the framework of the silica matrix. Full characterizations regarding the structure, surface properties, and micro morphology of the material were performed. The PMO material had a typical hexagonal pore arrangement (p6mm symmetry) and a high BET surface area (579.2m2/g). Due to the specific binding ability of the host units, the PMO material showed selective adsorption to strontium in the presence of various interfering metal ions. The material might potentially find applications in the field of metal separation in radioactive waste.

Strontium sorption by pectins isolated from the Sea grasses Zostera marina and Phyllospadix iwatensis

In this study, we determined the parameters of sorption of strontium ions by pectins that were isolated from the sea grasses Zostera marina and Phyllospadix iwatensis collected in the Peter the Great Bay of the Sea of Japan. The maximum strontium binding capacity and the coefficient of affinity with respect to strontium ions were significantly higher in sea grass pectins than in commercial pectin. These parameters were correlated with the etherification degree of pectins. The sea grasses studied here can be considered as a promising source of pectins for the binding and removal of strontium radioisotopes from the human body.

Strontium binding by calcium silicate hydrates

In the present study the binding of strontium with pure calcium silicate hydrates (C-S-H) has been investigated using batch-type experiments. Synthetic C-S-H phases with varying CaO:SiO 2 (C:S) mol ratios, relevant to non-degraded and degraded hardened cement paste, were prepared in the absence of alkalis (Na(I), K(I)) and in an alkali-rich artificial cement pore water (ACW). Two types of experimental approaches have been employed, investigating sorption and co-precipitation processes, respectively. The Sr(II) sorption kinetics were determined as well as sorption isotherms, the effect of the solid to liquid ratio and the composition (C:S ratio) of the C-S-H phases. In addition, the reversibility of the Sr(II) sorption was tested. It was shown that both the sorption and co-precipitation tests resulted in Sr(II) distribution ratios which were similar in value, indicating that the same sites are involved in Sr(II) binding. In alkali-free solutions, the Sr(II) uptake by C-S-H phases was described in terms of a Sr 2+–Ca 2+ ion exchange model. The selectivity coefficient for the Sr 2+–Ca 2+ exchange was determined to be 1.2 ± 0.3 .

NMR Study of Strontium Binding by a Micaceous Mineral

The nature of strontium binding by soil minerals directly affects the transport and sequestration/remediation of radioactive strontium species released from leaking high-level nuclear waste storage tanks. However, the molecular-level structure of strontium binding sites has seldom been explored in phyllosilicate minerals by direct spectroscopic means and is not well-understood. In this work, we use solid-state NMR to analyze strontium directly and indirectly in a fully strontium-exchanged synthetic mica of nominal composition Na(4)Mg(6)Al(4)Si(4)O(20)F(4). Thermogravimetric analysis, X-ray diffraction analysis, and NMR evidence supports that heat treatment at 500 degrees C for 4 h fully dehydrates the mica, creating a hydrogen-free interlayer. Analysis of the strontium NMR spectrum of the heat-treated mica shows a single strontium environment with a quadrupolar coupling constant of 9.02 MHz and a quadrupolar asymmetry parameter of 1.0. These quadrupolar parameters are consistent with a highly distorted and asymmetric coordination environment that would be produced by strontium cations without water in the coordination sphere bound deep within the ditrigonal holes. Evidence for at least one additional strontium environment, where proton-strontium couplings may occur, was found via a (1)H-(87)Sr transfer of populations by double resonance NMR experiment. We conclude that the strontium cations in the proton-free interlayer are observable by (87)Sr NMR and bound through electrostatic interactions as nine coordinate inner-sphere complexes sitting in the ditrigonal holes. Partially hydrated strontium cations invisible to direct (87)Sr NMR are also present and located on the external mica surfaces, which are known to hydrate upon exposure to atmospheric moisture. These results demonstrate that modern pulsed NMR techniques and high fields can be used effectively to provide structural details of strontium binding by phyllosilicate minerals.

High-field QCPMG NMR of strontium nuclei in natural minerals

The only stable NMR-active isotope of strontium, 87Sr, is a spin-9/2 quadrupolar nucleus that has a low gyromagnetic ratio, a low natural abundance, and a large nuclear electric quadrupole moment. In this work, we utilize the quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) pulse sequence and a 21.14 T NMR spectrometer at the Pacific Northwest National Laboratory to characterize the strontium sites in the natural minerals strontianite (SrCO 3) and celestine (SrSO 4). QCPMG at 21.14 T was found to provide sensitivity enhancements of roughly two orders of magnitude over Hahn-echo experiments at an 11.74 T magnetic field. We extracted the quadrupolar parameters for the strontium nuclei through iterative simulations of the experimental spectra with the SIMPSON program by Bak, Rasmussen, and Nielsen. The data show that the quadrupolar parameters of 87Sr appear to be highly sensitive to the symmetry of the strontium coordination environment and can thus provide information about the strontium binding environment in complex systems.

Determination of radioactive strontium in seawater

This paper describes the procedures of isolating strontium and yttrium from seawater that enable the determination of 89,90Sr. In one procedure, strontium is directly isolated from seawater on the column filled with Sr resin by binding of strontium to the resin from 3 M HNO 3 in a seawater, and successive elution with HNO 3. In others, strontium is precipitated from seawater with (NH 4) 2CO 3, followed by isolation on a Sr column or an anion exchange column. It is shown that strontium precipitation is optimal with concentration of 0.3 M (NH 4) 2CO 3 at pH = 11. In these conditions, 100% Y, 78% Sr, 80% Ca and 50% Mg are precipitated. Strontium is bound on to Sr column from 5 to 8 M HNO 3, separated from other elements by elution with 3 M HNO 3 and 0.05 M HNO 3. Strontium and yttrium are bound on to anion exchange column from alcoholic solutions of nitric acid. The optimum mixture of alcohols for sample binding is a mixture of ethanol and methanol with the volume ratio 1:3. Strontium and yttrium are separated from Mg, Ca, K, and other elements by elution with 0.25 M HNO 3 in the mixture of ethanol and methanol. After the separation, yttrium and strontium are eluted from the column with water or methanol. In the procedure of direct isolation from 1 l of the sample, the average recovery of 50% was obtained. In the remaining two procedures, the strontium recovery was about 60% for the Sr column and 65% for anion exchange column. Recovery of yttrium is about 70% for the anion exchange column. It turned out that the procedure with the Sr resin (direct isolation and isolation after precipitation) is simpler and faster in the phase of the isolation on the column in comparison with the procedure with the anion exchanger. The procedure with the anion exchanger, however, enables the simultaneous isolation of yttrium and strontium and rapid determination of 89,90Sr. These procedures were tested by determination of 89,90Sr on liquid scintillation counter and Cherenkov counting in 5 M HNO 3. Obtained results showed that activity of 50 mBq l −1 of 89,90Sr and higher can be simultaneously determined.

Metal–ion interactions with sugars. The crystal structure and FTIR study of an SrCl 2–fructose complex

The single-crystal structure of SrCl 2·2C 6H 12O 6·3H 2O was determined with M r=572.88, a=16.252(5), b=7.941(2), c=10.751(3) Å, β=127.652(4)°, V=1098.5(6) Å 3, C2, Z=2, μ=0.71073 Å and R=0.0296 for 1998 observed reflections. The fructose moiety of the complex exists as a β- d-pyranose. The strontium atom is surrounded by eight oxygen atoms, which are arranged in symmetry-related pairs that are derived from four sugar and two water molecules. Three nonvicinal hydroxyl groups of fructose are involved in strontium binding. All the hydroxyl groups and water molecules are involved in forming an extensive hydrogen-bond network. The Sr–fructose complex is isostructural with the Ca–fructose complex, and the crystal structures and FTIR spectra of the two complexes are compared in this article. The O–H, C–O, and C–O–H vibrations are shifted, and the relative intensities changed in the complexes IR spectra, which indicate sugar metalation. By studying the metal-binding properties of fructose, it is hoped that such would aid in the understanding of the structural chemistry of metal ions interacting with saccharides, as an actual biological system, and thereby aid in the interpretation of some particular biological processes.

Particle release and permeability reduction in a natural zeolite (clinoptilolite) and sand porous medium.

To evaluate clinoptilolite, a natural zeolite, as a candidate material for a permeable reactive barrier for removal of strontium from groundwater, we investigated particle release and permeability reduction in clinoptilolite and sand porous media. In flow-through column experiments, we tested the effects of solution chemistry, grain size, and clinoptilolite pretreatment on particle release and hydraulic conductivity. Permeability reduction occurred not in the clinoptilolite itself but only in finer-grained sand down-gradient of the clinoptilolite. Solutions of high ionic strength inhibited particle release and prevented clogging. Clinoptilolite of larger grain size produced slightly less particle release and clogging. Two pretreatments of the clinoptilolite, rinsing to remove fine particles and calcining to improve strength, reduced particle release and clogging. Calcining, however, significantly reduced the strontium binding strength of the clinoptilolite.

Strontium and Calcium Complexation by Humic Acid

The binding of strontium by humic acid was studied in systems designed to model soil leaching conditions. Ultrafiltration experiments were conducted for a wide range of CaCl and NaCl concentrations and for the pH range 4 to 7. It was found that the recovery of humic acid slowly decreased with increasing pH. This occurred because at neutral and basic solutions the increasing degree of ionization of the carboxylate groups leads to the full repulsion state of the humic acid and to increasing of the strontium binding (strontium recovery ∼60% at pH 7) with this natural polyelectrolyte. The strontium complexation in the presence of calcium was studied in different series of metal concentrations at pH 5 and the ionic strength 0.1M NaCl. The results were explained on the base of the metal ion charge neutralization model of humic complexation (CNM). Loading capacity (cca 40%) and complexation constants and Sr (3.51±0.27) and Ca (3.10±0.29) with Aldrich humic acid were found.

Crystal structure of acceptor stem of tRNA(Ala) from Escherichia coli shows unique G.U wobble base pair at 1.16 A resolution.

The acceptor stem of Escherichia coli tRNA(Ala), rGGGGCUA.rUAGCUCC (ALAwt), contains the main identity element for the correct aminoacylation by the alanyl tRNA synthetase. The presence of a G3.U70 wobble base pair is essential for the specificity of this reaction, but there is a debate whether direct minor-groove contact with the 2-amino group of G3 or a distortion of the acceptor stem induced by the wobble pair is the critical feature recognized by the synthetase. We here report the structure analysis of ALAwt at near-atomic resolution using twinned crystals. The crystal lattice is stabilized by a novel strontium binding motif between two cis-diolic O3'-terminal riboses. The two independent molecules in the asymmetric unit of the crystal show overall A-RNA geometry. A comparison with the crystal structure of the G3-C70 mutant of the acceptor stem (ALA(C70)) determined at 1.4 A exhibits a modulation in ALAwt of helical twist and slide due to the wobble base pair, but no recognizable distortion of the helix fragment distant from the wobble base pair. We suggest that a highly conserved hydration pattern in both grooves around the G3.U70 wobble base pair may be functionally significant.

A study of strontium binding to albumins, by a chromatographic method involving atomic emission spectrometric detection

A chromatographic method involving ICP-AES (inductively coupled plasma atomic emission spectrometry) detection has been successfully applied for the study of strontium-protein complexes. The chromatographic step involves the use of gel filtration—a large-zone Hummel and Dreyer method—which allows to dissociate the bound metallic ions and the free ones. This step is followed by an ICP-AES analysis of fractions collected throughout the chromatographic experiment: the concentration of ionic metallic species in solution can therefore be calculated. Two proteins have been tested: bovine serum albumin, which showed only weak interactions with Sr 2+ ions, and bovine α-lactalbumin: this protein, well-known for its calcium binding capacity, proved to interact strongly with strontium. The influence of various parameters on the formation of strontium-lactalbumin complexes were determined, namely temperature, pH. Competition experiments between Sr 2+ ions and, respectively Na + and Ca 2+ ions were also performed, by varying ionic strength of the medium, and by using both apo and native forms of bovine α-lactalbumin.

Strontium binding properties of inorganic adsorbents

A comparison of strontium sorption on synthetic inorganic ion exchangers and zeolites from 0.01M CaCl2 solution, sea water and an artificial foodstuff fluid has been carried out in batch and column experiments. It was found that some adsorbents among the groups manganese dioxides, mixed titanium-manganese oxides, antimony-based materials and synthetic zeolites had the highest selectivity to strontium (K d >1000). These adsorbents are interesting for further investigation concerning their use for radiostrontium decontamination of foodstuffs (if nontoxic), as well as for analytical purposes.

Binding of ADP to sarcoplasmic reticulum Ca 2+-ATPase in the absence of Mg 2+ is specifically inhibited by thapsigargin: observations on the ligand stoichiometry

The conditions of nucleotide binding to native, though partly purified, Ca 2+-ATPase from SR as well as the stoichiometry of nucleotide and strontium binding and the phosphorylation capacity was reevaluated. Binding of MgADP appeared to be aberrant whereas even high-affinity binding of [ 14C]-ADP took place in the absence of Mg 2+. Also low-affinity ATP binding was possible in the absence of divalent cations. A heterogeneity in ADP binding compatible with a two-component model in the absence of thapsigargin was changed to an apparent homogeneity of low-affinity receptors following a mole:mole interaction of enzyme and thapsigargin. Since the affinity of both components was reduced by thapsigargin, high- as well as low-affinity ADP binding seem to be specific and probably to the substrate receptor proper. Analysis of ADP binding isotherms in the absence of Mg 2+ according to a model of two independent populations of sites was compatible with a binding capacity of 8.49 ± 0.43 nmoles/mg protein corresponding to a molecular mass of 118 ± 6 kD per ADP site. The same total binding capacity was found for ATP. The phosphorylation capacity corresponded to more than one and less than two ≈P per two 110-kD peptides (formally one ≈P per 154 kD protein). Specific binding of Ca 2+ and the congener Sr 2+ to SR Ca 2+-ATPase was compatible with their interaction with a single population of sites. The binding capacity was equal to one divalent cation per nucleotide binding peptide. The binding of one nucleotide and one divalent cation per ≈110 kD peptide and the absence of cooperativity in divalent cation binding might imply that Ca 2+-ATPase works as a monomer.

The binding of strontium and europium by an aquatic fulvic acid—ion exchange distribution and ultrafiltration studies

The complexation of an aquatic fulvic acid, FA, with Sr 2+ and Eu 3+ was studied at 0.10 and O1.O1 M NaClO 4 using trace levels of metal ([Sr 2+] = 10 −9 M and [Eu 3+] = 10 −11 M) and a constant FA concentration (0.12 g/l) by an ultrafiltration technique (UF) and an ion exchange distribution method (IEDS). The overall complex formation function, β OV for the two metals was calculated and its dependence on pH, ionic strength and method was investigated. The absolute value of log β OV, the pH dependence and the influence of the ionic strength on the complexation differed depending on the metal ion and experimental technique employed. By considering the functional group heterogeneity of the FA molecule, it was possible to predict the most predominantly bound site (keto—enol) and resolve the complex formation function for this site and EU 3+ (IEDS: 9.43 ± 0.29 l/eq at 0.10 M and 10.58 ± 0.72 l/eq at 0.01 M; UF: 7.19 ± 1.51 l/eq at 0.01 M and 6.88 ± 0.91 l/eq at 0.01 M). The results are discussed in the light of possible intrinsic problems of the two experimental methods.

Strontium binding to sarcoplasmic reticulum Ca 2+-ATPase : Spectroscopic differentiation of the substeps involved

We investigated the consequences of Sr 2+ binding to the transport sites of sarcoplasmic reticulum (SR) Ca 2+-ATPase for two fluorescent conformational probes located in different regions of the ATPase. Using SR vesicles in which Lys-515 in the ATPase had been previously labeled with fluorescein 5'-isothiocyanate (FITC), we found that the Sr 2+-induced a drop in the fluorescein fluorescence of this FITC-labeled ATPase shifted toward lower Sr 2+ concentrations than the Sr 2+-induced rise in Trp fluorescence for the same FITC-labeled ATPase. The curve describing the Sr 2+-dependent rise in Trp fluorescence had a characteristic asymmetric shape, and the changes in Trp fluorescence occurred in parallel with the activation by Sr 2+ of pNPP hydrolysis by the ATPase. Analysis of these results in terms of the simplest scheme describing the sequential binding of the two Sr 2+ ions suggests that under the conditions of these experiments, i.e. at neutral pH in the presence of potassium, the Sr 2+-induced rise in the Trp fluorescence mainly reflected the formation of ATPase with two ions bound to the transport sites, whereas the binding of a single Sr 2+ ion was virtually sufficient to reduce the fluorescence of bound FITC to its minimal level.