Sorption of <Superscript>90</Superscript>Sr and <Superscript>90</Superscript>Y with stable and metastable modifications of calcium carbonate

A comparative study of sorption of ⁹⁰Sr²⁺ and ⁹⁰Y³⁺ cations from aqueous solutions saturated with respect to calcium carbonate on the metastable CaCO₃ modifications (aragonite and vaterite) and also on the stable CaCO₃ modification (calcite) was carried out at 20°C. The sorption of these radionuclides from the solutions containing Sr(NO₃)₂ carrier and without it was studied. Aragonite shows the highest sorption capacity for ⁹⁰Sr²⁺, and these parameters are the lowest for calcite. All the CaCO₃ metastable modifications irreversibly sorb ⁹⁰Sr²⁺ and ⁹⁰Y³⁺ cations and therefore show promise for sorptive decontamination of water from ⁹⁰Sr.     

Photoemission studies of polymorphic CaCO3 materials

The surface analysis of polymorphic calcium carbonate (CaCO₃) compounds, namely, calcite, aragonite, and vaterite were carried out by X-ray photoelectron spectroscopy (XPS). XPS results clearly demonstrate that vaterite is different from the other two and exhibit a low binding energy (BE) for all its constituent elements. It is attributed to the perpendicular orientation of CO₃²⁻ to ab plane in vaterite. Aragonite shows less calcium and more oxygen and indicates the surface is carbonate terminated. Intergrowth of calcite and aragonite and natural dolomite samples were also analysed and compared with the above CaCO₃ compounds. Ca:Mg=3 suggest that the dolomite surface is dominated by Ca.     

Doped nanocrystalline calcium carbonate phosphates

The formation of nanocrystalline calcium carbonate phosphates doped with Fe²⁺, Mg², Zn²⁺, K⁺, Si⁴⁺, and Mn²⁺ has been studied by X-ray diffraction, IR spectroscopy, differential thermal analysis, and energy dispersive X-ray fluorescence analysis. The results indicate that the synthesis involves the formation of hydroxy carbonate complexes from the three calcium carbonate polymorphs (calcite, vaterite, and aragonite) in a solution of ammonium chloride and ammonium carbonate, followed by reaction with orthophosphoric acid. This ensures the preparation of a bioactive material based on chlorophosphates, octacalcium hydrogen phosphate, and calcium chloride hydroxide phosphates containing cation vacancies. Particle-size analysis data show that the materials contain nanoparticles down to 10 nm in size. Heat treatment of the doped calcium carbonate phosphates produces calcium hydroxyapatite containing cation vacancies, which can be used as a bioactive ceramic.     

Kinetic model for radionuclide sorption

A mathematical kinetic model is developed for sorption of radionuclides, occurring via transition of the sorbate atoms initially to the subsurface layer of the particles of a polydisperse sorbent with a known particle size distribution function ?(R), and then to the bulk of these particles. The variance of the function ?(R) was demonstrated to have a considerable effect on the sorption rate. The simulated results are compared with the experimental data on the sorption of ⁹⁰Sr from an aqueous medium on polydisperse calcium carbonate in the forms of calcite and vaterite.     

Monohydrocalcite: a promising remediation material for hazardous anions

The formation conditions, solubility and stability of monohydrocalcite (MHC, CaCO₃·H₂O), as well as sorption behaviors of toxic anions on MHC, are reviewed to evaluate MHC as a remediation material for hazardous oxyanions. MHC is a rare mineral in geological settings that occurs in recent sediments in saline lakes. Water temperature does not seem to be an important factor for MHC formation. The pH of lake water is usually higher than 8 and the Mg/Ca ratio exceeds 4. MHC synthesis experiments as a function of time indicate that MHC is formed from amorphous calcium carbonate and transforms to calcite and/or aragonite. Most studies show that MHC forms from solutions containing Mg, which inhibits the formation of stable calcium carbonates. The solubility of MHC is higher than those of calcite, aragonite and vaterite, but lower than those of ikaite and amorphous calcium carbonate at ambient temperature. The solubility of MHC decreases with temperature. MHC is unstable and readily transforms to calcite or aragonite. The transformation consists of the dissolution of MHC and the subsequent formation of stable phases from the solution. The rate-limiting steps of the transformation of MHC are the nucleation and growth of stable crystalline phases. Natural occurrences indicate that certain additives, particularly PO₄ and Mg, stabilize MHC. Laboratory studies confirm that a small amount of PO₄ in solution (>30 μM) can significantly inhibit the transformation of MHC. MHC has a higher sorption capacity for PO₄ than calcite and aragonite. The modes of PO₄ uptake are adsorption on the MHC surface at moderate phosphate concentrations and precipitation of secondary calcium phosphate minerals at higher concentrations. Arsenate is most likely removed from the solution during the transformation of MHC. The proposed sorption mechanism of arsenate is coprecipitation during crystallization of aragonite. The arsenic sorption capacity by MHC is significantly higher than simple adsorption on calcite.     

Complex Radiochemical Analysis of Natural Waters and Nuclear Power Plant Wastewaters. Specific Features of the Method for Radiochemical Determination of <Superscript>90</Superscript>Sr

A procedure was developed for complex analysis of γ-emitting nuclides and ⁹⁰Sr in natural and wastewaters with two methods of preparation of counting samples for ⁹⁰Sr activity measurement: coprecipitation of ⁹⁰Sr with strontium sulfate and coprecipitation of ⁹⁰Y with yttrium hydroxide. Strontium-90 is concentrated on KU-2-8chs strongly acidic sulfonic cation-exchange resin in the Н+ form after removing γ-emitting radionuclides on complex selective sorbents, which are simultaneously counting samples for measuring the activity of γ-emitting nuclides. The main steps of the sorption procedure for the complex analysis are described, and the choice of the method for preparing a counting sample based on ⁹⁰Y for current radioecological monitoring is substantiated.     

Monohydrocalcite: a promising remediation material for hazardous anions

Abstract

The formation conditions, solubility and stability of monohydrocalcite (MHC, CaCO₃·H₂O), as well as sorption behaviors of toxic anions on MHC, are reviewed to evaluate MHC as a remediation material for hazardous oxyanions. MHC is a rare mineral in geological settings that occurs in recent sediments in saline lakes. Water temperature does not seem to be an important factor for MHC formation. The pH of lake water is usually higher than 8 and the Mg/Ca ratio exceeds 4. MHC synthesis experiments as a function of time indicate that MHC is formed from amorphous calcium carbonate and transforms to calcite and/or aragonite. Most studies show that MHC forms from solutions containing Mg, which inhibits the formation of stable calcium carbonates. The solubility of MHC is higher than those of calcite, aragonite and vaterite, but lower than those of ikaite and amorphous calcium carbonate at ambient temperature. The solubility of MHC decreases with temperature. MHC is unstable and readily transforms to calcite or aragonite. The transformation consists of the dissolution of MHC and the subsequent formation of stable phases from the solution. The rate-limiting steps of the transformation of MHC are the nucleation and growth of stable crystalline phases. Natural occurrences indicate that certain additives, particularly PO₄ and Mg, stabilize MHC. Laboratory studies confirm that a small amount of PO₄ in solution (>30 μM) can significantly inhibit the transformation of MHC. MHC has a higher sorption capacity for PO₄ than calcite and aragonite. The modes of PO₄ uptake are adsorption on the MHC surface at moderate phosphate concentrations and precipitation of secondary calcium phosphate minerals at higher concentrations. Arsenate is most likely removed from the solution during the transformation of MHC. The proposed sorption mechanism of arsenate is coprecipitation during crystallization of aragonite. The arsenic sorption capacity by MHC is significantly higher than simple adsorption on calcite.     

Sorption of Cs,Sr, U,and Pu radionuclides on natural and modified clays

The ability of natural and modified montmorillonite clays from Belgorod oblast to sorb Cs, Sr, U, and Pu radionuclides was studied. The clays were modified by treatment with metal (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe²⁺, Zn²⁺) chloride solutions or aqueous HCl. The natural and modified clays studied show high performance in sorption treatment of solutions to remove Cs radionuclides. The natural clay and the Na and Mg forms of clays show the best sorption characteristics with respect to Cs. The distribution coefficient K _d of ¹³⁷Cs in sorption on the above samples from a 0.1 M NaNO₃ solution is (1.1–1.4) × 10⁴ cm³ g⁻¹, which is 4–5 times higher compared to natural clinoptilolite. The Sr, U, and Pu radionuclides are sorbed on the examined clay samples to a considerably lesser extent. The K _d values in sorption of these radionuclides from tap water are lower by 2–3 orders of magnitude than in sorption of Cs. Addition of clay materials in the course of cementation of liquid radioactive wastes, including NPP bottom residues, allows the rate of radiocesium leaching from the hardened cement compounds to be decreased by a factor of 5–16. The most efficient sorption additive in cementation of NPP bottom residues is natural montmorillonite clay.     

A radionuclide-microscopic study of sorption of sodium [3H]succinate on textured nanohydroxyapatite

Specific features of sorption of sodium [³H]succinate on nanohydroxyapatite (HAP) and its hierarchic textures were studied by radionuclide-microscopic diagnostics. Chemisorption of sodium succinate leads to structural-morphological modification of HAP, manifested, in particular, in changes in the habit of individual nanocrystals and in the internal texture of HAP macrospheroids. The dynamics of sorption penetration of sodium succinate into macrospherical HAP granules was monitored by autoradiogarphy. The results obtained suggest that nanohydroxyapatite and textured products based on it can be used for the development of materials for drug transport and of complex succinic acid containing preparations for medical purpose.     

Using saponified olive oil to make cost effective calcium carbonate particles superhydrophobic

The main goal of this paper is to show that superhydrophobic particles of calcium carbonate can be synthesized at low cost from harmless biocompatible natural products like baking soda (NaHCO₃) and olive oil. Calcium carbonate particles were prepared by precipitating solutions containing CO₃^2? and Ca²⁺ ions at two different temperatures 30 °C and 80 °C. These particles were transformed into hydrophobic ones using soap made from olive oil from the region of Kabylie (Algeria). The olive oil was first saponified. Adjusted amounts of this soap were added to the sodium carbonate solution before the precipitation reaction. Several techniques were used to analyze the products of the reaction. Typical polymorphs of CaCO₃ (calcite, aragonite and vaterite) were identified by quantitative analysis of X-ray powder diffraction patterns, giving their crystallite size and atomic fractions. Scanning electron microscopy was used to probe the morphology of the CaCO₃ particles. FT-IR measurements indicated the presence of aliphatic chains in the hydrophobic particles, thereby proving that the calcium carbonate particles were functionalized by soap. The particles which formed were found to be superhydrophobic with a contact angle of up to 175°. Evidence of the efficiency of these particles in oil spill recovery is given.     

Sorption treatment of radioactively contaminated natural waters to remove <Superscript>90</Superscript>Sr and <Superscript>137</Superscript>Cs using bauxites and products of their processing

Sorption treatment of radioactively contaminated natural waters to remove ⁹⁰Sr and ¹³⁷Cs using an alumina production waste, red bauxite slimes, is considered. In radiostrontium sorption, they surpass both the initial bauxite ore (Boksitogorsk deposit) and adjoin rocks (limestone, dolomite, bentonite clay), and owing to porous structure they act as efficient filtering material.     

Effects of limestone powder on CaCO3 precipitation in CO2 cured cement pastes

The concept of using limestone powder as supplementary materials to accelerate cement hydration has been explored widely. There have also been interests in using CO₂ curing to enhance the early strength of concretes. This study investigated the effects of incorporating limestone powder on CO₂ curing of cement-based materials. The results showed that using limestone powder to partially replace cement could significantly increase the CO₂ curing degree of the cement pastes. QXRD analysis showed that calcite was the major reaction product, accompanied by amorphous calcium carbonate. The mass ratio of poorly crystallized calcium carbonate to highly crystallized calcium carbonate (DC/HC ratio) formed was affected by both the applied CO₂ pressure and the use of limestone powder. At low pressure, incorporating limestone powder led to an increase in the DC/HC ratio. However, a reversed trend was observed in the case of high CO₂ pressure.     

Beach sand from Cancun Mexico: a natural macro- and mesoporous material

Sand particles from Cancun, Mexico were studied using a number of advanced spectroscopic and microscopic techniques. The main chemical composition of sand particles was confirmed to be calcium carbonate by X-ray photoelectron spectroscopy and IR spectroscopic analysis. X-ray diffraction analysis revealed that the sand particles are aragonite, which has an Orthorhombic—Dipyramidal crystal structure. The morphological study of the sand particles by scanning electron microscopy and transmission electron microscopy revealed the presence of a highly porous channel-like structure in the sand particles. The sorption isotherm indicates that Cancun sand is a mesoporous material. The specific surface area of Cancun sand was determined to be 2.259 m²/g by BET measurement, which is significantly higher than that of Florida sand and other forms of natural aragonite and calcite. Furthermore, it was found that the porous sand particles can adsorb gold nanoparticles of the size of a few nanometers very efficiently. The distribution of gold nanoparticles demonstrated a channel-like porous inner structure of the sand particles. We also prepared a polymer composite material by mixing the sand particles with a poly(methyl methacrylate) matrix. SEM analysis of the composite materials showed a good interfacial adhesion between sand particles and polymer matrix. These results suggest that Cancun sand, as a natural macro- and mesoporous material, may find promising applications in filtration, pollution control, composite materials and biomaterials development. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Physical and nanomechanical properties of the synthetic anhydrous crystalline CaCO<Subscript>3</Subscript> polymorphs: vaterite,aragonite and calcite

The synthetic anhydrous crystalline CaCO₃ polymorphs—vaterite, aragonite and calcite—were tested using dilatometry and nanoindentation. Microstructural changes in the samples before and after measurements were observed under scanning electron microscope and their phase composition quantified with X-ray powder diffraction with the Rietveld method. The thermal expansion coefficients of vaterite and the hardness and elastic modulus of synthetic aragonite are reported for the first time. The physical and nanomechanical properties were measured under similar conditions for each CaCO₃ polymorph. Aragonite, calcite and vaterite showed volumetric thermal expansion coefficient at 303 K of 49.2(8), 48.6(2) and 44.1(3) 10^?6 K^?1, respectively. The elastic modulus increased from 5(4), 16(7) to 31(8) GPa for aragonite, calcite and vaterite, respectively. Average hardness was found lower than values from the literature, ranging from 0.3 to 1.3 GPa. The results are considered of interest for the design of CaCO₃-based materials for applications.     

Comparative study on nano-mechanics and thermodynamics of fish otoliths

Fish otolith is a kind of typical natural biomineral, which is composed of calcium carbonate and organic matrix. In fresh water carp otolith, the inorganic phase of lapillus is pure aragonite, and for asteriscus it is pure vaterite. In this research, the phase composition, phase transformation, mechanical property and solubility of lapillus and asteriscus were studied. And results showed that, the organic content of lapillus was higher than that of asteriscus; the phase-transition temperature of lapillus (aragonite–calcite) and asteriscus (vaterite–calcite) both happened between 520 and 640 °C; the nano-mechanical property of lapillus was better than that of asteriscus; the solubility of asteriscus powder was higher than that of lapillus powder.     

Sorption of 90Sr and 137Cs on Natural Sorbents in Model Environmental Systems

Sorption of ⁹⁰Sr and ¹³⁷Cs from both tap drinking water and water of Beloyarsk storage basin on samples of natural ion exchangers, glauconite concentrate and white alluvial clay, was studied under static conditions. The distribution coefficients of ⁹⁰Sr and ¹³⁷Cs between these sorbents and water were studied as influenced by the equilibration time and water type. With white alluvial clay and glauconite concentrate the sorption equilibrium is reached within 90 and 270 days, respectively. Sorption characteristics of these natural sorbents allow them to be recommended for water decontamination from ⁹⁰Sr and ¹³⁷Cs.     

Aragonite Nanorods in Calcium Carbonate/Polymer Hybrids Formed through Self‐Organization Processes from Amorphous Calcium Carbonate Solution

Nanostructured inorganic/polymer hybrid thin films comprising aragonite nanorods derived from aqueous suspensions of amorphous calcium carbonate (ACC) are prepared. For the formation of calcium carbonate (CaCO₃)/polymer hybrids, spincoated and annealed films of poly(vinyl alcohol) (PVA) that function as polymer matrices are soaked in aqueous colloidal solutions dispersing ACC stabilized by poly(acrylic acid) (PAA). In the initial stage, calcite thin films form on the surface. Subsequently, aragonite crystals start to form inside the PVA matrix that contains PVA crystallites which induce aragonite nucleation. Nanostructured hybrids composed of calcite thin films consisting of nanoparticles and assembled aragonite nanorods are formed in the matrices of PVA.     

The removal of lead ions of the aqueous solution by calcite with cotton morphology

In this paper, calcite with cotton morphology was prepared by co-precipitation method using cotton as templates. The effect of contact time, initial concentration, and solution temperature on calcite adsorption of lead ions was studied using a batch technique. The maximal adsorption capacity of calcite for Pb²⁺ was 3347.02 mg/g with a minimum contact time of 60 min. The pseudo-second-order kinetic model and Langmuir/Freundlich isotherms were proposed for modeling kinetic and equilibrium data, respectively. Thermodynamic parameters such as ?G ⁰, ?H ⁰, and ?S ⁰ were calculated to reveal the nature of sorption. Original calcite and the products of adsorption were characterized using SEM, TEM/EDAX, SAED, XRD, and BET. Dissolution of calcite followed by precipitation of lead carbonate was found to be the main operating mechanism for Pb²⁺ adsorption by calcite.     

Influence of temperature on the sorption properties of rocks from the Nizhnekansky massif

The kinetics of radionuclide sorption onto finely divided samples of rocks from the Nizhnekansky massif, taken from the level of the planned arrangement of deep radioactive waste repository, was studied, and the times in which the sorption equilibrium was attained at 20 and 90°С were determined. The distribution coefficients of actinides (²³³U, ²³⁹Pu, ²⁴¹Am) and fission products (¹³⁷Cs, ⁹⁰Sr) were determined. Increasing the temperature from 20 to 90°С intensifies the sorption of the radionuclides studied, except cesium for which the sorption slightly decreases.     

Effect of Inorganic Anions on the Physicochemical State of Radionuclides in Process Freshwater Basins at the Mayak Production Association

Published data on the speciation of microcomponents and radionuclides (calcium, strontium, plutonium, and americium) in the water and bottom sediments of selected process water basins at the Mayak Production Association were discussed in terms of the model of the sorption behavior of a sorbate existing in several forms. It was shown that distribution of calcium, ⁹⁰Sr, and natural isotopes of strontium does not contradict the views on formation of Ca^{2 +}, CaSO₄ ⁰, Sr^{2 +}, SrSO₄ ⁰, Sr[SO₄]₂ ^{2 -} ionic and molecular species and of colloidal sulfate (CaSO₄·2H₂O) at selected sites of the water basin. By combining these factors it is possible to explain the variation in the distribition coefficients of Ca, Sr, Pu, and Am in the water-bottom sediments system in terms of the model of ion-exchange sorption in the presence of nonsorbable nonequilibrium colloid.