Formation Of Carbonate Complexes Research Articles

Dissolution kinetics of rare earth metal phosphates in carbonate solutions of alkali metals

Treatment of apatite raw materials is associated with the formation of large-tonnage waste – phosphogypsum. The content of rare earth metals in such waste reaches 1 %, which makes it possible to consider it a technogenic source for obtaining rare earth metals and their compounds. Up to the present moment, there are neither processing plants, nor an efficient process flow to handle phosphogypsum dumps. It is rational to use a way that involves extraction of valuable components and overall reduction of phosphogypsum dumps. Such process flow is available with carbonate conversion of phosphogypsum to alkali metal or ammonium sulfate and calcium carbonate upon the condition of associated extraction of rare earth metal (REM) compounds. Associated extraction of REM compounds becomes possible since they form strong and stable complexes with hard bases according to Pearson, which among other things include carbonate, phosphate and sulfate anions. Formation of lanthanide complexes with inorganic oxygen-containing anions is facilitated by the formation of high-energy Ln-O bonds. The study focuses on the dissolution of lanthanide phosphates in carbonate media. It was established that formation of REM carbonate complexes from their phosphates is a spontaneous endothermic process and that formation of lanthanide carbonates and hydroxides serves as thermodynamic limitation of dissolution. A shift in equilibrium towards the formation of carbonate complexes is achieved by increasing the temperature to 90-100 °C and providing an excess of carbonate. The limiting stage of REM phosphate dissolution in carbonate media is external diffusion. This is indicated by increasing rate of the process with an intensification of stirring, first order of the reaction and the value of activation energy for phosphate dissolution from 27 to 60 kJ/mol. A combination of physical and chemical parameters of the process allowed to develop an engineering solution for associated REM extraction during carbonate conversion of phosphogypsum, which included a 4-5 h conversion of phosphogypsum at temperature of 90-110 °C by an alkali metal or ammonium carbonate solution with a concentration of 2-3 mol/l. As a result, a solution with alkali metal (ammonium) sulfate is obtained, which contains REMs in the form of carbonate complexes and calcium carbonate. The rate of REM extraction into the solution reaches no less than 93 %. Rare earth metals are separated from the mother liquor by precipitation or sorption on anion exchange resins, while the excess of alkali metal or ammonium carbonate is returned to the start of the process.

REE-Th mineralization in the Se-Chahun magnetite-apatite ore deposit, central Iran: Interplay of magmatic and metasomatic processes

The Se-Chahun Fe-P ore deposit is located in the Bafq mining district in the Central Iranian tectono-magmatic zone and contains REE and Th mineralization. The ore deposit consists of a magnetite-apatite massive ore that is significantly modified by late-stage metasomatic fluids that finally formed brecciated rocks within and at the margin of the ore deposit. Geochemical data from the rhyolitic rocks, Fe-P ore body and breccia as well as isotopic data from the breccia within the Se-Chahun Fe-P ore bodies, have been used to deduce the magmatic origin of the Fe-P ore zone and to assess the significance of post-magmatic processes with late-stage metasomatic fluids and the effects of these processes on remobilization of magmatic originated REE and concentration and deposition of Th. The chondrite-normalized REE patterns of the mineralized rocks in both the Fe-P ore and the Th mineralization zone demonstrate LREE enrichment and strongly negative Eu anomalies (Eu/Eu* = 0.25–0.91 for Th mineralization). δD and δ18O for actinolite paragenetic to Th-bearing silicates are estimated to be between −73.29 to −42.04 and 6.65 to 7.71, respectively, which lie within the both fields of magmatic and metamorphic fluids. Na-Ca metasomatism (which is evidenced by brecciation) has been a continuous process affecting the host rock, and the Fe-P ore body. Apatite, monazite, and Ti-La-Ce-Y-Nd-oxides are the main REE-bearing phases in the Fe-P ore body (REE≫Th) that are precipitated during magnetite crystallization whereas Th silicates (huttonite and thorite) occur within the breccia (Th ≫ REE) and were deposited from a late Th- and carbonate-rich fluid. Th mineralization occurred with late sodic-calcic fluids and Th-bearing minerals are paragenetic to actinolite, red albite, magnetite, titanite, calcite, and pyrite. Paragenetic association of Th minerals and actinolite, titanite, and calcite in carbonate veins and veinlets point to transportation of Th by carbonate complexes. The pH, fluid/rock ratio, Ca/Na ratio, and higher capability of Th (with respect to REE) in formation of carbonate complexes and lower concentration of Th (with respect to REE) in the source region are the most important geochemical parameters that triggered Th (Th ≫ REE) mineralization at the end of alkali metasomatism.

Facet dependent catalytic activities of anatase TiO2 for CO2 adsorption and conversion

Understanding the atomic-scale interaction mechanism of CO2 and H2O on TiO2 surface is crucial to establish a correlation between the catalytic efficiency with its exposed facet. Here, with the aid of a three-state model, nudged elastic band simulations, and DFT calculations, we examine the chemical restructuring of these molecules during the process of adsorption, coadsorption and conversion on (0 0 1) including (1 × 4)-reconstructed, (0 1 0), and (1 0 1) facets of anatase TiO2 and thereby, evaluate the step selective reactivity order. In addition, the results reveal the unexplored non-trivialities in the reaction mechanisms. For the most stable (1 0 1) facet, we show that the unfavorable carbonate complex formation becomes favorable by switching the reaction from endothermic to exothermic in the presence of water. Further, we find that the small binding energy does not necessarily imply physisorption. It can also give rise to chemisorption, where loss in energy due to repulsive Hartree and Madelung interactions is comparable to the energy gained through the chemical bonding. Such a scenario is demonstrated for the CO2 adsorption on (0 1 0) and (1 0 1) facets. Though (0 0 1) remains the most reactive surface, if it undergoes reconstruction, which happens at ultra high vacuum and high temperature, the number of active sites is reduced by three-fourth.

Chromium hazard and risk assessment: New insights from a detailed speciation study in a standard test medium.

Despite the consensus about the importance of chemical speciation in controlling the bioavailability and ecotoxicity of trace elements, detailed speciation studies during laboratory ecotoxicity testing remain scarce, contributing to uncertainty when extrapolating laboratory findings to real field situations in risk assessment. We characterized the speciation and ecotoxicological effects of chromium (CrIII and CrVI ) in the International Organization for Standardization (ISO) medium for algal ecotoxicity testing. Total and dissolved (< 0.22 μm) Cr concentrations showed little variability in media spiked with CrVI , whereas dissolved Cr concentration decreased by as much as 80% over a 72-h time period in medium amended with CrIII . Analyses by ion chromatography inductively coupled plasma mass spectrometry (IC-ICP-MS) highlighted the absence of redox interconversion between CrIII or CrVI both in the presence and absence of algal cells (Raphidocelis subcapitata). Furthermore, the concentration of ionic CrIII dropped below detection limits in less than 2 h with the corresponding formation of carbonate complexes and Cr hydroxides. Precipitation of CrIII in the form of colloidal particles of variable diameters was confirmed by nanoparticle (NP) tracking analysis, single particle ICP-MS, and single particle counting. In terms of time-weighted dissolved (< 0.22 μm) Cr concentration, CrIII was 4 to 10 times more toxic than CrVI . However, CrIII ecotoxicity could arise from interactions between free ionic CrIII and algae at the beginning of the test, from the presence of Cr-bearing NPs, or from a combination of the 2. Future ecotoxicological studies must pay more attention to Cr speciation to reliably compare the ecotoxicity of CrIII and CrVI . Environ Toxicol Chem 2018;37:983-992. © 2017 SETAC.

Trends in element incorporation in hyaline and porcelaneous foraminifera as a function of &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Abstract. In this study we analyzed the impact of seawater carbonate chemistry on the incorporation of elements in both hyaline and porcelaneous larger benthic foraminifera. We observed a higher incorporation of Zn and Ba when pCO2 increases from 350 to 1200 ppm. Modeling the activity of free ions as a function of pCO2 shows that speciation of some elements (like Zn and Ba) is mainly influenced by the formation of carbonate complexes in seawater. Hence, differences in foraminiferal uptake of these might be related primarily by the speciation of these elements in seawater. We investigated differences in trends in element incorporation between hyaline (perforate) and porcelaneous (imperforate) foraminifera in order to unravel processes involved in element uptake and subsequent foraminiferal calcification. In hyaline foraminifera we observed a correlation of element incorporation of different elements between species, reflected by a general higher incorporation of elements in species with higher Mg content. Between porcelaneous species, inter-element differences are much smaller. Besides these contrasting trends in element incorporation, however, similar trends are observed in element incorporation as a function of seawater carbonate chemistry in both hyaline and porcelaneous species. This suggests similar mechanisms responsible for the transportation of ions to the site of calcification for these groups of foraminifera, although the contribution of these processes might differ across species.

Uranium behavior during a tidal cycle in an estuarine system affected by acid mine drainage (AMD)

The Tinto River estuary is one of the most contaminated coastal systems in the world due to the high amounts of pollutants this acidic river transports. The tidal influence and the fluvial discharges control the water mixing, which include a salt-induced process (that is typical of all estuaries) and an acid neutralization process (pH-induced mixing). These processes affect the geochemical behavior of uranium, which has a non-conservative pattern. Despite the high concentrations of dissolved uranium in the Tinto River, 234U/238U shows that uranium stems mostly from seawater (between 80 and 100%). Riverine uranium is adsorbed onto both Fe and Al precipitates when the acidic river water is mixed with seawater. Distribution coefficients (Kd) show that dissolved and particulate uranium are controlled by adsorption–desorption processes and the formation of carbonate complexes, both depending on pH. At low pH, uranium tends to be dissolved, and the adsorption by suspended particles is low. As the pH increases, the adsorption processes onto Fe and Al particles are more intense, increasing particulate uranium which reaches a maximum at pH 5.5, where the uranium solubility minimum occurs. At higher pH values, dissolved uranium increases by carbonate complexation.

Diffusion and sorption of neptunium(V) in compacted montmorillonite: effects of carbonate and salinity

AbstractDiffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (De) and distribution coefficients (Kd) of237Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m−3were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO3).Devalues for carbonate-free conditions were of the order of 10−10–10−11 m2s−1and decreased as salinity increased, and those for carbonate conditions were of the order of 10−11–10−12 m2s−1and showed the opposite dependence. Diffusion-derivedKdvalues for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency ofDeandKdqualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO2+and NpO2CO3−for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies ofDefor salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.

Structural and Kinetic Study of Reversible CO2 Fixation by Dicopper Macrocyclic Complexes. From Intramolecular Binding to Self-Assembly of Molecular Boxes

A study of the reversible CO2 fixation by a series of macrocyclic dicopper complexes is described. The dicopper macrocyclic complexes [Cu2(OH)2(Me2p)](CF3SO3)2, 1(CF3SO3)2, and [Cu2(mu-OH)2(Me2m)](CF3SO3)2, 2(CF3SO3)2, (Scheme 1) containing terminally bound and bridging hydroxide ligands, respectively, promote reversible inter- and intramolecular CO2 fixation that results in the formation of the carbonate complexes [{Cu2(Me2p)}2(mu-CO3)2](CF3SO3)4, 4(CF3SO3)4, and [Cu2(mu-CO3)(Me2m)](CF3SO3)2, 5(CF3SO3)2. Under a N2 atmosphere the complexes evolve CO2 and revert to the starting hydroxo complexes 1(CF3SO3)2 and 2(CF3SO3)2, a reaction the rate of which linearly depends on [H2O]. In the presence of water, attempts to crystallize 5(CF3SO3)2 afford [{Cu2(Me2m)(H2O)}2(mu-CO3)2](CF3SO3)4, 6(CF3SO3)4, which appears to rapidly convert to 5(CF3SO3)2 in acetonitrile solution. [Cu2(OH)2(H3m)]2+, 7, which contains a larger macrocyclic ligand, irreversibly reacts with atmospheric CO2 to generate cagelike [{Cu2(H3m)}2(mu-CO3)2](ClO4)4, 8(ClO4)4. However, addition of 1 equiv of HClO4 per Cu generates [Cu2(H3m)(CH3CN)4]4+ (3), and subsequent addition of Et3N under air reassembles 8. The carbonate complexes 4(CF3SO3)4, 5(CF3SO3)2, 6(CF3SO3)4, and 8(ClO4)4 have been characterized in the solid state by X-ray crystallography. This analysis reveals that 4(CF3SO3)4, 6(CF3SO3)4, and 8(ClO4)4 consist of self-assembled molecular boxes containing two macrocyclic dicopper complexes, bridged by CO32- ligands. The bridging mode of the carbonate ligand is anti-anti-mu-eta1:eta1 in 4(CF3SO3)4, anti-anti-mu-eta2:eta1 in 6(CF3SO3)4 and anti-anti-mu-eta2:eta2 in 5(CF3SO3)2 and 8(ClO4)4. Magnetic susceptibility measurements on 4(CF3SO3)4, 6(CF3SO3)4, and 8(ClO4)4 indicate that the carbonate ligands mediate antiferromagnetic coupling between each pair of bridged CuII ions (J = -23.1, -108.3, and -163.4 cm-1, respectively, H = -JS1S2). Detailed kinetic analyses of the reaction between carbon dioxide and the macrocyclic complexes 1(CF3SO3)2 and 2(CF3SO3)2 suggest that it is actually hydrogen carbonate formed in aqueous solution on dissolving CO2 that is responsible for the observed formation of the different carbonate complexes controlled by the binding mode of the hydroxy ligands. This study shows that CO2 fixation can be used as an on/off switch for the reversible self-assembly of supramolecular structures based on macrocyclic dicopper complexes.

Actinide speciation in the environment

Nuclear test explosions and nuclear reactor wastes and accidents have released large amounts of radioactivity into the environment. Actinideions in waters often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is related to the form in which the nuclides are introduced into the aquatic system. Chemical speciation, oxidation state, redox reactions, and sorption characteristics are necessary in predicting solubility of the different actinides, their migration behaviors and their potential effects on marine biota. The most significant of these variables is the oxidation state of the metal ion as the simultaneous presence of more than one oxidation state for some actinides in a solution complicates actinide environmental behavior. Both Np(V)O2 + and Pu(V)O2 +, the most significant soluble states in natural oxic waters, are relatively noncomplexing and resistant to hydrolysis and subsequent precipitation. The solubility of NpO2 + can be as high as 10−4M while that of PuO2 + is much more limited by reduction to the insoluble tetravalent species, Pu(OH)4, (pKsp≥56) but which can be present in the pentavalent form in aqautic phases as colloidal material. The solubility of hexavalent UO2 2+ in sea water is relatively high due to formation of carbonate complexes. The insoluble trivalent americium hydroxocarbonate, Am(OH)(CO3) is the limiting species for the solubility of Am(III) in sea water. Thorium(IV) is present as Th(OH)4, in colloidal form. The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing the environmental behavior of actinides. Much is understood about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these radioactive species on marine biota, but much more understanding of the behavior of the actinides in the environment is needed to allow proper and reliable modeling needed for disposition of nuclear waste over many thousands of years.

CONSTRAINTS ON THE GENESIS OF YELLOW FLUORITE IN HYDROTHERMAL BARITE FLUORITE VEINS OF THE ERZGEBIRGE, EASTERN GERMANY: EVIDENCE FROM OPTICAL ABSORPTION SPECTROSCOPY, RARE-EARTH-ELEMENT DATA, AND FLUID-INCLUSION INVESTIGATIONS

The optical absorption spectra of yellow fl uorite crystals from hydrothermal barite–fl uorite veins of the Erzgebirge, eastern Germany, are characterized by a broad absorption band with a peak at 433 nm that shows a subtle vibronic structure. The optical absorption behavior of the crystals is attributed to the presence of O3 – centers in the structure; the O3 molecules were incorporated during crystallization. The presence of the O3 – centers implies that a very specifi c chemical reaction must have taken place during crystallization, under particular physicochemical conditions. Thermal stability constraints of the yellow coloration suggest that the fl uorite formed at low to moderate temperatures (below 250 to 400°C), which agrees with homogenization temperatures determined on fl uid inclusions entrapped in the crystals (below 150°C). The crystals have a middle-rare-earth-element-enriched signature. The depletion of the light rare-earth elements in the fl uorite-forming fl uids was caused by the precipitation of Ce-rich phases prior to the formation of fl uorite, whereas the incorporation of the heavy rare-earth elements into fl uorite was suppressed by the formation of carbonate complexes in the hydrothermal fl uids. Fluid-inclusion data show that fl uorite formation occurred in a comparably stagnant environment typifi ed by the mixing of fl uids of different salinity and composition, namely a highly saline, CaCl2- and NaCl-rich brine that contained some Na2SO4, Na2CO3, NaHCO3, CaSO4, and SrSO4, as well as a NaCl-dominated fl uid of distinctly lower salinity. Fluorite precipitation from the cooling hydrothermal fl uids occurred in a relatively reducing environment under near-neutral conditions. We propose a chemical reaction that may have led to the formation of O3 – centers in the structure, and that may also explain the formation of the yellow coloration of fl uorite in other hydrothermal systems. In particular, fl uorite precipitation in some Mississippi-Valley-type deposits apparently occurred under very similar environmental conditions.

Reactivity of palladium(II) methyl complexes towards CO2: formation of carbonate complexes

Treatment of a benzene solution of (tmeda)PdMe2 or (dppe)PdMe2 with carbon dioxide gives the corresponding methyl bicarbonate complex, (L–L)PdMe(O2COH). These were characterised by NMR spectroscopy and elemental analysis. Under strictly dry conditions no reaction was observed. Recrystallisation of the tmeda bicarbonate complex from acetone yields the corresponding η2-carbonate complex, which was characterised by X-ray crystallography. The reaction probably proceeds through attack by free carbonic acid on the dimethyl complex.

Sorption Characteristics of Actinium and Protactinium onto Soils

Sorption behavior of227Ac and 233Pa onto several kinds of soil has been studied by a batch sorption experiment and a sequential extraction technique. The sorbedform of 227Ac was mainly fractionated into a lattice-bound form in the soils, which suggests strong binding with a crystalline phase in the soils. On the other hand, most of 233Pa was fractionated into an association with amorphous Fe+Mn oxides in the soils. In addition, a linear relationship between the Kd values of 233Pa and the content of iron oxides in the soils was found. From these results, it was concluded that 227Ac and 233Pa are irreversibly sorbed to the crystalline phase and amorphous Fe+Mn oxide in the soils, respectively. Carbonate concentration dependence of the Kd values of 227Ac and 233Pa on soil was also studied to evaluate the influence of groundwater composition on the sorption behavior. The Kd values of 237Ac and 233Pa increased with increasing carbonate concentration because of the formation of carbonate complexes of 237Ac and 233Pa to be irreversibly sorbed on the soil surface.

On the influence of carbonate in mineral dissolution: II. The solubility of FeCO 3 (s) at 25°C and 1 atm total pressure

The solubility of siderite in acid, neutral, and alkaline medium was investigated. The experiments were performed as potentiometric titrations under two constant partial pressures of CO 2 (g). This yielded the following equilibrium constants at 25°C in a 1.0 M NaClO 4 medium (quoted errors through this work are given at 2σ confidence level). 1. 1) The solubility constant of FeCO 3(s) FeCO 3 (s) + 2H + = Fe 2 + CO 2 (g) + H 2O , with log K ∗ s0 = 7.59 ± 0.08. 2. 2) The formation constants for the two determined Fe(II) carbonate complexes: Fe 2+CO 2(g) + H 2O = FeCO 3(aq) + 2H + , with log β 11 = −12.9 ± 0.12 Fe 2+ + 2CO 2 (g) + 2H 2O = Fe(CO 3) 2 2− + 4H + , with log β 11 = − 28.4 ± 0.10 . By extrapolation to infinite dilution, the following reaction equilibria are estimated for the infinite dilution standard state: FeCO 3 (s) = Fe 2+ + CO 3 2− , with log K sp = − 10.8 ± 0.2 Fe 2+ + CO 3 2− = FeCO 3 (aq) , with log β 11 = 5.5 ± 0.2 Fe 2+ + 2CO 3 2− = Fe(CO 3) 2 2− , with log β 2 = 7.1 ± 0.2 . The determined solubility constant is in fair agreement with previously derived solubility data. The identified complexes and their formation constants emphasize their importance in various anoxic waters. However, carbonate complex formation does not account for the supersaturation conditions with regard to siderite equilibrium observed in many other present-day environments.

FAULT‐BLOCK TECTONISM IN THE DEVONIAN SUBSURFACE, WESTERN CANADA BASIN

An alternative model for the formation of Devonian carbonate reefs in the Western Canada Basin is proposed. This model is based on the epeiric clear‐water sedimentation model of Irwin (1965), with structural modfication throughout the Phanerozoic by repeated PreCambrian basement tectonism.The cratonic structural pattern is dominantly controlled by the global regmatic fracture‐system, which comprises a series of NW ‐ SE and NE ‐ SW vertical faults. These faults delineate a mosaic of fault blocks across the entire Western Canada Basin. Wrench faulting and vertical epeirogenic movements throughout the Paleozoic. Mesozoic, and Cenozoic subjected these blocks to further tectonic effects ‐ structural deformation and hydrothermal diagenesis. Preservation of down‐faulted carbonate blocks, and their subsequent isostatic recovery into overlying anhydrite or shale sequences, has led to the formation of carbonate complexes generally referred to as “organic reefs”.In addition, repeated emergence and submergence of the cratonic platform, particularly throughout the Middle and Upper Paleozoic, produced numerous regional and inter‐regional unconformities. These periods of erosion contributed to the leaching, and partial dolomitization, of the Devonian carbonates formed during periods of cratonic submergence. Repeated faulting is considered to have been a major factor in the diagenetic alteration of Devonian carbonates through associated hydrothermal activity, and a dominant control in the migration of hydrocarbons.

The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6–7.6 at 20 °C

Samples of 200 mg powdered enamel apatite were suspended in 10 ml of aqueous solutions made from phosphoric acid and calcium hydroxide for 60 days at 20 °C. Calcium and phosphate concentrations were determined by spectroscopy; pH and the concentrations of fluoride and carbonate by electrometry. The solubility product of hydroxyapatite varied linearly from 10 −56.9 at pH 4.6 to 10 −52.8 at pH 7.6. The carbonate concentration in the aqueous phase was too low to account for calcium carbonate complex formation of significance to the solubility product. After the equilibration, X-ray diffraction analysis of the powder showed that the salt was unchanged pure apatite with no trace of a transformation to brushite or any other calcium phosphate. A re-equilibration of some of the powder samples for another 60 days produced similar solubility products. Thus the change of the solubility product was most likely attributable to a pH-induced change of the ionic composition of the enamel crystal surface.

Studies on Metal Carbonate Equilibria. 11. Yttrium(III) Carbonate Complex Formation in Aqueous Perchlorate Media of Various Ionic Strengths.

Studies on Metal Carbonate Equilibria. 11. Yttrium(III) Carbonate Complex Formation in Aqueous Perchlorate Media of Various Ionic Strengths.