Experimental investigation of calcium carbonate mineralogy in past and future oceans

Inorganic marine calcium carbonate formation and mineralogy varies significantly concurrent with the solution composition. During the Phanerozoic, due to oscillations in the seawater composition, this resulted in the formation of either dominantly calcite or aragonite. Variations in seawater composition also appear to have influenced the evolution of biomineralizing organisms. Additionally, many organisms utilize amorphous calcium carbonate (ACC) during biomineralization. The occurrence of calcite and aragonite throughout the Phanerozoic and calcium carbonate biomineralization were investigated. This was done by determining the influence of solution chemistry (SO4 and Mg) on calcium carbonate formation, mineralogy and stability via a variety of laboratory and synchrotron based synthesis experiments. During the formation of aragonite and calcite, aqueous SO4 and the Mg/Ca ratio both affect the formation of calcite and aragonite. An increase in aqueous SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes dominant. These results suggest that the models relating seawater chemistry to calcium carbonate formation needs re-evaluation. Abiotic ACC crystallization to vaterite occurs in three stages. In the first stage, ACC crystallizes to vaterite via a spherulitic growth mechanism. The second stage is characterized by surface particle growth at the expense of ACC. Finally, particle growth via Ostwald ripening is the only remaining process. This process can be described as the inorganic analogue to biological ACC crystallization, which is adjusted by organisms to produce their preferred calcium carbonate polymorph and morphology. An increase in SO4 concentration only decreases the spherulitic growth rate and Ostwald ripening, even when rapidcreekite (as an intermediate) and gypsum crystallizes. Finally, SO4 promotes the formation of vaterite. Depending on the formation process this is caused by either the stabilization of vaterite and destabilization of calcite (slow heterogeneous formation), or by the destabilization and inhibition of calcite formation (spherulitic growth)

Controls on the fate and speciation of Np(V) during iron (oxyhydr)oxide crystallization

The speciation and fate of neptunium as Np(V)O2+ during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2+ to iron(III) (oxyhydr)oxide phases was reversible and, for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behavior of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long-term immobilization of radiotoxic Np in natural and engineered environments

Uranium fate during crystallization of magnetite from ferrihydrite in conditions relevant to the disposal of radioactive waste

Uranium incorporation into magnetite and its behaviour during subsequent oxidation has been investigated at high pH to determine the uranium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to U(VI)aq containing cement leachates (pH 10.5-13.1) and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of XRD, chemical extraction and XAS techniques provided direct evidence that U(VI) was reduced and incorporated into the magnetite structure, possibly as U(V), with a significant fraction recalcitrant to oxidative remobilization. Immobilization of U(VI) by reduction and incorporation into magnetite at high pH, and with significant stability upon reoxidation, has clear and important implications for limiting uranium migration in geological disposal of radioactive wastes. © 2016 by Walter de Gruyter Berlin/Boston

Ferrihydrite formation : the role of Fe13 Keggin clusters

Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for clean-up of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In-situ SAXS measurements supported by ex situ TEM indicate that initailly Fe13 Keggin clusters (radius ~0.45 nm) form in solution at pH 0.5 - 1.5, and are persistant for at least 18 days. The Fe13 clusters begin to aggregate above ~ pH 1, initially forming highly linear structures. Above pH ~ 2 densification of the aggregates occurs in conjunction with precipiation of low molecular weight Fe(III) speices (e.g. monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (~ 3 nm) in which the Fe13 Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core-shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered and industrial processes

Clean-up of divalent cobalt ions by massive sequestration in a low-cost calcium silicate hydrate material

Cobalt is a critical resource in industrial economies for the manufacture of electric-vehicle batteries, alloys, magnets, and catalysts, but has acute supply-chain risks and poses a threat to the environment. Large-scale sequestration of cobalt in low-cost materials under mild conditions opens a path to cobalt recycling, recovery and environmental clean-up. We describe such sequestration of cobalt by a widely available commercial calcium silicate material containing the mineral xonotlite. Xonotlite rapidly and spontaneously takes up 40 percent of its weight of cobalt under ambient conditions of temperature and pressure and reduces dissolved cobalt concentrations to low parts per million. A new Sharp Front experimental design is used to obtain kinetic and chemical information. Sequestration occurs by a coupled dissolution-precipitation replacement mechanism. The cobalt silicate reaction product is largely amorphous but has phyllosilicate features

Incorporation and retention of 99-Tc(IV) in magnetite under high pH conditions

Technetium incorporation into magnetite and its behavior during subsequent oxidation has been investigated at high pH to determine the technetium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to Tc(VII)(aq) containing cement leachates (pH 10.5-13.1), and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of X-ray diffraction (XRD), chemical extraction, and X-ray absorption spectroscopy (XAS) techniques provided direct evidence that Tc(VII) was reduced and incorporated into the magnetite structure. Subsequent air oxidation of the magnetite particles for up to 152 days resulted in only limited remobilization of the incorporated Tc(IV). Analysis of both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicated that the Tc(IV) was predominantly incorporated into the magnetite octahedral site in all systems studied. On reoxidation in air, the incorporated Tc(IV) was recalcitrant to oxidative dissolution with less than 40% remobilization to solution despite significant oxidation of the magnetite to maghemite/goethite: All solid associated Tc remained as Tc(IV). The results of this study provide the first direct evidence for significant Tc(IV) incorporation into the magnetite structure and confirm that magnetite incorporated Tc(IV) is recalcitrant to oxidative dissolution. Immobilization of Tc(VII) by reduction and incorporation into magnetite at high pH and with significant stability upon reoxidation has clear and important implications for limiting technetium migration under conditions where magnetite is formed including in geological disposal of radioactive wastes

Neptunium and manganese biocycling in nuclear legacy sediment systems

Understanding the behaviour of the highly radiotoxic, long half-life radionuclide neptunium in the environment is important for the management of radioactively contaminated land and the safe disposal of radioactive wastes. Recent studies have identified that microbial reduction can reduce the mobility of neptunium via reduction of soluble Np(V) to poorly soluble Np(IV), with coupling to both Mn- and Fe(III)- reduction implicated in neptunyl reduction. To further explore these processes Mn(IV) as δMnO2 was added to sediment microcosms to create a sediment microcosm experiment "poised" under Mn-reducing conditions. Enhanced removal of Np(V) from solution occurred during Mn-reduction, and parallel X-ray absorption spectroscopy (XAS) studies confirmed Np(V) reduction to Np(IV) commensurate with microbially-mediated Mn-reduction. Molecular ecology analysis of the XAS systems, which contained up to 0.2 mM Np showed no significant impact of elevated Np concentrations on the microbial population. These results demonstrate the importance of Mn cycling on Np biogeochemistry, and clearly highlight new pathways to reductive immobilisation for this highly radiotoxic actinide

Некоторые проблемы добычи полезных ископаемых на глубоких горизонтах недр

Cardiovascular screening may benefit middle-aged sportsmen, as coronary artery disease (CAD) is the main cause of exercise-related sudden cardiac death. Arterial stiffness, as measured by pulse wave velocity (PWV), may help identify sportsmen with subclinical CAD. We examined the additional value of PWV measurements to traditional CAD risk factors for identifying CAD.From the Measuring Athlete's Risk of Cardiovascular events (MARC) cohort of asymptomatic, middle-aged sportsmen who underwent low-dose Cardiac CT (CCT) after routine sports medical examination (SME), 193 consecutive sportsmen (aged 55 ± 6.6 years) were included with additional PWV measurements before CCT. Sensitivity, specificity and predictive values of PWV values (>8.3 and >7.5 m/s) assessed by Arteriograph were used to identify CAD (coronary artery calcium scoring ≥ 100 Agatston Units or coronary CT angiography luminal stenosis ≥ 50%) and to assess the additional diagnostic value of PWV to established cardiovascular risk factors.Forty-seven sportsmen (24%) had CAD on CCT. They were older (58.9 vs. 53.8 years, p<0.001), had more hypertension (17 vs. 4%, p=0.003), higher cholesterol levels (5.7 vs. 5.4 mmol/l) p=0.048), and more often were (ever) smokers (55 vs. 34%, p=0.008). Mean PWV was higher in those with CAD (8.9 vs. 8.0 m/s, p=0.017). For PWV >8.3m/s respectively >7.5 m/s sensitivity to detect CAD on CT was 43% and 74%, specificity 69% and 45%, positive predictive value 31% and 30%, and negative predictive value 79% and 84%. Adding PWV to traditional risk factor models did not change the area under the curve (from 0.78 (95% CI = 0.709-0.848)) to AUC 0.78 (95% CI 0.710-0.848, p = 0.99)) for prediction of CAD on CCT.Limited additional value was found for PWV on top of established risk factors to identify CAD. PWV might still have a role to identify CAD in middle-aged sportsmen if risk factors such as cholesterol are unknown