The role of citrate and phthalate during Co(II) coprecipitation with calcite

The influence of citrate and phthalate on Co coprecipitation with calcite was investigated using a combination of batch experiments, Fourier-transform infra-red (FT-IR) spectroscopy, and thermogravimetric analysis (TGA) over a wide range of precipitation rates. Steady-state growth conditions (at constant [Ca], [Co], DIC, and pH) were generally achieved within 3-5 h, after which Co(II) partitioning into calcite was evaluated. Only minor differences are observed in the partition coefficient (K_d) trends with and without citrate and phthalate as a function of calcite precipitation rate except at very low rates. Slight inhibition of calcite growth is observed in the presence of citrate or phthalate, which can be attributed to adsorption at surface sites. TGA curves for samples coprecipitated with citrate show a significant mass loss between 375 and 550 °C, whereas the weight-loss curves for the Co-phthalate coprecipitates are indistinguishable from those of the organic-free Co coprecipitates. This indicates that citrate is incorporated into calcite during calcite crystallization, whereas phthalate is excluded. FT-IR spectra for the sample with citrate show a broad absorption in the range 3700-3100 cm⁻¹, which is attributable to water molecules coordinated to citrate coprecipitated with calcite. The preferential incorporation of citrate over phthalate likely reflects differences in both aqueous speciation and conformation of the carboxylate groups. This new finding may provide new insight to the factors that control the behavior of macromolecules and their incorporation into the structure of calcium carbonate during biomineralization.     

Forecasting Solar Flares Using Magnetogram-based Predictors and Machine Learning

We propose a forecasting approach for solar flares based on data from Solar Cycle 24, taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) mission. In particular, we use the Space-weather HMI Active Region Patches (SHARP) product that facilitates cut-out magnetograms of solar active regions (AR) in the Sun in near-realtime (NRT), taken over a five-year interval (2012?–?2016). Our approach utilizes a set of thirteen predictors, which are not included in the SHARP metadata, extracted from line-of-sight and vector photospheric magnetograms. We exploit several machine learning (ML) and conventional statistics techniques to predict flares of peak magnitude \({>}\,\mbox{M1}\) and \({>}\,\mbox{C1}\) within a 24 h forecast window. The ML methods used are multi-layer perceptrons (MLP), support vector machines (SVM), and random forests (RF). We conclude that random forests could be the prediction technique of choice for our sample, with the second-best method being multi-layer perceptrons, subject to an entropy objective function. A Monte Carlo simulation showed that the best-performing method gives accuracy \(\mathrm{ACC}=0.93(0.00)\), true skill statistic \(\mathrm{TSS}=0.74(0.02)\), and Heidke skill score \(\mathrm{HSS}=0.49(0.01)\) for \({>}\,\mbox{M1}\) flare prediction with probability threshold 15% and \(\mathrm{ACC}=0.84(0.00)\), \(\mathrm{TSS}=0.60(0.01)\), and \(\mathrm{HSS}=0.59(0.01)\) for \({>}\,\mbox{C1}\) flare prediction with probability threshold 35%.     

The Remote Equatorial Nighttime Observatory of Ionospheric Regions Project and the International Heliospherical Year

We describe a new suite of instruments planned for deployment to Cape Verde as part of the International Heliospherical Year. The Remote Equatorial Nighttime Observatory of Ionospheric Regions (RENOIR) project consists of a bistatic Fabry–Perot interferometer system, an all-sky imaging system, a dual-frequency Global Positioning System (GPS) receiver, and an array of single-frequency GPS scintillation monitors. This instrumentation will allow for studying the low-latitude thermosphere/ionosphere (TI) system in great detail. Investigations to be conducted using this instrumentation while in Cape Verde include studying equatorial irregularity processes, the effects of neutral winds and gravity waves on irregularity development, the midnight temperature maximum, and ion-neutral coupling in the nighttime TI system. Initial observations from the RENOIR instrumentation during pre-deployment testing at the Urbana Atmospheric Observatory are presented, as is the deployment scenario for the project in Cape Verde.     

Uranyl and arsenate cosorption on aluminum oxide surface

In this study, we examined the effects of simultaneous adsorption of aqueous arsenate and uranyl onto aluminum oxide over a range of pH and concentration conditions. Arsenate was used as a chemical analog for phosphate, and offers advantages for characterization via X-ray absorption spectroscopy. By combining batch experiments, speciation calculations, X-ray absorption spectroscopy, and X-ray diffraction, we investigated the uptake behavior of uranyl, as well as the local and long-range structure of the final sorption products. In the presence of arsenate, uranyl sorption was greatly enhanced in the acidic pH range, and the amount of enhancement is positively correlated to the initial arsenate and uranyl concentrations. At pH 4-6, U L_III- and As K-edge EXAFS results suggest the formation of surface-sorbed uranyl and arsenate species as well as uranyl arsenate surface precipitate(s) that have a structure similar to trögerite. Uranyl polymeric species or oxyhydroxide precipitate(s) become more important with increasing pH values. Our results provide the basis for predictive models of the uptake of uranyl by aluminum oxide in the presence of arsenate and (by analogy) phosphate, which can be especially important for understanding phosphate-based uranium remediation systems.     

Coprecipitation of chromate with calcite: Batch experiments and X-ray absorption spectroscopy

Batch experiments, combined with in situ spectroscopic methods, are used to examine the coprecipitation of Cr(VI) with calcite, including partitioning behavior, site-specific distribution of Cr on the surface of calcite single crystals, and local coordination of Cr(VI) in the calcite structure. It is found that the concentration of Cr incorporated in calcite increases with increasing Cr concentration in solution. The calculated apparent partition coefficient, , is highest at low Cr solution concentration, and decreases to a constant value with increasing Cr solution concentration. DIC images of the surface of calcite single crystals grown in the presence of exhibit well-defined growth hillocks composed of two pairs of symmetrically nonequivalent vicinal faces, denoted as + and −, which reflect the orientation of structurally nonequivalent growth steps. Micro-XRF mapping of the Cr distribution over a growth hillock shows preferential incorporation of Cr into the—steps, which is considered to result from differences in surface structure geometry. XANES spectra confirm that incorporated Cr is hexavalent, and no reduction of Cr(VI) in the X-ray beam was observed up to 2 days at room temperature. EXAFS fit results show the incorporated Cr(VI) has the expected first shell of 4 O at ∼1.64 ± 0.01 Å, consistent with . Best fit results show that the second shell is split with ∼2.5 Ca at ∼3.33 ± 0.05 and ∼2.2 Ca at ∼3.55 ± 0.05 Å, which confirms the incorporation of chromate into calcite. Consideration of possible local coordination indicates that significant distortion or disruption is required to accommodate in the calcite structure.     

Pb (II) Sorption with Calcite: A Radiotracer Study

A ²¹⁰Pb radiotracer was used to monitor Pb solid-aqueousphase partitioning in sorption experiments at ambient temperature, pH = 8.2, and atmospheric PCO₂ in 0.15 M NaNO₃ solutions. A 24 h isotherm is linear up to Pb concentrations of 4 × 10^-6 M, above which an increase in slope suggests precipitation. The effect of Pb concentration, calcite loading, and ionic strength on Pb sorption with time was monitored. Sorption kinetics are rapid, followed by a slower sorption step.At a constant calcite loading500 mg L^-1,fractional sorptiondecreases with increasing initialPb concentration. The reverse isobserved for surface coverage, with0.6, 5.6 and 40.2% of availableCa²⁺sites occupied at10^-8,10^-7 and10^-6 MPb after 96 h. At a constant Pb concentration of10^-6 M,fractional sorption increases with increasing particleloading, however surface coverage decrease with72.5 and 22.1%Ca²⁺sites occupied at 100 and200 mg L^-1calcite after 96 h.The adsorption coefficient (K_d) is approximately 10³,increases with initial Pb concentration, but remains unaffected by variable calcite loading. Absence of an ionic strength effect on Pb sorption is interpreted as the dominance of inner-sphere complexation. For desorption experiments conducted over a range of initial sorption times, an average desorption index > 0.8 but < 1 indicates that sorption is largely reversible, but is accompanied by slight incorporation. Solid-solution formation increases with time, as observed by slower initial desorption rates for samples with longer sorption times. These findings indicate that Pb may be effectively sequestered by calcite; however re-release via desorption is likely as Pb does not become significantly incorporated into the mineral structure.