Diffusion Of Fluoride Research Articles

Performance modeling of layered double hydroxide incorporated mixed matrix beads for fluoride removal from contaminated groundwater with the scale up study

In this study, calcium aluminium based layered double hydroxide (LDH) was synthesized in a controlled manner and its efficiency as a potential adsorbent in fluoride removal was explored. The LDH basal spacing of 8.2°A confirmed the planar orientation of the intercalated NO3–. The maximum adsorption capacity of LDH was 247.1 mg/g (at 308 K) and the adsorption was facilitated by the lower crystal radius and higher electro-negativity of F- compared to NO3–. For practical application in adsorption column, the LDH was incorporated in polysulfone to form spherical mixed matrix beads (MMB) of average diameter 2.8 mm. The elongated macrovoids inside the MMB promoted the diffusion of fluoride to the adsorption sites. The maximum adsorption capacity of the MMB was 90 mg/g (at 308 K). A fundamental mass transfer-pore diffusion-adsorption model linking convection, diffusion and adsorption was used to predict the adsorption kinetics and the breakthrough behaviour in the batch and continuous column mode experiments with the synthetic solution and contaminated groundwater. Different model-estimated transport parameters, like, effective pore diffusivity, mass transfer coefficient and the axial dispersion coefficient were used for scaling up of the column filters. The spent adsorbent was regenerated using 0.1(M) NaOH and 1(M) NaNO3.

Peculiarities of ion mobility and conductivity in the (NH4)6LiHf2Zr2F23 compound

Ion mobility and electrophysical properties in (NH4)6LiHf2Zr2F23 compound have been studied by 1H, 19F NMR, and impedance spectroscopy. The types of ion motions in the fluoride and ammonium sublattices of the compound have been determined in the temperature range 150–450 ​K and their activation energy was estimated. The presence of the order-disorder phase transition in this compound in the temperature range 400–450 ​K with the formation of metastable high-temperature β–modifications, in which diffusion of fluoride and ammonium ions is the dominant type of ionic motion, is established. The value of conductivity at 450 ​K–4.5·10−3 ​S/cm indicates that the compound studied belongs to the class of superionic conductors.

Hierarchical hollow manganese-magnesium-aluminum ternary metal oxide for fluoride elimination

Rational designs and syntheses for advanced structures are of immense importance to enhance adsorption performances toward a variety of materials. In this study, the synthesis of a novel hierarchical hollow manganese-magnesium-aluminum ternary metal oxide (MMA) via a green hydrothermal strategy coupled with a calcination process serves as a robust adsorbent for fluoride elimination. Combining the strong affinities Mn, Mg, and Al species have toward fluoride into a 3D-hierarchical hollow structure with an adequately accessible adsorption surface can remarkably boost the migration and diffusion of fluoride and provide more mass diffusion pathways for fluoride elimination. Remarkably, the adsorption process follows the pseudo-second-order model and the Langmuir isotherm model with a considerable performance of 63.05 mg/g. Moreover, the adsorbent retained outstanding selectivity and recyclability. Overall, the results from the universal characterization techniques and batch experiments validate that the potential adsorption mechanisms were electrostatic attraction and ion exchange, and complexation. As such, the present method expands the current adsorbent toolbox by providing a rational design and synthesis of a highly efficient adsorbent material for use in managing environmental pollution.

Endcapping of high-power 3 µm fiber lasers.

Fiber tip photodegradation through OH diffusion currently limits the long term operation of high-power fiber lasers and amplifiers operating near 3 µm. To address this issue, we investigate the resistance to OH diffusion of fluoride and oxide endcaps manufactured out of ZrF$_4$4, AlF$_3$3, GeO$_2$2, SiO$_2$2 and Al$_2$2O$_3$3 fibers. To this extent, the endcaps are spliced at the output of a 20 W continuous-wave fiber laser operating at 2.8 µm and their degradation over a 100 h time period is monitored. While the fluoride-based endcaps underwent failure during the first 10 h, their oxide counterparts survived the experiment, although showcasing degradation which was reflected as an increase of the endface temperature over time. To overcome this issue, we propose a novel method to completely suppress OH diffusion which consists in sputtering a nanoscopic diffusion barrier film made of silicon nitride (Si$_3$3N$_4$4) on the output face of the endcap. The effectiveness of the approach is validated on Al$_2$2O$_3$3, ZrF$_4$4 and AlF$_3$3 endcaps which show no sign of degradation after being used for more than a 100 h at the output of a 3 µm high-power fiber laser.

Ion mobility, phase transitions and conductivity in the fluorides (NH4)6LiZr3HfF23 and (NH4)6LiZrHf3F23

Ion mobility, phase transitions, thermal and electrophysical properties in (NH4)6LiZr3HfF23 and (NH4)6LiZrHf3F23 fluorides have been studied by 1H, 19F NMR, X-ray diffraction, DSC and impedance spectroscopy. The types of ion motions in the fluoride and ammonium sublattices of the fluorides have been determined in temperature range of 150–450 K and their activation energy was estimated. The presence of order-disorder phase transition in these fluorides in the temperature range of 400–450 K with the formation of metastable high-temperature β–modifications in which diffusion of fluoride and ammonium ions is the dominant type of ionic motion is established. The values of conductivity at 453 K [(~ 9.4–6.6) × 10–3 S/cm] indicate that the studied fluorides belong to the class of superionic conductors.

Synthesis, Ion Mobility, and Phase Transition in the (NH4)6LiHf2Zr2F23 Compound

Ion mobility and a phase transition (PT) in the compound (NH4)6LiHf2Zr2F23 are studied using 1H NMR, 19F NMR, XRD, and DSC methods. The types of ionic motions in the fluoride and ammonium sublattices of this compound are determined at 150–450 K; their activation energies are estimated. It is shown that an order-disorder PT occurs in this compound at 400–450 K to form a metastable high-temperature modification, where ionic motions are represented mainly by the diffusion of fluoride and ammonium ions.

Thermal and Transport Properties, Ion Mobility, and Phase Transitions in Compounds (NH4)6CsZr4F23 and (NH4)6CsHf4F23

Thermal, electrophysical properties, ion mobility, and phase transitions (PT) in compounds (NH4)6CsZr4F23 (I) and (NH4)6CsHf4F23 (II) have been studied by 1H, 19F NMR, X-ray powder diffraction, DSC, and impedance spectroscopy. The types of ion motions in the fluoride and ammonium sublattices of both compounds have been determined in temperature range 150–450 K and their activation energy has been assessed. Phase transitions in compounds I and II has been revealed in temperature range 380–404 and 384–412 K to form high-temperature modifications with diffusion of fluoride and ammonium ions as the main type of ion motions. The high ionic conductivity of compounds I and II (higher 10–4 S/cm at 463 K) allows one to refer these fluoro complexes to the class of superionic conductors.

Synthesis of molten-electrolyte corrosion resistant MgAl2O4[sbnd]MgAlON sidewall materials by pressureless sintering

MgAl2O4MgAlON materials as potential candidate sidewalls were prepared by using AlN, MgO, Al2O3, and nanosized MgAl2O4 powder as starting materials. The reaction sintering of the MgAl2O4MgAlON materials was investigated by means of X-ray diffraction and scanning electron microscope. All the specimens were corroded in a Na3AlF6AlF3CaF2Al2O3 bath to assess the electrolyte corrosion resistance. The results show that reaction sintering occurs in the MgAl2O4MgOAl2O3AlN system at 1830 °C and 5 MPa in flowing nitrogen for 5 h. The MgAl2O4MgAlON specimens are obtained and have high bulk density. The flexural strength increases from 202 ± 6 MPa to 248 ± 8 MPa and the Vickers hardness increases from 9.5 ± 0.2 GPa to 12.6 ± 0.4 GPa with increasing MgAlON content from 10 wt% to 40 wt%. The MgAl2O4MgAlON materials exhibit a good corrosion resistance to the Na3AlF6AlF3CaF2Al2O3 bath owing to their high-density and the formation of the composite spinel in the specimens. The corrosion mechanisms of MgAl2O4MgAlON composites in molten electrolyte are mainly proposed by the dissolution of MgAl2O4 and the diffusion of fluoride. Corrosion kinetic obeys a nearly parabolic law and the corrosion rate decreases with increasing MgAlON content.

Phase transitions, ion mobility, and conductivity in fluoro complexes (NH4)6RbZr4F23 and (NH4)6RbHf4F23

Ion mobility, phase transitions (PT), and conductivity in compounds (NH4)6RbZr4F23 (I) and (NH4)6RbHf4F23 (II) were studied by 1H and 19F NMR spectroscopy, X-ray powder diffraction (XRD), DSC, and impedance spectroscopy. Ion movement modes in fluoride and ammonium sublattices of both compounds were determined in temperature range 150–480 K and their activation energy was assessed. Phase transitions in compounds I and II were revealed in temperature ranges 400–440 and 419–440 K resulting in formation of α modifications where diffusion of fluoride and ammonium ions becomes the dominant kind of ion movement. High ionic conductivity in fluoride II (σ ~ 7.7 × 10–3 S/cm at 463 K) indicates that this compound belongs to the class of superionic conductors.

Ion mobility, conductivity, structure, and phase transitions in K0.7M0.3SbF4 compounds with M=Rb, NH4

Ion mobility, phase transitions, structure, and conductivity in the K0.7M0.3SbF4 (M=Rb, NH4) compounds were studied by NMR spectroscopy, DSC, X-ray, and conductivity measurements. The predominant form in the ion motions resulting from the phase transition of high modification was diffusion of fluoride and ammonium ions above 450K. The high–temperature phases of K0.7M0.3SbF4 (M=Rb, NH4) are superionic, while their conductivity attains the values of ~10−2–10−4S/cm at 450–500K. The structures of α– and β–modifications of the K0.7Rb0.3SbF4 are monoclinic (space group P21/m). The main structural units in them are statistically substituting each other K+ and Rb+ cations and complex [SbF4]−∞1 anions linked into zigzag-like chains by bridge fluorine atoms. The nearest surrounding of each antimony atom contains five fluorine atoms, so that the antimony coordination polyhedron can be described, taking into account the lone electron pair, as a distorted SbF5E octahedron (ψ-octahedron).

Effects of fluoride and polymeric additives on the dissolution of calcite and the subsequent formation of fluorite

Fluoride is a problematic contaminant of the ground water arising from both natural and man-made origins. In this report, the fluoride uptake by calcite (calcium carbonate) was examined with emphasis on the effects of model organic compounds (polymers) and the possible adhesion of fluorite (calcium fluoride) on calcite. The primary route of the fluoride removal was fluorite formation through dissolution-and-recrystallization. While all model polymers affected the kinetics of fluorite formation, the positively charged poly(ethylene imine) had the most inhibiting influence. The observation of the calcite single crystals using atomic force microscopy revealed that the inhibitory effect was attributed to the retarded diffusion of fluoride to the calcite surface, which was also in accordance with the analysis of bulk experiments with calcite powders. It was also found that the oriented growth of fluorite was induced on the calcite (104) surface probably by the epitaxy with the (120) plane of fluorite. The current results could be of critical implications in the process design of fluoride removal by considering organic interferences and minimizing the need of flocculation step.

Ionic mobility and conduction in the 50PbF2 · 30BiF3 · 20NaF solid solution studied by NMR and impedance spectroscopy

Ionic mobility and electrical transport in the 50PbF2 · 30BiF3 · 20NaF fluorite solid solution have been studied by 19F and 23Na NMR and impedance spectroscopy. Analysis of the NMR spectra allowed us to assess the temperature effect on the nature of ion motion in the fluorine and sodium sublattices, to identify the types of ionic mobility, and determine the corresponding temperature ranges. The results demonstrate that, at temperatures above 390 K, the dominant type of ion motion in 50PbF2 · 30BiF3 · 20NaF is the diffusion of fluoride and sodium ions. The ionic conductivity of the solid solution reaches 4.4 × 10−4 S/cm at 535 K, suggesting that the 50PbF2 · 30BiF3 · 20NaF solid solution can be used as a key component for engineering new functional materials.

Ion Mobility in the Fluorite Solid Solutions 50PbF<sub>2</sub>–30BiF<sub>3</sub>–20K(Na)F According to <sup>19</sup>F, <sup>23</sup>Na NMR Data

Ion mobility in solid solutions of the fluorite structure 50Pb2–30BiF3–20KF (I) and 50Pb2–30BiF3–20NaF (II) was studied by NMR method. Analysis of 19F, 23Na NMR spectra made it possible to reveal the character of ion motions in the fluoride and sodium sublattices with temperature variation, to determine the types and temperature ranges in which they took place. It was found that the dominant form of ionic mobility in the samples I and II above 380 K was the diffusion of fluoride and sodium ions. According to preliminary results of electro-physical studies, the conductivity reached values of ~ 2×10–2 – 10–3 S/cm above 500 K. The solid solutions I and II can be recommended as a basis for use in the development of new functional materials.

Effect of pH on fluoride penetration into natural human plaque

Since recent studies have demonstrated that penetration profiles of fluoride into plaque falls from the saliva plaque interface towards the enamel, it was hypothesized that charged components may restrict the inward diffusion of fluoride i.e., protonated fluoride and may penetrate more effectively than fluoride ion. Therefore, in this study, we investigated the effects of pH on fluoride uptake and distribution in natural undisturbed human plaque formed in vivo by means of the Leeds in situ device. At pH 3 significantly less fluoride was present throughout the plaque layer compared with pH 7. Similar profiles were seen in the plaque produced over 1 week and 3 weeks. These results may be due to binding to components of the plaque matrix via hydrogen bonding. Protonation of bacterial cell surfaces may also lead to binding of the ionized portion of fluoride. In conclusion, low pH affected the short-term fluoride uptake from a 1,000 ppm fluoride solution.

Precise Test of the Diffusion-Controlled Wet Isotropic Etching of Silicon via Circular Mask Openings

Isotropic etching of silicon in -based solutions is expected to be controlled by the diffusion of fluoride to the silicon surface. In order to gain quantitative understanding of the process, we studied etching of silicon in via circular mask openings and compared the results with the theoretical expectations. The cavity edges due to etching under the mask were analyzed with a high precision by processing the optical microscope images. Dependence on the etching time and opening size was investigated. A small anisotropy was observed in perfect agreement with the crystal orientation symmetry. A weak effect of free convection induced by the gravitation was resolved. Importance of careful temperature control is stressed. The observed time dependence agrees perfectly well with the theoretical prediction. It was verified with 1% precision. Dependence on the opening size predicted theoretically is not fully supported by the experiment. There is a small (4%) but clearly observable deviation from the theory. It is demonstrated that both time and opening size dependencies can be predicted with 1% precision if one introduces an effective diffusion “constant” that changes slightly with the opening size.

Ion mobility, phase transitions, and ion transport in potassium-ammonium hexafluorozirconates

The mobility of fluoride and ammonium ions (180–480 K) in compounds K2− n (NH4)nZrF6 (0.2 ≤ n ≤ 1.70) was investigated by 19F and 1H NMR. Correlations have been found between the composition of the cation sublattice, the character of ion motions, and phase transition temperature in these compounds. The hightemperature modifications with n ≥ 0.85 of the compounds are characterized by translational diffusion of fluoride and ammonium ions and by uniaxial anisotropy of the 19F magnetic shielding tensor. The electrophysical characteristics of these compounds were studied in the temperature range 300–480 K.

On the ionic diffusive motion in some fluoride doped polymer electrolytes

The ionic diffusive motion (translational and rotational) of cations (NH 4 + / H +) and anions (F −/BF 4 −) in polyethylene oxide (PEO) based polymer electrolytes containing ammonium fluoride (NH 4F) and ammonium tetrafluoroborate (NH 4BF 4) has been studied by 1H and 19F NMR. Long range protonic translational motion has been observed at temperatures above the glass transition temperature ( T g) whereas rotational diffusion of fluoride has been observed in PEO–NH 4BF 4 at temperatures below T g. In PEO–NH 4BF 4, both cations and anions have been found to show translational motion above T g.

Quantifying the diffused benefit from water fluoridation in the United States.

To estimate the total contribution of water fluoridation to caries reduction by including the benefit from the diffusion of fluoride from fluoridated communities to surrounding nonfluoridated communities via the export of bottled beverages and processed foods. We analyzed data from the 1986-87 NIDR Children's Survey for 18,507 school children aged 6-17 years who had at least one permanent tooth and for whom a complete fluoride exposure history could be created. To measure water fluoridation exposure, we generated continuous and categorical exposure variables. Years of fluoridation exposure (YFE-continuous) measured the number of years the child lived at residences receiving fluoridated water. Lifetime fluoridation exposure (LFE-categorical) was high if the child lived at residences receiving fluoridated water more than 50% of his life and low, otherwise. We summed the proportion of state population receiving fluoridated water times the number of years the child had lived in each state and then divided this value by the child's age to measure diffusion exposure (DE). We grouped DE into three levels: low (DE<=0.25), medium (0.25<DE<0.55), and high (DE>=0.55). For each level of DE, we compared the age-adjusted mean DMFS for high and low LFE. In addition we used linear regression to measure the association between DMFS and YFE while controlling for DE, age, exposures to other fluoride sources, and sociodemographic variables. Reported results are significant at P<0.05. Comparison of mean DMFS scores found that the direct benefit of water fluoridation (DMFS(LFE=low) - DMFS(LFE=high)) was 1.44 surfaces among low DE children and 0 among high DE children. The diffused benefit (DMFS(LFE=low, DE=low) - DMFS(LFE=low, DE=high)) was 1.23 surfaces. The regression results were similar and indicated that the direct benefit would be 1.44 fewer DMFS for low DE children and the indirect benefit would be 1.09 fewer DMFS for high DE children. Failure to account for the diffusion effect may result in an underestimation of the total benefit of water fluoridation, especially in high diffusion exposure regions.

Remineralization of Human Dentine in vitro

In this paper the in vitro remineralization of human dentine with 0, 0.5, 2 and 10 ppm F in the remineralization solution is presented and analyzed by microradiography. Furthermore, the remineralization data of human and bovine dentine are quantitatively compared. The main results of this paper are: (1) the efficacy of human (and bovine) remineralization is about proportional to the square root of the F level in the remineralizing solution, and (2) the amount of mineral effectively deposited in the dentine is most likely controlled by the diffusion of fluoride into the tissue. This study shows furthermore that although numerical differences exist in the remineralization of human and bovine dentine, the general remineralization behaviour is quite similar. The presence of F- in the parts per million range is essential for the remineralization efficacy in vitro and also for the 'overremineralization' of the outer dentine surface.

Diffusion of fluoride through the rat enamel organ in vitro.

This study investigated the diffusion of fluoride through the enamel organ in vitro. The rat molar explants used were entirely in the secretory stage or predominantly in the maturation stage of enamel formation. The removal of the enamel organ or metabolic inhibition with iodoacetate caused significant increases in enamel fluoride uptake at both stages of enamel formation. Inhibition with dinitrophenol caused a significant increase only in the maturation phase. Uptake of fluoride in enamel was related to the fluoride concentration in the medium, except in the maturation stage explants, where increasing the medium fluoride concentration from 0.05 ppm to 0.08 ppm did not significantly increase fluoride uptake at any of the three observation times. The findings indicate that the enamel organ exists as a diffusion-limiting membrane to the movement of fluoride from the extracellular fluid compartment to the developing enamel.