Fluoride cycle

The harmful effects of air pollutants on human beings have been the major reason for efforts to understand and control their sources. During the past two decades, research on acidic deposition on water-based ecosystems has helped to reemphasize the importance of air pollutants in other receptors, such as soil-based ecosystems (1). When discussing the impact of air pollutants on ecosystems, the matter of scale becomes important. We will discuss three examples of elements which interact with air, water, and soil media on different geographic scales. These are the carbon cycle on a global scale, the sulfur cycle on a regional scale, and the fluoride cycle on a local scale. 

Morshina, T.N., Fluoride cycles in an estuarine ecosystem, Total Environ., 17, 223, 1980. 

The lanthanum fluoride cycle in which hexone extraction along with LaF3 as a carrier precipitate are used. 

Tampa Bay, Fla., may well represent another area that is ideal for stud)dng the impact of man on the fluoride cycle, particularly in an estuarine environment. Indeed, we believe that the present study provides... 

Lanthanum fluoride cycle. This classical procedure was first developed by McMillan and Abelson for the isolation of neptunium, but it is applicable elsewhere and is of great utility. For the U-Pu separation again, we have Scheme 28-3. The cycle shown is repeated with progressively smaller amounts of lanthanum carrier and smaller volumes of solution until plutonium... 

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

Fluorides. Uranium fluorides play an important role in the nuclear fuel cycle as well as in the production of uranium metal. The dark purple UF [13775-06-9] has been prepared by two different methods neither of which neither have been improved. The first involves a direct reaction of UF [10049-14-6] and uranium metal under elevated temperatures, while the second consists of the reduction of UF [10049-14-6] by UH [13598-56-6]. The local coordination environment of uranium in the trifluoride is pentacapped trigonal prismatic with an 11-coordinate uranium atom. The trifluoride is... 

Plot the observed e.m.f. values against the concentrations of the standard solutions, using a semi-log graph paper which covers four cycles (i.e. spans four decades on the log scale) use the log axis for the concentrations, which should be in terms of fluoride ion concentration. A straight line plot (calibration curve) will be obtained. With increasing dilution of the solutions there tends to be a departure from the straight line with the electrode combination and measuring system referred to above, this becomes apparent when the fluoride ion concentration is reduced to ca 0.2 mg L-1. 

We solved the first problem by bombarding large amounts of uranyl nitrate with neutrons at the cyclotrons at the University of California and Washington University plutonium concentrates were derived from these sources through the efforts of teams of chemists who used ether extractions to separate the bulk of the uranium and an oxidation-reduction cycle with rare earth fluoride carrier to concentrate the product. I managed to convince chemists trained in the techniques of ultramicrochemistry to join us to solve the second problem—Burris B. Cunningham and Louis B. Werner of the University of California and Michael Cefola from New York University. 

The reason for the ultramicrochemical test was to establish whether the bismuth phosphate would carry the plutonium at the concentrations that would exist at the Hanford extraction plant. This test was necessary because it did not seem logical that tripositive bismuth should be so efficient in carrying tetrapositive plutonium. In subsequent months there was much skepticism on this point and the ultramicrochemists were forced to make repeated tests to prove this point. Thompson soon showed that Pu(Vl) was not carried by bismuth phosphate, thus establishing that an oxidation-reduction cycle would be feasible. All the various parts of the bismuth-phosphate oxidation-reduction procedure, bulk reduction via cross-over to a rare earth fluoride oxidation-reduction step and final isolation by precipitation of plutonium (IV) peroxide were tested at the Hanford concentrations of... 

There are several examples in which metabolites that toxify the organism responsible for their synthesis are produced. The classic example is fluoroacetate (Peters 1952), which enters the TCA cycle and is thereby converted into fluorocitrate. This effectively inhibits aconitase—the enzyme involved in the next metabolic step—so that cell metabolism itself is inhibited with the resulting death of the cell. Walsh (1982) has extensively reinvestigated the problan and revealed both the complexity of the mechanism of inhibition and the stereospecihcity of the formation of fluorocitrate from fluoroacetate (p. 239). It should be noted, however, that bacteria able to degrade fluoroacetate to fluoride exist so that some organisms have developed the capability for overcoming this toxicity (Meyer et al. 1990). 

In the fifth paper of this chapter on cathodes, an investigation of thin-film oxide-hydroxide electrodes containing Cr, Ni, and Co compounds was authored by N. Vlasenko et al. The thin-films were produced by electrochemical deposition from transition metal aqueous fluorine-containing electrolytes onto steel substrates. These thin-films were tested in Li coin cells. Electrochemical activity appears to scale with the amount of fluoride used in the deposition the larger concentration of fluoride in the bath, the greater the capacity. One Ni oxide-hydroxide film electrode showed greater than 175 mAh/g reversible capacity on the 50th cycle with excellent coulombic efficiency. 

Following the reactions in Scheme 4, the olefin 13 would have to be generated before the formation of the 7r-crotyl species 2. Because the amount of the catalyst present is usually small as compared to the amount of product formed (1000 moles of product/mole catalyst), the quantity of 13 produced will not be significant and should be readily separable from the desired product. In the special case when crotyl chloride is used as the activator, hexadiene can be produced during the very first cycle of reaction, i.e., 13 = 1,4-hexadiene. Among the organic halides, the chloride derivatives are the most effective activators. Bromides are somewhat effective, while fluorides and iodide are rather ineffective (77). 

Catalytic sol-gel lipase immobilizates were rapidly commercialized (by Fluka) after their invention in 1995 because of their remarkably stable activity in esterification reactions (and also in the kinetic resolution of chiral alcohols and amines) along with unique stability (residual activity of 70% even after 20 reaction cycles is common). The original procedure for the encapsulation produced by the fluoride-catalysed hydrolysis of mixtures of RSi(OCH3)3 and Si(OCH3)4 has been improved... 

The ionic model, developed by Bom, Lande, and Lennard-Jones, enables lattice energies (U) to be summed from inverse square law interactions between spherically symmetrical charge distributions and interactions following higher inverse power laws. Formation enthalpies are related to calculated lattice energies in the familiar Bom-Haber cycle. For an alkali fluoride... 

Extension of the method to nonisostructural metal halides, some of which yield erroneous AHf values via Bom-Haber cycles, is shown in Fig. 1. All curves are nonlinear with the bow increasing in the expected order T1(I) < Pb(II) < Bi(III) < Ag(I). For the first transition metal dihalides, however, straight lines can be drawn within the limits of enthalpy errors except for Zn(II) or Mn(II) salts. Thus heats of formation of the fluorides can be extrapolated linearly from the other three halides to a first approximation. 

The next term in Equation (5) is the halide affinity of the metal halide, Ax (MtXJ. This is a very important term for the final value of AH4e and a term which, were it generally available, would facilitate worthwhile conclusions as to which metal halide would be the best anionogen. However, only two halide affinities for the metals of interest are known to us, viz. the fluoride affinities of BF3 and WF5 [26, 27]. In Scheme 3 the role of the halide affinity in the Born-Haber cycle for the BIE [Equation (iv)] is emphasised and an alternative route for the same energy contribution to AH4B is given. The individual BDE D(XBMt-X) is, like the halide affinity, known only for a very few examples of interest,... 

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