Thesis (Master of Science)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution,2008. / "June 2008." Bibliography: p. 45-47.

The use of cheap, sparingly soluble calcium phosphate fertilizers is increasitgly widespread, particularly in the extensive agriculture systems of the tropics where very high yields are not sought, and phosphate deficiency is a major limitation to crop production. At present there is little quantitative understanding of the factors determining the rates of dissolution of calcium phosphates in soils. Existing quantitative treatments are inadequate, being either empirical or based on oversimplified theory. By developing a precise model of the dissolution process, it should be possible to short-cut the usual practice of running extensive field trials to establish the responses over a wide range of soil conditions and management practices. In this thesis a model which makes no arbitrary assumptions is developed for predicting the rates of dissolution of dicalcium phosphate dihydrate (DCPD) in soils. DCPD is the initial reaction product of the dissolution of many phosphatic fertilizers, and is an important fertilizer in its own right; the mechanisms governing its dissolution in soils are basically the same for other, more complex calcium phosphates. The simple case of a planar layer of DCPD in contact with soil is considered first to introduce the principles of the model. This is the simplest system for measuring experimentally the solute concentration profiles close to the dissolving surface, in order to test the model. The model is then extended to describe the dissolution of granules of DCPD in soil. The model comprises numerical solutions of mathematical equations describing the diffusion and reaction of calcium, phosphate and base in soil. The concentrations of calcium, phosphate and hydrogen ions in the soil solution at the mineral/soil boundary are found (a) from the ion activity product of DCPD and (b) by equating the fluxes of calcium, phosphate and base across the boundary (1 mol of DCPD gives 1 mol each of calcium, phosphate and base). In the granular system, the diminution of the granules as they dissolve, and the effect of neighbouring particles on each other are allowed for. The solute concentration profiles predicted for the planar system agreed with experimentally measured profiles; and the predicted net rates of dissolution of granules of DCPD agreed with the rates determined by a radioactive-tracer technique, in which ⁴⁵Ca dissolved from labelled DCPD is recovered from the soil with an extractant, saturated with respect to DCPD. Thus all important processes have been accounted for in the model. Since the theory is non-specific, the model should apply equally well to most other soils. The model has nine input parameters : the concentrations of calcium and phosphate in the native soil solution, the native soil pH, the phosphate and lime potential buffer capacities of the soil, the moisture status, the diffusion impedance factor, and the rate of application and particle size of the DCPD. A sensitivity analysis of the model showed that the rate is particularly dependent on particle size, rate of application, and the pH and concentration of calcium in the soil solution. If the granules are so stages the rate of dissolution is independent of the soil buffer terms. But for typical rates and methods of application, neighbouring granules will influence each other, and the consequent interactions between the rate determining variables are complex. The extension of the model to describe the dissolution of carbonateapatites, and hence rock phosphates, is discussed.

This dissertation is comprised of three independent but interconnected studies with the scope of further understanding uranium and iodine partitioning between apatite and fluid. The studies herein presented investigated: 1) brushite to apatite crystallization method; 2) the degree of uranium incorporation into apatite; 3) the degree of iodine incorporation into apatite. The importance of this work is assessing the role of apatite in immobilizing these elements, where uranium is a major component of spent nuclear fuel and iodine is a chemical analog of its radioactive isotope (129I). Once we understand the incorporation mechanisms, we will provide data that can be used in development of engineering barrier systems via add-on of phosphate minerals. In the first chapter we evaluate a method for the crystallization of apatite (Ca10(PO4)6(OH, F, Cl)2) using brushite (CaHPO4·2H2O) as initial material. The solutions evaluated for this transformation were NaCl, NaF, and KOH. Result yielded 100% apatite transformation from brushite when pH is 3.5 or greater. At a pH lower than 3.5, transformation yields monetite-apatite mixtures. Crystal size is reduced during the transformation from ~10 micrometer to ~1 micrometer. In the second chapter, degree of uranium uptake by apatite was evaluated. Phosphate minerals were crystallized from U-bearing NaCl solutions at 25-350°C. After experimental runs uranium concentrations in experimental solids and fluids were analyzed using Inductively Coupled Plasma – Mass Spectrometry. Additionally, characterization of the solids was performed via X-Ray Diffraction, Scanning Electron Microscopy and Electron Microprobe to confirm the brushite to apatite conversion. Results show that >90% of uranium was extracted from solution. Moreover, that the partitioning of uranium between apatite and fluid decreases with increasing temperature. In the third chapter apatite was crystallized from iodine-bearing solutions. The crystallization was evaluated at 39 and 200°C. Iodine concentration in solids were acquired via Electron Microprobe Analysis (EMPA) whereas iodine concentration in fluids were acquired via UV-Visible Spectrophotometry. Iodine concentrations in fluids yielded minimum depletion (0.1M) from initial iodine added to the system (0.1M). Partitioning data suggest that iodate (the oxidized form of iodine) is more compatible with apatite compared to iodide (the reduced state of iodine).

Phosphorus (P) is a requirement for biological growth, but this vital nutrient is present at low or limiting concentrations across vast areas of the global surface ocean. Inorganic polyphosphate (poly-P), a linear polymer of at least three orthophosphate units, is one component of the marine P cycle that has been relatively overlooked as compared to other P species, owing in part to a lack of routine analytical techniques that cleanly evaluate it within samples. This thesis demonstrates that inorganic poly-P is a quantitatively significant and dynamic component of the global marine P cycle while also establishing two new techniques for its analysis in biological and environmental samples. In Chapter 2, experiments using the freshwater algae Chlamydomonas sp. and Chlorella sp. illustrate X-ray fluorescence spectromicroscopy as a powerful tool for the sub-micron scale assessment of poly-P composition in organisms. This method enabled the discovery, detailed in Chapter 3, of a mechanism for the long-term sequestration of the vital nutrient P from marine systems via the initial formation of poly-P in surface waters and its eventual transformation into the mineral apatite within sediments. The importance of marine poly-P is furthermore established in Chapter 3 by observations showing that naturally-occurring poly-P represents 7-11% of total P in particles and dissolved matter in Effingham Inlet, a eutrophic fjord located on Vancouver Island, British Columbia. In Chapter 4, a new fluorometric protocol based on the interaction of inorganic poly-P with 4',6-diamidino-2-phenylindole (DAPI) is established as a technique for the direct quantification of poly-P in environmental samples. Chapter 5 presents work from Effingham Inlet utilizing this method that show that inorganic poly-P plays a significant role in the redox-sensitive cycling of P in natural systems.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008. / Includes bibliographical references (p. 45-47). / Biogeochemical cycling of phosphate is a key component in the overall production rate of coastal ecosystems. Mineral phases in the near-shore sediments play a significant role in the return of phosphate remineralized in the upper sediments to the water column. Sequential Extraction (SEDEX) of the solid-phase associated P04-3 yielded reservoir profiles of phosphate at three sites off of the Massachusetts coast. These extractions found Fe-associated P04 to be the dominant phase associated with rapid porewater-solid P exchange. Additionally, a seasonal enrichment/depletion pattern of phosphate fluxes relative to total carbon was observed from the sediments. These observations established the behavior of phosphate in coastal sediments as interconnected with the ongoing Fe-cycling in the sediments as well. / by Richard Hunter Oates, Jr. / S.M.

Formamide is a solvent of great interest to prebiotic chemists because it is liquid over a wide range, it is less volatile than either water or HCN, and it possesses a versatile reactivity. When formamide is heated in the presence of minerals or inorganic catalysts, a variety of products including purine nucleobases are generated. Irradiation of formamide reaction solutions with ultraviolet light increases the yield and diversity of products, and eliminates the need for a mineral catalyst. We have also performed formamide reactions in the presence of pyrite, a mineral which is likely to have been available on the primordial Earth, under a variety of atmospheric conditions. Our results indicate the greatest yield and diversity of products result from the combination of a pyrite mineral catalyst, heat, UV irradiation, and a carbon dioxide atmosphere. Purine nucleobases are simple to synthesize in model reactions and they stack well in aqueous solution; it has been hypothesized that the first nucleic acids were composed of only purine bases, and that water-soluble, cationic, aromatic molecules with large stacking surfaces (“”molecular midwives””) may have aided the assembly of the earliest nucleic acid analogs. We have characterized the interactions of various intercalators with a standard DNA duplex as well as with an antiparallel homopurine DNA duplex and have determined that molecules which possess four or more rings and a curved shape interact selectively with all-purine DNA; such molecules can serve as models for putative prebiotic midwives.