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Solubility of Hydroxylapatite and Some Phosphate Rocks Donald Raymond Wier Iowa State University

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Solubility of Hydroxylapatite and Some Phosphate Rocks Donald Raymond Wier Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1968 Solubility of hydroxylapatite and some phosphate rocks Donald Raymond Wier Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agriculture Commons, and the Soil Science Commons Recommended Citation Wier, Donald Raymond, "Solubility of hydroxylapatite and some phosphate rocks " (1968). Retrospective Theses and Dissertations. 3522. https://lib.dr.iastate.edu/rtd/3522 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. This dissertation has been microfilmed exactly as received 69-4289 \VIER, Donald Raymond, 1940- SOLUBILiTY OF HYDROXYLAPATITE AND SOME PHOSPHATE ROCKS. Iowa State University, Ph.D., 1968 Agriculture, soil science University Microfilms, Inc., Ann Arbor, Michigan SOLUBILITY OF HYDROXYLAPATITE AÎID SOME PHOSPHATE ROCKS by Donald Raymond Wier A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Major Subject: Soil Chemistry Approved : Signature was redacted for privacy. In "Charge of Major Work Signature was redacted for privacy. Signature was redacted for privacy. Iowa State University of Science and Technology Ames, Iowa 1968 ii TABLE OF CONTENTS Page INTRODUCTION 1 PART I. SOLUBILITY OF HYDROXYLAPATITE 3a REVIEW OF LITERATURE 3b MATERIALS AND METHODS 14 EXPERIMENTS 24 SUMMARY 80 PART II. SOLUBILITY OF SOME PHOSPHATE ROCKS 8.3a REVIEW OF LITERATURE 83b INITIAL THEORY 93 MATERIALS AND METHODS 108 EXPERIMENTS 115 SUMMARY 167 LITERATURE CITED 173 ACKNOWLEDGMENTS 179 1 INTRODUCTION Van Wazer (1958) stated that the solubility of substances exhibiting the hydroxylapatite X-ray diffraction pattern has been the subject of a large amount of study because this information is thought to be needed to understand the physiology of bone growth and the equilibrium of bones with body fluids. He also stated that it is of considerable interest to the field of fertilizer technology and to the general problems of the interaction of calcium and phosphorus in soils. Sauchelli (1965) stated that the apatites are by far the most important phosphorus-bearing minerals. According to Sauchelli, originally all the phosphorus in the lithosphere must have been present in igneous rocks in the form of apatite. He stated that nearly all the phosphorus in sedimentary rocks is present in the form of apatites, and this mineral type makes up the larger part of the world's sedimentary deposits considered as phosphatic reserves. Apatite has been identified as a native mineral in the sand and silt fraction of a number of soils (Black, 1968). In addition, considerable apatite is applied to soils as a source of phosphorus for plants. In 1961-62 the total tonnage of phosphate rock applied direct to soil as a fertilizer was about 10 percent of the total world phosphate consumption (Sauchelli, 1965). Chaverri (1962) investigated the dissolution of a phosphate rock in relation to the solubility product principle with the ultimate goal of obtaining a general solubility relationship that could be used in evaluating the fertilizer effectiveness of the phosphate rock. Chaverri 2 stated that the solubility of the phosphorus-bearing constituent of the phosphate rock examined in his work could be explained on the basis of the theory of the"" solubility product of a solid solution of hydroxylapa- tite and fluorapatite after introducing the postulate that interaction of the solution with the solid may change the proportion of the two constituents in the solid solution. The author developed the theory of solubility of hydroxylapatite and fluorapatite beyond the stage to which it was carried by Chaverri and, as a result, obtained rzasons for questioning Chaverri's interpre­ tation of the results of his experiments. This theory is given in Part II. Furthermore, in reviewing literature, it was found that there was much controversy about the solubility of hydroxylapatite. One researcher felt that hydroxylapatite possessed a solubility product constant (Clark, 1955). Others felt that it did not possess a solubility product constant (Levinskas and Neuman, 1955), and still others felt that the solubility of hydroxylapatite was controlled by a complex that formed on the surface of the solid (Rootare et al., 1962; LaMer, 1962). After considering the importance of apatites in nature, and after considering the indefinite results obtained in previous solubility studies on hydroxylapatite and a phosphate rock, it was decided to study further the solubility of hydroxylapatite and phosphate rocks in relation to the solubility product principle. The over-all aim of the investigation was to obtain information that would aid in the understanding of the behavior of applied and native apatites in soils. It was felt that information might also be obtained that would be of interest to scientists working in other fields in which apatites are important. 3a PART I. SOLUBILITY OF HYDROXYLAPATITE 3b REVIEW OF LITERATURE Nature of Hydroxylapatite According to the International Union of Pure and Applied Chemistry, the complete chemical name for the salt Ca^(P0^)2(0H) is pentacalcium- hydroxide-tris-phosphate. The formula for the compound is also commonly written as Ca^^(P0^)^(0H)2, and the compound is called hydroxy&patite, hydroxylapatite, or hydroxide apatite (Sauchelli, 1965), According to Lehr et al. (1967a), hydroxylapatite crystallizes in O O the hexagonal system and has lattice constants of 9.415 A. and 6.878 A. Beevers and Mclntyre (1946) and Young and Elliot (1966) described the arrangement of atoms in a unit cell. Lehr et al. (1967a) published data on optical properties. X-ray diffraction, infrared absorption, and ocher properties of hydroxylapatite and also described a method of preparation and discussed the occurrence of this compound. Hill et al. (1954) found that the specific surface area of basic calcium phosphates depends on the method of preparation and tends to decrease with an increase in hydrogen-ion activity and temperature during formation. They obtained specific surface areas ranging from 4.5 to 142 2/ m. /g. Perloff and Posner (1960) described a method by which hydroxylapatite is prepared by adding 0.1 g. of calcium monohydrogen phosphate and 12 ml. of water to a platinum-lined hydrothermal bomb and heating at 300° C. for several days. According to Perloff and Posner, this procedure produces a very well-crystallized compound which has a high degree of purity. They 4 stated that all other methods previously reported produce either poorly crystallized or somewhat impure products. Brown (1964) stated that pure hydroxylapatite is extremely difficult to prepare. The crystals may look homogeneous under a microscope but, when they are immersed in a liquid with the appropriate index of refraction, one discovers that the index of refraction of the center of the hydroxylapatite particles is less than that of the outer part. Substitution of hydrogen and hydroxyl ions into abnormal positions in the hydroxylapatite structure has been suggested to occur. Posner et al. (1954), Posner and Perloff (1957), and Posner et al. (1960) have argued that a general formula for hydroxylapatite is '-^(]_o-x)^2x^^^4^6^^^^2 where M is sodium or hydrogen and the upper limit of x is probably 2. Winand et al. (1961) claimed to have established that the general formula of the apatitic calcium phosphate series is Ca^Q_^^H^(PO^)g(OH) where the value of x varies from 0 to 2. McConnell (1965) stated that "tetra- hedral hydroxyIs (H^O^)" substitute for phosphate groups. McConnell also claimed that the sites of the apatite structure which normally contain the fluoride or hydroxide ions may contain a water molecule as a con­ comitant effect of the substitution of (H^O^) for phosphate. A discussion of substitutions in naturally occurring apatites is given in Part II of this thesis. Substitution of hydrogen and hydroxide ions into abnormal positions in the hydroxylapatite structure could affect the solubility of hydroxyla­ patite in aqueous solutions. Such substitutions would cause the activity of hydroxylapatite and, consequently, the ion product in solution, to be a function of pH. 5 Mattson et al. (1951) investigated the electrophoretic behavior of a calcium phosphate compound that had a composition corresponding to the formula Caj.(PO.)- oo(OH)i oc- The charge on the material was found to be a function of the pH, and the isoelectric point was at pH 7.61. Above this pH the material had a negative charge and below this pH it had a positive charge. The pH at the isoelectric point was found to change with additions of either dibasic sodium phosphate or calcium hydroxide. After 2 milliequivalents of calcium hydroxidr. had been added to a quantity of the compound the isoelectric point was at a pH of approximately 10, and after 3 milliequivalents of dibasic sodium phosphate had been added to an equal quantity of the compound the isoelectric point was at a pH of approximately 6.5. Leach (1960) investigated the electrophoretic behavior of hydroxyla- patite in solutions buffered with 0.02 M diethylbarbituric acid and adjusted to pH 7.40 with potassium hydroxide and to a final ionic strength of 0.165 with potassium chloride. In the first experiment the solid to solution ratio was varied from 0.05 g./l. tc 40 g./l., and the correspond­ ing zeta potentials ranged from -16.6 to -1.4 millivolts. In the second experiment addition of calcium as calcium chloride increased the zeta potential in the positive direction, and addition of phosphate as potassium dihydrogen and monohydrogen phosphate increased the zeta poten­ tial in the negative direction.
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