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The Solubility and Thermodynamic Properties of Ettringite, Its Chromium Analogs, and Calcium Aluminum Monochromate

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The Solubility and Thermodynamic Properties of Ettringite, Its Chromium Analogs, and Calcium Aluminum Monochromate THE SOLUBILITY AND THERMODYNAMIC PROPERTIES OF ETTRINGITE, ITS CHROMIUM ANALOGS, AND CALCIUM ALUMINUM MONOCHROMATE (3CaO·Al2O3·CaCrO4·nH2O) by ROBERT BENJAMIN PERKINS A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in ENVIRONMENTAL SCIENCES AND RESOURCES: GEOLOGY Portland State University 2000 DISSERTATION APPROVAL The abstract and dissertation of Robert Benjamin Perkins for the Doctor of Philosophy in Environmental Sciences and Resources: Geology were presented May 5, 2000 and accepted by the dissertation committee and the doctoral program. COMMITTEE APPROVALS: _______________________________________ Michael Cummings, Chair _______________________________________ Carl Palmer _______________________________________ William Fish _______________________________________ Georg Grathoff _______________________________________ Jie Lin Representative of the Office of Graduate Studies DOCTORAL PROGRAM APPROVAL: ____________________________________ Roy Koch, Director Environmental Sciences and Resources Ph.D. Program i ABSTRACT An abstract of the dissertation of Robert Benjamin Perkins for the Doctor of Philosophy in Environmental Sciences and Resources: Geology presented May 5, 2000. Title: The Solubility and Thermodynamic Properties of Ettringite, Its Chromium Analogs, and Calcium Aluminum Monochromate (3CaO·Al2O3·CaCrO4·nH2O). Ettringite (Ca6[(Al(OH)6)]2(SO4)326H2O) is a naturally-occurring mineral and an important product of the hydration of Portland cements and fly ash. Substitution of Cr(III) for Al(III) and CrO4 for SO4 in the ettringite structure has been demonstrated and may be important with respect to the fate and transport of chromium in alkaline environments. Ettringite and its chromium analogs were synthesized and their solubilities measured via dissolution and precipitation experiments over a range of temperatures between 5 and 75ºC and pH values between 10 and 13. The log KSP,298 for the reaction 2+ - 2- - Ca6[Al(OH)6]2(SO4)3·26H2O = 6Ca + 2Al(OH)4 + 3SO4 + 4OH + 26H2O is –44.9 ±0.3.The enthalpy and entropy of reaction are 205 ±12 kJ mol-1 and 170 ±38 J mol-1 K-1, respectively. The free energy, enthalpy, and entropy of formation are –15211 ±20, –17550 ±16 kJ mol-1, and 1870±59 J mol-1 K-1, respectively, i based on the reaction parameters and partial molar properties for the dissolved ions. The log KSP, free energy, and enthalpy of reaction for the Cr(III) ettringite analog, bentorite, are KSP,298 = -52.9 ±0.8, ΔG°r,298 = 302 ±10 and ΔH°r = 320 ±76 kJ mol-1 based on an equivalent dissolution reaction. Dissolution of bentorite is incongruent due to precipitation of another phase tentatively identified as Ca2Cr2O5·8H2O. The log KSP, free energy and enthalpy of reaction for the dissolution of Ca6[(Al(OH)6)]2(CrO4)326H2O are KSP,298 = –41.46 ±0.30, ΔG°r,298 = 236.6 ±3.9 and -1 ΔH°r = 77.5 ±9.6 kJ mol . The log KSP varies with pH unless a CaCrO4(aq) complex is included in the speciation model. A log formation constant of K = 2.77 ±0.16 was 2+ 2- obtained for the reaction Ca + CrO4 = CaCrO4(aq) by minimizing the variance of the IAP for Ca6[(Al(OH)6)]2(CrO4)326H2O. A secondary precipitate, identified as calcium aluminum monochromate or (3CaO·Al2O3·CaCrO4·nH2O) was present in Cr(VI)-analog experimental residues. The log KSP for the reaction 2+ - 2- - 3CaO·Al2O3·CaCrO4·15H2O = 4Ca + 2Al(OH)4 + CrO4 + 4OH + 9H2O is –30.38 ±0.28. Ca6[(Al(OH)6)]2(SO4(1-x) , CrO4x)326H2O solids were synthesized and solid- solution aqueous-solution interactions were investigated through dissolution pathway studies. Although thermodynamic equilibrium was not achieved, dissolution pathways approximated stoichiometric saturation curves when plotted on Lippman diagrams. ii Acknowledgments I would first and foremost like to thank Dr. Carl Palmer for providing me with the opportunity to pursue this study and for showing me that, while the devil is in the details, good questions and further opportunities often reside there too. My completion of this study was greatly helped by Carl's boundless enthusiasm for research and patience with those of us new to the world of geochemistry. I would also like to express my sincere gratitude to Dr. Michael Cummings for his shrewd and practical advice and for his steady push to keep me going. I would certainly like to thank the other members of my committee; Dr. Bill Fish and Dr. Jie Lin for their constructive advice and interesting insights; and last, but not least, Dr. Georg Grathoff for his thorough review of this work and his constant encouragement. A project such as this requires a lot of help, which I found readily offered by many people here at Portland State. In particular, I would like to acknowledge Dr. Dennis Barnum for the generous use of his lab; the microbiologists on the 5th floor of SB-I for graciously allowing me use of their facilities (often unannounced and unexpected); Dr. Mary Taylor as well as the folks in the limnology lab for letting me fill my carboy when our water filter went out; Inge Wortman and Brent Schauer in Multimedia Production Services for making me look good (at the last second!) on several occasions; and Stuart Cowburn for making the basement geochemistry lab a more lively place to work. I would also like to say thanks to Dr. Alan Yeakley for his advice and encouragement. i Of course, none of this would have been possible without financial support. I would like to acknowledge the U.S. Environmental Protection Agency for initial funding, the Department of Geology and the Environmental Sciences and Resources Program, and the folks in the Graduate Studies Office and the members of the University Club for providing me sufficient freedom so that I could complete the bulk of this work. My experience here at Portland State University has been greatly enhanced by the many friends I have made - in and out of the Geology Department. I've not the room to list every individual, but thanks to you all! I must, however, say a word of thanks to Chris Hyatt for his good-humored grieve giving and for putting up with the clutter I kept during our Hawthorne day and a very special thanks to Dr. Ayna Alfadhli for her moral support and encouragement. Most of all, I thank my family, and especially my parents, Fyrl and Clayton Perkins, for their unending and unconditional love and support. ii TABLE OF CONTENTS Acknowledgments .......................................................................................................... i Table of Contents ......................................................................................................... iii List of Tables ............................................................................................................... vii List of Figures .............................................................................................................. ix Chapter 1. General Introduction ................................................................................ 1 1.1 BACKGROUND .......................................................................................... 1 1.2 OVERVIEW ................................................................................................. 2 Chapter 2. Solubility of Ettringite (Ca6[Al(OH)6]2(SO4)3·26H2O) At 5 - 75° C ...... 6 2.1 INTRODUCTION ........................................................................................ 6 2.2 PREVIOUS STUDIES ................................................................................. 7 2.3 METHODS ................................................................................................. 11 2.3.1. Synthesis of Ettringite .................................................................... 11 2.3.2. Characterization of Synthetic Ettringite ........................................ 12 2.3.3. Dissolution and Precipitation Experiments ................................... 13 2.4 EXPERIMENTAL RESULTS ................................................................... 15 2.4.1. Solid Characterization ................................................................... 15 2.4.2. Dissolution and Precipitation Experiments ................................... 23 2.5 DISCUSSION ............................................................................................. 27 iii 2.6 SUMMARY ............................................................................................... 39 Chapter 3. Solubility of Bentorite (Ca6[Cr(OH)6]2(SO4)3·26H2O) ......................... 41 3.1 INTRODUCTION ...................................................................................... 41 3.2 METHODS ................................................................................................. 43 3.2.1. Synthesis of Bentorite .................................................................... 43 3.2.2. Characterization of Synthetic Bentorite ........................................ 44 3.2.3. Dissolution and Precipitation Experiments ................................... 45 3.3 EXPERIMENTAL RESULTS ................................................................... 47 3.3.1. Solid Characterization ................................................................... 47 3.3.2. Dissolution and Precipitation Experiments ................................... 54 3.4 DISCUSSION ............................................................................................. 57 3.5 SUMMARY ............................................................................................... 74 Chapter 4. Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O),
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