MACROSCOPIC AND SPECTROSCOPIC INVESTIGATION OF INTERACTIONS OF ARSENIC WITH SYNTHESIZED PYRITE A Dissertation by EUN JUNG KIM Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2008 Major Subject: Civil Engineering MACROSCOPIC AND SPECTROSCOPIC INVESTIGATION OF INTERACTIONS OF ARSENIC WITH SYNTHESIZED PYRITE A Dissertation by EUN JUNG KIM Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Bill Batchelor Committee Members, Robin Autenrieth Richard H. Loeppert Kung-Hui Chu Head of Department, David V. Rosowsky December 2008 Major Subject: Civil Engineering iii ABSTRACT Macroscopic and Spectroscopic Investigation of Interactions of Arsenic with Synthesized Pyrite. (December 2008) Eun Jung Kim, B.S. University of Seoul; M.S., Pohang University of Science and Technology Chair of Advisory Committee: Dr. Bill Batchelor Sulfide minerals have been suggested to play an important role in regulating dissolved metal concentrations in anoxic environments. Pyrite is the most common sulfide mineral and it has shown an affinity for arsenic, but little is known about the arsenic retention mechanisms of pyrite. In this study, interactions of arsenic with pyrite were investigated in an anoxic environment to understand geochemical cycling of arsenic better and to predict arsenic fate and transport in the environment better. A procedure using microwaves was studied to develop a fast and reliable method for synthesizing pyrite. Arsenic-pyrite interactions were investigated using macroscopic (solution phase experiments) and microscopic (X-ray photoelectron spectroscopic investigation) approaches. Pyrite was successfully synthesized within a few minutes via reaction of ferric iron and hydrogen sulfide under the influence of irradiation by a conventional microwave oven. The SEM-EDX study revealed that the nucleation and growth of pyrite occurred on the surface of elemental sulfur, where polysulfides are available. Compared iv to conventional heating, microwave energy results in rapid (< 1 minute) formation of smaller particulates of pyrite. Higher levels of microwave power can form pyrite even faster, but faster reaction can lead to the formation of pyrite with defects. Arsenic removal by pyrite was strongly dependent on pH and arsenic species. Both arsenite (As(III)) and arsenate (As(V)) had a strong affinity for the pyrite surface under acidic conditions, but As(III) was removed more effectively than As(V). Under acidic conditions, arsenic removal continued to occur almost linearly with time until complete removal was achieved. However, under neutral to alkaline conditions, fast removal was followed by slow removal and complete removal was not achieved in our experimental conditions. A BET isotherm equation provided the best fit to arsenic removal data, suggesting that surface precipitation occurred at high arsenic/pyrite ratio. The addition of competing ions did not substantially affect the ultimate distribution of arsenic between the pyrite surface and the solution, but changing pH affected arsenic stability on pyrite. Xray photoelectron spectroscopy revealed that under acidic conditions, arsenic was removed and formed solid phases similar to As2S3 and As4S4 by reaction with pyrite. However, under neutral to alkaline conditions, arsenic was removed and formed As(III)-O and As(V)-O surface complexes, as well as As2S3/As4S4-like precipitates. As pH increases, the amount of arsenic that formed As2S3/As4S4-like precipitates decreased, while the amount that formed As(III)-O and As(V)-O surface complexes increased. Under alkaline conditions, a FeAsS-like phase was also detected. v ACKNOWLEDGEMENTS I am grateful to all those who have assisted me to finish this dissertation. Especially, I would like to thank my advisor, Dr. Bill Batchelor, for his guidance, support, and encouragement throughout the course of this research. I would also like to thank my committee members, Dr. Robin Autenrieth, Dr. Richard H. Loeppert, and Dr. Kung-Hui Chu, for their helpful discussion and advices. I would like to acknowledge the support and friendship of all my friends and colleagues. Their assistance and advice were invaluable resources to my research, and they made my time at Texas A&M University a great experience. Finally, special thanks to my family for their encouragement and support through my entire life and to my husband and best friend, Yoon E, for his support and love. vi TABLE OF CONTENTS Page ABSTRACT .............................................................................................................. iii ACKNOWLEDGEMENTS ....................................................................................... v TABLE OF CONTENTS .......................................................................................... vi LIST OF FIGURES ................................................................................................... viii LIST OF TABLES ..................................................................................................... xii CHAPTER I INTRODUCTION ................................................................................ 1 II MICROWAVE SYNTHESIS OF PYRITE AND CHARACTERIZATION ...................................................................... 5 2.1 Introduction .............................................................................. 5 2.2 Experimental Section ................................................................ 8 2.2.1 Pyrite Synthesis ........................................................... 8 2.2.2 Quantification of Synthesized Pyrite ........................... 9 2.2.3 Solid Characterization ................................................. 9 2.3 Results and Discussion ............................................................. 11 2.3.1 Pyrite Formation by Microwave Irradiation and Characterization ........................................................... 11 2.3.2 Mechanism of Pyrite Formation by the Reaction between Ferric Iron and Sulfide .................................. 17 2.3.3 Effect of Percent Time of Microwave Irradiation ....... 19 2.3.4 Effect of Reagent Concentration ................................. 23 III ARSENIC REMOVAL BY SYNTHESIZED PYRITE ....................... 25 3.1 Introduction .............................................................................. 25 3.2 Experimental Section ................................................................ 28 3.2.1 Materials ...................................................................... 28 3.2.2 Removal Experiments .................................................. 29 3.2.3 Xray Photoelectron Spectroscopy .............................. 31 vii CHAPTER Page 3.3 Results and Discussion ............................................................. 33 3.3.1 Removal Kinetics ........................................................ 33 3.3.1.1 Effect of pH ................................................. 33 3.3.1.2 Effect of As(III) Initial Concentrations ....... 41 3.3.1.3 Effect of Pyrite Dose ................................... 45 3.3.1.4 Effect of Sulfide ........................................... 46 3.3.2 Arsenic Removal Characteristics ................................. 48 3.3.2.1 Effect of pH on Extent of Removal ............. 48 3.3.2.2 Effect of Arsenic Concentration .................. 52 3.3.2.3 Effect of Competing Anions ........................ 58 3.3.2.4 Stability of Arsenic on Pyrite ...................... 60 3.3.3 Xray Photoelectron Spectroscopy Investigation ........ 65 3.3.3.1 As(III) Reacted Pyrite .................................. 65 3.3.3.2 Release Experiments .................................... 74 IV X-RAY PHOTOELECTRON SPECTROSCOPIC INVESTIGATION OF PYRITE AFTER REACTION WITH ARSENIC AS A FUNCTION OF pH .................................................. 81 4.1 Introduction .............................................................................. 81 4.2 Experimental Section ................................................................ 83 4.2.1 Materials ..................................................................... 83 4.2.2 Removal Experiments ................................................. 84 4.2.3 Xray Photoelectron Spectroscopy ............................. 85 4.3 Results and Discussion ............................................................. 87 4.3.1 Surface Characterization of Unreacted Pyrite ............ 87 4.3.2 Surface of Pyrite after Reaction with Arsenic at pH 4 ........................................................................ 98 4.3.3 Surface of Pyrite after Reaction with Arsenic at pH 7 ........................................................................ 106 4.3.4 Surface of Pyrite after Reaction with Arsenic at pH 10 ...................................................................... 114 4.3.5 Summary ..................................................................... 122 V SUMMARY AND CONCLUSION ..................................................... 124 LITERATURE CITED .............................................................................................. 127 VITA .......................................................................................................................... 136 viii LIST OF FIGURES Page Figure 2.1 SEM images of particles formed after (a) 4 min, (b) 6 min, (c) 8 min, and