THE FIXATION OF ARSENIC WASTES A thesis submitted for the degree of Doctor of Philosophy By Michael Andrew Leist BSc (Hons) February 2001 School of Life Sciences & Technology Faculty of Engineering & Science Victoria University Footscray Park Campus 232li^^t FTS THESIS 628.42 LEI 30001007176953 Leist, Michael Andrew The fixation of arsenic wastes II DECLARATION The data presented in this thesis is original material. The major findings of this research may, however, be encountered in a number of articles recently submitted for publication. These publications include: • The Management of Arsenic Wastes: Problems and Prospects, by M. Leist, R.J. Casey and D. Caridi, Journal of Hazardous Materials, 2000, volume 76, pp. 125-138. • The Leaching of Cement Immobilized Arsenic, by M. Leist, R.J. Casey and D. Caridi, in press • An FTIR Investigation into the Solidification/Stabilization of Arsenic Wastes, by M. Leist, R.J. Casey and D. Caridi, in press • The Fixation and Leaching of Cement Stabilized Arsenic, by M. Leist, R.J. Casey and D. Caridi, in press Throughout this thesis, where appropriate, work of other authors has been appropriately cited. Ill ACKNOWLEDGEMENTS I wish to thank my principal supervisor Associate Professor John Casey for his support and assistance throughout the three year period of my candidature. I would also like to thank my co-supervisor, Dr. Domenico Caridi for his, advice, encouragement, and helpful oversight. I would also like to thank Mr. Geoff Glew and Mr. Jason Cran of Chemsal Pty Ltd., for supplying arsenic wastes throughout the research period. This is greatly appreciated. Finally I would like to thank my family and fellow students, who have helped make this period of my life, a most enjoyable time. IV ABSTRACT Arsenic has found widespread use as a component in a variety of formulations designed to control or eliminate a variety of insect and fungicidal pests. Arsenical wastes are also often produced during the extraction of metals such as copper, gold, nickel and tin. Consequently, there are large numbers of sites contaminated with toxic arsenic residues. The environmental treatment of arsenic is complicated by the fact that it has a variety of valence states. This, coupled with the plurality of regulatory leaching test variants used, has made it impossible to gauge which of a number of Solidification/Stabilization (S/S) processes are the most effective. Cement based Solidification/Stabilization technology currently provides the most promising solution for the disposal of arsenic wastes. This thesis has shown that Solidification/Stabilization is a technology capable of significantly reducing arsenic leachate concentrations to well below regulatory limits. This thesis reports studies carried out to evaluate the process of Solidification/Stabilization (S/S) for rendering "safe" a variety of arsenic compounds in both of the common oxidation states, +111 and +V. The compounds studied included: • Arsenic trioxide, • Arsenic pentoxide, • Sodium arsenite, • Sodium arsenate, • Lead arsenate. The three S/S formulations, all containing approximately 10% arsenic, investigated were: • Cement only, • Cement + ferrous sulfate, • Cement + lime. The stability of the S/S formulations was evaluated with both current regulatory leaching tests (i.e.. Toxicity Characteristic Leaching Procedure (TCLP) and the Australian Bottle Leaching Procedure), sequential leaching tests, and column leaching tests. Clear differences in the efficacy of the S/S formulations in immobilizing arsenic were observed. The results did not merely reflect differences in the solubilities of the arsenic compounds formed, but were greatly affected by the interaction (positive or negative) that the arsenic compounds and/or additives had upon the cementation reactions. Microstructural analysis, using both FTIR and SEM, revealed the greatest changes to the matrices of the [cement + iron] formulations. The lower pHs of these formulations increased the extent of silicate polymerization, which is known to be destructive to cement matrices. Ettringite, which increases porosity, was also identified in these matrices. These detrimental changes to the matrices, coupled with their lower calcium content, explain the generally poorer performance of these matrices. The leaching of calcium has been shown to influence the leaching of arsenic. Those formulations containing large calcium contents were shown to be generally the most successful. The [cement + iron] formulations were shown to leach arsenic levels far in excess of those leached by the formulations containing greater calcium contents ([cement only] and [cement + lime]). Besides identifying that calcium rich S/S formulations are generally the most effective, regardless of the arsenic oxidation state, this research has identified the following. 1. Even if arsenic is present in the same oxidation state, the success of a given S/S process can vary greatly between one arsenic compound and another. 2. The S/S of arsenical wastes was shown to be the most effective when arsenic was present as the arsenate species (+V). VI 3. S/S formulations that behave most favorably in one type of leaching test do not always behave as satisfactorily when subjected to a different form of leaching test. This thesis, while reinforcing the general notion that the current regulatory tests are very severe and consequently not useful as a guide to contaminant levels that may be leached once the treated waste is placed in a landfill, questions the ability of current regulatory tests to positively identify those S/S matrices that will, indeed, behave most favorably. However, the results of this thesis work do not provide sufficient data to recommend a more appropriate alternative regulatory test. VII TABLE OF CONTENTS PAGE DECLARATION II ACKNOWLEDGEMENTS Ill ABSTRACT IV LIST OF FIGURES XII LIST OF TABLES XXX CHAPTER 1 1 1.0 Introduction 1 1.1 Background 3 1.1.1 Lead arsenate 5 1.2 Arsenic economics 6 1.3 Arsenic chemistry 8 1.4 Toxicity of arsenic 10 1.5 Treatment and removal of arsenic from waste waters 13 1.6 Solidification/Stabilization 20 1.6.1 Cement processes 20 1.6.1.1 Portland Cement 21 1.6.1.2 Types of Portland Cement 22 1.6.1.3 Portland Cement hydration 23 1.6.1.4 Chemical factors affecting solidification 26 1.6.2 Leaching/Extraction tests 27 1.6.2.1 Landfill disposal 30 1.6.2.2 General leaching mechanisms 32 1.6.3 Solidification/Stabilization (S/S) processes 34 vni PAGE 1.7 Research Direction 42 CHAPTER 2 44 2.0 Experimental 44 2.1 Materials 44 2.2 Solidification/stabilization procedure 45 2.2.1 Sodium peroxide fusion 52 2.3 Leaching procedures 54 2.3.1 Batch leaching tests 54 2.3.1.1 Australian Bottle Leaching Procedure (ABLP) 54 2.3.1.2 Toxicity Characteristic Leaching Procedure (TCLP) 55 2.3.1.3 Sequential leach tests 57 2.3.1.4 Continual leach tests 57 2.3.2 Column leaching 57 2.3.2.1 Column leaching (BLC) tests 59 2.3.2.2 Column leaching (rainfall) tests 60 2.4 Analysis of leachates 60 2.4.1 Vapor generation 62 2.4.1.1 Potassium Iodide/Ascorbic acid pretreatment 64 2.4.2 Inductively coupled plasma (ICP) 64 2.4.3 Interferences 65 2.5 Electrode measurements 69 2.5.1 pH 69 2.5.2 Redox potential 70 2.5.3 Conductivity 70 2.6 FTIR spectral data 70 2.7 Scanning Electron Microscopy (SEM) examination 72 IX PAGE CHAPTERS 73 3.0 Batch leaching 73 3.1 Regulatory leach tests 73 3.1.1 The effect of the arsenic oxidation state 75 3.2 Toxicity Characteristic Leaching Procedure (TCLP) 76 3.3 Modifications to regulatory leaching procedures 80 3.3.1 Sequential leaching 81 3.3.1.1 Sequential leaching (water) 81 3.3.1.1.1 Leaching mechanism 84 3.3.1.1.2 Precision of the sequential leaching (water) tests 90 3.3.1.2 Sequential leaching (acid) 95 3.3.1.2.1 Leaching mechanism 102 3.3.1.2.2 Precision of the sequential leaching (acid) tests 106 3.3.2 Continual leaching 112 3.4 Lime stabilization 121 3.4.1 Aqueous arsenic removal using calcium 121 3.4.2 Solidification/stabilization using calcium 124 3.4.3 Scanning Electron Microscopy 134 3.5 Iron stabilization 136 3.5.1 Aqueous arsenic removal using iron 138 3.5.2 The effect of the iron oxidation state on arsenic stabilization 142 3.5.2.1 Aqueous arsenic removal using iron (II) or iron (III) 142 3.5.2.2 Solidification/stabilization using either iron (II) or iron (III) 145 3.5.2.2.1 Arsenic stabilization using ferric chloride 148 3.6 Lead stabilization 151 X PAGE 3.7 pH 154 3.8 Redox potential 161 3.9 Conductivity 169 3.10 Batch leaching overview 176 CHAPTER 4 178 4.0 Column leaching 178 4.1 Column (BLC) tests 178 4.1.1 Calcium - arsenic decomposition 184 4.1.2 Leaching mechanism 189 4.1.3 Column (BLC) leaching versus sequential batch leaching tests 194 4.2 Column (rainfall) leaching tests 205 4.2.1 Leaching mechanism 213 4.2.2 Column (rainfall) leaching versus column (BLC) leaching 220 4.2.3 Column (rainfall) leaching versus batch leaching 230 4.3 Column leaching precision 233 4.4 pH 244 4.5 Redox potential 250 4.6 Conductivity 255 4.7 Column leaching overview 261 CHAPTERS 263 5.0 Matrix characterization 263 5.1 Molecular characterization 263 5.1.1 FTIR analysis of dry and hydrated Portland Cement 264 5.1.1.1 Arsenic doped cement 265 XI PAGE 5.1.1.2 The leaching of the stabilized wastes 272 5.2 Physical/Bulk characterization 275 5.2.1 Scanning Electron Microscopy (SEM) 275 5.2.1.1 The effect of sulfate addition 280 5.3 Matrix characterization overview.........