Simona Regenspurg Characterisation of Schwertmannite - Geochemical Interactions with Arsenate and Chromate and Significance in Sediments of Lignite Opencast Lakes Charakterisierung von Schwertmannit – Geochemische Wechselwirkungen mit Arsenat und Chromat und Bedeutung in Sedimenten von Restseen des Braunkohletagebaus Simona Regenspurg Characterisation of Schwertmannite - Geochemical Interactions with Arsenate and Chromate and Significance in Sediments of Lignite Opencast Lakes Dissertation aus dem Lehrstuhl für Hydrologie der Fakultät für Chemie, Biologie und Geowissenschaften, der Universität Bayreuth Promotionsgesuch eingereicht am 10.04.2002 Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth zur Erlangung des akademischen Grades eines Doktors des Naturwissenschaften eingereichte Dissertation. Wissenschaftliches Kolloquium am 31.10.2002 1. Gutachter Professor Dr. S. Peiffer 2. Gutachter Professor Dr. F. Seifert Table of Contents TABLE OF CONTENTS LIST OF TABLES IV LIST OF FIGURES V LIST OF ABBREVIATIONS AND DEFINITIONS VII ABSTRACT VIII KURZFASSUNG XI I. INTRODUCTION 1. STATE OF THE ART 1 2. OBJECTIVES 3 3. ARRANGEMENT OF THIS WORK 3 II. GEOCHEMICAL INTERACTIONS BETWEEN SCHWERTMANNITE, CHROMATE AND ARSENATE 1. MOTIVATION TO INVESTIGATE ARSENATE AND CHROMATE 6 2. EFFECTS OF ARSENATE AND CHROMATE INCORPORATION ON THE SCHWERTMANNITE STRUCTURE 8 2.1. Introduction 8 2.1.1. Schwertmannite Structure and Formation 8 2.1.2. Investigated Oxyanions 11 2.2. Materials and Methods 13 2.2.1. Mineral Synthesis 13 2.2.2. Analytical Methods 14 2.2.3. Stability Experiment 14 2.2.4. Adsorption Experiment 15 2.2.5. Natural Specimens 16 2.3. Results 17 2.3.1. X-Ray Diffraction Data 17 2.3.2. Processes and Products of Schwertmannite Synthesis 19 2.3.3. Changes in Schwertmannite Suspensions over Time at pH 2 and 4 24 - Release of Iron, Sulfate, Arsenate and Chromate 24 - Variation of the Crystal Structure 24 2.3.4. Adsorption Capacity for Arsenate and Chromate 26 2.3.5. Enrichment of As and Cr in Natural Schwertmannite 27 2.4. Discussion 28 2.4.1. Tunnel -Sulfate, -Arsenate or -Chromate in Schwertmannite 28 2.4.2. Solid Solution or Coprecipitation of a Secondary Phase 29 - Substitution of Sulfur in Minerals 29 - Structure and Structural Changes of Schwertmannite and its Solid Solutions 29 - Formation of Schwertmannite and Substitution Affinities 31 2.4.3. Influence of Arsenate and Chromate on Schwertmannite Stability 33 2.4.4. Adsorption Process 36 2.5. Conclusions 38 I Table of Contents 3. A FTIR SPECTROSCOPICAL STUDY TO EXPLAIN BONDING STRUCTURES OF OXYANIONS IN SCHWERTMANNITE 3.1. Introduction 39 3.2. Materials and Methods 41 3.3. Results 43 3.4. Discussion 46 3.4.1. Sulfate bound to Schwertmannite 46 3.4.2. Chromate bound to Schwertmannite 47 3.4.3. Arsenate bound to Schwertmannite 48 3.5. Conclusions 50 4. INCORPORATION OF AS(III) IN SCHWERTMANNITE 4.1. Introduction: Significance of Arsenite 51 4.2. Materials and Methods 52 4.3. Results and Discussion 53 4.3.1. Arsenite in the AMD Water of the Saalfelder-Feengrotten 53 4.3.2. Arsenite associated with Schwertmannite: Coprecipitation and Adsorption 53 4.3.3. Distinction of the Oxidation State of Arsenic by FTIR Spectroscopy 54 5. MOBILIZATION OF ARSENATE AND CHROMATE DURING MICROBIAL REDUCTION OF SCHWERTMANNITE 5.1. Introduction 56 5.2. Materials and Methods 57 5.2.1. Preparation of Schwertmannite Specimens 57 5.2.2. Microbial Reduction of Schwertmannite Samples 57 5.2.3. Analytical Techniques 58 5.3. Results 58 5.3.1. Characterization of uninoculated Schwertmannite Samples 58 5.3.2. Microbial Reduction of Schwertmannite Samples 61 5.3.3. Effect of microbial Fe(III) Reduction on the Release of Oxyanions 62 5.4. Discussion 62 5.5. Conclusions 64 6. SURFACE CHARACTERISTICS OF SCHWERTMANNITE 6.1. Surface- Size, -Morphology and -Charge 65 6.1.1. Introduction 65 6.1.2. Materials and Methods 66 6.1.3. Results and Interpretation 68 - Particle Morphologie and Surface Size 68 - Surface Charge and Point of Zero Charge 69 6.2. Acid-Base Titration 71 6.2.1. Introduction: Adsorption of Arsenate and Chromate 71 6.2.2. Method: Batch Experiment 73 6.2.3. Results and Discussion 74 - Titration Curves 76 - Adsorption Isotherms 80 6.3. Summary and Discussion 81 II Table of Contents III. FORMATION AND TRANSFORMATION OF SCHWERTMANNITE IN SEDIMENTS OF ACID LIGNITE OPENCAST LAKES 1. INTRODUCTION 83 2. MATERIALS AND METHODS 85 2.1 Study Sites 85 2.2. Sampling 86 2.3. Analytical Methods 87 2.4. Geochemical Modeling 89 2.5. Synthetic Samples 90 2.6. Stability Experiment 90 3. RESULTS 91 3.1. Hydrochemistry of Acidic Mining Lakes (AML) 91 3.2. Composition of the Upper Sediment Layers 93 3.3. Colloid Analysis of Surface Waters 96 3.4. Depth-Dependent Alteration of the Sediment Composition (ML 77) 96 3.5. Synthetic Schwertmannite 97 3.6. Stability of Schwertmannite in Dependence on Time 97 4. DISCUSSION 99 4.1. Occurrence of Schwertmannite in Acidic Mining Lakes (AML) 99 4.2. Processes regulating the mineralogy of iron in AML 99 4.3. The Regulation of the pH in AML 104 5. CONCLUSIONS 106 IV. SUMMARY AND CONCLUSIONS 107 V. REFERENCES 109 ACKNOWLEDGEMENTS 115 APPENDIX A) XRD-pattern Fe(III) hydroxides containing sulfate and/ or phosphate B) As and Cr in precipitates and in water samples of 2 former mines C) FTIR spectra of the schwertmannite-stability experiments (II.2 and III.) D) Scanning electron micrographs of synthetic Fe(III) precipitates E) Acid-base titration: data of II.6.2 F) Surface water analysis of the mining lakes G) PhreeqC: some calculated species in surface waters of the mining lakes H) Ultrafiltration I) Schwertmannite stability: Release of sulfate and iron during 362 days III List of Tables LIST OF TABLES Tab. II.1-1 Selection of some ecological-relevant characteristics of arsenic and chromium. 7 Tab. II.2-1 Crystallographic cell parameters of schwertmannite, akaganéite, goethite and chromated schwertmannite (sample Sh-Cr-10). 10 Tab. II.2-2 X-ray diffraction data of akaganéite and schwertmannite (Cornell & Schwertmann, 1996) compared to chromated schwertmannite (Sh-Cr-10); measured by H. Stanjek (TU- München). 11 Tab. II.2-3 Ionic charge and radius of certain anions (Wilkinson, 1987; Gmelin, 1954). 12 Tab. II.2-4 Schwertmannite-synthesis experiments: A) Precipitates with schwertmannite structure produced by “synthesis in Fe(III)solutions”. 22 B) Precipitates with no definable structure produced by “synthesis in Fe(III)solutions”. 22 C) Precipitates with schwertmannite structure produced by the “oxidative synthesis”. 23 Tab. II.2-5 Composition of the suspensions kept at constant pH of 2 or 4 in the beginning and in the end of the stability experiment. 24 Tab. II.2-6 Schwertmannite adsorption-capacity (maximum value) for arsenate and chromate, corresponding pH and release of sulfate in dependence on the concentrations of adsorbate and adsorbent. 26 Tab. II.2-7 Range of As- and Cr-concentrations determined in AMD waters and in associated schwertmannite containing precipitates sampled in 2 former mines. 27 Tab. II.2-8 Surface complexation constants of anions complexed with hydrous ferric oxide (Dzombak & Morel, 1990). 33 Tab. II.2-9 Calculation of the ionic activity product (IAP) of schwertmannite in dependence on its composition (reaction based on Bigham et al., 1996). 35 Tab. II.3-1 Position of absorption bands in the FTIR spectrum of schwertmannite: Measured data compared to literature data (Bigham et al., 1990). 40 Tab. II.3-2 Composition of the sample-synthesis solution. 41 Tab. II.3-3 IR absorption-band position of sulphate, arsenate and chromate in different phases. Measured data compared to literature data and interpretation. 50 Tab. II.4-1 As(III) and As(V) concentrations in the AMD water of the Saalfelder-Feengrotten. 53 Tab. II.5-1 Composition of the synthesis solution and of the solid schwertmannite specimens. 57 Tab. II.6-1 Specific surface area (mean value and standard deviation s) of schwertmannite (Sh) in dependence on the oxyanion content and of goethite (Gt). 68 Tab. II.6-2 pHiep of schwertmannite and goethite in dependence on the oxyanion content. 71 Tab. II.6-3 pHpzc of goethite determined from curves of acid-base titration (appendix E) in dependence on adsorbate- and adsorbent-concentration. 76 Tab. III-1 Site description of 18 AML (Peine, 1998; LMBV, 1997, Brand, 2001). 86 Tab. III-2 Equations to determine the solubility-product and the Fe(III) activity of 90 schwertmannite (Sh), ferrihydrite (Fh), K-jarosite (Jt) and goethite (Gt). Tab. III-3 Hydrochemical parameter of ML surface water samples, taken 1 m below surface in 91 summer 2000 (Brand, 2001) and in summer 1997 (ML 77, Peine, 1998). Tab. III-4 Identification of iron minerals in the upper sediment layer of AML by XRD, FTIR 95 spectroscopy and Fe:S ratio in the oxalate-extractable fraction. Tab. III-5 Calculation of pe-values of the minerals ferrihydrite (Fh), K-jarosite (Jt) and 100 goethite (Gt). The formula were created by equating the mineral-dissolution formula (Table III-2) and the oxidation of Fe2+ (Fe2+ à Fe3+ + e- log K = -13). Tab. III-6 Calculation of pH as result of an oxidation of a Fe(II) solution. 105 IV List of Figures LIST OF FIGURES Fig. II.1-1 Coordination of the molecules chromate (or sulfate) and arsenate (or phosphate). 6 Fig. II.2-1 Powder X-ray diffraction pattern of synthetic schwertmannite and akaganéite. 8 Fig. II.2-2 Schematic structure model of akaganéite (Stanjek, unpubl. in Cornell & Schwertmann, 1996) and schwertmannite. 9 Fig. II.2-3 Location map of the sampling sites: The two former mines “Saalfelder Feengrotten” in Germany (D) and “Prybyslav” in Czech Republic (Cz).