Mineral interactions in a gold mining environment: Change in oxidation rate of stibnite as affected by the addition of varying amounts of pyrite in an oxygenated flow through system

Jessica Adelman

Department of Natural Resource Sciences McGill University, Montréal January 2010

A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Jessica Adelman

© Jessica Adelman 2010

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Abstract

It is thought that a significant mechanism of stibnite (Sb2S3) oxidation in mine wastes is through contact with a mineral of greater rest potential (whereby the outer valence shell electrons operate at a higher energy level), such as pyrite (FeS2). The goal of this research is to determine if the oxidation of stibnite in an oxygenated flow through system was increased through contact with minerals of greater rest potential such as pyrite and arsenopyrite (FeAsS) and whether this electro-chemical reaction was affected by various ratios of stibnite to pyrite (or arsenopyrite) and slightly acidic (pH 5) to alkaline (7.5) pH conditions. Four column leaching experiments were carried out using various ratios of stibnite and pyrite (or arsenopyrite). When the two minerals were mixed in columns and leached with water, the treatment with the greatest amount of pyrite and least amount of stibnite (95% pyrite) had the highest cumulative amount of dissolved Sb

(Exp #2- 16 730 mg Sb/kg stibnite). X-ray absorption near-edge structure (XANES) analysis of the solid phase revealed that the 95% pyrite treatment had the highest proportion of total Sb as Sb(V)-O species compared to all other mixed treatments and

100% stibnite. These results indicate that galvanic interactions had occurred between stibnite and pyrite. Using a solution buffered to pH 7.5, it was possible to determine the effect of a high pH environment on these galvanic interactions between stibnite and pyrite. Under slightly alkaline pH conditions, the 95% pyrite treatment had the highest cumulative amount of dissolved Sb (Exp #3- 18 090 mg Sb/kg stibnite) and this amount was similar to the 95% pyrite treatment in experiment #2. XANES analysis revealed a smaller proportion of total Sb as oxide species in this experiment compared to experiment

i - #2, possibly due to the formation of a soluble Sb complex with HCO3 . In Experiment #4, the leaching with deionized water of a mixed system containing stibnite and arsenopyrite showed that the 95% arsenopyrite treatment (Exp #4- 10 311 mg Sb/kg stibnite) released less Sb into solution than the 95% pyrite treatment in experiment #2. This was due to the smaller difference between the rest potentials of stibnite and arsenopyrite. In the four experiments, when the solution data was measured in µmol/L, dissolved Sb was supersaturated in solution and precipitated only in the first out of six leaching cycles. For experiments #1, #2 and #4, dissolved Fe had precipitated throughout the leaching cycles.

While there is not a complete knowledge of the amount of dissolved metals in solution, galvanic interaction was still observed in all four experiments. The result of galvanic interactions between stibnite and pyrite/arsenopyrite is the accelerated release of dissolved Sb into water systems surrounding mine sites. Therefore, at mine sites where stibnite is associated with pyrite and arsenopyrite, dissolved Sb should be monitored along with other regulated metals to fully assess the impact of mine effluents on the environment and help design strategies to minimize its mobility if needed.

ii Jessica Adelman MSc Sciences de Resources Naturelles Résumé

On croit qu’un des mécanismes importants de dissolution de la stibnite (Sb₂S₃) dans les résidus miniers est par contact avec un minéral ayant un plus fort potentiel

électrique, tel que la pyrite (FeS₂). L’objectif de la recherche était de déterminer l’impact du ratio stibnite vs. pyrite et l’effet du pH sur le taux de dissolution de la stibnite. Quatre essais de lessivage en colonnes ont été réalisés. Les deux premiers essais de lessivage avec de l’eau déionisée ont montré que le traitement contenant la plus forte proportion de pyrite (95% pyrite) a résulté en la plus grand quantité cumulative de Sb dissous (Exp #2-

16730 mg Sb/kg stibnite). L’analyse spectroscopique par absorption des rayons-X

(XANES) de la phase solide a révélé que la proportion de Sb total sous forme Sb(V)-O

était plus importante dans le traitement 95% pyrite comparé à tous les autres traitements mixtes et au traitement contrôle de 100% stibnite. Ces observations indiquent que des interactions galvaniques ont eu lieu entre la stibnite et la pyrite. En utilisant une solution tamponnée à pH 7.5, il fut possible de déterminer l’effet d’un pH légèrement alcalin sur ces interactions entre la stibnite et la pyrite. Dans ces conditions, la quantité maximale de

Sb dissous a été atteinte dans le traitement 95% pyrite (Exp #3-18090 mg Sb/kg stibnite), ce qui est comparable au traitement 95% pyrite dans l’expérience #2 de lessivage à l’eau.

Cependant, l’analyse XANES a révélé une proportion moindre du Sb total sous forme d’oxide dans l’essai de lessivage à pH 7.5, possiblement en raison de la formation d’un complexe soluble entre Sb et HCO₃-. L’expérience #4 a testé l’impact de l’arsenopyrite sur la dissolution de la stibnite lors d’un lessivage à l’eau. Dans ce cas, le traitement 95% arsenopyrite (Exp #4-10311 mg Sb/kg stibnite) a libéré moins de Sb en solution comparé

iii au traitement 95% pyrite de l’expérience #2, et ce résultat s’explique par la plus petite différence de potentiel électrique entre la stibnite et de l’arsenopyrite. Les interactions galvaniques entre la stibnite et la pyrite/arsenopyrite favorisent la dissolution et la mobilisation de Sb vers les cours d’eau entourant les sites miniers. Pour les sites miniers où la stibnite est étroitement associée à la pyrite et l’arsenopyrite, il serait donc préférable d’inclure l’antimoine dans le monitoring des métaux dissous de façon à mieux cerner l’ impact de l’effluent dans l’environnement et adopter au besoin des mesures préventives pour réduire sa mobilité.

iv Contributions of Authors

The two manuscripts were co-authored by the candidate, and co-supervisors

Suzanne Beauchemin and William Hendershot. Laboratory instruction and guidance were given to the candidate by Helene Lalande (McGill University), Ted Mackinnon

(CANMET-MMSL) and Kumi Pandya (NSLS X11A, Brookhaven National

Laboratories). ICP-MS analysis was performed by the Analytical Services Group of

CANMET-MMSL and Helene Lalande. Assistance with the SEM and input into the development of the project was provided by John Kwong. Guidance in laboratory setup and editing the thesis came from William Hendershot. Overall direction in laboratory work, analysis and interpretation of results, and editorial assistance came from Suzanne

Beauchemin. The candidate performed the laboratory work, ICP-AAS analysis, statistics, interpretation of results and preparation of literature review and manuscripts.

v Acknowledgements

I would first and foremost like to thank my supervisor Dr. Suzanne Beauchemin

(CANMET-MMSL) who provided me with unwavering guidance and patience, making sure I understood the work I was doing every step of the way. She always made herself available and her kindness and understanding from the beginning was immensely appreciated.

Thank you to my supervisor Dr. William Hendershot (McGill University) for finding me this wonderful research opportunity and helping me broaden my research perspective. Thank you to committee member Dr. Jim Fyles (McGill University) for his good advice that contributed to the development of the project.

Thank you to Helene Lalande (McGill University) who helped me and taught me so much in the lab, for every step in my experiments she was always there to answer my many questions. Thank you to Ted MacKinnon (CANMET- MMSL) who guided me in the lab and made sure I understood the techniques I was using. Thank you to Dr. John

Kwong (CANMET-MMSL) for his help with the SEM.

Thank you to Dr. Kumi Pandya (NSLS) for her help in the setup at X11A. XAS spectroscopy was conducted at the beamline X-11A at the National Synchrotron Light

Source, which is supported by the US Department of Energy, Division of Material

Sciences and Division of Chemical Sciences, under contract number DE-AC02-

98CH10886. The beamline X-11 is supported by the Office of Naval Research and contributions from Participating Research Team (PRT) members.

vi Thanks are also extended to the Analytical Services Group of CANMET-MMSL for the ICP analysis. Financial support was provided by Natural Sciences and

Engineering Research Council of Canada (Grant G206184).

I want to thank my parents Reisa and Bernie and my brother Matt who kept me on track all the way from Winnipeg.

vii Table of Contents

ABSTRACT...... i

RÉSUMÉ...... iii

CONTRIBUTION OF AUTHORS...... v

ACKNOWLEDGEMENTS...... vi

TABLE OF CONTENTS...... viii

LIST OF TABLES...... xi

LIST OF FIGURES...... xii

GENERAL INTRODUCTION...... 1

1.0 LITERATURE REVIEW...... 2

1.1 BACKGROUND: THE PROCESS OF GOLD FORMATION...... 2

1.2 MINERALS ASSOCIATED WITH GOLD...... 3

1.3 ANTIMONY TOXICITY...... 4

1.4 GALVANIC INTERACTIONS: MECHANICS...... 5

1.5 GALVANIC INTERACTIONS: KINETICS AND THERMODYNAMICS...... 6

1.6 GALVANIC INTERACTIONS: MINERAL INTERACTION EXPERIMENT...7

1.6.1 HYDROMETALLURGICAL STUDIES...... 7

1.6.2 HYDROMETALLURGICAL STUDIES: FACTORS AFFECTING

THE MAGNITUDE OF GALVANIC INTERACTION...... 8

1.6.3 EXPERIMENTS IN AN OXYGENATED ENVIRONMENT...... 10

1.7 X-RAY ABSORPTION SPECTROSCOPY- DETERMINATION OF

SPECIATION AND OXIDATIVE STATE...... 11

viii 1.8 EH-PH DIAGRAMS OF Sb SPECIES...... 12

1.9 INTERACTION OF DISSOLVED Sb WITH OXYHYDROXIDES...... 13

MINERAL INTERACTIONS IN A GOLD MINING ENVIRONMENT: THE

CHANGE IN OXIDATION RATE OF STIBNITE AS AFFECTED BY PYRITE IN

AN OXYGENATED FLOW- THROUGH SYSTEM...... 20

ABSTRACT...... 20

INTRODUCTION...... 21

MATERIALS AND METHODS...... 24

MINERAL PREPARATION...... 24

FLOW- THROUGH EXPERIMENT...... 25

XANES AND SEM ANALYSIS...... 26

STATISTICAL ANALYSIS...... 27

RESULTS AND INTERPRETATION...... 28

FLOW- THROUGH EXPERIMENT WITH DIFFERING PYRITE TO

STIBNITE RATIOS: EXPERIMENTS #1 AND #2...... 28

XANES AND SEM ANALYSIS...... 33

DISCUSSION...... 35

CONCLUSION...... 38

CONNECTING STATEMENT...... 58

FACTORS AFFECTING THE OXIDATION RATE OF STIBNITE- A BUFFERED

PH 7.5 LEACHING SOLUTION AND THE SUBSTITUTION OF PYRITE WITH

ARSENOPYRITE IN A GALVANIC CELL...... 59

ABSTRACT...... 59

ix INTRODUCTION...... 60

MATERIALS AND METHODS...... 62

MINERAL PREPARATION...... 62

FLOW- THROUGH EXPERIMENT...... 62

XANES AND SEM ANALYSIS...... 63

STATISTICAL ANALYSIS...... 65

RESULTS AND INTERPRETATION...... 65

HIGH PYRITE TO STIBNITE FLOW- THROUGH EXPERIMENT USING A

LEACHING SOLUTION BUFFERED TO PH 7.5: EXPERIMENT #3...... 65

XANES ANALYSIS...... 68

HIGH ARSENOPYRITE TO STIBNITE FLOW -THROUGH EXPERIMENT-

EXPERIMENT #4...... 79

DISCUSSION...... 71

CONCLUSION...... 73

GENERAL CONCLUSION...... 92

REFERENCES...... 93

x List of Tables

Chapter 2 (Manuscript 1)

Table 1- Particle size distribution and surface area of stibnite and pyrite used in experiments 1(A) and 2(B)...... 40 Table 2: Mass ratio of stibnite to pyrite for experiments 1 and 2; total mixture of 2 g per sample...... 41 Table 3- Results from the multivariate analysis of variance for the effects of treatments and time on pH, Eh and dissolved Sb, Fe and sulphate concentrations in experiments 1(A) and 2(B)...... 42

Table 4- Concentration of dissolved Fe, and SO4 for pyrite alone (A) and Sb and SO4 for stibnite alone (B) in µmol/L on days 1 to 6 for experiment #1...... 43

Table 5- Molar concentrations of dissolved Sb, Fe, and SO4 for all treatments in µmol/L from days 1 to 6 for experiment #2...... 45

Chapter 3 (Manuscript 2) Table 1- Grain size and surface area of stibnite, pyrite and arsenopyrite used in experiments 3(A) and 4(B)...... 75

Table 2: Mass ratio of stibnite to pyrite and stibnite to arsenopyrite for experiments 3 and 4; total mixture of 2 g per sample...... 76

Table 3- Results from the multivariate analysis of variance for the effects of treatments and time on pH, and dissolved Sb, Fe, As and sulphate concentrations in experiments 3(A) and 4(B)...... 77

Table 4- Concentration of dissolved Fe, and SO4 for pyrite alone and Sb and SO4 for stibnite alone in µmol/L on days 1 to 6 for experiment #3...... 78

Table 5- Concentration of dissolved Sb, Fe, As and SO4 for all treatments in µmol/L from days 1 to 6 for experiment #4 with stibnite/arsenopyrite mixed systems...... 80

xi List of Figures

Chapter 1 (Literature Review)

Figure 1- Fermi Level in context of Electron Band of a semiconductor (Nave, 2005)...... 15

Figure 2- Current Densities of different flow rates overtime (1= 0.06 m/s, 2= 0.24 m/s, 3= 0.36 m/s) (Liu et al., 2008)...... 15

Figure 3- Proposed mechanisms for stibnite (anode) and pyrite (cathode) in an oxygenated environment (adapted from Ashley et al. 2003; Holmes and Crundwell 1995; Kwong et al. 2003)...... 16

Figure 4- Galena (gn)-sphalerite (sp)-pyrite (py) assemblage before and after the 26 week weathering period without bacteria (Kwong et al., 2003)...... 17

Figure 5- Preferential weathering of stibnite surrounding a grain of pyrite in a polished stibnite slab (Kwong et al., 2006)...... 17

Figure 6- Mobility of antimony in the environment (Ashley et al., 2003)...... 18

Figure 7- Eh-pH diagram of antimony species based on Gibbs Free Energy (Vink, 1996)...... 19

Chapter 2 (Manuscript 1)

Figure 1- pH of the leached solutions over the six leaching cycles- Experiment #1...47

Figure 2- pH of the leachates over the six leaching cycles- Experiment #2...... 48

Figure 3- Cumulative release of dissolved Sb over the six leaching cycles as affected by various amounts of pyrite in the mixture.- Experiment #1...... 49

Figure 4- Cumulative release of dissolved Sb over the six leaching cycles as affected by various high amounts of pyrite in the mixture.- Experiment #2...... 50

xii Figure 5- Cumulative release of dissolved Fe over the six leaching cycles as affected by various amounts of pyrite in the mixture - Experiment #1; Inset: Non cumulative concentration of dissolved Fe after each leaching cycle for three selected treatments...... 51

Figure 6- Cumulative release of dissolved Fe over the six leaching cycles as affected by various high amounts of pyrite in the mixture - Experiment #2; Inset: Non cumulative release of dissolved Fe after each leaching cycle for three selected treatments...... 52

Figure 7- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various high amounts of pyrite in the mixture. - Experiment #2...... 53

Figure 8- A. Sb K-edge XANES spectra for the reference compounds of

KSbO3·H2O, Sb2O3, Sb2S3 used as representative species for Sb(V)-O, Sb(III)-O and Sb(III)-S respectively in the fitting analysis. B. Sb K-edge XANES spectra for the selected treatments with 5%, 25% and 100% stibnite after six leaching cycles (solid lines = measured data; dashed lines = predicted data; Vertical line is an eye guide only)...... 54

Figure 9- Best fitting results from least-squares linear combination for stibnite before leaching, the 95, 75, 50% pyrite and 100% stibnite treatments after six days of leaching...... 55

Figure 10- SEM image of the 100% stibnite sample after leaching and corresponding X-ray maps for O, S and Sb (left). Sb, O and S elemental composition of point x in SEM image (right)...... 56

Figure 11- SEM image of the 75% pyrite sample after leaching and corresponding X-ray maps for O, S, Sb and Fe (top). Sb, O and S elemental composition of point x in SEM image (bottom right)...... 57

xiii Chapter 3 (Manuscript 2)

Figure 1- pH of the leached solution over the six leaching cycles- Experiment #3...... 82

Figure 2- Cumulative release of dissolved Sb over six leaching cycles as affected by various amounts of pyrite in the mixture and a buffered leaching solution, pH= 7.5- Experiment #3...... 83

Figure 3- Cumulative release of dissolved Fe over the six leaching cycles as affected by various amounts of pyrite in the mixture and a buffered leaching solution, pH=7.5- Experiment #3...... 84

Figure 4- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various amounts of pyrite in the mixture and a buffered leaching solution, pH= 7.5- Experiment #3...... 85

Figure 5- Sb K-edge XANES spectra for (A) two reference compounds of Sb2S3 [Sb(III)-S] and KSbO3·3H2O [Sb(V)-O], and (B) the selected treatments with 95% pyrite, and 100% stibnite after six leaching cycles using a buffered leaching solution, pH=7.5 (solid lines= measured data; dashed lines= predicted data; Vertical line is an eye guide only)- Experiment #3...... 86

Figure 6- Best fitting results from least-squares linear combination of stibnite before leaching and the 95% and 75% pyrite and 100% stibnite treatments after six days of leaching using a buffered leaching solution, pH=7.5- Experiment #3...... 87

Figure 7- pH of the leached solution over the six cycles- Experiment #4...... 88

Figure 8- Cumulative release of dissolved Sb over the six leaching cycles as affected by various amounts of arsenopyrite in the mixture- Experiment #4...... 89

Figure 9- Cumulative release of dissolved As over the six leaching cycles as affected by various high amounts of arsenopyrite in the mixture- Experiment #4...... 90

Figure 10- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various amounts of arsenopyrite in the mixture- Experiment #4...... 91

xiv General Introduction

Stibnite, Sb2S3, is a sulphide mineral often found in gold mines but discarded in waste rock piles along with other sulphide minerals. When stibnite is exposed to air it oxidizes to release Sb into the water system. Sb(III) and Sb(V) are toxic and known to be carcinogenic (Filella et al., 2002; Scheinost et al., 2006). It is thought that a significant way stibnite oxidizes is through contact with another electronically stronger mineral, such as pyrite (FeS2). This is known as a galvanic interaction where the mineral with greater rest potential (cathode) causes an increase in oxidation of the mineral with lower rest potential (anode). The objective of this study was to determine if the oxidation of stibnite in an oxygenated flow through system was increased through contact with minerals of higher rest potential such as pyrite and arsenopyrite (FeAsS) and whether this electro-chemical reaction was affected by various ratios of stibnite to pyrite (or arsenopyrite) and slightly acidic (pH 5) to alkaline (7.5) pH conditions. Currently Sb is not included in the Metal Mining Effluent Regulations form the industry has to fill out for monitoring the release of metals, the reason being not enough is known about the behaviour of stibnite (Department of Fisheries and Oceans, 2002). This study is part of a larger study headed by Natural Resources Canada to better understand the behaviour of stibnite in the mining environment.

1 1.0 Literature Review

1.1 Background: The process of gold formation

Gold mining creates environmental hazards in the mining and extraction processes. When the mining begins, minerals that have been buried within the earth for hundreds of millions of years are now being exposed to air, and chemical and physical processes begin to degrade and dissolve these minerals releasing metals into the environment. This study is part of a larger study that investigates the mobilization and attenuation processes of antimony, Sb, occurring at an inactive gold mine site.

This study will be investigating the behaviour of stibnite, Sb2S3, when in contact with other sulphide minerals of differing electrode potential, and whether the minerals with the higher electrode potential will cause stibnite, the weaker mineral, to oxidize and dissolve more quickly leading to the mobilization of Sb into the water system.

Gold often forms in an area of past subduction, with volcanism and subject to faulting. It is within fractures within faults that gold bearing mesothermal orogenic quartz- carbonate veins can form (Brouwer, 2005). Vein formation is associated with the late stages of magma movement and evolution. Early crystallization of magma leaves a siliceous fluid containing volatiles such as H2O and minor elements not found in common igneous rocks. The fluid migrates to the fractures and fissures formed from faulting and it is here that veins form containing gold ore deposits and associated sulphide minerals like stibnite, Sb2S3, pyrite, FeS2, and arsenopyrite, FeAsS. These minerals form under the same conditions as gold and are therefore often found with gold ore. The gold ore and associated sulphide minerals are then mined from the vein (Shuey, 1975).

2

1.2 Minerals associated with gold

Stibnite is a sulphide mineral composed of the metalloid antimony, Sb, and sulphur, S, and is mainly associated with low temperature hydrothermal or pegmatite veins containing gold and silver deposits (Shuey, 1975). Stibnite is closely associated with gold and other mineral deposits because it often mineralizes under similar conditions. When the gold ore is extracted, stibnite will be disposed of in waste rock piles or deposited as tailings where it is exposed to air and moisture causing stibnite to oxidize (Zhou et al., 2007). The oxidation of stibnite can release trivalent antimony

(Sb(III)) species which can further oxidize to form pentavalent antimony (Sb(V)) species

(Ashley et al., 2003). The oxidization of stibnite is of concern because Sb(III) is the most toxic of the Sb species and can cause serious harm to humans.

Pyrite, FeS2, is a sulphide mineral considered to be the main source of acid mine drainage (AMD). At low pH conditions, pyrite will oxidize and generate sulphuric acid into the environment (Huminicki and Rimstidt, 2009). The release of sulphuric acid causes the pH to drop resulting in the continued dissolution of metals found in tailings and waste rock. Once acid mine drainage is generated, it can last for hundreds of years.

This problem is very common at many mine sites. However, at mining sites where carbonates are present AMD is usually not an issue. Pyrite instead can play a role in the dissolution of sulphide minerals with lower rest potentials (Salonen et al., 2006).

Arsenopyrite, FeAsS, is a common sulphide mineral that often forms under the same conditions as gold, stibnite and pyrite. The oxidation of arsenopyrite is of great

3 concern due to the subsequent dissolution of arsenic. Arsenopyrite is usually implicated in mining extraction sites where arsenic levels are elevated (Craw et al., 2003).

At a pH greater than 7, the oxidation of arsenopyrite in natural waters can result in the formation of ferric hydroxide and scorodite deposits, FeAsO4·2H2O, deposits on the surface of the mineral. Below the pH of 7, following the oxidation of arsenopyrite, soluble iron species will form and the elemental sulphur can form at the surface of arsenopyrite (Craw et al., 2003; Urbano et al., 2008).

1.3 Antimony Toxicity

If microbiota contain small amounts of Sb in their system, predators such as fish who feed on these organisms in high amounts will accumulate Sb and if these fish are consumed by humans, the Sb levels in the fish could be toxic due to this bioaccumulation of antimony. In an Sb contaminated system, Sb levels of 1.50 μg/g in freshwater algae,

1.60 μg/g in marine phytoplankton and 2.05 μg/g in green algae have been reported

(Filella et al., 2007). Using brown trout, Salmo trutta L., as a biomarker to determine the effect of metals including antimony, it was observed that fish that were subjected to high

Sb concentrations in water draining from shooting ranges, displayed the accumulation of

Sb in the gills and liver (Heier et al., 2009). When Sb affects the health of aquatic organisms it can affect the health of the ecosystem as well as the health of the human population dependent on that aquatic ecosystem.

The drinking water limits established by the European Community (EC) for Sb is

5 µg L-1 whereas the United States Environmental Protection Agency (USEPA) adopts a drinking water standard of 6 µg L-1 (Filella et al., 2002; Kelepertsis et al., 2006). Pulido

4 and Parish (2003) determined that SbCl3, an Sb(III) compound, induces DNA damage.

The SbCl3 compound caused oxidative stress by reacting with oxygen compounds to form reactive oxygen species (ROS). ROS are oxygen species that have an unpaired electron and for this reason are highly reactive. The ROS can weaken the mitochondrial cell walls which will lead to cell death and the formation of cancer cells (Pulido and Parrish, 2003;

Wyllie and Fairlamb, 2006).

1.4 Galvanic Interactions: Mechanics

Due to the toxicity of Sb(III), the mechanisms by which dissolved Sb(III) enters the environment is extremely important. According to Liu et al. (2008), one of the main mechanisms by which the products of dissolved metal sulphides enter into hydrometallurgical processing stream is through galvanic interactions. Mehta and Murr

(1983) described galvanic interaction as a reaction that occurs when two sulphide minerals of different rest potential come into contact with each other. The sulphide with the higher rest potential, also known as the cathode, will cause the sulphide with a lesser rest potential, the anode, to increase its rate of oxidation. During this charge transfer the cathode is galvanically protected.

Before the process of galvanic interaction and how it relates to sulphide oxidation can be understood, it is necessary to understand the mechanics of the reaction itself.

Shuey (1975) was the first to describe semiconducting minerals and how there is a flow of electrons from mineral grains with a lower rest potential to grains with the higher rest potential. This process modifies the Fermi energy levels of both sulphides. The

5 electrons, known as Fermions, cannot exist in identical energy states. Therefore they press into layers of different electron energy states. The Fermi level refers to the top point of the collection of electron energy levels at absolute zero temperature. The Fermi level provides information on the velocities of electrons in different metals. Any temperature above absolute zero will allow a finite possibility that electrons can pass the

Fermi level and reach the conduction band as seen in Figure 1. The electrons that leave the lattice and the “hole” that is left behind contribute to a current flow (Nave, 2005).

With different Fermi energy levels, come different current flows and different rest potentials. Mehta and Murr (1983) were one of the first to discuss galvanic interactions among sulphides and how difference in rest potentials explains the mechanism for galvanic interaction. Rest potential, also known as corrosion potential, refers to the potential of a corroding surface in an electrolyte relative to the standard hydrogen electrode (SHE) which is 0v. It is the difference in rest potential that determine the galvanic interactions between two minerals.

1.5 Galvanic Interactions: Kinetics and Thermodynamics

Holmes and Crundwell (1995) and Liu et al. (2008) described galvanic interaction as having to do with electrically conducting minerals. They studied the kinetics and thermodynamics of galvanic interactions thereby devising a mathematical equation that describes the magnitude of galvanic interactions. Using copper-pyrite and galena-pyrite couples, Holmes and Crundwell (1995) developed a modified version of Kirchoff’s

Voltage Law and the Butler- Volmer equation to determine the magnitude of the difference between rest potentials, also known as the Cell emf. The equation explains

6 that the difference between the cathodic and anodic rest potentials, Ee,c- Ee,a, can be determined using cathode overpotentials which are determined from the current density and the surface area of the cathode, the solution voltage loss, IgR’solution, and the mineral- mineral contact voltage loss, IgR’contact. Over potential refers to the additional energy needed to allow the reaction to proceed at a required rate or current density (Holmes and

Crundwell, 1995; Liu et al., 2008).

Liu et al. (2008) described similar equations necessary to understanding the kinetics of current density including the significance of different rest potentials, while studying the interactions of pyrite-galena and pyrite-chalcopyrite in a flowing system of different rates. As the flow rate increased, the thickness of the diffusion layer decreased.

A thinner diffusion layer leads to a larger corrosion current density (Figure 2).

Liu at al. (2008) also determined that when there is an increase in the ratio of cathode to anode amount in the mixture of two minerals, there will be an increase in corrosion potential due to the greater difference between anode and cathode current densities. This means that the total corrosion current density will increase. When pyrite was increased 10-fold in a pyrite-galena system, there was an increase in corrosion current density meaning the oxidation of galena occurred at a greater rate.

1.6 Galvanic Interactions: Mineral interaction experiments

1.6.1 Hydrometallurgical studies:

Holmes and Crundwell (1995) explained the mechanisms of galvanic interaction based on a hydrometallurgical test involving galena and pyrite immersed in a ferric

7 solution. When galena is not in contact with the pyritic surface, the oxidation of galena is coupled with the reduction of the ferric ions present on its surface. However, when the surfaces of galena and pyrite come into contact, the oxidation of galena is coupled with the reduction of ferric ions present on the surface of pyrite through a charge transfer.

Because the reduction of ferric ions occurs at a greater rate on the surface of pyrite than that of galena, the rate of galena dissolution will thus increase during galvanic interaction

(Holmes and Crundwell, 1995).

Similar experiments were performed using pyrite with different anode impurities or unconsolidated mineral mixtures including pyrite whereby a solution was leached through the mineral mixtures then analyzed for dissolved metals and sulphates. In all experiments the dissolution rate of the anode minerals increased when in contact with pyrite, the cathode (Abraitis et al., 2004a; Cruz et al., 2005).

1.6.2 Hydrometallurgical studies: Factors affecting the magnitude of galvanic interaction

Mehta and Murr (1983) investigated various aspects of galvanic interaction: grain size of the minerals, and the effects of various pH levels. They also looked at the effect of bacteria, Thiobacillus ferroxidans on the rate of the oxidative dissolution of the anode, chalcopyrite. The chalcopyrite-pyrite couple that was not inoculated with the bacteria displayed an Eh level of 390 mV at the end of the experiment whereas the minerals that were inoculated with the bacteria displayed an Eh reading of 570 mV. This means that oxidation and subsequent copper dissolution was increased in the inoculated material

(Mehta and Murr, 1983).

8 With the larger mesh sizes of 100 and 200 µm, as more pyrite was added to a pyrite: chalcopyrite mixture, the dissolution of copper increased. However when smaller mesh sizes were used, 270 and 400 µm, the same amount of copper dissolution was not observed at the pyrite: chalcopyrite ratio of 5:5. Using the scanning electron microscope, they showed that the surfaces of the smaller particles were covered with hydroxide which reduced the contact between the two minerals thereby decreasing the galvanic interaction between the two minerals (Mehta and Murr, 1983).

Mahlangu et al. (2006) studied the galvanic interaction between stibnite, the anode, and iron shavings, the cathode, for ore recovery purposes and how the pH would affect the rate of reaction and dissolution of stibnite. A sample of stibnite flotation concentrate was placed in a reactor with various amounts of iron shavings and a solution consisting of hydrochloric acid, and distilled water was pumped into the reactor. It was determined that the oxidation of stibnite was strongly dependent on pH. As the pH decreased from 0.88 to 0.33, the dissolution rate increased. Mahlangu et al. 2006 used a low pH level of 0.88 and the high concentration of H+ ions within the solution greatly affected the behaviour of the iron shavings. A competing side reaction between H+ and iron was thought to have worked against the main reaction where the products, Fe2+ and

H+, from the reaction formed a porous layer around the unreacted surfaces of the minerals. Therefore, the formation of this porous layer around the reacting minerals decreased the pH thereby increasing the dissolution rate of stibnite (Mahlangu et al.,

2006).

While decreasing pH was seen to increase the dissolution rate of stibnite in highly acidic systems (Mahlangu et al., 2006), it may be possible for Sb to form complexes with

9 other compounds under neutral to alkaline conditions also increasing the dissolution rate as pH increases. Investigating the effect of pH on the oxidation of orpiment, As2S3, whose mineralization complex- forming behaviour is similar to stibnite, Lengke and

Tempel (2001) determined that at high pH levels, the oxidation rate of orpiment, As2S3, increased with increasing pH(Su et al., 2009). NaHCO3, sodium bicarbonate, and

Na2CO3, sodium carbonate, were used as buffers to regulate the pH at different levels between 6.3 and 10.3. The increasing oxidation rate of As2S3 with increasing pH values was believed to be influenced by the formation of an arsenocarbonate complex (Lengke and Tempel, 2001).

1.6.3 Experiments in an Oxygenated Environment:

Similar galvanic reactions can occur in an oxygenated system with O2 being the final electron acceptor, which will be reduced to water (Kwong et al., 2003). The oxidative dissolution of the anode sulphide, MS, (MS = M2+ + S0 + 2e-) is coupled with

+ the reduction of oxygen at the cathode surface (0.5O2 + 2H +2e- = H2O; Kwong et al.

2003). Figure 3 illustrates the proposed mechanisms for galvanic interaction between stibnite and pyrite in an oxygenated system, derived from the information provided in

Kwong et al. (2003), Holmes and Crundwell (1995) and Ashley et al. (2003).

When not in contact with each other, the rate of reaction is greater on the surface of pyrite compared to the reaction rate on the surface of stibnite alone. However, when stibnite is in contact with pyrite, the oxidative dissolution rate will increase at the anode, leading to enhanced dissolution.

10 To simulate an oxygenated environment, cyclic wetting- drying experiments were conducted whereby air and water were alternately pulled through the mineral assemblage.

Kwong et al. (2003) passed dry air through a closed chamber for three days followed by wet air flowing through the chamber for another three days. Figure 4 displays the oxidation that both galena and sphalerite have undergone after 26 weeks of wet/dry cycles. Out of the three minerals, pyrite appeared to have oxidized the least. Hita et al.

(2006) performed similar wetting-drying leaching experiments where an unconsolidated mineral mixture composed of sphalerite and pyrite grains were subjected to cycles of water and air being circulated through the mixture using a Centurion mechanical extractor (Hita et al., 2006).

1.7 X-ray absorption spectroscopy- Determination of speciation and oxidative state

XANES (X-ray absorption near edge structure) spectroscopy can distinguish the changes in speciation of a given element due to changes in the redox conditions. With increasing oxidation states of an element, there are less electrons within the valence shells and the fewer remaining electrons are held on more tightly around the nucleus.

Therefore, as the oxidation state increases, the energy level needed to eject an electron out of the atom increases and is translated in the XANES spectra by a shift to higher energy. This characteristic can be used to quantify the oxidation state of the element in a complex matrix without any pre-treatment. For example, Beauchemin and Kwong (2006) studied the change in As oxidation state in tailings subjected to reducing and oxidizing

11 conditions, and showed that As readily remobilized as As(III) during reduction

(Beauchemin and Kwong, 2006).

Kwong et al. (2006) used polished slabs of stibnite with pyrite inclusions to determine the pattern of weathering that occurred during a 37 day weathering period.

After the slabs were soaked for two hours then air dried, the slabs were analyzed using

XPS (X-ray photoelectron spectroscopy) and XANES and the solution was analyzed for dissolved metals. XPS and XANES analyses were used to determine in situ the Sb oxidation state of Sb and Fe at the edge of the two contacting minerals. XPS probes a very thin surface layer (1 to 3 nm) while XANES can penetrate deeper and provides insight on the 30 to 40 nm surface layer. Figure 5 displays the extensive weathering of stibnite that has occurred surrounding the grain of pyrite. Using the XPS and XANES technique, Kwong et al. (2006) determined that oxidative dissolution of stibnite occured preferentially at the interface with pyrite while the surface of pyrite was unaltered.

Kwong et al. (2006) was the first to study galvanic interactions between stibnite and pyrite. To build on that research, in this study, pyrite and stibnite grains in the form of an unconsolidated mineral mixture will be leached through alternating cycles of water and air. This unconsolidated mineral mixture reflects mineral mixtures that could be found in a tailings pond or waste rock piles at a gold mining site.

1.8 Eh- pH diagrams of Sb species

According to Ashley et al. (2003) there are several species of Sb that will result from the dissolution of stibnite in the natural environment. Figure 6 displays the

(III) theoretical progression of stibnite to valentinite, Sb 2O3, which can further dissolve into

12 (III)(V) stibiconite, Sb O6(OH). Ultimately, when in contact with oxygenated water, these

(V) - species will oxidize and dissolve into an oxyanion in solution, Sb O3 (aq).

After stibnite oxidizes, antimony will convert into other species, so it is important to determine under which pH conditions will certain species form especially if they are in the form of Sb(III). Vink (1996) revised an Eh-pH diagram based on newly available thermodynamics data that describes the different species of antimony that can form based on the Gibbs Free Energies of these species (Figure 7). Change in Gibbs Free Energy

(ΔG) refers to the measure of a tendency of a chemical change to take place which is dependent on the change in enthalpy, total heat, temperature and change in entropy, the amount of energy not available for work.

In Figure 7, Vink’s diagram predicts that in a highly oxic environment antimony

(V) - will remain mobile and occurs dominantly as an aqueous species, Sb O3 , regardless of the pH. Under a weakly oxidic environment, mixed valence Sb- oxide solid phases such as senarmontite and valentinite can precipitate. Under these conditions, the dominant

(III) 0 species is Sb (OH)3 (aq) within the pH range of 2 to 12 (also designated as the oxyanion

(III) HSb O2(aq)) (Krupta and Serne, 2002; Vink, 1996).

1.9 Interaction of dissolved Sb with oxyhydroxides

Al and Fe- oxyhydroxides have been widely used as mineral sorbents to decrease the mobility of metals and metalloids such as Sb from municipal waste and contaminated soils (Meima and Comans, 1998; Spuller et al., 2007). Belzile et al.(2001) explained that it is possible to remove toxic Sb(III) from the environment by having the antimony species

13 bind to an iron oxyhydroxide thereby forcing Sb(III) to oxidize and form an Sb(V) species

(Equations 1 and 2). This is a natural process that can decrease the mobility of dissolved

Sb. Natural Fe oxyhydroxides were collected and spiked with dissolved Sb(III). A molar ratio of 4500 for Fe:Sb was chosen to mimic the natural environment found in Poyang

Lake, China. The experiment demonstrated that iron oxyhydroxides can cause Sb(III) to

(V) oxidize into Sb within 5 to 6 days. When Sb(OH)3 is adsorbed onto the surface of the oxyhydroxides, the iron accepts an electron from Sb(III) causing it to oxidize to Sb(V).

During this reaction Fe(III) becomes Fe(II) (Belzile et al., 2001). Thanabalasingam and

Pickering (1990) recorded Sb(OH)3 adsorption capacity levels in different hydroxides.

At a pH of 6 to 7, MnOOH had the highest Sb sorption capacity followed by Al(OH)3 then FeOOH (Thanabalasingam and Pickering, 1990).

- + 2+ 2Fe(OH)3 + 2e + 6H → 2Fe +6H2O III V - Sb (OH)3 → Sb +2e V + Sb +4H2O → H3SbO4 +3H (1)

The half reactions above create the full reaction below (Belzile et al., 2001)

III V 2Fe(OH)3 +Sb (OH)3 → 2Fe(OH)2 +H3Sb O4 +H2O (2)

The use of oxyhydroxides could prevent Sb(III) mobilization into the water system (Belzile et al., 2001).

14

Figure 1- Fermi level in context of electron band of a semiconductor (Nave, 2005)

Figure 2- Current densities of different flow rates overtime (1= 0.06 m/s, 2= 0.24 m/s, 3= 0.36 m/s) (Liu et al., 2008)

15

Figure 3- Proposed mechanisms for stibnite (anode) and pyrite (cathode) in an oxygenated environment (adapted from Ashley et al. 2003; Holmes and Crundwell 1995; Kwong et al. 2003).

16

Figure 4- Galena (gn)-sphalerite (sp)-pyrite (py) assemblage before and after the 26 week weathering period without bacteria (Kwong et al., 2003)

Figure 5- Preferential weathering of stibnite surrounding a grain of pyrite in a polished stibnite slab (Kwong et al., 2006)

17

Figure 6- Mobility of antimony in the environment (Ashley et al., 2003)

18

Sb2O4 ‐ SbO3 ‐ SbO3

Eh Sb2S3

pH

Figure 7- Eh-pH diagram of antimony species based on Gibbs Free Energy (Vink, 1996)

19 Mineral interactions in a gold mining environment: the change in oxidation rate of stibnite, Sb2S3, as affected by pyrite, FeS2, in an oxygenated flow through system.

Abstract

Understanding the behaviour of stibnite when in contact with a mineral of higher rest potential, such as pyrite, is important in mine waste management because it is thought that galvanic interactions between the two minerals constitute a mechanism that can enhance stibnite oxidation and the release of antimony into the water system. The objective of the study was to determine the impact of pyrite on the dissolution rate of stibnite in unconsolidated mixtures of the two minerals subjected to leaching and the change in the solid- phase Sb speciation. Through leaching column flow- through experiments, water followed by air was pulled through the mineral mixture in a cycle of

24 hours, for a total of 6 leaching cycles. The treatment with 95% pyrite and 5% stibnite released the highest amount of Sb into solution (16 730 mg Sb/kg stibnite). However, considering the solution data, it is likely that Sb has reprecipitated on day 1 for all mixed treatments. Therefore the dissolved Sb solution data from the first leaching cycle underestimated the amount of Sb released into solution. From day 2 to 6, there was less

SO4 than metals (in molar ratio) in solution and this suggests that Sb was undersaturated in solution and did not reprecipitate. This means the dissolved Sb amounts were accurate.

XANES analysis of the solid phase of the leached mineral mixtures revealed the 95% pyrite treatment also contained the highest proportion of total Sb as Sb(V)-O species.

SEM images of the individual stibnite grains in the 75% pyrite and 100% stibnite

20 treatments revealed more oxygen on the surface of the grains in the 100% stibnite treatment caused by the slower oxidation of that treatment which allowed for the deposition of more O2 onto the mineral’s surface. The solution and solid phase data suggest that a galvanic interaction between the two minerals was the cause of the increased dissolution and oxidation of stibnite when in contact with pyrite in the mixed systems.

Introduction

Stibnite, Sb2S3, and pyrite, FeS2, are sulphide minerals often associated with gold in mesothermal carbonate-quartz veins, and are therefore commonly extracted at the same time as gold at mining sites (Shuey, 1975). Once the gold is recovered, these minerals are disposed of as waste rock or tailings where the storage conditions often favour the oxidation of sulphide minerals. The oxidation of stibnite will result in the release of dissolved antimony (Sb) into the surrounding waters. Antimony has three oxidation states, Sb(0), Sb(III) and Sb(V). Antimony is known to be carcinogenic, with Sb(III) being potentially more toxic than Sb(V) (Filella et al., 2002; Scheinost et al., 2006). When Sb(III) enters the human body it can react with oxygen compounds causing a redox reaction that will form reactive oxygen species. These reactive species can react with the nuclear and mitochondrial DNA which can weaken the cell walls of these structures and lead to cell death and the development of cancer cells (Pulido and Parrish, 2003).

Knowing the toxicity of antimony, it is important to understand better the behaviour of stibnite in mine wastes. Currently, antimony is not included in the annual report mining industries have to complete for the Mining Metals Effluent Regulations

21 (MMERs) (Environment Canada, 2002). One reason is that there is simply not enough known about the behaviour of antimony in the gold mining environment (Beak

International Inc., 2002). Under the lead of Natural Resources Canada, the goal of this research project is to determine the impact of pyrite on the dissolution rate of stibnite and to understand the subsequent oxidation species that form in the solid phase.

During the process of oxidation, stibnite combines with oxygen and water to form sulphate, an Sb(III) oxide, and protons (eq. 1). The Sb(III) oxide can further oxidize leading to its dissolution as an Sb(V) oxyanion along with protons and electrons (eq. 2) (Ashley et al., 2003).

(III) - (III) + Sb 2S3 + 3H2O + 6O2 = 3SO4 + Sb 2O3 + 6H (1)

(III) (V) - + - Sb 2O3 + 3H2O = 2Sb O3 + 6H + 4e (2)

The oxidation of sulphide minerals is affected by various factors including pH, Eh and galvanic interactions (Mehta and Murr, 1983). Galvanic interactions are processes occurring between two minerals when they come into contact and interact with each other at the electronic and elemental levels (Kwong et al., 2003; Mehta and Murr, 1983). When stibnite comes into contact with pyrite, a reaction will occur between the two minerals due to their difference in rest potentials (Kwong et al., 2006). Pyrite acts as the cathode because it has a higher rest potential than stibnite, the anode. Pyrite has a rest potential of

0.66 V compared to 0.12 V for stibnite (Abraitis et al., 2004b). Rest potential is related to the Fermi energy level of the mineral. The Fermi energy level refers to the highest electron energy state that can be reached by a specific element or mineral (Cruz et al.,

22 2005; Nave, 2005). Pyrite has a higher Fermi energy level than stibnite and therefore a higher rest potential and this means that the oxidative reactions occur at a greater rate on the surface of pyrite than on the surface of stibnite (Abraitis et al., 2004a). For example,

Holmes and Crundwell (1995) conducted a hydrometallurgical study on galvanic interactions where galena and pyrite were immersed in a ferric nitrate solution. When the anodic mineral, galena, came into contact with cathodic pyrite, the reactions occurring on the surfaces of galena were forced to occur at the reaction rate occurring on the surface of pyrite, thereby modifying the Fermi energy levels of both minerals and at the same time causing a greater dissolution rate of the anode mineral. The electrons released from the anode surface are transferred to the cathode where they will bond to the oxidant around the surface of pyrite preventing its oxidation. In the Holmes and Crundwell study, ferric ions acted as the oxidant that reacted with the released electrons thereby galvanically protecting pyrite from oxidation (Holmes and Crundwell, 1995).

Galvanic interactions can also occur in oxygenated environments such as tailings impoundments where mine wastes are disposed of. In the case of oxygenated environments, the oxidative dissolution at the anode (MS = M2+ + S0 + 2e-) is coupled

+ with the reduction of oxygen at the cathode surface (0.5O2 + 2H +2e- = H2O) (Kwong et al., 2003). Kwong et al. (2006) exposed thin slabs of a mineral assemblage consisting of pyrite, galena and sphalerite to short wet and dry air cycles. At the end of the cycle, pyrite had oxidized the least, and sphalerite oxidized at the greatest rate due to its lower rest potential compared to galena. Studies of galvanic interactions have also been conducted using unconsolidated mineral mixtures that simulated the leaching of mine wastes. An

23 increased dissolution rate of sphalerite due to galvanic interaction with pyrite was also reported in a mineral grain mixture of pyrite and sphalerite (Hita et al., 2006).

It is thought that galvanic interactions constitute an important mechanism responsible for the preferential release of specific metals at mining sites (Kwong et al.,

2003). Galvanic interaction between stibnite and pyrite has been observed in a polished section of a stibnite mineral containing impurities of pyrite (Kwong et al., 2006). The objective of this study is to determine the impact of pyrite on the dissolution rate of stibnite in unconsolidated mixtures of the two minerals subjected to leaching and determine the change in the solid- phase Sb speciation.

Materials and Methods

Mineral preparation

Mineral grains of stibnite and pyrite were purchased from Alfa Aesar and checked for purity by XRD. The stibnite and pyrite minerals were stored in an N2 purged glove box to preserve their oxidation state until the leaching experiments. For the column leaching experiment, both minerals were ground <250 µm in a glove box. The final particle size distribution and the specific surface area for the minerals used in each experiment are reported in Table 1. The specific surface area was determined using the single point N2 absorption and the Brunauer- Emmet - Teller (BET) method.

24 Flow through experiment

Two column leaching studies were carried out to assess the impact of various amounts of pyrite on the oxidation rate of stibnite. Each flow-through experiment was designed as complete randomized blocks with three replicates. The first study screened a large range of stibnite to pyrite ratios while the subsequent experiment refined the observations for samples in which the pyrite dominated (Table 2).

The minerals were weighed, mixed well and then placed into a 10 mL syringe between two frits. The syringes were covered in aluminum foil to maintain dark conditions as UV light can affect the oxidation rate of sulphide minerals (Lengke and

Tempel, 2001). The syringes were placed between two 50 mL syringes attached to a

Centurion mechanical extractor which regulates the flow of the distilled water through the mineral samples. A volume of 20 mL of double deionized water was then poured into the top syringe, pulled through the mineral mixture at a rate of 2.4 mL per hour, and collected in the bottom syringe. Water was pulled through the syringes for 9 hours and then air was pulled through for the remaining 9 hours to allow for the oxidative reaction to occur between the minerals (Hendershot et al., 2007). This cycle was repeated every

24 hours for 6 days.

After each leaching event, the pH, Eh and EC values were measured using a

Radiometer PH82, Horiba D-53, and Radiometer CDM 83 probe, respectively, and then recorded. The solution was filtered at 0.45 µm with a nylon membrane, acidified to 0.2 mL HNO3/ L and analyzed for dissolved Sb, Fe, As using ICP-MS or ICP-AAS. Sulfate concentrations were determined on unacidified filtered solution using high performance liquid chromatography (HPLC). For the purpose of comparison between treatments, the

25 dissolved Sb or Fe concentrations were corrected by the respective amount of stibnite or pyrite in each sample. Therefore, dissolved Sb is expressed in terms of mg Sb per kg of stibnite, and dissolved Fe as mg Fe per kg pyrite.

XANES and SEM analysis

To understand the change in the solid phase speciation of Sb, X-ray absorption near edge structure (XANES) spectroscopy was used to determine the average oxidation state and speciation of antimony in the final treated samples. The XANES analyses were conducted at the beamline X11A at the National Synchrotron Light Source (NSLS), the

Brookhaven National Laboratory (NY).

The following reference compounds were included in the study as end-members for Sb(V)-O, Sb(III)-O and Sb(III)-S species respectively, and analyzed by XRD for purity check: KSbO3·3H2O, Sb2O3 and Sb2S3. KSbO3·3H2O was used instead of Sb2O5 as a representative species for Sb(V)-O because commercially purchased Sb2O5 commonly contains a non negligible fraction of Sb2O3. All reference materials and samples were diluted in boron nitride (BN) to a concentration yielding an edge step of 1 in transmission mode and mounted behind Kapton tape in acrylic plexiglass holders

(Kelly et al., 2008). Data were collected in transmission mode using ion chambers. The

Si(311) monochromator was calibrated at the Sb K-edge using the first peak of the first derivative XANES spectrum for the Sb foil (30491 eV). The energy scale for each sample was also referenced to the edge in the spectrum for Sb(0) foil collected in transmission mode simultaneously with sample data. XANES data were corrected for baseline and normalized using Athena 8.053 (Ravel and Newville, 2005).

26 Least-squares linear combination fitting (LCF) was performed on the Sb K-edge

XANES spectra of the samples using all possible binary and ternary combinations of the three end-members, in the range from -40 to 80 eV (relative energy). During fitting, no constraints were imposed on weighting factors to be positive or to sum to 1, and no energy offset parameters were included. Combinations with negative weighting factors were rejected. R-factor, reduced chi-squared values and the sum of fraction before normalization were adopted as goodness-of-fit criteria.

To complement the bulk XANES analysis and get some insight at the grain scale, a restricted number of treated samples were examined using a Hitachi S-3200N scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer. The images were collected from loose, freeze-dried material that was dispersed in a thin layer and fixed onto a standard SEM sample platen.

Statistical analysis

To test the significance of the treatments and time factors on the Eh, pH and dissolved metal concentrations, a repeated measures MANOVA test was performed on the data from each experiment using SAS version 9.1 (SAS Institute Inc., 2009). The within subject effects test analyzed the change in time of each treatment. The between subject effects analyzed the difference between treatments in each time frame.

27

Results and Interpretation

Flow through experiment with differing pyrite to stibnite ratios: Experiments 1 and 2

In experiment #1 the pH of the leaching solution differs significantly among the treatments, especially in the first two days of the experiment (Figure 1, Table 3). The treatments with high amounts of pyrite (100% pyrite; 75% pyrite) had an initial pH around 5.7 compared to more acidic pH levels (~4) for the treatments dominated by stibnite in the mixture (90%; 95%; 100% stibnite). All treatments however, converged to a comparable pH around 4 from day 4 to the end of the leaching cycle. This decreased pH range between samples suggests the slowing of reactions for all treatments.

Similar to the pH trends, the redox potential (Eh) readings between treatments greatly varied ranging from 308 mV for 100% stibnite to 104 mV for 100% pyrite treatments (data not shown). By day 6, all samples had stabilized to a similar value, approximately 325 mV.

Compared to the pH levels in the initial experiment, there was a smaller range of pH in the first few leaching cycles between treatments in the second experiment

(excluding the 100% stibnite sample) (Figure 2). For all stibnite/pyrite mixtures with elevated amounts of pyrite (>75%), the pH at day 1 was approximately 6 and decreased to 3.8 after six days of leaching. The 100% stibnite and pyrite treatments both had a pH of 4.29 at the end of the experiment. The pH did decrease slightly with the treatments containing the mineral mixtures and this may be due to proton production during the oxidation of stibnite (Formula 1) (Lengke and Tempel, 2001). This trend was observed

28 for the Eh values as well where compared to the Eh levels on day 1 in the initial experiment, the treatments shared similar Eh values in a range between 70 mV for 95% stibnite and 114 mV for 80% pyrite (data not shown). The Eh values for all mixed treatments steadily increased to approximately 345 mV over the course of the experiment.

The 100% stibnite sample had an Eh reading of 357 mV on day 1 and this approximate value was observed for the remainder of the experiment.

When looking at the overall trend of pH and Eh over the six days of leaching, a decrease in pH is coupled with an increase in Eh, indicating that an oxidation reaction is occurring within the samples. Compared to the mixed treatments, the 100% stibnite sample has the smallest change in pH and Eh levels suggesting the oxidative reaction is less intense for the stibnite alone (Sparks, 2003). EC (data not shown), pH, and Eh (data not shown) all show a large change in values in the initial days of the experiment suggesting that the reactions occurring between the minerals were greatest during the initial part of the experiment. This trend is also observed in the dissolved Sb, Fe and SO4 values for both experiments.

There are two aspects to consider in the data analysis of the dissolved concentrations of Sb, Fe and SO4 in solution. The analysis of dissolved metals on a mass basis (mg/kg mineral) is needed to compare the dissolution of stibnite (or pyrite) between different treatments as the amount of the mineral varies among treatments. On the other hand, the non-cumulative molar concentrations of Sb, Fe and SO4 in µmol/L are also needed to determine whether reprecipitation has occurred in solution. Therefore, the excess of SO4 was calculated assuming a molar ratio of 1Fe:2S for the dissolution of pyrite (FeS2) and 2Sb:3S for the dissolution of stibnite (Sb2S3). If there is an excess of

29 SO4 in solution, Sb or Fe has precipitated out of solution leaving this imbalance in ratio between the metals and sulphate, also known as an incongruent ratio.

In the initial experiment over the six days of leaching, the cumulative release of dissolved Sb significantly increased with increasing amounts of pyrite in the mixture

(Figure 3, Table 3). The treatment with the highest amount of pyrite and least amount of stibnite (75% pyrite) produced the highest amount of dissolved Sb with a cumulative release of 6880 mg Sb/kg of stibnite compared to 1080 mg Sb/kg of stibnite for the treatment with stibnite alone (Figure 3). The same pattern of Sb dissolution was established in the second experiment: the addition of pyrite significantly affected the dissolution of stibnite (Table 3; Figure 4). In contrast to experiment 1 where solution pH differed among treatments, all mineral mixtures in experiment 2 (95% pyrite to 75% pyrite) had comparable solution pH and the difference in the stibnite oxidation rate among treatments is thus reflective of the impact of pyrite. The stibnite treatment with the highest amounts of pyrite (95% pyrite) released the highest amount of dissolved Sb with a cumulative amount of 16 730 mg Sb/kg of stibnite (Figure 4). The concentration of dissolved Sb expressed in µmol/L shows that the 50% pyrite and 75% pyrite treatments maintained the highest concentrations of Sb in solution (Table 4, 50% pyrite =

10.7 µmol/L, day 2: 75% pyrite = 15 µmol/L, day 2). These were also the only two treatments containing stibnite that had no excess of SO4 in solution in the first leaching cycle, suggesting no reprecipitation of Sb and/or Fe (Table 4). The total metal concentration over the 6 days of leaching also showed 75% pyrite with the highest level of total metal of the mixed treatments (µmol/L) (Table 4- total metal over 6 leaching days, 75% pyrite= 77.8 µmol/L).

30 In the initial experiment the mixed stibnite/pyrite treatments with the highest amounts of stibnite (95% stibnite; 90% stibnite) resulted in the highest levels of cumulative dissolved Fe over the six day leaching cycle (Figure 5, Table 3). By the final leaching cycle, the 95% stibnite treatment released a cumulative amount of dissolved Fe of 543 mg Fe/ kg of pyrite compared to 462 mg Fe/ kg of pyrite for the 75% pyrite treatment. The 100% pyrite treatment had a cumulative dissolved Fe level of 455 mg

Fe/kg of pyrite (Figure 5). Because the 100% pyrite treatment had the lowest levels of cumulative dissolved Fe, this would suggest that galvanic protection of pyrite was not occurring in the mixture containing stibnite. However when looking at the non cumulative amount of dissolved Fe per unit of pyrite for each leaching cycle the trend of galvanic protection is visible (Figure 5, inset). After day 3, the rate at which dissolved Fe increased was greater in the 100% pyrite treatment than the other mixed treatments whose dissolved Fe concentrations were beginning to level off. Therefore these data showed that galvanic protection is only visible by the third leaching cycle and this may be due to the difference in pH between treatments in the initial stages of the experiment where the

95% stibnite treatment had a pH of 4 compared to the 100% pyrite treatment that had a pH of 5.7 on day 1.

In experiment 2, dissolved Fe exhibited the same trend as in the first experiment where initially because of the lower pH levels for the treatments with stibnite, the concentrations of dissolved Fe were greater for the mixed treatments than for the pyrite alone in the first 3 leaching cycles. However in the final three leaching cycles, the rate of

Fe dissolution was greater for pyrite alone than for the mixed treatments with stibnite and this trend is more easily visible from the non cumulative amount of dissolved Fe after

31 each cycle (Figure 6, inset). For pyrite alone, the non-cumulative release of dissolved Fe concentration for day 6 reached 53 mg Fe/kg of pyrite compared to 42 mg Fe/kg of pyrite for the 95% pyrite treatment (Table 3; Figure 6, inset). However, on the basis of cumulative amount of Fe released in solution, the confounding effect of pH masked the trend overall and the cumulative amount of dissolved Fe over the six leaching cycles was higher in the mixed treatments with stibnite than in the treatment of pyrite alone (Figure

6).

The difficulty in viewing galvanic protection from the solution data is also explained by the dissolved Fe data in µmol/L. The 100% pyrite treatment exhibited excess SO4 levels from day 1 to day 4 in experiment #1 (Table 4) and from day 1 to day 5 in experiment #2 (Table 5). This means that the ratio of Fe to SO4 for these days was incongruent and Fe has reprecipitated. Therefore, the cumulative amounts of dissolved

Fe expressed in mg/kg pyrite are likely underestimated. This incongruent ratio is also visible in the total metal and sulphate over 6 leaching days for 100% pyrite (Table 4-

100% pyrite total metal and SO4 over 6 days= 16.4 µmol/L, 50.1 µmol/L). Considering the solubility chart for iron minerals depicting log Fe2+ in function of pe + pH reported in

Lindsay (1989) (graph not shown), the Fe precipitate would likely be in the form of fresh precipitate Fe3(OH) 8, based on the dissolved Fe concentrations and pe + pH measured in this treatment.

When looking at the cumulative amount of dissolved sulphate in experiment #2, the 95% pyrite displayed the highest level of SO4 in mg/kg mineral, which is in agreement with the treatment having the highest amount of pyrite allowing for the greatest dissolution rate of stibnite (Figure 7). For stibnite alone and all mixed treatments

32 a general trend is observed in µmol/L where excess SO4 is observed on day 1 (excluding

50% and 75% pyrite treatments) however from day 2 to day 6, the excess SO4 amounts become negative. This means that for all mixed treatments on day 1, the ratio of Sb and

Fe to SO4 were incongruent and Sb and/or Fe precipitated out of solution, allowing for the excess SO4 levels (Table 5). However, from days 2 to 6 the oxidation rates of the minerals must have decreased and this is exhibited in the low concentrations of SO4 in solution compared to the total SO4 over 6 days (100% stibnite Exp #2- Day 1= 11.2

µmol/L SO4: Day 6= 0.6 µmol/L SO4; total over 6 days= 16.7 µmol/L, Table 5) . This evidence of Sb and/or Fe reprecipitation in the first leaching cycle indicates that the cumulative amounts of dissolved Sb determined in mg/kg stibnite are likely underestimated in most treatments, except for the 50% and 75% pyrite treatments. This is visible in the 75% pyrite vs. 95% pyrite treatment where the 75% treatment had less total SO4 over 6 days than total metal (Table 5). The 95% pyrite treatment had more SO4 than total metal over the 6 days (Tables 5).

XANES and SEM analysis

XANES spectra were used to determine the relative oxidation states and speciation of Sb in the solid phase on a restricted number of samples. One characteristic of XANES is that spectra shift to higher energy as the oxidation state of the element increases (Figure 8A) because more energy is needed to excite the fewer electrons that are more tightly bound to the nucleus (Fendorf and Sparks, 1996; Kelly et al., 2008). In addition, XANES spectra from oxide species usually exhibit higher absorption peaks than their sulphide species (Figure 8A). Qualitative comparison of the XANES spectra

33 showed that the treatment containing 95% pyrite appears the most oxidized and had the spectra furthest to the right with the highest peak, followed by the 75% pyrite and 100% stibnite samples. Figure 8B illustrates the Sb XANES spectra for these three selected treatments along with the predicted data. Results from least-squares fitting for all treatments are presented in Figure 9.

All samples displayed the presence of Sb(V) oxidation species while only the

75% pyrite sample displayed the presence of an Sb(III)-O species (Figure 9). The sample with 95% pyrite contained the highest proportion of total Sb as Sb-O species whereas stibnite alone contained the lowest proportion of oxidized species. The sample of stibnite before the leaching process was already weakly oxidized with 12% of the total Sb as

Sb(V)-O so the 6 days of leaching for the 100% stibnite control resulted in a very small increase of the oxidation state of the stibnite treatments (15% of total Sb as Sb(V)-O;

Figure 9).

The XANES analyses provided an understanding of the solid phase in terms of bulk analysis. The leached mixtures were also examined at the mineral grain level using a

SEM to image the morphology of individual grains, as well as pinpoint the elemental composition on selected spots located on the grain. Samples containing 75% pyrite and

100% stibnite were viewed under SEM. Approximately 20 grains were viewed for each sample with 5 spots examined on each grain. The SEM images displayed are reflective of the trends observed.

Upon visual inspection of the grains, the backscattering image of the 25% stibnite sample shows fresh clean surfaces devoid of features while the leached 100% stibnite sample had rough and pitted surfaces with considerable microtopography (Figures 10,

34 11). Compared to the X-ray signal for the 75% pyrite treatment in which stibnite grains had very low oxygen levels but high counts of sulphur, the signal for 100% stibnite grains had higher counts in oxygen and was depleted in sulphur, indicating more oxidation of the grain surfaces.

Discussion

The leachate solution from the flow-through experiment revealed that the higher amount of pyrite added to the system, the greater the dissolution rate of stibnite due to the galvanic interaction between the two minerals. While this was demonstrated with the mass based data expressed in mg dissolved Sb per kg stibnite, the concentration of Sb in

µmol/L for stibnite alone and the mixed treatments revealed that in the first leaching cycle the solution was supersaturated and Sb had reprecipitated. This means that the solution data on day 1 does not provide an accurate amount of dissolved Sb and it is unclear whether there was an increased oxidation rate of stibnite due to contact with pyrite. However for all mixed treatments from day 2 to day 6 there was a negative value for excess SO4 levels meaning the oxidation rates of the mixed treatments decreased and

Sb likely was not supersaturated in solution. From day 2 to 6 the 95% treatment continued to release the highest amount of dissolved Sb in mg/kg stibnite.

Pyrite appeared to be galvanically protected in all mixed stibnite/pyrite treatments compared to pyrite alone. The galvanic protection of Fe in these mixed treatments was not visible until the third leaching cycle of the two experiments where the dissolution rate of the 100% pyrite treatment began to increase at a greater rate than in the other treatments which levelled off. The delay in the galvanic effect is likely due to the

35 differences in pH between the treatments with stibnite and the treatment with pyrite alone. On day 1 the 10% pyrite treatment had a pH of 4 compared to a pH of 5.8 for pyrite alone in the initial experiment. The lower pH for the 10% pyrite treatment may account for the increased levels of dissolved Fe on day 1 compared to pyrite alone. For the purposes of gold recovery, Kelsall and Welham (2000) compared the oxidation rate of gold plated pyrite in pH 2, 4 and 6 in sodium chloride. From pH 4 to 6, the rate of pyrite oxidation decreased fourfold (Welham and Kelsall, 2000), suggesting that the lower pH~4 associated with the 10% pyrite treatments would increase the Fe dissolution for pyrite compared to the pyrite alone (pH 5.7). The molar ratio of Fe concentration in solution to those of SO4 in solution for the treatment with pyrite alone was incongruent from day 1 to 5 in experiment #2 suggesting that an amount of Fe had reprecipitated out of solution for most of the leaching cycles and this makes it difficult to fully understand the pattern of pyrite oxidation between treatments.

The solid phase analysis of the mineral mixtures using XANES displayed the treatment with 95% pyrite as having the highest proportion of total Sb as Sb(V)-O species while the control of stibnite alone was the least oxidized. While the reprecipitation of Sb and/or Fe prevents us from clearly defining the ratio of stibnite to pyrite at which the highest dissolution rate of stibnite occurs, the solid phase analysis clearly shows that stibnite in the 95% pyrite treatment is the most oxidized. This suggests that the oxidation rate of stibnite was greatest in the treatment containing the highest amount of pyrite. The

75% pyrite treatment displayed overall amounts of oxidation between the 95% pyrite and

100% stibnite treatments however there was the presence of an Sb(III) species, suggesting

(III) (V) the possible formation of an Sb /Sb intermediary species such as Sb2O4. Xia et al.

36 (2009) studied the factors affecting the dissolution and replacement of pentlandite

[(FeNi)9S8] with the reprecipitation of violarite [(NiFe)3S4] including specific surface area and concentration of Fe3+ and Ni2+ ions in solution. Using a powder of pentlandite ground into three grain fractions, 65-150µm, 150-400 µm and 400-1000 µm, it was determined that with increased surface area of the total available original mineral, reprecipitation onto the surface of that mineral was greater (Xia et al., 2009). The greater surface area for stibnite in the 75% pyrite treatment compared to the 95% pyrite would be a factor that could favour the reprecipitation. Xia et al. (2009) also determined that with increasing Fe3+ ions and Ni2+ ions in solution, precipitation was inhibited meaning a greater dissolution rate would decrease the reprecipitation rate. The cumulative amounts of dissolved Sb in mg/kg of stibnite indicate more Sb dissolved in the 95% than the 75% pyrite treatment over 6 days of leaching. The dissolution rate of stibnite was thus slower in the 75% pyrite than in the 95% pyrite treatment which might have allowed for reprecipitation of the intermediary Sb(III) species. However, based on the solution data in

µmol/L, dissolved Sb concentrations were higher in the 75% than 95% pyrite and the absence of excess SO4 in the 75% pyrite treatment would not support reprecipitation of

Sb and/or Fe. Further investigations are needed to clarify these data. In particular, solubility phase diagrams need to be addressed given that pH levels were not comparable between treatments. When the Eh and pH results of the final leaching cycle for the 75% pyrite treatment were compared to the Eh-pH diagram of Vink (1996), the 75% pyrite treatment fit on the stability line of Sb2O4. This suggests that the solid phase of the 75%

(III) pyrite treatment could be an intermediary Sb species similar to Sb2O4.

37 The analysis of individual stibnite grains from the 75% pyrite and 100% stibnite samples revealed the presence of more oxygen on the surfaces of the grains in the 100% stibnite sample. The XANES analysis indicated the smallest proportion of total Sb as oxide species for this sample and therefore the slowest rate of dissolution which could allow for the accumulation and subsequent evolution of oxide products on the stibnite surface. In contrast, the stibnite in the 75% pyrite sample exhibited a higher dissolution rate and would see the oxide species being leached into solution with reprecipitation away from the stibnite surfaces. Domingo et al. (1993) had similar conclusions in a study on factors influencing the oxidation rate of Fe(OH)2 and conversion into FeOOH on the surface of Fe(OH)2. When the oxidation rate of Fe(OH)2 and subsequent rate of dissolution was slow, the low rates of reaction allowed for the nucleation and growth of a well crystallized oxide, FeOOH (Domingo et al., 1993; Xia et al., 2009). The trend observed in the study of Domingo et al. (1993) was similar to what was observed in the

XANES spectra and SEM images which explains why the 100% stibnite sample would have greater oxide deposition on the surface of the mineral with a slower dissolution rate than the 75% pyrite sample.

Conclusion

The dissolution of stibnite and subsequent release of toxic Sb species into the local water system is of concern to mining communities and other areas that depend on the water. This study shows that the oxidation of stibnite is enhanced when in contact with pyrite and that stibnite’s oxidation rate increases with increasing amounts of pyrite in the system. This was particularly evident in solid phase analysis of the leached

38 treatments however more in-depth analysis would need to be done on the solution data.

Because of the likely reprecipitation of dissolved Sb in the initial leaching cycle and pH variation among treatments, the amount of dissolved Sb in mg/kg of stibnite has left some questions about the initial interaction between stibnite and pyrite at the beginning of the leaching experiment. A next step would be to produce a solubility chart and determine the concentration and pH levels required to have undersaturated levels of Sb and Fe in solution so that accurate readings could be taken from start to finish of the column leaching experiment.

The overall results provide further evidence as to how Sb can be transferred from mine wastes to the aqueous system around mining operations. This study determined that under high dissolution rates of stibnite, part of the dissolved Sb may reprecipitate as Sb oxide species in the solid phase which represents an attenuation process that is likely to occur in the natural environment.

Further research is required to clarify the Sb species forming after the dissolution of stibnite in order to develop management options that would favour the stability of this metalloid in the mine wastes. The behaviour of stibnite under different pH conditions and when in contact with other sulphide minerals of higher rest potential such as arsenopyrite will also allow for a more comprehensive understanding of stibnite’s behaviour in varying mining conditions.

39 Table 1- Particle size distribution and surface area of stibnite and pyrite used in experiments 1(A) and 2(B).

A- Experiment 1 Grain size fraction/ Surface Stibnite (Sb2S3) Pyrite (FeS2) area

<53 µm (%) 32.4 22.8

> 53 to <100 µm (%) 38.3 40.8

>100 to <250 µm (%) 29.3 36.4

Specific Surface Area (m2/g) 0.176 ± 0.192 0.75 ± 0.075

B- Experiment 2 Grain size fraction/ Surface Stibnite (Sb2S3) Pyrite (FeS2) area

<53 µm (%) 15.0 11.9

> 53 to <100 µm (%) 50.9 31.6

>100 to <250 µm (%) 34.1 56.5

Specific Surface Area (m2/g) 0.190 ± 0.124 0.098 ± 0.036

40 Table 2: Mass ratio of stibnite to pyrite for experiments 1 and 2; total mixture of 2 g per sample

Treatment/ Exp. Exp. 2 Sb2S3 FeS2 Mass Ratio Specific Surface Ratio 1 (g) (g) (Stibnite-to- Area Ratio

(Sb2S3/FeS2) pyrite) (Stibnite-to- pyrite) 100/0 x x 2.00 0.00 100.0 100.0 95/5 x 1.90 0.10 19.0 44.6 90/10 x 1.80 0.20 9.0 21.1 75/25 x 1.50 0.50 3.0 7.0 50/50 x 1.00 1.00 1.0 2.4 25/75 x x 0.50 1.50 0.33 0.65 20/80 x 0.40 1.60 0.25 0.45 15/85 x 0.30 1.70 0.18 0.34 10/90 x 0.20 1.80 0.11 0.22 5/95 x 0.10 1.90 0.05 0.10 0/100 x x 0.00 2.00 0 0

41 Table 3- Results from the multivariate analysis of variance for the effects of treatments and time on pH, Eh and dissolved Sb, Fe and sulphate concentrations in experiments 1(A) and 2(B). A.

MANOVA Numerator- Denominator- pH Eh EC Sb Fe SO4 test Degrees of Degrees of Freedom Freedom Prob. >F (d.f.) (d.f.)

Treatment 6 14 <.0001 0.0002 0.0003 <.0001 <.0001 <.0001

Time 5 70 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001

Treatment 30 70 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 x Time

B.

MANOVA Numerator- Denominator- pH/Eh/EC Sb Fe SO4 test Degrees of Degrees of Freedom Freedom (d.f.) Prob. >F (d.f.)

Treatment 6 14 <.0001 <.0001 <.0001 <.0001

Time 5 70 <.0001 <.0001 <.0001 <.0001

Treatment 30 70 <.0001 <.0001 <.0001 <.0001 x Time

42 Table 4- Molar concentrations of dissolved Sb, Fe, and SO4 for all treatments in µmol/L from day 1 to 6 for experiment #1.

Treatment Day Sb Fe Total metal SO4 Excess † SO4 (µmol/L) 100% 1 6.9 N.A. †† 6.9 10.8 0.34 stibnite 2 6.7 N.A. 6.7 2.2 -5.2 3 6.5 N.A. 6.5 1.2 -5.7 4 6.2 N.A. 6.2 1.0 -6.0 5 6.3 N.A. 6.3 0.9 -5.7 6 5.9 N.A. 5.9 0.61 -5.5 Total of 38.5 N.A 38.5 16.7 N.A. 6 days

100% 1 N.A. 5.6 5.6 21.2 10 pyrite 2 N.A. 0.09 0.09 6.4 6.22 3 N.A. 1.0 1.0 5.3 3.3 4 N.A. 2.6 2.6 6.3 1.1 5 N.A. 3.3 3.3 5.5 -1.1 6 N.A. 3.8 3.8 5.4 -2.2 Total of N.A. 16.4 16.4 50.1 N.A. 6 days

5% pyrite 1 5.1 0.4 5.5 8.5 0.1 2 4.3 0.1 4.4 1.9 -4.7 3 4.5 0.1 4.6 1.4 -5.5 4 4.8 0.1 4.9 1.2 -6.2 5 4.5 0.1 4.6 0.9 -6.0 6 4.5 0.1 4.6 2.9 -4.0 Total of 27.7 0.9 28.6 16.8 N.A. 6 days

43 10% pyrite 1 4.5 0.8 5.3 9.7 1.4 2 5.2 0.2 5.4 2.2 -6.0 3 5.2 0.2 5.4 1.2 -7.0 4 5.4 0.2 5.6 1.1 -7.4 5 5.5 0.2 3.6 0.9 -7.7 6 5.4 0.2 4.1 0.8 -7.7 Total of 31.2 1.8 33 15.9 N.A. 6 days

25% pyrite 1 4.7 2.3 7.0 12.4 0.8 2 7.7 0.3 8.0 2.2 -9.9 3 7.7 0.4 8.1 1.3 -11.0 4 6.7 0.5 7.2 1.2 -9.8 5 6.8 0.5 7.3 1.1 -10.0 6 6.5 0.5 7.0 0.9 -9.8 Total of 40.1 4.5 44.6 19.1 N.A. 6 days

50% pyrite 1 9.2 4.5 13.7 14.8 -7.9 2 10.7 0.5 11.2 2.9 -14.1 3 10.7 0.6 11.3 2.1 -15.1 4 8.2 0.8 9.0 1.7 -12.1 5 8.0 0.8 8.8 1.2 -12.3 6 7.6 0.8 8.4 1.2 -11.7 Total of 54.4 8 62.4 23.9 N.A. 6 days

75% pyrite 1 10.6 6.5 17.1 18.5 -10.4 2 15.0 0.85 15.9 3.1 -8.7 3 15.0 0.96 16.0 3.1 -8.9 4 9.2 1.23 19.6 3.1 -13.1 5 8.0 1.4 9.4 3.0 -11.7 6 7.6 1.5 9.1 2.9 -5.2 Total of 65.4 12.4 77.8 33.7 N.A. 6 days † Excess: The excess of SO4 was calculated assuming a ratio of 1Fe:2S for the dissolution of pyrite (FeS2) and 2Sb:3S for the dissolution of stibnite (Sb2S3).

†† N.A.: Not applicable.

44

Table 5- Molar concentrations of dissolved Sb, Fe, and SO4 for all treatments in µmol/L from day 1 to 6 for experiment #2.

Treatment Day Sb Fe Total metal SO4 Excess † SO4 (µmol/L) 100% 1 3.5 N.A. †† 3.5 11.2 4.1 stibnite 2 4.2 N.A. 4.2 1.9 -2.9 3 4.1 N.A. 4.1 1.0 -3.4 4 3.8 N.A. 3.8 0.9 -3.2 5 3.6 N.A. 3.6 0.7 -1.1 6 3.1 N.A. 3.1 0.6 -2.7 Total of 22.3 N.A. N.A. 16.3 N.A. 6 days

100% 1 N.A. 4.7 4.7 20.7 11.3 pyrite 2 N.A. 0.01 0.01 3.9 3.88 3 N.A. 0.2 0.2 3.2 2.7 4 N.A. 1.1 1.1 4.1 1.9 5 N.A. 1.6 1.6 3.5 0.3 6 N.A. 1.9 1.9 3.8 -0.02 Total of N.A. 9.5 9.5 39.2 N.A. 6 days

75% pyrite 1 5.0 4.8 9.8 17.5 0.4 2 7.2 1.0 8.2 3.3 -9.4 3 5.5 1.2 6.7 2.2 -8.4 4 4.5 1.3 5.8 2.5 -6.5 5 4.4 1.3 5.7 1.9 -7.3 6 4.2 1.4 5.6 2.1 -7.0 Total of 30.8 11 41.8 29.5 N.A. 6 days

45

80% pyrite 1 4.9 4.9 9.8 17.3 0.2 2 7.6 1.0 8.6 3.4 -9.9 3 6.3 1.2 7.5 2.6 -9.2 4 5.2 1.3 6.5 3.2 -7.2 5 5.0 1.4 6.4 2.2 -8.1 6 4.7 1.6 6.3 2.5 -7.7 Total of 33.7 11.4 45.1 31.2 N.A. 6 days

85% pyrite 1 4.4 4.9 9.3 18.1 1.7 2 7.8 1.1 8.9 3.3 -10.5 3 5.9 1.2 7.1 2.6 -8.6 4 4.7 1.3 6.0 3.5 -6.1 5 4.5 1.4 5.9 2.6 -6.9 6 4.2 1.5 5.7 2.6 -6.7 Total of 31.5 11.4 42.9 32.7 N.A. 6 days

90% pyrite 1 4.2 4.7 8.9 17.6 1.9 2 6.3 0.6 6.9 4.4 -6.2 3 4.8 1.2 6.0 3.3 -6.3 4 3.3 1.1 4.4 4.0 -3.1 5 2.5 1.1 3.6 2.7 -3.2 6 2.5 1.2 3.7 3.0 -3.1 Total of 23.6 9.9 33.5 35 N.A. 6 days

95% pyrite 1 3.4 4.8 8.2 19.1 4.4 2 3.4 0.6 4.0 5.1 -1.2 3 2.2 0.9 3.1 4.2 -0.9 4 1.8 1.2 3.0 5.0 -0.1 5 1.6 1.3 2.9 4.2 -0.8 6 1.6 1.4 3.0 3.7 -1.5 Total of 14 10.2 24.2 41.3 N.A. 6 days † Excess: The excess of SO4 was calculated assuming a ratio of 1Fe:2S for the dissolution of pyrite (FeS2) and 2Sb:3S for the dissolution of stibnite (Sb2S3).

†† N.A.: Not applicable.

46

Figure 1- pH of the leached solutions over the six leaching cycles- Experiment #1.

47

Figure 2- pH of the leachates over the six leaching cycles- Experiment #2.

48

Figure 3- Cumulative release of dissolved Sb over the six leaching cycles as affected by various amounts of pyrite in the mixture - Experiment #1.

49

Figure 4- Cumulative release of dissolved Sb over the six leaching cycles as affected by various high amounts of pyrite in the mixture - Experiment #2.

50

Figure 5- Cumulative release of dissolved Fe over the six leaching cycles as affected by various amounts of pyrite in the mixture - Experiment #1; Inset: Non cumulative release of dissolved Fe after each leaching cycle for three selected treatments.

51

Figure 6- Cumulative release of dissolved Fe over the six leaching cycles as affected by various high amounts of pyrite in the mixture - Experiment #2; Inset: Non cumulative release of dissolved Fe after each leaching cycle for three selected treatments.

52

Figure 7- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various high amounts of pyrite in the mixture - Experiment #2.

53

Figure 8- A. Sb K-edge XANES spectra for the reference compounds of KSbO3·H2O, Sb2O3, Sb2S3 used as representative species for Sb(V)-O, Sb(III)-O and Sb(III)-S respectively in the fitting analysis. B. Sb K-edge XANES spectra for the selected treatments with 5%, 25% and 100% stibnite after six leaching cycles (solid lines = measured data; dashed lines = predicted data; Vertical line is an eye guide only).

54 Figure 9- Best fitting results from least-squares linear combination for stibnite before leaching, the 95, 75, 50% pyrite and 100% stibnite treatments after six days of leaching.

55 x

25 µm

Figure 10- SEM image of the 100% stibnite sample after leaching and corresponding X-ray maps for O, S and Sb (left). Sb, O and S elemental composition of point x in SEM image (right).

56 x 25 µm

Figure 11- SEM image of the 75% pyrite sample after leaching and corresponding X-ray maps for O, S, Sb and Fe (top). Sb, O and S elemental composition of point x in SEM image (bottom right).

57 Connecting Statement

As observed in the previous study, the mixed treatment with the highest amount of pyrite

(95% pyrite) produced the highest amount of dissolved Sb in mg/kg stibnite in solution while the

concentration of Sb and Fe in µmol/L revealed their reprecipitation making it difficult to

determine whether stibnite increased in oxidation rate due to the greater amount of pyrite in the

system. However, solid phase analysis revealed that the 95% pyrite treatment contained the

highest proportion of total Sb as Sb(V) oxide species suggesting that this is the case. The

following chapter discusses other factors that affect the galvanic interactions between stibnite

and a mineral of higher electrode potential. The first study examines the effect of a leaching

solution buffered to pH 7.5 and the second study examines the substitution of pyrite for

arsenopyrite which has an electrode potential lower than pyrite but higher than stibnite. These

studies will increase the understanding of stibnite oxidation in mining environments where

stibnite is exposed to different sulphide minerals under various pH conditions.

58 Factors affecting the oxidation rate of stibnite- A buffered pH 7.5 leaching solution and the substitution of pyrite with arsenopyrite in a galvanic cell.

Abstract

When stibnite comes into contact with pyrite, a galvanic interaction occurs where

stibnite, the mineral of lower rest potential oxidizes at a greater rate when in contact with pyrite,

a mineral of higher rest potential. The first objective of the study was to determine the effects of a high pH buffered solution on the interactions between these two minerals. A leaching solution buffered to pH 7.5 was used in a column leaching flow- through experiment. The leaching solution was pulled through the treatments containing different ratios of stibnite and pyrite

(ranging from 75% pyrite/ 25% stibnite to 95% pyrite/ 5% stibnite) every 24 hours for 6 leaching cycles. The treatment with the greatest amount of pyrite released the highest cumulative

amount of dissolved Sb (Exp #3- 18 090 mg Sb/kg stibnite) which was a similar amount of

dissolved Sb (Exp #2- 16 730 mg Sb/kg stibnite) released by the same treatment in the

corresponding experiment that used DI water as the leaching solution (pH~5). Solution data

expressed in µmol/L revealed the supersaturation and precipitation of dissolved Sb in the initial

leaching cycle indicating that the cumulative amounts of dissolved Sb expressed in mg Sb/kg

stibnite were underestimated for day 1 of the experiment. XANES analysis of the solid phase

revealed a smaller proportion of Sb as Sb(V)-oxide species - compared to the previous

experiment with deionized water as a leaching solution, possibly due to the formation of a

- soluble Sb- HCO3 complex. The second objective of the study was to determine the effect of

using a mineral with a rest potential lower than pyrite, arsenopyrite, on the oxidation rate of stibnite. The treatment with the greatest amount of arsenopyrite released the greatest amount of

59 dissolved Sb (Exp #4- 95% arsenopyrite= 10 311 mg Sb/kg stibnite) among all mixed treatments.

Dissolved Sb precipitated on day 1 and dissolved Fe precipitated throughout the leaching

experiment. However this cumulative release of dissolved Sb was lower than the 95% pyrite

treatment in experiment #2 because the current density between stibnite and arsenopyrite was lower than between stibnite and pyrite thereby reducing the galvanic effect between the two minerals.

Introduction

When stibnite comes into contact with pyrite, stibnite will dissolve at a greater rate.

This has been established as a galvanic interaction between the two minerals, where the reactions

occurring on the surface of the anode are forced to occur at the same rate as the reactions

occurring on the surface of the cathode (Mehta and Murr, 1983). In the case of oxygenated

environments, the oxidative surface dissolution at the anode (MS = M2+ + S0 + 2e-) is coupled

+ with the reduction of oxygen at the cathode surface (0.5O2 + 2H +2e- = H2O) (Kwong et al.,

2003). As seen in previous leaching experiments, the more pyrite added into this system, the

greater the dissolution rate of stibnite. However, the pH difference between the treatments was a

confounding factor which prevented our ability to clearly isolate the effect of the galvanic

reaction. In the initial leaching cycle the treatment of pyrite alone had a solution pH of 5.8

compared to pH~4 with the 5% pyrite treatment. Thus, there is a need to control the pH in the

following experiment to better understand the role of pH in the galvanic interaction on the

dissolution of stibnite.

In other experiments testing the galvanic interactions between anodic and cathodic

minerals, pH levels have affected the dissolution rates of the anode mineral. At varying pH

levels, Mahlangu et al. (2006) combined stibnite with iron shavings in a reactor while pumping

60 in HCl and distilled water. An increase in the dissolution rate of stibnite was observed between

the pH range from 3 to 6 compared to other pH levels (Mahlangu et al., 2006).

Arsenopyrite, FeAsS, like pyrite, FeS2, is a common sulphide mineral found in gold

mining sites. The rest potential of arsenopyrite is approximately 0.45 V, between that of stibnite,

0.12 V and pyrite, 0.66 V. Therefore with arsenopyrite acting as the cathode in the system, the

reaction between arsenopyrite and stibnite may be weaker than the reaction between stibnite and

pyrite due to the smaller difference in rest potentials between arsenopyrite and stibnite (Abraitis

et al., 2004b; Nicol and Guresin, 2003). Kwong et al. (2003) studied a mineral assemblage

consisting of pyrite, galena and sphalerite. The mineral slabs were subjected to moist and dry air

then immersed in distilled water to leach the oxidation products. The dissolution rates of both

galena and sphalerite increased due to contact with pyrite, however towards the end of the

experiment the dissolution of sphalerite was greater than that of galena due to the greater

difference between the rest potentials of sphalerite and pyrite (Kwong et al., 2003).

The objectives of this study were to (1) determine the effects of a buffered leaching

solution with a pH of 7.5 on the galvanic interactions between pyrite and stibnite and (2) assess

the impact of the substitution of pyrite for arsenopyrite in the galvanic cell on the dissolution of

stibnite. The results will help us to better understand the behaviour of stibnite and the factors

affecting its dissolution rate in wastes from gold mining environments.

61

Materials and Methods

Mineral Preparation

To continue the research from the first two experiments (experiments #1 and #2), two

column leaching studies were carried out. Natural grains of stibnite, pyrite and arsenopyrite were

purchased from Alfa Aesar and checked for purity by XRD. The minerals were stored in an N2 purged glove box to preserve their oxidation state until the leaching experiments. For the column leaching experiment, all minerals were ground <250 µm in a glove box. The final particle size distribution and the specific surface area for the minerals used in each experiment are reported in Table 1. The specific surface area was determined using the single point N2

absorption and the Brunauer- Emmett - Teller (BET) method.

Flow -through experiment

For the third experiment, ratios of high amounts of pyrite were combined with stibnite as

in the previous experiment #2 but the mixtures were leached with a buffered NaCl solution (pH

7.5) instead of DI water (Table 2). Experiment #4 also used the same ratios as in experiment #2

along with DI water as the leaching solution but in this case, pyrite was substituted by

arsenopyrite (Table 2). Each flow- through experiment was designed as complete randomized

blocks with three replicates.

The minerals were weighed and well mixed then placed into a 10 mL syringe between

two frits. The syringes were covered in aluminum foil to allow for dark conditions as UV light

62 can affect the oxidation rate of sulphide minerals (Lengke and Tempel, 2001). The small

syringes were placed between two larger syringes attached to a Centurion mechanical extractor

which regulates the flow of the solution that percolates through the mineral samples. A volume

of 20 mL of leaching solution consisting of 10 mM NaCl adjusted to pH 7.5 with added NaHCO3

for experiment #3 or double deionized water for experiment #4 was then poured into the top

syringe, pulled through the mineral mixture at a rate of 2.4 mL per hour, and collected in the

bottom syringe. The solution was pulled through the syringes for 9 hours and then air was pulled

through for the remaining 9 hours to allow for the oxidative reaction to occur between the

minerals (Hendershot et al., 2007). This cycle was repeated every 24 hours for 6 days.

After each leaching event, the pH, Eh and EC values were measured using respectively a

Radiometer PH82, Horiba D-53, and Radiometer CDM 83 probes, respectively, and then

recorded. The solution was filtered at 0.45 µm, acidified to a pH<2 using conc. HNO3 and

analyzed for dissolved Sb, Fe, As using ICP-MS or ICP-AAS. Sulfate concentrations were

determined on unacidified filtered solution using HPLC. For the purpose of comparison between

treatments, the dissolved Sb, Fe and As concentrations were normalized by the respective

amount of stibnite, pyrite or arsenopyrite in each treatment. Therefore, dissolved Sb is expressed

in terms of mg Sb per kg of stibnite, dissolved Fe as mg Fe per kg pyrite (exp. 3) or arsenopyrite

(exp. 4) and dissolved As as mg As per kg arsenopyrite.

XANES and SEM analysis

To understand the change in the solid phase speciation of Sb, X-ray absorption near edge

structure (XANES) spectroscopy was used to determine the average oxidation state and

speciation of antimony in the final treated samples. The XANES analyses were conducted at the

63 beamline X11A at the National Synchrotron Light Source (NSLS), Brookhaven National

Laboratory (NY).

The following reference compounds were included in the study as end-members for

Sb(V)-O, Sb(III)-O and Sb(III)-S species respectively, and analyzed by XRD for purity check:

KSbO3·3H2O, Sb2O3 and Sb2S3. KSbO3·3H2O was used instead of Sb2O5 as a representative

species for Sb(V)-O because commercially purchased Sb2O5 commonly contains a non

negligible fraction of Sb2O3. All reference materials and samples were diluted in boron nitride

(BN) to a concentration yielding an edge step of 1 in transmission mode and mounted behind

Kapton tape in acrylic plexiglass holders (Kelly et al., 2008). Data were collected in transmission

mode using ion chambers. The Si(311) monochromator was calibrated at the Sb K-edge using

the first peak of the first derivative XANES spectrum for the Sb foil (30491 eV). The energy

scale for each sample was also referenced to the edge in the spectrum for Sb(0) foil collected in

transmission mode simultaneously with sample data. XANES data were corrected for baseline

and normalized using Athena 8.053 (Ravel and Newville, 2005).

Least-squares linear combination fitting (LCF) was performed on the Sb K-edge XANES

spectra of the samples using all possible binary and ternary combinations of the three end-

members, in the range from -40 to 80 eV (relative energy). During fitting, no constraints were

imposed on weighting factors to be positive or to sum to 1, and no energy offset parameters were

included. Combinations with negative weighting factors were rejected. R-factor, reduced chi-

squared values and the sum of fraction before normalization were adopted as goodness-of-fit

criteria.

64 Statistical analysis

To test the significance of treatments and time factors on the pH and dissolved metal

concentrations, a repeated measures MANOVA test was performed on the data from each

experiment using SAS version 9.1 (SAS Institute Inc., 2009). The within subject effects test

analyzed the change in time of each treatment. The between subject effects analyzed the

difference between treatments in each time frame (Table 3).

Results and Interpretation

High pyrite to stibnite flow- through experiment using a leaching solution buffered to pH 7.5: Experiment #3

On day 1, all treatments had the same pH at approximately 6.4 except for stibnite alone

that had a pH of 5.2 (Figure 1). The treatments with high amounts of pyrite (100%, 95% and

90% pyrite) remained at this pH for the remainder of the leaching experiment whereas the

treatments with lower amounts of pyrite (75%, 80% and 85% pyrite) dropped by more than one

pH unit on day 2 to 5.5, 4.9 and 5.1, and then stabilized to a pH of approximately 6.5 on day 3

until the end of the experiment. The pH in the treatment of stibnite alone increased to pH 7 on

day 2 and reached pH 7.5 in the last two leaching cycles. The pH continuing to be lower than pH

7.5 throughout the experiment for all mixed treatments with pyrite suggests that there was a

greater oxidation of stibnite occurring and releasing protons in these treatments (Figure 1).

With increasing amounts of pyrite within the mineral mixture, the cumulative

concentration of dissolved Sb significantly increased (Figure 2, Table 3A). The treatment with

the highest amount of pyrite in the mineral mixture (95% pyrite) released the highest amount of

65 dissolved Sb with a cumulative amount of 18 090 mg Sb/kg stibnite compared to 975 mg Sb/kg

stibnite for the treatment with stibnite alone.

The analysis of treatments on a mass basis (mg/kg mineral) takes into account the amount

of stibnite and pyrite in each treatment. By looking at the non-cumulative concentrations of Sb,

Fe and SO4 in µmol/L, it is possible to determine whether reprecipitation has occurred in

solution (Table 4). Looking at the excess amount of SO4 explains whether there was more SO4

in solution than Sb or Fe. The excess of SO4 was calculated assuming a ratio of 1Fe:2S for the

dissolution of pyrite (FeS2) and 2Sb:3S for the dissolution of stibnite (Sb2S3). If there is an

excess of SO4 in solution, Sb or Fe has precipitated out of solution leaving this imbalance in ratio

between the metals and sulphate also known as an incongruent ratio. The concentration of

dissolved Sb on day 1 for stibnite alone was 3.9 µmol/L while the concentration of SO4 was 12.8

µmol/L for the same treatment (Total Sb and SO4 over 6 days for 100% stibnite= 16.1 µmol/L

and 18.1 µmol/L, Table 4). In this treatment there was an excess of 7.0 µmol/L of SO4 on day 1 indicating that the solution was supersaturated and Sb precipitated in the initial leaching cycle.

From days 2 to 6, the calculated amount of excess SO4 was negative and, thus, no Sb or Fe

precipitated out of solution. The dissolved Sb data expressed in µmol/L showed that the 80%

and 75% pyrite treatments maintained the highest concentrations of dissolved Sb over the six leaching cycles. The 80% pyrite treatment had the highest total metal concentration over the 6 days (42.3 µmol/L, Table 4). The 95% pyrite treatment had the highest level of SO4 for the total

of the 6 days for the mixed treatments (total sulphate over 6 days, 95% pyrite = 33.2 µmol/L).

The 95% pyrite treatment also had the most congruent ratio out of all of the mixed treatments for

the 6 days total (Table 4).

66 The cumulative amount of dissolved Fe in all mixed stibnite/pyrite treatments ranged between 418 and 478 mg Fe/kg pyrite at the end of the experiment (75%; 80% pyrite; Figure 3).

The treatment of pyrite alone had a cumulative dissolved Fe amount of 793 mg Fe/kg pyrite by day 6 of the experiment (Figure 3). This amount was greater than all other treatments with a stibnite/pyrite mixture suggesting that part of the pyrite in the mixed treatments was galvanically protected by stibnite from oxidation (Kwong et al., 2003). For pyrite alone, from days 1 to 6, there was more Fe than SO4 and Fe precipitation did not occur (Table 4). In terms of µmol/L, the

85% pyrite had the highest concentrations of dissolved Fe over the 6 leaching cycles. However, once corrected for the amount of pyrite in the system, the cumulative amount of dissolved Fe per kg of pyrite was not significantly affected by the various ratios of stibnite to pyrite (Figure 3;

Table 3A).

The treatment of pyrite alone had the highest level of cumulative dissolved SO4 with

2277 mg/kg of mineral (Figure 4). The high level of cumulative dissolved SO4 in this treatment

was related to the high level of dissolved Fe in the pyrite alone. The 95% pyrite treatment had

the second highest cumulative amount of dissolved SO4 with 1592 mg/kg of mineral and the

results are in agreement with the treatment having the highest amount of pyrite allowing for the

greatest dissolution rate of stibnite (Figure 4). This overall dissolved SO4 trend from experiment

#3 is similar to the dissolved SO4 trend in experiment #2 where pyrite alone had the highest

cumulative dissolved SO4 levels followed by the 95% pyrite treatment.

67 XANES analysis

XANES spectroscopy was used to determine the relative oxidation states and speciation

of Sb in the solid phase on a restricted number of samples from experiment #3. XANES spectra

typically shift to higher energy as the oxidation state of the element increases (Figure 5A). In

addition, oxide species of a given element usually tend to have higher absorption peak than their

corresponding sulphide species (Figure 5A). Qualitative comparison of the XANES spectra

showed that stibnite in the 95% pyrite treatment appears to be more oxidized, showing a higher

absorption peak than the 100% stibnite treatment (Figure 5B). The Sb K-edge XANES spectrum

for the 75% pyrite treatment was similar to that of the 100% stibnite (data not shown).

Results from least-squares fitting confirmed that the 95% pyrite treatment contained the highest proportion of total Sb as oxides in the solid phase with 19% of total Sb present as Sb(V)-

O (Figure 6). The 75% pyrite and 100% stibnite treatments after 6 days of leaching show no increase in the overall oxidation state of antimony (11% and 10% Sb(V)-O) compared to the stibnite before leaching, as the stibnite before leaching was already slightly oxidized and contained 12% of the total Sb as Sb(V)-O.

Even though the cumulative release of dissolved Sb in the 95% pyrite treatment leached with NaCl was similar to that of the 95% pyrite treatment leached with DI water in experiment

#2, all three treatments leached with a buffered NaCl solution with pH 7.5 had lower proportion of Sb as Sb-oxide precipitates compared to the same treatments leached with distilled water.

This suggests that the use of a pH 7.5 buffered solution, specifically with the presence of

NaHCO3, had an effect on the amount of dissolved Sb released from each leaching cycle, and

indirectly on the solid phase species (Lengke and Tempel, 2001).

68

High arsenopyrite to stibnite flow- through experiment- Experiment #4

All treatments containing the stibnite/arsenopyrite mixture had a pH~7.5 for the entire

leaching experiment. The pH of the solution after it was leached through the stibnite/arsenopyrite

mixture was higher than the pH of the leaching DI water (pH~5.8). The stibnite alone had a

pH~4 for all six leaching cycles (Figure 7) and maintained a higher Eh level of 391 mV

compared to 338 mV for all other treatments (data not shown). For all treatments the EC levels

were high on day 1 (~118 µS cm-1) then greatly decreased by day 2 (~30 µS cm-1) and maintained these levels for the remainder of the experiment (data not shown). The drop in EC by day 2 suggests that the oxidation rate of all treatments decreased by the second leaching cycle as well.

The 95% arsenopyrite treatment exhibited the greatest cumulative dissolution of Sb over the six leaching cycles with 10 311 mg Sb/kg stibnite compared to 4071 mg/kg stibnite for the

75% arsenopyrite treatment (Figure 8). However compared to experiment #2 with a pyrite/stibnite mixture, the cumulative amount of dissolved Sb is significantly lower in the 95%

arsenopyrite than in the 95% pyrite treatment (10 311 vs. 16 731 mg Sb/kg stibnite respectively).

This suggests that while a galvanic interaction has occurred between the two minerals, the oxidation rate of stibnite was not as great with arsenopyrite compared to pyrite due to the smaller difference in rest potentials between stibnite and arsenopyrite, than between stibnite and pyrite.

Looking at the solution data in µmol/L, the treatment with stibnite alone as seen also in experiment #2 had an excess of SO4 in solution on day 1, indicating that Sb precipitated out of

solution (Table 5). For days 2 to 6, there was no excess of SO4 meaning Sb did not precipitate.

69 The solution data in µmol/L also shows the 75% arsenopyrite treatment maintained the highest

concentrations of dissolved Sb. This was also visible when looking at the total metal over the 6

days, where the 75% pyrite had the highest total metal concentration (23.2 µmol/L, Table 5).

Throughout the course of the experiment, the cumulative amount of dissolved As was

slightly higher in the treatment with arsenopyrite alone than in the mixed treatments (Figure 9;

Table 3B). The cumulative dissolved As levels were comparable in all mixed treatments. By

day 6, the arsenopyrite alone had released 363 mg As/kg arsenopyrite compared to 335 mg As/kg

arsenopyrite in the 95% arsenopyrite treatment (Figure 9).

In µmol/L, the concentrations of dissolved As were significantly greater than those of

dissolved Fe for all treatments. Assuming the ratio should be 1As:1Fe during the dissolution of

arsenopyrite, these solution data suggest that Fe did precipitate out of solution from day 1 to day

6 for all treatments (Table 5). In line with these low Fe concentrations, all treatments exhibited

very low cumulative amounts of dissolved Fe (data not shown). Arsenopyrite alone had the highest level of cumulative dissolved Fe with 1.67 mg Fe/kg arsenopyrite on day 6. The 95% arsenopyrite treatment had the second highest value for cumulative dissolved Fe with 1.32 mg

Fe/kg arsenopyrite. The very low concentrations of dissolved Fe for all treatments may be

related to the high pH for the treatments with arsenopyrite.

The stibnite alone was the only treatment that had an increase in cumulative dissolved

SO4 over the six leaching cycles with 563.8 mg SO4/kg mineral on day 6 (Figure 10). In all

other mixed treatments, the cumulative amount of dissolved SO4 remained below approximately

250 mg/kg mineral for the entire leaching experiment and little increase in dissolved SO4 occurred after the first leaching cycle (Figure 10).

70 Discussion

In experiment #3, the concentrations of dissolved Sb expressed in µmol/L were the

highest in the 75% pyrite treatment. Once corrected for the amount of stibnite in each system,

however, the 95% pyrite treatment had the highest cumulative amount of dissolved Sb per kg of

stibnite compared to the other mixed treatments although pH was comparable among all mixed

treatments. These results confirm that, under similar pH conditions, increasing inputs of pyrite in

the mixed systems enhanced the dissolution of stibnite. For the treatment of stibnite alone, the

solution data in µmol/L indicated that Sb likely precipitated out of solution for the first day and

therefore, the cumulative amount of dissolved Sb in this treatment was underestimated.

The cumulative dissolved Fe amount on day 6 for pyrite alone was 800 mg Fe/kg pyrite

with a solution pH of 6.5. This amount was greater than the cumulative Fe amount for pyrite

alone leached with deionized water in experiment #2 with 250 mg Fe/kg pyrite on day 6 with a

solution pH of 4.2. The dissolved Fe solution data expressed in µmol/L also show higher

concentrations of dissolved Fe for all treatments in experiment #3 compared to experiment #2

(Exp #2, day 1, pyrite alone= 4.7 µmol/L ; Exp #3, day 1, pyrite alone= 9.9 µmol/L ). These

results apparently contradict the study of Kelsall and Welham (2000) where the rate of pyrite

oxidation increased fourfold as the pH decreased from 6 to 4. However in experiment #3 the

initial pH of the leaching solution was 7.5. A study done on the oxidation rate of pyrite as a

function of pH (2 to 12) determined that the concentrations of dissolved Fe were strongly

dependant on the initial pH of the solution. At pHs between 4 and 10, the pyrite oxidation rate

was greater than at pHs < 4 (Bonnissel-Gissinger et al., 1998). Similarly, Evangelou et al.

(1998) studied the effect of NaHCO3 at pH 8.5 on the oxidation rate of pyrite, and determined

that increasing the concentration of NaHCO3 increased the oxidation rate of pyrite. These results

71 (II) were explained by the formation of Fe CO3 complexes on the surface of pyrite which facilitated

(II) the transfer of electrons from Fe to O2. Another reason for the higher amount of dissolved Fe

in solution in the current experiment #3 compared to experiment #2 is likely due to less

secondary precipitation of Fe: the molar ratio of Fe to SO4 in solution was undersaturated for all

mixed treatments from day 1 to 6 in experiment #3 suggesting that more Fe stayed in solution

and less precipitated out.

Looking at the solid phase, the 95% pyrite treatment had the highest proportion of total

Sb as Sb(V)-O species, suggesting that the treatment with the highest amount of pyrite resulted in the greatest oxidation rate of stibnite. Comparing the solid phase of experiments #2 and #3,

all treatments from experiment #3 had a smaller proportion of Sb as Sb(V)-O species than the

corresponding treatments in experiment #2. It is possible that bicarbonate ions formed soluble

complexes with Sb, stabilizing it in solution and decreasing its precipitation. Tella and

Pokrovski (2008; 2009) determined that Sb(III) and Sb(V) formed stable, aqueous complexes with

adjacent carboxylic groups from low molecular weight organic ligands over a pH range of 3 to 9.

However, similar complexes between Sb and bicarbonate have not yet been documented.

Compared to experiment #3 with pyrite, the concentrations of dissolved Sb in all mixed

treatments were significantly lower in experiment #4 using arsenopyrite in the place of pyrite, for

both the solution data expressed as cumulative amount of dissolved Sb in mg Sb/kg stibnite or

dissolved concentrations in µmol/L. This is because the difference in rest potentials between

stibnite and arsenopyrite is smaller than the difference in rest potentials between stibnite and pyrite. A smaller difference between rest potentials results in a smaller current density between the reacting minerals and a smaller increase in oxidation rate for the anode as was observed with stibnite in the treatments with the mineral mixtures. A similar trend was observed by Kwong et

72 al. (2003) studying the mineral assemblage consisting of pyrite, galena and sphalerite. Again,

because Sb reprecipitation was possible on day 1 in the treatment of stibnite alone, the solution

data may have underestimated the dissolved Sb concentration for the initial leaching cycle of this

treatment.

Nesbitt et al. (1995) did a study on arsenopyrite oxidation using polished thin sections

exposed to air and air saturated distilled water. They determined that iron oxyhydroxides formed

on the surface of arsenopyrite. In experiment #4, high pH ~7.5 allowed for conditions where

iron oxyhydroxide might form on the surface of arsenopyrite (Craw et al., 2003). The low

concentration in µmol/L of Fe was incongruent with the ratio of Fe:As:S in arsenopyrite meaning

that it is likely that Fe precipitated out of solution. Nesbitt et al. (1995) and Craw et al. (2003)

observed that the precipitation of Fe oxyhydroxides onto the surface of arsenopyrite protected

the mineral against subsequent oxidation. Similar formation of a protective oxide coating might also explain the low amount of dissolved Fe observed in all treatments with arsenopyrite for experiment #4.

Conclusion

When stibnite and pyrite come into contact it is now evident that other factors such as a high pH affect their galvanic interaction. Compared to the leaching experiment with deionized

water, the leaching solution buffered to pH 7.5 had little effect on the amount of dissolved Sb in

solution but resulted in higher concentrations of Fe in solution for all treatments containing

pyrite. The results were tentatively explained by the presence of bicarbonate ions in solution but

this remains to be confirmed. In particular, the formation of a stable Sb-bicarbonate complex in

solution has not been documented in the literature and more research in this area is required.

73 Now that pH has been identified as a factor influencing the galvanic interactions between stibnite

and pyrite, further experiments at greater pH range and using various pH buffers would help

elucidate the effect of pH vs. bicarbonate ions on these interactions.

Compared to pyrite, the use of arsenopyrite, a mineral with a lower rest potential, led to

lower concentrations of dissolved Sb in the system and this was visible in both the solution data

expressed in mg Sb/kg stibnite and µmol/L. This strengthens the concept that the difference in

electrode potential between two minerals will determine the oxidation rate of the mineral with a

lower rest potential through galvanic interactions.

74

Table 1- Particle size distribution and specific surface area of stibnite, pyrite and arsenopyrite used in experiments 3(A) and 4(B). A- Experiment 3 Grain size fraction/ Surface Stibnite (Sb2S3) Pyrite (FeS2) area <53 µm (%) 18.1 16.6 > 53 to <100 µm (%) 59.1 51.9 >100 to <250 µm (%) 22.8 31.5 Specific Surface Area (m2/g ± 0.205 ± 0.210 0.106 ± 0.010 std. dev.)

B- Experiment 4 Grain size fraction/ Surface Stibnite (Sb2S3) Arsenopyrite (FeAsS) area <53 µm (%) 18.1 26.0 > 53 to <100 µm (%) 59.1 46.8 >100 to <250 µm (%) 22.8 27.2 Specific Surface Area (m2/g ± 0.205 ± 0.210 0.100 ± 0.098 std. dev.)

75 Table 2: Mass ratio of stibnite to pyrite or stibnite to arsenopyrite for experiments 3 and 4; total mixture of 2 g per sample Treatment/ Sb2S3(g) FeS2 Mass Ratio Specific Specific Surface Ratio (Exp 3) (stibnite-to- Surface Area Area Ratio (SSA) (Sb2S3/FeS2 or pyrite or Ratio (SSA) (stibnite to or Sb2S3/ AsFeS stibnite to (stibnite to arsenopyrite)- Exp 4 AsFeS) (Exp 4) arsenopyrite) pyrite) – Exp 3 (g) 100/0 2.00 0.00 100% stibnite 100% stibnite 100% stibnite 25/75 0.50 1.50 0.33 0.65 0.68 20/80 0.40 1.60 0.25 0.48 0.51 15/85 0.30 1.70 0.18 0.34 0.36 10/90 0.20 1.80 0.11 0.22 0.23 5/95 0.10 1.90 0.05 0.10 0.11 0/100 0.00 2.00 100% pyrite 100%pyrite 100% pyrite

76 Table 3- Results from the multivariate analysis of variance for the effects of treatments and time on pH, Eh, electrical conductivity (EC) and dissolved Sb, Fe, As and sulphate concentrations in experiments 3(A) and 4(B). A.

MANOVA Numerator- Denominator- Sb Fe SO4 pH Eh test Degrees of Degrees of Freedom Freedom Prob. > F (d.f.) (d.f.) Treatment 6 14 <.0001 0.1702 0.0038 0.0435 0.0838 Time 5 70 <.0001 <.0001 <.0001 <.0001 <.0001 Treatment 30 70 <.0001 0.0100 0.0506 <.0001 <.0001 x Time

B.

MANOVA Numerator- Denominator- Sb Fe As SO4 pH Eh EC test Degrees of Degrees of Freedom Freedom Prob. > F (d.f.) (d.f.) Treatment 6 14 <.0001 0.939 <.0001 <.0001 <.0001 0.0917 0.2790 Time 5 70 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 <.0001 Treatment 30 70 <.0001 0.1280 <.0001 0.8689 0.0554 0.0857 0.9985 x Time

77 Table 4- Concentration of dissolved Sb, Fe, and SO4 for all treatments in µmol/L from day 1 to 6 for experiment #3 with stibnite/pyrite mixed systems.

Treatment Day Sb Fe Total SO4 Excess † metal SO4 (µmol/L) 100% 1 3.9 N.A. †† 3.9 12.8 7.0 stibnite 2 3.1 N.A. 3.1 2.5 -2.1 3 2.7 N.A. 2.7 1.2 -2.8 4 2.2 N.A. 2.2 0 -3.3 5 2.2 N.A. 2.2 0.8 -2.5 6 2.0 N.A. 2.0 0.8 -2.2 Total of 6 16.1 N.A. 16.1 18.1 N.A. days

100% 1 N.A. 9.9 9.9 15.1 -4.7 pyrite 2 N.A. 3.7 3.7 7.1 -0.3 3 N.A. 4.5 4.5 6.5 -2.5 4 N.A. 3.7 3.7 6.5 -0.9 5 N.A. 3.4 3.4 6.0 -0.8 6 N.A. 3.3 3.3 6.3 -0.3 Total of 6 N.A. 28.5 28.5 47.5 N.A. days

95% 1 3.3 11.3 14.6 15.3 -12.2 pyrite 2 3.1 1.9 5.0 4.7 -3.7 3 2.7 0.8 3.5 4.0 -1.6 4 2.4 0.4 2.8 2.7 -1.7 5 1.9 0.04 1.9 3.0 0.1 6 1.8 0.5 2.3 3.5 -0.2 Total of 6 15.2 14.9 30.1 33.2 N.A. days

78

90% 1 2.9 10.9 13.8 13.9 -12.2 pyrite 2 4.6 1.2 5.8 4.0 -5.3 3 4.8 1.0 5.8 3.4 -5.8 4 4.3 0.5 4.8 3.6 -3.8 5 3.5 0.1 3.6 3.2 -2.2 6 3.5 0.6 4.1 3.4 -3.0 Total of 6 23.5 14.3 37.8 31.5 N.A. days

85% 1 3.0 11.8 14.8 15.9 -12.2 pyrite 2 5.5 1.9 7.4 6.0 -6.0 3 5.1 0.4 5.5 2.7 -5.7 4 4.8 0.1 4.9 2.6 -4.8 5 4.8 0 4.8 2.2 -5.0 6 3.8 0.3 4.1 2.5 -3.8 Total of 6 27 14.5 41.5 31.9 N.A days

80% 1 3.1 10.8 13.9 15.9 -10.3 pyrite 2 6.2 2.0 8.2 6.0 -7.3 3 5.5 0.5 6.0 2.4 -6.8 4 5.0 0.2 5.2 2.4 -5.5 5 4.6 0.001 4.6 2.6 -4.3 6 4.1 0.3 4.4 2.3 -4.4 Total of 6 28.5 13.8 42.3 31.6 N.A. days

75% 1 3.7 9.2 12.9 13.7 -10.2 pyrite 2 5.4 1.1 6.5 4.4 -5.9 3 4.9 0.4 5.3 2.9 -5.2 4 5.9 0.1 6.0 1.9 -7.1 5 4.8 0 4.8 2.0 -5.2 6 4.2 0.6 4.8 2.4 -5.1 Total of 6 28.9 11.4 40.3 27.3 N.A. days † Excess: The excess of SO4 was calculated assuming a ratio of 1Fe:2S for the dissolution of †† pyrite (FeS2), and 2Sb:3S for the dissolution of stibnite (Sb2S3). N.A.: Not applicable

79 Table 5- Concentration of dissolved Sb, Fe, As and SO4 for all treatments in µmol/L from day 1 to 6 for experiment #4 with stibnite/arsenopyrite mixed systems.

Treatment Day Sb Fe As Total SO4 Excess † metal SO4 (µmol/L) 100% 1 2.5 N.A. †† N.A. 2.5 5.3 1.6 Stibnite 2 2.5 N.A. N.A. 2.5 1.9 -1.8 3 2.3 N.A. N.A. 2.3 2.1 -1.3 4 2.6 N.A. N.A. 2.6 1.2 -2.7 5 2.7 N.A. N.A. 2.7 0.6 -3.4 6 2.5 N.A. N.A. 2.5 0.6 -3.1 Total of 15.1 N.A. N.A. 15.1 11.7 N.A. 6 days

100% 1 N.A. 0.01 2.6 2.6 1.6 -1.0 Arsenopyrite 2 N.A. 0.007 1.8 1.8 0.1 -1.7 3 N.A. 0.009 1.5 1.5 0 -1.5 4 N.A. 0.01 1.3 1.3 0 -1.3 5 N.A. 0.007 1.3 1.3 0.04 -1.3 6 N.A. 0.01 1.3 1.3 0.08 -1.2 Total of N.A. 0.053 9.8 9.9 1.82 N.A. 6 days

95% 1 2.6 0.003 2.1 4.7 2.3 -3.7 Arsenopyrite 2 2.2 0.006 1.3 3.5 0 -4.6 3 1.3 0.01 1.2 2.5 0 -3.2 4 1.0 0.002 1.1 2.1 0 -2.6 5 0.8 0.01 1.4 2.2 0.1 -2.5 6 0.6 0.01 1.4 2.0 0.1 -2.2 Total of 8.5 0.041 8.5 17 2.5 N.A. 6 days

80 90% 1 3.4 0.01 1.6 5 2.9 -3.8 Arsenopyrite 2 2.6 0.0002 1.0 3.6 0.4 -4.5 3 2.6 0.005 1.0 3.6 0 -4.9 4 1.9 0 1.1 3 0 -3.9 5 1.6 0.005 1.3 2.9 0.08 -3.6 6 1.2 0.01 1.4 2.6 0.1 -3.1 Total of 13.3 0.03 7.4 20.7 3.5 N.A. 6 days

85% 1 3.6 0.003 1.6 5.2 3.7 -3.3 Arsenopyrite 2 2.7 0.003 0.9 3.6 0.3 -4.6 3 2.4 0.0005 0.9 3.3 0 -4.5 4 2.6 0.0002 1.1 3.7 0 -5.0 5 2.2 0.007 1.3 3.5 0.1 -4.5 6 1.7 0.008 1.4 3.1 0.1 -3.8 Total of 11.6 0.02 7.2 18.8 4.2 N.A. 6 days

80% 1 3.8 0.0008 1.4 5.2 4.1 -3.0 Arsenopyrite 2 2.7 0.00003 0.8 3.5 0.6 -4.2 3 2.7 0.001 0.7 3.4 0 -4.7 4 2.7 0 1.1 3.8 0 -5.1 5 2.2 0.006 1.3 3.5 0.1 -4.5 6 1.7 0.01 1.3 3 0.9 -2.9 Total of 15.8 0.02 6.6 22.4 5.7 N.A. 6 days

75% 1 3.9 0.002 1.4 5.3 1.6 -5.6 Arsenopyrite 2 2.6 0.0001 0.9 3.5 0.1 -4.7 3 2.6 0 0.9 3.5 0 -4.8 4 2.8 0.001 1.1 3.9 0 -5.3 5 2.6 0.01 1.2 3.8 0.04 -5.1 6 2.0 0.02 1.2 3.2 0.08 -4.1 Total of 16.5 0.03 6.7 23.2 1.82 N.A. 6 days † Excess: The excess of SO4 was calculated assuming a ratio of 1Fe:1As:1S for the dissolution of †† arsenopyrite (AsFeS) and 2Sb:3S for the dissolution of stibnite (Sb2S3). N.A.: Not applicable

81

Figure 1- pH of the leached solution over the six leaching cycles- Experiment #3

82

Figure 2- Cumulative release of dissolved Sb over six leaching cycles as affected by various amounts of pyrite in the mixture and a buffered leaching solution, pH= 7.5- Experiment #3

83

Figure 3- Cumulative release of dissolved Fe over the six leaching cycles as affected by various amounts of stibnite in the mixture and a buffered leaching solution, pH=7.5- Experiment #3

84

Figure 4- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various amounts of pyrite in the mixture and a buffered leaching solution, pH= 7.5- Experiment #3

85

Figure 5- Sb K-edge XANES spectra for (A) two reference compounds of Sb2S3 [Sb(III)-S] and KSbO3·3H2O [Sb(V)-O], and (B) the selected treatments with 95% pyrite, and 100% stibnite after six leaching cycles using a buffered leaching solution, pH=7.5 (solid lines= measured data; dashed lines= predicted data; Vertical line is an eye guide only)- Experiment #3.

86 Figure 6- Best fitting results from least-squares linear combination for stibnite before leaching and the 95% and 75% pyrite and 100% stibnite treatments after six days of leaching using a buffered leaching solution, pH=7.5- Experiment #3

87

Figure 7- pH of the leached solution over the six cycles- Experiment #4

88

Figure 8- Cumulative release of dissolved Sb over the six leaching cycles as affected by various amounts of arsenopyrite in the mixture- Experiment #4

89

Figure 9- Cumulative release of dissolved As over the six leaching cycles as affected by various high amounts of arsenopyrite in the mixture- Experiment #4

90

Figure 10- Cumulative release of dissolved SO4 over the six leaching cycles as affected by various amounts of arsenopyrite in the mixture- Experiment #4

91 General Conclusion

The studies in this project have confirmed that there are galvanic interactions occurring between stibnite and pyrite (or arsenopyrite) during leaching of mixtures of the two finely ground minerals. At an alkaline pH these electro-chemical reactions between stibnite and pyrite are not hindered and continue at a similar rate. Although stibnite is not as common a mineral as other sulphides, the release of dissolved antimony can have serious effects on humans and ecosystems. Therefore, it is an element that should be monitored along with the other elements in mine wastes. This study is ongoing and has created further questions regarding the oxidation of stibnite due to contact with minerals that operate at higher energy levels (greater rest potentials) and the preventative measures that should be taken to minimize Sb release into the water system.

In the initial cycle of the experiments, Sb likely precipitated. The modelling of solution data using solubility phase diagrams would help determine whether supersaturation or undersaturation has occurred in solution, while giving an insight on the Sb and Fe phases susceptible to reprecipitate. Further analysis of the EXAFS data will allow for a more complete understanding of the speciation of the solid phase in each of the treatments.

92 References

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