The effects of galvanic interactions with pyrite on the generation of acid and metalliferous drainage

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1 Supporting Information The effects of galvanic interactions with pyrite on the generation of acid and metalliferous drainage Gujie Qian a,b, Rong Fan a, Michael D. Short a,b, Russell C. Schumann a,c, Jun Li a, Roger St.C. Smart a,d and Andrea R. Gerson d a b c d Natural and Built Environments Research Centre, School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Levay & Co. Environmental Services, Edinburgh, SA 5111, Australia Blue Minerals Consultancy, Wattle Grove, TAS 7109, Australia Present address: College of Science and Engineering, Flinders University, Bedford Park, SA 5042, Australia * Corresponding author: andrea@bluemineralsconsultancy.com.au; ph Cover Sheet Tables: Table S1 The elemental compositions (in ppm) of the natural sulfide and silicate minerals. S3 Figures: Fig. S1 Batch dissolution of (a) pyrite Fe, (b) pyrite S, (c) galena Pb, (d) galena S, (e) sphalerite Zn, and (f) sphalerite S, all cumulative, at ph 3.0, 5.0 and 7.4 as a function of time. The figure legend in (a) is common to all other figures and the y-axis scale is uniform for ease of comparison. S7 Fig. S2 Cumulative release of cations and S in mixed sulfide batch dissolution experiments at ph 3, 5 and 7 as a function of time. (a) and (b): Fe and Pb profiles for mixed pyrite galena. (c) and (d): Fe and Zn profiles for mixed pyrite S1

2 sphalerite. Figure legend in (a) is common to all other figures. The S profiles for the mixed sulfide batch dissolution are not included, as S is contributed from both pyrite and galena/sphalerite and does not give an indication of the wt% of each sulfide mineral dissolved. S8 Fig. S3 Cumulative amounts of Fe, Zn, Pb (ICP-OES data) in KLC leachates presented as percentages of the amount in the mineral initially added (i.e. Fe as pyrite, Zn as sphalerite, and Pb as galena) as a function of leach time for the single and mixed sulfides KLCs (a c), and the percentage of dissolved S as a function of time for all KLCs (d). Figure legend and line colours in (d) are common to (a c). Note the ICP-OES Fe data used for calculation of the leaching extents of pyrite in (a) were assumed to be from the dissolution of pyrite only, without considering the dissolution of iron-containing chlorite. S10 Fig. S4 SEM images of unleached K-feldspar (a), leached K-feldspar collected from pyrite-only KLC at 76 weeks (b), unleached chlorite (c), and leached chlorite collected from galena-only KLC at 76 weeks (d). Micro-particles found in (b) were possibly jarosite, with composition similar to that in Fig. 2b. Fecontaining precipitates (indicated by the circled area in (d)) were found on the leached chlorite surface. The inset in (d) is the enlargement of the circled area, showing the details of the Fe-containing precipitates. EDS analysis (red cross in the inset in (d)) revealed that the Fe precipitate contained: 61 at% O, 26 at% Fe, 6 at% Al, 5 at% Si, and 3 at% Mg. S11 S2

3 S1 Elemental concentrations of minerals, measured by ICP-OES using solutions from acid digestion of samples, used in batch and kinetic leach column tests Table S1 The elemental compositions (in ppm) of the natural sulfide and silicate minerals. Al As Ba Ca Cl Cu Fe K Mg Mn Quartz 40 <5 <5 <30 < <30 10 K-feldspar 96,000 <5 60 < < ,000 <30 8 Chlorite 105,000 <5 20 5,300 < , , Pyrite <5 100 < , <30 <5 Galena 40 <5 <5 <30 <60 < <30 <5 Sphalerite 100 < , <5 LOD Na Ni P Pb S Si Sr Ti V Zn Quartz 100 <3 <30 < ,000 <3 <5 <5 <5 K-feldspar 18,000 <3 <30 2, , <5 <5 <5 Chlorite 1, , , , Pyrite < ,000 <30 5 <5 <5 100 Galena 100 <3 <30 865, , <3 <5 <5 7 Sphalerite 70 < , ,900 <30 8 <5 <5 632,000 LOD LOD limit of detection S2 Details of the batch leach experiments For batch leaching experiments pyrite, sphalerite and galena were dry-crushed/pulverised and wet sieved to obtain a μm fraction. Ultrasonic treatment was then applied to remove fine particles adhering to the mineral surfaces in order to minimise variation in surface area. Each sample was then washed with 3 M HCl solution for several minutes (to remove oxidised surface layers possibly formed during sample preparation), cleaned with ethanol and dried in a vacuum oven at room temperature overnight. All batch leach tests were conducted using wide-mouth Nalgene HDPE bottles and 0.01 M KCl was added as a background electrolyte to maintain approximately constant solution ionic strength as per previous studies 1, 2. The ph was regularly monitored and adjusted to the target ph values using either HCl or NaOH solutions for ph 3 and 5 batch leach tests. For batch leaching controlled at ph 7, the ph was maintained by adding 1 g calcite particles ( μm; wrapped in nylon mesh) at the beginning of the experiments. S3

4 Assuming solution density is 1 g/ml for all batch leach experiments, the solution volume remaining in each batch leach test at each time of sampling was calculated by subtracting the initial weight of the empty Nalgene HDPE bottle plus the initial weight of each sulfide mineral (2 g) from the total weight of reaction bottle with solution and mineral particles. The total volume of added acid/base (up to only several ml in total throughout the whole leach process), all solution samples collected for ICP-OES, and the remaining solution at the end of experiment (645 days) for each batch leach test was approximately 95% of the original volume (1 L), suggesting that evaporative and other unaccounted volumetric loss was negligible under our experimental conditions. S3 Details of EDTA/HNO 3 extraction procedure An aliquot (0.5 g) of each sample from batch leaching or KLC tests was mixed with 45 ml of 0.05 mol/l disodium EDTA solution (ph 4.6) at room temperature and shaken for 3 h. The resulting mixtures were then filtered through 0.45 μm PTFE filters and residues were rinsed three times with Milli-Q water, with all filtrates collected for ICP-OES analysis. A second EDTA extraction was also carried out on the residues from the first EDTA extraction, to confirm whether or not oxidised surface products were completely dissolved in the first EDTA extraction. Precipitates on the wall of the plastic reaction containers (1 L HDPE bottles) from batch leaching experiments were dissolved using 2 wt% nitric acid (HNO 3 ) at 50 C, and solutions were filtered/collected for ICP-OES analysis. Extraction, using a mixed solution containing 15 wt% ammonium acetate plus 3 wt% acetic acid in Milli-Q water (hereafter AmAc), was also carried out on KLC leach residues (0.5 g solids in 25 ml of AmAc solution) based on Steger 3 for comparison with EDTA extraction. This AmAc solution has been shown to be effective in extraction of water-insoluble oxidation products of sulfide minerals, such as anglesite (PbSO 4 ) from the oxidation of galena. Similar results were obtained from AmAc and EDTA extractions for leach residues from the mixed pyrite galena KLC (e.g. 30.4% Pb vs. 29.7% Pb extracted by AmAc and EDTA solutions, respectively), suggesting that EDTA can also effectively dissolve water insoluble sulfate oxidation products. S4

5 S4 Details of instrumental analyses XRD data were collected using a Bruker D4 Endeavour across the 2θ range 5 90 with step size of 0.02 and 0.5 s per step. Each sample for XRD analysis was ground to particle size of <38 µm. Phase identification was carried out using the DiffracPlus EVA software (Bruker; Version 3.0) with application of the Crystallography Open Database (COD). Quantitative phase analysis was performed using the Rietveld method 4, 5 with the aid of the computer program TOPAS (Bruker; Version 4.2). The 2D XRD image (Debye rings) collected using the micro-diffractometer was converted to an intensity vs. 2θ profile using the computer software 2DP developed by Rigaku. Subsequent phase identification was carried out using the DiffracPlus Eva software (Version 3.0; Bruker). Bulk sample assay analyses (ICP-OES analysis of solutions from acid digestion of solids), including elemental S, were performed on the fresh and leached minerals (Levay & Co Environmental Services, Adelaide, South Australia). Elemental S was extracted from the leach residues using acetone and the extract analysed by high-performance liquid chromatography. ICP-OES analysis (Perkin Elmer Optima 5300 V) was carried out to assess the concentration (±10% analysis errors) of major ions in the leachates from both the batch and KLC leach experiments. KLC leachate (collected after each four-weekly flush) and batch leach solution ph and Eh were measured, using Ag/AgCl ph and E h electrodes (filled with saturated KCl) calibrated with standard ph buffers (TPS, Brendale, Queensland, Australia) and ORP standard solutions (EUTECH Instruments, Singapore). Conductivity measurements were carried out using a TPS conductivity metre (model: Aqua Cond) calibrated with a standard solution (2.76 ms/cm; TPS). Eh values are reported against the standard hydrogen electrode (SHE). Acidity measurements were carried out by titrating 40 ml of fresh leachates with 0.1 M standardised NaOH until the ph reached 8.3. The amount of NaOH consumed was then converted into the equivalent amounts of CaCO 3 (mg) normalised to the solution volumes (i.e. 40 ml). S5

6 S5 Batch leaching leachate analyses The release of Fe to solution from pyrite-only dissolution was less at ph 5 and 7 than at ph 3 (<2% cf. 5.4% of the total Fe at 645 days; Fig. S1a) most likely due to greater precipitation of iron (oxy)hydroxides at greater ph. In contrast the S data suggest that pyrite dissolution occurred more rapidly at the greater ph (Fig. S1b), in agreement with the rate law developed by Williamson and Rimstidt 6 for pyrite oxidation controlled by dissolved oxygen. The dissolution of galena-only resulted in reduced Pb and increasing S in solution as ph increased (Fig. S1c and S1d, respectively). The correlation between aqueous Pb and ph is in agreement with findings of Hsieh and Huang 7, and suggests possible formation of Pbcontaining precipitates. The reduced aqueous S and increased Pb at ph 3 and 5 as compared to ph 7 suggests that oxidised S may have precipitated or else the reaction product was insoluble, e.g. polysulfide S 2 n or elemental sulfur S 0 at the lower ph. For sphalerite-only dissolution the percentage of Zn present in solution decreased with increasing ph (Fig. S1e). However, in contrast to galena and pyrite, the amount of S in solution (Fig. S1f) was greatest at ph 3 over the entire experimental period. This suggests that sphalerite dissolution may occur more rapidly at ph 3 than at ph 5 and 7. A similar inverse relationship between sphalerite dissolution rate and ph has been found by Acero, et al. 8, but the ph range investigated ranged was only in that study. The amounts of S and Zn present in solution were smallest at ph 5 and ph 7, respectively, suggesting that S and Zn may precipitate (or in the case of S be present in an insoluble form) under different ph conditions. S6

7 Fig. S1 Batch dissolution of (a) pyrite Fe, (b) pyrite S, (c) galena Pb, (d) galena S, (e) sphalerite Zn, and (f) sphalerite S, all cumulative, at ph 3.0, 5.0 and 7.4 as a function of time. The figure legend in (a) is common to all other figures and the y-axis scale is uniform for ease of comparison. S7

8 Fig. S2 Cumulative release of cations and S in mixed sulfide batch dissolution experiments at ph 3, 5 and 7 as a function of time. (a) and (b): Fe and Pb profiles for mixed pyrite galena. (c) and (d): Fe and Zn profiles for mixed pyrite sphalerite. Figure legend in (a) is common to all other figures. The S profiles for the mixed sulfide batch dissolution are not included, as S is contributed from both pyrite and galena/sphalerite and does not give an indication of the wt% of each sulfide mineral dissolved. S6 Kinetic leach column leachate analyses As the wt% of each sulfide mineral in the single and mixed sulfide KLCs was different (to maintain total initial S at 2.0 wt%), the actual amounts of Fe, Pb, Zn and S in leachates were normalised against the total Fe in the pyrite, Pb in the galena, Zn in the sphalerite, and S in all sulfides, to gain a better understanding of their relative extents of reaction. Note that the aqueous Fe possibly included contributions from chlorite (18 wt% Fe). The increase in solution Fe was relatively more rapid in the pyrite-only KLC than in the mixed sulfide KLCs (Fig. S3a). In contrast, the presence of pyrite significantly increased the S8

9 release of Pb and Zn from the mixed pyrite galena and pyrite sphalerite KLCs (Fig. S3b,c), as compared to the galena- and sphalerite-only KLCs. The pyrite-only and mixed pyrite sphalerite KLCs gave rise to the greatest release of S, reaching approximately 7% of the total S by week 76, while sphalerite-only, galena-only and mixed pyrite galena KLCs generated less than 2% S in leachates over the same period of time. The variations of total Fe and S releases (Fig. S3a and 3d, respectively) for the mixed pyrite galena KLC test show notable increases from week 52, suggesting a likely increase in the pyrite oxidation rate. In comparison, the variations of total metals and S releases for other KLCs (Fig. S3) remained largely constant, suggesting relatively constant oxidation rates of sulfides in these KLCs (after 20 weeks). S9

10 Fig. S3 Cumulative amounts of Fe, Zn, Pb (ICP-OES data) in KLC leachates presented as percentages of the amount in the mineral initially added (i.e. Fe as pyrite, Zn as sphalerite, and Pb as galena) as a function of leach time for the single and mixed sulfides KLCs (a c), and the percentage of dissolved S as a function of time for all KLCs (d). Figure legend and line colours in (d) are common to (a c). Note the ICP-OES Fe data used for calculation of the leaching extents of pyrite in (a) were assumed to be from the dissolution of pyrite only, without considering the dissolution of iron-containing chlorite. S10

11 S7 SEM analysis of fresh and leached K-feldspar and chlorite from KLCs Fig. S4 SEM images of unleached K-feldspar (a), leached K-feldspar collected from pyriteonly KLC at 76 weeks (b), unleached chlorite (c), and leached chlorite collected from galenaonly KLC at 76 weeks (d). Micro-particles found in (b) were possibly jarosite, with composition similar to that in Fig. 2b. Fe-containing precipitates (indicated by the circled area in (d)) were found on the leached chlorite surface. The inset in (d) is the enlargement of the circled area, showing the details of the Fe-containing precipitates. EDS analysis (red cross in the inset in (d)) revealed that the Fe precipitate contained: 60 at% O, 26 at% Fe, 6 at% Al, 5 at% Si, and 3 at% Mg. References 1. Khare, N.; Hesterberg, D.; Beauchemin, S.; Wang, S.-L., XANES determination of adsorbed phosphate distribution between ferrihydrite and boehmite in mixtures. Soil Science Society of America Journal 2004, 68, (2), S11

12 2. Fan, R.; Short, M. D.; Zeng, S.; Qian, G.; Li, J.; Schumann, R.; Kawashima, N.; Smart, R. S. C.; Gerson, A. R., The formation of surface passivating layers on pyrite for reduced acid rock drainage. Environ. Sci. Technol. 2017, 51, (19) Steger, H. F., Oxidation of sulphide minerals - II. Determination of metal in the oxidation products of galena, sphalerite and chalcocite. Talanta 1977, 24, (4), Rietveld, H., Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Crystallogr. 1967, 22, (1), Rietveld, H., A profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr. 1969, 2, (2), Williamson, M. A.; Rimstidt, J. D., The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochim. Cosmochim. Acta 1994, 58, (24), Hsieh, Y. H.; Huang, C. P., The dissolution of PbS(s) in dilute aqueous solutions. J. Colloid Interface Sci. 1989, 131, (2), Acero, P.; Cama, J.; Ayora, C., Sphalerite dissolution kinetics in acidic environment. Appl. Geochem. 2007, 22, (9), S12