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The effects of galvanic interactions with pyrite on the generation of acid and metalliferous drainage Gujie Qian, Rong Fan, Michael D. Short, Russell Schumann, Jun Li, Roger St.C. Smart, and Andrea R Gerson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05558 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018
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The effects of galvanic interactions with pyrite on the
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generation of acid and metalliferous drainage
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Gujie Qiana,b†, Rong Fana, Michael D. Shorta,b, Russell C. Schumanna,c, Jun Lia,
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Roger St.C. Smarta,d and Andrea R. Gersond*
6 a
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Natural and Built Environments Research Centre, School of Natural and Built Environments,
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University of South Australia, Mawson Lakes, SA 5095, Australia b
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Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia c
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Levay & Co. Environmental Services, Edinburgh, SA 5111, Australia d
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Blue Minerals Consultancy, Wattle Grove, TAS 7109, Australia
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†
Present address: College of Science and Engineering, Flinders University, Bedford Park, SA 5042, Australia
*Corresponding author:
[email protected] 17
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Abstract
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Although the acid generating properties of pyrite (FeS2) have been studied extensively, the
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impact of galvanic interaction on pyrite oxidation, and the implications for acid and
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metalliferous drainage, remains largely unexplored. The relative galvanic effects on pyrite
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dissolution were found to be consistent with relative sulfide mineral surface area ratios with
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sphalerite (ZnS) having greater negative impact in batch leach tests (sulfide minerals only,
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controlled pH) and galena (PbS) having greater negative impact in kinetic leach column tests
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(KLCs, uncontrolled pH, >85 wt% silicate minerals). In contrast the presence of pyrite
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resulted consistently in greater increase in galena than sphalerite leaching suggesting that
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increased anodic leaching is dependent on the difference in anodic and cathodic sulfide
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mineral rest potentials. Acidity increases occurred after 44, 20 and 12 weeks in the pyrite–
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galena, pyrite–sphalerite and the pyrite containing KLCs. Thereafter acid generation rates
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were similar with the Eh consistently above the rest potential of pyrite (660 mV, SHE). This
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suggests that treatment of waste rocks or tailings, to establish and maintain low Eh conditions,
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may help to sustain protective galvanic interactions and that monitoring of Eh of leachates is
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potentially a useful indicator for predicting changes in acid generation behaviour.
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Keywords: Acid and metalliferous drainage; Acid generation; Batch leaching; Galvanic
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interaction; Kinetic leach columns; mixed sulfides
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1. Introduction
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Acid and metalliferous drainage (AMD) is a global environmental issue. AMD occurs
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primarily due to pyrite (FeS2) oxidation (Eqs. 1 and 2), generating acidic leachate often with
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associated toxic metals/metalloids e.g. As, Pb, Cu, Zn, Cd 1-3. Although AMD can occur due
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to natural weathering processes, it is largely manmade resulting from both operating and
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abandoned mine sites containing sulfidic mineralisation. Many previous studies have focused
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on the kinetics of pyrite oxidation
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microbes including iron-oxidisers and sulfate-reducers 8-12.
4-7
, with much attention paid to the interacting roles of
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FeS + 3.5O + H O → Fe + 2SO + 2H
(1)
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FeS + 14Fe + 8H ) → 15Fe + 2SO + 16H
(2)
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However, a further factor affecting pyrite oxidation in natural systems is its co-existence with
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other sulfide minerals
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potentials are in electrical contact, particularly in acidic solutions, electron transfer occurs
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from the sulfide with the smaller (anode) to the greater rest potential (cathode). A galvanic
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cell is formed in which the cathode is galvanically-protected and the anode is preferentially
5, 13-16
. When semi-conductive sulfide minerals with differing rest
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dissolved/oxidised
. Among common sulfide minerals, pyrite has a relatively high rest
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potential (660 mV SHE at pH 4; Table 7 in Li et al., 2013
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minerals such as sphalerite (460 mV SHE; pH 4) or galena (400 mV SHE; pH 4).
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Heidel, et al. 20 found that pyrite oxidation decreases in the presence of sphalerite and galena,
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due to galvanic interaction, during which sphalerite and galena are preferentially dissolved.
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However, the system investigated contained all three sulfide minerals such that any unique
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effect of sphalerite or galena could not be differentiated. Chopard, et al.
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geochemical
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pyrite−sphalerite mixtures (50 wt% quartz, 25 wt% of each sulfide). They suggested that
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galvanic coupling occurs between pyrite and chalcopyrite or sphalerite, resulting in reduced
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pyrite oxidation and increased chalcopyrite and sphalerite dissolution. However, it was
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acknowledged that the content of sulfide minerals (50 wt%) chosen was not typical of AMD
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mine wastes.
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Galvanic interaction has also been proposed in other studies to slow pyrite oxidation, or
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increase the oxidation of sphalerite or galena 22-25 but the impact on dissolution kinetics were
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not specifically examined. The variation of galvanic effect as a function of pH under AMD-
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relevant conditions is also unknown. Given that AMD encompasses a range of acidic pH
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conditions, understanding the effect of pH on galvanic coupling would enable better
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understanding of AMD processes within multi-sulfide systems. Accordingly, the aims of this
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study were:
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Aim 1 − to gain more detailed understanding of the effect of galvanic interaction on the
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dissolution of sulfides as a function of AMD-relevant pH. To achieve this the
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dissolution kinetics of both single- and bi-sulfide mineral systems was examined
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using batch leaching, under controlled conditions.
experiments,
using
weathering
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cells
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), as compared to other sulfide
with
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carried out
pyrite−chalcopyrite
or
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Aim 2 − to investigate the impacts of galvanic interaction on acid generation at low sulfide
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content representative of AMD wastes. For this purpose kinetic leach columns,
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using conditions which approximate those found in fully-oxygenated zones,
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containing single and mixed sulfide wastes were undertaken.
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2.1 Minerals
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Natural quartz, K-feldspar, chlorite (clinochlore), pyrite, galena and sphalerite (Geo
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Discoveries, New South Wales, Australia) were all confirmed to be > 90 wt% purity using
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powder X-ray diffraction (XRD). See Supporting Information S1 for elemental compositions.
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2.2 Batch leach experiments
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Fifteen batch leach tests consisting of five mineral systems (pyrite, sphalerite, galena, pyrite–
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galena, and pyrite–sphalerite) were carried out for 645 days (duplicate experiments were not
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carried out). For each batch leach experiment, two grams (38–75 μm; see Section 4.1 for
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calculation of the specific surface area of each sulfide) of either single sulfide minerals or
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thoroughly mixed sulfides with mass ratio of 1:1 were added into 1 L. Batch leach tests were
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maintained at room temperature (22±2 °C) under quiescent (non-stirred), air-saturated
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conditions at pH 3.0 (±0.1, controlled using HCl), 5.0 (±0.2, controlled using HCl) and 7.4
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(±0.4, calcite-saturated solution). Solution samples (10 mL) were collected periodically for
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analysis (see Figures S1 and S2 for sampling intervals). For mineral sample preparation and
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further experimental details see Supporting Information S2. The 1:1 sulfide ratio by weight
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was chosen such that the dissolution of galvanically-coupled mixed sulfides, rather than
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‘uncoupled’ sulfides, was ensured.
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2.3 Kinetic leach columns
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Kinetic leach columns were run for a period of 76 weeks (532 days). Sulfides, chlorite, K-
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feldspar and quartz, used for kinetic leach column (KLC) tests (compositions given in
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Table 1), were all dry-crushed to 20 wt% water (calculated as per 27) after each flush. This dropped
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to 2–7 wt% subsequent to cyclical heating. Leaching with ‘wet and dry’ cycles provides
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conditions that approximate those found in the field for an oxygenated zone containing
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sulfide materials. It was assumed on the basis of previous studies that oxygen was saturated
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throughout the KLCs 28, 29.
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2.4 Extraction of secondary precipitates
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EDTA can be used to dissolve oxidised metal ions as complexes providing an estimate of the
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extent of oxidation of all minerals. It does not cause the dissolution of metal ions from un-
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oxidised sulfide mineral surfaces 30 or from crystalline silicates and other bulk minerals 31.
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Leached solids (0.5 g; not further crushed) were collected from each batch leach residue for
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EDTA extraction. A representative amount of leached solids (50 g) collected at the end of
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each KLC test (week 76) was further dry-crushed and dry-sieved to
