Mineralogy and geochemistry of efflorescent minerals

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Environ Sci Pollut Res DOI 10.1007/s11356-015-5870-z

RESEARCH ARTICLE

Mineralogy and geochemistry of efflorescent minerals on mine tailings and their potential impact on water chemistry B. P. C. Grover 1 & R. H. Johnson 2 & D. G. Billing 1 & I.M. G. Weiersbye 3 & H. Tutu 1

Received: 15 July 2015 / Accepted: 23 November 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract In the gold mining Witwatersrand Basin of South Africa, efflorescent mineral crusts are a common occurrence on and nearby tailings dumps during the dry season. The crusts are readily soluble and generate acidic, metal- and sulphate-rich solutions on dissolution. In this study, the metal content of efflorescent crusts at an abandoned gold mine tailings dump was used to characterise surface and groundwater discharges from the site. Geochemical modelling of the pH of the solution resulting from the dissolution of the crusts was used to better understand the crusts’ potential impact on water chemistry. The study involved two approaches: (i) conducting leaching experiments on oxidised and unoxidised tailings using artificial rainwater and dilute sulphuric acid and correlating the composition of crusts to these leachates and (ii) modelling the dissolution of the crusts in order to gain insight into their mineralogy and their potential impact on receiving waters. The findings suggested that there were two chemically distinct discharges from the site, namely an aluminium- and magnesium-rich surface water plume and an iron-rich groundwater plume. The first plume was observed to originate from the oxidised tailings following leaching with rainwater while the second plume originated from the underlying unoxidised tailings with leaching by sulphuric acid. Both groups of minerals forming from the respective plumes were found to

significantly lower the pH of the receiving water with simulations of their dissolution found to be within 0.2 pH units of experimental values. It was observed that metals in a low abundance within the crust (for example, iron) had a stronger influence on the pH of the resulting solutions than metals in a greater abundance (aluminium or magnesium). Techniques such as powder X-ray diffraction (PXRD) and in situ mineral determination techniques such as remote sensing can effectively determine the dominant mineralogy. However, the minerals or metals incorporated through solid solution into bulk mineralogy that dominates the chemistry of the solutions upon their dissolution may occur in minor quantities that can only be predicted using chemical analysis. Their mineralogy can be predicted using geochemical modelling and can provide a set of hypothetical minerals that upon dissolution yield a solution similar to that of the actual crusts. This realisation has a bearing on decision-making such as in risk assessment and designing pollutant mitigation strategies. Keywords Efflorescent crusts . Geochemical modelling . Leaching experiments . Forward modelling

Introduction Responsible editor: Philippe Garrigues * H. Tutu [email protected] 1

Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa

2

SM Stoller Corporation, Grand Junction, CO 81503, USA

3

Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa

The Witwatersrand Basin is replete with surface tailings waste, a legacy from over 125 years of mining of the auriferous reefs. With most of the mining activities having been decommissioned, this surface waste remains a major environmental liability (Oelofse et al. 2007). Acid mine drainage (AMD), formed during the oxidation of remnant sulphides, mainly pyrite, in the waste, is a phenomenon that negatively impacts soil and water chemistry in the Witwatersrand Basin and has been reported in a number of studies over the years

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(Kempe 1983; Marsden 1986; Rosner and van Schalkwyk 2000; Tutu et al. 2008; McCarthy 2011). The resulting acidic plume from AMD leads to the leaching of metals held within the ores. Leaching of waste by rainwater sustains a continuous output of metals into surrounding water systems. Evaporation of near-surface seepage and surface water impacted by tailings leads to the formation of efflorescent crusts or salts, especially during the dry season. Efflorescent mineral crusts act as temporary sinks for metal pollutants released during oxidation of surface tailings. The crusts are generally highly soluble, dissolve during the first rainfall showers (a process termed the Bfirst flush^) and release episodic pulses of highly acidic and metal-laden solutions into receiving surface waters (Harris et al. 2003; Lottermoser 2005; Nordstrom 2008, 2011). The crusts in the Witwatersrand Basin are usually sulphate salts containing elevated metal contaminants and provide an indication or fingerprint of the suite of metals released from waste materials, a phenomenon that is often evidenced by the multitude of colours of crusts (Sutton 2008; Tutu et al. 2011). The findings in these studies confirmed that these crusts tend to store elevated contents of acidity, metals and sulphates. The study by Sutton (2008) explored the use of remote sensing to identify secondary minerals that could be used as indicators of the extent of pollution and AMD. In his work, jarosite (KFe(SO4)2(OH)6) and copiapite ((Fe5(SO4)6(OH)2.20H2O) were identified as the key minerals depicting the extent of AMD in tailings. Few studies have managed to identify the whole spectrum of efflorescent salts, largely due to limitations in analytical capability. This has led to the assumption that the predominant salts identified are the ones influencing water chemistry. For example, the dissolution of epsomite would be expected to have little impact on the pH of a solution. However, substitution of the cations in the mineral, for example, substitution of Mg for Fe, Co, Cu or Mn (Jambor et al. 2000), will change the impact of the dissolution of the crust and an acidic solution could occur. To this end, this study aimed at using geochemical modelling for comprehensively characterising efflorescent salts and determining their individual and combined influence on water chemistry. Processes leading to the formation of these salts from tailings and their impact on typical receiving surface waters have also been studied.

Methodology Sampling Sampling was conducted during the dry season at an abandoned tailings footprint in the Central Rand Goldfield of the Witwatersrand Basin (Fig. 1). The Central Rand Goldfield has hosted about 46 mines and the geology of the region has been

comprehensively documented (Pretorius 1964; McCarthy 2006). The study site was a tailings dump that was partially reprocessed and has since been abandoned, leaving a weathered tailings footprint (Fig. 1). Crust samples were scraped off from the side walls of the abandoned tailings dump; from the floor of an evaporated portion of the adjacent tailings pond and from an evaporated spillage from a monitoring borehole (Fig. 2). The crusts around the borehole have formed from the evaporation of groundwater that was pumped out during the last monitoring exercise, the borehole has since then been sealed. There were three samples taken from the side wall at different heights and each had a different colour. Samples of oxidised and unoxidised tailings were also collected. Oxidised tailings were ye llow i n co l our (showin g t he pres enc e of ir on oxyhydroxides) while unoxidised tailings were grey (showing the presence of pyrite). Water samples were collected from three different points within the pond, and on-site assessment of geochemical parameters including pH, electrical conductivity, redox potential and dissolved oxygen (ThermoScientific™ Orion™ Star) was conducted according to methods described by Wardencki and Namiesnik (2002). Laboratory procedures The water samples were vacuum filtered through a Prima PES 0.45-μm filter paper on the same day that they were collected. Crusts were separated from tailings and stored in a cool, dry, dark environment. Batch extractions were conducted on tailings to assess the resulting leachates in relation to the crusts. Unoxidised and oxidised tailings samples were leached with artificial rainwater and diluted sulphuric acid (pH 2.85) in order to investigate if there was a correlation between the efflorescent crust and the readily soluble or acid soluble fraction of the tailings. A 20:1 leaching ratio as used by the USGS Field Leach Test (FLT) (Hageman 2007) and USEPA Method 1312, Synthetic Precipitation Leaching Procedure (SPLP) leach test (U.S. Environmental Protection Agency 2002) was employed by leaching 20 g of air-dried solid material with 400 mL of leaching solution. The artificial rainwater solution was prepared according to the method described of Cocksedge (1988). Leaching experiments were conducted in triplicate at room temperature (approximately 20 °C) by shaking on an elliptical shaker for 24 h. The solutions were filtered using Prima PES 0.45-μm filter paper and electrode measurements of pH, oxidation-reduction potential (Eh), temperature and electrical conductivity (EC) were taken before and after leaching. During the dissolution of each crust, pH, Eh and EC measurements were taken. In total, 3 g of each crust sample were dissolved in six increments (0.5 g each) into 100 mL deionised water. After each incremental addition, the samples were

Environ Sci Pollut Res Fig. 1 Modified Google Earth image (dated 28 May 2013) of study site showing relative location of tailings dump, pond and monitoring borehole

stirred until the pH and conductivity readings were constant (more than 5 min). This allowed for complete dissolution of the readily soluble fraction before measurements were taken. Solutions were then filtered using Whatmann 11-μm pore-sized filter paper prior to further analysis for metals and anions. Characterisation and chemical analyses The mineralogy of the efflorescent crusts was determined by powder X-ray diffraction (PXRD) using a Bruker D2

Fig. 2 A conceptual section of the study site and sampling points

Phaser desktop diffractometer which was fitted with a cobalt X-ray source and a LynxEye 1-D detector (Bruker, Germany). The sampled pond water, dissolved crust solutions and leachate solutions for oxidised and unoxidised tailings were analysed using inductively coupled plasma-optical emission spectroscopy (ICP-OES) for quantification of metal content and chemically suppressed anion chromatography (IC) for quantification of anion content.

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Geochemical modelling Forward and inverse geochemical modelling was undertaken using PHREEQC (Parkhurst and Appelo 2013) based on the Wateq4f database. In forward modelling, the final composition of a solution after a reaction or equilibration is calculated (Zhu and Anderson 2002). The four-step procedure followed was similar to that by Chatin et al (2013) in their modelling of leaching of harbour sediments. The simulations of dissolution of crusts in this work involved definition of a pure initial solution (typically deionised water), equilibrating the solution with atmospheric oxygen and then selecting efflorescent minerals to be added to the solution. The pure end members of the minerals observed from PXRD were selected. For example, where pickeringite (a magnesium aluminium sulphate) was observed, magnesium sulphate and aluminium sulphate minerals were initially defined. Each forward geochemical model was iteratively improved by adding minerals until the modelled pH values and metal concentrations correlated with those of experimental solutions. Therefore, modelling provided a set of hypothetical minerals that upon dissolution would yield a solution of similar composition to that of the actual crusts. The minerals were added in a stepwise manner (adding one sixth of the total number of moles of crust required to give the appropriate concentration of the metal in the final solution) and then the composition of the resulting solution at each reaction step was determined. For example, in a solution which had a Mg concentration of 34 mmol L−1, the addition of a total quantity of 34 mmol of epsomite was modelled. The quantity of minerals was calculated from the chemical analyses of the dissolved crust solutions and in one case, through the use of inverse modelling. In inverse modelling, mass balance principles are employed to calculate the quantities of the reactants given the known compositions of both the initial and final solutions (Zhu and Anderson 2002). The Wateq4f database was updated by including compositional and thermodynamic data for the following ferric sulphate minerals that are known to occur in acid mine drainage environments: ferricopiapite (Fe 4.78(SO 4) 6(OH) 2.34 (H 2 O) 20.71), coquimbite ((Fe1.47Al0.53)(SO4)3(H2O)9.65) and rhomboclase ((H3O)1.34Fe(SO4)2.17(H2O)3.06). The log K values for these minerals were estimated from Gibbs free energy as calculated by Majzlan et al. (2006). Aside from the inclusion of these minerals,

Table 1 Wateq4f and calculated log K values used in dissolution modelling of efflorescent crusts

no further changes to the Wateq4f database were made. Table 1 summarises the log K values of minerals used in the modelling. It was assumed in the simulations that equilibrium between the solution and minerals was obtained.

Results and discussion Mineralogy and chemical composition of crusts From PXRD patterns, quartz (SiO2) was observed to be a major constituent in all of the crust patterns. The auriferous ore is hosted in quartzite reefs and as such the resulting tailings consist mostly of quartz. Additional minerals noted by others (Yibas et al. 2012) on similar tailings material include mica and chlorite/chloritoid with pyrite ranging between 2 and