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Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters, Macraes mine, New Zealand
Applied Geochemistry 17 (2002) 445–454 www.elsevier.com/locate/apgeochem
Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters, Macraes mine, New Zealand Alisa J. Roddick-Lanzilottaa, A. James McQuillana, Dave Crawb,* a
Chemistry Department, University of Otago, PO Box 56, Dunedin, New Zealand b Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand Received 5 October 2000; accepted 17 August 2001 Editorial handling by R. Fuge
Abstract Processing waters contain up to 10 mg l1 dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, Kads=(1.9 0.4)104 M1, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l1. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm1 (alkaline solutions) to 820 cm1 (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Arsenic is a common constituent of waters derived from mining areas, particularly Au mines which commonly contain arsenopyrite as an accessory mineral. This dissolved arsenic can be discharged from the mine area as a pollutant, or it may be chemically attenuated by precipitation or adsorption (Hem, 1977; Kimball et al., 1995). Adsorption of As on to Fe oxides and oxyhydroxides (HFO) is known to occur widely in nature (Bowell, 1994, Kimball et al., 1995; Swedlund and Webster, 1998), and other minerals such as Al oxides, clays, carbonates and organic material (Xu et al., 1991; Sadiq, 1997) have also been implicated. Adsorption of As on to HFO has been studied extensively in laboratory experiments (e.g.,
* Corresponding author. E-mail address: [email protected] (D. Craw).
Xu et al., 1991; Bowell, 1994, Swedlund and Webster, 1998), and much is known about the bulk chemistry of the adsorption process. In particular, the strong effects of pH, oxidation state, and associated solution composition on amounts of As adsorbed have been extensively studied (Xu et al., 1991; Bowell, 1994; Swedlund and Webster, 1998). This study builds on the above bulk chemistry database by examining As adsorption on the molecular scale at the surface of a number of mineral species including HFO. The study focuses on As adsorption on to natural materials. The materials used are those which occur within the discharge path of waters from the Macraes Au mine, New Zealand, where As adsorption is known to be occurring. From this combination of natural materials and detailed examination of surface chemistry phenomena, it is possible to identify the relative As adsorption efficiency of the various materials, and define some of the chemical processes which are occurring at
0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00116-0
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those surfaces. The Macraes mine waters have a narrow range of pH, but the adsorption processes are examined over a broader pH range to provide a more thorough chemical context for the natural adsorption processes which are occurring. 1.1. Macraes mine site The Macraes mine is developed in a mesothermal Au deposit in the Otago Schist of southern New Zealand (Craw et al., 1999a). Mineralised rock contains up to 5 wt.% As as arsenopyrite (FeAsS). Arsenopyrite-bearing ore is crushed to ca. 15 mm in the processing plant, and this material is discharged from the Au-extraction plant to a tailings impoundment system. The tailings impoundment (Fig. 1A) consists of a large debris dam constructed of fresh (unoxidised) waste rock schist excavated from the mine. The dam has an upstream facing zone (A zone, Fig. 1A) of partially oxidised schist stripped from the top of the mined area, added to reduce permeability in the dam structure. A porous chimney drain in the A zone (Fig. 1A), constructed with quartz pebbles, channels
water to the base of the dam. Tailings are discharged to the impoundment as a slurry from which the solids settle to form lake sediments, and water rises from the sediments to form a decant pond (pH=ca. 8) on the top of the sediments (Fig. 1A). Some of this water is recycled through the processing plant, and some escapes through the tailings sediments and the A zone, to reach the schist basement beneath the impoundment system (Fig. 1A). This escaping water mixes (approximately 1:1) with natural waters (pH=5.5–8) beneath the impoundment and discharges at the toe of the dam with a pH of 6.2–6.7 (Fig. 1A; Craw and Nelson 2000). Hence, unlike many mine sites around the world, which have acid discharges, the Macraes mine site is circumneutral throughout. 1.2. Mineralogy of site materials The schist basement is strongly segregated metamorphically, and consists of varying proportions of quartz, albite, muscovite, and chlorite, with minor amounts of epidote, titanite or rutile, and calcite or siderite. Mineralised rock also contains pyrite, arsenopyrite, scheelite,
Fig. 1. (A) Sketch of the tailings impoundment system at the Macraes mine, and water paths through the dam (summarised from Craw and Nelson, 2000). (B) Dissolved As concentrations (log scale) in water in the decant pond (black diamonds) on top of the tailings (Fig. 1A), and in the dam discharge waters (open circles), measured monthly as part of environmental monitoring programme of the dam site.
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and graphite, but these minerals constitute a small proportion (ca. 1–2%) of the rock mass in the tailings impoundment system. Oxidised schist is brown due to trace amounts ( <1%) of HFO coatings on joints and foliation surfaces, and contains some kaolinite and smectite clays (up to 10%). The tailings consist almost entirely of crushed unoxidised schist, although some layers of oxidised schist tailings do occur. Tailings contained negligible pyrite and arsenopyrite when the mine first opened in 1989, as these sulphide minerals were discharged to a separate impoundment structure (Craw et al., 1999b). All tailings were combined into the main impoundment (Fig. 1A) from mid 1993 to late 1998. Flocculating Fe3+ from the decant pond water (ca. 1–10 mg l1 Fe) contributes a very small amount (not visible) of HFO to the tailings sediments. Water discharging from the toe of the dam also deposits a coating of HFO (up to 2 mm thick) on the surrounding rocks. This latter HFO, which is amorphous (from X-ray diffraction), was used extensively in this study. 1.3. Arsenic adsorption in progress Arsenic concentrations of mine waters have been measured as part of a routine environmental monitoring programme. Analyses were conducted by Chemsearch (Chemistry Department, University of Otago), an International Accreditation New Zealand accredited laboratory. Arsenic analyses were obtained using graphite furnace atomic absorption spectrometry in the early stages of the monitoring period, with a detection limit of 0.005 mg l1. Flame atomic absorption spectrometry was used for most later As analyses, with detection limits ranging from 0.5 to 0.1 mg l1. Arsenic analyses are reproducible to better than 1% variation above 1 mg l1, decreasing to 20% variation near the detection limits, so all As uncertainties are contained within the plotted symbols on Fig. 1B. Dissolved As concentrations in the decant pond have increased with time, especially after incorporation of arsenopyrite-bearing tailings in 1993, and were commonly above 10 mg l1 (Fig. 1B), essentially all as As(V). In contrast, waters discharging from the dam toe have As concentrations typically below analytical detection limits. Dilution of dam waters by natural surface and ground waters (1:1 ratio; Craw and Nelson, 2000) explains some of this reduction in As concentration through the dam structure, but this alone would result in dissolved As near 5 mg l1 in the latter stages of the monitoring period. The rest of the As has clearly been removed from solution by some process such as adsorption or precipitation (see Section 1). The removal of As from the discharge waters represents trace element retardation in comparison to water flow through the dam (Freeze and Cherry, 1979).
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Observed water discharges of ca. 12 l s1 occur through ca. 10 m2 of dam material with hydraulic conductivity of ca. 0.01 m s1 at a gradient of 0.1. From these observations, water velocity, vwater, of ca. 80 m day1 can be calculated using Darcy’s law, implying that it takes water about 12 days to follow the ca. 1 km flow path. Detectable As has not emerged from the dam after 11 a (ca. 4000 days) of activity, so minimum retardation, vwater/varsenic, is ca. 330 for As (Freeze and Cherry, 1979). The true retardation value is higher than this by an unknown amount. Retardation is related to the distribution coefficient, Kd, for As between solid and solution by: vwater =varsenic ¼ 1 þ ðb =ÞKd ðFreeze and Cherry; 1979Þ where b is the bulk density, and is the porosity, of the medium through which the water passes. The ratio of b/ is about 6–8 for the Macraes dam, so minimum Kd is ca. 40–55 l kg1. These are order-of-magnitude calculations only, but they result in a minimum Kd that is similar to observed Kd values for As(V) in common soils (EPA, 1996). To investigate the As extraction process more closely, a set of materials from the Otago Schist, mainly from the Macraes mine area, have been used in a series of laboratory experiments. Fresh schist, fresh muscovite and chlorite separates, and quartz-albite rich and muscovote-chlorite rich metamorphic segregations were used to represent material in the tailings, the impounding dam, and the schist basement, along the water discharge path (Fig. 1A). Oxidised schist was used to represent the A zone and minor oxidised tailings layers. Most experiments were done on amorphous HFO deposits from the water discharge site, as this is representative of HFO formed at various stages in the water discharge path. Studies of the HFO were extended by comparing the results from this natural material to similar material formed in the laboratory.
2. Materials and methods 2.1. Infrared spectroscopy A BioRad Digilab FTS60 spectrometer with WinIR software was used to obtain infrared spectra. Spectra of arsenate (V) solution species were recorded from 0.1 M aqueous solutions prepared from Na2HAsO4.7H2O (BDH, AnalaR) and pH was adjusted with NaOH or HCl. Doubly distilled water was used for all solutions and preparations. Attenuated total reflection infrared (ATR–IR) spectra (Mirabella, 1985) of adsorbed arsenate(V) species were obtained from films deposited on single internal reflection 45 ZnSe prisms (Harrick Scientific Corporation) and in contact with aqueous arsenate (V) solutions. Reference spectra for solution were from water on a
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bare ATR prism while for adsorbed species were normally from a HFO film on a prism in contact with water. The experimental set-up is shown diagrammatically in Fig. 2. 2.2. Film preparation Natural mineral material from the Macraes mine area was used for most experiments in this study. Small pieces (ca. 1 g) of mineral material were finely ground with an agate mortar and pestle. A portion of the resultant powder (0.044 g) was then sonicated in 5.0 ml of water. The suspension was diluted 10 fold and 100 ml of the diluted suspension was withdrawn and placed on the ZnSe prism to occupy about 1 cm2 area and dried under water pump vacuum for 60 min. The films obtained by these methods were 5–10 mm thick. Most observations were made on HFO films (see below) so these films were prepared from 3 different forms of the material. Synthetic HFO was prepared by a reported procedure (Swedlund and Webster, 1998). About 0.2 g of Fe(NO3)3.9H2O (BDH, AnalaR) was dissolved in water and 1 M NaOH was added dropwise to give a pH 7. The HFO precipitate was then allowed to age overnight. The suspension was passed through a sintered glass filter (size 4) and the precipitate was washed with water. The HFO was then resuspended in water and sonicated for ca. 10 min. Films of natural amorphous HFO from the Macraes dam discharge site were prepared by two methods. Method A involved use of wet unground HFO directly from the sample bag; i.e., in its in situ form. Approximately 0.1 ml of sample was added to 5 ml of water. After 10 min sonication, 100 ml of the top of the suspension was removed and spread on a ZnSe prism to occupy
about 1 cm2 area. This was then dried under water pump vacuum for 60 min. Method B involved drying some wet HFO sample then grinding it in a mortar and pestle to a fine powder of which 0.044 g was sonicated in 5.0 ml of water. The suspension was diluted 10 fold and 100 ml of the diluted suspension was withdrawn and placed on the ZnSe prism as described in Method A. 2.3. STIRS procedure The surface titration by internal reflection spectroscopy (STIRS) experiment, which indicates how surface charge and adsorption vary with pH, has been described in detail previously (Dobson and McQuillan, 1997). In this procedure a series of solutions of different pH but constant ionic strength (0.005 M) containing both tetramethylamonium (TMA+) and perchlorate ions are flowed over the oxide film and a negative or positive surface charge is indicated in the spectra by a surface excess concentration of TMA+ or perchlorate ions respectively. Solution pH was controlled using mixtures of tetramethylammonmium hydroxide, tetramethylammonium perchlorate, and perchloric acid. Initially a 0.005 M NaOH (pH 11.5) solution is flowed over the film for ca. 30 min and is used to obtain the reference spectrum and subsequently spectra are recorded at stepwise decreased pH. The influence of arsenate adsorption on surface charge was determined by adding 0.001 M (4 ppm) arsenate(V) to the STIRS solutions. A flow rate of 2.5 ml min1 was used and spectra were recorded when constant absorbances were observed ( 40 min) indicating attainment of adsorption equilibrium. 2.4. Adsorption isotherm procedure An adsorption isotherm of arsenate on HFO was recorded in a simulated minewater consisting of 1000 1 Cl, 200 mg mg l1 Na+, 560 mg l1 SO2 4 , 263 mg l 1 1 2+ 1 l HCO3 , 150 mg l Ca and 70 mg l K+. The pH of this solution was adjusted from 7.9 to 7.2 using H2SO4. The reference spectrum was that of an amorphous HFO film in contact with the simulated mine water. Solutions of arsenate of increasing concentration (from 0.2 to 40 mg l1) were flowed over the film at a rate of 1.6 ml min1 and spectra for each concentration were recorded after 60 min flow.
3. Results Fig. 2. Attenuated total reflection infrared (ATR–IR) experimental set-up. Films of natural substrate, including HFO, were deposited on the ZnSe prism. As-bearing solutions of varying pH were passed through the glass cell. IR absorption is determined after internal reflection of the IR beam by the prism, with some IR penetration of the mineral substrate.
3.1. Spectra of arsenate(V) in aqueous solution The assignment of infrared spectra of aqueous arsenate species have been reported previously (Myneni et al., 1998) and will not be considered here in any detail.
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In the present work we show only the spectra in the As– O stretching region are shown. Fig. 3 shows the infrared spectra of 0.1 M arsenate(V) solutions at pHs of 3.0, 9.2 and 12.8. The pKa (25 C) values of H3AsO4 are 2.2, 6.9 and 11.5 (Greenwood and Earnshaw, 1984) indicating that the predominant species in these spectra are H2AsO 4, 3 HAsO2 4 and AsO4 , respectively. Clearly the degree of protonation of the arsenate species in solution affects its infrared spectrum, allowing different species to be distinguished. The IR spectrum of the solution at pH=12.8 (Fig. 3), which contains AsO34 as the predominant arsenate species, shows a major peak at 792 cm1. Some variation in the IR spectra of solution AsO3 has been noted in 4 other studies (Myneni et al., 1998 and references therein). The spectrum obtained for HAsO2 at 4 pH=9.2 (Fig. 3) shows a broad band which peaks at 858 cm1 corresponding well to that reported by Myneni et al. at pH=9.1. The spectrum obtained at pH=3.0 (Fig. 3), which is predominantly of H2AsO 4, gave major bands at 908, 878 and 738 cm1. 3.2. Arsenate(V) adsorption on particle films of minerals The adsorption of arsenate to particle films of minerals present in the Macraes tailings dam wall, namely muscovite, chlorite, quartz and feldspar, and kaolinitic schist, was investigated. No significant adsorption was observed on these films at pH=6.6 which indicates that the presence of these minerals is not important in arsenate (V) removal from Macraes minewater.
Fig. 3. Infrared spectra of 0.1 M arsenate(V) solution, indicating predominant arsenate species at various pH. Reference spectrum is that of water on ZnSe.
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3.3. Spectra from arsenate(V) adsorption to amorphous HFO The spectra of adsorbed species resulting from contact of 1103 M arsenate solution at pH=6.6 with 3 different HFO films are shown in Fig. 4. The 3 spectra have in common quite a strong broad positive band in the 900–750 cm1 region as well as several significant negative absorptions in the 1600–1300 cm1 region. The positive bands indicate that adsorption has occurred when the arsenate (V) solution has replaced the water in contact with each of the films while the negative bands show displacement/loss of species from the film surface by the adsorption process. The broad positive absorption band in Fig. 4A peaks at 820 cm1 and shows evidence of a composite structure with at least 3 component bands. This infrared absorption must arise from As–O stretch vibrations of adsorbed arsenate species. Furthermore, the shape of the absorption band is different from those of the arsenate species in aqueous solution (Fig. 3). This difference clearly indicates a perturbation on adsorption corresponding to coordination of the arsenate species to HFO film surface cations. The arsenate adsorption has resulted in absorption losses at 1577, 1380 and 1344 cm1. Most metal oxides in contact with the atmosphere readily adsorb Co2 dioxide to form surface carbonates (Dobson and McQuillan, 1997) with strong infrared absorptions in the 1600–1300 cm1 region. Natural HFO show
Fig. 4. Internal reflection infrared spectra of 1103 M arsenate adsorbed from an aqueous solution of pH=6.6 to a film of: (a) synthetic HFO; (b) natural sample of amorphous HFO using film preparation method B; and (c) natural sample of amorphous HFO using film preparation method A. Spectra (a) and (b) are offset on the absorbance scale; absorbances for (a) and (b) are amplified 15 and 3.7, respectively.
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spectra in this region that indicate carbonate adsorption, but the spectra are somewhat variable, dependent on the extent of hydration and thus on environmental history. The spectra of the natural HFO samples used in this study show several bands in this region, including a band at 1490 cm1, arising from more than one form of adsorbed carbonate. The bands at 1577, 1380 and 1344 cm1 are thus assigned to adsorbed carbonates. Although it is generally possible in adsorption studies to remove carbonate contaminants from metal oxide surfaces by alkaline washing (Dobson and McQuillan, 1997), such procedures were not employed in this work with its emphasis on processes occurring in the natural environment. Adsorption of arsenate onto a film of natural HFO prepared by Method B (above) is shown as spectrum (b) in Fig. 4. The positive absorption peak is again at 820 cm1 and of similar shape to spectrum (a) (Fig. 4). There are negative carbonate absorptions at 1560, 1490 and 1381 cm1 due to displacement by arsenate (V) and an additional negative band at 1080 cm1 which is probably due to loss of SO4 from the film as there is a significant concentration of SO4 in the mine water. Spectrum (c) in Fig. 4 is the result of arsenate adsorption onto a film of natural HFO prepared without grinding (Method A, above). This type of film gave the most intense spectra as can be seen by comparing the adsorbed arsenate bands in Fig. 4 [note amplified scales for (a) and (b)]. The adsorbed arsenate band peak in spectrum (c) is at 852 cm1 and the shape of the band differs somewhat from those in spectra (a) and (b) (Fig. 4). The negative absorptions from displaced carbonate at 1560, 1490 and 1390 cm1 and of displaced SO4 at 1080 cm1 are similar to those in spectrum (b) (Fig. 4). 3.4. pH dependence of arsenate(V) adsorption to HFO A STIRS experiment was used to investigate the pH dependence of arsenate adsorption, from solution containing 0.001 M arsenate, on natural amorphous HFO film (Method A). The resulting spectra recorded from high to low pH are shown in Fig. 5. At the initial pH of 11.6 the sharp bands at 1488 and 951 cm1 due to TMA+ ions indicate a surface excess of TMA+ arising from the negative surface charge. These bands are identical to those of TMA+ in bulk solution and the adsorption is entirely due to electrostatic forces. The TMA+ bands decrease in absorbance with decrease in pH and the 951 cm1 band is still detectable at pH=5.0. The adsorption of arsenate is detectable even at the highest pH but increases to become pronounced at the lowest pH of 2.6. The peak wavenumber of the adsorbed arsenate band shifts from 800 cm1 to 825 cm1 with some change in band shape as pH decreases and surface coverage increases. There is also a gradual loss with decreasing pH of carbonate bands at 1570 and
Fig. 5. Spectra from STIRS experiment for natural amorphous HFO prepared by method A (without grinding).
1390 cm1 indicating that not all of the adsorbed carbonate, present initially on the film, was removed by the 0.005 M NaOH wash procedure. The peak at 1630 cm1, which also becomes more prominent at lower pH in corresponding experiments on other oxides, is due to interfacial water (Connor et al., 1999). Following collection of the spectrum at pH=2.6, a 0.005 M NaOH solution was flowed over the film to test the permanence of the arsenate adsorption under conditions used to clean the film. After 30 min of flow with the high pH solution the absorbance of the arsenate band had decreased to 40% of its original value (at pH=2.6) and the band shape had become similar to that previously recorded at pH=8.0 (Fig. 5). Thus some of the adsorbed arsenate (peak 800 cm1) appears to be strongly adsorbed while under conditions of low pH/ high surface charge there is an additional more weakly adsorbed component. STIRS experiments carried out with only TMA+ and perchlorate ions present (apart from H+ and OH) usually result in the observation of significant surface excess concentrations of perchlorate ions at low pH due to the development of a positive surface charge under these conditions. The present results show little evidence of growth of the weak broad perchlorate signal at 1100 cm1 as pH is lowered. Thus it appears that the adsorption of the arsenate has resulted in the neutralisation of the positive surface charge expected at low pH. When it is also noted that there is TMA+ adsorption at pH=5, it appears that the arsenate adsorption under these conditions has resulted in surface charge reversal. Fig. 6 compares the absorbances of the adsorbed arsenate peak at 830 cm1 for a Method A film (Fig. 5
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Fig. 7. Adsorption isotherm for arsenate(V) adsorbed to HFO film (Method A) from pH=7.2 arsenate (V) solutions in simulated minewater. Fig. 6. Graph of absorbance due to arsenate adsorbed from a 1103 M solution to (A) a natural sample of amorphous HFO using film preparation method A and (B) a natural sample of amorphous HFO using film preparation method B.
data) with the corresponding data obtained from a Method B film over the experimental pH range. The trend of generally increasing adsorption with decreasing pH is shared by these two sets of data although the absorbances from the Method B film are lower, particularly at the higher pH values. These differences in infrared signal sensitivity are probably due to some loss of surface area, perhaps by particle agglomeration, as a result of the sample drying and grinding procedure used in Method B film preparation. It was also noted in the IR spectra of films made following the two methods that Method A yielded a film which was high in SO4 compared to that resulting from Method B. 3.5. Strength of arsenate(V) adsorption to amorphous HFO Determination of an adsorption isotherm provides an indication of the strength of adsorption and allows adsorption constant (K) to be calculated. Fig. 7 shows the adsorption isotherm obtained from absorbances at 803 cm1 of the arsenate (V) species adsorbed from simulated mine water (composition defined above). The isotherm shows that the oxide surface is almost saturated with adsorbed arsenate when in contact with mine water containing 103 M arsenate solution at pH=7.2. For adsorption following a Langmuir adsorption isotherm, the following relationship may be derived (Dobson and McQuillan, 1997): c=A ¼ c=Amax þ 1=ðAmax KÞ where c is the concentration of the adsorbate solution, A is the measured absorbance, Amax is the absorbance at
maximum coverage, and K is the equilibrium constant for adsorption. K is obtained from the slope and intercept of a c/A vs c plot. The present data give a linear c/A vs c plot and adsorption constant, K of (1.9 0.4)104 M1. This value of K is greater than that generally observed for electrostatic/ion exchange adsorption (Roddick-Lanzilotta et al., 1998) and very similar to that found for adsorption involving bidentate coordination e.g. phosphate on TiO2 (Connor and McQuillan, 1999).
4. Discussion 4.1. Analysis of spectra arising from adsorbed arsenate Arsenate(V) readily adsorbed to all HFO films examined in aqueous pH=6.6 solution. Under these conditions the predominant arsenate species is H2AsO 4 with spectrum corresponding to that in Fig. 3, although some HAsO24 is also expected to be present. The spectrum of the adsorbed arsenate species as seen in Figs. 4 and 5 has a maximum in the 852–820 cm1 range and does not correspond to that of any of the solution species. The absorption band is also somewhat broader than those of the solution arsenate species with evidence of some composite band structure. This spectral behaviour is similar to that for phosphate adsorption onto TiO2 (Connor and McQuillan, 1999) as might be expected for a Group V oxoanion. Analysis of the vibrational spectrum of an oxoanion adsorbed on a mineral surface can, in principle, reveal the geometry of the adsorption complex through symmetry considerations (Nakamoto, 1997). In practice, real systems often contain several different adsorption sites as well as mixtures of strongly and weakly adsorbed species and consequently there is a degree of uncertainty in these predictions. Myneni et al. (1998) have recently reviewed theoretical and experimental aspects
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of the vibrational spectroscopy of arsenate(V) ions in aqueous solutions and adsorbed on minerals. They concluded that it is often difficult to determine the type of coordination from the spectra of the adsorbed arsenate because both protonated and unprotonated arsenate ions may be present. In the present work the infrared spectral bands seen in Figs. 4 and 5 due to As–O stretching vibrations of the adsorbed arsenate species are distinctly different from those of the various solution arsenate species and appear to contain two main components. There is a broad adsorption which peaks at 800 cm1 for an adsorbate which exists at neutral to alkaline pH and its persistence under alkaline conditions indicates that it is strongly bound. Its relatively high Langmuir adsorption constant and comparisons with the adsorption behaviour of related oxoanions suggests that it arises from a species which is bound through a bidentate interaction with surface Fe3+ ions. A possible structure is shown in Fig. 8A. The amount of adsorbed arsenate increases significantly at lower pH, the band shape of the As–O stretching absorption changes, and the peak wavenumber (in Fig. 5) shifts from 800 cm1 to 825 cm1. Comparison with the spectra of the solution species in Fig. 3 suggests that a significant proportion of the adsorbate is behaving more like the HAsO24 species than AsO34 . Under acidic conditions the growth of positive charge is expected due to protonation of O2ions bridging surface Fe3+ ions. Thus the weakly adsorbed arsenate may be bound to protonated bridging oxide ions as shown in Fig. 8B. Such an interaction is expected to result in an arsenate spectrum more closely corresponding to that of HAsO34 . Similar considerations have been recently used to identify in infrared spectra the adsorption of both strongly and weakly bound oxalate ion to Cr oxide-hydroxide colloid films (Degenhardt and McQuillan, 1999).
4.2. Relationship of adsorbed arsenate to scorodite Scorodite, FeAsO4.2H2O, is the most common natural form of Fe(III) arsenate. A scorodite infrared spectrum was obtained from a pure coarse-grained (2 mm crystals) mineral specimen. This material showed a strong absorption band at 820 cm1 which reflects the arsenate component (Fig. 3). The structure of scorodite contains Fe(III) octahedra whose vertices are linked to the oxygens of arsenate tetrahedra. The structure also contains weaker linkages of arsenate tetrahedra to Fe(III) ions via H bonding through Fe–OH groups. Hence, each arsenate tetrahedron acts as bidentate bridge linking Fe octahedra (Kitahama et al., 1975). There are structural similarities between the arsenate adsorption to Fe in HFO inferred from the above experiments (Fig. 8A and B), and the structure of scorodite. The authors suggest that the relatively strongly-adsorbed component of the arsenate adsorbate on HFO, described above, has a scorodite-like structure and may be an intermediate stage in scorodite formation. Similar scorodite-like adsorption has been identified by extended X-ray absorption finestructure spectroscopy in HFO and jarosite, in weathered mine material in California (Savage et al., 2000). Scorodite forms in mine tailings at Macraes where arsenopyrite is oxidising (Craw et al., 1999b). Scorodite is most stable at a pH of about 4, and dissolution increases with increasing pH (Krause and Ettel, 1988). Scorodite dissolution produces about 1 mg l1 dissolved As at a pH of 6 (dam discharge water; above). Since the tailings waters have dissolved As as high as 10 ppm (Fig. 1B) at a pH near 8, some deposition of scoroditelike material should be expected in the dam structure as the pH is lowered. Some of the loss of dissolved As from the dam discharge waters (Fig. 1B) may be due to strongly bound adsorption of As on to HFO (as demonstrated in the laboratory, above) as a precursor to scorodite crystallisation. 4.3. Long-term As adsorption in the Macraes mine water system
Fig. 8. Possible geometries of adsorption between dissolved arsenate (top) and HFO substrate (bottom). (a) Bidentate adsorption of arsenate to surface Fe3+ ions. (b) Weak bidentate adsorption of arsenate to Fe via protonated oxide bridges.
HFO has clearly been effective at adsorbing As from mine waters at Macraes mine for the past 10 a. From an environmental point of view, it is important to know if this adsorption will continue into the future. The laboratory results described above, combined with field observations, allows some conclusions to be drawn on long-term adsorption, as outlined below. Preliminary calculations from the observed retardation of As in dam discharge waters suggest that Kd for As in the dam structure is >47 l kg1, although it is unknown how much above 47 the true Kd value lies (see above). The measured adsorption constant, Kads= (1.9 0.4)104 l mol1, can be converted at low concentrations to Kd:
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Kd ¼ cs =c where cs is the surface concentration in mol kg1 and c is the solution concentration of the adsorbing species in mol l1. Assuming the Langmuir isotherm model, cs =cs ðmaxÞ ¼ Kads c=ð1 þ Kads cÞ The limiting behaviour at low concentration is: cs =cs ðmaxÞ ¼ Kads c Thus under the same conditions,
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may be a precursor to scorodite formation. Dissolved As is adsorbed on to silicate minerals to a minor extent only. This adsorption is weak and the As is easily washed off. An adsorption isotherm for As on the Macraes HFO in simulated mine water at pH near 7 suggests an adsorption constant K of (1.9 0.4)104 M1 which for low solution As concentrations can be converted to a solid-water distribution coefficient Kd of ca. 105 l kg1. The isotherm suggests that saturation of the substrate with As occurs at above ca. 50 mg l1 As in solution. Waters from mine tailings have < 10 mg l1 dissolved As, so saturation is unlikely. Waters discharging from the mine site have dissolved As levels below detection, and these low levels will be maintained in the future.
Kd ¼ Kads cs ðmaxÞ: With these assumptions the conversion from Kads (l mol1) to Kd (l kg1) requires the maximum surface concentration (saturated coverage) in mol kg1. Using a specific surface area for HFO of 300 m2 g1 (Davis and Kent, 1990; Langmuir, 1997) and a site density of 20/ nm2 (Langmuir, 1997) a cs(max) of 10 mol kg1 is obtained. These calculations result in a Kd of c. 105 l kg1. This is nearly 4 orders of magnitude higher than the minimum estimate (above) and implies that retardation of As will be essentially infinite on human time scales. The full value of this high Kd will operate in the dam only if the medium through which the water passes has all surfaces coated with HFO and pH is similar to that of the present experiments. This is apparently true at the water discharge site, where HFO layers up to 2 mm have built up on all debris clasts, and pH is between 6 and 7. The isotherm determined for the amorphous HFO in the laboratory suggests that adsorption sites will be available for As until it starts to saturate above 0.0006 M dissolved As (Fig. 7). This is equivalent to about 50 mg l1 dissolved As, considerably greater than any observed As concentrations in the tailings waters of the dam (Fig. 1B). In addition, more HFO than dissolved As is being created in the tailings as the waters pass to the discharge site, so constant renewal of the substrate is occurring. From these observations, long-term As-free discharge appears likely.
5. Conclusions Dissolved As is strongly adsorbed on to HFO. Naturally occurring amorphous HFO deposited from Macraes mine, New Zealand, discharge waters is the most effective As adsorbing medium examined. This HFO has higher surface area than laboratory-made substrate. Adsorbed As has a bidentate structure which is tightly bound to the HFO and does not wash off readily. This adsorbed As
Acknowledgements This study was funded by the NZ Public Good Science Fund and University of Otago. Support from GRD Macraes, operators of the Macraes Mine is much appreciated, especially J. Bywater and D. Wilson. Constructive criticism by Jenny Webster-Brown substantially improved the clarity of the manuscript. References Bowell, R.J., 1994. Sorption of arsenic by iron oxides and oxyhydroxides in soils. Appl. Geochem. 9, 279–286. Connor, P.A., McQuillan, A.J., 1999. Phosphate adsorption onto TiO2 from aqueous solutions: an in situ internal reflection infrared spectroscopic study. Langmuir 15, 2916–2929. Connor, P.A., Dobson, K.D., McQuillan, A.J., 1999. Infrared spectroscopy of the TiO2/aqueous solution interface. Langmuir 15, 2402–2408. Craw, D., Windle, S.J., Angus, P.V., 1999a. Gold mineralization without quartz veins in a ductile-brittle shear zone, Macraes Mine, Otago Schist, New Zealand. Mineralium Deposita 34, 382–394. Craw, D., Chappell, D., Nelson, M., Walrond, M., 1999b. Consolidation and incipient oxidation of alkaline arsenopyrite-bearing mine tailings, Macraes Mine, New Zealand. Appl. Geochem. 14, 485–498. Craw, D. and Nelson, M. 2000. Geochemical signatures of discharge waters, Macraes Mine flotation tailings, east Otago, New Zealand. N. Z. Jour. Marine Freshwater Res., in press. Davis, J.A., Kent, D.B., 1990. Surface complexation models in aqueous geochemistry. In: Hochella, M.F., White, A.F. (Eds.), Mineral-water Interface Geochemistry. Min. Soc. Am. Rev. Mineralogy 23, 177–260. Degenhardt, J., McQuillan, A.J., 1999. Mechanism of oxalate adsorption on chromium oxide-hydroxide from pH dependence and time evolution of ATR–IR spectra. Chem. Phys. Letters 311, 179–184. Dobson, K.D., McQuillan, A.J., 1997. An infrared spectroscopic study of carbonate adsorption to zirconium dioxide sol-gel films from aqueous solutions. Langmuir 13, 3392–3396.
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EPA, 1996. Soil Screening Guidance: Technical Background Document. Environmental Protection Agency, USA, Document 540/R-95/128, Part 5: Chemical-Specific Parameters, 133–160. Freeze, R.A., Cherry, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ. Greenwood, N.N., Earnshaw, A., 1984. Chemistry of the Elements. Pergamon Press, Oxford. Hem, J.D., 1977. Reactions of metal ions at surfaces of hydrous iron oxide. Geochim. Cosmochim. Acta 41, 527–538. Kimball, B.A., Callendar, E., Axtmann, E.V., 1995. Effects of colloids on metal transport in a river receiving acid mine drainage, upper Arkansas River, Colorado, U.S.A.. Appl. Geochem. 10, 285–306. Kitahama, K., Kiriyawa, R., Baba, 1975. Refinement of the crystal structure of scorodite. Acta Crystallogr. B31, 322–332. Krause, E., Ettel, V.A., 1988. Solubility and stability of scorodite, FeAsO4.2H2O: new data and further discussion. Am. Mineral. 73, 850–854. Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice-Hall, Upper Saddle River, NJ. Mirabella, F.M., 1985. Internal reflection spectroscopy. Appl. Spectr. Rev. 21, 45–178. Myneni, S.C.B., Traina, S.J., Wachunas, G.A., Logan, T.J.,
1998. Experimental and theoretical vibrational spectroscopic evaluation of arsenate coordination in aqueous solutions, solids, and at mineral-water interfaces. Geochim. Cosmochim. Acta 62, 3285–3300. Nakamoto, K., 1997. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, 5th Edition. WileyInterscience, New York. Roddick-Lanzilotta, A.D., Connor, P.A., McQuillan, A.J., 1998. An in situ infrared spectroscopic study of the adsorption of lysine to TiO2 from aqueous solution. Langmuir 14, 6479–6484. Sadiq, M., 1997. Arsenic chemistry in soils: an overview of thermodynamic predictions and field observations. Water Air Soil Pollut. 93, 117–136. Savage, K.S., Tingle, T.N., O’Day, P.A., Waychunas, G.A., Bird, D.K., 2000. Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California. Appl. Geochem. 15, 1219–1244. Swedlund, P.J., Webster, J.G., 1999. Adsorption and polymerisation of silicic acid on ferrihydrite, and its effect on arsenic adsorption. Water Res. 33, 3413–3422. Xu, H., Allard, B., Grimvall, A., 1991. Effects of acidification and natural organic materials on the mobility of arsenic in the environment. Water Air Soil Pollut. 57-58, 269–278.
Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters, Macraes mine, New Zealand Alisa J. Roddick-Lanzilottaa, A. James McQuillana, Dave Crawb,* a
Chemistry Department, University of Otago, PO Box 56, Dunedin, New Zealand b Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand Received 5 October 2000; accepted 17 August 2001 Editorial handling by R. Fuge
Abstract Processing waters contain up to 10 mg l1 dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, Kads=(1.9 0.4)104 M1, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l1. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm1 (alkaline solutions) to 820 cm1 (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Arsenic is a common constituent of waters derived from mining areas, particularly Au mines which commonly contain arsenopyrite as an accessory mineral. This dissolved arsenic can be discharged from the mine area as a pollutant, or it may be chemically attenuated by precipitation or adsorption (Hem, 1977; Kimball et al., 1995). Adsorption of As on to Fe oxides and oxyhydroxides (HFO) is known to occur widely in nature (Bowell, 1994, Kimball et al., 1995; Swedlund and Webster, 1998), and other minerals such as Al oxides, clays, carbonates and organic material (Xu et al., 1991; Sadiq, 1997) have also been implicated. Adsorption of As on to HFO has been studied extensively in laboratory experiments (e.g.,
* Corresponding author. E-mail address: [email protected] (D. Craw).
Xu et al., 1991; Bowell, 1994, Swedlund and Webster, 1998), and much is known about the bulk chemistry of the adsorption process. In particular, the strong effects of pH, oxidation state, and associated solution composition on amounts of As adsorbed have been extensively studied (Xu et al., 1991; Bowell, 1994; Swedlund and Webster, 1998). This study builds on the above bulk chemistry database by examining As adsorption on the molecular scale at the surface of a number of mineral species including HFO. The study focuses on As adsorption on to natural materials. The materials used are those which occur within the discharge path of waters from the Macraes Au mine, New Zealand, where As adsorption is known to be occurring. From this combination of natural materials and detailed examination of surface chemistry phenomena, it is possible to identify the relative As adsorption efficiency of the various materials, and define some of the chemical processes which are occurring at
0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00116-0
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those surfaces. The Macraes mine waters have a narrow range of pH, but the adsorption processes are examined over a broader pH range to provide a more thorough chemical context for the natural adsorption processes which are occurring. 1.1. Macraes mine site The Macraes mine is developed in a mesothermal Au deposit in the Otago Schist of southern New Zealand (Craw et al., 1999a). Mineralised rock contains up to 5 wt.% As as arsenopyrite (FeAsS). Arsenopyrite-bearing ore is crushed to ca. 15 mm in the processing plant, and this material is discharged from the Au-extraction plant to a tailings impoundment system. The tailings impoundment (Fig. 1A) consists of a large debris dam constructed of fresh (unoxidised) waste rock schist excavated from the mine. The dam has an upstream facing zone (A zone, Fig. 1A) of partially oxidised schist stripped from the top of the mined area, added to reduce permeability in the dam structure. A porous chimney drain in the A zone (Fig. 1A), constructed with quartz pebbles, channels
water to the base of the dam. Tailings are discharged to the impoundment as a slurry from which the solids settle to form lake sediments, and water rises from the sediments to form a decant pond (pH=ca. 8) on the top of the sediments (Fig. 1A). Some of this water is recycled through the processing plant, and some escapes through the tailings sediments and the A zone, to reach the schist basement beneath the impoundment system (Fig. 1A). This escaping water mixes (approximately 1:1) with natural waters (pH=5.5–8) beneath the impoundment and discharges at the toe of the dam with a pH of 6.2–6.7 (Fig. 1A; Craw and Nelson 2000). Hence, unlike many mine sites around the world, which have acid discharges, the Macraes mine site is circumneutral throughout. 1.2. Mineralogy of site materials The schist basement is strongly segregated metamorphically, and consists of varying proportions of quartz, albite, muscovite, and chlorite, with minor amounts of epidote, titanite or rutile, and calcite or siderite. Mineralised rock also contains pyrite, arsenopyrite, scheelite,
Fig. 1. (A) Sketch of the tailings impoundment system at the Macraes mine, and water paths through the dam (summarised from Craw and Nelson, 2000). (B) Dissolved As concentrations (log scale) in water in the decant pond (black diamonds) on top of the tailings (Fig. 1A), and in the dam discharge waters (open circles), measured monthly as part of environmental monitoring programme of the dam site.
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and graphite, but these minerals constitute a small proportion (ca. 1–2%) of the rock mass in the tailings impoundment system. Oxidised schist is brown due to trace amounts ( <1%) of HFO coatings on joints and foliation surfaces, and contains some kaolinite and smectite clays (up to 10%). The tailings consist almost entirely of crushed unoxidised schist, although some layers of oxidised schist tailings do occur. Tailings contained negligible pyrite and arsenopyrite when the mine first opened in 1989, as these sulphide minerals were discharged to a separate impoundment structure (Craw et al., 1999b). All tailings were combined into the main impoundment (Fig. 1A) from mid 1993 to late 1998. Flocculating Fe3+ from the decant pond water (ca. 1–10 mg l1 Fe) contributes a very small amount (not visible) of HFO to the tailings sediments. Water discharging from the toe of the dam also deposits a coating of HFO (up to 2 mm thick) on the surrounding rocks. This latter HFO, which is amorphous (from X-ray diffraction), was used extensively in this study. 1.3. Arsenic adsorption in progress Arsenic concentrations of mine waters have been measured as part of a routine environmental monitoring programme. Analyses were conducted by Chemsearch (Chemistry Department, University of Otago), an International Accreditation New Zealand accredited laboratory. Arsenic analyses were obtained using graphite furnace atomic absorption spectrometry in the early stages of the monitoring period, with a detection limit of 0.005 mg l1. Flame atomic absorption spectrometry was used for most later As analyses, with detection limits ranging from 0.5 to 0.1 mg l1. Arsenic analyses are reproducible to better than 1% variation above 1 mg l1, decreasing to 20% variation near the detection limits, so all As uncertainties are contained within the plotted symbols on Fig. 1B. Dissolved As concentrations in the decant pond have increased with time, especially after incorporation of arsenopyrite-bearing tailings in 1993, and were commonly above 10 mg l1 (Fig. 1B), essentially all as As(V). In contrast, waters discharging from the dam toe have As concentrations typically below analytical detection limits. Dilution of dam waters by natural surface and ground waters (1:1 ratio; Craw and Nelson, 2000) explains some of this reduction in As concentration through the dam structure, but this alone would result in dissolved As near 5 mg l1 in the latter stages of the monitoring period. The rest of the As has clearly been removed from solution by some process such as adsorption or precipitation (see Section 1). The removal of As from the discharge waters represents trace element retardation in comparison to water flow through the dam (Freeze and Cherry, 1979).
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Observed water discharges of ca. 12 l s1 occur through ca. 10 m2 of dam material with hydraulic conductivity of ca. 0.01 m s1 at a gradient of 0.1. From these observations, water velocity, vwater, of ca. 80 m day1 can be calculated using Darcy’s law, implying that it takes water about 12 days to follow the ca. 1 km flow path. Detectable As has not emerged from the dam after 11 a (ca. 4000 days) of activity, so minimum retardation, vwater/varsenic, is ca. 330 for As (Freeze and Cherry, 1979). The true retardation value is higher than this by an unknown amount. Retardation is related to the distribution coefficient, Kd, for As between solid and solution by: vwater =varsenic ¼ 1 þ ðb =ÞKd ðFreeze and Cherry; 1979Þ where b is the bulk density, and is the porosity, of the medium through which the water passes. The ratio of b/ is about 6–8 for the Macraes dam, so minimum Kd is ca. 40–55 l kg1. These are order-of-magnitude calculations only, but they result in a minimum Kd that is similar to observed Kd values for As(V) in common soils (EPA, 1996). To investigate the As extraction process more closely, a set of materials from the Otago Schist, mainly from the Macraes mine area, have been used in a series of laboratory experiments. Fresh schist, fresh muscovite and chlorite separates, and quartz-albite rich and muscovote-chlorite rich metamorphic segregations were used to represent material in the tailings, the impounding dam, and the schist basement, along the water discharge path (Fig. 1A). Oxidised schist was used to represent the A zone and minor oxidised tailings layers. Most experiments were done on amorphous HFO deposits from the water discharge site, as this is representative of HFO formed at various stages in the water discharge path. Studies of the HFO were extended by comparing the results from this natural material to similar material formed in the laboratory.
2. Materials and methods 2.1. Infrared spectroscopy A BioRad Digilab FTS60 spectrometer with WinIR software was used to obtain infrared spectra. Spectra of arsenate (V) solution species were recorded from 0.1 M aqueous solutions prepared from Na2HAsO4.7H2O (BDH, AnalaR) and pH was adjusted with NaOH or HCl. Doubly distilled water was used for all solutions and preparations. Attenuated total reflection infrared (ATR–IR) spectra (Mirabella, 1985) of adsorbed arsenate(V) species were obtained from films deposited on single internal reflection 45 ZnSe prisms (Harrick Scientific Corporation) and in contact with aqueous arsenate (V) solutions. Reference spectra for solution were from water on a
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bare ATR prism while for adsorbed species were normally from a HFO film on a prism in contact with water. The experimental set-up is shown diagrammatically in Fig. 2. 2.2. Film preparation Natural mineral material from the Macraes mine area was used for most experiments in this study. Small pieces (ca. 1 g) of mineral material were finely ground with an agate mortar and pestle. A portion of the resultant powder (0.044 g) was then sonicated in 5.0 ml of water. The suspension was diluted 10 fold and 100 ml of the diluted suspension was withdrawn and placed on the ZnSe prism to occupy about 1 cm2 area and dried under water pump vacuum for 60 min. The films obtained by these methods were 5–10 mm thick. Most observations were made on HFO films (see below) so these films were prepared from 3 different forms of the material. Synthetic HFO was prepared by a reported procedure (Swedlund and Webster, 1998). About 0.2 g of Fe(NO3)3.9H2O (BDH, AnalaR) was dissolved in water and 1 M NaOH was added dropwise to give a pH 7. The HFO precipitate was then allowed to age overnight. The suspension was passed through a sintered glass filter (size 4) and the precipitate was washed with water. The HFO was then resuspended in water and sonicated for ca. 10 min. Films of natural amorphous HFO from the Macraes dam discharge site were prepared by two methods. Method A involved use of wet unground HFO directly from the sample bag; i.e., in its in situ form. Approximately 0.1 ml of sample was added to 5 ml of water. After 10 min sonication, 100 ml of the top of the suspension was removed and spread on a ZnSe prism to occupy
about 1 cm2 area. This was then dried under water pump vacuum for 60 min. Method B involved drying some wet HFO sample then grinding it in a mortar and pestle to a fine powder of which 0.044 g was sonicated in 5.0 ml of water. The suspension was diluted 10 fold and 100 ml of the diluted suspension was withdrawn and placed on the ZnSe prism as described in Method A. 2.3. STIRS procedure The surface titration by internal reflection spectroscopy (STIRS) experiment, which indicates how surface charge and adsorption vary with pH, has been described in detail previously (Dobson and McQuillan, 1997). In this procedure a series of solutions of different pH but constant ionic strength (0.005 M) containing both tetramethylamonium (TMA+) and perchlorate ions are flowed over the oxide film and a negative or positive surface charge is indicated in the spectra by a surface excess concentration of TMA+ or perchlorate ions respectively. Solution pH was controlled using mixtures of tetramethylammonmium hydroxide, tetramethylammonium perchlorate, and perchloric acid. Initially a 0.005 M NaOH (pH 11.5) solution is flowed over the film for ca. 30 min and is used to obtain the reference spectrum and subsequently spectra are recorded at stepwise decreased pH. The influence of arsenate adsorption on surface charge was determined by adding 0.001 M (4 ppm) arsenate(V) to the STIRS solutions. A flow rate of 2.5 ml min1 was used and spectra were recorded when constant absorbances were observed ( 40 min) indicating attainment of adsorption equilibrium. 2.4. Adsorption isotherm procedure An adsorption isotherm of arsenate on HFO was recorded in a simulated minewater consisting of 1000 1 Cl, 200 mg mg l1 Na+, 560 mg l1 SO2 4 , 263 mg l 1 1 2+ 1 l HCO3 , 150 mg l Ca and 70 mg l K+. The pH of this solution was adjusted from 7.9 to 7.2 using H2SO4. The reference spectrum was that of an amorphous HFO film in contact with the simulated mine water. Solutions of arsenate of increasing concentration (from 0.2 to 40 mg l1) were flowed over the film at a rate of 1.6 ml min1 and spectra for each concentration were recorded after 60 min flow.
3. Results Fig. 2. Attenuated total reflection infrared (ATR–IR) experimental set-up. Films of natural substrate, including HFO, were deposited on the ZnSe prism. As-bearing solutions of varying pH were passed through the glass cell. IR absorption is determined after internal reflection of the IR beam by the prism, with some IR penetration of the mineral substrate.
3.1. Spectra of arsenate(V) in aqueous solution The assignment of infrared spectra of aqueous arsenate species have been reported previously (Myneni et al., 1998) and will not be considered here in any detail.
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In the present work we show only the spectra in the As– O stretching region are shown. Fig. 3 shows the infrared spectra of 0.1 M arsenate(V) solutions at pHs of 3.0, 9.2 and 12.8. The pKa (25 C) values of H3AsO4 are 2.2, 6.9 and 11.5 (Greenwood and Earnshaw, 1984) indicating that the predominant species in these spectra are H2AsO 4, 3 HAsO2 4 and AsO4 , respectively. Clearly the degree of protonation of the arsenate species in solution affects its infrared spectrum, allowing different species to be distinguished. The IR spectrum of the solution at pH=12.8 (Fig. 3), which contains AsO34 as the predominant arsenate species, shows a major peak at 792 cm1. Some variation in the IR spectra of solution AsO3 has been noted in 4 other studies (Myneni et al., 1998 and references therein). The spectrum obtained for HAsO2 at 4 pH=9.2 (Fig. 3) shows a broad band which peaks at 858 cm1 corresponding well to that reported by Myneni et al. at pH=9.1. The spectrum obtained at pH=3.0 (Fig. 3), which is predominantly of H2AsO 4, gave major bands at 908, 878 and 738 cm1. 3.2. Arsenate(V) adsorption on particle films of minerals The adsorption of arsenate to particle films of minerals present in the Macraes tailings dam wall, namely muscovite, chlorite, quartz and feldspar, and kaolinitic schist, was investigated. No significant adsorption was observed on these films at pH=6.6 which indicates that the presence of these minerals is not important in arsenate (V) removal from Macraes minewater.
Fig. 3. Infrared spectra of 0.1 M arsenate(V) solution, indicating predominant arsenate species at various pH. Reference spectrum is that of water on ZnSe.
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3.3. Spectra from arsenate(V) adsorption to amorphous HFO The spectra of adsorbed species resulting from contact of 1103 M arsenate solution at pH=6.6 with 3 different HFO films are shown in Fig. 4. The 3 spectra have in common quite a strong broad positive band in the 900–750 cm1 region as well as several significant negative absorptions in the 1600–1300 cm1 region. The positive bands indicate that adsorption has occurred when the arsenate (V) solution has replaced the water in contact with each of the films while the negative bands show displacement/loss of species from the film surface by the adsorption process. The broad positive absorption band in Fig. 4A peaks at 820 cm1 and shows evidence of a composite structure with at least 3 component bands. This infrared absorption must arise from As–O stretch vibrations of adsorbed arsenate species. Furthermore, the shape of the absorption band is different from those of the arsenate species in aqueous solution (Fig. 3). This difference clearly indicates a perturbation on adsorption corresponding to coordination of the arsenate species to HFO film surface cations. The arsenate adsorption has resulted in absorption losses at 1577, 1380 and 1344 cm1. Most metal oxides in contact with the atmosphere readily adsorb Co2 dioxide to form surface carbonates (Dobson and McQuillan, 1997) with strong infrared absorptions in the 1600–1300 cm1 region. Natural HFO show
Fig. 4. Internal reflection infrared spectra of 1103 M arsenate adsorbed from an aqueous solution of pH=6.6 to a film of: (a) synthetic HFO; (b) natural sample of amorphous HFO using film preparation method B; and (c) natural sample of amorphous HFO using film preparation method A. Spectra (a) and (b) are offset on the absorbance scale; absorbances for (a) and (b) are amplified 15 and 3.7, respectively.
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spectra in this region that indicate carbonate adsorption, but the spectra are somewhat variable, dependent on the extent of hydration and thus on environmental history. The spectra of the natural HFO samples used in this study show several bands in this region, including a band at 1490 cm1, arising from more than one form of adsorbed carbonate. The bands at 1577, 1380 and 1344 cm1 are thus assigned to adsorbed carbonates. Although it is generally possible in adsorption studies to remove carbonate contaminants from metal oxide surfaces by alkaline washing (Dobson and McQuillan, 1997), such procedures were not employed in this work with its emphasis on processes occurring in the natural environment. Adsorption of arsenate onto a film of natural HFO prepared by Method B (above) is shown as spectrum (b) in Fig. 4. The positive absorption peak is again at 820 cm1 and of similar shape to spectrum (a) (Fig. 4). There are negative carbonate absorptions at 1560, 1490 and 1381 cm1 due to displacement by arsenate (V) and an additional negative band at 1080 cm1 which is probably due to loss of SO4 from the film as there is a significant concentration of SO4 in the mine water. Spectrum (c) in Fig. 4 is the result of arsenate adsorption onto a film of natural HFO prepared without grinding (Method A, above). This type of film gave the most intense spectra as can be seen by comparing the adsorbed arsenate bands in Fig. 4 [note amplified scales for (a) and (b)]. The adsorbed arsenate band peak in spectrum (c) is at 852 cm1 and the shape of the band differs somewhat from those in spectra (a) and (b) (Fig. 4). The negative absorptions from displaced carbonate at 1560, 1490 and 1390 cm1 and of displaced SO4 at 1080 cm1 are similar to those in spectrum (b) (Fig. 4). 3.4. pH dependence of arsenate(V) adsorption to HFO A STIRS experiment was used to investigate the pH dependence of arsenate adsorption, from solution containing 0.001 M arsenate, on natural amorphous HFO film (Method A). The resulting spectra recorded from high to low pH are shown in Fig. 5. At the initial pH of 11.6 the sharp bands at 1488 and 951 cm1 due to TMA+ ions indicate a surface excess of TMA+ arising from the negative surface charge. These bands are identical to those of TMA+ in bulk solution and the adsorption is entirely due to electrostatic forces. The TMA+ bands decrease in absorbance with decrease in pH and the 951 cm1 band is still detectable at pH=5.0. The adsorption of arsenate is detectable even at the highest pH but increases to become pronounced at the lowest pH of 2.6. The peak wavenumber of the adsorbed arsenate band shifts from 800 cm1 to 825 cm1 with some change in band shape as pH decreases and surface coverage increases. There is also a gradual loss with decreasing pH of carbonate bands at 1570 and
Fig. 5. Spectra from STIRS experiment for natural amorphous HFO prepared by method A (without grinding).
1390 cm1 indicating that not all of the adsorbed carbonate, present initially on the film, was removed by the 0.005 M NaOH wash procedure. The peak at 1630 cm1, which also becomes more prominent at lower pH in corresponding experiments on other oxides, is due to interfacial water (Connor et al., 1999). Following collection of the spectrum at pH=2.6, a 0.005 M NaOH solution was flowed over the film to test the permanence of the arsenate adsorption under conditions used to clean the film. After 30 min of flow with the high pH solution the absorbance of the arsenate band had decreased to 40% of its original value (at pH=2.6) and the band shape had become similar to that previously recorded at pH=8.0 (Fig. 5). Thus some of the adsorbed arsenate (peak 800 cm1) appears to be strongly adsorbed while under conditions of low pH/ high surface charge there is an additional more weakly adsorbed component. STIRS experiments carried out with only TMA+ and perchlorate ions present (apart from H+ and OH) usually result in the observation of significant surface excess concentrations of perchlorate ions at low pH due to the development of a positive surface charge under these conditions. The present results show little evidence of growth of the weak broad perchlorate signal at 1100 cm1 as pH is lowered. Thus it appears that the adsorption of the arsenate has resulted in the neutralisation of the positive surface charge expected at low pH. When it is also noted that there is TMA+ adsorption at pH=5, it appears that the arsenate adsorption under these conditions has resulted in surface charge reversal. Fig. 6 compares the absorbances of the adsorbed arsenate peak at 830 cm1 for a Method A film (Fig. 5
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Fig. 7. Adsorption isotherm for arsenate(V) adsorbed to HFO film (Method A) from pH=7.2 arsenate (V) solutions in simulated minewater. Fig. 6. Graph of absorbance due to arsenate adsorbed from a 1103 M solution to (A) a natural sample of amorphous HFO using film preparation method A and (B) a natural sample of amorphous HFO using film preparation method B.
data) with the corresponding data obtained from a Method B film over the experimental pH range. The trend of generally increasing adsorption with decreasing pH is shared by these two sets of data although the absorbances from the Method B film are lower, particularly at the higher pH values. These differences in infrared signal sensitivity are probably due to some loss of surface area, perhaps by particle agglomeration, as a result of the sample drying and grinding procedure used in Method B film preparation. It was also noted in the IR spectra of films made following the two methods that Method A yielded a film which was high in SO4 compared to that resulting from Method B. 3.5. Strength of arsenate(V) adsorption to amorphous HFO Determination of an adsorption isotherm provides an indication of the strength of adsorption and allows adsorption constant (K) to be calculated. Fig. 7 shows the adsorption isotherm obtained from absorbances at 803 cm1 of the arsenate (V) species adsorbed from simulated mine water (composition defined above). The isotherm shows that the oxide surface is almost saturated with adsorbed arsenate when in contact with mine water containing 103 M arsenate solution at pH=7.2. For adsorption following a Langmuir adsorption isotherm, the following relationship may be derived (Dobson and McQuillan, 1997): c=A ¼ c=Amax þ 1=ðAmax KÞ where c is the concentration of the adsorbate solution, A is the measured absorbance, Amax is the absorbance at
maximum coverage, and K is the equilibrium constant for adsorption. K is obtained from the slope and intercept of a c/A vs c plot. The present data give a linear c/A vs c plot and adsorption constant, K of (1.9 0.4)104 M1. This value of K is greater than that generally observed for electrostatic/ion exchange adsorption (Roddick-Lanzilotta et al., 1998) and very similar to that found for adsorption involving bidentate coordination e.g. phosphate on TiO2 (Connor and McQuillan, 1999).
4. Discussion 4.1. Analysis of spectra arising from adsorbed arsenate Arsenate(V) readily adsorbed to all HFO films examined in aqueous pH=6.6 solution. Under these conditions the predominant arsenate species is H2AsO 4 with spectrum corresponding to that in Fig. 3, although some HAsO24 is also expected to be present. The spectrum of the adsorbed arsenate species as seen in Figs. 4 and 5 has a maximum in the 852–820 cm1 range and does not correspond to that of any of the solution species. The absorption band is also somewhat broader than those of the solution arsenate species with evidence of some composite band structure. This spectral behaviour is similar to that for phosphate adsorption onto TiO2 (Connor and McQuillan, 1999) as might be expected for a Group V oxoanion. Analysis of the vibrational spectrum of an oxoanion adsorbed on a mineral surface can, in principle, reveal the geometry of the adsorption complex through symmetry considerations (Nakamoto, 1997). In practice, real systems often contain several different adsorption sites as well as mixtures of strongly and weakly adsorbed species and consequently there is a degree of uncertainty in these predictions. Myneni et al. (1998) have recently reviewed theoretical and experimental aspects
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of the vibrational spectroscopy of arsenate(V) ions in aqueous solutions and adsorbed on minerals. They concluded that it is often difficult to determine the type of coordination from the spectra of the adsorbed arsenate because both protonated and unprotonated arsenate ions may be present. In the present work the infrared spectral bands seen in Figs. 4 and 5 due to As–O stretching vibrations of the adsorbed arsenate species are distinctly different from those of the various solution arsenate species and appear to contain two main components. There is a broad adsorption which peaks at 800 cm1 for an adsorbate which exists at neutral to alkaline pH and its persistence under alkaline conditions indicates that it is strongly bound. Its relatively high Langmuir adsorption constant and comparisons with the adsorption behaviour of related oxoanions suggests that it arises from a species which is bound through a bidentate interaction with surface Fe3+ ions. A possible structure is shown in Fig. 8A. The amount of adsorbed arsenate increases significantly at lower pH, the band shape of the As–O stretching absorption changes, and the peak wavenumber (in Fig. 5) shifts from 800 cm1 to 825 cm1. Comparison with the spectra of the solution species in Fig. 3 suggests that a significant proportion of the adsorbate is behaving more like the HAsO24 species than AsO34 . Under acidic conditions the growth of positive charge is expected due to protonation of O2ions bridging surface Fe3+ ions. Thus the weakly adsorbed arsenate may be bound to protonated bridging oxide ions as shown in Fig. 8B. Such an interaction is expected to result in an arsenate spectrum more closely corresponding to that of HAsO34 . Similar considerations have been recently used to identify in infrared spectra the adsorption of both strongly and weakly bound oxalate ion to Cr oxide-hydroxide colloid films (Degenhardt and McQuillan, 1999).
4.2. Relationship of adsorbed arsenate to scorodite Scorodite, FeAsO4.2H2O, is the most common natural form of Fe(III) arsenate. A scorodite infrared spectrum was obtained from a pure coarse-grained (2 mm crystals) mineral specimen. This material showed a strong absorption band at 820 cm1 which reflects the arsenate component (Fig. 3). The structure of scorodite contains Fe(III) octahedra whose vertices are linked to the oxygens of arsenate tetrahedra. The structure also contains weaker linkages of arsenate tetrahedra to Fe(III) ions via H bonding through Fe–OH groups. Hence, each arsenate tetrahedron acts as bidentate bridge linking Fe octahedra (Kitahama et al., 1975). There are structural similarities between the arsenate adsorption to Fe in HFO inferred from the above experiments (Fig. 8A and B), and the structure of scorodite. The authors suggest that the relatively strongly-adsorbed component of the arsenate adsorbate on HFO, described above, has a scorodite-like structure and may be an intermediate stage in scorodite formation. Similar scorodite-like adsorption has been identified by extended X-ray absorption finestructure spectroscopy in HFO and jarosite, in weathered mine material in California (Savage et al., 2000). Scorodite forms in mine tailings at Macraes where arsenopyrite is oxidising (Craw et al., 1999b). Scorodite is most stable at a pH of about 4, and dissolution increases with increasing pH (Krause and Ettel, 1988). Scorodite dissolution produces about 1 mg l1 dissolved As at a pH of 6 (dam discharge water; above). Since the tailings waters have dissolved As as high as 10 ppm (Fig. 1B) at a pH near 8, some deposition of scoroditelike material should be expected in the dam structure as the pH is lowered. Some of the loss of dissolved As from the dam discharge waters (Fig. 1B) may be due to strongly bound adsorption of As on to HFO (as demonstrated in the laboratory, above) as a precursor to scorodite crystallisation. 4.3. Long-term As adsorption in the Macraes mine water system
Fig. 8. Possible geometries of adsorption between dissolved arsenate (top) and HFO substrate (bottom). (a) Bidentate adsorption of arsenate to surface Fe3+ ions. (b) Weak bidentate adsorption of arsenate to Fe via protonated oxide bridges.
HFO has clearly been effective at adsorbing As from mine waters at Macraes mine for the past 10 a. From an environmental point of view, it is important to know if this adsorption will continue into the future. The laboratory results described above, combined with field observations, allows some conclusions to be drawn on long-term adsorption, as outlined below. Preliminary calculations from the observed retardation of As in dam discharge waters suggest that Kd for As in the dam structure is >47 l kg1, although it is unknown how much above 47 the true Kd value lies (see above). The measured adsorption constant, Kads= (1.9 0.4)104 l mol1, can be converted at low concentrations to Kd:
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Kd ¼ cs =c where cs is the surface concentration in mol kg1 and c is the solution concentration of the adsorbing species in mol l1. Assuming the Langmuir isotherm model, cs =cs ðmaxÞ ¼ Kads c=ð1 þ Kads cÞ The limiting behaviour at low concentration is: cs =cs ðmaxÞ ¼ Kads c Thus under the same conditions,
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may be a precursor to scorodite formation. Dissolved As is adsorbed on to silicate minerals to a minor extent only. This adsorption is weak and the As is easily washed off. An adsorption isotherm for As on the Macraes HFO in simulated mine water at pH near 7 suggests an adsorption constant K of (1.9 0.4)104 M1 which for low solution As concentrations can be converted to a solid-water distribution coefficient Kd of ca. 105 l kg1. The isotherm suggests that saturation of the substrate with As occurs at above ca. 50 mg l1 As in solution. Waters from mine tailings have < 10 mg l1 dissolved As, so saturation is unlikely. Waters discharging from the mine site have dissolved As levels below detection, and these low levels will be maintained in the future.
Kd ¼ Kads cs ðmaxÞ: With these assumptions the conversion from Kads (l mol1) to Kd (l kg1) requires the maximum surface concentration (saturated coverage) in mol kg1. Using a specific surface area for HFO of 300 m2 g1 (Davis and Kent, 1990; Langmuir, 1997) and a site density of 20/ nm2 (Langmuir, 1997) a cs(max) of 10 mol kg1 is obtained. These calculations result in a Kd of c. 105 l kg1. This is nearly 4 orders of magnitude higher than the minimum estimate (above) and implies that retardation of As will be essentially infinite on human time scales. The full value of this high Kd will operate in the dam only if the medium through which the water passes has all surfaces coated with HFO and pH is similar to that of the present experiments. This is apparently true at the water discharge site, where HFO layers up to 2 mm have built up on all debris clasts, and pH is between 6 and 7. The isotherm determined for the amorphous HFO in the laboratory suggests that adsorption sites will be available for As until it starts to saturate above 0.0006 M dissolved As (Fig. 7). This is equivalent to about 50 mg l1 dissolved As, considerably greater than any observed As concentrations in the tailings waters of the dam (Fig. 1B). In addition, more HFO than dissolved As is being created in the tailings as the waters pass to the discharge site, so constant renewal of the substrate is occurring. From these observations, long-term As-free discharge appears likely.
5. Conclusions Dissolved As is strongly adsorbed on to HFO. Naturally occurring amorphous HFO deposited from Macraes mine, New Zealand, discharge waters is the most effective As adsorbing medium examined. This HFO has higher surface area than laboratory-made substrate. Adsorbed As has a bidentate structure which is tightly bound to the HFO and does not wash off readily. This adsorbed As
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