An attempt to use LA-ICP-SMS to quantify enrichment of traceelements on pyrite surfaces in oxidizing mine tailings
Björn Öhlander a,⁎, Barbara Müller a,b, Mikael Axelsson a,c, Lena Alakangas a
a Division of Applied Geology, Luleå University of Technology, SE-97187 Luleå, Swedenb Institute of Geotechnical Engineering, ETH Hönggerberg, 8093 Zürich, Switzerlandc Varian Analytical Instruments, Englundavägen 9, Box 1099, 17122 Solna, Sweden
Received 22 September 2005; accepted 17 June 2006Available online 1 September 2006
It has been shown by laboratory studies that thesurfaces of sulphide minerals have a strong affinity fordissolved metals (Jean and Bancroft, 1986; Wang et al.,
2 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
1989a; Kornicker and Morse, 1991), and pyrite formedin anoxic marine sediments has been suggested to be animportant sink for As, Hg, and Mo, moderately impor-tant for Co, Cu, Mn, and Ni, and less important for Cr,Cd, Pb, and Zn (Huerta-Diaz, 1992; Belzile and Lebel,1986). In many tailings deposits, the pyrite content ishigh below the oxidation front, and metal retention byadsorption to pyrite may potentially be important(Brown et al., 1979; Tingle et al., 1993; Al et al.,1997). However, the importance of adsorption to pyritein relation to the total secondary retainment of metalshas not been quantified in field scale.
Metals released by oxidation of sulphides are to a largeextent secondarily retained below the oxidation frontwhere pH is higher. In a study of the oxidizing sulphide-bearing mine tailings at the Laver mine in northern Swe-den, with unusually favourable conditions for estimationofmass balances, it was found that active oxidation occursin a sharp and distinct zone (Holmström et al., 1999a,b;Ljungberg and Öhlander, 2001). By a comparison of theweathering rate, estimated from both field and laboratorystudies, with the total amount of metals annually leavingthe tailings with drainage waters, it was concluded thatonly 5–10% of the total amounts of metals such as Cd,Co, Cu Ni and Zn, released by the weathering, reach thesurface-water systemdownstream themining area.Metalsreleased from oxidation and weathering of sulphideminerals are, thus, to a high degree retained at deeperlevels within the tailings themselves. It was concluded(Ljungberg and Öhlander, 2001) that the most importantretention mechanism was surface adsorption.
The importance of adsorption as a mechanism forsecondary metal retention in mine tailings has also beenstudied using sequential extraction. It was found (e.g., Linand Herbert, 1997; Dold and Fontboté, 2001; Carlssonet al., 2002) that the adsorbed/exchangeable fractioncarried a large part of the metals below the oxidation frontin pyrite-rich tailings. Secondary minerals such as Fe- andAl-oxyhydroxides have a high capacity for adsorbingtrace metals (Dzombak and Morel, 1990; Stumm, 1992;Palmqvist et al., 1997). In pyrite-rich tailings, the role ofFe-oxyhydroxides, in particular, can be expected to belarge in the oxidised zone and in downstream areas, butthe abundance of Fe-oxyhydroxides below the oxidationfront may be low. By using time-of-flight laser ionizationmass spectrometry on pyrite grains sampled from minetailings, Al et al. (1997) found a considerable adsorptionof elements such as Cu, Ag, Pb, Zn and Cd from the porewater. Müller et al. (2002) studied pyrite grains sampledfrom a profile through an oxidising tailings impoundmentby using laser ablation high-resolution ICP-MS (LA-ICP-SMS). At each spot hit by the laser, the surface layer was
analyzed in the first shot, and a second shot on the samespot gave the chemical composition of the pyriteimmediately below. A higher concentration of thedifferent elements in the surface analysis was interpretedas adsorption, which was found to be common. Thesurface adsorption of Cd, Cu and Zn on pyrite was mostimportant below the oxidation front, whereas elementssuch asAg, As, Au andBiwere preferably adsorbed to thelow amount of pyrite remaining in the oxidised zone.However, LA-ICP-SMS does not give a real surfaceanalysis, but the chemical composition of a volumedefined by the crater diameter and depth during ablation.The diameter is known, and if the crater depth for a lasershot could be determined, the adsorption could bequantified, assuming that the higher concentration in thefirst shot than in the second is caused by adsorption. Sinceprocesses other than adsorption may also lead to metalenrichments at grain surfaces, and the laser analysis givesno possibility to distinguish between different processes,the term surface enrichment is used in the following text.Examples of other processes are ion exchange andexchange of ions in monovalent complex forms (Nagyand Konya, 1988). In addition, pyrite grains that seemfresh and without surface coatings may still have a thinFe-hydroxide surface layer (Nicholson et al., 1990; Holm-ström et al., 1999a).
In this study, we determined a typical crater depthduring ablation on pyrite, and used the data of Mülleret al. (2002) to quantify in field scale the importance ofsurface enrichments of metals to pyrite below the oxida-tion front in oxidizing pyrite-rich tailings. Results areshown for As, Cd, Co, Cu, Ni and Zn.
2. Site description
TheKristinebergmine is located in the western part ofthe Skellefte ore district, approximately 175 km south-west of Luleå in northern Sweden (Fig. 1), and has beenoperated by Boliden since the operation was commis-sioned in 1940. The bedrock consists of ca. 1.9 Gametamorphosed ore-bearing volcanic rocks overlain bymetasedimentary rocks. The metasupracrustals display amarked foliation and extensive sericitization (Vivalloand Willdén, 1988). Pyrite-rich massive sulphide oresare intercalated within a stratigraphic unit consisting ofmainly basic volcanics and redeposited volcano-clasticrocks (Willdén, 1986).
The largest orebody in the area is the KristinebergZn–Cu deposit, which was brought into production in1940. In the past, ten different mines within 50 km ofKristineberg have supplied Kristineberg mill with ore,but today the Kristineberg mine is the only remaining
Fig. 1. Map showing the location of the Kristineberg mining area, and a map of Impoundment 1 showing the location of Profiles 1 and 4. 3B.Öhlander
etal.
/Journal
ofGeochem
icalExploration
92(2007)
1–12
Table 1Average composition of oxidised and unoxidised tailings atKristineberg, impoundment 1
Samples affected by secondary enrichment are excluded. All majorelements except S are expressed as oxides.
4 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
active mine in the area. The mine produces ca.450,000 tonnes of ore per year. The Kristineberg millwas closed in 1991, and today the mined ore is transpor-ted by highway trucks for processing in the Boliden mill,situated c. 100 km to the east.
Sulphide-rich tailings have been deposited in fiveimpoundments located along a small valley. Impound-ment 1 (Fig. 1), used as field site in this study, is theoldest, used until the early 1950s. The tailings impound-ments are in the final stage of remediation (Lindvallet al., 1999). Both dry covers with till as cover materialand flooding have been applied. Impoundments 1 and 1B(Fig. 1) were covered with till in 1996. A layer of crushedlimestone was distributed on the surface of the tailingsbefore the cover was applied. On Impoundment 1B, acomposite cover was applied, where 0.3m compacted tillwith a maximum hydraulic conductivity of 5 ·10−9 m/swas applied as a sealing layer. On top of that layer, aprotective layer of 1.2–1.5 m unspecified till was ap-plied. This system was also used on the northeastern partof Impoundment 1, covering approximately half the im-poundment area. On the remaining part of the impound-ment, 1 m of unspecified till was applied on the tailingsand the new concept of groundwater saturation wasimplemented. Groundwater saturation was maintainedby water management, whereby existing drainage dit-ches were backfilled to allow groundwater and surfacewater to enter the area from up-gradient areas. By co-vering the area with 1 m of unspecified till, this allowed afurther raising of the phreatic level. In this area, thegroundwater table is now shallow, completely coveringthe tailings and occasionally reaching the ground sur-face. The till surface was hydroseeded with grass, andtoday grass covers the surface.
The pyrite grains used in this study were sampledfrom Profile 1 in Impoundment 1 (Fig. 1), where theunderlying till was reached at a depth of 8.65 m. Thethickness of the tailings in Impoundment 1 ranges from afew up to 11 m (Holmström et al., 2001). The tailingswere oxidized in more than 40 years before the reme-diation, and a distinct zonation with an oxidised zoneabove the unoxidized tailing has developed. The thick-ness of the oxidised zone in Profile 1 is approximately1.05 m. The most common sulfide minerals in thetailings in decreasing order are: pyrite, pyrrhotite, spha-lerite, chalcopyrite, galena and covellite. Gangue miner-als include quartz, K-feldspar, chlorite, talc, plagioclase,muscovite, amphiboles, pyroxene, biotite and carbo-nates. The chemical composition of oxidized and unoxi-dized tailings (Table 1) clearly shows that the sulphideshave been largely lost in the oxidized zone. Sequentialextraction results (Carlsson et al., 2002) revealed that
significant fractions of As, Cd, Co, Cu, Mn, Ni, Pb andZn were adsorbed to mineral surfaces within the un-oxidised tailings. The geochemistry of tailings and porewater were studied in detail in Profile 4 (Holmströmet al., 2001), situated close to Profile 1, enabling usefulcomparisons.
The annual precipitation in the Kristineberg areavaries between 400–800 mm/a and the annual meantemperature is 0.7 °C. The vegetation in the Kristinebergarea mostly consists of coniferous forest with significantoccurrences of deciduous forest. Boglands are common.The major soil type in the area is a podzol produced fromweathered glacial till (Granlund andWennerholm, 1935;Granlund, 1943).
5B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
3. Methods
3.1. Sampling and treatment of pyrite grains
Sampling from Profile 1 was performed in February2000. Samples were taken from both the oxidized andunoxidized zones. During drilling, 19 samples between15 and 50 cm long were taken in a continuous columnfrom a depth of 130 cm, the bottom of the till cover, downto 600 cm. These drill core samples were stored in plasticbags and kept frozen until further treatment. From eachsample, between three and six larger grains of pyritewere sampled for analysis. Prior to analysis, the sampleswere thawed, sieved (0.5 mm sieve size) and repeatedlyrinsed with high-purity Milli-Q water in order to removethe clay fraction. After drying the samples at room tem-perature, the pyrite grains were separated from theremaining material by handpicking with wooden tooth-picks. The pyrite grains were optically selected using amicroscope. Only fresh grains without any visible cover,coating, inclusions or zoning were taken. This was theonly treatment applied before placing the grains in theablation chamber.
3.2. LA-ICP-SMS analysis
The analysis of the pyrite grains was performed usingan ELEMENT ICP-SFMS (sector field MS) instrumentequipped with a 266-nm Nd:YAG solid-state laser (bothfrom Thermo Finnigan MAT GmbH, Bremen, Germany),within a few weeks after sampling. A two-stage approachwas used. A first ablation aimed to remove all surface-associated elements, and a second ablation at the same spotsampled the interior of the pyrite grains. The LA para-meters used in this work, especially energy, were carefullyoptimized in order to be able to sample both the surface andthe interior of the grains. The pyrite grains were placed inthe ablation cell in groups of three to six grains with a sizeof approximately 0.5 mm. Each group represents a drillcore sample from a certain depth. Depending on the size ofthe grains, up to six different groups could be analyzedwithout opening the ablation cell. The sequence startedwith the analysis of the carrier gas without LA (argon gasblank). Gas blanks were repeatedly measured to determinethe limit of detection (LOD), based on the 3s criterion: theLOD is three times the standard deviation of a sample (thegas blank in this case),which contains no analyte. The laserwas then programmed to perform continuous raster abla-tion on one grain at a time. The ablation setting used was:
Energy 2.0 mJAblation diameter 10 μm
Number of shots in each crater 1Repetion rate 10 HzTime delay 0.1 s
After the first ablation sequence, a second sequence wasdirectly performed, using the same ablation setting, atexactly the same spots as the first one. This approachmakesit possible to study the difference between the surface andthe interior layers of the grains. Great care was taken toensure that an optimum focus of these rather irregularlyshaped pyrite grains was obtained, and that the pyrites wererepresentatively sampled by the probe. A potential problemof laser ablation arises from fractionation (mass removalbased on thermal properties). Fractionation does occurduring laser pulses and will depend on the sample and laserproperties (Outridge et al., 1996; Russo et al., 1998). Toavoid non-representative sampling, a low ablation energysetting was used to minimize enhanced ablation. To studyeventual fractionations, ablation of surface and interiorlayers was also performed on a polished pyrite standard,which was homogeneous in composition and contained noinclusions. Since no difference between the elementalcomposition of the two layers of the calibration standardcould be found, it was concluded that the elementalfractionation due to ablation effects and depletion of certainelements in the interior layer was negligible. The elementalconcentrations were calculated using external calibrationagainst a polished in-house pyrite standard from Rutje-bäcken, Sweden (Axelsson et al., 2001). In LA-ICP-SMSanalysis, normalization of the response of the analyte to thatof an internal standard (IS; usually an isotope of a majorelement) is frequently used (Pearce et al., 1997; Axelssonand Rodushkin, 2000). In this work Fe was used for thispurpose. The conversion of the intensity signals for aparticular element to concentration in ppm was carried outby comparing the intensity obtained during samplemeasurements with the intensity signals from the externalpyrite calibration standard after IS correction by using Fe(Axelsson andRodushkin, 2000).All signals collected afterLA analyses were first background-corrected using thevalues obtained during blank gas measurements.
3.3. Depth measurement procedure
The typical depth of a laser shot on a pyrite surfacewas determined by using scanning electron microscopy(SEM) on a polished section of pyrite to which laserablation (LA) had been applied. The depth was mea-sured in seven craters, which also gave some informa-tion about the precision. In order to study craters deepenough for reliable depth measurements, 60 laser shotswere directed towards each crater (Fig. 2). The average
6 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
depth for a single shot was obtained by dividing themeasured total depths with the number of shots (60). Byfocusing the SEM on a non-ablated area situated as closeas possible to the crater, noting the working distance,followed by measurement of the working distance in thebottom of the crater, the difference in working distanceswas used to determine the crater depths. The ablationsetting used was:
100×100 μm apartEnergy 2.0 mJAblation diameter 20 μmNumber of shots in each crater 60Repetion rate 20 HzTime delay 0.1 s
3.4. Calculation of the total enrichment on pyritesurfaces
If the depth of a laser crater is known as well as thedensity of the tailings, and the pyrite concentration andthe surface area of pyrite are known, then the enrichedmass of an element could be calculated by using thedifference in concentration between the first and thesecond laser shots on the pyrite grains. This difference inconcentration is multiplied by the total mass (PM) of thesurface layer of the pyrite grains (PM=crater depthmultiplied by the surface area of pyrite multiplied by thedensity of pyrite). The density of pyrite is assumed to be5.02 g/cm3.
Each sample represented a section of the tailings pile,corresponding to the length of the sampled section of the
Fig. 2. Typical laser craters obtained during the depth determination. To the leThe arrows show the approximate place were the SEM have been focused.
drill core. Sampling was performed to a depth of 600 cm,but the total depth to the underlying till was 865 cmincluding the till cover. The values for the section 660–865 cm were assumed to be equal to the average valuesof the three deepest samples. For the tailings, the averagevalues 2.04 g/cm3 for tailings bulk density, 26 wt.%pyrite in unoxidized tailings and 3.3 wt.% in oxidizedtailings were used (Holmström et al., 2001). The totalsurface area of pyrite potentially available for sorptionshould be the specific surface area, including both grainsurfaces and internal surfaces in fractures and otherinhomogeneities. According to BETanalysis, the specificsurface for the grain fractions 36–52 μm and 85–120 μmof pyrite was estimated at 0.66 m2/g and 0.75 m2/g,respectively (Mathews and Robins, 1974). For a tailingssample from the Boliden concentrator, with a pyritecontent of 24.8 wt.%, Gleisner and Herbert (2002)reported a specific surface area of the total tailings of3.72 m2/g, while pyrite grains separated with the aid ofheavy liquids had a surface area of 0.62 m2/g. Since theKristineberg tailings are similar to the tailings studied byGleisner and Herbert (2002), the value 0.62 m2/g wasused here. The values given by Mathews and Robins(1974) for specific size fractions, cannot be expected tocorrespond to values for pyrite in tailings, since althoughthe grain fractions 36–52 μm and 85–120 μm are com-mon in tailings, both smaller and larger grains occur, butsupport that the chosenBET value is of reasonably correctmagnitude.
The total mass (Mi) (in g) of an element enriched on thepyrite surfaces in a one-m2 cross section of the wholetailings pile is =Σ1
j LρP (PM) (C1−C2) where i=element,
ft the crater grid is shown, and to the right a single crater more in detail.
Fig. 3. Concentrations in ppm of As, Cd, Co, Cu, Ni and Zn vs. depth in Profile 1 in the surface layer (dots) and the interior (open circles) of pyrite grains.
7B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
j=sample, L=length of drill core sample (cm), ρ=densityof tailings (g/cm3), P=pyrite concentration (wt.%),PM=total mass of surface layer of pyrite in a drill coresample (g),C1=concentration of the first laser shot (ppm),C2=concentration of second laser shot (ppm).
4. Results
The average depth of a single shot, based on theseven crater measurements, was 0.20±0.06 μm. Thevalue 0.20 μm was used in the following calculations.
Within the upper part (130–220 cm) of the oxidizedzone, a limited number of fresh, small pyrite grains(close to 0.5 mm in size) were present. Other pyritesexhibited a skeletal form, as a result of oxidation. Lowerin the oxidized zone, the pyrite grains were generallylarger (up to 1 mm in size). Below the oxidation frontthe pyrite grains were not weathered. From 235 down to600 cm, the pyrite analyzed was fresh and occasionallyup to 1 mm in size, but generally approximately 0.5 mm.No pyrites from a depth below 6 m could be analyzed, asthe grains were too small, smaller than 0.5 mm.
Table 2Amounts of As, Cd, Cu and Zn enriched on pyrite surfaces in all samplesas well as the total amounts, in a one-m2 cross section of the tailings pile
Sample As (g) Cd (g) Cu (g) Zn (g)
130–160 cm 2.84 0 6.20 0.42160–200 cm 2.06 0.014 2.48 0.23200–220 cm 0.04 0.019 2.64 0.25220–235 cm 0.03 0.033 0.47 0.64Total in oxidized zone 4.97 0.066 11.79 1.54235–245 cm 2.03 0 12.87 1.40245–265 cm 1.58 0.014 13.80 0.96265–280 cm 13.30 0 12.40 0.37280–300 cm 7.50 0 17.67 1.21300–325 cm 14.12 0.017 86.49 8.17325–340 cm 6.26 0.005 118.0 5.84340–360 cm 4.91 0.006 70.22 3.33360–375 cm 5.47 0.070 38.91 17.37375–400 cm 6.09 0.067 60.30 5.72400–430 cm 27.59 0.40 8.06 6.73430–450 cm 16.32 0.053 13.33 1.61450–500 cm 36.15 0.50 111.6 42.27500–535 cm 8.56 0.034 14.73 17.87535–560 cm 6.20 0.011 10.08 9.34560–600 cm 6.73 0 36.27 19.96600–865 cm 58.4 0.087 153.1 122.3Total in unoxidized zone 221.2 1.26 797.8 264.5Total in tailings pile 226.1 1.33 809.6 266.0Mass lossa 314.4 42.90 1707 17,783Mass lossb 347.6 44.26 1870 18,980
Also shown are the total mass losses of these elements from the oxidizedzone during oxidation.a Just comparing concentrations in the oxidized and unoxidized zones.b Assuming Zr is immobile.
8 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
Concentrations obtained during the first and secondshot for samples from the whole profile are shown in Fig.3 for As, Cd, Co, Cu, Ni and Zn. Cobalt and Ni did notshow obvious enrichment in the surface layer of thepyrite grains in comparison to the interior layer. Cobalt isa common trace element in pyrite (Fleischer, 1955), andhigh concentrations are common. In the Kristinebergprofile the concentration of Co in pyrite variedconsiderably with depth, between 60 and 5774 ppm,but the difference between the first and the second lasershots was small. The varying Co concentrations probab-ly reflected that tailings from different ores weredeposited in Impoundment 1 (Holmström et al., 2001).Nickel was relatively difficult to determine precisely,because several parts of the ICP-SFMS equipment con-tain this element, and Ni can be released due to abrasionduring the mass flow. This resulted in fairly high back-ground signals for Ni (Müller et al., 2002).
Enrichment on pyrite wasmost obvious for Cu and As.For Cu, there was a distinct difference between the firstand second shots in all samples, but especially between300 and 400 cm depth, starting about 65 cm below theoxidation front. The highest Cu concentration was2539 ppm in the first shot at 325–340 cm depth. Znconcentration was also higher in the first than in thesecond shot, but the concentrations were lower than forCu, and the strongest surface enrichment occurred deeperdown in the tailings. The Cd concentrations were low, butthere was an obvious enrichment on pyrite surfacesbetween depths of 350 and 500 cm. Arsenic concentra-tions were high, and there was a strong enrichment onpyrite surfaces, except around the oxidation front.
The results of the calculations of the total amounts ofAs, Cd, Cu and Zn enriched on pyrite surfaces in a one-m2 cross section of the tailings pile is shown in Table 2,for all samples. These amounts were compared with thetotal loss from a one-m2 cross section of the oxidisedzone, 1.05 m thick, based on the average concentrationsin Table 1. The simplest way of calculating the total lossis just to use the bulk density 2.04 g/cm3 for bothoxidized and unoxidized tailing, and use the averageconcentrations. An alternative method for calculating themass loss of various elements during weathering is toassume that Zr is immobile (Holmström et al., 2001).Then, the mass change is
Mass changeð%Þ ¼ ððCoi =C
uni ÞðCun
Zr=CoZrÞ−1Þ � 100
whereCio=concentration of element i in oxidized tailings,
Ciun=concentration of element i in unoxidized tailings,
CZro =concentration of Zr in oxidized tailings, and CZr
un=concentration of Zr in unoxidized tailings.
Using the bulk density and the relative mass changegives the total amounts. The results of both methods ofcalculating the mass loss are included in Table 2. Ac-cording to the calculations presented in Table 2, as muchas between 65 and 72% of the As released from theoxidized zone has been secondarily retained on pyritesurfaces, mainly in the unoxidized zone. For Cu, a largepart of the mass released by weathering has also beenretained on pyrite, between 43 and 47%. Correspondingfigures for Cd and Zn are much lower, between 3.0 and3.1% and between 1.4 and 1.5%, respectively.
5. Discussion
The average Cu content in unoxidized tailings was956 ppm, but there was a distinct secondary enrichmentjust below the oxidation front, mainly explained by for-mation of covellite (Holmström et al., 2001). This isexemplified in Fig. 4 by Profile 4 (from Holmström et al.,2001), situated very close to Profile 1. The strongest Cuenrichment on pyrite surfaces occurred just below thepeak with high total concentration of Cu in the tailings.
Fig. 4. pH and redox vs. depth in Profile 4, and the concentrations of As and Cu in solid tailings (ppm) and pore waters in Profile 4 (from Holmströmet al., 2001).
9B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
Arsenic showed varying concentrations with depth in thetailings.
Measurements of pH in Profile 4 showed valuesbetween 6 and 7 in the till cover, around 6 in the upperpart of the oxidized zone, probably due to liming duringremediation, then between 4.5 and 5.3 until a sharpincrease to about 5.5 occurred at ca. 280 cm, and then anirregular increase to about 6 at depth (Fig. 4). Redoxdecreased from around 500 mV in the till cover to about200–300 mV at depth. pH was probably considerablylower around the oxidation front before remediation. Thepore water concentration of Cu in Profile 4 was high, c.790 μg/l, only in one sample, taken c. 30 cm below theoxidation front where pH was 4.6 (Fig. 4). Otherwise, theCu concentration was below 20 μg/l. The Zn concentra-tions varied, but were generally high through the profile.
Values up to 4500 mg/l were measured. Pore waterconcentrations of Cd were high between 280 cm and350 cm, up to 340 μg/l, but otherwise low. Arsenic washigh from the uppermost part of the oxidised zone down to380 cm, and at deeper levels the concentration was muchlower (Fig. 4). The highest As concentration was almost6 mg/l. The Co concentrations showed a pattern similar tothat observed for Cd, but the high concentrations, up to350 μg/l, reached deeper, down to 410 cm. The uppermostsample in the oxidized zone also had a rather highconcentration. Nickel showed a slow increase from c. 10–50 μg/l in the till cover, in the oxidized zone and just belowthe oxidation front to 260 μg/l at depth.
The strong pH dependence of adsorption is wellknown (Stumm, 1992) and experimentally described, forinstance by Jean and Bancroft (1986) for the adsorption
10 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
of Hg, Pb, Zn and Cd on sulfide mineral surfaces, byBalistrieri and Murray (1982) for the adsorption of Cu,Pb, Zn and Cd on goethite, byWang et al. (1989a) for theadsorption of Cu by pyrite, and by Kooner (1993) andPalmqvist et al. (1997) for the adsorption of Cu, Pb andZn onto goethite. There is a narrow pH range in which theadsorption of a cation goes from near-zero to almost100% adsorption.
Only one pore water sample with relatively low pHhad high Cu concentration. Otherwise, the Cu was im-mobilised by enrichment on pyrite and other processes.Enrichment of Cd on pyrite occurred just below the zonewith high pore water concentrations, indicating a pHcontrol. Before remediation, the upper part of theunoxidized zone was unsaturated. After remediation,the tailings in Profile 1 are saturated due to the raisedgroundwater level, and the high concentrations of Cu andCd in the pore water just below the oxidation front maybe a result of desorption due to the low pH. Zn enrich-ment on pyrite surfaces was lowest below the oxidationfront where pH is lowest, but otherwise there was nocorrelation between enrichment on pyrite and pore waterconcentration. There was no obvious correlation be-tween pore water concentrations of As and enrichmenton pyrite surfaces. Since Ni and Co do not appear to beenriched on pyrite, there was no correlation betweenenrichment and pore water concentrations.
The pH of point of zero charge for pyrite has beendetermined to 2.4 (Widler and Seward, 2002), suggest-ing that the pyrite surfaces in the tailings studied herehad a negative surface charge. Cations may thus havebeen adsorbed, but Cu is adsorbed at lower pH than Cdand Zn. Cu(II) has a stronger tendency than Cd(II) andZn(II) to replace Fe(II) in iron sulphides, which is also apossible mechanism for the Cu enrichment at the pyritesurfaces (Zouboulis et al., 1992).
Arsenic in aqueous solution occurs as oxyanions ofarsenate (As(V)) if conditions are oxidizing, and asarsenite (As(III)) in environments with lower redox(Smedley and Kinniburgh, 2002). If pH is >5, arsenite isstrongly adsorbed to pyrite (Bostick and Fendorf, 2003).No speciation of As in the pore waters in Kristineberghas been performed, but the possibility that arseniteoccurred below the oxidation front must be considered.
To the naked eye under the microscope, the pyritegrains used in this study appeared fresh and withoutsurface coatings, but the possibility that a thin layer ofFehydroxide occurred cannot be ruled out (cf. Nicholsonet al., 1990; Holmström et al., 1999a). The pore waters inProfile 4 had Fe concentrations up to 4000 mg/l at depth,and seem to have been oversaturated with both Fe-hydroxides and goethite (Holmström et al., 2001).
Sequential extraction performed on samples from Profile4 indicated that both amorphous Fe-oxyhydroxides andcrystalline Fe-oxides occurred below the oxidation front(Carlsson et al., 2002). However, the signal for Fe was ashigh in the first ablation as in the second on the pyritegrains (Müller et al., 2002). This suggests that if there is acoating of Fe-oxyhydroxides on the pyrite grains, thiscoating must be considerably thinner than the crater depth0.2 μm, since it is not at all reflected by the laser signal.
In general, adsorption of cations to mineral surfacesincreases with pH and anions are preferably adsorbed atlow pH (Stumm, 1992). Cu(II) is one of the divalentmetal ions that are most strongly adsorbed to ironoxyhydroxides, such as goethite and hydrous ferric oxide(Dzombak and Morel, 1990). It has been shown that amajor fraction of Cu(II) is adsorbed to goethite atpH>4.5 (Palmqvist et al., 1997). On the other hand,arsenate is strongly adsorbed to iron oxyhydroxides in avery wide pH range, extending from below pH 3 toabove pH 9 (Dixit and Hering, 2003). Thus, both cationicCu(II) and anionic As(V) may be simultaneouslyadsorbed to the same mineral particle.
According to the results shown in Table 2, Cu wasenriched on pyrite surfaces in the largest amount, followedby Zn and As. However, when comparing the totalamounts released from sulphide oxidation in the oxidizedzone, As has been retained on pyrite surfaces to the highestdegree followed by Cu, but only around 1.5% of the Zn.
According to the sequential extraction performed onsamples from Profile 4 by Carlsson et al. (2002), anaverage of 17.6% of the total Cu content in the tailingsbelow the oxidation front was released in the first leachingstep (adsorbed/exchangeable/carbonates), and 12% in theleaching step corresponding to amorphous Fe oxyhydrox-ides. The average Cu content in unoxidised tailings was956 ppm, and 17.6% corresponds to 2162 g in a one-m2
section of the tailings pile, more than the 798 g that appearto have been enriched on pyrite surfaces (Table 2). Varyingamounts, but generally less than 5% of the total As contentin the tailings below the oxidation front, was released in thefirst leaching step. Considering the average As content of183 ppm in unoxidized tailings, 5% corresponds to 116 gin a one-m2 section of the tailings pile, which is less thanthe amount suggested to have been enriched on pyritesurfaces (Table 2). The leaching step corresponding toamorphous Fe oxyhydroxides was most important for As.For Cd and Zn, the amounts released in the first extractionstep were much larger than the enrichments on pyritesurfaces. For Cd, Cu and Zn, there were no contradictionsbetween the calculated enrichments on pyrite surfaces andthe results of the sequential extraction, but for As therewas.
11B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
There were many uncertain assumptions used in thecalculation of the amounts of the various elements en-riched on the pyrite surfaces. One of the most uncertainparameters was the assumed BET surface of pyrite,which had a strong influence on the results. Also, theapproximation that the laser shots on the somewhatirregularly shaped pyrite grains from the tailings had thesame crater depth as the one determined during thecontrolled depth estimation was uncertain. The estimatedtotal amounts enriched on pyrite surfaces should, thus,not be taken too literally, but clearly indicate that sca-venging to pyrite surfaces is an important process belowthe oxidation front in pyrite-rich tailing, especially for Asand Cu of the studied elements. The laser technique usedin this study gives no information about the retentionmechanisms. Both adsorption to the pyrite directly or toFe-oxyhydroxides must be considered, and, in the caseof Cu, replacement of Fe(II) by Cu(II) in pyrite. It is alsopossible that Cu adsorption on pyrite to some extentalready occurred during the processing of the ore (Wanget al., 1989b). However, the obvious enrichment of Cuon pyrite surfaces just below the oxidation front indicatesthat the Cu source was downwards infiltrating acidleachates and not process water.
6. Conclusions
The laser ablation results clearly showed that there wasan enrichment of As, Cd, Cu and Zn on the pyrite surfacesbelow the oxidation front in the tailings, but not of Co andNi. Arsenic was also enriched on the pyrite grains thatsurvived in the oxidized zone. Copper has been enrichedon pyrite surfaces in the largest amount, followed by Znand As. However, according to the calculations presentedin Table 2, only 1.4 to 3.1% of the Cd and Zn released bysulphide oxidation in the oxidized zone has been enrichedon the pyrite surfaces in the unoxidized tailings, but for Asand Cu, corresponding figures are about 64 and 43%,respectively. There are many uncertainties in thesecalculations, and the results should not be taken tooliterally, but allow the conclusion that enrichment on pyritesurfaces is an important process for retention of As and Cubelow the oxidation front in pyrite rich tailings. This mustbe considered when trying to understand and quantify whytailings on the time scale of decades secondarily retainmost of the metals released by sulphide oxidation.
Laser ablation is not a surface-analysis technique, butmore of a thin-layer method, and gives no information onthe type of processes resulting in enrichment on thepyrite surfaces. Although only pyrite grains which ap-peared to be fresh and without surface coatings wereused in this study, the possibility that a thin layer of Fe-
hydroxides occurred must be considered. Both adsorp-tion to the pyrite directly or to Fe-oxyhydroxides mayexplain the enrichment of As, Cd, Cu and Zn on thepyrite surfaces, and, in the case of Cu, also replacementof Fe(II) by Cu(II) in pyrite.
Acknowledgements
We thank Erik Carlsson, Luleå, for assistance withpyrite sampling, and Lars Lövgren, Umeå, and WillisForsling, Luleå, for valuable discussions. This study wasfinanced by the MISTRA research programme MiMi(Mitigation of the Environmental Impact of MiningWaste), Norrbottens forskningsråd and Luleå Universityof Technology.
References
Al, T.A., Blowes, D.W., Martin, C.J., Cabri, L.J., Jambor, J., 1997.Aqueous geochemistry and analysis of pyrite surfaces in sulfide-richmine tailings. Geochimica et CosmochimActa 61, 2353–2366.
Axelsson,M.D., Rodushkin, I., 2000. Determination of major and traceelements in sphalerite using laser ablation double focusing sectorfield ICP-MS. Journal of Geochemical Exploration 72, 81–89.
Axelsson, M.D., Rodushkin, I., Petrov, P., Burman, E., Öhlander, B.,2001. Multielement analysis of sulfides by ICP techniques usingsolution nebulisation and laser ablation. Recent Research Develop-ments in Pure and Applied Analytical Chemistry, vol. 3, pp. 27–35.
Balistrieri, L.S., Murray, J.W., 1982. The adsorption of Cu, Pb, Zn, andCd on goethite from major ion seawater. Geochimica et Cosmochi-mica Acta 46, 1253–1265.
Belzile, N., Lebel, J., 1986. Capture of arsenic by pyrite in near-shoremarine sediments. Chemical Geology 54, 279–281.
Bostick, B.C., Fendorf, S., 2003. Arsenite sorption on troilite (FeS) andpyrite (FeS2). Geochimica et Cosmochimica Acta 67, 909–921.
Brown, J.R., Bancroft, G.M., Fyfe, W.S., McLean, R.A.N., 1979.Mercury removal from water by iron sulfide minerals. An electronspectroscopy for chemical analysis (ESCA) study. EnvironmentalScience and Technology 13, 1142–1144.
Carlsson, E., Thunberg, J., Öhlander, B., Holmström, H., 2002. Sequentialextraction of sulphide-rich tailings remediated by the application of tillcover, Kristineberg, northern Sweden. The Science of the TotalEnvironment 299, 207–226.
Dold,B., Fontboté, L., 2001. Element cycling and secondarymineralogy inporphyry copper tailings as a function of climate, primary mineralogy,andmineral processing. Journal ofGeochemical Exploration 74, 3–55.
Dixit, S., Hering, J.G., 2003. Comparison of arsenic(V) and arsenic(III) sorption onto iron oxide minerals: implications for arsenicmobility. Environmental Science and Technology 37, 4182–4189.
Dzombak, D.A., Morel, F.M.M., 1990. Surface Complexation Model-ling: Hydrous Ferric Oxide. John Wiley and Sons, New York.
Fleischer, M., 1955.Minor elements in some sulfide minerals. EconomicGeology, 50th Anniversary Volume, pp. 970–1024.
Gleisner, M., Herbert, R.B., 2002. Sulfide mineral oxidation in freshlyprocessed tailings: batch experiments. Journal of GeochemicalExploration 76, 139–153.
Granlund, E., 1943. Beskrivning till jordartskarta över Västerbottenslän nedanför odlingsgränsen. Swedish Geological Survey Ca, vol.26, pp. 1–165 (in Swedish).
12 B. Öhlander et al. / Journal of Geochemical Exploration 92 (2007) 1–12
Granlund, E., Wennerholm, S., 1935. Sambandet mellan moräntypersamt bestånds-och skogstyper i Västerbottens lappmarker. SwedishGeological Survey C, vol. 384, pp. 1–65 (in Swedish).
Holmström, H., Ljungberg, J., Ekström, M., Öhlander, B., 1999a.Secondary copper enrichment in tailings at the Laver mine,northern Sweden. Environmental Geology 38, 327–342.
Holmström, H., Ljungberg, B., Öhlander, B., 1999b. The role of carbo-nates for mitigation of metal release from mining waste: evidencefrom humidity cells tests. Environmental Geology 37, 267–280.
Holmström, H., Salmon, U.J., Carlsson, E., Petrov, P., Öhlander, B.,2001. Geochemical investigations of sulphide-bearing tailings atKristineberg, northern Sweden, a few years after remediation. TheScience of the Total Environment 273, 111–133.
Huerta-Diaz, M.A., Morse, J.W., 1992. Pyritization of trace metals inanoxic marine sediments. Geochimica et Cosmochimica Acta 56,2681–2702.
Jean, G.E., Bancroft, G.M., 1986. Heavy metal adsorption by sulfidemineral surfaces. Geochimica et CosmochimcaActa 50, 1455–1463.
Kooner, Z.S., 1993. Comparative study of adsorption behaviour ofcopper, lead and zinc onto goethite in aqueous systems. Environ-mental Geology 21, 242–250.
Kornicker, W.A., Morse, J.W., 1991. Interactions of divalent cationswith the surface of pyrite. Geochimica et Cosmochimica Acta 55,2159–2171.
Lin, Z., Herbert, R.B., 1997. Heavy metal retention in secondaryprecipitates from a mine rock dump and underlying soil, Dalarna,Sweden. Environmental Geology 33, 1–12.
Lindvall, M., Eriksson, N., Ljungberg, J., 1999. Decommissioning atKristineberg mine, Sweden. Sudbury ‘99, Mining and theEnvironment II, September 13–17, vol. 3, pp. 855–863.
Ljungberg, J., Öhlander, B., 2001. The geochemical dynamics ofoxidising mine tailings at Laver, northern Sweden. Journal ofGeochemical Exploration 74, 57–72.
Mathews, C.T., Robins, R.G., 1974. Aqueous oxidation of iron disulfideby molecular oxygen. Australian Chemical Engineering 15, 19–24.
Müller, B., Axelsson, M.D., Öhlander, B., 2002. Adsorption of traceelements on pyrite surfaces in sulfidic mine tailings from Kristine-berg (Sweden) a few years after remediation. The Science of the TotalEnvironment 298, 1–16.
Nagy, N.M., Konya, J., 1988. The interfacial processes between calcium-bentonite and zinc ions. Colloids and Surfaces 32, 223–235.
Nicholson, R.V., Gillham, R.W., Reardon, E.J., 1990. Pyrite oxidationin carbonate buffered solution. 2. Rate control by oxide coatings.Geochimica et Cosmochimica Acta 54, 395–402.
Outridge, P.M., Doherty, W., Gregoire, D.C., 1996. The formation oftrace element enriched particulates during laser ablation ofrefractory materials. Spectrochimica Acta B 51, 1451–1462.
Palmqvist, U., Ahlberg, E., Sjöberg, S., Lövgren, L., 1997. In-situdeterminations of metal ions in goethite suspensions: singlemetal ionsystems. Journal of Colloid and Interface Science 196, 254–266.
Pearce, N.J.G., Perkins,W.T.,Westgate, J.A., Gorton,M.P., Jackson, S.E.,Neal, C.R., Chenery, S.P., 1997. A compilation of new and publishedmajor and trace element data for NIST SRM 610 and NIST SRM 612glass reference materials. Geostandands Newsletter 21, 115–144.
Smedley, P.L., Kinniburgh, D.H., 2002. A review of the source,behaviour and distribution of arsenic in natural waters. AppliedGeochemistry 17, 517–568.
Stumm,W., 1992. Chemistry of the Solid–Water Interface. John Wileyand Sons, New York.
Tingle, T.N., Borch, R.S., Hochella, M.F., Becker, C.H., Walker, W.J.,1993. Characterization of lead on mineral surfaces in soils contami-nated by mining and smelting. Applied Surface Science 72, 301–306.
Vivallo, W., Willdén, M., 1988. Geology and geochemistry of an EarlyProterozoic volcanic arc sequence at Kristineberg, Skelleftedistrict, Sweden. Geologiska Föreningens i Stockholm Förhan-dlingar 110, 1–12.
Wang, X., Forssberg, K.S.E., Bolin, N.J., 1989a. Adsorption of copper(II) by pyrite in acidic to neutral pH media. Scandinavian Journalof Metallurgy 18, 262–270.
Wang, X., Forssberg, K.S.E., Bolin, N.J., 1989b. Pyrrhotite activationby Cu(II) in acidic to neutral pH. Scandinavian Journal ofMetallurgy 18, 271–279.
Widler, A.M., Seward, T.M., 2002. The adsorption of gold(I)hydrosulphide complexes by iron sulphide surfaces. Geochimicaet Cosmochimica Acta 66, 383–402.
Willdén, M., 1986. Geology of the western part of the Skellefte field andthe Kristineberg and Hornträsk sulphide deposits. In: Rickard, D.T.(Ed.), The Skellefte Field: 7th IAGODSymposium, ExcursionGuideNo 4. Sveriges Geologiska Undersökning CA, vol. 62, pp. 46–52.
