Metal mobilization under alkaline conditions in ash-covered tailings

Journal of Environmental Management 139 (2014) 38e49

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Metal mobilization under alkaline conditions in ash-covered tailings Jinmei Lu*, Lena Alakangas, Christina Wanhainen Division of Geosciences and Environmental Engineering, Luleå University of Technology, SE-97187 Luleå, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 July 2013 Received in revised form 10 December 2013 Accepted 12 December 2013 Available online

The aim of this study was to determine element mobilization and accumulation in mill tailings under alkaline conditions. The tailings were covered with 50 cm of fly ash, and above a sludge layer. The tailings were geochemically and mineralogically investigated. Sulfides, such as pyrrhotite, sphalerite and galena along with gangue minerals such as dolomite, calcite, micas, chlorite, epidote, Mn-pyroxene and rhodonite were identified in the unoxidized tailings. The dissolution of the fly ash layer resulted in a high pH (close to 12) in the underlying tailings. This, together with the presence of organic matter, increased the weathering of the tailings and mobilization of elements in the uppermost 47 cm of the tailings. All primary minerals were depleted, except quartz and feldspar which were covered by blurry secondary carbonates. Sulfide-associated elements such as Cd, Fe, Pb, S and Zn and silicate-associated elements such as Fe, Mg and Mn were released from the depletion zone and accumulated deeper down in the tailings where the pH decreased to circum-neutral. Sequential extraction suggests that Cd, Cu, Fe, Pb, S and Zn were retained deeper down in the tailings and were mainly associated with the sulfide phase. Calcium, Cr, K and Ni released from the ash layer were accumulated in the uppermost depletion zone of the tailings. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Ash cover Sulfide oxidation rate Metal mobilization Sulfide tailings

1. Introduction Acid Rock Drainage (ARD), resulting from the oxidation of sulfide-bearing mine waste, is one of the main pollution problems associated with mining activities. Problems arising from ARD and subsequent remedial measures have been studied extensively (INAP, 2009; Lottermoser, 2010). However, ARD does not include all mine drainage. If the waste material is poor in sulfides and rich in carbonates, drainage from the disposal area may have a neutral pH but could still contain elevated concentrations of heavy metals and sulfates; this is termed neutral mine drainage (NMD) (Balistrieri et al., 1999; Heikkinen and Räisänen, 2008; INAP, 2010; Jambor et al., 2003; Lindsay et al., 2009; Plante et al., 2010; Sracek et al., 2010). Metal-rich and sulfate-rich discharges from tailings may continue for hundreds of years after mine closure (Lottermoser, 2010). Thus, the prevention of contaminated mine drainage is an important task, not only over the operational lifetime of a mine, but also during decommissioning or remediation of abandoned mine sites, to reduce the environmental impacts and risks in the future (Deissmann et al., 2000). In Sweden, the two most common

* Corresponding author. Tel.: þ46 (0) 725355597; fax: þ46 920491199. E-mail addresses: [email protected], [email protected] (J. Lu). http://dx.doi.org/10.1016/j.jenvman.2013.12.036 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.

methods to prevent sulfide oxidation are dry cover and water cover (Cabral et al., 2000; Hallberg et al., 2005; Höglund et al., 2004). The dry covers can be simple or complex, ranging from a single layer of earthen material to several layers of different material types (Mylona et al., 2007). Engineered composite dry cover is assumed to be a long-term, low maintenance solution to the attenuation of ARD (INAP, 2009). Materials used for dry covers include till, lowsulfide tailings, oxide waste, clay subsoils, soils, organic wastes and neutralizing materials (Lottermoser, 2010). In recent years, research has focused on using alkaline industrial by-products as substitute for natural resources as dry cover over reactive mine waste to inhibit sulfide oxidation (Bellaloui et al., 1999; Cabral et al., 2000; Catalan and Kumari, 2005; Chtaini et al., 2001), thus solving two waste problems at the same time. Several case studies of alkaline industrial wastes used in dry covers have shown to be effective (Bellaloui et al., 1999; Chtaini et al., 2001; Greger et al., 2009; Hallberg et al., 2005; Perez-Lopez et al., 2007a,b). However, most of the applications are on wastes with high sulfide content, low carbonate content and acid-producing potential. The suitability of high alkaline industrial by-products as dry covers over wastes with low sulfide and high carbonate content has rarely been investigated. A study to evaluate the efficiency of four different dry covers consisting of paper mill ash and sludge over low-sulfide and carbonate-rich tailings at the Ryllshyttan tailings impoundment,

J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

Garpenberg, Sweden was carried out (Lu et al., 2013) and showed some interesting results from a profile with thickest ash application. The goal of this study, based on the previous study, was to explore the geochemical process of sulfide oxidation and element mobilization under extremely alkaline conditions (pH > 11). 2. Study area The Bergslagen ore district in south central Sweden is an intensively mineralized area with more than 6000 documented mineral deposits (Stephens et al., 2009). The Garpenberg ZnePbe Age(CueAu) deposit located north-west of Stockholm is the largest sulfide deposit in the district (Fig. 1) with reserves, resources and a mined tonnage of around 80 Mt (Jansson and Allen, 2011). Marble (metamorphosed limestone) within thick successions of metavolcanic rocks is the main host rocks to the Garpenberg ore body and to several minor sulfide ore-bodies in the 7 km long Garpenberg mineralized system (Gotthardsson and Sundberg, 2011). Mining from these deposits produces 1.5 Mt of complex sulfide ore annually. The ore, containing on average 5.6% Zn, 2.1% Pb, 0.1% Cu,

39

129 g/t Ag and 0.3 g/t Au (New Boliden, 2013), is processed in a concentrator located to the south of the Garpenberg mine, producing large quantities of tailings (Gotthardsson and Sundberg, 2011; Salmon and Destouni, 2001). The coarse fraction of the tailings is used for backfilling to the underground, whilst the fines are deposited in tailings impoundments (Gotthardsson and Sundberg, 2011; Salmon and Destouni, 2001). The tailings at Garpenberg have been deposited in impoundments since 1827. The Ryllshyttan tailings pond (Fig. 1) was opened in 1967 and consists of two parts, each of them with an area of about 35 ha. One part is still in use for the deposition of tailings; the other part is currently being decommissioned. Tailings in the Ryllshyttan impoundment consist of silicates (84%), carbonates (14%) and sulfides (2%), with metal contents of Fe (7%), Zn (0.5%), Pb (0.3%) and Cu (0.02%) (Gotthardsson and Sundberg, 2011). They contain large amounts of buffering minerals, such as talc, chlorite, calcite, rhodochrosite, dolomite and silicate, which can consume protons from sulfide oxidation. In the section being decommissioned, the central part is completely water-saturated, whilst limited areas along the west and

Fig. 1. The location of the Ryllshyttan tailings impoundment at Garpenberg, Sweden and the location of the dry covers in the test areas and samples selected for sequential extraction and mineralogical analysis from Ryllshyttan tailings impoundment. Area F has only a soil layer above tailings. The sludge and ash in test area B originate from the Fors paper mill.

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J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

south parts have a partly unsaturated top-soil (European Commission, 2004; Gotthardsson and Sundberg, 2011). In autumn 2000, a test program was started by Boliden and Fors paper mill in the unsaturated area along the west and south parts at the Ryllshyttan tailings pond. The aim was to prevent oxidation and weathering of the tailings within the unsaturated part and to establish vegetation on the ground. Four test areas (A, B, D and E) with dry covers over the tailings (Fig. 1) were constructed from autumn 2000 to autumn 2003. The materials used and the detailed design of each test area are described in a recent study by Lu et al. (2013). The present study aims to investigate the reactions that have occurred in test area B, with the thickest ash layer above tailings. 3. Materials and methods 3.1. Tailings profile sampling Two tailings profiles were drilled in September 2011. One profile had only vegetation soil on the tailings (profile F), the other profile had an ash layer with a thickness of 50 cm on the tailings (profile B), that was applied 10 years ago. The locations of the profiles are shown in Fig. 1. The drilling depth of the profiles was extended down below the unoxidized tailings (Fig. 1). Solid subsamples were collected from each layer at different intervals. In profile B, samples were collected at 20e30 cm intervals in the sludge layer, and at 15e20 cm intervals in the ash layer. In the underlying tailings samples were collected at 5 cm intervals in the top 20 cm, followed by 7e20 cm intervals to a depth of 205 cm. In profile F, two samples were collected in the soil cover, in the tailings layer at 5 cm intervals in the top 20 cm, followed by 5e15 cm intervals, based on the color and texture difference, to a depth of 137 cm. The collection scheme for samples is shown in Fig. 1. The samples were collected into two oxygen diffusion-free plastic bags and two plastic containers for analysis of paste pH, chemical composition, mineralogical composition, sequential extraction and pore-water extraction. All the samples were stored in a refrigerator at 18  C until analysis. Altogether, 29 solid samples were collected from the two profiles. 3.2. Sequential extraction The mobility of metals is strongly dependent on their specific chemical forms and methods of binding (Margui et al., 2004). Sequential extraction can provide information about the fractionation of metals in the different lattices of the solid sample (Margui et al., 2004). To better understand the transportation and retention mechanism of elements, seven tailings samples were selected for sequential extraction: 2 samples from the deepest part of the comparison profile F (F-T-11 and F-T-12) and 5 samples from profile B (3 in the upper 47 cm depletion zone B-T-1, B-T-3 and B-T-5, and 2 in the deepest part B-T-8 and B-T-9) (Fig. 1). The method of extraction was adapted from that of Tessier et al. (1979), Hall et al. (1996) and Dold (2003). Six fractions were extracted. The following extraction scheme was used. 1. Easily soluble and exchangeable fraction (F1): The samples were extracted with 1 M MgCl2 at L/S ratio 20 and initial pH 7. Add 20 ml 1 M MgCl2 to 1 g dried sample at pH 7 and room temperature, and it was continuously stirred for 1 h. 2. Carbonate fraction (F2): The residue from step 1 was extracted with 1 M NaOAc at L/S ratio 40 adjusted to pH 5 with HOAc. It was continuously stirred for 5 h at the room temperature. 3. Reducible or Fe and Mn oxide and hydroxide associated fraction (F3): The residue from step 2 was extracted with 1 M NH2OH.HCl at L/S ratio 40 in 25% (v/v) HOAc at pH 2. It was then occasionally stirred for 6 h at 96  C.

4. Organic and secondary sulfide fraction (F4): The residue from step 3 was extracted with 15 ml 30% H2O2 and 3 ml 0.02 M HNO3. It was then stirred for 2 h at 85  C. 5. Primary sulfide fraction (F5): The residue from step 4 was added 750 mg of KClO3 and 15 ml of 12 M HCl, cap and vortex. After 30 min, add 15 ml of water, cap, vortex and centrifuge for 10 min. Decant supernatant liquid into a labeled test tube. To the residue, add 10 ml of 4 M HNO3, cap and vortex. Place in a water bath at 90  C for 20 min. After digestion, transfer all contents to a Teflon pressure tube, vortex and centrifuge for 10 min. Decant supernatant liquid into the previous labeled test-tube. Rinse the residue with 5 ml of water, vortex and centrifuge again. Do this twice and add supernatant rinses to the test tube. Make up to 50 ml and analyze. 6. Residual fraction or silicate fraction (F6): The residue from the above extraction was sent to the accredited ALS Analytical laboratory for chemical analysis of the silicate fraction. The extraction was performed in 50 ml polyethylene tubes. After extraction 1e4, the residue was separated from the extract by centrifugation at 10,000 rev./min for 10 min, washed with 20 ml of Mili-Q water. Both the supernatant and wash solutions were collected into acid-washed plastic bottles and stored at 4  C in a refrigerator until analysis. 3.3. Mineralogical analysis Polished thin sections were prepared by Vancouver Petrographics Ltd, Canada, from 7 of the collected solid profile samples in order to reveal the mineralogical composition of the tailings material at different depths. A detailed optical examination of polished thin sections F-T-12 and B-T-3 was carried out in reflected and transmitted light using a conventional petrographic microscope (Nikon ECLIPSE E600 POL) and a Zeiss Merlin HR-SEM (0.8 nm) for textural analysis and phase identification. 3.4. Chemical composition analysis of solid and liquid samples All water samples were sent to the accredited ALS Analytical laboratory for analysis of As, Cd, Cr, Cu, Mn, Ni, P, Pb and Zn using ICPeSFMS and of Al, Ca, Fe, K, Mg, Na and S using ICPeAES. Before analysis, samples were acidified with super pure nitric acid using 1 ml of acid for 100 ml of sample. Analysis was carried out following the EPA modified method 200.8 for ICPeSFMS and 200.7 for ICPe AES. The solid samples were analyzed for major and minor elements by the accredited ALS Analytical laboratory. To analyze the elements As, Cd, Co, Cu, Ni, Pb, S and Zn, the samples were dried at 50  C and digested with 7 M nitric acid in closed Teflon vessels in a microwave oven. Other elements were determined after fusion with lithium metaborate followed by dissolution in diluted nitric acid. The solutions were centrifuged and diluted before analysis. For quality control of the data, two in-house reference materials were analyzed in parallel with the solid samples. The analysis results of the reference materials are shown in Table 1 of the supplementary materials. Two validation parameters (Limit of Detection (LOD) and Limit of Quantification (LOQ)) of the analytical method for both solid and water samples are shown in Table 2 of the supplementary materials. 3.5. Acid-neutralizing capacity (ANC) Acid-neutralizing capacity (ANC) is a measure for the overall buffering capacity in a material. ANC is calculated as the difference

J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

between the overall buffering capacity and the acid producing potential of a material. For tailings in our study, the buffering capacity is assumed to all originate from CaCO3, and CaCO3 is the only inorganic carbon (IC) source in the tailings. Therefore the total buffering capacity is calculated from the inorganic carbon. All S is assumed to exist as sulphides, and the acid producing potential can be calculated from the S content in the tailings. Thus the ANC of the tailings can be calculated from the inorganic carbon content and the S content in the tailings according to formula (1).

ANC ¼ IC=6  S=16

(1)

4. Results 4.1. Unoxidized tailings from the F profile 4.1.1. Chemical composition The tailings sample at the deepest part of the F profile F-T-12 (Fig. 1) was assumed to be unoxidized and didn’t change after deposition. Its chemical composition was compared with the Swedish guidelines for contaminated land in Table 1. Iron was the predominant element in the tailings, with content as high as 7.4%. The contents of CaO, K2O, MgO and MnO2 were also high, 4.6%, 1.2%, 5.1% and 1.2%, respectively. Compared with the Swedish guidelines, Cd, Pb and Zn in the tailings were one to two orders of magnitude higher than guideline values and As and Cu were two to five times higher. The contents of Cr and Ni in the tailings were low, with Ni one order of magnitude lower than the guideline values. The sulfur content was 3.6%. In summary, As, Ca, Cd, Cu, Fe, K, Mg, Mn, Pb and Zn constitute the dominant elements of the tailings. 4.1.2. Mineralogical composition Microphotographs from a polished thin section of a sample from the deepest unoxidized tailings in the F profile (sample F-T-12) are shown in Fig. 2. Minerals containing Ca, Fe, K and Mg, such as dolomite, K-mica (including muscovite, sericite, FeeMg biotite) and FeeMg chlorite were common in the sample. Silicate minerals containing Ca, K, Mg and Mn are generally associated with quartz. Iron was detected in all silicate minerals except quartz, muscovite and sericite. Carbonate and silicate minerals containing Ca, Fe, Mg and Mn, such as calcite, tremolite, epidote, and Mn-pyroxene, were also observed, but were not common. Pyrite and pyrrhotite were the dominant sulfide minerals in the tailings sample. Galena and sphalerite were also identified. Arsenic and Cu, identified in the chemical analysis, probably originate from arsenopyrite and chalcopyrite even if these two minerals were not observed in the studied thin section. These sulfide minerals are known to be present in the ore. No secondary phases were

Table 1 Chemical composition of the unoxidized tailings (F-T-12). Element

Unit

Tailings

CaO K2O MgO MnO2 Fe2O3 As Cd Cr Cu Ni Pb Zn

%TS %TS %TS %TS %TS mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg

4.59 1.15 5.1 1.23 7.38 51.3 10.5 31 181 2.77 3370 3650

Guidelines for contaminated land (Naturvårdsverket, 2009)

10 0.5 80 80 40 50 250

41

identified and sample F-T-12 can thus be regarded a good reference sample that has not been subject to alteration after deposition. 4.1.3. Sequential extraction results The results of the sequential extraction of the deepest samples from the tailings in F profile are listed in Table 2. Recovery rate of the extraction for each element was calculated and is shown in Table 3 of the supplementary materials. Most of the elements extracted showed high recovery rate, between 80 and 120%. The extracted fractions of As, Ca, Cd, Cu, Fe, K, Mg, Mn, Pb, Zn and S were analyzed since they dominated in the tailings. The sequential extraction suggests that Ca and Mn dominated in the silicate fraction (F6) and the carbonates fraction (F2), with silicate fraction accounting for about 57e66% for Ca and 78e79% for Mn of the total content, respectively. Ca carbonates, such as dolomite and calcite, were identified in the tailings. Magnesium was mainly present in the silicate phase (F6), which accounts for 86e87% of the total Mg content. Potassium and Pb was mainly found in the carbonate phase (F2), which account for 56e73% and 85e89% of total K and Pb, respectively, which is not consistent with the mineralogical analysis. A high K and Pb content in this phase indicates that most of the K and Pb minerals in the tailings are acid soluble. Soluble K minerals and galena are dissolved at this stage. Small percentages of K exist in the silicate phase (F6), which was also inferred from the mineralogical analysis. Some K and Mg exist in the primary sulfide fraction (F5), which indicates that some silicate minerals are dissolved in the fifth step since the existence of K and Mg as sulfides is not likely (Parviainen, 2009). Arsenic, Cd, Cu, Fe, S and Zn was found mainly in the fourth and fifth fraction (F4 and F5), as secondary and primary sulfides. Sulfide phases of Cu and Fe account for more than 80% and 40% of total element content. The mineralogical analysis showed that pyrite and pyrrhotite were common, which explain the association to the sulphide fraction. More than 40% of Fe was found in the silicate phase (F6), which was verified by mineralogical analysis, with Fe in almost all silicate phases. The correlation coefficients between elements in the unoxidized tailings were calculated and shown in Table 3. The correlation coefficients between Ca, Fe, Mg and Mn, and between As, Cd, Cu, Fe, S and Zn are high, which implies a coexistence of these elements in the tailings. This agrees with the results from the mineralogical analysis. The correlation coefficients between As and Fe, As and S are as high as 0.965 and 0.852, which indicates the existence of As in the tailings as arsenopyrite, although this mineral was not identified from mineralogical analysis. The correlation coefficients between Cd, Cu, Zn, and S are higher than 0.7, which indicates the existence of CdeCueZn sulfides in the tailings. Cadmium and Zn have almost the same form and similar geochemical behavior in the environment (Warrender et al., 2011). Substitution of Cd in the sphalerite structure is observed from the mineralogical analysis. 4.2. Composition of tailings in the profiles 4.2.1. Paste pH Because of the abundance of carbonate minerals, such as dolomite and calcite, and the relative scarcity of sulfide minerals, a nearneutral to alkaline condition has been formed throughout the tailings profile F (paste pH ¼ 6.0e7.5) (Lu et al., 2013). The presence of buffering minerals in the tailings plays a significant role in mitigating the effects of acid mine drainage. The acid-neutralizing capacity of the tailings is sufficient to consume the acidity generated by sulfide oxidation. An acidebase accounting (ABA) calculation from the S and inorganic carbon content in the tailings showed a positive net neutralization potential.

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J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

Fig. 2. Microphotograph of F-T-12 and B-T-3 in reflected light (a: F-T-12; c: B-T-3) and transmitted light (b: F-T-12; d: B-T-3).

In profile B, the paste pH in the tailings layer showed a distinct difference from profile F (without ash layer). The tailings layer can be divided into two distinct zones according to paste pH, which was about 12 in the uppermost 47 cm, much higher than in profile F, but gradually decreased to between 7 and 8 below 47 cm (Lu et al., 2013). The high pH in the uppermost tailings is due to the application of a thick fly ash layer, since the pH was much lower in the tailings in other thin fly ash or no fly ash applied test areas (Lu et al., 2013). Fly ash has a high neutralizing capacity and the paste pH of fresh ash is normally close to 12 (Nurmesniemi et al., 2008; Pöykiö et al., 2004). The application of a large amount of fly ash over the tailings significantly increased the pH of the underlying tailings. The magnitude of pH range in tailings, has shown dependency on the amount of fly ash applied and the sludge/fly ash ratio used (Sajwan et al., 2003). 4.2.2. Element content The element content vs. depth in the tailings profiles has been plotted for different elements in Figs. 3 and 4.

The two profiles show obvious differences. The tailings layer can be divided into two distinct zones based on the element content: the uppermost 47 cm and the part below 47 cm, which is consistent with the pH. In the uppermost 47 cm where pH is close to 12, the contents of elements such as As, Cd, Fe, Mg, Mn, Pb, S, Si and Zn were significantly lower than in the profile F. This indicates that these elements are extensively leached from this tailings zone. Below this layer where pH gradually decreases to between 7 and 8, most elements leached from the uppermost tailings were extensively accumulated. Furthermore, in the uppermost 47 cm, the contents of Ca, Cr, K and Ni were much higher than in the profile F, with Cr content consistently high throughout the tailings layer. Below 47 cm, the contents of Ca, Cr, K and Ni gradually decrease, to similar contents as in profile F in the deepest tailings. By looking at the element distribution in the two profiles (Fig. 4), it is clear that the contents of Ca, Cr, K and Ni in the ash are much higher than in the tailings. The enrichment of these elements in the tailings may thus originate from the ash layer above.

Table 2 Sequential extraction results of selected samples in F profile at Ryllshyttan impoundment at Garpenberg. Sample ID

Depth (cm)

Ca (%)

Mn (%)

F1

F-T-11 F-T-12

70e85 85e100

2.18 5.17

0.22 0.69

K (%) 0.49 0.15

F2

F-T-11 F-T-12

70e85 85e100

22.00 26.56

10.72 12.20

55.96 73.10

F3

F-T-11 F-T-12

70e85 85e100

7.39 8.41

7.09 6.13

F4

F-T-11 F-T-12

70e85 85e100

1.03 0.90

F5

F-T-11 F-T-12

70e85 85e100

F6

F-T-11 F-T-12

70e85 85e100

Mg (%)

Pb (%)

As (%)

Cd (%)

Cu (%)

4.28 2.88

2.16 2.29

11.20 16.82

0.52 0.82

0.09 0.087

2.22 2.77

3.92 7.59

1.70 2.45

85.42 89.11

5.67 3.91

20.28 26.22

8.42 19.95

0.83 1.44

27.54 39.68

1.68 0.76

13.84 10.66

3.12 3.11

6.73 5.58

6.68 11.86

8.81 7.82

0.30 0.18

4.82 4.87

10.53 9.66

0.86 0.51

0.21 0.19

0.38 0.18

0.23 0.23

0.51 0.31

1.26 1.03

29.43 28.31

65.10 58.02

8.01 9.97

29.55 26.54

38.10 29.61

1.85 2.15

2.36 2.85

14.17 7.92

7.67 8.10

2.08 1.48

83.28 80.09

29.94 20.61

24.85 20.09

39.34 38.63

29.03 20.50

50.86 58.28

65.56 56.81

79.40 77.94

15.16 7.98

87.28 86.11

0.97 0.63

0.95 0.83

0.34 0.22

0.82 0.94

46.91 45.02

1.13 0.85

4.58 3.24

0 0

Fe (%)

Zn (%)

S (%)

J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

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Table 3 Correlation coefficients of elements in the original tailings.

Ca Mn K Mg Pb As Cd Cu Fe Zn S

Ca

Mn

K

Mg

Pb

As

Cd

Cu

Fe

Zn

S

1 0.980 0.267 0.944 0.080 0.326 0.681 0.407 0.588 0.478 0.427

1 0.085 0.989 0.118 0.163 0.693 0.363 0.680 0.537 0.338

1 0.027 0.943 0.829 0.030 0.307 0.132 0.332 0.336

1 0.239 0.249 0.647 0.310 0.760 0.534 0.226

1 0.255 0.125 0.199 0.407 0.367 0.370

1 0.012 0.038 0.965 0.028 0.852

1 0.771 0.143 0.924 0.813

1 0.045 0.70 0.748

1 0.114 0.376

1 0.673

1

Note: Bold coefficients denote high correlation between elements.

The content of Cu in the uppermost 47 cm of tailings was similar as in profile F, but significantly accumulated in the deepest tailings in the B profile (Fig. 3). The accumulated Cu probably originates from the ash layer, since Cu content was high in the ash and had not leached significantly from the upper tailings. 4.2.3. Mineralogical composition of the uppermost tailings Microphotographs of a polished thin section of tailings samples from the middle section of the depletion zone in B profile (sample B-T-3 at 130e135 cm) are shown in Fig. 2. Quartz is accompanied by K, Na and Ca feldspars rimmed by blurry carbonates. Calcium, K, Mg and Mn silicates such as micas, chlorite, epidote, pyroxene and amphibole were not observed in this zone as in the unoxidized tailings. A few dolomite grains could be observed, but were not as common as in the unoxidized tailings. In respect of sulfides, no pyrrhotite, sphalerite or galena was observed. Only a few scattered FeO and pyrite grains were documented. 4.2.4. Sequential extraction results 4.2.4.1. Samples in the depletion zone. The concentration of each extracted fraction for As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, S and Zn in the selected samples from the B and F profiles is plotted and compared in Figs. 5 and 6. B-T-1, B-T-3 and B-T-5 are samples from the uppermost 47 cm depletion zone. The total extracted concentrations for As, Fe, Mg, Mn, Pb and S in this zone are significantly lower than in the unoxidized tailings, which is consistent with the total content decrease of these elements (Figs. 3 and 4). The total extracted concentrations for Cd, Cu and Zn were not significantly different from that in the unoxidized tailings, which is consistent with the difference in their total contents. Leaching of Cd, Cu and Zn in the depletion zone is not as significant as for As, Fe, Mg, Mn, Pb and S (Figs. 3 and 4). The total concentration of the six extracted fractions is significantly higher for Ca, Cr and Ni in this zone than in the unoxidized tailings, while this increase is not obvious for K. In the depletion zone, Ca is predominantly found in carbonates and in easily soluble and exchangeable phases (F2 and F1), which account for more than 67% and 15% of total Ca respectively. The concentrations of Cr and Ni were high in four and five of the six extracted fractions. Chromium and K dominate in the silicate fraction (F6), the concentration of which is higher than in the unoxidized tailings. Magnesium dominates in the carbonate phase (F2), which accounts for 67e79% of total Mg. The concentrations of Mn and Pb were low in all the extracted fractions. Compared with the unoxidized tailings, the concentrations of Ca and Mg were high in the carbonate phase (F2) and low in the silicate phase (F6). The concentrations of Fe and Mn were also low in the silicate phase (F6). The concentrations of K and Pb were low in the carbonate phase (F2) with a high concentration of K in the silicate phase (F6).

The concentrations of As, Cd, Cu, Fe, Pb, S and Zn were all low in the sulfide phase, which was almost depleted (F5). Secondary phases such as carbonates (F2) and hydroxides (F3) dominate as hosts for As, Cd and Cu. Some Ca and S were found in the easily soluble and exchangeable phase (F1), which indicates the formation of secondary gypsum. 4.2.4.2. Samples in the accumulation zone. B-T-8 and B-T-9 denote tailings in the deepest accumulation zone of the B profile. Calcium, Cr, K, Mg, Mn and Ni occur in similar forms as in the unoxidized tailings in profile F. The extraction concentrations of Ca, Cr, K, and Ni were much lower at this depth, similar to those in the unoxidized tailings, which suggests that the high concentrations extracted in the upper tailings probably originate from the fly ash addition. The total concentrations of the six extracted fractions for As, Cd, Cu, Fe, Mg, Mn, Pb, S and Zn are much higher in the accumulation zone than in the upper leaching zone and the unoxidized tailings, indicating an accumulation of these elements at this depth. The concentrations of As, Cd, Cu, Fe, Pb, S and Zn in sulfide fraction are high, indicating the precipitation of elements as sulfide minerals at this depth. Sulfate produced from the upper depletion zone is reduced to S2 and precipitated with metal irons as metal sulfides in a highly reducing environment. The Mg and Mn were high in the silicate phase (F6), indicating that Mg and Mn released from the upper zone precipitated as silicate minerals. 5. Discussion 5.1. Reactions in the depletion zone 5.1.1. Weathering of native silicates and carbonates at high pH Calcium, Fe, Mg and Mn carbonates and silicates, such as calcite, dolomite, micas, epidote, pyroxene or amphibole observed in the unoxidized tailings, were depleted in the uppermost 47 cm of the tailings of the B profile (Figs. 2, 5 and 6). Only resistant minerals such as quartz and feldspars remained, indicating a high weathering rate that was probably due to the high pH conditions created as a result of dissolution of the overlying ash layer, which commonly contains CaO and carbonates (Lahtinen et al., 2004). Due to the high concentration of carbonate ions, secondary carbonate was formed on the quartz and feldspar surface as blurry CaCO3. At  pH > 12, reactive hydrated hydroxyl ions (H3 O 2 or H2OOH ) might be present that strongly compete with anions such as O2. The reactivity of hydrated hydroxyl ion forces bond-breaking and dissolution reactions of many silicate minerals (Gates and Bouazza, 2010). The dissolution of native silicate minerals under extremely high pH conditions has been observed previously (Bayless and Schulz, 2003; Braney et al., 1993). For example, in northwestern Indiana, USA, native silicates underlying two slag disposals were

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Fig. 3. Comparison of contents of elements As, Cd, Cu, Fe, S and Zn in B and F profiles.

extensively weathered due to the hyper-alkaline drainage from the slag (Bayless and Schulz, 2003). Under room conditions, quartz becomes soluble at pH > 9.9 (Bayless and Schulz, 2003). The fluctuations in the contents of Ca, Fe, Mg, Mn and Si in the upper 47 cm of tailings are related to the dissolution of Ca, Fe, Mg and Mn

silicates and carbonates, consistent with a previous study (Heikkinen and Räisänen, 2008). Mineral precipitation and dissolution, as a result of hyper-alkaline fly ash drainage infiltrating into highly reactive tailings or native sediments, may have a significant effect on the water chemical composition.

J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

Fig. 4. Comparison of contents of elements Cr, Ni, Ca, K, Mg, Mn, Si and Pb in B and F profiles.

45

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Fig. 5. The concentration of each fraction for Ca, Cr, K, Mg, Mn, Ni and Pb Note: F1, F2, F3, F4, F5 and F6 denote the extracted phase in each step i.e. tailings existing as easily soluble and exchangeable forms, carbonate form, reducible form, organic and secondary sulfide, primary sulfide and residue respectively. B-T-1, B-T-3, B-T-5, B-T-8 and B-T-9 denote samples from the B profile at depths of 120e125 cm, 130e135 cm, 140e160 cm, 177e190 cm and 190e205 cm respectively. F-T-11 and F-T-12 denote samples from the F profile at depths of 107e122 cm and 122e137 cm respectively.

5.1.2. Weathering of sulfides at high pH Sulfides such as pyrrhotite, sphalerite, galena observed in the unoxidized tailings were not detected in the depletion zone (Fig. 2). Only scattered pyrite was observed in the depletion zone. Sequential extraction indicated that the concentrations of As, Cd, Cu, Fe, S and Zn were low in the sulfide fraction in the depletion zone (Fig. 6), and showed a decrease between 79 and 99.8% compared to those in the unoxidized tailings. The results indicate that weathering of sulfide minerals in depleted zone of the tailings has occurred over the last 10 years. The continuous chemical weathering observed in the uppermost 47 cm over the last 10 years has resulted in an average oxidation front movement of 4.7 cm per year. This is a rather high rate, compared to an earlier study of pyrrhotite-bearing tailings at Laver, Sweden with a similar sulfide content but lower carbonate content where the average rate was determined to be 2.8 cm per year (Alakangas et al., 2010). An average oxidation rate of 4.3 cm per year was, however, observed in newly deposited pyrite-rich (30%)

tailings (Alakangas et al., 2012). This indicates that, instead of preventing the weathering of sulfide minerals by adding an ash layer, the weathering rate increased due to the high pH condition (pH > 12) (Lahtinen et al., 2004). The high pH in the ash layer originates from the high CaO content. From a long-term perspective, when all the CaO in the ash layer is dissolved, the buffering will be dominated by carbonates, which can only keep the pH between 7 and 8 (Bone et al., 2003; Zhang et al., 2008). The weathering rate will thus decrease. An earlier study at the same site showed that the weathering rate was much lower in test areas with thinner fly ash (20 cm and 30 cm) (Lu et al., 2013). The results suggest that a thicker ash layer will increase the weathering and element release. For tailings with low sulfide and high carbonate content, application of a thick alkaline cover is unnecessary. 5.1.3. Addition of organic matter from the upper-lying ash The mobility of elements from the depletion zone may also be due to the presence of organic matter originating from the fly ash

J. Lu et al. / Journal of Environmental Management 139 (2014) 38e49

47

Fig. 6. The concentration of each fraction for As, Cd, Cu, Fe, S and Zn Note: F1, F2, F3, F4, F5 and F6 denote the extracted phase in each step i.e. tailings existing as easily soluble and exchangeable forms, carbonate form, reducible form, organic and secondary sulfide, primary sulfide and residue respectively. B-T-1, B-T-3, B-T-5, B-T-8 and B-T-9 denote samples from the B profile at depths of 120e125 cm, 130e135 cm, 140e160 cm, 177e190 cm and 190e205 cm respectively. F-T-11 and F-T-12 denote samples from the F profile at depths of 107e122 cm and 122e137 cm respectively.

and sludge layer. Loss on ignition (LOI) could be considered to reflect the organic matter content. The organic matter content (measured by LOI) in the ash and fiber sludge layer in the B profile is much higher than that in the tailings (Table 4). This LOI-measured content is higher in the tailings layer of the B profile than that in the tailings of the F profile (Table 4), which indicates the ash and fiber sludge cover had organic matter addition to the tailings. An extreme alkaline condition in the uppermost tailings (pH > 11) increased the solubility of added organic matter, which could mobilize metals in the tailings by forming organo-metallic complexes (Chirenje and Ma, 1999; Xiao et al., 1999). Elements such as Cu, Fe, Pb and Zn were leached from the depletion zone (Figs. 3 and 4). Organic matter has been observed to be less mobile when pH decreases from alkaline to near-neutral; soluble metaleorganic complexes may precipitate, reducing mobility (Chirenje and Ma, 1999; Xiao et al., 1999). The near-neutral pH in the deeper tailings facilitated precipitation of elements Cu, Fe, Pb, Zn leached from upper tailings (Figs. 3 and 4). This can be verified by the higher concentrations of Fe, Pb and Zn in the fourth extracted fraction (F4) from the deeper tailings (Figs. 5 and 6). This might be an additional

Table 4 LOI of B and F profile (%TS). B profile Sludge Ash Tailings

F profile 56.4e60.2 12e15.2 5.2e9.7

Subsoil

22.8e24.7

Tailings

4.4e7.2

explanation for the leaching of metals observed in the upper tailings. However, this has not been confirmed by analysis. An increase in dissolved organic matter could also increase the weathering of silicate minerals. The negative charge of dissolved organic matter increases as pH increases, which may allow for increased complexing capacity and silicate dissolution (Williams and Walter, 2004). Therefore, in locations with abundant dissolved organic matter and high pH, silicate weathering may be enhanced (Williams and Walter, 2004). Studies of deep methanogenic sediment showed increased dissolution of silicates due to high concentrations of dissolved organic carbon (Wallmann et al., 2008). Therefore, it might be likely that addition of dissolved organic matter from the fly ash layer is another reason for enhanced weathering of native Ca, Fe, Mg and Mn silicates in the tailings. 5.1.4. Formation of secondary minerals XRD analysis of fly ash from two paper mills in Sweden identified lime, calcite and quartz as the dominant minerals (Greger et al., 2006, 2009). The dissolution of fly ash will release carbonate ions to the underlying tailings, and the release of Ca and Mg from silicate weathering might result in the formation of secondary carbonates (Sherlock et al., 1995). This is in agreement with the observation of blurry and irregular CaCO3 on the surface of quartz and feldspar. The concentrations of Cd, Cu and Zn were rather unchanged in the uppermost depletion zone, indicated by the chemical composition and sequential extraction of the solid tailings (Figs. 3e6). Sequential extraction showed that Cd was dominant in the carbonate phase, and Cu in carbonate and oxide phases, indicating

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formation of secondary carbonate and oxides retaining these metals in this zone. A few scattered FeO and pyrite grains were observed in the mineralogical analysis (Fig. 2), indicating chemical weathering of pyrite and formation of secondary FeO. However, the concentration of Fe in the oxide phase in this zone is not high in the sequential extraction (Fig. 6), which implies that Fe oxide could not be retained in this zone at such high pH and is transported to the deeper accumulation zone. Sequential extraction showed that Ca and S existed in the easily soluble and exchangeable phase in the depletion zone, which indicates the formation of secondary sulfate mineral such as gypsum. Due to the high concentrations of Ca2þ and SO2 4 ions produced during the neutralization and sulfide oxidation process, gypsum (CaSO4.2H2O) is often present as a secondary mineral (Othmani et al., 2013), which can be transformed to jarosite hardpan later, after depletion of carbonates (Sracek et al., 2010). 5.1.5. Addition from the upper-lying ash layer The concentrations of Ca, Cr and Ni were much higher in the depletion zone than in the unoxidized tailings (Fig. 4), which suggests that the source of these elements was the ash layer. Sequential extraction of the upper-tailings showed that Ca and Cr were dominant in carbonate (F2) and silicate phase (F6) respectively, and Ni were associated to all phases (Fig. 5). The concentration of K was high in the silicate phase in the upper-tailings. In wood ash, alkaline and alkaline earth elements are primarily present as oxides, hydroxides or carbonates, but also as chlorides (Nurmesniemi et al., 2008). The application of large amount of fly ash will add these elements to the tailings. 5.2. Accumulation zone Elements released from the upper depletion zone and the overlying layers were accumulated in the deep tailings (Figs. 3 and 4). The sequential extraction showed that the concentrations of As, Cd, Cu, Fe, Pb, S and Zn were high in the sulfide fraction in the accumulation zone and even higher than in the unoxidized tailings (Figs. 5 and 6). This suggests that primary sulfides are relatively unoxidized and secondary sulfides might have precipitated. Copper and Pb were also found in the silicate (F6) and oxide fraction (F3), probably due to co-precipitation, adsorption or exchange processes. The total concentrations of sequential extracted Cu and the content of Cu were similar in the depletion zone as in the unoxidized tailings (Figs. 3 and 6). Thus, the high Cu content in the accumulation zone indicates that there is another source than the tailings. The source is assumed to be the overlying fly ash layer since the fresh fly ash had a rather high Cu content (Lu et al., 2013). Sequential extraction results showed that the concentrations of Ca, Cr, K, Mg and Ni were similar in the accumulation zone as in the unoxidized tailings (Fig. 5). This suggests that these elements were associated with primary minerals, with no additions from the upper-tailings. 6. Conclusions Extensive weathering of sulfides and gangue minerals was observed in the uppermost 47 cm of tailings, after 10 years of fly ash cover application. The high weathering rate was due to the high pH and presence of organic matter from the dissolution of the overlying fly ash ad fiber sludge. Most of the primary minerals were depleted in the upper tailings, except for quartz and feldspar which were covered by secondary carbonates. Secondary minerals such as gypsum and iron oxide were inferred by sequential extraction, but not identified in mineralogical studies. The weathering of Fe, Mg and Mn silicates and carbonates and As, Cd, Cu, Fe and Zn sulfides

led to extensive leaching of these elements from the depletion zone, where the pH was close to 12, and retention deeper down in the tailings, where the pH decreased to around 7e8. Dissolution of the fly ash layer released Ca, Cr, K and Ni into the tailings where they were accumulated in the uppermost depletion zone. Secondarily retained elements may be mobilized if the chemical conditions change due to prevailing oxidation and dissolution processes, resulting in downward movement to the groundwater. The results indicate that application of highly alkaline material, such as ash, on tailings should be carefully designed, since the dissolution of lime might increase the pH and thus the weathering. The landfill of highly alkaline waste should also be carefully undertaken to reduce weathering of underlying materials. Acknowledgment This research was supported by Boliden Mineral AB, Sweden. The authors are grateful to the staff at Boliden Mineral AB for their help in providing background information and assistance during sampling. Appendix A. Supplementary material Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2013.12.036. References Alakangas, L., Lundberg, A., Nason, P., 2012. Simulation of pyrite oxidation in fresh mine tailings under near-neutral conditions. J. Environ. Monitor. 14, 2245e 2253. Alakangas, L., Öhlander, B., Lundberg, A., 2010. Estimation of temporal changes in oxidation rates of sulphides in copper mine tailings at Laver, Northern Sweden. Sci. Total. Environ. 408, 1386e1392. Balistrieri, L.S., Box, S.E., Bookstrom, A.A., Ikramuddin, M., 1999. Assessing the influence of reacting pyrite and carbonate minerals on the geochemistry of drainage in the Coeur d’Alene mining district. Environ. Sci. Technol. 33, 3347e3353. Bayless, E.R., Schulz, M.S., 2003. Mineral precipitation and dissolution at two slagdisposal sites in northwestern Indiana, USA. Environ. Geol. 45, 252e261. Bellaloui, A., Chtaini, A., Ballivy, G., Narasiah, S., 1999. Laboratory investigation of the control of acid mine drainage using alkaline paper mill waste. Water Air Soil Pollut. 111, 57e73. Bone, B.D., Knox, K., Picken, A., Robinson, H.D., 2003. The effect of carbonation on leachate quality from landfilled municipal solid waste (MSW) incinerator residues. In: Proc. Sardinia 2003, 9th Int. Waste Management and Landfill Symp., 6e10 October 2003, S. Margherita di Pula, Cagliari, Italy. Braney, M.C., Haworth, A., Jefferies, N.L., Smith, A.C., 1993. A study of the effects of an alkaline plume from a cementitious repository on geological materials. J. Contam. Hydrol. 13, 379e402. Cabral, A., Racine, I., Burnotte, F., Lefebvre, G., 2000. Diffusion of oxygen through a pulp and paper residue barrier. Can. Geotech. J. 37, 201e217. Catalan, L.J.J., Kumari, A., 2005. Efficacy of lime mud residues from kraft mills to amend oxidized mine tailings before permanent flooding. J. Environ. Eng. Sci. 4, 241e256. Chirenje, T., Ma, L.Q., 1999. Effects of acidification on metal mobility in a papermillash amended soil. J. Environ. Qual. 28, 760e766. Chtaini, A., Bellaloui, A., Ballivy, G., Narasiah, S., 2001. Field investigation of controlling acid mine drainage using alkaline paper mill waste. Water Air Soil Pollut. 125, 357e374. Deissmann, G., Kistinger, S., Kirkaldy, J.L., Pettit, C.M., 2000. Predictive geochemical modeling of long-term environmental impacts from waste rocks. In: Proceedings of the 5th International Conference on Acid Rock Drainage (ICARD). 1, Chapter 6, Risk Assessment and Associated Tools, pp. 743e750. Dold, B., 2003. Speciation of the most soluble phases in a sequential extraction procedure adapted for geochemical studies of copper sulfide mine waste. J. Geochem. Explor. 80, 55e68. European Commission, July 2004. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities (BAT). European IPCC Bureau. www.jrc.es/pub/english.cgi/0/733169. Gates, W.P., Bouazza, A., 2010. Bentonite transformations in strongly alkaline solutions. Geotext. Geomembr. 28, 219e225. Gotthardsson, J., Sundberg, Å., 2011. Mine site reclamation using products from the pulp industry. Personal communication. Greger, M., Neuschütz, C., Isaksson, K.E., 2009. Influence of Vegetation and Sewage Sludge on Sealing Layer of Fly Ashes in Post-treatment of Mine Tailings Impoundments (Värmeforsk Report 1098 Q6-627) (in Swedish).

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