Applied Geochemistry 24 (2009) 475–482
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Applied Geochemistry
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Effects of sewage sludge on solution chemistry and plant uptake of Cu in sulphide mine tailings at different weathering stages
Lovisa Stjernman Forsberg a,*, Dan Berggren Kleja a, Maria Greger b, Stig Ledin a a Department of Soil and Environment, Swedish University of Agricultural Sciences, P.O. Box 7014, SE-750 07 Uppsala, Sweden b Department of Botany, Stockholm University, SE-106 91 Stockholm, Sweden article info abstract
Article history: This climate chamber experiment examines the effects of sewage sludge (SS) on sulphide mine tailings Received 4 June 2008 from the Aitik Cu mine in northern Sweden. The effects of SS were determined from Cu in solution Accepted 10 December 2008 and Cu uptake and growth of plants on tailings showing 3 different degrees of weathering. Possible rela- Available online 27 December 2008 tionships between Cu content in plants and Cu in solution measured in tailings (total dissolved Cu and free Cu) were also evaluated. Red fescue (Festuca rubra) was grown for 6 weeks in pots of the different Editorial handling by X.D. Li tailings treated with SS or NPK fertiliser. Soil solution was sampled with Rhizon tension lysimeters 2À and analysed for pH, dissolved organic C (DOC), free Cu, total dissolved Cu and SO4 . The effects of SS on Cu in solution and plants depended on the degree of weathering. In tailings with a low degree of sul- phide oxidation, SS application resulted in increased solubility and shoot accumulation of Cu compared with NPK-treated tailings, probably due to DOC forming soluble complexes with Cu. Sewage sludge also seemed to promote translocation of Cu to shoots in those tailings. In highly weathered tailings, lower contents of total dissolved Cu and free Cu in solution and lower Cu levels in shoots were found in SS-trea- ted samples than in NPK-treated. In the moderately weathered tailings, Cu concentrations in solutions were generally similar between treatments, but lower contents of Cu were found in shoots and roots of the fescue grown in the SS-treatment. Irrespective of degree of weathering and treatment, both free Cu and total dissolved Cu concentration in tailings correlated strongly with Cu levels found in fescue shoots. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction etation on the tailings deposit. Addition of sewage sludge (SS) has been shown to increase the survival and biomass production When metals are extracted from sulphide ores, tailings with of plants due to the contribution of nutrients and organic matter varying contents of sulphides are generated. Weathering sulphide (Pietz et al., 1989; Pichtel et al., 1994; Stoltz and Greger, 2002; tailings have a stronger tendency to leach metals and acid material Forsberg and Ledin, 2006). The original material for SS is the solid than other weathering tailings, as H2SO4 and metal ions are gener- waste that is separated through sedimentation from municipal ated during the oxidation of the metal sulphides (Singer and sewage water and the organic matter content usually ranges from Stumm, 1970; Kleinmann et al., 1981). Pyrite is the dominant me- 50% to 60% (Theodoratos et al., 2000). The interest in using SS on tal sulphide mineral in many sulphide deposits, but the tailings of- mine tailings is increasing worldwide, as sewage sludge consti- ten contain variable amounts of other metal sulphides, e.g. tutes a huge disposal problem, while the area of abandoned mine chalcopyrite, CuFeS2 (Ferguson and Erickson, 1988). tailings deposits is increasing. However, the effects of SS on mobil- Establishment of vegetation on a tailings deposit may decrease ity and plant availability of metals in the tailings are of great envi- water leaching, improve the quality of polluted soil solution, re- ronmental concern. Although solid phase organic matter is known duce wind and water erosion, increase stability of the land surface to immobilise metals (Bahaminyakamwe et al., 2006) several stud- and improve the aesthetic appearance of the mine area (Shetron ies have shown that there is a risk of increased solubility and plant and Duffek, 1970; Bradshaw and Chadwick, 1980; Clemensson-Lin- uptake of metals due to the formation of soluble complexes be- dell et al., 1992). However, mine tailings are sterile in terms of the tween metals and the dissolved organic C (DOC) in the SS (Minnich physical and chemical characteristics necessary for plant growth et al., 1987; McBride, 1989; Ahumada et al., 2001; Parker et al., and these conditions must be overcome in order to establish veg- 2001; Inaba and Takenaka, 2005; Schwab et al., 2006; Ashworth and Alloway, 2007). * Corresponding author. Tel.: +46 18 671264; fax: +46 18 672795. The affinity for metals of both the solid and soluble phase of SS E-mail address: [email protected] (L.S. Forsberg). is known to depend on pH and the presence of other ions, which
0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.12.030 476 L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482 may change the competition for reactive sites on the organic mol- posit, about one year after being pumped out onto the tailings ecules and/or adjust their effective charge (Temminghoff et al., deposit. Type A and Type B (later divided into B1 and B2) were 1994; Jordan et al., 1997; Impelliterri et al., 2002; Ashworth and homogenised separately and analysed with respect to total con- Alloway, 2007). This is of particular importance when using sew- tents of Cu, Fe and S to determine any differences in contents of age sludge on deposits with sulphide mine tailings, as great differ- metal sulphides between those two types. The analyses showed ences in pH and sulphide oxidation exist between different sites on that the A-tailings contained lower levels of those elements, which the same deposit due to variations in O2 and water conditions and suggested lower total sulphide content (Table 1). in sulphide content of the tailings (Peterson and Nielson, 1978; The SS used was collected at the Henriksdal treatment plant in
Swedish Environmental Protection Agency, 1998). However, there Stockholm, where it had been treated with FeSO4 (to precipitate P), is a lack of information concerning the influence of such variations dewatered to a water content of around 70% and digested at a tem- on the effects of SS on metals in plants and tailings. perature of 33–37 °C (Stockholm Vatten, 2002). The sludge con- The present paper deals with the effects of SS on Cu in tailings tained about 25% C, 4% P and 4% N and the content of metals did with different degrees of sulphide weathering. The tailings derived not exceed the permissible limits on total metal content in SS for from the Aitik Cu mine in the north of Sweden. Establishment of use on agricultural land (Swedish Environmental Protection vegetation on the Aitik mine tailings deposit is planned at closure Agency, 1996; Stockholm Vatten, 2002) (data not shown). The total of the mine, using sewage sludge as fertiliser. Red fescue (Festuca levels of Cu and S in the SS were similar to those found in the B- rubra) has been shown to adapt well to the cold climate in the area tailings, but Fe levels were higher in the SS than in both types of and is planned to be used in the reclamation of the tailings. Copper tailings, probably due to the FeSO4 used in the treatment process occurs in high amounts in both Cu mine tailings and sewage (Table 1). The SS was stored at 2 °C for 4 months before use. sludge, is easily released when the sulphides oxidise and is consid- ered toxic to plants and animals at high concentrations (Bowen, 2.2. Preparation and properties of A, B1 and B2 tailings 1979; Sohlenius and Öborn, 2004; Forsberg and Ledin, 2006). Many studies emphasise the importance of analysing the soil solution for For 3 months before the climate chamber experiment, the A- evaluation of plant uptake of metals from contaminated soils (Min- tailings and part of the B-tailings (B1) were kept frozen (À18 °C) nich et al., 1987; Sauvé et al., 1996; Göttlein et al., 1996; Datta and to avoid further oxidation. However, the remaining part of the B- Young, 2005). Rhizon lysimeters have been shown to be effective in tailings (B2) was kept unfrozen. One half of B2 was then mixed sampling soil pore solution for metal speciation and dissolved or- with SS (20% SS by volume) while the other half remained un- ganic C analysis, due to their simplicity and in situ extraction with treated. Both untreated and SS-treated B2-tailings were put into minimal disturbance (Datta and Young, 2005; Rais et al., 2006). pots (1 kg of substrate/pot, four replicates) and exposed to 12 Accordingly, the specific objectives of the present study were for- leaching and drying cycles in a greenhouse (25 °C) for 3 months mulated as follows: (1) determine the effects of SS on DOC, free to promote sulphide oxidation in the tailings. The leaching was Cu and total dissolved Cu in tailings solutions sampled by Rhizon performed by adding 0.25 L deionized water to each pot every lysimeters; (2) determine the effects of SS on growth and Cu con- week. Drainage water (about 0.05 L per pot) was not recycled back tent of roots and shoots of red fescue; and (3) explore possible rela- into the pots. tionships between Cu concentrations in soil solutions and plant In June 2006, A- and B1-tailings were thawed and moistened to uptake of Cu. a water content of 30% (v/v). The two types of tailings and the SS were homogenised separately. The SS was then mixed into sepa- 2. Material and methods rate samples of A- and B1-tailings (20% by volume), and the remaining part of each tailings was left untreated with SS. In the 2.1. Properties of tailings and sewage sludge same way as with B2, pots were filled with each of the two tailings without SS (A and B1) and with each of the two mixtures (A + SS, The Aitik Cu mine, located 20 km east of the town Gällivare, B1 + SS). 2À Norrbotten county, northern Sweden, is one of the largest Cu mines Before the climate chamber experiment, SO4 levels and pH in in Europe and the tailings deposit covers an area of more than solutions from the different tailings types were measured to check 2 2À 10 km . The tailings have the texture of fine sand and are pumped the degree of sulphide oxidation. Low pH and high SO4 levels were out as a sandy slurry onto the deposit. The sulphides in the tailings assumed to represent a high degree of sulphide oxidation. Accord- mainly consist of 95% pyrite and 5% chalcopyrite. The mine, pro- ing to these measurements (Table 2), no significant sulphide oxida- duction of the tailings and the deposit are described in detail in tion was taking place in the fresh A-tailings, as pH was high in both 2À Forsberg and Ledin (2003, 2006). NPK-treated and SS-treated samples, and SO4 levels were low in 2À In this study, different types of tailings from the Aitik Cu mine the non-treated tailings. High SO4 levels found in SS-treated A- were used (Table 1). Type A was sampled immediately after the flo- tailings were assumed to be due to the content of FeSO4 and S-con- tation process. Type B was collected at the eastern part of the de-
Table 2 Sulphate (mg LÀ1) and pH in solutions from untreated A-tailings, B1-tailings and B2- Table 1 tailings alone and in mixtures with sewage sludge (SS). Contents of Cu, Fe and S in A-tailings and B-tailings (later divided into B1 and B2) 2 from the Aitik copper mine, and sewage sludge from the Henriksdal treatment plant SO4 pH (mean values ± standard deviation, n = 4) at sampling. A 53.7 ± 17.0b,c,d 7.2 ± 0.1b,c B1 506.6 ± 139.6a,c 6.0 ± 0.3a,c Tot-Cu (mg kgÀ1) Tot-Fe (mg kgÀ1) Tot-S (%) B2 258.6 ± 96.0a,b 4.4 ± 0.2a,b Type A 180 ± 74 24,500 ± 577 0.11 ± 0.01 A + SS 317.6 ± 77.3 7.0 ± 0.2 TypeB 398 ± 29** 37,400 ± 1500*** 1.04 ± 0.08*** B1 + SS 404.5 ± 65.5 6.0 ± 0.5 Sewage sludge 398 ± 19 96,000 ± 6000 1.11 ± 0.08 B2 + SS 176.4 ± 52.0 4.3 ± 0.1
The metals were extracted by 7 M HNO3. a Significantly (p < 0.05) different from A-tailings in the same treatment. *p < 0.05, indicate significant differences between tailings types. b Significantly (p < 0.05) different from B1-tailings in the same treatment. ** p < 0.01, indicate significant differences between tailings types. c Significantly (p < 0.05) different from B2-tailings in the same treatment. *** p < 0.001 indicate significant differences between tailings types. d Different from the SS-treatment in the same tailings. L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482 477
Table 3 2.5. Statistical analysis Climatic data for the climate chamber. Night values in brackets. One period = 14 days. Ò Period Night length Light intensity Humidity Temperature Minitab statistical software version 14 was used for statistical (lM) (%) (°C) analyses. p-values were obtained from one-way analysis (ANOVA) 1 9.30 pm–7.00 am 500 (90) 50 (90) 13 (8) in order to distinguish different treatment effects and differences 2 9.30 pm–7.00 am 500 (90) 50 (90) 17 (13) between tailings types. For correlation analysis of the Cu uptake 3 9.30 pm–7.00 am 500 (90) 50 (90) 17 (11) by plants and Cu measured in solutions, the respective means of 4 7.00 pm–7.00 am 500 (90) 50 (90) 16 (10) all values of total dissolved Cu and free Cu measured in the four 5 7.00 pm–7.00 am 500 (90) 50 (90) 18 (13) 6 7.00 pm–7.00 am 500 (90) 50 (90) 18 (13) solution samples taken from the pots during the experiment were 7 7.00 pm–7.00 am 500 (90) 50 (90) 18 (13) used. Pearson correlation, which measures the extent to which two variables are linearly related, was applied to these data. taining organic matter in the sewage sludge. The B1-tailings had a 3. Results 2À pH around 6 and high SO4 levels in both untreated and SS-treated samples, suggesting that sulphide oxidation had started in those 3.1. Solution chemistry tailings, but was still not causing any acidity. The B2-tailings had 2À the lowest pH (around 4.5), which suggested that the sulphide oxi- Levels of pH, SO4 and DOC in solutions are shown in Fig. 1. 2À dation had proceeded longest in those tailings, although the SO4 Solution pH remained different among the tailings, with the levels were lower than in B1. Sulphate may have been washed unweathered tailings (A) displaying the highest pH (Fig. 1a) and out during leaching as the tailings were being oxidised. the most weathered tailings (B2) lowest pH (Fig. 1c). The pH in each tailings type did not decrease during the experiment and it 2.3. Layout of climate chamber experiment did not differ between SS and NPK treatments. 2À In NPK-treated tailings, SO4 remained lower in solutions from The pots with each of the three tailings (A, B1 and B2) and with unweathered tailings (A-tailings, Fig. 1d) than in those from each of the three mixtures (A + SS, B1 + SS and B2 + SS) were placed weathered tailings (B1 and B2, Fig. 1e and f, respectively) through- in a climate chamber for 6 weeks. The experiment had a random- out the experiment. In the unweathered tailings, significantly high- 2À ised complete block design with four replicates for each treatment. er levels of SO4 were found in solutions from SS-treated pots than The tailings without SS were mixed with a nutrient solution from NPK-treated pots on all sampling occasions. No such differ- (mg kgÀ1: 600 N, 250 P, 300 K). Red fescue (F. rubra, cv. Lovisa) ences were found for weathered tailings (B1 or B2). was sown densely (about 200 seeds per pot). The experiment also Dissolved organic C (DOC) levels were commonly (although not included pots with B1-tailings but without plants (NPK-treated always significantly) higher in solution from SS-treated tailings and SS-treated, respectively). than in solution from NPK-treated tailings (Fig. 1). Higher DOC lev- The pots were irrigated with deionized water twice a week, in els were found on the first sampling occasion in the fresh mixed amounts corresponding to water consumption, which was deter- tailings (A–SS and B1–SS) (Fig. 1g and h) than in the weathered mined by weighing the pots. The climate in the chamber was ad- B2-tailings (Fig. 1i). justed every second week according to the mean values of data The effect of SS on total dissolved Cu differed between tailings recorded in Gällivare during the corresponding 14 days in summer types (Fig. 2). Compared with NPK, SS increased the levels of total 2002 (SMHI, 2002, unpublished data; Table 3). dissolved Cu in unweathered tailings (A-tailings), whereas no ef- fect of sludge was observed in moderately weathered tailings 2.4. Sampling and analyses (B1) (Fig. 2a and b). In highly weathered tailings (B2), the applica- tion of sludge decreased Cu solubility (Fig. 2c). Soil solution was sampled with Rhizon tension lysimeters (Rhi- In contrast to total dissolved Cu, free Cu concentration was not zon SMS, Rhizosphere Research Products, Wageningen, the Nether- significantly affected by the addition of SS to unweathered tailings lands) at the beginning of the experiment and then every second (A) (Fig. 2d). In the weathered tailings (B1 and B2), however, the week, in total four times. The lysimeters (2.5 mm diameter, effect of SS on free ions was similar to its effect on total dissolved 100 mm long) are made of a porous polymer with a mean pore size Cu, although somewhat less significant (Fig. 2e and f). of 0.1 lm. The lysimeters were installed diagonally in the pots and As indicated in Fig. 2, in both treatments the fraction of free Cu attached to a 10 mL pre-evacuated sterile sampling tube upon found in solution increased with weathering and constituted about sampling. 0%, 10% and 20% in A, B1 and B2, respectively. Thermodynamic All solutions were analysed to determine their pH, free Cu activ- equilibrium calculations using Visual MINTEQ (Gustafsson, 2006) 2À ity, total-Cu, total-Al, total-SO4 and dissolved organic C (DOC). showed that inorganic complexes made up a minor fraction of Cations and anions were analysed by an Elan 6100 ICP-MS (Induc- the total amount of inorganic Cu (data not shown). The major inor- tion Coupled Plasma Mass Spectrometry) instrument equipped ganic complex was CuSO4, which contributed an increasing pro- with a DRC (Dynamic Reaction Cell) and an ion chromatograph portion with increasing degree of weathering (<1% in A, about 4% (Dionex, MODULE/SP). Levels of DOC were determined using a Shi- in B1 and 8% in B2) in both NPK and SS treatments. Furthermore, matzu 5000A TOC-analyser. The free Cu ion concentration (Cu) was solutions were all strongly undersaturated with respect to mineral measured potentiometrically using a Cu ion selective electrode phases such as CuO, Cu(OH)2 and CuCO3 (data not shown). (Cu-ISE Orion 96–29, Thermo Electron Corporation, Beverly, MA). The shoots of the fescue were harvested after 6 weeks by cut- 3.2. Growth and Cu content of plants ting the plants at the soil surface. The roots were then removed from the pots and rinsed. Both shoots and roots were dried at Biomass of shoots and roots and shoot:root biomass ratios are 50 °C, weighed and milled. Two grams of each shoot sample and shown in Table 4. There was a tendency for root biomass to de-
1 g of each root sample were boiled in 10 mL 7 M HNO3. After dilu- crease with increased degree of weathering of tailings, as shown tion to 2 M HNO3 the Cu levels in the extracts were determined by by a significant correlation between pH and root biomass ICP-MS with DRC. (r = 0.97, data not shown). However, shoot biomass did not differ 478 L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482
A-tailings B1-tailings B2-tailings 9.0 9.0 9.0 8.0 a 8.0 bc8.0 7.0 7.0 7.0
pH 6.0 6.0 6.0 5.0 5.0 5.0 4.0 4.0 4.0 3.0 3.0 3.0 0246 0246 0246 800 800 800 def 600 600 600 400 ] (mg/L) 400 400 2- 4 200 200 200 [SO 0 0 0 0246 0246 0246 250 250 250 200 g 200 h 200 i 150 150 150 100 100 100 50 50 50 [DOC] (mg/L) 0 0 0 0246 0246 0246 Weeks Weeks Weeks
2À Fig. 1. pH and concentrations of SO4 and dissolved organic carbon (DOC) in solutions from tailings treated with NPK fertiliser (black symbols) and sewage sludge (white symbols) during 6 weeks. Tailings were unweathered (A), moderately weathered (B1) or highly weathered (B2) and vegetated with red fescue. Mean ± SD (n = 4).
A-tailings B1-tailings B2-tailings 3.0 15.0 100 2.5 12.5 75 2.0 10.0 1.5 7.5 50 1.0 5.0 25 2.5
[Tot Cu](µM) [Tot 0.5 0.0 0.0 0 0 642 062064 42 3.0 1500 10000 2.5 7500 2.0 1000 1.5 5000 1.0 500 2500 0.5 [Free Cu](nM) 0.0 0 0 0642 062064 42 Weeks Weeks Weeks
Fig. 2. Concentrations of total dissolved Cu and free Cu in solutions from tailings treated with NPK fertiliser (black symbols) and sewage sludge (white symbols) during 6 weeks. Tailings were unweathered (A), moderately weathered (B1) or highly weathered (B2) and vegetated with red fescue. Mean ± SD (n = 4). Note the different scales. significantly between unweathered tailings (A) and moderately treated with SS. The shoot:root ratio of biomass increased with de- weathered tailings (B1). Compared with NPK, SS increased the root gree of weathering of the tailings and was generally higher in the biomass in all tailings types studied, but only increased the shoot NPK treatment than the SS treatment. biomass in plants grown on highly weathered tailings (B2). Com- Fescue grown in SS-treated unweathered tailings (A) contained paring treatments of the B2-tailings, roots were only about 2 mm slightly higher Cu levels in shoots than fescue grown in the corre- long in the NPK-treated pots, but reached at least 20 mm in pots sponding NPK-treated tailings (Fig. 3a). In the moderately and L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482 479
Table 4 Shoot and root biomass (g dry weight) of red fescue grown in unweathered (A), moderately weathered (B1) and highly weathered (B2) tailings treated with NPK or sewage sludge (SS). Shoot:root biomass ratio, biomass of shoots in relation to biomass of roots, (g DW shoot potÀ1/g DW root potÀ1), n = 4, ±SD.
Tailings Treatment Shoot biomass gDW potÀ1 Root biomass gDW potÀ1 Shoot:root biomass ratio A NPK 8.4 ± 1.1c 3.4 ± 0.5b,c 2.5 ± 0.6c,d A SS 9.0 ± 1.2c 4.7 ± 0.63b,c,d 2.0 ± 0.5 B1 NPK 7.9 ± 1.1c 1.5 ± 0.31a,c 5.2 ± 1.4a,c B1 SS 8.8 ± 0.8c 3.1 ± 0.62a,c,d 2.9 ± 0.5c,d B2 NPK 2.2 ± 0.6a,b 0.0 ± 0.0a,b >1000a,b B2 SS 6.2 ± 0.3a,b,d 0.4 ± 0.3a,b,d 24.1 ± 14.1a,b,d
a Significantly (p < 0.05) different from A-tailings in the same treatment. b Significantly (p < 0.05) different from B1-tailings in the same treatment. c Significantly (p < 0.05) different from B2-tailings in the same treatment. d Different from the NPK-treatment in the same tailings.
A-tailings B1-tailings B2-tailings 120 a. Cu in shoots 120 b. Cu in shoots 600 c. Cu in shoots 100 100 500 80 80 400 60 ** 60 300 40 40 200 ** 20 ** 20 100 ** 0 0 0 Conc. Amount Conc. Amount Conc. Amount k(mg/kg) (µg) k(mg/kg) (µg) (mg/kg) (µg) 2500 2500 2000 d. Cu in roots 2000 e. Cu in roots 1500 1500 * 1000 ** 1000 500 ** 500 * 0 0 Conc. Amount Conc. Amount (mg/kg) (µg) (mg/kg) (µg) 0.20 0.20 f. S/R-ratio g. S/R-ratio 0.15 0.15 0.10 ** 0.10 0.05 ** 0.05 ** 0.00 0.00 Conc. Amount Conc. Amount
Fig. 3. Concentrations (mg/kg DW) and content (lg per pot) of Cu (means ± standard deviations, n = 4) in shoots and roots of red fescue grown in various tailings treated with
NPK fertiliser (black bars) and sewage sludge (white bars) during 6 weeks. S:R-ratio, Cu accumulated in shoots in relation to Cu accumulated in roots, (Conc.: [Cu]shoot/[Cu]root; * ** *** Amount: lgCushoot/lgCuroot). Tailings were unweathered (A), moderately weathered (B1) or highly weathered (B2). (p < 0.05), (p < 0.01), (p < 0.001) indicate significant differences between treatments. highly weathered tailings (B1 and B2, respectively), however, low- the correlation coefficients. The log scale plots follow quadratic er Cu concentrations were found in SS-treated plants than in NPK- relationships and indicate that the data followed the same curved treated (although differences were not significant for the B1-tail- line irrespective of treatment. Corresponding regression analyses ings, Fig. 3b and c). The highest Cu levels in shoots (between 133 were not carried out for the root data, as the number of root sam- and 203 mg kgÀ1) were found in plants grown in NPK-treated ples was too low. highly weathered tailings (B2) (Fig. 3c). Roots grown in those tail- ings were not analysed because of the low biomass. In all cases, higher Cu content and concentration were found in roots than in 4. Discussion shoots (Fig. 3a–e). The shoot to root ratio (S/R-ratio) for Cu concen- trations accumulated in the plant was generally higher in SS-trea- This work shows that sewage sludge addition influenced the ted plants than in NPK-treated, although the total amount of Cu in solution chemistry of Cu in the tailings, and that the effects were plants, including both roots and shoots, was always higher in the different among tailings with different degrees of weathering. NPK-treated plants (Fig. 3f and 3g). Since the mine tailings solutions were all strongly undersaturated
As shown in Fig. 4, there was generally a strong correlation be- with respect to mineral phases such as CuO, Cu(OH)2 and CuCO3, tween Cu concentration in shoots and soil solution concentration the Cu in solution was probably controlled by surface complexa- of total Cu (r2 = 0.95) or of free Cu (r2 = 0.88). Log transformation tion mechanisms. Organic compounds originating from the sludge improved the data distribution (approximately normal distribu- or the plants were the major ligands present. In such a system the tion) for shoots versus total Cu in solution and free Cu in solution, Cu2+ concentration in solution is likely to be controlled by the and therefore log–log relationships were used when calculating following factors: (i) solution pH, (ii) the amount of ‘geochemically 480 L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482
ab1.0 1.0
0.5 0.5 y = 0.45x2 + 0.11x - 1.08 y = 0.12x2 + 0.64x - 0.37 0.0 R2 = 0.95 0.0 R2 = 0.88
-0.5 -0.5
-1.0 -1.0 Log Cu shoot in DM) (mmol/kg Log Cu in shoot DM) (mmol/kg -1.5 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 Log tot-Cu in solution (µM) Log free Cu in solution (µM) Fig. 4. The dependence of log concentration Cu in shoot tissues on: (a) log total Cu in solution and (b) log free Cu in solution. Equations and correlation coefficients of quadratic regressions for the whole set of data are shown. Black symbols represent NPK treatment, white symbols represent sewage sludge treatment. Squares = unweathered tailings (A), circles = moderately weathered tailings (B1) and triangles = highly weathered tailings (B2). active’ Cu, and (iii) the number of sites available on the solid mate- Growth and Cu content of plants differed between tailings types rial for complex formation. The term ‘geochemically active’ can be and treatments (Table 4, Fig. 3). It is likely that the sulphide weath- defined as the amount of metal in the system that can equilibrate ering decreased plant growth due to its effect on pH and/or metal with the soil solution within the time scale of the experiment concentrations (Parker et al., 2001). Higher root biomass in SS- (Lofts et al., 2004). Since pH did not differ between SS and NPK treated tailings was probably caused by improved nutrient status treatments (Fig. 1), it is likely that the effect of SS on Cu in these and physical conditions (Epstein et al., 1976; Forsberg and Ledin, systems was related to changes in the amount of ‘geochemically 2003, 2006), and/or on lower metal levels in solution for the most active’ Cu and to the addition of organic matter (giving more sites weathered tailings (B2). The very low root biomass found in the for Cu binding). The former originated both from the weathering B2-tailings (Table 4) suggests that the Cu levels had reached toxic sulphides and from the sludge itself. values for red fescue grown in both treatments (170 mg kgÀ1 and In the unweathered tailings (A-tailings) the low concentration 30 mg kgÀ1 in plants grown in NPK-treated and SS-treated tailings, of free Cu was due to low sulphide weathering in this material respectively) (Fig. 3c). However, at the low pH occurring in these (Fig. 2d). The immobilising effect of SS on Cu through adding more highly weathered tailings (Fig. 1c), several metals may have con- binding sites was counteracted by the contribution of Cu2+ from tributed to the toxicity, not only Cu. Levels of Al in the tailings solu- the ‘geochemically active’ Cu pool in the sludge. The higher con- tion (about 200 lM in both treatments, data not shown) were far centration of total dissolved Cu in the SS-treatment (Fig. 2a) at high above levels in solution culture that have caused reduced yields pH values is consistent with findings in a previous column study of red fescue shoots by 50% (10–20 lM) (Wheeler et al., 1992). (Forsberg et al., 2008) and probably depended on Cu–DOC-com- The shoot level of Cu found in the NPK-treated tailings in the plexes being released from the sludge. It has been shown that Cu acidic conditions exceeded the maximum tolerable Cu level of complexation by SS–DOC is greatest at around neutral pH (Ash- dry forage for daily intake by cattle (100 mg kgÀ1, NRC, 1980). worth and Alloway, 2007), which was the pH in the unweathered The corresponding Cu level in shoots of SS-treated fescue did not tailings (Fig. 1a). Below this pH, H+ compete with metal ions for an- exceed that limit in this study (Fig. 3c). This is relevant to consider, ionic binding sites, while above this pH hydroxyl ions compete as the deposit may become a significant pasture for reindeer, with organic ligands for metal ions (Ashworth and Alloway, moose and hares living in the area (Forsberg and Ledin, 2006). 2007). In the more weathered B1-tailings, the content of ‘geochem- In a field study conducted adjacent to the tailings deposit of the ically active’ Cu in the sludge also counteracted any immobilising Aitik Cu mine in the year 2001, levels of Cu found in red fescue effect of the sludge. One exception was the last leaching occasion, shoots after 3 months did not agree with the levels found in any when Cu concentration had increased in the NPK-treated samples of the corresponding treatments on any of the tailings types used but not in the SS-treated samples (Fig. 2b and e). An immobilising in the climate chamber experiment (Forsberg and Ledin, 2006). In effect of the SS was also observed on all leaching occasions in the the field study, red fescue grown in sludge-treated plots contained B2-tailings (Fig. 2c and f). These systems probably contained a significantly higher levels of Cu (55 mg kgÀ1) compared to the NPK- higher concentration of ‘geochemically active’ Cu produced by sul- treated plots (30 mg kgÀ1). There are a number of differences be- phide weathering and the Cu levels in solution then became more tween these two studies that could explain the different results. dependent on the binding of Cu to the solid phase organic matter. In the field trial the decrease in pH occurred some time during Compared to Cu concentration in the SS, the binding site of the SS the growing season, but exactly when is not known. The pH de- surfaces were unsaturated and could complex Cu from the mine crease may also have occurred at different times in the different tailings. It should be remembered that the degree of decomposition treatments. In the climate chamber, however, the plants in each probably differed between the SS in the weathered B2-tailings and tailings type were grown under similar pH conditions throughout the SS in the A-tailings and B1-tailings, as the B2-SS mixture had the experiment, with small changes and small differences between been leached over several weeks before the first sampling occasion. treatments. Other differences between the two experiments in- The organic matter was degraded fast after the tailings and SS were cluded the NPK fertilisation, plant growth and climatic conditions. mixed and rewetted, causing high DOC levels in solutions from the The higher S/R-ratio found in plants grown in SS-treated tailings A- and B1-tailings on the first sampling occasion (Fig. 1g and h). In compared with NPK-treated (Fig. 3f and 3g) may be due to in- the B2-tailings, however, the DOC levels had become steady at that creased transport of Cu from roots to shoots in those plants due time (Fig. 1i). to enhanced formation of uncharged complexes with organic com- L.S. Forsberg et al. / Applied Geochemistry 24 (2009) 475–482 481 pounds (Bell and Biddulph, 1963; Momoshima and Bondietti, References 1990; Tatár et al., 1998; Rauser, 1999). When metals are bound in neutrally charged complexes with organic compounds, their Ahumada, I., Mendoza, J., Escudero, P., Ascar, L., 2001. Effect of acetate, citrate and lactate incorporation on distribution of cadmium and copper chemical forms in affinity for the negative charges in the cell walls are reduced soil. Commun. Soil. Sci. Plant. Anal. 32, 771–785. (White, 2001; Cataldo et al., 1988) and the complexes may there- Ashworth, D.J., Alloway, B.J., 2007. 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The higher bio- London. mass due to SS application may have caused a dilution effect on Bradshaw, A.D., Chadwick, M.J., 1980. The Restoration of Land. Blackwell, Oxford. Cataldo, D.A., McFadden, K.M., Garland, T.R., Wildung, R.E., 1988. Organic Cu level in the plants that was greater for roots than shoots. This constituents and complexation of nickel (II), iron (III), cadmium (II), and may have been the case especially in the B1-tailings, where no ef- plutonium (IV) in soybean xylem exudates. Plant Physiol. 86, 734–739. fect of SS on the Cu distribution could be confirmed statistically Clemensson-Lindell, A., Borgegård, S-O., Persson, H., 1992. Reclamation of mine waste and its effects on plant growth and root development – a literature when calculating the S/R-ratio as amounts (Fig. 3g). review. ISSN 0348-422x, Report 47, Department of Ecology and Environmental The Rhizon lysimeters appeared to extract representative soil Research. 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