The influence of Magnafloc10 on the acidic, alkaline, and electrodialytic desorption of metals from mine tailings

Journal of Environmental Management 224 (2018) 130–139

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Research article

The influence of Magnafloc10 on the acidic, alkaline, and electrodialytic desorption of metals from mine tailings

T

Kristine B. Pedersena,∗, Helena C. Reinardyb, Pernille E. Jensenc, Lisbeth M. Ottosenc, Juho Junttilad, Marianne Frantzena a

Akvaplan-niva AS, Fram Centre - High North Research Centre for Climate and the Environment, Hjalmar Johansens Gate 14, 9007, Tromsø, Norway Department of Arctic Technology, University Centre in Svalbard, Longyearbyen, Svalbard, Norway c Arctic Technology Centre, Department of Civil Engineering, Technical University of Denmark, Building 118, 2800, Lyngby, Denmark d Department of Geosciences, UiT – the Arctic University of Norway in Tromsø, Dramsveien 201, 9037, Tromsø, Norway b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrokinetic remediation Copper Magnafloc10 Sequential extraction Metal extraction Mine tailings

Repparfjorden in northern Norway has been partly designated for submarine mine tailings disposal when the adjacent Cu mine re-opens in 2019. In order to increase sedimentation, the flocculant, Magnafloc10 is planned to be added to the mine tailings prior to discharge into the fjord. This study investigated the feasibility of reducing the Cu concentrations (375 mg/kg) in the mine tailings by applying electrodialytic extraction, including potential optimisation by adding Magnafloc10. In the acidic electrodialytic treatment (pH < 2), Magnafloc10 increased the extraction of Cu from the mine tailings particles from 76 to 86%, and the flocs with adsorbed metals were separated from the tailings solids by the electric field (1 mA/cm2). The electric energy consumption increased with the use of Magnafloc10 (from 17 to 30 kWh/g Cu extracted), due to lower conductivity in the liquid phase and clogging of the membrane by the flocs. In the alkaline electrodialytic treatment (pH > 12), Magnafloc10 reduced the extraction of Cu from 17% to 0.7%, due to the flocs remaining in the tailing slurries. The electric energy consumption per extracted Cu was similar in the acidic and alkaline electrodialytic treatments without the addition of Magnafloc10. In the alkaline electrodialytic treatment, the extraction of other metals was low (< 2%), however longer treatment time is necessary to achieve similar Cu extraction as in the acidic electrodialysis. Depending on the target and timescale for treatment, acidic and alkaline electrodialysis can be employed to reduce the Cu concentration in the mine tailings thereby reducing the metal toxicity potential.

1. Introduction Mining of minerals generates large volumes of waste materials (Cooke and Johnson, 2002) that are most commonly deposited on land (> 99%) and to a lesser degree marine or riverine disposal (Vogt, 2012). The main produced waste includes non-processed rock and tailings (Jamieson et al., 2015). The tailings are a by-product from the separation of targeted metal(s) from the mined ore and are often a slurry of fine material (crushed rock) with a high water content (Kline and Stekoll, 2001). There is a large, unexplored potential for turning the mine tailings waste products into a resource. Research and technology development for using mine tailings as a resource has increased in recent years, the reuse of mine tailings has however still found limited use in practice (Lottermoser, 2011). Reuse options for mine tailings have included backfilling of mines, in construction materials

∗

such as tiles, glass, and ceramics (Edraki et al., 2014; Lottermoser, 2011), and to fill in harbours (van Leeuwen and Ratsma, 1997). Mine tailings may contain high concentrations of metals of environmental concern (Ramirez-Llodra et al., 2015) and in such cases, removal of the metals could be necessary prior to re-using the mine tailings as a nonpolluting material. An acknowledged method for removal of metals from mine tailings is bioleaching, in which bacteria is used to extract metals by oxidation or reduction of sulphide minerals. After the leaching, precipitation separates the metals from the leachate (Falagán et al., 2017). Another method that has shown promise of separating metals from mine tailings in one step is electrodialysis (Hansen et al., 2007), based on the principles of electrokinetics (Acar and Alshawabkeh, 1993). An electric field of low intensity is applied to the mine tailings suspended in liquid and, depending on the cell design; electrolysis reactions at the

Corresponding author. E-mail address: [email protected] (K.B. Pedersen).

https://doi.org/10.1016/j.jenvman.2018.07.050 Received 16 March 2018; Received in revised form 30 June 2018; Accepted 15 July 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.

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2. Material and methods

electrodes promote acidic or alkaline conditions. This ensures desorption and subsequent transport of metals and/or metal complexes from the suspended material to the electrode of opposite charge. Ion-exchange membranes are employed to control the transport of ions between the material and the electrolyte/electrode (Ottosen et al., 2000). High removal efficiencies of metals from mine tailings have been reported, while maintaining low levels of energy consumption (Pedersen et al., 2017a). Enhancement methods of electrokinetic metal removal have involved addition of surfactants in the form of salts (anionic), organic acids and chelating agents (Lee and Kim, 2010; Masi et al., 2016; Yuan and Weng, 2006). Of chelating agents, anionic surfactants are less likely to adsorb to the negatively charged mine tailings particles. Flocculants are used in mineral processing to separate minerals from one another, to separate fine solids from liquid in order to re-use the process water, and as agents for enhancing the sedimentation of submarine tailings (Dao et al., 2016; Skei and Syvitski, 2013). Flocculation increases the settling rate, decreases turbidity, and decreases the risk of re-suspension upon settling. In wastewater treatment, flocculants have been used as agents for adsorbing metals, thereby transferring the metals from the water to solid phase (Bratby, 2006; Lee et al., 2014). In the mineral processing industry, polyacrylamide based flocculants are most commonly used (> 90%) and the advantage of polyacrylamide is that the non-ionic polymer can be designed according to solid-liquid separation conditions, such as differing contents of suspended solids, dissolved solids, pH, temperature, and mineral/metal composition (Pearse, 2005). Anionic flocculants could potentially increase the electrodialytic extraction of metals and were tested as an enhancement agent for the first time in this study. The mine tailings used in the study were attained from a Cu mine in Kvalsund, northern Norway, planned to be in operation in the 1970s. Cu was mined in the area in the 1970s. During the years of active mine operations, approximately one million tons of tailings were discharged into the inner part of the fjord, Repparfjorden (Kvassnes and Iversen, 2013). Environmental investigations from 2013 to 2015 showed that an estimated 2.5–10 tons of Cu in the mine tailings had dispersed to the outer fjord, partly due to dispersion of mine tailings particles and partly due to desorption of Cu from the mine tailings (Pedersen et al., 2017b; Sternal et al., 2017). The mining company Nussir ASA have plans of re-opening the mine in 2019 and submarine mine tailings disposal of approximately 1–2 million tons per year for an operational period of 30 years is planned. In 2015, Nussir ASA received a permit from the Norwegian Environment Agency for discharging the mine tailings into Repparfjorden and the permit includes the use of Magnafloc10 as an anionic flocculant to be added to the mine tailings prior to discharge to increase sedimentation in the fjord (Miljødirektoratet, 2015). As an agent for enhancing the electrodialytic removal of metals from the mine tailings, Magnafloc10 was used. The flocculant has a broad range of commercial use, including as an agent to increase sedimentation (Berge et al., 2014), for flocculation of microalgae (Uduman et al., 2010), and removal of metals in wastewater treatment (Janin et al., 2009), the effect of Magnafloc10 on the desorption of metals under acidic and alkaline conditions has however not previously been investigated. The main objective of this study is to evaluate Magnafloc10 as a flocculant and surfactant to increase the extraction of metals during electrodialytic remediation of mine tailings. In addition, the effect on acidic and alkaline desorption and metal partitioning in the mine tailings was investigated. The influence of Magnafloc10 on electrodialytic processes and energy consumption as an indicator of the cost-efficiency of the method was included in the evaluation.

2.1. Mine tailings Mine tailings were purchased from Nussir ASA and consisted of rock sourced from the Nussir (90%) and Ulverryggen (10%) ores in Repparfjorden, Norway. The rock was processed at SGS Mineral Services, Canada, to simulate the commercial mining metal extraction processes planned to take place at the mine in 2019 (Kleiv, 2011). 2.2. Magnafloc10 stock solution According to the Nussir ASA discharge permit, addition of Magnafloc10 will be in the ratio 30 mg to 1 kg of mine tailings. Stock solutions of 30 mg/l were made by adding 30 mg Magnafloc10 to 1 L of distilled water. Addition of the Magnafloc10 stock solution to the mine tailings was made in the ratio 1 mL/1 g of dry weight tailings. According to the manufacturer technical document for Magnafloc10 (BASF Corporation, Tucson, Arizona, USA), the stock solution has a storage life of 1–2 days, therefore stock solutions were made daily. For the electrodialysis experiments (section 2.4), stock solutions of 3.6 g/l were made by adding 90 mg of Magnafloc10 to 50 mL of distilled water. 2.3. Mine tailings characterisation The mine tailings were characterised by analysis of metal concentrations, metal partitioning, pH, conductivity, grain size distribution, and content of carbonate, organic matter, chloride, and sulphate. Unless stated otherwise, triple determinations were made. The effect of addition of Magnafloc10 stock solution to the mine tailings was investigated by analysis of metal concentration, partitioning, and desorption. The carbonate content was measured by adding HCl (3 M; 20 mL) to dry mine tailings (5.0 g) and the developed CO2 was measured volumetrically in a Scheibler apparatus, calibrated with CaCO3. The content of organic matter was based on loss on ignition (mass) of dry mine tailings (2.5 g) heated to 550 °C for one hour. pH (KCl) was measured using a radiometer analytical electrode (SenION+ MM374) in the liquid phase after dry mine tailings (5.0 g) were agitated with KCl (1 M, 12.5 mL) for one hour. Conductivity was measured using a radiometer analytical electrode (SenION+ MM374) in the liquid phase after dry mine tailings (5.0 g) were agitated with distilled water for an hour. The content of chloride and sulphate was measured in the liquid phase after agitation of dry mine tailings (10 g) with micropore water (40 mL) for 20 h; solid particles were removed by 0.45 μm vacuum filtration and the concentrations were measured by ion chromatography. Grain size distribution was measured within the range of 0.017–2000 μm with a Laser Diffraction Particle Size Analyzer (Beckman Coulter LS 13 320) with Polarisation Intensity Differential Scattering (PIDS); the measurements were performed in 10 analyses (three 60 s runs/analysis) from which the average grain size was calculated. For metal analysis (Al, Ba, Ca, Fe, K, Mg, Mn, As, Cd, Cr, Cu, Ni, Pb, and Zn) tailings were digested (Norwegian standard NS4770); dry mine tailings (1.0 g) and HNO3 (9 M, 20 mL) were autoclaved (200 kPa, 120 °C, 30 min). Solid particles were removed by vacuum filtration through a 0.45 μm filter and the liquid was diluted with distilled water to 100 mL. Metal concentrations in the liquid were measured by Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) on a Varian 720-ES with standards and internal controls PlasmaCAL from SCP Science (Courtaboeuf, France), at the Arctic Technology Centre at The Technical University of Denmark. Metal partitioning analysis was conducted by sequential extraction in four steps based on the improvement of the three-step method (Rauret et al., 1999) described by Standards, Measurements and Testing Program of the European Union (Quevauviller et al., 1994). In the first step, acetic acid (0.11 M, 20 mL, pH 3) was agitated with dry mine 131

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Six experiments were conducted. In all experiments the mine tailings slurry consisted of 120 g mine tailings (dry weight) and 300 mL distilled water. The electrodialytic experiments had a duration of 28 days at a current of 50 mA. The experimental set-up (acidic/alkaline) and addition of Magnafloc10 varied between the experiments and are given in Table 2. The influence of Magnafloc10 was investigated by adding 1 mL of stock solution (section 2.2) in the beginning of the experiments (0 h, experiments 2 and 5) and on a daily basis (exp. 3 and 6). The power consumption in Wh (E) was calculated as:

tailings (0.5 g) for 16 h. In the second step, the solid particles were agitated with hydroxylammonium chloride (0.1 M, 20 mL; pH 2) for 16 h. In the third step, the solid particles were agitated with hydrogen peroxide (8.8 M, 5 mL) for 1 h, subsequently heated at 85 °C for 1 h, evaporation of liquid at 85 °C, followed by agitation of the cooled solid fraction with ammonium acetate (1 M, 25 mL, pH 2) for 16 h. In the fourth step, the remaining solid particles were digested as described above. The liquids from each step were analysed for metals by ICP-OES. Tests of desorption of metals as a function of pH were made by agitating 10 samples of dry mine tailings (5.0 g) with HNO3 (25 mL) or NaOH (25 mL) at different concentrations (0.01, 0.05, 0.1, 0.5 and 1.0 M). Two reference tests were made for which mine tailings (5.0 g) were agitated with distilled water (25 mL). The suspensions were agitated for a week on a horizontal shaker. Samples were subsequently allowed to settle for 15 min and the pH of the fluid was measured using a radiometer analytical electrode (SenION+ MM374). The sediment was vacuum-filtered through a 45 μm filter. To determine the percentage of metals desorbed into the liquid fraction, metal concentrations were measured in the solid particle and liquid fractions (digestion and ICP-OES methods outlined above). The influence of Magnafloc10 on the mine tailings was investigated by analysis of metal concentration, partitioning, and desorption after adding Magnafloc10 stock solution (section 2.2) to the mine tailings in the ratio 1 mL/1 g of dry weight tailings. For the metal concentration analysis, 1 mL Magnafloc10 stock was added with 9 M HNO3 to the mine tailings prior to autoclaving the samples. For the metal partitioning analysis, 0.5 mL of the Magnafloc10 stock solution was added in each step of the sequential extraction. For the desorption of metals as function of pH, 5 mL of the Magnafloc10 stock solution was added to each suspension on a daily basis.

t

∫ E = VI dt t=0

where V is the voltage between the electrodes (V), I is the current (A) and t is the treatment time (h). After the electrodialysis experiments, the suspensions were filtered and the metal concentration in both the suspension liquid and solids were measured by ICP-OES. The stirrer, membranes, and electrodes were rinsed in HNO3 (5 M) overnight and the metal concentrations in the rinsing and electrolyte liquids were measured by ICP-OES. 2.5. Statistical analyses The F-test in one-way ANOVA analysis was used to evaluate whether there was a significant difference between the desorption of metals in the different electrodialytic extraction experiments, using Microsoft Excel 2016. 3. Results and discussion 3.1. Mine tailings characteristics

2.4. Electrodialytic remediation The characteristics of the mine tailings are summarised in Table 1. The mine tailings are calcareous with a high pH as would be expected in a 9:1 mixture of the Nussir and Ulveryggen ores, characterised by the use of same characterisation methods in Kleiv (2011). The low content of organic matter, sulphate, and chloride has previously been observed in mine tailings from the two ores (Kleiv, 2011; Pedersen et al., 2017a). The mine tailings mainly consist of silt, with the soil texture of a silt

Two different electrodialysis cells were employed for this study, both based on a 2-compartment cell design of which the set-up and principles have been described in detail (Pedersen et al., 2017a). The two compartments contained an electrode and electrolyte liquid, and a suspension of distilled water and mine tailings, respectively. The anode was placed in the suspension compartment to ensure acidic conditions in the acidic set-up and the cathode was placed in the suspension compartment to ensure alkaline conditions in the alkaline set-up. Ionexchange membranes were used to control transport of ions between the two cell compartments, and application of a cation exchange membrane in the acidifying electrodialysis cell prevented the hydroxyl ions from the electrolysis reaction on the cathode from entering the suspension. By applying an anion exchange membrane in the alkaline electrodialysis cell, protons from electrolysis reactions on the anode were similarly prevented from entering the compartment with the suspension. The two-cell design consisted of an electrolyte compartment (anolyte or catholyte) and a compartment containing the suspension. The cell compartments were manufactured from Plexiglas and the dimensions were: length of electrolyte compartments 3.5 cm; length of sediment suspension compartment 10 cm; inner diameter of all compartments 8 cm. Ionics (Watertown, Massachusetts, USA) supplied the anion exchange membrane (204 SZRA B02249C) for the alkaline electrodialysis cell and the cation exchange membrane (CR67 HUY N12116B) for the acidic electrodialysis cell. The electrolyte was NaNO3 (0.01 M) adjusted to pH 2 by HNO3 (5 M, acidic cell) or adjusted to pH 12 by NaOH (5 M, alkaline cell). The electrolyte liquids (500 mL) were circulated (Pan World pumps) at flow rates of 30 mL/min. Platinum coated titanium electrodes were used in each electrolyte compartment and a power supply (Hewlett Packard E3612A) maintained a constant DC current. A RW11 Basic lab-egg (IKA 2830001) with a stirrer consisting of plastic flaps (4 cm × 0.5 cm) fastened to a glass rod stirred the suspension.

Table 1 Analysed characteristics of the Nussir ASA mine tailings.

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Characteristic

Mine tailings

Carbonate (%) Organic matter (%) pH Conductivity (mS/cm) Sulphate (mg/kg) Chloride (mg/kg) Grain size (% vol) Clay (< 2 μm) Silt (2–63 μm) Very fine sand (63–125 μm) Sand (125 μm-1 mm) Metals (mg/kg dry weight) Al Ba Ca Fe K Mg Mn As Cd Cr Cu Ni Pb Zn

20.2 ± 0.4 0.3 ± 0.02 9.2 ± 0.01 0.09 ± 0.001 43.9 ± 3 43.0 ± 5 8.5 ± 2 63.3 ± 2 22.0 ± 2 6.7 ± 2 6130 ± 100 200 ± 8 59,400 ± 1100 8300 ± 180 5460 ± 90 20,800 ± 480 2300 ± 40 2.5 ± 0.3 0.3 ± 0.02 64 ± 0.9 375 ± 8 22 ± 0.4 23 ± 0.6 23 ± 0.9

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can be explained by the difference in extraction chemicals, the total extraction time, the liquid-solid ratio, type of agitation, and liquid-solid separation (Gleyzes et al., 2002). The opposite trend was observed in the mine tailings with added Magnafloc10. The amount of metals desorbed during digestion was either similar (Ba, Mn and Cu) or higher (single factor ANOVA for Al, Fe, Mg – F [13–71] > Fcrit [7.7]) than the total amount of metal desorbed during sequential extraction. This could be due to flocs with metals being discarded after the centrifugation and decanting in steps 1–2 of the sequential extraction.

loam according to the European soil classification system (ISO 14688). Approximately 71% of the mine tailings was fine grained clay and silt; previous investigations have shown that during electrodialytic remediation, low content of fine grained fraction (< 10%) increased the energy consumption (Thöming et al., 2000), and higher removal efficiencies of metals have been observed when solely treating the clay and silt particles (Sun et al., 2012). The mine tailings characteristics differ from the natural sediments in Repparfjorden (Pedersen et al., 2017b; Sternal et al., 2017), with 5–40 times higher concentration of Ba, Cu, Mg, and Mn in the tailings compared with the background sediment levels. In order to evaluate whether the mine tailings should be managed as a polluted or non-polluted material (NFFA, 2015); the metal concentrations were compared to the Norwegian quality criteria for soil (Miljødirektoratet, 2009). The concentrations of As, Cd, Cr, Ni, Pb, and Zn were below levels shown to potentially result in adverse environmental effects, i.e. below class 2 in the Norwegian classification of soil. The concentration of Cu exceeded the class 2 criteria, classifying the mine tailings as a moderately polluted material (class 3). Remediation of the mine tailings is hence a possible strategy to reduce the hazardous classification and subsequent potential environmental impacts of submarine disposal, or re-use of the tailings. The targeted metal for remediation is Cu and is the focus of this study.

3.3. Influence of Magnafloc10 on metal partitioning and desorption The metal partitioning results (Fig. 1) are comparable to a previous study of a different batch of mine tailings from the same source ore (Pedersen et al., 2017a). Less than 20% of the total metal content was bound in the fractions that can be made available by changing geochemical properties (exchangeable, reducible, and oxidisable) for Al and Fe. For Ba, Cu, and Mg 45–60% were found in the available fractions while more than 90% of Mn was found in these fractions. By using the metal partitioning as an indicator for desorption potential under changing geochemical conditions, the relative order of desorption would be Mn > Mg > Cu > Ba > Fe > Al. Desorption experiments (Fig. 2) largely confirm these observations, in that the relative amounts desorbed under acidic conditions follow the order Mn > Mg≈Cu > Fe > Al > Ba. The reason that Ba deviates from the preliminary assessment of desorption potential could due to the formation of barite (barium sulphate), with low solubility, that will only dissolve in strong acid and hence be found in the residual fraction of the tailings (Gonneea and Paytan, 2006). The mine tailings contain sulphate (Table 1) that could react with Ba ions in the desorption experiments. This could occur if the formation of barite is solubility controlled. The liquid-solid ratio in the sequential extraction (40 mL/g) was higher than the desorption experiments (5 mL/g) with the potential of shifting the chemical equilibrium of Ba desorption. The retention time in the desorption experiments was longer; 7 days compared to 3 days in first three steps of the sequential extraction procedure, providing longer time for precipitation of barite. Without addition of Magnafloc10, desorption of Ba from the mine tailings occurred below pH 8, with 9–28% of the total content of Ba present in the liquid phase (Fig. 2). Up to 30% Al and 60% Fe desorption occurred at pH values below 2. The desorption of Mg, Mn, and Cu occurred at pH levels below 7, increasing with decreasing pH, the highest desorption, in the range 78–83%, found at pH < 1. Under alkaline conditions, desorption was only observed for Ba and Cu. At pH 12.5 limited dissolution of Ba (maximum 2%) was observed while up to 30% Cu was found in the liquid phase; the desorption of Cu could be related to the dissolution of Cu sulphides under alkaline conditions (Acres et al., 2010; Todd et al., 2003). Magnafloc10 decreased the metal observed in the liquid phase by 40–50% for Fe, Mg and Mn, and by 70% for Cu with no change in desorption of Al and Ba (Fig. 2). This is an indication that Magnafloc10 adsorbed Fe, Mg, and Mn under acidic conditions, and adsorbed Cu under both acidic and alkaline conditions and in the subsequent filtration (0.45 μm), the flocs with adsorbed metals remained in the solids. The adsorption kinetics of Magnafloc10 hence differs depending on the type of metal, in line with previous studies of flocculants with different sorption capacity for different metals (Kurniawan et al., 2006; Wang and Chen, 2014).

3.2. Influence of Magnafloc10 on the digestion of mine tailings The addition of Magnafloc10 increased the amount of Al, Fe, Mg, Mn, and Cu desorbed by 9–27% during the digestion of the mine tailings (single factor ANOVA, n = 3, F [28–195] > Fcrit [7.7]). This indicates that flocculation affected the chemical equilibrium of these metals. Previous studies of wastewater treatment with anionic flocculants showed that flocs with adsorbed metals separated from the liquid phase during filtration and sedimentation (Fu and Wang, 2011). Lower amounts of metals would be expected in the filtered liquid; however, the opposite was observed (Fig. 1). This suggests that temperature and/ or high pressure during the digestion broke up the flocs, and adsorbed metals passed through the 0.45 μm filter in the separation process. This is in line with other studies that have found that flocs have broken under the influence of increasing temperatures (Mpofu et al., 2004) and high pressure (Dickinson and Pawlowsky, 1996), and after treatment at high temperature (> 25 °C) the flocs did not reform to their original size (Fitzpatrick et al., 2004). The percentage increase of each metal in the liquid phase differed from each other caused by the differences in the aqueous-solid metal chemistry and floc-metal chemistry. For Ba, Magnafloc10 did not influence the total concentration (single factor ANOVA, n = 3, F [0.01] < Fcrit [7.7]). This could be due to Ba not being as susceptible to desorption under different pH and redox conditions compared to the other analysed metals (Chow et al., 1978; González-Munoz et al., 2003; Salminen et al., 2005). In the following, the sum of concentration refers to the total amount of metals desorbed in the four steps of the sequential extraction, while total metal concentration refers to the metal desorption during digestion. The sum of concentration was lower for Al, Fe, Mn and Cu in the mine tailings with added Magnafloc10 (single factor ANOVA F [10–24] > Fcrit [7.7]), the opposite trend to that observed in the total metal concentrations (Fig. 1). This could be due to limited breakage of flocs in the first two steps of the sequential extraction. Flocs with adsorbed metals could thus have been discarded in connection with centrifugation and subsequent decanting for separation of solids from liquid. In the mine tailings without added Magnafloc10, the sum of concentrations exceeded the total metal concentration by 5–25% (single ANOVA analysis, n = 3, F [8.0–128] > Fcrit [7.7]). This is within the range of higher sum of concentrations in sequential extraction compared to total metal concentrations analysed after digestion (Davidson et al., 1998). The difference in concentrations between the two methods

3.4. Influence of Magnafloc10 on the electrodialytic extraction process In the alkaline electrodialytic experiments, the pH increased to 12 within the first day of treatment and stabilised at around pH 12.5 after 140 h (6 days) of treatment (Fig. 3), in line with the desorption experiments (Fig. 2). In the desorption experiments, substantial amounts of metals were desorbed below pH 2. It took 13 days to reach this level in the acidic electrodialytic experiments. The pH continued to decrease 133

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Fig. 1. Metal concentrations of the Nussir ASA mine tailings and mine tailings added Magnafloc10. The metal concentrations based on the sequential extraction analysis include concentrations in the exchangeable, reducible, oxidisable and residual fractions. 'Total conc.' refers to total metal concentrations after digestion and analysed by ICP-OES.

This is due to the short storage life of Magnafloc10 of 1–2 days (section 2.2). Acidic electrodialysis consumed more energy compared to alkaline electrodialysis; however, energy consumption levels were still within the lower range of other electrodialytic remediation tests (Pedersen et al., 2015; Sun and Ottosen, 2012). As observed for conductivity, it was only in exp. 3 and 6 that the influence of Magnafloc10 was observed, the higher energy consumption was most distinct for the acidic treatment (exp. 3). The increase is likely due to lower conductivity in the suspension liquid, but might also be due to larger particles (flocs) clogging the membrane. The desorption of metals after the electrodialytic treatments is summarised in Table 2. The highest desorption was observed for Mg, Mn, and Cu in line with the desorption experiments (Fig. 2). The desorption of Al and Fe was lower and Mg in the electrodialytic experiments with final pH around 0.8–0.9 (exp. 1–3) was in the same range as the desorption experiments at the same pH level. The desorption of Ba, Mn, and Cu was higher in the acidic electrodialytic treatments compared to the desorption experiments. This is related to the continuous

to pH 0.8–0.9 after 400 h (∼18 days) and remained at this level throughout the remaining time (up to 10 days) of the experiments. In both the acidic and alkaline experiments, no influence of Magnafloc10 on the development of pH was observed (Fig. 3). For both the acidic and alkaline electrodialytic experiments, an increase in the electric conductivity with time was observed (Fig. 3), in line with observations in a previous study (Ebbers et al., 2015). The increase is largely related to the generation of ions from the electrolysis reaction on the electrode in the mine tailings suspension compartment. The increase in conductivity is higher in the acidic electrodialytic treatments due to twice as high molar ion conductivity of protons compared to hydroxyl ions (Adamson, 1979). There was a decrease in conductivity in the experiments with daily addition of Magnafloc10 (exp. 3 and exp. 6), indicating that generated flocs adsorbed ions in the suspension liquid. This observation is in line with a previous study in which conductivity decreased when treated with an anionic flocculant (Reddy et al., 2006). The addition of Magnafloc10 in the beginning of the experiments (exp. 2 and 5), the conductivity was in the same range as for the experiments with no addition of Magnafloc10 (exp. 1 and 4). 134

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Fig. 2. Percentage of metals desorbed (%) as function of pH. The desorption curves were based on treatments of mine tailings slurry (liquid-solid ratio of 5 mL/g) with different strengths of acid and base. The metal desorption curves of mine tailings and Magnafloc10 was based on daily addition of Magnafloc10 to the slurry.

have not included Al, Ba, Fe, Mg and Mn (not environmental focus) Magnafloc10 has not been used as an enhancement agent. Other desorbing agents that have been investigated for enhancing the removal of metals from polluted soil and sediments include salts, acids, organic acids, and chelating agents such as ethylenediaminetetraacetic acid (EDTA) (Song et al., 2016). Efficiency of desorbing agents varies depending on the metals and characteristics of the material, e.g. type of soil/sediment. In a stationary set-up, the removal of Cu was for instance observed to improve from 2% to 50% by the use of EDTA, while not affected by citric acid (Masi et al., 2016). In a stirred set-up the removal of Cu from harbour sediments was observed to improve from 50% to 70% by using lactic acid as a desorbing agent, while not improving with the use of other desorbing agents (Nystroem et al., 2006). The effect of Magnafloc10 on the extraction of Cu in this study is assessed as within the range of efficient desorbing agents found in literature, when taking into regard the high removal efficiency in the treatment without the addition of Magnafloc10 (exp.1, 76%). In the alkaline electrodialytic treatment, the desorption of all metals

shifting of the chemical equilibrium during electrodialysis due to the transport of metal ions to the electrolyte compartment and is in line with previous investigations of acid- and electrodialytic desorption of metals from harbour sediments (Pedersen et al., 2017c). The most distinct effect of Magnafloc10 was observed for Cu, improving the desorption from 76% to 86%. The desorption of Ba and Fe also improved with the addition of Magnafloc10. The higher desorption of Ba could be related to desorbed Ba ions adsorbing to flocs instead of reacting with sulphate to form barite. For Cu and Fe, the higher desorption could be related to a shift in equilibrium, increasing the desorption of some of the more available metals when Magnafloc10 was added. The observation that greater desorption of Ba, Cu, and Fe occurred with the addition of Magnafloc10 indicates that the flocs with adsorbed metals were transported to the electrolyte compartment, and were thus separated from the mine tailing solids. The desorption of Mg and Mn may have reached a maximum for the tested tailings slurry, and no effect of Magnafloc10 was observed. In previous electrokinetic remediation studies, the targeted metals 135

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Table 2 Desorption (%) of metals in the electrodialytic treatments. Experiment

Addition of Magnafloc10

Acidic electrodialytic treatment 1 None 2 1 mL 30 μg/kg at t = 0 h 3 1 mL 30 μg/kg daily Alkaline electrodialytic treatment 4 None 5 1 mL 30 μg/kg at t = 0 h 6 1 mL 30 μg/kg daily

Desorption (%) Al

Ba

Fe

Mg

Mn

Cu

20 15 17

23 24 31

40 43 50

71 68 74

92 96 97

76 83 86

0.7 0.4 0.5

0.5 1.2 1.6

0.6 0.7 0.1

< 0.1 < 0.1 < 0.1

< 0.1 < 0.1 0.1

17 18 0.7

indication of desorbed Cu adsorbed to flocs that did not deposit on the cathode. In the separation of the suspension liquid and mine tailings, the flocs are likely to remain in the solids fraction. It is hence not possible to directly compare the effect of Magnafloc10 with the observed improved removal of Cu by using ammonium citrate as an enhancement agent under alkaline conditions in previous electrodialytic experiments (Ottosen et al., 2001). The distribution of metals in the cell after the acidic electrodialytic experiments is illustrated in Fig. 4. The relative amounts of all the metals in the suspension liquid were lower in exp. 3 (daily addition of Magnafloc10), an indication that metals adsorbed to flocs were measured as solids at the end of the experiment. Apart from Mn, the addition of Magnafloc10 prior to initiation of the experiment did not influence the relative amounts of metals in the suspension liquid (exp. 1 and 2). More metal was transported from the suspension to the electrolyte compartment (cathode and electrolyte in Fig. 4) in exp. 3 (daily addition of Magnafloc10). The distribution between electrode and electrolyte differed and less metal was deposited on the cathode in exp. 3 compared to exp.1 and 2. This is an indication that more of the metal in the electrolyte compartment was associated with Magnafloc10 and suggests that the flocs were transported to the electrolyte compartment either due to charged flocs or by electroosmosis. The difference in distribution of metals in the electrolyte compartment between exp. 1 and exp.2 (addition of Magnafloc10 prior to electrodialysis) was most pronounced for Mg, Mn, and Cu. In exp. 2 there was relatively more metal in the electrolyte for Mg and Mn while the opposite was observed for Cu. This could be related to the difference in pH levels at which desorption began to occur. For both Mg and Mn, desorption occurred at higher pH than Cu (Fig. 2). In the electrodialytic treatment, Magnafloc10 was still effective in adsorbing metals when the desorption of Mg and Mn initiated, subsequently transporting some of these metals as flocs to the electrolyte compartment. Some breakage of flocs may have occurred, however it appears that Mg and Mn were still associated with flocs at the end of the experiment. Desorption of Cu was initiated at lower pH (Fig. 2) and hence at a later time in the electrodialytic remediation. At this time point, breakage of flocs could have occurred to such a degree that adsorption to the flocs was limited, and more Cu was deposited onto the cathode. The higher energy use in exp. 3 was likely related to clogging of flocs in the membrane. The distribution of Fe supports this because seven times more Fe was found in the membrane in exp. 3 compared to exp 1 (no addition of Magnafloc10). Approximately twice as much metal (except Fe) was found in the membrane in exp. 3 compared to exp. 1, but the amounts found in the membrane were < 2% (Fig. 4). In the alkaline electrodialytic experiments, the only metal to be desorbed more than 2% was Cu and for this reason the distribution in the electrodialytic cell has only been included for Cu in Fig. 5. The addition of Magnafloc10 prior to treatment (exp. 5) does not have an effect on the distribution of Cu after the experiments. In the three experiments the major part of the Cu was found to have deposited onto the cathode in the suspension compartment indicating that Cu was

Fig. 3. Development in pH, conductivity and energy consumption during the electrodialytic experiments.

apart from Cu was below 2%. The desorption of Cu was below the level observed to be in the liquid phase of the desorption experiments (24%, Fig. 2) at similar pH level (12.5). In exp. 6, with daily addition of Magnafloc10, the desorption of Cu was below 1% and could be an 136

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Fig. 4. Distribution of metals at the end of the electrodialytic experiments, calculated as the fraction of total metals in the different parts of the electrodiaytic cell. This includes metals remaining in the mine tailing suspension compartment (mine tailings, suspension liquid and precipitated on the anode), metals precipitated on the membrane and metals transported to the electrolyte compartment (electrolyte liquid and precipitated on the cathode).

highest desorption of Cu (86%) from the mine tailings was observed in exp. 3 (daily addition of Magnafloc10). The Cu was, however, not deposited onto the cathode to the same extent as exp.1 (no addition of Magnafloc10), accordingly separation the Cu from the electrolyte liquid has to be done after electrodialytic treatment. In addition, the daily addition of Magnafloc10 increased the electric energy consumption (Fig. 3); the energy required to remove the same amount of Cu was 30 kWh/g, compared to 17 kWh/g in exp. 1. In the alkaline electrodialytic treatment without addition of Magnafloc10, the energy required to remove the same amount of Cu was also 17 kWh/g (exp. 4). Compared to acidic electrodialysis, the alkaline treatment provides the possibility of desorbing Cu while limiting the removal of other metals (< 2%, Table 2). Applying alkaline electrodialysis however requires optimisation (e.g. increasing current intensity or treatment time) to achieve similar desorption levels as observed in the acidic electrodialytic extraction experiments. The choice of whether to proceed with the acidic or alkaline electrodialytic extraction would depend on whether limiting desorption of other metals or the experimental settings (current, treatment time) are important factors. Another limiting factor could be the desired final pH, depending on the final use of the tailings.

Fig. 5. Distribution of Cu at the end of the electrodialytic experiments, calculated as the fraction of total metals in the different parts of the electrodiaytic cell. This includes metals remaining in the mine tailing suspension compartment (mine tailings, suspension liquid and precipitated on the cathode), metals precipitated on the membrane and metals transported to the electrolyte compartment (electrolyte liquid and precipitated on the anode).

4. Conclusion +

available as positive charged complex at this pH, possibly as Cu(OH) or Cu2(OH)22+ (Woods et al., 1987). In exp. 6 with daily addition of Magnafloc10, no desorption of Cu was observed, related to adsorption of the positive charged Cu complexes to Magnafloc10, subsequently measured in solid particles. In the Nussir ASA mine tailings, the targeted metal in the mining process is Cu. This is the only metal that exceeds class 2 of the Norwegian soil and sediment quality criteria, i.e. with the potential for causing adverse effects on human health and the environment. After the acidic electrodialytic experiments (exp. 1–3), the concentration of Cu in the mine tailings were within the limits of class 2 soil and sediment. The

The capacity of Magnafloc10 to adsorb metals in solution was observed to depend on the type of metal. In acidic conditions with high desorption percentages, Magnafloc10 most efficiently adsorbed Cu, Mg and Mn, while not adsorbing Al and Ba. The targeted metal for remediation in the studied mine tailings was Cu and Magnafloc10 increased the electrodialytic extraction of Cu from the mine tailings particles from 76% to 86%, in the acidic cell design, indicating transport of flocs with adsorbed metals under the influence of an electric field. However, Magnafloc10 increased the electric energy to remove Cu from the tailings by 76% and less Cu was deposited on the cathode. Considerations as to whether the increased Cu removal outweighs the 137

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increased energy consumption will have to be made prior to initiating further optimisation efforts. In the alkaline electrodialytic treatment the flocs remained in the tailings suspension compartment, and were not deposited on the cathode, reducing the separation of Cu from the mine tailings from 17% to 0.7%. The energy consumption in the alkaline electrodialytic treatment without the addition of Magnafloc10 was lower than the acidic treatment. In addition there was low mobilisation of other metals (< 2%), enabling targeting the removal of Cu, while limiting the removal of other metals. The choice of which method to proceed with will hence depend on other factors, such as acceptable time ranges for the remediation, acceptable desorption of other metals and the final pH in the mine tailings.

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