Impacts of climate change on metal leaching and partitioning for submarine mine tailings disposal.

At present, there are no standardised tests to assess metal leaching during submarine tailings discharge. In this study the influence of variables known to affect metal mobility and availability (dissolved organic carbon (DOC), pH, salinity, temperature, aerated/anoxic conditions) along with variables affected by the discharge conditions (flocculant concentration, suspension) were studied in bench-scale experiments. The leaching tests were developed based on the case of a copper mine by Repparfjorden, northern Norway, which is planned to re-open in 2022. The experiments, which had three week duration, revealed low (<6 %) leaching of metals. Multivariate analysis showed that all variables, apart from DOC, highly influenced leaching and partitioning of at least one metal (Ba, Cr, Cu, and/or Mn). The high quantity of the planned annual discharge of mine tailings to the fjord (1-2 million tonnes) warranted estimation of the leached quantity of metals. Multivariate models, using present-day conditions in the fjord, estimated leaching of up to 124 kg Ba, 154 kg Cu and 2400 kg Mn per year during discharge of tailings. Future changes in the fjord conditions caused by climate change (decreased pH, increased temperature) was predicted by the multivariate models to increase the leaching up to 55 %, by the year 2065. The bench-scale experiments demonstrated the importance of including relevant variables (such as pH, salinity, and temperature) for metal leaching and -partitioning in leaching tests. The results showed that metal leaching during discharge is expected and will increase in the future due to the changed conditions caused by the foreseen climate change, and thereby underline the importance of monitoring metal concentrations in water during operations to determine the fate of metals in the fjord.

Comparison of 2- and 3-compartment electrodialytic remediation cells for oil polluted soil from northwest Russia.

Electrodialytic remediation is a method based on electrokinetics, in which an electric field of low intensity increases the availability of pollutants in solid waste materials. The electric field induces processes that mobilise and transport inorganic and organic pollutants. The transport of ions in the electrodialytic cell is controlled by employing ion-exchange membranes, allowing separation of the electrodes from the solids. In this study, using a two cell design, electrodialytic experiments were conducted to compare remediation of a heavily oil-polluted soil from Arkhangelsk, Russia. The 2-compartment cell has not previously been employed for electrodialytic removal of organic pollutants and was tested along with the traditional 3-compartment design. The influence of experimental variables (current density, remediation time, stirring and light) and settings on the two cell designs was investigated. The highest removal (77%) of total hydrocarbons (THC) was observed in the 3-compartment cell at high current density (0.68 mA/cm2), longer remediation time (28 days), stirring and exposure to daylight. High current density and stirring increased the removal efficiencies in both cell designs. Within the studied experimental domain, the removal efficiencies in the 3-compartment cell (10-77%) were, however, higher than those observed in the 2-compartment cell (0-38%).

Lability of toxic elements in Submarine Tailings Disposal: The relationship between metal fractionation and metal uptake by sandworms (Alitta virens).

Mine tailings from the iron ore mine Sydvaranger Gruve have been deposited in Bøkfjorden, Northern Norway, for >40 years. This study investigates the environmental impacts of submarine tailings disposal one year after the cessation of the mine by combining analyses of total metal concentrations (nitric acid digestion) and metal fractionation patterns (sequential extraction procedure) of sediments with the metal uptake (bioassays) by sandworms (Alitta virens) exposed to Bøkfjorden sediment. The analyses focused on metals (As, Cd, Cr, Cu, Fe, Ni, Pb, Zn) in sediment cores retrieved from a horizontal transect of the fjord. High concentrations of the target metal iron was found in the fjord (>13,000 mg/kg). According to Norwegian sediment quality guidelines (SQG), cadmium (2.55 mg/kg) and lead (174 mg/kg) were the only metals to exceed good sediment quality at the source area of mine tailings discharge. Despite general low total metal concentrations, labile (summarized exchangeable, acid soluble, reducible fractions) and potentially labile (summarized labile and oxidisable fractions) pools of mine tailings-related metals of cadmium (10-82%), copper (25-88%) and lead (10-67%) were detected while iron were characterized as inert (residual fraction: 89-97%). Despite spatial trends in total metal concentrations presumed to originate from mine tailings (Cd, Cu, Fe, Pb), there were no correlations between total and labile fractions. This can be explained by the high concentrations of iron, which under oxidised forms are suggested to inhibit a high lability of other metals. The metal uptake of sandworms was generally high compared to similar studies and exceeded moderate contamination classes for all metals compared to Norwegian standard values. Both labile metal fractions (cadmium, copper and iron) and total metal concentrations (copper, iron) showed positive correlations (r > 0.6) with metal uptake. Thus, both the total and labile metal fractions can pose an environmental risk.

Effects of mine tailings exposure on early life stages of atlantic cod.

In Norway, mine tailings waste can be deposited by coastal submarine dispersal. Mine tailings slurry includes fine particles <10 µm with elevated levels of metals (e.g., copper, iron) from residual mineral ore. Prolonged suspension of small particles in the water column may bring them into contact with locally spawned pelagic fish eggs, including Atlantic cod, Gadus morhua. Newly fertilized cod embryos were exposed to suspended mine tailings particles up to 3.2 mg/L in flow-through aquaria for a total of 21 d. Significantly more particles adhered to the surface of the chorion from the high treatment after 11-d exposure, and dissolved Cu concentrations increased in the water (up to 0.36 ± 0.06 µg/L). There was no adverse effect on embryo mortality but an 8% elevation in larval mortality. There were no differences with treatment on timing of hatching, embryo and larva morphometrics, abnormalities, or cardiac activity. There was a treatment-dependent up-regulation of stress marker genes (hspa8, cyp1c1) but no indication of metal-induced activation of metallothionien (mt gene transcription). Transcription markers for DNA and histone methyltransferases did show treatment-related up-regulation, indicative of altered methylation in larvae when developmental methylation patterns are determined, indicating some level of chronic toxicity that may have longer-term effects. Environ Toxicol Chem 2019;38:1446-1454. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Applying multivariate analysis for optimising the electrodialytic removal of Cu and Pb from shooting range soils.

Multivariate analysis was applied to simultaneously evaluate the influence of soil properties and experimental variables on electrodialytic removal of Cu and Pb from three shooting range soils. Both stationary and stirred set-ups in laboratory scale were tested, representing in-situ and ex-situ remediation conditions, respectively. Within the same experimental space, higher removal of the targeted metals, Cu and Pb, were observed in the stirred set-up (9-81%) compared to the stationary set-up (0-41%). Multivariate analysis (projections onto latent structures) revealed that the influence of soil type on the remediation efficiency was dependent on the metal and varied in the stationary and stirred set-ups. Optimising the removal of Cu by adjusting the experimental settings was easier to achieve in the stirred set-up and could be done by increasing the current density. Optimising the removal of Pb could be done by prolonging the treatment and in the stirred set-up also by increasing the current density.

Comparison of two- and three-compartment cells for electrodialytic removal of heavy metals from contaminated material suspensions.

Electrodialytic remediation can be applied to remove heavy metals from contaminated particulate materials in suspension. The applied electric current is the cleaning agent and the heavy metals are removed by electromigration. In this study, a two-compartment cell was compared to a three-compartment cell, for several contaminated materials such as soils, sediments, mine tailings and ashes and totally 20 experiments were conducted. The pH decrease was faster in the two-compartment cell, but the metal removal was higher in the three-compartment cell since anionic metal species are removed from the material suspension in this cell set-up. For materials with relatively high chloride content, fly ash and harbour sediments, up to 38% of the metals were found in the filtrate in the two-compartment cell. Up to 9% of the current was used to transport heavy metal ions in the experiments and the current was mainly carried by H+ and Ca2+. Even with the lower pH in the two-compartment cell experiments, there was little difference in the percentage of the current carried by the metal ions between the two set-ups. Multivariate analysis showed that the choice of cell set-up depends on the metals targeted by remediation and the material characteristics.

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

Influence of electrode placement for mobilising and removing metals during electrodialytic remediation of metals from shooting range soil.

Electrodialytic remediation was applied to a shooting range soil to investigate the influence of electrode placement on the removal and binding of metals during the treatment. The set-up was based on a 2-compartment cell, in which the cathode was separated from the soil by a cation exchange membrane and the anode was placed directly in the soil, thereby introducing protons and oxygen directly in the soil. Mobilisation of metals from less available fractions (oxidisable and residual) in the soil occurred, due to oxidation/dissolution of insoluble/soluble organic matter and possibly metal oxides in the residual fraction. The transport via electromigration out of the soil and/or re-precipitation in other fractions of the soil (oxidisable, reducible, exchangeable) depended on the metal. More than 30% of the initial content of Mn, Cd, Cu, Pb and Zn and less than 20% of the initial content of Al, Fe, K, Mg, As, Cr and Ni was transported out of the soil. By decreasing the distance between the electrodes from 3.0 to 1.5 cm, the removal of the targeted metal for remediation, Pb, was improved by more than 200%, from 14 to 31%. A similar removal could be achieved in experiments with long distance between electrodes (3.0 cm) by increasing the current intensity from 4 to 10 mA and/or the remediation time from 7 to 35 d. The experiments showed that the design and optimisation of electrodialytic remediation depends on the targeted metal and metal partitioning.

Long-term dispersion and availability of metals from submarine mine tailing disposal in a fjord in Arctic Norway.

Mining of Cu took place in Kvalsund in the Arctic part of Norway in the 1970s, and mine tailings were discharged to the inner part of the fjord, Repparfjorden. Metal speciation analysis was used to assess the historical dispersion of metals as well as their potential bioavailability from the area of the mine tailing disposal. It was revealed that the dispersion of Ba, Cr, Ni, Pb and Zn from the mine tailings has been limited. Dispersion of Cu to the outer fjord has, however, occurred; the amounts released and dispersed from the mine tailing disposal area quantified to be 2.5-10 t, less than 5% of Cu in the original mine tailings. An estimated 80-390 t of Cu still remains in the disposal area from the surface to a depth of 16 cm. Metal partitioning showed that 56-95% of the Cu is bound in the potential bioavailable fractions (exchangeable, reducible and oxidisable) of the sediments, totalling approximately 70-340 t, with potential for continuous release to the pore water and re-precipitation in over- and underlying sediments. Surface sediments in the deposit area were affected by elevated Cu concentrations just above the probable effect level according to the Norwegian sediment quality criteria, with 50-80% Cu bound in the exchangeable, reducible and oxidisable fractions, potentially available for release to the water column and/or for uptake in benthic organisms.

Simultaneous electrodialytic removal of PAH, PCB, TBT and heavy metals from sediments.

Contaminated sediments are remediated in order to protect human health and the environment, with the additional benefit of using the treated sediments for other activities. Common for many polluted sediments is the contamination with several different pollutants, making remediation challenging with the need of different remedial actions for each pollutant. In this study, electrodialytic remediation (EDR) of sediments was found effective for simultaneous removal of heavy metals and organic pollutants for sediments from Arctic regions - Sisimiut in Greenland and Hammerfest in Norway. The influence of sediment properties and experimental settings on the remediation process was studied by employing multivariate analysis. The importance of the variables studied varied with the pollutant and based on these results it was possible to assess removal processes for the different pollutants. Desorption was found to be important for the removal of heavy metals and TBT, while photolysis was significant for removal of PAH, PCB and TBT. In addition, dechlorination was found to be important for the removal of PCB. The highest removal efficiencies were found for heavy metals, TBT and PCB (>40%) and lower removal efficiencies for PAH (<35%).

Degradation of oil products in a soil from a Russian Barents hot-spot during electrodialytic remediation.

A highly oil-polluted soil from Krasnoe in North-West Russia was used to investigate the degradation of organic pollutants during electrodialytic remediation. Removal efficiencies were up to 70 % for total hydrocarbons (THC) and up to 65 % for polyaromatic hydrocarbons (PAH). Relatively more of the lighter PAH compounds and THC fractions were degraded. A principal component analysis (PCA) revealed a difference in the distribution of PAH compounds after the remediation. The observed clustering of experiments in the PCA scores plot was assessed to be related to the stirring rate. Multivariate analysis of the experimental settings and final concentrations in the 12 experiments revealed that the stirring rate of the soil suspension was by far the most important parameter for the remediation for both THC and PAH. Light was the second most important variable for PAH and seems to influence degradation. The experimental variables current density and remediation time did not significantly influence the degradation of the organic pollutants. Despite current density not influencing the remediation, there is potential for degrading organic pollutants during electrodialytic removal of heavy metals, as long as a stirred set-up is applied. Depending on remediation objectives, further optimisation may be needed in order to develop efficient remediation strategies.

Applying multivariate analysis as decision tool for evaluating sediment-specific remediation strategies.

Multivariate methodology was employed for finding optimum remediation conditions for electrodialytic remediation of harbour sediment from an Arctic location in Norway. The parts of the experimental domain in which both sediment- and technology-specific remediation objectives were met were identified. Objectives targeted were removal of the sediment-specific pollutants Cu and Pb, while minimising the effect on the sediment matrix by limiting the removal of naturally occurring metals while maintaining low energy consumption. Two different cell designs for electrochemical remediation were tested and final concentrations of Cu and Pb were below background levels in large parts of the experimental domain when operating at low current densities (<0.12 mA/cm(2)). However, energy consumption, remediation times and the effect on naturally occurring metals were different for the 2- and 3-compartment cells.

Screening of variable importance for optimizing electrodialytic remediation of heavy metals from polluted harbour sediments.

Using multivariate design and modelling, the optimal conditions for electrodialytic remediation (EDR) of heavy metals were determined for polluted harbour sediments from Hammerfest harbour located in the geographic Arctic region of Norway. The comparative importance of the variables, current density, remediation time, light/no light, the liquid-solid ratio and stirring rate of the sediment suspension, was determined in 15 laboratory-scale EDR experiments by projection to latent structures (PLS). The relation between the X matrix (experimental variables) and the Y matrix (removal efficiencies) was computed and variable importance in the projection was used to assess the influence of the experimental variables. Current density and remediation time proved to have the highest influence on the remediation of the heavy metals Cr, Cu, Ni, Pb and Zn in the studied experimental domain. In addition, it was shown that excluding the acidification time improved the PLS model, indicating the importance of applying a limited experimental domain that covers the removal phases of each heavy metal in the specific sediment. Based on PLS modelling, the optimal conditions for remediating the Hammerfest sediment were determined; operating in the experimental domain of 0.5-0.8 mA/cm(2) and a remediation time after acidification of 450-570 h met acceptable levels according to Norwegian sediment quality guidelines.