Please cite this article as: J. Almeida, R. Craveiro, P. Faria, et al., Electrodialytic removal of tungsten and arsenic from secondary mine resources -Deep eutectic solvents enhancement, Science of the Total Environment (2018), https://doi..pt (A. B. Ribeiro) HIGHLIGHTS  Proof of concept in coupling DES + ED for As and W removal from mining residues  Best As and W extraction achieved with ChCl:MA (1:2) and ChCl:OA (1:1)  DES and electrical current use improved extraction for As (22 %) and W (11 %)  From total As and W extracted, electromigration achieved 82 % for As and 77 % for W Journal Pre-proof J o u r n a l P r e -p r o o f 2 ABSTRACTTungsten is a critical raw material for European and U.S. economies. Tungsten mine residues, usually considered an environmental burden due to e.g. arsenic content, are also secondary tungsten resources. The electrodialytic (ED) process and deep eutectic solvents (DES) have been successfully and independently applied for the extraction of metals from different complex environmental matrices. In this study a proof of concept demonstrates that coupling DES in a two-compartment ED set-up enhances the removal and separation of arsenic and tungsten from Panasqueira mine secondary resources. Choline chloride with malonic acid (1:2), and choline chloride with oxalic acid (1:1) were the DES that in batch extracted the average maximum contents of arsenic (16 %) and tungsten (9 %) from the residues. However, when ED was operated at a current intensity of 100 mA for 4 days, the extraction yields increased 22 % for arsenic and 11 % for tungsten, comparing to the tests with no current.From the total arsenic and tungsten extracted, 82 % and 77 % respectively were successfully removed from the matrix compartment, as they electromigrated to the anolyte compartment, from where these elements can be further separated. This achievement potentiates circular economy, as the final treated residue could be incorporated in construction materials production, mitigating current environmental problems in both mining and construction sectors. KeywordsCritical raw material, harmful compound, secondary resource, electro-based technology, choline chloride/malonic acid, choline chloride/oxalic acid Abbreviations Journal Pre-proof J o u r n a l P r e -p r o o f 3 "W" -Tungsten; "As" -Arsenic; "ED" -Electrodialytic; "DES" -Deep Eutectic Solvents; "ChCl:OA" -Choline Chloride/ Oxalic Acid; "ChCl:MA" -Choline Chloride/ Malonic Acid; "ChCl:LA" -Choline Chloride/ Latic Acid; "PA:U" -Propionic Acid/ Urea the individuals exposed (Coelho et al., 2014).The electrodialytic (ED) process is a treatment technology, that has been studied over the last three decades, to remove heavy metals from several environmental matrices: (1) soil (Ribeiro Journal Pre-proof W that electromigrated into the anode compartment (anolyte) were determined according to Eq. 3 (in section 2.3.3) and presented in Table 6. The high migration percentages were obtained with ChCl:MA at 100 mA for As (82.3 ± 1.3 %) and 50 mA for W (76.5 ± 8.6 %).This ...

Highlights• Self-produced H2 from electrodialytic treatment of environmental matrices collected • Collected H2 average purity (% mol/mol) of ≈ 98%• A fuel cell used to produce electricity from the self-produced H2 (~1 V)• Experimental self-generated energy promotes savings on electroremediation (≈ 7%) AbstractElectrodialytic technologies are clean-up processes based on the application of a low-level electrical current to produce electrolysis reactions and the consequent electrochemically-induced transport of contaminants. These treatments inherently produce electrolytic hydrogen, an energy carrier, at the cathode compartment, in addition to other cathode reactions. However, exploring this by-product for self-energy generation in electroremediation has never been researched. In this work we present the study of hydrogen production during the electrodialytic treatment of three different environmental matrices (briny water, effluent and mine tailings), at two current intensities (50 and 100 mA). In all cases, hydrogen gas was produced with purities between 73% to 98%, decreasing the electrical costs of the electrodialytic treatment up to ≈ 7%. A protonexchange membrane fuel cell was used to evaluate the possibility to generate electrical energy from the hydrogen production at the cathode, showing a stable output (~1 V) and demonstrating the proof of concept of the process.

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Electrodialytic technologies are defined as treatment processes that promote the removal/recovery of substances in a matrix, based on the application of low-level current intensities. Additionally, along these processes hydrogen is self-generated, allowing them to operationally produce clean energy. This energy carrier is produced due to electrolysis reactions occurring at the cathode end of the electrodialytic reactor, when using inert electrodes. Herein, hydrogen production during the electrodialytic treatment of sewage sludge and mining residues suspensions (coupled with effluent or sewage sludge), at 50 and 100 mA, was assessed. During the electrodialytic treatment of sewage sludge, hydrogen purity production achieved 33%. When effluent or sewage sludge were used as enhancements in mining residues suspensions, hydrogen purity reached 71% and 34%, respectively. Furthermore, a proton-exchange membrane fuel cell was connected to the cathode compartment of the electrodialytic reactor. The electrical energy generated from self-hydrogen produced at 100 mA achieved ≈1 V in all performed experiments. Simultaneously, critical raw materials extraction, namely phosphorus and tungsten, was evaluated. When the process was applied to mining residue suspensions combined with sewage sludge, the highest extraction ratio of phosphorus (71%) and tungsten (62%) was observed.

Electroremediation and deep eutectic solvents are well-documented clean-up processes for metals extraction from solid matrices. Depending on the purpose, these treatments may generate a residue free of pollutants and critical raw materials. Several studies were conducted to re-insert treated secondary resources in building materials. However, there is a research gap in the improvement of reactive properties of these secondary resources. In addition, there is a lack of pozzolans that can optimize cementitious materials. This study investigates the pozzolanic reactivity of tungsten mining residues after receiving electrodialytic treatment in the presence of natural deep eutectic solvents. In all cases, thermal treatment after electroremediation potentiated the pozzolanic reactivity of tungsten mining residues, between 64% to 87%. The introduction of these pozzolanic resources in cementitious-based materials may increase their performance, enlarge the range of applications in the construction industry, reduce the environmental impact, and contribute to a circular economy. Keywords: electro-based technology, tungsten mining waste, construction material, pozzolanicity.