Prerpints.org logo
Preprint
Article

Simultaneously Recovery of Thorium and Tungsten by Hybrid Electrolysis–Nanofiltration Processes

Altmetrics

Downloads

134

Views

132

Comments

0

A peer-reviewed article of this preprint also exists.

Submitted:

17 December 2023

Posted:

19 December 2023

You are already at the latest version

Alerts
Abstract
The recovery and recycling of metals that generate toxic ions in the environment is of particular importance, especially when these are tungsten and, in particular, thorium. The radioactive element thorium has unexpectedly accessible domestic applications (filaments of light bulbs and electronic tubes, welding electrodes, working alloys containing aluminum and magnesium), which lead to its appearance in electrical and electronic waste from municipal waste management platforms. The current paper proposes the simultaneous recovery of waste containing tungsten and thorium from welding electrodes. The simultaneous recovery is done by applying a hybrid membrane electrolysis technology coupled with nanofiltration. An electrolysis cell with sulphonated polyether–ether–ketone membranes (sPEEK) and a nanofiltration module with chitosan-polypropylene membranes (C–PHF–M) are used to carry out the hybrid process. The analysis of welding electrodes led to a composition of: W (tungsten) 89.4%; Th 7.1%; O2 2.5% and Al 1.1%. Thus, the parameters of the electrolysis process were chosen according to the speciation of the three metals suggested by the superimposed Pourbaix diagrams. At a constant potential of 20.0 V and an electrolysis current of 1.0 A, the pH is varied and the possible composition of the solution in the anodic workspace is analyzed. Favorable conditions for both electrolysis and nanofiltration were obtained at pH from 6 to 9, when the soluble tungstate ion, the aluminum hydroxide and solid thorium dioxide were formed. Through a first nanofiltration, the tungstate ion is obtained in the permeate, and thorium dioxide and aluminum hydroxide in the concentrate. By adding a pH 13 solution over the two precipitates, the aluminum is solubilized as sodium aluminate which will be found after the second nanofiltration in permeate, the thorium dioxide remaining integrally (within an error of ±0.1ppm) on the C–PHF–M membrane.
Keywords: 
Subject: Chemistry and Materials Science  -   Applied Chemistry

1. Introduction

The performances proven by membrane separation processes in various ecological and greening technologies constitute the important arguments for their involvement in the recovery and recycling of various urban wastes, especially those that generate toxic products [1,2,3,4]. Thus, in order to develop a technology for the separation and valorization of some metallic elements from specific urban waste, baromembrane processes can be approached, when the elements are in colloidal (nanometric) or suspension (micrometric) state, or processes with electric and/or concentration gradient, if the metals are found in the forms of ions [5,6,7,8].
The development of new technologies for the recovery of metals from urban waste requires solid arguments regarding the value of the separated metals or the negative impact on the environment if such metals are not separated and recycled [9,10,11]. A special case, reported recently [12], shows that metals such thorium or tungsten that come from electric and electronic equipment or materials used in construction (refractory bricks, welding electrodes) can appear in the urban waste [13,14]. The recovery of both metals is of technical importance because they have high practical utility (Figure 1) [11,15], but also because they have a particularly undesirable impact on the environment, especially thorium [16].
Both thorium and tungsten are obtained from ores by established processes such as: hydrometallurgical [17,18,19], separation [20,21,22,23,24], treatment of alloys [25]. Moreover, tungsten has been the focus of process engineers for its recovery from various residues through electrolytic processes, acid or basic solubilization processes [26,27,28,29,30]. On the other hand, thorium has been the focus of researchers in the field of membranes, for concentration, separation or purification, especially when it comes from ores in which uranium or rare earths can also be found [31,32,33,34,35,36,37,38,39].
This work studies the simultaneous recovery of tungsten and thorium, from waste coming from the welding industry with W–Th electrodes, through a hybrid technology of membrane electrolysis and nanofiltration. The membrane used in the electrolysis cell is based on sulfonated polyether–ether–ketone (sPEEK–M), and the membrane used for nanofiltration is chitosan deposited on propylene hollow fiber (C–PHF–M).

2. Materials and Methods

2.1. Reagents and Materials

All reagents and organic compounds used in the presented work were of analytical grade. Th(NO3)4·5H2O, KSCN, NaCl, NaOH pellets, Ca(OH)2, HNO3 (68%), H2SO4 (96%), HCl 35% suprapure and NH4OH 25% (analytical grade) were purchased from Merck KGaA Darmstadt, Germany.
Aluminon, Torin and glacial acetic acid (analytical grade, Sigma-Aldrich Chemie GmbH, Steinheim, Germany) were used in the studies.
Materials: polyether–ether–ketone (PEEK), Mw=30,000 g/mol, ρ=1.24 g/cm3 powder, average particle size 80 micron; chitosan, high molecular weight, (Merck KGaA, Darmstadt, Germany), red band welding electrodes (ProConstruct Distribution SRL, Balotești, Romania).
The hollow fibers polypropylene support membranes (PHF–M) were provided by GOST Ltd., Perugia, Italy [40,41].
The purified water characterized by 18.2 μS/cm conductivity was obtained with a RO Millipore system (MilliQ® Direct 8 RO Water Purification System, Merck, Darmstadt, Germany).
Table 1. The characteristics of the used polymer compounds.
Table 1. The characteristics of the used polymer compounds.
Preprints 93613 i001

2.2. Procedures and methods

2.2.1. Preparation of sPEEK–M membranes from PEEK solution in sulfuric acid

In a conical glass of 250 cm3, introduce 30 mL H2SO4 of concentration 96%, after which 2.5 g polymer is gradually added, stirring continuously with a glass rod to avoid agglomeration of the polymer. After about 2 hours of stirring, the glass is covered with a thin layer of parafilm and is kept without stirring for up to 48 hours from the moment of covering the solution, to favor the complete dissolution of the polymer in acid. In this time interval the sulfonation of the polymer occurs. A clear brick-colored solution with a concentration of approx. 4.5% PEEK is obtained. In order to obtain the actual support membranes, the polymer solution is introduced for gelation and washing in 300 mL of water, in portions of 100 mL each, separated and applied on a spectral glass plate with the help of a chromatographic scraper knife having slit opening between 0.5 and 0.6 mm. The pellicle is dried at 60 °C for 2 hours, is washed well with distilled water and placed in a Ca(OH)2 bath where it is kept for 48 hours, then removed and washed repeatedly in ultrapure water (Figure 2). The removal of the sulfate ion was followed by the identification reaction with a 2 mol/L barium chloride solution. The main macroscopic observations, which can be highlighted, in the case of incomplete washing are [42,43]:
  • undulation of the membrane;
  • the occurrence of defects (microcracks, holes);
  • local re-solubilizations (transparencies, gelations).
sPEEK–M membranes were characterized by scanning electron microscopy (SEM) and Energy Dispersive X–ray Spectroscopy (EDAX).

2.2.2. Preparation of Chitosan–Polypropylene Hollow Fibers membranes (C–PHF–M)

In a round-bottomed flask, a chitosan solution is prepared under continuous magnetic stirring by dissolving 10 g of chitosan powder in 990 mL of 10% acetic acid solution. After mixing for about two hours, a yellowish-white solution is obtained, which is the feed phase of the ultrafiltration membrane module (Figure 3) equipped with polypropylene hollow fiber membrane. With the help of a recirculation pump, a pressure of 6 bars is ensured in the space between the body of the module and the outside of the membranes, in a closed circuit. The permeate, which is made up of the acetic acid solution, is collected inside the membranes. A layer of chitosan is deposited on the polypropylene hollow fiber membrane [44,45].
The C–PHF–M membranes were characterized by scanning electron microscopy (SEM), Energy Dispersive X–ray Spectroscopy (EDAX) and from the perspective of performance in the nanofiltration process.

2.2.3. Electrolysis of tungsten and thorium-based electrodes in acidic aqueous solution

250 mL volume of each solution was introduced, in turn, into an electrolysis cell (Figure 4) of the MASTECH HY3005D-3 potentiostat equipment provided with three electrodes: tungsten welding electrode as anode, platinum wire as cathode, and a reference electrode. The potentiostat was operated at a potential of +20.0 V and an electrolysis current of 1.0 A. The pH in the anodic space was imposed at 0, 2, 4, 6, 8 and 10, 12, and in the cathodic space at pH 13, using hydrochloric acid and sodium hydroxide, respectively. The experimental procedure took place at room temperature. After six hours of work at the cell base, both solutions (anodic and cathodic) were collected, and subjected to nanofiltration in order to recover of thorium as thorium dioxide and tungsten as tungstate ion. The chloride anion concentration was followed up with a combined selective chloride electrode (HI 4107, Hanna Instruments Ltd., Leighton Buzzard, UK) and a multiparameter system (HI 5522, Hanna Instruments Ltd., Leighton Buzzard, UK).
For the recovery of thorium dioxide, both the solution from the anodic space and the one from the cathode space are nanofiltered, both of which constitute the feed solutions (FS) of the membrane mode in the installation diagram in Figure 3. This time the module is equipped with a chitosan–polypropylene nanofiltration membrane. A separate module is used for each of the two feeding solutions, so that the components in the anodic and cathodic space can be identified.

2.3. Equipment

The membranes microscopy studies, SEM and HFSEM, were performed on a Hitachi S4500 system (Hitachi High-Technologies Europe GmbH, Germany) [46].
The UV-Vis analyses of the solutions were done on a Spectrometer CamSpec M550 (Spectronic CamSpec Ltd., Leeds, UK) [47].
The UV-Vis studies on the nanoparticles samples composition were performed on dual-beam UV equipment – Varian Cary 50 (Agilent Technologies Inc., Santa Clara, California, USA) at a resolution of 1 nm, spectral bandwidth 1.5 nm, and 300 nm/s scan rate. The UV-Vis spectra of the samples were recorded for a wavelength from 200 to 800 nm, at room temperature, using 10 mm quartz cells [48].
MASTECH HY3005D-3 (San Jose, CA, USA) is a working power supply which has an electrical output, continuously adjustable, between 0–30 V DC and 0–5 A.
The validation of the electrochemical processes was followed up with a PARSTAT 2273 Potentiostat (Princeton Applied Research, AMETEK Inc., USA). A glass cell with three electrodes setup was used [47,48].
The pH of the medium was followed up with a combined selective electrode (HI 4107, Hanna Instruments Ltd, UK) and a multiparameter system (HI5522, Hanna Instruments Ltd, Leighton Buzzard, UK) [49].
To assess and validate the content in metal ions, the atomic absorption spectrometer AAnalyst 400 AA Spectrometer (Perkin Elmer Inc., Waltham, Massachusetts, USA) with WinLab32–AA software (Perkin Elmer Inc., Waltham, Massachusetts, USA), with a single-element hollow-cathode lamp was used [50].
In the case of the present work, for the validation of the SEM and EDAX analyses, the tungsten electrodes samples subjected to the analysis were visualized with the help of the FESEM–FIB workstation (scanning electron microscope with field emission electron and focused beam of ions) model Auriga (Carl Zeiss SMT, Oberkochen, Germany), by means of the secondary electron/ion detector (SESI) in the sample chamber for the topography/morphology of the surface of the analyzed samples [51].
The verification of the chemical composition was carried out with the help of the EDS probe (energy dispersive spectrum for the characteristic X–ray) produced by Oxford Instruments, UK – energy dispersive spectrometer model X–MaxN with the Aztec acquisition and processing software integrated on the FESEM–FIB Auriga working station [52].
Secondary electron (SE) topography images were acquired and analyzed from the sample chamber with the Everhart Thornley SESI secondary electron detector with Faraday cup (SESI)/in-column annular SE in-lens detector, at a voltage acceleration of 5 kV for sample visualization and 10 kV for EDS spectroscopy, for the following types of analyzes carried out by the energy dispersive probe, on the surface of the analyzed sample:
  • semi-quantitative spot analyses located at certain intervals in the same micro-area, for the distribution of elements from the point of view of composition on the surface of the material as well as the variational verification of the composition of the investigated micro-area with the points from which the respective spectra were acquired;
  • elemental mapping analyses, i.e. obtaining spectral images – where the distribution of the elements on the surface swept by the electron beam is superimposed and possible compositional differences are highlighted – increasing the area of the present element or the appearance/disappearance of an analyzed and identified element.

3. Results and discussion

The interest in thorium is argued both because it is a material with huge potential for energy generation, including the nuclear energy [53] but also because, although it is radioactive, it is the object of some domestic applications that capitalize on the resistance to high temperatures of thorium dioxide (lamps, shields, crucibles, welding electrodes) [54] or special refractive index (lenses, glasses, high resolution opto-electronic apparatus) [12]. It becomes obvious that, with the use of these materials, but also because thorium has a natural abundance similar to that of lead [55] it will be found in urban industrial waste arriving in various forms on municipal integrated processing platforms [56].
In this paper, the possibility of recovering thorium from waste from the welding industry is studied based on a hybrid technology consisting of membrane electrolysis coupled with nanofiltration.

3.1. Characterization of the sPEEK–M membrane

Membranes of sulfonated polyether–ether–ketone (sPEEK) [42,43] or sPEEK polymer composites [44] are used in ultrafiltration, nanofiltration or reverse osmosis processes [42,45]. To carry out the electrolysis process of the tungsten electrodes, a specific sPEEK cation exchange membrane was prepared by forming it from sulfonated polyether–ether–ketone gel, drying in a vacuum oven at 60 °C for 2 hours and fixing in a bath with calcium hydroxide solution. Here it is kept for 48 hours, then it is removed and washed repeatedly in ultrapure water. Three samples of 1 cm2 each from the membrane obtained after fixing with calcium hydroxide and after keeping in ultrapure water are introduced in a Dewar vessel with liquid nitrogen, and fractured. After coating with a superficial layer (50 nm) of gold, the membrane obtained from the fixing bath with calcium hydroxide (Figure 5 and Figure 6) and the one obtained after washing and keeping in pure water (Figure 7 and Figure 8) are SEM and EDAX characterized.

3.2. Characterization of C–PHF–M membrane

The membranes based on chitosan have been frequently used to carry out membrane processes for the separation of ions and molecules [57,58,59,60], but in this work chitosan membranes are obtained on a polypropylene hollow fiber (C–PHF–M) support. Practically, the polypropylene hollow fiber (PHF) support, which has previously presented ultrafiltration performances [43,44,45], is transformed in nanofiltration composite membranes (C–PHF–M) by ultrafiltration of a chitosan solution to obtain in situ a selective layer of chitosan. Samples of 3 cm polypropylene hollow fiber membrane and chitosan-polypropylene hollow fiber membrane were fractured in liquid nitrogen and covered with a superficial layer (50 nm) of gold and characterized by scanning electron microscopy (SEM) (Figure 9 and Figure 10).
The support membrane (Figure 9a) with pores of 0.002–0.2 µm specific for micro and ultrafiltration is transformed after the coating with chitosan (Figure 9b) into the composite membrane (C–PHF–M) for nanofiltration with pores smaller than 0.002 µm.

3.3. Electrolysis of tungsten electrodes for the recovery of thorium and tungsten

The raw material required for the hybrid thorium recovery process is the remains and ends of the welding electrodes at high temperatures. These materials can be selectively collected on construction sites or industrial production halls and undergo membrane electrolysis followed by nanofiltration, when thorium is recovered as thorium dioxide and tungsten as a tungstate solution.

3.3.1. SEM and EDAX analysis of welding electrodes

The SEM and EDAX characterization of the raw material, the welding electrodes, used in this work highlighted the morphology, but also the surprising composition (Figure 11 and Figure 12). Macroscopically, the welding electrode has a relatively smooth surface (Figure 11a), but with high probability, tungsten filaments appear in the section (Figure 11b), joined by thorium dioxide cubes (Figure 11c,d).
In order to confirm the assessments made by analyzing the electron microscopy, the Energy Dispersive X–ray Spectroscopy (EDAX) analysis was performed in the electrode fracture (Figure 12). Surprisingly (Figure 12a,b), apart from tungsten, thorium and oxygen appear in the fracture, as well as a measurable amount of aluminum.
The elemental distribution (Figure 12c,d,f) confirms the assumption that tungsten forms filaments and plates that are joined with nanometric cubes of thorium dioxide. Aluminum, which is distributed over the entire fracture surface of the electrode (Figure 12f), mainly covers the tungsten filaments and plates, suggesting that it is an element with a binding role. The quantitative composition (Figure 12b) is: W 89.4%; thorium 7.1%; oxygen 2.5% and aluminum 1.0%.

3.3.2. Membrane electrolysis of welding electrodes

In order to avoid the loss of thorium as thorium dioxide of nanometric dimensions from the membrane electrolysis process, this was coupled with the nanofiltration of both the solution from the cathode space and the solution in the anode space (Figure 13).
The probable reactions in the anodic space (1–4) and cathodic space (5,6) follow:
ANODE
W + 12H2O ⇌ WO42− + 8H3O+ + 6e−
Th + 6H2O ⇌ ThO2 + 4H3O+ + 4e−
Al ⇌ Al3+ + 3e−
3H2O ⇌ 2H3O+ + 2e− + 1/2 O2
CATHODE
2H2O + 2e− ⇌ H2 + 2HO−
2H3O+ + 2e− ⇌ H2 + 2H2O
However, the choice of the potential of the working electrode (anode) as well as the pH in the anodic space largely determine the speciation of each component element of the welding electrodes considered in this study: tungsten, thorium and aluminum (Table 2 and Figure 14). Thus, superimposing the Pourbaix diagrams [61,62,63] of the three elements (Figure 14) it is possible to specify, at constant potential, the chemical species for each pH in the anodic space (Table 2).
At the potential of 20.0 V and pH=0 in the anodic space, and pH=13 in the cathodic space, following electrolysis, tungsten will be found in the form of tungstic acid WO3·H2O(s), thorium as Th4+(aq) and aluminum as Al3+(aq) (Figure 14). The two cations, Th4+ and Al3+, will migrate to the cathode, and the tungstic acid will remain in the anodic space. Tungsten in the anode space will be recovered by nanofiltration through the module attached to this space. Thorium will be immobilized as a solid in cathode space in the form of thorium dioxide (ThO2), and the aluminum ion will pass into sodium aluminate (AlO2⁻), soluble. By nanofiltration [64,65,66] of the solution from the cathode space through the module attached to this space, thorium dioxide will be retained on the membrane (concentrate) and aluminate ion as permeate.
At the same potential value of the anodic space, but at pH between 1 and 3.5, the chemical species of the three elements will be: tungsten as WO3 H2O(s), thorium as ThO2(s), and aluminum as Al3+(aq). In this case, the solution in the anodic space is nanofiltered after treatment with ammonia solution, when ammonium tungstate will pass into the permeate, and thorium dioxide remains on the membrane, in concentrate. The solution in the cathode space will contain sodium aluminate, which, being in solution, does not require nanofiltration. However, for the safety of the hybrid process, the solution from the cathode space also undergoes nanofiltration, when we will find sodium aluminate in the permeate, and any other solid impurity is retained on the membrane.
If the anode has a potential of 20.0 V and the pH in anodic space stays between 3.5 and 6, the chemical species obtained for the three elements are: tungsten as WO3·H2O(s), thorium as ThO2(s), and aluminum as Al(OH)3 (s). In this case, the solutions from the anodic and cathodic spaces obtained after electrolysis are mixed together and subjected to nanofiltration, when thorium dioxide is retained in the concentrate, and the tungstate and aluminate ions are found in the permeate. To separate the two anions, the pH is adjusted to 5 when the aluminum hydroxide precipitates, which is retained by nanofiltration in the second module of the installation.
If the anode has a potential of 20.0 V, and the pH in the anodic space is kept between 6 and 12, then the chemical species obtained for the three elements are: tungsten as WO42⁻(aq), thorium as ThO2(s), and aluminum as Al(OH)3(s).
In this case, the nanofiltration of the anodic space takes place in two stages:
  • the first stage at the pH (6–12) from this space when a permeate, containing the soluble tungstate ion, and a concentrate containing thorium dioxide and aluminum hydroxide, are obtained;
  • the second stage when the concentrate is mixed with the pH 13 solution from the cathodic space, when the aluminum is solubilized as aluminate, passing into the permeate, and thorium dioxide remains in the concentrate.
If the anode has a potential of 20.0 V, and the pH in the anodic space is above 12, then the chemical species obtained for the three elements are: tungsten as WO42⁻(aq), thorium as ThO2(s) and aluminum as AlO2⁻(aq).
In this case the nanofiltration of anodic space takes place in two stages:
  • the first stage at the pH in this space (above 12), when a permeate containing soluble tungstate and aluminate ions is obtained, and a concentrate containing thorium dioxide and aluminum hydroxide;
  • the second stage when the permeate is brought to a pH between 6 and 9, when the aluminum precipitates as hydroxide and is separated from the tungstate ion by nanofiltration, in the second module.
Tungsten recovery after electrolysis means nanofiltration of the solution in order to purify it, because it is obtained as tungstate either by neutralizing tungstic acid or directly, at a pH higher than 6.
The recovery of thorium requires additional attention, firstly because thorium is a radioactive element, and secondly because being obtained as a thorium dioxide dispersion, it will have to be nanofiltered under optimal conditions so as not to be lost in the permeate.
The results obtained during the nanofiltration of nanodispersions, containing thorium dioxide, depending on the pH and at 8 bar working pressure, show flows between 8.5 and 14 L/m2·h and a removal of thorium reaching below 1 ppm in the permeate, at pH higher than 4, with a determination error of ±0.1 ppm (Figure 15). This concentration of thorium is below the natural background in various locations in the world, especially those where rare earths are also found [67,68,69,70,71,72,73].
The higher concentration of thorium in the permeate at pH below 3 is due to the existence of thorium dioxide in dispersion and as Th4+ ion, which is partially retained by the C–PHF–M membrane, which is in cationic form (+H3N–R). At pH above 3, the concentration of thorium in the permeate drops sharply to 0.1 ppm, which shows that thorium is found almost entirely as thorium dioxide ThO2, and any thorium ions are retained by the membrane in cationic form.
What is interesting is the evolution of the permeate concentration during nanofiltration depending on the morphological nature of thorium dioxide (Figure 14). Thus, over the entire pH range, the concentration of thorium in the permeate for thorium dioxide obtained from electrolysis (Figure 15 – green circles) is lower than that of the same element precipitated from Th(NO3)4 solution (Figure 15 – blue squares).
There can be two reasons for this behavior:
  • the existence of thorium in ionic form and as thorium dioxide of different morphology (Figure 16);
  • ionic charge of the nanofiltration membrane.
The first argument would probably have a greater influence on the thorium concentration in the permeate. Thus, the thorium dioxide obtained by the precipitation of Th(NO3)4 in amorphous form (Figure 16a) is retained to a lesser extent on the membrane, than the crystalline form of thorium dioxide released from the interstices of the tungsten wires (plates), through electrolysis of the welding electrode (Figure 16b, Figure 11c,d).
The permeate flow performances of nanofiltration of thorium dioxide nanodispersions, from the complex system obtained by electrolysis, depending on pH and working pressure, are presented in Figure 17.
Over the entire pH range (Figure 17) the fluxes increase with increasing operating pressure from 6 to 10 bar. Each time, from pH 0 to 2, the values of the permeate flows increase up to about 30 L/m2h which is obtained at 10 bars. Starting at pH 4, the flow values increase slightly until pH 10, after which there is a slight decrease.
The recorded flux results are consistent with the speciation of thorium and of the C–PHF–M membrane. At low pH, below 2, thorium is found as ion, and after pH 2 thorium dioxide is formed. It is likely that the size of thorium dioxide nanoparticles increases with the pH, which improves filterability. However, after pH 8 the viscosity of the alkaline solution causes a slight decrease in the permeate flow.
From the point of view of the nanofiltration results, the pH in the electrolysis installation can be up to 2, when we have thorium ions and tungstic acid in the solution, or between 8 and 10 when we have thorium dioxide and tungstate ions in the solution.
The presence of aluminum in the system, even in traces, could impose electrolysis with the anodic space at strongly alkaline pH when aluminum is found as aluminate, tungsten as tungstate, both in solution, and thorium as insoluble precipitated of thorium dioxide. In the latter case, the separation of the anodic and cathodic spaces through the sPEEK membrane becomes unnecessary, the recovery of thorium dioxide being done through a first nanofiltration. The separation of aluminum is done by the second nanofiltration, after adjusting the pH between 5 and 9 when the aluminum will be retained on the membrane in the form of insoluble aluminum hydroxide.

4. Conclusions

The recovery and recycling of metals that generate toxic ions in the environment is of particular importance, especially when these are tungsten, aluminum and, above all, thorium.
This paper presents the separation and recovery of tungsten, thorium and aluminum from welding electrodes at high temperature, in a hybrid installation of membrane electrolysis and nanofiltration. The membranes used are made from sPEEK for the separation of anodic and cathodic spaces in the electrolysis cell, and from chitosan deposited on polypropylene hollow fiber membrane (C–PHF–M) in the nanofiltration module.
Through electrolysis at 20.0 V and variable pH, the speciation of the three studied elements could be separated. Favorable conditions for both electrolysis and nanofiltration were obtained at pH from 6 to 9 when the soluble tungstate ion, solid aluminum hydroxide and solid thorium dioxide were formed. Through a first nanofiltration, the tungstate ion is obtained in the permeate, and thorium dioxide and aluminum hydroxide in the concentrate. By adding a pH 13 solution over the two precipitates, aluminum is solubilized as soluble sodium aluminate, which after the second nanofiltration will be found in the permeate, the thorium dioxide remaining integral (within an error of ±0.1 ppm) on the C–PHF–M membrane.
Variants of the operation procedure of the hybrid electrolysis and nanofiltration installation were suggested by superimposing the Pourbaix diagrams of tungsten, thorium and aluminum.
The increase of the working potential above the value of 20.0 V brings only the increase of molecular oxygen in the anodic space as a chemical speciation modification.

Author Contributions

Conceptualization, G.T.M., G.N., and V.-A.G.; methodology, A.C.N., A.R.G. and D.I.P.; validation, D.I.P., V.E.M., and A.R.G.; formal analysis, A.C.N., A.R.G. and D.I.P.; investigation, G.T.M., G.N., V.E.M., P.C.A., V.-A.G.; A.C.N., A.R.G. and D.I.P.; resources, G.T.M., G.N., D.I.P. and, P.C.A.; data curation, A.C.N., A.R.G., V.-A.G. writing—original draft preparation, G.T.M., G.N., A.C.N. and, V.-A.G.; writing—review and editing, G.T.M., G.N., and V.-A.G. All authors have read and agreed to the published version of the manuscript.

Funding

Part of this research was conducted under Program 1 – Development of the national research and development system, Subprogram 1.1. Institutional performance – Projects to finance excellence in RDI, Contract No. 19PFE/12.30.2021, funded by the Ministry of Research, Innovation and Digitization (for the work of Geani Teodor Man and Diana Ionela Popescu (Stegarus)). The partial financial support was provided by the Ministry of Research, Innovation and Digitization through contract PN 23140201/42N–2023 (for the work of Virgil Emanuel Marinescu). The partial financial support was provided by the Ministry of Research, Innovation and Digitization through contract PN 21 02 03 (for the work of Paul Constantin Albu).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The authors gratefully acknowledge the valuable help and friendly assistance of Eng. Roxana Truşcă for performing the membrane scanning microscopy analysis.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Deng, P.; Cheng, L.; Li, A.; Zeng, Z.; Liao, C. Recent Advances in the Utilization of Tungsten Residue: A Mini Review of China. Metals 2023, 13, 1481. [Google Scholar] [CrossRef]
  2. Huang. M.; Hu, K.; Li, X.; Wang, Y.; Ouyang, J, Zhou, L.; Liu, Z. Mineralogical Properties of a Refractory Tantalum-Niobium Slag and the Effect of Roasting on the Leaching of Uranium-Thorium. Toxics 2022; 10(8), 469. [CrossRef]
  3. Han, Z.; Golev, A.; Edraki, M. A Review of Tungsten Resources and Potential Extraction from Mine Waste. Minerals 2021, 11, 701. [Google Scholar] [CrossRef]
  4. Mahmoud, A.E.D.; Mostafa, E. Nanofiltration Membranes for the Removal of Heavy Metals from Aqueous Solutions: Preparations and Applications. Membranes 2023, 13, 789. [Google Scholar] [CrossRef] [PubMed]
  5. Abdusamadzoda, D.; Abdushukurov, D.A.; Duliu, O.G.; Zinicovscaia, I. Assessment of the Toxic Metals Pollution of Soil and Sediment in Zarafshon Valley, Northwest Tajikistan (Part II). Toxics 2020, 8, 113. [Google Scholar] [CrossRef] [PubMed]
  6. Safiulina, A.M.; Lizunov, A.V.; Semenov, A.A.; Baulin, D.V.; Baulin, V.E.; Tsivadze, A.Y.; Aksenov, S.M.; Tananaev, I.G. Recovery of Uranium, Thorium, and Other Rare Metals from Eudialyte Concentrate by a Binary Extractant Based on 1,5-bis[2-(hydroxyethoxyphosphoryl)-4-ethylphenoxy]-3-oxapentane and Methyl Trioctylammonium Nitrate. Minerals 2022, 12, 1469. [Google Scholar] [CrossRef]
  7. Ushakov, S.V.; Hong, Q.-J.; Gilbert, D.A.; Navrotsky, A.; Walle, A.v.d. Thorium and Rare Earth Monoxides and Related Phases. Materials 2023, 16, 1350. [Google Scholar] [CrossRef] [PubMed]
  8. Kukkonen, E.; Virtanen, E.J.; Moilanen, J.O. α-Aminophosphonates, -Phosphinates, and -Phosphine Oxides as Extraction and Precipitation Agents for Rare Earth Metals, Thorium, and Uranium: A Review. Molecules 2022, 27, 3465. [Google Scholar] [CrossRef] [PubMed]
  9. Arabi, H.R.; Milani, S.A.; Abolghasemi, H; Zahakifar, F. Recovery and transport of thorium (IV) through polymer inclusion membrane with D2EHPA from nitric acid solutions. Journal of Radioanalytical and Nuclear Chemistry, 2021, 327, pp.653–665. [CrossRef]
  10. Houessionon, M.G.K.; Ouendo, E.-M.D.; Bouland, C.; Takyi, S.A.; Kedote, N.M.; Fayomi, B.; Fobil, J.N.; Basu, N. Environmental Heavy Metal Contamination from Electronic Waste (E-Waste) Recycling Activities Worldwide: A Systematic Review from 2005 to 2017. Int. J. Environ. Res. Public Health 2021, 18, 3517. [Google Scholar] [CrossRef]
  11. Lu, J.; He, K.; Wang, Y.; Chen, G.; Weng, H.; Lin, M. An Effective Process for the Separation of U(VI), Th(IV) from Rare Earth Elements by Using Ionic Liquid Cyphos IL 104. Chin. Chem. Lett. 2022, 33, 3422–3428. [Google Scholar] [CrossRef]
  12. Man, G.T.; Albu, P.C.; Nechifor, A.C.; Grosu, A.R.; Tanczos, S.-K.; Grosu, V.-A.; Ioan, M.-R.; Nechifor, G. Thorium Removal, Recovery and Recycling: A Membrane Challenge for Urban Mining. Membranes 2023, 13, 765. [Google Scholar] [CrossRef]
  13. Zhang, H.; Gao, Y. Polymeric Materials for Rare Earth Elements Recovery. Gels 2023, 9, 775. [Google Scholar] [CrossRef]
  14. Pathapati, S.V.S.H.; Free, M.L.; Sarswat, P.K. A Comparative Study on Recent Developments for Individual Rare Earth Elements Separation. Processes 2023, 11, 2070. [Google Scholar] [CrossRef]
  15. Saleh, T.A.; Sarı, A.; Tuzen, M. Development and characterization of bentonite-gum arabic composite as novel highly-efficient adsorbent to remove thorium ions from aqueous media. Cellulose 2021, 28, 10321–10333. [Google Scholar] [CrossRef]
  16. Ault, T.; Krahn, S.; Croff, A. Comparing the environmental impacts of uranium- and thorium-based fuel cycles with different recycle options. Prog. Nucl. Energy 2017, 100, 114–134. [Google Scholar] [CrossRef]
  17. Elbshary, R.E.; Gouda, A.A.; El Sheikh, R.; Alqahtani, M.S.; Hanfi, M.Y.; Atia, B.M.; Sakr, A.K.; Gado, M.A. Recovery of W(VI) from Wolframite Ore Using New Synthetic Schiff Base Derivative. Int. J. Mol. Sci. 2023, 24, 7423. [Google Scholar] [CrossRef] [PubMed]
  18. Xu, L.; Zhao, B. Extraction of Sodium Tungstate from Tungsten Ore by Pyrometallurgical Smelting. Metals 2023, 13, 312. [Google Scholar] [CrossRef]
  19. Kumar, R.; Kariminejad, A.; Antonov, M.; Goljandin, D.; Klimczyk, P.; Hussainova, I. Progress in Sustainable Recycling and Circular Economy of Tungsten Carbide Hard Metal Scraps for Industry 5.0 and Onwards. Sustainability 2023, 15, 12249. [Google Scholar] [CrossRef]
  20. Han, Z.; Golev, A.; Edraki, M. A Review of Tungsten Resources and Potential Extraction from Mine Waste. Minerals 2021, 11, 701. [Google Scholar] [CrossRef]
  21. Katiyar, P.K.; Randhawa, N.S. A comprehensive review on recycling methods for cemented tungsten carbide scraps highlighting the electrochemical techniques. Int. J. Refract. Met. Hard Mater. 2020, 90, 105251. [Google Scholar] [CrossRef]
  22. Das, S.K.; Nagesh, C.H.R.V.S.; Sreenivas, T.; Kundu, T.; Angadi, S.I. A treatise on occurrence, beneficiation and plant practices of tungsten-bearing ores. Powder Technology 2023, 429, 118938. [Google Scholar] [CrossRef]
  23. Sabbatovskii, K.G. The effect of the adsorption of multicharge cations on the selectivity of a nanofiltration membrane. Colloid J. 2003, 65, 237–243. [Google Scholar] [CrossRef]
  24. Kaptakov, V.O.; Milyutin, V.V.; Nekrasova, N.A.; Zelenin, P.G.; Kozlitin, E.A. Nanofiltration Extraction of Uranium and Thorium from Aqueous Solutions. Radiochemistry 2021, 63, 169–172. [Google Scholar] [CrossRef]
  25. Ilaiyaraja, P.; Deb, A.K.S.; Ponraju, D. Removal of uranium and thorium from aqueous solution by ultrafiltration (UF) and PAMAM dendrimer assisted ultrafiltration (DAUF). Journal of Radioanalytical and Nuclear Chemistry, 2015, 303, 441–450. [Google Scholar] [CrossRef]
  26. Katiyar, P.K.; Randhawa, N.S.; Hait, J.; Jana, R.K.; Singh, K.K.; Mankhand, T.R. Anodic Dissolution Behaviour of Tungsten Carbide Scraps in Ammoniacal Media. Adv. Mater. Res. 2014, 828, 11–20. [Google Scholar] [CrossRef]
  27. Kuntyi, O.I.; Ivashkin, V.R.; Yavorskii, V.T.; Zozulya, G.I. Electrochemical processing of WC-Ni pseudoalloys in sulfuric acid solutions to ammonium paratungstate and nickel(II) sulfate. Russ. J. Appl. Chem. 2007, 80, 1856–1859. [Google Scholar] [CrossRef]
  28. Ubaldini, A.; Cicconi, F.; Rizzo, A.; Salvi, S.; Cuzzola, V.; Gennerini, F.; Bruni, S.; Marghella, G.; Gessi, A.; Falsini, N. Preparation and Characterization of Isostructural Na2MoO4 and Na2WO4 and a Study of the Composition of Their Mixed System. Molecules 2023, 28, 6602. [Google Scholar] [CrossRef] [PubMed]
  29. Liu, H.; Liu, H.; Nie, C.; Zhang, J.; Steenari, B.M.; Ekberg, C. Comprehensive treatments of tungsten slags in China: A critical review. Journal of Environmental Management, 2020, 270, p.110927. [CrossRef]
  30. Cao, Y.; Guo, Q.; Sun, W.; Chelnokov, G.A. Efficient and Fast Removal of Aqueous Tungstate by an Iron-Based LDH Delaminated in L-Asparagine. Int. J. Environ. Res. Public Health 2022, 19, 7280. [Google Scholar] [CrossRef]
  31. Leal Filho, W.; Kotter, R.; Özuyar, P.G.; Abubakar, I.R.; Eustachio, J.H.P.P.; Matandirotya, N.R. Understanding Rare Earth Elements as Critical Raw Materials. Sustainability 2023, 15, 1919. [Google Scholar] [CrossRef]
  32. Aziman, E.S.; Mohd Salehuddin, A.H.J.; Ismail, A.F. Remediation of thorium (IV) from wastewater: Current status and way forward. Separation & Purification Reviews, 2021, 50(2), pp.177-202. [CrossRef]
  33. Gomaa, H.; Emran, M.Y.; Elsenety, M.M.; Abdel-Rahim, R.D.; Deng, Q.; Gadallah, M.I.; Saad, M.; ALMohiy, H.; Ezzeldien, M.; Seaf El-Nasr, T.A.; El-Gaby, M.S. Detection and Selective Removal Strategy of Thorium Ions Using a Novel Fluorescent Ligand and Hybrid Mesoporous γ-Al2O3-like Nanoneedles. ACS Sustainable Chemistry & Engineering 2023, 11, 2127–2138. [Google Scholar] [CrossRef]
  34. Rahman, I.U.; Mohammed, H.J.; Siddique, M.F.; Ullah, M.; Bamasag, A.; Alqahtani, T; Algarni, S. Application of membrane technology in the treatment of waste liquid containing radioactive materials. Journal of Radioanalytical and Nuclear Chemistry, 2023, 1-14. [CrossRef]
  35. Xiong, L.; Lyu, K.; Zeng, Y.; Yang, C.; Chi, F.; Hu, S.; Long, X. Stable and high-flux polyacrylonitrile/hafnium phosphonate nanofibrous membranes for efficient removal of actinides from strong acidic solutions. Journal of Environmental Chemical Engineering, 2023,11(2), .109619. [CrossRef]
  36. Abbasizadeh, S.; Keshtkar, A.R.; Mousavian, M.A. Preparation of a novel electrospun polyvinyl alcohol/titanium oxide nanofiber adsorbent modified with mercapto groups for uranium (VI) and thorium (IV) removal from aqueous solution. Chem. Eng. J. 2013, 220, 161–171. [Google Scholar] [CrossRef]
  37. Tsezos, M.; Volesky, B. Biosorption of uranium and thorium. Biotechnol. Bioeng. 1981, 23, 583–604. [Google Scholar] [CrossRef]
  38. Hritcu, D.; Humelnicu, D.; Dodi, G.; Popa, M.I. Magnetic chitosan composite particles: Evaluation of thorium and uranyl ion adsorption from aqueous solutions. Carbohydr. Polym. 2012, 87, 1185–1191. [Google Scholar] [CrossRef]
  39. Zolfonoun, E.; Yousefi, S.R. Sorption and preconcentration of uranium and thorium from aqueous solutions using multi-walled carbon nanotubes decorated with magnetic nanoparticles. Radiochim. Acta 2015, 103, 835–841. [Google Scholar] [CrossRef]
  40. Nechifor, A.C.; Goran, A.; Grosu, V.-A.; Bungău, C.; Albu, P.C.; Grosu, A.R.; Oprea, O.; Păncescu, F.M.; Nechifor, G. Improving the Performance of Composite Hollow Fiber Membranes with Magnetic Field Generated Convection Application on pH Correction. Membranes 2021, 11, 445. [Google Scholar] [CrossRef] [PubMed]
  41. Nechifor, G.; Păncescu, F.M.; Grosu, A.R.; Albu, P.C.; Oprea, O.; Tanczos, S.-K.; Bungău, C.; Grosu, V.-A.; Pîrțac, A.; Nechifor, A.C. Osmium Nanoparticles-Polypropylene Hollow Fiber Membranes Applied in Redox Processes. Nanomaterials 2021, 11, 2526. [Google Scholar] [CrossRef]
  42. Paun, G.; Neagu, E.; Parvulescu, V.; Anastasescu, M.; Petrescu, S.; Albu, C.; Nechifor, G.; Radu, G.L. New Hybrid Nanofiltration Membranes with Enhanced Flux and Separation Performances Based on Polyphenylene Ether-Ether-Sulfone/Polyacrylonitrile/SBA-15. Membranes 2022, 12, 689. [Google Scholar] [CrossRef]
  43. Cimbru, A.M.; Rikabi, A.A.K.K.; Oprea, O.; Grosu, A.R.; Tanczos, S.-K.; Simonescu, M.C.; Pașcu, D.; Grosu, V.-A.; Dumitru, F.; Nechifor, G. pH and pCl Operational Parameters in Some Metallic Ions Separation with Composite Chitosan/Sulfonated Polyether Ether Ketone/Polypropylene Hollow Fibers Membranes. Membranes 2022, 12, 833. [Google Scholar] [CrossRef]
  44. Păncescu, F.M.; Rikabi, A.A.K.K.; Oprea, O.C.; Grosu, A.R.; Nechifor, A.C.; Grosu, V.-A.; Tanczos, S.-K.; Dumitru, F.; Nechifor, G.; Bungău, S.G. Chitosan–sEPDM and Melatonin–Chitosan–sEPDM Composite Membranes for Melatonin Transport and Release. Membranes 2023, 13, 282. [Google Scholar] [CrossRef] [PubMed]
  45. Pașcu, D.; Nechifor, A.C.; Grosu, V.-A.; Oprea, O.C.; Tanczos, S.-K.; Man, G.T.; Dumitru, F.; Grosu, A.R.; Nechifor, G. Hydrogen Sulphide Sequestration with Metallic Ions in Acidic Media Based on Chitosan/sEPDM/Polypropylene Composites Hollow Fiber Membranes System. Membranes 2023, 13, 350. [Google Scholar] [CrossRef]
  46. Dimulescu, I.A.; Nechifor, A.C.; Bǎrdacǎ, C.; Oprea, O.; Paşcu, D.; Totu, E.E.; Albu, P.C.; Nechifor, G.; Bungău, S.G. Accessible Silver-Iron Oxide Nanoparticles as a Nanomaterial for Supported Liquid Membranes. Nanomaterials 2021, 11, 1204. [Google Scholar] [CrossRef]
  47. Nechifor, A.C.; Cotorcea, S.; Bungău, C.; Albu, P.C.; Pașcu, D.; Oprea, O.; Grosu, A.R.; Pîrțac, A.; Nechifor, G. Removing of the Sulfur Compounds by Impregnated Polypropylene Fibers with Silver Nanoparticles-Cellulose Derivatives for Air Odor Correction. Membranes 2021, 11, 256. [Google Scholar] [CrossRef] [PubMed]
  48. Nechifor, G.; Păncescu, F.M.; Grosu, A.R.; Albu, P.C.; Oprea, O.; Tanczos, S.-K.; Bungău, C.; Grosu, V.-A.; Pîrțac, A.; Nechifor, A.C. Osmium Nanoparticles-Polypropylene Hollow Fiber Membranes Applied in Redox Processes. Nanomaterials 2021, 11, 2526. [Google Scholar] [CrossRef] [PubMed]
  49. Bărdacă Urducea, C.; Nechifor, A.C.; Dimulescu, I.A.; Oprea, O.; Nechifor, G.; Totu, E.E.; Isildak, I.; Albu, P.C.; Bungău, S.G. Control of Nanostructured Polysulfone Membrane Preparation by Phase Inversion Method. Nanomaterials 2020, 10, 2349. [Google Scholar] [CrossRef]
  50. Pavel, E.; Prodan, G.; Marinescu, V.; Trusca, R. Recent advances in 3- to 10-nm quantum optical lithography. Journal of Micro/Nanolithography, MEMS, and MOEMS 2019,18(2), 020501 (7 May). [CrossRef]
  51. Prioteasa, P.; Marinescu, V.; Bara, A.; Iordoc, M.; Teisanu, A.; Banciu, C.; Meltzer, V. Electrodeposition of Polypyrrole on Carbon Nanotubes/Si in the Presence of Fe Catalyst for Application in Supercapacitors. Rev. Chim. 2015, 66, 820–824. [Google Scholar]
  52. Zamfir, L.-G.; Rotariu, L.; Marinescu, V. E.; Simelane, X.T.; Baker, P.G.L.; Iwuoha, E. I.; Bala, C. Non-enzymatic polyamic acid sensors for hydrogen peroxide detection. Sensors and Actuators B: Chemical, 2016, 226, 525–533. [Google Scholar] [CrossRef]
  53. Cazalaà, J.B. Radium and thorium applications for the general public: unexpected consequences of the discovery from Pierre and Marie Curie. The Journal of the American Society of Anesthesiologists 2012, 117(6), 1202–1202. [Google Scholar] [CrossRef]
  54. Pohjalainen, I.; Moore, I.D.; Geldhof, S.; Rosecker, V.; Sterba, J.; Schumm, T. Gas cell studies of thorium using filament dispensers at IGISOL. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2020, 484, 59–70. [Google Scholar] [CrossRef]
  55. Takaki, N.; Mardiansah, D. Core Design and Deployment Strategy of Heavy Water Cooled Sustainable Thorium Reactor. Sustainability 2012, 4, 1933–1945. [Google Scholar] [CrossRef]
  56. Sidik, P.; Takaki, N.; Sekimoto, H. Feasible region of design parameters for water cooled thorium breeder reactor. J. Nucl. Sci. Technol. 2007, 44, 946–957. [Google Scholar] [CrossRef]
  57. Aranaz, I.; Alcántara, A.R.; Civera, M.C.; Arias, C.; Elorza, B.; Heras Caballero, A.; Acosta, N. Chitosan: An Overview of Its Properties and Applications. Polymers 2021, 13, 3256. [Google Scholar] [CrossRef]
  58. Jiménez-Gómez, C.P.; Cecilia, J.A. Chitosan: A Natural Biopolymer with a Wide and Varied Range of Applications. Molecules 2020, 25, 3981. [Google Scholar] [CrossRef]
  59. Ibrahim, M.A.; Alhalafi, M.H.; Emam, E.-A.M.; Ibrahim, H.; Mosaad, R.M. A Review of Chitosan and Chitosan Nanofiber: Preparation, Characterization, and Its Potential Applications. Polymers 2023, 15, 2820. [Google Scholar] [CrossRef]
  60. Radu, E.-R.; Pandele, A.M.; Tuncel, C.; Miculescu, F.; Voicu, S.I. Preparation and Characterization of Chitosan/LDH Composite Membranes for Drug Delivery Application. Membranes 2023, 13, 179. [Google Scholar] [CrossRef] [PubMed]
  61. Nave, M.I.; Kornev, K.G. Complexity of Products of Tungsten Corrosion: Comparison of the 3D Pourbaix Diagrams with the Experimental Data. Metall. Mater. Trans. A 2017, 48, 1414–1424. [Google Scholar] [CrossRef]
  62. Berlanga-Labari, C.; Biezma-Moraleda, M.V.; Rivero, P.J. Corrosion of Cast Aluminum Alloys: A Review. Metals. 2020, 10(10), 1384. [Google Scholar] [CrossRef]
  63. Han, K.N. Characteristics of Precipitation of Rare Earth Elements with Various Precipitants. Minerals 2020, 10, 178. [Google Scholar] [CrossRef]
  64. Ma, H.; Shen, M.; Tong, Y.; Wang, X. Radioactive Wastewater Treatment Technologies: A Review. Molecules 2023, 28, 1935. [Google Scholar] [CrossRef]
  65. Rybak, A.; Rybak, A. Characteristics of Some Selected Methods of Rare Earth Elements Recovery from Coal Fly Ashes. Metals 2021, 11, 142. [Google Scholar] [CrossRef]
  66. Chang, H.; Zhang, G. Separation Techniques for the Efficient and Green Recovery of Metal Minerals. Separations 2023, 10, 520. [Google Scholar] [CrossRef]
  67. Nisbet, H.; Migdisov, A.; Xu, H.; Guo, X., van Hinsberg, V.; Williams-Jones, A.E.; Boukhalfa, H.; Roback, R. An experimental study of the solubility and speciation of thorium in chloride-bearing aqueous solutions at temperatures up to 250 C. Geochimica et Cosmochimica Acta 2018, 239, 363–373. [CrossRef]
  68. Kusumkar, V.V.; Galamboš, M.; Viglašová, E.; Daňo, M.; Šmelková, J. Ion-Imprinted Polymers: Synthesis, Characterization, and Adsorption of Radionuclides. Materials 2021, 14, 1083. [Google Scholar] [CrossRef] [PubMed]
  69. Atanassova, M. Thenoyltrifluoroacetone: Preferable Molecule for Solvent Extraction of Metals—Ancient Twists to New Approaches. Separations 2022, 9, 154. [Google Scholar] [CrossRef]
  70. Peng, C.; Ma, Y.; Ding, Y.; He, X.; Zhang, P.; Lan, T.; Wang, D.; Zhang, Z.; Zhang, Z. Influence of Speciation of Thorium on Toxic Effects to Green Algae Chlorella pyrenoidosa. Int. J. Mol. Sci. 2017, 18, 795. [Google Scholar] [CrossRef]
  71. Vakanjac, B.; Naunovic, Z.; Ristić Vakanjac, V.; Đumić, T.; Bakrač, S.; Štrbački, J.; Gajić, V.; Alzarog, T.M. Distribution of Potentially Toxic Elements in the City of Zintan and Its Surroundings (Northwestern Libya) by Surface Soil Sampling. Minerals 2023, 13, 1048. [Google Scholar] [CrossRef]
  72. García, A.C.; Latifi, M.; Amini, A.; Chaouki, J. Separation of Radioactive Elements from Rare Earth Element-Bearing Minerals. Metals 2020, 10, 1524. [Google Scholar] [CrossRef]
  73. Kaptakova, V.O.; Milyutina, V.V.; Nekrasovaa, N.A.; Zelenina, P.G.; Kozlitina, E.A. Nanofiltration Extraction of Uranium and Thoriumfrom Aqueous Solutions. Radiochemistry, 2021, 63(2),169–172. ISSN 1066-3622, © Pleiades Publishing, Inc., 2021. Russian Text © The Author(s), 2021, published in Radiokhimiya, 2021, 63(2), 139–142.
Preprints 93613 g001
Figure 1. Some of the individual and common applications of tungsten and thorium: (blue) tungsten; (yellow) thorium; (green) common.
Figure 1. Some of the individual and common applications of tungsten and thorium: (blue) tungsten; (yellow) thorium; (green) common.
Preprints 93613 g001
Preprints 93613 g002
Figure 2. Preparation of sPEEK–M membranes from PEEK solution in sulfuric acid.
Figure 2. Preparation of sPEEK–M membranes from PEEK solution in sulfuric acid.
Preprints 93613 g002
Preprints 93613 g003
Figure 3. Schematic of the nanofiltration installation for the preparation of C–PHF–M membrane: MM – membrane module; P – permeate; FS – feed solution; RP – recirculation pumps.
Figure 3. Schematic of the nanofiltration installation for the preparation of C–PHF–M membrane: MM – membrane module; P – permeate; FS – feed solution; RP – recirculation pumps.
Preprints 93613 g003
Preprints 93613 g004
Figure 4. The schema of the electrolysis cell of tungsten electrodes for the recovery of thorium, and tungsten.
Figure 4. The schema of the electrolysis cell of tungsten electrodes for the recovery of thorium, and tungsten.
Preprints 93613 g004
Preprints 93613 g005
Figure 5. The images obtained by scanning electron microscopy (SEM) for: (a) the section of the membrane after keeping it in the calcium hydroxide bath; and (b) the detail of the section.
Figure 5. The images obtained by scanning electron microscopy (SEM) for: (a) the section of the membrane after keeping it in the calcium hydroxide bath; and (b) the detail of the section.
Preprints 93613 g005
Preprints 93613 g006
Figure 6. Energy Dispersive X–ray Spectroscopy (EDAX) of the membrane after keeping it in the calcium hydroxide bath.
Figure 6. Energy Dispersive X–ray Spectroscopy (EDAX) of the membrane after keeping it in the calcium hydroxide bath.
Preprints 93613 g006
Preprints 93613 g007
Figure 7. The image obtained by scanning electron microscopy (SEM) for: (a) section of the washed membrane; and (b) the detail of the section.
Figure 7. The image obtained by scanning electron microscopy (SEM) for: (a) section of the washed membrane; and (b) the detail of the section.
Preprints 93613 g007
Preprints 93613 g008
Figure 8. Comparative Energy Dispersive X–ray Spectroscopy (EDAX): (a) membrane before washing; and (b) membrane after keeping in ultrapure water.
Figure 8. Comparative Energy Dispersive X–ray Spectroscopy (EDAX): (a) membrane before washing; and (b) membrane after keeping in ultrapure water.
Preprints 93613 g008
Preprints 93613 g009
Figure 9. The image obtained by scanning electron microscopy (SEM) for: (a) polypropylene hollow fiber membrane (PHF–M) surface; (b) chitosan-polypropylene hollow fiber membrane (C–PHF–M).
Figure 9. The image obtained by scanning electron microscopy (SEM) for: (a) polypropylene hollow fiber membrane (PHF–M) surface; (b) chitosan-polypropylene hollow fiber membrane (C–PHF–M).
Preprints 93613 g009
Preprints 93613 g010
Figure 10. The image obtained by scanning electron microscopy (SEM) of the chitosan-polypropylene hollow fiber membrane (C–PHF–M) section: (a) general section; (b) detail of the section highlighting the active layer; and (c) detail of the polypropylene hollow fiber membrane (PHF–M) substrate.
Figure 10. The image obtained by scanning electron microscopy (SEM) of the chitosan-polypropylene hollow fiber membrane (C–PHF–M) section: (a) general section; (b) detail of the section highlighting the active layer; and (c) detail of the polypropylene hollow fiber membrane (PHF–M) substrate.
Preprints 93613 g010
Preprints 93613 g011aPreprints 93613 g011b
Figure 11. Images obtained by scanning electron microscopy (SEM) for: (a) electrode surface; (b) the section of the electrode 20,000×; (c) the section of the electrode 50,000×; and (d) the detail at the electrode section 50,000×.
Figure 11. Images obtained by scanning electron microscopy (SEM) for: (a) electrode surface; (b) the section of the electrode 20,000×; (c) the section of the electrode 50,000×; and (d) the detail at the electrode section 50,000×.
Preprints 93613 g011aPreprints 93613 g011b
Preprints 93613 g012
Figure 12. The elemental distribution obtained by Energy Dispersive X–ray Spectroscopy (EDAX) for the fracture of the welding electrode: (a) general distribution of the elements in colors; (b) quantitative elemental composition; (c) tungsten distribution; (d) thorium distribution; (e) oxygen distribution; and (f) aluminum distribution.
Figure 12. The elemental distribution obtained by Energy Dispersive X–ray Spectroscopy (EDAX) for the fracture of the welding electrode: (a) general distribution of the elements in colors; (b) quantitative elemental composition; (c) tungsten distribution; (d) thorium distribution; (e) oxygen distribution; and (f) aluminum distribution.
Preprints 93613 g012
Preprints 93613 g013
Figure 13. Scheme of the hybrid installation for membrane electrolysis and nanofiltration of tungsten electrodes.
Figure 13. Scheme of the hybrid installation for membrane electrolysis and nanofiltration of tungsten electrodes.
Preprints 93613 g013
Preprints 93613 g014
Figure 14. Superimposed Pourbaix diagrams of tungsten (purple), thorium (red), aluminum (black) and the blue dotted lines delimiting the stability domain of water.
Figure 14. Superimposed Pourbaix diagrams of tungsten (purple), thorium (red), aluminum (black) and the blue dotted lines delimiting the stability domain of water.
Preprints 93613 g014
Preprints 93613 g015
Figure 15. Variation of thorium concentration in the permeate during nanofiltration of thorium dioxide nanodispersion, depending on pH: (blue): thorium dioxide obtained from the electrolysis process of the welding electrode; and (green) thorium dioxide obtained by precipitation from the solution.
Figure 15. Variation of thorium concentration in the permeate during nanofiltration of thorium dioxide nanodispersion, depending on pH: (blue): thorium dioxide obtained from the electrolysis process of the welding electrode; and (green) thorium dioxide obtained by precipitation from the solution.
Preprints 93613 g015
Preprints 93613 g016
Figure 16. The morphological aspect highlighted through scanning electron microscopy (SEM) for thorium dioxide obtained by: (a) precipitation of Th(NO3)4; (b) electrolysis of the welding electrode.
Figure 16. The morphological aspect highlighted through scanning electron microscopy (SEM) for thorium dioxide obtained by: (a) precipitation of Th(NO3)4; (b) electrolysis of the welding electrode.
Preprints 93613 g016
Preprints 93613 g017
Figure 17. Variation of the nanofiltration flow of nanodispersions containing thorium dioxide, depending on pH and the working pressure.
Figure 17. Variation of the nanofiltration flow of nanodispersions containing thorium dioxide, depending on pH and the working pressure.
Preprints 93613 g017
Table 2. The speciation of tungsten, thorium and aluminum elements depending on the potential and the variable pH in the anodic space, and pH 13 in the cathodic space.
Table 2. The speciation of tungsten, thorium and aluminum elements depending on the potential and the variable pH in the anodic space, and pH 13 in the cathodic space.
Metallic element or membrane reactive group Metal element speciation at variable pH in the anodic space, at pH 13 in the
cathodic space and 20.0 V anodic potential
0–1 1–3 3–6 6–12 >12
Wolfram WO3·H2O(s) WO3·H2O(s) WO3·H2O(s) WO42⁻(aq) WO42⁻(aq)
Thorium Th4+(aq) ThO2(s) ThO2(s) ThO2(s) ThO2(s)
Aluminum Al3+(aq) Al3+(aq) Al(OH)3(s) Al(OH)3(s) AlO2⁻(aq)
sPEEK–M *) HSO3–Ar HSO3–Ar ⁻SO3–Ar ⁻SO3–Ar ⁻SO3–Ar
C–PHF–M **) +H3N–R +H3N–R +H3N–R H2N–R H2N–R
*) Ar – aromatic group; **) R – alkyl group.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
Prerpints.org logo

Preprints.org is a free preprint server supported by MDPI in Basel, Switzerland.

facebook logotwitter logolinkedin logo
MDPI Initiatives
Important Links
Subscribe

Disclaimer

Terms of Use

Privacy Policy

© 2024 MDPI (Basel, Switzerland) unless otherwise stated