Mohammed F Hamza

Khalid A M Salih

Kanggen Zhou

Yuezhou Wei
email: yzwei@usc.edu.cn

Hanaa A Abu Khoziem

Saad H Alotaibi

Eric Guibal
email: eric.guibal@mines-ales.fr

Effect of bi-functionalization of algal/polyethyleneimine composite beads on the enhancement of tungstate sorption: Application to metal recovery from ore leachate

Bi-functionalization of algal/PEI beads for W (VI) enhanced sorption Sorption isotherms and uptake kinetics Selective recovery of W(VI) and Nb(V) from complex solutions Remarkable stability in desorption efficiency and sorbent recycling Efficient application to ore leachate

Introduction

Historically, tungsten was used for manufacturing incandescent lamps, making profit of the high melting point of the metal. The poor energy efficiency of these lamps progressively caused their replacement with more efficient systems (like led, fluorescent, halogen lamps, etc.). Nowadays, tungsten is essentially used for designing alloys for hightemperature applications. Tungsten carbide is also frequently employed for manufacturing hard cutting and drilling tools for metallurgy, mining, and petroleum industries. The main mineral resources for tungsten are scheelite (CaWO 4 , associated with Mo, Nb and Ta impurities) and wolframite (iron, manganese, magnesium tungstate). The main producers and stock holders are China, Russia, Bolivia and USA, which concentrate more than 80% of resources. The relative supply risk is evaluated to 9.5 (the highest risk being ranked 10) [START_REF]RSC, Periodic Table[END_REF]. The rate of recycling is estimated between 10% and 30%. This may explain that These beneficial effects of bi-functionalization are investigated in this work comparing the sorption properties of A L PEI (as the reference and raw support), A L PEI-P (phosphorylated derivative of raw sorbent), A L PEI-Q (quaternized beads), and A L PEI-PQ (bi-functional sorbent based on the quaternization of phosphorylated derivative) for W(VI) recovery. The sorption mechanisms are investigated using FTIR and XPS spectrometry; in addition, textural, TGA, and titration analyses are used for characterizing the materials. The sorption properties are compared through the study of the effect of pH, uptake kinetics and sorption isotherms to demonstrate the enhancing effects of the bi-functionalization. The effect of competitor ions is carried out for evaluating metal selectivity (in function of pH). Tungstate desorption from metal-loaded sorbent is investigated, including kinetic profiles and sorbent recycling. The final part of the work focuses on the application of the sorbent for tungstate recovery from solvent extraction residues (applied to alkaline leachate of Egyptian W-bearing ore).

Materials and methods

Materials

Laminaria digitata (brown algae) was purchased from Setalg (Pleubian, France). PEI (branched; 50%, w/w in water), sodium tungstate dihydrate (≥99%) and glutaraldehyde were supplied by Sigma-Aldrich (Taufkirchen, Germany). Calcium chloride (CaCl 2 ), zinc oxide (used in selectivity tests) and sodium carbonate (Na 2 CO 3 ) were purchased from Chem-Lab NV (Zedelgem, Belgium). Glycidyltrimethylammonium chloride (≥95%) and poly(ethylene glycol diglycidyl ether) (PEGDGE) were supplied by Shanghai Makclin Biochemical Co., Ltd. (Shanghai, China). Nd 2 (SO 4 ) 3 (used in selectivity tests) was supplied by National Engineering Research Centre of Rare Earth Metallurgy and Functional Materials Co., Ltd (Baotou, China). Tributyl phosphate (TBP, C 12 H 27 O 4 P), was provided by Aladdin Industrial Corporation, (Shanghai, China). Aluminum sulfate octadecahydrate, sodium hydroxide, and absolute ethanol were supplied by Guangdong Guanghua Sci-Tech Co. (Guangzhou, China). Phosphoric acid was provided by Xilong Scientific Co., (Guangdong, China). All other reagents are Prolabo products, which were used as received.

Synthesis of sorbents

The synthesis procedures are detailed in Supplementary Information (Section S1, Scheme S1). The synthesis of A L PEI beads (pristine support) was already described [51]: alginate was (partially) thermally extracted from algal biomass with sodium carbonate before reacting with PEI (amine groups from PEI with carboxylate groups of alginate) before dropping the mixture through a nozzle into CaCl 2 solution (for the ionotropic gelation of carboxylate groups of the biopolymer).

The quaternization of A L PEI beads (A L PEI-Q) was performed by reaction of pristine beads with glycidyltrimethyl ammonium chloride [START_REF] Hamza | Quaternization of algal/PEI beads (a new sorbent): characterization and application to scandium recovery from aqueous solutions[END_REF]. The synthesis of A L PEI-P consisted of the synthesis of dibutyl-(3chloro-2-hydroxy)-propyl phosphate, followed by the grafting of this TBP-derivative onto A L PEI. The bi-functionalization of A L PEI (to obtain A L PEI-PQ) was obtained by quaternization of A L PEI-P using the same procedure described for direct quaternization of pristine beads. Scheme 1 reports the suggested structures of synthesized sorbents.

Characterization of sorbents

The elemental analysis of the sorbents (i.e., A L PEI, A L PEI-P and A L PEI-PQ beads) was performed for C, N, O, P contents using an element analyzer-Vario EL cube (Elementar Analysensysteme GmbH, Langenselbold, Germany). An ESCALAB-250XI+-instrument (Thermo Fischer Scientific, Inc., Waltham, MA, USA) was used for XPS analyses. FTIR spectrometry was performed on dried samples through IRTracer 100 (Shimadzu, Tokyo, Japan) after being lixed with KBR and conditioned as KBr discs. A Netzsch STA-449 F3 Jupiter (NETZSCH-Gerätebau GmbH, Selb, Germany) was used for thermogravimetric analyses (TGA) under N 2 atmosphere, with temperature ramp: 10 • C/min. The pH-drift strong incentive have been enacted for promoting the recycling of strategic resources and for decreasing the environmental impact of metal uses [START_REF] Welz | Environmental impacts of lighting technologies -Life cycle assessment and sensitivity analysis[END_REF][START_REF] Mishra | Recycling of secondary tungsten resources[END_REF][START_REF] Shemi | Recycling of tungsten carbide scrap metal: a review of recycling methods and future prospects[END_REF][START_REF] Tkaczyk | Sustainability evaluation of essential critical raw materials: cobalt, niobium, tungsten and rare earth elements[END_REF][START_REF] Zeiler | Recycling of tungsten: current share, economic limitations, technologies and future potential[END_REF].

Tungsten extraction from ores and wastes may involve leaching processes [START_REF] Zhang | Coordination leaching of tungsten from scheelite concentrate with phosphorus in nitric acid[END_REF][START_REF] Chen | Eco-friendly H2O2 leaching for noblemetals Re and W selective recovery from waste thermoelectric materials[END_REF][START_REF] Dimitrijevic | Recovery of tungsten trioxide from waste diamond core drilling crowns by nitric acid leaching[END_REF][START_REF] Shen | Tungsten extractive metallurgy: a review of processes and their challenges for sustainability[END_REF] (eventually associated with pyro-metallurgical techniques [START_REF] Liu | Comprehensive treatments of tungsten slags in China: a critical review[END_REF]). Several techniques have been reported for concentrating and extracting tungsten from leachates, including liquid membranes [START_REF] Won | Extraction of tungsten from acetic tungstate complex solutions by liquid membrane[END_REF], and solvent extraction systems [START_REF] Li | Separation of tungsten from molybdate using solvent extraction with primary amine N[END_REF][START_REF] Sasaki | Various metal extraction from hydrochloric acid into n-dodecane by methylimino-dioctylacetamide (MIDOA)[END_REF][START_REF] Yang | Extraction and separation of tungsten from acidic high-phosphorus solution[END_REF][START_REF] Zhang | A novel process for tungsten hydrometallurgy based on direct solvent extraction in alkaline medium[END_REF]. Playing with the characteristics of ionic liquids and precipitation, Papaiconomou and Viboud [START_REF] Papaiconomou | Extraction of polyoxometallate anions containing tungsten towards ionic liquids[END_REF] showed the possibility to separate tungstate from polytungstate species. The solvent extraction techniques are highly efficient and competitive for the treatment of effluent containing high concentrations of target metals. However, sorption processes are generally more appropriate for diluted effluent. This is typically the case for the residues of solvent extraction operations, where sorption allows recovering additional amounts of metals and decreasing environmental impact in the global process. A wide range of materials may be used for recovering valuable or toxic metal ions (including tungsten) from dilute solutions including mineral-based sorbents [START_REF] Vucina | Sorption of tungsten on alumina in dynamic conditions[END_REF][START_REF] Cao | Tungstate removal from aqueous solution by nanocrystalline iowaite: an iron-bearing layered double hydroxide[END_REF][START_REF] Li | Adsorption of tungstate on kaolinite: adsorption models and kinetics[END_REF][START_REF] Li | Tungstate adsorption onto oxisols in the vicinity of the world's largest and longest-operating tungsten mine in China[END_REF][START_REF] Li | Adsorption behavior of tungstate on montmorillonite as a function of pH, ionic strength and competitive anion, Desalin[END_REF][START_REF] Petruzzelli | Adsorption, desorption and bioavailability of tungstate in Mediterranean soils[END_REF][START_REF] Sen Tuna | Evaluation of the adsorption of mono-and polytungstates onto different types of clay minerals and pahokee peat[END_REF][START_REF] Cui | Microwave-assisted onepot rapid synthesis of mesoporous silica-chitosan composites for efficient recovery of rhenium(Ⅶ)[END_REF], activated carbon [START_REF] Alcaraz | Application of activated carbon obtained from spent coffee ground wastes to effective terbium recovery from liquid solutions[END_REF], carbon nanotubes [START_REF] Ihsanullah | First investigations on the removal of tungsten species from water using multi-walled carbon nanotubes[END_REF], functionalized synthetic fibers [START_REF] Wei | Preparation of PP non-woven fabric with good heavy metal adsorption performance via plasma modification and graft polymerization[END_REF], and metalorganic framework [START_REF] Zhang | Adsorption behavior and mechanism of Hg (II) on a porous core-shell copper hydroxy sulfate@MOF composite[END_REF]. Chelating resins [START_REF] Durisova | Mutual separation of vanadium and tungsten from aqueous solution via electrochemical reduction and sorption onto chelating resin[END_REF][START_REF] Uchikoshi | Anion-exchange behavior of Mo(V) and W(VI) in HCl solutions[END_REF][START_REF] Nguyen | Separation of molybdenum(VI) and tungsten(VI) from sulfuric acid solution by ion exchange with TEVA resin[END_REF][START_REF] Masry | Sorption behavior of tungsten and molybdenum on TVEX-TOPO resin from nitric acid solution[END_REF] and ion-exchangers [START_REF] Uchikoshi | Anion-exchange behavior of Mo(V) and W(VI) in HCl solutions[END_REF][START_REF] Blokhin | Sorption purification of aqueous ammonium molybdate to remove tungsten impurities with a cation exchanger containing pyrocatechol functional groups[END_REF][START_REF] Gott | Radiochemical study of Re/W adsorption behavior on a strongly basic anion exchange resin[END_REF][START_REF] Lu | Separation of macro amounts of tungsten and molybdenum by ion exchange with D309 resin[END_REF][START_REF] Alcaraz | Microporous adsorbent from winemaking waste for the recovery of Mn(VII) in liquid solutions[END_REF] have also retained a great attention for the last decades. The possibilities of biosorbents have also been explored for metal binding [START_REF] Li | Pregrafting effect on improving adsorption efficiency of cellulose based biosorbent for Hg (II) removal from aqueous solution[END_REF][START_REF] Nicomel | Selective and enhanced nickel adsorption from sulfate-and calcium-rich solutions CRediT authorship contribution statement Mohammed F. Hamza: Conceptualization, Methodology, Software, Formal analysis[END_REF] and for tungsten recovery [40][41][42][43]; playing with the presence of reactive groups at the surface of biosourced materials (functional groups analogues to those present on synthetic resins). The biosorbents can be also functionalized to improve metal binding through grafting of complementary reactive groups, such as aminophosphonate [44].

Recently, a "collection" of new sorbents has been developed based on the functionalization of a composite material constituted of brown algal biomass and polyethyleneimine (PEI) (i.e., A L PEI). The interactions of carboxylate groups on the algal biomass (alginate fraction) and protonated amine groups (on PEI), eventually completed by the glutaraldehyde crosslinking of PEI, leads to the formation of stable interpenetrated networks, conditioned as hydrogel beads [45]. Both alginate and PEI have shown attractive intrinsic potentialities for metal binding and/or complexation [46,47]. In the composite, the coexistence of hydroxyl, carboxyl, and amine groups makes the material very reactive for metal ions but also readily available for functionalization. For example, the sorbent was amidoximated [48], phosphorylated [49], quaternized [50], and sulfonated [51] for the recovery of a series of valuable metal ions. These functionalized sorbents are enriched with specific reactive groups though some sites present on the raw material remain available for metal sorption and/or for complementary functionalization.

Several studies have shown the interest of bi-functionality for improving sorption properties, expanding the pH range for efficient metal sorption, or for enhancing separation properties [52-57]. For example, Alexandratos and Natesan [START_REF] Alexandratos | Ion-selective polymer-supported reagents: the principle of bifunctionality[END_REF] reported the usefulness of associating a recognition ligand to an access ligand for promoting rare earth sorption. Alexandratos [START_REF] Alexandratos | Intraligand cooperation in metal-ion binding by immobilized ligands: The effect of bifunctionality[END_REF] also identified the beneficial impact of intraligand cooperation on sorption performance. Modulating the acidity/basicity of the sorbent may increase the sorption performance [START_REF] Alexandratos | High-affinity ion-complexing polymer-supported reagents: Immobilized phosphate ligands and their affinity for the uranyl ion[END_REF]; for example, the vicinity of hydroxyl groups with phosphoryl oxygen is favorable for uranyl sorption. Alexandratos and Zhu [START_REF] Alexandratos | High-affinity ion-complexing polymer-supported reagents: Immobilized phosphate ligands and their affinity for the uranyl ion[END_REF] claim the possibility to tune metal ion affinity playing with polarizability for designing efficient and selective sorbents. Wei et al. [START_REF] Wei | Bifunctional polymeric microspheres for efficient uranium sorption from aqueous solution: synergistic interaction of positive charge and amidoxime group[END_REF] demonstrated that the synergistic effects of protonation and amidoximation of the resin improve both the kinetics and selectivity of the sorbent for U(VI) sorption. Piechowicz et al. [START_REF] Piechowicz | Design, synthesis, and characterization of a bifunctional chelator with ultrahigh capacity for uranium uptake from seawater simulant[END_REF] also reported the enhanced reactivity of bi-functional resin for U(VI): carboxyl group and amidoxime ligands cooperate for metal binding. method was used for determining the pH PZC values. One hundred mg of sorbent were mixed with 0.1 M NaCl solutions (50 mL), with pH 0 initially adjusted between 1 and 11. The final pH (pH eq ) was measured using a Mettler Toledo-pH meter (Mettler, Colombus, OH, USA). The pH PZC is determined as the value corresponding to pH 0 = pH eq . Morphological characterization (through SEM) and semi-quantitative energy dispersive X-ray analysis (EDX) were performed using Phenom ProX scanning electron microscope (Thermo Fisher Scientific, Netherlands) equipped with EDX facilities. Textural analysis of A L PEI and A L PEI-PQ beads was performed on Micromeritics TriStar II (Norcross, GA, USA), after pre-treatment (degassing at 120 • C for 12 h, before analysis). The BJH method was used for the evaluation of both specific surface area and pore size distribution.

Sorption tests 2.4.1. Experimental procedures

Sorption experiments were performed in batch system using an agitated tank reactor (agitation speed, v, 210 rpm). A fixed volume of solution (V, L) at given initial metal concentration (C 0 , mmol W L -1 ) and fixed initial pH value (pH 0 ) was mixed with a fixed amount of sorbent (m, g; sorbent dose, SD = m/V, g L -1 ) for given contact times (t, min). After fixed contact time, a sample of solution was collected and filtrated on a 1.2 µm pore size membrane and the filtrate was analyzed for residual metal concentration (C(t) or C eq , mmol W L -1 ) using ICP-AES (ICPS-7510-Shimadzu, Japan), after filtration using 1.2 µm pore size filter membrane; the equilibrium pH was systematically monitored (pH eq , no pH-control during sorption process). The sorption capacity (q eq , mmol W g -1 ) was deduced by the mass balance equation: q eq = (C 0 -C eq ) × V/m. Similar procedures were used for investigating metal sorption from multi-component equimolar solutions (and tests on real complex solutions). The selectivity coefficient (SC Me1/Me2 ) was calculated as the ratio D Me1 /D Me2 (where D is the distribution ratio, D = q eq / C eq ):

SC Me1/Me2 = D Me1 D Me2 = q eq,1 × C eq,2 C eq,1 × q eq,2 (1)
Desorption studies were also performed in batch systems. Metalloaded sorbents were collected from uptake kinetics. After careful rinsing with demineralized water, the sorbents were mixed with the eluent solution (0.2 M HCl solution) for a given contact time, before analyzing the concentration of released metal ions. Before recycling the sorbent, the material was rinsed with demineralized water and the sorption properties were compared using the same experimental conditions for five successive cycles of sorption and desorption.

Note: experimental conditions are systematically reported in the caption of relevant figures. Experiments were duplicated. The experimental profiles are reported in the core of the manuscript or in the Supplementary Information to show the reproducibility of sorption performances.

Modeling

Uptake kinetic and sorption isotherms were fitted using conventional equations:

(a) the pseudo-first order and pseudo-second order rate equations (PFORE and PSORE), and the resistance to intraparticle diffusion (RIDE) [START_REF] Ho | Pseudo-second order model for sorption processes[END_REF][START_REF] Crank | The Mathematics of Diffusion[END_REF], (b) the Langmuir, Freundlich, Sips and Temkin equations [START_REF] Tien | Adsorption Calculations and Modeling[END_REF][START_REF] Buema | Performance assessment of five adsorbents based on fly ash for removal of cadmium ions[END_REF]. Scheme 1. Suggested structures for A L PEI, A L PEI-Q, A L PEI-P, and A L PEI-PQ.

The facilities of non-linear regression analysis of Mathematica® software were used for determining the parameters of the different models. The relevant equations are reported in Supplementary Information. The quality of fits was evaluated (and compared) using the determination coefficient (i.e., R 2 ) and the Akaike information criterion (AIC) [START_REF] Falyouna | Magnetic zeolite synthesis for efficient removal of cesium in a lab-scale continuous treatment system[END_REF].

Application to ore treatment 2.5.1. Ore treatment

Annex A (in SI) describes the location of ore deposit (Gabal Qash Amir area, Southwestern Desert, Egypt, Figure A1), the geological context (invaded quartz vein / muscovite granite), and the geochemical characteristics of the investigated ore. The grinded and sieved ore (size < 1 mm) was pre-treated through successive concentration steps using shaking and magnetic tables; the hubnerite concentrate (MnWO 4 , more than 87% w/w, see Composition in Table A1, Figures A2 andA3) was used for hydrothermal leaching steps: (a) first, HCl acid leaching for Fe and Mn extraction (more than 99% and 94%, respectively, see composition of residue in Table A2), and (b) NaOH alkaline leaching (99 % W leaching). The detailed characterization of concentrates (and their treated residues) are reported in Annex A.

The composition of the alkaline leachate is reported in Table A3 (XRD diffractogram in Figure A4). The alkaline leachate was treated by solvent extraction using Alamine 336 (trioctylamine), in the presence of octanol as the modifier. The extraction efficiency (under selected experimental conditions) reached 94%. The stripping efficiency was close to 98%, using ammonia solution. This concentrate was valorized by evaporation at 50 • C and crystallization of pure ammonium paratungstate (99.97%, converted to WO 3 at 750 • C); the recovery yield attained 90% during the crystallization step.

Residual tungsten-loaded Alamine 336 phase was scrubbed with demineralized water for removing Na and Si impurities. The raffinate and scrubbed solutions were mixed for investigating: (a) the sorption properties of A L PEI-PQ on complex solutions, (b) complementary W(VI) valorization, and (c) decontamination of residues. Table A4 reports the composition of the effluent tested for W(VI) sorption.

Tungstate recovery from residues of liquid/liquid extraction of alkaline leachates

The sorption of W(VI) (main target) was investigated using the batch process. Sorbent dose was set to 1 g L -1 ; the contact time fixed to 10 h. Experiments were performed at room temperature (T: 22 ± 1 • C), under standard agitation (i.e., 210 rpm). The pH 0 was adjusted to different pH values (between 3 and 7). The residual concentrations were analyzed for calculating the sorption capacities, the distribution ratios (D,), and the selectivity coefficients (SC W/Metal ).

Results and discussion

Summary of characterization of sorbents

The detailed characterization of the sorbents is presented and discussed in the Supplementary Information (Section 2). Optical microscopy allowed evaluating the average size of sorbent beads: 2.25 ± 0.02 mm, 2.12 ± 0.10 mm, and 2.06 ± 0.16 mm for A L PEI, A L PEI-P, and A L PEI-PQ, respectively. Figure S1 shows the roughly spherical shape of the beads and the irregular surface (resulting from shrinking during the sorption steps). Figure S2 shows N 2 sorption and desorption curves for the BET determination of textural properties: the specific surface area (S BET ) increases with the functionalization of the beads: 13.1 m 2 g -1 (for A L PEI), 24.7 m 2 g -1 (for A L PEI-P), and up to 42.0 m 2 g -1 (for A L PEI-PQ).

FTIR and XPS spectral methods allow characterizing both the functional groups progressively grafted onto pristine beads and their interactions with tungstate species. First, the phosphorylation of pristine beads (A L PEI-P) is demonstrated by the appearance of characteristic phosphonate bonds such as P = O asymmetric stretching and P-O-C stretching bands (at ≈ 1320 cm -1 and 876 cm -1 , respectively), and the strong band at 1425 cm -1 (C-H asymmetric stretching of P-O-C 4 H 9 ) (Figure S3, Table S1). The quaternization of the sorbent (A L PEI-PQ) induces: (a) the decrease of the relative intensity of the phosphonate bands, (b) the appearance of bands at 1463 and 1409 cm -1 (C-H stretching from quaternary ammonium salt moieties), and (c) the appearance of a new band at 974 cm -1 . Figure S4 and Tables S2-S4 show the effect of metal sorption and sorbent recycling on FTIR spectra and the assignment of relevant bands (see the main conclusions in section 3.2.4. and extensive discussion in Supplementary Information). The bifunctionalization of pristine beads is also confirmed by XPS analysis (Fig. 1, Figure S5 and Table S5). The phosphorylation is characterized by the appearance of P 2p signal in the survey XPS spectra, and in high resolution spectra by the changes in the binding energies and atomic fractions of O 1s (phosphonate grafting), N 1s signals (modification of the chemical environment of amine groups). The effective quaternization is also confirmed by the reinforcement of the signal associated with quaternary ammonium salt: a strong band appears at BE: ≈402 eV. Metal binding is characterized by the appearance of W 4f, W 4d and W 4p signals in Fig. 1 (XPS survey curves), while Figures S6-S8 (and Table S5) compare the high-resolution XPS spectra of selected signals for A L PEI and A L PEI-PQ sorbents before and after tungstate sorption. Elemental analysis (Table S6) shows the phosphorylation of A L PEI (1.39 mmol P g -1 in A L PEI-P), the quaternization of A L PEI (increase of N content from 3.13 to 5.98 mmol N g -1 in A L PEI-P). The bifunctionalization (in A L PEI-PQ) increases N content from 3.13 to 5.67 mmol N g -1 (comparable to A L PEI-Q), and P content up to 1.35 mmol P g -1 (comparable to A L PEI-P). Figure S9 shows the SEM photographs and semi-quantitative EDX analysis of the surface of sorbent beads; A L PEI-P seems to be more porous (largest surface holes). The sorption of W(VI) onto A L PEI-PQ is confirmed by the semi-quantitative EDX analysis (Figure S10), while after metal desorption tungsten totally disappears from sorbent surface (the elemental composition at the surface is restored). The thermogravimetric analysis of the materials is reported in Figure S11: the phosphorylation increases the stability of the material (total weight loss does not exceed 64%, contrary to 86.7% for A L PEI). Grafting phosphorous element is a well-known method for increasing the thermal stability of fire-retardant materials. However, the bifunctionalization decreases again when the intermediary phosphonated derivative is quaternized (total weight loss reaches 72.8%). The titration of the sorbents (to determine the pH PZC values) shows the scaling in the acidification of the sorbents (Figure S12). Hence, the pH PZC values range according to: A L PEI (7.36) > A L PEI-P (6.74) > A L PEI-PQ (5.78) > A L PEI-Q (5.24). This means that all the sorbents are protonated below pH 5.2; this is favorable to the electrostatic attraction of metal anions. The protonation increases with quaternization while phosphonate groups bring additional chelating properties.

Sorption properties 3.2.1. Effect of pH

The pH is a critical parameter for designing a sorption process because this parameter may control both the surface properties of the sorbent (protonation/deprotonation of reactive groups, with attraction or repulsion effects) and the speciation of metal ions (formation of precipitates, hydrolyzed species and complexes, and/or formation of polynuclear species). Fig. 2 shows the evolution of W(VI) sorption capacity vs. equilibrium pH for A L PEI, A L PEI-P, A L PEI-Q and A L PEI-PQ sorbents (under comparable experimental conditions). Duplicate experiments confirm the good experimental reproducibility of sorption tests. This figure clearly demonstrates the benefits of successive functionalization steps. The sorption of tungstate with A L PEI is hardly affected by pH eq (in the range 3-6.5): the sorption capacity linearly increases from 0.08 to 0.15 mmol W g -1 . The reactive groups present on the sorbent, which is positively charged on the full pH range (with pH PZC : 7.36), may explain the possibility to bind tungstate anions. However, the presence of carboxylic acid groups (from alginate) with pK a values of 3.68-3.35 and amine groups (from PEI) with pK a values close to 4.5, 6.7 and 11.6 mean that tungstate anions will be bound to free amine groups. The relatively weak sorption may be explained by the limited availability of these amine groups that are engaged in interactions with carboxylate groups. The grafting of phosphonate moieties strongly increases W(VI) sorption ability; under selected experimental conditions, at the optimum pH eq (i.e., around 5) the sorption capacity reaches 0.6 mmol W g -1 . The pH PZC value of A L PEI-P is close to 6.74, meaning that in the whole range of pH investigated for W (VI) uptake the global positive charge of the sorbent remains favorable to the electrostatic attraction of tungstate anions. In acidic solutions, the competition of nitrate anions (HNO 3 has been used for pH control) may explain the low sorption efficiency of the phosphorylated sorbent. With the increase of the pH, the competition effect decreases and the binding of W(VI) is enhanced. However, at pH eq above 5, the sorption capacity progressively decreases; this means that sorption is controlled by another mechanism or criterion. The speciation diagram of W(VI) (under the experimental conditions selected for pH study, and calculated in terms of pH variation (probably due to acid-base properties and pH PZC values, and to the weaker sorption affinity for W(VI)): the equilibrium pH varies by less than 0.2 pH unit (except at pH 7). The other sorbents show largest pH variations (increasing with the functionalization of the sorbent and consistently with pH PZC values). Below pH 0 4.5, the pH tends to increase, while above, a reciprocal trend is observed. At pH 6, the pH decrease reaches 0.6 pH unit for A L PEI-P and up to 0.85 pH unit for A L PEI-PQ: metal sorption is followed by proton release and/or OH binding (possibly through the binding of hydrolyzed species around pH 5 (such as W 6 O 20 (OH) 5-). It is noteworthy that pH variation is the same (superposition of curves) for A L PEI-Q and A L PEI-PQ; the predominant impact of quaternary ammonium groups is clearly demonstrated against phosphonate moieties regarding proton exchange.

Uptake kinetics

The kinetics of sorption may be controlled by a series of diffusion mechanisms, including resistance to bulk diffusion and film diffusion (whose effects may be limited by appropriate agitation) and resistance to intraparticle diffusion. Preliminary tests showed that adopting a velocity of agitation of 210 rpm allows preventing the settling of sorbent particles, insuring the concentration of the metal (and sorbent) to be homogeneous in the reactor, and minimizing the effect of resistance to film diffusion. The uptake kinetics for the three sorbents are compared under the same experimental conditions in Fig. 3. The equilibrium time varies with the sorbent: ≈120 min for A L PEI, ≈90 min for A L PEI-P, and ≈60 min for A L PEI-PQ. The increase in the density of reactive groups and the diversity in functional groups tend to enhance kinetic rate. Hubbe et al. [START_REF] Hubbe | Implications of apparent pseudo-second-order adsorption kinetics onto cellulosic materials: A review[END_REF][START_REF] Simonin | On the comparison of pseudo-first order and pseudo-second order rate laws in the modeling of adsorption kinetics[END_REF] discussed the different experimental conditions requested for appropriately applying the pseudo-first order and pseudosecond order rate equations (Table S7a). Among the criteria that are frequently not enough taken into account they reported the variability of solute concentration in the solution. They remind that the correct interpretation of PSORE model requires the concentration remaining constant to be able to correlate mechanism interpretation (chemisorption vs. physisorption) with good mathematical fit of experimental profiles with the relevant equation. Hubbe et al. [START_REF] Hubbe | Implications of apparent pseudo-second-order adsorption kinetics onto cellulosic materials: A review[END_REF] concluded that, when the conditions are not satisfied, the PSORE fit can be more appropriately correlated to a system controlled by resistance to intraparticle diffusion. Herein, except for A L PEI (where concentration variation does not exceed 10%), the other sorbents show large concentration using the stability constants reported by Baes and Mesmer, [START_REF] Baes | Hydrolysis of Cations[END_REF]) shows that up to pH 4.5 the predominant species is W 12 O 39 6-(a polynuclear species) (Figure S13). However, above pH 4.5, the fraction of WO 4

2-(free mononuclear species) progressively increases (and predominates above pH 5). Sorption is favored by the presence of polynuclear anionic species. Binding a single polynuclear mole allows binding 12 W(VI) moles. Similar affinity for polynuclear species has already been reported in the case of molybdate and vanadate species using chitosan-based sorbents [START_REF]Interactions of metal ions with chitosan-based sorbents: a review[END_REF]. The sorption may engage the formation of polytungstate/phosphonate complexes [START_REF] Sethuraman | Heteropolypentatungstobisphosphonates -cyclopentane-like pseudorotation of an oxometalate structure[END_REF]. Apparently, this mechanism is not active in acidic solution: the sorption capacity at pH 3 is identical to the sorption level reached with raw material (A L PEI). With the direct quaternization of A L PEI (A L PEI-Q), the sorption in acidic solutions is higher than with the raw material (i.e., 0.2 mmol W g -1 ) and progressively increases to 0.47 mmol W g -1 at pH eq ≈ 4.7 before slightly decreasing at pH eq 5.2. Quaternary ammonium groups essentially contribute in the acid pH range to W(VI) binding. In the case of A L PEI-PQ, the quaternization offers complementary reactive groups to phosphonate moieties; quaternary ammonium moieties, which have strong affinity for W(VI) anions that are probably associated with an ionexchange mechanism. The bi-functional sorbent cumulates ionexchange from amine and quaternary ammonium groups and chelating abilities of phosphonate moieties. At the optimum pH (identical to A L PEI-P; i.e., pH eq ≈5), the sorption capacity increases up to 1.14 mmol W g -1 . The predominant species are bearing 6 anionic charges per polynuclear species, three times more than the predominant species at pH eq > 5 (i.e., for WO 4

2-

). This may be correlated with the stronger decrease in the sorption capacity at near neutral pH (compared with A L PEI-P sorbent). This is confirmed by the analysis of the log 10 plots of the distribution ratios against pH eq (Figure S14a). The combination of different mechanisms (especially for the bi-functional sorbent) makes complex the interpretation of the slopes (frequently used for analyzing the stoichiometric ratio between metal ions and reactive groups, and/or the proton exchanges). However, these plots allow highlighting the sensitivity of the different systems against pH. The dual functionalization clearly enhances W(VI) sorption by synergistic (a) complexation of tungstate with phosphate groups, and (b) ion exchange of phosphonate anions with quaternary ammonium groups.

Figure S14b shows the pH variations associated with the different sorbents during W(VI) sorption. A L PEI sorbent shows the weakest effect × 10 -8 m 2 min -1 , [START_REF] Marcus | Ion Properties[END_REF]). This means that the resistance to intraparticle diffusion does not play a critical role in the overall control of W(VI) sorption; this is directly correlated to the large porosity of the sorbents (Section 3.1.1.). This conclusion is particularly meaningful since the size of tungstate ions in solution may be drastically increased due to the formation of polynuclear species in moderately acidic solutions (W 12 O 39 6-, Figure S13).

Sorption isotherms and thermodynamics

The sorption isotherms report the distribution of the solute for different concentrations between solution and sorbent at fixed temperature and under similar pH conditions. Two important criteria can be deduced from experimental profiles: (a) the sorption capacity on the saturation plateau, and (b) the initial slope of the isotherm (which can be correlated to the affinity coefficient). Fig. 4 compares the sorption isotherms at T: 22 ± 1 • C and pH 0 : 5. This figure combines the duplicates and the modeling of experimental profiles with the Sips equation; duplicate experimental curves show the good reproducibility of sorption tests (Figure S16). Figures S17 shows the fitting of cumulated duplicated series with Sips and Temkin equations. Table 2 compares the modeling of experimental curves for A L PEI, A L PEI-P, and A L PEI-PQ (modeling of individual curves are reported in Table S8). For the three sorbents, the Sips equation (a combination of Langmuir and Freundlich equations) best fits sorption isotherms. In the case of A L PEI, the calculated sorption capacity at saturation drastically exceeds the experimental value (1.071 mmol W g -1 vs. 0.459 mmol W g -1 . Considering N content (i.e., 3.13 mmol N g -1 ), the molar ratio at saturation reaches 6.8 mol N per mole W (VI). Assuming that the predominant specie (W 12 O 39 6-

) is sorbed, the molar ratio tends to 0.57 mol N per mole of polynuclear specie; this molar ratio is not consistent with charge balance (contrary to the charge ratio: 6.8 protonated amine per W 12 O 39

6-

). After the functionalization of A L PEI (A L PEI-P and A L PEI-PQ), the calculated sorption capacities at equilibrium are more consistent with experimental data (overestimation by 17% for A L PEI-P and 4% for A L PEI-PQ). The functionalizations strongly increase sorption performances, both in terms of maximum sorption capacity (0.46 < 1.09 < 1.77 mmol W g -1 ) and affinity coefficient (0.345 < 2.96 < 8.32 L mmol -1 ), while the exponential term hardly changes (in the range 1.13-1.24). While the phosphorylation of A L PEI maintains N-content, P content reaches 1.39 mmol P g -1 (as -P(=O)(O-C 4 H 9 ) 2 ). The increase in maximum sorption capacity (i.e., 0.63 mmol W g -1 ) represents only 45% of grafted phosphonate moieties. The stoichiometric ratio between new functional groups and tungstate approaches 2:1. Obviously, the environment of phosphonate groups with amine groups (under different configurations) and the change in acid-base properties (pH PZC values) do not allow the direct comparison of sorption properties on the simple basis of addition of individual densities of reactive groups. The same features and explanation can be applied to A L PEI-PQ sorbent. The maximum sorption capacity increases by 0.68 mmol W g -1 , while for constant P content, the increase in N content (as quaternary ammonium salt) reaches 2.6 mmol N g -1 . The molar ratio on additional quaternary ammonium and W(VI) sorption capacity tends to 3.8; meaning that the sorption mechanism does not involve a simple and single ion-exchange mechanism for the binding of polynuclear tungstate species. In order to approach the effect of multi-functionality the sorption isotherms for A L PEI-P and A L PEI-PQ have been tentatively fitted with the Langmuir Dual Site equation:

q eq = q m,L1 C eq 1 + b L1 C eq + q m,L2 C eq 1 + b L2 C eq (2)
where the parameters are similar to those reported for the Langmuir equation (Table S7b) for the two types of reactive sites (Site #1 and Site #2). Table S8 summarizes the parameters (and statistical criteria), while Figure S18 reports the modeling of the isotherms with the Langmuir Dual Site model (Table S9). In the case of A L PEI-P, the introduction of the second reactive group does not improve the quality of the fit; the Langmuir fit remains the most appropriate. In addition, Figure S19 shows that the contribution of the Site #2 is limited to 0.125 mmol W g -1 (almost equally on the whole range of concentrations, and less than 12% of the maximum sorption capacity). In the case of A L PEI-PQ, the variations and the interpretation of mathematical fits must be considered just as a tool for comparing the systems. Table 1 summarizes the parameters of the models (and their statistical criteria) for the three sorbents. Globally, the kinetic profiles are best fitted by the PFORE: the calculated equilibrium sorption capacities are close to the experimental values (variation < 7.6%). Except for A L PEI, both R 2 and AIC criteria show that the PFORE describes better W(VI) sorption. Consistently with the initial slopes of the kinetic curves (Fig. 3), the apparent rate coefficient k 1 slightly increases with sorbent quaternization (from 3.46 to 3.62 × 10 -2 min -1 to 4.06-4.20 × 10 -2 min -1 ). It is important to remind that PFORE and PSORE have been initially designed for describing the kinetics of homogeneous chemical reaction; however, when applied to heterogeneous systems the rate parameters must be considered as apparent rates (which implicitly integrate the contribution of diffusion mechanisms). Though the RIDE is not the best equation for fitting experimental profiles (Figure S15), it may be used for comparing the systems and approaching the order of magnitude of effective diffusivity coefficient (D e , m 2 min -1 ) of W(VI) into the three sorbents. Table 1 reports the calculated values of D e : a greater variation is reported for the duplicate values in the case of A L PEI (8.2-5.2 × 10 -9 m 2 min -1 ) compared with the values reported for A L PEI-P (6.1-6.8 × 10 -9 m 2 min -1 ). The quaternization of the sorbent leads to little decrease in the size of the beads; contrary to the trends observed in Fig. 3, D e values are significantly decreased (i.e., 3.0-2.9 × 10 -9 m 2 min -1 ). Anyway, the values of D e (i.e., 8.2-2.9 × 10 -9 m 2 min -1 ) are only one order of magnitude lower than the self-diffusivity of tungstate in water (i.e., 5.51 relative contributions of the two sites vary with the temperature. More specifically, the Site #1 contributes to maximum sorption capacity in the range 1.4-1.7 mmol W g -1 , while the contribution of Site #2 reaches a maximum sorption capacity in the range 0.57-1.05 mmol W g -1 . In most cases, the affinity coefficient of Site #2 is much lower than that of Site #1. It is noteworthy that the quality of isotherm fits (with the LDS model) decreases with increasing the temperature. The thermodynamic parameters (changes in enthalpy, ΔH 0 , and Gibbs free energy, ΔH 0 , in kJ mol -1 , and entropy change, ΔS 0 , in J K -1 mol -1 ) were calculated using the van't Hoff equation:

lnK 0 eq = -ΔG 0 RT = -ΔH 0 RT + ΔS 0 R (3)
where K eq 0 is the standard thermodynamic equilibrium constant. Tran et al. [START_REF] Tran | Thermodynamic parameters of liquid-phase adsorption process calculated from different equilibrium constants related to adsorption isotherms: A comparison study[END_REF] discussed the application of a series of sorption isotherms with their relevant thermodynamic approach. They reported the importance of converting the parameters of equilibrium constant (K model , for example affinity constant in the case of Langmuir equation) to dimensionless form. In addition, they listed the relationship between K 0 eq and K model .

In the case of the Langmuir equation, the relationship reduces to:

K 0 eq = b * L × C 0 sorbate γ sorbate (4)
where b L * is the converted form of the Langmuir affinity coefficient (dimensionless), C 0 sorbate is the unitary standard concentration of the sorbate (~1 mol L -1 ), and γ sorbate is the activity coefficient of the sorbate (~1, for diluted solution).

Fig. 5 reports W(VI) sorption isotherms at different temperatures, with the Langmuir and the Sips fits (Table 3). Both maximum sorption capacity and affinity coefficient (correlated to the initial slope) increase with the temperature. The sorption of W(VI) is endothermic for A L PEI-PQ sorbent. Figure S20 shows the corresponding van't Hoff plot applied to the affinity coefficient from Langmuir equation (the relevant parameters for duplicated series are all joined). The dispersion of data leads to poor determination coefficient; the thermodynamic parameters reported in Table S10 must be considered as indicative values. The enthalpy change is positive meaning that sorption is endothermic and relatively low (≈ 2.1 kJ mol -1 ), while the entropy changes is close to 150 J K -1 mol -1 . The positive entropy means that with W(VI) sorption the randomness of the system increases. This increase in the entropy of the global systems is associated with the dehydration of metal ions and the release of exchanged ions. The negative value of the change of Gibbs free energy means that the sorption process is spontaneous (and becomes more spontaneous with thermal activation).

Table 4 compares the sorption properties of a series of sorbents for W n.s.: non-significant.

(VI). Taking into account the Langmuir-related parameters and the equilibrium contact time, A L PEI-PQ is part of the most efficient sorbents, comparable to acid-treated carbon cloth [START_REF] Afkhami | Mo(VI) and W(VI) removal from water samples by acid-treated high area carbon cloth[END_REF]; just surpassed by alumina [START_REF] Vucina | Sorption of tungsten on alumina in dynamic conditions[END_REF]. Most of these sorbents have been tested in the pH range 4-7. In strong acid solutions, the sorption capacities are negligible; except for TVEX-TOPO resin (a resin impregnated with trioctylphosphine oxide) that accommodates 0.5 M HNO 3 solutions.

Binding mechanisms

The FTIR analysis of sorbents before and after metal sorption confirms suspected binding mechanisms. In the case of A L PEI, the poor sorption levels hardly alter FTIR spectrum: the most significant change is observed in the region where protonated amine groups are identified (i. e., 1530-1300 cm -1 ), meaning that protonated amine groups bind tungstate anions (Figure S4, Table S2). The sorption of W(VI) onto phosphorylated A L PEI beads is followed by a significant decrease of the relative intensity of the typical bands of phosphonate groups, while after metal desorption the peaks are restored: as expected, phosphonate groups are involved in tungstate binding. For bi-functionalized sorbent (i.e., A L PEI-PQ), the sorption of tungstate induces numerous additional changes to those observed for A L PEI-P (assigned to phosphonate moieties), such as the disappearance of the quaternary ammonium salt peak (at 974 cm -1 ), the superposition of W-O stretching with the P-O stretching (at 1251 cm -1 ), and the appearance of a shoulder at 1730 cm -1 (attributed to carboxylic acid groups). Contrary to A L PEI-P, in the case of A L PEI-PQ metal desorption does not completely restore the FTIR spectrum. However, despite these chemical modifications the sorption/ desorption performances are still maintained for a minimum of five cycles (see Section 3.2.6.). These conclusions are confirmed by the substantial changes observed in the XPS spectra of A L PEI and A L PEI-PQ (reported in Figures S6-S7 and Table S5): amine groups (protonated form) are involved in W(VI) binding for A L PEI, while metal binding affects amine-based groups (not only quaternary ammonium salt sites), and O-based groups (from phosphonate moieties). In addition, the analysis of W 4f signal apparently shows that a small fraction of tungstate is reduced to W(V) for both A L PEI and A L PEI-PQ sorbents. This information combined with the effect of pH on metal sorption (protonation/deprotonation of reactive groups in relation with both overall charge surface and pH PZC , and metal speciation) allows proposing different sorption mechanisms for the binding of tungstate anions onto A L PEI-PQ sorbent (Scheme 2). The reactive groups that may significantly contribute to metal binding are hydroxyl, amine groups (present on quaternary ammonium moiety), and phosphonate chelating groups. These hypotheses are supported by the study of the effect of pH and the evaluation of pH PZC values (electrostatic attraction and ionexchange process depends on the pH). At optimum pH value, the partial protonation of reactive groups and the presence of quaternary ammonium groups support the anion exchange mechanism between R 4 N + groups and tungstate anions (including polynuclear species:

W X O Y Z-
). On the other hand, phosphonate groups may contribute to chelate tungstate through electron-donating groups (in addition to hydroxyl and nitrogen from free amine groups, depending on the pH). These different modes of interactions can be correlated to the optimum pH values, the comparison of FTIR and XPS spectra. The combination of ion-exchange and chelating modes for the different reactive groups enhances the binding of mononuclear or polynuclear species; their respective contributions depend on the pH (metal speciation and the extent of protonation and deprotonation of reactive groups).

Effect of competitor metal ions -Selectivity

In order to anticipate on the possibility to use A L PEI-PQ for the treatment of complex solutions, the sorption properties of W(VI) have been explored in multi-component equimolar solutions containing Ca (II), Mg(II), Fe(III), Al(III), Zn(II), and Nd(III) (metal ions usually found in the leachates of W-bearing Egyptian ores). Different pH values (from 3 to 6) have been tested. Fig. 6a shows that the selectivity for W(VI) increases with decreasing the pH; this conclusion is confirmed by the semi-quantitative EDX analysis of sorbent surface (Figure S21). The sorption of metal cations increases with pH, contrary to tungstate anions. It is noteworthy that the presence of competitor ions reverse the effect of the pH on the distribution ratio (Figure S22): D w increases with the pH in single-component solutions, contrary to multi-component solutions. This inversion is probably associated with the increasing competitor effect of other metal ions when increasing the pH; though W(VI) sorption is less favorable in acidic solutions, the competition of metal cations at higher pH values limits the number of reactive groups available for W(VI) binding. Fig. 6a clearly confirms the highest affinity of the sorbent for W(VI) at pH eq 3.4. The selectivity for W(VI) against metal cations follows the trend: Mg(II) (31.9) > Ca(II) (17.8) > Al(III) (15.1) > Zn(II) (9.3) > Fe(III) (8.4) > Nd(III) (5.3).

Tungsten is part of the hard acids according the hard and soft acidic and base theory [START_REF] Pearson | Acids and bases[END_REF], as most of the competitor metals (except Zn(II)). Since a significant part of tungstate binding occurs through ionexchange mechanisms on quaternary ammonium salt moieties, the preference of the sorbent for metal anions can explain its good selectivity for tungstate (or other metal-anion forming species). The weaker selectivity for Nd, its high-value, and presence in Egyptian ores from Units: Time (min); q m : mmol W g -1 ; b L : L mmol -1 .

Scheme 2.

Tentative mechanisms for the sorption of tungstate anions onto A L PEI-PQ.

Table 3 Effect of temperature on W(VI) sorption isotherms for A L PEI-PQ -Parameters of Langmuir, Sips and Temkin models (cumulating duplicated series).

Eastern Desert made pertinent the supplementary analysis of selectivity profiles for Nd(III) against other metal anions. Fig. 6b shows that the selectivity of A L PEI-PQ for Nd(III) is hardly affected by the pH (though little better at pH 5. The selectivity for Nd(III) depends on the pH only for Ca(II) (increasing from 2.7 to 5.5).

Figure S23 reports the cumulative sorption capacity that increases with the pH to reach a maximum (close to 3.1 mmol g -1 ) at pH 5.5-5.8. The relative fraction of W(VI) in the sorbent progressively decreases with the pH (from 38% to 15%), while that of Nd(III) remains stable (around 18%) below pH 4.8, before slightly increasing up to 22% at pH 5.8. The fractions of the other metal ions in the sorbent are poorly affected by the pH: Fe(III) (15.2 ± 1.5 %) > Zn(II)≈Al(III) (12.7 ± 2.3 %) > Ca(II) (8.1 ± 0.9 %) > Mg(II) (6.4 ± 1.5 %).

In acidic solutions, the sorbent selectively recovers W(VI) over metal cations; increasing the pH to 5.8 (after tungstate removal) would not allow separating neodymium from residual metals but could contribute to enriching the concentration the rare earth element in the sorbent.

Metal desorption and sorbent recycling

The evaluation of a sorbent requires investigating desorption of metal-loaded material, not only for recovering toxic or valuable metal but also for analyzing the possibility to recycle the sorbent. Metal desorption is highly effective using 0.2 M NH 4 Cl solution (Figure S24). Tungstate is fully desorbed within 30 min of contact. The same eluent can be used indifferently with A L PEI, A L PEI-P, and A L PEI-PQ, without significant differences in the kinetic profiles. Five cycles of sorption and desorption were tested for evaluating the variation in both sorption efficiency and desorption yield. The FTIR analysis of the sorbents after these five cycles has shown significant changes compared with their original spectra. Therefore, the comparison of sorption and desorption performances is of critical importance. Table 5 shows that the sorption efficiency progressively decreases with the number of re-uses. Since desorption efficiency is not significantly changed during the re-uses, the loss in sorption is not associated with the progressive saturation of sorption sites. Therefore, the decrease in sorption performance is probably due to the physical or chemical degradation of the sorbents. XPS analyses have shown a weak but detectable reduction of W(VI) to W (V); corollary to this reduction of the metal, the sorbent is probably partially oxidized (consistently with observed FTIR changes). It is noteworthy that the loss in sorption performance (between first and fifth cycle) significantly decreases with the functionalization of the sorbent: 17% for A L PEI, 10% for A L PEI-P, and down to 6% for A L PEI-PQ. The bifunctional sorbent is remarkably stable in terms of re-use. In the case of A L PEI, W(VI) desorption process should be improved. Masry et al. [START_REF] Masry | Sorption behavior of tungsten and molybdenum on TVEX-TOPO resin from nitric acid solution[END_REF] successfully desorbed tungstate in the presence of ascorbic acid in the case of extractant-impregnated resins.

Application to real effluent

Ore pre-treatment (ore concentrate, leaching, and solvent extraction processing)

Annex A provides information on the location of ore collection (with geochemical and mineralogical relevant characteristics). The ore was pre-treated with density and magnetic concentration. The concentrate is essentially constituted of hubnerite (manganese tungstate that may contain traces of other metal tungstates). Table A1 reports the composition of the concentrate. The acidic leaching of the concentrate allows recovering 95% of manganese and 99% of iron without removing valuable elements such as W (less than 0.1%) and Nb (negligible); the composition of the acidic leachate is reported in Table A2 (XRF characterization of produced tungstic acid). The next step in the process consisted in the alkaline leaching of the tungstic acid. This step allows releasing about 99% of W(VI) and only 10% of Nb(V). The residue is washed with water and the aqueous phase is mixed with the alkaline leachate to produce the pregnant alkaline sodium tungstate solution (PLS, 1 L, pH 12.3). The composition of PLS is reported in Table A3.

The treatment of PLS (pH adjusted to 8 with HCl solution) is processed using solvent extraction (Alamine 336 in kerosene, with octanol as the modifier) [START_REF] Abu Khoziem | Sequential recovery processes of tungsten from hubnerite mineral concentrate of Qash Amir mineralized quartz veins, South Eastern Desert, Egypt, Physicochem[END_REF][START_REF] Paulino | Recovery of tungsten by liquid-liquid extraction from a wolframite concentrate after fusion with sodium hydroxide[END_REF]. Tungstate extraction yields 94%. Organic phase is scrubbed with demineralized water for removing sodium and silica impurities. The W-loaded organic phase is stripped with ammonium hydroxide; stripping yields 97.9%. The strip solution is evaporated to produce pure ammonium paratungstate (grade: 99.97%; with a rate of 90%). Calcination at 750 • C allows producing pure tungsten oxide (i.e., WO 3 ). The composition of the raffinate (i.e., residue of solvent extraction completed with scrub solution) is reported in Table A4: tungstate is one of the major elements, with concentration as high as 374.9 mg W L -1 (i.e., 2.04 mmol W L -1 ), apart Na at 5.58 g Na L -1 (i.e., 0.243 mol Na L -1 ).

Note: The experimental conditions, for each individual step, are fully reported in Annex A.

Tungstate recovery from raffinate

Consistently with the methodology followed for the study of selectivity, the raffinate was adjusted to different pH values in the range 3.3-6.3, before processing to sorption tests. Under selected experimental conditions (i.e., more specifically, SD: 1 g L -1 ), the sorption process allows recovering almost 96% of tungstate at pH eq 4.74 (optimum pH for sorption efficiency, Fig. 7a), while for other metals the recovery does not exceed 58%. The maximum sorption efficiency for the other metals is generally obtained at pH eq 5.85-6.28, consistently with the trends observed in the study of selectivity (Section 3.2.5.). The curve for Na(I) is not reported; the weak removal (lower than 3%) is explained by the huge level of sodium in the raffinate. Figure S25 reports the distribution ratios (log 10 plot) against equilibrium pH. This figure confirms these trends: (a) in most cases, the distribution ratios are poorly influenced by the pH (except for tungstate), (b) all the elements are grouped (except sodium, which large excess leads to poor concentration effect into the sorbent), and (c) tungstate uptake shows a maximum in the distribution ratio at pH eq 4.74, about one order of magnitude greater in D value compared with those of other metals.

Fig. 7b plots the SC W/metal values vs. pH eq . As expected (from Figure S25), the highest values for tungstate selectivity are obtained at pH eq 4. [START_REF] Marcus | Ion Properties[END_REF]. The sorbent allows outstanding separation (selective recovery) for W(VI) against: Na(I) (927) > Si(IV) (47.7) > Ti(IV) (42.8) > Ca(II) (35.2) > Al(III) (33.2) > Ta(V) (28.7) > Nb(V) (15.9).

Despite the complexity of the raffinate with huge excess of Na(I) and the presence of other metal cations, A L PEI-PQ allows recovering W(VI) with high efficiency at pH eq close to 4.7. Under these conditions, Figure S26a reports the percentage of metals on loaded sorbent. Consistently with the huge excess of Na(I) in the raffinate, this element represents about 56 % of the metals bound to A L PEI-PQ. Similarly, W(VI) represents about 19%, Si(IV) about 13.5% and Al(III) ≈5.5%; other elements count for less than 6%. Figure S26b gives another visualization of these data through the plot of the concentration factor (CF: q eq /C 0 , L g -1 ). The concentration effect is significantly higher for the metals that form anionic complexes (such as tungstate, niobiate). Apart the direct effect of their relative concentration in the raffinate, the strong affinity of A L PEI-PQ for metal ions that form anionic complexes makes it highly selective for tungstate, with a good concentration effect, suitable for valorization of the metal from the residues of solvent extraction processing. Figure S27 reports the semi-quantitative EDX analysis of the sorbent after treating the raffinate at pH eq 3.28 and 6.28. These analyses clearly show the wide spectrum of metal bounds to A L PEI-PQ. It is especially interesting to observe the affinity of the sorbent for strategic metals such as niobium, tantalum, titanium or nickel.

Cost analysis and comparison of performance with commercial resins

The cost analysis of A L PEI-PQ synthesis is made difficult by the labscale production: cost of reagents, operating costs, investment amortization would considerably change with a large-scale application. In Supplementary Information (Annex B), a very simple evaluation of the production costs (only based on the compilation of reagent expenses) is provided. The sorption capacity of A L PEI-PQ is compared for tungstate recovery from the residues of solvent extraction (in the processing of ore leachate) with the performances obtained with two commercial resins (i. e., Amberlite IRA402 and Dowex 1X8 200-400 Mesh). Under comparable experimental conditions, the sorption capacities of A L PEI-PQ reaches 0.487 mmol W g -1 ; meaning significantly higher than those obtained with Amberlite IRA402 (i.e., 0.070 mmol W g -1 ), or Dowex 1X8 (i.e., 0.145 mmol W g -1 ). Referred to the uptake of 1 mmol W, the cost of material in metal recovery is significantly lower for the new sorbent (i.e., 75 €) than for commercial resins (i.e., 156-193 €). Obviously, the incomplete evaluation of the production cost for A L PEI-PQ requires considering with caution this analysis; however, this preevaluation confirms that the new sorbent offers promising perspectives.

Conclusion

The poor efficiency of A L PEI sorbent for binding tungstate from

Table 5

Metal sorption and sorbent recycling -Sorption (SE, %) and desorption (DE, %) efficiencies for five successive cycles for A L PEI, A L PEI-P, and A L PEI-PQ.

weakly acidic solutions was incentive to functionalize the support using phosphorylation. Despite the benefit of phosphonate grafting that increases the maximum sorption capacity by ≈0.6 mmol W g -1 , the subsequent quaternization of the modified sorbent allows increasing even more the sorption capacity (by 0.68 mmol W g -1 for A L PEI-PQ). The study of pH effect also compared the behavior with directly quaternized A L PEI beads; these results show that the synergistic effect of quaternary ammonium groups and phosphonate moieties benefits to W(VI) sorption performance as a justification of bi-functionalization. With a maximum sorption capacity close to 1.77 mmol W g -1 , the bi-functional sorbent is part of the most efficient sorbents reported in the literature. Both, the strong affinity of quaternary ammonium salt and phosphonate groups and the large meso-porosity of the material explain the fast uptake (equilibrium reached within 60 min). The synergistic effects of phosphonate and quaternary ammonium salt groups strongly enhance sorption isotherms and uptake kinetics. The sorption of tungstate involves both complexation (with phosphonate groups) and ion-exchange with quaternary ammonium groups. The sorption isotherms can be fitted using the Sips equation and the Langmuir Dual Site model, while the uptake kinetics are efficiently described by the pseudo-first order rate equation. XPS and FTIR spectrometric analyses confirm the chemical modification of the sorbent and contribute to explain the interaction mechanisms. Despite the non-reversible modification of the sorbents during sorption/desorption cycles and the partial reduction of tungstate on the sorbents, the sorption properties remain remarkably stable (especially for the bi-functional sorbent). Ammonium chloride solution (0.2 M) allows achieving the complete desorption of tungstate at a relatively fast rate (less than 30 min of contact). The bi-functional sorbent shows remarkable selectivity for tungstate in multi-component solutions (synthetic equimolar solutions). This is also confirmed by the efficient recovery of the metal from the raffinate produced during W(VI) solvent extraction from the leachates of hubnerite concentrate (in the processing of Egyptian ores from Southwestern Desert).

Fig. 1 .

 1 Fig. 1. XPS core level spectra of A L PEI and A L PEI-PQ sorbents before and after W(VI) sorption.

Fig. 2 .

 2 Fig. 2. Effect of pH on W(VI) sorption using A L PEI, A L PEI-P, A L PEI-Q, and A L PEI-PQ (C 0 : 0.275 mmol W L -1 ; Sorbent dose, SD: 0.4 g L -1 ; v: 210 rpm; time: 48 h; T: 22 ± 1 • C, duplicated series: #1 and #2).

Fig. 3 .

 3 Fig. 3. W(VI) sorption using A L PEI, A L PEI-P, and A L PEI-PQ at pH 0 5 -Modeling with the PFORE (C 0 : 0.565 mmol W L -1 ; SD: 0.4 g L -1 ; v: 210 rpm; T: 22 ± 1 • C; duplicated series).

Fig. 4 .

 4 Fig. 4. W(VI) sorption isotherm at pH 0 5 using A L PEI, A L PEI-P, and A L PEI-PQ -Modeling with the Langmuir equation (C 0 : 0.05-2.8 mmol W L -1 ; SD: 0.4 g L -1 ; v: 210 rpm; T: 22 • C; time: 48 h; pH eq : 5.09-5.14 for A L PEI, 4.91-4.82 for A L PEI-P, and 4.95-4.76 for A L PEI-PQ; duplicated series).

Fig. 5 .

 5 Fig. 5. Effect of temperature on W(VI) sorption isotherm at pH 0 5 using A L PEI-PQ -Modeling with the Langmuir equation (a) and the Sips equation (b) (C 0 : 0.05-2.8 mmol W L -1 ; 0.4 g L -1 ; v: 210 rpm; time: 48 h; duplicated seriesmodeling with cumulated series).

  8): Mg(II) (5.5) ≈ Ca(II) (5.5) > Al(III) (2.6) > W(VI)≈Zn(II)≈Fe(III) (2.3-2.0).

Fig. 6 .

 6 Fig. 6. Effect of pH on sorption selectivity using A L PEI-PQ: (a) SC W/Metal , and (b) SC Nd/Metal (equimolar concentrations, C 0 : 1 mmol L -1 ; SD: 1 g L -1 ; time: 600 min; v: 210 rpm; T: 22 • C).

Fig. 7 .

 7 Fig. 7. Effect of the pH on sorption efficiency (%) and selectivity coefficient SC W/Metal (b) for A L PEI-PQ used for the recovery of W(VI) from raffinate produced during the treatment of hubnerite leachates (SD: 1 g L -1 ; T: 22 ± 1 • C; v: 210 rpm; time: 10 h; numbers represent outstanding out-of-range SC values for pH eq 3.28-4.74).

Table 1 W

 1 (VI) uptake kinetics using -Parameters of the models.

	Model	Parameter	Unit	Sorbent
				A L PEI		A L PEI-P		A L PEI-PQ
				#1	#2	#1	#2	#1	#2
	Exper. PFORE	q m,exp. q m,1 k 1 × 10 2 R 2	mmol W g -1 mmol W g -1 min -1 -	0.142 0.135 3.68 0.979	0.145 0.156 1.94 0.981	0.571 0.606 3.62 0.977	0.603 0.630 3.46 0.989	1.146 1.211 4.06 0.978	1.158 1.222 4.20 0.974
	PSORE	AIC q m,2 k 2 × 10 2 R 2	-mmol W g -1 g mmol -1 min -1 -	-154 0.160 27.1 0.984	-158 0.209 7.88 0.973	-108 0.741 5.05 0.948	-118 0.775 4.56 0.968	-83 1.459 3.00 0.947	-86 1.158 3.12 0.942
	RIDE	AIC De × 10 9 R 2	-m 2 min -1 -	-162 8.19 0.990	-155 5.19 0.963	-99 6.76 0.948	-105 6.07 0.965	-78 3.02 0.933	-76 2.88 0.928
		AIC	-	-171	-148	-93	-98	-69	-68

Table 2 W

 2

	Model	Parameter	Unit	Sorbent
				A L PEI	A L PEI-	A L PEI-
					P	PQ
	Experimental Langmuir	q m,exp. q m,L b L R 2	mmol W g -1 mmol W g -1 L mmol -1 -	0.459 0.849 0.484 0.982	1.091 1.167 4.808 0.989	1.771 1.736 16.11 0.990
	Freundlich	AIC k F	-mmol 1-1/n g -1 L 1/ n	-162 0.265	-133 0.892	-115 n.s.
		n F R 2	--	1.569 0.979	3.146 0.959
	Sips	AIC q m,S b S	-mmol W g -1 (mol L -1 ) nS	-169 1.071 0.345	-118 1.276 2.956	1.839 8.320
		n s R 2	--	1.133 0.982	1.240 0.991	1.216 0.993
	Temkin	AIC a T b T R 2	-L mol -1 kJ kg mol -2 -	-170 15.47 21.78 0.911	-151 97.45 12.07 0.985	-121 308.4 8.624 0.992
		AIC	-	-141	-143	-121

(VI) sorption isotherms using A L PEI, A L PEI-P, and A L PEI-PQ -Parameters of Langmuir, Sips and Temkin models (cumulating duplicated series).

Table 4

 4 Comparison of tungstate sorption properties for selected sorbents.

	Model	Parameter	Unit	Temperature ( • C)
					22	40	50	60
	Experimental Langmuir	q m,exp. q m,L b L R 2		mmol W g -1 mmol W g -1 L mmol -1 -	1.771 1.736 16.11 0.990	2.108 2.084 17.81 0.989	2.170 2.206 22.64 0.990	2.421 2.358 49.58 0.984
	Sips	AIC q m,S b S		-mmol W g -1 (mol L -1 ) nS	-115 1.839 8.320	-104 2.235 7.826	-102 2.316 10.98	-86 2.499 14.81
		n s R 2		--	1.216 0.993	1.289 0.993	1.254 0.993	1.385 0.991
	Temkin	AIC a T b T R 2		-L mol -1 kJ kg mol -2 -	-121 308.4 8.624 0.992	-118 494.4 8.256 0.986	-124 840.3 8.597 0.978	-109 1909 8.848 0.983
		AIC		-	-121	-106	-89	-86
	Sorbent		pH	Time	q m,	q m,L	b L	Ref.
					exp.
	Alumina		4	-	2.0	2.1	540	[18]
	Acid-treated carbon-	7	30	1.35	1.99	0.670	[76]
	cloth
	Composite soil		5	6000	-	0.011	0.035	[21]
	Kaolinite		5	2880	0.04	0.036	0.0035	[20]
	Diamino resin		6	60	0.344	0.369	3.13	[80]
	Nanocrystalline		≈6	480	-	0.294	32.2	[19]
	iowaite
	Multi-walled carbon	6	120	0.096	0.112	17,292	[27]
	nanotubes
	Soil		4.7	1080	0.027	0.035	0.259	[23]
	TVEX-TOPO resin	0.5 M	75	-	0.194	6.43	[33]
			HNO 3
	A L PEI		5	60	0.459	0.849	0.484	This
								work
	A L PEI-P		5	60	1.09	1.17	4.81	This
								work
	A L PEI-PQ		5	60	1.77	1.74	16.11	This
								work

Acknowledgements S. H. Alotaibi acknowledges Taif University for Researchers Supporting Project (TURSP-2020/83), Taif University, Saudi Arabia. W.Y acknowledges the support of National Natural Science Foundation of China [U1967218, and 11975082]. E. Guibal, and M. F Hamza acknowledge the support of IMHOTEP Program (MetalValor project).

Funding acquisition. Eric Guibal: Conceptualization, Methodology, Data curation, Investigation, Software, Writingreview & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi. org/10.1016/j.seppur.2022.120893.


Whelsy Boungou
email: w.boungou@psbedu.paris.

Alhonita Yatié
email: alhonita.yatie@u-bordeaux.fr.

The impact of the Ukraine-Russia war on world stock market returns *

Keywords: H56, G11, G14, G15 Stock index, War, Ukraine, Russia

As a topical issue, this paper studies the responses of world stock market indices to the ongoing war between Ukraine and Russia. Using daily stock market returns in a sample of 94 countries and covers the period from 22 January 2022 to 24 March 2022, we consistently document a negative relationship between the Ukraine-Russia war and world stock market returns. Our results point to a larger impact at the onset of war, especially during the first two weeks after the invasion of Ukraine on 24 February 2022. The reaction of global stock markets was weaker in the weeks that followed. Furthermore, we find that these effects were most pronounced for countries bordering Ukraine and Russia, as well as for those UN member states that demanded an end to the Russian offensive in Ukraine. Overall, we provide the first empirical evidence of the effect of the Ukraine-Russia war on world stock market returns.

Introduction

"The war is expected to have a considerable impact on the global economy."

Christine Lagarde, President of the ECB, 27 March 2022.

Several months after the deployment of military bases close to the border with Ukraine, Russia officially attacked Ukraine on 24 February 2022. The ongoing military action raises concerns about: (i) the duration of the conflict; (ii) how

Russia will respond to Western sanctions; (iii) its potential impact on the global economy and in particular on the reaction of global financial markets. In this paper, we focus on the latter. In tandem with the loss of life and destruction of property, this attack has introduced new uncertainty into world stock markets (in addition to those related to the COVID-19 pandemic). Although anecdotal, the New York Times1 noted that on the day of the attack, the S&P 500 index recorded its first correction since October 2020, i.e. it fell by more than 10% from its recent peak2 . To the best of our knowledge, there are no empirical studies yet exploring the impact of the Ukraine-Russia war on world stock markets. This paper is an attempt to fill this gap.

For our empirical investigation, we use daily data of stock market returns in a sample of 94 countries over the period from 22 January 2022 to 24 March 2022.

Our results highlight a negative and significant effect of the armed conflict between Ukraine and Russia on world stock returns. Although tensions between Ukraine and Russia have existed for a long time, the responses of stock market indices to the conflict were more pronounced starting with Russia's invasion of Ukraine on 24 February 2022. However, the reaction of global stock markets was weaker in the weeks following the invasion. Finally, we notice that the performance of the stock market indices was weaker for the countries bordering Ukraine and Russia, as well as for the UN member countries that demanded an end to the Russian offensive in Ukraine.

The results of our analysis are important on at least two aspects. First, they allow us to understand the financial impacts of the ongoing conflict so that investors, portfolio managers and policy makers can design effective financial strategies. Second, by considering the Ukraine-Russia war, we extend the previous studies on the relationship between wars and stock markets, which focused mainly on the Second World War (such as [START_REF] Frey | Wars and markets: How bond values reflect the second world war[END_REF][START_REF] Hudson | War and stock markets: The effect of World War Two on the British stock market[END_REF][START_REF] Goel | How vulnerable are international financial markets to terrorism? An empirical study based on terrorist incidents worldwide[END_REF][START_REF] Richard | Armageddon and the stock market: US, Canadian and Mexican market responses to the 1962 Cuban Missile Crisis[END_REF].

Data and model

The level of our analysis is country-day and covers the period from 22 January 2022 to 24 March 2022 (without weekends). Although the start of the war is 24

February 2022, many of the world's major forces had already been planning for it for some time and considered it only a matter of time. The daily world stock index data was obtained from the https://www.investing.com/, consisting of almost 3750 daily observations. We use the daily log return of each stock market index. Our sample is based on stock market returns in 94 countries around the Russia and Ukraine (reported in Tables with "Ukraine-Russia war").

The regression controls for time fixed-effects, , absorbing a level shift for each day, capturing the overall trend. The country fixed-effects, , absorbs the different fixed and time-invariant levels of search intensities across countries.

The standard errors are robust and clustered at the country level. Our empirical analysis captures the impact of the Ukraine-Russia war on world stock market returns.

Results

In this section, we present the results of our analysis on the impact of the war between Ukraine and Russia on world stock market indices. We organize our study in several steps by investigating: (i) the impact of the war on global stock market returns (Table 1); (ii) how global stock market responses evolved from the invasion (Table 2) and several weeks afterwards (Table 3); (iii) whether the effects differed according to geographical proximity to Ukraine and Russia (Table 4); (iv) whether stock market responses differed between neutral and condemning countries (Table 5).

Table 1 presents our main results considering several specifications. Our results highlight a negative and statistically significant relationship at the 1% level between the current conflict and stock market indices. Indeed, our results show that tensions between Ukraine and Russia have a significant negative impact on the performance of world stock market indices 5 . This result highlights the sensitivity of world markets to the Ukraine-Russia war 6 . This result is consistent with previous analyses that have also found a negative relationship between conflict and stock market indices (e.g. [START_REF] Frey | Wars and markets: How bond values reflect the second world war[END_REF]Hudson andUrquhart, 2015, 2022;[START_REF] Goel | How vulnerable are international financial markets to terrorism? An empirical study based on terrorist incidents worldwide[END_REF][START_REF] Richard | Armageddon and the stock market: US, Canadian and Mexican market responses to the 1962 Cuban Missile Crisis[END_REF]. For instance, exploring the effect of the Second World War (WWII) on the British stock market, [START_REF] Hudson | War and stock markets: The effect of World War Two on the British stock market[END_REF] find that the British stock market reacted negatively to WWII 7 .

Table 1. Ukraine-Russia war and stock market index

To deepen our discussion, we investigate how the stock market returns evolved before and after the invasion of the Russian armies in Ukraine, i.e. on 24 February 2022. To do so, we organize our sample into two subsets from: (i) 22

January 2022 to 23 February 2022 (Pre-invasion); (ii) 24 February 2022 to 24

March 2022 (Post-invasion). The results of this analysis are reported in Table 2.

We still observe a negative relationship between the Ukraine-Russia war and global stock market indices. However, we highlight that this negative impact was significantly greater from the time of Russia's invasion of Ukraine compared to the period prior to 24 February 2022.

7 Taking the case of terrorist attacks, [START_REF] Arin | The price of terror: The effects of terrorism on stock market returns and volatility[END_REF] 3). To do so, we analyze the reactions of the stock markets week by week. While we reveal a stronger impact at the beginning of the war (especially two weeks after the invasion), this impact becomes weaker three to four weeks after, thus highlighting a recovery in global stock markets. ***, **, and * stand for significance at the 1%, 5%, and 10% levels, respectively.

Table 3. The impact several weeks after the invasion Furthermore, we complete our analysis by assessing whether the impact is stronger in countries geographically close to Ukraine and Russia. We find that the stock market indices of countries geographically close to the conflict have been the most impacted by the war (Table 4). Finally, we examine whether the impact of the war was stronger for those UN member countries that condemned the invasion compared to countries that remained neutral (e.g. China, India, South Africa) 8 .We still observe a negative stock market reaction for both the countries that condemned the invasion and those that remained neutral. However, the impact was significantly greater for the countries that condemned the invasion (Table 5).

world 3 .

 3 Data on search volumes related to the Ukraine-Russia war was downloaded from Wikipedia Trends 4 . Using panel data, we analyze the reaction of world stock market returns to the Ukraine-Russia war. The panel data describes a worldwide sample of 94 countries and is estimated as follows: where denotes the log of the stock market index of country c on day d. represents the log of Wikipedia Trends search data in country c on day d, related to the war between Ukraine and Russia (including words like conflict, war, Ukraine, Russia, Vladmir, Putin). It therefore measures the intensity (or anxiety) of internet searches related to the current armed conflict between

  also find that terrorism has a significant and negative impact on global stock markets.

			Stock market index
		(I)	(II)	(III)	(IV)
	Ukraine-Russia war	-1.412***	-0.889***	-1.466***	-0.858***
		[0.42]	[0.33]	[0.46]	[0.26]
	Number of observations	3747	3747	3747	3747
	Number of countries	94	94	94	94
	Country fixed-effect	No	No	Yes	Yes
	Day fixed-effect	No	Yes	No	Yes
	R2 (within)	0.001	0.017	0.001	0.017

Notes: Robust stantard errors clustered at the country level are presented in brackets. ***, **, and * stand for significance at the 1%, 5%, and 10% levels, respectively.

Table 2 .

 2 The impact before and during the war To better understand the dynamics of the propagation of global shocks, we

investigate how global stock market indices have adjusted their responses as the war persists (Table

  : Robust stantard errors clustered at the country level are presented in brackets.

			Stock market index
		All periods Pre-invasion Post-invasion
	Ukraine-Russia war	-0.858***	-0.224	-1.001***
		[0.26]	[0.36]	[0.20]
	Number of observations	3747	1945	1802
	Number of countries	94	94	94
	Country fixed-effect	Yes	Yes	Yes
	Day fixed-effect	Yes	Yes	Yes
	R2 (within)	0.017	0.142	0.016

Notes

Table 4 .

 4 Nearby vs. geographically distant countries

			Stock market index
			The weeks after the invasion:
		1 week	2 weeks	3 weeks	4 weeks
	Ukraine-Russia war	-0.610***	-1.121*	-0.888**	-0.858***
		[0.19]	[0.57]	[0.36]	[0.26]
	Number of observations	2375	2811	3266	3747
	Number of countries	94	94	94	94
	Country fixed-effect	Yes	Yes	Yes	Yes
	Day fixed-effect	Yes	Yes	Yes	Yes
	R2 (within)	0.215	0.017	0.017	0.017
	Notes: Robust stantard errors clustered at the country level are presented in brackets. ***, **, and * stand
	for significance at the 1%, 5%, and 10% levels, respectively.

Notes: Robust stantard errors clustered at the country level are presented in brackets. ***, **, and * stand for significance at the 1%, 5%, and 10% levels, respectively.

Table 5 .

 5 Neutral countries vs. countries that condemned the invasionThis paper provides the first empirical evidence of the impact of the war between Ukraine and Russia on world stock market returns. Using daily data on stock returns for a sample of 94 countries over the period from 22 January 2022 to 24 March 2022, our results show significant negative effects of the Ukraine-Russia war on global stock indices. The results of this analysis are8 For more information on the UN resolution demanding an end to the Russian offensive in Ukraine: https://news.un.org/en/story/2022/03/1113152

		Stock market index
		Neutral	Condemn
		countries	countries
	Ukraine-Russia war	-0.266*	-0.944***
		[0.14]	[0.29]
	Number of observations	711	3036
	Number of countries	19	75
	Country fixed-effect	Yes	Yes
	Day fixed-effect	Yes	Yes
	R2 (within)	0.106	0.021
	4. Conclusion

Notes: Robust stantard errors clustered at the country level are presented in brackets. ***, **, and * stand for significance at the 1%, 5%, and 10% levels, respectively.

nytimes.com/2022/03/07/business/stock-market-today.html

Other stock indices also fell after the Russia attack, such as IMOEX (-33.28%), Wig Poland Index (-10.53%), DAX (-3.96%), FTSE Italia (-4.04%).

We recognize that a longer period of analysis would have been preferable in order to capture long-term effects. However, we believe that our results will go some way to initiating the debate on the financial impact of the war.

As a robustness check, we lag our stock market variable by one day, assuming that stock market responses are not immediate[START_REF] Kaplanski | Sentiment and stock prices: The case of aviation disasters[END_REF][START_REF] Hudson | War and stock markets: The effect of World War Two on the British stock market[END_REF]. We find similar results, not reported to save space (but available on request).

important for at least one reason: To understand the financial impacts of the ongoing conflict so that investors, portfolio managers and policy makers can design effective financial strategies.