Sorption of Perrhenate Ions by a New Anion Exchanger Based on an Oligomer of Epichlorohydrin and 4-vinylpyridine

In this study, oligomer epichlorohydrine (OECH) was crosslinked with 4-vinylpyridin (VP) present initiator of peroxide benzoyl (BP). The resulting anionite (OECH-VP) was characterized by scanning electron microscopy (SEM) and tested for perrhenate ions sorption. The new macropore anion exchange resin was synthesized by polycondensation of epichlorohydrin oligomer and 4-vinylpyridine, the static exchange capacity (SEC) of which is equal to 6.75 mg-equiv∙g-1 in 0.1 M HCI solution and the sorption of perrhenate ions was studied. The influence of the concentration and pH of the model ammonium perrhenate solutions, contact time on the sorption activity of new anion exchangers (ECHO-VP) to perrhenate ions were studied. When studying the concentration effect of NH4ReO4 on the sorption of perrhenate ions by anionite ECHO-VP, the recovery degree at the content of 0.1-0.7 g/L of rhenium remained virtually unchanged and varied between 91and 92%. When increasing the concentration of rhenium up to 1.02 g/L, the recovery degree (A) is reduced to 86%. The time to reach the equilibrium between the anion exchanger ECHO-VP and the solution of NH4ReO4, containing 0.94 g/L of rhenium and having a pH of 5.1 is 6 hours. Thus in the first 15 minutes 81% perrhenate ions are recovered. Structure of the surface anionite before and after sorption of rhenium ions were studied by electronic microscopy method. The results revealed that the anionite ECHO-VP has a folded surface with a developed system of macropores and the size of macropores was found to vary between 0.70-1.76 mc, and individual pores reach 2.59 mc. It was established that ion exchanger based on oligomers of epichlorohydrin and 4-vinylpyridine have better sorption properties for rhenium (VII) ions that a wide range of commercial sorbents. It was found that the sorption capacity and the extent of new anion extracting the perrhenate ions and reach their maximum values are respectively 371.6 mg∙Re/g and 99% at pH 5.1.


Introduction
At the present time, there is a worldwide trend of rapid growth in consumption of rhenium by expanding its use in superalloys used in space technology, as well as in the manufacture of platinum-rhenium catalysts for high-octane gasoline production [1].Rhenium is widely used in alloys with tungsten, molybdenum, nickel, cobalt, chromium, dramatically improving their properties and making them particularly heat resistant.These alloys are also used for manufacturing elastic elements, having high strength and vibration resistance, electronics, electrical engineering and instrumentation.Promising areas of application for rhenium-containing catalysts are hydrocracking processes, manufacture of filters-neutralizers car exhaust converters.In this regard, the need for rhenium is growing.
Rhenium is a typical scattered element.Rhenium does not have own minerals, and it is produced simultaneously in the complex processing of copper, lead, uranium, molybdenum concentrates, ore tailings and waste waters, where its content ranges from fractions to tens of milligrams per liter [2][3][4].Low rhenium content in the feed causes the difficulty of obtaining it.Improvement of existing technological schemes and creating new efficient processes associated rhenium from non-traditional raw materials is an important issue.The hydrometallurgical processing of polymetallic rhenium containing raw material is recovered by ion exchange [1,5].Therefore of great scientific and practical importance is the creation of ion-exchange resins with improved absorption properties with respect to the perrhenate ions.
In this study, we synthesized and characterized of anionit based on epichlorohydrin oligomer (ECHO) and 4 -vinylpyridine (VP).Benzoyl peroxide (BP) (Linear formula

Study of the Sorption of Perrhenate Ions by Anionites ECHO-VP
Sorption of perrhenate anionite ECHO-VP in OH-form (grain size 0.5-1 mm) was studied in static conditions at a ratio of sorbent: solution 1:400, at room temperature of 20±2 °С, varying the concentration of rhenium in the solution NH 4 ReO 4 from 0.102 to 1.024 g/L and a pH range of 1.2 to 6.2 with 0.1 N solution H 2 SO 4 .Contact time of sorbent with solution is from 15 minutes to 7 days.
Sorption capacity (SC) was calculated from the difference between the initial and equilibrium Ammonium perrhenate NH 4 ReO 4 (Aldrich, Germany).
Epichlorohydrin oligomer (ECHO) was obtained in the presence of the M-14 catalyst, activated aluminosilicate (H + +Al 3+ ), taken in an amount of 1% of the monomer weight.The reaction mixture was heated for 2 h at 30-50 °C and for 5-6 h at 60-80 °C with stirring at a constant rate and then cooled down.The reaction product was dissolved in benzene, precipitated with water-ethanol mixture (2:1), and filtered off.The resulting viscous brown product was dried at room temperature under vacuum to constant weight.
Anion exchange resin was synthesized in an optimal condition by polycondensation of epichlorohydrin oligomer (ECHO) and 4-vinylyridine (VP) in the presence of 0.1-0.5 wt.% benzoyl peroxide at 80 °С for 5 hours in a weight ratio ECHO:VP equal to 10:4.Then the reaction mixture is cured at 120 °С for 16 hours.It was then ground to give a particle size of 0.5-1.0mm.As a result, a new anion exchanger ECHO-4VP was synthesized.The spatial structure of the anion exchanger resin: concentration of the solution, which was determined by the method of classical polarography against 0.5 N H 2 SO 4 solution on the recovery in Re 7+ (Е 1/2 = − 0.50 V).The polarogram was filmed on a polarograph universal PU-1 cell thermostated at 25±0.5 °С using a dropping mercury electrode.Oxygen from the test solutions was removed by blowing argon for 5 minutes.A saturated calomel electrode was used as reference electrode.
The structure of the surface anion before and after the sorption of rhenium was examined by electron microscopy scanning electron microscope JSM 6510LA (JEOL, Japan) at a resolution of the microscope of 30 Å•cm -1 .As it is seen from Fig. 2, increasing the content of rhenium in the solution of NH 4 ReO 4 , the SC of the anion exchanger increases, reaching 352.8 mg/g during the recovery of perrhenate ions from a solution containing 1.02 g/L of rhenium (pH 6.2).
It is shown [7] that increasing the concentration of the external solution lead to increasing the amount of metal ions that penetrate into the polymer skeleton, while the speed of penetration of exchanging ions in the deeper layer of the sorbent also increases.
When removing the perrhenate ions from a solution of NH 4 ReO 4 , containing 0.1 g/L of rhenium anion exchanger ECHO-VP is equal to 37.2 mg of Re/g.Under similar conditions, industrial medium and low-basic anion condensation type EDE-10P, AN-2F and macroreticular adsorbents polymerization of AN-21, AN-22 have a lower absorbency and recovers 18.9: 20.5: 25.5 and 28 3 mg of Re/g respectively [8].When the concentration the stock solution is 0.5 g/L of rhenium, industrial highly basic gel anionite AMP and a porous anion AM-2b have similar sorption properties and reaches a SC of 47 mg of Re/g [9].Anionite ECHO-VP perrhenate when removing ions from a solution of NH 4 ReO 4

Effect of Concentration
Sorption of perrhenate ions by anionite ECHO-VP was performed from model solutions of ammonium perrhenate, which is normally used in industrial applications in the production of rhenium and its products [6,7].When studying the concentration effect of NH 4 ReO 4 on the sorption of perrhenate ions by anionite ECHO-VP, the recovery degree at the content of 0.1-0.7 g/L of rhenium remained virtually unchanged and varied between 91 and 92% (Fig. 1).When increasing the concentration of rhenium up to 1.02 g/L, the recovery degree (A) is reduced to 86%.

Effect of pH
The effect of the acidity of the solution on the sorption of perrhenate ions by anionite ECHO-VP is of particular interest, since the processing of copper and polymetallic ores produces rhenium-containing sulfuric acid solutions.As it is seen from Fig. 3, in the pH range 1.8-6.2this effect is minimal.At pH 5.1 the SC and the degree of recovery of the ECHO-VP anion exchanger reach maximum values of 371.6 mg of Re/g and 99%, respectively.It is known [1] that in neutral, acidic and alkaline solutions rhenium is in the form of a simple monovalent anion ReO 4 − .The sorption of perrhenate ion anion exchanger in the OHform can be schematically represented as follows: By varying the pH the capacity is influenced by factors such as changes in the degree of protonation of the amino groups and the competitive adsorption of anions of acidic residues.These reasons seem to cause the reduction of anionite ECHO-VP at pH 6.2 (by reducing the degree of protonation of the amine) and pH 1.2 (by competitive adsorption of ions SO 4 2-).with the same content of rhenium has a higher absorbency and SC, equal to 216.8 mg of Re/g.

Effect of time
It follows from Fig. 4 that the time to reach the equilibrium between the anion exchanger ECHO-VP and the solution of NH 4 ReO 4 , containing 0.94 g/L of rhenium and having a pH of 5.1 is 6 hours.Thus in the first 15 minutes 81% perrhenate ions are recovered.Equilibrium sorption on the anion derived from lignin oligomer epichlorohydrin and 2-vinylpyridine was determined for 12 hours [1], for hot on anion exchangers AMP and AM-2b for 30 min [9].The duration of establishing equilibrium in an anionite-solution system by an industrial anion exchanger AN-21×16 depends on the degree of cross-linking and, consequently, resin with a DVB Content from 2 to 20% varies from 2 to 77 hours [1].
When studying the sorption of rhenium ions from a solution of ammonium perrhenate, containing 0.01 mol/dm 3 of rhenium at pH 5.45-5.85found that the capacities of the AN-21 anion exchanger gel structure is 49.2-81.1 mg/g [10].SC of anion exchange of lignin-based oligomer of epichlorohydrin and 2-vinylpyridine is 111.6 mg of Re/g [1].SC of synthesized anion exchanger ECHO-VP is much higher and is 371.6 mg of Re/g.The high sorption properties of the anion exchanger ECHO-VP are apparently due to the microstructure of its surface.Pictures taken from electronmicroscopy (Fig. 5) show that the anionite ECHO-VP has a folded surface with a well developed system of macropores.
As it can be seen from Fig. 5b,c, the size of pores is in the range of 0.70-1.76mc, and the individual pores reach 2.59 mc.The large pore size helps to absorb the large ReO 4 − ions, with an ionic radius of 0.28 nm [11].The surface layer of the anion exchanger ECHO-VP contains 5.91-17.11%rhenium.

Conclusions
The sorption activity of new anion exchangers based on oligomers of epichlorohydrin and 4-vinylpyridine towards perrhenate ions was studied, looking into the dependency on the concentration and pH of the model ammonium perrhenate solutions and the contact time.It was established that it has a high sorption and good kinetic properties with respect to the perrhenate ions when extracting rhenium from the individual solutions.It was found that at a pH of 5.1 the SC and the degree of recovery of the ECHO-VP anion exchanger reach maximum values of 371.6 mg of Re/g and 99% respectively.

Fig. 1 .
Fig. 1.The degree of recovery of perrhenate ions on ECHO-VP anion exchanger as function of the concentration of NH 4 ReO 4 solution.The contact duration is 7 days.

Fig. 4 .
Fig. 4. Sorption capacity SC of ECHO-VP anion exchanger for perrhenate ions as function of the contact duration in the NH 4 ReO 4 solution (C Re = 0.94 g/L, pH 5.1).

Fig. 5 .
The microstructure of the surface anion exchanger ECHO-VP before (a, b, c) after (d) sorption of perrhenate ions.ab