Modern methods of rhenium determination
O.V. Evdokimova, N.V. Pechishcheva, K.Yu. Shunyaev
Institute of Metallurgy of UB RAS,
101, Amundsen st., Ekaterinburg, Russia
Introduction
Rhenium due to its unique properties is the promising metalwidely used in various industries. At present day, the main areas of application of rhenium is the production of catalysts for the petroleum refining industry and refractory alloys, used for turbines manufacturing [1].
The great demand for this element requires large amounts of its production. Thereis a need extracting rhenium even from industrial waste water, from plants [2] due to the high cost and its low content in natural materials.
This situation stimulates the development (or modification) of methods of analytical control of various nature materials.
The contentof rhenium in rhenium-containing materials, both natural and technogenic, and contect ofaccompanying to rhenium elementsvary in a wide range of concentrations from 10-7 to tens of percent.
Earlier, the following methods were used for the determination of rhenium: spectrophotometry, gravimetry, kinetic, electrochemical, extraction-fluorimetricmethods, X-ray fluorescence analysis [3]. The main disadvantages of mostlymethods for determining rhenium are thelow sensitivity, the badreproducibilityof results, the influence of accompanying elements: Ag, W, Mo, Pt, Cu, Fe and etc.
Inmodern analytical practicethe following methods for the rhenium determination are used: inductively coupled plasma atomic emission spectroscopy (AES ICP), inductively coupled plasma - mass spectrometry (ICP-MS) [4], electrochemical methods [1]. X-ray fluorescence analysis and spectrophotometric methods do not lose their relevance [1], they have undergone significant modifications recently.
Inductively coupled plasma atomic emission spectroscopy (AES ICP) is widely used for the rhenium determination in mineral raw materials and products of metallurgy production. This method allows to determine up to 10-4% rhenium. The advantage of AES ICP is the high stability and reproducibility of results, absence of chemical influences.
However, analysis of more complex objects such as metallurgical products is a not easy task, because the lines of rhenium emission are overlaped with the lines of accompanying elementsin samples. So, the lines of Mo (221,427 nm), W (221,431 nm), Fe (227,519 nm), which may be present in the samples in large quantities, are overlaped to the most intense lines of rhenium (221.426 nm and 227.525 nm). This problem requires the development of new methods of sample preparation and selection of optimal conditions for determination of rhenium by atomicemission spectrometres.
Also, a significant disadvantage of this method is the small range of certificated reference materials. So there are a limited number of Russian rhenium standard materials with certified value of the rhenium content. It is molybdenum and copper-molybdenum ores and concentrates, in which the rhenium content is in the range of concentrations from 0.00047 to 0.0221%.
In most cases, analysts develop the synthetic mixture to monitor the rhenium content in the analysis of specific samples of complex composition. This mixture is similar to composition to the matrix of the analyzed samples, consisting of rhenium ions and other ions with a given concentration. For example, the authors [5] to develop a technique for rhenium determining together with platinum and palladium in the samples of spent catalysts by AES-ICP applied a synthetic mixture, prepared on the basis of aluminum oxide and standard solutions of Pt (IV), Pd (II), Re (VII).
One of modern methods and the most sensitive methods for the determination of rhenium is inductively coupled plasma - mass spectrometry (ICP MS) [4, 6, 7, 8]. These days, ICP MS with separation and concentration allows to measure rhenium at lower than several ng/g. However, ICP MS performance in analyses of complex samples is commonly affected by matrix effects and polyatomic interference and signal drift. High levels of salt solutions content cause plugging of sampling orifice, with decrease in analytical signal, in addition many spectral interferences may occur[6].
For the rhenium determination in molybdenite by ICP MS should beuse large dilution of sample to reduce the matrix influence and reduce the salts influence. However, this approach is not feasible in the case of high levels of molybdenum and relatively low levels of rhenium in the analyzed objects. The most effective way to minimize the matrix effects is separation of rhenium from the matrix. Often for this purpose extraction by organic solvents [6], sorption by anion-exchangers [8] are used.
Recently, X-ray fluorescence analysis becomes more popular. It is rapid and is often used for mass analysis. The advantage of this method is the possibility of direct determination of rhenium in the solid samples, in water solutions [9, 10],in the biological samples (plants) [2].
However the method is not without disadvantages: firstly, the detection limit of rhenium by X-ray fluorescence analysis is low and is only 0,05-0,1%, secondly, there are only fewthe standard materials with a high rhenium content, and thirdly, the influence of interfering elements in the sample related to determination of rhenium.
Using the concentration can not only reduce the detection limit, but also in the same time, solve and reduce the influence of interfering ions. For the concentration of rhenium in X-ray fluorescence analysisis often used sorption of rhenium in the form of perrhenate-ions [9, 10].
The authors [11] describes a problem related to the development of rhenium-containingstandard materialsby traditional high temperature approach for X-ray fluorescence analysis. Thus, high-temperature studies of MoO3-ReO3, which could be served as comparison materials for the rhenium determination by X-ray fluorescence analysis, showed that 50-90% of rhenium is lost during calcination of mixtures, it indicates the impossibility to use them for development of standard materials. In the paper [11] the method of preparing rhenium glassyreference samples (1,0 - 5,0%) on the basis of Bi2O3and B2O3is described. The developed method allows to determine rhenium in the range of 0,01-10% [11]
Electrochemical methods, in particular the electrostripping voltammetry (ESV), occupy a significant place in the analytical chemistry of rhenium [12, 13]. This method allows to determine up to 10-6-10-5% of rhenium.
To avoid the effects of many electropositive components (Mo, W, Cu, Ag, Au), which may interfere to the rhenium determination by ESV, it has been proposed the sorption concentration of perrhenate ions onthe surface of activated charcoal (BAU) [12, 13].
The most widely used techniques determine the 10-2 - 10-5 % of rhenium is spectrophotometric method. The advantages of this method are simplicity, low cost equipment and a relatively high sensitivity. Spectrophotometric method is based on the formation of colored complex compounds of rhenium with organic and inorganic ligands [1]. Photometric methods with thiocyanate ion, thiourea are widely spread [14, 15, 16]. Development of spectrophotometric methods for rhenium determination is largely due to the searching and using of new reagents. In [17] for the extraction-photometric determination of perrhenate ions in the form of ion associates, the basic polymethine dyes, derivatives of 1,3,3-trimethyl-3H-indolehave been offered,but the influence of oxyanions of tungsten and molybdenum is not excluded [17].
The disadvantage of the spectrophotometric methods is the need for prior separation of rhenium from a number of interfering elements (Mo, W, Cu), that it is achieved by concentrating perrhenate-ions by sorption or extraction.
Over the past decade, main changes in the methods of rhenium determination related with the improvement stadium of sample preparation, transfer the sample into an analytical form, modification of known methods and reagents (eg, creation of new facilities, development of new reagents for measurements), and conditions of analysis.
In general, in the literature a large number of works are related with the separation of rhenium from the analyzed solutions and the separation of rhenium (VII) from interfering elements by using new types of extractants and new sorbents, is given. Used extractants and sorbents, as well as the optimal conditions for extraction and sorption of rhenium are presented in Table 1 and 2, respectively.
Extraction plays a dominant role in the methods of separation and concentration of rhenium.
In most cases in the hydrometallurgical processing of rhenium-containing products in the acidic solutions ReO4- are formed. For perrhenate ions extraction the anion-exchange reagents or extractants of neutral type are often used. The literature contains information on the extraction of rhenium (VII) by various amines and quaternary ammonium compounds [18, 19, 20]. Efficient extractants of rhenium from acidic solutions are neutral organophosphorus compounds (tributyl phosphate, alkylphosphineoxides,their derivatives) [21, 22], a variety of solvent mixtures (tributyl phosphate + trioctylamine [23]), the extractants of neutral type, such as ketones and aliphatic alcohols [16, 24, 25].
Alcohols, ketones and ethers are more selective, having higher speed separation of organic and aqueous phases, as well as higher chemical resistance and lower cost compared with amines and organophosphorus compounds, but inferior to them in the extraction capacity for rhenium (VII) [16].
Thus, forperrhenate ions extraction aliphatic alcohols with 7-10 carbon atoms in the aliphatic chain are well proven, that can extract more than 98% of rhenium from sulfuric acid and hydrochloric acid solutions. In the case of alcohol there is no need to use solvents and modifiers, what simplifies their use in extraction processes[16].
The efficiency of rhenium extraction intoorganic phase by amines decrease as follow: quaternary> tertiary>secondary>primary.Among them, secondary and tertiary amines are widely used as efficient extractants of rhenium from acidic solutions. Perrhenate ions are extracted by amines in a wide range of pH. For systems of amine - low-polar diluent - H2SO4-ReO4-H2O the formation inverse micelles istypicalin the organic phase. Acid ions and anionic complexes are located inside the aqueous core of the micelle, with the metal ion coordinates the polar functional group of amine [19, 20].
It should be noted that the extraction by amines is complicated by the use of solvents, the nature of which depends on the solubility of amines and their extraction capacity. So, low-polarity solvent toluene, in contrast to the non-polar kerosene, enhances the polarity of anionic salts of amine, which increases the reactivity of the extractant to the anion exchange of inorganic acid to extractable anionic rhenium complexes [18].
Tertiary amines are the most effective extractants for rhenium (VII). However, in paper [18] it is shown that the secondary amine (diisododecylamine)gives advantageto the tertiary amines on the rhenium extraction efficiency from sulfuric acid media. It can be explained by the influence of steric factors and smaller rival extraction of mineral acids by secondary amines [1].
Most papers are related to the rhenium extraction from acidic solutions, but the extraction of rhenium from alkaline medium, which are formed after leaching of ores, concentrates, also represents a difficult problem. In the paper [23] rhenium extraction from alkaline solutions containing also molybdenum by solvent extraction using a mixture of tributylphosphate (TBP) and trioctylamine (N235) is described. Molybdenum, which is also extracted by solvents in small amounts, interferes to the extraction of rhenium
Over the last decade most works refer to the development of fundamentally new classes of extractants for perrhenate ions [26, 27, 28, 29], such as encapsulating ligands (cryptands and podands), macrocycles , crown ethers.These ligands can interactwith ReO4− by both the electrostatic interaction between ReO4− and protonated ligand and the hydrogen bond formation, compared with simple open-chain ligands.If the complex between ReO4− and ligand has high hydrophobicity, ReO4− in an aqueous solution may be separated effectively by a solvent extraction technique [30].
Crown ethers extract rhenium (VII) in the presence of potassium or sodium in the form of K(Na)LReO4 (L-crown-ether) into the organic phase (1,2 - dichloroethane, chloroform) [31, 32]. In the paper [31] the extraction perrhenate-ions by 3m-crown-m-ethers (m = 5,6) ether and its mono-benzo-derivatives in 1,2-dichloroethane are described.
Podands areanalogues of crown ethers, containing terminal phosphoryl ligands in their polyether chains, they are used for the extraction of rhenium (VII). The efficiency of extraction by phosphoryl podands depends of the following factors: the number of oxygen atoms in the polyether chain molecules, the number of donor centers in the molecule of podands, hydrophobicity of the reagent molecule, the size of forming cycles, the nature of substituent at the phosphorus atom. Studies have shown that phosphoryl podands with three oxygen atoms in the aromatic polyether chain, combined with the phosphoryl group by dimetilen or o-phenylene fragments, have high extraction ability for rhenium from sulfuric acid solutions [32].
In the paper [30] authors mark another type of podands, such as podands with nitrogen donor ligand -N, N, N `,N`-tetrakis (2-pyridymethyl) -1,2-ethylendiamine (TREN) and its hydrophobic analogs, which also allow to extract perrhenate ions from highly acidic environments.
Perrhenate is characterized by its ability to undergo a change in geometry, specifically from tetrahedral to hexagonal, in the presence of donor ligands (e.g., acetonitrile, triphenylphosphine). Protonation changes the electron density present on the oxygen atoms. Beer et al. [33] suggested that the tripodal ligandL1would be suitable for the binding and extraction of perrhenate anion. This ligand (Fig. 1), based on the combination of tris(2-aminoethyl)amine and crown ether motifs,was found to complex sodium cations and to extract perrhenate anions from aqueous solutions into an organic phase.
Atwood and co-workers developed calixarene-type ligandL2 (Fig. 1) that specifically extracts perrhenate from water solution into an organic phase. The selectivity for extractions decreases as follow: TcO4− ≥ReO4− > ClO4−NO3− >SO42− >Cl−.This selectivity pattern is attributed to a combination of charge, size and shape. Efficient extraction is observed at high and neutral pH, the molar ratio of ligand:perrhenate ion = 1:4[33].
L1L2
Fig. 1. Tripodal ligand L1andcalixarene-type ligand L2 for perrhenate extraction.
Schiff-base macrocycles are used as a new conjugated macrocycles for perrhenate ions. Thus, a series of amino-azacryptands (L3–L16) for encapsulation and extraction of the oxoanions perrhenate(Fig. 2) from aqueous solution were proposed by the authors [34].The complexation amino-azacryptands L to ReO4-is via hydrogen-bonded interactions.
Fig.2. Amino-azacryptands (L3–L16) for encapsulation and extraction of the oxoanions perrhenate.
Thus, the main characteristics of the compounds for the effective perrhenate ions extraction as follows:
- Energy coordination of ligand with ReO4- should be higher than the energy of perrhenate ion hydration
- The interaction between the ligand and perrhenate ions: an electrostatic interaction or the formation of hydrogen bonds.
- Functional ligands to be a suitable size (volume of the cavity should be more than 73,6 Å3), shape, electronegativity, and hydrophobicity.
- Ligand should beprotonated.
Table 1. Characteristics of extractants for rhenium extraction
№ / Extractant / Analysis object Composition of the initial solution / Extracton conditions / Interferinginfluences
1 / AliphaticalcoholswithC 7-10
1-Heptanol, 4-Heptanol, 1-octanol, 1-decanol, 4-decanol, 2-Heptanol, 3-Heptanol, 3-octanol
back-extractant NH4OH / Solutions HCl and H2SO4 / Т=293К
Time of phase contact
tex = 5 min
organic phase to aqueous
(O:L = 1:1)
4 steps of extraction, 2 stripping / Coextraction of mineral acids, incomplete re-extraction of Re (VII)
Octanol / Solutions of HNO3 and H2SO4 / Т=286-290К
tex = 10 minO:L = 1:1 / Coextraction of HNO3, H2SO4
2 / Basic polymethine dyes (derivatives of 1,3,3-trimethyl-3H-indole) astrazon violet / Aqueous and aqueous-organic solution / Т=293К
рН=6
tex = 10-30 sec
extractant: mixture toluene + dichloroethane
(1: 1) / do not interfere:
3000-5000 fold excess of S042-, CO32-, 300- HPO42-, MoO42-, WO42-,
10-20 S2O32-. Cr2O72-. IO3-; metal ions as sulfates
3 / Secondary (diisododecylamine) and tertiary amines (dioctylamin and trioctylamine) / Solutions H2SO4 / Т=293К
A wide range of pH.
tex=5-7 min
diluent - toluene / -
4 / N-benzoyl-N –phenyl-
hydroxylamine / Molybdenite, dissolved in HCl, HNO3 / HCl 0.5 mol/l
tex=15 min
diluentchloroform / -
Table 1 (continued)
№ / Extractant / Analysis object Composition of the initial solution / Extracton conditions / Interferinginfluences
5 / Phosphoryl podands
back-extractant H2O / СReinitial= 2·105mol/l;
aqueous solutions of salts of alkali metals, solutions of mineral acids / Т=286-291К
О:L=1:1,
tex= 60 min
diluent:nitrobenzene, 1,2-dichloroethane, chloroform, toluene / -
6 / Triotylamine (N235)+ tributyl phosphate(TBP),
back-extractant
18% NH4OH / Alkaline solutions after leaching, containing Mo
СRe 0,1-16,5 g/l / T=293 К,
рН =9.0, O:L=1:1
tex=10 мин
20% triotylamine+ 30% tributyl phosphate, diluent kerosene / -
7 / Podand-type nitrogen donor ligand –N,N,N`,N`-tetrakis(2-pyridymethyl)-1,2-ethylendiamine (TREN) / Aqueous solutionNH4ReO4
С =10-4M, / Ionicstrength 0.1M
pH=1-6.5
diluent chloroform
О:L=1:1,
tex=24 h / -
8 / 3m-crown-m-ethers (m=5,6), mono-benzo-derivates
1,2-dichloroethane / СReO4-=
0.057-0.060М / T=291-295K
tex=2h / -
Table 1 (continued)
№ / The range of Re concentrations Recovery / Methods for determination / Ref.1 / Recovery >99% / Determination from back-extract.
Spectrophotometric method with thiourea, reductant-Sn (II); wavelength of 390 nm. / [16, 24]
>98% / Spectrophotometric method / [25]
2 / The range of Re concentrations 0,01-5,50 mcg/ml / Determination from extract. Spectrophotometric method;
wavelength of 540 nm / [17]
3 / - / AES-ICP
Spectrophotometric method
withthiourea / [18, 19, 20]
4 / Mo, W, Fe are extracted 97% into the organic phase / Determination from aqua phase after extraction
ICP-MS / [6]
5 / - / AES-ICP
Spectrophotometric method / [21, 22]
6 / 96,8% / Spectrophotometric method with butyl rhodamine / [23]
7 / - / AES-ICP / [30]
8 / - / AES-ICP
Spectrophotometric method / [31]
9 / - / ICP-MS / [32]
Table 2. Characteristics of sorbents for rhenium sorption
№ / Sorbent / Analysis object Composition of the initial solution / Conditions of sorption / Interferinginfluences
1 / Activated carbons (BAU)
Eluent;
hot soda solution / nitrate media
gold ore raw / static conditions, а)рH =2-3,
б) рH =1,5-2,5
volume of solution 10 ml, mass of sorbent0,3 g
(S:L=1:33,3)
t=10 minUV / a) electro-positive components (Mo, W, Cu, Ag, Au).
b)1000 fold excess of Mo, W do not interfere
2 / Activated carbons- CN-G, CN-P, CU, developed from waste wood and grain processing industries / sulfuric acid solutions with CRe = 0,02 g/l, pH =2 / solid phases: liquid S:L==1:0,5
t=5-7 days / -
3 / 2. Carbon fibrous materials modified with chitosan / neutral aqua solutions of rhenium / static conditionsТ=286-289 К
S:L=1:1000 / -
4 / 3. Weakly basic
anion-exchangers
АН-105, Purolite A 170 / mineralized sulphite solution, simulating rinsing water
(С Re=0,01-0,02 g/l, Mo, Cu, Fe, As) / static and dynamic conditions
S:L = 1:500
t = 150-200 min / -
5 / Strongly-basic anion-exchangers
АВ-17
(sorbent PAN-АВ-17) / neutral or slightly acid
solutions / dynamic conditions
t = 20 min
The disks of polyacrylonitrile fiber filled resin / 1000 fold excess of Fe, Cu, Zn, Pb, Cd do not interfere
6 / Lignin anion-exchangers / solutions NH4ReO4 / static conditions S:L=1:400,
t=15min-2 h. / -
Table 2 (continued)
№ / Notes / Methods for determination / Ref.1 / а) Sorption capacity of BAU for Re СЕ=14,175 mg/g AC. Detection< 10%
б) СЕ=0,0763 mmol/g or 14,2 mg/g.
The concentrations range of Re
0,50 ... 100 mg/L in standard solutions,
0,25 ... 5,0 mg/l in the presence of Mo and W (1:1000). / a)Electrostripping voltammetry.
b) X-ray fluorescence analysis / a) [12]
b) [9, 10]
2 / - / Spectrophotometric method / [35]
3 / СЕ=17,9-18,5mg/g / Spectrophotometric method withammonium thiocyanate / [38, 39]
4 / Full dynamic exchange capacity
11,4 mg/g / Spectrophotometric method withammonium thiocyanate,kinetic method / [36]
5 / - / Determination of Re by the diffuse reflectance spectra at 420 nm;
rhenium thiocyanate complex; in the presence of tin (II). / [15]
6 / СЕ=34,27-232,8 mg/g / Traditional polarography / [37]
Sorption is one of the methods for separation of rhenium from various solutions.