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Separation Science and Technology

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Separation of rhodium(III) and iridium(IV)chlorido complexes using polymer microspheresfunctionalized with quaternary diammoniumgroups

Avela Majavu, Adeniyi S. Ogunlaja & Zenixole R. Tshentu

To cite this article: Avela Majavu, Adeniyi S. Ogunlaja & Zenixole R. Tshentu (2017) Separationof rhodium(III) and iridium(IV) chlorido complexes using polymer microspheres functionalizedwith quaternary diammonium groups, Separation Science and Technology, 52:1, 71-80, DOI:10.1080/01496395.2016.1235051

To link to this article: http://dx.doi.org/10.1080/01496395.2016.1235051

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Separation of rhodium(III) and iridium(IV) chlorido complexes using polymermicrospheres functionalized with quaternary diammonium groupsAvela Majavu, Adeniyi S. Ogunlaja, and Zenixole R. Tshentu

Departmemt of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa

ABSTRACTMerrifield resin functionalized with different quaternary diammonium groups derived from ethy-lenediamine (EDA), tetramethylenediamine (TMDA), hexamethylenediamine (HMDA), 1,8-diami-nooctane (OMDA), 1,10-diaminodecane (DMDA) and 1,12-diaminododecane (DDMDA) wereinvestigated for the separation of [RhCl5(H2O)]

2− and [IrCl6]2−. Selective loading of [IrCl6]

2− in 6M HCl medium onto the column was achieved in the presence of [RhCl5(H2O)]

2− by the synthe-sized sorbents. The iridium loading capacities were 3.80, 6.49, 13.07, 19.29, 27.09 and 4.36 mg/gfor EDA, TMDA, HMDA, OMDA, DMDA and DDMDA-functionalized microspheres, respectively. Thematerials showed great potential for application in separating rhodium and iridium from aqueousHCl solutions.

ARTICLE HISTORYReceived 1 September 2015Accepted 7 September 2016

KEYWORDSAnion exchanger; iridium(IV); kinetics; rhodium(III);separation

Introduction

Platinum group metals (PGMs), silver and gold occurin nature together with base metals such as iron, cobalt,nickel and copper, and alongside a wide range of minorelements such as lead, tellurium, selenium and arsenic.Base metals and other impurities are extracted from thePGM ore bodies before the processing of the preciousmetals.[1] Preconcentration and separation methodssuch as distillation for OsO4 and RuO4, solvent extrac-tion, precipitation and ion exchange have been used toachieve highly pure added value metals from low-gradeore solutions.[2,3] PGMs have similar chemical behaviorbut with some subtle differences. The most importantfor separation of rhodium and iridium is the exploita-tion of their oxidation states [Rh(III) and Ir(IV)] andtheir chloride chemistry[4–8] with [IrCl6]

2− and [RhCl5(H2O)]

2− being the main species for their separationwith diammonium centers. du Preez et al.[9] hasdemonstrated the affinity of [IrCl6]

2− for the diammo-nium centers in an HCl medium, and this phenomenonwas attributed to the doubly charged quaternary dia-mmonium cation which results in a higher loadingcapacity of [IrCl6]

2− than a singly charged cation andthe lack of affinity of the aquated complex species forammonium centers.

The high demand for PGMs, especially in the oil andautomotive industries, has warranted a continued

interest in the development of new extractants fortheir separation. The design of materials, especiallyanion exchange solid phase materials, to improve theloading capacity and separation factors for the PGMsremains an interesting area of research. Ion exchange isa preferred technology due to the high separation effi-ciency, high loading capacity and ease of operation.[10]

Ion exchange, however, has its own disadvantages mostnotable of which is the slow kinetics of metal ion orcomplex anion uptake, but this drawback can be over-come by increasing the accessibility of the functionalgroups within the material.[11] Types of ion exchangeresins which can be used are cation, anion and ampho-teric exchange resins depending on the form of thetargeted metal ion(s). Anion exchange resins are classi-fied into two categories; namely, weak base and strongbase anion exchangers.[12] Primary, secondary and ter-tiary amino groups are classified as weak-base anionexchangers, while quaternary amino groups are classi-fied as strong-base anion exchangers. Anion exchangerswith ammonium or quaternary ammonium functional-ities are known to interact with anionic chlorido com-plexes of the PGMs.[13,14]

This article presents the modification of anionexchange materials (microspheres) functionalized withquaternary diammonium groups derived from ethyle-nediamine (EDA), tetramethylenediamine (TMDA),

CONTACT Zenixole R. Tshentu zenixole.tshentu@nmmu.ac.za Departmemt of Chemistry, Nelson Mandela Metropolitan University, P.O. Box 77000,Port Elizabeth 6031, South Africa.Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst.

Supplemental data for this article can be accessed on the publisher’s website.

SEPARATION SCIENCE AND TECHNOLOGY2017, VOL. 52, NO. 1, 71–80http://dx.doi.org/10.1080/01496395.2016.1235051

© 2017 Taylor & Francis

hexamethylenediamine (HMDA), 1,8-diaminooctane(OMDA), 1,10-diaminodecane (DMDA) and 1,12-dia-minododecane (DDMDA). The effect of the methylenespacers between the two cationic diammonium centerswas investigated. The designed materials are directedtowards improving the loading capacities and betterseparation of rhodium and iridium, i.e. for maximumuptake of [IrCl6]

2− and poor uptake of [RhCl5(H2O)]2−

in a column experiment. The adsorption equilibriumand kinetic studies[15] for the uptake of [RhCl5(H2O)]

2−

and [IrCl6]2− were also investigated through batch

experiments.

Experimental

Reagents

Merrifield resin (1% crosslinked with divinylbenzene,[Cl]: 1.2 mmol/g, 40–60 mesh), EDA (75–80%), TMDA(99%), HMDA (98%), OMDA (99%), DMDA (99%)and DDMDA (99%), 2,6-lutidine (≥99%), iodomethane(purity ≥ 99%), iridium(III) chloride hydrate (99.9%),rhodium(III) chloride hydrate (98%), hydrochloric acid(32%), ferric chloride (99%), sodium chlorate (99%),sodium metabisulfite (≥97.0%) and sodium iodide(≥99.5%) were purchased from Sigma-Aldrich. All sol-vents were purchased from Merck Chemicals.

Instrumentation

The identity of the rhodium(III) and iridium(IV) chlor-ido species was determined by a Perkin-Elmer UV–visspectrophotometer. A Perkin-Elmer 400 FTIR was usedto confirm the presence of the expected functionalgroups during the syntheses steps. The scanning elec-tron microscopy TESCAN Vega TS 5136LM (SEM),operated at 20 kV and at a working distance of20 mm, was used to image the morphology and tomeasure the diameter of the polymer materials. Eachsample of polymer microspheres was coated with goldusing a gold sputter machine before imaging to preventelectrostatic charging. Elemental analysis was carriedout with a Vario Elementary ELIII Microcube CHNSanalyzer to determine the nitrogen percentage of thefunctionalized microspheres. The metal ions analyses(Rh and Ir) were carried out with a Thermo Electron(iCAP 6000 Series) inductively coupled plasma (ICP)spectrometer equipped with an OES detector at 343.4nm for Rh and 215.2 nm for Ir. A Micromeritics ASAP2020 Surface area and Porosity Analyzer was used forsurface area analysis by carbon dioxide adsorption.Carbon dioxide was employed for surface area mea-surement at 237 K. Each sample of polymer

microspheres was degassed for 8 days at 50°C to com-pletely remove the adsorbed impurities before conduct-ing the surface area analysis. X-ray photoelectronspectroscopy (XPS) measurements were performedwith a Kratos Axis Ultra X-ray PhotoelectronSpectrometer equipped with a monochromatic Al Kαsource (1486.6 eV). A custom-made glass column withthe following dimensions was used for the column(dynamic) studies; 10 cm length, internal diameter of3.5 mm and a tip diameter of 1 mm. The thermaldegradation of microspheres were studied with a ther-mogravimetric analyzer (TA instrument Q600-0345SDT) in the temperature range 25–800°C under nitro-gen at a flow rate of 50 mL/min and at a heating rate of10°C/min.

Preparation of the solutions

Rhodium and iridium complex speciesRhodium solution. The rhodium starting material wasobtained as a black solid rhodium(III) chloride salt(RhCl3). An quantity of 0.47 g salt was weighed, and50 mL of 6 M HCl was added. The solution was heatedto reflux for an hour at 70°C and a dark red solutionwas obtained. A peak corresponding to [RhCl5(H2O)]

2−

was observed at 516 nm using a UV–visspectrophotometer[16] (Fig. S1).

Iridium solution. The iridium starting material wasobtained as a black solid iridium(III) chloride hydratesalt (IrCl3·xH2O). An quantity of 0.22 g salt wasweighed, and 50 mL of 6 M HCl was added. Thesolution was heated to reflux at 70°C for an hour anda brown solution was formed. The solution was allowedto cool, and then 4.36 g of sodium chlorate (NaClO3)was added to oxidize Ir(III) to Ir(IV) during which adark brown solution was obtained. A peak correspond-ing to [IrCl6]

2− was observed at 488 nm using a UV–visspectrophotometer[8,17] (Fig. S1).

Preparation of the metal stock solutions (industrialfeed solution). An amount of 0.045 M rhodium and0.015 M iridium solutions were prepared to give amolarity ratio of 3:1 rhodium-to-iridium solution in 6M HCl. This synthetic mixture corresponds well withthe real industrial feed solution.[17]

Functionalization and quaternization ofmicrospheres

Three grams of polymer microspheres were reactedwith 0.15 mol, 18 g of diamines (EDA, TMDA,HMDA, OMDA, DMDA and DDMDA) in 60 mL dry

72 A. MAJAVU ET AL.

ethanol. A catalytic amount of sodium iodide wasadded, and the reaction was refluxed for 2 days. Thevarious functionalized resins were filtered and washedwith water and ethanol. Quaternization of the variousresins were carried out by reacting 10 g of functiona-lized resin with a solution containing 0.525 mol methyliodide in and 0.3 mol lutidine in 60 mL dry ethanol(Scheme 1). The mixture was refluxed for 2 days fol-lowed by filtration and washing with water and ethanol.The resins were then treated with excess 0.1 M iron(III)solution in 6 M HCl followed by a hot solution ofsodium metabisulfite (~0.05 M) in distilled water andfinally with 32% concentrated HCl to remove anyiodide or tri-iodide ions on the resin. The resins werethen collected, washed with ethanol via Soxhlet extrac-tion and dried in air. The nitrogen content was deter-mined by microanalysis and the data are presented inTable 1.

Column adsorption studies

Single element studiesMilliQ water (5 mL) and 6 M HCl (5 mL) were passedthrough a packed glass column containing 0.3 g of eachbead, in order to condition the sorbent materials(beads). After the sorbent conditioning, 2.5 mL ofeach metal solution was loaded onto the column andallowed to equilibrate for 12 h. The solution was thenallowed to flow using an optimized flow rate of 0.5 mL/h. An amount of 10 mL of 6 M HCl was used to washoff the un-adsorbed metal complex, while 0.05 M (5mL) of aqueous sodium metabisulfite was employed asa stripping agent, and this process reduces Ir(IV) to Ir(III).[18] Finally, an elution step was carried out with 10mL of 20% aqueous HCl solution. An amount of 0.5mL eluted fractions were collected throughout the pro-cess, and the eluent diluted for ICP-OES analysis.

Binary mixtureA volume of 1.25 mL [RhCl5(H2O)]

2− solution wasadded to 1.25 mL of [IrCl6]

2− solution and the resultingmixture (2.5 mL) was loaded onto the conditioned

column. Thereafter, the un-adsorbed metal complexspecies were washed off the column using 6 M HCl,followed by stripping with 0.05 M sodium metabisulfitesolution and then elution with 20% HCl. The collectedfractions were diluted appropriately and analyzed forthe metal content by ICP-OES.

Adsorption studiesAdsorption isotherm for [IrCl6]

2− and [RhCl5(H2O)]2−.

The amount of adsorption at equilibrium (qe [mg/g]) ofthe complex species was calculated using Eq. (1), andmetal ion uptake was calculated using Eq. (2).[19]

qe ¼ C0 � Ce � VW

(1)

where C0 is the initial metal concentration (mg/mL), Ce

is the equilibrium metal concentration (mg/mL), Vbeing the volume of the metal solution (mL) and W isthe mass of the sorbent (g).

Adsorption ð%Þ ¼ C0 � Ce

C0� 100 (2)

Scheme 1. Functionalization of Merrifield beads with quaternary diammonium groups.

Table 1. The microanalyses data (%) for the microspheres(before and after quaternization).Sorbent materials* C (%) H (%) N (%) C:N

B-EDA 83.42 8.47 6.05 16:1B-QUAT EDA 63.94 7.55 3.44 21:1B-TMDA 83.04 9.11 5.47 18:1B-QUAT TMDA 58.82 7.75 3.55 20:1B-HMDA 82.48 9.30 5.04 19:1B-QUAT HMDA 63.70 9.06 3.47 21:1B-OMDA 82.76 8.35 4.85 17:1B-QUAT OMDA 63.30 7.89 3.43 18:1B-DMDA 80.25 9.65 4.03 20:1B-QUAT DMDA 66.15 8.40 3.04 22:1B-DDMDA 79.99 9.42 4.40 18:1B-QUAT DDMDA 66.12 8.76 3.28 20:1

*B-EDA: Ethylenediamine functionalized microspheres; B-QUAT EDA: qua-ternized ethylenediamine functionalized microspheres; B-TMDA: tetra-methylenediamine functionalized microspheres; B-QUAT-TMDA:quaternized tetramethylenediamine functionalized microspheres;B-HMDA: hexamethylenediamine functionalized microspheres; B-QUAT-HMDA: quaternized hexamethylenediamine functionalized microspheres;B-OMDA: 1,8-diaminooctane functionalized microspheres; B-QUAT-OMDA:quaternized 1,8-diaminooctane functionalized microspheres; B-DMDA:1,10-diaminodecane functionalized microspheres; B-QUAT-DMDA: quater-nized 1,10-diaminodecane functionalized microspheres; B-DDMDA: 1,12-diaminododecane functionalized microspheres; B-QUAT-DDMDA: quater-nized 1,12-diaminododecane functionalized microspheres.

SEPARATION SCIENCE AND TECHNOLOGY 73

Resin regenerationResin regeneration for several usages was carried out bywashing the resin (0.3 g) after use with 6 M solution ofhydrochloric acid (HCl).

Adsorption isothermal studiesThe studies were carried out at 25°C equilibrating 0.1 gof sorbent material with 10 mL of 6 M HCl solution. Avolume of 2.5 mL metal ion solution was shaken in amechanical shaker at 120 rpm followed by aliquot with-drawal for analysis at every minute interval. A concen-tration range of 923–2447 mg/L for [IrCl6]

2− and1408–4193 mg/L for [RhCl5(H2O)]

2− was employedfor the adsorption studies. The metal ion concentrationwas analyzed using an ICP-OES. An adsorption iso-thermal study was carried out on two well-known iso-therms, Langmuir adsorption isotherm and Freundlichadsorption isotherm, represented by Eqs. (3) and (5).The Langmuir isotherm can be written in linear form asin the following equation[20]:

Ce

Qe¼ Ce

Q0þ 1bQ0

(3)

where Q0 and b are Langmuir constants, Qe is the loadingcapacity and Ce is an equilibrium concentration. TheLangmuir isotherm can be expressed by a separationfactor (RL), which is given by the following equation:

RL ¼ 11þ bC0

(4)

RL indicates the isotherm slope according to the follow-ing assumption characteristics RL > 1 is unfavorable, RL

= 1 is linear adsorption, 0 < RL < 1 is favorable. C0 isthe concentration of each metal solution and b is theLangmuir constant.

The Freundlich isotherm is expressed as follows:

log qe ¼ log kf þ 1nlogCe (5)

where Ce and qe are equilibrium concentration andadsorption capacity at equilibrium stage. The quantitiesof kf and n can be determined from a linear plot of logQe versus log Ce. The magnitude of the exponent 1=ngives an indication of the favorability of theadsorption.[21]

Adsorption kineticsAdsorption kinetics using the various quaternized micro-spheres were carried out by employing the methoddescribed as follows. A 0.1-g quantity of beads wasweighed and added into conical flasks, in which the 10-mL of 6 M HCl solution was added with shaking in amechanical shaker. A volume of 2.5 mL metal solution

was added (0.045 M Rh and 0.015 M Ir), and the mixturewas placed on a rotary shaker at an agitation speed of 120rpm. Equal amount of aliquots were sampled at fixed timeintervals, diluted appropriately, filtered through a 0.45-μm pore size filter and analyzed by ICP-OES. Two sim-plified kinetic models, i.e. pseudo first-order and pseudosecond-order were tested to investigate the adsorptionmechanisms using Eqs. (6) and (7), respectively.

log qe � qtð Þ ¼ log qe � k12:303

t (6)

The plot of log(qe − qt) versus t (min) gives a straightline and k1 and qe are determined from the slope andintercept, respectively. The pseudo second-order modelcan be represented as in Eq. (7).

tqt

¼ 1k2q2e

þ 1qtt (7)

where k2 (g/mg min) is the rate constant of pseudosecond-order adsorption.

Results and discussion

Absorbent characterization

Elemental analysis of microspheresThe elemental analyses were carried out to determine thenitrogen content (%) on the materials before and afterquaternization. A decrease in the nitrogen content afterquaternization was observed and this was evidence of thesuccess of the quaternization reaction. Elemental analysisof the quaternized microspheres are provided in Table 1.

FT-IR spectroscopyThe FT-IR spectrum for the unfunctionalized micro-spheres (Fig. 1) showed a strong peak at 670 cm−1

which is due to the ν(C–Cl) and a strong peak at1260 cm−1 which is assigned to the CH2–Cl bendingvibrations. Upon functionalizing the polymer micro-spheres, the ν(C–Cl) and CH2–Cl vibration band dis-appeared, hence confirming that functionalization tooplace via the Cl atom of the polymers. New peaks suchas the N–H stretch at 3153 cm−1 and N–H bendingmode at 1573 cm−1 appeared to show the presence ofthe amine. A strong peak at 1265 cm−1 for ν(C–N)appeared after the quaternization step.

SEM imagesThe SEM images showed that the morphology of themicrospheres remained the same after functionalizationwith no noticeable defects after the quaternization step(Fig. 2). Nonetheless, a slight increase in the averagediameter of quaternized microspheres from 188

74 A. MAJAVU ET AL.

(average diameter) to 209 μm upon functionalization ofthe microspheres was observed (Fig. 2). The resultingincrease in microsphere diameter was attributed to theincorporation of quaternized ligands unto the polymermatrix.

XPSXPS was employed to investigate the near surfacechemistry of the anion exchangers in their chlorideform since this technique characterizes the elementsby measuring the energies of the emittedphotoelectrons.[22] The presence of chlorine from C–

Cl and CH2–Cl bonding in the unfunctionalized micro-spheres is evident from the Cl 2p and Cl 2s peaksappearing around 200 and 270 eV, respectively(Fig. 3). These peaks, however, disappeared after func-tionalization with amines, and the quaternary diammo-nium functionalities are shown by the two N 1s peakswith the binding energies of 397 and 405 eV for themicrospheres. No iodine peaks were observed in thematerials as they were repeatedly washed with iron(III)chloride solution in 6 M HCl and sodium metabisul-phite solution.[16] The solutions helped in eliminatingthe iodide and tri-iodide which may have formed uponquaternization of the functionalized polymers.

BET surface areaBET surface area analysis was carried out as it is one ofthe factors contributing to the uptake of metal complexspecies in the study. For adsorption to occur, adsorbentmust offer a reasonable surface area. Single point surfacearea measurements using CO2 were carried out as nitro-gen adsorption/desorption was not possible for thesematerials due to their high affinity for nitrogen.[22,23]

In Table 2, we observed a drop in the surface area offunctionalized microspheres as compared to the unfunc-tionalized microspheres. The reduction in surface areafurther indicated that the various quaternized amineswere successfully functionalized unto the pores withinthe surface of the material.

Adsorption studiesSingle element elution profiles. An investigation ofthe single element loading studies was carried outin order to establish how well the metal ions chloridocomplexes were adsorbed at varying HCl

Figure 1. FT-IR spectra of (A) unfunctionalized 1% crosslinkedMerrifield beads (1% beads), (B) tetramethylenediamine(TMDA), (C) TMDA-functionalized beads (1% beads: TMDA),and (D) quaternized TMDA beads (B-QUAT TMDA beads).

(a)

(b) (d)(c)

(e) (f) (g)

Figure 2. SEM images of (a) unfunctionalized 1% Merrifield beads (average diameter = 188 μm), (b) B-QUAT EDA beads (208 μm), (c)B-QUAT TMDA beads (209 μm), (d) B-QUAT HMDA beads (210 μm), B-QUAT OMDA (212 μm), B-QUAT DMDA (215 μm) and B-QUATDDMDA (218 μm).

SEPARATION SCIENCE AND TECHNOLOGY 75

concentrations (Fig. S2). This study exploits thechloride medium and the synthesis to ensure thatchloridation occurs to the species of interest forboth Rh(III) and Ir(IV), viz. [RhCl5(H2O)]2− and[IrCl6]

2−.[8] The column study was carried out at 3,6 and 8 M HCl (Fig. S2). The 8-M chloride mediumdoes not lower the uptake of [IrCl6]

2− and it in factincreases, but the 6-M is still attractive since at 8 MHCl chloridation results in the formation of [RhCl6]

3

− species which also has an affinity for the cationicphases compared with [RhCl5(H2O)]2− which doesnot load in an HCl medium. The species are thenstripped in the same fractions as the stripping of[IrCl6]

2−. At 3 M HCl, the species [IrCl6]2− is not

formed in significant quantities as compared to theconditions employed in the formation of [IrCl6]

2− (at6 M HCl), resulting in a low loading capacity. Theselectivity or specificity for iridium separation fromrhodium is, therefore, successfully achieved withoutcontamination at 6 M HCl. According to previousstudy reported by du Preez et al.[18], the synthetic

mixture (i.e. in 6 M HCl) corresponds well with thereal industrial feed solution. This means that control-ling the acidity of the solution improvement of theselectivity of [IrCl6]

2− over [RhCl5(H2O)]2− sorptionprocess can be achieved in 6 M HCl.

The difference in the loading capacities of the dif-ferent functional materials for [IrCl6]

2− in 6 M HClwith the nature of the diammonium center wasevident.[18] An increase in the methylene spacersbetween the diammonium groups resulted in anincrease in Ir(IV) loading capacity. The aquachloridocomplexes remained in solution due to hydrogen bond-ing of the water ligands with water molecules keepsthem solvated rather than being adsorbed on the sur-face of the resin.[15] All the materials did not adsorb[RhCl5(H2O)]

2− in a 6-M chloride medium, but loaded[IrCl6]

2− (Fig. S3), i.e. the materials had a higher affi-nity for [IrCl6]

2−. The results showed that functionali-zation with 1,10-decamethylediamine group, which hasa lower charge density, gave the highest loading capa-city (Fig. 4). The size and the charge density of thecation are the most dominating factors as they arerelated to the affinity for an anion.[9] The observediridium loading capacities were 3.80, 6.49, 13.07,19.29, 27.09 and 4.36 mg/g for EDA, TMDA, HMDA,OMDA, DMDA and DDMDA microspheres, respec-tively. The separation process presented is fast, simple,and economical as compared to some studies reportedin the literature.[24–26]

Adsorption isotherms. The parameters of theLangmuir and Freundlich isothermal models obtainedare presented in Table 3 and Fig. 5. The results showed

600 500 400 300 200 100 0

Binding Energy (eV)

(a)

(b)

C (1s)

(c)

Cl 2p1

N (1s)

(d)

N (1s)

(e)

(f)

(g)

B

Figure 3. XPS spectra of (a) unfunctionalized, (b) B-QUAT EDA,(c) B-QUAT TMDA, (d) B-QUAT HMDA, (e) B-QUAT OMDA, (f)B-QUAT DMDA and (g) B-QUAT DDMDA microspheres.

Table 2. BET surface area analysis of the various microspheres.Microspherical beads Surface area (m2/g)

1% Merrifield beads 124.01B-QUAT EDA 18.08B-QUAT TMDA 18.77B-QUAT HMDA 19.61B-QUAT OMDA 23.56B-QUAT DMDA 28.23B-QUAT DDMDA 35.96

No.of carbon spacers

2 4 6 8 10 12

Load

ing

capa

citie

s (m

g/g)

0

5

10

15

20

25

30

3.80

6.46

19.29

13.07

4.36

27.09

Figure 4. Loading capacities of Ir(IV) calculated from the col-umn study results for B-QUAT EDA, B-QUAT TMDA, B-QUATHMDA, B-QUAT OMDA, B-QUAT DMDA and B-QUAT DDMDAon microspheres at a flow rate of 0.5 mL/h.

76 A. MAJAVU ET AL.

that the adsorption process for [IrCl6]2− could be

described well with the Freundlich isothermal modelas the experimental equilibrium data fitted with thecorrelation coefficient value very close to 1 when com-pared to Langmuir adsorption model. The Freundlichparameter, n, indicates the favorability of the adsorp-tion. If the adsorption intensity, n is less than 1, itindicates that adsorption intensity is good (or favor-able) over the entire range of concentration studied, butif the n value is more than 1, it means that adsorptionintensity is good (or favorable) at high concentration,but much less at lower concentration.[27–29] All adsor-bents showed n values more than 1 indicating that theadsorption intensity is favorable at high concentration.The calculated values for 1=n showed that all materialsare heterogeneous adsorbents. The increase in kf withthe increase of carbon spacer between the diaminegroups introduced within the sorbents suggested anincrease in the effective sorption sites with the excep-tion of B-QUAT DDMDA.

Adsorption kinetics. In these experiments, the effect oftime on the adsorption of [RhCl5(H2O)]

2− and [IrCl6]2−

onto the resin was studied, respectively. The kineticstudies result showed that the rate of ion exchange of

these anionic metal ions complexes (RhCl5(H2O)]2−

and [IrCl6]2−) by the anion exchangers were very

rapid initially due to the availability of surface adsorp-tion and thereafter slowed down as a result of surfacesaturation, adsorption equilibrium was attained after 5min. Pseudo first-order and pseudo second-order wereemployed in order to understand the behavior of themicrospheres beads and also to examine the rate con-trolling mechanism of the adsorption process. The fit-ting validity of these models were checked by linearplots of log(qe − qt) versus time and (t/qt) versus timefor the pseudo-first and pseudo second-order, respec-tively, for the adsorption of the chloride metal com-plexes unto the microspheres. The plot of log(qe − qt)versus time, pseudo first-order adsorption model gave alinear regression (R2 ~ 0.9) (Table 4, Fig. 6). While, theplot of (t/qt) versus time did not fit as all the regres-sions (R2) were less than 0.9 for the adsorption of[IrCl6]

2− on microspheres (Table 4, Fig S4). It may beconcluded that [IrCl6]

2− adsorption on the sorbentmaterials is mainly controlled by mass transfer insolution.

Binary system. The 1,10-decamethyldiamine diqua-ternary ammonium center was chosen since it presentedthe highest iridium loading capacity (Fig. 4). Theabsence of rhodium after the washing step of the columnbed adsorption suggested that [RhCl5(H2O)]

2− was notloaded in the presence of a high chloride medium as wasobserved in single-element studies. It was also observedthat the colour of the earlier fractions appeared intensered which was indicative of rhodium coming off imme-diately upon allowing the column to flow (Fig. 7). Theintensity of the red color gradually decreased uponwashing with a 6-M HCl solution. Since iridium wasloaded in excess (in order to determine the loadingcapacity), its presence was also detected in the earlier

Table 3. Langmuir and Freundlich isothermal parameters foradsorption of [IrCl6]

2− on microspheres.Isotherm model

Langmuir Freundlich

Sorbent materialsQ0

(mg/g)b

(L/mg) RL R2kf

(mg/g) n R2

B-QUAT EDA 48.54 −0.03 −0.01 0.73 106.38 8.32 0.99B-QUAT TMDA 47.85 −0.03 −0.01 0.80 112.33 7.71 0.99B-QUAT HMDA 47.39 −0.03 −0.01 0.79 116.25 7.36 0.99B-QUAT OMDA 45.66 −0.02 −0.18 0.86 130.98 6.39 0.99B-QUAT DMDA 45.46 −0.03 −0.02 0.83 132.10 6.33 0.99B-QUAT DDMDA 50.25 −0.42 0.01 0.91 96.05 9.85 0.99

3.5

4.0

4.5

5.0

5.5

6.0

Ce/

q e

Ce

(a)(b)(c)

(d)

(e)

(f)

(A) (B)

200 220 240 260 280 300 2.28 2.30 2.32 2.34 2.36 2.38 2.40 2.42 2.44 2.46 2.48 2.50

1.725

1.730

1.735

1.740

1.745

1.750

log

q e

log Ce

(c)

(e)

(d)

(b)

(f)

(a)

Figure 5. Plots of Langmuir and Freundlich isotherms of (A): (a) B-QUAT EDA, (b) B-QUAT TMDA, (c) B-QUAT HMDA, (d) B-QUATOMDA, (e) B-QUAT DMDA and (f) B-QUAT DDMDA for [IrCl6]

2− adsorption on microspheres and (B): (a) B-QUAT EDA, (b) B-QUATTMDA, (c) B-QUAT HMDA, (d) B-QUAT OMDA, (e) B-QUAT DMDA and (f) B-QUAT DDMDA for [IrCl6]

2− adsorption on microspheres.

SEPARATION SCIENCE AND TECHNOLOGY 77

fractions (Fig. 7). At the stripping and elution steps, ayellow color was observed, indicating the presence ofiridium in the fractions (above fraction number 40,Fig. 7). As the fraction number was increased above 40,the original yellow color gradually disappeared.

Iridium(IV) exists as two possible species, [IrCl5(H2O)]

− and [IrCl6]2−, in 6 M HCl solutions with the

former species only present in very small quantities.[8] Onthe other hand, rhodium(III) exists as [RhCl5(H2O)]

2− and[RhCl6]

3− in 6 M HCl with the latter in small quantities.-[7,18] Therefore, the small peaks just above fraction 30 maybe due to the presence of small quantities of these species inthe synthetic feed solution at 6 M HCl. The metals withinthese fractions could readily be recovered and transferredback to the feed solution for reprocessing.

The separation demonstrated in Fig. 7 has also beenshown in a form of breakthrough curves (Fig. 8) formicrospheres. Breakthrough curves generally permit agood description of the continuous flow process in ion-exchange columns.[30] The breakthrough point for[RhCl5(H2O)]

2− appeared at the start of the columnoperation and that of [IrCl6]

2− on microspheresoccurred when the volume was about 7 mL (about1.33 h after the operation started). It means that it ispossible to separate these metals only after 1.3 h afterthe operation of the column has started. A break-through volume of about 12 mL was observed inbeads with the stripping/elution regime that wasapplied to achieve the separation observed in Fig. 8.

Table 4. Parameters of the pseudo first-order and pseudo sec-ond-order rate law for the adsorption of [IrCl6]

2− onmicrospheres.

Sorbent material

Pseudo first order Pseudo second order

k1 (1/min) R2qe calc.(mg/g)

k2 (mg/g min)× 10−6 R2

B-QUAT EDA 0.49 0.99 55.87 7.40 0.91B-QUAT TMDA 0.37 0.97 53.76 7.31 0.80B-QUAT HMDA 0.42 0.96 53.76 6.98 0.81B-QUAT OMDA 0.33 0.99 53.48 7.62 0.80B-QUAT DMDA 0.40 0.98 52.91 7.84 0.81B-QUAT DDMDA 0.48 0.99 56.82 6.57 0.81

Figure 6. Plots of pseudo first-order kinetic model of (A): (a)B-QUAT EDA, (b) B-QUAT TMDA, (c) B-QUAT HMDA, (d) B-QUATOMDA, (e) B-QUAT DMDA and (f) B-QUAT DDMDA for [IrCl6]

2−

adsorption on microspheres.

0

200

400

600

800

1000

1200

1400

1600

Con

c (p

pm)

B-QUAT DMDA Ir B-QUAT DMDA Rh

Fraction numbers

0

200

400

600

800

1000

Con

c (p

pm)

Fraction numbers

B-QUAT OMDA Ir B-QUAT OMDA Rh

0

5000

10000

15000

20000

25000

30000

Con

c (p

pm)

Fraction Numbers

B-QUAT EDA Rh B-QUAT EDA Ir

0

1000

2000

3000

4000

5000

B-QUAT HMDA Rh B-QUAT HMDA Ir

Con

c (p

pm)

Fraction Numbers

0 10 20 30 40 50 600 10 20 30 40 50 60

0 5 10 15 20 25 300 10 20 30 40 50 600 5 10 15 20 25 30

0

5000

10000

15000

20000

25000

30000

Con

c (p

pm)

Fraction Numbers

B-QUAT TMDA Rh B-QUAT TMDA Ir

(A) )A()A(

(B)(A) (A)

(B)(B)

(B)

(B)

0

100

200

300

400

500

600

700

800

900

1000

Con

c (p

pm)

Fraction numbers

B-QUAT DDMDA Ir B-QUAT DDMDA Rh

0 10 20 30 40 50 60

(A) (B) (C)

(C)(C)

(C)

(C)(C)

Figure 7. A separation profile of rhodium and iridium at 6 M HCl for B-QUAT EDA, TMDA, B-QUAT HMDA, B-QUAT OMDA, B-QUATDMDA and B-QUAT DDMDA at 6 M HCl in 0.3 g of microspheres, loading of 5 mL binary mixture of ([IrCl6]

2−/[RhCl5(H2O)]2−), (A)

washing with 10 mL of 6-M HCl, (B) stripping with 10 mL of 0.05-M sodium metabisulphite and (C) elution with 15 mL of 20% HCl.

78 A. MAJAVU ET AL.

Resin regeneration capacities. Reusability studies wascarried out on the HMDA functionalized microspheressince it presented the highest loading capacity for [IrCl6]

2

−. The HMDA microspheres presented adsorption capaci-ties for iridium were 27.1 ± 0.8, 26.8 ± 0.9, and 26.5 ± 1.1mg/g for the first, second and third cycles, respectively(Fig. 9). DMDAmicrospheres were at least fully recyclableup to the two stages of regeneration that were investigated.The slight drop in loading capacities may be ascribed to theunavailability of some adsorption sites on the HMDAmicrospheres upon re-uses.

Conclusions

The microspheres were successfully functionalized withquaternary diammonium groups derived from EDA,

TMDA, HMDA, OMDA, DMDA and DDMDA. XPSstudies were conducted to confirm the presence of nitro-gen and other atoms on the sorbents. It was observed thatrhodium in the form of [RhCl5(H2O)]

2− was unabsorbedby the microspheres in the column study operated undera high chloride medium. Therefore, the study has provento be suitable for the separation of rhodium and iridiumas rhodium in the form of [RhCl5(H2O)]

2− was notloaded while iridium [IrCl6]

2− was highly loaded. Theresults showed that the adsorption behavior of all sorbentmaterial for [IrCl6]

2− was not affected by physical adsorp-tion, but was mainly dependent on the chemical interac-tion. Equilibrium isothermal data fitted the Freundlichmodel. The kinetics of the adsorption process was foundto follow the pseudo first-order kinetic model. It wasobserved that as the methylene spacers between thecationic centers increased to DMDA, the iridium loadingcapacities also increased. The selectivity of the quaternarydiammonium functional groups towards [IrCl6]

2− wasdependent on the charge density of the cationic centerssince the charge diffuse [IrCl6]

2− was loaded in higherquantities on the sorbent materials as the size of thequaternary diammonium center was increased. The func-tionalized microspheres were recoverable without loss ofactivity even after three cycles of usage.

Acknowledgement

We would like to thank the Electron Microscopy Unit(Rhodes University) for their SEM facilities.

Funding

We are grateful to the NRF (SA) for funding [Grant Number:CPRR 20100406000010238]. We also thank the NMMUResearch Themes Grant for funding.

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