avela majavu paper

Minerals Engineering xxx (2015) xxx–xxx

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Minerals Engineering

journal homepage: www.elsevier .com/locate /mineng

Functional nanofibers for separation of rhodium(III) and iridium(IV)chlorido species

http://dx.doi.org/10.1016/j.mineng.2015.10.0090892-6875/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail addresses: [email protected], [email protected]

(Z.R. Tshentu).

Please cite this article in press as: Majavu, A., et al. Functional nanofibers for separation of rhodium(III) and iridium(IV) chlorido species. Mine(2015), http://dx.doi.org/10.1016/j.mineng.2015.10.009

Avela Majavu, Adeniyi S. Ogunlaja, Eric C. Hosten, Zenixole R. Tshentu ⇑Chemistry Department, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa

a r t i c l e i n f o

Article history:Received 6 July 2015Revised 8 October 2015Accepted 9 October 2015Available online xxxx

Keywords:Rhodium(III)Iridium(IV)Diammonium cationsNanofibersSeparation

a b s t r a c t

Three quaternary diammonium-functionalized polyvinylbenzyl chloride nanofibers were prepared usinghexamethylenediamine (HMDA) quaternized with methyl, ethyl and benzyl groups to produce F-QUATHMDA-methyl, F-QUAT HMDA-ethyl and F-QUAT HMDA-benzyl, respectively. The synthesized nanofi-bers were characterized by means of FTIR, XPS, SEM, BET surface area, thermogravimetric analysis andelemental analysis. The materials were used to investigate the adsorption and separation of[RhCl5(H2O)]

2� and [IrCl6]2�. Adsorption isothermal batch studies were conducted on both metal ion

complexes ([RhCl5(H2O)]2� and [IrCl6]2�) using single metal aqueous solutions. The Langmuir isothermconfirmed monolayer adsorption for the uptake of both [RhCl5(H2O)]2� and [IrCl6]2� using F-QUATHMDA-ethyl, F-QUAT HMDA-methyl and F-QUAT HMDA-benzyl. The adsorption process of these mate-rials obeyed the pseudo second-order kinetics, suggesting that chemisorption was be the rate-determining step. Column studies were conducted for a binary mixture of the metal ions chlorido species,and the iridium loading capacities of 15.2(0.8) mg/g, 9.7(0.7) mg/g and 42.9(0.7) mg/g were obtained forF-QUAT HMDA-methyl, F-QUAT HMDA-ethyl and F-QUAT HMDA-benzyl, respectively. This order wasinterpreted on the basis of inductive effects of the three quaternizing groups. The results provided impor-tant insight into the effect of derivatizing the cationic center on the loading capacity for [IrCl6]2�. Thisstudy presents truly iridium-specific sorption materials which can be applied to solutions of low gradeores or secondary raw materials of PGMs.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Platinum group metals (PGMs) play a significant role as cata-lysts in the chemical, petrochemical and automobile industries(Bonnie and Gavin, 2010). PGMs, silver and gold occur in naturetogether with base metals such as iron, copper, nickel and cobalt,and alongside a wide range of minor elements such as lead, tel-lurium, selenium and arsenic (van Averbeke, 2003; Liddell andAdams, 2012). The high demand for PGMs, the depletion of the nat-ural PGMs deposits and the increasing scope to process secondaryraw materials (such as spent catalysts and electronic scraps) sug-gest the need to develop new sorption methods that can processlow concentrations of the PGMs (Laplante and Xiao, 2004; Jones,1999). The similar chemical behavior of the PGMs makes their sep-aration difficult (du Preez and Naidoo, 2005a), and it therefore isessential to develop more simple, efficient and selective methodsthrough the exploitation of their subtle chemical differences. Most

important for the separation of rhodium and iridium is theexploitation of their oxidation states [Rh(III) and Ir(IV)] and theirchlorido chemistry with [IrCl6]2� and [RhCl5(H2O)]2� being themain species of interest (du Preez and Naidoo, 2005a).

Various processes such as distillation (OsO4 and RuO4), precip-itation, solvent extraction, ion exchange and electrodepositionhave been employed for the separation of PGMs (Musina et al.,2014). Amongst these methods, precipitation and ion exchangemethods were previously considered because of their simplicityand economic viability (Fujiwara et al., 2007). Ion exchange is stilla preferred technology due to the high separation efficiency, highloading capacity and ease of operation (Hubickin et al., 2007;Hubicki and Wolowicz, 2009). The design of materials, especiallyanion exchange solid phase materials, to improve the loadingcapacity and separation factors for the PGMs remains aninteresting area of research. The solid support used to host anionexchangers in this study is electrospun nanofibers. Electrospinningis the most suitable technique for production of nanofibers (Teoand Ramakrishna, 2006). The advantages include its relative ease,low cost, high speed, vast materials selection, and versatility.Additionally, the technique allows control over fiber diameter,

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2 A. Majavu et al. /Minerals Engineering xxx (2015) xxx–xxx

microstructure, and arrangement (Teo and Ramakrishna, 2006;Greiner and Wendorff, 2007; Reneker and Yarin, 2008). This workdetails the development of new materials (nanofiber-based sor-bents) for the separation of PGMs. The introduction of an ionicfunctional groups into the nanofibers is a promising option to pro-vide novel ion exchangers with a high ion exchange capacity. Thecombination of high surface area of the nanofibers, and surfacefunctionalization should result in improvement of ion exchangers(e.g., high adsorption capacity and rapid kinetics of adsorption),and will lead to a significant expansion of nanofibers applications(Cao et al., 2012; Matsumoto and Tanioka, 2011). We have demon-strated this concept in one of our previous publications where acomparison of the performance of functional nanofibers was madeagainst microspheres containing the same functional groups(Fayemi et al., 2013).

This article presents the fabrication of anion exchange nanofi-bers functionalized with quaternary diammonium groups derivedfrom hexamethylenediamine (HMDA) that was quaternized bymethyl, ethyl and benzyl groups. It had been shown by du Preezand Naidoo (2005a, 2005b) that hexamethylenediamine (HMDA)quaternized with methyl groups and hosted on silica gave a goodseparation and loading capacity for [IrCl6]2�, hence our studyexploits a different host material and the effect of quaternizinggroups. The evaluation of the separation efficiency of the materialswas studied using a column (dynamic) experiment. The study ofthe underlying chemistry was studied by using computationalmethods based on Density Functional Theory (DFT) to investigatethe interaction of the cations and the anionic chlorido species ofrhodium(III) and iridium(IV). The adsorption isotherms and kinet-ics for the uptake of [RhCl5(H2O)]2� and [IrCl6]2� on functionalnanofibers was studied through batch experiments.

2. Experimental

2.1. Reagents

Hexamethylenediamine (98%), 2,6-lutidine (P98%), iodo-methane (purumP 99%), iodoethane (99%), and benzylbromide(98%) were used as obtained from Sigma–Aldrich. Iridium in theform of iridium(III) chloride hydrate (99.9%) and rhodium as rho-dium(III) chloride (98%) were also obtained from Sigma–Aldrich.Hydrochloric acid (32%), 4-vinylbenzylchloride (VBC) (98%), azobi-sisobutyronitrile (AIBN) (98%), tetrahydrofuran (99.5%), ferric chlo-ride (99%), sodium chlorate (99%), sodium metabisulphite(P97.0%) and sodium iodide (P99.5%) were also purchased fromSigma–Aldrich. Toluene was purchased from Merck Chemicals.DMF (99%) was purchased from Lab Chem. Ethanol and methanolwere sourced from NCP Alcohol and Protea Chemicals, respectively,and were distilled before use. Calibration standards for the ICP-MSanalyses were prepared from a mixed PGM Spectrascan standardSS-018313. High purity water as obtained from a Millipore Sim-plicity system using singly distilled water.

2.2. Instrumentation

The identity of the rhodium(III) and iridium(IV) chlorido specieswas determined by a Perkin Elmer UV–Vis spectrophotometer.Fourier Transform Infrared Spectroscopy (FTIR) was carried outusing a Perkin Elmer 400 to confirm the structural information ofthese anionic exchangers. The morphology and diameter of theelectrospun PVBC nanofiber mats before and after the introductionof the ion exchange groups were observed using a Scanning Elec-tron Microscope TESCAN Vega TS 5136LM (SEM), operated at20 kV and at a working distance of 20 mm. Each sample of polymernanofiber mats was coated with gold using a gold sputter machine

Please cite this article in press as: Majavu, A., et al. Functional nanofibers for(2015), http://dx.doi.org/10.1016/j.mineng.2015.10.009

before imaging to prevent electrostatic charging. The carbon,hydrogen and nitrogen content were determined using a Vario Ele-mentary ELIII Microcube CHNS analyser. A Perkin Elmer SCIEXElan-6100 ICP-MS with an AS-90 autosampler was used for con-centration determinations of rhodium and iridium solutions usinga plasma power of 1.2 kW and a nebulizer flow rate of 1.0 L/min.Calibration standards were prepared in the 0.1–1000 lg/L rangeand rhodium and iridium were analyzed at mass 103 and 193respectively. The surface area was determined by the BET methodusing a Micromeritics ASAP 2020 Surface area and Porosity Anal-yser. Carbon dioxide adsorption isotherm was measured at 77 K.Each sample was degassed for 8 days before analysis at 100 �C tocompletely remove the adsorbed impurities. Thermogravimetricanalysis (TGA) was conducted with TGA Q50 from TA Instrumentsat a heating rate of 20 �C/min under nitrogen.

2.3. Synthesis and electrospinning of poly(vinylbenzylchloride) (PVBC)

The method reported by Fayemi et al. (2013) was followed forthe synthesis and electrospinning of poly(vinylbenzylchloride)(PVBC).

2.4. Functionalization and quaternization of PVBC nanofibers

The functionalization and quaternization procedures describedby du Preez and Naidoo (2005b) were followed with little modifi-cations (Scheme 1.1). 0.1 g of nanofibers was reacted with 18 g(0.15 mol) of hexamethylenediamine in 60 mL of dry ethanol. Acatalytic amount of sodium iodide was added, and the reactionwas heated under reflux for 2 days. The aminated nanofibers werefiltered and washed with water and ethanol. The fibers were thenplaced in a round bottom flask and to this was added 0.525 mol ofeither methyl iodide, ethyl iodide or benzyl iodide (prepared frombenzyl bromide) in 60 mL dry ethanol followed by 0.3 mol of luti-dine. The mixture was then heated under reflux for 2 days followedby filtration and washing with water and ethanol. The materialswere then repeatedly treated with excess iron(III) (0.1 M) in 6 MHCl followed by a solution of hot sodium metabisulphite(�0.05 M) in distilled water and finally concentrated HCl toremove any iodide or tri-iodide ions on the resin. The materialswere then collected, washed with ethanol via Soxhlet extractionand dried in air. The nitrogen content (%) was determined bymicroanalysis and the data is presented in Table 1 in the resultssection.

2.5. Preparation of the metal stock solutions

Stock solution containing 0.015 M of rhodium and 0.045 M irid-iumwere prepared from RhCl3 and IrCl3�xH2O in 6 M HCl under thereflux for an hour at 70 �C. Iridium(III) solution was further oxi-dized with sodium chlorate to obtain Ir(IV). The resulting solutionshad a brown color for Ir(IV) and dark red color for Rh(III). The solu-tions were confirmed by UV–visible spectrophotometric analysis(Fig. S1 in the supplementary information).

2.6. Adsorption studies

2.6.1. Adsorption dynamics for [IrCl6]2� and [RhCl5(H2O)]

2�

The amount of adsorption at equilibrium (qe, mg/g) of the com-plex species was calculated using Eq. (2.1), and metal ion uptakewas calculated using Eq. (2.2) (Jadhav et al., 2015; Putra et al.,2014; Akkaya et al., 2013; Goel et al., 2005).

qe ¼ðCo � CeÞ � V

Wð2:1Þ

separation of rhodium(III) and iridium(IV) chlorido species. Miner. Eng.


Scheme 1.1. Scheme for functionalization of PVBC nanofibers.

Table 1The microanalyses data (%) for the fibers (before and after quaternization).

Nanofibers C H N C:N

F-HMDA 82.48 9.30 5.04 16:1F-QUAT HMDA-methyl 63.70 9.06 3.47 18:1F-QUAT HMDA-ethyl 61.05 7.58 2.68 23:1F-QUAT HMDA-benzyl 66.63 7.79 2.56 26:1

A. Majavu et al. /Minerals Engineering xxx (2015) xxx–xxx 3

where Co is the initial metal concentration (mg/mL), Ce is the equi-librium metal concentration (mg/mL), V being the volume of themetal solution (mL) and W is the mass of the sorbent (g).

Adsorption ð%Þ ¼ ðCo � CeÞCo

� 100 ð2:2Þ

2.6.2. Adsorption isothermal studiesBatch adsorption experiments were carried out using the

concentration range of 923–2447 ppm for [IrCl6]2� and 1408–4193 ppm for [RhCl5(H2O)]2�. This was done by equilibrating0.1 g of the nanofiber material with 2.5 mL of metal ion solutionfor 40 min with shaking at 120 rpm at room temperature(25 ± 1 �C) using a mechanical shaker. The metal ion concentrationwas analyzed using an ICP-MS.

The adsorption data for [IrCl6]2� and [RhCl5(H2O)]2� on nanofi-bers were analyzed in terms of Langmuir and Freundlich isothermmodels presented by Eqs. (2.3)–(2.5) (Hasan et al., 2008; Min et al.,2012; Anaia et al., 2014; Boparai et al., 2011).

Ce

Q e¼ Ce

Qoþ 1bQo

ð2:3Þ

where Qo and b are the Langmuir constant which are related to theadsorption capacity and energy of adsorption, respectively and canbe calculated from the intercept and slope of the linear plot withCe/qe versus Ce. The essential features of the Langmuir isothermcan be express in terms of the dimensionless equilibrium parameterRL, which is define as

RL ¼ 11þ bCo

ð2:4Þ

where b is Langmuir constant and Co is the initial concentrationof Rh(III) and Ir(IV). RL value within the range 0 < RL < 1, indicatefavorable adsorption (Min et al., 2012; Thilagavathy and Santhi,2014).

Log qe ¼ log kf þ 1nlogCe ð2:5Þ

where qe and Ce are the equilibrium concentration of [IrCl6]2� and[RhCl5(H2O)]2� in the adsorbed (mg/g) and liquid phases (mg/L),respectively. Kf and n are Freundlich constants which are relatedto the adsorption capacity and intensity, respectively. These


constants can be calculated from slope and intercept of linear plotwith logqe versus logCe (Goel et al., 2005; Hasan et al., 2008; Minet al., 2012).

2.6.3. Kinetics of adsorptionThe kinetics of the adsorption process were studied by carrying

out a set of adsorption experiments at a constant temperature(25 ± 1 �C) and monitoring the amount adsorbed with time. Theadsorption kinetic data of [IrCl6]2� and [RhCl5(H2O)]2� was ana-lyzed in terms of pseudo first-order and pseudo second-orderkinetic equations. Assuming the pseudo-first order kinetics, therate of the adsorptive interaction can be calculated by using theLagergren equation (2.6).

logðqe � qtÞ ¼ log qe �k1

2:303t ð2:6Þ

where qe and qt are the amount of [IrCl6]2� or [RhCl5(H2O)]2� (mg/g)adsorbed at equilibrium and at time t, respectively, and k1 (min�1)is the rate constant of pseudo first-order adsorption. The qe and therate constant (k1) were calculated by plotting the log (qe � qt) ver-sus t.

The pseudo-second-order model can be represented as in Eq.(2.7) (Goel et al., 2005; Min et al., 2012; Boparai et al., 2011).

tqt

¼ 1k2q2

eþ 1qt

t ð2:7Þ

Thus, k2 (g/mg min) is the rate constant of pseudo second-orderadsorption. If the pseudo second-order equation is applicable, theplot of t

qtversus t shows a linear relationship. qe and k2 can be

determined from the slope and intercept of the plot (Min et al.,2012; Thilagavathy and Santhi, 2014).

2.7. Binary column studies

A custom-made columnwith an internal diameter of 0.7 cm anda length of 10 cmwas used for the column studies. Prior to the sep-aration procedure, 0.1 g of nanofibers was packed on the column.The column was conditioned with 5 mL distilled water followedby 6 M HCl. Solutions were delivered through the column at anoptimized flow rate of 0.5 mL/h. After loading the metal solutions,the column was washed with 15 mL of 6 M HCl and then strippedusing 15 mL of a solution of 0.05 M sodium metabisulphitefollowed by elution with 20 mL of 20% HCl. From the outlet ofthe column 0.5 mL fractions of the effluent were collected. Thefractions from the flow-through, wash, strip and the elution werethen analyzed for rhodium and iridium by ICP-MS.

2.8. Resin regeneration

The re-usability of the materials was tested after each cycle bywashing the nanofibers (0.1 g) with 6 M HCl solution after the




separation protocol discussed in Section 2.7. The materials werethen respectively taken through another cycle while still packedon the column.

2.9. Molecular modelling studies

Density Functional Theory (DFT) for molecular modelling wasemployed to understand the mode of interactions between the var-ious quaternary diammonium cations and metal complex anions([RhCl5(H2O)]2� and [IrCl6]2�). Prior to analysis, geometry opti-mizations and vibrational analyses of quaternary diammoniumcations and metal complex anions ([RhCl5(H2O)]2� and [IrCl6]2�)were performed using the DFT methods with B3LYP functionaland SDD basis set on a Gaussian program (Frisch et al., 2009).The molecular orbital energies characterized by the highest occu-pied molecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO) positions of adducts were analyzed using B3LYP/SDD method. A brief description of these orbital energies are pro-vided below:

2.9.1. Molecular orbital energiesInteraction between atoms or molecules happens most likely

between the HOMO of one molecule and the LUMO of the othermolecules. The amount of energy required to add or remove elec-trons in a molecule can be obtained from the HOMO and LUMOenergy values. HOMO characterizes the nucleophilicity of a species,that is, its tendency to donate electron, while LUMO characterizesthe electrophilicity of a species, that is, its tendency to receiveelectron.

2.9.2. Orbital energy gapFrom the HOMO and LUMO energy values, the HOMO–LUMO

energy gap can be determined. Large HOMO–LUMO gap indicateshigh stability and resistance to charge transfer and changes in elec-tron number and distribution. Therefore, hard molecules have alarge HOMO–LUMO gap. Meanwhile, small HOMO–LUMO gap indi-cates high polarizability since they only require small amount ofenergies to get them to the excited states. Small HOMO–LUMOgap is, therefore, indicative of soft molecules (Ogunlaja et al.,2014; Hizaddin et al., 2013).

The enthalpies of formation (DDHadduct), Gibb’s free energies(DDGadduct) and entropies of formation (DDSadduct) for each adduct(ion-pair) formed were calculated by using Eqs. (2.8) and (2.9).Charge transfer studies were carried-out by running the naturalbond orbital (NBO) program Thilagavathy and Santhi, 2014.

DDHadduct ¼ DHadduct � ðmDHQDC þ tDHMetal anionsÞ ð2:8Þ

where m and t are the stoichiometric amounts of quaternarydiammonium cations and metal complex anions ([RhCl5(H2O)]2�

and [IrCl6]2�) involved in ion-pair formation.

DDGadduct ¼ DDHadduct � TDDSadduct ð2:9Þ

DDG, T and DDS are the Gibbs free energy for the adduct formation,temperature (298 K) and entropy for adduct formation at standardconditions (i.e. 1 M concentration for solvents and 1 atm pressure),respectively. The Gibbs free energy (DG�) gives information aboutthe feasibility of anion–cation interaction (adduct formation);entropy (DS�) describes the spontaneous nature of interactionwhereas the sign of DH� reflects the endo- or exothermic natureof the process.


3. Results and discussion

3.1. Elemental analysis

In order to gain an understanding of the chemical compositionof nanofibers, elemental analysis was performed to reveal the per-centage of C, H, and N in the synthesized nanofibers (Table 1). Thecarbon:nitrogen ratios suggested that the diamines were con-nected to two benzyl groups in the formation of the functionalmaterials (as shown in Scheme 1.1).

3.2. Scanning Electron Microscopy (SEM) images of functionalizedmaterials

3.2.1. SEM images of nanofibersScanning Electron Microscopy (SEM) was used to monitor

changes in morphology of the nanofibers from their fabricationto the functionalization steps. The samples were coated with a thinlayer of gold by sputtering before the SEM image. The SEM micro-graphs of the nanofibers before and after functionalization aregiven in Fig. 1. The unfunctionalized PVBC nanofibers showedsmooth fibers with no beads or beads-on-strings. The electrospunnanofibers show no morphological changes or damage after thefunctionalization steps. The SEM images of the nanofibers whichwere synthesized with different quaternary diammonium groupsshowed fiber diameters in the range 532–895 nm compared withan average of 508 nm for the unfunctionalized fibers. The biggerthe quaternizing group the larger the diameter.

3.2.2. Energy-dispersive spectroscopic analysisEDS analysis was carried out in order to verify surface function-

alization of the nanofibers. The EDS spectra are presented inFigs. S2–S4 in the supplementary information. The spectra showedthat the nanofiber materials dominantly consisted of N, O and Cl,respectively. It was interesting to note the presence of the Cl in thisbulk analysis of this material which was evidence of incompletefunctionalization, and perhaps a suggestion that functionalizationmight have occured on the surface of the materials. This was fur-ther corroborated with XPS analysis.

3.3. X-ray Photoelectron Spectroscopy (XPS)

The surface chemistry analysis of the nanofiber-based anionexchangers was carried out with XPS (Fayemi et al., 2013), andthe spectra are presented in Fig. S5. The binding energies at200 eV and 270 eV, which were assigned to the Cl 2p and Cl 2srespectively, were observed on unfunctionalized nanofibers repre-senting the chloride group (C–Cl) in PVBC. The disappearance ofthese peaks was noticed upon functionalization and the presenceof N 1s peaks on functional nanofibers, with the binding energiesof 397 eV, confirmed the functionalization with amines. The XPSanalysis revealed that only surface functionalization occurred sincethere was no Cl observed by XPS but the bulk analysis by EDSshowed the presence of Cl (Figs. S2–S4).

3.4. FTIR spectroscopic analysis

The chemical composition of the unfunctionalized andfunctionalized nanofibers also confirmed by comparing the FTIRspectra of PVBC before and after the reaction with diamines(Fig. S6 (i) and (ii)). In all spectra, the absorption band around3153 cm�1 was present revealing the stretching vibration of theN–H group and the band at 1573 cm�1 was representative of theN–H bending. The unfunctionalized nanofibers showed a strong



Fig. 1. SEM images of (A) unfunctionalized PVBC fiber (average diameter = 508 nm), (B) F-QUAT HMDA-methyl (532 nm) (C) F-QUAT HMDA-ethyl (565 nm), (D) F-QUATHMDA-benzyl (895 nm).


peak at 670 cm�1 which was due to the m(C–Cl) and a strong peakat 1260 cm�1 which can be assigned to the CH2–Cl bending. Thesepeaks disappeared or got diminished after functionalization. Apeak at 1487 cm�1 for F-QUAT HMDA-methyl corresponds to thequaternary diammonium groups, specifically to the symmetricbending of the methyl groups on the methylated diammoniumgroups. A change of the quaternizing group from methyl to ethyland benzyl groups gives two bands near 1433–1468 cm�1 whichcould be attributed to the bending of the CH3 and CH2 groups.Moreover, the peak at 1413 cm�1 was ascribed to as the m(C–N)stretching vibration of the quaternary diammonium groups.

3.5. BET surface area

The surface area of unfunctionalized and functionalized nanofi-ber materials were obtained by the Brunauer–Emmett–Teller (BET)method and the results are presented in Table 2. The samples weredegassed at 100 �C for 3 days. The surface area was determined byusing single point CO2 gas adsorption isotherm. It can be observedthat the surface area of the original PVBC fiber was 341.89 m2 g�1

and it decreased in functional materials in the order the decrease inthe size of the quaternizing group (benzyl > ethyl > methyl). Thiscould be rationalized on the basis that the smaller groups canpenetrate within the material with ease and result in effective

Table 2Single point BET surface area for the functional nanofibers.

Nanofibers Surface area (m2/g)

PVBC 341.83F-QUAT HMDA-methyl 36.93F-QUAT HMDA-ethyl 39.95F-QUAT HMDA-benzyl 89.36


functionalization thereby decreasing the surface area. It isexpected that F-QUAT HMDA-benzyl will give a higher loadingcapacity provided that the ion-pairing chemistry is suitable.

3.6. Thermogravimetric analyses (TGA)

The thermal stability of the samples was carried out by thermo-gravimetric analysis under N2 atmosphere (Fig. S7). Samples wereheated from room temperature to 800 �C at a heating rate of10 �C/min. The TG curves for the functionalized nanofibers showedan initial weight loss of �5% which occurred up to 100 �C and wasattributed to intramolecular solvents. The TG curves of F-QUATHMDA-methyl, F-QUAT HMDA-ethyl and F-QUAT HMDA-benzylgave clear indication that a three step degradation process isobserved. A weight loss of �30% (second step) at about 200–300 �C is due to the partial collapse of functional groups on thepolymer while the final step of �40% weight loss was probablydue to the collapse of the polymer at about 350–400 �C. The func-tional nanofibers, therefore, showed thermal stability up to 180 �C.

3.7. Anion–cation interactions studies by molecular modelling

Interactions between quaternary diammonium compounds andmetal complex anions depend on the following properties, theHOMO and LUMO energies and charge transfer as well as thermo-dynamic properties. High ionization potential is indicated by lowHOMO energy (better electron donor), while high electron affinityis indicated by high LUMO energy (better electron acceptor). TheHOMO and LUMO energy locations of QUAT-HMDA-methyl,QUAT-HMDA-ethyl and QUAT-HMDA-benzyl are presented inFigs. 2–4, respectively. The HOMO (EH) and LUMO (EL) energies ofquaternary diammonium compounds and metal complexes([RhCl5(H2O)]2� and [IrCl6]2�) before and after adduct formation



Fig. 2. HOMO and LUMO locations of QUAT HMDA-methyl: (A) HOMO and (B) LUMO. Blue, gray and light gray colors represent nitrogen, carbon and hydrogen atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. HOMO and LUMO locations of QUAT HMDA-ethyl: (A) HOMO and (B) LUMO. Blue, gray and light gray colors represent nitrogen, carbon and hydrogen atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. HOMO and LUMO locations of QUAT HMDA-benzyl: (A) HOMO and (B) LUMO. Blue, gray and light gray colors represent nitrogen, carbon and hydrogen atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


are depicted in Table 3. All the calculations were done in gas phase.Single point water phase calculations at the gas phase equilibriumgeometry were done using these basic set. For the gas phase struc-ture (DFT-computed) there is freedom of rotation around thebonds, allowing for different forms and subsequently an ideal con-formation representing lowest-energy geometry of all these possi-bilities is acquired. The HOMO and LUMO are both destabilized inwater than the gas phase because the compounds contain N-atoms(Kürs�at Efil1 and Yunus Bekdemir, 2014).

Upon formation of adducts with the metal ion species,[RhCl5(H2O)]2� and [IrCl6]2�, the HOMO and LUMO positions ofthe various quaternary diammonium compounds, QUAT HMDA-methyl, QUAT HMDA-ethyl and QUAT HMDA-benzyl shifted(Figs. 2–4). Molecular interaction between the various quaternarydiammonium compounds and metal complex anions ([RhCl5(H2O)]2�

and [IrCl6]2�) caused a decrease in the various HOMO–LUMOenergy gaps (Table 3). [IrCl6]2�-quaternary diammonium com-pounds adducts presented lower HOMO–LUMO energy gap, ascompared to the adducts formed with [RhCl5(H2O)]2�, hence con-firming the possibility of adduct formation between [IrCl6]2� andthe various quaternary diammonium compounds (Figs. 5–10).


The interaction between [IrCl6]2� and quaternary diammoniumcompounds increased as the HOMO–LUMO energy gap decreasedin the order; QUAT HMDA-methyl < QUAT HMDA-ethyl < QUATHMDA-benzyl.

Charge transfer within the various adducts was investigated byemploying natural bond orbital (NBO) analysis. The partial chargesof Ir in [IrCl6]2� changed upon interaction with the various quater-nary ammonium compounds and an increase from �0.991 to�0.378, �0.377 and �0.377 was observed for QUAT HMDA-methyl, QUAT HMDA-ethyl and QUAT HMDA-benzyl respectively.A slight increase in the partial charge of Rh was observed when[RhCl5(H2O)]2� interacted with the various quaternary ammoniumcompounds from �0.207 to �0.135, �0.132 and �0.125 for QUATHMDA-methyl, QUAT HMDA-ethyl and QUAT HMDA-benzylrespectively (Table 4). A significant amount of charge was trans-ferred from Ir in [IrCl6]2� to the chloride atoms upon interactionwith quaternary diammonium cations as compared to Rh in[RhCl5(H2O)]2� herein most of the charge was retained on themetal ion.

Thermodynamic parameters such as enthalpy (DDH), entropy(DDS) and free energies (DDG) for the various adducts formed



Table 3HOMO (EH) and LUMO (EL) energies for quaternary diammonium cations and metal ion chlorido species, and their adducts.

Compounds EH (a.u.) EL (a.u.) Orbital energy gap (EG) (a.u.) Orbital energy gap (EG) (eV)

[IrCl6]2� �0.36362 �0.31798 0.05564 1.51404[RhCl5(H2O)]2� �0.31179 �0.27705 0.03474 0.94532QUAT HMDA-methyl �0.34587 �0.02988 0.31599 8.59853QUAT HMDA-ethyl �0.10363 0.06028 0.16391 4.46022QUAT HMDA-benzyl �0.14155 �0.04622 0.09533 2.59406QUAT HMDA-methyl-[RhCl5(H2O)]2� �0.23302 �0.08930 0.14372 3.91082QUAT HMDA-ethyl-[RhCl5(H2O)]2� �0.23657 �0.09217 0.1444 3.92932QUAT HMDA-benzyl-[RhCl5(H2O)]2� �0.23806 �0.09408 0.14398 3.91790QUAT HMDA-methyl-[IrCl6]2� �0.29439 �0.25380 0.04056 1.10415QUAT HMDA-ethyl-[IrCl6]2� �0.29356 �0.25309 0.04047 1.10125QUAT HMDA-benzyl-[IrCl6]2� �0.27335 �0.25313 0.02022 0.55021

Fig. 5. HOMO and LUMO locations of QUAT HMDA-methyl-[IrCl6]2�: (A) HOMO and (B) LUMO. Blue, green and gray colors represent nitrogen, chloride and carbon atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. HOMO and LUMO locations of QUAT HMDA-methyl-[RhCl5(H2O)]2�: (A) HOMO and (B) LUMO. Red, blue, green and gray colors represent oxygen, nitrogen, chlorideand carbon atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


are presented in Table 4. The higher enthalpy values observed for[RhCl5(H2O)]2�-quaternary diammonium compounds furtherexplained the less favorable interaction of [RhCl5(H2O)]2� withthe various quaternary diammonium cations at low temperatures,while the [IrCl6]2�-quaternary diammonium cations interactionswere favorable at low temperatures due to the observed highernegative enthalpy values (Table 4).

3.8. Adsorption studies

3.8.1. Adsorption isothermsThe parameters of the Langmuir and Freundlich isothermal

models for [IrCl6]2� and [RhCl5(H2O)]2� are presented inTables 5 and 6 respectively, and the plots are provided in the


supplementary section (Figs. S8 and S9). Langmuir equation plotsfrom Fig. S9, a graph of Ce vs Ce/qe for [IrCl6]2� and [RhCl5(H2O)]2�

respectively gave a larger correlation coefficient, R2 as compared tothe Freundlich isothermal model. This showed that the adsorptionprocess for [IrCl6]2� and [RhCl5(H2O)]2� using the various adsor-bents could be described well with the Langmuir isothermal modelas the experimental equilibrium data fitted with the correlationcoefficient value very close to 1. Langmuir equation obtained indi-cated a proof of chemical adsorption, and usually mean monolayeradsorption on the surface of adsorbents (Park and Kim, 2010;Blanchard et al., 1984; Qi et al., 2014). The isotherm confirms theion-pairing mechanism of interaction between the metal ion([IrCl6]2� and [RhCl5(H2O)]2�) and the various adsorbents,F-QUAT HMDA-methyl, F-QUAT HMDA-ethyl and F-QUAT HMDA-benzyl.



Fig. 7. HOMO and LUMO locations of QUAT-HMDA-ethyl-[RhCl5(H2O)]2�: (A) HOMO and (B) LUMO. Red, blue, green and gray colors represent oxygen, nitrogen, chloride andcarbon atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. HOMO and LUMO locations of QUAT HMDA-ethyl-[IrCl6]2�: (A) HOMO and (B) LUMO. Blue, green and gray colors represent nitrogen, chloride and carbon atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. HOMO and LUMO locations of QUAT HMDA-benzyl-[RhCl5(H2O)]2�: (A) HOMO and (B) LUMO. Red, blue, green and gray colors represent oxygen, nitrogen, chlorideand carbon atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


Please cite this article in press as: Majavu, A., et al. Functional nanofibers for separation of rhodium(III) and iridium(IV) chlorido species. Miner. Eng.(2015), http://dx.doi.org/10.1016/j.mineng.2015.10.009


Fig. 10. HOMO and LUMO locations of QUAT HMDA-benzyl-[IrCl6]2�: (A) HOMO and (B) LUMO. Blue, green and gray colors represent nitrogen, chloride and carbon atomsrespectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 4Electrostatic charge transfer between quaternary diammonium ion and metal ion chlorido species.

Atomic charges

Cl(1) Cl(2) Cl(3) Cl(4) Cl(5) Cl(6) Ir or Rh

[RhCl5(H2O)]2� �0.041 �0.041 �0.035 �0.035 �0.061 – �0.207QUAT HMDA-methyl-[RhCl5(H2O)]2� �0.413 �0.414 �0.387 �0.441 �0.436 – �0.135QUAT HMDA-ethyl-[RhCl5(H2O)]2� �0.428 �0.423 �0.390 �0.431 �0.423 – �0.132QUAT HMDA-benzyl-[RhCl5(H2O)]2� �0.431 �0.413 �0.393 �0.417 �0.434 – �0.124[IrCl6]2� 0.511 0.391 0.391 0.391 0.390 �0.083 �0.991QUAT HMDA-methyl-[IrCl6]2� �0.097 �0.091 �0.092 �0.113 �0.106 �0.108 �0.378QUAT HMDA-ethyl-[IrCl6]2� �0.093 �0.098 �0.098 �0.111 �0.103 �0.104 �0.377QUAT HMDA-benzyl-[IrCl6]2� �0.099 �0.100 �0.101 �0.116 �0.105 �0.102 �0.377

DDG (kcal mol�1) DDS (kcal mol�1) DDH (cal mol�1)

Thermodynamic data (DDH, DDG and DDS) for the formation of adducts (quaternary diammonium cations and metal ion chlorido species)QUAT HMDA-methyl-[RhCl5(H2O)]2� �312.58231 �0.03138 �321.78536QUAT HMDA-ethyl-[RhCl5(H2O)]2� �312.56739 0.01930 �320.41845QUAT HMDA-benzyl-[RhCl5(H2O)]2� �312.25448 0.02763 �321.85329QUAT HMDA-methyl-[IrCl6]2� �715.38662 �0.00628 �717.59168QUAT HMDA-ethyl-[IrCl6]2� �715.01354 �0.01324 �715.49234QUAT HMDA-benzyl-[IrCl6]2� �713.76995 �0.01223 �712.67473


3.8.2. Adsorption kineticsIn these experiments the effect of time on the adsorption of

[IrCl5(H2O)]2� and [IrCl6]2� onto the materials was studied, respec-tively (Fig. S10). The results showed that the rate of ion exchangeof these anionic metal ions complexes by the anion exchangerswere very rapid and reached equilibrium state within 15 min ofshaking, and it remained at equilibrium up to 40 min.

In order to obtain further insight into the mechanism of adsorp-tion of [RhCl5(H2O)]2� and [IrCl6]2� onto nanofibers, the experi-mental data were tested against pseudo first-order and pseudosecond-order kinetic models (Figs. S11 and S12). It has been shownthat the experimental qe values are not in good agreement withtheoretical values calculated from the pseudo first-order model,and the correlation coefficients also support the lack of fit of thismodel. Application of the pseudo second-order model providesmuch better correlation coefficients, with the R2 values beinggreater than 0.999, respectively. Moreover, there is an excellentcorrelation between the calculated qe(cal) values from the pseudosecond-order model and the experimental qe(exp) values given inTables 7 and 8. Therefore, it may be concluded that [RhCl5(H2O)]2�

and [IrCl6]2� adsorption onto the adsorbents proceeds via chemicaladsorption (Blanchard et al., 1984; Qi et al., 2014; Achmad et al.,2012).

In comparing the adsorption of [RhCl5(H2O)]2� and [IrCl6]2� onF-QUAT HMDA-methyl, F-QUAT HMDA-ethyl and F-QUATHMDA-benzyl, it was observed that the adsorption capacity of


F-QUAT HMDA-benzyl was higher than that of F-QUAT HMDA-methyl and F-QUAT HMDA-ethyl (Tables 7 and 8), which wasattributed not only to the larger surface area of F-QUAT HMDA-benzyl but also to the suitable HOMO–LUMO energies, electrostaticcharge transfer and the thermodynamic parameters detailed inSection 3.7.

3.9. Column studies

The column studies were performed firstly by equilibrating thenanofiber with 6 M HCl and then a binary metal solution([IrCl6]2�/[RhCl5(H2O)]2�) was passed through the column bed atan optimized flow rate of 0.5 mL/h. Thereafter, the column waswashed with 6 M HCl to remove unabsorbed metal complexanions. [IrCl6]2� was retained in relation to [RhCl5(H2O)]2� whichcomes off the column upon washing suggesting that the high chlo-ride medium displaces this anion while [IrCl6]2� was retainedunder these conditions (around fractions 40–45 in Figs. 11 andS13 and S14 for F-QUAT HMDA-benzyl, F-QUAT HMDA-ethyl andF-QUAT HMDA-methyl, respectively). There was no rhodiumdetected after washing the column bed suggesting that[RhCl5(H2O)]2� was not loaded in the presence of a high chloridemedium as was observed in this study. It was also observed thatthe color of the earlier fractions appeared intense red which wasindicative of rhodium coming off immediately upon allowing thecolumn to flow (earlier fractions, Fig. 11). The mass balance for



Table 5Langmuir and Freundlich isothermal parameters for adsorption of [IrCl6]2� on functional nanofibers.

Sorbent materials Isotherm model

Langmuir Freundlich

Qo (mg/g) b (L/mg) RL R2 Kf (mg/g) n R2

F-QUAT HMDA-methyl 44.4 �0.0192 �0.0217 0.9998 141.8 5.87 0.9972F-QUAT HMDA-ethyl 50.2 �0.0477 �0.0085 1.0000 96.4 9.79 0.9992F-QUAT HMDA-benzyl 49.5 �0.0412 �0.0098 0.9999 200.5 9.10 0.9962

Table 6Langmuir and Freundlich isothermal parameters for adsorption of [RhCl5(H2O)]2� on functional nanofibers.

Sorbent materials Isotherm model

Langmuir Freundlich

Qo (mg/g) b (L/mg) RL R2 Kf (mg/g) n R2

F-QUAT HMDA-methyl 45.9 �0.0021 �0.1279 0.9980 271.3 1.98 0.9956F-QUAT HMDA-ethyl 90.9 �0.0052 �0.0459 1.0000 146.9 1.38 0.9981F-QUAT HMDA-benzyl 93.4 �0.0750 �0.0031 0.9999 436.3 1.70 0.9907

Table 7Parameters of the pseudo first-order and pseudo second-order rate law for the adsorption of [IrCl6]2� on nanofibers.

Sorbent material qe exp (mg/g) Pseudo 1st order Pseudo 2nd order

qe calc (mg/g) K1 (1/min) R2 qe calc (mg/g) K2 (g/mg min) h (mg/g min) R2

F-QUAT HMDA-methyl 55.5 1.88 � 1044 �0.1672 0.3708 52.9 0.2381 666.7 0.9997F-QUAT HMDA-ethyl 55.4 1.99 � 1048 �1.3196 0.2587 55.9 0.1105 344.8 0.9997F-QUAT HMDA-benzyl 55.4 1.71 � 1045 �0.1564 0.7955 56.2 0.0754 238.1 0.9999

Table 8Parameters of the pseudo first-order and pseudo second-order rate law for the adsorption of [RhCl5(H2O)]2� on nanofibers.

Sorbent material qe exp (mg/g) Pseudo 1st order Pseudo 2nd order

qe calc (mg/g) K1 (1/min) R2 qe calc (mg/g) K2 (g/mg min) h (mg/g min) R2

F-QUAT HMDA-methyl 71.0 2.24 � 1025 �1.2118 0.8309 76.3 0.0114 66.7 0.9995F-QUAT HMDA-ethyl 97.5 3.08 � 1089 �0.0781 0.1822 98.0 0.1301 1250 0.9999F-QUAT HMDA-benzyl 99.3 1.53 � 1092 �0.1662 0.2864 100.0 0.2500 25,000 0.9999


rhodium was fully observed with the amount loaded matching thetotal of fractions 1–15 for rhodium. The intensity of the red colorgradually decreased upon washing with a 6 M HCl solution. Sinceiridium was loaded in excess (in order to determine the loadingcapacity) its presence was also detected in the earlier fractions(Fig. 11). Furthermore, the stripping solution of sodium metabisul-fite was added onto to the columns to reduce [IrCl6]2� to form[IrCl6]3� which could be eluted and elution steps a yellow colorwas observed, indicating the presence of iridium in the fractions(above fraction number 40, Fig. 11). As the fraction number wasincreased above 40, the original yellow color gradually disap-peared. [IrCl6]3� was then eluted with 20% HCl and all the fractionswere determined by ICP-MS. The un-absorbed [RhCl5(H2O)]2�/[RhCl6]3� and the overloaded [IrCl6]2� came off the column at theearly stages of the column process (up to around fraction 15).

It is appreciated that a fair proportion of [RhCl6]3� exists in 6 MHCl solutions (Gerber et al., 2010). However, there are two factorsthat result in poor sorption of this species, and it is the charge andcharge density. The order in which metal-chlorido complexes aremore likely to form ion pairs with anion-exchangers is dependenton the charge of the anion, for example, the doubly-charged com-plexes are found to be strongly sorbed by dicationic resins whereastriply-charged are weakly bound (Bernardis et al., 2005). The


[IrCl6]2� selectivity over [RhCl5(H2O)]2� is due to the charge den-sity effect and polarity.

It was interesting to observe that the loading capacity for[IrCl6]2� was affected by the change in the quaternizing group(Table 9). The results showed that F-QUAT HMDA-benzyl had thehighest loading capacity of 43 mg of iridium per gram of material,while F-QUAT HMDA-methyl (15 mg/g) had a higher loadingcapacity than F-QUAT HMDA-ethyl (10 mg/g). This was attributedto the electron-donating nature of the ethyl group and the increaseof hydrophobicity when replacing a methyl group with the ethylgroup. The benzylated quaternary diammonium group was foundto be a good quaternary ammonium center to yield the highestloading capacity. The effect of the benzyl group can be interpretedon the basis of inductive effects. Introducing electron withdrawingsubstituents such as benzyl group serves two functions: it greatlylowers the charge density of the quaternary diammonium center,and the charge delocalizing ability of the benzyl group results ina stronger ionic strength. The separation demonstrated in Fig. 11has also been shown in a form of breakthrough curves (Fig. 12)for the three functional nanofibers as column bed materials.

Breakthrough volumes represent the evolution of the concen-tration of a metal ion eluted as a function of parameters such ascontact time between metal ion and solid phase (quaternized



0 10 20 30 40 500

100020003000400050006000700080009000

10000110001200013000

Con

c (p

pm)

Fraction numbers

F-QUAT HMDA-benzyl Rh F-QUAT HMDA-benzyl Ir

A B C

Fig. 11. A separation profile of rhodium and iridium at 6 M HCl for F-QUAT HMDA-benzyl, 6 M HCl in 0.1 g of nanofibers, 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 of0.05 M sodium metabisulphite, and (C) elution with 15 mL of 20% HCl.

Table 9Loading capacities calculated from the column study results for HMDA quaternizedwith methyl, ethyl and benzyl group for functionalized nanofibers at a flow rate of0.5 ml/h.

Nanofibers Loading capacities (mg/g)

F-QUAT HMDA-methyl 15.2(0.8)F-QUAT HMDA-ethyl 9.7(0.7)F-QUAT HMDA-benzyl 42.9(0.7)

Volume (mL)0 2 4 6 8 10 12

Ce/C

o

0.0

0.2

0.4

0.6

0.8

1.0

Ir

Rh

Fig. 12. Breakthrough curves for (a) N F-QUAT HMDA-benzyl, (b)j F-QUAT HMDA-methyl and (c) d F-QUAT HMDA-ethyl nanofibers.

A

600 500 400 300 200 100Binding energy (eV)

F-QUAT HMDA-A-W F-QUAT HMDA-A-SIr 4p1

Ir 4p3

Rh 3p3N 1s

Ir 4d5

Rh 3d5Cl 2p

Cl 2s

Fig. 13. XPS spectra of F-QUAT HMDA-A-W (quaternized nanofibers after washing)and F-QUAT HMDA-A-S (quaternized nanofibers after stripping).


nanofibers), solvent concentration and temperature (Fayemi et al.,2013). Breakthrough curves generally permit a good description ofthe continuous flow process in ion-exchange columns. The break-through curves of Ce/Co against volume the metal ion solutionsare presented in Fig. 12. Co was the initial concentration of themetal ion and Ce was the eluted concentration of the metal ion.From the breakthrough curve, [RhCl5(H2O)]2� was eluted almostimmediately in the presence of HMDA-methyl, HMDA-ethyl andHMDA-benzyl functionalized nanofibers. On the other hand,[IrCl6]2� was held on the HMDA-methyl, HMDA-ethyl and


HMDA-benzyl functionalized nanofibers, hence confirming inter-actions between the various solid sorbents and [IrCl6]2�. [IrCl6]2�

began to elute at around 6 mL for all three materials, and thisrelates to about 12 h from the time of operation (at 0.5 ml/h).

3.10. XPS study of the adsorption of the binary mixture on nanofibermaterial

The process of adsorption of [IrCl6]2� and [RhCl5(H2O)]2� on thesurface of the materials was studied by using XPS on the F-QUATHMDA-methyl material, a representative nanofiber material(Fig. 13). It was observed from ICP-MS analysis of the fractionsfrom the column that [IrCl6]2� tends to be retained more than[RhCl5(H2O)]2� which comes off the column upon washing(Fig. 11). The Ir(III) speciation in a chloride medium reveals that[RhCl5(H2O)]2� is a predominant species in 6 M HCl solution while[RhCl6]3� is the minor species. Furthermore, the stripping solution(sodium metabisulphite solution) was added onto to the columnsto reduce [IrCl6]2� to form [IrCl6]3� which could be eluted. Rh3p3 was observed in F-QUAT HMDA-methyl after washing(F-QUAT HMDA-A-W) around 496.3 eV region. Rh 3d5 was alsoobserved in F-QUAT HMDA-A-W at a region of 297 eV and theywere not observed after stripping the metal complexes from thenanofiber material. This confirmed that there was no trace of anyrhodium(III) species before stripping (after washing) as seen inFig. 13 (F-QUAT DMDA-A-S). The Rh peaks disappeared and onlythe Ir 4d5 (298 eV) response was dominant. Other peaks that wereobserved were Ir 4p1 (578 eV), Ir 4p3 (495 eV), N 1s (401 eV), Cl 2p(210 eV) and Cl 2s (200 eV).

3.11. Nanofiber regeneration studies

Reusability studies were carried out on the F-QUAT HMDA-methyl, F-QUAT HMDA-ethyl and F-QUAT HMDA-benzyl nanofibermaterials since it presented the highest loading capacity for[IrCl6]2�. The F-QUAT HMDA-methyl nanofibers presented adsorp-tion capacities for iridium of 15.2(±0.8) mg/g, 14.9(±0.7) mg/g, and14.8(±0.7) mg/g for the 1st, 2nd and 3rd cycles, respectively(Fig. 14). The F-QUAT HMDA-ethyl showed the following iridiumloading capacity; 9.7(±0.5) mg/g, 8.9(±0.4) mg/g, and 8.5(±0.3)mg/g for the 1st, 2nd and 3rd cycles, respectively and theF-QUAT HMDA-benzyl for iridium loading capacity; 42.9(±0.5)



No. of cycles

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Load

ing

capa

city

(mg/

g)

0

10

20

30

40

50

60

F-QUAT HMDA-methylF-QUAT HMDA-ethylF-QUAT HMDA-benzyl

Fig. 14. Adsorption capacities of F-QUAT HMDA-methyl, F-QUAT HMDA-ethyl andF-QUAT HMDA-benzyl nanofibers, for [IrCl6]2� uptake, up to three cycles ofoperation.


mg/g, 41.9(±0.4) mg/g, and 41.6(±0.4) mg/g for the 1st, 2nd and 3rdcycles, respectively (Fig. 14). These nanofibers materials were atleast fully recyclable up to the three stages of regeneration thatwere investigated.

4. Conclusions

The polymer nanofibers were successfully functionalized withquaternary diammonium groups. The nanofiber materials werecharacterized by spectroscopy, microscopy and elemental analysis.Both batch adsorption and column (dynamic) studies were carriedout. The Langmuir model showed the best fit for the equilibriumadsorption processes for the uptake of both [RhCl5(H2O)]2� and[IrCl6]2� as F-QUAT HMDA-ethyl, F-QUAT HMDA-methyl andF-QUAT HMDA-benzyl. Kinetic studies suggested a pseudosecond-order kinetic model. The results showed that the adsorp-tion behavior of all sorbent material for [IrCl6]2� was not affectedby physical adsorption, but was mainly dependent on the chemicalinteraction.

The loading capacities of the materials for [IrCl6]2� were evalu-ated and the effect of the quaternizing group was investigated. Itwas observed that rhodium(III) in the form of [RhCl5(H2O)]2� wasunabsorbed by the functional materials nanofibers in the column(dynamic) study operated under a high chloride medium. There-fore, the column studies proved to be satisfactory since rhodiumwas not loaded while iridium was highly loaded. The iridium load-ing capacities increased in the order: F F-QUAT HMDA-ethyl, F-QUAT HMDA-methyl and F-QUAT HMDA-benzyl (15.2 mg/g,9.7 mg/g and 42.9 mg/g). These findings were explained on thebasis of inductive effects and size of the quaternizing group, andthey agreed well with the theoretical studies suggesting that theatomic charges upon anion–cation interaction (in adduct forma-tion), and the HOMO–LUMO energies are diagnostic to drivetoward [IrCl6]2�-specificity. These results provide importantinsight into the effect of the quaternizing group, and suggesthow the loading capacity for [IrCl6]2� can be improved on cationicdiammonium centers.

Author contributions

The manuscript was written through contributions of allauthors. All authors have given approval to the final version.


Acknowledgements

We would like to thank the HRTEM Centre at NMMU and theElectron Microscopy Unit (Rhodes University) for the SEMfacilities. We are grateful to the NRF (SA) for funding (CPR20100406000010238). We also thank the NMMU Research ThemesGrant for funding. We would also like to thank CHPC in Cape Townfor access to the modelling facilities.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.mineng.2015.10.009.

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