The development of novel nickel selective amine extractants: 2,2′-Pyridylimidazole functionalised chelating resin

Minerals Engineering 54 (2013) 88–93

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The development of novel nickel selective amine extractants: 2,20 -Pyridylimidazole functionalised chelating resin Adeleye I. Okewole a,b, Edith Antunes a, Tebello Nyokong a, Zenixole R. Tshentu a,⇑ a b

Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa Department of Chemical Science, Yaba College of Technology, Yaba-Lagos, Nigeria

a r t i c l e

i n f o

Article history: Available online 16 May 2013 Keywords: Base metals 2,20 -Pyridylimidazole Microspheres Nickel-selective

a b s t r a c t A chelating ion exchanger, prepared by functionalising Merrifield resin with 2,20 -pyridylimidazole, was utilized to selectively adsorb and separate nickel from other base metal ions in synthetic sulfate solutions. The sorbent material was characterized by scanning electron microscopy (SEM), microanalysis, infrared (IR), X-ray photoelectron spectroscopy (XPS) and BET surface area. The distribution ratio (D) and the sorption capacity of the microspheres toward Ni(II), Cu(II), Co(II) and Fe(II) ions was studied by using the batch and column methods, respectively. Ni(II) followed by Cu(II) showed the highest distribution ratio (D) and the highest sorption efficiency of nickel(II) ions around pH 2. The binary separation of nickel(II) from copper(II), cobalt(II) and iron(II) respectively, undertaken in a column study, through loading the metal ions at pH  2 followed by selective decomplexation, demonstrated the selectivity of the sorbent material for nickel(II). Thus, 2,20 -pyridylimidazole can be regarded as a nickel-specific extractant. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Separation of nickel from cobalt and other base metals such as copper and iron is known to have numerous challenges (Reddy et al., 2009). The application of solvent extraction (SX) no doubt has yielded, to some extent, the successful separation of nickel and cobalt with the application of organophosphorus reagents (Flett, 2005). Similarly, the hydroxyoximes have proved to be versatile extractants in the separation of copper with SX technique (Flett et al., 1973). However, these extractions are carried out at high pH values, and this necessitates for the precipitation of Fe(III) prior to these extractions since these extractants are oxygen donors which tend to co-extract ferric ions and other impurities (Reddy et al., 2009). For these reasons, there has been blossoming activity in the area of base metal separations in search for new adsorbents and liquid extractants (Gross and Cecille, 1991). It is, however known that solvent extraction has limited applicability when metals in pregnant solutions are present in low concentrations which is currently the case with nickel processing (Green and Hancock, 1982). Ion exchange is being revived as an alternative technique for the processing of such low grade ore feed solutions (Alexandratos, 2009). Ion exchange offers numerous advantages among which is their insolubility which renders them environmentally friendly, and the cycle of loading/regeneration/ ⇑ Corresponding author. Tel.: +27 46603 8846; fax: +27 46622 5109. E-mail address: [email protected] (Z.R. Tshentu). 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.04.019

reloading makes them reusable for many years owing to high mechanical strengths and toughness of the exchange particles (Agrawal et al., 2004). Chelating ion exchangers have the ability to selectively chelate metal ions of interest through suitable functional group(s) (Gregor et al., 1952). Various polymeric support materials have been used in the design of chelating solid phase systems such as silica gel, cellulose, functionalised polymers such as chloromethylated polystyrene cross-linked with divinylbenzene resins (Merrifield resins) (Merrifield, 1963). Microporous resins are versatile in chelating divalent metal ions due to their numerous pores as shown by Lindsay et al. (1987). In the application of chelating resins, the amine containing resins have shown excellent adsorption property for metal ions and are widely applied in the separation and removal of metal ions. Chanda and Rempel (1993) showed that polyethylene amine on poly(vinylbenzaldehyde) has high selectivity for Fe3+ over Cu2+, Ni2+, Co2+, Fe2+, Zn2+ and Mn2+. Similarly, chelating resin containing benzoylacetanilide groups was successfully applied in the separation of Ti3+ and Fe3+ from Cu2+, Co2+, Ni2+, Na+, K+, Ca2+, Mg2+ and Al3+ in the pH range 1.3–2.0 by Majee et al. (1988). Not much has been achieved, to the best of our knowledge, on the selective adsorption and separation of nickel in the presence of other base metal ions in a sulfate medium even with earlier works with the Dow resins and other chelating systems (Rosato et al., 1984; Grinstead, 1984). In our earlier work with 2,20 -pyridylimidazole as an extractant (1-octyl-2,20 -pyridylimidazole (OPIM)) and dinonylnaphthalene

A.I. Okewole et al. / Minerals Engineering 54 (2013) 88–93

sulfonic acid (DNNSA) as a bulky counterion in a solvent extraction system (Fig. 1), we have demonstrated that this extractant can separate nickel from other base metals in an acidic sulfate medium (Okewole et al., 2012). The low pKs of this ligand allowed for interactions with borderline metal ions in the low pH range and the aromatic nitrogenous groups did not interact with the hard ions such as ferric ions in this pH range. We have extended this study to a solid phase system via the functionalization of the Merrifield beads with 2,20 -pyridylimidazole. The beads were characterized by scanning electron microscopy (SEM), microanalysis, infrared (IR), X-ray photoelectron spectroscopy (XPS) and BET surface area. The sorption capacity of the resin was assessed in the batch study and the selective sorption and separation of nickel from the binary metals sulfate solutions containing cobalt, copper and iron was carried out in a column study.

89

2. Experimental

Microcube ELIII. The polymer microspheres were imaged using a TESCAN Vega TS 5136LM scanning electron microscope (SEM). The infrared spectra were recorded on either a Perkin Elmer 100 ATRFTIR or Perkin Elmer 400 FT-IR spectrometer. Surface area analysis was performed by carbon dioxide adsorption at 77 k using a Micromeritics ASAP 2020. Before analysis the beads were degassed for 7 days at 150 °C. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos Axis Ultra X-ray Photoelectron Spectrometer equipped with a monochromatic Al Ka source (1486.6 eV). The base pressure of the system was below 3  107 Pa. XPS experiments were recorded with 75 W power source using hybrid-slot spectral acquisition mode and an angular acceptance angle of ±20°. The Labcon micro-processor controlled orbital platform shaker model SPO-MP 15 was used for contacting the contents of the vials during batch studies. A custom-made glass column with a 10 cm length, 3.5 mm internal diameter and a tip diameter of 1 mm was used for the column studies.

2.1. Materials

2.3. Preparation of 2,20 -pyridylimidazole functionalised microspheres

Pyridine-2-aldehyde (99%, Sigma–Aldrich), glyoxal (40% in water, Sigma–Aldrich), ammonia (28%, Merck Chemicals), ethanol (99%, Sigma–Aldrich), diethylether (99%, Merck Chemicals) and ethylacetate (98%, Sigma–Aldrich) were reagent grade chemicals used as received for the synthesis of 2,20 -pyridyl-1H-imidazole (PIMH). NiSO46H2O (98%), CuSO45H2O (99%) and Fe(SO4)7H2O (70%) were obtained from Merck Chemicals. CoSO47H2O (97.5%) was obtained from Fluka. Merrifield chloromethylated polystyrene–divinylbenzene resin (capacity [Cl]: 1.2 mmol g1 resin, 40–60 mesh) was obtained from Sigma–Aldrich and used as received. The radical initiator azobisisobutyronitrile (AIBN, Riedel de Haën) was recrystallized twice from methanol and dried under vacuum before use. All other chemicals and solvents were purchased from commercial sources and used as received.

5.0 g of the chloromethylated resin was swollen in 15.0 mL of dry dimethylformamide (DMF) overnight at room temperature, and then 10.0 g (0.070 mol) of 2,20 -pyridylimidazole (Gerber et al., 2006) was added (Scheme 1). The mixture was heated to reflux at 70 °C for 24 h. Thereafter the resin was washed with excess methanol, filtered and further cleansed with a Soxhlet extraction system using diethyl ether as a solvent. The beads were then collected and dried in an oven at 60 °C overnight. Anal. Found (%): C, 80.31; H, 7.46; N, 6.52. Calcd. (%): C, 79.55; H, 8.80; N, 7.05.

2.2. Instrumentation The metal ion analyses were carried out using a Thermo Electron (iCAP 6000 Series) inductively coupled plasma (ICP) spectrometer equipped with an OES detector. Wavelengths with minimum interferences were chosen; 231.6 (Ni2+), 237.80 (Co2+), 324.75 (Cu2+) and 259.99 (Fe2+), and three repeats were performed at each run. Microanalysis was carried out using a Vario Elementar

100 Ni 2+ 2+ Co Cu 2+ Fe 2+ Zn 2+ Mn 2+ Mg 2+ Cd 2+ Fe 3+ Ca 2+

80

%E

60

40

2.4.1. Batch adsorption studies 5 mL of 0.02 M solution of the metal sulfate solutions of different pH values were transferred with a micropipette into each of the five 100 ml flasks and diluted to 25 mL with deionised water. 0.5 g of the dried resin was weighed into each of the flasks which were then stoppered. The contents of the flask were shaken with the aid of the orbital platform shaker at a speed of 100 rpm for a period of 5 h and the pH for adsorption was evaluated from these results. In a kinetic experiment which was carried out at the optimal pH, 0.5 ml from the solutions of the respective metals was taken out at 30 min intervals to determine the optimum time required for sorption of the metal ions. Each of the metal ion solutions was diluted appropriately for analysis on ICP-OES. Thus the distribution ratio (D) of the metal ions between the resin and the solution was calculated according to: nþ

D¼

20

0 0

2.4. Sorption of metal ions with the functionalised resins

1

2

3

4

pHi Fig. 1. A plot of % E vs initial pH of equimolar concentration (0.001 M) of Ni2+, Co2+, Cu2+, Fe2+, Zn2+, Fe3+, Mn2+, Mg2+, Cd2+, and Ca2+ extracted with OPIM (M:L ratio of 1:25) and 0.015 M DNNSA in 2-octanol/Shellsol 2325 (8:2) from dilute sulfate medium. Copyright requested from Elsevier (Okewole et al., 2012).

Con of M in 1 g of Dry resin nþ Con of M in 1 ml of the solution

ð1Þ

2.4.2. Column studies 2.4.2.1. Effect of pH on the loading of nickel on the resin. The capacity of the resin for nickel was determined as a function of pH in a column test. 0.02 M nickel sulfate solution was loaded into the column after a prior conditioning of the column loaded with 0.4 g of resin with water followed by pH 1.0, 1.5, 2.0 and 2.5 respective solutions of sulfuric acid. The column was left for 24 h after which it was washed with 5 mL of distilled deionised water to remove metals that did not adhered to the surface of the resin. The extracted nickel ion was eluted from the loaded resin with varying concentration of sulfuric acid solution in the pH range of 0.5–1.5 through the column. The eluent (1.5 mL fractions) was diluted appropriately and analyzed with ICP-OES. The resin loading

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H N DMF 70 oC

+ N

N N

Cl

N

N

Scheme 1. Synthesis scheme for 2,20 -pyridylimidazole functionalized resin.

capacity was calculated as millimoles of nickel per gram of the resin as given by:

qe ¼

ðC i  C f ÞV M

ð2Þ

where qe is the loading capacity (mmol g1), Ci and Cf are the initial and final metal ion concentrations (mol/L) respectively, V is the solution volume in mL and M is the mass of adsorbent (g) used. 2.4.2.2. Effect of flow rate on the adsorption of nickel. In an initial study conducted with 0.02 M nickel sulfate solution at a pH  2.0, various flow rates (0.5–2.5 mL/h) were used in contacting the solution of nickel ion with the resin using a programmable syringe pump. The optimum flow-rate was found to be 1.5 mL/h and was thereafter applied for the study of all the other metal ions. 2.4.2.3. Binary separation of nickel from cobalt, copper and iron. The binary separation of nickel(II) from cobalt(II), nickel(II) from copper(II), and nickel(II) from iron(II) was undertaken by loading a pre-conditioned column with a mixed metal sulfate solution containing 0.02 M of the respective metal ions at a pH of ca. 2. The column was eluted with sulfuric acid with the exception of copper which was eluted with 1.0 M ammonium hydroxide solution before the elution of Ni(II) with sulfuric acid. The separation factor of nickel from each of the metal ion was calculated as follows:

bNi=M ¼

½Nir½Ms ½Nis½Mr

ð3Þ

where [Ni]r and [Ni]s or [M]r and [M]s refers to the concentration of nickel or the metal ions on the resin and in the solution at equilibrium respectively. 3. Results and discussion 3.1. Characterization of the functionalised microspheres The SEM micrographs of the corresponding 2,20 -pyridylimidazole functionalised beads showed little or no morphological differences from the unfunctionalised beads as depicted by Fig. 2. The average diameter of the unfunctionalised and functionalised beads as determined by SEM was 223 and 225 lm respectively (n = 25). There was a slight increase in diameter upon functionalisation which could be indicative of significant surface functionalization or swelling of the beads. In the infrared spectrum of the precursor (unfunctionalized beads) there are two peaks appearing at 1264 cm1 and 670 cm1 (Fig. 3a) corresponding to the CH2Cl wagging and CACl stretching frequencies respectively. These disappeared completely upon functionalization with pyridylimidazole (Huang et al., 2008), and new bands at 1671 cm1 and 1018 cm1 (Fig. 3c) appeared

corresponding to the m(C@N) and m(CAN), respectively, confirming the effective anchoring of the ligand onto the beads. An investigation of the surface chemistry of this adsorbent by XPS also confirmed the presence of nitrogen, along with the other expected elements (Fig. 4). In the unfunctionalised beads a Cl 1s peak appeared around 200 eV and this peak was diminished significantly upon functionalization. The N1s peak appeared at 398 eV confirming the presence of pyridylimidazole on the surface of the beads (Uguzdogan et al., 2009). The O 1s peak probably represents some water molecules on the surface of the beads. The specific surface area of the functionalised beads was determined using the Brunauer–Emmet–Teller (BET) method and was found to be 46.82 m2 g1, on a single surface area while the average multiple area was determined as 123.34 m2 g1. This surface area was, however much lower that of the unfunctionalised beads (324.34 m2 g1), indicating that the presence of the ligand reduced the available surface area. 3.2. Sorption of metal ions with functionalised microspheres 3.2.1. Effect of pH A preliminary assessment of the distribution ratio of the metal ions on the pyridylimidazole-functionalized resin was investigated as a function of pH in a batch study prior to determining the loading capacities at the optimum pH values. The results depicted a higher value of D around pH 1.5–2.5, for nickel(II) and copper(II) (Fig. 5 and Table 1), hence the column studies were conducted around these pH values. Furthermore, it can be predicted that nickel would have a higher loading capacity around pH 2.0 than cobalt(II) indicating a possible separation of the two metals as was observed in the solvent extraction system (Fig. 1) (Okewole et al., 2012). 3.2.2. Effect of contact time The effect of the contact time on the percentage sorption of Ni2+, 2+ Co , Cu2+ and Fe2+ at pH 2.0 showed that cobalt has the fastest kinetics of uptake of all the metals with the percent sorption reaching as high as 85% within 60 min, followed by copper (Fig. 6). It can also be observed that all the metal ions adsorbed significantly at pH 2.0 within 3 h while iron(II) showed the slowest kinetics and was the least adsorbed. The contact time of 3 h was therefore considered sufficient for adsorption of these base metals. 3.3. Binary separation of nickel from other base metals Separation of nickel(II) from cobalt(II) was undertaken since the removal of nickel(II) from base metals was the primary focus in this study. This became necessary in view of the limited achievements made thus far in solvent extraction of this duo because of their similar aqueous chemistry. Secondly, the separation of nickel(II) from copper(II) and from iron(II) was also examined. A binary

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Fig. 2. Scanning electron micrographs of (a) unfunctionalized beads and (b) functionalized beads.

Table 1 Sorption capacity of the sorbent for the metal ions (mmol g1) as a function of pH.

a 670 1264

b

pH

1.0

1.5

2.0

2.5

3.0

Nickel(II) Cobalt(II) Copper(II) Iron(II)

0.25 0.09 0.23 0.02

0.28 0.11 0.25 0.25

0.95 0.55 1.20 0.05

0.65 0.40 0.77 0.08

0.23 0.12 0.22 ND

%T

ND – Not detectable.

c 200 1671

Ni2+ Co2+ Cu2+ Fe3+

1018 150

1800

1600

1400

1200

1000

800

D

2000

Wavenumber (cm-1) Fig. 3. The infrared spectra of (a) unfunctionalised beads, (b) 2,20 -pyridylimidazole, and (c) functionalized beads.

100

50

0 1.0

C 1s

700

600

N 1s

500

Intensity

2.5

3.0

Fig. 5. Metal ions distribution ratio on the resin conducted in a batch study as a function of pH.

Merrified beads

800

2.0

pH

O 1s

PIM- fuctionalized beads

1.5

400

Cl 1s

300

200

Binding energy (eV) Fig. 4. Wide scan XPS spectra of the unfunctionalised (Merrifield beads), and pyridylimidazole-functionalised (PIM-functionalised) beads.

synthetic solution containing equimolar concentrations (0.02 M) of nickel and cobalt as sulfates was loaded sequentially into the preconditioned resin in the column. This was allowed for equilibration for a period of 3 h. After washing the un-adsorbed metal ions off the resin with 15 mL of distilled deionised water, the resin was eluted by gradient elution with 0–0.02 M sulfuric acid until complete elution was achieved (5 mL of each of deionised water, 0.01 M, 0.0125 M, 0.015 M, 0.0175 M and 0.02 M, respectively). The separation procedure involving copper(II) included a 1.0 M ammonia solution elution step (10 mL), washing with water (10 mL), and then stripping and elution of nickel(II) with 10 mL of 0.02 M sulfuric acid. The column bed volume was 0.5 mL, and

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80

20

Concentration (mM)

25

% Sorption

100

60 Cu2+ Co2+ Ni 2+ Fe 2+

40

15

10

Cu2+ Ni2+

5

20

0

0 0

50

100

150

200

250

300

0

350

10

5

Fig. 6. The effect of contact time on the percentage sorption of nickel(II), cobalt(II), copper(II) and iron(II) by pyridylimidazole-functionalized beads at pH 2.0.

20

25

Fig. 8. Elution profile of mixed sulfate solution of 0.02 M nickel(II) and copper(II) in a binary column study. Copper (II) was eluted with a 1.0 M ammonia solution (10 mL), followed by washing with 10 mL of water, and then stripping and elution of nickel(II) was carried out with 10 mL of 0.02 M sulfuric acid.

Table 2 The separation factors calculated from the binary separation of the metals at pH 2.0 under the column study. Ni2+/Co2+

Ni2+/Cu2+

Ni2+/Fe2+

Separation factor (b)

22

5

15

25

20

20

Concentration (mM)

Metal pair

Concentration (mM)

15

Fraction number

Time (min)

15

Fe2+ Ni2+ 10

5

15 0 10

2+

Co Ni 2+

0

5

10

15

20

Fraction number Fig. 9. Elution profile of mixed sulfate solution of 0.02 M nickel(II) and iron(II) in a binary column study. The column was eluted by gradient elution with 0–0.02 M sulfuric acid until complete elution was achieved (5 mL of each of deionised water, 0.01 M, 0.0125 M, 0.015 M, 0.0175 M and 0.02 M, respectively).

5

0

0

5

10

15

20

Fraction number Fig. 7. Elution profile of mixed sulfate solution of 0.02 M nickel(II) and cobalt(II) in a binary column study. The column was eluted by gradient elution with 0–0.02 M sulfuric acid until complete elution was achieved (5 mL of each of deionised water, 0.01 M, 0.0125 M, 0.015 M, 0.0175 M and 0.02 M, respectively).

it therefore takes several bed volumes to recover nickel from the resin bed. The resin was removed from the column after elution, washed and dried for the next determination, and the regeneration efficiencies were over 98%. Table 2 gives the calculated separation factors according to Eq. (3). The data were obtained after the optimum contact time of about 3 h. The separation factor is fairly good with respect to Ni2+/Co2+, however a better selectivity is required for Ni(II)/Cu(II) and the Ni(II)/Fe(II) binary system. The corresponding plots of concentration of the metal ions as a function of the fraction number on Figs. 7–9 gives the respective elution profiles of Ni2+/Co2+, Ni2+/Cu2+ and Ni2+/Fe2+. The separation of nickel(II) from cobalt(II) proved to be successful on this pyridylimidazole-functionalized sorbent. We have also noted a similar observation to exist with this ligand in a solvent extraction system (Okewole et al., 2012). The first nine fractions of the eluent collected contain cobalt(II) ions predominantly while fractions

11–19 are dominantly nickel(II)-containing. The break observed in the elution profile in fractions 9–11 give credence to the effective separation of nickel(II) and cobalt(II) by this system (Fig. 7) and the high level of purity of the separated metals. The results obtained are further backed-up with highest separation factor of 22 obtained for Ni2+/Co2+ (Table 2). Separation of nickel from copper however shows a lower separating factor of 5 in Ni2+/Cu2+ and the elution profile in Fig. 8 buttresses this result, and a similar observation was noted for Ni2+/ Fe2+ (Fig. 9). Selective decomplexation was employed to achieve separation of nickel(II) from other base metals since the nickel(II)-pyridylimidazole complex is more stable (at lower pH) compared with other complexes, and therefore each of copper(II), cobalt(II) and iron(II) can be fully separated from nickel by simply employing the less concentrated sulfuric acid leach solutions. These column separation results suggested that nickel can be separated from a synthetic leach solution containing these four metals ions since it is retained in the column until it is stripped and eluted with a more concentrated sulfuric acid solution. The higher affinity of nickel(II) for 2,20 -pyridylimidazole and the higher pH-metric stability of the nickel(II)-pyridylimidazole complex, therefore, renders this ligand to be a nickel-selective extractant. The loading capacities

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were 56.1 mg Ni/g resin for Ni/Co, 60.1 mg Ni/g resin for Ni/Cu, and 79.5 mg Ni/g resin for Ni/Fe separations. 4. Conclusions The 2,20 -pyridylimidazole-functionalized microspheres were successfully fabricated and characterized using FTIR, SEM, XPS, BET surface area and microanalysis to show evidence of the functionalization as well as the surface properties. This material was found to be a versatile adsorbent for the sorption and separation of nickel(II) from base metals. Even though the contact time for adsorption of Co(II) and Cu(II) under batch conditions is better than that of Ni(II), the loading of the metal ions at pH 2 followed by selective decompexation under column dynamic conditions effects a separation in favor of Ni(II). The high sorption capacity for nickel at pH 2 underscores the fact that even in highly acidic sulfate solution, this adsorbent can perform at reasonably high efficiency with loading capacities in the range 56–79 mg Ni/g resin. The solidsolution system investigated here for 2,20 -pyridylimidazole also addressed the problem of poor phase transferability of the sulfate complexes experienced in the solvent extraction system, thereby eliminating the need for a bulky anion such as dinonylnaphthalene sulfonic acid (DNNSA) which was employed (Okewole et al., 2012). Acknowledgements We would like to thank the Rhodes University Microscopy Unit for the SEM facilities. We are also grateful to the South African NRF (CPR20100406000010238) for funding. References Agrawal, A., Sahu, K.K., Pandey, B.D., 2004. Separation and removal of cobalt and zinc from chloride solution by Indion BSR-A chelating resin. Separation Science and Technology 39 (10), 2373–2388.

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