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Chemical Engineering Journal 174 (2011) 58– 67

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

olid phase pre-concentration of cobalt(II) using pyridine–thiocyanate reagents:-ray crystal structure of the extracted ternary cobalt(II) complex

aja Sahaa, Animesh Sahanaa, Sudipta Dasa, César J. Pastorb, Elena Torres Lopezb, Debasis Dasa,∗

Department of Chemistry, The University of Burdwan, Burdwan 713104, IndiaDepartamento de Quimica Inorganica, Universidad Autónoma de Madrid, 28049 Madrid, Spain

r t i c l e i n f o

rticle history:eceived 12 May 2011eceived in revised form 12 August 2011ccepted 16 August 2011

eywords:o(II)yridine-2,6-dimethanolhiocyanaterystal structure

a b s t r a c t

Mixture of pyridine-2,6-dimethanol (PDM) and thiocyanate (SCN−) (1:1, mole ratio) immobilized onsilica served as a very efficient sorbent for selective retention of Co(II) from other associated metal ions attrace level. The maximum sorption capacity for Co(II) was found to be 0.203 mmol g−1 at pH 9.0. SorbedCo(II) was completely eluted by 3.5 mL of 3 mol L−1 HCl and measured using flame atomic absorptionspectrometer (FAAS). The structure of the extracted Co(II) complex was confirmed by single crystal X-ray structure and Fourier transform infrared (FTIR) spectroscopy. Thermo gravimetric analysis (TGA) ofthe chelated Co(II) complex revealed its stability at the optimum extraction temperature (55 ◦C). Themethod was reproducible with a relative standard deviation (RSD) of 0.6% (N = 10) with three sigmadetection limit (N = 10) of 0.6 �g g−1. A pre-concentration factor, 94 was achieved. Interferences from

2+ 2+ 2+ − 2+

helationlame atomic absorption spectrometry

Mn and Cu ions were eliminated by prior oxidation of Mn by KIO4 to MnO4 and masking of Cuwith NH4SCN, respectively. A plausible mechanism for the selective extraction of Co(II) was attributed tothe formation of a first order water insoluble inner–metallic complex as confirmed by the single crystalX-ray structure analysis. The developed method has been tested for trace level separation and estimationof cobalt in some certified reference materials. Analyses of some biological and environmental sampleswere performed.

. Introduction

Like all essential elements, trace level Co(II) is necessary (asitamin-B12) for human beings [1–3]. The major sources for theroduction of cobalt are certain oxide and sulfide ores or wastes.ydrometallurgical dissolution of such materials using hydrochlo-

ic acid results in solutions containing cobalt along with somempurities; also, cobalt readily forms alloys with other metals, suchs chromium, nickel, copper, and tungsten, which have specialpplications as materials for cutting tools. Cobalt (along with otheretals) is recovered from alloys by treatment with acids. There-

ore, a simple and selective separation of cobalt from other metalons has paramount importance. Due to chemical similarities, it haslways been difficult to separate Co(II) from Ni(II), Cd(II) and otherd metal ions. Determination of cobalt and nickel in a mixturesing spectrophotometric [4] or graphite furnace atomic absorp-ion spectrometric [5] methods has always been problematic due

o their mutual interference. Among various methods of separa-ion of Co(II) [6–8], prior to analytical measurement, liquid–liquidxtraction (LLE) [9–12] is widely studied. But it suffers from several

∗ Corresponding author. Tel.: +91 342 2533913; fax: +91 342 2530452.E-mail address: ddas100in@yahoo.com (D. Das).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.08.045

© 2011 Elsevier B.V. All rights reserved.

limitations, viz. it is laborious and time-consuming, requires largevolume of organic solvent, and tends to form emulsion.

Solid phase extraction (SPE) is much greener than other tech-niques due to minimal waste generation, eco-friendliness, rapidity,simplicity, high pre-concentration factor, stability as well asreusability of the sorbent, reduced/no use of organic solvents andreduced cost. Several authors have used a number of solid sor-bents viz. activated carbon [13,14], activated bentonite [15], DuoliteXAD-761 [16], chromosorb 102 [17], Amberlyst 36 [18], AmberliteXAD series [19,20] for the enrichment of cobalt(II) from dilute solu-tions prior to determination by a variety of analytical techniques.Among several types of solid support/sorbents employed in SPE,silica immobilized with organic chelating compounds has receivedgreat interest for its non-swelling properties, large specific surfacearea, fast kinetics, good mechanical, thermal and chemical stability[21,22]. Report on Co(II) selective solid phase extractors [23] arevery less. Herein we report thiocyanate assisted solid phase reten-tion and pre-concentration of Co(II) using PDM. FTIR supports thebinding of the sorbent to Co(II). To the best of our knowledge, noneof the works as reported till date confirmed the structure of the

extracted cobalt complex by single crystal X-ray structure analysis,which nowadays provide a very strong evidence of metal complexformation. The newly developed methodology is verified by anal-ysis of certified reference materials and applied to the analysis of

R. Saha et al. / Chemical Engineerin

Nomenclature

RSD relative standard deviationSPE solid phase extractionPDM pyridine-2,6-dimethanolA.R. analytical reagentFW formulae weightZ number of molecules present per unit cella crystallographic distance along ‘x’ axis, in a unit cell,

(A)b crystallographic distance along ‘y’ axis, in a unit cell,

(A)c crystallographic distance along ‘z’ axis, in a unit cell,

(A) crystallographic angle in a unit cell between b and

c, (◦) crystallographic angle in a unit cell between c and

a, (◦)� crystallographic angle in a unit cell between a and

b, (◦)V volume, (A3)� density in g cm−3.� (MoK�) molybdenum K� radiation in wave number used

in X-ray diffraction (cm−1)F (0 0 0) It is crystallographic (0 0 0) plane

a

eo

2

2

toaaDm2Faodo9tsmtuapmtb

2

w

R1 wR2 I > 2�(I): represents goodness-of-fit;� = standard deviation

nvironmental and biological samples. Different analytical figuresf merit are also reported.

. Experimental

.1. Instrumentation

A VARIAN (Spectra AA 55) flame atomic absorption spectropho-ometer (FAAS) (Australia) was used for measuring concentrationf Co(II). All measurements were performed using integratedbsorbance. Hollow cathode lamp for Co was operated at 7.0 mAnd at wave length of 240.7 nm with a slit width of 0.2 nm.2-back ground correction was performed in all the measure-ents. Air and acetylene flow rates were maintained at 10 and

L min−1 respectively. FTIR spectra were recorded on a JASCOTIR spectrophotometer (model: FTIR-H20). Thermogravimetricnalysis was done on a Perkin Elmer TGA lab system 1 (Technol-gy by SII). pH measurements were performed with a Systronicsigital pH meter (model 335). A domestic Samsung microwaveven (model CE2933) with a 2450 MHz frequency magnetron and00 W maximum power and a polytetrafluoroethylene (PTFE) reac-or (115 mL internal volume, 1 cm cell wall thickness and hermeticcrew caps) was used for digestion. Extraction temperature wasonitored by using Biochemical Oxygen Demand (B.O.D) incuba-

or (YONA, India). The X-ray crystal data were collected at 93 K bysing a Rigaku MM007 High brilliance RA generator/confocal opticsnd Mercury CCD system. Intensities were corrected for Lorentzolarization and absorption. The structures were solved by directethods. Hydrogen atoms bound to carbon were idealized. Struc-

ural refinements were obtained with full-matrix least-squaresased on F2 using the program SHELXTL [24].

.2. Chemicals and reagents

A stock solution of Co(II) having concentration of 1000 �g mL−1

as prepared by dissolving appropriate quantity of Co(II)

g Journal 174 (2011) 58– 67 59

chloride hexahydrate (British Drug House Ltd., UK) in deionisedwater. A working solution containing 50 �g mL−1 Co(II) was pre-pared by appropriate dilution. Potassium periodate (Merck, India)was used as received. Spectroscopic grade potassium bromide(SRL, India) was used for making pellets for FTIR studies. Pyridine-2,6-dimethanol (PDM) (Aldrich, USA), ammonium thiocyanate andsilica (80–120 mesh) (SRL, India) were used as received. Referencematerials like NIES No. 1, pepperbush; NIES No. 2, pond sedimentand NIES No. 3, chlorella were supplied by the National Institute ofEnvironmental Studies (NIES, Japan). Several real samples viz. vita-min B12 (Medipaams India Pvt. Ltd., India), vitamin B complex (ZenLabs India), tap water (Durgapur, India) and waste water (Durga-pur, India) samples were collected. Deionised water from a Milli-QMillipore® 18.2 M� cm−1 conductivity purification system (Bed-ford, MA, USA) was used to prepare all solutions. Buffer solutionsof pH 2–3, pH 4–6, and 7–10 were prepared by mixing appropri-ate ratios of H3PO4 with KH2PO4, citric acid with potassium citrate,acetic acid with sodium acetate and ammonia with NH4Cl solu-tions, respectively [25]. All other chemicals and reagents used wereof analytical reagent (A.R.) grade.

2.3. Preparation of column

The impregnation of PDM and NH4SCN was done by mixingthem in appropriate quantities (1:1, mole/mole) in methanol withsilica beads. The mixture was then stirred till the solvent evapo-rated. The modified silica beads (sorbent) thus obtained were keptovernight at ambient temperature.

2.4. General procedure

Sorption and desorption studies were carried out both by batchand column method. Air-dried sorbent (1.0 g) was either taken ina standard joint conical flask (100 mL) or packed in a glass col-umn (100 mm × 10 mm) and was allowed to be in contact withthe sorbent. For column method, both sorption and desorptioncharacteristics for Co(II) were studied at the optimum flow rate.The sorbent (bed, in case of column) was thoroughly washed with0.1 mol L−1 HNO3 followed by deionised water till the effluent wasfree from acid. A sample solution containing 50 �g mL−1 Co(II) waspassed through the column. Any adhering metal ions (not sorbed)were completely washed out by using solutions of appropriate pH.Sorbed Co(II) was completely eluted with suitable eluent. ElutedCo(II) was measured by FAAS. Different experimental parameterssuch as sample volume, flow rate, pH, equilibration time, effect offoreign ions, varying nature and concentration of eluents, werestudied to optimize the sorption and desorption conditions forCo(II). Effect of temperature (30–60 ◦C) on the sorption of Co(II) bythe sorbent was monitored by batch method. 0.1 g of the sorbentwas taken in a 100 mL standard joint conical flask and appropri-ate amount of Co(II) solution was added to the sorbent at optimumpH. Then it was placed inside the B.O.D. incubator and shaken atthe required temperature for optimum period of time. Finally, theconical flask containing sorbent immersed with Co(II) solution wastaken out and filtered through G4 Gooch crucible. The amount ofCo(II) in the filtrate as well as in the sorbent (after elution withsuitable eluent) was measured by FAAS. To prove that there was nodecomposition of the Co(II) complex at this elevated temperature,thermal stability of the Co(II) complex was determined coveringthe temperature range. LOD, defined as that analyte concentrationgiving a signal equal to three times standard deviation of blanksignal [26] was estimated. Separation of Co(II) from other accom-

panying elements (binary mixture) was performed. The developedmethodology was verified by analyzing certified reference materialwhich was brought into solution by modifying the literature pro-cedure [27]. 0.5 g of the certified reference materials were treated

6 neering Journal 174 (2011) 58– 67

i3aoStwtmp(taa

2

maCt

2

eudc

2

bI

2

t(ab

2t

cwmwi

tspt

3

3

u

10987654321

0.00

0.05

0.10

0.15

0.20

Sorp

tion

(mm

ol g

-1)

pH

Fig. 1. Sorption (mmol g−1) of Co(II) as a function of pH.

1 2 3 4 5 6

0.13

0.14

0.15

0.16

0.17

0.18

0.19

0.20

Sorp

tion

(mm

ol g

-1)

Fig. 2 clearly showed that with increasing time sorption capacityincreases, and after 5 h, no further change in sorption has beenobserved. Fig. 3 demonstrated that sorption of Co(II) gradually

0.18

0.19

0.20

0.21

Sorp

tion

(mm

ol g

-1)

0 R. Saha et al. / Chemical Engi

n a hermitically sealed PTFE (115 mL) reactor in sequence (450 W,.0 min each) with 2 mL of 15.8 M HNO3 and 0.2 mL of 11.6 M HClO4cid. The volume of the solution was made up to 50 mL. The pHf the above sample was adjusted to 9.0 using NH3–NH4Cl buffer.imilarly, 5 mL of the vitamin C samples were brought into solu-ion by microwave digestion technique as described above. Wasteater (at the industrial outlet to river Damodar of Durgapur indus-

rial area of West Bengal, India) was filtered through a celluloseembrane filter (Millipore) of pore size 0.45 �m. Any organic com-

onent of the water sample was oxidized by the mixture of H2O21%) and concentrated nitric acid. The water samples were filteredhrough a filter paper and stored in polyethylene bottles undercidic conditions (1% nitric acid). Tap water samples were directlynalyzed on basis of our new method without any prior treatment.

.5. Equilibration time

In batch method, each 25 mL of 50 �g mL−1 Co(II) solution wasixed with appropriate amount of sorbent at optimum pH and

llowed to stand for different times viz. 1, 2, 3, 4, 5, 6 and 24 h.oncentration of Co(II) in the filtrate as well as after elution fromhe sorbent was measured by FAAS.

.6. Kinetic studies

Amount of Co(II) sorbed by the sorbent under the optimizedxperimental conditions was monitored at different time intervalssing batch technique as described above (Section 2.5). From thatata, rate constant for the sorption of Co(II) on the sorbent wasalculated.

.7. Effect of foreign ions

Several binary synthetic mixtures were prepared and equili-rated with the sorbent at optimum conditions (batch method).

n mixtures, foreign ions were taken 200 fold excess to Co(II).

.8. Desorption of metal ions

Sorbent obtained after uptake of Co(II) ion in the above men-ioned batch technique was shaken with 25 mL of different eluents1 mol L−1 to 5.0 mol L−1 HNO3, 1 mol L−1 to 3.0 mol L−1 HCl) for 6 hnd filtered. The concentration of Co(II) in the filtrate was measuredy FAAS.

.9. FTIR and single crystal X-ray structural characterization ofhe extracted complex

After removal of moisture at 100 ◦C in hot air oven followed byooling at room temperature in a desiccator, the sample was mixedith the spectroscopic grade KBr. The mixture was grinded withortar and pestle to make very fine powder. A pellet was preparedith that powder using a hydraulic system and placed in the FTIR

nstrument to record the FTIR spectrum.Single crystals of the extracted Co(II) complex were grown from

he methanol solution obtained after elution of Co(II) from theorbed material by slow evaporation technique. After one week,ale pink crystals were formed. Data collected from the diffrac-ometer were refined as described in Section 2.1.

. Results and discussion

.1. Sorption studies

Fig. 1 indicated that with increasing pH, sorption of Co(II) grad-ally increased to a level of pH 9.0. Decreased sorption capacity

Time (h)

Fig. 2. Sorption (mmol g−1) of Co(II) as a function of time.

at higher acidity may be attributed to the protonation of donorsites. Hence pH 9.0 was preferred throughout the entire study.

605550454035

Temper ature ( °C )

Fig. 3. Sorption (mmol g−1) of Co(II) as a function of temperature.

R. Saha et al. / Chemical Engineering Journal 174 (2011) 58– 67 61

of th

idt

tsflrH

aw

ct(s

Fig. 4. TGA graph

ncreased with temperature, to a maximum value at 55 ◦C. Noecomposition of the Co(II) complex was observed at this elevatedemperature (Fig. 4).

The sample flow rate should be optimized to ensure quantita-ive retention along with minimization of the time required forample processing. From Fig. 5, it was found that with increasingow rate from 0.5 mL min−1 to 5.5 mL min−1, the recoveries of Co(II)emained unchanged up to 2.5 mL min−1, after which it decreased.ence, sample flow rate of 2.5 mL min−1 was selected.

It was observed from Fig. 6 that the percent of sorption droppedfter 330 mL of Co(II) solutions were passed through the sorbenthich might be due to the saturation of the sorbent with Co(II).

The rate of exchange of metal ion by a sorbent has beenontrolled by a second order kinetic equation [28]. Using the equa-

ion ln Z = 2kQ0 (Q0 − Q˛)t/Q˛, where Z = [Qt(Q0 − 2Q˛) + Q0Q˛]/Q0Q� − Qt) developed by Turse and Riemen [29]. The rate con-tant k was calculated from the slope S of the equation:

6543210

20

30

40

50

60

70

80

90

100

110

% r

ecov

ery

of C

o (I

I)

Sample flow rate (mL mi n -1)

Fig. 5. Effect of sample flow rate (mL min−1) on the recovery of Co (II).

e Co(II) complex.

S = 2kQ0(Q0 − Q˛)/Q˛, where Qt was the amount (mmol) of Co(II)exchanged at time t, Q˛, was the maximum sorption capacity(mmol) at equilibrium and Q0 was the amount (mmol) of sorbent,in terms of millimoles of Na+ exchanged after 24 h. The values ofQ˛ and Q0 were found to be 0.203 and 2.9 mmol g−1, respectively.A plot of ln Z versus t should be linear through zero. Fig. 7 providedthe values for slope (S) and rate constant (k) as 0.869 and 2.561(mmol−1 min−1), respectively.

In presence of 200 fold excess of diverse metal ions (binary mix-tures) like alkaline, alkaline earth and transition metal ions, morethan 90% recovery for Co(II) was observed. Little interferences fromMn(II) and Cu(II) were eliminated by prior oxidation/complexationby KIO4 and NH4SCN respectively. Among various anions, VO3

−

interferes to some extent. The results were presented in Table 1.

Extraction recovery could be represented by the equation:

E(%) = (C0 − C∞)C0

× 100

5004003002001000

30

40

50

60

70

80

90

100

110

% r

ecov

ery

of C

o (I

I)

Sample volume (mL)

Fig. 6. Sample breakthrough volume for the sorption of Co (II).

62 R. Saha et al. / Chemical Engineering Journal 174 (2011) 58– 67

1 2 3 4 5 6

3

4

5

6

7

8

lnZ

Time (h)

ln Z = 0.869 t + 2.562

Fig. 7. Second order plot for the sorption of Co (II).

Table 1Effect of foreign ions (binary mixture) on sorption of Co (II).

Foreign ionsa % recovery of Co(II)

Ag(I) 99.5Cu(II) 94.3Hg(II) 99.4Zn(II) 101.4Mg(II) 97.9Ni(II) 99.8Mn(II) 98.8Cr(III) 97.2Ca(II) 99.6Fe(III) 99.9Na(I) 99.7Cl− 98.2AsO4

3− 100.4VO3

− 97.4PO4

3− 100.4CrO4

2− 98.9

wt1

3

n

TEs

765432

60

65

70

75

80

85

90

95

100

105

% R

ecov

ery

of C

o (I

I)

cyanate stretching [30]. Similarly, other characteristics frequenciesof the free PDM ligand were also shifted. Selected IR frequencies ofthe free PDM ligand and its Co(II) complex are presented in Table 3.

MnO4− 99.4

a Coexisting ions are 200 fold to [Co(II)].

here E (%) represents the extraction percentage, C0 and C∝ werehe initial and equilibrium concentrations of Co(II). We found00.0 ± 0.3% extraction of Co(II) at the optimized conditions.

.2. Elution studies

Quantitative recovery of Co(II) from the sorbed material wasecessary for repeated use of the sorbent. Table 2 showed the

able 2ffect of different eluents on the recovery of sorbed Co(II) on the PDM impregnatedilica.

Eluents (mol L−1) % recoverya

HNO3 (1) 13 ± 0.2HNO3 (2) 24 ± 0.4HNO3 (3) 38 ± 0.3HNO3 (4) 59 ± 0.5HNO3 (5) 61 ± 0.7HCl (1) 36 ± 0.8HCl (2) 69 ± 0.6HCl (3) 100 ± 0.3HCl (4) 100 ± 0.9HCl (5) 101 ± 0.5

a Average of three replicate measurements ± % RSD.

Volume of HCl (mL)

Fig. 8. Desorption of Co(II) as a function of eluent volume.

efficiency of different eluents for the recovery of Co(II) from Co(II)loaded sorbent. Recovery of Co(II) was 100.0 ± 0.3% with 3 mol L−1

HCl.The effect of eluent volume for quantitative recovery of

Co(II) was investigated separately. It was found that 3.5 mL HCl(3.0 mol L−1) was useful for quantitative recovery (Fig. 8). The effectof elution flow rate (Fig. 9) on the recovery of Co(II) was investi-gated within the range of 0.2–4.0 mL min−1. 100.0 ± 0.3% recoveryof Co(II) was observed at a flow rate range of 0.2–2.0 mL min−1.Consequently, elution flow rate of 2.0 mL min−1 was maintained.

3.3. FTIR and single crystal x-ray structural confirmation of theextracted complex

FTIR spectra of the PDM loaded silica (Fig. 10) was compared tothat of the Co(II) complex loaded on silica (Fig. 11). The O–H bandof the free PDM ligand at 3360.81 cm−1 had been red shifted to2867 cm−1 in the Co(II) complex, indicating the “O” donor site ofPDM was involved in cobalt(II) binding. Appearance of a new sharpband in Fig. 11 at 2121 cm−1 was assigned unambiguously to thio-

4.54.03.53.02.52.01.51.00.50.0

60

70

80

90

100

110

% R

ecov

ery

of C

o (I

I)

Elution flow ra te (mL min -1)

Fig. 9. Effect of flow rate of eluent (mL min−1) on the recovery of sorbed Co (II).

R. Saha et al. / Chemical Engineering Journal 174 (2011) 58– 67 63

Fig. 10. FTIR spectrum of PDM immobilized on silica.

d coba

Mwpcbr

TC

Fig. 11. FTIR spectrum of the chelate

ore powerful evidence of the Co(II) binding ability of PDM/SCN−

as provided by the single crystal X-ray structure of the Co(II) com-

lex (Fig. 12). Crystal data and structure refinement for the Co(II)omplex was presented in Table 4. The selected bond lengths andond angles of the complex have been presented in Figs. 12 and 13,espectively. Detailed bond lengths and bond angles of the complex

able 3omparison of FTIR data of free PDM ligand with that of Co(II) complex.

�O–H (cm−1) �NCS (cm−1)

Free PDM ligand 3360.81 –Co(II) complex 2867 2121

lt(II) complex immobilized on silica.

have been presented in Table 5. Fig. 12 clearly showed that PDM inthe chelated Co(II) complex acted as a bidentate ligand while oneof its arm remained uncoordinated (in O–H form) and Co(II) washexa coordinated (CoN4O2 chromophore). Co(II) being a border linelewis acid, could easily bonded with hard donor “O” and border linepyridine N-sites (from PDM part). As a result of oxygen bonding toCo(II), it became harder so that N-site but not the S-site of thio-cyanate bonded to Co(II). Methanol could also be used to elute Co(II)complex from the column but methanol unloaded the chelating lig-

and PDM from the column. So, reloading of the column with thechelating ligand was necessary. So, choice of methanol as elutingagent was rejected. Chelated Co(II) complex was an inner–metalliccomplex of the first order (both charge and coordination numbers

64 R. Saha et al. / Chemical Engineering Journal 174 (2011) 58– 67

Fr

aodtp

4

ptv(

TC

Table 5Bond lengths [A] and angles [◦] for Co(II)–PDM–SCN− complex.

Co(1)-N(2) 2.0443(16)Co(1)-N(2)#1 2.0443(16)Co(1)-O(2) 2.1037(12)Co(1)-O(2)#1 2.1037(12)Co(1)-N(1) 2.2012(14)Co(1)-N(1)#1 2.2012(14)O(2)-C(7) 1.419(2)O(1)-C(1) 1.429(2)N(1)-C(6) 1.341(2)N(1)-C(2) 1.348(2)N(2)-C(8) 1.152(2)C(2)-C(3) 1.384(2)C(2)-C(1) 1.501(2)C(3)-C(4) 1.378(3)C(5)-C(4) 1.379(3)C(5)-C(6) 1.385(2)C(6)-C(7) 1.504(2)C(8)-S(1) 1.6416(18)

N(2)-Co(1)-N(2)#1 92.01(9)N(2)-Co(1)-O(2) 173.55(5)N(2)#1-Co(1)-O(2) 87.60(6)N(2)-Co(1)-O(2)#1 87.60(6)N(2)#1-Co(1)-O(2)#1 173.55(5)O(2)-Co(1)-O(2)#1 93.50(7)N(2)-Co(1)-N(1) 109.58(6)N(2)#1-Co(1)-N(1) 90.94(6)O(2)-Co(1)-N(1) 76.87(5)O(2)#1-Co(1)-N(1) 83.13(5)N(2)-Co(1)-N(1)#1 90.94(6)N(2)#1-Co(1)-N(1)#1 109.58(6)O(2)-Co(1)-N(1)#1 83.13(5)O(2)#1-Co(1)-N(1)#1 76.87(5)N(1)-Co(1)-N(1)#1 150.68(7)C(7)-O(2)-Co(1) 114.67(10)C(6)-N(1)-C(2) 118.13(15)C(6)-N(1)-Co(1) 113.88(11)C(2)-N(1)-Co(1) 127.84(12)C(8)-N(2)-Co(1) 173.25(15)

ig. 12. Single crystal X-ray structure of the extracted Co(II) complex (H atoms areemoved for clarity). Selected bond lengths are marked.

re satisfied by the PDM and SCN− ligand) and insoluble in aque-us solution and remained stacked on the silica column. Powderiffraction (Fig. 14) pattern of the Co(II) complex proved its crys-allinity and purity. Different optimized sorption parameters wereresented in Table 6.

. Analytical performance

The pre-concentration factor was one of the most important

arameters to evaluate the performance of solid phase extrac-ion methodologies. It was calculated as the ratio of sampleolume to the volume of eluent used for quantitative recovery100.0 ± 0.3%) of Co(II). A pre-concentration factor of 94 (330/3.5)

able 4rystal data and structure refinement for Co(II)–PDM–SCN− complex.

Empirical formula C16H18CoN4O4S2

Formula weight 453.39Temperature 100(2) KWavelength 0.71073 ACrystal system OrthorhombicSpace group PbcnUnit cell dimensions a = 12.5684(13) A = 90

b = 9.1972(9) A = 90c = 16.5043(17) A � = 90

Volume 1907.8(3) A3Z 4Density (calculated) 1.579 Mg/m3Absorption coefficient 1.148 mm−1F(000) 932Crystal size 0.20 × 0.15 × 0.05 mm3Theta range for data collection 2.47 to 27.87.Index ranges −16 ≤ h ≤ 16, −12 ≤ k ≤ 12,

−20 ≤ l ≤ 21Reflections collected 18448Independent reflections 2278 [R(int) = 0.0584]Completeness to theta = 27.87◦ 100.0%Absorption correction Semi-empirical from equivalentsMax. and min. transmission 0.9448 and 0.8028Refinement method Full-matrix least-squares on F2Data/restraints/parameters 2278/0/128Goodness-of-fit on F2 1.028Final R indices [I > 2sigma(I)] R1 = 0.0278, wR2 = 0.0623R indices (all data) R1 = 0.0416, wR2 = 0.0687Largest diff. peak and hole 0.360 and −0.350 e.A−3

N(1)-C(2)-C(3) 121.99(17)N(1)-C(2)-C(1) 116.21(15)C(3)-C(2)-C(1) 121.79(16)C(4)-C(3)-C(2) 119.50(17)C(4)-C(5)-C(6) 119.03(17)N(1)-C(6)-C(5) 122.60(16)N(1)-C(6)-C(7) 116.69(15)C(5)-C(6)-C(7) 120.69(16)N(2)-C(8)-S(1) 179.13(17)O(1)-C(1)-C(2) 111.79(14)C(3)-C(4)-C(5) 118.74(17)

O(2)-C(7)-C(6) 111.62(15)

Symmetry transformations used to generate equivalent atoms: #1 −x + 2, y, −z + 3/2.

could be achieved. Under the optimum conditions with the useof 25 mL sample solution, a linear calibration range 3–80 �g mL−1

was obtained. Precision of the method was investigated usingoptimum conditions for sorption and desorption of Co(II) andexpressed in terms of the relative standard deviation (RSD). In tenreplicate experiments, RSD value of 0.6% was achieved for Co(II)

Table 6Different sorption parameters.

Maximum sorption capacity(at pH 9.0)

0.203 mmol g−1

Elutiing agent 3.5 mL of 3 mol L−1 HClDetection limit (N = 10) 0.6 �g g−1

Pre-concentration factor (%) 94Sample flow rate 2.5 mL min−1

Sample volume (50 �g mL−1

Co(II))330 mL

Elution flow rate 2.0 mL min−1

Equilibrium time 5.0 hInterference Cu(II), (94.3%); Mn(II), (98.8%); VO3

− ,(97.4%)

R. Saha et al. / Chemical Engineering Journal 174 (2011) 58– 67 65

Table 7Comparison of the present method with some other existing Co(II) sorption methods.

Matrix LOD Eluent Pre-concentrationfactor

Sample analyzed

Amberlite XAD-1180-o-aminophenolchelating resin (XAD-o-AP) [31]

0.9–4.3 mg L−1 4 mol L−1 HNO3 92–100 Boiler feeding water and waste watersamples

2-(2-quinolinylazo)-5-dimethylaminobenzoic acid on C18

cartridge [32]

0.03 mg L−1 Ethanol containing 5% acetic acid 100 Drinking water

Chloromethylatedpolystyrene, functionalized with N,N-bis(naphthylideneimino)diethylenetriamine(PS-NAPdien) [33]

0.75 �g L−1 2.0 mol L−1 HCl 110 Spiked water samples

Aliquat 336 chloride immobilized inpoly(vinyl chloride) [23]

NAa Deionised water NAa Fe(III) and Cd(II) containing syntheticmixture

1-(2-pyridylazo)-2-naphthol on amembrane filter [34]

0.36 ng No eluent but wet ashing of the Co(II)complex was done

NAa Tap water, apple juice, plum liqueur,white wine and red wine

Ethyl xanthate modifiedbenzophenone [27]

0.3 �g L−1 No eluent but dimethyl formamide todissolve the Co(II) complex along withsorbent

200 NIES CRM Human Hair No. 5 and IAEAReference Hair HH-1 certifiedreference materials and human hairs ofAndhra Pradesh and Tamil Nadu, India

1-(2-pyridylazo)-2-naphtholsupported on �-cyclodextrincross-linked polymer (�-CDCP) [35]

5.84 ng L−1 p-octylpolyethylene-glycolphenylether (Triton X-100)

10 National reference materials (China).

Amberlite XAD-1180 resin loaded with4-(2-pyridylazo)-resorcinol (PAR)[36]

3.3 �g L−1 20 mL of 3 mol L−1 HNO3 95 Tap water, stream water, salt andstreet dust samples

Co(II)-PAR (cobalt chelates with4-(2-Pyridylazo) resorcinol) [16]

0.36 �g L−1 4 mL ethanol 150 Water samples, such as city line,geothermal, river and lake

Amberlite XAD-7 [19] 0.24 mg L−1 2 mol L−1 HNO3, 7 mL (in ethanol) 200 Water samples1:1 Mixture of pyridine 2,6-dimethanol

and thiocyanate immobilized onsilica [Present method]

0.6 �g mL−1 3.0 mol L−1 HCl 94 NIES CRMs, waste water, tap water,human hair, vitamin B12 (cobalamine),vitamin B complex

a NA = Not available.

Table 8Comparison of sorption characteristics of the present method with other Co(II) sorbents loaded on silica or alumina.

Matrix Capacity (mmol g−1) LOD (ng mL−1) Eluent Pre-concentrationfactor

Samples analyzed

2-nitroso-1-naphthol [37] – 0.02 Ethanol 125 Water samples, vitamin B12 and B-complex.1,8-dihydroxyanthraquinone [38] 0.25 3.3 2 mol L−1 HCl 150 Underground, tap, and river water samples and

pharmaceutical samples1-(2-Pyridylazo)-2-naphthol [39] 0.85 2.0 Acetone (9 mL) 100 Mineral water and polluted river waterAminothioamidoanthraquinone

[40]0.12 0.0095 1% HNO3 – Pond water, tap water and drinking water

1:1 Mixture of pyridine 0. 203 0.0006 3 mol L−1 HCl 94 NIES CRMs, waste water, tap water, humanhair, vitamin B12 (cobalamine), vitamin Bcomplex

dftratwtaso

4

4

dRp

p

Table 9Results of analysis of certified reference materials.

Sample Certified value (�g g−1)/Reference methodb

Founda(�g g−1)(% recovery)

NIES No. 1 23.0 ± 3.0 22.4 ± 0.1 (97%)NIES No. 2 26.7 ± 0.5 25.9 ± 0.4 (97%)NIES No. 3 0.87 ± 0.8 0.88 ± 0.2 (101.1%)

a Average of five determinations ± standard deviation.b Ref. [41].

Table 10Results of real sample analysis.

Sample Certified value (�g g−1)/Reference methodb

Founda(�g g−1)(% recovery)

Vitamin B12 50.5 ± 2.3 51.4 ± 2.1 (101.7%)Vitamin B complex 452.8 ± 1.4 453.5 ± 1.5 (100.1%)Tap water 6.5 ± 0.5b 6.4 ± 0.3 (98.4%)Pond water 8.4 ± 0.3b 8.3 ± 0.4 (98.4%)

2,6-dimethanol and thiocyanateimmobilized on silica [Presentmethod]

etermination. Limit of detection (LOD) of the proposed methodor the determination of Co(II) was studied under its optimal sorp-ion conditions and found as 0.6 �g g−1. The stability and potentialeusability of the column packed with impregnated sorbent wasssessed by monitoring the maximum sorption capacity of Co(II)hrough several sorption–desorption cycles. No significant changeas observed up to 10 cycles. The sorption characteristics of

he present method were compared with other cobalt sorbentsvailable in the literature (Table 7) [16,19,23,27,31–36]. Also theorption characteristics of the present method are compared withther cobalt sorbents loaded on silica or alumina in Table 8 [37–40].

.1. Application

.1.1. Analyses of certified and real samplesNIES samples were brought into solution by the method

escribed in Section 2.4 and analyzed by the developed procedure.

esults presented in Table 9 clearly indicated the accuracy of theresent method.

Similarly, real samples (waste river water, tap water, well water,ond water and vitamin samples) were analyzed and compared

River water 9.4 ± 0.3b 9.3 ± 0.4 (98.9%)Well water 5.3 ± 0.2b 5.2 ± 0.3 (98.1%)

a Average of five determinations ± standard deviation.b Ref. [40].

66 R. Saha et al. / Chemical Engineerin

Fig. 13. Selected bond angles of the extracted Co(II) complex.

w[t

5

oatbfcSdbbtmwtwfa

[

[

[

[

[

[

[

[

[

[

[

[

[

Fig. 14. Powder diffraction pattern of the extracted Co(II) complex.

ith a standard method using nitroso-R salt as a coloring reagent41] (Table 10). A fair degree of agreement between our results withhat of standard method was observed.

. Conclusion

The proposed method was based upon the preferential sorptionf Co(II) at pH 9.0 using mixture of pyridine 2,6–dimethanol (PDM)nd SCN− impregnated on silica. The method was simple and selec-ive for separation as well as pre-concentration of Co(II). As PDMear hard donor sites ‘O’, and border line donor sites like pyridine-N,or binding border line Co(II) ion, the water insoluble inner–metallicomplex formation was facilitated with the use of ambidentateCN− ion, which contained both soft donor site, S and border lineonor site N to coordinate Co(II). But as Co(II) became harder oninding with oxygen from PDM, it was not the S donor site of SCN−

ut the N-site which bonded to Co(II). For the first time, the struc-ural characterization of the extracted Co(II) chelate complex was

ade by single crystal X-ray analysis. Finally, the proposed methodas verified by analyzing standard reference materials and used for

he analysis of real samples like waste water, human hair, red wine,hite wine and vitamin B12 (cobalamine). High pre-concentration

actor (94), reusability (10 cycles) and good LOD (0.6 �g g−1) werechieved.

[

[

g Journal 174 (2011) 58– 67

Acknowledgements

The authors are grateful to IUC-DAE, Kolkata Centre for fund-ing. Animesh Sahana is grateful to CSIR, New Delhi for providingfellowship.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2011.08.045.

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