14 Chapter 5

Chapter 5

37,38,39,40,41,42-hexa hydroxy

8,13,23,31-tetranitro 1,19 (bis

N-phenyl

benzo)calix(6)arenehydroxamic

acid(NPCHA) for kinetic

extraction, sequential separation

and trace determination of

uranium(VI) and thorium(IV) by

inductively coupled plasma mass

spectrometry (ICP-MS)

107

5.1 Abstract

5.1 Abstract

The kinetic extraction, sequential separation and trace determination of ura-

nium(VI) and thorium(IV) with p-nitro calixarene hydroxamic acid (NPCHA)

is reported. Uranium(VI) and thorium(IV) are extracted at pH 6.0 and 4.5, re-

spectively in dicholoromethane. The influence of NPCHA, pH and diverse ions

of uranium(VI) and thorium(IV) was examined. The uranium(VI)-NPCHA com-

plex has a maximum absorbance at 407 nm with molar absorptivity 3.69 104 lmol1 cm1. The system obeys Beers law over the range of 0.94-4.74 g ml1

of uranium(VI) at 407 nm. The thorium(IV)-NPCHA complex has a maximum

absorbance at 372 nm with molar absorptivity 1.46 104 l mol1 cm1. The sys-tem obeys Beers law over the range of 0.92-4.62 g ml1 of thorium(IV) at 452

nm. For trace determination the extract were directly inserted into plasma for

inductively coupled plasma mass spectrometry (ICP-MS)measurement for ura-

nium(VI) and thorium(IV), which enhances the sensitivity to several times and

obeys Beers law in the range of 47-230 ng ml1 for uranium and 40-200 ng ml1

for thorium. The method is applied for determination of uranium(VI) and tho-

rium(IV) in real standard samples and environmental samples.

108

5.2 Introduction

5.2 Introduction

Uranium and thorium are the two most vital elements for nuclear energy pro-

grammes. Their natural sources generally contain a sizeable fraction of lan-

thanides which in their own right have diverse technological applications. Thus

the methodology adopted for the separation of these metal ions from different

ores has always attracted the attention of separation scientists. It may be used

to contaminate rare earth concentrates from uranium and thorium radioactiv-

ity. These separations are significantly important also from the point of view of

nuclear fuel processing1. kinetics extraction has become a very attractive field

in the study of solvent extraction. Information about kinetics extraction can be

useful in choosing optimum condition for carrying out the reaction. Kinetic in-

formation on the extraction mechanism and rate equation allow the prediction of

order of reaction and enhancements which could be used to improve and monitor

the extraction, preconcentration, speciation and trace determination of uranium

and thorium. For the last two decades a large number of the macrocyclic com-

pounds have been synthesized and used for host guest complexation of several

cations. Calixarene derivatives, which are synthesized easily and functionalized

in various ways have been noted as useful extractant of metal ions.28 These

novel compounds can recognize a target metal ion by the cavity size of the cyclic

molecule together with the chelating effect of their functional groups. Therefore,

functionalized calixarene is one of the most promising extractant for the inno-

vative solvent extraction process and has been tried out for the separation of

vanadium in recent years.912 Introduction of the hydroxamic acid group in the

macrocycle may enhance the complexing ability towards the metal ions. Hydrox-

amic acids are versatile metal extractants that have been the subjects of large

number of physicochemical investigations because of their wide applications in

analytical,13 agriculture14 and biological fields.15 In recent years few macrocycles

bearing hydroxamic acid as a functional group have been synthesized and used

for complexation studies.1621. However, no kinetic study concerning the extrac-

tion of uranium and thorium with a NPCHA has been carried out previously.

In the present work, extraction studies of uranium and thorium with NPCHA

were carried out. The influence of NPCHA, pH and diverse ions of uranium and

109

5.3 Experimental

thorium was examined. Kinetics extraction studies of uranium and thorium with

NPCHA were carried out. The extracts were inserted directly into the plasma

for ICP-MS measurements of uranium and thorium.

5.3 Experimental

5.3.1 Chemicals

All the chemicals were of analytical grade of Fluka, B.D.H or E.Merck. Glass

distilled and de-ionized water was used throughout the experiments.

5.3.2 Metal solution

A 9.97105 M standard uranium solution was prepared by dissolving 2.109 g ofuranyl nitrate hexa hydrate in one litre of double distilled water and standardized

spectrophotometrically.22 It was further diluted as and when required. A 1.0104M standard stock solution of thorium(IV) was prepared by dissolving 2.5 g of tho-

rium nitrate tetra hydrate in one litre of double distilled water and standardized

spectrophotometrically.21

5.3.3 Reagents

37,38,39,40,41,42-hexa hydroxy 8,13,23,31-tetra nitro 1,19(bis N-phenyl benzo)calix(6)arene

hydroxamic acid was synthesized as described in Chapter 2. Its 1.0 104 M(0.1%w/v) stock solution of NPCHA was prepared in dicholoromethane.

5.3.4 Apparatus

Electronic spectra were recorded on a JASCO-980 UV-VIS-NIR spectrophotome-

ter with matching 10mm quartz cell. pH measurements were performed with

Lab India pH meter Model 6E488, equipped with a combined glass and calomel

electrode. A VG Plasmaquad 2+ Inductively Coupled Plasma-mass spectropho-

tometer, controlled by software provided by the manufacture and running on a

110

5.3 Experimental

compag Deskpro 286e computer was used for analysis. The instrumental param-

eters and detection limits of uranium and thorium are given in Table 1.

Table 1 ICP-MS opreating conditions

ICP Plasma Argon

Forward power 1.35 kW

Reflected power 10 W

Coolant gas flow 16 l/min

Carrier gas flow 0.70 l/min

Auxiliar gas flow 0.30 l/min

Nebuliser pressure 2 bar

Solution uptake rate 0.8 ml/min

Sample cone aperture 1 min

Skimmer cone aperture 0.7 mm

uranium mass number 238

thorium mass number 233

Detection limits of uranium 1 ng dm1

Detection limits of thorium 2 ng dm1

111

5.3 Experimental

Sample preparation

The samples were digested with a mixture of conc. HNO3 and HClO4 (1:1)

and evoprated to dryness. The residue was redissolved in 0.1 M HClO4 and diluted

to 250 ml with distilled water. An aliquot of solution was taken for the extraction

and determination of uranium and thorium. The concentration obtained are in

good agreement with the certified values(Tables-9 and 10).

Separation and determination of Thorium in Rare Earths

The thorium was separated and determined in the monazite sand and rare

earths magnesium-silicon alloy. A weight quantity of 0.l-0.2 g of powder was

mixed with 1 - 2 ml hydrofluoric acid in a platinum crucible and then heatde to

dissolve it. Then the contents were heated with 2 ml of H2SO4 to dryness, and

residue was dissolved in 5 ml of 2 M HCl. Finally it was diluted to 100 ml with

distilled water. The data is in Table 10.

5.3.5 Extraction procedure

An aliquot of uranium(VI) or thorium(IV) solution containing (0.94-4.74 or 0.92-

4.62 g ml1, respectively) was transferred into a 60-ml separatory funnel and pH

6.0 or 4.5 was adjusted with 10 ml of buffer solution. The mixture was shaken with

10 ml dicholoromethane solution of NPCHA. The organic phase was seprated,

dried over anhydrous sulphate and transferred into 25-ml volumetric flask. To

ensure the complete recovery of uranium(VI) or thorium(IV) the extraction was

repeated with 5 ml reagent solution. The combined extract and washing were

collected and finally diluted to 25 ml with dicholoromethane. The absorbance of

the extract was measured against the reagent blank.

112

5.4 Result and Discussion


The uranium(VI)-NPCHA complex has a maximum absorbance at 407 nm with

molar absorptivity 3.69 104 l mol1 cm1. The reagent blank does not absorbat this wave length. The system obeys Beers law over the range of 0.94-4.74 g

ml1 of uranium(VI) at 407 nm.

The thorium(IV)-NPCHA complex has a maximum absorbance at 372 nm

with molar absorptivity 1.46 104 l mol1 cm1. The reagent blank does notabsorb at this wave length. The system obeys Beers law over the range of 0.92-

4.62 g ml1 of thorium(IV) at 452 nm.

For trace determination the extract were directly inserted into plasma for

inductively coupled plasma mass spectrometry (ICP-MS) measurement for ura-

nium(VI) and thorium(IV), which enhances the sensitivity to several times and

obeys Beers law in the range of 47-230 ng ml1 for uranium and 40-200 ng ml1

for thorium.

5.4.1 Effect of pH

The optimum pH for maximum extraction was determined by carrying out the

extraction with varying concentration of uranium(VI), thorium(IV) and NPCHA.

The pH of the aqueous phase was varied using diffrent buffer solution. The ex-

traction of uranium(VI) or thorium(IV) increased with the increase in pH until it

leveled off at pH 6.0 for uranium(VI) and pH 4.5 for thorium(IV). Thus, the op-

timum pH for efficient extraction lies within the range of 5.0-5.8 for uranium(VI)

and 4.0-4.8 for thorium(IV), respectively. The low extraability at lower pH value

may be attributed to the proton extraction into organic phase rather than the

metal ion itself(Tables-2 and 3).

5.4.2 Effect of reagent concentration

The influence of the NPCHA was studied by extracting a fixed amount of ura-

nium(VI) or thorium(IV) with varying concentration of NPCHA at pH 6.0 and

4.5. A 10 ml of 6.48104 solution of NPCHA is quite adequate for the quantita-tive extraction of uranium(VI) or thorium(IV). Lower concentration of NPCHA

113


reduce the percentage extraction. While an excess of reagent can be used without

any adverse result(Tables-4 and 5).

114


Table 2 Effect of pH on the extraction of uranium(VI) with NPCHA

Solvent : Dicholoromethane

Uranium : 4.74 g ml1

NPCHA : 10 ml, 6.48104 Mmax : 407 nm

pH %Extraction molar absorptivity lmol1cm1()

4.5 92.8 3.44104

5.0 100 3.70104

5.5 100 3.70104

5.8 100 3.70104

6.0 100 3.70104

6.5 92.4 3.42 104

115


Table 3 Effect of pH on the extraction of thorium(IV) with NPCHA


Thorium : 4.62 g ml1

NPCHA : 10 ml, 6.48104 M(0.1%)max : 452 nm

pH %Extraction molar absorptivity lmol1cm1()

3.5 46.1 0.677104

4.0 100 1.36104

4.5 100 1.36104

4.8 100 1.36104

5.0 58.9 .863104

116


Table 4 Effect of reagent concentration on uranium(VI) with NPCHA


Uranium : 4.74 g ml1

pH : 6.0

max : 407 nm

NPCHA 104 log NPCHA [U ]org 105 [U ]aq 105 logD

0.51 4.28 0.38 1.60 0.61

1.03 3.98 1.05 0.93 0.05

1.55 3.80 1.49 0.49 0.47

2.07 3.68 1.79 0.19 0.95

2.59 3.58 1.99 0.01 2.29

3.11 3.41 1.99 0.01 2.29

117


Table 5 Effect of reagent concentration on thorium(IV) with NPCHA


Thorium : 4.62 g ml1

pH : 4.5

max : 452 nm

NPCHA 104 log NPCHA [Th]org 105 [Th]aq 105 logD

0.51 4.28 0.10 1.88 1.26

1.03 3.98 0.91 1.07 0.07

1.55 3.80 1.30 0.68 0.28

2.07 3.68 1.71 0.27 0.79

2.59 3.58 1.99 0.01 2.29

3.11 3.41 1.99 0.01 2.29

118


5.4.3 Stoichiometry of complex

The extraction of uranium(VI) or thorium(IV) with NPCHA at pH 6.0 or 4.5

was adjusted with 10 ml of buffer solution can be given by

UO2(NO3)2 + 2HA [{UO2A2}] + 2HNO3 (5.1)

ThO2 + 2HA [{ThOA2}] + H2O (5.2)The equilibrium constant Kex can be given by

Kex =[{UO2A2}](o)[H]2(aq)

[UO2(NO3)2](aq)[HA]2(o)

(5.3)

or

Kex =[{ThOA2}](o)[H]2(aq)[ThO2](aq)[HA]

2(o)

(5.4)

where subscripts (aq) and (o) are aqueous and organic phase respectively.

HA is for hydroxamic acid group of NPCHA and distribution of uranium(VI) or

thorium(IV) is given by :

D =[{UO2A2}](o)

[UO2(NO3)2](aq)(5.5)

D =[{ThOA2}](o)

[ThO2](aq)(5.6)

From equation (5.3), (5.4), (5.5) and (5.6)

Kex =D[H]2(aq)[HA]2(o)

(5.7)

logKex = logD 2pH 2log[HA]O (5.8)To established the stoichiometry of complex, the method of slope ratio was

employed, viz. by plotting a graph of logarithm of the distribution ratio of metal

(logD) against the negative logarithm of ligand concentration (log NPCHA)(Figs. 5.1 and 5.2). The extraction was carried out by taking fixed amount

119


of uranium(VI) or thorium(IV) with varying amount of NPCHA, which gives

metal:NPCHA ratio as 1:1.

5.4.4 Kinetics extraction

Kinetics extraction studies of uranium(VI) and thorium(IV) with NPCHA were

carried out with respect to hydrogen ion and reagent concentration. It indi-

cates that the reaction is of first order with respect to the uranium(VI) and

thorium(IV). The value of rate constants 0.299 mol1 l1 s1 and 0.211 mol1

l1 s1 were calculated from the slope and the half life period were found 2.31

second and 3.28 second for uranium and thorium respectively.

120


! " # $ % & '

Figure 5.1: Plot of (log D) against (-log NPCHA) for uranium complex with

NPCHA.

121


! " " " # " " $ # % & ' # $( ) * + ,

Figure 5.2: Plot of (log D) against (-log NPCHA) for thorium complex with

NPCHA.

122


Determination of uranium(IV) and uranium(VI))

A mixture of uranium(IV) and uranium(VI) of 25 g ml1 each was trans-

ferred to 60-ml separatory funnel and uranium(IV) was extracted with NPCHA

after adjusting the pH 6.0 with the buffer. The dichloromethane layer was sepa-

rated and uranium(IV) was determined. Into the aqueous layer a 1 M potassium

persulphate solution was added to oxidize uranium(IV) to uranium(VI) and then

extracted with dichloromethane solution of NPCHA(Table 6).

Effect of diverse ions

In order to examine the utility of the present method effect of various cations

and anions in the separation and determination of uranium and thorium was

studied. Interference studies were made by measuring the absorbance of the

extracted organic phase and also by making measurement by ICP-MS of both

extract as well as aqueous phase. The uranium or thorium was extracted in the

presence of large number of competitive ions at 6.0 and 4.5 pH respectively, and

none of them affected the absorbance of uranium and thorium complexes(Tables-7

and 8).

Sequential separation of uranium and thorium

An aliquot of uranium(VI) or thorium(IV) solution containing (0.94-4.74 or

0.92-4.62 g ml1, respectively) was transferred into a 60-ml separatory fun-

nel. Then 10 ml of the reagent NPCHA solution was added and pH 4.5 was

adjusted with 10 ml of buffer solution. The contents were shaken with 10 ml

of dicholoromethane and the organic phase was allowed to separate and thorium

concentration was determined spectrophotometrically. The pH of aqueous layer

was adjusted 6.0 with buffer and contents were shaken with 10 ml of the reagent

NPCHA, the organic layer was separated and uranium was estimated by spec-

trophotometrically(Fig. 5.3).

123


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5.5 Conclusion

5.5 Conclusion

The present method is very simple, selective and sensitive. Uranium and Thorium

are first pre-concentrated by solvent extraction technique and then subjected to

ICP-MS estimation with detection limit 1-2 ng dm1. NPCHA has shown high

affinity towards uranium and thorium in presence of large quantities of associated

metal ions.

132

Bibliography

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istry., 2002, 251, 451.

[2] Izatt, R. M., Lamb, J. D., Hawkins, R. T., Brown, P. R., Izatt, S. R., Chris-

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[22] Lobinski, R., Marczenko, Z. Spectrochemical Trace Analysis for Metals and

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134

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