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Class Ion ExchangeCHE 712

22 OCT 08

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Chromatographic Extraction

• It is possible to realize the liquid-liquid extraction of metallic ions by another technique: Ion exchange resin.

• An ion exchange resin is an insoluble matrix (or support structure) normally in the form of small (1-2 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and released ions.

• The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion exchange. There are multiple different types of ion exchange resin which are fabricated to selectively prefer one or several different types of ions.

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• Advantages of chromatographic extraction vs. liquid –liquid extraction are:– Simplicity of use– Realization of an important number of

successive equilibria in the chromatographic column

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• There are four main types differing in their functional groups:

• strongly acidic (sulfonic acid groups, eg. sodium polystyrene sulfonate or polyAMPS)

• strongly basic, (trimethylammonium groups, eg. polyAPTAC)

• weakly acidic (carboxylic acid groups)

• weakly basic (amino groups, eg. polyethylene amine)

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• TBP absorbed on porous silica that is hydrophobic by adding methyl groups onto its surface is used to extract U(VI) and Pu(IV) from nitric acid solutions.

• Very sensitive separation such as Es 3+ and Fm3+ can be possible, by chromatographic extraction on column where the stationary phase is composed of HDEHP/Celite.

• Even though the separation factor KdFm3+/Kd Es3+ = 2.2, an excellent separation is achieved.

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Es(III)/Fm(III) separation by chromatographic extraction

• Column = HDEHP 8.8% mass/celite,

• S = 0.062 cm2, • H = 10cm• Eluent = 0.41M

HNO3

• Flow = 1.1mL/cm2/mm

• T = 60°C

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Ion Exchange resins

• Generalities

• Ion exchange resins are organic polymer containing polystyrene chains linked between themselves by divinyl benzene bridges (DVB).

• On the polymer chains, sulfonic groups SO3H or quaternary ammonium groups R4N+ can be added.

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• In the case of sulfonic resins, the proton can be replaced by metallic actions; the resin is cationic exchanger.

• In the case of quaternary ammonium resins, the positive charge must be neutralized by an anion X- (R4N+, X-), this anion can be constituted of an anionic metallic complex MXn

(n-m)- with – n: number of ligands linked to the metal – m: metallic ion charge,

these resins are anionic exchangers

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Other important parameters

• Exchange capacity– Expressed in eq/g (dry resin) of monovalent ions H+ (cationic

resin) or anions X- (anionic resin) which enables the determination of the limiting quantity of metallic ion absorbed by gram of resin.

– If q0 is the maximum exchange capacity for a monovalent ion, for a divalent ion, the saturation will be obtained for q0/2. etc…

• Bridging Rate– Percentage of DVB in the resin which influences the ion

exchange kinetics between phases. The Kinetics to obtain equilibrium is more rapid when X is low.

• Particle size analysis– Expressed in mesh (inversely proportional to the diameter of the

spherical grains of the resin ). Partition equilibrium are reached faster for resins with low particle size (high value of mesh).

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KD

• The partition of a metallic ions M between an aqueous phase and the ion exchange resin is characterized by the partition coefficient KD

• KD = CMR * CM

a-1

• With CMR = concentration of M in the resin (Mole

for a gram of resin)

• CMa-1 = concentration of M in the aqueous phase

in mole/L

• The dimension of KD is L/g

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Capacity• Capacity is defined as the number of counter-ion equivalents in a specified amount

of material. Capacity and related data are primarily used for two reasons:- for characterizing ion-exchange materials, and for use in the numerical calculation of ion-exchange operations. Capacity can be defined in numerous ways:

• 1. Capacity (Maximum capacity, ion-exchange capacity) Definition : Number of inorganic groups per

• specified amount of ion-exchanger• 2. Scientific Weight Capacity Units : meq/g dry H+ or Cl− form• 3. Technical Volume Capacity Units: eq/liter packed bed in H+ or Cl− form and fully

water-swollen• 4. Apparent Capacity (Effective Capacity) Definition : Number of exchangeable

counter ions per specified amount of ion exchanger. Units : meq/g dry H+ or Cl form (apparent weight capacity). Apparent capacity is lower than maximum capacity when inorganic groups are incompletely ionized ; depends on experimental conditions (pH, conc. ,etc)

• 5. Sorption Capacity. Definition : Amount of solute , taken up by sorption rather than by exchange, per specified amount of ion exchanger

• 6. Useful Capacity Definition : Capacity utilized when equilibrium is not attained Used at low ion exchange rates Depends on experimental conditions (ion-exchange rate, etc.)

• 7. Breakthrough Capacity ( Dynamic Capacity) Definition : Capacity utilized in column operation, Depends on operating conditions

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Characteristics of a chromatographic column

• Diameter: Φ

• Height H

• Optimal ratio H/ Φ ~ 10

• Interstitial volume or dead volume which corresponds to the volume around the resin grains.

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2 paths for separation by chromatography

• Development by elution for small amount of metallic ions to be separated

• Development by displacement in the case of important quantity of matter to be separated

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Cationic Resins

• Actinides elements are absorbed onto the cationic exchange resins (sulfonic, strong acid) as a function of the charge. The affinity of the cationic resin is:

MO2+<MO2

2+ <M 3+ <M 4+ The reaction equation is

With M n+ = actinide ion, HR: resin under acidic form, MRn is the metallic compound formed in the resin

nHMRnHRM nKexn

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• In the case of the absorption of tetravalent actinides or trivalent actinides, we observe an extreme sensibility of the partition coefficient KD to the pH of the aqueous solutions.

• Consequently, to master the partition of ions between the 2 phases, the resin is often used under the form NH4+, the equilibrium is no more dependent on the pH:

44 nNHMRRnNHM nn

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• The used of cationic resins is used especially for the investigation of An(III) behavior.

• This method is at the origin of the discovery of the transplutonium elements which exist exclusively in aqueous solution as ions M(III).

• This method also is used to study the formation of complexes between M n+ and ligands in aqueous solution.

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Absorption characteristics of Am(III) and Cm(III) and Lanthanides (III) towards the resin DOWEX 50X4 (under

H+ form)

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• One can notice that the reactions occur because of the strong associated entropic variations.

• Two actinides (III) have the same affinity towards the resin (G is quite similar).

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Distribution of Am(III), Pu(III)

and Pm(III) with cationic resins.Influence of the

acid concentration

on KD

a,c = Resin DOWEX

b= resin C 50

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• For acidic concentration <3M, the increase of the acidity implies a decrease in KD (exchange M3+/3H+)

• The KD values for a metal are very close and are independent of the nature of the acid. This is du to the fact that the nitrato and chloro complexes of actinides (III) have a weak stability.

• Furthermore the KD do not depend on the Z of the element

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• These systems are not favorable for a separation of actinides between themselves or the separation of actinides and lanthanides.

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Hydroxocarboxylic acids

• Particularly studied for the separation of An(III)

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Separation factor for the An(III) from transplutonium (Am to Md) elements for the

system resin DOWEX 50 * 12 with ammonium hydroxycarboxylate solutions

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Anionic Resin

• The absorption of metallic ions by a anionic resin is possible if the metallic ion M n+ forms with the anionic ligand X- one or several anionic complexes MXn

(m-n)- . The anion X- is often = Cl-, SCN-, NO3

-, SO42-.

• Since only few metallic ions can form such complexes, extremely selective separation can be realized.

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-Not Absorbed+ Absorbed

++ Strongly Absorbed

Medium Chloride Nitrate Sulfate

Actinide HCl MCl HNO3 MNO3 H2SO4 M2SO4

M(III) - ++ - ++ - -

M(IV) ++ ++ ++ - + +

M(V) - - _ ++ - -

M(VI) ++ ++ _ ++ + ++

Affinity of actinides for anion exchange resin as a function of the oxidation state and acid or acid salt

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• From the previous table we wee that:– Actinides M(IV) and M(VI) are the most

susceptible to be sorbed as anionic complexes.

– The absorption of M 3+ ions is not possible from solution HCl, HNO3 and H2SO4, on the other side actinides M 3+ can be sorbed by the salts MCl and MNO3 in concentrated solutions.

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• Among the most important systems, the absorption of U(VI) from sulfate medium or Pu(IV) from concentrated HNO3 are going to be presented because they present an industrial interest

• U(VI), purification of U from the sulfuric liquors used to attack the minerals

• Pu(IV), final purification of Pu in certain reprocessing plants

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U(VI) in sulfate medium (1)

• U(VI) can exits in sulfate medium as• Find the complexes of U sulfate.• The 2 anionic forms of U(VI) sulfate can be

absorbed on an anionic resin as:

• For a ionic strength of 0.3, • K1 = 230 and K2 = 262

243424

2434242

242422

224242

2)()(2

)()( 1

SOSOUORSOUOSOR

SOSOUORSOUOSOR

K

K

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U(VI) in sulfate medium (2)

• A reaction is competing and is not in favor for the formation of the sulfato U(VI) complexes, the absorption of the bisulfate ions HSO4

- whose the quantity increases with the increase of H2SO4 concentration:

• With K3 = 17.5 (for ionic strength of 0.3)

244442 )(22 3 SOSORHHSOSOR

K

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INFLUENCE OF [H2SO4] OR [(NH4)2SO4] ON KD

U(VI)

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• In H2SO4, the increase of the acidity of the medium, corresponds to a decrease of KD

U(VI).

• If the absorption of U(VI) is excellent for H2SO4 = 0.1M (KD = 103 mL/g) , it becomes mediocre for H2SO4 = 1M (KD = 6 mL/g).

• This is due to the competition with the HSO4

- ions for the resin sites.

• For (NH4)2SO4, the effect is not as strong.

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• The behavior of Th(IV) is quite similar to U(VI) but displaced with 2 order of magnitudes for KD values.

• Separation of U(VI)/Th(IV) are consequently possible with a selective absorption of U(VI).

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• The extraction of U(VI) by anion resins in sulfate medium is a very selective method which separates U(VI) from numerous metallic ions: M+ (alkalines), Tl+, Be 2+ , Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Al 3+ , Sb 3+ , Ln 3+ (rare earth)…

• After its absorption on the anionic resin (SO4 2- ),

uranium can be eluted from the chromatographic column by the seepage of an aqueous solution that contains anions which have a bigger affinity for the resin than the ions SO4

2- have.

• The ions are Cl-, NO3-, ClO4

-

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Elution of U(VI) from an anionic resin (SO42- )

Eluent: 0.9M NaCl, 0.1M HClFlow: 8.4 mL/cm2/mn

Column diameter = 5cm, H = 122 cm

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• The elution peak of SO42-

ions is obtained for a volume of eluent = 1V while the elution peak for U(VI) is obtained for a volume of eluent = 2.5V.

• The elution peak of the U(VI) is large, which is probably due to the greater eluent speed onto the column

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Pu(IV) in HNO3 medium (1)

• Tetravalent Pu has a tendency to form anionic complexes with NO3

- ions in very concentrated HNO3 solutions or in concentrated nitrates solutions (LiNO3, Ca(NO3)2, Al(NO3)3

• Important ions NO3- concentrations are

necessary because the stability constants of Pu(IV) nitrate complexes are generally weak.

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Pu(IV) in HNO3 medium (2)

36.02

223

13

3

10,)(

10,)(

KNOPu

KNOPu

This property (weak stability constant) is unique for M(IV) in concentrated HNO3 medium

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Pu(IV) in HNO3 medium (3)

• The absorption reaction of Pu(IV) by anion resins (NO3

- form) can be written as:

• Which means that the anionic nitrato complex of Pu(IV) is formed in the resin

63234

3 )(42 NOPuRNOPuRNO

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Influence of [NO3-] on the extraction of Pu(IV)

by the resin DOWEX 1X4 (50 to 100 mesh)

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• In every cases, we observe a strong increase of KD with NO3

-, the curves have a maximum for NO3

- = 7 to 7.5M• Ca(NO3)2 is a more favorable

medium for the extraction of Pu(IV) than HNO3 medium, because of the formation of compounds such as HPu(NO3)6

- and H2Pu(NO3)6 in the aqueous solutions

• Increase of temperature does not favor a good absorption of Pu onto DOWEX 1X4

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Pu(IV) in HNO3 medium (4)

• The absorption of Pu(IV) by anion resins is an extreme slow process, it can take several months at ambient temperature to reach the equilibrium.

• The desorption of Pu absorbed on anionic resins column can take place by– Seepage of diluted HNO3

– Reduction of Pu(IV) by hydroxylammonium nitrate (NH3OHNO3)

– Displacement of anions by percolation of HClO4 solutions

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Pu(IV) in HNO3 medium (5)

• By an absorption/desorption cycle on anionic resins (NO3

-), Pu can be separated from a big variety of contaminants.

• Next table is presenting the performances of a cycle of purification

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Separation of Pu from impurities by anionic exchange at 60°C

element Initial Pu in ppm Final Pu in ppm Decontamination Factor

Ag 105 <2 >5*104

Al 105 <13 >7.7*103

Ca 105 <5 >2*104

Cr 105 5 2*104

Cu 105 10 104

Fe 2*106 45 4.4*104

K 105 <5 >2*104

Li 105 <1 >105

Mg 105 20 5*103

Mn 104 2 5*103

Na 104 20 5*102

Ni 105 <10 >104

Column: 0.28 cm2, H = 90cm, Resin DOWEX 1X4, Wash: 15 volumes, 7.2M HNO3, Flow: 10 mL/cm2/mn