Plutonium chemistry and other actinides in aqueous solutions Part 2a Homogeneous system

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1 Plutonium chemistry and other actinides in aqueous solutions Part 2a Homogeneous system Ph. MOISY CEA/DEN/DMRC ; Marcoule philippe.moisy@cea.fr PAGE 2

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Artificial Good Américium Z= 95 Curium Z= 96 Protactinium Z= 91 Natural Low Berkelium Z= 97 Artificial Bad (very

3 Actinide family: 15 (14?) elements From Actinium Ac (Z=89) to Lawrencium Lr (Z=103) (Ac: 7s 2 6d 1 : 14 An?) Element Natural/Artificial Knowledge Thorium Uranium Z= 90 Z= 92 Natural Good Neptunium Z= 93 Plutonium Z= 94 Artificial Good Américium Z= 95 Curium Z= 96 Protactinium Z= 91 Natural Low Berkelium Z= 97 Artificial Bad (very bad) Lawrencium Z= 103 Einsteinium Z= 99 Last elements prepared in ponderable scale Fermium (Z=100) Lawrencium (Z=103) : Trace Chemistry

4 Actinide family: 15 (14?) elements Oxidation State (Valency) of Actinide in Aqueous Solution Outlook 1- Actinide ions in solution 2- Structural data 3- Thermodynamic data

5 Hydration sphere Actinide ions in aqueous solution: M(III) and M(IV): M(H 2 O) n 3+ and M(H 2 O) n 4+ with n = 8 or 9 Am(H 2 O) 8 3+, Pu(H 2 O) 8 4+ M(V) and M(VI): MO 2 (H 2 O) n + and MO 2 (H 2 O) n 2+ with M = U, Np, Pu and Am PuO 2 2+ (H 2 0) 5, AmO 2 2+ (H 2 0) 5 For Pa(V): PaOOH(H 2 O) x 2+ M(VII): NpO 3 (H 2 O) y + (in acid media), Pu(VII) and Am(VII) are unstable MO 4 (OH) 2 (H 2 O) z 3- with M = Np, Pu and Am (in basic media)

6 Special case for Oxidation State +6 and +5

7 Structural Data: Actinyl (Oxidation state +V and +VI) M(VI) in carbonate media (2 M) by Raman spectrocopy U(VI)-aquo by cristallography Np(V) and Np(VI) acetate by cristallography

8 Plutonium

9 U(VI) fluorescence Uranium

10 Neptunium HClO 4 (1M)

11 Americium and Curium

12 The case of An(VII) Acidic media: MO 3 (H 2 O) x + (only Np) Basic media : MO 4 - (Np, Pu and Am) 13

13 Protactinium: an irregularity in the Mendeleev Table Pa(V) = PaO 3+ (PaO(OH) x (H 2 O) y 3-x ) PaF z 5-z (z = 6 and 7) Polymerisation of Pa(V) (colloïdal form) Adsorption (glass wall, ) Chemical analog: Ta(V) and Nb(V) Pa(IV) =? 14

14 Kinetic control of water electroactivity 2 H 2 O - 4 e - O 2(g) + 4 H + E 0 = V/ESH 2 H e - H 2(g) (P H2 = 1 atm.) E 0 = 0 V/ESH (by convention) Irreversible processus (slow) due to phase changement: liq/gaz! Kinetic control

15 Stability of Lanthanide in aqueous solution Stability of Ce(IV) and «Metastability» of Pr(IV), Tb(IV) and Eu(II) Electroactivity domain 2 H 2 O - 4 e - O 2(g) + 4 H + 2 H e - H 2(g) Oxidation of Ln(0) Reduction of Ln(IV) Stability of Ln(III) only! Ln(0) + 3 H + Ln aq /2 H 2(g)

16 Stability of Actinide in aqueous solution 2 H 2 O - 4 e - O 2(g) + 4 H + Electroactivity domain Oxidation of An(0) Oxidation of U(III) Reduction of Am(IV) Stability of An: +III, +IV etc 2 H e - H 2(g) Pu(a) + 3 H + Pu aq /2 H 2(g)

17 Redox behavior of An: Thermodynamic or Kinetic control Only one charge transfert: An n+ / An (n+1)+ and AnO 2 n+ / AnO 2 (n+1)+ Reversible process (rapid): Thermodynamic control Chemical reaction coupled with charge transfert: An 4+ / AnO 2 + Irreversible process (slow): Kinetic control

18 Latimer diagram for Actinide (aqueous solution, HClO 4 (1M), V/ESH) Rapid Slow

19 Stability (metastability) in acidic media (HClO 4 1M) Stability Metastability

20 Disproportionation of Pu(IV) and Pu(V) PuO 2 2+ Pu 4+ PuO 2 + HClO 4 (1M) PuO 2 + Pu 3+ Pu 4+ 2PuO H + Pu 4+ + PuO H 2 O PuO 2 2+ PuO 2 + Pu 4+ PuO 2 + Pu 4+ Pu 3+ Low acidity PuO H + + 2e - Pu H 2 O 2Pu H 2 O Pu 3+ + PuO H +

21 Disproportionation of Pu(V) (and redox reaction) 2PuO H + Pu 4+ + PuO H 2 O slow PuO Pu 4+ PuO Pu 3+ rapid 3PuO H + Pu PuO H 2 O Disproportionation of Pu(V) in HClO 4 (1M)

22 Disproportionation of Pu(IV) (and hydrolysis) 2Pu H 2 O Pu 3+ + PuO H + Pu 4+ + PuO 2+ Pu 3+ + PuO 2 2+ slow rapid 3Pu H 2 O 2Pu 3+ + PuO H + Kinetic law:

23 Chemical Behavior of Actinide in aqueous solution: outlook 1- Hydrolysis 2- Complexation 3- Pourbaix diagram

while for others it is dominated by their Electronegativity 2- Electrostatic interaction (ionic potential z/r) 3-

24 Chemical Behavior of Actinide in aqueous solution: basic knowledge 1- Actinide ions are hard acid cations (Pearson (1963) Hard and Soft Acids and Bases (HSAB) For some metal ions, their chemistry is dominated by size and charge, while for others it is dominated by their Electronegativity 2- Electrostatic interaction (ionic potential z/r) 3- For Actinyl, z are the effective charge: Oxidation state : M 4+ > MO 2 2+ > M 3+ > MO 2 + Effective charge :

25 Hydrolysis Hydrolysis = special case of complexation (in water solvent) Ligand = OH - «hydroxyl anion» When ph increase: structure aquo ions is modified One H + are expelled from inner water sphere to unbound water (into the bulk) M(H 2 O) n Z+ + H 2 O MOH(H 2 O) n-1 (z-1)+ + H 3 O + M z+ + H 2 O MOH (z-1)+ + H + As hydroxyl anion is a bridger ligand, Hydrolysis produced hydroxopolynuclear compounds (and sometime polymer): xm z+ + yh 2 O M x OH y (xz-y)+ + yh +

26 Hydrolysis of Pu(IV) Technical difficulties: 1) disproportionation 2) occurence of polymers 3) slow kinetics log K xy Pu 4+ + H 2 O PuOH 3+ + H + -0,5 Pu H 2 O Pu(OH) H + -2,3 Pu H 2 O Pu(OH) H + -5,3 Pu H 2 O Pu(OH) 4 + 4H + -9,5 For largest [Pu 4+ ] concentrations: polymer with molecular mass 10 3 to g Pu(OH) 4 green ; [Pu 4+ ] [OH - ] 4 = solubility product 10-55!! Pu(OH) 4 soluble if freshly prepared but insoluble if aged (PuO 2 nh 2 O)

27 Pu(IV) polymeric form -Low acidity (less than 0.5M, without complexing anions) -Quick formation (dilution of an acidic Pu(IV) solution with water will frequently cause polymerisation in localized areas of low acidity, even when the final acidity of the solution is too high for polymerisation occur) -Irreversibility (hight stability, no precipitation,.) -No reactivity (very slow with strong complexant anion such as F -, or oxydo-reduction)

28 Hydrolysis of Pu(IV) Ion (0.01 M) Hydrolysis (10%) at ph PuO Pu 3+ 7 PuO Pu For the same actinide, the hydrolysis increase with: M 4+ > MO 2 2+ > M 3+ > MO 2 + no steric constraint with OH - (the same size as H 2 O)!

29 Complexation Solution chemistry: Ionic potential (z/r), Acido-Basicity of Ligand (pka) and Cation (hydrolysis), stoechiometry, speciation, activity coefficient, Coordination chemistry: inner-sphere or outer-sphere, atom donnor, coordination number, steric constraints, polyhedre,. Redox chemistry (ligand and cation): Nernst law, intra-molecular, temperature effect,

30 Electrolyte theory Electrolyte theory provides a useful theory for thermodynamic application Debye Huckel models (and also more complicated model as Pitzer theory and others )

31 Hard and Soft donnor -Hard acids interact with Hard bases through ionic bonding - Soft acids and bases interactions favour covalent bonding

32 Water Radiolysis product: H 2 O 2 Chemical properties in aqueous media: Acido-basic H 2 O 2 HO H + pk 1 = 11,6 HO 2 - O H + pk 2 very high Coordination M n+ + m H 2 O 2 M(O 2 ) m (n-2m)+ + 2mH + Bidentate, «bridger», no steric constraint Redox H 2 O H e 2H 2 O E 0 = 1.77 V/ENH H 2 O 2-2 e O 2 + 2H + E 0 = 0.68 V/ENH

The case of Pu H 2 O 2 (1) 2 Pu 4+ + H 2 O 2 + H 2 O (Pu) 2 (O 2 )(OH) 5+ + 3H + K 1 = 8,8 x 10 6 mol/l brun (complex 2:1) 2

33 The case of Pu H 2 O 2 (1) 2 Pu 4+ + H 2 O 2 + H 2 O (Pu) 2 (O 2 )(OH) H + K 1 = 8,8 x 10 6 mol/l brun (complex 2:1) 2 Pu H 2 O 2 (Pu(O 2 )) H + K 2 = 6,3 x 10 8 mol/l rouge (complex 2:2 (1:1)) 34 Solutions Pu 4+ (HCl 0.5 M) + H 2 O 2

34 The case of Pu H 2 O 2 (2) Pu H 2 O 2 + H 2 O Pu(O 2 ) 2 + 4H + unsoluble complex 1:2 But Pu(IV) reduction to Pu(III) at low acidity (0.5 M) produce (Pu IV Pu III )(O 2 ) 7/2 Calcination of this complex can be used to prepare PuO 2, but it is difficult to control the redox chemistry of Pu! Reduction of Pu(VI) to Pu(IV) with H 2 O 2, but U(VI) can be complexed to form an unsoluble compounds: UO 2 (O 2 ) (industrial application in front-end cycle) 35

35 Thermal treatment: oxide/dioxide (ThO 2, U 3 O 8, NpO 2, PuO 2, AmO 2, CmO 2, ) The case of An(III and IV) with C 2 O 4 2- : speciation - Acido-basicity H 2 C 2 O 4 H + + HC 2 O 4 - pk a1 = 1.2 HC 2 O 4 - H + + C 2 O 4 2- pk a2 = No redox reaction (in normal pressure and temperature conditions) -Bidendate, bridger, steric constraint: Stoechiometry: 1:1 to 1:5 for An(III and IV) 1:1 to 1:3 for An(V and VI) Unsoluble compounds for neutral species: M 2 (C 2 O 4 ) 3.10H 2 O, M(C 2 O 4 ) 2.6H 2 O, MO 2 (C 2 O 4 ).xh 2 O,

36 Polyaminocarboxylate complex of An(III and IV) DTPA (Diethylene triamine pentaacetic acid, H 5 Y) is perhaps the most effective for Pu excretion as anionic Pu(IV) chelate complexes (stoechiometry 1:1). CN = 8, but steric constraint Redox behavior (slow reductant at ambiant temperature - 5 carboxylate functions and 3 nitrogen (CN = 8) - Reduction of Pu(VI) to Pu(IV) Pu(IV)Y - log K = + 33 M n+ + H 5 Y MY (n-5) + 5H + An(III)Y 2- log K = + 22 to + 23 DTPA is a medicine for the treatment of contaminated with Pu workers

37 For the same actinide, the complexation increase with: M 4+ > M 3+ > MO 2 2+ > MO 2 + For the same oxidation state, the complexation increase with Z (atomic number) according to ionic potential (z/r) But steric constraint modify this order! General law for complexation of An in aqueous solution

38 Carbonate complexes of An(V and VI ): An = U, Np, Pu and Am 1) Acido-basicity: 2) Not Redox: CO 2 + H 2 O HCO H + CO H + 3) Complexation: mono/bidendate, no steric constraint UO CO 2-3 UO 2 CO 3 UO 2 CO 3 + CO M + (or M 2+ ) M 2 UO 2 (CO 3 ) 2 AUPuC (RNR fuel) Conversion at C under N 2 /H 2 atm. of (NH 4 ) 4 (U-Pu)O 2 (CO 3 ) 3 To (U-Pu)O 2 UO 2 (CO 3 ) CO M + (or 2M 2+ ) M 4 UO 2 (CO 3 ) 3 Concentration in natural water: M, and 10-2 M in underground water AnO 2 (CO 3 ), CaAnO 2 (CO 3 ) 2 and Ca 2 AnO 2 (CO 3 ) 3 compounds are expected

39 40

40 Carbonate complexes of An(III and IV): An = Np, Pu and Am - Bidentate mode and bridger - Soluble and unsoluble form (Am 2 (CO 3 ) 3.nH 2 O, NaAm(CO 3 ) 2.nH 2 O, ) - Competition between OH - and CO 3 2- (An(OH)(CO 3 ) for An(III) and An(OH) 2m (CO 3 ) n for An(IV)) - Limit complexes better known than intermediate forms

41 Coordination sphere [C(NH 2 ) 3 ] 6 [Th(CO 3 ) 5 ] Hexagonal bipyramid [Na 6 Pu(CO 3 ) 5 ] 2.Na 2 CO 3.33H 2 O Na 6 BaTh(CO 3 ) 6.6H 2 O Icosahedron Tuliokite Na 6 BaTh(CO 3 ) 6.6H 2 O, has been discovered in the Khibinski region, Russia, in 1990

42 Pourbaix diagram: the case of U(IV) (0<pH<4) U 4+ + H 2 O UOH 3+ + H + log Q 11 = -1,16 (ph = 1.16) U 3+ U 4+ + e E= log[u 4+ ] / [U 3+ ] UO 2 + UO e E= log[uo 2 2+ ] / [UO 2+ ] U H 2 O UO H + + e E= pH log[uo 2+ ] / [U 4+ ] U 3+ + H 2 O U(OH) 3+ + H + + e E= pH log[u(oh) 3+ ] / [U 3+ ] UOH 3+ + H 2 O UO H + + e E= pH log[uo 2+ ] / [UOH 3+ ] U H 2 O UO H + + 2e E= pH log[uo 2+ 2 ] / [U 4+ ] UOH 3+ + H 2 O UO H + + 2e E= pH log[uo 2+ 2 ] / [UOH 3+ ] 43

43 E ph Uranium 44

44 E ph Plutonium 45

45 E ph Neptunium 46

46 E ph Americium 47

47 Gas phase chemistry of actinide Liquid media - Solvation (hydration) - Redox behaviour - Hydrolysis - Complexation Solid compounds - Coordination chemistry Gas phase - ElectroSpray Ionisation Mass Spectroscopy (ESI MS)

48 Plutonium chemistry and other actinides in aqueous solutions Part 2b Biphasic System Ph. MOISY CEA/DEN/DMRC ; Marcoule philippe.moisy@cea.fr PAGE 49

49 Actinide behaviour in aqueous media with diphasic system Solid / Liquid Separation - Precipitation - Co-precipitation Liquid / Liquid Extraction - Neutral complexes - Ionic species (cation and anion complexes) Ionic exchange on solid support - Cationic exchange - Anionic exchange

50 Solide/Liquid distibution Coprecipitation - Very useful for heavy actinides (at trace scale): Coprecipitation of 256 Md with LaF 3 Md 3+ Coprecipitation of 256 Md with BaSO 4 (Eu II SO 4 ) in presence of Cr(II) Md 2+ - Interesting for decontamination and for liquid waste treatment: Fe(OH) 3, (Fe(OH) 2, Fe(OH) 3 ), Precipitation - Very useful for actinide dioxide conversion: thermal decomposition of neutral compounds (under air or controled atmopsphere) An III 2(C 2 O 4 ) 3.10 H 2 O, An IV (C 2 O 4 ) 2.6 H 2 O, An(O 2 ) 2, An VI O 2 (O 2 ), M I (1+2x).An V O 2 (CO 3 ) (1+x), An VI O 2 (CO 3 ), 51

51 Two phases distibution: liquid/liquid system Interest - Fundamental research of Actinide behaviour in aqueous media - Reprocessing of Nuclear Fuel General consideration: «One aqueous phase and One organic phase» - Good affinity between An and extractant with hard donnor as oxygen and nitrogen (Pearson model) - An extractant is a ligand (complexant) only soluble in organic phase - Competition between cation and acidity for extractant - Reactivity (extractibility) of An: high for An 4+ ; An 3+ ; AnO 2 2+ but low for AnO Mainly electrostatic interaction but also covalent bonding (selectivity) - Competition between extraction (ligand soluble in organic phase) and complexation in aqueous media

52 Liquid/Liquid extraction: supramolecular organisation of organic phase - «Organic chain» Lipohily (solubility in organic phase «apolar tail» and immiscibility in aqueous phase) - «Polar head» Actinide interaction (water, mineral acid, ) An An Species in Aqueous Phase Interface Species in Organic Phase - Extraction of water and mineral acid: micellisation in organic phase (reversed micelles) - Interfacial structure?

Actinides (f-block) 1-1

Actinides (f-block) 1-1 Actinides (f-block) Actinide Chemistry Speciation Role of Oxidation State Complexation Specific Actinides U, Pu, Am Example: Am and Cm transport at Oak Ridge Use of laboratory data to determine chemical