We describe concentration changes caused by chemical and alpha-induced radiolytic reactions in various oxidation state pure solutions of Pu(VI), Pu(V), or Pu(IV) colloid or mixtures of these oxidation states at pH values > 1 for a period of nearly two years. The rates of approach to steady-states and the resulting experimental concentration quotient values were determined in order to find the conditions under which equilibrium in 2PuO{sub 2}{sup +} + PuO{sub 2}{sup 2+} + PuO/sub 2(coll)/ reaction might be attained and to learn about the underlying reactions. Computer calculations were used to compare the data with the results required from proposed reaction schemes

Hobart, D. E.

Newton, T. W.

Palmer, P. D.

UNT Digital Library

LA-UR -85-3111.cohif-w147--/LM Alamos BMtIOnU LaDOrmtOrVIS opwmod by MO Unwwslly o! CSlllOfnl~ fw lM UnMd Slmtoa DOPWImMt of Energy under conlrmcl W-7405 -EN9.MLA-UR--85-3111DE85 017524TITLE The Formation of Plutonium(IV) Colloid by theALPHA-RF9UCTION of Aqueous Solutions of Pu(V)and ~u(VI).AUTHOR(S): D. E. Hobart, T. W. Newton, and P, D. PalmerSUBMITTEDTO Proceedings ACTINIDES ’85 ConferenceAix-en-Provence, FranceDISCLAIMERThbrapor! wuprwpmd nsnn nccountofworknpontoredbymrmSencyofthe UnitcdStntaOovurnment Nellkrlhe Unitd Sl~luWernment nurmiym~ncy lherd, noranyof theiremployeus,mokmmrywnrrnnty,expreuor Implbd, or mum= ●ry bgmlIbbillty or ruspnsi-billty for Wmxuracy,mrmplotenou,or IMulnuuof nnyInfommtlom,a~rmtum, product,or~ dl~l~. w mpromnlsIhmt1!sUM wouldnot infringeprlvmelyownodrlshls. Refer-ow homhr10●ry -wMIc commercblproduct,p~, or Mrvlw by Irade name,trnderwk,mtnufschmor,or rrlkwiw doa not necouarilycamtilute or Imply IM endorwmenl,rearm.mendatkcm,or fworlng by ttm United SImI- Oovamrnantor ●ny Wncy tharecf, The VleWId oplnbtu uf nulhon exprossodherein do not ~rlly daIe or rdloct Ihaa of theUnhodSlw4 QOwmnwnlor MIy qency Ihord.By acceplanc c o! lh,~ wIIcla, Ihn pulJl!Ihar !Kogn!mo Ihal Il?o U S Oovnrnmonl rolcms c nononclumlw ,oyally.lrea IlcOnC. to publlon of r.produc~Ilw pbbl,shad Iofm of ‘hw Conlf, buhon of b allow olnmn 10 do 80, Iof U S Qovwnnwnl Durposwrhc Log Almmos Nwonnl L~bor8tory fmowm Ihml ma publww ,denllty mm ●lIcle ● worh per fofmad undw Iho wapticm 01 lh@ U S Dopsrtm@nl 01 EnwQy—~~~~b~~~ LosAlamos,Ne.Mexico87545Los Alamos National Laboratory.~For publication in the Proceedings of the ACTIN;DES’85 Conference, Aix-en-Provence,France, September 1-6, 1985.]THE FOikMAT?ON OF PLUTONIUM COLLOID BY 3=THE ALPHA-REi~UCTION OF AQUEOUS SOLUTIONSOF Pu(V) AND pu(V~)*/D. E. Hobart, T. W. Newton. and P. D. PalmerIsotope and Nuclear Chemistry DivisionLos Alamos National LaboratoryLo.: Alamos, New Mexico 87545 USA‘ Re~,earch sponsored by the Division of Chemical Scicncea, Office of Baoic Energysc lenccs, 11.S. Depivtmellt of Energy and the Nevada Nuclear Waste Storage Investi-gf,tions (N31WSI).THE FORMATION OF PLUTONIUM COLLOID BY THETHE ALPHA-REDUCTION OF AQUEOUS SOLU~IONSOF Pu(V) AND Pu(VI)D. E. Hobart, T. W. Newton, and P. D. PalmerIsotope and Nuclear Chemistry DivisionLos A Iamos National LaboratoryLos Alamos, New Mexico 87545 USA1NTRODUCTIO!NThe mast common isotope of plutonium, 23*Pu, emits alpha particles with111For pluto-an average energy of 5.15 Me\: and has a half life of 24,36CI y~ars.nium in solution, this self-irradiation has important effects in any long term ex-periment, The:te effects, for acid solutions, have been reviewed elsewhere.12’31 Thealpha- radiolysis of water produces H202 and the radicals H, OH, and H02, all ofwhich are capable of reacting witn the various oxidation states of plutonium. inperch loric acid solut ions 23{’Pu(VI) is reduced to Pu(V) at a rate of about ].s% ofthe total plutonium concentration per day. The Pu(V) formed disappears by dis-proporticmat ion; there is no evidence that it is reduced by the radiolysis products,141Only fragmentary results on alpha-particle-ind oced reactions of plutonium havebeen reported for solutions with pH values near 2.151Further work is needed becausethis is the region in which the most reliable determination of the solut)ility of PU02has been carie(.1 out.lc: In addition, the effects of alpha radiation in this pH regionare expected to differ from those in more acidic solutions because Pu(V) is muchmore stable and the Pu(l\’) colloid forms relatively rapidly from Pa, ~7-9~Thus ~he overall reaction2PUO: -- Puo:’ + Puo~ (,.,]) (1)and possibly its reverse may be vxpected to be important in describing alpha-particleeflects under conditions where colloidal Pu(IV) can form,The equilibrium for reaction (1) i~ given by[Puo;”]——-Q’=p,,q]2 (2)and is relatml to the volubility product, [Pu4+]/OH’ \’, for colloidal Pu(IV) by1.way of the disproportionatim equilibrium2PuO~ + 4H+ — PU4+ + I%@;+ + 2H20 (4)Log Q3 for reaction (3) is calculated to be 4.290, haed on the formal potentials11~1and 1.893 at zero ionic strength, based on the ionic strengthin 1 M HC104dependence found for the analogous uranium reaction.{’ 1We shall define Q’ es an experimental concentra! ion quo( ient having the formof equation (2) but not necessarily reflecting an equilibrium condition. Severalprevious reports contain data that can be ~sed to calculate Q’, for reaction (1).Due to the slowness of the reaction and possible radiation effects, it is not clear thatequilibrium was reached in these experiments. The results of these calculations aresum]narized in Table 1. The values calculated for log Q’ range from 1.41 to 4.10.TABLE IL.*EQ’ ValueF Calculated from Literat urc DataTotal Pu TimeP [’one,(M) (days) pH log Q’ Author \Ref. ~—. -1 1.53 Y 10-~ 4.2 1.28”8.312.$18.8-() !!, (J2 ~ lo :1 0.5 I,0’0.625012 9,4 k 10- g 0,5 122”0.6?50,1 9,3 h 10-~ 0.5 A,4(Y’0625(i k ]()-” 83 1.01.21451 no2.002.:u-l2803,103,4010 3.7II1:$1$’2010 (),04’1R&i /():3.75 Kamha 112~3.803.83387256 (’o~lanso2.54 ●1 al., 17~2.882.85Ii>4;* 4“2!1H I4.193nn3350i.m4103832 ,,3.75:~,?~141’ Ms4ir1,56 ?1 al., 11311.781.97230dI2.In this paper we describe concentration changes caused by chemical and alpha-induced rac!iolytic reactions in various oxidation state pure solutions of Pu (VI),Pu(V), or Pu(lV) colloid or mixtures of these oxidation states at pH values > ]for a period of nearly two years. The rates of approach to steady-states and theresulting Q’ values were determined in order to find the crmditims under whichequilibrium in reaction (1) might be attained and to learn about the underlyingreactions. Computer calculations were used to compare the data with the r=ultsrequired from proposed reaction schemes.EXPERIMENTALSOLUTIONSPlutonium(lll) solutionsreduction of higher oxidationwere prepared by acid dissolution of Pu metal or bystates of Pu with zinc amalgam. Plutonium(VI) S*lutions were prepared by perch loric acid fuming or electrolytic oxidation of loweroxidation states. Rcw-colored Pu(V) solutions were made by electrolytic reductionof Pu(V1) at a platinum screen cathode (+ 0.79 \’/NHE, pH > 2). Pu(IV) solu-tions were prepared by mixing stoichiometric amounts of Pu(\’1) and Pu(I1l) c:by electrolytic oxidation of Pu(lll) at a platinum anode (- ].] V,/NHE, pH < O).Colloidal SOISof Pu(l\’) were made by d]lution of concentrated solutions of Pu(ll’)in 3 M HCI to < 0.01 M HCI. These solutions were left standing for two weeks atroom temperature followed by purificat ion by ion exchange or centrifugal ion.ANALYSISThe \“arious oxidation states of plutonium were determined spectrophatomet. ri-cally, Pu(\’1) was determined in HC104 solution b) using the sharp peak near 83(]nm, Pu(l”) was determined from the increase of the Pu(I’1) peak after addition ofexcess Ce(I\” J perch iorate. The Pu(IV) colloid reacts quitr slowly with Ce(l\’) soit does not interfere with the Pu(V) analysis,The presence of colloid was indicated b~l the characteristic green color. ? hiswas confirmed spectrophotometrica lly for some samples.INSTRUMENTATIONA Cary Model 17D spectrophotom~ter was used for spectral analysis. Ekc-trochemical procedures were performed using an E, G.&G, PARC Model 173/]79Potentiostat ‘Coulometer. Values for pH were determined using a Corning combi-nation pH ek-trode and either a Corning Model 130 or a Beckman Model 4500 pHMeter.PROCEDURESOxidation state pure solutions or appropriate mixtures were made volumet-rically and stored At room temperature. Periodically, samples were removed foranalysis as described above.3RESULTS AND DISCUSS1ONThequalitative behavior of the plutonium solutions isconvenient]y shownonternary plots, since Pu(VJ), Pu(V), and Pu(IV)-colloid are the principal substancespr-ent. No evidence for the presence of Pu(III) was found in these solutions. Theresults of typical runs are shown in Figures 1, 2, and 3. Initially pure solutions ofPu(l’1) form Pu(V) at first and then form the colloid. Initially pure solutions ofPu(V) form both Pu(VI] and Pu(lV)-colloid with increasing concentrations of thecolloid as time goes on. Suspensions of colloidal Pu(lV) form small concentrationsof Pu(”tiI) and PIJ(~). After sufficient time, all the mixtures appear to becomepredominantly Pu(lV)-colloid.%tV)Figure 1. Ternary compo~itlon plot of ini-tially pure Pu(V) solutions.l%(v)//,// _ A %(IV)cowFigure 2. Ternuy composition plot of ini-tially pure Pu(\’1) solution~,con.Pu(vl)Pu(v)AAFiguret, Ternary composition pIot of ini-tin!ly pure Pu(l\”) colloid solutions ●nd vari-ous rtlixt~ws,Jyti.(iv)cOm,.The overall ●ffect of alpha-irradiation is to change the average oxidation state,6%, defined as31Pu(111)] + 4; P11(1v)] + 51Pu(V)] + 6! Pu(V1)}03 =Total[Pu](5)Figures 4 and 5 show typical = versus time data. for various runs.Because of the importance of the equilibrium in reaction (l), it is of interest toexamine the values of Q’, the experimental concentration quotient, versus time. Atypical plot is shown in Figure 6. For many of the runs, the values of log Q’ are closeto 4 after long time periods. Additional information is displayed in concentrationprofile plots (Figures 7 and 8) of Pu(VI) and Pu(V) versus time.A careful and exhaustive consideration of the data has not resulted in a con-vincing reaction scheme with a consistent set of rate parameters. However a modelthat approximates the observed behavior would be helpful in understanding thevery complicated system involved, including the significance of the steady-state logQ’ values, The model is described briefly below. The following assumptions aremade: (a) alpha particles react with water to form hydrogen peroxide or radicalsthat are rapidi~ :unsumed in the following reactions; (b) only a single reactive in-termediate is important; and (c) this intermediate reacts with other species in thesolution as an oxidizing agent as wei’ -s a reducing agent. The pro~’ision~l modelconsists of the following reactions and rate terms:H20 = R (5)R + Pu(VI) = pu(~) + R’ (6)2PU(V) = PU(VI) + PU(IV)(.,)ll) (7)R + Pu(\’) = PV(IV)(CC,II) + R’ (8)2R + PU(l~)(COll) = Pu(vl) + 2R’ (9)Equation no. Rate Expression.——-. —.(5) o(\Pu(vl); +! Pu(v)}) + clc[coll)(6) k(i; R;i Pu(\’ l)\(7) k7!Pu(V)]2j{’ !-k_7[Pu(VI)][H+]Iralll) :coll::f%l(vf)l(8)(9) kg~”R]\CoH”;— —-—. —..-— . ..— ——... ..—.—-— ——————where:R standm for hydrogen p~roxide or a radical,R’ ie the appropriate reaction product.[Coil] stands for the concentration of colloidal Pu(IV) in mole~ of plutonium perliter.5.T’his reaction model and modiftcatiortsof it were investigated by making theusual steady-state approximation for the concentration of R and solving the result-ing rate equations for the concentrations of Pu(V1) and Pu(V) versus time using the14: The calculated plots are shown as dashed lines in FiguresRunge-Kutta method’m 7 awd 8.L,oo_— -. ,— ..1.. ._—– Jlb m =0IM 10s,.)Figure 4. Average oxidation state vcrr.ustime plots for initially pure Pu(V1) and forPu(Vl)-Pu(V) mixtures.Figure 5. Average oxida(ion slate versustime plots for initially pure Pu(lV) colloid so-Iulions.1 I I 1 r 1 I/c1so 1co 240 Wo 400 480 680 040TME (Owd .Figure 6, Log Q’ vermus time for ● number of variou? plutonium solutions,69— R---● .6? -m----- ---o- —— A.—_o PmFigur~ 7. Pu(VJ) and Fu[V) concentra-tion versus tinle plots for an initially pure Pu(V])Po]ulion,?!“ 00 L.——.—..—L.. . .0 VW *WFigure 8. Pu(VI) and Pu(V) concentra-tion versus lime plots [or a Pu(Vl)-Pu(V] mix-ture.CONCLUSIONSThe following is a brief summary of the conclusions resulting from this study.The simple computer model. although not completely satisfactory, does approxi-mate the experimental data in a qualitative manner with regards to concentrationprofiles, 6x, and Q’. Any value for log Q’ greater than about 3 is consistent withthe data. Stead;’-state concentrations, which may be far from equilibrium, dependon total plutonium concentration, Some of the data suggest that the kinetic prop-erties of the colloid formed are not constant, but depend on such factors as the ageof the colloid and the concentration at which it was formed. Our work with rolloidsuspensions shows that conventional long term volubility experiments with PUOZwill not reach equilibrium because of alpha self irradiation. A complete report afthis work wilt be published elsewhere.1’5;.4CKNOVt’LEDGE.MENTSThis research is sponsored by the Division of Chemical Sciences, Office of BasicEnergy Scier,ccs, U.S. Department of Energy and the I+levada Nuclear Waste Storage(’nvestigtations (N NWSI), The authors extend their appreciation to G. G. Miller forexpertise and technical amlstance in the computer generation of this manuscriptand to A. Gonzales, C. Baca, and others of E. G.& G. Los Alamos for expertise ingraphic arts and illustrations.7.REFERENCES11j F. W. ~’a]ker, G. J. Kjrou~, and F. hl. Rourke, Chart Of the ~uckks, Twelfth‘ Ed., Knolls Atomic Power Laboratory, U.S. Department of Energy (]977)[2] J. M. Cleveland, The Chemistry of p/utonium, Gordon and Breach, New York,1970, pp. 35-39.131 T. W. Newton, The Kinetics of the Oxhfat.ion Reduction Reactions of Uranium,INeptunium, Plutonium and Americium in Aqueous Solutions, U.S. EnergyResearch and Development Administration Report TID-26505 (1975), pp. 55-57.14] S. W. Rabideau, J. Am. Chem. Sot., 79,6350 (1957).~5\ G. E. Moore, Clinton Laboratories Report No. CL-P-427, June 1945./6; D. Rai. Radiochim. Acts, 35, 97 (1984)[7] D. A. Costanzo, R. E. Biggers, and J. T. Bell, J. lnorg. IVIJCI. Chem,, 35.609(1973).[8/ L. M. Toth, H. A, Friedman, and M. M. Osborne, J. lnorg. A’ucl. Chem., 43,2929 (1981).[9j T. W. Newton and V. L. Rundberg, Disproportjonation and Polymerization ofPlutonium(IV) in Dilute Aqueous Solution, ,Mat. Rec. Sot. Symp. Proc., Vol.26, Elscvier Science Pub]. (1984).~10j J. Fuger and F. L. Oetting, The Chemical ~hermocl~namics of Actinide E/-emenis and Compounds. Part 2. The .4c(inide Aqueous Ions, InternationalAtomic Energy Agency, Vienna, 1976.[Ill F. Nelson and K. A. Krause, J. Am. Chern. Sot., 73,2157 (1951).]12] M. Kasha, Reactions Between Plutonium Ions in Perchloric Acid Solution:Rates, hfechanisms, and Equilibria, in The Transuranium Elements, Part 1, G.T. Seaborg , J. J. Katz, and W. M. Manning, Eds., McGraw-Hill, New York,1949, p. 295.113] C. Madic, G. M. Begun, D. E, Hobart, and R. L. Hahn, lnorg. Chem,, 23,1914 (1984),~14] H. Margenau and G. M. Murphy, The Mathematics of Physics and Chemistry,D. Van Nostrand Co., New York, 1943.[15] T. W. l$ewton, D. E. Hobart, and P. D. Palmer, The Formation of Pluto-nium(ll’) Colloid by the Alpha-Recfuction of Aqueous Solutions of Pu(V) andPu(VI) (to be published).

Formation of plutonium(IV) colloid by the alpha-reduction of aqueous solutions of Pu(V) and Pu(VI)

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Formation of plutonium(IV) colloid by the alpha-reduction of aqueous solutions of Pu(V) and Pu(VI)

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