ISSN 1066-3622, Radiochemistry, 2007, Vol. 49, No. 4, pp. 419! 425. + Pleiades Publishing, Inc., 2007. Original Russian Text + A.B. Khasanova, N.S. Shcherbina, S.N. Kalmykov, Yu.A.Teterin, A.P. Novikov, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 4, pp. 367!372. ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ Sorption of Np(V), Pu(V), and Pu(IV) on Colloids of Fe(III) Oxides and Hydrous Oxides and MnO2 A. B. Khasanovaa, N. S. Shcherbinaa, S. N. Kalmykova, Yu. A. Teterinb, and A. P. Novikovc a Chemistry Faculty, Lomonosov State University, Moscow, Russia b Russian Research Centre Kurchatov Institute, Moscow, Russia c Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia Received March 20, 2006 Abstract-Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (a-FeOOH), hematite a g ( -Fe2O3), maghemite ( -Fe2O3), and amorphous MnO2 was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced a a g upon sorption on -FeOOH, but not on -Fe2O3 and -Fe2O3. On the MnO2 surface, Pu occurs as Pu(VI). + From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O2 and + Pu(V)O2 with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with a-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order K > K= > MnO2 -FeOOH K= ~ KC . -Fe2O3 -Fe2O3 PACS numbers: 68.43.-h, 82.70.Dd, 89.60.Ec DOI: 10.1134/S1066362207040170 The modern approaches to long-term safety assess- demonstrated by the methods of micro- and ultrafiltra- ment of radioactive waste (RW) repositories in surface tion that Pu and Am are associated with colloid par- and underground geological formations are based on ticles 253 450 and <2 nm in size, respectively. How- data on the radionuclide sorption on components of ever, no information on the nature of these colloids engineered and geochemical barriers (rocks and min- was reported. erals), and also on the solubility of radionuclides in Naturally occurring minerals differ considerably in underground waters. Repositories for RW should meet their sorption capacity for cations, and the available the criterion that radionuclides from RW must not data on the preferential sorption of radionuclides on enter the human habitation zone in tens and hundreds colloid particles of various kinds are contradictory. thousand years. According to most of the existing Duff et al. [9] studied the Pu sorption on tuff with in- estimates, tri- and tetravalent actinides and Tc(IV) re- clusions of zeolite, smectite, and Fe and Mn oxides by present no hazard, as having strong tendency to sorp- various microanalytical methods. They have demon- tion on components of engineered barriers and host strated that Pu is sorbed on smectites and Mn oxides rocks and also as having extremely low solubility. only, and in the latter case, it is oxidized to Pu(VI). However, recent data suggest the possibility of col- loidal migration of these radionuclides in groundwater A great deal of work has been made to model [13 6]. Kersting et al. [7] reported that, at the test site radionuclide sorption on various minerals, including in Nevada, 239Pu was carried with smectite colloid to oxides and hydrous oxides of Fe [10 312] and Mn a distance of 1.3 km in 30 years. Penrose et al. [8] [13315], clay [16], calcite [15], etc. Most authors em- have studied the behavior of Pu and Am in the Mor- ploy the distribution coefficient Kd as a parameter tandad canyon (Los Alamos National Laboratory) characterizing the sorption. However, the use of this and found that these radionuclides have migrated with parameter is limited to specific experimental condi- colloid particles to a distance of 3390 m from the tions (pH, complexation, etc.). An alternative ap- source. By the way, the theoretical estimates for proach to description of the sorption involves calcula- the migration rates of these radionuclides suggested tion of the equilibrium constants of one or another that their path is limited to several meters. It was sorption reaction occurring on the surface of colloid 419 420 KHASANOVA et al. Table 1. Characteristics of colloid samples ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ ³ a ³ a ³ g ³ Parameter -FeOOH -Fe2O3 -Fe2O3 MnO2 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄ Total specific surface area, m2/g* ³ 41.7 ³ 6.4 ³ 20.4 ³ 182 Specific micropore surface area, m2/g* ³ 7.23 ³ 3.02 ³ 9.8 ³ 32.9 Mean particle size, mm** ³ 0.30 + 0.05 ³ 0.35 + 0.05 ³ 3 ³ 3 pH(IEP) *** ³ 8.8 ³ 9.4 ³ 8.9 ³ 1.5 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄ * From BET analysis. ** From SEM data. *** From potentiometric titration data. particles. From the set of the equilibrium constants of was dissolved in deionized water, concentrated HCl all possible reactions occurring both in solution and was added to pH 1.5, and the solution was heated to on the surface of colloid particles, one may estimate 100oC. The resulting hot solution was diluted with Kd for any system. the estimated volume of hot deionized water, placed This study is aimed to examine the sorption of in a Teflon vessel, and heated in a pressure vessel at 3 o Np(V), Pu(V), and Pu(IV) on various Fe(III) oxides 90 100 C for a week. After cooling, the suspension and hydrous oxides and MnO , to determine the was separated by centrifugation, multiply washed with 2 o valence states of the actinides, and to estimate the double-distilled water, and air-dried at 80 C. g equilibrium constants for sorption reactions. Colloidal Colloidal maghemite ( -Fe2O3) was synthesized by iron and manganese oxides and hydrous oxides may the method described in [21]. A weighed portion of . be formed under conditions of both near-by and dis- FeCl3 6H2O was dissolved in deionized water, 2.5 M tant zones of RW repositories, representing a factor NaOH was added, and the mixture was heated at 70oC strongly influencing the behavior of radionuclides. for 2 days. Then the mixture was heated in a pressure a o Furthermore, such iron oxides as hematite ( -Fe2O3) vessel at 350 C for 1 h. The suspension was separated a and magnetite ( -Fe3O4) can be formed in contour by centrifugation, multiply washed with double-dis- water of nuclear reactors under the action of water tilled water, and dried. radiolysis products [17]. Transformation of magnetite Colloidal manganese dioxide was synthesized by to hematite and sorption of radionuclides are the the method described in [22]. To a solution of KMnO factors determining to a considerable extent the deci- 4 in NaOH, MnCl was added stepwise with magnetic sion on the methods for contour water management. 2 stirring. The resulting suspension was settled for 24 h, As mentioned above, a great body of information is filtered off, multiply washed with deionized water, gained on the radionuclide sorption on Fe and Mn and air-dried. oxides and hydrous oxides. However, data on the equilibrium constants of sorption reactions are either The samples were characterized by X-ray diffrac- lacking at all or contradictory [9, 14, 16, 18]. tion (XRD), scanning electron microscopy (SEM), and potentiometric titration [21]. The BET surface EXPERIMENTAL area was determined from the nitrogen adsorption. No foreign phases were found in Fe-containing samples. Preparation and Characterization of Colloid Particles Manganese dioxide samples are amorphous. Charac- of Fe(III) and Mn(IV) Oxides and Hydrous Oxides teristics of the samples are summarized in Table 1. Colloidal goethite (a-FeOOH) was synthesized by Aqueous solutions for the sorption experiments the procedure described in [19]. A weighed portion of were prepared with deionized water (Milli-Q) treated . Fe(NO3)3 9H2O was dissolved in deionized water, to remove CO2 by purging ultrapure grade N2. Solu- 2.5 M KOH was added to pH 12, and the reaction tion pH values were measured with a combined glass mixture was vigorously stirred for 24 h at 60oC. electrode [Mettler Toledo InLab (Switzerland)]. The resulting suspension was dialyzed until the total nitrate concentration in the outer solution was 34 Preparation and Purification of Pu(IV), Pu(V), <10 M. Finally, the colloid suspension was multiply and Np(V) washed with acetone and dried at 40oC for 48 h. a Colloidal hematite ( -Fe2O3) was synthesized ac- Since the Pu sorption experiments were planned to . cording to [20]. A weighed portion of FeCl3 6H2O carry out at various total Pu concentrations, Pu iso- RADIOCHEMISTRY Vol. 49 No. 4 2007 SORPTION OF Np(V), Pu(V), AND Pu(IV) ON COLLOIDS 421 topes of different specific activity were used (242Pu, 239 238 An + colloid Pu, and Pu). The radioactivity of solutions was 2 measured using liquid scintillation spectrometry Dilution (1 M HClO4,pH1.5) (TriCarb 2700TR, Packard Ind.). 2 7 Filtration 7 Plutonium in the oxidation state 4+ was stabilized 2 2 Dilution (0.6 M HNO ). Dilution (1.0 M HCl). by adding a small amount of NaNO2 to the stock 3 nitric acid solution [23] and was monitored by the Extraction (0.5 M TTA Extraction (0.05 M HDEHP UV spectra (Shimadzu UV-3100). in cyclohexane) in heptane) 2222 Solutions of Pu(V) were prepared by electrolytic Aqueous Organic Aqueous Organic method [23]. The completeness of Pu(IV) oxidation to phase: phase: phase: phase: Pu(V) was controlled by the UV spectra. Just prior to An(V), An(VI) An(IV) An(V) An(IV), An(VI) the sorption experiments, the Pu(V) solution was treated with 0.05 M HDEHP (Merck) in heptane to Fig.