Mechanism of Am(III) oxidation with ozone in carbonate solutions

The reaction of ozone with Am(III) in bicarbonate and carbonate solutions was studied by spectrophotometry. On adding ozone-saturated water to a 2 × 10⁻⁴ M Am(III) solution in 1 M NaHCO₃, about 1/3 of Am remains in the trivalent state and 2/3 is converted to Am(V), with no accumulation of Am(IV). The reaction of Am(III) with ozone involves replacement of H₂O molecules in the coordination sphere of Am(III) by O₃ molecule, followed by elimination of the O₂ molecule. The third O atom remains bonded with Am, converting it to the pentavalent state.     

Specific features of reactions of Am(V) with reductants in weakly acidic solutions

In EDTA solutions with pH ??5 at 25°C, Am(V) in a concentration of 5 × 10^?4?3 × 10^?3 M slowly transforms into Am(III). The Am(V) reduction and Am(III) accumulation follow the zero-order rate law. In the range 60?C80°C, the reaction is faster. In some cases, an induction period is observed, disappearing in acetate buffer solutions. In the range pH 3?C7, the rate somewhat increases with pH. In an acetate buffer solution, an increase in [EDTA] accelerates the process. The activation energy is 47 kJ mol^?1. Zero reaction order with respect to [Am(V)] is observed in solutions of ascorbic and tartaric acids, of Li₂SO₃ (pH > 3), and of hydrazine. The process starts with the reaction of Am(V) with the reductant. The Am(III) ion formed in the reaction is in the excited state, *Am(III). On collision of *Am(III) with Am(V), the excitation is transferred to Am(V), and it reacts with the reductant: *Am(V) + reductant ?? Am(IV) + R₁ and then Am(IV) + reductant ?? *Am(III) + R₁, Am(V) + R₁ ?? Am(IV) + R₂. A branched chain reaction arises. The decay of radicals in side reactions keeps the system in the steady state; therefore, zero reaction order is observed.     

Redox Behavior of Am(IV) and Methods of Its Preparation in Alkaline Media

A scheme was suggested for Am(OH)₄ isolation by treatment of Am(OH)₃ suspension in 0.1–1.0 M NaOH with ozone (3.5–5 vol %)-oxygen mixture (4–5 l h⁻¹ flow rate) at 20°C for 40 min, followed by ultrasonic treatment of the resulting Am(VI) (44 kHz, 1 W cm⁻³) for 45 min. The separated precipitate of Am(III) hydroxy peroxide was treated with 1–2 mlof 7–10 M NaOH to form Am(OH)₄. Mixing suspensions of equivalent amounts of Am(III) and Am(V) hydroxides in NaOH also gives Am(IV) hydroxide in a >98% yield. The reproportionation Am(III) + Am(V) = 2Am(IV) in 1 M NaOH starts on heating above 70°C, whereas at NaOH concentration higher than 7 M it is completed even at room temperature. The reaction of Am(III) with Am(VI) in alkaline solutions, Am(VI) + 2Am(III) → 3Am(IV), occurs during mixing the reactants. The equilibrium reaction of Am(OH)3 with [Fe(CN)₆]³⁻ in alkaline solutions was studied. It was shown that increasing the alkali concentration to 2 M NaOH promotes formation of Am(OH)₄. At further increase in the alkali concentration, Am(V) is formed.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 234–238.Original Russian Text Copyright © 2005 by Nikonov, Gogolev, Tananaev, Myasoedov, Clark.     

Reduction of Am(IV) with Water in KHCO3 + K2CO3 Solutions

Reduction of Am(IV) with water in KHCO₃, K₂CO₃ solutions (pH 8.5-10.5) was studiedspectrophotometrically at 54-70°C. The Am(IV) concentration decreases, following the first-order rate law.The reduction rate increases with pH (logk/pH = 0.4), but decreases with increase in (HCO₃ ^- + CO₃ ^{2 -}) concentration. It was assumed that the thermally excited Am(IV) ion forms a dimer with unexcited Am(IV). The dimer dissociates into two Am(III) ions and H₂O₂. Hydrogen peroxide reduces two more Am(IV) ions. In this process, the excited Am(III) ion appears, which transfers the excitation to Am(IV) at collision. Thus, a chain process is initiated. This scheme can also explain the kinetics of Am(VI) and Np(VII) reduction in carbonate solutions.     

Electrochemical Oxidation of Am(III) Ions in HNO3 Solutions

Formal oxidation potentials (E0 ^p) of the Am(IV)/Am(III) couple were measured and the kinetics of electrochemical oxidation of Am(III) on platinum electrode in concentrated solutions of nitric acid (1-6 M) containing potassium phosphotungstate K_{1 0}P₂W_{1 7}O_{6 1} (KPW) was studied. The formal potential E0 ^p only slightly depends on the concentration of HNO₃ and is shifted toward the negative region by 1.0 V as compared with the standard values. The extent of Am(III) oxidation increases with increasing KPW concentration and decreasing concentration of nitric acid. Electrochemical oxidation of Am(III) is accompanied by radiochemical reduction of Am(IV) and is described by the equation -dC _{A m ( I I I )}/dt = (k + k ₁)C _Am(III) - k ₁ C ₀ - k ₀, where k is the apparent rate constant of electrochemical oxidation of Am(III), k ₁ is the apparent rate constant of Am(IV) reduction, and k ₀ is the constant of radiation-chemical reduction of Am(IV).     

Sorption-barrier properties of granitoids and andesite-basaltic metavolcanites with respect to Am(III) and Pu(IV): 2. Absorption of Am and Pu from groundwater on crushed samples of granitoids and andesite-basaltic metavolcanites and also on their rock-forming minerals

A model study is made of the sorption-barrier properties of crushed samples of granitoids and andesite-basaltic metavolcanites with respect to Am(III) and Pu(IV). In sorption from simulated groundwater (pH 8.3), the volume distribution coefficient K _d of Am was determined to be (0.8–1.6) × 10³ and (3.4–7.0) × 10³ cm³ g^?1, and that of Pu, (0.5–1.7) × 10³ and (1.0–10.0) × 10² cm³ g^?1 for metavolcanites and granitoids, respectively, suggesting good sorption-barrier properties of these rocks. The sorption power of the basic rock-forming minerals of granitoids decreases in the order biotite > feldspar > quartz. The results obtained in this study can be used as input data in predicting the rates of Am and Pu migration with groundwater.     

Use of ozone for dissolving high-level plutonium dioxide in nitric acid in the presence of Am(V,VI) ions

Plutonium dioxide recovered in the course of reprocessing of SNF from WWER reactors (so-called high-level PuO₂) was subjected to dissolution in 0.6–3.0 M HNO₃ in the presence of Am(III) ions under ozonation with an ozone-oxygen mixture containing 30–180 mg l⁻¹ O₃. Measurements of the rate of the PuO₂ dissolution in 3 M HNO₃ in the temperature interval from 30 to 80°C showed that, with an increase in the ozone concentration in the ozone-oxygen mixture from 30 to 180 mg l⁻¹, the dissolution rate increases by a factor of 4–5. The acceleration of the PuO₂ dissolution is attributed to the formation of Am(V,VI) by homogeneous oxidation of Am(III) ions with ozone dissolved in HNO₃. The Am dioxocations formed act as PuO₂ oxidants and are continuously regenerated by the oxidation of Am(III) with ozone. This assumption is confirmed by an additional increase in the dissolution rate, observed on introducing Am(III) into the initial electrolyte for the PuO₂ dissolution.     

Behavior of Am(V) in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The Am(V) disproportionation in (2–10) × 10^?3 M K₁₀P₂W₁₇O₆₁ (KPW) solutions in 1–7 M HNO₃ was studied spectrophotometrically. Under the experimental conditions, Am(VI) and Am(III) are the final products of Am(V) transformation; the process is described by the rate equation of a reversible reaction. The direct reaction follows the first-order rate equation with respect to Am(V) concentration, and the reverse reaction, the zero-order equation.     

Mechanism of cerium(III) oxidation with ozone in sulfuric acid solutions

The kinetics of Ce(III) oxidation with ozone in 0.1–3.2 M H₂SO₄ solutions was studied by spectrophotometry. The reaction follows the first-order rate law with respect to each reactant. The rate constant k slightly increases with an increase in the acid concentration, which is associated with an increase in the O₃/O ₃ ^? oxidation potential. The activation energy in the range 17–35°C is 46 kJ mol^?1. With excess Ce(III), the stoichiometric coefficient Δ[Ce(IV)]/Δ[O₃] increases from 1.6 to 2 in going from 0.1 to 1–3.2 M H₂SO₄. The extent of the Ce(III) oxidation is 78% in 0.1 M H₂SO₄ and reaches 82% in 1 M H₂SO₄. The ozonation involves the reactions Ce(III) + O₃ → Ce(IV) + O ₃ ^? , O ₃ ^? + H⁺ → HO₃, HO₃ → OH + O₂, OH + HSO ₄ ^? → H₂O + SO ₄ ^? , OH + Ce(III) → OH^? + Ce(IV), and SO ₄ ^? + Ce(III) → SO_4/2? + Ce(IV). Low stoichiometric coefficient of the Ce(III) oxidation is associated with the hydrolysis of Ce(IV). The excited Ce(IV) ion arising from oxidation of Ce(III) with OH radical forms with the hydrolyzed Ce(IV) ion a dimer whose decomposition yields Ce(III) and H₂O₂. After the ozonation termination, Ce(IV) is relatively stable in sulfuric acid solution, with only 5–7% of Ce(IV) disappearing in 24 h.     

Sorption-barrier properties of granitoids and andesite-basaltic metavolcanites with respect to Am(III) and Pu(IV): 1. Absorption of Am and Pu from groundwater on monolithic samples of granitoids and andesite-basaltic metavolcanites

A model study is made of the sorption-barrier properties of intact monolithic samples of granitoids and andesite-basaltic metavolcanites with respect to Am(III) and Pu(IV). In sorption from simulated groundwater (pH 8.3), the surface distribution coefficient in surface sorption K _a was determined to be 8–37 and 4–80 cm for Am and Pu, respectively. The mineral components of the rocks responsible for the radionuclide sorption were identified by autoradiography. The rocks tested are characterized by high retention capacity for Am and Pu.     

Effect of ionic liquids on the extraction of Am from HNO3 solutions with diphenyl(dibutylcarbamoylmethyl)phosphine oxide in dichloroethane

Addition of small (1?C5 vol %) amounts of ionic liquids (ILs) increases the Am(III) distribution ratio in the system with diphenyl(dibutylcarbamoylmethyl)phosphine oxide (Ph₂Bu₂) by more than 2 orders of magnitude. With 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄mim]⁺Tf₂N^?) used as IL, the Am(III) extraction is possible even from 8 M HNO₃, which is important in radiochemical analysis of process and environmental samples, because many procedures are based on the transfer of solid samples into 8 M HNO₃. The extraction data show that the extractable Am(III) complex contains three Ph₂Bu₂ ligands, an IL anion, and two NO ₃ ^? anions.     

Speciation of Cr(III) radionuclides in solutions

The speciation of Cr(III) radionuclides in solutions within the concentration range 10^?6–10^?2 M at pH 1–12 was studied. Adsorption on the glass surface, ion exchange, migration in the electric field, centrifugation, dialysis, and ultrafiltration were used. It was found that, at a concentration of 10^?6 M at pH 1.0–4.4, Cr(III) exists in the solution as a monomeric species, and at pH > 4 it exhibits colloidal properties. At pH 1.0–3.4, Cr(III) exists in form of hydrated cations Cr(H₂O) ₃₊ ⁶ . At pH > 3.4, they are hydrolysed with formation of mononuclear hydroxo complexes Cr(OH)²⁺ [K _h (3.35 ⊥ 0.15) × 10^?5]. At the Cr(III) concentration exceeding 1 × 10^?4 M, binuclear hydroxo complexes Cr₂(OH)_q are formed.     

Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe₂O₃, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe₂O₃ was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.     

Catalytic decomposition of organic anions in alkaline radioactive waste: II. oxidation of N-(2-Hydroxyethyl)ethylenediaminetriacetate

Decomposition of N-(2-hydroxyethyl)ethylenediam inetriacetate (HEDTA) in alkaline solutions containing H₂O₂, S₂O ₈ ^2? , and ClO^? and under the action of ozone was studied titrimetrically. HEDTA decomposes in the presence of cobalt or copper salts at stepwise addition of H₂O₂. In the reaction between HEDTA and persulfate ion, an induction period was observed. It shortens with increasing alkali concentration, or temperature, or with decreasing the initial HEDTA concentration. Nitrite ion does not affect the complexone decomposition. The process is initiated by thermal dissociation of persulfate ions into radical ions followed by chain reaction. Hypochlorite ions effectively decompose HEDTA. Porducts of HEDTA decomposition are oxidized by ozone and persulfate mainly to oxalate and carbonate.     

Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

Extraction and sorption preconcentration of U(VI), Am(III), and Pu(IV) from nitric acid solutions with alkylenediphosphine dioxides

The extraction of U(VI), Am(III), and Pu(VI) from nitric acid solutions in the form of complexes with alkylenebis(diphenylphosphine) dioxides and their sorption with POLIORGS F-6 sorbent prepared by noncovalent immobilization of methylenebis(diphenylphosphine) dioxide (MDPPD) on a KhAD-7M? polymeric matrix were studied. The preconcentration conditions and distribution coefficients of U(VI), Am(III), and Pu(IV) in their sorption from 3 M HNO₃ were determined. The possibility of concentrating actinides from multicomponent solutions was demonstrated. The composition and nature of complexes of U(VI) with MDPPD were determined from the ³¹P NMR data.     

Precipitation of Am(III) and REE peroxides from alkaline solutions containing complexing agents

Precipitation of Am(III) and REE peroxides from alkaline-citrate solutions was studied spectrophotometrically. Hydrogen peroxide (0.25–0.4 M) quantitatively precipitates Am(III) and Nd(III) ions from a solution containing 0.1 M NaOH and 0.2–0.4 M citrate. After H₂O₂ decomposition, the precipitates formed are dissolved in the mother liquor. At lower alkali concentration, 4f and 5f metal ions are precipitated incompletely. In the systems studied, Am and Nd form citrate and hydroxo-citrate complexes in a solution and peroxides in the solid phase.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 524–526.Original Russian Text Copyright © 2004 by Gogolev, Nikonov, Tananaev, Myasoedov.     

Catalytic decomposition of organic anions in alkaline radioactive waste: III. Oxidation of oxalate and glycolate

Decomposition of oxalate and glycolate ions in alkaline solutions under the action of O₃, H₂O₂, and Na₂S₂O₈ was studied spectrophotometrically and titrimetrically. At 20°C, ozone slowly decomposes oxalate in 0.05 M NaOH. In 1 M NaOH, heating at 90°C is required to oxidize oxalate with ozone. Glycolate is readily oxidized with ozone at 20°C in 0.05–1 M NaOH, predominantly into oxalate. Hydrogen peroxide is ineffective reagent for oxalate and glycolate decomposition. Persulfate oxidizes oxalate ion in 0.5–5 M NaOH at 90°C. The reaction of persulfate with glycolate proceeds at 50°C and higher temperatures and is characterized by an induction period, which shortens with increase in concentration of S₂O ₈ ^2? , OH^?, and temperature, or in the presence of AgNO₃ and K₄Fe(CN)₆. Oxidation involves thermal dissociation of persulfate ions into radical ions followed by a chain reaction.     

Tc(VII)-catalyzed oxidation of U(IV) with nitric acid in solution of TBP in <Emphasis Type="Italic">n</Emphasis>-dodecane

Oxidation of U(IV) with nitric acid in 30% solution of TBP in n-dodecane is catalyzed by Tc ions; the rate-determining steps are 3U(IV) + 2Tc(VII) → 3U(VI) + 2Tc(IV) and Tc(IV) + Tc(VII) → Tc(V) + Tc(VI). Oxidation of U(IV) is inhibited by the reaction product, HNO₂, which partially binds Tc(IV) ions (TcO²⁺) in an inert complex. The overall rate equation of U(IV) oxidation is-d[U(IV)]/dt = k ₁[U(IV)][Tc][HNO₃]^?3 ? k ₄[U(IV)]²[HNO₂]²[HNO₃]^?1, where k ₁ = 4.8 ± 1.0 mol²1^?2 min^?1 and k ₄ = (2.4 ± 1.0) × 10⁵ 1² mol^?2 min^?1 at 25°C, [H₂O] = 0.4 M ([Tc] is the total Tc concentration in the reaction mixture). Water and U(VI) have no effect on the reaction rate.     

Disproportionation of Pu(V) in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

In solutions of unsaturated heteropolytungstate K₁₀P₂W₁₇O₆₁, Pu(V) disproportionates in a wide pH range; it is a first-order reaction with respect to Pu(V), and its rate only slightly changes in the pH range from 0.7 to 4.0. The activation energy E _a of Pu(V) disproportionation was determined as 78.6±2.0 and 64.2±3 kJ mol^?1 at pH 2.0±0.1 and 4.0±0.2, respectively. The thermodynamic parameters of activation ΔH ^≠ and ΔS ^≠ were evaluated. Published data on disproportionation of Np(V) and Am(V) in K₁₀P₂W₁₇O₆₁ solutions were analyzed.