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Photochemical reactions involving Np(VI) and Np(V) in 0.5–17.3 M CH3COOH (HAc) solutions have been studied. It was found that irradiation with UV light causes reduction of Np(VI) to Np(V). The initial rate of Np(VI) reduction is at a maximum in a 0.5–1 M solution of HAc and decreases with increasing HAc concentration, which is due to the formation of higher acetic acid complexes with Np(VI). After a partial reduction of Np(VI) as a result of irradiation, its concentration continues to decrease in the dark reaction with products formed in the oxidation of HAc. The reduction of Np(V) occurs with an induction period, after which the reaction becomes faster. This is due to the appearance and accumulation of Np(IV), which forms a cation-cation complex with Np(V). The complex absorbs light, which leads to the reaction of Np(V) with HAc. Np(VI) is oxidized in acetic acid solutions by oxygen to Np(V).

Spectrophotometry has been used to examine the kinetics of Np(V) disproportionation in HCOOH solutions containing H2O and HClO4 at temperatures of 20–45°C. The reaction rate is described in the initial portion by the equation –d[Np(V)]/dt = k[Np(V)]2. The rate constant k sharply grows with increasing [HClO4] (order n = 3.5) and with decreasing content of H2O (n = –2.30. The appearing Np(VI) is slowly reduced to Np(V) by formic acid. The activation energy of the disproportionation reaction is 54 kJ mol–1. The mechanism includes the elimination of an H2O molecule from the H+aq ion and addition of a proton to NpO2+. The protonated Np(V) ions form a dimer which is an activated complex. The H transfer in this complex yields Np(IV) and Np(VI).

The properties and stability of the S2O82– ion and mechanisms of its reaction with ions of f-elements in aqueous solutions in a wide range of pH values are considered. S2O82– are converted to SO4 ion radicals. The decomposition is accelerated by Н+, ОН–, Ag+, Co2+ ions, and others. The decomposition in solutions with [H+] > 0.5 or [OH–] > 0.5 M yields monoperoxysulfuric acid, with its hydrolysis generating Н2О2. The SO4– radicals oxidize ions of f-elements and convert ОН– and СО32– ions to oxidizing radicals ОН and СО3– and transform H2PO4– and NO3– ions into reducing compounds. In aerated alkaline solutions, the ОН radicals are transformed to ion-radicals О3–,which oxidize Np(VI), Pu(VI), and Am(VI) to the heptavalent state. In weakly acidic solutions of heteropolycompounds, SO4– radicals transform Tb(III), Am(III), Cm(III), Cf(III) to the tetravalent state. The mixture of Ag+ and S2O82– is used to oxidize Np(VI) to Np(VII) in an alkaline medium, Pu4+ to PuO22+ in acid solutions, Am(III) to Am(IV) in Н3РО4 solutions and in the presence of heteropolycompounds in 5 M HNO3 or 3 M H2SO4 solutions, Am3+ to AmO22+ in acid solutions. The decomposition of S2O82– into radicals occurs under exposure to light or radiation. The reactions of oxidation of f-element ions by SO4– radicals occur by the zeroth-order rate law. To one more group of reactions belongs the interaction of S2O82– with NpO2+, Pu3+, Am3+ ions. The reactions occur with a first-order rate with respect to the f-ion. In the case of U4+, Np4+ in acid media and, Pu(IV) in a solution of K2СО3, complexes of these ions with S2O82– are formed. The complexes decompose by the first-order rate law. Studies are planned of studying reactions involving S2O82– and ions of f-elements, including those in fluoride solutions containing actinides and fission products, and reactions of SO4– radicals at low temperatures to give actinides in higher oxidation state.

Spectrophotometric method has been used to examine valence transformations of neptunium ions in formi acid aqueous solutions under the action of light from a mercury lamp. It was shown that Np(VI) ions are reduced in ~10 min to Np(IV) containing a small amount of (Np(V). Np(V) ions in solutions and suspensions are also effectively reduced to Np(IV). Np(III) appears in a photolysis of Np(IV) ions. In 99.7% HCOOH, the Np(IV) → Np(III) conversion occurs nearly fully. Np(III) ions are unstable and are transformed to Np(IV) in storage. The hydrolysis and polymerization of Np(IV) hinder the reduction of Np(IV) in1 M HCOOH.

Specific features of the interaction of O3 in aqueous media with f-elements in a wide range of pH values are considered. In neutral solutions, O3 splits-off an O atom, which becomes the beginning of a chain process of O3 decomposition. NO3–, HSO4–, OH– anions initiate the decomposition by forming complexes with O3. The products of the subsequent transformations are OH, HO2, HO3, and H2O2. Phosphate and carbonate ions and HNO3 react with OH radicals and block the chain process. The peroxynitric acid formed from HNO3 serves both as an oxidizing agent and, as a result of hydrolysis, as reducing agent. In solutions of 1 M NaHCO3 or Na2CO3, ozone forms a complex with CO32– and its stability grows. In the alkaline medium, O3 is converted to O3–, but reducing agents are formed in the alkaline solution at a high concentration of O3 in the gas phase. The reactions of O3 with Ce, U, Np, Pu, Am, Bk, Ce, Tb, and Pr ions are analyzed. The reactions occur via a preliminary formation by cations of anion complexes and then complexes with O3. The formation constant of the Pu(IV) complex with O3 in a 4 M HNO3 solution is estimated to be 4.5 × 105 M–1. U(IV) and Pu(III) in an HClO4 solution, Np(IV) and Pu(IV) in an HNO3 solution, Np(IV) and Am(III) in a NaHCO3 solution are oxidized by ozone with transfer of the O. atom, Ce(III) and Bk(III) in an HNO3 solution, U(IV) and Am(III) in a solution of heteropolyanions, Np(IV, V) in a Na2CO3 solution, Np(V,VI), Pu(V, VI), and Am(V,VI) in alkaline media are oxidized with charge transfer to O3 or O3–.

The spectrophometric method was used to examine the stoichiometry of the reaction of Pu(VI) reduction by EDTA ions in 0.05–0.5 M HClO4 solutions at a temperature of 23°C at an excess of Pu(VI), 1 mol of EDTA reduces 3.5 ± 0.5 mol of Pu(VI) in air, with the main reaction product being Pu(V). At a solution pH 4.5, 1 mol of EDTA reduces 5 ± 0.5 mol of Pu(VI). The reaction of Pu(VI) reduction by EDTA ions hardly occurs in 0.1 M HClO4 in an inert atmosphere, whereas at pH 4.5, the reaction is complete in ∼1 min. In 0.05–0.5 N HClO4 solutions (ionic strength of 1.0 was maintained by addition of LiClO4) at temperatures of 23–45°C, Pu(VI) is consumed by the first-order rate law. The reaction rate is of the first order in EDTA and −2 order in H+ ions. An activated complex is formed with loss of two protons. The activation energy is 128 ± 10 kJ/mol. Assumptions are made about the possible involvement of oxygen in the reaction mechanism.

Spectroscopic method was used to examine the stoichiometry of the reaction of No(VI) with hexamethylenediaminetetraacetic acid (HMDTA, H4hmdta) in a 0.05 M HClO4 solution. At an excess of Np(VI), 1 mole of complexon reduces about 4 moles of Np(VI) to Np(V). In 0.1–1.0 M HClO4 solutions (the ionic strength of 1.0 was maintained with LiClO4) containing 2–20 mM HMDTA, neptunium(VI) with concentration of 0.3–3.0 M decreases at 35–55°C by the first-order rate law until the instant when less than 20% Np(VI) remains. The initial reaction rate has the first order in [HMDTA] and –2 order in [H+]. An activated complex is formed with loss of two H+ ions. The activation energy is 102 ± 7 kJ/mol.

Np(V) acetate in glacial acetic acid and aqueous acetic acid solutions is resistant to disproportiona-tion and occurs in equilibrium with the NpO2(Ac)·H2O precipitate. Within the time of dehydration with excess acetic anhydride, Np(V) disproportionates. The inverse reproportionation process occurs after adding excess water to anhydrous acetic acid containing acetic anhydride, Np(IV), and Np(VI). Addition of HClO4 causes rapid disproportionation of Np(V) in anhydrous acetic acid and in concentrated acetic acid solutions. The spectra of suspensions containing Np(V) and potassium acetate in concentrated acetic acid contain absorption bands with maxima at 990 and 1020 nm, assigned to NpO2(Ac)32- anions and K2NpO2(Ac)3 particles, respectively.