Vladimir Sladkov

Interaction of uranyl with Se(IV) and Se(VI) in aqueous acid solutions by means of capillary electrophoresis

The uranyl–selenium(IV) and uranyl–selenium(VI) interactions were studied by CE in aqueous acid solutions, containing U(VI) and Se(IV) or Se(VI) at different concentrations, at pH 1.5, 2.0 and 2.5. The method proposed in this paper allows one with the use of CE data on metal ion mobilities at different pHs to establish the ligand species interacting with metal ion and complex species formed. In the case of Se(VI) a selenate, as demonstrated, interacts with uranyl ions, in the case of Se(IV) this is a hydroselenite. It was also shown that the equilibria for the U(VI)–Se(VI) and U(VI)–Se(IV) systems can be established from CE data. The formation of UO2SeO4, UO2(SeO4)  22− , UO2HSeO  3+ and UO2(HSeO3)2 species is demonstrated. The stability constant values were measured at different ionic strengths (from 0.02 to 0.2 mol/L). The logarithms of the stability constant values (β°) extrapolated to ionic strength 0 by the specific ion interaction theory (SIT) are found to be log β°1=2.93±0.06 for UO2SeO4 formation, log β°2=4.030.18 for UO2(SeO4)  22− formation, log β°1=3.270.15 for UO2HSeO  3+ formation and log β°2=5.510.11 for UO2(HSeO3)2 at 25°C. The results for the first constant values for each of systems are consistent with the published values. For UO2(SeO4)  22− formation, a new constant stability value is given. The existence of UO2(HSeO3)2 complex species is demonstrated and its constant stability value is given for the first time.

Uranyl complexation with acetate studied by means of affinity capillary electrophoresis

The interaction of uranyl with acetate is studied by affinity capillary electrophoresis in aqueous acid solutions at the pH values 2.0 and 2.5. The use of data on metal ion mobilities at different pHs allows to establish the ligand species interacting with metal ion and complex species formed. The formation of two complex species UO2CH3COO(+) and UO2(CH3COO)2 is observed (acetic acid concentration is up to 0.8M). In the case of uranyl-acetic acid system, the viscosity of solution is significantly changed with an increase of acid concentration. For calculation of ion mobilities the viscosity changes are taken into account. The stability constants are calculated at the ionic strengths 0.02 and 0.05 mol L(-1). The logarithms of the thermodynamic stability constants (β°) calculated with Davies equation for the activity coefficients of the ions are log β1(°)=2.94±0.08 and log β2(°)=5.50±0.15 at 25 °C. Obtained values are compared with literature data.

Uranyl complexation with selenate at variable temperatures studied by affinity capillary electrophoresis.

The uranyl-selenate system is studied in aqueous acid solutions (pH 2.5) at different values of the ionic strength (from 0.02 to 0.1 molL(-1)) in the temperature range from 15°C to 55°C by affinity capillary electrophoresis. Hydrodynamic transfer of the sample through the non-thermostated inlet into the thermostated region of capillary is used to avoid the influence of non efficiently thermostated short inlet. The formation of two complex species UO(2)SeO(4) and UO(2)(SeO(4))(2)(2-) is observed. Thermodynamic parameters (the molar Gibbs energy of reaction (Δ(r)G(m)), the molar enthalpy of reaction (Δ(r)H(m)) and the molar entropy of reaction (Δ(r)S(m))) are calculated and extrapolated to zero ionic strength with aid of specific ion interaction theory. Results show that complex species of uranyl with selenate become stronger as the temperature is increased. The complexation is endothermic and entropy-driven, showing typical characteristics of inner-sphere complexation between hard acceptor and hard donor.

Uranyl-Se(IV) interaction in aqueous acid solutions studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and UV-Vis spectrophotometry

The uranyl-selenium(IV) interaction was studied by time-resolved laser-induced fluorescence spectroscopy in aqueous perchloric acid solutions containing 1 x 10(-4) M U(VI) and Se(IV) at different concentrations (from 0 up to 0.3 M) at pH 1 and 2. The quenching of uranyl fluorescence is observed. HSeO(3)(-) is demonstrated to be responsible for the fluorescence quenching. Stern-Volmer analysis gives the dynamic quenching rate constant value (k(q)) (5.0 +/- 0.4) x 10(9) L M(-1) s(-1) at ionic strength (mu) 0.05 M. The bimolecular excited-state process is shown to be diffusion-controlled as k(q) is practically identical to the diffusion rate constant as calculated for uranyl and hydroselenite charged species. With an increase of HSeO(3)(-) concentration, static quenching occurs in addition to the dynamic quenching. The hypothesis of the formation of a weak fluorescent ground state complex between uranyl and HSeO(3)(-) is supported. The logarithm of the stability constant value (beta) is found to be 3.35 +/- 0.12 (mu= 0.05 M) and 3.32 +/- 0.15 (mu = 0.2 M). These constants are confirmed by UV-Vis spectrophotometry. The modelling factor analysis of absorption spectra gives logbeta = 3.35 +/- 0.17 (mu = 0.035 M). Extrapolation to mu = 0 by the specific ion interaction theory (SIT) gives logbeta degrees = 3.62 +/- 0.15 at 20 degrees C.

Affinity capillary electrophoresis in studying the complex formation equilibria of radionuclides in aqueous solutions.

Interaction of radionuclides with inorganic and organic species present in natural environment plays an important role in their eventual dispersion. The complex equilibria established in the aqueous phase cause significant changes in the migration properties of radionuclides. Affinity capillary electrophoresis (ACE) can be fruitful in studying these equilibria. This paper reviews the recent methodological advances of the use of ACE in studying the complex equilibria of radionuclides in aqueous solutions. Special attention is paid to the determination of a number of species involved in equilibrium, species constituents (number of ligands, protonated, deprotonated), the influence of ionic strength and temperature on stability constants of complex species formed. Use of ACE for the determination of the main thermodynamic parameters (the molar Gibbs energy (ΔrGm), the molar enthalpy (ΔrHm) and the molar entropy (ΔrSm)) of complex formation reactions is also discussed. These data are essential to predict dispersion of radionuclides in the natural environment.

Modified Square-Wave Cathodic Stripping Voltammetry for Determination of Se(IV) at Trace and Ultratrace Levels

A method for the improvement of the signal-to-noise ratio in square-wave (SW) voltammetry is proposed. It is based on the modification of a square-wave waveform. This method is applied to cathodic stripping voltammetric (CSV) determination of selenium(IV) in nitric acid solutions. Hanging mercury drop electrode (HMDE) was used in the presence of Cu(II) ions. The optimal conditions were chosen: square-wave waveform mode, pH, time and potential of electrodeposition. Kinetic processes of copper selenide reduction with the use of SWCSV, applying different square-wave waveforms, are studied. Quasi-reversible processes are observed. The standard reaction rate constants are evaluated for different square-wave waveforms. It is shown that the determination of Se(IV) by this modified SWCSV technique is possible with an excellent sensitivity (the detection limit 8×10−12 mol L−1 only for 5 min of electrodeposition) and good reproducibility (Sr<8%) in a wide range of concentration (1×10−11−1×10−6 mol L−1) of Se(IV). The presence of Mo(VI), Pb(II), Ni(II), Zn(II), Cr(VI) metal ions with molar excess around 1000 do not prevent the detection of Se(IV) signals.