- Equilibrium and kinetic studies of Se(VI) removal by Mg-Al layered double hydroxide doped with Fe2+
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Mg-Al layered double hydroxide (Mg-Al LDH) doped with Fe2+ was found to be superior to undoped Mg-Al LDH in the removal of Se(vi) from aqueous solutions. For both systems, Se(vi) as SeO42- was removed through anion exchange with intercalated Cl-. In the Fe2+-doped Mg-Al LDH, however, some of the Se(vi) was reduced to Se(iv) upon oxidation of Fe2+ to Fe3+ in the LDH host layer to produce SeO32-, which was also adsorbed by the Fe2+-doped Mg-Al LDH through anion exchange. The reduction of Se(vi) to Se(iv) is advantageous for Se(vi) removal by Fe2+-doped Mg-Al LDH due to the larger charge density of SeO32-. The Fe2+-doped Mg-Al LDH effectively removed Se(vi) from an aqueous solution because of the anion exchange properties of Mg-Al LDH and activity of Fe2+ as a reducing agent. Se(vi) removal occurs through Langmuir-type adsorption, where the maximum adsorption and equilibrium adsorption constant were 1.4 mmol g-1 and 1.6, respectively. Se(vi) removal is well expressed as a pseudo second-order reaction. The apparent rate constants at 10, 30, and 60°C were 1.2 × 10-3, 1.5 × 10-3, and 2.2 × 10-3 g mmol-1 min-1, respectively, and the apparent activation energy was 10.0 kJ mol-1. The rate-determining step is chemical adsorption through anion exchange of SeO42- and SeO32- with intercalated Cl-.
- Kameda, Tomohito,Kondo, Eisuke,Yoshioka, Toshiaki
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- SELENIUM(V). A PULSE RADIOLYSIS STUDY
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Two Se(V) species, assumed to be SeO3(1-) and HSeO4(2-), in equilibrium with each other were observed in pulse radiolysis of aqueous selenite and selenate solutions.The following reactions and equilibria were observed: SeO3(1-) + OH(1-) HSeO4(2-), K= 0.78 dm3 mol-1; SeO3(2-) + OH HSeO4(2-), kf= (3.5 +/- 0.2) x 1E9 dm3 mol-1 s-1, kr= (7.3 +/- 0.5) x 1E5 s-1; O(1-) + H2O + SeO3(2-) HSeO4(2-) + OH(1-), kf= (1.1 +/- 0.1) x 1E7 dm3 mol-1 s-1, kr(3+/-1)*1E5 dm3mol-1s-1; HSeO3(1-) + OH -> (H2SO4(1-)) -> SeO3(1-) + H2O, k= (1.6 +/- 0.1) x 1E8 dm3 mol-1 s-1; H2SeO3 + OH -> (H2SeO4) -> SeO3(1-) + H3O(1+), k=1.0 +/-0.1)*1E9 dm3 mol-1 s-1; 2SeO3(1-) -> Se(IV) + Se(VI), k= (5.2 +/- 0.5) x 108 dm3 mol-1 s-1; 2HSeO4(2-) -> SeO3(2-) + SeO4(2-) + H2O, k ca. 5 x 1E8 dm3 mol-1 s-1; HSeO4(2-) + SeO3(1-) SeO3(2-) + SeO4(2-) + H2O, k ca. 1E9 dm3 mol-1 s-1; CO3(2-) + SeO3(1-) -> CO3(1-) + SeO3(2-), k= (6 +/-) x1E6 dm3 mol-1 s-1; SeO4(2-) + eaq- HSeO4(2-) (SeO3(1-) + OH(1-)), k= 1.1 x1E9 dm3 mol-1 s-1; HSeO4(1-) + H -> (H2SeO4(1-)) -> SeO3(1-) + H2O, k ca. 106 dm3 mol-1 s-1.The standard Gibbs energy of formation of SeO3(1-) and HSeO4(2-), ΔfGao0(SeO3(1-)) = -201.6 kJ mol-1 and ΔfGao0(HSeO4(2-)) = -358.2 kJ mol-1, and the standard reduction potentials Eao0(SeO3(1-)/SeO3(2-)) = 1.68 +/- 0.01 V and Eao0(HSeO4(2-)/SeO3(2-)) = 1.69 +/- 0.01 V were determined from the rate constants and the standard Gibbs energy of formation of OH, OH(-), H2O, and SeO3(2-).The relatively small values for ΔfGao0(SeO3(1-)) and ΔfGao0(HSeO4(2-) agree with the observation that one-equivalent oxidation of Se(IV) or reduction of Se(VI) oxoacids and oxoanions generally are fast processes and indicate that direct electron transfer to selenate or from selenate feasible processes.
- Klaening, U. K.,Sehested, K.
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- Reductive reaction of selenate with hydrazine over TiO2-supported Pt catalyst in aqueous solution
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Although there has been no report on the catalytic reaction of selenate over heterogeneous catalysts in aqueous solution, we found that titania-supported Pt catalysts exhibited significantly high activity for the reduction of selenate with hydrazine. The Pt catalysts were deactivated during the reaction because of the deposition of Se metal, but showed excellent reusability. Therefore, it was firstly demonstrated that heterogeneous catalysts are effective for the removal of selenate from aqueous solution.
- Zhao, Jinxian,Matsune, Hideki,Takenaka, Sakae,Kishida, Masahiro
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p. 1563 - 1565
(2015/11/24)
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- Analogue reaction systems of selenate reductase
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Analogue reaction systems of selenate reductase, which reduces substrate in the overall enzymatic reaction SeO42- + 2H+ + 2e- → SeO32- + H2O, have been developed using bis(dithiolene) complexes of MoIV and WIV. On the basis of the results of EXAFS analysis of the oxidized and reduced enzyme, the minimal reaction MoIVOH + SeO42- → MoVIO(OH) + SeO32- is probable. The square pyramidal complexes [M(OMe)(S2C2Me2)2]1- (M = Mo, W) were prepared as structural analogues of the reduced enzyme site. The systems, [ML(S2C2Me2)2]1-/SeO 42- (L = OMe, OPh, SC6H2-2,4,6-Pri3) in acetonitrile, cleanly reduce selenate to selenite in second-order reactions whose negative entropies of activation implicate associative transition states. Rate constants at 298 K are in the 10-2-10-4 M-1 s-1 range with ΔS? = -12 to -34 eu. When rate constants are compared with previous data for the reduction of (CH2)4SO, Ph3AsO, and nitrate by oxygen atom transfer, reactivity trends dependent on the metal, axial ligand L, and substrate are identified. As in all other cases of substrate reduction by oxo transfer, the kinetic metal effect k2W > k2Mo holds. A proposal from primary sequence alignments suggesting that a conserved Asp residue is a likely ligand in the type II enzymes in the DMSO reductase family has been pursued by synthesis of the [MoIV(O2CR)(S2C2Me2) 2]1- (R = Ph, But) complexes. The species display symmetrical η2-carboxylate binding and distorted trigonal prismatic stereochemistry. They serve as possible structural analogues of the reduced sites of nitrate, selenate, and perchlorate reductases under the proposed aspartate coordination. Carboxylate binding has been crystallographically demonstrated for one nitrate reductase, but not for the other two enzymes.
- Wang, Jun-Jieh,Tessier, Christian,Holm
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p. 2979 - 2988
(2008/10/09)
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- Haloselenate(IV) formation and selenous acid dissociation equilibria in hydrochloric and hydrofluoric acids
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Raman spectroscopy has shown that the initial stages of haloselenate(IV) formation equilibria are the same in HF and HCl solutions. The constants K4 and K5, evaluated by quantitative Raman measurements, for HCl solutions are 0.015 (+/- 0.001) L2mol-2 and 0.9 (+/- 0.1)E-5 L2mol-2, respectively.These constants yield the minimum average deviation in normalized molar scattering intensity for the species formed in three regions of the spectra, 690 and 890 cm-1 (SeO stretching) and 100-450 cm-1 (deformation and SeCl stretching).Before the oneset of haloselenate(IV) formation, changes in the SeO stretching band intensities with HX concentrations occur due to repression of ionic dissociation of selenious acid Quantitative Raman measurements yield K1 = 9.8 (+/- 0.3)E-3 mol L-1.Reasons for the high value of this constant, compared to reported values, are discussed.
- Milne, John
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p. 316 - 321
(2007/10/02)
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