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- Author or Editor: P. Pathak x
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Abstract
The effects of ionic strength and of ethylenediamin et etraacetic acid (EDTA) on the sorption of uranyl ion, UO2 2+, to SiO2·xH2O (silica gel) were investigated. It was observed that pH and the ions present in the supporting electrolytes influence the ionic strength effects. The presence of different sodium salts in the concentration range (0.20 to 1.40M) suppressed the sorption of UO2 2+ in the order: NaNO3 < NaClO4 < NaCl < NaOCOCH3 < Na2SO4 [pH 2.75(±0.05)], while the presence of perchlorate salts of Li+, Na+ and Ca2+ (0.20 to 1.40M) promoted the sorption of UO2 2+ on silica gel in the order: LiClO4∼NaClO4<Ca(ClO4)2 at pH 2.80(±0.05). The ionic strength effect on UO2 2+ sorption was studied in presence of EDTA (0–1.00·10−3M) in the pH range 2.90 to 5.57. The sorption data and speciation calculation suggest negligible complexation of UO2 2+ with EDTA at I≥1.00M NaClO4.
Summary
Batch sorption experiments of cesium, Cs+, on SiO2 . xH2O (silica gel) have been conducted with variable times of equilibration, amounts of silica gel (0.10-1.00 g), cesium concentrations (5.00 . 10-5-2.40 . 10-3M), ionic strengths (0.20-1.40M NaClO4), pH (2.50-7.70), and temperatures (273-333 K). The diffusion coefficient of Cs+ ion was calculated to be (9.19±0.86) . 10-11 m2 . s-1 under particle diffusion-controlled conditions. The sorption rate was (3.94±0.65) . 10-3 s-1 at 298 K, pH 7.70±0.05 in 0.20M NaClO4. The sorption data fits the Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Cesium sorption on 0.20 g silica gel decreased with ionic strength from (40.42±0.34)% in 0.20M NaClO4 to (6.35±0.40)% in 1.40M NaClO4, at pH(initial) 8.20±0.05. A gradual decrease in pH with increased ionic strength is consistent with a cation-exchange mechanism. Sorption of Cs+ on silica gel decreased with increased temperature, indicating an exothermic enthalpy. The presence of anions such as fluoride, carbonate, phosphate and oxalate in the aqueous medium did not influence the cesium sorption profile.
Abstract
Complexation behavior of NpO2 + with ortho-silicic acid (o-SA) has been studied using solvent extraction at ionic strengths varying from 0.10 to 1.00M (NaClO4) at pcH 3.68±0.08 and 25 °C with bis-(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stability constant value (log β1) for the 1:1 complex, NpO2(OSi(OH)3), was found to decrease with increase in ionic strength of the aqueous phase [6.83±0.01 at I=0.10M to 6.51±0.02 at I = 1.00M]. These values have been fitted in the SIT model expression and compared with similar values of complexation of the metal ions Am3+, Eu3+, UO2 2+, PuO2 2+, Np4+, Ni2+ and Co2+. The speciation of NpO2 +-o-silicate/carbonate system has been calculated as a function of pcH under ground water conditions.
Abstract
Sorption of NpO2 + on silicate (10.00 g/l) particles dispersed in sodium perchlorate media was studied as a function of pcH and ionic strength at 298 K. The sorption increased with increased pcH in the range of ∼6.5 to 9.2, above which saturation was observed. An increase in ionic strength from 0.20M to 1.00M (NaClO4), increased the NpO2 + sorption, which then decreased at 1.50M (NaClO4) for 7<pcH<8.5. The effects of different types of ligands on the sorption of NpO2 + to suspended silicate were investigated. The types of ligands included: (i) inorganic anions (fluoride, carbonate, phosphate (ii) N-donors (ethylenediamine, 1,10 phenanthroline (iii) carboxylic acids (oxalic acid, citric acid, iminodiacetic acid, ethylenediaminetetraacetic acid) and (iv) humic acid. A synergistic enhancement in sorption to the suspended silicate was observed for phosphate, oxalate, ethylenediaminetetraacetic acid, ethylenediamine, 1,10 phenanthroline (5<pcH<8) and humic acid (6.5<pcH<8.8). This behavior was attributed to the formation of ternary NpO2 +/silicate/ligand complexes. The effects of Ca(II) (1.00·10−3M) and Eu(III) (1.00·10−4 and 1.00·10−3M) ions on NpO2 + sorption to suspended silicate were also investigated.
Abstract
Sorption of tracer Am3+ to silicate particles was studied as a function of pcH (4 to 9) and of ionic strength [0.20M to 1.50M (NaClO4)] at 298 K. The sorption increased with increased pcH from 4 to 6 above which saturation was observed. The insensitivity of Am3+ sorption to increased ionic strength indicates inner-sphere complexation with the surface silicate sites. The effects of different complexing anions such as carbonate, acetate, oxalate, phosphate, citrate, EDTA and humic acid, on Am3+ sorption were investigated. Synergistic enhancement in Am3+ sorption was observed in the presence of phosphate (4≤pcH≤7) and acetate (4≤pcH≤5) ligands at 0.20M NaClO4. The presence of the other ligands inhibited Am3+ sorption in the order: EDTA > citrate > oxalate > carbonate. Am3+ sorption in the presence of HA (25.00 mg/l) increased in the pcH range of 4.0 to 5.5, then decreased. Increased ionic strength enhanced Am3+ sorption in the presence of 25.00 mg/l HA for 4≤pcH≤9. The sorption increased in the presence of a mixture of HA (25.00 mg/l) and phosphate (1.00·10−3M) as compared to that of HA (25.00 mg/l) alone. The presence of Fe3+ (1.00·10−4M) enhanced Am3+ sorption at pcH∼4 but suppressed it from pcH of 5 to 9; 1.00·10−4M of Ca2+ and of UO2 2+ ions had no effect on the sorption profile.
Summary
Sorption behavior of europium, Eu3+, on SiO2 . xH2O (silica gel) has been investigated as a function of time, the amount of silica gel, Eu3+ concentration, the ionic strength, and pH (in absence and in presence of carbonate). The sorption data were fitted to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. The sorption capacity of silica gel was determined to be in the range of (2.62-8.00) . 10-7 mol/g at pH 5.30±0.05 and 0.20M NaClO4. The mean energy of sorption was calculated to be 13.50±0.05 kJ/mol from the D-R isotherm, suggesting the involvement of ion-exchange reactions in the sorption process. Sorption of Eu3+ decreased with increased ionic strength. A gradual decrease in pH with increased ionic strength supports the involvement of an ion-exchange mechanism in the sorption process. The diffusion coefficient of Eu3+ ion on silica gel was calculated as (3.98±0.12) . 10-13 m2 . s-1 under the particle diffusion-controlled conditions.
Summary
Sorption of nickel, Ni2+, on SiO2 . xH2O (silica gel) has been studied as a function of time conditions: amount of silica gel: 0.10-1.00 g, nickel concentration: 5.00 . 10-5-1.20 . 10-3M, ionic strength: 0.20-1.40M NaClO4, pH 6.50 to 8.50, and temperature 273-318 K. From the kinetic data, the diffusion coefficient of Ni2+ ion was calculated to be 1.28(±0.07) . 10-11 m2 . s-1 under particle diffusion-controlled conditions. The sorption rate was determined as 3.79(±0.35) . 10-3 s-1 at 298 K, pH 8.40 in 0.20M (NaClO4). The sorption data were fitted to Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Nickel sorption on 0.20 g silica gel decreased with ionic strength from 77.70±0.70% (0.20M NaClO4) to 16.12±0.37% (1.40M NaClO4) at intial pH of 8.50±0.05. A gradual decrease in pH with increasing ionic strength suggests an ion exchange mechanism and the sorption of Ni2+ on silica gel increasing with temperature indicates an endothermic enthalpy. The effect of different ligands such as fluoride, carbonate, phosphate and oxalate on Ni2+ sorption on silica gel was studied.
Summary
The complexation behavior of Co2+with ortho-silicic acid (o-SA) has been studied as a function of ionic strength (I) from 0.20 to 1.00M (NaClO4) at pH 4.96±0.03 and 25 °C by solvent extraction with bis(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stoichiometry of the extracted species was determined to be Co(DEHP)2(HDEHP)2. Co2+forms a 1:1 complex, CoOSi(OH)3 +, as the predominant species witho-SA concentrations of 3.00. 10-4to 4.00. 10-3M. The stability constant (logb1) values for CoOSi(OH)3 +complex decrease with the increase in ionic strength. These values were fitted with the extended Debye-Huckel expression to obtain the value of logb1at I=0.00M. The effect of aging time of the o-SA solution on logb1values for CoOSi(OH)3 +complex was investigated and compared with those of the UO2OSi(OH)3 +complex.
Abstract
The distribution behavior of uranium and thorium has been investigated in a biphasic system of different aqueous nitric acid concentrations and a solution of tris(2-ethylhexyl) phosphate (TEHP) inn-dodecane at 25°C. The effect of different uranium and thorium concentrations in the aqueous phase on the extraction of these metal ions is evaluated. These results indicate that TEHP is a better choice than tri-n-butyl phosphate (TBP) for the separation of233U from the irradiated thorium matrix.
Abstract
Extraction of uranium from tissue paper, synthetic soil, and from its oxides (UO2, UO3 and U3O8) was carried out using supercritical carbon dioxide modified with methanol solutions of extractants such as tri-n-butyl phosphate (TBP) or N,N-dihexyl octanamide (DHOA). The effects of temperature, pressure, extractant/nitric acid (nitrate) concentration, and of hydrogen peroxide on uranium extraction were investigated. The dissolution and extraction of uranium in supercritical CO2 modified with TBP, from oxide samples followed the order: UO3 ≫ UO2 > U3O8. Addition of hydrogen peroxide in the modifier solution enhanced the dissolution/extraction of uranium in dynamic mode. DHOA appeared better than TBP for recovery of uranium from different oxide samples. Similar enhancement in uranium extraction was observed in static mode experiments in the presence of hydrogen peroxide. Uranium estimation in the extracted fraction was carried out by spectrophotometry employing 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) as the chromophore.
