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  • Author: K. L. Ramakumar x
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Abstract

Various β-diketones viz. acetylacetone (AA), trifluoroacetylacetone (TFA), hexafluoroacetylacetone (HFA), thenoyltrifluoroacetylacetone (TTA) and heptafluorobutanoylpivaroylmethane (FOD) were evaluated as chelating agents for the supercritical fluid extraction of thorium from tissue paper matrix. In-situ chelation mode was found to be more effective than online chelation mode. A combination of tributyl phosphate (TBP) and β-diketones further enhanced the extraction efficiency. The extraction efficiency trend observed in all the cases was: TTA>FOD>HFA>TFA>AA. A correlation was observed between extraction efficiency and degree of fluorination in the side arms of β-diketones. This could be attributed to the fact that in supercritical CO2, higher fluorination results in higher percentage of enol content, greater dissociation into enolate ion and higher solubility and stability of β-diketones as well as of Th-β-diketone chelates. Highest extraction efficiency with TTA was probably due to the presence of aromatic thenoyl group. The spectra of the extracted Th-chelates displayed peaks in the visible region, which shifted towards UV region with increasing fluorination. In the Th-TTA chelate spectrum, red shift was observed.

Abstract

Supercritical carbon dioxide (SC CO2) extraction of uranium from nitric acid medium employing various crown ethers was studied. CO2-phillic C–F bonds in pentadecafluoro-n-octanic acid (HPFOA) counter ion enhanced the extraction efficiency. Pressure (100−300 atm) and temperature (323−353 K) were found to influence extraction efficiency by affecting SC CO2 density as well as due to large size cluster formation of supercritical fluid near critical point thereby resulting in reduced interaction with solute. Pressure of 200 atm and temperature of 323 K were found to be optimum. The trend in efficiency for benzo substituted crown ethers was due to the combined effect of cavity size and number of ether oxygen atoms. The extraction efficiency among 18-crown-6 series is influenced by the extent of basicity of ether oxygen, which in turn is dictated by the substituent group. Efficiency also decreased beyond 2 M nitric acid due to co-extraction of crown-nitric acid complex. Uranium: crown: HPFOA mole ratio influenced extraction efficiency, having optimum value at 1:10:100. Under optimized conditions with ditertiarybutyldicyclohexano-18-crown-6 the efficiency was found to be (86±5) %.

Abstract

In the supercritical fluid extraction of uranium from acidic medium employing TBP as co-solvent, effects of various parameters on extraction efficiency were studied. Variation in pressure (80–300 atm), temperature (308–353 K), CO2 flow rate (0.5–3 mL/min), co-solvent percentage (1–10%), molarity of nitric acid (0.5–10 M) were found to influence uranium extraction efficiency. The uranium extraction efficiency depends on distribution ratio and kinetics of transport of U-TBP complex into supercritical CO2. In the 150–300 atm pressure range, variation in extraction efficiency was similar to that of uranium distribution ratio under equilibrium conditions. Whereas below 150 atm, it closely followed supercritical CO2 density variation which could be attributed to non-equilibrium behavior that eventually attained equilibrium. In the non-equilibrium region, increased supercritical CO2 density with pressure favored enhancement in solubility as well as extraction kinetics of the U-TBP complex. Increase in temperature generally resulted in enhanced volatility of U-TBP complex and a decrease in supercritical CO2 density which in turn affected the extraction efficiency. Up to 333 K temperature, extraction efficiency gradually increased due to enhancement in volatility of U-TBP complex which more than compensates for decrease in the supercritical CO2 density. Beyond this temperature, the steep fall in the extraction efficiency is attributed to combined effect of saturation in volatility of the U-TBP complex and significant decrease in the density of supercritical CO2 approaching the critical value at which supercritical CO2 tends to form large clusters thereby resulting in steep decrease in its solvating power. Extraction efficiency was found to increase with nitric acid molarity up to 7 M and afterwards showed small decrease possibly due to competitive co-extraction of HNO3. Up to CO2 flow rate of 1 mL/min increase in extraction efficiency was observed which attained saturation afterwards. Linear increase in extraction efficiency was observed with the amount of TBP. Extraction efficiency was found to increase linearly with logarithm of extraction time. Under optimised conditions (150 atm, 333 K, 1 mL/min CO2 flowrate, 10% co-solvent, 7 M nitric acid and 30 min dynamic extraction time) extraction efficiency was found to be (98±2) was observed. 30 min dynamic extraction mode was found equivalent to 40 min static mode. Online complexation mode was more efficient than in situ mode.

Summary

Feasibility of supercritical CO2 extraction of thorium from tissue paper matrix was studied and various organophosphorus reagents (TBP, TOPO, TPP, TPPO, TBPO) were evaluated as co-solvent. Effects of pressure (100–300 atm), temperature (313–353 K), flow rate (0.5–3 mL min−1), static time (0–40 min) and extraction time (5–40 min) on thorium extraction efficiency were investigated. Pressure of 200 atm, temperature of 333 K, CO2 flow rate of 2 mL min−1, 20 min static period followed by 20 min of extraction were found to be optimum. Under optimum conditions employing various organophosphorus reagents as co-solvent, extraction efficiency indicated the trends: (i) Among phosphates, if aliphatic group attached to P was replaced by aromatic group, then extraction efficiency decreased e.g. TBP (53%±8%) and TPP (39%±5%). (ii) Among the phosphine oxides, higher the aliphatic chain length attached to P, higher was the extraction efficiency e.g. TOPO (68%±4%) and TBPO (50%±9%). (iii) In case of phosphine oxides, if phenyl group was attached to P i.e. TPPO (59%±15%) then extraction efficiency was intermediate to TOPO and TBPO. (iv) Between phosphates and phosphine oxides, for aliphatic hydrocarbons, the extraction efficiencies were comparable e.g. TBP and TBPO. For aromatic ring, extraction efficiency was higher in phosphine oxides e.g. TPP and TPPO. Both TBPO and TPP have shown higher extraction efficiency in combination with TBP. The solvents dissolving oraganophosphorus reagents were found to influence extraction efficiency. The extraction efficiency was found to decrease slightly with increasing aliphatic chain length of the alcohol. The maximum extraction efficiency was obtained with 0.2 M TOPO in methanol where thorium was extractable with 68%±4 efficiency.

Complexing behaviour of three dihydroxybenzenes (DHB) namely, hydroquinone, resorcinol and catechol with Th(IV) along with their protonation constants were studied in 1 M NaClO4 medium at 23 ± 0.5 °C, using pH titration technique. Both protonation and metal-ligand equilibrium constants of dihydroxybenzenes were computed using advance software suite of program HYPERQUAD. Logarithmic values of overall protonation constants (log β1H and log β2H) were found to be 11.47 ± 0.05 and 21.45 ± 0.06, for hydroquinone, 11.76 ± 0.04 and 20.98 ± 0.04 for resorcinol and 12.58 ± 0.03 and 21.87 ± 0.08 for catechol respectively. Complex formation has been investigated in the pH range 2 to 4. The logarithmic values of metal-ligand equilibrium constants obtained in the present work were β13-1=48.51 ± 0.67 and β14-1=64.86 ± 1.25 for hydroquinone, β110=16.98 ± 0.16, β13-1=46.46 ± 0.18 and β14-2=59.65 ± 0.20 for resorcinol and β11-1=14.06 ± 0.10 for catechol. The literature values were reviewed.

The determination of fluoride from diverse matrices at front and back end of nuclear technology and some studies from this laboratory on optimizations of different experimental parameters differing with multiple fuels and reactor materials, have been reviewed. The most useful techniques such as fluoride ion selective electrode (F-ISE) and ion-chromatography (IC) widely adopted as routine methods for fluoride determination in nuclear industry have been discussed. The effect of various buffer strengths on the response of the fluoride ion selective electrode has been examined. The ion chromatographic studies on mobile phase concentration, medium of sample, sample injection volume etc. to get distinct fluoride peak within optimum time in presence of other anionic species in diverse concentrations have been reported. The results of various sample matrices such as UO2, PuO2, (U,Pu)O2, Pu-alloy, thoria, zircaloy, slag, HLLW, LLLW etc., analyzed after matrix separation using pyrohydrolysis setup, for both solid and liquid samples and without matrix separation by masking potentially interfering ions of liquid samples, have been presented.