S. C. Tripathi

Nano-cerium vanadate: a novel inorganic ion exchanger for removal of americium and uranium from simulated aqueous nuclear waste.

Cerium vanadate nanopowders were synthesized by a facile low temperature co-precipitation method. The product was characterized by X-ray diffraction and transmission electron microscopy and found to consist of ∼25 nm spherical nanoparticles. The efficiency of these nanopowders for uptake of alpha-emitting radionuclides (233)U (4.82 MeV α) and (241)Am (5.49 MeV α, 60 keV γ) has been investigated. Thermodynamically and kinetically favorable uptake of these radionuclides resulted in their complete removal within 3h from aqueous acidic feed solutions. The uptake capacity was observed to increase with increase in pH as the zeta potential value decreased with the increase in pH but effect of ionic strength was insignificant. Little influence of the ions like Sr(2+), Ru(3+), Fe(3+), etc., in the uptake process indicated CeVO4 nanopowders to be amenable for practical applications. The isotherms indicated predominant uptake of the radioactive metal ions in the solid phase of the exchanger at lower feed concentrations and linear Kielland plots with positive slopes indicated favorable exchange of the metal ions with the nanopowder. Performance comparison with the other sorbents reported indicated excellent potential of nano-cerium vanadate for removing americium and uranium from large volumes of aqueous acidic solutions.

Potassium cobalthexacyanoferrate–gel beads for cesium removal: kinetics and sorption studies

The hydrogel–sorbent composite beads of potassium cobalthexacyanoferrate (KCoHCF) were synthesized for separation of cesium from waste generated in the nuclear industry. Cross-linked sodium alginate, containing PVA, was used as a binding matrix for the synthesis of spherical beads of 2 mm diameter. The characterization of these beads was carried out, using TGA, SEM and BET surface area analysis techniques, which indicated the highly porous and hydrophilic nature of the beads. The sorption of cesium ions onto these synthesized beads was studied, using a radiotracer technique. To determine equilibrium sorption and kinetic parameters for different initial cesium ion concentrations, all the studies were conducted by a batch method. The experimental data were analyzed, using the different sorption isotherms, and found to be described the best by a Langmuir sorption isotherm. The monolayer capacity of KCoHCF–gel beads is found to be 15 mg g−1 of the dry beads. Sorption kinetics data were analysed, using different kinetic models. The results show that the sorption follows pseudo second-order reaction kinetics. The initial sorption rate and the rate constants for the pseudo first-order, pseudo second-order and intraparticle diffusion were evaluated, and discussed for different initial concentrations in the range 1–20 mg L−1. The mechanism of the sorption of cesium ions onto the synthesized beads was also investigated and film diffusion was found to be the rate determining step.

Studies on the Separation and Recovery of Uranium from Phosphoric Acid Medium Using a Synergistic Mixture of (2-Ethylhexyl)phosphonic Acid Mono 2-Ethyl Hexyl Ester (PC-88A) and Tri-n-octylphosphine Oxide (TOPO)

This paper deals with studies on the extraction of uranium(VI) from phosphoric acid medium using (2-ethylhexyl)phosphonic acid mono 2-ethylhexyl ester and tri-n-octylphosphine oxide individually as well as from their synergistic mixture. Different extraction parameters were investigated. With an increase in phosphoric acid concentration in the aqueous phase, the distribution ratio (Du) was found to decrease in all the cases. Synergism was observed when a mixture of PC-88A and TOPO was used. The synergistic mixture in the mole ratio of 4:1 (1.80 M PC-88A: 0.45 M TOPO) in xylene was found to be most suitable for uranium extraction. Among the various strip liquors used, 5% (w/v) solution of (NH4)2CO3 was found to be the most suitable. Using a mixture of 1.8 M PC-88A and 0.45 M TOPO as the extractant system and 0.5 M ammonium carbonate as the stripping agent, uranium recovery was found to be better than 97% ± 3% in multiple contacts, (n = 2) from actual Davies Gray Waste while in case of wet phosphoric acid more than 52% ± 3% (n = 3) only could be recovered where n is the number of contacts.

Transport of Thorium(IV) Across a Supported Liquid Membrane Containing N,N,N′,N′-Tetraoctyl-3-oxapentanediamide (TODGA) as the Extractant

The transport behavior of Th4+ was investigated from a feed containing 3.0 M HNO3 into a receiver phase containing 0.1 M oxalic acid across a PTFE flat sheet supported liquid membrane (SLM) which contained TODGA (N,N,N′,N′-Tetraoctyl-3-oxapentanediamide) in n-dodecane as the extractant. Effects of the nature of the strippant, extractant concentration, Th concentration in the feed, and feed acidity on the transport rates were investigated. The transport behavior apparently depended on the rate of extraction of the metal ion at the feed-membrane interface and was not diffusion controlled. Influence of Th concentration on flux was also investigated. Transport mechanism was elucidated and the diffusion coefficient was calculated to be 2.13 × 10−7 cm2/s. Solvent extraction studies at varying feed acidity and TODGA concentration were also carried out.

Removal of americium from aqueous nitrate solutions by sorption onto PC88A—Impregnated macroporous polymeric beads

The removal of Am (III) ions from aqueous solutions was studied by solid-liquid extraction using indigenously synthesized Extractant Impregnated Macroporous Polymeric Beads (EIMPBs). These beads were prepared by an in situ phase inversion method using polyethersulfone (PES) as base polymer and 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A) as an extractant. The synthesized EIMPBs were characterized by FTIR, TGA and SEM techniques. The batch equilibration study using these beads for the uptake of Am (III) was carried out as a function of parameters, like pH, equilibration time, Am (III) concentration, etc. The blank polymeric beads, without PC88A, have shown negligible sorption of Am (III) under the experimental conditions. The experimental data on the sorption behavior of Am (III) on the polymeric beads fitted well in the pseudo-second-order kinetics model. The synthesized polymeric beads exhibited very good sorption capacity for Am (III) at pH 3. The reusability of the beads was also ascertained by repetitive sorption/desorption of Am (III) up to 10 cycles of operation, without any significant change in their sorption characteristics.

Controlled Pertraction of Plutonium(III) Under Reducing Conditions From Acidic Feeds Using TODGA as the Carrier Extractant

Transport of Pu3+ across PTFE flat sheet supported liquid membranes (SLM) using N,N,N′,N′-tetraoctyl-3-oxapentanediamide (TODGA) in n-dodecane as the carrier extractant was investigated. The feed solution was usually 3.0 M HNO3 while the receiver phase contained 0.1 M HNO3. In order to model the transport data, the two-phase liquid-liquid extraction experiments are also reported. A series of reducing agents viz. hydrazinium nitrate (HN), phenyl hydrazine (PH), hydroxyl ammonium nitrate (HAN), and ascorbic acid (AA) were evaluated to maintain plutonium in the +3 state in the feed, out of which phenyl hydrazine was found to be the most effective. Effectiveness of various strippants for quantitative and rapid transport of Pu was also evaluated. Additionally, the effects of feed acidity, carrier concentration, membrane thickness, effect of Pu carrier concentration, etc. were investigated. Membrane diffusion co-efficients were calculated and found to be 1.47 × 10−6 cm2/s for Pu(III). The stability of the SLM, investigated as a function of contact time, was found to be reasonably good even after 20 days of continuous operation.

Studies on the Separation and Recovery of Thorium from Nitric Acid Medium using (2-ethyl hexyl) Phosphonic Acid, Mono (2-ethyl hexyl) Ester (PC88A)/N-Dodecane as Extractant System

This paper deals with the studies on the separation and recovery of thorium and 233-uranium from nitric acid medium using (2-ethyl hexyl) phosphonic acid, mono (2-ethyl hexyl) ester/n-dodecane as an extractant system. The different extraction parameters were investigated. The distribution ratio of thorium decreased with increase in nitric acid concentration. The optimum solvent concentration for quantitative separation of thorium from aqueous feed solution was 0.75 M of PC88A whereas dodecane was the most suitable of diluents with an organic to aqueous phase ratio of 1:1. Among the various strippants used, 2 M solution of (NH4)2CO3 was found to be the most suitable for back extraction of thorium. The developed method was used to recover thorium and 233U from radioanalytical waste generated during thorium analysis by ethylene diamine tetraacetic acid (EDTA) titremtric method and recoveries for both Th and U were more than 85%.

Carrier Facilitated Transport of Europium(III) Across Supported Liquid Membranes Containing N,N,N′,N′-tetra-2-ethylhexyl-3-oxapentane-diamide (T2EHDGA) as the Extractant

Studies on the solvent extraction and pertraction behavior of europium(III) was carried out from acidic feed solutions using N,N,N′,N′-tetra-2-ethylhexyl-3-oxapentane-diamide (T2EHDGA) in n-dodecane as the solvent. The nature of the extracted species from the solvent extraction studies conformed to Eu(NO3)3 · 3T2EHDGA which is in variance with the analogous Eu(III) – TODGA (linear homolog of T2EHDGA) extraction system. The transport behavior of Eu(III) was investigated from a feed containing 3.0 M HNO3 into a receiver phase containing 0.01 M HNO3 across a PTFE flat sheet supported liquid membrane (SLM) containing 0.2 M T2EHDGA in n-dodecane as the carrier solvent and 30% iso-decanol as the phase modifier. Effects of feed acidity, carrier extractant concentration, membrane pore size, and Eu concentration in the feed on the transport rates of Eu(III) were also investigated. Membrane diffusion coefficient (D o) for the pertracted species was calculated using the Wilke-Chang equation as 4.25 × 10−6 cm2 · s−1. The influence of Eu concentration on the flux was also investigated. The role of temperature on the transport rates was investigated and the thermodynamic parameters were calculated.

Removal of 241Am from aqueous nitrate solutions by liquid surfactant membrane containing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as ion carrier

A liquid surfactant membrane (LSMs) containing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (H(2)A(2)) was tested for the extraction of americium(III) from aqueous nitrate solutions of different compositions. Span 80 a surface-active agent and 0.5M HNO(3) were used as emulsion stabilizer and internal phase respectively in the LSM system. Influence of some important experimental parameters such as pH of the exterior phase, ionic impurities in the exterior phase, concentration of H(2)A(2) and Span80 in liquid membrane phase on the LSM permeation process was systematically studied. The maximum efficiency of Am(III) extraction among group of experiments was 93+/-2% with modified permeability coefficient=1.21+/-0.02min(-1) and the corresponding concentration factor of Am(III) in the receiving phase was 10.5+/-0.2. Extraction of commonly associated fission product elements such as (137)Cs, (152,154)Eu, (90)Sr, (95)Zr, (144)Ce, (95)Nb and (103)Ru was also investigated from feed solution in the exterior phase adjusted at different pH. In a single batch of extraction, more than 90% removal and about 10 times concentration of Am(III) was obtained from uranium and plutonium depleted (process of Plutonium Uranium Recovery by Extraction) waste solution. A complete demulsification of metal loaded emulsion was obtained by chemical using 2-ethylhexanol and physical using freeze thaw method.

Separation of Carrier-Free 90Y from 90Sr by SLM Technique Using D2EHPA in N-Dodecane as Carrier

Solvent extraction studies on Sr2+ and Y3+ are carried out from varying concentrations (0.01–6.0 M) of nitric acid using di-(2-ethylhexyl)phosphoric acid (D2EHPA) as extractant. Extraction of yttrium is observed to be higher than that of strontium at all the acidities and is found to increase substantially with decreasing concentration of nitric acid. Practically negligible extraction (D < 10−3) of Sr2+ is observed from feed solutions containing nitric acid in the range of 1.0 to 4.0 M. These solvent extraction data are used to optimize the transport of 90Sr and 90Y across the supported liquid membrane (SLM) individually as well as from their mixture (due to insitue growth) under different experimental conditions. Selective separation of 90Y (>90%) from 90Sr is obtained in 6 h, when the concentration of nitric acid in feed is kept at 1.0 M and that of receiving phase is maintained at 4.0 M. 20% D2EHPA in n-dodecane is found to be the optimum carrier concentration for the efficient transport of 90Y in SLM mode. Under these conditions transport of strontium is found to be negligible. Radiochemical purity of the product 90Y is checked by following its decay as well as by extraction paper chromatography. The contamination of 90Sr in 90Y product is found to be < 0.001%. Based on the experimental results, a single stage SLM system for the generation of carrier-free 90Y from 90Sr source is described. The system is amenable for automation and scale up.