V. K. Manchanda

N,N,N′,N′‐Tetraoctyl Diglycolamide (TODGA): A Promising Extractant for Actinide‐Partitioning from High‐Level Waste (HLW)

Abstract N,N,N′,N′‐Tetraoctyl diglycolamide (TODGA) has been evaluated as an extractant for the partitioning of minor actinides from simulated high level nuclear waste solutions. Acid uptake studies suggested that TODGA is more basic (KH: 4.1) as compared to TRUEX and DIAMEX solvents viz. CMPO (KH: 2.0) and DMDBTDMA (KH: 0.32), respectively. TODGA molecules display a tendency toward aggregation in n‐dodecane at lower acidities. Effect of diluent on the distribution behavior of Am(III) was studied employing diluents of different dielectric constants. N,N‐dialkyl amides with different alkyl groups viz. dibutyl decanamide (DBDA), di(2‐ethylhexyl) acetamide (D2EHAA), di(2‐ethylhexyl) propionamide (D2EHPRA), di(2‐ethylhexyl) isobutyramide (D2EHIBA), dihexyl octanamide (DHOA) and dihexyl decanamide (DHDA) were evaluated as phase modifiers. Distribution behavior of various metal ions viz. Am(III), Pu(IV), U(VI), Eu(III), Fe(III), Sr(II) and Cs(I) was studied from pure nitric acid solution as well as from simulated high level waste solution.

SOLVENT EXTRACTION STUDIES ON U(VI), Pu(IV), AND FISSION PRODUCTS USING N,N-DIHEXYLOCTANAMIDE

ABSTRACT The extraction behaviour of nitric acid, uranium(VI), and plutonium(IV) has been investigated using N,N-dihexyloctanamide (DHOA) in n-dodecane. Results indicate that, similar to TBP, the amide extracts HNO3, U(VI), and Pu(IV) as HNO3.DHOA, UO2(NO3)2 2DHOA and Pu(NO3)4.2DHOA, respectively, from moderately acidic solutions (3·5M HNO3). On the other hand at higher acidities (8·5 M HNO3), similar to tertiary amines, anionic species, UO2(NO3)3 − and Pu(NO3)6 2− are extracted as [UO2(NO3)− 3] [HDHOA+] and [Pu(NO3)6 2−] [HDHOA+]2, respectively, by ion-pair formation. The basicity of the amide (KH= 0·188), indicated by the equilibrium constant for the uptake of nitric acid is marginally larger than that of TBP (0·160) The third phase formation in H2O-HNO3-O·5 M DHOA-n-dodecane system occurs when nitric acid concentration in the aqueous phase approaches a value of 9 M. The equilibrium constant values (log Kex) for the formation of UO2(NO3)2 2DHOA and Pu(NO3,)4 2DHOA solvates are evaluated as 1·49 ± 0·01 and 3·55± 0·02 respectively. Preliminary data on temperature effect, extraction and stripping from fission products are also included.

Exchanges of uranium(VI) species in amidoxime-functionalized sorbents.

Amidoxime (AO)-functionalized polymer sorbents used in this study were prepared by two different routes involving UV grafting and electron-beam grafting of acrylonitrile (AN) into poly(propylene) fibrous and microporous sheets, and subsequent conversion of AN to AO groups by reacting the precursor sorbent with hydroxylamine. The values of self-diffusion coefficient (D(s)) of UO(2)(2+) in fibrous and sheet AO sorbents were found to be 1.1 x 10(-6) and 2.3 x 10(-10) cm(2) s(-1), respectively. The higher diffusion mobility of UO(2)(2+) in the fibrous AO sorbent was attributed to its higher free volume as observed in scanning electron microscopic studies. The water content was also found to be maximum in AO-fibrous sorbent (165-200 wt %) and minimum in AO-sheet sorbent (70 wt %). In fibrous AO sorbent, the values of D(s) for Na(+) and Sr(2+) were found to be comparable to their self-diffusion coefficients in the aqueous medium. This indicated that the retardation in diffusion mobility of the ions was a minimum in the fibrous AO sorbent. However, D(s) of UO(2)(2+) in the fibrous membrane was found to be significantly lower than that of Sr(2+), which has a self-diffusion coefficient comparable to that of UO(2)(2+) in aqueous medium. This could be attributed to stronger binding of UO(2)(2+) with AO groups as compared to Sr(2+). To understand the parameters affecting the U(VI) sorption from seawater, the U(VI) exchange rates between fibrous AO sorbent (S) and seawater (aq) involving (H(+)/Na(+))(S) right harpoon over left harpoon ([UO(2)(CO(3))(3)](4-))(aq) and (UO(2)(2+))(S) right harpoon over left harpoon ([UO(2)(CO(3))(3)](4-))(aq) systems were experimentally measured. The exchange profiles thus obtained were found to be non-Fickian and much slower than (H(+))(S) right harpoon over left harpoon (UO(2)(2+))(aq) and (UO(2)(2+))(S) right harpoon over left harpoon (UO(2)(2+))(aq) exchanges. This seems to suggest that the reaction kinetics involved in decomplexation of [UO(2)(CO(3))(3)](4-) into UO(2)(2+), which forms a complex with AO groups, is the rate-determining step in sorption of U(VI) from seawater. The kinetics of U(VI) sorption in AO-gel and AO-fibrous sorbents followed the pseudo-second-order rate equation. The density of AO groups in the sorbents and their conditioning were found to influence the U(VI) sorption from seawater.

Evaluation of polymer inclusion membranes containing crown ethers for selective cesium separation from nuclear waste solution

Transport behaviour of (137)Cs from nitric acid feed was investigated using cellulose triacetate plasticized polymer inclusion membrane (PIM) containing several crown ether carriers viz. di-benzo-18-crown-6 (DB18C6), di-benzo-21-crown-7 (DB21C7) and di-tert-butylbenzo-18-crown-6 (DTBB18C6). The PIM was prepared from cellulose triacetate (CTA) with various crown ethers and plasticizers. DTBB18C6 and tri-n-butyl phosphate (TBP) were found to give higher transport rate for (137)Cs as compared to other carriers and plasticizers. Effect of crown ether concentration, nitric acid concentration, plasticizer and CTA concentration on the transport rate of Cs was also studied. The Cs selectivity with respect to various fission products obtained from an irradiated natural uranium target was found to be heavily dependent on the nature of the plasticizer. The present work shows that by choosing a proper plasticizer, one can get either good transport efficiency or selectivity. Though TBP plasticized membranes showed good transport efficiency, it displayed poor selectivities. On the other hand, an entirely opposite separation behaviour was observed with 2-nitrophenyloctylether (NPOE) plasticized membranes suggesting the possible application of the later membranes for the removal of bulk (137)Cs from the nuclear waste. The stability of the membrane was tested by carrying out transport runs for nearly 25 days.

Extraction of actinides using N, N, N′, N′-tetraoctyl diglycolamide (TODGA): a thermodynamic study

Summary The effect of temperature on the extraction behaviour of Am(III), Pu(IV) and U(VI) from nitric acid medium was studied employing N,N,N′,N′-tetraoctyl diglycolamide (TODGA) in n-dodecane. The two-phase equilibrium constants (log K′ex) were calculated and compared with those of other extractants proposed for actinide partitioning, viz. octyl-(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and N,N,N′,N′-dimethyl dibutyl tetradecyl malonamide (DMDBTDMA). Thermodynamic parameters, viz. ΔG, ΔH and ΔS for the extraction of actinides by TODGA were also compared with those of CMPO and DMDBTDMA. These studies indicate that the extraction processes of Am(III) and U(VI) are enthalpy driven whereas entropy factor counteracts the extraction. However, in the case of Pu(IV), the extraction process is enthalpy as well as entropy favoured. Role of diluent on the loading of Nd(III) in 0.1 M TODGA has also been investigated.

Separation of Americium(III) and Europium(III) from Nitrate Medium Using a Binary Mixture of Cyanex‐301 with N‐donor Ligands

Abstract Separation behavior of Am3+ and Eu3+ was studied from aqueous nitrate medium using the “soft‐donor” Cyanex‐301 (bis(2,4,4‐trimethylpentyl) dithiophosphinic acid) in toluene as the extractant. The separation factor (S.F.=DAm/DEu) value of 5000 (obtained with pure Cyanex‐301) increased about eight times when a binary mixture of Cyanex‐301 and N‐donor ligands (such as 2,2′‐bipyridyl (bipy) or 1,10‐phenanthroline (phen)) was used as the extractant. The S.F. values of >40,000 are the highest reported to date. Nitrate ion played an important role in the separation behavior of the trivalent lanthanide and actinide ions. The extracted species, established by the slope analysis method, are Am(A)3 · Bo and Eu(A)2(X) · Bo, respectively where A=deprotonated form of Cyanex‐301, B=phen or bipy and X=NO3 −.

Phytoremediation: role of terrestrial plants and aquatic macrophytes in the remediation of radionuclides and heavy metal contaminated soil and water

Nuclear power reactors are operating in 31 countries around the world. Along with reactor operations, activities like mining, fuel fabrication, fuel reprocessing and military operations are the major contributors to the nuclear waste. The presence of a large number of fission products along with multiple oxidation state long-lived radionuclides such as neptunium (237Np), plutonium (239Pu), americium (241/243Am) and curium (245Cm) make the waste streams a potential radiological threat to the environment. Commonly high concentrations of cesium (137Cs) and strontium (90Sr) are found in a nuclear waste. These radionuclides are capable enough to produce potential health threat due to their long half-lives and effortless translocation into the human body. Besides the radionuclides, heavy metal contamination is also a serious issue. Heavy metals occur naturally in the earth crust and in low concentration, are also essential for the metabolism of living beings. Bioaccumulation of these heavy metals causes hazardous effects. These pollutants enter the human body directly via contaminated drinking water or through the food chain. This issue has drawn the attention of scientists throughout the world to device eco-friendly treatments to remediate the soil and water resources. Various physical and chemical treatments are being applied to clean the waste, but these techniques are quite expensive, complicated and comprise various side effects. One of the promising techniques, which has been pursued vigorously to overcome these demerits, is phytoremediation. The process is very effective, eco-friendly, easy and affordable. This technique utilizes the plants and its associated microbes to decontaminate the low and moderately contaminated sites efficiently. Many plant species are successfully used for remediation of contaminated soil and water systems. Remediation of these systems turns into a serious problem due to various anthropogenic activities that have significantly raised the amount of heavy metals and radionuclides in it. Also, these activities are continuously increasing the area of the contaminated sites. In this context, an attempt has been made to review different modes of the phytoremediation and various terrestrial and aquatic plants which are being used to remediate the heavy metals and radionuclide-contaminated soil and aquatic systems. Natural and synthetic enhancers, those hasten the process of metal adsorption/absorption by plants, are also discussed. The article includes 216 references.

Demonstration of T2EHDGA Based Process for Actinide Partitioning Part II: Counter-Current Extraction Studies

Abstract Counter-current mixer-settler studies for actinide partitioning were carried using N,N,N′,N′-tetra-2-ethylhexyl diglycolamide (T2EHDGA) as the extractant. The feed solution was Simulated High Level Waste of Pressurized Heavy Water Reactor (PHWR-SHLW) origin spiked with 241Am, 244Cm, 152Eu, 137Cs, 85,89Sr, 59Fe, 106Ru, 109Pd, 95Zr, and 99Mo tracers. The organic stream was 0.1 M T2EHDGA + 5% isodecanol in n-dodecane. Extraction, scrubbing, and stripping experiments were performed by maintaining an organic to aqueous phase ratio of 1. More than 99.9% of the trivalent actinides and lanthanides were extracted in four stages, and the decontamination factors (D.F.) values were >103 obtained for most fission products. The co-extraction of Zr and Pd was prevented by the addition of oxalic acid and N-(2-hydroxyethyl)-ethylenediamine-triacetic acid (HEDTA) into the feed solution. However, ∼20% Ru and 10% Mo was extracted into the organic phase, which was successfully scrubbed using a mixture of 0.2 M oxalic acid and 0.1 M HEDTA in 5 M HNO3. Finally, the extracted actinides and lanthanides were quantitatively stripped with 0.2 M HNO3. Raffinate of the extraction cycle was found to be free from any alpha activity.

Development of T2EHDGA Based Process for Actinide Partitioning. Part I: Batch Studies for Process Optimization

Abstract N,N,N′,N′-tetra-2-ethylhexyl diglycolamide (T2EHDGA) has been used for the extraction of actinides, lanthanides, fission products, and structural elements from Pressurized Heavy Water Reactor High Level Waste (PHWR-HLW). In view of third-phase formation under loading conditions, iso-decanol, tri-n-butyl phosphate (TBP), and N,N-di-n-hexyl octanamide (DHOA) were evaluated as the phase modifiers along with T2EHDGA. The loading of lanthanides (e.g. Nd(III), used as surrogate for trivalent minor actinides, Am(III) and Cm(III) and rare earth elements) in the organic phase sharply increased with the addition of iso-decanol as compared to other modifiers (TBP and DHOA). A minimum concentration of phase modifiers needed to avoid third-phase formation for 0.1 M T2EHDGA was 5% iso-decanol, 20% TBP or 20% DHOA. The most promising system, viz. 0.1 M T2EHDGA and 5% (v/v) iso-decanol was evaluated as extractant for actinide partitioning. The distribution behavior of various metal ions, viz. Am, Pu, U, Eu, Sr, Pd, Cs, Tc, Fe, and Mo has been studied from nitric acid as well as from synthetic PHWR-HLW using the optimized organic phase. Extraction of Am (tracer) from synthetic PHWR-HLW suggested a quantitative recovery of minor actinides in four contacts, while stripping (with 0.01 M HNO3) was quantitative in two contacts at O/A = 1. The testing of the optimized extraction system in mixer-settlers with simulated HLW is in progress.

Separation Studies of Uranium and Thorium Using Di-2-Ethylhexyl Isobutyramide (D2EHIBA)

The extraction behavior of di-2-ethylhexyl isobutyramide (D2EHIBA) in dodecane medium for U(VI), Th(IV), and fission products such as Zr, Ce, Eu, and Cs, and the structural material Fe, has been investigated over a wide range of nitric acid concentrations. It has been observed that whereas D U varies from < 10−3 (pH 2.0) to 4.4 (6 M HNO3) with 1 M ligand, the corresponding D Th values are 1.5 × 10−3 and 4 × 10−2. In the presence of 250 g/L of Th, D U values are 8.6 (pH 2.0) and 2.2 (6 M HNO3). D2EHIBA has been found to be a promising extractant of trace concentrations of U in the presence of macro amounts of Th. The extraction of fission products and Fe is found to be negligible. D2EHIBA is found to extract nitric acid predominantly as a 1:1 species (K H = 0.156 ± 0.048). U(VI) is extracted as a disolvate UO2(NO3)·2D2EHIBA (K ex = 0.87 ± 0.08).