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 ((237)Np), plutonium ((239)Pu), americium ((241/243)Am) and curium ((245)Cm) make the waste streams a potential radiological threat to the environment. Commonly high concentrations of cesium ((137)Cs) and strontium ((90)Sr) 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.

Radiochemistry in India: A saga of five decades

Radiochemistry in India essentially blossomed under the auspices of the Department of Atomic Energy (DAE) for the last 55 years or so. Major activities in this area are centred at Bhabha Atomic Research Centre, Mumbai (BARC) and Indira Gandhi Centre for Atomic Research, Kalpakkam (IGCAR). Though there were several centers of excellence which were established by renowned radiochemists during the 1960s at the academic institutions in different parts of the country and nurtured by their close associates during the eighties and nineties, their glamour did not last long and only very few have sustained the challenges presented by social and technological upheaval of last five decades. Board of Research in Nuclear Sciences (BRNS), an organ of DAE has been in the forefront for promotion of education and research in nuclear sciences at academic institutions. It sponsors symposia in Nuclear and Radiochemistry (NUCAR), Nuclear Analytical Chemistry (NAC) and Applications of Radioisotopes in Chemistry, Environment and Biology (ARCEB) which are organized periodically to provide a platform for interaction of the radiochemists within and outside DAE. A professional body, viz. Indian Association of Nuclear Chemists and Allied Scientists (IANCAS), formed in early eighties at BARC, Mumbai has been spearheading the campaign to popularize the subject of radiochemistry in schools and colleges through workshops and publishing monographs and thematic bulletins regularly in the area of interest to the radiochemists.

During the last five decades, radiochemistry programme at BARC has centered around attaining excellence in basic research utilizing radiations and radioisotopes to unravel various nuclear and chemical phenomena, related to actinides and fission products. This programme encompassed a number of research and development areas such as nuclear fission, nuclear reactions, nuclear probes for materials study, nuclear and chemical properties of actinides, actinide spectroscopy, separation science of actinides, thermodynamics and characterization of fuels, post irradiation examination, chemical and non destructive assay techniques for nuclear materials. Production and application of radioisotopes in societal benefit activities in agriculture, industry and health science was another facet of the radiochemistry programme at BARC. The radiochemistry programme at IGCAR has been focused on chemistry of fast reactor and fuel cycle related materials such as sodium and boron, in addition to the chemistry of actinides, fission products and pyrochemical studies related to processing of spent fuels.

Publication of about 2000 peer reviewed papers in international journals of repute and award of Ph.D. degrees to more than 150 scientists is an evidence of the front line research activities pursued under this programme. All along the 55 years, sustained efforts were made to meet the growing challenges of closed nuclear fuel cycle (both thermal as well as fast). After five decades of continuous research and development perhaps one can feel satisfied that the programme could fulfil not only the dreams of its founders but is also ready to take on the future challenges related to the second and third stage of Indian nuclear power programme.

Modified synthesis scheme for N,N´-dimethyl-N,N´-dioctyl-2,(2´-hexyloxyethyl) malonamide (DMDOHEMA) and its comparison with proposed solvents for actinide partitioning

N,N´-dimethyl-N,N´-dioctyl-2,(2´-hexyloxyethyl) malonamide (DMDOHEMA) has been synthesized by a relatively simpler route with a better yield than the conventional procedures for the synthesis of pentaalkyl substituted diamides. The proposed route replaces the use of acid chlorides and brominating agents (corrosive in nature) by acid diester and p-toluene sulphonyl (tosyl) chloride, respectively. The later easily replaces hydrogen from primary OH group of the 3-oxanonyl side chain and relatively less corrosive. The synthesized product has been tested for the extraction behavior of 241Am, Np, Pu as pure tracers as well as under Pressurized Heavy Water Reactor (PHWR) Simulated High-Level Waste (SHLW) conditions. The stoichiometries of extracted species of Np(IV), Pu(IV), and Am(III) from 3 M HNO3 using varying concentrations of DMDOHEMA in n-dodecane has been determined. The effect of the nature of mineral acid (viz. HCl, HNO3, and HClO4) on the extraction of Am(III) has also been investigated employing DMDOHEMA and N,N´-dimethyl-N,N´-dibutyl-tetradecyl malonamide (DMDBTDMA) as extractants and explained in terms of the aggregation behavior of diamides. The performance of DMDOHEMA has been evaluated vis-à-vis other proposed extractants for actinide partitioning under PHWR-SHLW conditions.

Liquid-liquid extraction and flat sheet supported liquid membrane studies on Am(III) and Eu(III) separation using 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine as the extractant

Solvent extraction and supported liquid membrane transport studies for the preferential removal of Am(3+) from feeds containing a mixture of Am(3+) and Eu(3+) was carried out using 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-Pr-BTP) as the extractant. Diluent plays an important role in these studies. It was observed that the distribution coefficients deteriorate significantly for both Am(3+) and Eu(3+) though the separation factors were affected only marginally. The transport studies were carried out at pH 2.0 in the presence of NaNO(3) to result in the preferential Am(3+) transport with high separation factors. Effect of different experimental parameters, viz. feed composition, stripping agents, diluents of the organic liquid membrane and membrane pore size was studied on the transport and separation behaviour of Am(3+) and Eu(3+). The supported liquid membrane studies indicated about 85% Am(3+) and 6% Eu(3+) transport in 6h using 0.03 M n-Pr-BTP in n-dodecane/1-octanol (7:3) diluent mixture for a feed containing 1M NaNO(3) at pH 2 and a receiver phase containing pH 2 solution as the strippant. Consequently, a permeability coefficient of (1.75 ± 0.21) × 10(-4)cms(-1) was determined for the Am(3+) transport. Stability of the n-Pr-BTP and its SLM was also studied by carrying out the distribution and transport experiment after different time intervals.

Effect of humic acid on sorption of technetium by alumina

Sorption of technetium by alumina has been studied in absence as well as in presence of humic acid using (95)Tc(m) as a tracer. Measurements were carried out at fixed ionic strength (0.1M NaClO(4)) under varying pH (3-10) as well as redox (aerobic and reducing anaerobic) conditions. Under aerobic conditions, negligible sorption of technetium was observed onto alumina both in absence and in presence of humic acid. However, under reducing conditions (simulated with [Sn(II)] = 10(-6)M), presence of humic acid enhanced the sorption of technetium in the low pH region significantly and decreased at higher pH with respect to that in absence of humic acid. Linear additive as well as surface complexation modeling of Tc(IV) sorption in presence of humic acid indicated the predominant role of sorbed humic acid in deciding technetium sorption onto alumina.

Studies on the selective Am3+ transport, irradiation stability and surface morphology of polymer inclusion membranes containing Cyanex-301 as carrier extractant

Transport behaviour of Am(3+) across cellulose triacetate (CTA) based polymer inclusion membranes (PIM) containing Cyanex-301 (bis(2,4,4-trimethylpentyl)dithiophosphinic acid) as the carrier extractant and tri-n-butyl phosphate (TBP) or 2-nitrophenyloctylether (NPOE) as the plasticizer was investigated from different feed and strip conditions. The TBP plasticized membrane resulted back transport of Am when alpha-hydroxy iso-butyric acid was used as the complexing agent in the strip phase while no such effect was seen when ethylene diamine tetraacetic acid (EDTA) was used as the complexant. Effect of varying Cyanex-301 concentration and bipyridyl (bipy) concentration on Am transport was also investigated. Long term reusability of the membrane was studied by measuring the permeability coefficient (P) after exposing the PIMs to a maximum gamma ray dose of ∼ 200 kGy. The surface morphology of the membranes was analyzed by atomic force microscopy and the roughness parameter was correlated to transport efficiency.

Extraction of caesium(I) from HNO3 medium using room temperature ionic liquid containing calix[4]crown ligands as the selective extractants

Studies on the extraction of Cs(I) from nitric acid medium were carried out using three commercial calix-crown ligands, viz. calix[4]arene-bis-crown-6, calix[4]arene-bis-1,2-benzocrown-6 and calix[4]arene-bis-2,3-naphthocrown-6 dissolved in different room temperature ionic liquids (RTILs), viz. 1-butyl-3-methyl-imidazolium hexafluorophosphate (C4mim·PF6), 1-hexyl-3-methyl-imidazolium hexafluorophosphate (C6mim·PF6) and 1-octyl-3-methyl-imidazolium hexafluorophosphate (C8mim·PF6). The distribution ratio of Cs(I) decreased with increased alkyl chain length of RTILs. The effect of nitric acid concentration on distribution behaviour of Cs(I) by calix[4]arene-bis-1,2-benzocrown-6 was investigated. The extraction of Cs(I) from HNO3 was found to be exothermic and extraction was found to be enthalpy driven. Selective extraction of Cs(I) could be achieved from a mixt ure of various fission product elements like Tc, Mo, Zr, Ce, Ru, La, Ba, and Sr.

Facilitated transport of uranium(VI) across supported liquid membranes containing T2EHDGA as the carrier extractant

Facilitated transport of uranyl ion from nitric acid feed solutions was investigated across PTFE supported liquid membranes using N,N,N',N'-tetra-2-ethylhexyl-3-pentane-diamide (T2EHDGA) in n-dodecane as the carrier extractant containing 30% iso-decanol as the phase modifier. Solvent extraction studies indicated extraction of species of the type, UO(2)(NO(3))(2)·T2EHDGA. The distribution coefficients increased in the presence of NaNO(3) as compared to equivalent concentration of HNO(3) which was exactly the opposite of what was reported for Am(III)-TODGA extraction system. Supported liquid membrane studies indicated about 11h were required for quantitative transport of U(VI) from a feed of 3M HNO(3) using 0.2M T2EHDGA in n-dodecane containing 30% iso-decanol as the carrier extractant. Effect of various parameters such as feed acidity, T2EHDGA concentration, and nature of the strippant on the transport rate was investigated. The transport was found to be diffusion controlled in the membrane phase and the permeability coefficient was calculated to be (3.20 ± 0.13)× 10(-4)cm/s for the feed composition of 3M HNO(3), receiver phase composition of 0.01 M HNO(3) and membrane carrier phase of 0.2M T2EHDGA in n-dodecane containing 30% iso-decanol. The present results may be useful for the separation of U from lean solutions or radioactive wastes considered hazardous due to the presence of alpha-particle emitting radionuclides.

Sorption of curium by silica colloids: effect of humic acid

Sorption of curium by silica colloids has been studied as a function of pH and ionic strength using (244)Cm as a tracer. The sorption was found to increase with increasing pH and reach a saturation value of ∼95% at pH beyond 5.3. The effect of humic acid on the sorption of (244)Cm onto silica was studied by changing the order of addition of the metal ion and humic acid. In general, in the presence of humic acid (2 mg/L), the sorption increased at lower pH (<5) while it decreased in the pH range 6.5-8 and above pH 8, the sorption was found to increase again. As curium forms strong complex with humic acid, its presence results in the enhancement of curium sorption at lower pH. At higher pH the humic acid present in the solution competes with the surface sites for curium thus decreasing the sorption. The decrease in the Cm sorption in presence of humic acid was found to be less when humic acid was added after the addition of curium. Linear additive model qualitatively reproduced the profile of the Cm(III) sorption by silica in presence of humic acid at least in the lower pH region, however it failed to yield quantitative agreement with the experimental results. The results of the present study evidenced the incorporation of Cm into the silica matrix.

Thermodynamic study of Th(IV) complexes with dicarboxylates by potentiometry and calorimetry

The thermodynamic quantities (ΔG c, ΔH c and ΔS c) for formation of Th(IV) complexes with dicarboxylic ligands, namely, malonate, succinate, glutarate and adipate were determined using potentiometry and calorimetry. The protonation constants of the dicarboxylate ligands were determined by potentiometric titration of the ligand solution, while the corresponding enthalpy values were taken from the literature. In the case of Th(IV)-malonate, multiple species (ML j , j=1–3) were revealed from the potentiometric data, while in the case of higher homologues, the data for only 1:1 complex could be obtained owing to precipitation at higher ligand concentration. The effect of chain length on the thermodynamic parameters of Th(IV)-dicarboxylate complexation was studied. All the complexation reactions were found to be highly entropy driven, which is the characteristic of the hard acid (metal ion) and hard base (ligand) interactions. The TΔS c values for 1:1 complexes were found to be nearly constant while the ΔH c values increased from malonate to glutarate and then leveled off in adipate. The thermodynamic data of Th(IV) have been compared with corresponding data for U(VI) and rare earths.

Selective separation of radio-cesium from acidic solutions using supported liquid membrane containing chlorinated cobalt dicarbollide (CCD) in phenyltrifluoromethyl sulphone (PTMS)

A supported liquid membrane method was developed using chlorinated cobalt dicarbollide (CCD) in phenyltrifluoromethyl sulphone (PTMS) as the carrier, impregnated in PTFE flat sheet membranes for the selective separation of Cs(I) from nitric acid feed solution. Solvent extraction studies were carried out for optimizing the feed as well as strip conditions. >95% Facilitated transport of Cs(I) was observed in about 3h when 1M HNO(3) and 8M HNO(3) were used as the feed and strip solutions, respectively while 0.025M CCD in PTMS was used as the carrier extractant. Selectivity studies, carried out using a mixture of radiotracers viz. (51)Cr, (59)Fe, (99)Mo, (99m)Tc, (106)Ru, (137)Cs, (152)Eu and (241)Am, indicated selective transport of Cs(I) with DF values >100. Though reproducibility of the transport data was excellent when carried out in two successive transport experiments with freshly loaded carrier solvent, the stability of the membrane was poor which restricts its long term use.

Selective caesium transport using hollow fibre-supported liquid membrane containing calix[4]arene-bis-naphthocrown-6 as the carrier extractant

Polypropylene hollow fibre-based supported liquid membrane (HFSLM) containing calix[4]-bis-2,3-naphtho-crown-6 in 20% (v/v) n-dodecane+80% (v/v) 2-nitrophenyl octyl ether (NPOE) as the carrier solvent was employed for the first time to investigate the transport behaviour of radio-caesium. The carrier solution was 1 mM calix[4]-bis-2,3-naphtho-crown-6 in 20% (v/v) dodecane+80% (v/v) NPOE+0.4% alamine 336 and the feed was 3 M HNO3 while distilled water was used as the receiver phase. In spite of high viscosity of the carrier solvent, quantitative (>99%) transport was possible in about 6 h. Significant amount of acid transport was also noticed which was ascribed to a co-transport mechanism involving encapsulated hydronium ion. Selectivity studies were carried out by taking a mixture of radiotracers, viz. 51Cr, 59Fe, 85,89Sr, 137Cs, 152,154Eu and 241Am, which represent some of the elements present in the high level waste, in the feed solution. The decontamination factors for various metal ions were found to be in the range 540–890. High decontamination factors as well as possibility of working over several days without deterioration of the liquid membrane suggested possible application of the system for the recovery of radio-caesium from high level waste. Studies on the membrane stability indicated reproducible transport data over 40 h.

Sorption of 137Cs, 133Ba and 154Eu by synthesized sodium aluminosilicate (Na-AS)

Sodium aluminosilicate has been synthesized by solution route for use as a sorbent for various radionuclides. It was characterized by XRD, zeta potential, BET surface area, FTIR spectroscopy and site density measurement. Sorption studies of (137)Cs, (133)Ba and (154)Eu on synthesized sodium aluminosilicate have been carried out at varying pH (3-10). Sorption of all the metal ions was found to increase with pH of suspension with the saturation value increasing with the oxidation state of metal ion. Effect of Aldrich humic acid (2mg/l) on sorption was also investigated. In case of (137)Cs, the sorption was not affected by the presence of humic acid, while in case of (133)Ba and (154)Eu, sorption was enhanced at lower pH and decreased at higher pH in presence of HA. A series of experiments were carried out for (154)Eu sorption on sodium aluminosilicate at various conditions. Sorption of europium was analyzed by different sorption isotherms, viz., Freundlich and D-R isotherm. Thermodynamic data reveal sorption phenomena as endothermic and spontaneous. Studies were further extended to find out effect of diverse ions (Ca(2+) and CO(3)(2-)) on sorption of europium.

Selective recovery of uranium from THOREX feed by hollow fibre supported liquid membrane technique containing di(2-ethylhexyl) isobutyramide (D2EHIBA) as the carrier

Hollow fibre supported liquid membrane (HFSLM) studies were carried out for the selective recovery of uranium from THOREX feed solution (0.2 g/L U+200 g/L Th+0.03 M HF+0.1 M Al(NO3)3 at 4 M HNO3) using di(2-ethylhexyl) isobutyramide (D2EHIBA) as the carrier and distilled water as the strippant. About 85% of uranium was selectively transported into the receiver phase in 30 min with <1% contamination from thorium. On the other hand, under identical conditions of feed and strip solutions, about 90% of uranium was recovered in the same time when 5% TBP (tri-n-butyl phosphate) was used as the carrier, but a relatively large amount of thorium (>10%) was also co-transported to the product side. Strip phase acidity analysis revealed significant amount of acid co-transport with both the carriers. Acid transport was found to increase with the total salt content of the feed solution. The transport of metal ions was explained with the help of a theoretical model. Stability of the liquid membrane was also studied over about 5 d for long term reusability.