Release of long-lived radioactivity to the aquatic bodies from various nuclear fuel cycle related operations is of great environmental
concern in view of their possible migration into biosphere. This migration is significantly influenced by various factors
such as pH, complexing ions present in aquatic environment and sorption of species involving radionuclides on the sediments
around the water bodies. 241/243Am are two major radionuclides which can contribute a great deal to radioactivity for several thousand years. In the present
study, 241Am sorption on natural sediment collected from site near a nuclear installation in India, has been investigated under the
varying conditions of pH (3–10) and ionic strength [I = 0.01–1 M (NaClO4)]. The sorption of Am increased with pH of the aqueous medium [10% (pH 2) to ~100% (pH 10)], which was explained in terms
of the increased negative surface charge on the sediment particles. There was marginal variation in Am(III) sorption with
increased ionic strength (within error limits) of the aqueous medium suggesting inner-sphere complexation/sorption process.
Sediment was characterized for its elemental composition and structural phases using Energy Dispersive X-Ray (SEM-EDX) and
X-Ray Diffraction (XRD) techniques. Zeta-potential measurement at I = 0.1 M (NaClO4) suggested that Point of Zero Charge (pHPZC) was ~2, indicating the presence of silica as major component in the sediment. Kurabtov plot using sorption data as a function
of pH at fixed I = 0.1 M (NaClO4) indicated the presence of multiple Am(III) species present on the surface. Potentiometric titration of the suspension indicated
the presence of mineral oxide like behavior and assuming a generic nature (≡XOH) for all types of surface sites, protonation–deprotonation
constants and total number of sites have been obtained. The sorption data has been modeled using 2-pK Diffuse Double Layer
Surface Complexation Model (DDL-SCM). ≡XOAm2+ has been identified as the main species responsible for the sorption profile.
Attapulgite has been applied in the sorption of metal and radionuclide ions since its discovery. Herein, radionuclide Am(III)
sorption onto attapulgite was carried out at 25 °C in 0.01 mol/L NaNO3 solutions. Effects of contact time, Am(III) initial concentration, pH, humic acid and temperature on Am(III) sorption onto
attapulgite were investigated. The sorption of Am(III) increases with increasing contact time and reaches a maximum value
within 24 h at different Am(III) initial concentration. The fast sorption velocity indicates that strong chemical sorption
or strong surface complexation contributes to the sorption of Am(III) onto attapulgite under the experimental conditions.
The experimental data can be described well by the pseudo-second-order rate model. The sorption of Am(III) onto attapulgite
is strongly dependent on pH values and surface complexation is the main sorption mechanism. The presence of HA enhances the
sorption of Am(III) onto attapulgite at pH < 8.5, whereas, at pH > 8.5, little effect of HA on Am(III) sorption is observed.
The Langmuir, Freundlich and D-R models were used to simulate the sorption data at different pH values and the results indicated
that Langmuir model simulates the experimental data better than Freundlich and D-R models. The thermodynamic parameters indicates
that the sorption of Am(III) onto attapulgite is an endothermic and spontaneous process. The results suggest that the attapulgite
is a suitable material as an adsorbent for preconcentration and immobilization of Am(III) from aqueous solutions.
Sorption behavior of Am(III) onto granite was investigated. The distribution coefficient (Kd) of Am(III) onto granite was determined in the solution of which pH was ranged from 2.9 to 11.4 and ionic strength was set
at 10−2 and 10−1. TheKd values were found to increase with increasing pH and with decreasing ionic strength. The obtained data were successfully
analyzed by applying an electrical double layer model. The optimum parameter values of the double layer electrostatics and
adsorption reactions were obtained, and the selective adsorption behavior of Am(III) onto the granite was discussed.
The Am(III) adsorption isotherms on natural hematite, on a red earth and on three kinds of treated red earth samples were
determined and compared. The treatment was performed to remove iron oxides, organic matter (OM) and both. The batch technique
and aqueous Am(III) solutions with molarities less than 3 . 10-9 mol/l were used. A very high adsorbability of Am(III) on hematite was found, and in order to decrease the adsorption percentage,
stable Eu3+ as a hold back carrier was added to the aqueous solution. All the isotherms were found to be linear. The strong effects of
pH, ionic strength and fulvic acid (FA) on the Am(III) adsorption on natural hematite were demonstrated. A positive contribution
of OM and an unexpectedly high negative influence of iron oxides on the Am(III) adsorption by the untreated red earth were
found too. The average distribution coefficients of Am(III) adsorption on natural hematite, on red earth and on three kinds
of treated red earth samples were determined, respectively, from these linear isotherms. The very high adsorbability of Am(III)
on hematite and the very strong negative influence of iron oxides on the Am(III) adsorption on untreated red earth were distinctly
demonstrated as well. It appears that the adsorbability of composed natural materials cannot be predicted from the adsorbabilities
of each of the mineral components alone, due to possible interactions between the mineral components and the different characteristics
of the composite.
The unsymmetrical diamide methylbuthylmalonamide has been synthesized and used in the extraction of U(VI), Pu(IV) and Am(III) in benzene medium. The distribution ratio for the three cations was found to increase with increasing aqueous nitric acid concentration. U(VI) and Pu(IV) were found to be extracted as disolvates while Am(III) as a trisolvate. The thermodynamic parameters determined by the temperature variation method showed the extraction reactions to be mainly enthalpy-controlled. Am(III) was found to be back-extracted with dilute nitric acid, while Pu(IV) by dilute nitric acid-hydrofluoric acid mixture and U(VI) by dilute sodium carbonate solution.
Benzyldimethyldodecylammonium nitrate and benzyltrioctylammonium nitrate were used for the extraction of Am(III) from aqueous
nitrate solutions. The dependence of the extraction performance for Am(III) on the concentration of nitric acid, the kind
and concentration of salting-out agents in the aqueous phase, and the kind of solvent was investigated. Americium is extracted
by the above quarternary salts as a R4NAm(NO3)4 associate. The extraction of Am(III) is compared with the extraction of lanthanides. The high differences in the distribution
coefficients for lanthanides and americium can be utilized for the separation of lanthanides and americium.
Binding constants of Eu(III)- and Am(III)-complexes with soil-derived humic acid were determined by solvent extraction at various pH and ionic strength. Based on the dependence of binding constants on pH and ionic strength, stabilities of the humate complexes in land water and seawater were estimated. Speciation calculation based on the binding constants indicated that Am(III) could combine with humic substances in natural water system.
The adsorption and desorption of Am(III) on a calcareous soil (sierozem) and its parent material (loess) were studied by batch technique. The molarities of the Am(III) aqueous solutions were less than 5 . 10-9 mol/l. High adsorbability was found of Am(III) on the calcareous soil and its parent material. In order to decrease the adsorption and, hence, to investigate the adsorption characteristics properly, stable Eu3+ as hold back carrier and analogue was added to the aqueous solution. The relative contributions of CaCO3, organic matter (OM) to the Am(III) adsorption on calcareous soil and its parent material were investigated. The adsorption and desorption isotherms of Am(III) on untreated soil and loess and the three kinds of treated soils and three kinds of treated loesses to remove CaCO3, OM and both CaCO3 and OM were determined, respectively. It was found that all isotherms were linear, the average distribution coefficients (K-d) for the untreated soil and for the untreated loess were almost equal, while there was an obvious difference between the values of the average distribution coefficients (K-d) for the treated soil and the treated loess to remove CaCO3 or OM. The adsorption-desorption hysteresis on the untreated and treated soils and loesses actually occurred and there was an obvious difference between the hysteresis coefficients on both the corresponding treated soil and loess. It can be concluded that the adsorbability of Am(III) on calcareous soil is similar to that on its parent material, and that the contributions of CaCO3 and OM to the Am(III) adsorption by the untreated soil are different from those by the untreated parent material.
For the selective extraction of Am(III) and Eu(III), quadridentate divalent phenolic Schiff bases-bis-salicylaldehyde ethylenediamine
(H2salen) was investigated as a kind of extractant. The influences of alkaline cation, inorganic anion, ionic strength, pH and
the concentration of H2salen on the distribution ratio of Am(III) and Eu(III) were investigated in detail. As a result, Am(III) and Eu(III) made
anionic 1:1 complexes with the ligand (H2salen) and could be extracted into nitrobenzene as ion-pairs with a suitable monovalent counter anion in the aqueous solution,
the extracted species were possibly of the type Am(H2salen) Eu(salen)Cl and Eu(H2salen)Cl3, respectively. The extractability of Eu(III) was significantly stronger than that of Am(III) and the maximum separation factor,
SF(Am/Eu), was 96 at pH 4.0. The results indicated that H2salen had good selectivity for Am(III) and Eu(III).
The oxidation of americium in HNO3, H2SO4 and HClO4 solutions by a mixture of potassium persulfate with silver salt in the presence of potassium phosphotungstate has been investigated. The influence of acid and its concentration, of (NH4)2S2O3, K10P2W17O61 and silver salt on Am(III) oxidation rate, yield and stability of Am(IV) and Am(VI), has been studied. The complexation of Am(III), Am(IV) and Am(VI) with phosphotungstate ions has been investigated. It has been established that Am(III) and Am(IV) form ML2 complexes and their apparent stability constants have been estimated. The oxidation mechanism is discussed. A method for preparing of Am(IV) in 0.1–6M HNO3, O.1–3M H2SO4, 0.1–1M HClO4 solutions is proposed. The oxidation of Am(III) to Am(IV) by KBrO3 and K2Cr2O7 in HNO3, H2SO4, HClO4 solutions in the presence of K10P2W17O61 has been investigated.