Sorption of U(VI) onto granite surfaces: A kinetic approach

Surface sorption experiments of U(VI) onto the surfaces of a Korean granite rock are carried out in order to investigate the kinetics and reversibility of U(VI) sorption as a function of pH and surface types such as fresh intact surfaces and natural fracture surfaces. It was shown that the effect of pH is significant in the sorption of U(VI) onto both types of the granite surfaces. However the sorption rates do not greatly depend upon the pH regardless of the surface types. A two-step first order kinetic behavior dominates onto both the intact surfaces and natural fracture surfaces of granite and that the linearization approach of the kinetic model agrees well with experimental sorption data. The desorption results showed that the sorption process of U(VI) was a little irreversible for the two types of granite surfaces regardless of pH and surface types. This kinetic approach could give a better understanding of U(VI) sorption onto granite surfaces depending on pH and surface types. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sorption of U(VI) on granite

The sorption-desorption of uranium (VI) on Grimsel granite of Switzerland was studied under oxidizing conditions with an initial uranium concentration range of between 9.7·10^–7 and 4.5·10^–4M, using a batch technique. The sorption coefficients varied between 8.0 and 0.4 ml/g and sorption was not fully reversible. The data could be fitted to a Freundlich isotherm. By fitting the data with the Dubinin-Radushkevich equation, a mean energy of sorption of 10.7 kJ/mol was calculated, which corresponds to the energy of ion exchange reactions. The kinetic data could be interpreted by assuming diffusion into the crushed granite particles. The calculated pore diffusion coefficient was between 2.0·10^–11–7.7·10^–11 m²/s.     

Sorption and desorption characteristics of Eu(III) on red earth

The batch method and the column method were simultaneously employed to study the sorption and desorption of Eu(III) on red earth as a function of pH (4.6–6), the presence of a well-characterized fulvic acid (FA) and the iron oxides content of red earth. The results from both methods were consistent qualitatively. The Eu(III) sorption showed significant dependences on pH and FA, the sorption was increased with increasing pH and by addition of FA to the solutions, while the iron oxides content of the red earth had a negative contribution to the sorption of Eu(III). Additionally, the sorption-desorption hysteresis of Eu(III) on red earth occurred at a pH range of 4.6–6. Therefore, the humic substance and high pH have a great tendency to immobilize Eu(III) on red earth.     

Sorption studies of U(VI) on Amberlite XAD-2 resin impregnated with Cyanex272

U(VI) sorption from nitric media using Cyanex272 impregnated on Amberlite XAD-2 resin has been studied using batch method. The influence of different experimental parameter such as aqueous acidity, effect of time, influence of eluting agents on U(VI) uptake was evaluated. The maximum sorption capacity of 0.168?mmol?g^?1 of U(VI) evaluated based upon these studies. Sorption of U(VI) follows both the Langmuir and Freundlich adsorption isotherms.     

Adsorption thermodynamic and desorption studies of U(VI) on modified silica gel (SiAPMS-HL)

In this study, the adsorption process was examined by various isotherm models Langmuir, Freundlich and Dubinin–Radushkevich and equilibrium data were successfully described by Langmuir model. Adsorption thermodynamics of uranium (VI) on modified silica gel (SiAPMS-HL) has been studied within a temperature range from 293 to 333 K and the thermodynamic parameters, such as equilibrium constant (K _D), standard free energy changes (ΔG°), standard enthalpy change (ΔH°) and standard entropy change (ΔS°), have been obtained. The desorption studies were conducted in batch system to investigate the kind, concentration and volume of the eluent.     

Sorption of U(VI) onto a decarbonated calcareous soil

Sorption of U(VI) from aqueous solution to decarbonated calcareous soil (DCS) was studied under ambient conditions using batch technique. Soil samples were characterized by XRD, FT-IR and SEM in detail and the effects of pH, solid-to-liquid ratio (m/V), temperature, contact time, fulvic acid (FA), CO₂ and carbonates on U(VI) sorption to calcareous soil were also studied in detail using batch technique. The results from experimental techniques showed that sorption of U(VI) on DCS was significantly influenced by pH values of the aqueous phase, indicating a formation of inner-sphere complexes at solid–liquid interface, and increased with increasing temperature, suggesting the sorption process was endothermic and spontaneous. Compared to Freundlich model, sorption of U(VI) to DCS was simulated better with Langmuir model. The sorption equilibrium could be quickly achieved within 5 h, and sorption results fitted pseudo-second-order model well. The presence of FA in sorption system enhanced U(VI) sorption at low pH and reduced U(VI) sorption at high pH values. In absence of FA, the sorption of U(VI) onto DCS was an irreversible process, while the presence of FA reinforced the U(VI) desorption process reversible. The presence of CO₂ decreased U(VI) sorption largely at pH >8, which might due to a weakly adsorbable formation of Ca₂UO₂(CO₃)₃ complex in aqueous phase.     

Determination of U(IV) and U(VI) by ion chromatography-inductively coupled plasma mass spectrometry and its application to kinetic studies

The ions normally formed by actinides in aqueous solutions by the oxidation states III-VI are M3-, M4+, MO2+ and MO2(+2), respectively. Oxidation state representatives such as Am3+, Th4+, NpO+ and UO+, which resist oxidation state changes, were used to investigate a method to separate the oxidised species (MO2 and MO2(2+)) from the reduced species (M3+ and M4+). With this method the hexavalent state of uranium could be separated from the tetravalent state of uranium in aqueous media in less than 8 min. Uranium concentrations down to 10(-9) M could be analysed without changing the redox composition during the separation. The oxidation kinetics of the tetravalent uranium for different hydrochloric acid concentrations was investigated. The measurements showed good agreement with values found in the literature, although the uranium concentrations were one million times lower.     

Sorption of U(VI) on magnetic sepiolite investigated by batch and XANES techniques

Additional experimental cross sections were deduced for the long half-life activation products (¹⁷²Hf and ¹⁷³Lu) from the alpha particle induced reactions on ytterbium up to 38 MeV from late, long measurements and for ¹⁷⁵Yb, ¹⁶⁷Tm from a re-evaluation of earlier measured spectra. The cross-sections are compared with the earlier experimental datasets and with the data based on the TALYS theoretical nuclear reaction model (available in the TENDL-2014 and 2015 libraries) and the ALICE-IPPE code.

     

Sorption of U(VI) ions on sol-gel-synthesized amorphous spherically granulated titanium phosphates

The ion-exchange-precipitation mechanism of sorption was established for amorphous spherically granulated titanium phosphates. Under optimal conditions (pH 5.0?C6.0) uranium undergoes sorptive accumulation in amounts substantially exceeding the ion-exchange capacity of the sorbents. Under acidic and alkaline conditions, where ion exchange cannot proceed, uranium sorption follows the precipitation mechanism, which allows treatment of real uranium-containing solutions without optimized conditions. The ion-exchange-precipitation mechanism also operates in sorption of some heavy metals on amorphous spherically granulated titanium phosphates.     

Adsorption of U(VI) onto the carboxymethylated chitosan/Na-bentonite membranes: kinetic,isothermic and thermodynamic studies

The carboxymethylated chitosan (CMC)/Na-bentonite (Na-Bt) composite membranes were prepared and throughly characterized. The Na-Bt/CMC mass ratio was optimized, and CB10 (membrane with Na-Bt/CMC mass ratio of 10%) was selected as the best membrane for U(VI) sorption. XPS analysis indicates that the main mechanism for UO₂²⁺ sorption onto CB10 is through inner-surface complexation. The sorption kinetics followed pseudo-second order model, indicating the chemisorption as the rate-controlling step. The U(VI) sorption on CB10 is endothermic and spontaneous, with the maximum mono-layer adsorption capacity of 115.6 mg/g at pH 5.0 and 298 K. Finally, the U(VI)-loaded CB10 can consecutively desorbed and reused for several cycles.     

Pyrogallol red and bromopyrogallol red in new optical methods for the determination of molybdenum(VI) and tungsten(VI)

The complexation of molybdenum(VI) and tungsten(VI) with pyrogallol red (PR) and bromopyrogallol red (BPR) in the presence of a cationic surfactant, cetylpyridinium bromide was studied. Conditions of the preconcentration of molybdenum(VI) and tungsten(VI) as complexes with PR and BPR on Silochrom S-120 were found. The concentration coefficients were no lower than 67 for a volume of the aqueous phase of 20 mL and a mass of the sorbent of 0.3 g. Chromaticity characteristics of the complexes in solutions and on the sorbent were determined. It was demonstrated that the complex of molybdenum(VI) with BPR in the presence of cetylpyridinium bromide should be used in the analysis of materials with low concentrations of molybdenum.     

Adsorption of Cr(VI) using activated neem leaves: kinetic studies

In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The dimensionless equilibrium parameter, R _L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R _L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg) (min).     

U(VI) sorption on kaolinite: effects of pH,U(VI) concentration and oxyanions

U(VI) sorption on kaolinite was studied as functions of contact time, pH, U(VI) concentration, solid-to-liquid ratio (m/V) by using a batch experimental method. The effects of sulfate and phosphate on U(VI) sorption were also investigated. It was found that the sorption kinetics of U(VI) can be described by a pseudo-second-order model. Potentiometric titrations at variable ionic strengths indicated that the titration curves of kaolinite were not sensitive to ionic strength, and that the pH of the zero net proton charge (pH_PZNPC) was at 6.9. The sorption of U(VI) on kaolinite increased with pH up to 6.5 and reached a plateau at pH >6.5. The presence of phosphate strongly increased U(VI) sorption especially at pH <5.5, which may be due to formation of ternary surface complexes involving phosphate. In contrast, the presence of sulfate did not cause any apparent effect on U(VI) sorption. A double layer model was used to interpret both results of potentiometric titrations and U(VI) sorption on kaolinite.     

Sorption mechanism of U(VI) on a reference montmorillonite: Binding to the internal and external surfaces

Batch type experiments of U(VI) sorption on a reference montmorillonite(SWy-2) were carried out over wide ranges of pH, ionic strength, and totalU(VI) concentration. The influences of these factors on the sorption behaviorof U(VI) were analyzed to gain a macroscopic understanding of the sorptionmechanism. The sorption of U(VI) on montmorillonite showed a distinct dependencyon ionic strength. When it was low (0.01 or 0.001M), almost all of the totalU(VI) was sorbed over the whole pH range studied, therefore, the dependencyon pH was not clear. But the sorption of U(VI) on montmorillonite showed asorption pH edge in the high ionic strength condition (0.1M), like those onother clay minerals, kaolinite and chlorite. A mechanistic model was establishedby considering the mineral structure of montmorillonite together with ourprevious EPR result, which successfully explained the U(VI) sorption on montmorilloniteover the whole range of experimental conditions. The model describes the U(VI)sorption on montmorillonite as simultaneous and competitive reactions of ionexchange and surface complexation, whose relative contribution to the totalsorption depends on pH and ionic strength. At low ionic strength and low pHconditions, ion exchange was the dominant mechanism for U(VI) sorption onmontmorillonite. At high ionic strength and high pH conditions, surface complexationwas the dominant     

Sorption studies of radionuclides on a modified mesoporous cerium(IV) silicate

Five different samples of a new sorbent, modified mesoporous cerium(IV) silicate have been prepared with various mole ratios of Si/Ce and Cetyltrimethylammonium bromide (CTMABr) as template. XRD, nitrogen sorption, SEM, IR, thermogravimetry and sorption of radionuclides have been studied. Separation of Hg(II)-Th(IV), Hg(II)-Zr(IV) and Rb(I)-Zr(IV) have been developed on columns of this novel sorbent.     

Sorption of Mo(VI), W(VI), Cr(VI), and V(V) anions on a silica gel modified with immobilized polyionene

The sorption of polyionene 1,4-MePh on the silica gel surface was studied. The silica gel modified with polyionene sorbed was used for sorption preconcentration of MoO ₄ ^2? , WO ₄ ^2? , Cr₂O ₇ ^2? , and VO ₃ ^? anions from aqueous solutions. The sorption isotherms of these anions on the initial and modified silica gels were obtained and analyzed.     

Biosorption studies of uranium (VI) on cross-linked chitosan: isotherm,kinetic and thermodynamic aspects

The cross-linked chitosan (CS) gels synthesized by using glutaraldehyde (GLA), epichlorohydrin (EC), and ethylene glycol diglycidyl ether (EGDE) as cross-linkers respectively were used to investigate the adsorption of U(VI) ions in an aqueous solution. The pure chitosan (PCS) and the cross-linked chitosan gels were characterized by FTIR and SEM analysis. The kinetic, thermodynamic adsorption and adsorption isotherms of U(VI) ions onto unmodified and modified cross-linked chitosan were studied in a batch adsorption experiments. The effect of pH, contact time and temperature on the adsorption capacity were also carried out. At the optimum pH, the maximum adsorbed amount of PCS, GLACS, ECCS and EGDECS were 483.05, 147.05, 344.83 and 67.56 mg/g, respectively. The uranium (VI) adsorption process of PCS and ECCS followed better with pseudo-second-order kinetic model, while GLACS and EGDECS followed pseudo-first-order kinetic model well. The results obtained from the equilibrium isotherms adsorption studied of U(VI) ions were analyzed in two adsorption models, namely, Langmuir and Freundlich isothms models, the results showed that the Langmuir isotherm had better conformity to the equilibrium data. The thermodynamic parameters such as enthalpy (ΔHo), entropy (ΔSo), and Gibbs free energy (ΔGo) showed that the adsorption process was both spontaneous and endothermic.     

Sorption and desorption of radioselenium on red earth and its solid components

The effect of organic matter and iron oxides as solid components of the red earth on the retention of SeO₃ has been investigated by a batch technique and selective extraction method. The sorption and desorption isotherms of SeO₃ on the untreated red earth and the three treated soils were determined at 20°C, pH 6.8 or 7.2 and in the presence of 0.01M CaCl₂. It was found that the sorption-desorption hysteresis for untreated an treated soils is obvious and the clays play an important role in the sorption-desorption hysteresis, and that the retention of SeO₃ on red earth is attributed to the iron oxides to a great extent.