Sorption of Uranium ( VI ) and Thorium ( IV ) by Jordanian Bentonite

Puri�cation of raw bentonite was done to remove quartz. is includes mixing the raw bentonite with water and then centrifuge it at �50 rpm; this process is repeated until white puri�ed bentonite is obtained. �RD, �RF, FTIR, and SEM techniques will be used for the characterization of puri�ed bentonite. e sorption behavior of puri�ed Jordanian bentonite towards UO2 2+ and  metal ions in aqueous solutions was studied by batch experiment as a function of pH, contact time, temperature, and column techniques at 25.0C and pH = 3. e highest rate of metal ions uptake was observed aer 18 h of shaking, and the uptake has increased with increasing pH and reached a maximum at pH = 3. Bentonite has shown high metal ion uptake capacity toward uranium(VI) than thorium(IV). Sorption data were evaluated according to the pseudosecond-order reaction kinetic. Sorption isotherms were studied at temperatures 25.0C, 35.0C, and 45.0C. e Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) sorption models equations were applied and the proper constants were derived. It was found that the sorption process is enthalpy driven for uranium(VI) and thorium(IV). Recovery of uranium(VI) and thorium(IV) ions aer sorption was carried out by treatment of the loaded bentonite with different concentrations of HNO3 1.0M, 0.5M, 0.1M, and 0.01M. e best percent recovery for uranium(VI) and thorium(IV) was obtained when 1.0M HNO3 was used.


Introduction
Jordan is rich in industrial rocks and minerals.e main two areas containing bentonite deposits in Jordan are Al Yamaniyya and Al Azraq areas.Al Yamaniyya is located about 10 km south of Aqaba, while Al Azraq area is divided into two parts: Q'a Al Azraq and Ein Al Bayda areas.
Bentonite is suitable to isolate nuclear wastes and groundwater because of its low hydraulic conductivity, high swelling property, good self-sealing capacities, and so forth [1], Bentonite is also used as buffer material or back�ll material according to its function and location in the nuclear waste disposal �eld [2].
e presence of uranium and thorium in the environment not only originates from the nuclear industry but also from other anthropogenic activities such as lignite burning in power stations, ore processing, and the use of fertilizers [3].
Uranium is enriched in the Phosphorite and Chalk Marl Units of central Jordan (Daba-Siwaqa area 60 km south of Amman).e area is currently under investigation by Areva Co. and huge reserves are expected.UO 2 concentrations range between 140 and 2200 ppm in central Jordan.orium is associated with the Dubaydib sandstone formation, southern Jordan, where the level of thorium oxide reaches 400 ppm [4].
Wyoming montmorillonite, <2-/xm particle size, saturated with Na, K, Mg, Ca, and Ba ions, was studied with uranyl nitrate solutions in the concentration range 1-300 ppm uranium.With constant amounts of clay and solution volume, the adsorption isotherms of uranyl ions on the clay followed Langmuir-type curves with increasing concentration of uranium.e maximum adsorption derived from linear Langmuir plots corresponds to the exchange capacity of the clay.Experiments with solutions of constant volume and constant ionicity, but with variable proportions of uranyl and other cations, showed that uranyl ions were strongly preferred by the clay to Na + and K + , but less strongly than Mg 2+ , Ca 2+ , and Ba 2+ [5].Adsorption of uranyl ions to SWy-1 montmorillonite was evaluated experimentally and the results were modeled to identify likely surface complexation reactions responsible for the removal of uranyl ions from solution.Uranyl ions were contacted with SWy-1 montmorillonite in an NaC1O 4 electrolyte solution at three ionic strengths (    ), at pH 4 to 8.5, in an N 2 atmosphere.At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8, while at higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of �xed-charge sites and edge sites analogous to gibbsite and silica.e concentration of �xedcharge sites was estimated as the cation exchange capacity, and nonpreference exchange was assumed in calculating the contribution of �xed-charge sites to total uranyl ions adsorption.e concentration of edge sites was estimated by image analysis of transmission electron photomicrographs.Adsorption constants for uranyl ions binding to gibbsite and silica were determined by �tting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results.e best simulations were obtained with an ionization model in which AlOH 2 + was the dominant aluminol surface species throughout the experimental range in pH [6].
Sorption interactions with SAz-1 montmorillonite of U(6+) is potentially important to understand the mechanisms and the mobility of U(6+) and other radionuclides at pHs typical of natural waters (pH ≈6 to ≈9) through the subsurface environment.e results showed that U(6+) sorption on montmorillonite is a strong function of pH, reaching a maximum at near-neutral pH (≈6 to ≈6.5) and decreasing sharply towards more acidic or more alkaline conditions.At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH.A Diffuse-Layer model (DLM) assuming aluminol (>AlOH ∘ ) and silanol (>SiOH ∘ ) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H 2 O-CO 2 -montmorillonite system at an ionic strength of 0.1 M and pH > 35 [7].
e adsorption of uranium and thorium on surface-modi�ed bentonite under hydrothermal conditions was studied using bentonite isolated from southern clay (USA).ey found changes in the speciation of uranium(VI) and thorium(IV) ions which have been observed in exchanged clays as a function of hydrothermal conditioning and/or surface modi�cation.ere are indications of aggregation of both of these ions within the interlayer spacing of the clay [8].
e adsorption and thermodynamic behavior of uranium(VI) onto bentonite composite adsorbent, isolated from Aegean Sea in Izmir Bay (Turky) composite adsorbent, was investigated.It was found that bentonite composite adsorbent is an economical and effective sorbent for uranium(VI) ions and the composite adsorbent exhibited excellent sorption selectivity for uranium(VI) [9].Extended X-ray absorption �ne structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO 2 2+ ) onto Wyoming montmorillonite.At low pH (∼4) and low ionic strength (10 −3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange.At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites.Linear-combination �tting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites.Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH) 6 ] octahedral sites over [Al(O,OH) 6 ] sites [10].
e adsorption of (IV) on MX-80 bentonite as a function of pH, ionic strength, (IV) concentration, and temperature was studied by using batch technique.e results indicated that the adsorption of (IV) on bentonite depended on pH, ionic strength, and temperature.e adsorption of (IV) decreased with increasing temperature, indicating that the adsorption process of (IV) on bentonite was exothermic.e sorption isotherms were obtained at   293, 303, 313, and 323K and were analyzed with the Langmuir and Freundlich models, showing that the Langmuir model �tted the adsorption data better than the Freundlich model [11].
Recently, we reported the sorption of some heavy metals by Jordanian bentonite [12] and the sorption of uranium and thorium by Jordanian zeolitic tuff Tulul al-Shabba [13].
e primary objective of this study is the sorption of uranium and thorium by puri�ed Jordanian bentonite from Al-Azraq at different pH = 1.0, 2.0, and 3.0, at different temperature 25.0 ∘ C, 35.0 ∘ C, and 45.0 ∘ C, and different contact time.e puri�cation of raw bentonite will be done by the removal of quartz.is includes mixing the raw bentonite with water and then centrifugation at 750 rpm, this process is repeated until white puri�ed bentonite is obtained.
XRD, XRF, FTIR, and SEM techniques will be used for the characterization of bentonite.
e sorption properties of the bentonite toward uranium and thorium in aqueous solutions will be examined under various experimental conditions (batch and column experiment).e data will be analyzed based on sorption models such as: Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) sorption isotherms.In order to determine the nature and characteristic of the sorption process, thermodynamic and kinetic functions will be determined during this work.

Materials and Instrumentation.
All reagents used in this study were of analytical grad reagents.(NO 3 ) 4 ⋅5H 2 O is from Riedel DeHäen, UO 2 (NO 3 ) 3 ⋅H 2 O, sodium perchlorate, sodium acetate, 65% Nitric acid are from Merck, 35% Hydrochloric acid from analytical Rasayan, Arsenazo (III) Indicator from BDH Chemicals Ltd.Samples of Jordanian bentonite from Azraq were obtained from Natural Resources Authority (NRA-Amman-Jordan).ese natural bentonite rocks were grounded gently using glass rod, and they were then sieved to remove particles greater than 120 mesh (<250 μm).e bentonite portions of particle size less than 120 mesh were used in further puri�cation to be used in this investigation.
Infrared spectra were recorded using a ermonicollet Nexus 870 FTIR.e thermal gravimetric analysis (TGA) was recorded using NETZSCH STA 409 PC. e main components and minor components present in bentonite were studied by X-ray powdered diffraction (Shimadzu PXRD-6000).e XRF measurements were carried out by a sequential wavelength dispersive X-Ray Fluorescence spectrometer (Shimadzu XRF-1800).Cyberscan waterproof PC 300 meter was used for pH measurements.e shape and surface morphology of bentonite samples were also studied with (Shimadzu-SEM SUPER SCAN SSX Series).e analytical balance that was used is Shimadzu and its type is AW120 (0.1 mg).Shaking of samples was done using Clion Shaker equipped with a thermostat.UV-VIS Spectrophotometer was from Spectroscan model 80DV with soware UV Win5 v5.0.5.

Puri�cation of Al-Azra�
Bentonite.e main aim of this puri�cation is to remove quartz.is process was done using the following steps: �rstly, the bentonite samples of particle size less than 250 m were mixed with distilled water and then the suspension was centrifuged at 750 rpm to obtain particles less than 2 m.ese particles were carefully separated in large bottles.e centrifugation was repeated for further �ve times [14].e centrifuged (pure) bentonite samples were dried in an oven at 105 ∘ C for 4 hours and stored in polyethylene bottles.

Spectrophotometric Procedure for (IV) and U(VI) 2.3.1. Preparation of Arsenazo (III) Indicator Solution.
A 0.10% aqueous solution of Arsenazo (III) (Figure 1) was used as a spectrophotometric reagent in the determination of thorium(IV) and uranium(VI) ion concentration [15].e spectrophotometric determination of uranium(VI) and thorium(IV) ions in the aqueous solution was carried out as mentioned in our previous work [16].

Metal Ion-Uptake by Bentonite Using Batch Sorption.
Batch sorption was carried out using Pyrex glass �asks.Experiments were performed for determination of the equilibrium time for the sorption process involving 0.05 g ± 0.1 mg of the bentonite with 50.0 mL of metal ion solution added; the mixed solutions were mechanically shaken.e contact time was varied from 0.15 hour to 72 hours at 25 ∘ C, the concentration of the metal ion remaining in solution was determined with UV-VIS.Similar experiments were also carried out at different pH 1.0, 2.0, and 3.0.
e mass of the sorbed metal per unit mass of the bentonite was calculated [12,13] using the following equation: metal ion uptake by bentonite in (mg M/g bentonite),   : initial metal concentration (ppm),   : the residual concentration of metal ion in solution at equilibrium in (ppm), and : mass of bentonite (g).e percentage of metal ion loading by bentonite expressed as percentage uptake was calculated (2) where Distribution Coefficient (  ) is a standard parameter in the assessment of the physiochemical behavior of metal ion between solid and liquid phase.It can be used to evaluate the sorption and retention of the metal ion in bentonite.  is calculated using the following equation: metal ion on the polymer mg × volume of the solution (L) metal ion in solution mg × mass of the polymer g .
is relationship assumes that Δ * is independent on , where  is the absolute temperature (Kelvin).
Change in Gibbs-free energy (Δ * ) was calculated using the following equation: (5)

Sorption Isotherm
Studies.An accurate mass of 0.05 g of bentonite measured to the nearest 0.1 mg was shaken with 50.0 mL of metal ion solution at different concentrations, in thermostatic shaker for 24 h (which had been found sufficient to ensure equilibrium) at 25.0 ∘ C, 35.0 ∘ C, and 45.0 ∘ C. e sorption isotherms were studied using similar conditions at different pH = 1.0, 2.0, and 3.0.

Metal Ion-Uptake by Bentonite Using Column Experiment.
Glass column of 150 mm length and 10 mm inner diameter was used in this experiment.e column was packed with 1.00 g ± 0.0001 g of bentonite; a sample volume of 50.0 mL containing either uranium(VI) or thorium(IV) of 2000 ppm was passed through the column at a �ow rate of 1.0 mL/1.5 min.e eluate was collected in a 50 mL volumetric �ask, and concentration of the metal ion was then determined by using UV-VIS.

Desorption Studies.
Desorption of the uranium(VI) or thorium(IV) was carried out under column condition, where bentonite was loaded with each metal ion as described.A 50.0 mL of the following four different concentration from the same eluting agent, 1 M, 0.5 M, 0.1 M and 0.01 M HNO 3 , were used for metal ion recovery from sorbed bentonite, keeping the �ow rate of elution at 1 mL/1.5 min.e concentration of metal ion in the eluate was collected in ten −5 mL portions, and then determined by using UV-VIS.

Characterization and Analysis of Bentonite Samples.
Bentonite samples were characterized using X-ray diffraction A comparison between raw bentonite and puri�ed bentonite is illustrated in Figure 2. is �gure shows that the percent of SiO 2 decreases signi�cantly a�er puri�cation process while the montmorillonite content remains unchanged; as a result, the percent of montmorillonite in puri�ed bentonite is increased.
e XRD patterns for raw bentonite and puri�ed bentonite (stirring with distilled water) are shown in Figure 2.Although stirring process reduces the base line due to the removal of dusts and soluble impurities, the percent of quartz and montmorillonite did not change.e major peaks in Jordanian bentonite disagree alot with those of commercial untreated bentonite from Saudi Arabia, which contains mainly montmorillonite and kaolinite [17].

XRF. e XRF analysis for Jordanian bentonite (Table 2)
shows that this type of clays consists mainly for aluminum silicate, with Na and Ca as exchangeable cations, so it is considered as intermediate bentonite with moderate swelling capacity.Upon puri�cation the percentage of SiO 2 decreased due to the removal of quartz, since quartz is not the only source of SiO 2 in bentonite, while the percentages of other metal oxides increased.Jordanian bentonite contains more SiO 2 , less Al 2 O 3 , less Fe 2 O 3 , and less CaO than Saudi bentonite [17].

FTIR Spectra.
Bentonite was characterized by FTIR.is technique could be used to identify the major functional groups present in bentonite.From the various wave numbers of the molecular vibrations modes, a good explanation of the chemical structure could be obtained.As illustrated in Figure 3, the characteristic IR band of montmorillonite appeared at 3468 cm −1 .is band represents the fundamental stretching vibrations of different -OH groups present in Mg-OH-Al, Al-OH-Al, and Fe-OH-Al units in the octahedral layer [18].e strong peak appearing at 1035 cm −1 is related to the stretching vibrations of Si-O groups, while the bands at 533 cm −1 and 476 cm −1 are due to Al-O-Si and Si-O-Si bending vibrations, respectively [19].e peak at 848 cm −1 is assigned to O-Si-O asymmetric stretching, while the peak at 450 cm −1 is due to O-Si-O bending mode.On the other hand, the peak at 1647 cm −1 is for H-O-H bending whereas the peak at 890 cm −1 is due to OH bending bounded to Fe 3+ and Al 3+ [19].
e comparison between raw bentonite and puri�ed bentonite using FTIR spectra is illustrated in Figure 4.
Figure 4 gives strong indication that the aim of puri�cation process is achieved.According to this �gure, the intensity of absorbance at 1035 cm −1 peak, which relates to Si-O groups, decreased aer puri�cation.is can be explained by the fact that the amount of SiO 2 decreases aer puri�cation process.

SEM Observations.
To observe the surface morphology of raw bentonite, Scanning Electron Microscope (SEM) of gold-coated samples were taken under liquid nitrogen.Figure 5 clearly illustrates the presence of cavities in the raw bentonite.e images indicate that there are abundant pores distributed on the surface.e existence of these pores would provide convenient diffusion channels for metal ions into the interior of bentonite when it is immersed with metal ions from aqueous solutions.
Scanning electron microscope is used to study bentonite shape.Figure 5 gives good indication about the puri�cation process and the shape of bentonite crystal.It is clearly that the quartz crystals are reduced.�entonite appears as corn �ake like crystals with �uffy appearance revealing its extremely �ne platy structure [18,20].

Rate of Metal Ion Sorption by
Bentonite.e rate of metal ions uptake by bentonite was determined at different times (0.15, 0.25, 0.5, 1.0, 2.0, 4.0, 6.0, 18.0, 22, 24, 48 and 72 h), with a concentration of 50 ppm at 25 ∘ C to �nd the effect of time of the percentage of uptake other experiments were also conducted varying the pH values (1.0, 2.0, and 3.0).e results are of these experiments are shown in plots are shown in Figures 6 and 7.
It can be noticed from Figures 6 and 7, that saturation takes place aer 6 h, because of this we choose 24 h to be sure that we have reached equilibrium since it is a slow one.Also the % uptake increased as the pH increased, which is due to the increase on the surface negative charge or decrease in its positive charge.e idea for using low acidic pH is to avoid hydrolysis of thorium(IV) and uranium(VI) ions and to simulate the removal or puri�cation of both from their ores.

Kinetics of Sorption.
Sorption kinetics is used in order to explain the sorption mechanism and sorption characteristics.3.8.Pseudo-First-Order Reaction Kinetic.�seudo-�rst-order reaction kinetic data were calculated using the following equation: where  1 is the rate constant for the �rst-order sorption,   is the amount of heavy metal sorbed at time  (mg/g), and   is the amount of heavy metal sorbed at saturation (mg/g).Rate constants ( 1 ) were calculated from the slopes of the curves (Tables 3 and 4).
3.9.Pseudo-Second-Order Reaction Kinetic.Sorption data was also evaluated according to the pseudo-second-order reaction kinetic proposed by following equation: e plot of   against time for each metal ion gives a linear relationship, where the values of   and  2   2 are obtained from the slope and intercept of   against time plots (Tables 3 and 4).As the difference between calculated and experimental   values is considered, it is seen that the removal of thorium(IV) and uranium(VI) with bentonite is well described by the second-order reaction kinetic.Moreover, all the correlation coefficients of second-order reaction kinetic are higher than that of the �rst-order reaction kinetic [17,21].

Metal Sorption Studies on
Bentonite.e sorption of thorium(IV) and uranium(VI) on bentonite were studied in order to determine the effect of concentration variation from 10 to 100 ppm and pH on the isotherms of uranium(VI) and thorium(IV).e sorption isotherms were determined for thorium(IV) and uranium(VI) at different pH values (1.0, 2.0, and 3.0) and different temperatures (25  Where   parameter is related to the strength of the sorbed ion adsorbent binding (i.e., orium(IV) ions-bentonite),   is the saturation sorption capacity,   is a parameter related to the sorption capacity, and n is a measure of the sorption intensity [22,23].e estimated parameters of the sorption isotherms, calculated from the intercepts and slopes of the corresponding linear plots of linearized Langmuir and Freundlich for orium(IV) and Uranium(VI), sorption onto bentonite at different temperatures, together with their correlation coefficients ( 2 ), are given in Tables 5 and 6 at different pH values (1.0, 2.0, and 3.0).Example plots are shown in Figures 8 and 9.
In case of Langmuir, some deviation from linearity is shown; the proposed explanation of this deviation is that when all of the available monolayer sites are saturated, some fresh internal sites can be created.e creation of the additional surface arises from the pressure of metal ions being forced into the macropore and micropore structures [24].
Since the  2 values for Freundlich and Langmuir models are above 0.95, the surface of bentonite is a mixture of homogeneous and heterogeneous sites and this agrees very well with SEM results.
In Freundlich isotherm, the value of  denotes the type of sorption whether it is favorable or not.A value of  ranging from 2 to 10 is considered favorable, a value ranging from 1 to 2 is considered moderately difficult, while a value of 0.5 or less indicates very poor sorption [25].erefore, based on the  values from Tables 5 and 6, sorption of thorium and uranium are moderately difficult.Based on the values of   in Tables 5  and 6, sorption of thorium(IV) is greater than uranium(VI) metal ions on bentonite.
is observation can be explained in terms of the following three factors.
(1) Hydration Energy.e adsorptivity of metal ions (  ) on bentonite was found to be directly proportional to the ionic radius.is is due to the decrease in enthalpy hydration (−Δ ℎ ) as the ionic radius increases [26].Increasing the hydration energy due to increase in the hydration shell makes it more difficult for metal ion to discharge the water of hydration.e formation of aqua complex [M(OH 2 )  ] + takes place (where  is larger than six, perhaps eight or nine), the aqua complex, having  H 2 O molecules surrounding the central ion, has a de�nite structure, and the cloud of water molecules (hydration shell) has another geometry than the rest of the water.us, when say M(NO 3 )  salt is dissolved in water, there will be very little attraction between [M(OH 2 )  ] + and the solvated NO 3 − ion.Unless the other ions or ligands have a strong structure breaking in�uence, the sheath of water molecule will protect the metals ions from the in�uence of other anions or ligands.When complexes are formed, the approach of a ligand will interfere with the hydration shell and the ordered geometry will break down [27].A stronger hydration shell will surround small metal ion, which has smaller radius than the metal ion with larger radius; the adsorptivity of ion of large radius is larger than small radius.
When it depends on the charge-to-size ratio, a large ratio results in an increase in hydration energy, which means that the hydrated ion prefers the solution phase, where it may satisfy its hydration requirements.Ions with lower hydration energy prefer the bentonite phase.Table 7 shows that thorium has lower hydration energy than uranium.is means that thorium can exchange easily at the bentonite surface.
(2) Hydrolysis Reaction.Hydrolysis reaction can be represented by a hydrolysis constant ( ℎ ) From the value of  ℎ in Table 5, the following sequence was observed: is indicates that at lower  ℎ value the metal ion can diffuse easily and has a stronger binding strength at bentonite surface.orium(IV) has a lower  ℎ , so a lower resistance to reach the active sites.(3) Deprotonating the SiOH and AlOH Groups.As the pH increase, the amount of deprotonated SiOH and AlOH groups increases, so the negative charge on the surface and edge sites increases.is will increase the interaction between the metal ions and the bentonite (Tables 5 and 6).e higher the metal charge causes a stronger interaction.is agrees very well with previous studies [5,6,11].e correlation coefficient is a mathematical expression, which reveals the favorability of the sorption process, since the values of  2 for Langmuir isotherm and Freundlich are very good.
As mentioned before, the maximum sorption capacity (  ) is determined form Langmuir sorption isotherm.In bentonite, the metal ions seem to reach saturation which means that the metal ion had �lled the possible available sites, so the sorption efficiency increases to a certain level and then remained constant with concentration.

Dubinin-Radushkevich. e linear form for Dubinin-Radushkevich (D-R) isotherm has the following expression:
where  is the ideal gas constant (8.3145J ⋅mol 1 ⋅ K 1 ) and  is the absolute temperature (Kelvin).e values of  and  max are evaluated from the slope and intercept of the linear plot of lnq versus  2 , where  max is related to the sorption capacity and  is the constant related to the sorption energy.e sorption-free energy () is de�ned as the free energy change required for transferring one mole of ions from solution to solid surface; this energy is calculated as following: e value of  gives information about the physical and chemical features of the sorption.Low values of sorption energy () show that the sorption has a physical nature.e results of the Dubinin-Radushkevich concentration variation isotherms of orium(IV) and Uranium(VI) are shown in Table 8. e D-R isotherm model is more general than Langmuir isotherm as it rejects the homogenous surface or constant sorption potential.As illustrated in Table 8, the values of  are less than 8.00 kJ/mol, this indicates that physical forces affect the sorption [28].It is interesting to note that the difference of  max derived from the Langmuir and   / derived from D-R models is quite large.e difference may be attributed to the different de�nition of  max in the two models.In Langmuir model,  max represents the maximum sorption of metal ions at monolayer coverage, whereas in D-R model it represents the maximum sorption of metal ions at the total speci�c micropore volume of the sorbent.ereby, the value of  max derived from D-R model is lower than that derived from Langmuir model [29].e values of ,   / , and  for thorium(IV) and uranium(VI) are summarized in Table 8.

3.
12. ermodynamics of Sorption on Bentonite.In order to understand the possible sorption mechanism involved in the removal process, thermodynamic functions for the system, including changes in Gibbs-free energy (Δ * ), change in enthalpy of sorption (Δ * ), and changes in entropy of sorption (Δ * ), were calculated using the following equation: where   is the equilibrium constant,  is the gas constant, and  is the temperature in Kelvin.e plot of ln   against 1/ for each metal ion (Figures 10 and 11) gives a linear relationship, where the values of enthalpy (Δ * ) and entropy (Δ * ) are obtained from the slope and intercept of ln   versus 1/ plots.Δ * was calculated at each temperature using the following equation: From the Vant Hoff equation Δ * , and Δ * were calculated for (IV) and U(VI) as shown in Table 9. e adsorptive process is enthalpy driven for thorium(IV) and uranium(VI).e negative values of enthalpy show that the sorption of thorium(IV) and uranium(VI).on bentonite is an exothermic process.is exothermic effect can be explained by the forces of interaction between the bentonite and metal, which are stronger than those existing in both bentonite and metal alone, which means that it would prefer the product than reactant [30].e almost zero values of Δ * suggests that the entropy of the system is decreased or stayed constant.On the other hand, these almost zero values of entropy indicate that the sorption process is irreversible and favors the stability of value [29].e low to negative values for Δ * indicate that the sorption process is more energy favorable at higher pH values, since we are talking about a heterogeneous and not a homogeneous system.

Column Experiments
4.1.Metal Ion Uptake by Bentonite.e investigation of metal ion uptake by bentonite using column experiments for thorium(IV) and uranium(VI) was determined at the optimum pH for each metal ion with initial concentration of 2000 ppm at 25 ∘ C and �ow rate of 1 mL/2 min.e results are expressed as percent metal uptake by the column and are presented in Table 10.
It can be seen that the uptake capacities of the metal ions fall in the order (IV)>U(VI).
It is observed that the % uptake in column experiment is smaller than those obtained in batch experiment, because in order to achieve complete saturation, long time of contact is required.On the other hand, the shaking process in batch technique is a fundamental factor in addition to the large contact time, which increases the % uptake of metal ions compared to column technique.

Desorption Studies.
e main aim of this study is to determine the capability of bentonite to be regenerated for further uses.50 mL of 1.0 M HNO 3 , 0.5 M HNO 3 , 0.1 M HNO 3 , and 0.01 M HNO 3 were used for the removal of metal ions and regeneration of bentonite, keeping �ow rate of elution 1 mL/2 min.e elute was collected in ten portions, 5 mL for each portion; the results are expressed as percent recovery and represented in Table 11.
Depending on the values of percentage of cumulative recovery, in Table 11, the following trends were observed for eluting agents of (IV) and U(VI) from bentonite: Table 11 shows desorption yields of uranium and thorium aer ten desorption stages.Desorption yield for bentonite decreased with increasing the desorption stages.orium recovery decreases with decreasing acidity.When concentration of HNO 3 is increased, desorption of uranium(VI) and thorium(IV) is increased due to cation exchange mechanism.It is clearly observed that the efficiency of Jordanian bentonite to adsorb metal ions for the second time is expected to   decrease signi�cantly when desorption of the �rst time is done; this fact indicated that the washing of bentonite with HNO 3 regenerates bentonite but the regeneration is not complete, since bentonite loses signi�cant efficiency to adsorb metal ions for the second sorption process.ere are other studies showing the effectiveness of bentonite sorption of heavy metals from aqueous solution Table 12; the heavy metals which have been studied are Pb(II), Cr(III), Zn(II), and Mn(II).e sorption of these heavy metals on puri�ed bentonite was studied as a function of contact time, pH, temperature, and concentration variation [31].e estimated parameters,   and   , of the sorption isotherms for the three forms of linearized Langmuir equations were calculated from the intercept and slopes of the corresponding linear plots for Zn(II), Pb(II), Cr(III), and Mn(II) sorption onto bentonite at different temperatures and pH values.e values of these parameters for Langmuir Model form (II) with their correlation coefficients ( 2 ) at 140 ppm are given in Table 12.
From the following Tables 5, 6, and 12, it is clear that the sorption capacity of thorium and uranium in bentonite is slightly greater than the capacity of bentonite to sorb heavy metals except for Pb(II).

Conclusion and Recommendations
e present work has focused on the puri�cation of Jordanian bentonite and then studying its sorption characteristics toward (IV) and U(VI).Raw and puri�ed bentonites were characterized by XRD, XRF, and SEM.e sorption characteristics for (IV) and U(VI) ions from aqueous solutions were examined under various experimental conditions using both batch and column techniques.e effective desorption of the metal ions was also studied and the coefficients of recovery of sorption ability were also investigated.e following remarks are concluded.
(1) Jordanian bentonite showed a relatively high uptake for thorium(IV) and lower uptake for uranium(VI).
(2) e in�uence of different pH on metals uptake showed that the metal ion uptake by bentonite increased with pH and reached a maximum at pH = 3 for  4+ and UO 2 2+ .
(3) e maximum sorption capacity (  ) of bentonite was high for  4+ and the extent of metal ions uptake followed the order:  4+ > UO 2 2+ at pH = 3 and 25 ∘ C.
(4) e equilibrium for each metal ion on the surface of bentonite occurs at 18 hours to achieve maximum uptake level.
(6) A glass column was packed with bentonite and exhibits good sorption properties toward metal ions.e regeneration process of bentonite was done using different concentration of HNO 3 and the results show that this process is effective for environmental applications.
(7) e best percent recovery for thorium(IV) and uranium(VI) was obtained when 1.0 M HNO 3 was used.
(8) orium (IV) and uranium (VI) removal with bentonite is well described by the second-order reaction kinetic.
(9) is study strongly recommends the use of Jordanian bentonite to remove radioactive wastes; such wastes arise from technologies producing uranium and from laboratories working with radioactive materials.

F 5 :
e S�� images for Raw bentonite and �uri�ed bentonite.�agni�cation � 300 times.T 3: Comparison of sorption rate constants, experimental and calculated   values for the pseudo-�rst-and -second-order reaction kinetics of removal for thorium(IV) by bentonite.
∘ C, 35 ∘ C, and 45 ∘ C). e following linearized Langmuir, Freundlich isotherm forms, analyzed the sorption isotherms results erefore, a plot of  versus  gives a straight line of slope (1  ) and intercept 1(    ), a plot of log  versus log  gives a straight line with a slope 1 and intercept log   .
T 4: Comparison of sorption rate constants, experimental, and calculated   values for the pseudo-�rst-and second-order reaction kinetics of removal for uranium (VI) by bentonite.  , ,   ,  2 ,   , and   values obtained from Langmuir and Freundlich plots for orium(IV).
T 11: Desorption of U(VI) and (IV) ions from bentonite.

T 12 :
2, and   ,   values obtained for heavy metals from Langmuir form (II) plots.