The Removal of Uranium and Thorium from Aqueous Solutions Onto by-pass Cement Dust (BCD)

The adsorption behavior of uranium and thorium from aqueous solutions by By-pass cement dust (BCD) has been investigated by a batch technique. The uranium and thorium adsorption on BCD was studied as a function of initial concentration, weight of BCD, pH, shaking time and temperature. The uptake of uranium and thorium at the determined optimum conditions follows Freundlich isotherm. The adsorption control of both thorium and uranium are first order and uptake energy of activation E a =10 and 15 kJ/mol for thorium and uranium respectively. Thermodynamic parameters such as ∆H o , ∆S o and ∆G o were also investigated.


Introduction
Pollution with radioactive elements has been a matter of great concern for the last decades for hu-man health and animals. Uranium and thorium are the most important natural radioactive elements af-fecting the environment. Therefore the removal of uranium and thorium from water is very important [1][2][3].
Various techniques are employed for the removal of uranium ions from wastewaters and radioactive wastes. Chemical precipitation, membrane process-es, ion exchange, solvent extraction, photo catalysis and adsorption are the most commonly used meth-ods [4][5][6][7][8][9]. Adsorption of uranium (VI) onto various solids is important from purification, environmenntal and radioactive waste disposal points of view [10,11]. In recent years, attention has been focused on various adsorbents with metal-binding capacities and low-cost, such as chitosan, zeolites, clay or cer-tain waste products [12]. 24 different materials have been studied for the remediation of U-and Mo-containing groundwater. It was found that the best results were achieved with hydroxyapatite (HAP), fly ash, TiO 2 , BaCl 2 , hydrat-ed lime, peat and lignite [13].
HAP is readily available from natural resources as well as a synthetic product from chemical indus--try [14]. In general removal of U from ground-wa-ter in contact with HAP can occur by sorption onto HAP surfaces or by precipitation of uranyl phosph-ates [15].
The adsorption of uranium (VI) from aqueous solutions onto activated carbon using a batch tec-hnique indicated that the optimum parameters that affect the uranium (VI) adsorption are (contact time 240 min; pH 3.0 ± 0.1; initial uranium concentration 100 mg/L; temperature 293.15 K), also the Freund-lich model can be applied and the activation energy = 7.91 kJ/mol [16].
The adsorption behavior of thorium from aqueous solutions by Polyacrylonitrile PAN/zeolite composi-te has been investigated. It was found that the most important parameters on the sorption system, which can influence the sorption behavior of thorium are: initial thorium concentration, pH value of the solut-ion is important factor and the optimum value of pH is 4, the sorption of thorium slightly increases with increasing shaking time and the thorium sorption yi-eld was increased with increasing temperature [17].
It was found that the using of By-pass cement dust (BCD) enhances significantly the percentage removal of the heavy metals and it has a great pote-ntial to be used as low cost raw material for removal of metal pollutants from industrial wastewaters dis-charged by industries [18][19].
The aim of this work is to investigate the adso-rption and the mechanism of by-pass cement dust (BCD) to remove uranium and thorium from aque-ous solutions.
BCD was brought from National Cement Co. Egypt. The chemical composition were determent by XRF and is illustrated in table 1.and the X-Ray diffraction pattern is shown in Fig.1 where it is clear that BCD mainly consists of calcite, calcium sulph-ate, mono calcium silicates, calcium carbonate, qua-rtz, sodium chloride.
The received BCD was put in a glass container which is in turn was kept in a desiccator all the time of the experiments.

Reagents
Standard individual metal ion solutions (1000 ppm) for U(VI) and Th(IV) were prepared from UO--2(NO 3 ) 2 •6H 2 O and Th(NO 3 ) 4 •5H 2 O by dissolving 2.10 and 2.46 g in 1 L of double distilled water. Ar-senazo III was obtained from Aldrich Chem. Co. All other reagents used were analytical reagent grade.

Adsorption experiments
The adsorption experiments have been studied by a batch technique in a thermostat>s shaker bath Stuart model SBS,U.K. The shaking rate was cons-tant for all the experiments. The adsorbent material (BCD) (0.01 g) and standard aqueous solution of uranium and thorium (50 ml) were shaken at diffe-rent temperatures for various mixing time. The pH of the aqueous solutions was maintained by Thiel buffer solution in the pH range (2 -6) [20] since the adsorbent is affected by strong acidic or basic solutions. Filtration/separation of solid phase from liquid was followed by centrifuging at 4000 rpm for 15 min. The uranium and thorium were determined spectrophotometrically using Arsenazo III method Table1. Major Chemical constituents of BCD Fig.1. X-ray diffraction of by-pass cement dust as complexing agent at 655 nm and 659 nm, respecti-vely against reagent blank [21] employing Shimadzu UV-Vis160A Spectrophotometer. For calculating the uranium or thorium concentration, the absorbance of complex solution was compared with a calibration curve plotted of absorbance versus standard conce-ntration of uranium or thorium at fixed λmax for the colored complexes of the metal ions the amount of adsorbed uranium or thorium was estimated from the difference of the concentrations in the aqueous soluti-on before and after the adsorption process.

Calculation
The Uptake percentage (%) and distribution con-stant KD (ml/g) was calculated from the equations where C 0 and C f are the concentration of the met-al ion in the initial and final solutions (mg/l), Mads and Msol are the amount of metal ion on adsorbent and in solution (mg), V is the volume of the solution (ml), and W is the weight of the adsorbent (g). The amount of metal ion uptake qt at time t, was calculated from the mass balance equation Where t is the equilibrium contact time, C t = C e , q t = q e , and the amount of metal ion sorbed at equil-ibrium, q e , is calculated using Eq. (3).

Adsorption kinetics
To examine the controlling mechanism of the ad-sorption process, several kinetic models are used to test experimental data. A simple kinetic analysis of adsorption is the Lagergren pseudo firstnorder equantion in the following form [22] Where k 1 is the rate constant of firstnorder adsnorption and qe denotes the amount of adsorption at equilibrium. After definite integration by applying the initial conditions q t = 0 at t = 0 and q t = q t at t = t, Eq. (4) becomes The slopes of the plots of log (q e −q t ) versus t were used to determine the first order rate constannt k 1 that it can be used to determine the activation energy of the adsorption process using Arrhenius equation.

Desorption experiments
Desorption of uranium and thorium from BCD was performed also by batch technique. Some acids e.g. HNO 3 , HCl and H 2 SO 4 were treated with loaded adsorbent to recover the metals from the adsorbent as a function of desorptive reagents concentration, time and temperature.

Adsorption isotherm
The adsorption isotherms were obtained by an-alyzing solutions in contact with adsorbent before and after equilibrium and plotted in terms of the eq-uivalent fraction of metal ion on the adsorbent solid phase against the equivalent fraction in the aqueous phase.

Thermodynamic parameters
0.01 g of adsorbent was treated with metal ion solution of 100 mg/l at constant pH 4 and 3 for ura-nium and thorium, respectively for constant shaking time 7 min for both elements in different temperatu-res. The thermodynamic parameters (∆H o , ∆G o and ∆S o ) were calculated from the adsorption results.

Adsorption experiments
The parameters which affect the uranium and th-orium uptake on by pass cement dust, such as initial concentration of metal ion, contact time, adsorbent weight, pH and temperature were investigated.

Effect of initial concentration of metal ion
The effect of the initial concentration of uranium and thorium on the adsorption rate was studied by contacting a fixed weight of BCD (0.01g) at a fixned temperature (293.15 K) and initial pH (3.0±0.1) for thorium and pH (4.0±0.1) for uranium with va-ried initial concentrations (50 -600 mg/l) in volu-me 50 ml of UO 2 2+ or Th 4+ ions aqueous solutions. The results are given in Fig.2 which reveals that the adsorption percentage of uranium is increased with decreasing the initial uranium concentration, but in case of thorium the percentage is decreased with increasing initial thorium concentration up to 400 mg/l. This may be due to the geometrical shape and size of UO 2 2+ which is greater than that of Th 4+ , thus as the concentration of UO 2 2+ increases the adsorpt-ion percentage decreases.

Adsorbent weight
Effect of adsorbent weight on the adsorption pro-cess of the investigated metal ions is shown in Fig.3. The adsorption experiments were carried out for st-udy this effect with different amounts of adsorbent (BCD) ranging from 0.01 g to 0.05 g at fixed tempenrature 293.15 K using metal ion concentration of 100 mg/l at pH (3±0.1) for thorium and pH (4.0±0.1) for uranium in volume 50 ml and 7 min shaking time. It was found that the uptake percentage of both met-al ions increased with increasing the amount of ads-orbent up to 0.02 g and stays nearly constant with no significant increase in its value. The reason that the uptake reaches 100% at 0.02 g .so no more uptakes and there is no uptake >100%.

Effect of pH
The adsorption experiments are carried out for study this effect with fixed amounts of adsorbent (BCD) (0.02 gm) at fixed temperature 293.15 K usning metal ion concentration of 100 mg/l and 7 min shaking time. Fig.4 shows the influence of pH on the adsorption of uranium and thorium on BCD. The percentage of adsorption increases with increa-sing pH to a maximum value at pH (4±0.1) and then becomes constant. The influence of pH on uranium adsorption can be explained in the following way: hydrolysis of uranyl ion takes place as the pH varies from 2 to 6, Uranium exists in hydrolyzed form and the following ionic species have been exist UO 2 +2 , [(UO 2 ) 2 (OH) 2 ] 2+ dimer, [(UO 2 ) 3 (OH) 5 ] + trimer that are adsorbed on the surface of BCD. The optimum removal took place at pH (4±0.1).
In case of thorium below pH 3, the predomina-nt thorium ion would be the positively charged Th 4+ [17].Thorium uptake on the adsorbent reached a maximum of 100% at pH (3±0.1). Above pH 3, the uptake percentage decreases with increasing pH.

Effect of temperature and Adsorption dynamics
The relation between the uptake percentage of uranium and thorium by BCD and time for differ-ent temperature are illustrated in Figs.5-6 using co-ncentration of metal ion 200 mg/l, pH (3±0.1) and 0.02 gm BCD. it is clear that the adsorption rate very high at the initial time then the rate decreased. The analyses of the curves relating the uptake percentage and time show that each curve has 3 different slopes indicating 3 different values of adsorption rates. The first value is high, while the second is somewhat slnower and the third is the slowest one. Also it can be observed that both elements uptake was increased with increasing temperature. The increase of adsorption percentage with rise of temperature may be due to the increase of numb-er of reacting moles having excess of energy which leads to the increase of adsorption rate, also the rai-se of temperature leads to an increase of the rate of mass transfer of the diffusion and rat of adsorption [23][24].     The natural logarithms of K 1 were used accordi-ng to the Arrhenius equation to calculate the activa-tion energy of adsorption reaction. A plot of log K 1 and 1/T gives a straight line as shown in Fig. 8, from which it is clear that the activation energy for first order E a1 = 15 kJ/ mole.  The natural logarithms were used according to the Arrhenius equation to calculate the activation energies of adsorption reaction. A plot of Log K 1 and 1/T gives a straight line as shown in Fig. 10 from which it is clear that the activation energy of first order for thorium E a1 = 9.8 kJ/ mole.

Thermodynamic parameters
The thermodynamic parameters obtained for the adsorption process for both uranium and thorium were calculated using the equations The values of ∆H 0 , ∆S 0 and ∆G 0 were calculated from the slopes and intercepts of linear regression of ln K D versus 1/T as shown in Figs.11-12. The values are tabulated in table 3. In fact, the positive value of enthalpy change ∆H 0 for the processes further confinrms the endothermic nature of the process, the posi-tive entropy of adsorption ∆S 0 reflects the affinity of the adsorbent material toward both elements.
It is clear that the free energy for both element ad-sorptions are negative and the negative free energy values ∆G 0 indicate the feasibility of the process and its spontaneous nature without an induction period.

Sorption isotherms
The sorption data have been subjected to diffe-rent sorption isotherms namely Langmuir and Fre-undlich. The simple Freundlich isotherm was able to describe the adsorption over all the concentration range used according to the equation: Log q e = log K + 1/n log C e Where qe is the amount of solute adsorbed per unit weight of BCD at equilibrium (mg/g) and C e is the equilibrium concentration of metal ion in solu-- Table 2.
Thermodynamic parameters for adsorption of thorium(IV) and uranium(VI) on BCD  tion (mg/l). K and 1/n are the Freundlich constants related to sorption capacity and sorption intensity of the sorbent, respectively. A plot of log q e vs. log C e as seen in Fig.13 would result in a straight line with a slope of (1/n) and intercept of log K as in table 2. It was found that the slope value ranges between 0 and 1. It can be concluded that the sorption of the in-vestigated elements takes place through the format-ion of a single monolayer of the sorbed species. The Freundlich equation was found to fit the data in the whole range of Th 4+ and U 6+ concentrations tested. The Langmuir isotherm was applied for the sorp-tion equilibrium of BCD: Where C e is the equilibrium concentration of me-tal ion in solution (mg/l), qe is the amount of solu-te adsorbed per unit weight of BCD at equilibrium (mg/g), and Q 0 and b are the Langmuir constants related to adsorption capacity and energy of adsorp-tion, respectively.
According to the Langmuir model, adsorption occurs uniformly on the active sites of the sorbent, and once a sorbate occupies a site, no further sorpti-on can take place at this site. A plot of C e /q e vs. C e would result in a straight line with a slope of (1/Q 0 ) and intercept of 1/bQ 0 as seen in Fig.14. The values of the slope and intercept of this plot are in table 2.

Desorption experiments
The loaded BCD is treated with different acid solutions e.g. HNO 3 , HCl and H 2 SO 4 of different co-ncentrations. The concentration of the metal ion is determined after the desorption reaction and the stri-pping percentage is calculated. The factors affecting the desorption process are studied as following.

Effect of desorption time
The desorption from loaded adsorbent has been investigated as a function of equilibrium time in the range of 2-20 min. 50 ml of 0.005 M acid soluti-on were used in the desorption process that is carr-ied out at 30 o C. Fig.15 shows that desorption yield increases with increasing shaking time and attains equilibrium within 5 min for uranium, while as the shaking time in case of thorium increases up to 5 min the desorption decreases. Therefore, in further experiments 5 min equilibrium time was used.

Effect of temperature
Effect of temperature in desorption process was investigated from 30 to 65 o C employing 5 min sh-aking time with 50 ml of 0.005 M acid solution. Fig.18 shows influence of temperature on desorptinon of metal ion from the adsorbent material. Desor-ption yield increases with increasing temperature up to 60 o C in case of desorption of uranium, while in case of thorium desorption is constant to 60 o C and then decreases.

Conclusions
• The adsorption of uranium and thorium from liquid solutions by BCD has been shown to depend on initial concentration of metal ion, BCD dose, contact time, pH and temperature. The adso-rption of these radionuclides can be represented by Freundlich adsorption isotherms. It can be conclud-ed that this BCD can be utilized for removal of rad-ioisotopes from waste solutions.
• Uranium and thorium recovery is the more important step, owing to its low affinity towards the adsorbent. With acid solutions desorption of U (VI) and Th (IV) is high. Desorption percent for this ads-orbent was ranged from (20-90 %). This may result from creating different physical forms of metal ion with some components of the adsorbent. Some other mechanisms are involved in the observed process.
• The thermodynamic parameters ∆H 0 , ∆S 0 , and ∆G 0 values of uranium (VI) and thorium (IV) adsorption onto BCD show endothermic heat of ad-sorption, favored at high temperatures. The positive entropy value is an indication of the probability of