Kinetics and Thermodynamics Studies on the Recovery of Thorium

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Kinetics and thermodynamics studies on the recovery of thorium ions using amino resins with magnetic properties Mahmoud Osman Abd El-Magied, Emad Elshehy, El-Sayed A. Manaa, Ahmad A. Tolba, and asem ali atia Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02977 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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Industrial & Engineering Chemistry Research

Kinetics and thermodynamics studies on the recovery of thorium ions using amino resins with magnetic properties Mahmoud O. Abd El-Magieda, Emad A. Elshehya, El-Sayed A. Manaaa, Ahmad A. Tolbaa and Asem A. Atiab,∗ a

Nuclear Materials Authority, P.O. Box 530, El Maadi, Cairo, Egypt

b

Chemistry Department, Faculty of Science, Menoufia University, Menoufia, Egypt

Abstract Magnetic polymeric matrices were synthesized from glycidyl methacrylate, N,N'methylene bisacrylamide (MBA) and nano-magnetite particles. The obtained polymers were modified by ethylenediamine (DA) and diethylenetriamine (TA) to produce two aminomagnetic resins named R-DA and R-TA. The recovery of Th(IV) ions from their aqueous solutions by R-DA and R-TA were studied in the pH range 1-4. Maximum adsorption capacity values of 60 and 84 mg/g of Th(IV) ions on R-DA and R-TA, respectively were obtained at pH 3.5 and 293 K. At a solid/liquid ratio (S/L) of 2 g/L, recovery efficiency values of 86 and 95% were achieved from initial thorium ion concentration of 100 mg/L using R-DA and R-TA, respectively. Adsorption isotherms, kinetic and thermodynamic parameters of the adsorption process were obtained and analyzed. Regeneration of the resins was tested by eluting the loaded Th(IV) ions on the spent resins using 0.2 M HNO3 followed by washing with dilute NaOH.

Keywords: Thorium; Magnetic Resins; Adsorption; Kinetics; Thermodynamics.

∗

Correspondence: [email protected]; [email protected] 1 - Environment ACS Paragon -Plus


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1. Introduction Thorium is a radioactive actinide element presents in the earth crust at about 3-4 times as abundant as uranium. Thorium may be used as an alternative fuel in a nuclear reactor.1,2 The high-temperature properties of thorium oxide enabled it useful in the manufacture of high-strength, high-temperature alloys and in heat-resistant ceramics.3,4 Different industries, such as extraction of thorium and uranium from ores and production of phosphate fertilizer, contaminate surface and ground waters by radioactive wastes including thorium.5,6 Thorium has a harmful nature even at trace level with increased risk of blood, pancreas, and lung cancer, liver diseases, and kidney failure.1,7 Recovery of thorium from different aqueous systems has been considered of a strategic value in view of economic and environmental interest. Recently, different technologies, including precipitation, solvent extraction, membranes, and ion exchange resins have been developed for thorium recovery. Adsorption at solid/liquid interfaces is one of the most effective technologies used in removing different pollutants from water sources.7-9 Various adsorbents such as styrene divinylbenzene,6,10 clays,11 silica,12 activated carbon,13 zeolite,14 and graphene oxide15 have been widely used for the recovery of thorium ions from liquors. In the last decades, magnetic resins became of great interest due to their ease of separation using an external magnetic field improving the separation process. The magnetoresponsive polymeric beads benefit comes from the combination of their components features; magnetic particles and polymer.16 Nano and micromagnetic particles are important for many technological applications like separation of metal ions, medical applications and oil industry.16,17 The aim of this work was to synthesize two magnetic-resins functionalized with ethylenediamine and diethylenetriamine active moieties. The magnetite core allows more spreading of the active groups on the surface of magnetite particles enhancing the adsorption efficiency compared to the non-magnetite core resins. The prepared magnetite-cored resins have an advantage toward the filtration process. The magnetite particles impart the magnetic properties to the beads that allow rapid and easy separation of beads by the application of an external magnetic field avoiding some technical problems arising due to using the traditional filter papers. The resins obtained were used to recover thorium ions from their acidic solutions at different adsorption conditions. The kinetics, thermodynamics, and capacities of the adsorption of Th(IV) ions on the resins obtained were investigated and the data obtained were discussed.

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2. Materials and methods 2.1. Materials Glycidyl methacrylate (GMA), N,N'-methylene bisacrylamide (MBA) and 2,2azobisisobutyronitrile (AIBN) were Aldrich products. Th(NO3)4.5H2O was Merck product and was used as a source of Th(IV) ions. A stock solution of 1000 mg/L of Th(IV) ions was prepared by dissolving 2.46 g of Th(NO3)4.5H2O in a liter of 0.5% nitric acid solution. Other concentrations of thorium ions were prepared through dilution using double distilled water. The concentration of Th(IV) ions was estimated by Thoron I method using double beam PC scanning spectrophotometer UV/VIS (LABOMED, INC., U.S.A).18 Diluted solutions of NaOH and/or HNO3 were used to adjust the desired solutions pH. 2.2. Preparation of resins Nano magnetite particles were prepared as described in our previously published work.

17

GMA/MBA copolymers beads containing 85 mole% of GMA were prepared by

suspension radical copolymerization of GMA (monomer) and MBA (cross-linker) in the presence of AIBN (initiator, 2%), nano-magnetite (Fe3O4) and 2-ethyl-1-hexanol as a diluent. The polymerization reaction was carried out at 348-358 K for 8 h with a stirring rate of 300 rpm. After completion of the reaction, the beads formed were decanted using an external magnet and washed several times with an excess of water and ethanol, respectively. Two portions of the magnetic-GMA/MBA resin (7 g each) were added portion-wise with stirring to 42 mL ethylenediamine or diethylenetriamine in proper flasks. The mixtures were placed in an oil bath at 348-358 K for 72 h. After completion of the reaction, the formed beads were simply decanted using an external magnet, washed several times then dried and marked as R-DA and R-TA, respectively. The concentration of the amino active sites of the obtained resins was determined using the volumetric method described in our previously published work.19 2.3. Adsorption studies 2.3.1. Effect of pH Portions of 50 mg of the resin were added to a series of flasks each contains 100 mL of Th(IV) ions at an initial concentration of 100 mg/L. The acidity of the contents of the flasks was adjusted at different pH values in the range 1-4 and shake at 293 K and 300 rpm



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for 2 h. Afterward, the residual concentration of Th(IV) ions was determined. 2.3.2. Kinetic experiments The kinetics of recovery of Th(IV) ions was studied by placing portions of 50 mg of R-DA or R-TA in a series of flasks each contains 100 mL solution of 100 mg/L Th(IV) ions at pH 3.5. The flasks were shaken on a shaking water bath for the required time period. 2.3.3. Adsorption isotherms Adsorption isotherms of Th(IV) ions on R-DA and R-TA were carried out at definite concentrations in the range of 20-120 mg/L and pH 3.5. The contents of the flasks were equilibrated on a shaking water bath at different temperatures. After equilibration, the concentration of the remaining Th(IV) ions was measured. 2.3.4. Effect of adsorbent dose The effect of the dose of R-DA and R-TA on the recovery of Th(IV) ions was studied by varying the amount of the resin from 0.025 g to 0.25 g and keeping the rest of the adsorption parameters constant at 100 mL (100 mg/L) of Th(IV) ions, pH 3.5, 293 K and 2 h equilibration time. The recovery percentage (R%) of Th(IV) ions by R-DA and R-TA was calculated using the concentration balance equation17 R% =

( )

x 100 (1)

3. Results and discussions 3.1. Resins structure and characterization The representative structure of resins R-DA and R-TA is shown in Scheme 1. The FTIR spectra of magnetic-GMA/MBA resins showed a stretching band of the epoxy group at 910 cm-1 which disappeared in the spectra of resins R-DA and R-TA. Moreover, the spectra of R-DA and R-TA were characterized by bands of νNH2 at 3443 cm-1. This indicated the success of modification of the resins with amino groups. The concentration of the amino active sites was found to be 3.8 and 5.4 mmol/g for RDA and R-TA, respectively. These noticeably high values of N active sites may indicate the formation of extended thin film of the resin over the magnetic iron oxide particles. This allowed the accessibility of DA and TA to react with the epoxy groups of the parent resins giving the adsorbents with the immobilized amine moieties (R-DA and R-TA). 4 - Environment ACS Paragon -Plus

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3.2. Effect of pH on adsorption process The acidity of the medium is an important factor that determines the species of the metal ion and the surface charge of the adsorbent. Studying the effect of the pH on the adsorption is essential to understand the operating mechanism. The adsorption of Th(IV) ions on R-DA and R-TA as a function of pH was studied and the amount adsorbed of the metal ions was calculated using the equation q =

( )

V

(2)

where Ci and Ce are the initial and equilibrium thorium concentrations (mg/L), V is the adsorption volume (L), W is the weight of the resin used (g) and qe is the adsorption capacity at equilibrium (mg/g).19,20 The data obtained for the adsorption of Th(IV) ions on R-DA and R-TA at different pHs are represented in Fig. 1. Obviously, the maximum capacities for the adsorption of Th(IV) ions on both resins are noted at pH 3.5 and decrease as the pH of the medium decreases. The appreciable higher adsorption capacity of R-TA than R-DA may be attributed to the higher concentration of N active sites of the former as indicated from the data of active site concentration of the resins. At all concentration ranges, the uptake of R-TA exceeded that of R-DA. The adsorption of Th(IV) ions on R-DA and R-TA may be explained to proceed via chelation mechanism between the positively charged Th(IV) ions and the non-protonated N active sites on the resin surface as shown in Scheme 2. The Th(IV) ion may form a stable five-chelating rings with the N active sites located at the surface of R-DA and R-TA. As the number of the chelating rings increases, the adsorption becomes favored as noticed in the case of R-TA relative to R-DA. Decreasing the acidity of the medium to pH < 3.5, the N sites become more protonated. The protonated N sites repel the positively charged Th(IV) ions and consequently the adsorption capacity decreases. At pH < 1, the amino active sites of R-DA and R-TA may become highly protonated and consequently, electrostatic repulsion operates between the protonated amino active sites and Th(IV) cations giving no appreciable uptake of thorium ions. Below pH 3.5, the predominant thorium ion would be positively charged Th+4. At pH > 3.5, Th(IV) ions undergo extensive hydrolysis with water including monomeric, oligomeric and later polymeric species according to the equation21-23 mTh + nH O → Th (OH) + nH (3)



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Adsorption studies at pH > 3.5 were avoided due to the formation of such hydrolytic species. At pH 3.5 and 293 K, maximum loading capacity values of 60 and 84 mg/g of Th(IV) ions were achieved by R-DA and R-TA, respectively. These values are much better compared to that achieved by the unmodified magnetic-GMA/MBA (qe = 5 mg/g). 3.3. Kinetic studies The study of kinetics is important for gaining an understanding of the physical chemistry of the adsorption process and the design of the adsorption systems. In general, the adsorption of an adsorbate on a stationary phase (adsorbent) from solutions may proceed through multi-steps including, bulk diffusion; film diffusion; pore diffusion and interaction of the adsorbate with the surface active sites on the interior surface of pores.24,25 The rate determining step may be one of the above steps or combination of them. For gaining perception on the adsorption kinetics, the adsorption of Th(IV) ions on RDA and R-TA as a function of time is shown in Fig. 2a. Clearly, the adsorption capacity was increased steadily until reaching a plateau at 30 min for the two resins. Within 10 minutes, the recovery efficiency of 73 and 88% was achieved by R-DA and R-TA, respectively. The adsorption time-data were analyzed according to various kinetics models: (i) Order of adsorption reaction To verify the order of the adsorption reaction, the adsorption data represented in Fig. 2a were tested according to pseudo-first (Eq. 4) and pseudo-second (Eq. 5) order models19 (

) log (q# − q% ) = log (q&'% ) − .+,+ t

%

./

=(

&

0 0 .012

&

+. t

(4) (5)

where qt, is the experimental adsorption capacity at time (t); q1st and q2nd are the calculated adsorption capacity from the pseudo-first and the pseudo-second order models (mg/g), respectively; k1 (min-1) and k2 (g/mg.min) are the rate constant of pseudo-first and pseudosecond order models, respectively. Plotting log (qe-qt) versus (t) of Eq. 4 gave straight lines, where the values of q1st and k1 were calculated. Plotting (t/qt) against (t) of Eq. 5, gave straight lines with slopes and intercepts (Fig. 2b) from which the values of k2 and q2nd were calculated and reported in Table 1. The goodness of the model may be tested from the value of regression (R2) and the consistency of the qcalc value calculated with the experimental one.


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The results indicated that the adsorption of Th(IV) ions onto R-DA and R-TA followed the pseudo-second order rather than pseudo-first order kinetics. This implies that the rate of adsorption of Th(IV) ions on R-DA and R-TA depends on the properties of the metal ion and the resin. (ii)

Intraparticle diffusion model In this model, the adsorption of an adsorbate on an adsorbent surface varies

proportionately with t0.5 according to Weber-Morris equation26 q% = K 45 t ,.6 + I

(6)

where Kip is the intraparticle diffusion rate constant and I is a constant proportional to the boundary layer (mg/g.min0.5). The larger I value, the greater contribution of the boundary layer.26 Kinetic parameters for the Weber-Morris model were determined from the slope and intercept of plots presented in Fig. 2c, and the results are listed in Table 1. The regression coefficient (R2) for Weber–Morris model has values ranged from 0.9942 to 0.9782. The intraparticle diffusion rate constant for the first linear portion (Kip) has values of 0.0409 and 0.0582 mg/g.min0.5 for R-TA and R-DA, respectively indicating the higher rate in the case of R-TA relative to R-DA. For the second linear portion, the (Kip) values are 0.0067 and 0.0021 mg/g.min0.5 for R-DA and R-TA, respectively which refer to lower rates compared to the first portion of the line. These values are in agreement with the trend of the experimental rates for the two resins. Moreover, the plots almost have a zero intercept indicating that the intraparticle diffusion may be the rate controlling step in the adsorption process and the boundary layer has a negligible effect. (iii) Bangham model The model of Bangham is applicable to the adsorption systems in which the intraparticle diffusion is the rate-determining step. The adsorption data of Fig. 2a were treated according to Bangham kinetic model27 log log 8

C4 mK ; : = log 8 : + α log t (7) C4 − mq% 2.303 V

where V is the volume of solution (mL), m is the weight of resin per liter of solution (g/L), α and Kb (mL/g/L) are Bangham constants. Fig. 2d shows the plots of the adsorption time data of Th(IV) ions on the two resins according to Eq. 7. The plots of the left-hand side against



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(log t) gave straight lines where the values of α and Kb were calculated from their slopes and intercepts and reported in Table 1. The straight lines obtained refer to that the pore diffusion may be the rate determining step in the case of the two resins. The overall above results show that the pseudo-second order, Bangham, and intraparticle diffusion models describe the adsorption of thorium ions on R-DA and R-TA better than the other studied models. While Pseudo-first order plots, Elovich equation, and Liquid film diffusion models found not suitable to describe the adsorption process (Supporting Information, SI). This ﬁnding indicates that the diffusion of thorium ions into the pores of the resin beads may be the rate determining step. 3.4. Equilibrium studies The equilibrium studies are generally useful for both the design of adsorption process and understanding the adsorption mechanism. An adsorbate may be adsorbed from aqueous media onto the surface of an adsorbent by several interaction modes. The effective mechanism is related to the nature of adsorption sites, surface properties, affinities of the adsorbent, the type of the adsorbate and the adsorption conditions. Generally, the temperature increases the speed of the adsorption and the amount adsorbed. The adsorption isotherms of Th(IV) ions on R-DA and R-TA were obtained at different temperatures and are shown in Fig. 3. Clearly, as the temperature increases, the amount adsorbed of Th(IV) ions on both resins increases with a maximum adsorption capacity of 90 and 110 mg/g for R-DA and RTA, respectively at 323 K. The increase in the adsorption capacity for both resins as the temperature increases indicates an endothermic adsorption process. The values of the adsorption capacity of Th(IV) ions on R-DA and R-TA are better compared to that reported by other investigators using different adsorbents (SI). The interaction between Th(IV) ions and the amine-active sites on R-DA and R-TA surface may be explained to proceed via the chelation mechanism. The Th(IV) ions are reported to have four, five or six coordination.1,28 Both N surface atoms, as well as H2O molecules, may participate in completing the chelation number of thorium ions as shown in the graphical abstract. The observed differences between the values of the experimental adsorption capacities (0.258 and 0.362 mmol/g for R-DA and R-TA, respectively) and the concentration the amine-active sites of the resins (3.8 and 5.4 mmol/g for R-DA and R-TA, respectively) may be attributed to the non-accessibility of all active sites for coordination with Th(IV) ions. This behavior confirms the effect of textural properties of resins on the 8 - Environment ACS Paragon -Plus

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adsorption capacity of thorium ions and the validity of chelation as the operating mechanism of interaction of Th(IV) ions with the N surface active sites. Moreover, the results obtained for the adsorption isotherms were treated according to different adsorption models to verify the adsorption parameters that drive the adsorption process: (i) Langmuir adsorption isotherms model Langmuir adsorption isotherm is one of the most famous well-adopted models used to describe the adsorption systems from solutions. It assumes a reversible adsorption process, the maximum adsorption capacity of a monolayer, energy identical active sites, the heat of adsorption is constant.19 The linear form of Langmuir equation may be given by the equation C# C# 1 = + (8) q# q?@ K A Q?@ where Qmax is the maximum theoretical adsorption capacity (mg/g), and KL is the Langmuir constant (L/mg). Plotting of Ce/qe against Ce gives a straight line with slope and intercept equal to 1/Qmax and 1/KLQmax. The adsorption data of Th(IV) ions on R-DA and R-TA represented in Fig. 3 were subjected to analysis according to Eq. 8. The values of KL and Qmax at different temperatures were obtained from Fig. 4a,b and are reported in Table 2. The calculated values of Qmax are comparable to that experimentally obtained. This indicates that the adsorption of Th(IV) ions on the two resins follows Langmuir model with a maximum surface monolayer coverage and energetically identical active sites. The separation factor constant (RL) is a factor indicates the suitability of a resin towards a targeted metal ion and may be calculated from the equation RA =

1 (9) 1 + K A C4

The value of RL gives an indication for the possibility of the adsorption process to proceed, RL> 1.0 unsuitable; RL = 1 linear; 0 < RL < 1 suitable; RL = 0 irreversible.1 The value of RL, as shown in Figs. 4c, were found to lie between 0.086 and 0.482 indicating the suitability of the R-DA and R-TA as adsorbents for the recovery of Th(IV) ions from nitric acid solutions. Another factor can help understanding the behavior of the adsorption of Th(IV) ions on R-DA and R-TA, is the Langmuir surface coverage (θ) equation, which relates the surface



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coverage (θ) of the resin to the initial concentration of Th(IV) ions.29 θ=

K A C4 (10) 1 + K A C4

Rearranging gives K A C4 =

θ (11) 1−θ

where θ is the surface coverage percentage. Fig. 4d depicts the change in the surface coverage (θ) as a function of initial concentration (Ci) of Th(IV) ions. Obviously, the adsorption of Th(IV) ions on R-DA and R-TA in the early stages of adsorption was very fast (low coverage of resin surface and plenty of free active sites are available for binding with the metal ions) then tends to a plateau at higher surface coverage where most of the N active sites are occupied. This implies the applicability of Langmuir model to describe the adsorption of Th(IV) ions on R-DA and R-TA. (iii) D-R isotherm model Dubinin-Radushkevich (D-R) isotherm is a temperature-dependent model used to estimate the apparent energy of adsorption.30,17 The model is represented by the following equations lnq# = lnq' − K ?F ε (12) where ε = RT ln (1+1/Ce), qs is the theoretical isotherm saturation capacity (mg/g), Kad is the D-R isotherm constant (mol2/kJ2), ε is the D-R isotherm constant, R is the gas constant and T is the absolute temperature (K). The adsorption data of Figs 3 were subjected to analysis according to the D-R model (Eq. 12). The purpose is to verify the nature of the interaction between Th(IV) ions and the surface active sites of the resin to be physisorption or chemisorption. The maximum adsorption capacity (Qmax, mg/g), Polanyi potential (ε) and the activity coefficient related to adsorption mean free energy (Kad, mol2/kJ2) were calculated from plotting (ln qe) against ε2 as shown in Fig. 5a. The apparent energy of adsorption, E, can be computed using the following relation E = I(J

&

K2 )

L.M

N

(13)

For E < 8 kJ/mol, physisorption may dominate whilst values of E in the range 8-16 kJ/mol,

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indicate chemisorption process. Qmax, Kad and E values for the adsorption of Th(IV) ions on R-DA and R-TA were obtained and listed in Table 2. The calculated values of Qmax are 60.1 and 85.2 mg/g for R-DA and R-TA, respectively which are in a good agreement with the experimental values obtained for the resins. The E value was calculated to be 5.0 and 7.07 kJ/mol for R-DA and R-TA, respectively, referring to a process of physisorption nature. The overall data analysis indicates that the adsorption data of Th(IV) ions on the studied resins may be described by the D-R, and Langmuir's models indicating a maximum monolayer surface coverage with a physisorption nature. A comparison between the experimental and calculated values using different models also showed that the Langmuir and D-R models produce the best fitting parameters. Freundlich and Temkin adsorption models were tested and found not suitable to describe the adsorption process of thorium ions on the studied resins (SI). 3.5. Thermodynamic studies Evaluation of the thermodynamic parameters is of great importance in optimizing the process application, to show the feasibility of the adsorption process. Thermodynamic parameters for the adsorption of Th(IV) ions onto R-DA and R-TA were calculated by substituting KL (obtained from Eq. 9) into Van't Hoff equation ln K A =

∆P° QR

+

∆S° Q

(14)

Plotting ln KL against 1/T gives a straight line with intercept and slope equal ∆S°/R and ∆H°/R, respectively. Plotting ln KL vs. 1/T of Eq. 14 for the adsorption data of Th(IV) ions on the two resins gave straight lines with slopes and intercepts (Fig. 5b) from which the values of ∆So and ∆Ho were calculated and reported in Table 3. The positive values of ∆H° in both cases of R-DA and R-TA confirm the endothermic nature of the adsorption of Th(IV) ions and the metal ion-resin binding increases at elevated temperatures (Table 3). The positive value of ∆S° may be explained by the increased degree of randomness due to the liberation of free water molecules in the chelation sphere as a consequence of chelation of Th(IV) ions with the amine-active sites (see the graphical abstract). The Gibbs free energy of adsorption (∆G°) was calculated using the Gibbs free energy relation19

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∆G° = ∆H° − T∆S° (15) The values of ∆G° for the adsorption of Th(IV) ions on R-DA and R-TA were obtained and reported in Table 3. The values of ∆Go are in the range from -22.29 to -31.46 kJ/mol indicating the spontaneity of the adsorption process and become more favorable at higher temperatures.19 Moreover, the data shows that │∆H°│

Kinetics and Thermodynamics Studies on the Recovery of Thorium

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