Uranium and thorium adsorption from aqueous solution using a novel polyhydroxyethylmethacrylate-pumice composite

Journal of Environmental Radioactivity 120 (2013) 58e63

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Uranium and thorium adsorption from aqueous solution using a novel polyhydroxyethylmethacrylate-pumice composite Recep Akkaya* Cumhuriyet University, Vocational School of Health Services, 58140 Sivas, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 March 2012 Received in revised form 26 November 2012 Accepted 27 November 2012 Available online 14 February 2013

Poly(hydroxyethylmethacrylate-pumice), [P(HEMA-Pum)], composite was synthesized by free radical polymerization in aqueous solution. The adsorptive features of P(HEMA-Pum) composite were investi4þ using a range of pH, concentration, time (kinetics), temperature (thermodygated for UO2þ 2 and Th namics), ionic strength and selectivity, and the related parameters were derived from the obtained results. These results indicated that all adsorbents had high affinity to the uranium and thorium ions. The parameters obtained from Langmuir, Freundlich and DubinineRadsushkevich models fit the data well. The values of enthalpy and entropy changes showed that the overall adsorption process was endothermic (DH > 0) and increasing entropy (DS > 0), and it was spontaneous (DG < 0) as expected. The adsorption kinetics following the pseudo-second order model indicated that the rate-controlling step was chemical adsorption that occurred by ion exchange process. Reusability of P(HEMA-Pum) was also investigated, and it was found that the composite could be used at least 5 times. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Adsorption Composite Pumice Polyhydroxyethylmethacrylate Thorium Uranium

1. Introduction Radioactive uranium and thorium ions have often pollute the groundwater and some surface water sources, which causes serious environmental hazards (Ju et al., 2009). Several authors have reported studies on various low cost adsorbents such as cellulose (Fischer and Lieser, 1993; Metilda et al., 2004), pumice (Turan et al., 2011), coal (Dubey et al., 1998; Mahramanlioglu et al., 2007), peat (Rachkova et al., 2010), chitosan (Akkaya and Ulusoy, 2008), hydroxyapatite (Ulusoy and Akkaya, 2009), or natural zeolites (Ulusoy and Simsek, 2005), natural clay (Veli and Alyüz, 2007) and orange residue (Khormaei et al., 2007) for the removal of soluble metals from wastewaters. Because of their low efficiency, there is a recent focus on the development of alternative methods. Due to the relatively large external specific surface areas, various nanomaterials have attracted attention as adsorbents. Chemical precipitation, ion exchange, solvent extraction and sorption on the solid are the commonly used methods for removal toxic or radioactive metal ions from waste solutions or wastes. Therefore, in the previous studies, polymer composites were used to remove the metal ions from the waste solutions.

* Tel.: þ90 3462191010/1341; fax: þ90 3462191256. E-mail address: [email protected]. 0265-931X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2012.11.015

There has been several studies on modified Poly (hydroxyethylmethacrylate), P(HEMA), which is a non-adsorbent and hydrophilic polymer (Saglam et al., 2001; Denizli et al., 2004, 2005). Intra- or inter-chain reactions occurring between the side groups and resulting in crosslinking or cyclization can be used for modifying the structure of the polymer (Samia et al., 2008). Pumice, Pum, is frothy, light weight, highly porous (pore volumes up to 82%) with a density (generally about 0.6e1 kg L1) low enough to float on water (Kitis et al., 2005). The properties such as mechanical behavior, solubility, absorbability, and swelling can be changed by the use of composite materials (Karadag et al., 2002). The polymerbased composite adsorbents have been used for recovery of metal ions in water, groundwater, wastewater, and sea water (Ozer et al., 1997; Sahiner et al., 1999; Pekel and Guven, 2002; Erdogan et al., 2004; Ozay et al., 2009; Anirudhan and Sreekumari, 2010; Bozkurt et al., 2011). In a previous study, several polyacrylamide-based composite materials were produced and used for the selective removal of 4þ 2þ from aqueous media (Akkaya and Ulusoy, UO2þ 2 , Th , and Pb 2008; Ulusoy and Akkaya, 2009; Akkaya, 2009). The main objective of this work was to study the feasibility of uranium and thorium adsorption onto the novel P(HEMA-Pum) adsorbent composite. The adsorptive capacity of P(HEMA-Pum) was then 4þ ions, and many of the physicochemical tested with UO2þ 2 and Th

R. Akkaya / Journal of Environmental Radioactivity 120 (2013) 58e63

59

parameters (pH, concentration, contact time and temperature) for the adsorption were optimized.

points, 278, 288, 298, 308, and 313 K, to determine the effect of temperature on ion adsorption.

2. Description of experiment

2.7. Reusability

2.1. Reagents

The 100 mg of the P(HEMA-Pum) were equilibrated with 10 mL 3 mol L1) and Th4þ (4.3  103 mol L1) soluof UO2þ 2 (3.7  10 tions for 6 h (adequate period of time for completion of the adsorption after kinetic studies). The amount of adsorbed ions was derived from the contents of solutions at equilibrium. P(HEMAPum) composite material was precipitated after the adsorption procedure and was then placed in a column to remove the bound ions by washing with 20 mL of 1 mol L1 HCl with a flow rate of 0.5 mL min1. The reusability experiments were performed 5 times by using duplicate samples.

HEMA, ethylene glycol dimethacrylate (EGDMA), N,N,N0 ,N0 -tetramethylethylenediamine, UO2(NO3)2.6H2O, and Th(NO3)4.4H2O were purchased from Sigma Aldrich. Arsenazo III (disodium salt) was obtained from Acros. Merck was the supplier of 4-(20 -pyridylazo)-resorcinol (PAR). Pumice (Pum) was obtained from Ucler mining co. ltd. in Nevsehir, Turkey. Ammoniumperoxodisulfate; APS (H8N2O8S2), were obtained from Aldrich. 2.2. Preparation of P(HEMA-Pum) The samples were prepared by suspension polymerization in a solution by a procedure previously described in detail (Akkaya, 2012). Ten grams P(HEMA) and 2 g Pum was suspended in 20 mL of distilled water by stirring for 15 min. All manipulations and polymerization reactions took place at room temperature. The polymerization product, P(HEMA-Pum) composite, was washed with distilled water until the effluent attained neutral pH. The composite was then dried, ground, sieved to a particle 707 microns size, and stored in polypropylene containers.

2.8. Competitive adsorption 4þ Equivalent concentrations, 5  103 mol L1, of UO2þ 2 and Th were prepared and used for assessment of the ion selectivity of P(HEMA-Pum). Ten milliliter aliquots were taken from the suspension solution, and after 24 h incubation, ion contents were 4þ were determined with measured. Concentrations of UO2þ 2 and Th and in 0.04% Arsenazo III in HCl to provide pH ¼ 1.5 for UO2þ 2 2 mol L1 HClO4 for Th4þ analysis (Rohwer et al., 1997; Khan et al., 2001).

2.3. The relationship between adsorption and pH UO2þ 2

4þ

and Th adsorption onto P(HEMAThe effects of pH on Pum) composite were investigated at pH points ranging from 1 to 5 (1.0, 2.0, 3.0, 4.0, and 5.0). pH adjustments were performed using 0.1 mol L1 HCl solution. 4þ 2.4. UO2þ adsorption 2 and Th

Amounts of 0.1 g adsorbent were dispersed in 10 mL UO2þ 2 or Th4þ ions at concentrations ranging from 1  104 to 8  103 mol L1 in solution and agitated for 24 h at 298 K before the dispersions were centrifuged. Arsenazo-III was used for colorimetric determination of Th4þ by a procedure previously described in detail (Akkaya and Ulusoy, 2008). PAR was used as a complexin the forming reagent to determine the concentration of UO2þ 2 suspensions by a procedure previously described in detail (Ulusoy and Akkaya, 2009). To confirm the accuracy of the concentration, values obtained 4þ from the ion-dye detection, UO2þ 2 and Th , in some of the selected samples were also determined by using a gamma spectrometer [NAI(Tl) detector combined with a EG&G ORTEC multi-channel analyzer and software, MAESTRO 32, MCA Emulator, USA]. 2.5. Time dependence of adsorption One hundred milligrams of solid P(HEMA-Pum) were added to 4þ 10 mL solutions of each of the UO2þ 2 and Th . Fifty micromilliliter aliquots of the reagenteion complex solution were withdrawn, starting immediately after the solution-solid contact and continued at time intervals of 1, 2, 5, 10, 15, 30, 60, 120, 240, and 480 min. 2.6. Temperature dependence of adsorption One hundred milligrams of P(HEMA-Pum) equilibrated with (3.7  103 mol L1) and Th4þ (4.3  103 mol L1) UO2þ 2 (1000 mg L1) were incubated for 24 h at 5 different temperature

4þ concentration 2.9. Dependency of adsorption to UO2þ 2 and Th

As was described in my previous publications (Akkaya, 2009; þ4 ions were studied in 1000 mg L1 deriva2012) UO2þ 2 and Th tion range. The samples were incubated for 24 h at room temperature. Concentrations of the equilibrated solutions were then assessed by spectrophotometry (Shimadzu 160A, Japan). 3. Results and discussion 3.1. AdsorptionepH relationship 4þ (Q, mol kg1) were The adsorbed amounts of the UO2þ 2 and Th calculated using the formula Q ¼ [(Ci  Ce)V/w], in which Ci and Ce are the initial and equilibrium concentrations (mol L1), w is the mass of adsorbent (kg), and V is the solution volume (L). Due to the inertness of P(HEMA), the values of Q were calculated with reference to the Pum content of adsorbents (0.035 g of 0.1 g P(HEMA-Pum)). 4þ ions onto the P(HEMAThe amounts of adsorbed UO2þ 2 and Th Pum) increased (0.28 mol kg1) with increasing pH, and reached a plateau at around pH 3.0 (Fig. 1). As can be seen, the recovery of 4þ ions from P(HEMA-Pum) was strongly dependent UO2þ 2 and Th on pH (Fig. 1). The amounts of adsorbed ions were close to the maximum adsorption capacity of P(HEMA-Pum) at pH 3.0. The pH 4þ ions dependence of adsorption also confirmed that UO2þ 2 and Th were adsorbed via an ion-exchange mechanism. It has been shown that P(HEMA-Pum) was able to form complex ion associations with uranium at pH 3. The increase in adsorption with increasing pH levels should be explained by the chemical features of Th4þ ions in aquatic solutions. Thorium is always found in þ4 oxidation states in solution, is the largest of the cations with the same valence, and is hardly hydrolyzed in acidic solutions. According to Zhao et al. (2008), the surface of aluminosilicates is also hydrolyzed by means of the mechanisms listed below; the occurrence of the latter dominates with increasing pH:

Q / mol kg

60

R. Akkaya / Journal of Environmental Radioactivity 120 (2013) 58e63 0,3

0,2 pH Th pH UO

0,1 0

1

2

3

4

5

6

pH 4þ Fig. 1. The effect of final solution pH on UO2þ adsorption onto the studied 2 and Th composite P(HEMA-Pum).

amounts of ions recovery for P(HEMA-Pum) were 1 of 0.430 mmol g1 of composite for UO2þ 2 , and 0.210 mmol g composite for Th4þ, respectively. The XL value of the composite was higher than Pum, indicating that the encapsulation with P(HEMA) increased the adsorption of Pum. The increase in adsorption could be attributed to catalytic contribution of P(HEMA) in the adsorption process, and the fine dispersion of mineral particles in P(HEMA) could be accounted for by the high adsorption capacity. The values of “RL” were always found to be 0 < RL < 1, indicating and Th4þ that P(HEMA-Pum) could be suggested for UO2þ 2 adsorption (Table 2). The calculated mass of P(HEMA-Pum) for the and Th4þ from a hypothetical solution removal of 50% of UO2þ 2 2þ 1 containing 100 mg L UO2 and Th4þ was found to be 0.21 mg L1 and 0.14 mg L1. A t-test was applied to obtain the significance of regression coefficients (R2) for the compatibility of the present data to the Langmuir, Freundlich, and DeR models, and for the linearity of kinetic equations (Miller and Miller, 1989). 3.3. Adsorption kinetics

hSOH%hSO þ Hþ ðin basic solutionsÞ Based on the facts mentioned above, the adsorption of Th can be explained by the following mechanisms [40]:

Th4þ þ H2 O%ThðOHÞ3þ þHþ

Equations related to the pseudo-second order kinetic and intraparticle diffusion models were t=Qt ¼ 1=ðkQe2 Þ þ t=Qe and Qt ¼ kit1/2, where Qt and Qe are the adsorbed amounts (mol kg1) at

Q/mol kg

hSOH þ Hþ %hSOHþ 2 ðin acidic solutionsÞ

0,5

0,4

4ðhSOHÞ þ Th4þ %ðhSOÞ4 Th þ 4Hþ   3 hSO þ ThðOHÞ3þ %ðhSOÞ3 ThOH

0,3

    4 hSO þ Th4þ % hSO Th

0,2

4

Th UO

0,1

Langmuir Freundlich

3.2. Concentration dependence of adsorption 0,0 0

1

2

3

4 C x10 /mol L

0,4 Q/mol kg

The Langmuir and Freundlich models, defined with Q ¼ (KLXLCe)/(1 þ KLCe), where Ce is the concentration at equilibrium, XL is the monolayer sorption capacity (mol kg1), KL is the adsorption equilibrium constant (L mol1) related to the adsorption energy, and Q ¼ XFCeb, XF and ‘b’ are empirical Freundlich constants associated with the capacity and intensity of adsorption, fitted the experimentally obtained isotherms. The isotherms were also evaluated with reference to the DubinineRadushkevich (DR) model to determine KDR (mol2 K J2) related to the sorption energy from 2 Q ¼ XDR eKDR ε , where XDR is the sorption capacity (mol kg1) and ε is the Polanyi potential, given with ε ¼ RTln (1 þ 1/Ce), in which R and T represent the ideal gas constant (8.314 J mol1 K1) and absolute temperature (298 K). Free energy change (E; J mol1) required to transfer 1 mol of ion from the infinity in the solution to the solid surface was then derived from E ¼ (2KDR)1/2. Langmuir isotherms were further considered to predict whether P(HEMAPum) was “favorable” in view of the dimensionless factor (RL) and to calculate the weight of the composite (w; kg) for removing UO2þ 2 and Th4þ from hypothetic solutions with “V”(L) volume: RL ¼ 1/ (1 þ KLCe) as suggested (Dogan and Alkan, 2003). Adsorption isotherms obtained were of L or H types, specified in Giles classification (Fig. 2). The parameters derived from the Langmuir, Freundlich, and DeR models are shown in Table 1. The

0,3

0,2

0,1 Th UO Dubinin Radushckevic fit 0,0 0

2

4

6

8 2

e x108 Fig. 2. Experimentally obtained isotherms and their compatibility to Langmuir, 4þ onto Freundlich and DubinineRadushkevich models for adsorption of UO2þ 2 and Th the studied composite P(HEMA-Pum).

Table 1 and Th4þ adsorption on to Langmuir, Freundlich and DeR parameters for UO2þ 2 P(HEMA-Pum). Langmuir a

UO2þ 2 Th4þ

KL

c 2

XF

b

c 2

XDR

KDR  109

c 2

0.947 0.989

2.30 1.89

0.35 0.34

0.873 0.923

2.41 1.06

7.74 3.40

0.991 0.982

R

R

R

mol kg1. L mol1. statistically significant correlation (p < 0.05).

3.4. Temperature dependence of adsorption To determine the temperature dependence of adsorption, the distribution coefficients (Kd) were derived from Kd ¼ Q/Ce for each temperature and “ln Kd” was depicted against 1/T to provide adsorption enthalpy (DH, kJ mol1), and entropy (DS, J mol1 K1) from the slopes (DH/R) and intercepts (DS/R) of the depictions with reference to ln Kd ¼ DS/R  DH/(RT). After DH and DS values were obtained, DG values were calculated using the formula DG ¼ DH  TDS for 298 K. Temperature dependence of the Table 2 4þ adsorption on to P(HEMA-Pum), derived Kinetic parameters, for UO2þ 2 and Th from the second order and WebereMorris models and coefficients of regression (R2) for the compatibilities. Pseudo second order model

UO2þ 2 Th4þ a b c d e f

k

2.74 3.71

b

c

d

f 2

e

f 2

0.30 0.29

0.30 0.29

0.344 0.291

8.71 4.21

0.999 0.999

2.21 4.32

0.944 0.879

Q

H

2,0

0,5

Th UO

0,0 0

200

400

600

800 t/dk

0,4

0,3

0,2

0,1 Th UO 0,0 0

5

10

15

20

25

30 t

/min

4þ Fig. 3. The compatibility of UO2þ adsorption kinetics to pseudo second order 2 and Th model and WebereMorris model.

adsorption was represented as a function of ln Kd (Fig. 4). Thermodynamic parameters, derived from the depictions and free energy change (EDeR), derived from DeR model, and are presented in Table 3.

6,0

5,5

5,0

Th UO

WebereMorris model

b

QMod

2,5

1,0

time t; k and ki are the rate constants applied to the results of kinetic studies to envisage the controlling mechanism of the adsorption process. Initial adsorption rate (H) was calculated by using the equation H ¼ kQe2 , which relates to time required for adsorption of half of the concentrations (t1/2) given by t1/2 ¼ 1/(kQe) ¼ Qe/H (Sun and Wang, 2006; Basha and Murthy, 2007). and Th4þ adsorption data to the The compatibility of UO2þ 2 pseudo-second order kinetics and intra-particle diffusion (Webere Morris) models was also evaluated (Table 2) with reference to the linearity obtained from “t  t/Qt” and “t0.5  Qt” plots (Fig. 3). Experimental adsorption data have been analyzed using sorption kinetic models such as the intra-particle diffusion (WebereMorris) and pseudo-second order kinetic models. It has been observed that pseudo-second order kinetic model provided a high degree of correlation with experimental data for the adsorption of uranium and thorium ions on P(HEMA-Pum) from aqueous solutions. The sorption kinetics is best described by the pseudo-second-order 4þ is present in the model. Intra-particle diffusion of UO2þ 2 and Th sorption process, but it is not the rate-limiting step. Furthermore, the identicalness of the values of adsorbed amounts at equilibrium, obtained from the model (Qmod) and from the experiment (Qe), confirmed that the nature of adsorption was concentration dependent (Weber and Morris, 1963). Qt values plotted as a function of t0.5 (Weber and Morris model) did not provide intercepts at the origin, defining the adsorption process controlled by diffusion (Fig. 3), but it was a curve that could be evaluated in two linear parts (Ho and McKay, 1999).

a

3,0

1,5

-1 Qt / mol kg

c

b

726 527

DubinineRadushkevich

61

ln K

a b

XL

0.314 0.363

Freundlich

-1 (t/Qt)x10 / mol kg min

R. Akkaya / Journal of Environmental Radioactivity 120 (2013) 58e63

t1/2

mol1 kg dk1. mol kg1. mol kg1 dk1. dk. mol kg1 dk0.5. Statistically not significant (p > 0.05).

R

ki  102

R

4,5

4,0 3,2

3,4

3,6 T x 10 K

4þ Fig. 4. Temperature dependence of UO2þ adsorption and its compatibility to 2 and Th linearity.

62

R. Akkaya / Journal of Environmental Radioactivity 120 (2013) 58e63

Table 3 Thermodynamics parameters obtained from Vant-Hoff and DeR models. Vant Hoff

DS/j

a 2

E/kj mol1

a 2

mol1 K1

DG/kj mol1

28.7 3.6

28.7 49.8

11.9 12.3

0.992 0.998

10.5 11.6

0.991 0.982

mol1 UO2þ 2 Th4þ a

DeR

DH/kj

R

R

Statistically significant, p < 0.05.

Acknowledgment

Table 4 Metal selectivity of P(HEMA-Pum) from solutions containing possible combinations of studied ions at equivalent concentrations (5  103 mol L1). Combinations (binary)

Adsorption %

UO2þ 2 Th4þ

81a (74)b 34 (26)

a b

also found to be positive. The regeneration tests showed that P(HEMA-Pum) could be used at least 5 times. Selectivity studies also showed that P(HEMA-Pum) had higher selectivity for UO2þ 2 . P(HEMA-Pum) has a considerable potential as an adsorbent of metal ions, dyes and other pollutants in a commercial system because of being cheap and efficient.

As percentage of the amount of ion added to the solution. As percentage of total ion adsorption onto the adsorbent.

Table 3 also shows that the rate increase: Th4þ < UO2þ holds for all the temperatures of the study. The sorption occurs spontaneously for all the uranium and thorium ions, the spontaneity increasing with temperature (Table 3). For all temperatures, the spontaneity changes in a similar uranium and thorium order as the sorption capacity. The sorption is endothermic, the DH values of sorption increasing in the series: Th4þ < UO2þ. The sorption is endothermic and is accompanied by an increase in entropy. The same trend of increase is found for the DS values of sorption. 3.5. Reusability 4þ 5 The reusability of P(HEMA-Pum) was tested for UO2þ 2 and Th times, 1 initial and 4 regenerations, and the resulting adsorption percentages were calculated. The adsorption means of the 4 regenerations and their S.E.M values were found to be 93  1.2% 4þ for UO2þ 2 and 81  2.4% for Th , respectively. The mean values were found not to be significantly different from that of the initial adsorption experiment (p < 0.05).

3.6. Competitive adsorption The ion selectivity of P(HEMA-Pum) was tested for a mixture 4þ ions (5  103 mol L1) (Table 4). solution of UO2þ 2 and Th The new adsorbent was found to be more selective toward 4þ UO2þ 2 , even in the presence of Th . This can be explained by the magnitude of the ionic diameter and by the parameters specified for UO2þ 2 adsorption (KL and b, see Table 1). 3.7. The effect of ionic strength It was observed that the ionic strength of CaCl solutions did not have any significant effects on the adsorption capacity of adsorbents. 4. Conclusion The study showed that P(HEMA-Pum) can be used for removal and Th4þ ions from aqueous solutions. The adsorbed of UO2þ 2 4þ ions increased with increasing pH for amounts of UO2þ 2 and Th and Th4þ ions P(HEMA-Pum). The adsorbed amount of UO2þ 2 increased with increase in temperature of P(HEMA-Pum). The 4þ were found to be compatible adsorption kinetics for UO2þ 2 and Th with the pseudo-second order model, and the adsorption process was chemical. Changes in the values of enthalpy and entropy were

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