Sorption of uranium from aqueous solutions using palm-shell-based adsorbents: a kinetic and equilibrium study

Journal of Environmental Radioactivity 126 (2013) 115e124

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Sorption of uranium from aqueous solutions using palm-shell-based adsorbents: a kinetic and equilibrium study Shilpi Kushwaha, Padmaja P. Sudhakar* Department of Chemistry, The M. S. University of Baroda, Vadodara 390002, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 April 2012 Received in revised form 15 January 2013 Accepted 31 July 2013 Available online

In this study adsorbents based on palm shell powder as well as modified and activated palm shell powder were studied to analyze their behavior in sorbing U6þ by both batch and fixed column modes. Seven different two-parameter isotherm models were applied to the experimental data to predict the sorption isotherms. The DG0 values from Langmuir and thermodynamic calculations indicate physisorption as the major mechanism for adsorption of uranium. Usefulness of various kinetic models like pseudo first order, pseudo second order, intraparticle diffusion, Bangham, Elovich and Liquid film diffusion were tested. The adsorption capacities were found to be greater than 200 mg/g for all the adsorbents under study. The column data were fitted by Thomas, Yoon and Nelson as well as Wolborska models. The Thomas and Yoon and Nelson models were best to fit the breakthrough curves under the experimental conditions studied. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Uranium Kinetics Isotherms Biosorption

1. Introduction Metal contamination of the aquatic environment has become an issue of importance with respect to environmental and human health. Recently studies sought to quantify the adsorption of metals from industrial wastes and subsurface environment through sorption (Veglio et al., 2003). Processing steps in the nuclear fuel cycle generate wastewater streams containing a variety of dissolved metals, including lanthanide and actinide elements. The problem lies in the continuous production of high-volume, often low activity, waste requiring continuous treatment. Remediation of radionuclides from aqueous solutions has been studied extensively, because of their toxicity even at low concentration (Zhang et al., 2002; Bezrodny et al., 2002; Griffin, 1999). Special interests are raised when dealing with uranium contamination of the environment. Major concerns with uranium are its mobility as it is highly soluble in the pH ranges of acid rain and occurs in hexavalent form as a mobile hydrated uranyl ion. Uranium can cause transient chemical damage to the kidneys if present at a level above 0.1 mg/ kg of body weight (Mehra et al., 2007). Several physical, chemical, and biological methods are available for the removal of uranium(VI) from contaminated water. However, these methods include toxic

* Corresponding author. Tel./fax: þ91 265 2795552. E-mail addresses: [email protected], (P.P. Sudhakar).

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0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.07.021

byproducts, some do not work in acidic pH, and some are proven to be relatively costly (Torresdey et al., 2002). Biosorption is a term that describes the removal of metals ions by the binding to living/ non-living biomass from an aqueous solution. Unlike physical and chemical treatments, biosorption can reduce the operational costs and many potential sources of biological material are cheaply and readily available (Wood, 1992; Hobson and Poole, 1988). Many biosorbents have been used for removal of uranium from aqueous solutions, acid mine drainage and process solutions (Bayramoglu et al., 2006; Kalin et al., 2005; Khani et al., 2008; Li et al., 2004; Parab et al., 2005). As a local agro-waste we have used palm shell, which is available throughout coastal Asia (Kushwaha et al., 2008, 2009, Sreelatha and Padmaja, 2008, 2010). The Borassus flabellifer species of palm is available in coastal India. In this study, emphasis was on the use of palm shell powder as well as modified and activated palm shell powder as cost effective adsorbents for the removal of uranium. The efficiency of uranium adsorption was assessed using the best fit with the adsorption data of the two-parameter models, namely Freundlich, Langmuir, Temkin, Dubinin-Radushkevich (DR), Flory-Huggins, Elovich and Halsey isotherms. Usefulness of various kinetic models like Pseudo First order, Pseudo Second order, Intraparticle diffusion, Bangham, Elovich and Liquid film diffusion were tested. The applicability of various isotherms and kinetic models were determined by their r2 value and error analysis. The objectives of the study were: 1) to explore the potential use of palm shell and palm shell based adsorbents for adsorption of

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S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

uranium, and 2) to study the influence of various parameters on adsorption.

Thermodynamic parameters of the adsorption process (DG0, DH0 and DS0) could be determined from the experimental data obtained at various temperatures using following equations:

2. Materials and methods 2.1. Preparation of adsorbent material The palm shell powder (PSP) was treated with sulphuric acid (APSP) for oxidation of surface functional groups and then further treated with: i) steam (SAPSP) at 14psi pressure in presence of air ii) persulfate (PAPSP) for further oxidation and iii) thermal treatment at 900  C (9AAC) for increasing porosity and surface area. The palm shell powder was modified with formaldehyde (MPSP) to protonate the functional groups present and to polymerize the color imparting components of PSP. As prepared and characterized PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP (Kushwaha et al., 2012a, b) were tested for uptake studies of U6þ.

DS0 DH 0 ln⁡qe  ¼ Ce R RT

(2)

DG0 ¼ DH0  T DS0

(3)

where R is the gas constant (8.314 J mol1 K1) and T is the absolute temperature (K). Values of correlation coefficients and standard deviation were used to compare the models. SD was calculated using the equation.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u1 X SD ¼ t ðx  xÞ2 N i¼1 i

(4)

2.2. Batch uptake

2.3. Desorption studies

A stock solution of U6þ was prepared by dissolving 2.11 g of uranyl nitrate (Sulab) in slightly acidified double distilled water and making up to 1 L to give 1000 mg/L of uranyl solution. Working standards were prepared by diluting different volumes of the stock solution to obtain the desired concentration. Batch adsorption experiments were conducted at 30  C by agitating 0.1 g of adsorbent with 25 mL of uranyl ion solution of desired concentration maintained at pH 1.0 (using 0.1N HNO3) in 100 mL stoppered conical flasks in a thermostated rotary mechanical shaker at 180 rpm for 4 h (except for the contact time experiments). The effect of the initial concentration (100e 1000 mg/L) was also studied in order to determine the effect of the parameter on the adsorption of metal from the solution. The optimum equilibrium time was determined as the contact time required for the concentration of metal in the solution to reach equilibrium and was obtained by varying the contact time in the range 30e240 min. At the end of the predetermined time intervals, the suspensions were filtered and the uranium content in the filtrate was analysed using arsenazo III (Lopez-de-Alba et al., 1997) by spectrophotometer. The uptake of uranyl ion by the adsorbents under study (qe) was calculated from the difference between the initial and final concentration as follows:

0.1 g of uranium loaded adsorbents under study was treated with 10 mL of different desorbing solutions like 0.1M EDTA, 0.1M NH3 and 0.1M HCl for a period of 30 min in a thermostated rotary mechanical shaker. After 30 min the amount of metal desorbed from the adsorbents under study was determined by spectrophotometer. The adsorption-desorption experiments were repeated for three cycles.

qe ¼ ðCi  Ce Þ=m;

2.4. Column studies Column experiments were conducted in a glass column with an internal diameter of 1 cm and a length of 30 cm packed with the adsorbent under study. U6þ solution of known concentration (1000 mg/L) at pH 1 was passed through the column of adsorbent at a flow rate of 1 mL/min. Samples from the column effluent were collected at regular intervals and analyzed by atomic absorption spectrometry. The break-through time has been chosen when the concentration of the effluent is 1 mg/L. Adherence of the column data to three different models (Thomas, Yoon and Nelson and Wolborska models) was studied using equations as described in Table 1. 3. Results and discussion

(1)

where, Ci e initial concentration of metal ion mg/L; Ce e Equilibrium concentration of metal ion mg/L; m e Mass of adsorbent g/L; qe e Amount of metal ion adsorbed per gram of adsorbent. Each experimental result was obtained by averaging the data from three parallel experiments. Adsorption isotherm experiments were also performed by agitating 0.1 g of the adsorbent under study with a series of 25 mL solutions at pH 1.0, containing different initial concentrations of (100e1000 mg/L) at 30  C. After the established contact time (4 h) was attained, the suspension was filtered, and supernatant was analyzed for the metal concentration. The adherence of the equilibrium isotherm and data obtained to different adsorption isotherms models as given in Table 1 was tested. Similarly the uranium adsorption data obtained after agitating solution containing 100 mg/L of uranium for various contact times with the adsorbents under study at pH 1 were calculated to determine the order of reaction rate and the adherence to different kinetic models as given in Table 1 was tested.

3.1. Uptake studies 3.1.1. Contact time variation Contact time variation shows that equilibrium is achieved faster (30 and 80 min) when 9AAC and APSP were used as the adsorbents as compared to other adsorbents under study (PSP, SAPSP, PAPSP and MPSP) where equilibrium was achieved in 180 min (Fig. 1). The rate of adsorption is very fast initially with about 96% of total uranium being removed within few minutes followed by a decreased rate with the approach of equilibrium. The removal rate is high initially due to the presence of free binding sites which gradually become saturated with time resulting in decreased rate of adsorption as equilibrium approaches. This indicates that the adsorption is mainly through surface binding. Similar observations were made by Das et al. (2007). 3.1.2. Amount of adsorbent variation The effect of dose of adsorbents under study on the removal of uranium is shown in Fig. 2, which illustrates the adsorption of

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

117

Table 1 Isotherm, Kinetic and Column Models. Eqn.

Isotherm

Functional form

Plotting

I

Freundlich

qe ¼

1=n K f Ce

II

Langmuir

qe qm

¼

KL Ce 1þ KL Ce

III

Elovich

qe qm

¼ KE Ce exp

IV

Temkin

qe qm

¼

V

Dubinin-Radushkevich

qe qm

¼ expðbε2 Þ with b ¼

VI

Florry-Huggins

Log Cq0 ¼ LogKFH þ nFH Logð1  qÞ

Log qe vs Log Ce



VII

Halsey

qe ¼

Kinetics VIII

Pseudo 1st order

dq ¼ dt

RT DQ

1 qe

  qe qm

vs

1 Ce

Ln Cqee vs qe

LnðKT Ce Þ 1 E2

qe vs LnCe   2 Lnqe vs Ln CCes

and ε ¼ RTLn CCes

Log Cq0 vs Logð1  qÞ

1=n KH =Ce H

Log qe vs LogCe

K0i ðqe  qt Þ

Log (qe  qt) vs t

q2e K20 t 1þqe Kt

t qt

IX

Pseudo 2nd order

qt ¼

X

Elovich

qt ¼ 1b LnðabÞ þ 1b Lnt

qt vs Lnt

XI

Intraparticle Diffusion

XII

Liquid Film Diffusion

qt ¼ Kit0.5   Ln 1  qqet ¼ KFD t

qt vs t0.5   Ln CCo ¼ bNCoo t  bUZ

Column I

Thomas

Ln

II

Yoon & Nelson

Ln

III

Wolborska

Ln



C0 C



C C0 C

 

uranyl ion with change of the adsorbent dose from 200 to 1000 mg. As inferred from Fig. 1, for a fixed metal initial concentration, increasing the adsorbent dose provided greater surface area and availability of more active sites (Xiao and Thomas, 2005), thus leading to the enhancement of metal ion uptake. Adsorption increased from 98 to >99% with increase in adsorbent dose. 3.1.3. Temperature variation Temperature studies showed (Fig. 3) almost the same trend for all the adsorbents under study. From the figure it can be seen that uptake decreases as temperature increases indicating that the mechanism of adsorption is exothermic in nature. 25.0

1 

C Co



¼

kTH q0 m Q

vs t

  1 vs t   Ln C0CC vs t   Ln CCo vs t

 kTH C0 t

Ln

¼ kYN t  t0:5 kYN

¼ bNCoo t  bUZ



C0 C

3.2. Adsorption kinetics Fig. 4 shows the adsorption kinetics conducted at pH 1 for U6þ removal by PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP (detailed study on effect of pH is described in our earlier work) (Kushwaha et al., 2012a, b). Adsorption of U6þ ions onto the adsorbents under study was carried out for 240 min to ensure attainment of equilibrium. The kinetic models of Pseudo First order, Pseudo Second order, Intraparticle diffusion, Bangham, Elovich and Liquid film diffusion models were studied and the kinetic constants for the adsorption of uranium by all the adsorbents under study are presented in Table 2. The pseudo second order kinetics provided the best fit for the kinetic data (Table 2). The qe values were very close to the experimental qe value and correlation coefficient values were 0.990e 1.000 for PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP suggesting rate

24.5

24.0

180

qe (mg/g)

qe (mg/g)

200

23.5

160

140

23.0 0

50

PSP PAPSP

100

150

Time (min) APSP 9AAC

200

250

120

SAPSP MPSP

0.02

PSP

0.04

APSP

0.06

0.08

Dose (g) SAPSP PAPSP

0.10

9AAC

MPSP

, Fig. 1. Effect of time on adsorption of U6þ.

Fig. 2. Effect of dose on adsorption of U6þ.

118

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

25.0

The curves for Liquid Film Diffusion Model (Table 1) did not pass though origin as required by the model for all the adsorbents under study but had very small intercepts (curves not shown) indicating that diffusion of uranium from the liquid phase to the adsorbent surface might be having some role in deciding the rate processes. The Weber and Morris adsorption kinetic model was plotted using the equation given in Table 1. The plots obtained for PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP do not pass through origin implying that intraparticle diffusion is not the only operative mechanism. The intraparticle diffusion rate is fastest in PSP and APSP, the order being APSP w PSP > MPSP > PAPSP > SAPSP>9AAC as evident from intraparticle diffusion rate constants. Finally, in order to further confirm the occurrence of intraparticle diffusion, Bangham equation (Table 1) was applied to the adsorption data. The double logarithmic plots obtained with very less correlation coefficients (>0.455) indicated very less contribution of pore diffusion towards adsorption process. The lowest r2 value for 9AAC is in agreement with the results of intraparticle diffusion. The not so linear curves indicated the diffusion of adsorbate into pores of the sorbent is not the only rate controlling step. Adsorption capacity of the adsorbents for uranium was highest at pH 1 (Kushwaha et al., 2012a, b) hence kinetics was studied at pH1, which is consistent with results of others (Sarma et al., 2008). Initial rapid uptake implies the binding of adsorbate ions on the surface of adsorbents under study through proton exchange at pH 1. Uranium occurs primarily as UOþ2 2 ion at pH 1 in aqueous medium (Kushwaha et al., 2012a, b). Later on slower adsorption might be due to intraparticle diffusion, and diffusion of uranium from the aqueous phase to the adsorbent due to weak acidic and basic groups such as carboxyl, hydroxyl, amino groups at pH > 1.

qe (mg/g)

24.9

24.8

24.7

24.6

30

40

50

60

70

0

Temperature ( C) PSP

APSP

SAPSP

PAPSP

9AAC

MPSP

Fig. 3. Effect of temperature on uptake of uranyl ions by PSP, MPSP, APSP, SAPSP, PAPSP and 9AAC.

limiting step in adsorption of uranium could be chemisorption involving valence forces through the exchange of electrons between sorbent and sorbate, complexation, coordination and/or chelation (Febrianto et al., 2009; Vieira and Beppu, 2006). In pseudo first order model the qe(exp) values were much higher than qe fitted values showing large discrepancies demonstrating that the reaction cannot be classified as pseudo first order although this plot has reasonably good correlation coefficient from the fitting process. This underestimate of the amount of binding sites is probably due to the fact that qe was determined from the y-intercept (0, 1). This has been observed by several other workers (Ho and McKay, 1998; Schiewer and Patil, 2008; Reddad et al., 2002; Vijayaraghavan et al., 2006). This could be due to boundary layer controlling the beginning of the adsorption process (Vijayaraghavan et al., 2006). 25 20 15 10

qt (mg/g)

For modeling of uranium uptake Freundlich, Langmuir, Temkin, Dubinin-Radushkevich (DR), Flory-Huggins, Elovich and Halsey isotherm models were employed. Adsorption isotherms of the type qe vs Ce were also used to verify the isotherm models. Model fits for all the isotherms along with experimental data for adsorption of

25

25

20

20

15

15

10

PSP

10

APSP

0

50

100

150

200

250

0

0 0

50

100

150

200

250

25

25

25

20

20

20

15

15

15

10 5 0

50

100

150

200

qe (exp)

250

PS2

0

50

100

150

200

250

MPSP

9AAC 5

5

0

0

10

10

PAPSP

SAPSP

5

5

5 0

3.3. Adsorption isotherms

0 0

50

100

150

Time (min) PS1 IP

Fig. 4. Kinetic studies.

200

250

Elovich

0

50

LDM

100

150

200

Bangham

250

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

119

Table 2 Kinetics parameters for uranium adsorption. pH1 PSP

APSP

SAPSP

PAPSP

9AAC

MPSP

24.714

25.000

24.920

24.996

25.000

24.802

22.629 0.154 0.999 0.017

25.056 0.104 1.000 0.005

25.044 0.031 1.000 0.008

21.286 0.047 1.000 0.007

62.112 0.094 0.999 6.24E-04

24.944 0.028 0.999 0.016

Lagergren qe (mg g-1) K1 (min1) r2 SD

4.604 0.035 0.795 0.082

1.223 0.043 0.970 0.067

1.971 0.023 0.942 0.101

1.546 0.021 0.989 0.075

1.197 0.129 0.986 0.098

1.545 0.018 0.954 0.111

Intra particle diffusion Kip (mg/g/min) r2 SD

0.079 0.951 0.072

0.008 0.806 0.016

0.049 0.976 0.030

0.054 0.967 0.039

0.000 0.000 0.000

0.070 0.921 0.082

Elovich b (g.mg1) a (mg g1 min1) r2 SD

1.907 3.788Eþ41 0.989 0.065

3.478 3.66Eþ81 0.859 0.107

1.969 4.32Eþ43 0.964 0.119

2.204 3.83Eþ49 0.971 0.096

1.865 7.70Eþ45 0.711 0.046

2.158 8.89Eþ47 0.981 0.079

Liquid film diffusion model KFD (min1) r2 SD

0.017 0.979 0.095

0.043 0.970 0.085

0.023 0.942 0.097

0.021 0.989 0.103

0.129 0.987 0.106

0.018 0.954 0.163

114.160 1.02E-04 0.881 0.004

113.179 4.43E-05 0.616 0.004

113.807 9.15E-05 0.798 0.005

113.635 8.20E-05 0.811 0.004

112.841 7.28E-06 0.455 0.001

113.918 8.88E-05 0.863 0.004

qe (exp) Pseudo 2nd order qe (mg g-1) K2 (g mg min1) r2 SD

Bangham KBM (mL g1 L1)

a

r2 SD

U6þ on PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP are presented in Fig. 5. The values of model constants along with their correlation coefficients, r2 and SD values for all the systems studied are presented in Table 3.

qe (mg/g)

50000

PSP

250000

Freundlich equation describes adsorption (possibly multilayer in nature) on a highly heterogeneous surface consisting of non identical and energetically non uniform sites. The value of n z 1 for the Freundlich model indicates favorable adsorption. The Langmuir

200000

40000

150000

30000

100000

20000

50000

10000

2000

APSP

SAPSP

1500 1000

500 0

0

0 0

200

400

600

800

0

1000

200

400

600

800

200000

30000

150000

20000

100000

10000

50000

400

600

800

1000

MPSP

9AAC

PAPSP

40000

200

20000

250000

50000

0

1000

15000 10000

0

5000 0

0 0

200

400

600

800

1000

0

200

400

600

800

1000

0

200

Ce (mg/L) qe (exp)

Freundlich

Langmuir

Temkin

Fig. 5. Adsorption Isotherms for U6þ.

Halsey

400

600

800

1000

120

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

Table 3 Isotherms parameters for different adsorbents. PSP

APSP

SAPSP

PAPSP

9AAC

MPSP

248.440

248.280

248.364

248.415

248.427

248.280

3.739 0.714 0.989 0.045

4.618 0.893 0.977 0.068

5.989 1.248 0.991 0.042

5.207 1.008 0.988 0.048

4.226 0.770 0.961 0.089

5.474 1.188 0.998 0.020

Langmuir KL (dm3 mg1) qm (mg g1) DG (kJ mol1) r2 SD

0.157 249.376 4.810 0.986 1.90E-05

0.985 253.807 0.039 0.989 9.72E-04

0.269 252.525 3.407 0.972 0.001

0.395 232.558 2.416 0.999 2.48E-04

0.067 244.498 7.017 0.993 0.002

0.216 235.849 3.979 0.958 9.91E-04

Temkin DH (kJ mol1) KT (dm3 mg1) r2 SD

3.614 0.639 0.995 0.078

4.551 0.867 0.995 0.081

6.399 0.570 0.994 0.085

4.774 0.869 0.989 0.053

3.715 0.748 0.994 0.099

5.822 0.725 0.978 0.086

DR qm (mg g1) E0 (KJ) r2 SD

284.292 14.041 0.991 0.095

256.466 17.330 0.985 0.023

214.862 12.883 0.964 0.093

232.758 12.918 0.973 0.087

304.904 1.498 0.998 0.044

207.265 15.377 0.953 0.022

Halsey KH (mg g1)(dm3/mg)1/n nH r2 SD

0.114 0.714 0.989 0.094

0.035 0.949 0.977 0.056

0.006 1.248 0.991 0.098

0.022 1.008 0.988 0.098

0.078 0.770 0.961 0.094

0.010 1.176 0.998 0.046

Flory-Huggins KFH (mg g1)(dm3/mg)1/n NFH r2 SD

0.956 0.994 1.000 4.97E-04

0.983 0.998 1.000 4.06E-04

1.025 0.997 1.000 3.69E-04

1.026 0.997 1.000 7.78E-04

1.016 0.998 1.000 2.57E-04

1.038 0.995 1.000 6.08E-04

Elovich qm (mg g1) KE (dm3 mg1) r2 SD

237.461 0.846 0.986 7.39E-04

232.758 0.092 0.996 0.012

208.513 0.018 0.974 0.030

217.022 0.419 0.978 0.079

262.434 0.080 0.983 0.030

198.3434 0.238 0.989 0.037

qe(exp) (mg/g) Freundlich KF (mg g1)(dm3/mg)1/n n r2 SD

isotherm model is basically developed for gas-phase adsorption on homogeneous surfaces of glass and metals and predicts a single maximum binding capacity. The parameters KL (equilibrium sorption constant) and qmax were calculated from the intercept and slope of the plot of Ce/qe versus Ce. Based on the correlation coefficient r2 and

standard deviations for Langmuir it varied from 0.958 to 0.999 with very less SD values. Langmuir monolayer adsorption capacity was found in good agreement with the experimental adsorption capacities. The adsorption capacity was found to be in the order of APSP w SAPSP > PSP>9AAC > MPSP > PAPSP corresponding to

Table 4 Agro-based adsorbents with their maximum adsorption capacities for uranium. Sr. No.

Adsorbent

Adsorbate (U6þ)

Reference

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Activated Carbon Aspergillus Niger Penicillium Chrysogenum R. Arrhizus Date pits Catenellarepens-red alga Cross-linked chitosan AC’s-benzoylthiourea-anchored anchored Chitosan/clinoptilolite composite Chitosan Penicillium citrinum Bi-functionalized biocomposite Palm Shell Powder (PSP) Acid treated palm shell powder (APSP) Steam treated palm shell powder (SAPSP) Persulfate treated palm shell powder (PAPSP) Acid activated carbon (9AAC) Modified Palm shell powder (MPSP)

28.5 mg g1 29.0 mg g1 70.0 mg g1 220.0 mg g1 10.0 mg g1 303.0 mg g1 72.5 mg g1 113.8 mg g1 562.6 mg g1 482.7 mg g1 274.7 mg g1 43.2 mg g1 249.4 mg g1 253.8 mg g1 252.5 mg g1 232.6 mg g1 244.5 mg g1 235.9 mg g1

(Kutahyali and Eral, 2004) (Ahluwalia and Goyal, 2007)

(Saad et al., 2008) (VikasBhat et al., 2008) (Wang et al., 2009) (Zhao et al., 2010) (Humelnicu et al., 2011) (Pang et al., 2011) (Aytas et al., 2011) This study This study This study This study This study This study

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124 Table 5 Thermodynamic Parameters for uranyl ions by PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP.

DS (kJ mol1 K1)

DH (kJ mol1)

r2

SD

11.862 10.134 10.076 10.201 10.941 10.852

0.033 0.031 0.030 0.030 0.032 0.031

1.854 0.881 0.856 0.981 1.396 1.355

0.945 0.986 0.997 0.996 0.977 0.997

0.001 0.003 0.002 0.002 0.006 0.002

252.5e253.8 > 249.4>244.5 > 235.8>232.6 mg/g. The adsorption capacities of the adsorbents were found to be higher than those reported in literature as shown in Table 4. The correlation coefficients of Temkin Model (0.978e0.995) indicate a satisfactory fit of the model to the experimental data. The variation of adsorption energy, DQ ¼ (3.614, 4.551, 6.399, 4.774, 3.715 and 5.822) is positive for all the sorbents under study, which indicated the adsorption process to be exothermic. The DubinineRadushkevich model is applied in the form of linear equation as given in Table 1. The adsorption energy values calculated for U6þ on PSP (14.041 K Jmol1), APSP (17.330 K Jmol1), SAPSP (12.883 K Jmol1), PAPSP (12.918 K Jmol1), 9AAC (1.498 K Jmol1) and MPSP (15.377 K Jmol1). The magnitude of E is useful for estimating the mechanism of the sorption reaction. In case of E < 8.0 kJ/mol, physical forces may affect the sorption; for E in the range 8e16 kJ/mol, ion exchange is the working mechanism, while for E > 16 kJ/mol sorption may be dominated by particle diffusion (Gerente et al., 2007; Gupta et al., 2005). It is thus evident from D-R model that for 9AAC physisorption is the predominant mechanism, while for APSP, SAPSP, PAPSP and MPSP ion-exchange is the predominant mechanism. Multilayer adsorption is generally discussed by the Halsey equation and is found to fit well with the experimental data having r2 (0.971) (Febrianto et al., 2009) indicating that the mechanism may be multilayer sorption for adsorbents under study while the low values of KH suggest that multilayer sorption might be playing only a small role.

1.0

0.8

C/C0 (mg/L)

PSP APSP SAPSP PAPSP 9AAC MPSP

DG (kJ mol1)

0.6

0.4

0.2

0.0 0 PSP

50

100 150 200 250 Number of Bed Volumes APSP

SAPSP

PAPSP

300 9AAC

350 MPSP

Fig. 7. Column studies.

The Flory-Huggins model was used to assess the isotherm data. From the linear plots of log(q/C0) versus log(1  q) (Figure not shown) for uranium adsorption on the adsorbents under study and the correlation coefficient values (r2 ¼ 1.00), it is apparent that the model shows good fits for the adsorbent under study. However the negative values of n and low values of kFH imply that the model cannot be used to describe the adsorption data. 3.4. Thermodynamic parameters The thermodynamic parameters of the sorption process could be determined from the experimental data obtained at various temperatures using the equations:

DG0 ¼ DH 0  T DS0

(5)

The values of DH and DS (Table 5) can be calculated from the slope and intercept of the plots of DG0 against T (Fig. 6). The negative value of DH0 indicates that the adsorption of U6þ on PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP is exothermic. Generally the absolute magnitude of the change in energy for physisorption is between (20 and 0 kJ/mol); chemisorption has a range of (400 and 80 kJ/mol). The negative value of DG0 in Table 5 indicates that sorption of uranyl ion by PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP to be physisorption and is spontaneous and thermodynamically favorable (Torresdey et al., 2002). Also the DG0 values become less negative with increase in temperature suggesting that adsorption is favored at lower temperatures and hence is exothermic. The positive values of DS0 suggest increased randomness during adsorption and a high affinity of the adsorbent towards the adsorbate. 0

-3.40

-3.35

0

delta G (KJ/mol)

121

-3.30

-3.25

0

3.5. Column 40

30 PSP

APSP

50

Temperature ( 0C) SAPSP

60

PAPSP

Fig. 6. Thermodynamic studies.

70 9AAC

MPSP

The column breakthrough curves for uranium adsorption by adsorbents under study are shown in Fig. 7. The effluent concentration is seen to have the typical ‘S’ shape. A total of w2.8 L of 1000 mg/L metal ion solution was passed through the column containing 5 g of adsorbent under study. As seen from Fig. 7 the breakthrough for uranium is seen to take place at 279.3, 147.2, 135.1, 129.1, 179.3 and 239 bed volumes for PSP, APSP, SAPSP, PAPSP, 9AAC and MPSP respectively.

122

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Table 6 Column Isotherms parameters for different adsorbents. PSP

APSP

SAPSP

KTH (dm3/(mg min)) q0 (mg g1) r2 SD

0.00000443 184.198 0.980 0.074

0.00000435 67.310 0.882 0.085

Yoon & Nelson model KYN (min1) t0.5 (exp) (min) t0.5 (cal) (min) r2 SD

4.43E-03 950 920.993 0.980 0.073

4.35E-03 335 336.551 0.882 0.088

0.917 348775.700 0.936 0.063

0.423 229873.900 0.724 0.067

PAPSP

9AAC

MPSP

Thomas model 0.00000389 54.498 0.877 0.086

0.00000424 37.981 0.818 0.093

0.00389 292 272.494 0.877 0.086

0.00000453 88.609 0.915 0.766

0.00424 189 191.038 0.818 0.093

0.00453 451 443.046 0.915 0.076

0.00000503 158.767 0.995 0.091 0.00503 789 794.035 0.996 0.081

Wolborska model

b (min1)

N0 (mg dm3) r2 SD

0.346 231018.700 0.697 0.068

Thomas and YooneNelson models were also applied to the column adsorption data at a flow rate of 1 mL/min at an initial metal ion concentration of 1 g/L and bed height 5 cm with all the adsorbents under study. From the linear plots of ln[(Co/Ct)  1] versus Veff (Figure not shown) Thomas rate constant (kTh) and bed capacity (qTh) were calculated and are presented in Table 6. The theoretical predictions based on the model parameters are compared in Fig. 8 with the observed data. Similarly, from the plot of sampling time (t) versus ln[Ce/(C0  Ce)], the Yoon and Nelson constant KYN and ô (the time necessary to reach 50% of the retention) were calculated and are shown in Table 6. The well fit of the experimental data on to the Thomas and Yoon-Nelson model indicate that external and internal diffusion will not be the limiting step. From the equations in Table 1 it is evident that the characteristic parameter associated with Thomas and Yoon and Nelson models vary but both the models predict essentially same uptake capacity and C/C0 values for a particular

0.509 241528.663 0.774 0.095

3.6. Desorption Solutions of 0.1 M HCl, 0.1 M EDTA and 0.1 M NH3 have been studied as eluents for desorption of U6þ. From Fig. 9 it is evident that desorption of U6þ from the metal-loaded adsorbents with 0.1M EDTA resulted in >99% recovery. This indicates that ion exchange is involved in the adsorption process (Xiaomin et al., 2008). However, the use of 0.1M NH3 and 0.1 M HCl resulted in >48 and >20% recovery of U6þ respectively. It was observed that U6þ was easily desorbed within 30 min, which would prove highly advantageous for metal recovery. From Fig. 9 it is evident that the removal capacity of adsorbents shows insignificant changes in the second and third cycle. Thus regeneration and reuse of the adsorbents under study is an economical and efficient method for removal of U6þ from water.

1.0 SAPSP

0.8

0.8

0.8

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0.0 0

0.0 500

1000

1500

2000

0

500

1000

1500

2000

0.0

0

1.0 PAPSP

1.0 9AAC

1.0

0.8

0.8

0.8

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0.0 0

500

1000

1500

2000

0.0 0 qe (exp)

500

0.894 293228.900 0.973 0.069

experimental set of data. Hence same r2 and SD values were obtained as also suggested by Baral et al. (2009).

1.0 APSP

1.0

C/C0

PSP

0.337 199769.200 0.646 0.073

1000

1500

Time (min) Thomas Yoon & Nelson

Fig. 8. Column Modeling studies.

2000

500

1000

1500

2000

500

1000

1500

2000

MPSP

0.0 0 Wolborska

S. Kushwaha, P.P. Sudhakar / Journal of Environmental Radioactivity 126 (2013) 115e124

(c.) 100

80

80

80

60

60 40

PSP

APSP EDTA

SAPSP PAPSP 9AAC HCL

NH3

MPSP

0

60 40 20

20

20 0

% Desorption

100

qe(mg/g)

(b.)

100

% Desorption

(a.)

40

123

PSP

APSP st

1 Cycle

SAPSP PAPSP 9AAC nd

2 Cycle

MPSP

0

PSP

rd

3 Cycle

Desorption

APSP SAPSP PAPSP 9AAC MPSP st

1 cycle

2

nd

cycle

rd

3 cycle

Fig. 9. Desorption and cycles of adsorption (a) Effect of desorbents (b) Cycles of adsorption (c) Cycles of desorption.

4. Conclusions The adsorption potential of PSP, APSP, SAPSP, PASP, 9AAC and MPSP towards adsorption of uranium was evaluated. The parameters influencing the adsorption process like dose of adsorbent and agitation time were optimized. Sorption isotherms of U(VI) on the adsorbents under study were evaluated using Freundlich, Langmuir, Temkin, Dubinin-Radushkevich (DR), Flory-Huggins, Elovich and Halsey isotherms. The values of E0 derived from DR model suggest ion exchange as the working mechanism for APSP, SAPSP, MPSP and PAPSP, particle diffusion for PSP while for 9AAC the value suggests physisorption. The DG0 values from Langmuir and thermodynamic calculations indicate physisorption as the major mechanism for adsorption of uranium. The adsorption capacity of the adsorbents under study was found comparable to those reported in literature. The adherence to pseudo second order kinetic model and Halsey model indicate multilayer and ion exchange as the mode of adsorption thus justifying our hypothesis that a range of mechanisms are involved in the adsorption process to different extents for the adsorbents under study. The sorption process was found to be exothermic, spontaneous and accompanied by decrease in entropy. Furthermore data also suggest that the use of adsorbents under study, particularly APSP and PAPSP may be an inexpensive and viable means to remediate UO2þ 2 from contaminated acidic solutions as till date adsorbents with good adsorption capacity at low pH(w1) are not available in literature. Acknowledgement This work has been funded by the Board of research in Nuclear Sciences, INDIA. The authors thank The M. S. University of Baroda and Head Department of Chemistry, The M. S. University of Baroda, for laboratory facilities. References Ahluwalia, S.S., Goyal, D., 2007. Microbial and plant derived biomass for removal of heavy metals from wastewater. Biores. Technol. 98, 2243e2257. Aytas, S., Turkozu, D.A., Gok, C., 2011. Biosorption of uranium(VI) by bifunctionalized low cost biocomposite adsorbent. Desalination 280, 354e362. Baral, S.S., Das, N., Ramulu, T.S., Sahoo, S.K., Das, S.N., Chaudhury, G.R., 2009. Removal of Cr(VI) by thermally activated weed Salviniacucullata in a fixed-bed column. J. Hazard. Mater. 161, 1427e1435. Bayramoglu, G., Celik, G., Arica, M.Y., 2006. Studies on accumulation of uranium by fungus Lentinus sajor-caju. J. Hazard. Mater. 136, 345e353. Bezrodny, S., Bakarzhiyev, Y., Pesmenny, B., 2002. Purification of Waste Effluents from Uranium Mines and Mills in Ukraine. IAEA, Vienna, p. 51. IAEATECDOC-1296.

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