Sorption and desorption studies of radioiodine onto silver chloride via batch equilibration with its aqueous media

Journal of Environmental Radioactivity 150 (2015) 9e19

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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Sorption and desorption studies of radioiodine onto silver chloride via batch equilibration with its aqueous media M. Mostafa*, H.E. Ramadan, M.A. El-Amir Radioactive Isotopes and Generators Department, Hot Laboratories Center, Atomic Energy Authority, P.O. Box 13759, Cairo, Egypt

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2015 Received in revised form 19 July 2015 Accepted 20 July 2015 Available online xxx

The uncontrolled spread out of radioiodine (especially 131I) produced from nuclear activities or accidents, due to its high volatility, to the biosphere represents an environmental impact because of its concentration in the thyroid gland and accumulation on soil surface. This work represents a simple method for isolation of radioiodine from aqueous solution in the form of insoluble solid compound and further recovery of it in aqueous phase for any further controlled use. Crystalline silver chloride was prepared and characterized. Batch sorption of 131I onto the prepared AgCl was studied from different aqueous media (H2O and NaOH of different concentrations) and at different I
:Ag molar ratios (from alkaline media) for different times at 25  C. It was found that the
sorption yield of 131I from 2.5 M NaOH solution (at I
:Ag and S2O2
3 :I molar ratios of 0.025 and 2, respectively) reached 97.7% after 6 h and only slightly increased to 98.6% after 16 h of contact time. The presence of H2O2 adversely affected the batch sorption process. The included REDOX and precipitation reactions were discussed. Batch desorption of the sorbed 131I from AgCl into aqueous phase was studied with NaOCl solutions of different concentrations and different contact times at 25  C. Desorption yield of 131 I was found to be 94.5% with 10 mL of 0.5 M NaOCl solution after contact time of 16 h. Kinetic analysis has been performed for both batch sorption and desorption processes. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Silver chloride Radioiodine Batch sorption Batch desorption Kinetic analysis

1. Introduction Different fission-radioiodine isotopes are produced in a large scale as a result of the nuclear fuel fission during research and power nuclear reactor operations, radioisotope production of 99Mo via 235U(n, f)99Mo nuclear reaction and nuclear accidents. The most important fission-iodine isotopes are 131I (t1/2 ¼ 8.02 d) and 129I (t1/ 7 129 I can mainly 2 ¼ 1.57  10 y). Contamination of soil surface with be produced from long-distance dispersion of fuel reprocessing releases (Muramatsu et al., 2008). The presence of 131I could be determined in tap water and human breast milk after Fukushima power plant accident (Uno et al., 2012). One significant example of health risk is the possible adult infertility as a result of childhood radioiodine exposure (Stone et al., 2013). Efficient isolation of radioiodine from nuclear waste effluents via sorption on common minerals is difficult because of its high volatility and complex chemical behavior exhibited by the existence of a wide variety of chemical species in different conditions (Kozar, 2001;

* Corresponding author. E-mail address: [email protected] (M. Mostafa). http://dx.doi.org/10.1016/j.jenvrad.2015.07.022 0265-931X/© 2015 Elsevier Ltd. All rights reserved.

Bruchertseifer et al., 2003). Silver and silver containing matrices were used for sequestration of radioiodine, via forming insoluble compounds, from liquid and gaseous effluents for the purpose of avoiding its impact on health (Stone et al., 2013) or further recovery for use in nuclear medicine applications (Mostafa et al., 2014). A chromatographic column of porous silver could be used for retention of 97% of fission 131 I from alkaline solution during the production process of fission 99 Mo (Mondino et al., 1999). Freshly precipitated silver iodide could be used to sorb 70% of the residual trace 131I impurity from fission 99 Mo in ammonia solution (Deptuła et al., 1974). Silver-converted synthetic zeolites have been used for removal of I2 and CH3I from off-gas streams of nuclear fuel reprocessing plants with the advantages of the ability of working at elevated temperature, the absence of explosion hazards and resistance to poisoning (Jubin, 1979). In addition, silver zeolites have achieved high decontamination factors (ratios of contaminant activities in the contaminated streams to those in the decontaminated ones) for elemental iodine and organic iodides ranging from 103 to 105 (Haefner and Tranter, 2007). Silver-containing amphoteric solid oxides or amphoteric composites could be used for radioiodine sorption. Silver nitrate-

10

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

impregnated silica and alumina substrates have been used for removal of iodine and alkyl iodides from fuel re-processing streams (Jubin, 1979). Sorption of 131I (as iodide) has been studied from the aqueous phase on hydrated ferric oxide containing 1% m/m of silver, along with investigating the dependence of sorption on pHvalue of the medium and the effect of both the surface active k, 1966). The kinetics of iodine substance and temperature (Kepa adsorption on the amphoteric calcium alginateesilver chloride composite (with a point of zero charge at pH 7.2) have been studied, and it was found that the process could be suitably described by the pseudo-first order kinetic model and was not affected by the initial solution pH in the range of 1e6 achieving a sorption capacity of 1.1 mmol g
1 (Zhang et al., 2011). Silver-loaded commerciallyavailable resins achieved high decontamination of large-volume solutions from radioiodine. A mixed bed column containing silver-loaded CL resin (TrisKem International) and XAD-4 resin (SigmaeAldrich) has been developed and optimized for iodine removal from the process effluents of National Institute for Radioelement (IRE), Belgium showing efficient removal of iodine (>90%) from large volume solutions (>10 L) even at high flow rates (>180 mL/min) (Decamp and Happel, 2013). Nanotechnology could be involved in the radioiodine sorption process utilizing the extremely large surface area of nanoparticles, as compared to that of bulk particles, to achieve much higher sorption capacities. Silver-doped carbon nanotubes have been used for sequestration of 129I achieving a capacity of 458 ± 73 mg I/g (Pishko et al., 2011). Macroporous monolithic columns containing anchored silver nanoparticles have been used for the elimination of excess radioiodine from m-iodobenzylguanidine pharmaceutical labeled with 125I. The column loading capacity was found to be 1.42 mg/cm of the column length for iodine and 0.99 mg/cm for sodium iodide (Sedlacek et al., 2014). Recovery of radioiodine from silver and silver-containing solid materials could be achieved by few methods. For instance, radioiodine could be eluted from a chromatographic column of silvercoated alumina with Na2S solution, achieving an elution yield of 85e90 % (Wilkinson et al., 2003). Iodine-125 could be quantitatively desorbed from 125I-loaded silver particles via batch contact with a mixture of zinc dust and ammonia solution (Mostafa et al., 2014). Carrier-free iodine could be eluted from silver chloridesilica gel column with calcium hypochlorite solution (pH 7e8) (Lieser and Hild, 1967). In one method, 0.01 M CaCl2 was used for desorption of I
and IO
3 from different types of soils achieving desorption yields in the range of 12.6e35.3 % for I
and 8.1e24.7 % for IO
3 (Hong et al., 2012). This work aims at the preparation and characterization of AgCl, studying batch sorption of radioiodine from aqueous solution onto the prepared AgCl (under variable conditions), studying batch desorption of radioiodine and kinetic analysis for both sorption and desorption reactions. This work includes, to some extent, a detailed discussion (e.g., indicating the accompanied REDOX and precipitation reactions and interference between them) which is rarely simultaneously included in most literature related to sorption/ desorption of radioiodine onto and from silver and silvercontaining materials. The REDOX reactions including H2O2 and S2O2
3 and their effect on radioiodine sorption were investigated. In addition, desorption studies of radioiodine using OCl
was investigated. It is worth mentioning that AgCl was selected for performing sorption and desorption studies in this work because of its much lower toxicity than that of AgNO3 (Ratte, 1999), so it is preferred on AgNO3- impregnated materials in this aspect. In addition, AgCl sorbent was prepared using a much simpler method than many of those mentioned in literature and was used without any support (e.g., calcium algenate, silica gel, etc) to avoid additional preparation steps and minimize the amount of the used

sorbent. Using such simple-structure sorbent facilitated the study of radioiodine sorption and desorption mechanisms more clearly. 2. Experimental 2.1. Characteristics of the used instruments Radioactivity measurements were done by a gamma-ray spectrometer, which has a p-type coaxial HPGe detector (GX2518 model), Canberra, USA, with 29.4% relative efficiency and 1.66 keV FWHM at 1332.5 keV of 60Co. The detector was coupled with a multichannel analyzer (MCA), power supply and amplifier that were contained in one unit (Inspector 2000 model, Canberra Series, made in USA). Relative efficiency curve of the HPGe detector was obtained by using 152,154Eu point source. The absolute efficiency curve of the detector was obtained by using the standard point sources of 137Cs (S/N: C-145-8, Oxford) and 60Co (S/N: C-142-10, Oxford). X-ray diffraction (XRD) analysis was performed using Shimadzu X-ray diffractometer (Model XD-490, Japan) with a nickel filter and CueKa radiation. Thermal analysis (TGA and DTA) was carried out under nitrogen atmosphere using Shimadzu DTG-60H thermal analyzer, Japan. The morphology was characterized using a field emission scanning electron microscope (FESEM) (JSM-6510A, Japan). 2.2. Preparation and characterization of AgCl All chemicals used in this work were of AR Grade. Distilled water was used for the preparation of all solutions and for washing purposes. The method of AgCl preparation used by Chattopadhyay et al. (2002) was used with some modifications. 3 g of AgNO3 was dissolved in 10 mL of distilled H2O. 20 mL of NaCl saturated solution (360 g/L) was added dropwise to the AgNO3 solution. The mixture was centrifuged and the supernatant was decanted. The formed AgCl precipitate was washed thoroughly with distilled H2O, the mixture was centrifuged and the washing supernatant was decanted. The washed AgCl precipitate was dried in an electric oven at 50  C for 24 h. The prepared AgCl was characterized by XRD, thermal analysis (TGA and DTA) and FESEM. 2.3.

131

I radiotracer

The no-carrier added (NCA) 131I radiotracer stock solution (131I
in NaCl solution adjusted to pH 11) was supplied from Radioisotope Production Facility, ETRReII Complex, Egyptian Atomic Energy Authority. 2.4. Batch sorption studies All of batch sorption studies mentioned below were conducted via equilibration of 0.1 g of the prepared AgCl with 10 mL of solutions (spiked with 74 MBq 131I from the radioiodine stock solution) in 20 mL glass vials without shaking at 25  C, i.e., solid: solution ratio was 100 mL/g. The chemical composition of 131I solution was varied from one study to another, as mentioned below. The 131I remained in the solution contacted with AgCl was radiometrically checked by withdrawing 100 mL aliquots, using a micro-pipette, from that solution at different times, counting them by using the gamma-ray spectrometer and returning them again to the main solution contacted with AgCl, by a slowly disposal on the inner wall of the vial. Thus, 131I sorption yield at different contact times could be calculated by using the following equation:

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

2.5. Batch desorption studies At the end of batch contact time of 131I solution (10 mL of
1.7  10
3 M I
solution containing Na2S2O3;S2O2
molar 3 :I ratio ¼ 2) and 0.1 g AgCl, the equilibrated solution was carefully decanted. The 131I-loaded AgCl was thoroughly washed three times with 5-mL aliquots of distilled H2O with discarding the washing effluent each time by careful decantation. Then, batch desorption was conducted via batch equilibration of the 131Iloaded AgCl (0.1 g) with 10 mL of alkaline NaOCl solution (pH 12) at 25  C, i.e., solid: solution ratio was 100 mL/g. The Effect of NaOCl solution concentration (in the range from 0.1 to 1 M) on 131I desorption yield was studied for contact time of 24 h. Thereafter, 131 I desorption yield was followed with time for the optimum NaOCl concentration over 16 h (by periodically checking 100 mL aliquots from the contacted NaOCl solution radiometrically, as mentioned above for the sorption case) and could be calculated from the following equation: 0

Ydes ðt Þ ¼

Cc  100 ð%Þ Cr  Ys ðte Þ=100

(II)

2.6. Kinetic modeling The reaction and diffusion kinetic models were investigated for the 131I batch sorption (from the additive-free solution and in the presence of Na2S2O3) and desorption (in NaOCl solution) to

400

Where: 0 Cc : count rate of 131I released in 100 mL of the NaOCl solution after contact time t with AgCl. Ys(te): sorption yield of AgCl at the end of contact time. 0

20

40

60

422

2.4.3. Effect of H2O2 and Na2S2O3 The 131I sorption yield was followed with time from solutions containing 1.7  10
3 M I
in the presence of (i) H2O2 (H2O2:I

molar ratio ¼ 3.6) and (ii) Na2S2O3 (S2O2
3 :I molar ratio ¼ 2) and in the absence of both H2O2 and Na2S2O3 (additive-free solution) for 16 h.

The prepared AgCl was a fine powder. All the peaks appeared in the XRD pattern shown in Fig. 1 (the peaks at 27.8, 32.2, 46.3, 54.8, 57.5, 67.4, 74.4, 76.7 and 85.3  C corresponding to hkl values of 111, 200, 220, 311, 222, 400, 331, 420 and 422, respectively) were related to the face-centered cubic (FCC) phase of AgCl (according to JCPDS card no.31e1238) indicating the high purity of the prepared AgCl (Tiwari and Rao, 2008; Kim et al., 2010). The melting point of AgCl is 455  C (Lide, 2004). According to the obtained TGA curve (Fig. 2), there was not a measurable weight loss until 493  C. At 608  C, there was only a weight loss of 1.67%. Weight losses of ~10.11, 1.99 and 15.85% were found in the temperature ranges of 608e704  C, 704e847  C and 847e1000  C, respectively. The total weight loss was 29.62%. Since the weight loss due to release of all chlorine is only 24.74% (as deduced from Ag/Cl stoichiometry in AgCl; 1 mol Ag: 1 mol Cl), the remaining weight loss of 4.88% may be assigned to the partial evaporation of silver, which is significant at temperatures above 900  C (Lowe, 1964; Hu and Liu, 1996). Three main endothermic peaks were obtained on the accompanied DTA curve at ~304  C, assigned to thermal expansion of AgCl crystals (Nicklow and Young, 1963), 490  C, attributed to phase transition accompanying AgCl melting, and 667  C, due to thermal decomposition of AgCl and release of Cl2 gas. Thus, the thermal analysis confirmed the high purity of the prepared AgCl and indicated its fairly high thermal stability. Consequently, there is a possibility for the prepared AgCl to be used at high temperatures, up to 490  C, for sorption of iodine from aqueous or gas phase in accidental conditions, since the formed AgI as a result of sorption (as discussed below) has a higher melting point of 558  C (Lide, 2004). According to the field emission scanning electron microscope (FESEM) images (Fig. 3), the prepared AgCl was arranged in large aggregate islands (Fig. 3a) consisting of distorted cubic and nearspherical particles with sizes in the range of ~1.5e3.5 mm (Fig. 3b

331 420

2.4.2. Effect of I
carrier:Ag molar ratio The 131I sorption yield was followed with time in 2.5 M NaOH at different I
carrier:Ag molar ratios in the range from 0 to 0.2 for 23 h. It is worth mentioning that the 131I solution was spiked with different volumes of 0.1 M KI solution, to obtain the different I
carrier:Ag molar ratios, before being in contact with AgCl.

3.1. Characterization of the prepared AgCl

311 222

2.4.1. Effect of sorption media The NCA 131I sorption yield was followed with time from H2O, 0.1 M NaOH and 2.5 M NaOH solutions for 72 h.

3. Results and discussion

220

Where: Ys(t): sorption yield of 131I after contact time t. Cr: count rate of 100 mL aliquot from reference solution (131I solution without contact with AgCl) at time t. Cc: count rate of 100 mL aliquot withdrawn from 131I after contact time t with AgCl. For each 131I batch sorption process, studying the effect of different factors as mentioned below, 131I reference solution was the one had the same chemical composition as that contacted with AgCl.

determine the most suitable model(s) for each process (Qiu et al., 2009).

200

(I)

111

Cr
Cc  100 ð%Þ Cr

Intensity (a.u.)

Ys ðt Þ ¼

11

80

2θ (degree) Fig. 1. XRD of the prepared AgCl with hkl values of the corresponding peaks (JCPDS card no.31e1238).

12

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

100

90

and c). The morphology resembles to some extent that of AgCl prepared by using the hydrothermal method in the presence of ionic liquids (Lou et al., 2011). So, the simple AgCl preparation method used in this work was preferred for the present study, while the method of Lou et al. (2011) was more suitable for their study on photocatalytic applications of AgCl crystals, where they controlled AgCl crystal sizes by changing the hydrothermal reaction time. 3.2.

131

I batch sorption studies

70

H2O

60

0.1 M NaOH 2.5 M NaOH

50

131

Fig. 2. TGA and DTA of the prepared AgCl.

I sorption yield, %

80

40

30

20

3.2.1. Sorption from H2O and NaOH media As indicated from the study of batch sorption of NCA 131I onto 0.1 g AgCl as a function of time from different aqueous media (Fig. 4), it was found that the sorption yield from H2O in the first 27 h was markedly higher than those from both NaOH media, while it was slightly higher from 0.1 M NaOH than from 2.5 M NaOH. However, as time proceeded, the sorption yields became closer reaching values around 95% after 72 h of contact time. In water, the following equilibria are thought to occur (Clough and Starkie, 1985; Kulyukhin et al., 2011): I2 þ H2O # HIO þ I
þ Hþ

(1)


þ 3I2 þ 3H2 O#IO
3 þ 5I þ 6H

(2)

In alkaline media, the following equilibrium is likely to occur (Kaplan and Serne, 2000):

3I2 þ 6OH
#5I
þ IO
3 þ 3H2 O

(3)

Thus, it is thought that direct 131I sorption onto AgCl (Ksp ¼ 1.77  10
10) surface was proceeded via forming AgI precipitate (Ksp ¼ 8.52  10
17), and not AgIO3 (Ksp ¼ 3.17  10
8) precipitate, since AgI has a lower Ksp-value than those of both AgIO3

0

10

20

30

40

50

60

70

80

t, h Fig. 4. Batch sorption of NCA 131I as a function of time onto 0.1 g AgCl from 10 mL of H2O, 0.1 M NaOH and 2.5 M NaOH.

and AgCl, while AgIO3 has a higher Ksp-value than that of AgCl (Lide, 2004). It is worth mentioning that sorption includes three mechanisms; adsorption, surface precipitation and absorption (Hubbard, 2002). Precipitation reaction is defined as the formation of a solid in a solution or inside another solid via a chemical reaction or diffusion in another solid (Figueroa et al., 2015). In case of water, Cl
of the solid AgCl is displaced with I
(aq) according to (Hoskins et al., 2002):
AgClðsÞ þ I
ðaqÞ #AgIðsÞ þ ClðaqÞ

(4)

AgIO3(s) can also be formed indirectly by interaction of IO
3 (aq) with AgI(s) via REDOX reaction mentioned below (Reaction 5). Generally, oxidation-reduction (REDOX) reactions can be defined as transfer of electrons between the reacting chemical species,

Fig. 3. Field emission scanning electron microscope (FESEM) images of the prepared AgCl.

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

electrons are lost in oxidation and gained in reduction. REDOX reactions can be formulated as the sum of the half-oxidation and halfereduction reactions. At definite conditions, the corresponding REDOX potentials can be used to predict the favorability of the overall reaction of any combination of half-reactions (Tratnyek et al., 2011). Such predictions were used in this work. In some cases, REDOX reactions include the formation of solid product(s) as shown below:

100

I sorption yield, %

80

131

13


Oxidation half
reaction: AgðsÞ þ IO
3 #AgIO3ðsÞ þ e
 ðEox ¼
0:354 V

60

Reduction half
reaction: AgIðsÞ þ e
#AgðsÞ þ I


ðEred ¼
0:15224 V

-

I carrier:Ag molar ratio: 0 0.025 0.05 0.075 0.1 0.15 0.2

40

20

0 0

5

10

15

20

25

t, h Fig. 5. Batch sorption of 131I onto 0.1 g AgCl from 10 mL of 2.5 M NaOH as a function of time at different I
carrier:Ag molar ratios (from 0 to 0.2).

(5.1)




(5.2)



Total reaction : AgIðSÞ þ IO
ðEtot ¼
0:506 V 3 #AgIO3ðsÞ þ I (5.3)


The I released from Reaction 5.3 can be further sorbed onto AgCl according to Reaction 4. In the case of alkaline media, Cl
eI
displacement is interfered with and slowed down by the formation of Ag2O (Ksp ¼ 2  10
8) (Lyu and Huang, 2011), despite its higher Ksp-value than those of AgI and AgCl, according to (Willbanks, 1953; Lyu and Huang, 2011; Wang et al., 2012):
2AgClðsÞ þ 2OH
ðaqÞ #2AgOHðsÞ þ 2ClðaqÞ

(6.1)

2AgOH(s) # Ag2O(s) þ H2O (spontaneous)

(6.2)

100

100

80

I desorption yield, %

60

60

40

131

40

131

I sorption yield, %

80

No additives 2- S2O3 :I molar ratio = 2

20

20

-

H2O2:I molar ratio =3.6 0 0

2

4

6

8

10

12

14

16

18

t, h Fig. 6. Batch sorption of 131I as a function of time onto 0.1 g AgCl from 10 mL of 2.5 M NaOH containing 1.7  10
3 M I
(I
carrier:Ag molar ratio ¼ 0.025) (i) without additives and in the presence of (ii) Na2S2O3 and (iii) H2O2. No. of replicates ¼ 3.

0 0.0

0.2

0.4

0.6

0.8

1.0

-

Concentration of OCl , M Fig. 7. Batch desorption of 131I from 0.1 g AgCl in 10 mL of NaOCl solution (pH 12) as a function of OCl
concentration at a contact time of 24 h. No. of replicates ¼ 3.

14

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

100

Reduction half
reaction: 2AgClðsÞ þ 2e
#2AgðsÞ þ 2Cl

 ðEred ¼ 0:22233 v (6.3.2)

80



ðEtot ¼
0:120

Total reaction: Reaction: 6:3




I desorption yield, %

131

As indicated by the high I sorption yield in alkaline media (Fig. 1), Reactions 4 and 5 are likely also to occur in such media in addition to sorption of I
and IO
3 onto Ag2O according to the following reactions (Pretty et al., 2008; Zhang et al., 2008; Bo et al., 2013):

60

- Sorption of I
onto Ag2O: 40

131


Ag2 OðsÞ þ 2I
ðaqÞ þ H2 O#2AgIðsÞ þ 2OHðaqÞ

(7)

(Ksp-values of AgI and Ag2O are 8.52  10
17 and 2  10
8, respectively, and Gibbs-free energy for Reaction 7 is
32 kJ/mol) and

20

- Sorption of AgIO3 onto Ag2O: 0 0

2

4

6

8

10

12

14

16

18

t, h


Oxidation half
reaction: 2AgðsÞ þ 2IO
3 #AgIO3ðsÞ þ 2e
 ðEox ¼
0:354

(8.1)

Fig. 8. Desorption of 131I from 0.1 g AgCl as a function of time by batch equilibration with 10 mL of 0.5 M NaOCl solution (pH 12). No. of replicates ¼ 3.


Total Reaction : 2AgClðsÞ þ 2OH
ðaqÞ #Ag2 OðsÞ þ H2 O þ 2ClðaqÞ

Reduction half
reaction: Ag2 OðsÞ þ H2 O þ 2e
#2AgðsÞ þ 2OH

 ðEred ¼ 0:342 (8.2)

(6.3) The rate of Ag2O formation increased with increasing NaOH concentration. This could be visually ascertained, where the color darkening rate of the AgCl surface (i.e., formation rate of the dark brown/black Ag2O) increased with increasing NaOH concentration. Formation of Ag2O is favored as predicted from the electrical potentials of the REDOX reactions:

Oxidation half
reaction: 2AgðsÞ þ 2OH
#Ag2 OðsÞ þ H2 O þ 2e

 ðEox ¼
0:342 V (6.3.1)


Total reaction: Ag2 OðsÞ þ 2IO
3 þ H2 O#2AgIO3ðsÞ þ 2OH
 ðEtot ¼
0:012 v

(8.3) However, the alkaline media (2.5 M NaOH) was chosen for further studies, to avoid the formation of the volatile HOI species (Reaction 1) (Selinus et al., 2005). In addition, the radioactive waste solutions are expected to be alkaline to ensure the avoidance of corrosion of the storage tanks.

Table 1 Mathematical expressions of pseudo first order, pseudo second order and film diffusion mass transfer kinetic models for batch Process Batch

131

Kinetic model I sorption

Pseudo first order Pseudo second order Film diffusion mass transfer

Batch

131

I desorption

Pseudo first order Pseudo second order Film diffusion mass transfer

Mathematical expression logðqe
qt Þ ¼ log qe


kp1 2:303 t

t qt

¼ V10 þ q1e t   Rd t log 1
qqet ¼
2:303 0

0

0

0

k

p1 t logðqe
qt Þ ¼ log qe
2:303

t 0 qt

¼ V10 þ q10 t e 0   0 0 Rd qt log 1
q0 ¼
2:303 t

131

I sorption and desorption processes. Slope

Intercept

kp1
2:303 1 qe Rd
2:303

logqe

k

0

p1
2:303

1 0 qe R

0

d
2:303

e

Where. qe and qt: are the sorption capacities at equilibrium and at time t (mmol g
1), respectively. 0 0 qe and qt : are the desorbed solute quantity at equilibrium and at time t (mmol L
1), respectively. kp1 and Rd: are the pseudo first order rate constant (h
1) and liquid film diffusion constant (h
1), respectively, for the sorption processes. 0 0 kp1 and Rd : are the pseudo first order rate constant (h
1) and liquid film diffusion constant (h
1), respectively, for the desorption process. V0: is the initial sorption rate in the pseudo second order model (mmol g
1 h
1). 0 V0 : is the initial desorption rate in the pseudo second order model (mmol L
1 h
1).

1 V0

0 0

log qe 1 0 V0

0

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

3.2.2. Sorption at different I
carrier:Ag molar ratios In the study of batch sorption of 131I onto 0.1 g AgCl from 10 mL of 2.5 M NaOH as a function of time at different I
carrier:Ag molar ratios (Fig. 5), it was observed that the 131I sorption rates in the presence of I
carrier with different molar ratios were markedly higher than that for NCA 131I. The sorption yield increased with increasing I
carrier:Ag for the first 3 h. The sorption yields in the case of different I
carrier:Ag molar ratios (from 0.025 to 2) approached a constant value with time (~98%) after 23 h, while in the case of NCA 131 I it was 60.0%. Since the sorption yields in case of using I
carrier were very close, especially in the time range of 15e23 h, the lowest I
carrier:Ag molar ratio (0.025) was chosen for further studies.

15

3.2.3. Sorption in the presence of H2O2 and Na2S2O3 Fig. 6 shows batch sorption of 131I onto 0.1 g AgCl from 10 mL of 2.5 M NaOH containing 1.7  10
3 M I
(I
carrier:Ag molar ratio ¼ 0.025) as a function of time without additives and in the presence of Na2S2O3 and H2O2. After 0.25 h of contact time, 131I sorption yields were 8.7, 26.7 and 38.0% for the cases of the additive-free solution, S2O2
3 and H2O2, respectively. Then, sorption yields continued increasing in the cases of the additive-free solution and S2O2
3 (reaching 42.1 and 83.9%, respectively, after 2 h), while it decreased in the case of H2O2 reaching 42.6% (a beginning of a plateau) after 2 h. After 6 h, the sorption yield in the case of S2O2
(96.8%) reached a plateau, while it reached 78% for the 3 additive-free solution. After 16 h, the sorption yields reached 97.1,

100

0.1

t/qt , h g/mmol

qe - qt , mmol/g

80

(a) 0.01

60

(b) 40

20

1E-3 0

2

4

6

8

10

12

14

16

18

t, h

0

2

4

6

8

10

12

14

16

18

t, h

1- ( qt / qe )

1

(c) 0.1

0.01 0

2

4

6

8

10

12

14

16

18

t, h Fig. 9. Kinetic models of (a) pseudo first order, (b) pseudo second order and (c) film diffusion mass transfer of batch sorption of 131I onto 0.1 g AgCl from 10 mL of 1 M NaOH solution initially containing 1.7  10
3 M I
(additive-free solution). No of replicates ¼ 3.

16

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

100

Total reaction: 2H2 O2 þ Ag2 OðsÞ #2AgðsÞ þ H2 O þ 2HO
2
 ðEtot ¼
1:153 V (10.2)

80

t/qt , h g/mmol

- AgX:


Oxidation half
reaction: H2 O2 #Hþ þ HO
2 þe
 ðEox ¼
1:495 V Reduction half
reaction: AgXðsÞ þ e
#AgðsÞ þ X


60

(11.1)


  ðEredAgCl ¼ 0:222 V; EredAgI ¼
0:152 V (11.2)

40

Total reaction: H2 O2 þ AgXðsÞ #AgðsÞ þ X
þ Hþ þ HO
2


  ðEtotAgCl ¼
1:273 V; EtotAgI ¼
1:647 V (11.3)

20

The iodate formed in alkaline solutions (Reaction 3) can be reduced on adding thiosulfate according to (Liu, 2012):

2IO
3 0

2

4

6

8

10

12

14

16

18

Fig. 10. Pseudo second order kinetic model of batch sorption of 131I onto 0.1 g AgCl from 10 mL of 1 M NaOH solution initially containing 1.7  10
3 M I
and 2

3.5  10
3 M S2O
2 3 (S2O3 :I molar ratio ¼ 2). No. of replicates ¼ 3.

98.6 and 41.4% for the cases of the additive-free solution, S2O2
3 and H2O2, respectively.
In alkaline media, H2O2 reduces IO
3 to I according to (Lide, 2004):

(9.1)




Reduction half
reaction: IO
3 þ 3H2 O þ 6e #I þ 6OH
 ðEred ¼ 0:26 V

(9.2)





Total reaction: 6H2 O2 þIO
3 #I þ3H2 Oþ6HO2 ðEtot ¼
1:24V




(9.3) þ

H2O2 also reduces Ag in Ag2O and AgX (X ¼ Cl, I) to Ag according to (Hsu, 1996; Ganguly, 2012): - Ag2O:


Oxidation half
reaction: 2H2 O2 #2Hþ þ 2HO
2 þ 2e
 ðEox ¼
1:495 V

(10.1) Reduction half-reaction: Reaction 8.2



þ 3S2 O2
3 ðaqÞ þ 3H2 O#2IðaqÞ þ 6HSO3 ðaqÞ

(12)

On the other hand, AgX (X ¼ Cl, I) dissolves in thiosulfate according to (Svehla, 1979):

t, h


Oxidation half
reaction: 6H2 O2 #6Hþ þ 6HO
2 þ 6e
 ðEox ¼
1:495 V

ðaqÞ

 3

AgXðsÞ þ 2S2 O2
3 ðaqÞ # AgðS2 O3 Þ2 ðaqÞ þ XðaqÞ

(13)

But silver iodide is barely soluble (Svehla, 1979; Jakubke and Jeschkeit, 1993). According to the sorption results (Fig. 6), it can be concluded
that the reduction rate of IO
3 to I (Reaction 9.3) was fast as indicated by the highest sorption yields in the case of H2O2 after 0.5 h. Further decrease in sorption yield with time may be due to the release of I
from AgI (Reaction 11.3) which occurred after Reaction 9.3 due to the lowest reduction potential of AgI than that of IO
3 (Reactions 11.2 and 9.2, respectively). It is clear from Fig. 6 that the
reduction rate of IO
3 to I was slower in the case of thiosulfate than that in the case of H2O2 (as shown for the first 0.5 h). It is also obvious that the sorption rate of I
/IO
3 from the additive-free so
lution was slower than that in the case of I
from S2O2
3 , where I could be sorbed onto both AgCl(s) and Ag2O(s) (Reactions 4 and 7) while, in case of additive-free solution, IO
3 could be only sorbed onto Ag2O(s) (Reaction 8.3) or indirectly by interaction with AgI(s) (Reaction 5.3) to form AgIO3(s). 3.3.

131

I batch desorption studies

On studying the desorption of 131I from 0.1 g AgCl with 10 mL of alkaline NaOCl solution (pH 12) of different concentrations (in the range of 0.1e1M) by batch contact for 24 h (as indicated in Fig. 7), it was found that the desorption yield abruptly increased from 54.2 to 80.2 % as the OCl
increased from 0.1 M to 0.15 M and then gradually increased to 94.5% as the OCl
concentration increased to 0.5 M reaching a plateau, where the yield only very slightly increased to 94.8% with 1 M OCl
. However, 0.5 M NaOCl solution was used for the further study. Thereafter, on following the 131I batch desorption yield with time from 0.1 g AgCl in 10 mL of 0.5 M NaOCl (Fig. 8), it was found that the desorption yield increased gradually with time until reaching 94.5% after a contact time of 16 h.

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

17

1000

0.01

t/q't , h l/ mmol

(qe'-q't), mmol/l

800

(a) 1E-3

600

(b)

400

200

1E-4 0

2

4

6

8

10

12

14

t, h

0

2

4

6

8

10

12

14

16

18

t, h

1

1 - ( q't / q'e )

(c) 0.1

0.01 0

2

4

6

8

10

12

14

t, h Fig. 11. Kinetic models of (a) pseudo first order, (b) pseudo second order and (c) film diffusion mass transfer of batch desorption of solution (pH 12). No. of replicates ¼ 3.


On reviewing REDOX reactions of I
, IO
3 and OCl (Lide, 2004; Rich, 2007), it was concluded that the desorption of 131I in OCl
solution may be interpreted by two subsequent reactions; oxidation reaction and displacement reaction as follows:


Oxidation: AgIðsÞ þ 3OCl
ðaqÞ #AgIO3ðsÞ þ 3ClðaqÞ
Displacement: AgIO3ðsÞ þ Cl
ðaqÞ #AgClðsÞ þ IO3

(14) ðaqÞ

(15)

3.4. Kinetic studies Kinetic models of pseudo first order, pseudo second order and

I from 0.1 g AgCl in 10 mL of 0.5 M OCl


131

film diffusion mass transfer were found to be applicable for both the batch 131I sorption from additive-free solution and desorption processes. Pseudo second order was found to be the only kinetic model applicable to the batch 131I sorption from S2O2
3 solution. Table 1 compiles the mathematical expressions of the applied kinetic models (Mohapatra, 2009; Qiu et al., 2009). Figs. 9e11 graphically represent the applicable models for sorption and desorption processes, while Table 2 compiles data obtained from these graphs, namely intercepts, slopes and correlation factor (R2) values. 3.4.1. Kinetic analysis of sorption processes 3.4.1.1. Additive-free solution. According to Fig. 9(a and b), the batch 131 I sorption process from the additive-free solution can be

18

M. Mostafa et al. / Journal of Environmental Radioactivity 150 (2015) 9e19

Table 2 Slope, R2 and intercept values of the applicable kinetic models for batch sorption and desorption of Reaction

131

I onto and from AgCl.

Kinetic models Reaction models

Diffusion model

Pseudo 1st order

131

I onto AgCl from Sorption of additive-free solution Sorption of 131I onto AgCl from 2.5 M NaOH solution in the presence of S2O23 Desorption of 131I from AgCl in 0.5 M NaOCl solution

Pseudo 2nd order

Film diffusion mass transfer

Slope

R2

Intercept

Slope

R2

Intercept

Slope

R2

Intercept


0.11 Fig. 9a

0.994


0.78

4.81 Fig. 9b 5.50 Fig. 10 53.96 Fig. 11b

0.995

16.36


0.11 Fig. 9c

0.998


1.20  10
2

0.998

3.36

0.984

129.39


0.11 Fig. 11c

0.972


3.55  10
2


0.11 Fig. 11a

0.987


1.80

described by pseudo first order and pseudo second order kinetic models, with R2 values of 0.994 and 0.995, respectively (Table 2). On the other hand, using mathematical expressions (Table 1) and intercept values (Table 2), it was found that the experimental and calculated values of qe in the case of the pseudo 1st order kinetic model (0.17 and 1.66  10
1 mmol g
1, respectively) were closer than the experimental and calculated values of 1/qe in the case of the pseudo 2nd order kinetic model (5.90 and 4.81 g mmol
1, respectively). Thus, the sorption from the additive-free solution is much better described by the pseudo 1st order kinetic model than by the pseudo 2nd order one. In addition, the sorption process can also be described by film diffusion mass transfer model (Fig. 9c) with R2 value of 0.998 (Table 2). 2
3.4.1.2. S2O2
3 solution. In the case of S2O3 , the sorption can be described by the pseudo 2nd order kinetic model (Fig. 10) with R2 of 0.998. The experimental and calculated values of 1/qe were found to be 5.81 g mmol
1 and 5.50 g mmol
1, respectively.

3.4.2. Kinetic analysis of desorption process The batch 131I desorption from AgCl in OCl
solution can be described by pseudo 1st order, pseudo 2nd order and film diffusion mass transfer kinetic models (Fig. 11a, b and c, respectively) with R2 values of 0.987, 0.984 and 0.972, respectively. In the case of the pseudo 1st order model, the experimental and the calculated qe values were 1.64  10
2 mmol L
1 and 1.58  10
2 mmol L
1, respectively. In the case of the pseudo 2nd order model, the experimental and the calculated 1/qe values were 61.03 L mmol
1 and 53.96 L mmol
1, respectively. Thus, the desorption process is much better described by the pseudo 1st order model than by the pseudo 2nd order one, since the former has closer experimental and calculated values. 4. Conclusion 131 I sorption The presence of I
carrier and S2O2
3 increased the rate onto AgCl particles. H2O2 was found to have adverse effect on 131 I sorption under the pre-mentioned experimental conditions. Efficient recovery of 131I from AgCl could be achieved using NaOCl solution. Interpretation of the obtained results was done in the light of the REDOX and/or precipitation reactions and interference between them. Both reaction and diffusion kinetic models were tested for both batch 131I sorption and desorption processes, and the suitable ones were indicated. The mentioned optimum conditions for the sorption and desorption processes can be used efficiently for (i) separation and recovery of 131I (as a radioisotope product) from fission products, (ii) pre-concentration and recovery of 129,131I from contaminated liquid effluents, (iii) determination of radioiodine concentration in environmental samples etc. Many future studies can be done such as studying both radioiodine

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