Investigation of radioiodine isotope adsorption on activated carbon

(1)

INVESTIGATION OF

RADIOIODINE ISOTOPE

ADSORPTION ON

ACTIVATED CARBON

Nilgün Karatepe, Sevilay Hacıyakupoğlu,

Nesrin Altınsoy ,

Nilgün Baydoğan, Beril Tuğrul

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14-17 Oct. 2008 N.Karatepe, S.Hacıyakupoğlu N. Altınsoy, V. Eurosian Conference N. Baydoğan, B. Tuğrul of Nuclear Science and Its Application

Introduction



The radionuclide of greatest concern from a

radiation protection viewpoint is radioiodine

which is regularly used in nuclear medicine

procedures.



Various methods have been evaluated for the

removal of radioiodine from actual liquid

waste received from the isotope.



Adsorption is an important technique in

separation and purification processes which is

used in water and waste water industry.

(3)

The objectives of this research



The objectives of this research were to

investigate the adsorption of iodine on

activated carbon and contribute the storage

problem of medical radioactive liquid waste.



In this frame, the adsorption phenomenon of

iodine and radioiodine from aqueous solutions

on activated carbon was studied.



Activated carbon samples were prepared from

(4)

Experimental Processes



Preparation of Activated Carbon



Adsorption Process



Irradiation and Gamma Spectroscopic

(5)

Preparation of

Activated Carbon



Activated carbon samples produced from olive stone with chemical

activation method were used in this study.



This material was initially stirred for 1 h at 100 °C in a general-purpose

oven.



Then, the sample was sieved to separate the seeds from the unwanted

cortex material.



They were washed to remove oil residues and dried.



Then they were grounded subsequently and the samples were

impregnated with phosphoric acid of 50 % concentration, dried in air at

about 220°C and then carbonized in a quartz reactor at a temperature of

400°C for 120 min.



The carbonization was done in a flow of nitrogen (300 ml/min). After

carbonization, the carbon was cooled down to room temperature in a flow

of nitrogen.



To remove the excess of H3PO4, the carbons after carbonization were

extensively washed with hot water until neutral pH. Then the samples

were dried in an oven at 110°C.

(6)

BET surface area and pore volume of the

activated carbon sample.

Sample BET surface area (m2_/g)

Total pore volume (P/P₀= 0.95) (cm3_/g)

(7)

Adsorption Process



To represent the iodine specimen’s sodium iodide and

radioactive sodium iodide samples were prepared and the

adsorption performance of activated carbon for each condition

was evaluated.



The batch iodine adsorption experiments were performed on a

shaker by varying the parameters such as pH (3.0 and 9.0),

initial iodine concentration (100 mg/L and 200 mg/L),

temperature (25 °C and 75 °C), and shaking time (60 min and

120 min).



The batch radioiodine (128I) adsorption experiments were also

performed on a shaker by varying the parameters such as pH

(3.0, 6,0 and 9.0) and initial radioiodine concentration

(100 mg/L, 150mg/L and 200 mg/L), at temperature of 75 °C

for shaking time of 60 min.

(8)

The results of iodine adsorption

experiments

Sample

No. Temp.(°C) Iodine Conc.(mg/L) pH Time(min) (mg iodine/ gr AC)Adsorption Adsorption( %) 1 25 100 3 60 9.12 87.5 2 120 8.9 83.8 3 9 60 9.12 85.5 4 120 8.80 83.5 5 200 3 60 20.82 94.48 6 120 20.80 94.38 7 9 60 20.30 92.13 8 120 18.04 89.54 9 75 100 3 60 9.12 85.81 10 120 8.96 84.00 11 9 60 8.25 77.34 12 120 8.29 78.22 13 200 3 60 19.89 91.15 14 120 19.83 89.38 15 9 60 20.15 91.45 16 120 19.73 89.53

(9)

The results of radioactive iodine

adsorption experiments.

Sample

No. Iodine Conc.(mg/L) pH _{Count rate} (γ/s/ml) (± %8) 17 100 3 696113 18 6 812576 19 9 944001 20 150 3 1091857 21 6 1552185 22 9 1735619 23 200 3 2753254 24 6 3601212 25 9 3843929

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Adsorption Process (Continue)



The iodine solutions were prepared by

dissolving known quantities of sodium iodide

and radioactive sodium iodide (produced by

irradiation of sodium iodide in ITU TRIGA

Mark II nuclear research reactor) in distilled

water.



The pH was adjusted using dilute hydrochloric

acid or sodium hydroxide solutions by pH

meter.



All chemicals used were analytical reagent

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Irradiation and Gamma

Spectroscopic Process



Neutron activation analysis method was used to

determine the amount of iodine in adsorption process.



For this purpose the samples which taken from the

adsorption solutions were irradiated at ITU TRIGA Mark

II nuclear research reactor at Energy Institute of

Istanbul Technical University.



ITU TRIGA Mark-II reactor was equipped with a

pneumatic system of rabbit transfer that permits the

use of short-lived radioisotopes.



The system is located in the outer ring of fuel element

positions that has region of high neutron flux.



The sample container is conveyed to a receiver-sender

station via polyethylene tubing with 2.2 cm outer

diameter and 12 cm height.

(12)

Irradiation and Gamma

Spectroscopic Process (Continue)



Neutron bombardment of the iodine

sample activates

127 I to the radioactive

isotope

128

I. 

The

127 I (n,γ)

128 I reaction provides the

enough activity to measure

128 I

radioisotopes (cross section of

127 I is 6.2

barn) [7].



The half life of this radioisotope is

(13)

Nuclear properties of

128

I isotope

γ-ray Energy (keV) 27.20 31.00 31.70 442.89 * 526.62 743.32 969.53 Abundance (%) 1.30000 0.70000 0.15000 16.90000 1.59000 0.16900 0.43000 † Generating reactions 127_{I (n} th, γ) and 128Xe (nfa, γ) * Useful energy

(14)



41.5 µl solutions of aqueous phase for different adsorption sets were

transferred onto filter paper and sealed in the suitable films.



After this stage only sodium iodide samples were packed together in

polyethylene rabbit container and irradiated at a reactor power level

of 250 kW for 60 seconds at TRIGA Mark II reactor of the institute.



After appropriate cooling time, irradiated samples were transferred to

the gamma-ray spectroscopy system.



The

128

I radioactivity for 41.5 µl solutions of aqueous phase obtained

by sodium iodide and radioactive sodium iodide adsorption sets were

measured separately.



For determination of

128

I content samples were counted for 120 and

240 seconds.



Counting rates of the

128

I isotope of aqueous phase for different

adsorption sets were derived from the -rays spectra of the irradiated

samples and corrected to the end of the irradiation. The 442.89 keV

-ray peak from

128

I isotope was counted each time [6].

Irradiation and Gamma

(15)

The gamma spectrometry

system

 The gamma spectrometry

system consists of a n-type high purity germanium detector (with relative efficiency of 48.7 %) and Canberra Eagle Plus MCA multichannel analyzer installed in 4096 channel.

 The gamma spectrometry

system was calibrated by using 60Co and 137Cs

radionuclides standards.

 The count rate resolution of

the detector at 1333 keV is 2.1 keV. The gamma- ray spectra of the irradiated samples were analyzed by the Genie 2000 software.

(16)

Result and Discussion



The adsorption behavior of iodine on

activated carbon samples was

investigated and the results are

discussed as follows with respect to the

effects of temperature, pH, initial iodine

concentration, adsorption contact time

and radioactivity.

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Effect of pH

The removal of iodine as a function of pH at different temperatures and initial iodine

concentrations (Shaking time : 60 min).

(18)

Effect of Temperature

The removal of iodine as a function of temperature at different pH and initial

iodine concentrations (Shaking time : 60 min).

(19)

Effect of Initial Iodine

Concentration

The removal of iodine as a function of initial iodine concentration at different pH

and temperature (Shaking time : 60 min).

(20)

Effect of Adsorption

Contact Time

The removal of iodine as a function of shaking time at different initial iodine concentrations

and different pH (Temperature: 25°).

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Statistical Analysis Results



In this study, a statistical design technique described

in detail elsewhere [8, 9] was also applied by use of a

two level factorial design matrix to interpret the

experimental results.



A major advantage of the statistical model over the

analytical ones is that they do not use rough

approximations and allow for a greater number of

factors.



In two level factorial design experiments, process

variables were selected as temperature (25 °C and 75

°C), pH (3.0 and 9.0), shaking time (60 min and 120

min) and initial iodine concentration (100 mg/L and

200 mg/L).

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The regression equation



The variance test of the parameters for the two samples

showed that some of the variables are not statistically

significant. Therefore, their respective terms can be rejected as

given in the following proposed models:

Y= 87.357– 1.497 X

1

-1.456 X

2

+ 4.148 X

3

-0.813 X

4

+ 0.613X

2

X

3

+ 1.224 X

1

X

2

X

3

(2)

where Y is the amount of iodine adsorption in % , X

₁

is coded

value of temperature in °C, X

2

is coded value of pH, X

3

is

coded value of initial iodine concentration in mg/L and X

4

is

coded value of shaking time in min. The correlation coefficients

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The relationship between the coded values (X

k

) and the actual

values can be given as follows:



X

₁

= (T- 50)/25

(3)



X

₂

= (pH- 6)/3

(4)



X

₃

= (C- 150)/50

(5)



X

₄

= (t- 1.5)/0.5

(6)

where T is the temperature in °C, t is the shaking time in min and

C is the initial iodine concentration in mg/L.

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Effect of Radioactivity

The count rate of aqueous phase as a function of initial radioiodine concentration

at 75°C for different pH.

(25)

Conclusion



Active carbon which was prepared with chemical activation method from olive

stone is an effective adsorbent for the removal of iodine from aqueous

solutions.



In adsorption process, it was found that increasing the temperature, pH and

shaking time cause a decrease in iodine adsorption.



On the contrary, increasing the initial iodine concentration induce an increase in

iodine adsorption.



A statistical design technique was also applied by use of two-level factorial

design matrix to measure the main effects due to the variables in iodine

adsorption and to optimize the process conditions.



The multi-linear mathematical model developed to predict the amount of iodine

adsorption on activated carbon was found to be successful with correlation

coefficient above 0.974.

