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ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 2  |  Page : 77-79  

Effect of humidity on thoron adsorption in activated charcoal bed


1 Radioecology Research Laboratory, University Science Instrumentation Centre, Mangalore University, Mangalagangothri, Mangalore, Karnataka, India
2 Radiological Physics and Advisory Division, BARC, Mumbai, Maharashtra, India
3 Department of Chemical Engineering, IIT Bombay, Mumbai, Maharashtra, India

Date of Web Publication18-Dec-2014

Correspondence Address:
N Karunakara
Radioecology Research Laboratory, University Science Instrumentation Centre, Mangalore University, Mangalagangothri, Mangalore, Karnataka 574 199
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.147279

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  Abstract 

Activated charcoal is a well-known adsorber of 222 Rn and 220 Rn gases. This property can be effectively used for remediation of these gases in the workplaces of uranium and thorium processing facilities. However, the adsorption on charcoal is sensitive to variation in temperature and humidity. The successful designing and characterization of adsorption systems require an adequate understanding of these sensitivities. The study has been carried out towards this end, to delineate the effect of relative humidity on the efficacy of 220 Rn mitigations in a charcoal bed. Air carrying 220 Rn from a Pylon source was passed through a column filled with coconut shell-based granular activated charcoal. The relative humidity of the air was controlled, and the transmission characteristics were examined at relative humidity varying from 45% to 60%. The mitigation factor was found to decrease significantly with an increase of humidity in the air.

Keywords: 220 Rn, adsorption, granular activated charcoal, humidity, mitigation


How to cite this article:
Kumara K S, Karunakara N, Yashodhara I, Sapra B K, Sahoo B K, Gaware J J, Kanse S D, Mayya Y S. Effect of humidity on thoron adsorption in activated charcoal bed . Radiat Prot Environ 2014;37:77-9

How to cite this URL:
Kumara K S, Karunakara N, Yashodhara I, Sapra B K, Sahoo B K, Gaware J J, Kanse S D, Mayya Y S. Effect of humidity on thoron adsorption in activated charcoal bed . Radiat Prot Environ [serial online] 2014 [cited 2022 May 20];37:77-9. Available from: https://www.rpe.org.in/text.asp?2014/37/2/77/147279


  Introduction Top


Activated charcoal is a well-known adsorber of 222 Rn and 220 Rn gases at room temperature. [1],[ 2] Based on this important property of charcoal, there is a potential to develop simple and inexpensive systems, for the continuous removal of 222 Rn and 220 Rn from off-gas streams of uranium and thorium processing facilities. [3] India has vast deposits of thorium rich monazite which can be exploited for the nuclear power generation programme. One of the most commonly associated problems in thorium processing plants is the build-up of 220 Rn in workplaces and its release into the environment through the stacks. It should be noted that adsorption of 222 Rn and 220 Rn on charcoal is sensitive to the temperature and humidity of the carrier gas. [4] The successful designing and characterization of adsorption systems require an adequate understanding of these sensitivities. In view of this, a study was carried out to evaluate the effect of humidity on the mitigation of 220 Rn in charcoal bed. The results are presented and discussed in this paper.


  Materials and methods Top


The experimental arrangement for studying the adsorption of 220 Rn on activated charcoal, and the effect of humidity is as shown in [Figure 1]. Coconut shell-based granular activated charcoal was filled in commercially available polyvinyl chloride  (PVC) cylinder and used as adsorber beds. The length of the adsorber bed was 0.15 m and the diameter was 0.016 m. The cylinders were completely filled with charcoal ensuring minimum possible void space. The packing density was 603 kg/m 3 and the weight of the charcoal filled in the cylinder was 17.3 g. 220 Rn gas was drawn from the Pylon source and injected into the adsorber bed using an air pump at a constant flow rate of 1 L/min. The ambient air inside the laboratory room was used as the carrier gas for the 220 Rn generated in the Pylon source. The concentration of 220 Rn in the inlet and outlet of the adsorber bed was measured simultaneously using scintillation cell-based thoron monitors (STM, developed by RP and AD, BARC). [5] The experiments were carried out in an air-conditioned room so that the effect of humidity on 220 Rn adsorption on charcoal could be studied. The humidity in the ambient air could be increased by switching OFF the air-conditioner and it could be decreased by switching ON the air-conditioner.
Figure 1: Experimental arrangement for studying 220Rn mitigation in charcoal bed

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  Results and discussion Top


Mitigation of 220 Rn using activated charcoal bed

As discussed earlier, the 220 Rn rich air from the Pylon source was pumped through the charcoal bed, and the 220 Rn concentration, both at the inlet and outlet of the charcoal bed were monitored continuously. [Figure 2] presents the results of 220 Rn concentration both at the inlet and outlet of the charcoal bed. As seen from the figure, the concentration of 220 Rn in the inlet remained fairly constant (~11 kBq/m 3 ) throughout the experiment. The outlet concentration was reduced to the normal level (<12 Bq/m 3 ) which indicates that the 220 Rn present in the inlet air was considerably mitigated. Even after pumping the 220 Rn rich air through a charcoal bed for several hours, the concentration in the outlet did not increase, which indicates stability in the mitigation ability of the charcoal bed. The mitigation factor, which is the ratio of concentration in the inlet to that in the outlet, was 10 3 for the results presented in [Figure 2]. The humidity of the carrier air was maintained at a nearly constant value (~45%, which represents a lower relative humidity) throughout these experiments and this was achieved by the use of air-conditioner during the experiments. Extensive studies were carried out using 220 Rn sources of higher strength under ambient humidity conditions (varying in the range 80-90%) and these studies have indicated that the mitigation factor of > 10 5 was easily achievable using adsorber beds of appropriate dimensions. [Figure 3] presents the typical results of 220 Rn mitigation studies under high input concentration and ambient humidity conditions for a adsorber bed of diameter = 0.063 m, length = 0.61 m at a flow rate of 10 L/min. These results indicate that charcoal based mitigation systems can be effectively used for 220 Rn mitigation in workplaces.
Figure 2: Demonstration of stability of 220Rn mitigation in activated charcoal bed (at relative humidity 45%)

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Figure 3: 220Rn adsorption characteristics of activated charcoal at high input concentration and ambient humidity conditions. Concentration values shown with an axis break for a better visualisation of the variation of concentration of 220Rn in outlet air of charcoal bed (adsorber dimension: Diameter = 0.063 m, length = 0.61 m, flow rate = 10 L/min)

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Effect of humidity on 220 Rn adsorption in charcoal bed

[Figure 4] represents typical results that demonstrate the effect of humidity on 220 Rn adsorption. As discussed earlier, humidity is one of the important factors affecting the adsorption of 220 Rn in the charcoal bed. [6] This is because the water molecules present in the air occupy the pore space of the charcoal and upon continuous use, the charcoal gets saturated with humidity, and this reduces the efficiency of 220 Rn adsorption. When the air-conditioner was switched ON, the humidity was maintained at nearly constant (~45%) and the 220 Rn concentration in the outlet remained close to zero [Figure 4]. When the humidity in the carrier air was increased by switching OFF the air-conditioner, the 220 Rn concentration in the outlet increased gradually with the increase of humidity. At 60% relative humidity, the concentration at the outlet increased to about 2 kBq/m 3 indicating that the mitigation factor got reduced to about 6 from the original value of ~ 1000, as evident from [Figure 5] which presents the variation of 220 Rn mitigation factor with the humidity. When the air-conditioner was switched ON again, the 220 Rn concentration in the outlet decreased. However, it should be noted from [Figure 4] that even if the humidity in the input air is decreased to the original values of 45%, the concentration at the outlet has not reduced to the initial value. This is because of the reasons explained earlier in this section, when the adsorber bed is operated at higher humidity it would result in accumulation of water molecules in activated charcoal that reduces the 220 Rn adsorption efficiency. These results demonstrate that humidity is one of the important factors to be considered while designing 220 Rn mitigation systems using charcoal beds.
Figure 4: Effect of humidity on 220Rn adsorption in charcoal bed

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Figure 5: Variation of mitigation factor with the humidity of the air

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  Conclusion Top


Activated charcoal is a very good adsorber of 220 Rn and it can be easily used for 220 Rn mitigation applications in workplaces. Under normal circumstances, the mitigation factor >10 5 was easily achieved using charcoal beds. Humidity is an important factor to be considered while designing charcoal based mitigation systems as increase in humidity in the ambient air would significantly reduce the efficiency of such mitigation systems.


  Acknowledgments Top


The investigators would like to thank BARC, Mumbai and Mangalore University for implementing the MoU programme to undertake this research study. The investigators are thankful to Dr. D. N. Sharma, Director, HS and EG, BARC, Dr. A.K. Ghosh and Shri. H. S. Kushwaha, former Director, HS and EG, BARC for their help, encouragement, and co-operation in undertaking the studies.

 
  References Top

1.
Darwish Al-Azmi., Karunakara, N. A simple radon chamber for use with soil gas for calibration of radon measuring devices and instruments. Int. J. of Low Radiation, 2011 Vol. 8, No. 5/6, pp. 429-439.  Back to cited text no. 1
    
2.
Ackley, R. D. Removal of 220 Rn from HTGR Fuel reprocessing and refabrication off- gas streams by adsorption, Oak Ridge National Laboratory. 1975. ORNL-TM-4883.  Back to cited text no. 2
    
3.
Sudeep Kumara., Yashodhara, I., Karunakara, N., Mayya, Y, S., Sapra, B, K., Sahoo, B, K., Gaware, J, J., Kanse, S, D. Studies on radon and thoron Mitigation using Charcoal based systems. 19 th NSRP, Chennai, December 12-14, 2012.  Back to cited text no. 3
    
4.
Pojer, P, M., Peggie, J, R., O'Brien, R, S., Solomon, S, B., Wise, K, N. Performance of a diffusion barrier charcoal adsorption 222 Rn monitor under conditions of varying humidity and temperature. Health Phys 1990; 58:13-9.  Back to cited text no. 4
    
5.
Gaware J, J., Sahoo, B, K., Sapra, B, K., Mayya, Y, S. Development of ZnS (Ag) detector based portable continuous radon-thoron monitors for multiple applications. Proceedings of, Synergy in Physics and Industry Held at BARC, Mumbai during January 21-22, 2013. p. 80.  Back to cited text no. 5
    
6.
Sudeep Kumara, K., Karunakara, N., Yashodhara, I., Sapra, B, K., Sahoo, B, K., Gaware, J, J., Kanse, S, D. and Mayya, Y, S. Effect of humidity on thoron adsorption in charcoal bed. Book of abstracts, IARPNC; 2014.  Back to cited text no. 6
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]



 

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