Nonconjugated conductive polymers for protection against nuclear radiation including radioactive iodine

Mrinal Thakur

doi:10.4103/rpe.RPE_33_20

ORIGINAL ARTICLE

Year : 2020 | Volume : 43 | Issue : 3 | Page : 148-153

Nonconjugated conductive polymers for protection against nuclear radiation including radioactive iodine

Mrinal Thakur
Photonic Materials Research Laboratory, Auburn University, Auburn, Alabama, USA

Date of Submission	27-Jun-2020
Date of Decision	25-Aug-2020
Date of Acceptance	07-Sep-2020
Date of Web Publication	6-Jan-2021

Correspondence Address:
Mrinal Thakur
Photonic Materials Research Laboratory, Auburn University, Auburn, Alabama 36849
USA

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/rpe.RPE_33_20

Abstract

Nonconjugated conductive polymers have unique characteristics for providing protection against nuclear radiation including radioactive iodine (carcinogenic) which remains in vapor phase at and above room temperature and is difficult to contain. When iodine comes in contact with such a polymer, a charge-transfer occurs between the double bond and iodine, and as a consequence, the iodine atoms become bound to the polymer chain. Large films/sheets of these polymers covering nuclear reactors and waste storage facilities will act as effective shields against radioactive iodine since iodine atoms will be captured by these polymers and will not be able to escape to the environment. In addition, apparels made of these polymers will reduce exposure to radioactive iodine for doctors, nurses, attendees, and visitors during and after radioiodine therapy of thyroid patients. Thus, these polymeric shields should protect lives and the environment and reduce or avoid the exposure to humans in case of iodine releases from nuclear reactors: in normal day-to-day operations, due to accidents, and in disasters up to the magnitude of Fukushima Daiichi and Chernobyl.

Keywords: Nonconjugated conductive polymers, nuclear reactor, protection against nuclear radiation including radioactive iodine

How to cite this article:
Thakur M. Nonconjugated conductive polymers for protection against nuclear radiation including radioactive iodine. Radiat Prot Environ 2020;43:148-53

How to cite this URL:
Thakur M. Nonconjugated conductive polymers for protection against nuclear radiation including radioactive iodine. Radiat Prot Environ [serial online] 2020 [cited 2023 May 30];43:148-53. Available from: https://www.rpe.org.in/text.asp?2020/43/3/148/306278

Introduction

Nuclear radiation from reactors and explosives based on fission (uranium or plutonium) includes radioactive iodine (I-131 and I-129) which causes thyroid cancer and other illnesses when exposed to high levels. The worse part of radioactive iodine is that it is volatile at room temperature and can leak out of reactors and cracked/spent fuel rods to the atmosphere. Iodine is partially soluble in water, and common chemical solvents, therefore, can mix in and be absorbed in food, milk, plants, and in living organisms leading to long-term health hazards.^{[1],[2],[3],[4],[5]} The half-lives of I-131 and I-129 are 8 days and 15.7 million years, respectively. About 3% of the fission product is radioactive iodine, and it decays by gamma and beta emission. Clearly, the capture/containment of radioactive iodine is of great importance and a challenging task. Although iodine pills (potassium iodide) can be administered to saturate the thyroid to avoid the effect of radioactive iodine, such treatment is cumbersome, has many side effects, and may not always be effective. In addition, an individual who is unaware of whether he/she is going to be exposed would not be able to take this precautionary measure. Besides the nuclear power plants, thyroid patients treated with I-131 emit radioiodine and associated radiation – then the potential victims of exposure are doctors, nurses, visitors, and attendees.

While a nuclear disaster can cause extensive health issues/fatalities over a short period of time, the nuclear power generators in normal day-to-day operations may release controlled radioiodine that may accumulate in the proximity to cause health hazards. Recent reports indicate that the state of New York and the vicinity (New Jersey and Pennsylvania) where there are 13 nuclear power plants in operation has significantly (~ 66%) higher thyroid cancer cases within a 90-mile radius compared to the rest of the United States.^[6] As has been reported, there were about 20,000 thyroid cancer cases in the Chernobyl disaster alone (UNSCEAR (2018), Evaluation of data on Thyroid cancer in region affected by the Chernobyl accident: A white paper to guide the Scientific Committee's future programme of work).^[7] What the final figure going to be for the Fukushima Daiichi disaster is yet to be known. Therefore, having appropriate shields that would capture and hold the radioiodine before it leaks out and contaminate the environment is of critical need.

Experimental Measurements and Results

In the present report, specific polymers with isolated double bonds [nonconjugated conductive polymers, examples shown in [Figure 1]] are discussed regarding their application as protective shields against radioactive iodine.^[8] When iodine interacts with such a polymer, electron-transfer occurs from the isolated double bonds in the polymers to the iodine atoms. Thus radical cations are formed which hold the iodine atoms (counter-ions) immobile close to the double-bond sites [Figure 5], discussed later]. This process/interaction (called doping) leads to many orders of magnitude increase in electrical conductivity of these polymers,^{[9],[10],[11]} although this rise of conductivity may not be of significant relevance to the present application. The major outcome of using films/sheets of these polymers is that iodine and its isotopes are captured/absorbed and held immobile within these polymeric chain molecules. Any enclosure/shield made of these polymers will stop leakage of radioactive iodine – thus providing protection. The polymer in appropriate forms can also be sprayed in air or water as cleansing agents of radioactive species. These shields in appropriate form can be used for nuclear reactors on land, in ships, and in other transportation systems.

Figure 1: Some examples of nonconjugated conductive polymers (with isolated double bonds): (a) cis-1,4-polyisoprene, (b) trans-1,4-polyisoprene, and (c) poly(β-pinene)

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Figure 2: Optical absorption spectra of cis-1,4-polyisoprene^[10] for different doping levels (molar concentrations, y of iodine)

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Figure 3: Solid-state13C nuclear magnetic resonance spectroscopic results^[12] for 1,4-cis-polyisoprene with different doping levels (y = molar concentration) of iodine. (a) Corresponds to undoped state. (b-d) Correspond to polymer with increasing molar concentration of iodine. The resonances corresponding to the double-bonded carbon atoms (α and β) decrease in intensity with doping due to formation of radical cations. Formation of any conjugation upon doping is ruled out since double-bond concentration decreases with doping. The carbon atoms in the radical cations contribute to the resonance peaks around the aliphatic carbons (γ, δ, and ε)

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Figure 4: Fourier transform infrared spectra of poly(β-pinene) before and after iodine doping. (a) Before doping and (b) after doping. The = C-H vibration band loses intensity as the C = C transforms into radical cation upon doping with iodine

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Figure 5: Schematic of interaction of iodine with nonconjugated conductive polymer involving charge-transfer from double bond to iodine, formation of radical cation, and consequent capture of iodine. Top: Polyisoprene after interaction with iodine;Bottom poly(β-pinene) before and after interaction (doping) with iodine

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The fact these polymers capture and render iodine immobile has been established as discussed in the following. The chemical structures and characteristics of these polymers after iodine doping have been investigated using various microscopic, electrical, and spectroscopic (optical absorption, Fourier transform infrared [FTIR], ¹³C nuclear magnetic resonance [NMR], electron paramagnetic resonance, and Raman) measurements.^{[9],[10],[11],[12],[13],[14],[15],[16]} Electrical conductivity increases by about eleven orders of magnitude upon high level of doping with iodine, although this increase of conductivity is not of importance for the application discussed in this report. The optical absorption spectra of polyisoprene for different doping levels of iodine are shown in [Figure 2]. Thin films of nonconjugated conductive polymers can be easily cast from solution on any substrate. These films have been used to thoroughly characterize by spectroscopic methods such as optical absorption and FTIR.

The solid-state¹³C NMR spectroscopic measurements have provided a clear insight regarding the effect of iodine doping on nonconjugated conductive polymer, cis-polyisoprene.^[12] The measurements have been made by detailed cross polarization and magic angle spinning techniques, and the effects of doping on the backbone carbon atoms were elucidated. The results of such measurements are shown in [Figure 3]. As these results have shown, the intensity of resonances for double-bond carbon atoms (α and β) decreases as the dopant concentration is increased. No new resonance appears in the downfield region, showing that new double bonds do not form or conjugation does not occur. The bands corresponding to the aliphatic region broaden with doping because of the formation of radical cations.

FTIR spectra of nonconjugated conductive polymer show a decrease of the double bonds upon iodine doping as the double bonds interact with iodine and transform into radical cations. An example of FTIR spectra recoded before and after doping of iodine is shown in [Figure 4]. This is for poly(β-pinene). The = C-H bending vibration band at 728/cm decreases in intensity as poly(β-pinene) is doped with iodine. The C = C stretching band at 1610/cm, which is weak, also decreases upon iodine doping.

As all these results have shown, the dopant iodine interacts with the double bond in the polymer backbone and forms a polaronic state (radical cation) as an electron is transferred from the double bond to the dopant creating a hole/positive charge carrier at the double-bond site. The iodine atoms as I₃ become bound to the radical cations because of Coulomb attraction [Figure 5]. The presence of I₃^-has been identified/confirmed by resonance Raman and Mossbauer spectroscopies. The color of the polymer becomes darker (absorption increases) as more iodine atoms interact and get absorbed within the polymer [Figure 6]. Before interaction with iodine, these polymers are transparent in color.

Figure 6: The color of the nonconjugated conductive polymer changes from transparent to dark as more iodine interacts and becomes bound. The molar concentration of iodine corresponding to the darkest color (extreme right) is about 0.8

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The following experiment was performed to show that these polymers can capture/extract iodine from an iodine-water solution. In a solution of iodine in water which is brown in color, a small piece of a nonconjugated conductive polymer was introduced. After a few hours, the solution became colorless while the polymer became darker in color, showing that iodine has been absorbed by the polymer [Figure 7]. Therefore, radioactive iodine in water produced in nuclear reactors can be cleansed using these polymers. The radioactive iodine leaked out to seawater/groundwater during the disaster in Japan could have been cleansed using these polymers.

Figure 7: Capture of iodine by a nonconjugated conductive polymer from iodine-water solution (brown color). Left: Iodine-water solution; Right: Iodine is captured by nonconjugated conductive polymer leaving colorless water

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Long-Term Retention of Iodine

In order to investigate how long the iodine captured by the polymers is retained within the polymer, the following experiments were performed. Polymer films were cast from solutions (e.g., in toluene for cis- and trans-polyisoprenes) on glass substrates. These films were dried, and the weights were recorded. Then, the films were exposed to vapor of iodine at room temperature in a closed container. After a brief exposure, the weight uptakes of iodine were measured (~10% of the weight of the polymer). Subsequently, the iodine-doped films were left under ambient condition (room temperature and pressure in air) for a long period of time (about 100 days). The weights of the samples were regularly monitored throughout this time. No significant decrease of weight was noticed, indicating that iodine was retained within the polymers [Table 1]. The color of the polymers was slightly faded due to low-level chemical reaction in air/moisture.

Table 1: Weight of polymer film before and after treatment with iodine (an example)

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Elemental analyses performed (by Galbraith Laboratories, Inc.) on such iodine-doped samples showed that the iodine contents remained unchanged (~ 10% of the weight of the polymer) after the samples were left under ambient conditions for 100 days. These are important results, showing that radioactive iodine will be retained within these polymers for long term so that the environment would remain unharmed.

Capture/Intake of Iodine by These Polymers

These polymeric materials interact instantly with iodine holding the iodine atoms immobile. A maximum of up to about 0.8 molar concentration of iodine is absorbed. According to the federal safety standard for radioactive iodine (in drinking water), a molar concentration of more than about 10^-21 per day (equivalent to about 3 pCi/L) is considered hazardous. However, this lower limit is many orders of magnitude less than the saturation molar concentration (0.8) of these polymers – therefore, safety can be assured.

As more iodine is captured/absorbed, these polymers (originally transparent) become darker in color. A photograph showing the color change of one such plastic sheet for a different duration of interaction with iodine is shown in [Figure 6]. The darkest picture on the right corresponds to the highest concentration of iodine (about 0.8 molar).

Significance of the Results – Comparison with other Materials

The nuclear reactor facilities for power generation are usually designed to minimize leakage of radioactive species. However, if any flaw exists, an accident occurs, or a disaster strikes, then the designed structures may fail leading to leakage of radiation causing health hazards, as have been observed many times in the past.^{[1],[2],[3],[4],[5],[6],[7]} As stated earlier, even in normal day-to-day operations, there is a finite leakage of radioiodine.^[6],[7] These polymer barriers/absorbers will act as safety shields for workers at nuclear power plants, people living close to nuclear reactors, and operators of nuclear-powered vehicles against exposure to radioactive iodine.

The most attractive feature of these polymeric/plastic materials is that these can be easily processed into any desired shape, size, and structure. In addition, these are exceptionally inexpensive. At present, specific solid adsorbents (e.g., activated charcoal) and wet scrubbing with specific chemicals and liquids are used for capture of radioactive iodine in nuclear power plants.^[17] The solid absorbents including specific silver-containing zeolites^[17],[18] as used are brittle, expensive, and have other shortcomings and cannot be used for fabrication of films, sheets, and large structures. The adsorption process in materials such as charcoal involves weak binding and not selective to specific molecular species. A slight rise in temperature (~50°C) leads to rapid desorption of iodine from charcoal. Smoke particles adhere to charcoal better than iodine does. The plastics discussed here are flexible/processible, capture and retain iodine efficiently, and provide the opportunity of large-scale applications such as covering the entire waste storage areas and nuclear reactor facilities (both inner and outer walls) with thin sheets of the material to prevent the leakage of radioiodine. The charge-transfer process is selective to iodine, and the overall interaction with the double bonds leads to strong binding of iodine to the polymer molecule. These plastics can be fabricated as a barrier/wall between the nuclear reactor and the space designated for workers at the plant facility to prevent unwanted exposure. These materials can also be used as paints on the outside walls of the buildings of nuclear power plants to prevent leakage to the environment. In addition, the materials can be sprayed as a microparticulate mist into the affected environment (air or water) to cleanse radioactive species.

Although nuclear reactor facilities with more advanced designs are now significantly safer than before, concerns of accidental leakages and potential disasters, as occurred (2011) in Japan and previously in Chernobyl, are always present. Radioactive iodine emitted from Fukushima crossed the Pacific to reach California, USA, and has been suspected of causing illnesses in babies.^[4] These polymers will provide reliable safety shields against such emission and can protect lives. Appropriate designs to place the polymeric shields at a distance from the nuclear reactor building should be made so that any high temperature that may be generated in the reactor does not affect the integrity and performance of the polymer.

In hospitals and clinics, the doctors, nurses, and attendees are often worried of being exposed to radioactive iodine during and after radioiodine therapy of patients having thyroid cancer and hyperthyroidism. Patients emit radiation for a number of days after taking a capsule containing radioiodine. The use of apparels made of these nonconjugated conductive polymers will reduce the potential exposure to radioiodine and lessen the anxiety. The apparels presently used in hospitals and clinics do not absorb iodine, therefore are not effective shields/barriers against radioiodine.

Conclusions

Nonconjugated conductive polymers, natural and synthetic, have unique characteristics for providing protection against nuclear radiation including radioactive iodine. When iodine comes in contact with such a polymer, a charge-transfer occurs between the double bond of the polymer and iodine, and radical cations are formed. As a result of this, the iodine atoms become bound to the double-bond site because of Coulomb attraction. These polymers are inexpensive and easily processible. Large films/sheets of these polymers covering nuclear reactors and waste storage facilities will act as effective shields against radioactive iodine since iodine atoms emitted from these facilities will be captured by these polymers – preventing leakage to the environment. Apparels made of these polymers will reduce exposure to radioactive iodine for doctors, nurses, attendees, and visitors during and after radioiodine therapy of thyroid patients. Thus, these polymeric shields should protect lives and the environment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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