Preparation and characterization of a double-layered porous film to assay for surface radioactive contamination

Journal of Membrane Science 223 (2003) 59–67

Preparation and characterization of a double-layered porous film to assay for surface radioactive contamination Myeong-Jin Han a,∗ , Kune Woo Lee b , Bum-Kyoung Seo b a

b

Department of Chemical Engineering, Kyungil University, Kyungbuk 712-701, South Korea Division of Decommissioning Technology Development, Korea Atomic Energy Research Institute, Daejon 305-353, South Korea Received 28 February 2003; accepted 30 June 2003

Abstract Double-layered polysulfone (PSF) membranes, containing cerium activated yttrium silicate (CAYS) as a fluor, were prepared from double casting of two polymeric solutions, and their morphology and radioactive capacity were compared with those of single-layered membranes. The backing, the bottom layer of double-layered membranes, was made of a binary casting solution of polysulfone and methylene chloride (MC), its cast film being solidified by vacuum evaporation. The second casting solution with dimethylformamide (DMF) as solvent was cast over the solidified backing film and coagulated by being immersed into a nonsolvent bath of water or methanol. The prepared membranes revealed two distinct, but tightly attached, double layers, their attachment being identified by the morphology of the interface between the two layers. Membranes prepared with CAYS in the casting solution have more developed macropores than those prepared without CAYS. In the radionuclide detection test of the CAYS-impregnated membranes, the membranes solidified by water precipitation showed better detection efficiency than those solidified by methanol precipitation. Its superior efficiency is not due to better holding of the radioisotopes, but due to greater density of CAYS in the membrane surface region. In the comparison with single-layered membranes, the double-layered membranes showed a greater ability in holding the radionuclides, spotted on the surface, as well as an improvement in physical strength because of the dense backing layer. © 2003 Elsevier B.V. All rights reserved. Keywords: Detection; Radionuclide; Polymer membrane; Fluor; Radioactive waste

1. Introduction Radioactive materials can be applied in various fields, including the preparation of special medicine for tracing drug metabolism. In particular, radioactive ␤-emitters such as 3 H, 14 C and 35 S are typically used as tracers for measuring kinetic and dynamic properties of drugs or specific ingredients. Radioactivity of radionuclides, or radiolabelled molecules, can be detected by measuring the electric current in an ionized ∗ Corresponding author. Fax: +82-53-850-7613. E-mail address: [email protected] (M.-J. Han).

chamber, the darkness of a photographic emulsion, or the number of electron pulses [1]. The commonly used electron pulse detection or radiotrace method estimates pulses formed by the ionization of scintillator interacting with the radioactive particles. This kind of radiotrace method, which requires radiolabelled materials, has several advantages that include high sensitivity, sharp locality and nondestructive character in describing surface and colloidal phenomena [2]. Long and extensive use of the radiolabelled materials in a place, however, may cause radioactive contamination on working surfaces such as table tops and floors. According to US Nuclear Regulatory

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00308-9

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Commissioners [3], the potentially contaminated area has to be monitored regularly, whether or not cleaning of the contaminated area is decided by measuring the degree of radioactive contamination on the surface. To measure the surface contamination, two different types of detection methods can be applied. One is the probing method, a direct measurement where a detecting device is put on the contaminated surface and the level of contamination is probed by the gauge’s reading. The other is a smear or wipe test, an indirect method where the surface is smeared using a wiping medium, whose radioactivity is quantified using a proper measuring device. Even though the direct detection method is easily applied, being affected by circumferential effect is a major weakness. Therefore, the indirect wipe test is the preferred and recommended technique to monitor radioactivity on work surfaces. In the conventional wipe test [4], a large volume of radioactive waste consisting of both the radionuclides and the organic materials from the cocktail can be produced because of diffusion of the picked-up radionuclides into the cocktail. The storage and disposal of the radioactive waste, which causes an environmental problem, has therefore been of a major concern to the researchers using radioisotopes. In an attempt to overcome this concern, solid scintillation proximity membranes or film, which can be used as a medium for the wipe test, have been investigated [5–8]. The underlying idea for these membranes is to impregnate in a solid matrix with fluor, which can illuminate by absorbing energy and emitting a portion of the energy within the ultraviolet, visible or infrared region. Thus the activity of radioisotopes adsorbed on the matrix during the wipe test can be detected without dissolving the collected isotopes in the scintillation cocktail, with the effect of radioactive waste reduction. One of the scintillation proximity membrane developed now consisted of polysulfone (PSF) as the polymer matrix and cerium activated yttrium silicate (CAYS) as the inorganic fluor [8,9]. Polysulfone was chosen because the aromatic rings in the molecular structure show good ionic transmittance or energy transfer. CAYS was selected for its high radioactive detection efficiency. The CAYS-impregnated PSF membranes were prepared by the phase inversion method [10]. Precipitating a CAYS-dispersed casting solution resulted in a membrane with a highly porous structure in a

nonsolvent medium. But, single-layered scintillation proximity membranes prepared in this way showed some weakness that made it improper for use as a radioactive detecting medium. Firstly, membranes could not retain all the radionuclides collected on the membrane because their pores extended from the top to the bottom, resulting in the leakage of radionuclides from the membrane structure. Secondly, a large amount of the reactant, or the fluor, was required to get effective counting, because the collected radionuclides were spread across the membrane structure [11]. The high portion of inorganic fluor in the membrane, in turn, induced the physical weakness such as brittleness or fragile property. Also, the highly dispersed fluor particles make them inefficient for radioactive counting. To improve the structure stability of the membrane and the efficient use of fluor, solid scintillation proximity membranes of a double-layered structure was prepared. In this research the CAYS-suspended polymer solution was cast on top of a first, dense membrane and coagulated, subsequently making CAYS particles to condense in an active second layer. The structure and radioactive detection ability of the double-layered membranes were compared with those of the single-layered membranes. Also, in the preparation of the membranes through the phase inversion process using a nonsolvent coagulant, the effect of two nonsolvent types on the membrane’s morphology and detection efficiency is reported. 2. Experimental methods 2.1. Membrane preparation Casting solutions of two different compositions were prepared. One was a binary solution of PSF/ methylene chloride (MC) (25 g/80 g) for the support layer, and the other a PSF/dimethylformamide (DMF) (4.5 g/25 g) solution for the active layer. To formulate a solution for CAYS-impregnated membranes, 5 g of CAYS (P-47, SPI Supplies) was dispersed in the fully mixed PSF/DMF solution. PSF (Mn 22,000), MC and DMF were purchased from Aldrich. The particle size of CAYS ranged from 3 to 5 ␮m, with an average diameter of 3.42 ␮m. Single-layered membranes were produced by spreading the DMF-containing solutions with a

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Fig. 1. Schematic diagram for membrane preparation.

200 ␮m clearance gap made on a clean glass plate and immersing the cast film into a coagulation bath holding either methanol or water. As shown in Fig. 1, doublelayered membranes were produced through two separate castings. First, the PSF/MC solution was cast in a 300 ␮m clearance gap on a clean glass plate and dried in a vacuum oven at 30 ◦ C. After 48 h, the glass plate, holding the PSF film solidified from the first cast, was removed from the oven. The second casting solution was spread over the PSF film in a 200 ␮m clearance gap and immediately the glass plate immersed into a water or methanol bath to coagulate the composite film. After keeping the bath for 24 h, the coagulated films were removed, dried and stored at ambient conditions before testing. 2.2. SEM The morphology of the solidified membranes was observed by using a scanning electron microscope (SEM, Philips XL30W). Samples were freezefractured under cryogenic conditions using liquid nitrogen and coated with a gold and palladium (60:40) alloy before observation. 2.3. Radiation detection test For the radiation detection test of the prepared membranes, a radioisotope solution of aliquots of 30 ␮l was deposited on the prepared membranes. The radioactive solution consisted of a common ra-

dionuclide, oleoyl-coenzyme A (14 C) (Amersham Pharmacia Biotech, UK), a low energy ␤-emitter, diluted in isopropanol. Its radioactive concentration was 2.3 Bq/␮l. After being allowed to dry overnight at room temperature, the amount of radioactivity spotted on the membrane surface was quantified by using both photomultiplier tube (PMT, Hamamatsu Photonics K.K.) and low background (LB, ␣/␤ counter, Canberra S5XLB). Their radioactivity measurement was recorded as radioactive counts per minute (CPM). In the PMT test, the membrane containing the radioactivity deposited on the surface was put into a test tube, and its detection capacity was measured directly without any help of any auxiliary agent such as a scintillation cocktail. In the LB test, the membrane holding the isotopes were directly applied to the counter and its CPM values were measured. 3. Results and discussion Firstly, mono-layered membranes were prepared both with and without CAYS, as compared for doublelayered membranes. As shown in Fig. 2(a) and (b), the mono-layered membranes prepared in the absence of CAYS are dense in skin and porous in substructure which is typical of a diffusion-induced phase inversion membrane that shows an asymmetric structure. Large macrovoids or macropores are evident in the membrane substructure, and the membrane solidified in a methanol bath has a little larger macropores

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Fig. 2. Cross-sections of mono-layer membranes prepared by methanol (a) and water (b) immersion without CAYS; and by methanol (c) and water (d) immersion with CAYS.

than the membrane solidified in a water bath. The graded structure is a little bit clearer in the membrane prepared with water as the nonsolvent coagulant. When prepared with CAYS (Fig. 2(c) and (d)), the morphology of the membranes shows asymmetric structures similar to that of the membranes prepared without CAYS. But the development of macropores in the CAYS-embedded membranes is very different from those in the CAYS-free membranes. In the CAYS-embedded membranes, macropores are initiated just under the skin, being distinguished from those initiated deep in the middle of the CAYS-free membranes. In particular, in the CAYS-embedded membrane solidified with nonsolvent water (Fig. 2(d)), the enlarged macropores developed from just below

the skin region and reach to the bottom of the membrane, showing a highly continuous path of the pores throughout membrane cross-section. According to Smolders et al. [12,13], the onset of the formation of macrovoids can be enhanced by localized fluctuation on the surface of a casting solution. Therefore, it is assumed that the enhancement of macropore arises from the thermodynamic immiscibility of CAYS against components such as solvent, nonsolvent and polymer. The immiscibility makes CAYS particles work as an inhibitor for constraining mutual diffusion between components in a coagulation process, resulting in the diffusion front’s fluctuation. This fluctuation, in turn, may induce nonhomogeneous phase separation in the solution, subsequently working

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as the driving force for structure irregularity through not uniform collapsing. The CAYS particles embedded in the prepared membranes seem to adhere to the polymer structure throughout the membrane. Traces of CAYS are observed not on the top surface, or the air side during the cast, but on the bottom side contacting the glass plate, irrespective of nonsolvent coagulant used. These results indicate that the CAYS particles stick to the polymer matrix on the skin region, while some of them are loosened from polymer matrix on the bottom. The loosening or separation of CAYS from the matrix is due to CAYS expelled from the nucleated polymer-lean phases during the liquid–liquid phase

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separation near the thermodynamic equilibrium. Even though their impregnation is very similar with each other, the CAYS density near the skin region seems to be a little higher in water than in the methanol precipitated membranes. As shown in Fig. 3(a) and (b), when prepared in the absence of CAYS, the second layer of the doublelayered membranes show that macropore formation is relatively suppressed, compared to those shown in Fig. 2(a) and (b). And also, the interface between two layers has a relatively well-developed sponge structure, compared to the weakly developed one that can be found near the bottom of mono-layered membranes. In the specific region between two layers, this kind

Fig. 3. Cross-sections of double-layer membranes prepared by methanol (a) and water (b) immersion without CAYS; and by methanol (c) and water (d) immersion with CAYS.

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of sponge structure formation can be explained by the following concept. Just after the second cast, the PSF concentration near the interface rapidly increases because of the solvent’s penetration into the first layer. With the nonsolvent’s chemical potential for demixing already reduced through the mixing with solvent, the diffusions between components becomes significantly weakened, and, in turn, the demixing of the mixed solution of high polymer concentration happens near the equilibrium. Consequently, the distinctive sponge structures near interface are formed, proving the two layers are tightly interconnected. As the second layer is formed from a CAYS-containing solution in (Fig. 3(c) and (d)), the CAYS in the casting solution has made the macropores in the double-layered structures enlarged and distinct just the way it did those in the mono-layered structures in Fig. 2(c) and (d). But, rather than extending from the top to the bottom of the second layer, the macropores are limited well over the border between two layers. Like the CAYS-free membranes, the limitation on the macropore development underscores the formation of highly concentrated polymer solution near interface, which is also related with well-developed sponge structures. Fig. 4 shows that CAYS particles are evenly dispersed on the membrane surfaces, irrespective of nonsolvent type, being safely embedded in the polymer matrix near the skin. The CAYS particles held in the structure indicates that the precipitation of the

Fig. 5. Schematic diagram of CAYS-impregnated membranes; shaded parts for porous area and black dots for CAYS.

casting solutions by nonsolvent occurs rapidly without CAYS being separated from the polymer matrix. If the casting solution is phase separated near thermodynamic equilibrium, the CAYS, immiscible with polymer, should be leached out from polymer matrix during coagulation process, as explained in Fig. 5.

Fig. 4. Surfaces of double-layer membranes prepared by methanol (a) and water (b) immersion with CAYS.

M.-J. Han et al. / Journal of Membrane Science 223 (2003) 59–67

Both PMT and LB can be used for radiation detection, but they are different from each other in the detection technique. Even though LB counts the radiation with direct interaction between radiation and detector, it has a problem in that it cannot be applied to detect low energy radionuclides such as 3 H. But, if the loss of radionuclides is induced at any rate, it can be identified by the loss in LB counting rates, because it is not affected by the scintillation reaction. On the other hand, PMT can increase the detection capacity of low energy radionuclides, detecting the amount of photons released by the scintillation interaction between radiation and a fluor such as CAYS. The radioactivity detection using a CAYS-impregnated membrane brings upon some reduction of the activity in measurement, as compared with the original radioactivity deposited on the membrane. The reduction of radioactive counting can be caused both by the leakage of the deposited radionuclides and by the imperfect detection of the activity collected in the membrane. The leakage reduction can be related to the radioactive materials leaving the membrane through pores, while the imperfect reduction is correlated with impact of membrane morphology on detecting. In the case of PMT measurement, the imperfect detection can be due to both the membrane morphology and the impregnation characteristics of CAYS in the polymer matrix. The polymer enveloping over CAYS cannot help but drop the detection efficiency of PMT, causing the decrease in the interaction yield. Therefore, the further the distance between CAYS and the radionuclides becomes, the higher becomes the degree of counting reduction. The other effect, quenching, is largely due to the geometric shape of a membrane. The photon released by the de-excitation of the reacted CAYS needs to reach a counting device for quantification. Passing through a membrane matrix, the emitted light pulse becomes absorbed, or

Table 1 Comparison of radioactive counts per minute (CPM) of Coagulant

LB counts (CPM) PMT counts (CPM)

14 C-oleoyl

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scattered. Therefore, the radioactive counts quantified using PMT reflects both the decrease in scintillation interaction due to the polymer enveloping over CAYS and the quenching effect due to membrane geometry. In the case of LB counting, in contrast, the imperfect detection is largely due to self-absorption corresponding to radiation scattering or absorption inside a membrane. The self-absorption in the LB counting is equivalent to quenching in the PMT measurement. To determine the detection capacity of membrane to radionuclides, their activity deposited in the membranes is measured using PMT and LB. As shown in Table 1, LB CPM values of double layer membranes are higher than those of mono-layered membranes. The higher values of the double-layered membranes correspond to their dense support layer and porous but suppressed macropores, which can block the leakage of radionuclide solution from the composite film. LB counting values also indicate that the membranes prepared with water coagulation have lower values than those prepared with methanol, irrespective of being mono- or double-layered. In the mono-layered membranes, the relatively low CPM value of the watercoagulated membrane to the methanol-coagulated one represents that the membranes prepared with water are more porous and have a higher probability of loss of radionuclides during the spotting. Overall LB CPM values reveal that with single-layered membranes, the water-coagulated membranes have more porous structures than the methanol-coagulated ones, causing the high leakage loss of radionuclides, but that the porous effect reduces on the double-layered membranes because of the dense support layer. In the PMT measurement, the CPM values are lower than those measured with LB, because of the effect of polymer enveloping the scintillation reaction. The CPM values of the double-layered membranes are lower than those of the single-layered membranes,

coenzyme A detected following spot test on the prepared membranes

Single-layered (standard deviation, %)

Double-layered (standard deviation, %)

Water (A)

Methanol (B)

A/B

Water (A)

Methanol (B)

A/B

95.7 (3.8) 54.8 (3.7)

118.0 (3.4) 55.3 (3.2)

0.81 0.99

123.9 (3.3) 47.7 (2.8)

132.6 (1.8) 38.9 (3.3)

0.93 1.22

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which is not because of the detection efficiency but because of overall decrease in the CAYS amount due to the lower casting thickness of the active site in the double-layered membranes. In the comparison between nonsolvent coagulants, however, PMT counts reveal an opposite trend to LB counts, showing that the membranes prepared by water coagulation have much improved values compared with those prepared by methanol coagulation, irrespective of being monoor double-layered. For the double-layered membranes, in particular, even though the LB count of the water-coagulated membrane is lower than that of the methanol-coagulated membrane, the PMT counts of those membranes show the reversed effect. The scintillation reaction is taking place not just on the surface but also inside the membrane. High porosity can induce an increase in a scintillation yield only to be offset by quenching, even though it can contribute to the radionuclide’s holding inside membrane. And density and dispersity of CAYS particles in the top region of the membrane is more important than that inside the membranes. Therefore, in the PMT measurement, as compared to the LB measurement, the improvement of the ratio of the water-coagulated membrane’s CPM over the methanol-coagulated membrane’s indicates that the radioactive detection near the skin region is high enough to overweight the loss from the radionuclides leakage or geometrical disadvantage. The overall morphological and the radioactive detection show the advantages of the double-layered membranes over the mono-layered membranes. The double-layered membranes including a tight and dense supporting layer improve the morphological disadvantage of the single-layered membranes. This structure improvement gives the membranes the definite advantage of secure holding of radionuclides, a better stability of eliminating their possible loss from the membrane matrix. In the scintillation detection, the membranes prepared with the water coagulation are better than those prepared with the methanol coagulation.

4. Conclusions The polysulfone membranes containing CAYS as an inorganic fluor can be prepared to be used as the

scintillation assay medium for monitoring the surface contamination. When prepared in the existence of CAYS in a casting solution, the membranes have much enlarged macropores, compared to the membranes prepared in the absence of CAYS. The direct counting of radioactivity using LB shows that the radionuclides can be secured in the membrane matrix with the double-layered structure, its dense support layer being compared with the highly porous and open structure of mono-layered membranes. With the double-layered structure, the membranes’ stability is much improved, significantly reducing the possible loss of the deposited radioisotopes through the pores. In the scintillation assay using PMT, the detection efficiency of membranes prepared with water precipitation is better than those of membranes prepared with the methanol, its improvement being due to higher CAYS density in the membrane skin region. References [1] C.G. Potter, G.T. Warner, Scintillation counting of harvested biological samples with low energy beta emitters, using solid scintillant filters, in: H. Ross, J.E. Noakes, J.D. Spaulding (Eds.), Liquid Scintillating Counting and Organic Scintillators, Lewis Publishers, Chelsea, MI, 1991. [2] M. Muramatsu, Radioactive tracers in surface and colloid science, Surf. Colloid Sci. 6 (1973) 101. [3] US Nuclear Regulatory Commission, Radiation Safety Surveys at Medical Institutions, NRC Regulatory Guide 8, vol. 23, Washington, DC, 1 January 1981. [4] R.C. Klein, L. Linins, E.L. Gershey, Detecting removable surface contamination, Health Phys. Soc. 62 (1992) 186. [5] S.W. Wunderly, J.F. Quint, Sample Counting Support with Solid Scintillator for Use in Scintillation Counting, US Patent No. 4,916,320 (1990). [6] K.A. Schellenberg, Solid Phase Scintillation Counting Method, US Patent No. 4,562,158 (1985). [7] L.F. Costa, D.C. Harrington, R.S. Miller, Solid Scintillator Counting Compositions, US Patent No. 4,692,266 (1987). [8] M.J. Han, P.M. Bummer, M. Jay, Solid scintillation proximity membranes. II. Use in wipe test assays for radioactive contamination, J. Membr. Sci. 148 (1998) 223. [9] M.J. Han, M. Jay, Radioactivity measurement of radionuclides using solid scintillation proximity membranes prepared from polysulfone and an inorganic fluor, Korea Polym. J. 6 (4) (1998) 341. [10] M.J. Han, P.M. Bummer, M. Jay, Solid scintillation proximity membranes. I. Characterization of polysulfone-inorganic fluor morphologies precipitated from NMP solutions, J. Membr. Sci. 140 (1998) 235.

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[13] I.M. Wienk, R.M. Boom, N.A.M. Beerlage, A.M.W. Bulte, C.A. Smolders, H. Strathmann, Recent advances in the formation of phase inversion membranes made from amorphous or semicrystalline polymers, J. Membr. Sci. 113 (1996) 361.