富山大学水素同位体科学研究センター研究報告 32：35－43，2012.

Research Note
繰り返し使用した MCM-41 からのトリチウム水の脱着
田口 明，鳥養 祐二，松山 政夫

富山大学水素同位体科学研究センター〒930-8555

富山市五福 3190

Desorption of tritiated water from reused MCM-41

Akira Taguchi, Yuji Torikai, Masao Matsuyama

Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555(Received January 31, 2013; accepted April 30, 2013)
Abstract

The mesostructure and tritium desorption properties of as-synthesized MCM-41 and that after 5 cycles ofsequential H2O impregnation/desorption have been compared. The XRD and tritium desorption
experiments showed good reusability and reproducibility in the enrichment of tritiated water.

Research note

The enrichment of tritiated water is an important and indispensable task in tritium (T)recycling and environmental protection. Although there are several methods for deuterium
enrichment that are practically available and used in operations, these methods, such as gas
diffusion, distillation, and electrolysis [1], are not applicable to T enrichment because of their35

田口明・鳥養祐二・松山政夫

high cost and low efficiency. The development of a novel method for the enrichment of Tfrom tritiated water has long been required. Selective sorption and separation of T withcertain adsorbents is an attractive method. Zeolites, a family of microporous aluminosilicates,
are one of the candidates for adsorbents, and much effort has been devoted in optimizing theoperation conditions [2].
Mesoporous silicas, such as MCM-41 [3,4] and SBA-15 [5], which possess large surface
areas (up to ca. 1200 m2/g) and pore volumes (larger than ca. 0.8 cm3/g) with uniform
mesopores (larger than ca. 2.0 nm), are interesting adsorbents since they have large water
adsorption capacities [6]. We have investigated the desorption of T from several porous
materials including MCM-41, SBA-15, LTA(5A) zeolite, and amorphous SiO2 [7]. Amongthem, MCM-41 was found to have a superior intrinsic T desorption property as comparedwith LTA(5A). It was also found that such T enrichment could be reproduced after 5 cycles ofwater impregnation and desorption [7]. In this research note, we report the details of T
desorption behavior of MCM-41 after 5 cycles of water impregnation/desorption treatments.
MCM-41 was prepared by the procedure given in the literature with slight modifications[8]. The starting materials were colloidal silica (Snowtex-20, Nissan Chemical Industries,Ltd.) as the silica source, dodecyltrimethylammonium bromide (Tokyo Kasei) as the template,
NaOH (Kanto Chemical), and distilled water. The mixed solution with the molar compositionof 1.00 SiO2 : 0.50 DTAB : 0.28 NaOH : 54.5 H2O was hydrothermally treated at 130°C for
48 h in a stainless-steel autoclave. The white precipitate was post-treated by dispersion in
distilled water (solid/water = 1:10 by weight) at 130°C for 24 h. Calcination in order to36繰り返し使用した MCM-41 からのトリチウム水の脱着

110200denoted as MCM(1).

2102°C/min. The as-synthesized material was

Intensity /a.u.

flow at 550°C for 10 h with a ramp rate of100

burn-off the template was done under an air

MCM(1)The obtained MCM(1) was dehydrated atMCM(5)180 °C for more than two days, and thencooled to room temperature under a vacuum.
Then,distilledwater

impregnation

was02462 / degree

810Fig. 1 XRD patterns of MCM-41. MCM(1):as-synthesized, MCM(5): after 5 cycles of water
impregnation/desorption.

performed by the incipient wetness method. The sample in a quartz boat was placed in thedesorption apparatus equipped with an oven under an Ar flow of 50 cm3/min at 25°C for 24 h.
Subsequently, the sample was placed in a quartz tube in a furnace. The temperature of thefurnace was increased to 100°C at a ramp rate of 5°C/min, and this temperature wasmaintained for 60 min. Then, the temperature was raised to 150°C for 30 min. By repeatingthis stepwise heating, temperature was raised in increments of 50°C up to 550°C. The
impregnation and desorption cycles were repeated 5 times, and the resulting material wasdenoted as MCM(5).
The periodic mesostructure of MCM(1) and MCM(5) was confirmed by X-ray diffraction(Philips, PW1825/00). Figure 1 shows the XRD patterns of MCM(1) and MCM(5). The XRD
patterns of both samples exhibited well-resolved diffraction signals assignable to the 100, 110,
200, and 210 reflections of hexagonal symmetry (p6mm). This indicates that the structural
collapse is not critical in MCM(5). The unit cell parameter (a0 = 2d100/√3) was estimated to be37

田口明・鳥養祐二・松山政夫

3.88 and 3.89 nm for MCM(1) and MCM(5), respectively. Nitrogen sorption isotherms(Autosorb-1MP, Quantachrome Instruments) showed type-IV curves with a steep H1
hysteresis loop in the relative pressure range of 0.2-0.3 in both MCM(1) and MCM(5),indicating uniform tubular pore channels [9]. The Brunauer-Emmett-Teller (BET) surface
area, pore diameter by Non-Local Density Functional Theory (NLDFT [9]), and pore volumewere estimated, respectively, to be 1109 m2/g, 3.13 nm, and 0.79 cm3/g for MCM(1), and1114 m2/g, 3.18 nm, and 0.78 cm3/g for MCM(5). These results indicate that mesostructure ismore stable under the present experimental conditions than that in previous studies, whichwere carried out under steam [10], hydrothermal [11,12], and high temperature conditions[13].
T desorption was studied using the apparatus [14] in the radioisotope laboratory ofHydrogen Isotope Research Center, University of Toyama. Experimental procedures were thesame as those mentioned above, except that tritiated water was used instead of distilled water.
Briefly, a dried (180 °C and evacuation) sample was impregnated with tritiated water (231.1kBq/mL) by the incipient wetness method. The total amount of T impregnated was 462 and410 kBq/g for MCM(1) and MCM(5), respectively (Table 1). The sample (0.1–0.3 g asadsorbent) in a quartz boat was placed in the T desorption apparatus equipped with an ovenunder an Ar flow of 50 cm3/min at 25 °C. Here, under an Ar flow, T was allowed to desorb
spontaneously from the sample (mainly as HTO) and was collected in water bubblers in an icebath. Then, water-insoluble species such as HT were led into a CuO bed heated at 527°C,
where HT was oxidized to HTO and collected in a separate water bubbler. The amount of T

38

繰り返し使用した MCM-41 からのトリチウム水の脱着

Table 1 Summary of T impregnation and desorption for MCM(1) and MCM(5)T impregnatedMCM(1)
MCM(5)

wt%
66
65

kBq/g
449
410

Spontaneous desorption
T desorbed
Weight loss
wt%
kBq/g
kBq/mL
65.5
424.6
216.5
62.8
382.2219.3

Thermal desorption
T desorbed
Weight loss
wt%kBq/g
kBq/mL
1.10
9.57
290.0
1.18
9.27
282.4

collected was measured by using a liquid scintillation counter (Tri-Carb 2100TR, Packard)
with a liquid scintillator (Ecoscint XR, National Diagnostic). It should be noted that morethan 98% of T was collected as HTO in the desorption experiments.
Spontaneous desorption of T from MCM-41 was observed under Ar flow at 25°C. The
desorbed T was collected over an adequate period up to 24 h. Figure 2 shows the T desorption
profiles of MCM(1) and MCM(5). Here, the amount of T desorbed is normalized with respect
to the weight of the adsorbent and time (kBq/gh). It was revealed that a large amount of T
was desorbed in the early stages up to 5 h. The desorption rate decreased after 5 h, andbecame about 0.2 kBq/gh after 24 h. As listed in Table 1, the cumulative amount of T
desorbed during the first 24 h was estimated to be 424.6 and 382.2 kBq/g for MCM(1) andMCM(5), respectively, which were almost 90 % of the quantities impregnated. A significant

Table 1; more than 95 % of the tritiated water
was released. On the basis of the amount of Tdesorbed

andtheweightloss,

theconcentration of the tritiated water released

Amount of T desorpbed [kBq/gh]weight loss was also observed as shown in
100

1010.1was calculated to be 216.5 and 219.3 kBq/mL,

MCM(1)
MCM(5)

05

10152025

Time / hFig. 2 Change in the amount of T desorbed fromrespectively. These values were clearly lower MCM(1) and MCM(5) at 25 ºC. Bars along thex-axis represent the periods of T collection.

39

田口明・鳥養祐二・松山政夫

10impregnation, suggesting that a certain
amount of T remained in the samples.
Subsequently, the T remaining in theTritium desorbed [kBq/g]than those of the tritiated water used for1

MCM(1)
MCM(5)

0.1

samples

werethermallydesorbed.

The

0100

200
300
400
Temperature / ºC

500

600

sample was placed in a quartz tube in a Fig. 3 Tritium desorption profile from MCM(1) andMCM(5) at 25 – 550 ºC. Error bars represents thefurnace under an Ar flow (100 cm3/min). period of T collected.

Then, the temperature of the furnace was increased to 100°C at a ramp rate of 5°C/min, andthis temperature was maintained for 60 min. After the water bubblers were changed, thetemperature was raised to 150°C for 30 min. By repeating this stepwise heating, temperature
was raised in increments of 50°C up to 550°C.
Figure 3 shows the change in the amount of T desorbed depending on temperature. The
amount of T desorbed was normalized to the weight of the adsorbent as unity. The T
desorption profiles of MCM(1) and MCM(5) were very similar to each other, as seen inFigure 3. Both in MCM(1) and MCM(5), T desorption decreased when temperature rose from25 to 150 °C, and showed a minimum of about 0.2–0.3 kBq/g at 100–150 °C. Then, thedesorption amount increased again above 150 °C. An almost constant amount of T of 0.8–1.1
kBq/g was observed at temperatures from 200 to 550 °C. Such desorption profile can beexplained by the state of adsorbed HTO molecule [7]. Namely, a large number of physisorbed
HTO molecules desorbed below 100°C. Thereafter, desorption of HTO via condensation ofsurface silanol groups dominated above 100°C [7].

40繰り返し使用した MCM-41 からのトリチウム水の脱着

The cumulative amount of T desorbed from 25 to 550 °C was estimated to be 9.57 and9.27 kBq/g for MCM(1) and MCM(5), respectively. The sum of the T desorbed at 25 °C andunder thermal conditions corresponded to 434.2 and 391.5 kBq/g for MCM(1) and MCM(5),respectively, showing more than 95 % of mass balance (Table 1). Besides, after thermal
desorption, a weight loss of 1.10 wt% for MCM(1) and 1.18 wt% for MCM(5) were observed.
On the basis of the amount of T desorbed and the weight loss, the concentration of tritiatedwater was calculated to be 290.0 and 282.4 kBq/mL, respectively. It is obvious that theconcentration of tritiated water increased as compared with that used for impregnation. The
concentration ratio, defined as the ratio of the concentration of T desorbed under thermal
conditions to that derived from spontaneous desorption, was 1.34 and 1.29, respectively.
These indicate good reproducibility and reusability of MCM41 in the enrichment of tritiatedwater. Although the volume of tritiated water obtained under thermal conditions was small,the optimization of desorption and especially adsorption conditions might improve theeffective enrichment of tritiated water.
In conclusion, the structural stability and T desorption behavior of MCM-41 wasinvestigated. The mesostructure of MCM-41 was found to be maintained up to 5 cycles ofsequential impregnation and spontaneous and thermal desorption. The T desorption behaviorof reused MCM-41 was very similar to that of as-synthesized one, showing a good reusability
and reproducibility.

The results suggest that MCM-41 is one of the attractive adsorbents forthe enrichment of tritiated water.
References41

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