Hydrogen isotope adsorbent with differential binding properties to hydrogen isotopes, manufacturing method thereof and method for separating hydrogen isotopes using the same

Abstract

Provided is a hydrogen isotope adsorbent with differential binding properties and including mesoporous silica doped with fluorine.

Claims

1. A manufacturing method of a hydrogen isotope adsorbent comprising the following steps: (a) adding mesoporous silica and a fluorine compound to a solvent to generate a mixture; and (b) heat-treating the mixture, wherein in step (b), the mixture is first heat-treated at 100 to 140° C. for 20 to 28 hours, and then the mixture is second heat-treated at 100 to 140° C. for 3 to 7 hours.

2. The manufacturing method of the hydrogen isotope adsorbent of claim 1, wherein the mesoporous silica is MCM-41.

3. The manufacturing method of the hydrogen isotope adsorbent of claim 2, wherein the solvent is isopropanol.

4. The manufacturing method of the hydrogen isotope adsorbent of claim 2, wherein the fluorine compound is ammonium fluoride (NH.sub.4F).

5. The manufacturing method of the hydrogen isotope adsorbent of claim 2, wherein in step (a), the content of the fluorine compound is 20 to 100 parts by weight with respect to 100 parts by weight of the MCM-41.

6. The manufacturing method of the hydrogen isotope adsorbent of claim 5, wherein in step (a), the content of the fluorine compound is 40 to 60 parts by weight with respect to 100 parts by weight of the MCM-41.

7. The manufacturing method of the hydrogen isotope adsorbent of claim 2, wherein in step (a), 1.5 to 1.8 g of the MCM-41 per 100 mL of the solvent is added.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 5 are diagrams illustrating experimental results of adsorbents according to the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(2) Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Further, terms to be described below, as terms which are defined in consideration of functions in the present disclosure may vary depending on the intention or custom of a user or an operator. Accordingly, definitions of the terms need to be described based on contents throughout this specification.

(3) 1. Manufacture of Hydrogen Isotope Adsorbent

(4) Tritium, deuterium and light hydrogen are considered to be chemically identical in classical mechanics, but show different rates of chemical reactions depending on the mass of isotopes due to the difference in quantum mechanical zero point energy (ZPE). In general, binding sites with significant differences in bonding enthalpy (LH) between hydrogen isotopes tend to preferentially bind to heavier isotopes.

(5) This concept is also associated with a chemical isotope exchange reaction (e.g., Girdler sulfide; H.sub.2O.Math.H.sub.2S/CECE; H.sub.2.Math.H.sub.2O) on which a currently commercialized hydrogen isotope separation process is based.

(6) The separation of isotopes in such an isotope exchange reaction may be predicted through an isotopic reduced partition function (IRPF; s/s′f (T/H)) value. When hydrogen isotopes X (e.g., .sup.3H, .sup.2H.sub., .sup.1H) are distributed in two different chemical environments (e.g., X.sub.2O(l), X-A(s)), heavy hydrogen isotopes preferentially move toward a chemical environment having a higher IRPF(s/s′f (T/H)) value by an isotope exchange reaction, and it is generally known that in a chemical environment having a strong bond, an IRPF value is larger.

(7) As can be seen in Table 1 below, according to the research results of Biseleisen and Mayer, since fluorine shows a high IRPF value in diatomic hydride (HX), it may be considered that an adsorbent bearing fluorine as a chemical reactant has differential binding properties to hydrogen isotopes, and is effective in separating hydrogen isotopes contained in water such as radioactive wastewater, tritiated water or deuterium water.

(8) TABLE-US-00001 TABLE 1 Isotopic reduced partition function of various diatomic hydrides (HX) Molecule s/s′f(T/H) In s/s′f(D/H) H.sub.2 5.283 1.215 HF 28.50 2.359 HCl 10.52 1.669 HBr 7.935 1.469 HI 5.714 1.245

(9) Therefore, in the present disclosure, an adsorbent capable of separating hydrogen isotopes contained in water such as radioactive wastewater, tritium water or deuterium water is manufactured by doping fluorine on the surface of MCM-41, a mesoporous silica material having a high surface area.

(10) Representative mesoporous silica materials MCM-41, SBA-15, and KIT-6 have high potential as framework materials due to a large surface area. However, due to their low hydrothermal stability, the structure is easily broken during calcination, substitution, and ion exchange processes, resulting in the limitations to be used as a catalyst and an adsorbent.

(11) Considering these characteristics, when a synthetic method requiring high heat treatment, such as hydrothermal synthesis and a sol-gel method, is selected, chemical species need to be functionalized using a solvent other than water.

(12) Accordingly, isopropanol was used as a solvent, and basic ammonium fluoride (NH.sub.4F) is used as a fluorine compound so that fluorine is easily doped on the surface of a mesoporous silica material MCM-41.

(13) Specifically, various amounts of NH.sub.4F (n=0.0 g, 0.2 g, 0.5 g, and 1.0 g) were added to 1 g of MCM-41 in 60 mL of isopropanol at 25° C., and the mixture was vigorously stirred for 1 hour.

(14) Thereafter, the generated mixture was first heat-treated at 120° C. for 24 hours. In this process, isopropanol is gradually removed from the synthesis mixture by evaporation due to heating, thereby increasing the concentration of fluorine in the synthesis mixture.

(15) Then, the generated solid product was mixed well and second heat-treated again at 120° C. for 5 hours.

(16) MCM-41 that was not subjected to the above-described synthesis process was denoted as Pristine MCM-41, each MCM-41 adsorbent doped with different amounts of fluorine was denoted as (n)F-MCM-41_NCs (0.0F-MCM-41 NC is added to a solvent and subjected to the synthesis process without adding a fluorine compound), and MCM-41 in which each (n)F-MCM-41_NCs is calcined at 250° C. for 5 hours was denoted as (n)F-MCM-41_C.

(17) 2. Adsorption Experiment

(18) The separation performance of an adsorbent developed to separate hydrogen isotopes contained in water such as radioactive wastewater, tritium water or deuterium water was evaluated through a batch experiment using 10000 Bq/mL of deionized tritium water (solid to solution ratio=50 g/L), and the isotope separation factor was evaluated through Equation 1 below.

(19) $\begin{array}{cc}\alpha =\frac{{\left[T/H\right]}_{F-\mathrm{MCM}-41}}{{\left[T/H\right]}_{\mathrm{water}}}\cong \frac{{\left[\frac{\mathrm{HTO}}{H_{2}O}\right]}_{F-\mathrm{MCM}-41}}{{\left[\frac{\mathrm{HTO}}{H_{2}O}\right]}_{\mathrm{water}}}=\frac{\frac{c_0{V}_i-c_e{V}_e}{V_{\mathrm{ads}}}}{\frac{c_e{V}_e}{V_e}}& \left[\mathrm{Equation}1\right]\end{array}$

(20) 3. Experimental Result

(21) Referring to Table 2, as a result of the batch experiment, fluorinated 0.5F-MCM-41_NC shows a significantly improved tritium isotope separation factor α compared to Pristine MCM-41, and both materials show higher tritium isotope separation factors (α.sub.pristine, 25° C.=1.03, α.sub.Pristine, 6° C.=1.20, α.sub.0.5F, 25° C.=1.20, and α.sub.0.5F, 6° C.=3.29) at low temperature conditions. This indicates that an isotope-selective chemically active site is formed by fluorine doping, and the formed chemically active site can more efficiently separate tritium under low-temperature conditions.

(22) TABLE-US-00002 TABLE 2 Tritium separation performance of adsorbent before and after fluorine doping under two temperature conditions Removal Sorption Liquid fluorine Temperature efficiency performance Isotope separation After reaction concentration after [° C.] [%] [Bq/g] factor[—] pH reaction[mmol] Pristine 25 0.04 170 1.03 5.67 0 MCM-41 6 0.58 1181.11 1.20 5.61 0 0.5F-MCM-4 25 0.09 646.67 1.20 2.56 2.254 1_NC 6 3.94 7726.67 3.29 2.55 2.595

(23) According to the X-ray diffraction (FIG. 1A), Fourier transform infrared spectroscopy (FIG. 1B), and solid-state .sup.19F MAS NMR results (FIG. 3), a main chemical species of (n)F-MCM-41_NCs is (NH.sub.4).sub.2SiF.sub.6. Moreover, with the increasing concentration of NH.sub.4F in the synthesis mixture, each signal shows higher intensity. In addition, the X-ray fluorescence (XRF) analysis result shows that the fluorine weight percent of 0.2F-MCM-41_NC, 0.5F-MCM-41_NC, and 0.5F-MCM-41_NC is 8 wt %, 19 wt %, and 32 wt %, respectively.

(24) Referring to Table 3, isotope separation factors evaluated through an equilibrium batch adsorption experiment using Pristine MCM-41, 0.2F-MCM-41_NC, and 0.5F-MCM-41_NC show positive correlation with the content of (NH.sub.4).sub.2SiF.sub.6.

(25) TABLE-US-00003 TABLE 3 Tritium separation performance of adsorbent according to content of (NH.sub.4).sub.2SiF.sub.6 Isotope Removal Sorption separation After Liquid fluorine efficiency performance factor reaction concentration after 6° C., 2 d [%] [Bq/g] [—] pH reaction[mmol] Pristine MCM-41 0.58 1181.11 1.20 5.61 0 0.2F-MCM-41_NC 1.35 2686.07 1.56 1.85 1.078 0.5F-MCM-41_NC 3.94 7726.67 3.29 2.55 2.595 0.5F-MCM-41_C 0.24 290.00 1.06 2.01 — 1.0F-MCM-41_NC 0.93 1840.00 1.59 2.85 3.861

(26) In order to evaluate an effect of (NH.sub.4).sub.2SiF.sub.6 on the tritium separation efficiency, the tritium isotope separation factors were comparatively tested before and after calcination at 250° C. In 0.5F-MCM-41_C after calcination, (NH.sub.4).sub.2SiF.sub.6 is decomposed into gaseous states (NH.sub.3, HF, and SiF.sub.4) and fully removed (FIG. 1), and the tritium isotope separation factor is also reduced from 3.29 before calcination to 1.06 after calcinations (Table 3). This suggests that the presence of (NH.sub.4).sub.2SiF.sub.6 is closely related with tritium separation.

(27) However, 1.0F-MCM-41_NC showed a lower tritium isotope separation factor than 0.5F-MCM-41_NC despite a higher content of (NH.sub.4).sub.2SiF.sub.6 (α.sub.1.0F, 6° C.=1.59). This is because a large amount of fluorine is dissolved in a solution during the sorption reaction with tritium as a pore structure of MCM-41 is damaged during the synthesis process of 1.0F-MCM-41_NC.

(28) Referring to Table 4, as a result of N.sub.2 adsorption desorption analysis, 0.2F-MCM-41_NC and 0.5F-MCM-41_NC are observed to have very similar BET surface areas of about 540 m.sup.2/g, but 1.0F-MCM-41_NC shows significantly the reduced BET surface area to 300 m.sup.2/g, indicating partial destruction of the MCM-41 pore structure.

(29) TABLE-US-00004 TABLE 4 Pore characteristics of each adsorbent using results of nitrogen adsorption desorption analysis Pore diameter Pore volume BET Suface area [nm] [cm.sup.3 g.sup.−1] [m.sup.2 g.sup.−1] Pristine MCM-41 2.3020 0.5818 714.50 0.0 F-MCM-41_NC 2.3020 0.6264 774.79 0.2 F-MCM-41_NC 2.6572 0.4908 540.07 0.5 F-MCM-41_NC 2.8548 0.4183 541.49 1.0 F-MCM-41_NC 3.0671 0.2727 301.49

(30) The destruction of such a pore structure is also confirmed by the result of TEM analysis FIG. 2. According to previous studies about a fluorine doping mechanism of other silica materials, fluorine doping is performed by formation of octahedral fluorosilicate species by the fluorinative opening of the siloxane bridge on the silica surface.

(31) Since the opening of the siloxane bridge makes the pore wall of MCM-41 thin, the pore structure of MCM-41 may be partially destroyed when an excessive amount of NH.sub.4F is present in the synthesis mixture as in the synthesis process of 1.0F-MCM-41_NC.

(32) In addition, according to a result of quantitative analysis through the integrate signal of solid-state .sup.19F MAS NMR (FIG. 3), the pore structure integrity of MCM-41 represented by BET surface area and a ratio of dissolved fluoride show a negative correlation (FIG. 4), and in the case of 1.0F-MCM-41_NC, a solid-state .sup.19F MAS NMR signal assigned to fluoride adsorbed in the pores after the reaction is significantly reduced, unlike other adsorbents (FIG. 3).

(33) This means that as the pore structure of 1.0F-MCM-41_NC is damaged by the synthesis process, the solid-state fluorine retention capacity is reduced, and a large amount of fluorine is dissolved in the solvent during the reaction with tritium.

(34) The above results suggest the importance of not only the content of (NH.sub.4).sub.2SiF.sub.6 but also the integrity of the MCM-41 pore structure and the dissolution amount of fluorine according to structural damage when separating tritium.

(35) 0.5F-MCM-41_NC presents an excellent compromise between the amount of isotope-selective chemically active sites and the dissolution amount of fluorine according to the integrity of the MCM-41 pore structure. The tritium isotope separation factor (α.sub.0.5F, 6° C.=3.29) of 0.5F-MCM-41_NC is much higher than that of the chemical isotope exchange reaction (α=2.33; H.sub.2O(l)+HDS(g).Math.HDO(l)+H.sub.2S(g)) of the Girdler sulfide process, and comparable to that of the chemical isotope exchange reaction (α=3.7; H.sub.2O(l)+HD(g).Math.HDO(l)+H.sub.2(g)) of the CECE process, which shows the highest efficiency and processing capacity among existing separation processes.

(36) FTIR spectra (FIG. 5) and solid-state .sup.19F MAS NMR analysis results (FIG. 3) before and after the experiment indicate that the chemical hydrogen isotope exchange reaction at F . . . OH hydrogen bonding sites following the ion exchange reaction between NH.sub.4.sup.+ and H.sub.3O.sup.+ in (NH.sub.4).sub.2SiF.sub.6 is a main tritium separation mechanism of (n)F-MCM-41_NC.

(37) After the reaction, all (n)F-MCM-41_NCs show an upfield shifting of the .sup.19F MAS NMR signal about −5 ppm (FIG. 3), and according to previous studies, this indicates a change in coordination environment of .sup.19F nucleus by a complexation reaction of fluorine with H.sub.3O.sup.+ or H.sup.+ and the formation of F . . . OH hydrogen bond interaction.

(38) FTIR analysis results before and after the reaction of (n)F-MCM-41_NCs (FIG. 5) show that NH.sub.4.sup.+ ions of (NH.sub.4).sub.2SiF.sub.6 were exchanged with H.sub.3O.sup.+ through the reaction with tritium, and accordingly, it is known that generated H.sub.2SiF.sub.6.Math.4H.sub.2O crystals are formed by F . . . OH hydrogen bonds.

(39) In addition, even in the solid-state .sup.19F MAS NMR analysis result after the reaction of (n)F-MCM-41_NCs, a new octahedral species (−125 ppm) allocable to H.sub.2SiF.sub.6.Math.4H.sub.2O crystals is formed (FIG. 3), and the ratio of an integrated signal area of a peak corresponding to H.sub.2SiF.sub.6.Math.4H.sub.2O shows a positive correlation with the tritium isotope separation factor of (n)F-MCM-41_NCs (FIG. 4).

(40) Therefore, it may be concluded that the chemical isotope-selective exchange reaction at the F . . . OH hydrogen bonding site with the formation of H2SiF6.Math.4H2O from (NH4)2SiF6 is a main tritium separation mechanism of (n)F-MCM-41_NCs developed in the present study.

(41) Therefore, as can be seen from the experiments, (n)F-MCM-41_NCs according to the present disclosure have high tritium separation efficiency (hydrogen isotope separation factor: 3.29) as being comparable with the chemical isotope exchange reaction of the CECE process with the highest efficiency and processing capacity among existing separation processes. As a result, it is natural that hydrogen isotopes included in water such as radioactive wastewater, tritiated water or deuterium water may be separated with high processing capacity and efficiency by using (n)F-MCM-41_NCs according to the present disclosure.

(42) Hereinabove, in this specification, although the present disclosure has been described with reference to the embodiments illustrated in the drawings so as to be easily understood and reproduced by those skilled in the art, it is just illustrative, and it will be understood to those skilled in the art that various modifications and other equivalent embodiments can be made from the embodiments of the present disclosure. Therefore, the scope of the present disclosure should be determined by the appended claims.