Sorption properties of hybrid organic-silica material towards 137 Cs,90 Sr and 90 Y radionuclides

Abstract

Hybrid organic-silica material modified by amino groups has been investigated as adsorbent of radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ from aqueous solutions. The sorption ability of the organic-silica material in the pH range of 2–9 with thermostating is studied by static sorption method. Analysis of sorption kinetic curves of radionuclides for aminoxerogel has shown that sorption equilibrium is established in 3–3.5 h. It is shown that the mechanisms of radionuclides sorption on the organic-silica material are combined by the complex formation and ion-exchange mechanism. The amino containing organic-silica material can be recommended for the cleaning of surfaces and purification of water solutions contaminated with ${\hspace{0.17em}}^{90}\mathit{Sr}$ and ${\hspace{0.17em}}^{90}Y$ .

Introduction

One of the most important challenges of the modern chemistry is search of agents for purification of potable and industrial water from radionuclide and toxic metals (Lujaniene et al., 2008; Myasoedova, 2005; Tarasevich and Dabrowski, 1999; Wu et al., 2008). The most appropriate ways to solve this problem is use of sorbents (Krasnopyorova et al., 2007, 2008, 2013; Mokhodoeva et al., 2011). Hybrid organic-silica materials which contain organic moieties are used in adsorption, catalysis, and chromatography (Boury, 2002; Laughlin et al., 2000; Morosanova et al., 1998; Shilova, 2002). The main idea of creating such materials is to combine their inorganic and organic parts, which allows to control their mechanical, thermal and structural stability as well as to provide specific material properties through chemical properties of organic modifiers (Shilova, 2002). The sol–gel technology of hybrid organic-silica materials production is referred as nanotechnology due to the possibility of regulating the material structure at the molecular level. The widespread sol–gel method allows to produce the materials with predictable properties and different morphology. These are organic mineral monoliths and films, sorbents, catalysts, fibers, ceramics, chemo- and biosensors and others (Boury, 2002; Laughlin et al., 2000; Morosanova et al., 1998; Shilova, 2002; Zub and Parish, 1996). In addition, among the materials obtained by the sol–gel method are widely adopted aminocontaining organic-silica materials (xerogels) (Dash et al., 2008; e.g. hybrid silica-organic; Tkachenko et al., 2013). The goal of this study is to investigate the sorption and concentration of radionuclides ( ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ ) on the surface of the organic-silica hybrid material modified by amino groups. Moreover, the appropriate conditions of the sorption are determined.

Experimental

Synthesis of hybrid organic-silica material

Amino containing hybrid silica-organic material was obtained by sol–gel method according to the procedure reported in (Morosanova et al., 1998). To the mixture of equal volumes of γ-aminopropyltriethoxysilane (C₂H₅O)₃Si(CH₂)₃NH₂ (APTES) and tetraethyl orthosilicate Si(OC₂H₅)₄ (TEOS) (both Merck, 98%), ethanol ((Dubovyazivka Distillery, Ukraine, 96%) and aqueous solution of gelation catalyst (ammonium hexafluorosilicate, Reachim, Ukraine) were added. The ratio of volumes of the components in the mixture silane:ethanol:water was 2:5:2. The mixture was stirred until the gel formation and then was dried in the microwave at 70 W (10 min) and 150 W (5 min) (Figure 1). The details of structure and properties of hybrid organic-silica were described earlier in Baraban et al. (2012). The specific surface area of the material was 70 ± 5 m²/g, while specific concentration of amino groups was 4.02 mmol/g was calculated from the reagent mixing conditions. This value was in agreement with the one which is obtained by the maximal absorption of H⁺ ions (4.04 ± 0.02 mmol/g).

Figure 1.

Amino containing xerogel formation scheme.

Sorption of radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ on amino containing xerogel

Sorption ability of amino containing xerogel to absorb radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ was studied by the static sorption methods in the range of pH 2–9 under thermostatic conditions at 298 K. The solutions of radionuclides of ${\hspace{0.17em}}^{90}\mathit{Sr}$ ( $1.8·{10}^7$ Bq/dm³), ${\hspace{0.17em}}^{90}Y$ ( $1.8·{10}^7$ Bq/dm³) and ${\hspace{0.17em}}^{137}\mathit{Cs}$ ( $8.6·{10}^7$ Bq/dm³) and amino containing xerogel were used in the experiments; 0.05 g of the sorbent was contacted with 20 ml of appropriate solution until the equilibrium was achieved. Activity of the model solutions was determined according to the standard procedure in active dry residue of 0.2 ml solution aliquot before and after sorption equilibrium achievement. Radioactivity of the dry residue was measured by $\alpha -\beta -$ automat NRR-610 “Tesla.” The radioactivity measurements were carried out in the equal geometrical conditions. Statistic error of the experiments did not exceed 5%. In order to determine the sorption equilibrium in 1, 2, 4, 6, 24 h, an aliquot of the solution was taken and radioactivity was measured. Stability of two successively taken samples showed that the sorption equilibrium was attained in the system. Time that corresponds to the solution activity equilibrium was determined graphically according to the dependence of solution activity on time of the solution contact with the sorbent. The equilibrium was achieved in 3–3.5 h for the studying sorbent.

Results and discussion

Radioactivity of the model solutions before and after establishment of the thermodynamic equilibrium was determined by the standard method, i.e. by estimating the activity of the solid rest of an aliquot of the solution. Quantitative characteristics of the sorbent interaction with radionuclides were revealed with help of the radionuclide distribution coefficients (K_d, cm³/g) and sorption coefficients (K_s, %) (Krasnopyorova et al., 2007, 2008) (Table 1).

Table 1.

Distribution coefficients K_d and sorption coefficients K_s values for amino containing xerogel are different for various radionuclides ${\hspace{0.17em}}^{90}Y,{\hspace{0.17em}}^{90}\mathit{Sr}$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ .

	${\hspace{0.17em}}^{90}Y$		${\hspace{0.17em}}^{90}\mathit{Sr}$		${\hspace{0.17em}}^{137}\mathit{Cs}$
pH	Kd	Ks	Kd	Ks	Kd	Kd
2	169	16	27	8	17	6
4	353	42	296	28	31	10
6	532	75	362	56	101	12
9	611	89	449	67	135	17

Analysis of the kinetic curves of the radionuclides sorption on the xerogel at pH 7 showed that the sorption equilibrium in the system radionuclide sorbent for ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ was attained in 3–3.5 h (Figure 2).

Figure 2.

Kinetic curves of radionuclides sorption on amino containing xerogel at pH 7.

The mechanism of the sorption process is rather complicated. Diffusion in the solution makes an impact on initial speed of the sorption process. The literature analysis shows that the main contribution to sorption of radionuclides on the hybrid material surface gives complexation (Myasoedova and Nikashina, 2006). Different functional groups of available sorbent lead to slower diffusion on its surface, and the absorption of radionuclides is achieved due to the formation of complexes on the surface (1) and ions exchange (2).

$$\overline{Q}+M^{2+}\to \overline{{\mathit{QM}}^{2+}}$$

(1)

$$\begin{array}{c}\multicolumn{1}{c}{(\equiv \mathit{Si}-\mathit{OH}{)}_2+M^{2+}\to \equiv \mathit{Si}-O-M-O-\mathit{Si}\equiv +2H^{+}}\end{array}$$

(2)

The pH of an environment significantly affects the quantitative sorption characteristics. Sorption of radionuclides is minimum at pH = 2. With increasing pH, the sorption increases and reaches the maximum at pH 8–9 (Figure 3).

Figure 3.

Sorption coefficients K_s values for amino containing xerogel are different for various radionuclides ${\hspace{0.17em}}^{90}Y,{\hspace{0.17em}}^{90}\mathit{Sr}$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ .

Such dependence of the sorption from pH is typical for weakly acidic ion-exchangers, which is modified xerogel, due to the presence of silanol groups. It should be noted that the sorption ${\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{90}\mathit{Sr}$ for all pH values is much higher than ${\hspace{0.17em}}^{137}\mathit{Cs}$ sorption. Especially this difference increases with increasing pH, which indicates a significant effect of pH on the sorption of some cations. The complex nature of sorption can be explained by change in the condition of sorption centers on the hybrid material surfaces depending on pH of the aqueous phase. The obtained results are quite consistent with the results of Boury (2002), Dash et al. (2008) and Wu et al. (2008), which based on the analysis of effective diffusion coefficients for the exchange of monovalent and divalent cations. It is assumed that the exchange rate depends on the sorbent selectivity, energy and cation hydration radius. The data suggest that the dominant mechanism of radionuclides sorption on the surface of organo-silica material is the ions exchange.

Conclusion

The distribution of radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr},{\hspace{0.17em}}^{90}Y$ and ${\hspace{0.17em}}^{137}\mathit{Cs}$ in the system “aqueous radionuclide solution – amino containing xerogel” has been studied, depending on the environment pH. The thermodynamic equilibrium of the system at different pH is attained in 3–3.5 h. The high sorption ability of aminoxerogel has been revealed for radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr}\hspace{1em}{\text{\hspace{1em}and\hspace{1em}}}^{90}Y$ in the pH range of 7–9. It is shown that the sorption of radionuclides occurs by the combined mechanism: ions exchange and complexes formation. The investigated organic-silica material can be recommended for water purification from radionuclides ${\hspace{0.17em}}^{90}\mathit{Sr}$ and ${\hspace{0.17em}}^{90}Y$ .