Low-sintering-temperature borosilicate glass to immobilize silver-coated silica-gel with different iodine loadings

https://doi.org/10.1016/j.jhazmat.2020.123588Get rights and content

Highlights

  • Borosilicate glass presents a uniform distribution for different elements.

  • Silver-coated silica gel with varies iodine loadings was successfully immobilized.

  • Low sintering temperature (600 ℃) was conducted to immobilize AgIs.

  • Composition design confronted various I loaded silver-coated silica-gel.

  • Leaching rate of I in glass cured AgIs is 2.51 × 10-5 g•cm-2•d-1 in 7th day.

Abstract

To better deal with the radioactive iodine generated during the development of nuclear energy, B2O3, Bi2O3, ZnO, and SiO2 were used to sinter borosilicate glass for the immobilization of iodine. The effect of B2O3 on glass formation was discussed by changing the molar ratio of B2O3 in the matrix. When B2O3 content is 50 mol% and sintering temperature is 600 ℃, the amorphous degree of quaternary glass is the highest. The sintered body with the highest degree of amorphous was selected to study radioactive iodine. Then, the effects of different iodine loading concentrations for sintering borosilicate glass in terms of microstructure and phase change were discussed. With the increase in iodine content on silica-gel, the degree of amorphous of the specimens presented a decreasing trend, and there are obvious SiO2 peaks. When the content was 20 wt.%-30 wt.%, a large number of new phases were generated. When the iodine content is 20 wt.%, in addition to the enrichment of Si and O elements, the elemental distribution for B, Bi, Zn, I, and Ag was even. TEM results also showed that there was a crystalline phase in the sinter.

Introduction

Radioactive iodine, including long-lived radionuclide 129I (half-life 1.7 × 107a), is an important kind of gas wastes produced in the process of nuclear fuel reprocessing (Lebel et al., 2016). To mitigate the emission of iodine into the environment, adsorbents are commonly used for solid phase adsorption (Jie et al., 2017; Kim et al., 2017; Wang and Chu, 2018; Zhang et al., 2017). In the choice of adsorbent, silver-based materials are widely used for their excellent adsorption of radioactive iodine (Kim et al., 2017), moreover, porous sorbents is seriously considered for the capture and storage of radioactive iodine compounds (Huve et al., 2018). Thus, silver-coated silica-gel was focused for its silver-based component and porous structure as solid sorbent (Riley et al., 2016). However, the application of silver-coated silica-gel is hard to storage due to the instable property of silica-gel, i.e. water sucking. Therefore, it is necessary to explore the immobilizing technology of radioactive iodine adsorbents, such as iodine loaded silver-coated silica-gel (AgIs).

Due to the mature process and good structural inclusiveness, glass is taken as potential material for curing iodine loaded silver - coated silica-gel (Reynes et al., 2001). However, a traditional glass formula design and the sintering process have a low inclusive rate of iodine, which may be attributed to the volatilization of iodine at high temperature in the melting process (Han et al., 2014; Joffrey et al., 2018; Lobanov et al., 2018; Mao et al., 2016; Nur et al., 2018). Considering that AgI will be dissociated at approximately 558 ℃, a low-temperature sintering method (below 650 ℃) should be adopted for the curing treatment (Garino et al., 2011).

Glass composition is essential for the sintering temperature and for the curing of iodine adsorbents. Oxides like ZnO, PbO, or Bi2O3 along with B2O3 or SiO2 are normally taken into account. Considering both Bi and Pb are glass intermediates during glass formation (Chaudhry, 2013; Dyamant et al., 2005; Mustafa et al., 2017; Singh and Karmakar, 2011), Bi2O3 has been chosen to replace PbO (Ling et al., 2010) since traditional leaded glass would harm the environment. In addition, B2O3 and SiO2 are the primary raw materials for forming the glass network, while ZnO is a glass modifier. Moreover, B2O3 can also reduce sintering temperature, which is consistent with the original intention of low-sintering-temperature glass.

In this work, iodine loaded silver-coated silica-gel was immobilized at a relatively low temperature via borosilicate glass. ZnO, Bi2O3, B2O3 and SiO2 were used as raw material of glass, and B2O3 was taken as a variable factor to obtain an optimized proportion. Moreover, the quaternary glass was sintered at varied temperatures by traditional sintering method to select an optimal glass process in a method with the simplest operation and lowest cost. Based on the optimized proportion of B2O3 and process, different amounts of iodine on the silver-coated silica-gel were considered as varied adsorption capacity, while the total iodine content in each sample was fixed by corresponding component adjustment. Meanwhile, the phase, structure, iodine distribution and basic performance of the iodine-immobilized matrix were studied under different adsorption efficiencies of silica-gel with the aim to provide meaningful data for the waste form development community.

Section snippets

Experimental design

B2O3 not only participates in the formation of glass network, but also plays a cooling role in glass curing, so the content of B2O3 was chosen as the variable to explore the curing matrix. Table 1 shows the formula design of the four-element pure glass in this study. Combined with the formulation design and sintering concept of pure quaternary glass, the iodine capacity in the silver-coated silica-gel varied from 10 ∼ 50 wt.%. The iodine content was fixed as 8 wt.% in each sample by changing

Phase transition

Fig. 2(a), Fig. 3(a), and Fig. 4(a) show the XRD results of the samples sintered at 500 ℃, 550 ℃, and 600 ℃ with B2O3 contents from 0 to 50 mol%. The phase of the sintered body at the three temperatures show a decreasing trend with the increase of B2O3 content, the amorphous index also shows an upward trend. The B2O3-free samples sintered at different temperatures are composed of Bi12ZnO20, ZnO, and SiO2 phases, Bi12ZnO20 is the main object phase. When the content of B2O3 increased to 10 mol%,

Glass-AgIs phase transition

Fig. 7 shows the XRD patterns of sintered samples with 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, and 50 wt.% iodine loadings in silver-coated silica-gel. The sintering temperature was 600 ℃ (based on the amorphous index analysis before). It’s mainly SiO2 and ZnB4O7 with a small amount of ZnO in the sintered matrix at a low iodine loading amount (10wt.% to 20 wt.%). As the iodine loading increased to 30 wt.%, the main phase of the sintered sample changed to the ZnB4O7 phase, and the phase of the

Conclusions

In this study, borosilicate glass was successfully sintered by raw materials Bi2O3, B2O3, SiO2, and ZnO at a low sintering temperature (500 ℃, 550 ℃, 600 ℃). The content of B2O3 is the variable, and the amorphous degree of sintered body generally increases with the increase of temperature, and with the increase of boron content, the maximum amorphous index is 0.693. On this basis, the following conclusions were obtained by studying the glass solidified silver-coated silica-gel. When the iodine

Declaration of Competing Interest

This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal’s policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.

CRediT authorship contribution statement

Yi Liu: Conceptualization, Formal analysis, Writing - original draft. Bingsheng Li: Conceptualization, Methodology, Writing - review & editing. Xiaoyan Shu: Investigation, Writing - review & editing. Zhentao Zhang: Methodology, Writing - review & editing. Guilin Wei: Data curation. Yi Liu: Data curation. Shunzhang Chen: Data curation. Yi Xie: Investigation. Xirui Lu: Conceptualization, Investigation, Supervision, Data curation.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The authors appreciate the Project of State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology (grant number 18FKSY0213) and Targeted poverty alleviation of undergraduate innovation on fund project by southwest university of science and technology (No. JZ19-021).

References (37)

Cited by (21)

View all citing articles on Scopus
View full text