Elsevier

Chemosphere

Volume 241, February 2020, 125069
Chemosphere

Microwave irradiation assisted sodium hexametaphosphate modification on the alkali-activated blast furnace slag for enhancing immobilization of strontium

https://doi.org/10.1016/j.chemosphere.2019.125069Get rights and content

Highlights

  • A chemical modification-MW irradiation system was set to enhance Sr incorporation.

  • Hexametaphosphate (Na6O18P6) modified BFS was designed to enhance Sr2+ stabilization.

  • Microwave irradiation was employed to strengthen the binder of blast furnace slag.

  • A surface wash-off mechanism and a diffusion-stage process were combinedly determined.

  • This work presents a novel method for improving immobilization and solidification.

Abstract

An inadvertent leakage of 90Sr into the environment can induce an easy accumulation in biosphere and cause a continuous radiation to the surrounding ecosystem. In this study, sodium hexametaphosphate (Na6O18P6) was employed to modify the blast furnace slags (BFS) to enhance the chemical stabilization of Sr2+ ions in the BFS-based cementitious materials. Microwave irradiation (MW) was used to further increase the binder activity of BFS samples and strengthened the mechanical strengths and durability of BFS-based blocks. A combination of experimental factors including the mass ratio of Na6O18P6 to BFS–Sr0.1 of 15%, the ratio of solid to liquid of 1:4 mg/L, the output power of 650 W, and the activation time of 3 min was most conductive to achieving an optimal microwave-irradiation process. Four extraction solutions were sorted by their leaching abilities following as MgSO4 solution > H2SO4 solution > CH3OOH solution > deionized (DI) water based on their leaching results. Compared with microwave irradiation, an addition of Na6O18P6 to BFS samples obtained a better compressive strength for BFS-based blocks. However, a microwave-irradiation treatment was more effective in improving the resistances of blocks to gamma irradiation and thermal-thaw changes. Exposing to gamma irradiation over 6 months and enduring to thermal-thaw tests over 15 cycles, the microwave-treated blocks only lost 3.29% and 2.23% of leaching removal efficiencies in deionized water, respectively. Microwave irradiation increased the mechanical strengths of BFS-based blocks and inhibited leaching of Sr2+ ions from matrices mainly by strengthening hydration reactions and Sr2+ encapsulation.

Introduction

An obviously industrial expansion in nuclear industries has been achieved in decades in China, which causes a large amount of radioactive wastewater from the operation process (Su-Xia et al., 2014; Zhang et al., 2019a). Progress has been slow in dealing with nuclear waste and waste fluids containing radionuclides (Gonzalez et al., 2013; Carey et al., 2018). 90Sr2+ is one of some radioactive metal ions commonly existing in the radioactive wastewater, has approximately one third of the lifetime of 238Pu and relatively a lower density. 90Sr has been widely applied in radioisotope thermoelectric generators (RTGs) as a power source and used in 90Sr/90Y generator for obtaining 90Y for medical applications (Dixon et al., 2016; Chakravarty et al., 2012). Recycling 90Sr ions from radioactive wastes is desirable (Ashworth et al., 2018; Vicente Vilas et al., 2018). However, the recovery of Sr2+ from the low and medium-level radioactive wastes or wastewaters is high-cost and easily produces secondary pollutants, thereby increasing the amount of low-to-medium radioactive waste to be disposed of (Tangestani et al., 2017; Feng et al., 2018; Lupa et al., 2018; Ghalami et al., 2019). 90Sr is also considered to one of the most hazardous radiotoxic ions due to its high solubility. An inadvertent leakage into the environment can induce an easy accumulation in biosphere and cause a continuous radiation to the surrounding ecosystem (Ashworth et al., 2018; Pittet et al., 2019).

A deep disposal of immobilized high-level radioactive wastes for a long-term isolation from the surface geochemical environments has been accepted at large by some main nuclear use countries (Disposal of high-level radioactive wastes in geologic repositories, 1979; Roh et al., 2015; Kurihara et al., 2018; Wu et al., 2018). The development of new cementitious matrices and super-inert wrapping materials has been considered key for avoiding the premature breach of radionuclides from barrier systems, optimizing radioactive management, and upgrade the disposal technique (Ei-Eswed et al., 2017; Shiota et al., 2017; Ashworth et al., 2018; Huang et al., 2019; Zhang et al., 2019a). Furthermore, an unprecedented release of radionuclides with a high radioactivity from Fukushima accident not only makes people understand the terrible nature of radioactive pollution also emphasizes the need to find efficient binder materials in case of unforeseen unclear accidents (Hancock et al., 2019; Nakama et al., 2019; Ohira et al., 2019; Zhang et al., 2019b). Blast furnace slag (BFS) produced from smelting iron procedure has become one of hot-spot materials for the preparations of geopolymer and cementitious materials. Most of silicon-aluminum compounds in BFS are formed by a dramatic cooling process (Colangelo and Cioffi, 2013; Vilaplana et al., 2013; Salihoglu, 2014; Meena et al., 2015). BFS-based geopolymers and cements have been fully characterized and also prepared for the capture of some divalent heavy metal (HM) ions such as Pb2+, Cd2+, and Cu2+, etc. (Koplik et al., 2016; Tae and Morita, 2017; Bae et al., 2018). Smaller leaching values and better mechanic performances are observed in BFS-based solidification compared with the ordinary Portland cement (Ozkan et al., 2007; Zhou et al., 2017; Gijbels et al., 2019). Therefore, BFS has potential for Sr2+ immobilization as a binder precursor.

However, the early-strength formation of BFS-based synthesis is clearly inadequate in rapidly processing for Sr-contaminated wastes. Some vitreous structures retained in BFS further restrict the incorporation of Sr2+ into the newly formed matrix during hydration process (Ozeki et al., 2014; He et al., 2016). To strength the immobilization of Sr in the solidified products, increase the durability of BFS-based cement and geopolymer, and improve irradiation tolerance, a suitable activation method should be sought and explored. Microwave irradiation activates materials by the dipole rotation, molecule vibration, and ionic conduction (Hildago-Oporto et al., 2019; Na et al., 2019; Zhou et al., 2019). It can significantly increase the temperatures inside substances using quite a short time. Microwave irradiation has been used for the preparation of the inorganic thin films, electrode catalyst, and porous carbon materials (Luo et al., 2013; Surendran et al., 2017; Nguyen et al., 2018; Yuan et al., 2018; Lee et al., 2019). Its heating mechanism makes it easier to achieve a more uniform heating and dramatic temperature increases between inorganic molecules and inside the mineral structures at the same time. Severe temperature changes can further enhance the instability of amorphous silicon-aluminum compounds in BFS, which facilitates the dissolution of raw microstructures and induces a generation of nucleation and grain crystallization (Salihoglu, 2014; Goodarzi and Movahedrad, 2017; Wang et al., 2018; Kuo et al., 2019). Hence, the feasibility of microwave irradiation to change the cementitious characteristics of BFS samples is worth exploring. Besides, the potential advantages of microwave irradiation for the inorganic minerals have been rarely explored from the solidification mechanism, which requires a further study.

Strontium apatite (i.e., Sr10(PO4)6(OH)2) is an insoluble substance, which is formed by incorporating Sr2+ ions into apatite like minerals (Rabone and De Leeuw, 2006; Ozeki et al., 2014; Panpisut et al., 2019). Calcium ions can be easily substituted by strontium ions in the structures of apatite minerals due to their similar electron configurations (Li et al., 2007; Fredholm et al., 2012; He et al., 2016). The formation of strontium apatite undoubtedly will enhance the capture of Sr2+ in the matrix and increase the durability of blocks. In this study, sodium hexametaphosphate was added to BFS samples to enhance the chemical stabilization of Sr2+ ions in the BFS-based cementitious materials. Microwave irradiation was used to increase the binder activity of BFS samples. Four relevant factors of microwave irradiation were adjusted and optimized in an orthogonal experiment. The influence of each factor on the immobilization capacity of Sr2+ (wt.%) in the matrix was quantitatively determined based on the significance analysis and marginal means. The leaching values, compressive strengths, gamma irradiation degradation, and thermal durability of microwave-irradiation treated solidified blocks were comprehensively investigated. X-ray diffraction (XRD, PANalytical B.V., Holland) and a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS, Carl Zeiss AG, Germany) were used to further analyze the solidification characteristics of BFS-based cementitious materials and explore the specific immobilization mechanisms of Sr2+ ions. This work will present a feasible method for the modification of potential binder materials and exploration of new materials to achieve a fast immobilization of Sr2+ from the nuclear wastewater.

Section snippets

Materials

Blast furnace slag (BFS) samples were obtained from a steel mill located in Changzhou, Jiangsu, China. The BFS samples were dried at 100 °C for 12 h (h), grinded using a ball-grinding machine for 2 h, and sieved through a 200-mesh sieve. The chemical composition of BFS is listed in Table S1 in the supporting information (SI). Sodium hexametaphosphate (Na6O18P6, Analytical grade) used in the experiments was purchased from Chongqing CHUANDONG chemical Inc., China. The water glass with a modulus

Optimization of microwave irradiation

Microwave irradiation can dramatically increase temperatures inside mineral structures, which will change the amorphous activity of BFS samples and affect the interactions between Na6O18P6 and Sr2+ ions. To achieve a maximum irradiation performance, the influences of some relevant factors are necessary to be investigated. As shown in Table 1, changes in the experimental factors clearly affected the immobilization capacities of Sr2+ ions in the MA-BFS-Sr0.1 based solidified blocks. Among 16

Discussion on activation mechanisms

XRD peak patterns of the mortars and binder matrices with 0.1 M incorporation concentration of Sr2+ ions are shown in Fig. 4. Sr2+ ions were chemically immobilized in the internal matrices of P–BFS–Sr0.1-based blocks mainly in the forms of strontium hydroxide precipitations (Kuenzel et al., 2015; Jang et al., 2017; Vandevenne et al., 2018) and strontium substituted hydroxyapatites. Sr2+ ions reacted with the alkali activators to form some hydroxide precipitations (Eq. (7) and Eq. (8), e.g.,

Conclusion

An excessive incorporation of Sr2+ ions into BFS-based cements or geopolymer would result in a rapid release of Sr2+ and a dramatic reduction of compressive strengths. An appropriate addition of sodium hexametaphosphate to BFS obviously enhanced the chemical stabilization of Sr2+ ions incorporated into the BFS-based cementitious materials through the hydroxyapatite formation and Sr2+ substitutions. Microwave irradiation further increased the mechanical strengths of BFS-based blocks and

Declaration of competing interest

None.

Acknowledgements

The author has received no financial support for the research, authorship, and/or publication of this article.

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