Controlled preparation of cerium oxide loaded slag-based geopolymer microspheres (CeO2@SGMs) for the adsorptive removal and solidification of F− from acidic waste-water
Graphical abstract
Introduction
Geopolymers (GPs) are inorganic polymers with three-dimensional network gel structure initially proposed by Davidovits in 1978 (Davidovits, 1989). Accredited to their high mechanical strength (Dong et al., 2020; Wang et al., 2017a), high and low temperature stability (Wang et al., 2017b; Fu et al., 2019) and vacuum stability (Wang et al., 2016, 2017c), they have been widely applied in construction and building materials. In addition, due to their rapid development, GPs have found widespread applications in the field of water treatment attributed to their characteristic pore structure and elemental composition. In this regard, Na+ and K+ covered by GP tetrahedron has the ability to ion-exchange with other cations in wastewater (Wang et al., 2019a). Furthermore, the alkali-activator is generally excessively used during the preparation of GPs, and is widely used for the remediation of heavy metals (Yu et al., 2019; Tang et al., 2018; Luukkonen et al., 2016) and solidification of radioactive elements (Jia et al., 2020; He et al., 2020; Fu et al., 2020; He et al., 2019).
Although GPs have earned widespread applications with promising results in the field of heavy metal ions adsorption, still some anions like F−, PO43- and IO3− offer great resistance to their effective remediation. In addition to this, the rapid development of fluorine based chemical industry has resulted in a concomitantly increased release of F− in the wastewater crossing its permissible limits set by World Health Organization (WHO) regulations i.e. 0.5–1.5 mg/L in water (Jia et al., 2015; Wang et al., 2013; Grabow et al., 2001; Zhang et al., 2017a). Fluorine though is an essential trace element in human body, however its excessive amount causes dental fluorosis or crippling fluorosis (Jia et al., 2015; Wang et al., 2013). Therefore, various strategies and great amount of research has been reported for the effective and fast removal of fluorine content from drinking water recently.
The major types of F− purification technology include chemical precipitation (Huang et al., 2017; Turner et al., 2005), ion-exchange (Vaaramaa and Lehto, 2003), membrane separation (Zhang et al., 2017b) and adsorption (Zhang et al., 2017a). Chemical precipitation approach is simple in operation with greater industrial feasibility, however it produces secondary pollution and hence has been strictly criticized. Similarly, ion-exchange strategy is simple, but the exchange capacity is limited and regeneration processing is complicated involving complex and convoluted steps. Membrane separation though, requires low energy for operation, has high efficiency, and is conveniently operated, but the process suffers from high cost. Contrary to these strategies, adsorption is deemed superior accredited to its fast and simplified operation, high adsorption capacity, and low cost of production. The key factors in the adsorption approach is to design an adsorbent with high adsorption capacity and selectivity, high mechanical strength and low cost. For the remediation of F− from wastewater, a variety of adsorbents have been reported including single metal oxide, polymetallic oxides, natural mineral and composite materials like magnesium (Maliyekkal et al., 2010; Jin et al., 2015), yttrium (Takenouchi et al., 2017), alumina (Tian et al., 2017), iron (Zhang et al., 2017a; Kumar et al., 2009), magnetic Fe3O4/γ-MnO2 (Zhao et al., 2018), Fe-Al-Ce trimetal hydrous oxide (Wu et al., 2013), nature diatomite (Xu et al., 2015), graphene (Wang et al., 2015) and SA/CMC-Ca-Al (Wu et al., 2016). In addition, some novel anion-exchange materials have also been reported, such as molecular organic frameworks (MOFs) (Sheng et al., 2019, 2017; Zhu et al., 2017) and cationic polymeric network (Li et al., 2018). Although these materials exhibited good adsorption performance for F− removal, but most of these are nano-powder or block particles, possess poor resistance to acidic and alkaline media, and have high preparation costs, which greatly hinder their large-scale industrial production and column-based applications. Therefore, the development of microsphere-based adsorbent with simplified preparation, cost effectiveness and chemical and mechanical stability for the remediation of F− from wastewater is greatly needed.
GPs have the fast solidification characteristic under certain temperature (Wang et al., 2017b, a) and this feature was utilized in this study to prepare the slag-based geopolymer microspheres (SGMs) through the dispersion-suspension-solidification strategy. This approach is simplified and cost-effective and awarded the resulting SGMs with good sphericity and uniform particle size (75−300 μm). The fabricated SGMs were then dispersed in cerium solution where cerium hydroxide was generated on the surface of SGMs using geopolymer reaction under excess of alkali finally resulting in CeO2@SGMs. Attributed to the excellent corrosion resistance of CeO2 against acidic-alkaline media and high adsorption performance for F− (Wang et al., 2019b), the chemical stability and F− adsorption capacity of CeO2@SGMs were greatly improved. CeO2@SGMs could be directly used in column based experimental set up to achieve continuous treatment of F− containing waste-water, and hence can promote the large-scale application of SGMs for wastewater treatment.
Section snippets
Raw materials
In order to prepare CeO2@SGMs, the slag having chemical composition shown in Table 1 was obtained from Beihai Chengde company. The industrial-grade liquid water glass was purchased from Guangxi Chunxu chemical company having solid content 33.70 % (wt.%) and n(SiO2)/n(Na2O) = 3.31 M ratio which was adjusted by the addition of NaOH. CeCl3·7H2O, H2O2, F− solution (NaF) and solution of various competitive anion (NaNO3, Na3PO4, NaCl and Na2SO4) were purchased from Guangdong Guanghua Co., Ltd.,
Optimal experimental parameters for the preparation of SGMs
Fig. 2 shows the optical and SEM images of SGMs prepared by different water glass modulus (molar ratio: SiO2/Na2O). Fig. 2(A and A1) show good sphericity of the fabricated SGMs with a non-uniform particle size and some visible adhesion at a water glass contents of 1.5 M. The SGMs prepared by 1.7 M water glass show better sphericity and much uniform particle size as visible from Fig. 2(B and B1). With further increasing the number of water glass modulus, the alkalinity of the geopolymer slurry
Conclusions
In summary, we prepared slag-based geopolymer microspheres (SGMs) with identical PSD (75−300 μm) and good sphericity, and were then modified with CeCl3·7H2O solution to obtain CeO2@SGMs. Characterization of SGMs and CeO2@SGMs were performed by optical images, SEM/EDS, PSD, BET, XRD and TG/DSC which concluded the loading of Ce on the SGMs while 0.02CeO2@SGMs was confirmed as the best adsorbent. ICP-AE and pH values before and after adsorption of F− solution showed that the adsorption effect and
Credit author statement
The experiments were designed by Huiye Lei and Kaituo Wang. Most of the experiments were performed by Huiye Lei and only a small part of the experimental test were done by Fan Chen and Kaituo Wang. Yaseen Muhammad helped revise the paper and analyzed the experimental data. Feng Gao, Yuezhou Wei and Toyohisa Fujita were responsible for reviewing and editing. Kaituo Wang wrote the paper.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
This work was financially supported by the Guangxi Natural Science Fund (Grant: 2018GXNSFBA281064 and 2019GXNSFAA245015), Dean/Open Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Science and Technology Major Project of Guangxi Province (AA18118052) and National Natural Science Foundation of China (Grants: 51772055 and 11675102).
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