Synthesis of geopolymer-supported zeolites via robust one-step method and their adsorption potential
Graphical abstract
Introduction
In recent years, heavy metals contamination has become a crucial environmental and safety issue [[1], [2], [3]]. Lead (Pb) is one of the most common and toxic heavy metals (with permissible limit of only 0.015 mg/L in drinking water), which can cause damage to central nervous system, kidney, or heart [4,5]. Adsorption is a widely researched technique for removal of such hazardous contaminants due to high efficiency and protocol simplicity [6,7]. Zeolites, inorganic nano-crystalline minerals with tetrahedral aluminosilicate framework, are efficient adsorbents owing to their high surface area and cation exchange capacity (CEC) [[8], [9], [10]]. Zeolites are normally synthesized under hydrothermal conditions by alkali-activation of alumina and silica source materials [11,12]. However, granular (pulverized) zeolites often require porous supports for their wide industrial application such as zeolitic membranes, which increase the complexity and cost of manufacturing [11,13]. Moreover, granular adsorbents cannot be easily retrieved after use, thus solid bulk-type adsorbents can provide better alternative [1,14].
Geopolymers, another class of inorganic aluminosilicates, consist of amorphous geopolymeric gel, and are often considered analogous to crystalline zeolites due to their similar chemical structure [15,16]. Several studies have reported the detection of zeolitic structures in geopolymeric gels [[17], [18], [19]]. However, in contrast to zeolites, geopolymers usually possess high mechanical strength and durability due to dense mesoporous geopolymeric gel [[20], [21], [22]]. The inherited beneficial properties of geopolymers (i.e., strength and potential of conversion to zeolite) can be utilized to synthesize hybrid geopolymer-zeolite bulk materials [14,22,23]. The amorphous geopolymeric gel will serve as a support for zeolites while higher surface area of zeolites will contribute in increasing the surface area, pore volume, porosity, and CEC of bulk geopolymers. These can be ultimately used for a number of potential applications such as bulk-type adsorbents or self-supporting zeolitic molecular sieves [14,22,24,25]. Moreover, benefits of using industrial by-products (fly ash and slag) for synthesis of these high value materials will be twofold: low production cost and efficient waste management.
Studies reported on the synthesis of geopolymers incorporating zeolites are limited [23,26]. Several factors such as mineralogical properties of starting materials, amount and type of alkali activator, and synthesis conditions affect the zeolite type and yield in geopolymeric gel [25,27,28]. In our previous works [14,22], the hybrid geopolymer-zeolite bulk materials (hereafter referred to as geopolymer-supported zeolites) were successfully synthesized and showed high potential for removal of cesium. Synthesis of geopolymer-supported zeolites followed a typical two-step route: (1) synthesis of bulk geopolymers (24 h curing) and (2) nano-structural conversion of some of the geopolymeric gel into zeolites by hydrothermal treatment [22]. The synthesized geopolymer-supported zeolites showed excellent adsorption characteristics for Cs+ (>90% removal) [14].
Here, we report a robust one-step method for synthesis of high strength geopolymer-supported zeolites in an effort to increase the zeolite yield and make it more energy and time efficient. The proposed method aims to increase the zeolite yield by direct nucleation of zeolite crystals from aluminosilicate oligomers as well as conversion of amorphous geopolymeric gel to zeolites, simultaneously. Effects of synthesis parameters such as hydrothermal treatment time, autoclave temperature/pressure, water content, molarity of alkali activator, and mix proportion of starting materials were investigated. Finally, batch adsorption experiments were conducted to assess the adsorption capacity of synthesized geopolymer-supported zeolites for lead (Pb2+).
Section snippets
Design philosophy
Deventer et al. [29] elucidated the reaction process involved in geopolymerization, which includes four main steps i.e., dissolution, oligomerization, polymerization/nucleation, and gelation/crystallization. Depending on the starting mix proportion and curing conditions, aluminosilicate oligomers can either nucleate to nano-crystalline zeolites or polymerize to amorphous geopolymeric gels [29]. However, direct synthesis of zeolites require a higher water/Na2O molar ratio (20–100) compared to a
Compressive strength
3 and 28-day strength of geopolymer-supported zeolites is shown in Fig. 1. Since fly ash and slag were used in these samples, two types of gels i.e., low-Ca (zeolite like N-A-S-H geopolymeric type) and high-Ca (aluminum substituted C-A-S-H type) were coexisting and giving strength to the samples [33,34]. The control sample C showed compressive strength of 12.8 and 14.4 MPa at 3 and 28 days, respectively. The 28-day compressive strength of sample C–W, with higher water (used in autoclave for
Concluding remarks
In the present study, robust one-step method is proposed to synthesize geopolymer-supported zeolites using industrial by-products (fly ash and slag). The effects of different hydrothermal treatment conditions, molarity of NaOH solution, and slag content on the microstructural characteristics of geopolymer-supported zeolites were studied. Followed by detailed characterization, their adsorption potential for Pb2+ removal was explored. The test results clearly enable several conclusions to be
Acknowledgements
This study was supported by a grant from the National Research Foundation of Korea (NRF) (2015R1A2A1A10055694) funded by the Korean government.
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