Research paperCTAB modified large surface area nanoporous geopolymer with high adsorption capacity for copper ion removal
Introduction
Geopolymers, also called polysialates, are a class of amorphous aluminosilicate materials formed near ambient temperature. Chemically, geopolymers consist of cross-lined units of AlO4− and SiO4 tetrahedra, where charge-balancing cations are provided by alkali metal cations such as Li+, Na+, K+, and Cs+. It has already been demonstrated in the previous studies that geopolymers could be prepared from metakaolin or wastes, such as slag, fly ash and tailing (Duxson et al., 2007; Goretta et al., 2007). This makes geopolymer an important material as these precursors are highly cost effective and can be easily obtained in bulk. Geopolymers have applications such as fire- and heat-resistant coatings and adhesives, medicinal applications, high-temperature ceramics, new binders for fire-resistant fiber composites, toxic and radioactive waste encapsulation and as cementing components to make concrete (Davidovits, 2015). In addition to these applications, geopolymers are being used as a substrate in adsorption process due of their low cost, excellent mechanical and physical properties, low energy consumption and green synthesis process. In regards to adsorption, they have been mainly explored for removing heavy metals from wastewater (Cheng et al., 2012; Medpelli et al., 2015). The removal of heavy metals from the environment is necessary because of their extreme toxicity and tendency for bioaccumulation in the food chain even in relatively low concentration (Bansal et al., 2009). Although removal of heavy metals from waste water can be done by using a large number of techniques, such as, ion exchange (Kang et al., 2004), reverse osmosis (Mohsen-Nia et al., 2007), chemical coagulation (Chang and Wang, 2007), chemical precipitations (Ku and Jung, 2001) and solvent extraction (Černá, 1995), adsorption however remains a very attractive method (Bhattacharyya and Gupta, 2008; Jusoh et al., 2007; Li et al., 2010) due to its simplistic methodology, effectiveness, and low cost for heavy metal wastewater treatment. Adsorption also offers flexibility in design and operation and in many cases produces high-quality treated effluent. Another advantage of adsorption process is that it is sometimes reversible, and hence adsorbents can be regenerated by suitable desorption process. Methods other than adsorption may possibly be very effective but have high capital and operation costs and the problem of residual disposal.
Among the heavy metals, copper metal is very hazardous to human health and the environment. Copper is generated in metal cleaning and plating baths, paper, paperboard mills, wood-pulp production, tire manufacture, and fertilizer industries and is accumulating in their waste streams (Wang et al., 2007). Adsorption of Cu2 + has short and long term effects on human health. Many effective methods are used for copper removal from wastewater including chemical precipitation, ion exchange, reverse osmosis, electrochemical treatment, evaporative recovery, and adsorption. Cheng et al. (2012) recently demonstrated the adsorption capacity of metakaolin based geopolymer toward Pb(II), Cu(II), Cr(III) and Cd(II). Kara et al. (2017) reported also Metakaolin based geopolymer as an effective adsorbent for adsorption of zinc(II) and nickel(II) ions from aqueous solutions. The removal of copper ions has been achieved by fly ash based geopolymer (Al-Harahsheh et al., 2015; Wang et al., 2007). The effects of zeolitic tuffs implemented as a filler on the mechanical performance and adsorption capacity of geopolymer products has been investigated by Yousef et al. (2009) for methylene blue and Cu2 +. Porous geopolymers have been used to adsorb copper ions with good adsorption capacity (Andrejkovičová et al., 2016; Cheng et al., 2012; Ge et al., 2015; López et al., 2014; Tang et al., 2015). Similarly, there are many studies devoted to the use of geopolymer for copper ion removal (Duan et al., 2016; El-Eswed et al., 2012; Ge et al., 2017; Taskin et al., 2016; Yunsheng et al., 2007; Zhang et al., 2008). In addition to copper, geopolymers have also been used to adsorb of Ca2 + and Mg2 + for softening the water (Naghsh and Shams, 2017).
In spite of many reports in the literature there are very few reports where the use of high surface area nanoporous geopolymer has been used for removal of metals (Medpelli et al., 2015). Hence, in the present work we have demonstrated the synthesis of metakaolin-based high surface area nanoporous geopolymers using CTAB as the surfactant. We further demonstrate its efficiency for the adsorption of copper ions. Nanoporous geopolymer will be important as its high surface area will lead to high adsorption capacity toward metal ions. We have focused on the removal of copper ions as they are one the major pollutants present in the surface and ground water coming from industries.
Section snippets
Synthesis
14 g of KOH is dissolved in 32 mL of distilled water in a polypropylene beaker. To the aqueous solution of KOH, 15.43 g of fumed silica is slowly added and then stirred with mechanical stirrer for 30 min at 800 rpm. To the obtained clear solution, 10 g of metakaolin is slowly added and dissolved properly. Metakaolin is prepared by calcining kaolinite at 750 °C for 10 h to cause dehydration, and thus increasing reaction activity. Resin thus formed is cured in oven at 60o C for 24 h. A light brown colored
Characteristics of geopolymer
X-ray diffraction patterns of kaolinite, metakaolin (obtained by calcining kaolinite at 750 °C for 10 h), geopolymer synthesized without using CTAB and geopolymer using CTAB is shown in Fig. 1. Kaolin exists in three main crystalline phases - kaolinite, illite, and quartz. The three major peaks at 12, 25 and 27° in Fig. 1(a), matched exactly with the kaolinite phase, whereas, the small peaks at around 20° and between 30 and 40° were mixture of all the three phases of kaolin (Konan et al., 2009).
Conclusions
In conclusion, highly nanoporous geopolymer using simple CTAB modification has been synthesized. Addition of CTAB significantly changes the porosity and surface area. Surface area increases from 137 to 216 m2/g and the pore volume increases from 0.19 to 0.22 cm3/g. SEM of geopolymer with and without CTAB distinctly shows the change in the morphology from plate like nonporous structure to spherical particle like porous morphology. High porosity of CTAB modified geopolymer is harnessed by using it
Acknowledgements
The financial support (seed funding) of Ahmedabad University is gratefully acknowledged by AS in undertaking this work (AU/SG/SEAS/2016-17/03). AS would like to acknowledge Prof. Sudhanshu Sharma (IIT Gandhinagar) for providing instrumentation facility.
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2022, Journal of Environmental ManagementCitation Excerpt :In the quest to improve their textural properties and adsorption efficiencies, several adjuvants from mineral resources, agricultural wastes, and chemical additives have been reported. For example, the incorporation of Cetyl trimethylammonium bromide (CTAB) during synthesis improved the surface area, porosity, and copper adsorption capacity of a metakaolin-based geopolymer (Singhal et al., 2017). The use of bivalent metallic ions (Ni2+, Zn2+, and Ba2+) for chemical modification of slag-based geopolymer adsorbents have also been reported (Sarkar et al., 2018; Hanna et al., 2016).