CTAB modified large surface area nanoporous geopolymer with high adsorption capacity for copper ion removal

Highlights

• High surface area geopolymer using CTAB

• Surface area and pore volume is 216 m²/g and 0.22 cm³/g.

• High adsorption capacity; 1.65 meq/g

Abstract

Nanoporous geopolymer was synthesized with and without using Cetyl trimethylammonium bromide (CTAB) by condensing the mixture of metakaolin and alkali solution at a fixed ratio at room temperature. The surface area of CTAB-geopolymer was found to be more (216 m²/g) as compared to without CTAB added geopolymer (137 m²/g). The experimental results verified that the geopolymer could adsorb copper ions completely at lower concentrations and partially at higher concentrations. Pseudo second order model fits well at all the concentration from 55 to 1700 ppm as the values of the correlation coefficient lies between 0.96 and 0.99. Intraparticle diffusion model at the concentration 55 ppm explains that there is only boundary layer diffusion (instantaneous) and after this step, all the Cu ions are exchanged by the nanoporous geopolymer. At 120 ppm, intraparticle diffusion model shows multilinearity. Different adsorption models - Langmuir, Freundlich and Tempkin were also tested to evaluate the most appropriate model and it was found that adsorption follows Langmuir model. The adsorption capacity and pseudo second order rate constant is estimated to be 1.65 meq/g which is significantly higher than the fly ash based nonporous geopolymer.

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 Cu^2 + 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 Cu^2 +. 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 Ca^2 + and Mg^2 + 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.