EDTA-functionalized KCC-1 and KIT-6 mesoporous silicas for Nd3+ ion recovery from aqueous solutions

https://doi.org/10.1016/j.jiec.2018.06.031Get rights and content

Highlights

  • Ethylenediaminetetraacetic acid (EDTA) grafted KIT-6 and KCC-1 was prepared.

  • KIT-6-EDTA showed high adsorption capacities for Nd3+ ions in water.

  • Adsorption equilibrium, isotherm, and kinetics were investigated.

  • KIT-6-EDTA reused for 5 cycles without deterioration in adsorption capacities.

Abstract

Ethylenediaminetetraacetic acid (EDTA)-functionalized KIT-6 and KCC-1 mesoporous silicas were prepared via post-synthesis grafting and examined for their ability to promote the recovery of rare earth metal ions such as Nd3+ from an aqueous medium. The obtained adsorption isotherms were fitted to the Langmuir model, which gave a maximum adsorption of Nd3+ ions of 109.8 and 96.5 mg/g for KIT-6-EDTA and KCC-1-EDTA, respectively, at 25 °C and pH 6. The adsorption kinetic profile of KIT-6 was faster than KCC-1. KIT-6 was also proved to be more stable against desorption under acidic regeneration conditions.

Introduction

The limited mineral deposits and increasing demand for rare earth elements (REEs) from high technology industries over the last decade have made it increasingly important to recover REEs from various sources including industrial wastes [1]. Among these, neodymium has been widely used to produce alloys, permanent magnets, electronic components and others, and its recovery has been receiving a special attention [2].

Several different methods can be applied for the recovery of REEs including solvent extraction [3], [4], ion exchange [5], adsorption [6], [7], [8], biosorption [9], chemical precipitation [10], and hydrometallurgy [11]. Among these, adsorption process offers potential advantages over the others such as simple operation, compact equipment, low solvent utilization, negligible waste accumulation, and low capital cost, which make it useful for the recovery of REEs at low concentrations [12]. For this purpose, various adsorbent materials have been reported so far including porous silica [13], [14], [15], carbons [15], metal oxides [16], metal–organic frameworks [17], [18], polymers [19], and biomaterials [20], [21], [22]. Metal oxides and metal–organic frameworks are generally comprised of micropores with potential diffusion limitations and often exhibited weak chemical stability during the recycle operation [23]. Carbon materials, on the other hand, possess higher stability but only limited surface functionalization schemes of the organic moieties that are necessary to achieve high adsorption of the REE ions are available. Consequently, mesoporous silicas have been preferred as an adsorbent material due to their large surface area, mesopores with a narrow pore size distribution, and the presence of high surface hydroxyl groups that enabled easy immobilization of organic functionalities on their surface. For the organic functional groups that effectively bind to the Lewis acidic REE ions, Lewis bases containing N, P, or COO, such as ethylenediaminetetraacetic acid (EDTA) [24], N-octyl-N-tolyl-1,10-phenanthroline-2-carboxamide [14], and benzene triamido-tetraphosphonic acid [25] have been reported.

For Nd3+ recovery, Awual et al. used an organic-functionalized silica monolith, which showed a high adsorption capacity of 162 mg/g with good reusability; however, the preparation of the functionalized silica monolith required a long sequence of synthetic steps [14]. Naser et al. reported a silica-based urea–formaldehyde composite material for the sorption of REE ions, which exhibited a 5 mg/g adsorption capacity for Nd3+ [26]. Melnyk et al. developed SiP-grafted silica materials and employed them for Nd3+ separation, attaining a 45 mg/g adsorption capacity [27]. Ogata et al. reported adsorption behavior of REEs on silica gel particles modified with diglycol amic acid, which exhibited a 33 mg/g adsorption capacity for Nd3+ [28]. Polida Legaria et al. developed an adsorbent bearing iminodiacetic acid incorporated over Fe3O4/SiO2 core-shell spherical particles, which captured 33 mg/g of Nd3+ [29]. Most of these silica-based adsorbents showed relatively low Nd3+ adsorption capacities, probably because of the active sites having a relatively low affinity for Nd3+ on the silica surface.

Many previous studies on metal ions adsorption in liquid phase [30] established that an appropriate porosity of the adsorbents must be realized so that (1) a sufficient amount of functional organic groups can be accommodated into the porous network and (2) the target ions can easily diffuse into the matrix. In addition, the stability of the support materials and the incorporated organic functional groups during the adsorption and recycle steps should be sufficiently high to prevent the loss of active sites during these operations. Among the mesoporous silicas, KIT-6 has been reported to offer significantly enhanced mass transfer rates in liquid phase chemical processes due to its 3-D arrangement of large mesopores in the size range of 5–8 nm [31]; its superiority over SBA-15 with a pseudo 1-D pore arrangement has been reported, where the adsorption is expected to take place inside the mesopores [32]. More recently, KCC-1 with a unique fibrous morphology was prepared via reverse micelle templating and its superior performance in many mass transfer-controlled processes was reported [33], [34]. The functionalized sites in KCC-1 are expected to be located on the outside of its fibrous surface, which can reduce the mass transfer resistance during ion transfer. However, these two very promising support materials with effective adsorption sites have not yet been evaluated as an adsorbent host material for REEs.

Herein, two representative mesoporous silica, KIT-6 and KCC-1 were functionalized with EDTA groups via a simple post-synthesis grafting procedure (Fig. 1) and employed them in the REE ions separation using Nd3+ as a model species. The kinetics and adsorption isotherms of the functionalized materials were systematically studied and compared with other adsorbent materials reported in the literature. The robustness of the adsorbents during recycle was also examined for the stability of the functionalized materials under acidic conditions.

Section snippets

Materials

Tetraethyl orthosilicate (TEOS), cetylpyridinium bromide, Pluronic P123 (EO/20:PO/70:EO/20), n-butanol, cyclohexane, 1-pentane, ethanol, methanol, toluene, acetic acid, 3-aminopropyltriethoxysilane (APTES), ethylenediaminetetraacetic dianhydride (EDTA anhydride), and neodymium(III) nitrate hexahydrate were purchased from Sigma–Aldrich and used as received.

Synthesis of KIT-6 and KCC-1 mesoporous silica

KIT-6 was synthesized using a procedure reported earlier [31]. In a typical synthesis, 4.6 g of P123 was dissolved in 167 g of deionized water

Materials characterization

Fig. S1a shows the low angle XRD patterns of KIT-6, KIT-6-APTES, and KIT-6-EDTA. The three major peaks of the XRD pattern for KIT-6 could be indexed to the (211), (220) and (322) reflections at 1.08, 1.25 and 2.03 2θ values, respectively, which are in line with the highly ordered 3D cubic structure with Ia3d pore symmetry [31]. After surface modification, the peak intensities of the functionalized materials (KIT-6-APTES and KIT-EDTA) showed slight decreases compared with KIT-6, the peak

Conclusions

EDTA-functionalized KIT-6 and KCC-1 were synthesized via post-synthesis grafting and exhibited high adsorption capacities for Nd3+ ions. The adsorption equilibrium data fitted well with the Langmuir model, which revealed a 109.8 and 96.5 mg/g Nd3+ adsorption capacity for KIT-6-EDTA and KCC-1-EDTA, respectively, at the optimal pH of 6. The adsorption equilibrium of Nd3+ was reached within 60 min of contact time and well fitted to a second-order kinetic model. Overall, KIT-6-EDTA and KCC-1-EDTA

Acknowledgments

This work was financially supported by the National Strategic Project Carbon Upcycling of the National Research Foundation of Korea (NRF), which was funded by the Ministry of Science and ICT (MSIT), Ministry of Environment (ME), and Ministry of Trade, Industry and Energy (MOTIE). Grant number: NRF-2017M3D8A2086050.

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