Elsevier

Applied Clay Science

Volume 50, Issue 2, October 2010, Pages 260-265
Applied Clay Science

An ion-imprinted polymer based on palygorskite as a sacrificial support for selective removal of strontium(II)

https://doi.org/10.1016/j.clay.2010.08.007Get rights and content

Abstract

Surface ion-imprinting technique combined with a sacrificial-support process was established to synthesize Sr(II) ion imprinted polymer (S-IIP) palygorskite which acted as the sacrificial support. FT-IR, TEM, nitrogen gas adsorption and laser particle size measurements were employed for the characterization of S-IIP and non-imprinted polymer (NIP). Factors for the adsorption of Sr(II) ions were investigated. Under the optimum condition, adsorption and selective recognition of Sr(II) ions followed the order S-IIP > NIP. The adsorption equilibrium data were fitted by the Langmuir isotherm model and the monolayer adsorption capacity of S-IIP was 45.0 mg g 1 at 298 K, 53.5 mg g 1 at 308 K and 58.5 mg g 1 at 318 K. Adsorption was described by the pseudo-second-order kinetics, and the thermodynamic parameters also indicated that the adsorption process was spontaneous but endothermic. S-IIP was a promising adsorbent for the selective removal of traces of Sr(II) ions. It could be reused four times with only about 5% loss for adsorption and 17% loss for desorption.

Graphical Abstract

► Attapulgite is a hydrated octahedral layered magnesium aluminum silicate mineral which have siloxane groups in the bulk and silanol groups on its surface. It is available to be a support in surface-imprinting technique because of its particular intensity, special structure, stable chemical property and abundant raw materials. ► Then a new approach was used to synthesize a novel ion-imprinted polymer (IIP) based on surface ion-imprinting technique with attapulgite as a sacrificial-support material and chitosan incorporated sol–gel process. During this process, attapulgite as a support material was coated with a pre-polymerization ion-imprinting mixture. In a following step, the attapulgite was dissolved and removed, resulting in hollow spherical beads (S-IIP) with greater surface area and pore volume. Subsequently, the adsorption behaviors of S-IIP and non-imprinted polymer (NIP) for Sr2+ (containing adsorption parameters, isotherms, kinetics, thermodynamics, and selective recognition) were investigated, the structural characteristics and imprinting mechanism of S-IIP were discussed in detail. ► A model for selective removal of target radioelements could be set up according to the results.

Introduction

In the latest decades, the ion-imprinting technique was recognized as a convenient and powerful method for synthesizing an artificial receptor-like recognition polymer by co-polymerizing suitable monomers in the presence of desired template ions. After removing the template ions, imprinted cavities providing tailor-made binding sites for these ions are left on the ion-imprinted polymer (IIP) (Zhang et al., 2007). However, most of these traditional IIP exhibit high selective recognition but poor site accessibility to the target ions, as the template ions and functional groups are totally embedded inside the polymer network and the mass transfer is low (Janiak et al., 2009).

Surface-imprinting technique first proposed by Takagi's group in 1992 utilized an amphiphilic functional monomer forming a stationary complex with the template at the interface of emulsion droplets (Tsukagoshi et al., 1993). Subsequently, various surface-imprinting systems with faster mass transfer kinetics were established, such as emulsion (suspension) polymerization and surface-imprinting process based on support materials like SiO2 (Kodakari et al., 1997), TiO2 (Wu, et al., 2007), α-Al2O3 (Pérez et al., 2001), CdS (Diltemiz et al., 2008), ZnS (Wang et al., 2009a, Wang et al., 2009b), carbon nanotubes (Suzuki et al., 2002), silica gel (Li et al., 2006), and magnetic Fe3O4 (Wang et al., 2009a, Wang et al., 2009b). Palygorskite can be used as a support in the surface-imprinting technique because of its chemical, mechanical and thermal stability, and low cost.

Recently, the design and synthesis of hollow functional materials with surface-imprinting technique and sacrificial-support concept arose intense interest because of their novel applications in advanced separation technologies. Yilmaz et al. (2002) prepared spherical molecular imprinted polymer (MIP) by using common C4-silica particles as a sacrificial support. He et al. (2008) prepared imprinted silica with testosterone as template molecule. The surface-imprinting technique in combination with the sacrificial-support process is a versatile method to prepare the uniformly sized adsorbents, and to improve the kinetics of the adsorption and desorption by increasing the specific surface area, pore volume and active binding sites of the adsorbents.

Strontium-90 (90Sr) usually accumulates in human liver, lung and kidney along with the increase of the food chain and is difficult to remove from body in metabolism. 90Sr has a long half-life time (28 years) and could substitute calcium in the body, increasing the probability of leukemia and other diseases. The main purpose of this work was to present a new approach to synthesize a novel IIP (S-IIP) based on surface ion-imprinting technique in the presence of palygorskite as a sacrificial support (Fig. 1). Palygorskite was coated with a pre-polymerization ion-imprinting mixture. In a following step, palygorskite was dissolved and removed, resulting in hollow spherical beads (S-IIP). Subsequently, the adsorption behaviors of S-IIP for Sr2+ were studied.

Section snippets

Instruments and apparatus

Infrared spectra (4000–400 cm 1) were recorded on a Nicolet NEXUS-470FT-IR apparatus (U.S.A.). Transmission electron microscope (TEM) analysis was performed by using JEM-2010HR at 200 kv. The mean diameter of the particles was measured by BIC-90 laser particle size instrument. The specific surface area and pore volume of adsorbents were measured by nitrogen adsorption based on the Brunauer–Emmett–Teller (BET) model using the single point analysis (Flowsorb II 2300, Micromeritics Instrument

Imprinted polymers

FT-IR spectra of raw materials and polymers were described in Fig. 2. A strong and overlapped band around 3450 cm−1, from stretching vibrations of N–H and O–H in CTS (Fig. 2b), was shifted to 3415 cm−1 and narrowed in S-IIP before palygorskite dissolution (Fig. 2e). Compared with CTS, the absorption band of C–OH decreased from 1090 cm−1 to 1070 cm−1 (Fig. 2e), and its shape became sharp. Moreover, the characteristic feature of δsN–H at 1650 cm−1 and νasC–N at 1380 cm−1 showed no obvious change. All

Conclusions

A new type of Sr(II)-imprinted polymer was prepared by a surface-imprinting technique combined with a sacrificial-support process. S-IIP formed hollow microspheres with enhanced specific surface area and pore volume. Adsorption and selective recognition of Sr2+ followed the order S-IIP > NIP. Sr2+ adsorption onto the S-IIP and NIP fitted to the Langmuir model, indicating monolayer adsorption. ΔHo, ΔGo, and ΔSo showed that the adsorption of Sr2+ ions on S-IIP was endothermic and spontaneous. The

Acknowledgments

We are thankful to the financial support by the National Natural Science Foundation of China (No. 20877036), National Natural Science Foundation of China (No.30970309), and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20093227110015).

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