Abatement of aqueous anionic contaminants by thermo-responsive nanocomposites: (Poly(N-isopropylacrylamide))-co-silylanized Magnesium/Aluminun layered double hydroxides

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Abstract

A series of novel thermo-responsive composite sorbents, were prepared by free-radical co-polymerization of N-isopropylacrylamide (NIPAm) and the silylanized Mg/Al layered double hydroxides (SiLDHs), named as PNIPAm-co-SiLDHs. For keeping the high affinity of Mg/Al layered double hydroxides towards anions, the layered structure of LDHs was assumed to be reserved in PNIPAm-co-SiLDHs by the silanization of the wet LDH plates as evidenced by the X-ray powder diffraction. The sorption capacity of PNIPAm-co-SiLDH (13.5 mg/g) for Orange-II from water was found to be seven times higher than that of PNIPAm (2.0 mg/g), and the sorption capacities of arsenate onto PNIPAm-co-SiLDH are also greater than that onto PNIPAm, for both As(III) and As(V). These sorption results suggest that reserved LDH structure played a significant role in enhancing the sorption capacities. NO3 intercalated LDHs composite showed the stronger sorption capacity for Orange-II than that of CO32−. After sorption, the PNIPAm-co-SiLDH may be removed from water because of its gel-like nature, and may be easily regenerated contributing to the accelerated desorption of anionic contaminants from PNIPAm-co-SiLDHs by the unique phase-transfer feature through slightly heating (to 40 °C). These recyclable and regeneratable properties of thermo-responsive nanocomposites facilitate its potential application in the in-situ remediation of organic and inorganic anions from contaminated water.

Introduction

Layered double hydroxides (LDHs), also referred to as hydrotalcite-like compounds, have received considerable attention in aqueous environment remediation due to their special layered structure and high affinities to anionic contaminants [1], [2], [3]. A generic formula for LDHs is: [M2+1−xM3+x(OH)2][An]x/n·zH2O, where M2+ and M3+ are divalent and trivalent cations consisting of the hydrotalcite-like layers; An is a non-framework inorganic or organic anion for charge compensating [1]. LDHs can effectively remove the ionic chemicals, such as lead [4], radiocobalt [5], uranium(VI) [6], arsenate [7], as well as organic anions [8], dyes [9], [10] by ion-exchange, chelation and adsorption. However, the application of LDHs in wastewater treatment is restricted by the difficulty of recycling of the LDHs from water [1]. For improving the recycle properties of LDHs, magnetic composites prepared by hybridizing magnetic cores such as Fe3O4 [11], [12], magnesium ferrite [13] and cobalt ferrite [14] were developed. These magnetic composites showed good recycling performance, whereas there are difficulties in thoroughly dispersion of granules in water. Therefore, the design of the unique composites based on LDHs that could be effectively recycled and easily regenerated is still crucial for the application of LDHs in waste water treatment.

LDHs have shown a good flexibility to hybrid with polymers and show the enhancement of mechanical or thermal stability properties to polymeric materials [12], [15], [16]. In these studies, the layered structures of LDHs are normally destroyed or exfoliated [17], [18], [19], [20], [21] and the naturally chemical and physical properties of LDHs crystals were ignored. However, keeping the layered structure of LDHs is worthwhile to the functional application of hybrid materials, since the physical and chemical properties of LDHs can greatly impact on the sorption capability of LDH/polymer composites for the abatement of environmental contaminants. Therefore, how to protect the layered structure of LDH in the polymeric reaction is meaningful to improve the function of LDH/polymer composites.

Poly(N-isopropylacrylamide) (PNIPAm) The equilibrium sorption capacities of Orange-II onto thermo-responsive polymer [22], [23], [24] and it has been widely applied in drug control and release [25], [26], sensors [27], actuators [28], catalysts [29], smart hydrogels [30], smart hybrid hydrogel especially with clays[31], [32], [33], [34], [35], etc. PNIPAm can show a fast and sharp coil-globule phase transition with the change of the ambient temperature [36], [37]. We thus, supposed that covalently incorporation of PNIPAm into LDH could produce a new thermo-responsive nanocomposite. Placing this composite in water, it would spread out like a sponge and promote the sorption due to its dispersion into LDHs. And this composite could be easily removed from water because of its polymer-like nature. The thermo-responsive feature would be utilized to improve the desorption efficiency of the LDH materials. Taking advantages of the temperature-induced aggregation and de-aggregation behavior like a sponge, the regeneration process for the LDH/polymer composites after sorption of the aqueous contaminants would be more easily and simply achieved.

In this study, a series of new thermo-responsive nanocomposites were prepared by a free-radical co-polymerization of NIPAm and the silylanized Mg/Al LDH with various mass ratios. The silanization process was induced by the hydrolysis of silane coupling agent (γ-Methacryloxypropyl trimethoxy silane, MPTS) on the surface of the wet LDH plates in order to protect the layered structure of LDH in co-polymerization reaction. Two precursors, NO3 and CO32− intercalated LDHs were used to reveal the influences of interlayer anions of LDH on the surface characteristics and the sorption properties of the nanocomposites. The microstructure of nanocomposites were characterized by X-ray powder diffraction, scanning electron microscope and Fourier transform infrared spectroscopy. p-(2-Hydroxy-1-naphthylazo) benzenesulfonic acid sodium salt (Orange-II) and arsenate anions (As(III) or As(V)) were selected as the target contaminants to investigate the sorption capacity of the nanocomposite. The regeneration experiment of the nanocomposite was conducted basing on its thermo-responsive properties. As expected, the prepared nanocomposites (PNIPAm-co-SiLDHs) with a gel-like feature can be easily picked out from the wastewater pools. And the used material can be regenerated by slightly heating to above 40 °C. This thermo-responsive material has the potential application in the in-situ remediation of anions polluted water.

Section snippets

Materials

The chemicals: Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, NaHCO3, NaOH, ethanol, 2,2′-Azo-bis-iso-butyronitrile (AIBN) and N,N′-Methylene bisacrylamide (MBA) were all purchased from Sinopharm Chemical Reagent Co., Ltd. γ-Methacryloxypropyl trimethoxy silane (MPTS, the silane coupling reagent) was purchased from NanJing Pinning Coupling Agent Co., Ltd. All chemicals were used as received. The N-isopropyl acrylamide (NIPAm) used for polymerization and p-(2-Hydroxy-1-naphthylazo) benzenesulfonic acid

Evidence from FT-IR for the synthetic steps of PNIPAm-co-SiLDH

The Fourier transform infrared spectroscopy (FT-IR) spectra of samples formed at every step of the synthetic route (Scheme 1a and b) are shown in Fig. 1. In Fig. 1a and b, the bridge band in ∼3450 cm−1 is the stretching vibration of –OH groups for CO3–Mg/Al LDH or NO3–Mg/Al LDH. The band observed at ∼1635 cm−1 is the bending vibration of the interlayer water. The strong and sharp band at ∼1380 (∼1384) cm−1 is attributed to the CO32− (NO3) located in interlayer space, and the band at ∼660 cm−1 is

Conclusions

The novel thermoresponsive nanocomposites PNIPAm-co-SiLDHs were prepared from NIPAm monomer and the silanized layered double hydroxides (SiLDH) by free-radical polymerization. The sorption capacities of P/SN-2 for Orange-II, As(III) and As(V) from water were all greater than that of PNIPAm. NO3 intercalated LDHs composite showed the stronger sorption capability for Orange-II than that of CO32− which due to the larger exposed external surface of the NO3 intercalated LDH nanocomposite. After

Acknowledgments

This project is supported by National Nature Science Foundation of China (No. 20907029), the Natural Science Fund Projects of Shanghai Municipal Science and Technology Commission (No. 14ZR1415600), the Innovative Project of Shanghai Municipal Education Commission (No. 10YZ07), and the Key Subject of Shanghai Municipality (No. S30109).

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