Functional, mesoporous, superparamagnetic colloidal

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Jul 25, 2012 - that anhydride groups reacting with primary amines and. EDTA are attached to the mesoporous silica wall. The synthesis of thiol functionalized ...
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Functional, mesoporous, superparamagnetic colloidal sorbents for efficient removal of toxic metalsw Downloaded by Indian Association for the Cultivation of Science on 20 August 2012 Published on 25 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CC33893A

Arjyabaran Sinha and Nikhil R. Jana* Received 31st May 2012, Accepted 25th July 2012 DOI: 10.1039/c2cc33893a c-Fe2O3 incorporated mesoporous silica particles of 50–100 nm size have been synthesized which are functionalized with chelating agents of metal ions. These particles are water dispersible but aggregate in response to the external magnetic field and have been used for high performance and selective removal of Cd, Pb, Hg and As. Mesoporous silicas are widely used for drinking water treatment due to their high surface area and easier separation options for removal of various pollutants.1 Incorporation of magnetic nanoparticles into porous materials provides high surface area in combination with magnetic property.2 These magnetic mesoporous silicas (MMS) have high performance in separation of toxic material and purification of contaminated water,2 as compared to magnetic nanoparticles3 or mesoporous silica.1 However, MMS used in previous studies are generally large in size, typically >0.5 mm diameter with few exceptions. The large particle size inhibits their stable colloidal dispersion that decreases their capturing ability as sorbents. In addition removal of different toxic materials requires specific functionalization of MMS.4 Although there are many reports on functionalization of magnetic nanoparticles by silica coating,3 there are limited reports on functional MMS based water purification,4 particularly using 50–100 nm size dispersible and functional MMS.4b,c Thus functional and dispersible MMS are highly desirable which can efficiently capture specific toxic material from an aqueous or a biological environment and are easily separable by the magnetic field. Here we report colloidally dispersed MMS of B50–100 nm size which are functionalized with ethylenediaminetetraacetic acid (EDTA) or thiols. These MMS efficiently capture toxic metals from water in their colloidal dispersion and in response to the external magnetic field the MMS aggregate, offering easier removal of captured metals. Scheme 1 demonstrates the synthesis approach for MMS. Superparamagnetic g-Fe2O3 of 8–12 nm size is synthesized first via a high temperature thermal degradation method5a and then

Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata 700032, India. E-mail: [email protected]; Fax: +91-33-24732805; Tel: +91-33-24734971 w Electronic supplementary information (ESI) available: Experimental procedure for the synthesis of different functional MMS, details of characterization and procedure for toxic metal removal. See DOI: 10.1039/c2cc33893a

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Scheme 1 Synthesis strategy for MMS–EDTA and MMS–SH.

converted into water soluble and silica coated g-Fe2O3 with a silica shell thickness of 2–5 nm.5b Next, MMS are synthesized via silane hydrolysis in ethanol–water medium in the presence of silica coated g-Fe2O3.2 The silica shell present on the surface of g-Fe2O3 ensures that nucleation of silica is initiated at the surface of g-Fe2O3 and thus offers very effective encapsulation of every g-Fe2O3 by mesoporous silica. In the synthesis of EDTA functionalized MMS (MMS–EDTA), primary amine terminated MMS are reacted with dianhydride of EDTA so that anhydride groups reacting with primary amines and EDTA are attached to the mesoporous silica wall. The synthesis of thiol functionalized MMS (MMS–SH) has been achieved by hydrolyzing 1 : 6 molar ratio mixtures of mercaptopropyltrimethoxysilane and tetraethoxysilane and in the reaction process thiols are incorporated into the porous silica wall. The synthesis conditions are optimized so that uniform and isolated MMS of B50–100 nm size are produced, each having magnetic particles inside (Fig. 1a and b and Fig. S1, ESIw). In a typical procedure 45 mL of aqueous silica coated g-Fe2O3 is mixed with 5 mL of CTAB (0.15 M) and 1.5 mL of ammonia solution. The reaction is initiated by adding 10 mL of ethanolic solution of TEOS under stirring, followed by other silanes whatever necessary. This journal is

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Fig. 1 TEM image of MMS–EDTA (a) and MMS–SH (b), field dependent magnetization curves of MMS–EDTA at 2 K and 300 K (c) and temperature dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves (d).

Isolated MMS are reacted with the DMF solution of EDTAanhydride for the preparation of MMS–EDTA. Magnetic measurement study shows that MMS are superparamagnetic in nature (Fig. 1c and d). The hysteresis curve for MMS shows that they are superparamagnetic at room temperature and exhibit hysteresis at 2 K with a coercivity of 250 Oe. The saturation magnetization value is B1 emu g1 at 300 K and is B2 emu g1 at 2 K. These values are significantly lower than those of pure nanocrystalline g-Fe2O3 which is attributed to the presence of a large percentage of silica in MMS. The zero field cooled (ZFC) and field cooled (FC) magnetization curves of MMS show a distinct blocking temperature of 25 K (measured at 500 Oe), which is also a characteristic of superparamagnetism. The porous structure of MMS has been investigated by nitrogen adsorption isotherm study, showing the commonly observed type IV adsorption– desorption isotherm with pore size distribution between 2 and 3.5 nm. The BET surface area of these MMS varied between 200 and 250 m2 g1 (Fig. 2a and b). Functionalization of MMS with EDTA is motivated from strong complexing property of EDTA with various metal ions.3d,6 The presence of EDTA in MMS–EDTA has been confirmed by FTIR study (Fig. 2c) and the percentage of EDTA has been estimated by a complexometric titration method (ESIw).6 FTIR data show that EDTA functionalization of MMS leads to the shift of N–H deformation peaks from 1472 cm1 to 1408 cm1 and a relative increase in the intensity of the 1633 cm1 peak (either due to N–H and/or carbonyl groups), signifying the reaction of primary amine groups of MMS with anhydride groups of EDTA. Titration data show that the amount of EDTA is B0.8  103 mole g1, suggesting that MMS–EDTA prepared by our approach has a significant percentage of EDTA with complexing ability to different metal ions. Thiol functionalization of MMS has been inspired by their efficient complexation with Hg and As ions.4a The amount of thiol present in MMS–SH has been quantified using This journal is

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Fig. 2 (a) Representative N2 adsorption–desorption isotherm of MMS–SH and (b) pore size distribution curve with a BET surface area of B230 m2 g1, (c) FTIR spectra of MMS before and after EDTA functionalization showing the peak shift from 1472 cm1 to 1408 cm1 and relative increase in intensity of the 1633 cm1 peak and (d) UV-visible spectrum of MMS–SH before and after reaction with 5,5 0 -dithiobis-2-nitrobenzoic acid (DTNB) with the characteristic peak at 410 nm.

5,5 0 -dithiobis-2-nitrobenzoic acid (DTNB) as the spectrophotometric reagent7 (Fig. 2d and ESIw). DTNB reacts with thiol forming a characteristic absorbance peak at 410 nm and this absorbance has been used for quantitative estimation of thiols. The estimated thiols present in MMS–SH are B2.4  104 mole g1, suggesting that sufficient thiols are present for binding with Hg and As. The advantage of the presented synthetic method is that the MMS–EDTA and MMS–SH can be prepared as solid powders on the gram scale and this synthesis can be further extended to larger scale synthesis (Fig. 3 and ESIw). The most important feature of these materials is that they are highly dispersible in water and in different aqueous buffer solutions of pH 5–9 (Fig. S2, ESIw) but aggregate within few minutes in response to the external magnetic field (Fig. 3). Thus high water solubility would offer easier access to MMS bound EDTA and thiols for complexation/binding with water soluble metal ions and magnetic response can be used for easier separation of metal bound MMS. Application potential of these materials towards water purification has been tested via removal efficiency of different toxic metals from water. The metals that we have tested for separation include Cd, Pb, Hg and As. These metals are

Fig. 3 Digital image of solid MMS powder, aqueous colloidal dispersions and magnetic response of colloidal dispersions.

Chem. Commun., 2012, 48, 9272–9274

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Fig. 4 MMS–EDTA and MMS–SH based toxic metal removal from waste water down to drinking water standard. MMS–EDTA is used for Pb and Cd removal and MMS–SH is used for As and Hg removal.

considered as extremely toxic and their permissible limits in drinking are typically o0.1 mg L1.3d,8 The removal of Cd and Pb was performed using MMS–EDTA and the removal of Hg and As was performed using MMS–SH. The purification procedure involves mixing of solid MMS–EDTA or MMS–SH with contaminated water having respective metal salt solution followed by shaking of the mixture for a few hours to overnight. Next, metal bound dispersed MMS–EDTA/MMS–SH were collected by using a laboratory based bar magnet. The supernatant was collected as purified water and the presence of metals was quantified by inductively coupled plasma–atomic emission spectrometry (ICP-AES) measurement. Fig. 4 summarizes the result showing that removal efficiencies of all the tested metals are very high and metals can be removed beyond the permissible limit allowed for drinking water. Tested metal concentration ranges from 0.05 mg L1 to 10 mg L1 which are usually found in contaminated water and in most cases the removal efficiencies are >90%. However, the removal efficiency of As is relatively low and varies between 60 and 95% depending on the concentration of As present in water and the solution pH. This result is consistent with earlier observation and is due to moderately stable complexation of thiols with As(III).9 We have also estimated the iron present in purified water by ICP-AES and found that it always remains below 0.01 mg L1, meaning that all the MMS are removed from water. Control removal experiments have been performed using MMS without any EDTA or thiol functionalization. It is generally observed that MMS without functionalization have poor separation efficiency (typically o10–50%) in a similar concentration range which arises mainly due to non-specific binding and weaker chelating properties of amines present on the MMS surface.10 This result suggests that the observed high separation efficiencies for MMS–EDTA and MMS–SH are specific effects due to functionalization. The presented MMS offer high colloidal dispersity and high separation performance as compared to commercially available functional magnetic beads4e (Table S1, ESIw). This is because commercial beads are larger in size (1–5 mm as compared to our size of 50–100 nm), having lower surface area (B100 m2 g1 as compared to our surface area of 200–250 m2 g1) and lower functional group

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density (B50–65 mM g1 as compared to 0.24–0.8 mM g1 in our case). We found that typically one gram of functional MMS can purify one liter of contaminated water with the option that MMS can be re-used (Fig. S3, ESIw). In conclusion we have synthesized 50–100 nm size magnetic mesoporous silica particles which are functionalized with EDTA and thiols. These materials produce stable colloidal solution in water but rapidly aggregate in response to the external magnetic field. The colloidal form of MMS and EDTA–thiol functionality offers efficient capture of toxic metal ions from an aqueous environment and their magnetic response offers easier separation of captured metal ions. Application potential of these materials has been demonstrated by purifying water contaminated with Cd, Pb, Hg and As up to the label of drinking water quality. This work is supported by DST and CSIR, Government of India. AS thanks CSIR, India, for research fellowship.

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