Hydrophilic Superparamagnetic Nanoparticles: Synthesis

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Ind. Eng. Chem. Res. 2011, 50, 382–388

Hydrophilic Superparamagnetic Nanoparticles: Synthesis, Characterization, and Performance in Forward Osmosis Processes Qingchun Ge,† Jincai Su,† Tai-Shung Chung,*,† and Gary Amy‡ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, Singapore 117576, and Water Desalination & Reuse (WDR) Center, King Abdullah UniVersity of Science and Technology, Saudi Arabia 23955-6900

Forward osmosis (FO) is an emerging technology for desalination and water reuse. However, a big challenge is finding suitable draw solutes. In this work, we have synthesized magnetic nanoparticles (MNPs), investigated their potential as draw solutes in FO systems, and explored their recovery and reusability. A series of poly(ethylene glycol)diacid-coated (PEG-(COOH)2-coated) MNPs with different size distributions have been synthesized by means of the thermal decomposition method. The physical properties and chemical compositions of the resultant MNPs are fully characterized. Transmission electron microscopy (TEM) analyses show the characteristics of spherical morphology with narrow size distribution, and a mean size from 4.2 to 17.5 nm depending on the ratio of the two starting materials of PEG-(COOH)2 to ferric triacetylacetonate (Fe(acac)3). Vibrating sample magnetometer analyses confirm the magnetic behavior of the PEG-(COOH)2 MNPs. The PEG-(COOH)2 layer on the MNPs ascertained from Fourier transform infrared (FTIR) analysis and thermogravimetric analysis demonstrates a hydrophilic surface composition. The as-prepared PEG-(COOH)2 MNPs exhibit good dispersibility and generate high osmotic pressures in aqueous solutions. Water fluxes of >10 L m-2 h-1 are achieved across Hydration Technologies Inc. flat sheet membranes when deionized water is used as the feed solution. The MNPs can be easily recovered from draw solutions by applying a magnetic field. The MNPs remain active after nine runs of recycle but with a total water flux decrease of 21% due to slight aggregation. Results have demonstrated that using PEG-(COOH)2 MNPs as draw solutes is feasible in the FO process. 1. Introduction Freshwater shortage has caused serious worldwide problems and is expected to grow worse in the future.1,2 To alleviate water scarcity and ensure potable water quality, many technologies have been developed for water reuse and seawater desalination.3-8 Among these technologies, reverse osmosis (RO) is the most mature. RO, however, is relatively expensive and energy intensive because of its pressure-driven characteristics. Recovery limitation and environmental impact further disfavor its application in water treatment.9 Therefore, other technologies with less expense and higher recovery are needed for water reuse. Forward osmosis (FO) may be viewed as a promising alternative to current technologies. Unlike pressure-driven membrane processes, FO is an osmosis-driven membrane process.6 The prominent advantages of this process over others include high feedwater recovery, brine discharge minimization, and relatively low energy requirement and cost.6 Although the FO technology has shown great potential in desalination and water reuse, some problems still exist. The development of FO membranes with high flux, the minimization of internal concentration polarization (ICP), and the exploration of easily separable draw solutes are still constraints. Considered as a very important factor for advancing the FO process, a variety of draw solutes have been developed over the past 30 years and sugars or salt solutions were extensively used as draw solutions.6,10,11 Such draw solutions have provided high osmotic pressures and created good water fluxes. Despite the progress, the application of the reported draw solution systems is limited by high expense * To whom correspondence should be addressed. Telephone: +6565166645. Fax: +65-67791936. E-mail: [email protected]. † National University of Singapore. ‡ King Abdullah University of Science and Technology.

and energy waste during the draw solute recycle. In some cases, the draw solutes are even toxic12 or consumed in the process of separation,13 which makes it difficult to obtain pure potable water. Therefore, exploration of nontoxic draw solutions that can generate high osmotic pressures and be easily separated from the product water is crucial. Superparamagnetic nanoparticles are materials consisting of a magnetic core and polymer shell. The magnetic core contains small magnetic nanoparticles (MNPs) and can be separated from mixtures under an external magnetic field. The polymer shell surrounds the magnetic core to prevent particle aggregation and also enables surface modification. MNPs can be purposely designed and synthesized for specific applications. Such characteristics make MNPs an appropriate option for draw solutes. Although MNPs have been extensively used in catalysis,14 magnetic resonance imaging,15 and biomedical areas,16,17 they have not been applied in FO processes for water treatment before the pioneering trial in our group.18 The results from the application of MNPs in water treatment are quite encouraging. In this work, a series of superparamagnetic nanoparticles using PEG-(COOH)2 with different molecular weights as surface ligands have been synthesized by the thermal decomposition approach via a facile one-pot reaction. PEG-(COOH)2 is a combination of a chemically inert hydrophilic polyether chain with two reactive carboxylic acid moieties. The carboxylic group can readily react with the iron metal ion and attach onto the Fe3O4 particle surface. The PEG-(COOH)2 ligand has the characteristics of nontoxicity, good water solubility, and high boiling point. All these advantages make it a competitive candidate in the design of hydrophilic MNPs for FO purposes. The as-prepared PEG-(COOH)2-coated MNPs have proven to satisfy the criteria of being an ideal draw solute10 and are separable with recyclable characteristics. Systematic perfor-

10.1021/ie101013w  2011 American Chemical Society Published on Web 11/22/2010

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

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mance experiments were carried out through the FO process by using the resultant MNPs as draw solutes. The MNPs can be easily separated from the dilute draw solution via an external magnetic field. The recovered MNPs can be redispersed in water to reproduce the draw solution and used again. These characteristics favor MNPs as draw solutes in water reuse via FO processes. 2. Materials and Methods 2.1. Starting Materials for Synthesizing MNPs. Unless otherwise stated, all chemicals are commercial reagents and used as supplied. All reactions were carried out under an argon atmosphere with the use of standard Schlenk techniques. Triethylene glycol (TEG, 98%), Fe(acac)3 (99%), poly(ethylene glycol)diacid (PEG-(COOH)2 250, Mn ) 250, 99%), and PEG-(COOH)2 600 (Mn ) 600, 99%) were obtained from Aldrich and used as-received. PEG-(COOH)2 4000 (Mn ) 4000) was prepared using the method reported by Feng et al.19 Ethyl acetate (99%) was obtained from Acros Organics. Deionized (DI) water from a Milli Q (Millipore) system was used in all experiments. 2.2. Experimental Section: Synthesis of PEG-(COOH)2Coated MNPs (PEG-(COOH)2 MNPs). MNPs capped with PEG-(COOH)2 were synthesized by a thermal decomposition method established elsewhere.20,21 (PEG-(COOH)2 250) MNPs (11.7 nm, 1:2) were obtained as follows. A mixture of 30 mL of TEG and 1.0 mmol of PEG-(COOH)2 (Mn ) 250, 0.25 g) was added to a three-necked flask containing 2.0 mmol of Fe(acac)3 (0.71 g). The mixture was first purged with argon for 30 min to remove oxygen. Then it was heated to 280 °C and held at 280 °C for 30 min under vigorous stirring to form a black solution. After the reaction solution was cooled to room temperature, 100 mL of ethyl acetate was added to precipitate the resultant MNPs. The precipitated MNPs were separated by centrifugation and washed three times with water/ethyl acetate (v/v 1:3). Subsequently, MNPs were dispersed in water. The resultant aqueous solution was neutralized with dilute NaOH (0.1 M) until pH 7 and dialyzed (MWCO 2000) for 2 days to remove the species with lower molecular weights. The last step was to add ethyl acetate to the colloid solution after dialysis and reprecipitate the MNPs. The resultant solid particles had a mean diameter of 11.7 nm and were ready for characterization and performance tests. In a similar way, [PEG-(COOH)2 600] MNPs (1:2) and [PEG-(COOH)2 4000] MNPs (1:2) with mean diameters of 13.5 and 17.5 nm, respectively, were obtained. [PEG-(COOH)2 600] MNPs with smaller diameters, that is, 5.5 and 4.2 nm, were obtained by increasing the molar ratio of PEG-(COOH)2 600 to Fe(acac)3 to 2:1 and 4:1, respectively. 2.3. Characterization of MNPs. The size distribution and morphology of the resultant MNPs were characterized with a nanoparticle size analyzer (Nano ZS, ZEN3600) and transmission electron microscope (TEM, JEOL TEM-2010) at an accelerating voltage of 200 kV, respectively. The magnetic behavior of the particles was evaluated through a vibrating sample magnetometer (VSM, LakeShore 450-10) from -17 to 17 kOe at room temperature. The saturation magnetization values were normalized to the mass of nanoparticles to yield the specific magnetization, M (emu g-1). Fourier transform infrared measurements were performed using a Perkin-Elmer FT-IR Spectrometer 2000 to determine the functional groups of MNPs. The scan range was from 4000 to 400 cm-1. The samples were dried overnight under vacuum at 80 °C before any measurements. The spectra were obtained by the solid KBr

Figure 1. Graphical scheme of magnetic separator.

Scheme 1. Synthesis of PEG-(COOH)2 MNPs

method. The weight loss of MNPs (10 mg) was characterized by thermogravimetric analysis (TGA) with a TGA 2050 themogravimetric analyzer (TA Instruments, New Castle, DE) during thermal oxidation. The measurement was conducted under air from 50 to 750 °C at a heating rate of 10 °C/min. The osmotic pressures of draw solutions prepared from MNPs were measured using a model 3250 osmometer (Advanced Instruments, Inc.). This device uses the technique of freezing-point depression to measure osmolality, which is the total solute concentration of an aqueous solution. The freezing-point technique follows the principles of “colligative property” that the freezing point is depressed and the osmotic pressure is increased when a solute is dissolved in a pure solvent.22,23 2.4. FO Process. FO experiments were conducted through a laboratory-scale circulating filtration unit as depicted elsewhere.24 The water flux and the reverse draw solute leakage of flat sheet membranes from Hydration Technologies Inc. (HTI, Albany, OR; Batch No. 060327-3) were measured. The crossflow permeation cell was of a plate and frame design with a rectangular channel (8.0 cm in length, 1.5 cm in width, and 0.25 cm in height) on each side of the membrane. During FO tests, the feed and draw solutions flowed cocurrently through respective cell channels at the same velocity of 6.4 cm s-1. The

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Figure 2. TEM images and size distribution of (a) [PEG-(COOH)2 250] MNPs (1:2), (b) [PEG-(COOH)2 600] MNPs (1:2), (c) [PEG-(COOH)2 4000] MNPs (1:2), (d) [PEG-(COOH)2 600] MNPs (2:1), (e) [PEG-(COOH)2 600] MNPs (4:1), and (f) [PEG-(COOH)2 600] MNPs (1:2) (after first run of recycle).

temperatures of the feed and draw solutions were maintained at 23 ( 1 °C. Draw solutions were prepared from MNPs based on different surface ligands such as PEG-(COOH)2 250, PEG-(COOH)2 600, and PEG-(COOH)2 4000. DI water was used as the feed solution in all FO experiments. The water permeation flux, Jv (L m-2 h-1, abbreviated as LMH), is calculated from the volume change of the feed solution using eq 1: Jv ) ∆V/(A∆t)

(1)

where ∆V (L) is the volume change of the feed solution over a predetermined time ∆t (h) and A is the effective membrane surface area (m2). The concentration of surface ligands surrounding each magnetic nanoparticle, C (mol L-1), is calculated using the formula below: C)

(F - Fw)w Mw

(2)

where F is the density of the solution containing MNPs (g L-1) while the water density, Fw, is assumed to be 1000 g L-1, Mw is the molecular weight of the surface ligands (g mol-1), and w is the weight fraction of the surface ligands on MNPs. The value of w was obtained by TGA via weight percentage change after thermal oxidation of MNPs at high temperatures. Here we assume no additional volume change in the draw solution after adding MNPs, except their own individual volumes. 2.5. Recycle of Draw Solution. After a FO test, the draw solution containing MNPs is diluted. To recover the MNPs from the solution, MNPs were separated from water through a Frantz Canister Separator (Model L-1CN, S.G. Frantz Co. Inc., Trenton, NJ). This recycle process is graphically represented in Figure 1. The iron gauze is packed into a plastic column and magnetized under a magnetic field (i.e., between parallel bars of induced magnets). When the solution containing MNPs is introduced into the plastic column, the magnetized iron gauze tends to attract MNPs and allow water to pass through. Upon removal of the magnetic field, the iron gauze demagnetizes and


Figure 3. Mass magnetization M as a function of applied external field H measured for (9, black) pure Fe3O4, (b, red) [PEG-(COOH)2 250] MNPs (1:2), (2, blue) [PEG-(COOH)2 600] MNPs (1:2), and (1, teal) [PEG-(COOH)2 4000] MNPs (1:2). Inset: Mass magnetization of [PEG-(COOH)2 600] MNPs with different ratios of PEG-(COOH)2 600 and Fe3O4 (9, magenta) 1:2, (b, green) 2:1, and (2, purple) 4:1.

the trapped MNPs can be washed away by fresh DI water. In this way, a fresh draw solution is prepared with the recycled MNPs. The magnetic separator used in our study was operated at 110 V and 1.7 A at the most. It should be noted that this separator is not specially designed for our study and only a very small fraction of this energy is consumed by the separation of MNPs. The strength of the magnetic field is between 0 and 15500 G depending on the working power. It took