Electrosteric Stabilization and Its Role in Cooperative

Article pubs.acs.org/Langmuir

Electrosteric Stabilization and Its Role in Cooperative Magnetophoresis of Colloidal Magnetic Nanoparticles Swee Pin Yeap,† Abdul Latif Ahmad,† Boon Seng Ooi,† and JitKang Lim*,†,‡ †

School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang 14300, Malaysia Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

‡

S Supporting Information *

ABSTRACT: A detailed study on the conflicting role that colloid stability plays in magnetophoresis is presented. Magnetic iron oxide particles (MIOPs) that were sterically stabilized via surface modification with poly(sodium 4-styrene sulfonate) of different molecular weights (i.e., 70 and 1000 kDa) were employed as our model system. Both sedimentation kinetics and quartz crystal microbalance with dissipation (QCM-D) measurements suggested that PSS 70 kDa is a better stabilizer as compared to PSS 1000 kDa. This observation is mostly attributed to the bridging flocculation of PSS 1000 kDa decorated MIOPs originated from the extended polymeric conformation layer. Later, a lab-scale high gradient magnetic separation (HGMS) device was designed to study the magnetophoretic collection of MIOPs. Our experimental results revealed that the more colloidally stable the MIOP suspension is, the harder it is to be magnetically isolated by HGMS. At 50 mg/L, naked MIOPs without coating can be easily captured by HGMS at separation efficiency up to 96.9 ± 2.6%. However, the degree of separation dropped quite drastically to 83.1 ± 1.2% and 67.7 ± 4.6%, for MIOPs with PSS 1000k and PSS 70k coating, respectively. This observation clearly implies that polyelectrolyte coating that was usually employed to electrosterically stabilize a colloidal system in turn compromises the magnetic isolation efficiency. By artificially destroying the colloidal stability of the MIOPs with ionic strength increment, the ability for HGMS to recover the most stable suspension (i.e., PSS 70k-coated MIOPs) increased to >86% at 100 mM monovalent ion (Na+) or at 10 mM divalent ion (Ca2+). This observation has verified the conflicting role of colloidal stability in magnetophoretic separation.

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INTRODUCTION Nanomaterials have been rapidly emerging as the fastest growing and most promising candidate for various engineering applications. In this regard, magnetic iron oxide particles (MIOPs) have shown huge potential. In addition to the unique feature of a large surface area to volume ratio1 shared by their relatively small dimension, MIOPs also possess attractive superparamagnetic property.2 In fact, it has been widely recognized that iron-based nanoparticles are the first generation nanomaterial used in nanorelated environmental technologies.3,4 Recently, there is ample experimental evidence showing the feasibility of MIOPs as an excellent nanoagent tailor toward removal of heavy metals,5,6 colorants,7 chlorinated hydrocarbon,8 organic pollutants,9 suspended solids,10,11 as well as aquatic organisms12 from water resources. Usually, the cleanup activities were carried out by first seeding the target pollutants with MIOPs either through electrostatic interaction or a specific binding group, followed by magnetic removal of the pollutantsbound particles. In most cases, the iron oxide nanoparticles were surface grafted with polymers or surfactants so as to develop specific functionalities12 and induce better particle delivery in soil medium.8 Magnetic nanoparticles that were deliberately added for water treatment purposes must eventually be separated out to © 2012 American Chemical Society

avoid the pollutants-bound particles from returning into the environment.5 Upon its inevitable contribution to severe eco(toxicity),13 releasing a large amount of magnetic nanoparticles into the environment for remediation without proper recovery system is somehow unacceptable. Thereby, effective clarification technologies are needed14 and must be the frontier area of research15 prior to any large-scale applications of nanomaterials for water reclamation. Because each MIOP exists as a single domain16 with huge magnetic moment,17 it could be easily magnetized and removed by an external applied magnetic field.18 Here, MIOPs show the advantage of easy isolation via magnetic separation, which is more selective and rapid as compared to both centrifugation and filtration.5,11,19 In fact, many studies have elucidated the effectiveness of separating MIOPs either through low gradient magnetic separation (LGMS)5,12,20,21 or through high gradient magnetic separation (HGMS).5,22 However, even though such technologies have been implemented for on-site applications,10,23 many factors that might influence magnetic collectability of MIOPs such as Received: August 4, 2012 Revised: September 20, 2012 Published: October 1, 2012 14878

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Aldrich. The monomer unit for this anionic polymer is shown in Figure 1. Sodium chloride, NaCl, was purchased from Merck Sdn.Bhd.

the effect of surface modification employed have not yet been completely understood. Bare MIOPs are prone to fast agglomeration,24 preponderantly due to both long-range van der Waals and magnetic attraction experienced among the particles25 and the tendency of nanoscaled materials to aggregate into bulk to lower their surface energy.26 Agglomerated particles exhibit low chemical reactivity upon reduction in their exposed specific surface area27 and subjection to rapid sedimentation, rendering bad water purification efficiency. Under this scenario, polyelectrolyte molecules are usually being used for the surface modification of MIOPs. The formation of polyelectrolyte adlayer introduced electrosteric repulsions among particles28 to overcome the attractive interaction(s) when two particles approach each other at distances less than twice the adlayer thickness.25,29 Although strengthening the colloidal stability is a pivotal criterion to ensure effective applications of magnetic nanoparticles, conflict arose as its role is still unclear for any application relying on HGMS. Here, we demonstrated that the alteration of the surface chemistry with stabilizers would eventually diminish the ability of MIOPs to be isolated by HGMS later. It has been reported that the magnetophoretic motion of magnetic particles varied in great degree depending on the particle size,26,30 shape,25 and surface coating.22 For instance, longer time is needed for silicacoated iron oxide particles to achieve reversible aggregation as compared to bare particles.21 This phenomenon was mostly caused by the relatively low magnetic attraction among individually dispersed particles, which is directly scaled to power sixth of the particles radius.31 Furthermore, Zhao and coworkers have also reported the shell effect on the saturation magnetization value (Ms) of magnetic particles, at which the Ms of Fe3O4−silica core shell nanoparticles is lower than that of Fe3O4 alone.32 Those literature findings have indirectly indicated that the surface modification of magnetic core with a nonmagnetic shell eventually affect their magnetic property. Because application of HGMS strongly relies on the magnetophoretic responsiveness of the targeted particles, thus, to some extent, an adverse effect induced by colloidal stability on the HGMS efficacy should be expected. The present study is dedicated to identifying the role that colloidal stability plays on magnetic separation of both naked and polymers-coated MIOPs. First, commercially available MIOPs were stabilized with anionic poly(sodium(4)styrenesulfonate) (PSS) of different molecular weights (i.e., 70 kDa and 1000 kDa). Successively, the influences of colloidal stability on magnetic separation were evaluated through HGMS study. In addition, we interpreted our experimental results on colloidal stability of MIOPs in terms of an extended Derjaguin−Landau−Verwey−Overbeek (DLVO) analysis of the interparticle interactions. Four major interaction energies we considered are van der Waals and magnetic dipole−dipole attraction, and electrostatic double layer and steric repulsion. Later, the stability ratio measurement is employed to give a more quantitative indication of both colloidal stability and magnetophoretic collectability of MIOPs.

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Figure 1. Sodium styrene sulfonate, monomer unit of PSS. Dissociation of this hydrophilic polymer in water releases the Na+ ion, leaving a strongly polyanionic structure.

Calcium chloride, CaCl2, was obtained from HmbG. Sodium hydroxide NaOH (Analytical reagent grade) and hydrochloric acid HCl (Analytical reagent grade, 37%) were both obtained from Fisher Scientific. Cylindrical shaped N50-graded neodymium boron ferrite (NdBFe) permanent magnet with surface magnetic field of ∼6000 G was purchased from Ningbo YuXiang E&M Int’l Co., Ltd. All of the chemicals were used as received without further modification or purification. Preparation of PSS-Coated MIOPs. A 2500 mg/L of MIOPs (Fe3O4, NanoAmor) suspension in deionized water was prepared by ultrasonication to disperse the existing aggregates. Similarly, 0.005 g/ mL of PSS 70k and PSS 1000k solution was prepared and ultrasonicated for at least 60 min to assist their dissolution as well as to promote good dispersity of the polymeric solution. The polymer concentration is chosen to make sure the available polymer molecules are at least 500 times excess the estimated amount needed to form monolayer on the particles surface.25 PSS solutions were kept overnight in a water bath at 40 °C prior to use. The pH of MIOPs suspension and PSS solution was then adjusted to 3.5−4 before the former was added into the PSS solution. The pH adjustment is necessary to ensure oppositely charged condition happened between MIOPs and PSS in which the physisorption of PSS on MIOPs via electrostatic attraction was favored. This scenario is mainly due to the fact that Fe3O4 is amphoteric with isoelectric point ∼6.30; thus, at pH 3.5−4, MIOPs suspension is positively charged, while PSS solutions remained negatively charged at all pH values in the range. The physisorption was allowed to occur for at least 1 day in an end-to-end rotating rack set at 37 rpm rotation speed. Because adsorption of polymer molecules on oppositely charged solid surfaces is rapid, strong, and irreversible with short relaxation times,28 1 day of adsorption time is suffice to achieve equilibrium. The PSS-coated MIOPs were then separated with a permanent magnet and prewashed before final dispersion in Mili-Q water. The washing step is vital because the presence of free polymer molecules in the suspension might lead to the occurrence of depletion flocculation that later promotes particle aggregation into large flocs.29 In the present work, two PSS of different molecular weights (i.e., 70 and 1000 kDa) have been employed to identify the effect of polymer size on colloidal stability and subsequently the magnetophoretic isolation by HGMS. The radius of gyration, Rg, for both of the PSS polymers was estimated on the basis of Flory’s mean field approach:33

EXPERIMENTAL SECTION

Materials. Iron oxide nanoparticles, Fe3O4 (APS, 98+% purity), were purchased from Nanostructured & Amorphous Materials, Inc. Water-soluble poly(sodium(4)styrenesulfonate) polyelectrolyte with average molecular weight ∼70 000 and ∼1 000 000 Da (hereafter detonated as PSS 70k and PSS 1000k), were supplied by Sigma-

Rg ≈ Nv

(1)

log[R g ] = v log[N ] + log a

(2)

N= 14879

Mp Mm

(3) dx.doi.org/10.1021/la303169g | Langmuir 2012, 28, 14878−14891

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where N is the degree of polymerization, a is a constant that depends on local properties of polymer, v is 3/5 for good solvent and 1/3 for bad solvent, Mp is the molecular weight of polymer, while Mm is the molecular weight of monomer. The solvency is determined by the extent of ionic strength. In good solvent, polymer tends to swell into extended conformation and favor steric stabilization, while in bad solvent, polymers shrink into collapsed conformation.34,35 High Gradient Magnetic Separation (HGMS) Study. For magnetophoretic study, a lab-scaled HGMS was constructed as described in the literature.11 Briefly, a cylindrical column with diameter of around 1 in. was randomly packed with 5 g of fine stainless steel mesh wool (see Figure S1 in the Supporting Information). The void volume of this packing is as high as ∼92%, thus creating low pressure drop and relatively large holding capacity for magnetically isolated particles. This packed column was then mounted between two NdBFe magnet blocks with surface field strength ∼6000 gauss. Here, the packing materials were magnetized by the permanent magnet, generating relatively high and localized magnetic field gradient near the wire surfaces36 that are capable of magnetically trapping magnetically susceptible particles from fluids. Experiment was carried out by presonicating the prepared colloidal suspensions for 2 min to allow full dispersion of particles before pumping them through the magnetized column by a peristaltic pump (Watson Marlow) at constant rate (1 mL/min). The effluent from the HGMS column was collected, and the iron oxide content was determined by checking the turbidity using a UV−vis spectrophotometer (Cary 60) at absorbance 532 nm. The efficiency of HGMS to capture the particles was determined by the magnitude of turbidity reduction from inlet suspension to outlet suspension. After the experiments, both permanent magnets were detached, and the packed column was flushed thoroughly with Mili-Q water. Because the wool mesh is no longer magnetized upon the absence of external magnetic field, the retained particles can slowly be released out by cleaning with flowing water. For thorough washing, the wool mesh was immersed in water and ultrasonicated for at least 1 min before being dried in an oven. The standard calibration curve for naked MIOPs, PSS 1000kcoated MIOPs, and PSS 70k-coated MIOPs measured at abs 532 nm are provided in the Supporting Information (Figures S2−S4). The magnitude of absorbance was found to be directly proportional to the particle concentration, which fit pretty well in a straight line with the value of coefficient of determination R2 higher than 0.99. Sedimentation Kinetics. The ability of particles (naked MIOPs, PSS 70k-coated MIOPs, and PSS 1000k-coated MIOPs) to remain suspended in water was determined and compared through sedimentation kinetics measurement. For this study, 3 mL of particle suspensions at 50 mg/L was prepared and ultrasonicated for 2 min, before being inserted into a UV−vis spectrophotometer for kinetics study. The optical intensity of absorbance observed at 532 nm was recorded every 2.5 min for 24 h. All measurements were made at 25 °C. Later, a sedimentation kinetics profile was plotted to show the suspension opacity (a.u.) change with time. Here, suspension opacity was calculated as:

opacity =

It I0

(or correlogram), which shows the correlation factor decays exponentially with time. The sharper was the decay curve, the smaller were the particles as smaller particles diffuse faster and cause the intensity of correlogram to change quickly. Analyses of this correlogram using CONTIN algorithm eventually give the translational diffusion coefficient needed for calculating the Stoke Einstein equation.25 Particle size measured while dispersed in solution using DLS is known as hydrodynamic diameter/radii because the magnitude is based on both the hydro (dispersant medium) and the dynamic (shape) effect. Thus, to avoid contradicting results arising from hydro effect, all of the samples were dispersed in the same medium prior to the size measurements. Electrophoresis measurement was carried out by measuring the velocity of particles moving between oppositely charged electrodes. This electrophoretic mobility was then converted to zeta potential using the Helmholtz−Smoluchowski approximation. Here, the Helmholtz−Smoluchowski approximation was chosen as it is more appropriate for colloidal system with size >0.2 μm with polar solvent as dispersant medium. In the present study, ξ is mainly used to verify the successful attachment of polymer on the particles surface. In addition, ξ can also serve as an indication for the colloidal stability. Particle suspension with a magnitude of ξ higher than 30 mV is considered stable because the particles exhibit strong electrostatic repulsion. Polymer Adsorption. Because colloidal stability is strongly dependent on the adlayer conformation, the attachment of PSS onto iron oxide surface was investigated by using a quartz crystal microbalance with dissipation (QCM-D) system with single-sensor entry level (Västra Frölunda, Sweden). QCM-D is a fast and accurate tool that allows one to simultaneously measure the mass and structural changes with respect to time during polymers adsorption. In our study, the structural changes upon formation of PSS polymeric thin film (nm) were monitored using this system. First, an AT-cut piezoelectric quartz crystal coated with thin Fe3O4 electrodes was carefully installed in the unit. The baseline was calibrated by flushing Milli-Q water through the QCM-D sensor crystal until steady state was achieved. The experiment was then started by pumping Milli-Q water (pH adjusted near to 4) for 2 min. Next, 2 mL of the same PSS solutions used for particles coating was introduced into the system at constant flow rate (0.15 mL/min). Once the steady state was achieved, the peristaltic pump was stopped and the adsorption was allowed to continue under quiescent condition for 1 h. The dissipation data were then collected using QTools software.

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RESULTS AND DISCUSSION Stability of Naked MIOPs and Polymer-Stabilized MIOPs. The flocculation kinetics of the prepared colloidal suspensions was determined through time lapse hydrodynamic size measurement using DLS (Figure 2). For this kinetics study, particles were dispersed in 1 mM NaCl dispersion and were subjected to ultrasonication prior to measurements. As expected, naked MIOPs were extremely unstable even at low ionic strength (1 mM NaCl) and low concentration (6 mg/L). The particles start to flocculate into large clusters within a few minutes. As shown in Figure 2, naked MIOPs initially appeared as small clusters with average hydrodynamic diameter at around 331.2 nm during the first DLS measurement done exactly 2 min after ultrasonication. Because magnetic attraction among particles increases with particles radius to the sixth power,31 stronger magnetic attraction existed among the small clusters, and thus further induces flocculation. Note that the slight size reduction after 200 min does not indicate the detachment of particles from the clusters formed; instead, it is due to the detection of smaller clusters upon complete settlement of those larger clusters with an average hydrodynamic size ∼2000 nm. However, the growth rate decreased over time because higher surface potential of bulk particles enhances the energy barrier.38

(4)

where It and I0 are the intensities of absorbance at time t and during the starting time, respectively. All of the samples were tested at the same concentration. However, it should be noted that the direct colloidal stability comparison of this kind is highly dependent on the surface chemistry of iron oxide nanoparticles used. Because the extents of adsorption or polymers conformation are affected by the nature of the nanoparticle surface, contradicting results might be observed in certain cases.31,37 Particles Characterization. Malvern Instruments Nanosizer ZS (173° optical arrangement) was employed to determine the hydrodynamic size, electrophoretic mobility (UE), as well as zeta potential (ξ) of all of the particles studied in this work. During size measurement, the intensity of light scattered by the colloidal suspension will automatically be fitted into an autocorrelation curve 14880


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Table 1. Details on Zeta Potential, Electrophoretic Mobility, and Fraction of Particles with Magnitude of Zeta Potential ≥30 mV for All MIOP Suspensions (with and without PSS Modification) Employed in This Work colloidal suspension naked MIOPs PSS 70kcoated MIOPs PSS 1000kcoated MIOPs

average zeta potential (mV)

electrophoretic mobility (μm·cm/V·s)

% fraction particles with magnitude of zeta potential ≥30 mVa

+25.8

+2.157

−31.0

−2.424

58.57%

−30.5

−2.384

60.87%

a The % fraction of particles with magnitude of zeta potential ≥30 mV was calculated on the basis of the area under the graph.

Figure 2. Flocculation kinetic profile showing the average hydrodynamic size (nm) of naked MIOPs (●), PSS 1000k-coated MIOPs (□), and PSS 70k-coated MIOPs (+) detected by dynamic light scattering plotted versus time. All samples were standardized at 6 mg/ L with 1 mM NaCl dispersant medium. Data were collected every 5 min for 5 h with 15 runs being carried out during each measurement.

polymers coating that also provides significant stability enhancement. In this study, PSS was found to attach on small clusters instead of on individual particle. This can be seen from the DLS measurement that detected an average size of ∼300 nm instead of the core size (20−40 nm) of one single particle observed under transmission electron microscope (TEM) as indicated in Figure 3. This indicates the presence of MIOPs in small clusters

As described, the flocculation tendency of this naked MIOPs was attributed to both the long-range van der Waals and the magnetic attraction24,25 experienced among the particles during collisions. Because of the higher number of nanosized particles as compared to micrometer-sized particles with the same mass concentration,39 the collision and interaction among nanoscaled structure are almost inevitable in a colloidal system with no surface modification. Here, we successfully promote the colloidal stability of the MIOPs through surface modification with PSS anionic polymers. As shown in Figure 2, the average hydrodynamic size of both PSS 70k-coated MIOPs and PSS 1000k-coated MIOPs was almost constant throughout the measurement time. Similar size was observed from the same samples after 1 month storage indicated that the PSS (0.005 g/mL) used in this study is suffice to promote both steric and electrostatic stabilization, rendering the limited interaction among the coated particles. From DLS measurements on particles dispersed in pure deionized water, the intensity-weighted distribution of hydrodynamic diameter of naked MIOPs shifted to larger size indicates real-time formation of particle clusters. Both PSScoated MIOPs do not exhibit any obvious clustering behavior within the same time scale (see Supporting Information, Figure S5). Adsorption of PSS on MIOPs was confirmed by the zeta potential (ξ) measurement of the particles before and after surface coating (Table 1). The ξ of naked MIOPs was +25.8 mV at pH 4, but after being coated with PSS it has experienced drastic charge reversal at which the zeta potentials shifted to −31.0 and −30.5 mV, for PSS 70k- and PSS 1000k-coated MIOPs, respectively. Besides verifying the successful attachment of PSS polymer chain on MIOPs, ξ with absolute value larger than 30 mV is an indication of good colloidal stability.40 In our experiments, the magnitude of zeta potential for both coated samples does not show a significant difference; this is mostly due to the electrophoretic mobility of the poly(styrene sulfonate) increasing with the size of molecule but staying almost constant when the degree of polymerization >20.34 The degree of polymerization of PSS 70k is 339.81 and PSS 1000k is 4854.37, calculated on the basis of eq 3. After all, ξ can only be used to indicate stability originated from electrostatic repulsion but can not account for steric effect contributed from the

Figure 3. TEM micrographs of PSS 70k-coated MIOPs. This image showed the core size of MIOPs, but should not indicate the actual clustering size as measured by DLS. This is because further aggregation occurred when particles were deposited and dried on the carbon grid. PSS polymeric layer could not be shown here due to the low contrast of the molecules to that of the carbon substrate.

during polymer coating. It is difficult to separate the iron oxide suspension into individual particles even under prolonged ultrasonication (>1 h), probably due to the drying process carried out by the manufacturer to solidify the as-synthesized iron oxide particles, which eventually makes some of the particles permanently attach to each other. However, for ease of explanation, we regard this cluster of particles as an individual entity throughout this Article. In fact, many studies have previously investigated the cluster−cluster interaction by treating a cluster consisting of many subunits as a single unified structure.41 14881


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Small clusters are more favorable than individually dispersed nanosized particles especially for environmental related field applications because colloids in micrometer size are more transportable in groundwater due to better rejection by the smaller grain pore spaces.42 Similarly, Ditsch and co-workers26 have also suggested the wider applicability of MIOP cluster with size larger than 50 nm. In this study, PSS-coated MIOPs show a net negative charge in the entire pH range (Figure 4),

Figure 5. (a) Sedimentation kinetics profile of all colloidal suspensions at 50 mg/L (without salt), and (b) attached photos are (i) PSS 70kcoated MIOPs, (ii) PSS 1000k-coated MIOPs, and (iii) naked MIOPs taken a few days after the sedimentation. (Note: For all three samples, sedimentation was observed during the first 300 min and hereafter stayed almost constant.) Figure 4. Effect of pH on the electrophoretic mobility of naked MIOPs (○), PSS 1000k- (□), and PSS 70k-coated MIOPs (△) suspensions. The isoelectic point of naked MIOPs falls around 6.30. All samples were dispersed in 1 mM NaCl solution for this measurement. Standard error was obtained from triplicate measurements.

presence of sintered aggregates that are unable to be stabilized by any modifier43 and (ii) bridging flocculation. The slope of the kinetics profile can be used to describe the sedimentation flux. Table 2 listed the slope of the sedimentation kinetic profile Table 2. Data Extraction from Sedimentation Kinetics Study

which (1) facilitates their mobility in soil that is also negatively charged upon sorption of natural organic matter such as humic acid and fulvic acids, and (2) allows constant functionalities of the particles in any pH of soil and underground water. So from the surface functionality point of view, the PSS-coated MIOPs tailored made in this study are thereby physically feasible for both soil and underground water remediation. Sedimentation Kinetics of Particles. As discussed previously, physically unstable particles are subjected to fast agglomeration within a few minutes. Because the sedimentation flux increased with increment of particles size,24 large and heavy clusters rapidly respond to the gravitational force. In short, sedimentation kinetics could be used to compare the colloidal stability of colloidal suspensions at which stable particles are expected to stay dispersed in suspension, while unstable particles settle out from suspension rapidly. Figure 5 shows the sedimentation kinetics profile of both naked and coated suspensions. Our results clearly implied that PSS 70k-coated MIOPs is the most stable suspension with 58.67% of the initial suspension remaining suspended after 1 day in a standard 1 × 1 × 4 cm disposable cuvette. This was followed by PSS 1000kcoated MIOPs, which retains about 29.57% of the initial suspension. While only 5.08% of naked MIOPs stayed dispersed after 1 day, this observation is consistent with the DLS measurement showing fast agglomeration of naked MIOPs. If experimental time was prolonged, complete settlement was observed for naked MIOPs suspension, while both PSS-coated samples were still present as a darkish suspension for months. Unlike naked MIOPs where the aggregates formed are predominantly due to particles−particles van der Waals and magnetic attraction, we believe the discernible sediments in both PSS-coated suspensions were mostly due to (i) the

type of suspensions naked MIOPs PSS 1000kcoated MIOPs PSS 70kcoated MIOPs

slope of the kinetics profile (mg/ L·min)a

normalized sedimentation flux (mg/ m2·min)

transmittance (%)b

suspension retained after 24 h (%)

−0.2176

−6.53

92.73

5.08

−0.1485

−4.46

68.22

29.57

−0.0667

−2.00

55.28

58.67

Slope was determined for the first 120 min suspensions standing time. bTransmittances (%) were calculated when steady state was achieved as an indication of the intensity of light that has passed through the sample as compared to the intensity of light when it entered the sample.

a

for different suspensions. Because sedimentation flux is directly proportional to the square of particles radius,31 the size of aggregates formed by both coated MIOPs is generally smaller than that of naked MIOPs as could be deducted from the profile slope. Here, for the most stable (PSS 70k-coated MIOPs) and the less stable suspensions (naked MIOPs), we have carefully withdrawn the supernatants and resuspended the bottom sediments by gently turning the cuvette upside down. Later, the size of the sediments samples was characterized by DLS measurement. Results showed that the average hydrodynamic size of sediments in PSS 70k-coated MIOPs falls as 595.6 nm (Supporting Information, Figure S6). On the other hand, no particles could be detected for the sediments formed by naked MIOPs; this means the aggregates formed are too large and out of the measurement limit of DLS apparatus (10 000 nm). 14882


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Figure 6. Schematic diagram illustrates two possible bridging flocculation mechanisms. It is believed that low molecular weight polymer (PSS 70k) mostly results in type “a” flocculation, while high molecular weight polymer (PSS 1000k) can result in both types of bridging flocculation mechanisms.

where Edissipated is the energy dissipated per oscillation cycle, while Estored refers to the energy storage in the oscillating system. The dissipation−time curve (Figure 7a) showed that both PSS 70k and 1000k actually form viscous, soft, and elongated film on the iron oxide surface, at which the D-shifts are all positive. The saturate value (ΔDsat) was listed in Table 3,

In this work, PSS 1000k was found to poorly stabilize MIOPs as compared to PSS 70k. This observation was in line with literature that also reported that small PSS chains were better stabilizers than large PSS chains for magnetic nanoparticles.31 This phenomenon was attributed to the occurrence of bridging flocculation as PSS 1000k was used as stabilizing agent. It have been reported that besides inducing steric forces and perturbing the existing van der Waals and electrostatic forces, adsorbed polymer chains also might introduce bridging forces,28 at which high molecular weight polymers tend to bridge between particles.26 Bridging is almost inevitable in our case because the PSS polymeric coil prefers to stay in a more extended conformation when dissolved in good solvent29 upon the selfrepulsion existing among the strongly charged repeating unit.34,44 In this study, PSS 1000k, which has a radius of gyration about 4.93 times larger than that of PSS 70k (estimated on the basis of eqs 1−3), was expected to induce more bridging because the collision radii among particles increased with larger adsorbed layer,45 rendering less stable particles. Thus, huge polymer is not an ideal solution to promise formation of highly stable suspensions.26 Figure 6 graphically illustrates the possible bridging mechanism between PSS-coated particles. QCM-D Study on the Conformation of Adsorbed Layer. The conformation of the adlayer directly affects the ultimate colloidal stability of particles. Hence, in this study, QCM-D technique was utilized to detect the structure of the PSS adsorb film, and the obtained results can be employed to rationalize the stability introduced by PSS of different molecular weights. Here, the dissipation data (D) recorded as functions of time were presented to describe the rigidity of the polymer thin layer. It have been reported that the dissipation changes are directly related to the changes in the film structure.46 Hence, observed energy dissipation serves as a good indication of the viscoelastic state of adsorbed polymer layer(s).47 Polymer forms (i) rigid/compressed/flat film when ΔD ≈ 0, and (ii) soft/viscous/floppy/elongated film when ΔD > 0. The high D-factor in the latter case is assigned to the large energy dissipated during oscillatory motion that is induced within the film.48 In this case, soft films that attached onto the flat crystal surface tend to deform, thus damping the sensor’s oscillation. The D-factor is defined as the reciprocal of quality factor (Q) as shown below:49

D=

Edissipated 1 = Q 2πEstored

Figure 7. (a) Dissipation curves describe the conformation of polymer adsorption on the quartz crystal. Polymer forms (i) rigid/compressed/ flat film when ΔD ≈ 0, and (ii) soft/viscous/floppy/elongated film when ΔD > 0. Here, it was obvious that both PSS solutions form the second type of film on the Fe3O4 quartz crystal. The PSS 1000k film is softer than PSS 70k upon its relatively large and extended size of the polymer chain. (b) ΔD versus Δf curve fittings for the first 1600 s of QCM-D measurement at which PSS adsorption happened and reached steady state. The initial linear line was attributed to the pumping of Mili-Q water (pH adjusted to 4) for the first 120 s, predominantly to equilibrate the crystal oscillation, as well as to make the iron oxide surface positively charged.

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Table 3. Data from QCM-D Measurement

a

polymer

molecular weight (Da)

ΔDsat (10−6)a

Qsat (105)b

PSS PSS

1 000 000 70 000

6.3 4.7

1.57 2.13

Saturation dissipation value. bSaturation quality factor.

typically around 6.3 × 10−6 and 4.7 × 10−6, for PSS 1000k and PSS 70k, respectively. Adsorption of large polymer chains on iron oxide surface caused more energy dissipated during every oscillation cycle, thereby leading to a larger D-factor as compared to adsorption of the smaller polymer chain. Figure 7b illustrates another useful way to scrutinize the relationship between ΔD and Δf by eliminating the time dependencies. This result showed the dissipation changed upon a unit mass added. More information such as (i) adsorption kinetics and (ii) absolute viscoelastic properties of the adlayer could be extracted especially from the slope (∂D∂f). The fitted curve clearly illustrated that both PSSs displayed almost similar adsorption behavior with PSS 1000k adsorbing faster as indicated by its higher ∂D∂f gradient. It was reported that the ratio ΔD/Δf increased with the viscoelasticity of the polymeric adlayer;50 thus, the adlayer film formed by PSS 1000k film is much more viscous and less dense than that of PSS 70k. Because formation of less dense film is hypothesized as favored toward bridging flocculation,25 the QCM-D results indirectly suggest enhanced bridging phenomenon when PSS 1000k is used as particle stabilizer. The result is in good agreement with the sedimentation kinetics study that also significantly implied poor stabilizing properties of PSS 1000k as compared to PSS 70k. Effects of Colloidal Stability on Magnetophoretic Behavior of MIOPs in HGMS. In the previous section, enhanced stability induced by PSS coating on MIOPs has been experimentally demonstrated and compared to the results finding that PSS 70k is a better stabilizer for MIOPs. Subsequently, the impact of colloid stability on the magnetophoretic behavior of MIOPs is further studied through the HGMS process. Because HGMS was reported to be the ratelimiting step in the separation of magnetically susceptible nanoparticles,36 using HGMS as the model system is therefore crucial for particles that are tailored for environmental engineering applications especially if magnetic separation will be implemented as the downstream process. The main purpose of this Article is to investigate the contribution of colloidal stability in affecting HGMS in capturing particles. All factors that were known to influence the HGMS performance such as column volume, inlet flow velocity,36 magnetic field strength,5 packing height, type of packing, as well as the concentration of suspensions, were made constant, except for the colloidal nature of the suspensions to be treated. Under these circumstances, all of the suspensions were subjected to nearly identical magnetic force (Fmag), drag force (Fd), and gravitation forces (Fg) with very similar fluid dynamic condition. Theoretically, Fmag needs to be higher than the summation of Fd and Fg to magnetically retain the particles inside the column.51 In our case, the fine mesh wool packed in the column allows intimate contact with the colloidal droplets, generating high magnetic field gradient (T/m) suffice to overcome the sum of Fd and Fg. Figure 8a shows the percent of particles successfully isolated by HGMS for all three different suspensions. We found that the particle isolation efficiency was in reverse order of the colloidal

Figure 8. (a) The amount of particles (%) successfully isolated by HGMS (standard error was obtained from three independent experiments), and (b) photos of effluents from HGMS for naked MIOPs (left), PSS 1000k-coated MIOPs (middle), and PSS 70kcoated MIOPs (right). Result is in the reverse order of colloidal stability, at which polymer-stabilized suspensions are hard to recover by HGMS, leaving brownish effluents.

stability, at which the more stable are the particles, the harder it is for them to be magnetically isolated. Naked MIOPs with saturation magnetization value of 74.61emu/g (see Figure S7 in the Supporting Information for the M−H curve) were effectively removed by our HGMS setup to 96.9 ± 2.6%. However, on the other hand, HGMS separation of PSS 1000kcoated MIOPs was 83.1 ± 1.2%, while PSS 70k-coated MIOPs (i.e., the most stable suspensions prepared in this study) only were able to be removed up to 67.7 ± 4.6%. Here, formation of large particle clusters due to bridging flocculation when PSS 1000k is used as stabilizer could be the primary reason that leads to better magnetophoretic separation as compared to PSS 70k.52The brownish color found in the discharged effluents indicated the presence of iron oxide nanoparticles that failed to be separated by HGMS (Figure 8b). For statistical comparison, the concentration of MIOPs remaining in the effluents from HGMS column was compared to the allowed discharge limit by the Environmental Quality (Sewage and Industrial Effluents) Regulations under Environmental Quality Art (EQA) 1974 of Malaysia and the Secondary Maximum Contaminant Level (SMCL) standard for iron in drinking water coupled with the Environmental Protection Agency United States (US EPA) (Table 4). Obviously, the effluents from HGMS separation are about 1.6 and 3.1 times beyond the EAQ discharge limit, and 26.9 and 51.3 times beyond the US EPA standard, for PSS 1000k- and PSS 70k-coated MIOPs, accordingly. Problems arose where stable particles that failed to be removed are themselves a hazardous contaminant to the environment. 14884


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Table 4. Statistical Comparison between Concentration of MIOPs Escaping from Magnetic Separation (HGMS) with Maximum Allowed Discharge Limit concentration of inlet suspension (mg/L)

concentration of outlet suspension (mg/L)

naked MIOPs

50

1.7 ± 1.5

under discharge limit

PSS 1000k-coated MIOPs PSS 70k-coated MIOPs

50

8.1 ± 0.1

50

15.4 ± 2.2

beyond discharge limit by 1.6 times beyond discharge limit by 3.1 times

type of suspensions

a

EQA 1974a

EPA 1974a beyond standard by 5.6 times beyond standard by 26.9 times beyond standard by 51.3 times

The approved discharged limits of iron (Fe) are 5 and 0.3 mg/L according to EQA 1974 and US EPA 1974, respectively.

ranging from 25 to 100 mg/L, PSS 70k-coated MIOPs always showed the poorest separation efficiency, while naked MIOPs displayed the best capturing results. This observation again suggests an inversely proportional effect between enhanced colloidal stability and magnetophoretic collection. Nevertheless, we do not rule out the possibility of complete collection of all of the polymer-stabilized particles via HGMS with the introduction of higher magnetic fields, extension of the packing height, or recycling of the untreated effluents for further separation. Knowing that colloidal stability is the main reason leading to poor magnetic separation of polymer-coated MIOPs, we hypothesize that destroying this stability should enhance the HGMS performance. In this section, monovalent electrolyte NaCl and divalent electrolyte CaCl2 were added into the colloidal systems, forming resultant suspensions with different ionic strengths, ranging from 5 to 100 mM of salt concentration. The aim of this salt adjustment is to progressively destroy the colloidal stability of the coated samples, and test the subsequent separation efficiency under HGMS. Usually, it is difficult for particles to maintain colloidal stability at elevated ionic strength, at which under this kind of environment, the electrical double layer thickness (k−1) was easily screened and becomes insignificant.25,42 As a result, there is high tendency for most of the particles to overcome the energy barrier50 and thus further aggregate into permanent precipitates that might no longer be redispersed.26 These large clusters composed of flocculated particles experience collective magnetophoretic forces and thus could be easily captured by the applied field.7 Figure 10 shows the influence of ionic strength adjustment on the performance of HGMS. Obviously, the presence of either type of salt successfully induces better capturing of both PSS-coated MIOPs via our HGMS setup with more particles being magnetically isolated when the ionic strength increased. The results were in good agreement with particles flocculation experiments showing severe particle aggregation at elevated ionic strength condition (see Figures S8−S10 in the Supporting Information). The stability of coated suspensions is suppressed due to the bridging attraction among particles that was reported to be dominant when the particles approached each other.28 Even though polymer coating provides steric repulsions that are strong and robust at elevated ionic strength,56 the effect will be weaken once the solvency is destroyed.29 In fact, steric effects dropped as a function of the dispersion solvency, which will be further discussed in the following section. As compared to monovalent Na+, divalent Ca2+ was found to even heavily suppress particles stability57 at which under the same concentration, divalent showed a more drastic impact on the efficiency of HGMS isolation.

This observation has revealed the conflicting role of polymer coating in enhancing colloidal stability but suppressing magnetic separation. It is very likely that the magnetophoresis-induced separation of MIOPs in our study is a cooperative phenomenon53 in which aggregation of particles enhanced the removal efficiency.20 The tendency of particles aggregation that aided the HGMS removal has been shown previously under elevated ionic strength condition36 and is consistent with our observation. Because thick aggregates are magnetically more responsive than thin aggregates,53 the small colloidally stable MIOPs are difficult to isolate by HGMS as shown in our case mainly due to the influence of thermal displacement energy.54 Effect of Suspension Concentration and Ionic Strength on HGMS Collection. On the basis of colloidal stability of MIOPs, which indicate that particle interactions have a very positive influence on the removal efficiency of HGMS, experiments were performed to see how the variation of suspension concentration influences capture. Approximately 96−99% of unmodified MIOPs could be effectively collected by HGMS at concentrations ranging from 25 to 100 mg/L. On the other hand, HGMS isolation of PSS 1000k- and PSS 70kcoated MIOPs suspensions improved to 92.7 ± 0.5 and 83.6 ± 6.1, correspondingly, when the concentration of suspensions increased to 100 mg/L (Figure 9). Because the rate of collision between particles is directly proportional to their concentration,55 MIOP suspensions with higher concentration usually are less stable and tend to aggregate much faster within a given time frame.52 At all levels of particle concentration investigated,

Figure 9. Influence of initial suspension concentration on amount of particles successfully isolated by HGMS. The markers assigned are naked MIOPs (○), PSS 1000k-coated MIOPs (□), and PSS 70kcoated MIOPs (△). All of the experiments were repeated three times to obtain the standard error. The line is a means to guide the readers’ eyes. 14885


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theory, which sums all of the attractive and repulsive potential energies:58 Uoverall = UudW + Umag + Uelec + Usteric

(6)

where UudW is long-range van der Waals, Umag is magnetic attraction, Uelec is electrostatic repulsion, and Usteric is steric energy due to the presence of polymer coating. Calculating the overall interaction energy Uoverall provides a useful way to predict the nature of interaction between the particles either attractive (Uoverall = negative) or repulsive (Uoverall = possitive) with respect to interparticles surface-to-surface distance h. All of these interactions were depicted in Figure 11.

Figure 11. Schematic showing the interaction energies between two identical particles coated with polymers adlayer to a thickness δ. Note that we treat a cluster of particles as a single entity during DLVO modeling. r is the particle radius (taken as 50 nm), while h is the interparticle distance.

Figure 10. Effects of increasing suspension ionic strength through addition of (a) monovalent salt (NaCl) and (b) divalent salt (CaCl2) on the amount of particles isolated by HGMS. Markers assigned are naked MIOPs (●), PSS 1000k-coated MIOPs (□), and PSS 70kcoated MIOPs (△). Note the capturing of magnetic particles by HGMS much more sensitive to divalent ion than monovalent ion. Excellent isolation of naked MIOPs can be observed even at low ionic strength condition upon its unstable nature. Standard error was obtained from triplicate measurements. The lines between markers are means to guide the readers’ eyes.

In the present study, the magnitude of UudW is taken to be the sum of attraction between all atomic pairs59 of radii r. Because we are working with polymer-coated colloidal suspension, here, we employed the modified formulation suggested by Vold, 1961,60 which includes the Hamaker constant of the adlayer to show the direct influence of adsorbed layer on particle attraction:

As shown in Figure 10, more than 96% of HGMS isolation of naked MIOPs was achieved at low ionic strength condition ( 1/2 that usually leads to negative Uosm (attraction).64 We take χ as 0.45 for the model calculations.31 Note that according to both eqs 12 and 13, an increase in χ value upon salt addition renders low or even reverse steric effects of the PSS layer. Figure 12 demonstrates the predicted steric effects under different solvency conditions. The effective volume fraction (Φa) or layer density was determined from:31

−6

Usteric =

Mp

−1

where kB is Boltzmann constant (1.3806 × 10 J K ), T is the absolute temperature, ε is the dielectric constant, hp is Planck’s constant, n is the refractive index, and Ve is the absorption frequency of iron oxide (∼3.00 × 1015 s−1).62 The existence of magnetic attraction among MIOPs is mainly due to the intrinsic permanent magnetic dipole moment.63 With saturation magnetization (Ms) of the particle at 380 511 A m−1 (see Figure S7 in the Supporting Information), the magnetic dipole moment μ is 3.114 × 10−18 A m2. Umag could be expressed as:24 Umag =

2πrδ 2ρp (Φa)

⎛ h ⎞⎤ ⎛1 ⎞⎡ h 4πrδ 2(Φa)2 1 kBT ⎜ − χ ⎟⎢ − − ln⎜ ⎟⎥ ⎝ ⎠ ⎝ δ ⎠⎦ ⎣ vs 2 2δ 4 (13) 14887


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In this work, the stability ratio of both PSS 70k- and PSS 1000k-coated MIOPs was experimentally determined on the basis of the particle aggregation kinetic under different ionic strength altered by monovalent salt. DLS was employed to trace the change in particles hydrodynamic radius Rh within the first 2000 s. Note that the measurement was started immediately upon the addition of NaCl solution into the sample. When the hydrodynamic radius change is plotted with respect to time, the Rh and t relationship could be easily determined by fitting the plotted curve with a second-order polynomial equation.67 The Ksystem can the be determined by:

⎛ dR h ⎞ ⎜ ⎟ = K systemno ⎝ dt ⎠ t → 0

(17)

in which no is the colloid number concentration that was kept constant for all measurements. This is to omit the contribution of suspension concentration toward particle aggregation. Thus, by substituting eq 17 into eq 16, the stability ratio could be redefined as: Wexp

dR h dt t → 0rapid

( ) = ( )

dR h dt t → 0system

(18)

In this study, the temporal evolution of hydrodynamic radius was obtained for both naked MIOPs and PSS-coated MIOPs at electrolyte concentration ranging from 0 mM (in deionized water) up to a few hundred millimolar until the size increments overlap each other. Here, PSS 70k-coated MIOPs, which is more colloidally stable, was observed to display the highest critical coagulation concentration (ccc) at around ∼200 mM, while PSS 1000k-coated MIOPs exhibit a ccc value ∼75 mM. As for unmodified MIOPs, the flocculation rate starts to overlap at electrolyte concentration as low as ∼5 mM (see Figures S8− S10 in the Supporting Information). Figure 14 displays log Wexp

Figure 13. Extended DLVO analysis showing all of the potential energy involved in two extreme cases, which are (a) naked MIOPs (worst stability) and (b) PSS 70k-coated MIOPs (best stability). Models were drawn by taking an average cluster size of 100 nm.

naked MIOPs observed even in low ionic strength suspension (Figure 2). As for the MIOPs modified with PSS 70k, good stability results obtained in both flocculation kinetics and sedimentation kinetics nicely explained the net particles− particles repulsion (positive Uoverall) observed. As shown in Figure 13, the net repulsion was mostly contributed from steric interaction induced by the polymer adlayer. Furthermore, the shallow attractive well of less than 1 kT (Figure 13b) suggests the aggregation, if happened, is generally weak and should be disrupted easily by thermal energy. Experimental Determination of Stability Ratio. Stability ratio, Wexp, has been widely utilized to quantitatively represent the coagulation rate and thus colloidal stability of a colloid suspension. It is experimentally defined as: K rapid Wexp = K system (16)

Figure 14. Stability ratio graph plotted as log[Wexp] versus log[NaCl]. The markers indicated the stability results obtained experimentally with eq 18. Standard error was obtained from replicate measurements.

the ratio of fast aggregation kinetic constant when all collisions result in particles aggregation (Krapid) to the kinetic constant for the prepared suspension at different ionic strengths (Ksystem).65 The Krapid constant obtained when the formation of aggregates from individual particles is unchallenged. This is the worst case scenario where only attractive potential (i.e., UudW and Umag) exists among the particles. Thus, a colloid suspension showing Wexp ≈ 1 indicates that the system is extremely unstable and subjected to rapid flocculation. On the other hand, stable particles have no or a slow aggregation rate, and hence result in higher Wexp.

versus log[NaCl] for both PSS-coated MIOPs where the plateau of fast aggregation regime is located at the right-hand side and vice versa. It is clearly seen that the increase of electrolyte concentration, and thus ionic strength of the suspensions, substantially reduces particle stability (log Wexp ≈ 0), particularly through suppression of the electrosteric interaction. However, PSS 1000k-coated MIOPs were more sensitive to electrolyte addition as compared to PSS 70k-coated 14888


Langmuir MIOPs due to its higher collision radii. In short, MIOP suspension with high stability ratio is more stable and hence would be much more difficult to separate magnetophoretically. From our results, a stability ratio greater than 1.8 is sufficient to show an adverse effect on magnetic separation.

CONCLUSIONS PSS with molecular weight of 70k Da is effective on imparting colloidal stability to a small cluster of MIOPs as compared to the same polymer of higher molecular weight, that is, 1000k Da. The significant difference observed was attributed to the longer polymer chains in the latter case that lead to the formation of less dense and extended PSS layer as verified by QCM-D study. This extended polymeric chains favored bridging flocculation, which makes the coated particles less stable. We have experimentally illustrated the contradicting role of colloidal stability on the magnetophoretic behavior of MIOPs in HGMS. Here, the most colloidally stable suspension (PSS 70k-coated MIOPs) exhibited the worst magnetophoretic collectability in a HGMS column, with low separation efficiency at only around 68%, whereas unstable naked MIOPs can be easily captured by HGMS up to 97%, leaving almost a crystal clear solution. By artificially perturbing the suspension colloidal stability with ionic strength increment, the previously stable PSS-coated MIOPs can be easily removed by HGMS up to 96%. Both extended DLVO and stability ratio analysis provide a useful way to predict both the colloidal stability and the magnetophoretic collectability of MIOPs employed. The interrelation between colloidal stability and magnetophoresis has highlighted the conflicting issue where the pivotal role of colloidal stability necessary to maintain a high surface to volume ratio of nanomaterial for engineering applications was in fact limiting the particle removal by HGMS.

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REFERENCES

(1) Lu, H. A.; Salabas, E. L.; Schüth, F. Magnetic nanoparticles: Synthesis, protection, functionalization, and application. Angew. Chem., Int. Ed. 2007, 46, 1222−1244. (2) Bean, C. P.; Livingstone, J. D. Superparamagnetism. J. Appl. Phys. 1959, 30, 120S−129S. (3) Sun, Y.; Li, X.; Cao, X. J.; Zhang, W.; Wang, H. P. Characterization of zero-valent iron nanoparticles. Adv. Colloid Interface Sci. 2006, 120, 47−56. (4) Tratnyek, P. G.; Johnson, R. L. Nanotechnologies for environmental cleanup. Nano Today 2006, 1, 44−48. (5) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Low-field magnetic separation of monodisperse Fe3O4 nanocrystals. Science 2006, 314, 964−967. (6) Gregorio-Jauregui, K. M.; Guadalupe Pineda, M.; Rivera-Salinas, J. E.; Hurtado, G.; Saade, H.; Martinez, J. L.; Ilyina, A.; López, R. G. One-step method for preparation of magnetic nanoparticles coated with chitosan. J. Nanomater. 2012, 2012, 1−8. (7) Kong, L. P.; Gan, X. J.; Ahmad, A. L.; Hamed, B. H.; Evarts, E. R.; Ooi, B. S.; Lim, J. K. Design and synthesis of magnetic nanoparticles augmented microcapsule with catalytic and magnetic bifunctionalities for dye removal. Chem. Eng. J. 2012, 197, 350−358. (8) Saleh, N.; Sirk, K.; Liu, Y.; Phenrat, T.; Dufour, B.; Matyjaszewski, K.; Tilton, R. D.; Lowry, G. V. Surface modifications enhance nanoiron transport and NAPL targeting in saturated porous media. Environ. Eng. Sci. 2007, 24, 45−57. (9) Zhang, X.; Niu, H.; Pan, Y.; Shi, Y.; Cai, Y. Modifying the surface of Fe3O4/SiO2 magnetic nanoparticles with C18/NH2 mixed group to get an efficient sorbent for anionic organic pollutants. J. Colloid Interface Sci. 2011, 362, 107−112. (10) Kakihara, Y.; Fukunishi, T.; Takeda, S.; Nishijima, S.; Nakahira, A. Superconducting high gradient magnetic separation for purification of wastewater from paper factory. IEEE Trans. Appl. Supercond. 2004, 14, 1565−1567. (11) Hirschbein, B. L.; Brown, D. W.; Whitesides, G. M. Magnetic separations in chemistry and biochemistry. CHEMTECH 1982, 12, 172−179. (12) Lim, J. K.; Chieh, D. C. J.; Jalak, S. A.; Toh, P. Y.; Yasin, N. H. M.; Ng, B. W.; Ahmad, A. L. Rapid magnetophoretic separation of microalgae. Small 2012, 8, 1683−1692. (13) Nel, A.; Xia, T.; Madler, L.; Li, N. Toxic potential of materials at the nanolevel. Science 2006, 311, 622−627. (14) Limbach, L. K.; Bereiter, R.; Müller, E.; Krebs, R.; Gälli, R.; Stark, W. J. Removal of oxide nanoparticles in a model wastewater treatment plant: Influence of agglomeration and surfactants on clearing efficiency. Environ. Sci. Technol. 2008, 42, 5828−5833. (15) Ambashta, R. D.; Sillanpäa,̈ M. Water purification using magnetic assistance: A review. J. Hazard. Mater. 2010, 180, 38−49. (16) Faraji, M.; Yamini, Y.; Rezaee, M. Magnetic nanoparticles: Synthesis, stabilization, functionalization, characterization, and applications. J. Iran. Chem. Soc. 2010, 7, 1−37. (17) Mathew, D. S.; Juang, R.-S. An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions. Chem. Eng. J. 2007, 129, 51−65. (18) Vatta, L. L.; Sanderson, R. D.; Koch, K. R. Magnetic nanoparticles: Properties and potential applications. Pure Appl. Chem. 2006, 78, 1793−1801. (19) Chang, S. C.; Anderson, T.; Bahrman, S.; Gruden, C. L.; Khijniak, A. I.; Adriaens, P. Comparing recovering efficiency of

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: Schematic drawing of the HGMS setup. Figures S2− S4: Calibration curve of naked MIOPs, PSS 1000k-coated MIOPs, and PSS 70k-MIOPs measured with Cary 60 spectrophotometer at abs wavelength 532 nm. Figure S5: Time trace on intensity-weighted distribution of size of particles suspended in deionized water. Figure S6: Average hydrodynamic size of sediments. Figure S7: VSM for MIOPs. Figures S8−S10: Ionic strength effects on particles aggregation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ABBREVIATIONS

MIOPs, magnetic iron oxide particles; PSS, poly(sodium 4styrene sulfonate); HGMS, high gradient magnetic separation; LGMS, low gradient magnetic separation; DLS, dynamic light scattering; QCM-D, quartz crystal microbalance with dissipation; DLVO, Derjaguin−Landau−Verwey−Overbeek

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +60-4-599-6423. Fax: +60-4-599-1013. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is based on materials supported by the Exploratory Research Grant Scheme (ERGS) (Grant no.: 203/ PJKIMIA/6730012), FRGS Grant from MOHE (Grant no.: 203/PJKIMIA/6071180), and International Foundation for Science Grant (Grant no.: 304/PJKIIMA/6050232/I100). S.P.Y. is financially assisted by MyPhD. We are all affiliated with the Membrane Science and Technology cluster USM. 14889


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(40) Sherman, P. Rheology of dispersed systems. Industrial Rheology; Academic Press Inc.: London, 1970. (41) Calbi, M. M.; Gatica, S. M.; Velegol, D.; Cole, M. W. Retarded and nonretarded van der Waals interactions between a cluster and a second cluster or a conducting surface. Phys. Rev. A 2002, 67, 1−5. (42) Vance, D. B.; Jacobs, J. A. Particulate transport in groundwater− Bacteria and colloids. Water Encycl. 2005, 349−352. (43) Phenrat, T.; Kim, H.-J.; Fagerlund, F.; Illangasekare, T.; Tilton, R. D.; Lowry, G. V. Particle size distribution, concentration, and magnetic attraction affect transport of polymer-modifier Fe0 nanoparticles in sand columns. Environ. Sci. Technol. 2009, 43, 5079−5085. (44) Evans, D. F.; Wennerström, H. The Colloidal Domain Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed.; Wiley-VCH: New York, 1999. (45) Yu, X.; Somasundaran, P. Role of polymer conformation in interparticle-bridging dominated flocculation. J. CoIloid interface Sci. 1996, 177, 283−287. (46) Bishop, D. J.; Reppy, J. D. Study of the superfluid transition in two-dimensional 4He films. Phys. Rev. Lett. 1978, 40, 1727−1730. (47) Dultsev, F. N.; Kolosovsky, E. A. Identifying a single biological nano-sized particle using a quartz crystal microbalance. A mathematical model. Sens. Actuators, B 2009, 143, 17−24. (48) Rodahl, M.; Hook, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Quartz crystal microbalance setup for frequency and Q-factor measurements in gaseous and liquid environments. Rev. Sci. Instrum. 1995, 66, 3924−3930. (49) Höök, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. Energy dissipation kinetics for protein and antibody-antigen under shear oscillation on a quartz crystal microbalance. Langmuir 1998, 14, 729− 734. (50) Höök, F. Development of Novel QCM Technique for Protein Adsorption Studies; Department of Biochemistry and Biophysics and Department of Applied Physics, Chalmers University of Technology, 1997. (51) Bhakdi, S. C.; Ottinger, A.; Somsri, S.; Sratongno, P.; Pannadaporn, P.; Chimma, P.; Malasit, P.; Pattanapanyasat, K.; Neumann, H. P. Optimized high gradient magnetic separation for isolation of Plasmodium-infected red blood cells. Malar. J. 2010, 9, 38. (52) Helseth, L. E.; Skodvin, T. Optical monitoring of low-field magnetophoretic separation of particles. Meas. Sci. Technol. 2009, 20, 1−8. (53) Faraudo, J.; Camacho, J. Cooperative magnetophoresis of superparamagnetic colloids: theoretical aspects. Colloid Polym. Sci. 2010, 288, 207−215. (54) Lim, J. K.; Tan, D. X.; Lanni, F.; Tilton, R. D.; Majetich, S. A. Optical imaging and magnetophoresis of nanorods. J. Magn. Magn. Mater. 2009b, 321, 1557−1562. (55) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. A. Particle Deposition and Aggregation: Measurement, Modeling and Simulation; Butterworth-Heinemann: Woburn, MA, 1995. (56) Biesheuvel, P. M. Ionizable polyelectrolyte brushes: Brush heightand electrosteric interaction. J. Colloid Interface Sci. 2004, 275, 97−100. (57) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; pp 62−104. (58) Mefford, O. T.; Vadala, M. L.; Goff, J. D.; Carroll, M. R. J.; Mejia-Ariza, R.; Caba, B. L.; Pierre, T. G. St.; Woodward, R. C.; Davis, R. M.; Riffle, J. S. Stability of polydimethylsiloxane-magnetite nanoparticle dispersions against flocculation: interparticle interactions of polydisperse materials. Langmuir 2008, 24, 5060−5069. (59) Sato, T.; Ruch, R. Stabilization of colloidal dispersions by polymer adsorption. Surfactant Science Series; Marcel Dekker: New York and Basel, 1980; Vol. 9, p 47. (60) Vold, M. J. The effect of adsorption on the van der Waals interaction of spherical colloidal particles. J. Colloid Interface Sci. 1961, 16, 1−12. (61) Pierre, A. C. Introduction to Sol-Gel Processing; Kluwer: Norwell, MA, 1998.

immunomagnetic separation and centrifugation of mycobacteria in metalworking fluids. J. Ind. Microbiol. Biotechnol. 2005, 32, 629−638. (20) De Las Cuevas, G.; Faraudo, J.; Camacho, J. Low-gradient magnetophoresis throught field induced reversible aggregation. J. Phys. Chem. C 2008, 112, 945−950. (21) Benelmekki, M.; Caparros, C.; Montras, A.; Gonçalves, R.; Lanceros-Mendez, S.; Martinez, L. M. Horizontal low gradient magnetophoresis behavior of iron oxide nanoclusters at the different steps of the synthesis route. J. Nanopart. Res. 2011, 13, 3199−3206. (22) Moeser, G. D.; Roach, K. A.; Green, W. H.; Hatton, T. A.; Laibinis, P. E. High gradient magnetic separation of coated magnetic nanoparticles. AIChE J. 2004, 50, 2835−2848. (23) Chiba, A.; Okada, H.; Tada, T.; Kudo, H.; Nakazawa, H.; Mitsuhashi, K.; Ohara, T.; Wada, H. Removal of arsenic from geothermal water by high gradient magnetic separation. IEEE Trans. Appl. Supercond. 2002, 12, 952−954. (24) Phenrat, T.; Saleh, N.; Sirk, K.; Tilton, R. D.; Lowry, G. V. Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions. Environ. Sci. Technol. 2007, 41, 284−290. (25) Lim, J. K.; Majetich, S. A.; Tilton, R. D. Stabilization of superparamagnetic iron oxide core-gold shell nanoparticles in high ionic strength media. Langmuir 2009, 25, 13384−13393. (26) Ditsch, A.; Laibinis, P. E.; Wang, D. I. C.; Hatton, A. Controlled clustering and enhanced stability of polymer-coated magnetic nanoparticles. Langmuir 2005, 21, 6006−6018. (27) Schrick, B.; Hydutsky, B. W.; Blough, J. L.; Mallouk, T. E. Delivery vehicles for zerovalent metal nanoparticles in soil and groundwater. Chem. Mater. 2004, 16, 2187−2193. (28) Runkana, V.; Somasundaran, P.; Kapur, P. C. A population balance model for flocculation of colloidal suspensions by polymer bridging. Chem. Eng. Sci. 2006, 61, 182−191. (29) Seebergh, J. E.; Berg, J. C. Depletion flocculation of aqueous, electrosterically-stabilized latex dispersions. Langmuir 1994, 10, 454− 463. (30) Lim, J. K.; Lanni, C.; Evarts, E. R.; Lanni, F.; Tilton, R. D.; Majetich, S. A. Magnetophoresis of nanoparticles. ACS Nano 2011, 5, 217−226. (31) Phenrat, T.; Saleh, N.; Sirk, K.; Kim, H.-J.; Tilton, R. D.; Lowry, G. V. J. Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: Adsorbed anionic polyelectrolyte properties and their effect on aggregation and sedimentation. Nanopart. Res. 2008, 10, 795−814. (32) Zhao, X.; Shi, Y.; Wang, T.; Cai, Y.; Jiang, G. Preparation of silica-magnetite nanoparticle mixed hemimicelle sorbents for extraction of several typical phenolic compounds from environmental water samples. J. Chromatogr., A 2008, 1188, 140−147. (33) De Gennes, P. G. Scaling Concepts in Polymer Physics, 1st ed.; Cornell University Press: Cornell, 1979. (34) Böhme, U.; Scheler, U. Hydrodynamic size and electrophoretic mobility of poly(styrene sulfonate) versus molecular weight. Macromol. Chem. Phys. 2007, 208, 2254−2257. (35) Chremos, A.; Glynos, E.; Koutsos, V.; Camp, P. J. Adsorption and self-assembly of linear polymers on surfaces: a computer simulation study. Soft Matter 2009, 5, 637−645. (36) Ditsch, A.; Lindenmann, S.; Laibinis, P. E.; Wang, D. I. C.; Hatton, A. High-gradient magnetic separation of magnetic nanoclusters. Ind. Eng. Chem. Res. 2005b, 44, 6824−6836. (37) Cirtiu, C. M.; Raychoudhury, T.; Ghoshal, S.; Moores, A. Systematic comparison of the size, surface characteristics and colloidal stability of zero valent iron nanoparticles pre- and post-grafted with common polymers. Colloids Surf., A 2011, 390, 95−104. (38) Shen, L.; Stachowiak, A.; Fateen, S. E. K.; Laibinis, P. E.; Hatton, T. A. Structure of alkanoic acid stabilized magnetic fluids a small-angle neutron and light scattering analysis. Langmuir 2001, 17, 288−299. (39) Hydutsky, B. W.; Mack, E. J.; Beckerman, B. B.; Skluzacek, J. M.; Mallouk, T. E. Optimization of nano- and microiron transport through sand columns using polyelectrolyte mixtures. Environ. Sci. Technol. 2007, 41, 6418−6424. 14890


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(62) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press Inc.: London, 2011. (63) McCurrie, R. A. Ferromagnetic Materials: Structure and Properties; Academic Press Inc.: London, 1994. (64) Tadros, T. F. Colloidal Stability: The Role of Surface Forces-Part I, 1st ed.; Wiley-Vch Verlag FmbH & Co. KGaA: New York, 2006; Vol. 1. (65) Romero-Cano, M. S.; Martin-Rodriguez, A.; De Las Nieves, F. J. Electrosteric stabilization of polymer colloids with different functionality. Langmuir 2001, 17, 3505−3511. (66) Napper, D. H. Polymeric Stabilization of Colloidal Dispersion; Academic Press Inc.: London, 1983. (67) Di Marco, M.; Guilbert, I.; Port, M.; Robic, C.; Couvreur, P.; Dubernet, C. Colloidal stability of ultrasmall superparamagnetic iron oxide (USPIO) particles with different coatings. Int. J. Pharm. 2007, 331, 197−203.

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