Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: adsorbed anionic polyelectrolyte layer properties and their effect on aggregation and sedimentation
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- Phenrat, T., Saleh, N., Sirk, K. et al. J Nanopart Res (2008) 10: 795. doi:10.1007/s11051-007-9315-6
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Nanoscale zerovalent iron (NZVI) particles are 5–40 nm sized Fe0/Fe-oxide particles that rapidly transform many environmental contaminants to benign products and are a promising in situ remediation agent. Rapid aggregation and limited mobility in water-saturated porous media limits the ability to deliver NZVI dispersions in the subsurface. This study prepares stable NZVI dispersions through physisorption of commercially available anionic polyelectrolytes, characterizes the adsorbed polymer layer, and correlates the polymer coating properties with the ability to prevent rapid aggregation and sedimentation of NZVI dispersions. Poly(styrene sulfonate) with molecular weights of 70 k and 1,000 k g/mol (PSS70K and PSS1M), carboxymethyl cellulose with molecular weights of 90 k and 700 k g/mol (CMC90K and CMC700K), and polyaspartate with molecular weights of 2.5 k and 10 k g/mol (PAP2.5K and 10K) were compared. Particle size distributions were determined by dynamic light scattering during aggregation. The order of effectiveness to prevent rapid aggregation and stabilize the dispersions was PSS70K(83%) > ≈PAP10K(82%) > PAP2.5K(72%) > CMC700K(52%), where stability is defined operationally as the volume percent of particles that do not aggregate after 1 h. CMC90K and PSS1M could not stabilize RNIP relative to bare RNIP. A similar trend was observed for their ability to prevent sedimentation, with 40, 34, 32, 20, and 5 wt%, of the PSS70K, PAP10K, PAP2.5K, CMC700K, and CMC90K modified NZVI remaining suspended after 7 h of quiescent settling, respectively. The stable fractions with respect to both aggregation and sedimentation correlate well with the adsorbed polyelectrolyte mass and thickness of the adsorbed polyelectrolyte layers as determined by Oshima’s soft particle theory. A fraction of the particles cannot be stabilized by any modifier and rapidly agglomerates to micron sized aggregates, as is also observed for unmodified NZVI. This non-dispersible fraction is attributed to strong magnetic attractions among the larger particles present in the polydisperse NZVI slurry, as the magnetic attractive forces increase as r6.
KeywordsNZVINanoscale zerovalent iron particlesPolyelectrolyteDispersion stabilityColloidSurface modificationAggregation and sedimentationSteric stabilizationExtended DLVOEnvironmental nanotechnologyGroundwater remediationNanoparticle environmental fate and transport
Nanoscale zerovalent iron
Reactive nano-scale iron particles
Dynamic light scattering
The high specific surface area and resulting high reactivity of nanoscale zerovalent iron (NZVI) makes it a promising and flexible technology for in situ remediation of groundwater contaminants that are amenable to reduction by Fe0, e.g. chlorinated organic contaminants and heavy metals (Kanel et al. 2007; Liu and Lowry 2006; Liu et al. 2005; Ponder et al. 2000; Wu et al. 2005; Xu et al. 2005; Zhang 2003). Early field demonstrations indicate NZVI as a promising in situ groundwater remediation technology (Elliott and Zhang 2001; Henn and Waddill 2006), however, high reactivity alone is not sufficient to make NZVI a good in situ remediation agent. NZVI particles must also be readily dispersible in water such that they can migrate through water-saturated porous media to the contaminated area. Thus, for the purpose of in situ environmental remediation, colloidal stability of an aqueous NZVI dispersion is a critical property.
Colloidal stability is defined operationally as the ability of a particle dispersion to resist aggregation for a specified time (Hiemenz and Rajagopalan 1997; Strenge 1993). Colloidal stability requires an energy barrier of at least several kT in the interparticle interaction potential. According to the DLVO theory, the net interaction energy between particles is the sum of van der Waals attraction and electrostatic double layer repulsion and depends on the size, Hamaker constant, and surface potential of the particles, and on the solution ionic strength (Elimelech et al. 1995; Evans and Wennerstrom 1999; Hiemenz and Rajagopalan 1997). NZVI exposed to water acquires an oxide shell, resulting in Fe0/Fe-oxide core/shell particles. Reactive nano-scale iron particles (RNIP) manufactured by Toda Kogyo, Japan, specifically for in situ groundwater remediation, have a Fe0 core and a primarily magnetite (Fe3O4) shell (Liu et al. 2005; Nurmi et al. 2005), both of which are magnetic (McCurrie 1994; Rosenweig 1985). We recently reported the rapid aggregation and gelation of chain-like aggregates in aqueous bare (unmodified) RNIP dispersions. The rapid aggregation resulted from the magnetic attractive force between particles due to their intrinsic magnetic moments (Phenrat et al. 2007). Gelation of the chain-like aggregates not only increases the size of NZVI clusters being transported, which might cause pore plugging during the intended groundwater application (Phenrat et al. 2007; Saleh et al. 2007), but also enhances gravitational sedimentation (Allain and Cloitre 1993; Phenrat et al. 2007) which might promote particle deposition (Elimelech et al. 1995). For this reason, particle modification to introduce long range repulsive forces that overcome the attractive forces is required to enhance NZVI colloidal stability and thus its mobility in groundwater.
Surface modification by physisorption of charged macromolecules has received a great deal of attention (Goddard and Vincent 1984; Sato and Ruch 1980), especially in industrial applications (Duro et al. 1999), as a mean to enhance colloidal stability of both magnetic (Viota et al. 2005; Williams et al. 2006) and non-magnetic dispersions (Kim and Sigmund 2004; Singh et al. 2005). Adsorption of charged macromolecules onto the particle surface is governed by the molecular weight, ionization and charge density of the macromolecule, the charge density and polarity of the solid surface, the solvent quality, and ionic strength (Fleer et al. 1993; Holmberg et al. 2003). The mass adsorbed and the configuration of the adsorbed layer is dictated by a balance between electrostatic attraction to the surface and repulsions among neighboring ionized monomer units, a loss of chain entropy upon adsorption, and also nonspecific dipolar interactions between the macromolecule, the solvent and the surface (Fleer et al. 1993; Holmberg et al. 2003).
Our recent studies have demonstrated the enhancement of NZVI colloidal stability (Saleh et al. 2005) and mobility through saturated porous media (Saleh et al. 2007) by adsorbing a novel series of triblock copolymers to the NZVI surface. However, these experimental triblock copolymers are synthesized in small quantities and have not yet been scaled up for large scale field application. Therefore, one objective of this study is to evaluate the ability of inexpensive and commercially available anionic polyelectrolytes, including polystyrene sulfonate (sodium salt) (PSS), carboxymethyl cellulose (sodium salt) (CMC), and polyaspartate (sodium salt) (PAP), to stabilize NZVI dispersions against aggregation and sedimentation. The PAP monomer unit is aspartate, one of the 20 natural amino acid building blocks of proteins, making PAP of interest as potentially an environmentally benign modifier. A modified biopolymer, CMC may also be a potentially benign modifier. The use of anionic polyelectrolytes is particularly relevant to groundwater mobility, since most mineral and natural organic matter surfaces encountered in groundwater are negatively charged. Thus, the coating must also provide electrostatic repulsion from these surfaces in order to minimize adhesion and filtration. The second objective is to characterize the properties of the absorbed polyelectrolyte layers, including adsorbed mass and layer thickness, and correlate these properties with the ability of each modifier to stabilize the NZVI dispersions. Therefore, we measure an adsorption isotherm for each polyelectrolyte on NZVI and characterize each adsorbed polyelectrolyte layer in terms of electrical potential, thickness, and a softness parameter that relates to solvent drainage through the layer (Ohshima 1995a). Although dynamic light scattering (DLS) is sometimes used to estimate the thickness of polymer layers adsorbed on colloidal particles, the RNIP in this study is far too polydisperse to allow an accurate thickness determination by DLS. Therefore, we used Ohshima’s approach, which is not very sensitive to particle polydispersity for the size of particles (5 nm to 40 nm) and measured layer thicknesses used here (see Appendix). Finally, extended DLVO theory that includes both magnetic attraction and steric repulsions in addition to the usual van der Waals and electrostatic double layer forces is used to semi-quantitatively support the experimentally observed effects of each adsorbed polyelectrolyte on the aggregation and sedimentation of NZVI.
Nanoscale zerovalent iron particles
Reactive nanoscale iron particles (RNIP), commercially available reactive Fe0/Fe3O4 core-shell NZVI particles, were obtained from Toda Kogyo, Japan. The physical and chemical properties of RNIP were reported previously (Liu et al. 2005; Nurmi et al. 2005; Phenrat et al. 2007). RNIP consists of a polydisperse suspension of irregularly shaped particles, with primary particles ranging in size from 5 nm to 40 nm with a median radius of ∼20 nm. Prior to use, RNIP had been stored as an aqueous slurry (pH 10.6) at approximately 300 g/L under anaerobic conditions in a glove box for 6 months. From this slurry, an aqueous stock dispersion (10 mL at 120 g/L) was prepared in 1 mM NaHCO3 using an ultrasonic probe (550 sonic dismembrator, Fisher Scientific, Howell, NJ) at power level 3 for 30 min to break aggregates that formed during storage. This sonicated stock dispersion was then diluted again with 1 mM NaHCO3 to ∼6 g/L. The specific surface area of RNIP was determined by the BET method to be 15 ± 0.5 m2/g.
Chemical properties and structures of polyelectrolyte surface modifiers
Average monomer MW (g/mol)
Poly(styrene sulfonate)/strong anionic polyelectrolyte
Carboxymethylcellulose/weak anionic polysaccharide
Polyaspartate/weak anionic polypeptide
The adsorption isotherms of the polyelectrolytes on RNIP were measured by the solution depletion method with the following conditions: All suspensions contained 3 g/L RNIP and 1 mM NaHCO3. Polyelectrolyte stock solution aliquots were added to provide equilibrium polyelectrolyte concentrations ranging from 5 to 1,000 mg/L. The pH of the mixtures was from 9.5 to 10.5 due to the oxidation of Fe0 in the particles and subsequent production of OH− (Liu and Lowry 2006). The isoelectric point of bare RNIP is pH 6.3. Therefore, bare RNIP is negatively charged in this study. The samples were mixed at 25°C for 48 h using an end-over-end rotator at 30 rpm. Afterward, they were centrifuged at 27,500 rpm for 80 min (Sorvall® Ultracentrifuge OTD65B) to separate the particles from the supernatant. The equilibrium concentration of non-adsorbed PSS in the supernatant was measured by the absorbance at 225 nm (extinction coefficient = 0.04 L mg−1 cm−1) using a UV–vis spectrophotometer (Varian, Palo Alto, CA). The equilibrium concentration of PAP and CMC were measured using a total organic carbon analyzer (OI Analytical). The adsorbed mass was calculated from the difference between the initial and equilibrium concentrations. Each isotherm point was measured from duplicate samples. Isotherms are plotted using the average sorbed mass and equilibrium concentration and multi-directional error bars showing the standard deviation of the measurements.
Adsorbed polyelectrolyte layer characterization, aggregation, and sedimentation
Adsorbed polyelectrolyte layer characterization measurements, aggregation, and sedimentation were conducted on samples prepared with 3 g/L RNIP and 1 g/L polyelectrolyte in 1 mM NaHCO3. After equilibration, the dispersions were centrifuged at 27,500 rpm for 80 min, washed to remove free polymer from solution, and re-suspended in 1 mM NaHCO3. This process was repeated three times followed by ultrasonication for 10 min prior to measuring aggregation, sedimentation, or electrophoretic mobility as described next.
Adsorbed polyelectrolyte layer characterization
The procedure for extracting the characteristics of the polyelectrolyte layer from EM data involves fitting Eq. 4, with terms defined as in Eqs. 5–8 to the experimental electrophoretic mobility vs. concentration of a symmetrical electrolyte (NaCl in this study) to obtain the best fit N, λ, and d (Ohshima 1994, 1995a, 1995b; Ohshima et al. 1992; Ramos-Tejada et al. 2003; Tsuneda et al. 2004; Viota et al. 2004, 2005). All other parameters in Eqs. 4–8 are fixed for a given salt concentration. A MATLAB (the Mathworks, Novi, MI) code employing iterative least squares minimization was used for this fitting the EM data (Fig. 4).
Calculated surface excess, Γmax, for each adsorbed polyelectrolytes
2.1 ± 0.3
1. 8 ± 0.2
74 ± 12
1.9 ± 0.2
0.1 ± 0.01
6 ± 0.7
1.0 ± 0.2
0.6 ± 0.1
33 ± 5
2.0 ± 0.1
0.2 ± 0.01
9 ± 0.4
2.3 ± 0.2
55.1 ± 5.6
2767 ± 279
2.2 ± 0.4
13.2 ± 2.1
665 ± 105
Dynamic light scattering (Malvern Zetasizer, Southborough, MA) was used to monitor the time-dependent hydrodynamic diameter of aggregates during the early stage of aggregation (first 60 min). All measurements were conducted at 25 °C. Dilute samples (5, 30, and 60 mg/L) of the washed, polyelectrolyte-coated RNIP dispersions as well as the bare RNIP dispersions were used in order to avoid multiple scattering effects. The CONTIN algorithm was used to convert intensity autocorrelation functions to intensity-weighted particle hydrodynamic diameter distributions, assuming the Stokes-Einstein relationship for spherical particles.
To determine the fraction of the particle population that aggregates, comparisons between the intensity averaged DLS data and number averaged DLS data were made. Large particles scatter much more light than small particles because the scattering intensity is proportional to the sixth power of the particle diameter according to Rayleigh’s approximation (Hiemenz and Rajagopalan 1997). Therefore, the intensity- averaged particle size distributions are particularly sensitive to aggregation even if only a small fraction of particles in the population aggregate. The number-averaged particle size distribution is less sensitive to aggregation. The intensity-weighted particle hydrodynamic diameter distributions were converted to volume- or number-weighted particle size distributions using the refractive index of the magnetite shell to represent the inhomogeneous refractive index of the Fe0/Fe3O4 core-shell structure. The volume- and number-weighted particle size distributions of RNIP were insensitive to the choice of refractive index, i.e. using refractive index values for either Fe0(n = 2.87 + 3.35i) or Fe3O4 (n = 2.42) yielded similar particle size distributions (the difference is <5%).
It should be noted that although these experiments are conducted ex situ (i.e. in a DLS cuvette), the fundamental physics governing aggregation (i.e. all particle–particle interactions including van der Waals attraction, magnetic attraction, electrical double repulsion, and electrosteric repulsion) in these experiments are also operable in porous media and the trends observed here should be useful for predicting aggregation behavior in porous media. Ex situ DLS aggregation experiments are a well-established method to understand nanoparticle (colloid) transport in porous media (Chen and Elimelech 2006; Dunphy Guzman et al. 2006; Wiesner et al. 2006).
The sedimentation of bare RNIP and the washed, polyelectrolyte-coated RNIP was determined for three different initial RNIP concentrations (100, 400, and 800 mg/L) by monitoring the optical absorbance at 800 nm as a function of time by UV–vis spectrophotometery (Varian, Palo Alto, CA). All measurements were made at 25 °C in duplicate.
Results and discussion
Polyelectrolyte adsorption to RNIP
Characteristics of the adsorbed polyelectrolyte layers
Characteristics of the adsorbed polyelectrolyte layers at pH 8.5 ± 0.1 as estimated by Ohshima’s soft particle analysis
Stable fraction with respect to aggregationc
Stable fraction with respect to sedimentationd
(% by vol.)
(% by mass)
0.17 ± 0.09
67 ± 7
41 ± 11
2.2 ± 1.1
1.1 ± 0.6
0.13 ± 0.08
198 ± 30
55 ± 20
1.7 ± 0.1
0.83 ± 0.2
0.35 ± 0.20
7.2 ± 3.2
9.2 ± 4.2
4.5 ± 2.5
19.7 ± 4.0
0.33 ± 0.07
40 ± 6.5
24 ± 0.6
4.2 ± 0.9
2.7 ± 0.1
0.37 ± 0.12
40 ± 12
24 ± 0.6
4.7 ± 1.5
2.9 ± 0.4
0.36 ± 0.21
44 ± 13
26 ± 5
4.6 ± 2.6
2.7 ± 1.0
The measured layer thicknesses (d) for each polymer are consistent with expectations. Comparing the thicknesses of adsorbed PSS and CMC layers with comparable molecular weights, i.e. comparing PSS70K with CMC90K and PSS1M with CMC700K, the thickness of adsorbed PSS is in general several folds greater than for CMC, yet the saturation surface excess concentrations for these polyelectrolytes are less than two fold different. This implies that adsorbed PSS chains are more extended than the CMC chains. This observation is consistent with a recent report that CMC adsorbed in a flat conformation on a talc surface (Wang and Somasundaran 2005). Furthermore, the CMC carboxyl groups can protonate to reduce the intra-layer charge density, and carboxyl groups are known to specifically adsorb to iron oxide surfaces (Chibowski and Wisniewska 2002); each factor would tend to favor a less extended chain conformation at the surface. In contrast, PSS is a strong polyelectrolyte that tends to have a constant degree of ionization in adsorbed layers (Biesheuvel 2004) which would favor an extended conformation.
In all cases, the thickness of the adsorbed layer increased with increasing molecular weight of the polyelectrolyte. Notably, the thickness of adsorbed PAP2.5K and PAP10K layers is as large as that of CMC700K even though PAP has a significantly lower molecular weight compared to CMC700K. The contour lengths of PAP2.5K and PAP10K are 10 and 39 nm, respectively, as roughly estimated from the molecular structure of PAP. Therefore, the relatively large estimated layer thicknesses of both PAP are a result of multilayer adsorption, as implied by the adsorption isotherm.
For the measured EM, the ψ0 values for polyelectrolyte-modified RNIP calculated from Ohshima’s soft particle theory (Table 3) are significantly smaller (∼ −3 mV) than the ζ-potential that would be calculated from Smoluchowski’s formula (∼ −50 mV) which assumes a hard particle. This is typical for Ohshima’s soft particle theory because the EM of soft particles is insensitive to the position of the slip plane so the hard particle assumption is invalid (Nakamura et al. 1992; Viota et al. 2004, 2005). The low electrical surface potential for polyelectrolyte-modified RNIP implies that electrostatic double layer repulsion does not play a major role in stabilizing polyelectrolyte-modified RNIP, and it is the electrosteric component that enhances the colloidal stability of polyelectrolyte-modified RNIP as discussed later.
Effect of surface modification on RNIP aggregation
Adsorbed polyelectrolyte layers can provide electrosteric repulsions to enhance the NZVI dispersion stability with respect to aggregation and sedimentation. In this section, we qualitatively examine the effect of the adsorbed polyelectrolyte layers on the rate and extent of aggregation of bare and surface-modified RNIP. The effect of the adsorbed polyelectrolyte layer on sedimentation is discussed subsequently.
For all surface modified RNIP, adsorbed polyelectrolyte layers affect the rate of aggregation and the fraction of the particle population that aggregates. The aggregating, unstabilized fraction is the fraction that has a continuously increasing particle size distribution over time. In contrast, the stabilized fraction is that with a relatively stable particle size distribution over time and a size range in good agreement with the individual particle size distribution as observed by TEM (RH = 5–40 nm) (Nurmi et al. 2005).
Of the six polyelectrolytes evaluated, PSS1M modified RNIP was an outlier. Although the PSS1M modified RNIP has high surface excess (∼2 mg/m2) and a highly extended layer (∼198 nm), only 35 vol% remained stable with RH ranging from 13 to 110 nm (average RH = 30 nm). A possible explanation is that PSS 1M does not provide steric stabilization due to inhomogenous surface coverage, a result of its large degree of polymerization and the small number (6) of polymer chains adsorbed to each particle compared to the other polyelectrolytes evaluated (Table 2). During a Brownian collision of two PSS1M modified RNIP particles the PSS1M can move laterally, resulting in displacement coagulation (Napper 1983). This displacement coagulation is not expected for smaller polyelectrolytes such as PSS70K, CMC90K, PAP2.5K and PAP10K in this study because the number of the polyelectrolyte chains at the RNIP surface is quite high (Table 2); therefore, lateral movement is unlikely for these modifiers. As for CMC700K, although the average number of chains per RNIP particle (∼9 chains) is lower than the smaller polyelectrolytes, the possibility of the lateral movement is lower than PSS1M because CMC has carboxylic groups which strongly anchor it to the Fe3O4 surface. This hypothesis is also supported by the small layer extension for CMC (d = 40 nm) compared to PSS1M (d = 198 nm).
Effect of surface modification on RNIP sedimentation
A dispersion of nanoparticles can remain stable for very long time if the diffusion flux of nanoparticles, which is inversely proportional to the particle size and works in the opposite direction to the gravity, overcomes the sedimentation flux which is proportional to the square of the particle radius. When nanoparticles aggregate to micron-sized clusters, they tend to settle to the bottom of the container because the diffusion flux becomes smaller than the sedimentation flux. Thus, the sedimentation rate is a good indicator of colloidal stability of bare and polyelectrolyte-modified RNIP.
It should be noted that aggregation and sedimentation kinetics are concentration-dependent, i.e. second order with respect to particle population (Evans and Wennerstrom 1999; Hiemenz and Rajagopalan 1997). However, the main objective of this study was to compare the efficacy of different polymeric surface modifiers. Thus, all other parameters including particle type and particle concentration were kept constant and the aggregation and sedimentation kinetics as a function of initial particle concentrations are not reported.
Extended DLVO theory to explain the influence of adsorbed polyelectrolyte layers on RNIP stabilization
The lower rate and extent of aggregation (Figs. 6–8) and increased unsedimented fraction (Figs. 9 and 10) of the surface modified RNIP compared to bare RNIP can be semi-quantitatively explained using DLVO theory extended to include magnetic and steric forces.
Measured Fe0 of each surface modified RNIP and the estimated saturation magnetization, Ms
Fe0 content (%)
Estimated MS (kA/M)a
26 ± 0.5
741 ± 10
15 ± 0.0
589 ± 1
15 ± 0.7
576 ± 15
29 ± 1.4
779 ± 25
24 ± 0.3
706 ± 5
25 ± 1.3
727 ± 24
24 ± 0.6
712 ± 11
Applying this simplified calculation to PSS1M modified RNIP predicts the largest stable fraction. This contrasts to both aggregation and sedimentation results which reveal that only relatively small fraction remains stable. This disagreement can be explained by non-thermodynamically limited steric stabilization (Napper 1983) as noted above. For Eqs. 15 and 16 to be valid, the lateral movement of adsorbed polyelectrolytes onto RNIP surface is not allowed.
This study investigated the stabilization of polydisperse nanoscale zerovalent iron (NZVI) particles (5–40 nm in radius) using commercially available anionic polyelectrolytes, PSS, CMC, and PAP. The maximum surface excess concentration for each of these polyelectrolytes adsorbed on NZVI was ∼1–2 mg/m2. The layer thicknesses estimated using electrophoretic mobility and Ohshima’s soft particle analysis are ∼7 nm for CMC90K, ∼40 nm for CMC90K, PAP2.5K, and PAP10K, ∼67 nm for PSS70K, and ∼198 nm for PSS1M. The stable fractions with respect to both aggregation and sedimentation correlate with the adsorbed mass and layer thickness, indicating that both of these properties must be considered when predicting the stability of polyelectrolyte-stabilized nanoparticles. A fraction of the polydisperse particles used in this study could not be stabilized regardless of the modifier used and rapidly aggregated to micron sized fractal aggregates as observed for bare NZVI. This sedimented fraction is attributed to the larger particles present in the polydisperse NZVI suspension as the magnetic attractive forces increase as r6. According to the aggregation and sedimentation measurements, PSS70K, PAP2.5K, and PAP10K perform best in NZVI stabilization, i.e. 40, 32, 34 wt% of the PSS70K, PAP2.5K, and PAP10K modified NZVI remain suspended for more than several months.
This research was funded in part by the Department of Defense through the Strategic Environmental Research and Development Program (W912HQ-06-C-0038), the Office of Science (BER), U.S. Department of Energy, (DE-FG07-02ER63507), the U.S. EPA (R830898), the U.S. National Science Foundation (BES-0608646), and the Royal Thai Government through a fellowship to Tanapon Phenrat.