Forces of Interactions between Bare and Polymer-Coated Iron and

Article pubs.acs.org/est

Forces of Interactions between Bare and Polymer-Coated Iron and Silica: Effect of pH, Ionic Strength, and Humic Acids Erica Pensini,† Brent E. Sleep,*,† Christopher M. Yip,‡ and Denis O’Carroll§ †

Department of Civil Engineering, University of Toronto, Toronto (ON), M5S 1A4, Canada Department of Chemical Engineering, University of Toronto, Toronto (ON), M5S 3E1, Canada § Department of Civil and Environmental Engineering, Western University, London (ON), N6A 5B9, Canada ‡

S Supporting Information *

ABSTRACT: The interactions between a silica substrate and iron particles were investigated using atomic force microscopy-based force spectroscopy (AFM). The micrometer- and nanosized iron particles employed were either bare or coated with carboxymethyl cellulose (CMC), a polymer utilized to stabilize iron particle suspensions. The effect of water chemistry on the forces of interaction was probed by varying ionic strength (with 100 mM NaCl and 100 mM CaCl2) or pH (4, 5.5, and 8) or by introducing 10 mg/L of humic acids (HA). When particles were uncoated, the forces upon approach between silica and iron were attractive at pH 4 and 5.5 and in 100 mM CaCl2 at pH 8, but they were negligible in 100 mM NaCl buffered to pH 8 and repulsive in water buffered to pH 8 and in HA solutions. HA produced electrosteric repulsion between iron particles and silica, likely due to its sorption to iron particles. HA sorption to silica was excluded on the basis of experiments conducted with a quartz-crystal microbalance with dissipation monitoring. Repulsion with CMC-coated iron was attributed to electrosteric forces, which were damped at high ionic strength. An extended DLVO model and a modified version of Ohshima’s theory were successfully utilized to model AFM data.

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INTRODUCTION Injection of nanoscale zerovalent iron (nZVI) is a potentially effective remediation technology to treat groundwater contaminants, in particular chlorinated compounds.1,2 Achieving adequate subsurface transport of iron particles to target treatment zones is of paramount importance in applying nZVI for remediation.3−5 Subsurface transport is influenced by a variety of factors,6 including groundwater velocity7,8 and particle retention by the geological substrates constituting the aquifer.8 The forces of interaction between particles and substrates significantly influence retention.9 In particular, attractive interactions will favor particle retention, but in the presence of repulsive or neutral interactions, the particle may not be retained by the substrate. In general, iron particles are coated with surfactants and polymers,10−12 including carboxymethyl cellulose (CMC),13,14 to enhance their stability and limit their otherwise rapid aggregation. This study analyzes the effects of polymer coatings on the forces of interaction, which include electrostatic, van der Waals, Born, hydration, and steric forces. Hydration forces are repulsive and their origin has been ascribed to different causes, including the following: changes in the water structure at the interface,15 polarization of water molecules,16 adsorption of ions at the solid−liquid interface, variations in the dielectric constant of the liquid, and formation of hydrogen bonds between the water molecules and the surface.17 © 2012 American Chemical Society

Hydration forces depend on the nature of the interacting surfaces as well as on the water chemistry. It was previously observed that cations could reduce hydration of silica surfaces, as they could bind to the silanols at the silica surface and disrupt the structured water layer at the silica surface.18 Steric forces can arise at short separation distances due to increasing osmotic pressure when polymer strands are compressed between the approaching surfaces. In this study, steric forces are expected in the presence of CMC polymer coatings and in humic acid solutions. Humic acids are macromolecules that can adsorb to nanoscale zerovalent iron19 and to iron oxides,20−26 forming complexes due to their hydroxyl and carboxyl functional groups.27 Studies have examined the impact of different stabilization methods on iron particle transport,10,13,28 but to date the effect of steric forces29−31 and of polymer coatings32 on the interactions between iron particles and silica has not been investigated extensively, with only one previous report examining the effect of poly(ethylene oxide)/poly(propyleneoxide)/poly(ethylene oxide) triblock copolymer coatings on iron particle adhesion to silicon wafers.33 Moreover, Received: Revised: Accepted: Published: 13401

September 10, 2012 November 13, 2012 November 19, 2012 November 19, 2012 dx.doi.org/10.1021/es3036779 | Environ. Sci. Technol. 2012, 46, 13401−13408

Environmental Science & Technology

Article

sorption onto silica. The principles of this technology are briefly discussed in the Supporting Information (cf. section S3) and in greater detail elsewhere.36−42 AFM Cantilever Characteristics and Functionalization. Tipless silicon nitride AFM cantilevers were purchased from Bruker (model NP-O10). Their average spring constant (kc) was determined with the thermal resonance end-mass method43 and varied depending on the wafer used, ranging between 0.064 and 0.096 nN/nm. Micron-sized carbonyl iron particles were affixed onto these cantilevers using a micromanipulator. At the end of each experiment, scanning electron microscopy was used to determine the actual dimensions of the affixed particles. Conventional cantilevers with integral pyramidal tips with an average experimentally determined kc of 0.051 nN/nm were also purchased from Bruker (model NP10). Single iron nanoparticles were directly electrodeposited onto the apex of the cantilevers as described above. Carboxymethyl Cellulose Coating of Iron Particles. The sodium salt of CMC was obtained from Sigma Aldrich (product ID 419273). It had a molecular weight of 90 000 g/ mol and a degree of substitution per mole of cellulose of 0.7. CMC is a neutral molecule only in acidic environments, but it hydrolyzes at higher pH values, acquiring a negative charge. The charge of this polymer depends on the degree of ionization, i.e., the extent to which the Na+ counterion dissociates from the polymer molecule, leaving it negatively charged. The degree of ionization is a function of both the pH of the solution and ionic strength. It was found that around pH 6 dissociation was 100% at ionic strengths of 0.01 M NaCl solutions for a CMC with a degree of substitution of 0.75 and a molecular weight of about 100 000 g/mol.44 The degree of ionization of this CMC decreased with decreasing pH.44 CMC polymer coatings were obtained by immersing probes modified with carbonyl iron particles in a bath of 10 g/L CMC solution in pH 4 acetate buffer. The probes were then gently rinsed with Milli-Q water and immediately used. AFM Working Conditions and Data Analysis. All AFM data were collected using a Nanoscope Multimode AFM equipped with a Nanoscope IIIA controller, utilizing a glass fluid cell and a J-scanner, and version 5.30a of the Nanoscope software. All force curves were collected using a maximum applied load of 18.3 nN at a 1 Hz scan rate. All AFM curves were registered relative to the point of contact (defined as x = 0 in the following discussion) and normalized with respect to the experimentally determined iron particle radius. The adhesive force (F) was calculated as F = kcΔz, where Δz = tip deflection. The adhesive force was measured at the minimum of the retraction curve when uncoated iron particles were used, whereas it equaled the force required to separate the last polymer strand from the silica surface when CMC-coated CIP was employed. On average, 100 approach and retraction curves were measured for each water chemistry with different probes and were acquired in different areas of the silica substrate. Confidence intervals of 95% were computed using a two-tailed Student’s t-distribution. Extended DLVO (XDLVO) and Modified Ohshima’s Model. XDLVO theory was employed to model the interactions between bare iron particles and silica substrates. This theory describes the forces of interaction between surfaces in terms of DLVO and non-DLVO forces. DLVO forces include van der Waals (FvdW) and electrostatic forces (Felectro) as well as the Born force (FBorn), while non-DLVO forces include

no previous studies have analyzed the role of hydration in the interaction between silica and iron particles.

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MATERIALS AND METHODS Carbonyl Iron Particles. Micron-sized carbonyl iron particles (CIP) were purchased from Alfa Aesar (product number 10214). The purity of the particles was assessed in a previous study by scanning electron microscopy−energy dispersive spectroscopy.34 The nominal size of CIP was 6−10 μm. At the end of each AFM experiment, the particles were imaged using scanning electron microscopy to determine their exact size. The point of zero charge (PZC) is expected to be pH >10, consistent with another study in which the same types of particles employed in the present work were utilized.34 Nanosized Iron Particles. Single iron nanoparticles having a size ranging from 0.1 to 0.2 μm were directly electrodeposited onto the apex of the AFM cantilevers using a previously described protocol.35 Briefly, cantilevers passivated with silicone oil and paraffin and rinsed with Milli-Q water were subjected to a ∼75 s 1.2 mA current pulse in a 0.01 M Fe2SO3/0.05 M H2SO4 solution using a Pt wire counter electrode (Figure 1). The apex of tip was exposed prior to deposition by briefly imaging a stainless steel surface with the coated tip.

Figure 1. SEM image of an iron nanoparticle electrodeposited at the AFM tip apex. The edge of the iron nanoparticle is indicated by the arrows.

Silica Substrate. Substrates sputter-coated with silica were purchased from Q-Sense (product id. QSX 303) and employed for AFM and QCM-D experiments. Solutions. The solutions utilized in this study were prepared using water from a Millipore system, the resistivity of which was 18.2 mΩ·cm (Milli-Q water). The pH of Milli-Q water as employed was slightly acidic (5.5), due to equilibration with atmospheric CO2. Milli-Q water was used as such or buffered to pH 8 or 4 with NaHCO3 or acetate buffers, respectively; 100 mM NaCl and CaCl2 solutions were also used, and their pH was either 5.5 or 8 (when the solutions were buffered with NaHCO3). The humic acid sodium salt (Aldrich, H16752) solutions had concentrations of 10 mg/L and a pH of 6.75. All solutions were stored at room temperature (23 °C) for 24 h prior to conducting experiments. Quartz-Crystal Microbalance with Dissipation Monitoring (QCM-D). A QCM-D system (Q-Sense, Biolin Scientific, Sweden) was employed to analyze humic acid 13402

dx.doi.org/10.1021/es3036779 | Environ. Sci. Technol. 2012, 46, 13401−13408


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Table 1. Mean Normalized Forces of Interaction upon Approach between Bare Iron Particles and Silica with 95% Confidence Intervals force upon approach, x = 0 (nN/μm)

a

solution

pH

CIP

nanosized iron

Milli-Q acetate 100 mM NaCl 100 mM CaCl2 Milli-Q + NaHCO3a 100 mM NaCl + NaHCO3 100 mM CaCl2 + NaHCO3 HAa

5.5 4 5.5 5.5 8 8 8 6.75

−0.12 ± 0.02 −0.16 ± 0.05 −0.13 ± 0.04 −0.12 ± 0.04 0.35 ± 0.08 0.01 ± 0.05 −0.18 ± 0.09 0.77 ± 0.14

−0.74 ± 0.44 −0.61 ± 0.22 −0.61 ± 0.27 −0.47 ± 0.16 1.25 ± 0.48 −0.05 ± 0.26 −0.59 ± 0.16 not measured

adhesive force (nN/μm) CIP −1.41 −1.65 −1.10 −0.85 −0.40 −0.57 −1.84 −1.27

± ± ± ± ± ± ± ±

nanosized iron 0.44 1.01 0.39 0.40 0.29 0.24 0.29 0.46

−2.58 ± 0.89 −2.00 ± 0.32 −1.92 ± 0.75 −1.82 ± 0.74 −0.36 ± 0.28 −1.22 ± 0.09 −2.55 ± 0.56 not measured

Adhesive forces were negligible in some cases: confidence intervals were found on the basis of detected values.

where Γ is the grafting density of the polymers. This expression is valid only for L0/d < 1, as steric forces are zero when the polymer strands are not compressed, i.e., when d ≥ L0. Since only iron particles were polymer-coated and silica was not, eq 2 was modified by substituting L0 for 2L0. The optimal parameters for either XDLVO or modified Ohshima’s models were obtained by minimizing the sum of the squared differences between the estimated and the predicted values. This was accomplished using the Solver tool in Microsoft Excel. A sensitivity analysis was also conducted by perturbing by ±30% the optimal values of the parameters.

hydrophobic and hydration forces. Hydrophobic interactions are not of importance in this study, whereas hydration forces should play a significant role because both iron oxides45−50 and silica are strongly hydrated. The hydration of silica is widely recognized, although the reasons for this are debated.15,51 Some researchers hypothesize the existence of a “gel layer” of polymeric silicates at the silica surface,52,53 while others speculate that water molecules form hydrogen bonds with the silanol groups and that this structured water layer acts as a steric barrier.15,18,54 Hydration forces are modeled using empirical exponential equations. The XDLVO forces between a sphere and a planar surface are given in the Supporting Information (eqs S2−S4). The assumption underlying the XDLVO model is that the interacting surfaces are hard; therefore, XDLVO theory could be utilized only to describe AFM data collected with uncoated iron particles. A modified Ohshima’s model was employed when CMC-coated particles were used. This model describes the surface potential of a particle comprised of a hard core coated with a charged polymer layer in terms of the zeta potential of the particle’s hard core [Ai(t)] and of the electrostatic potential of the polymer, known as the Donnan potential (ψDonnan). The surface potential can be described as55,56 ψ (d) = ψDonnan + (ψ0 − ψDonnan)ekmx + ⎡ ⎢ 1 + tanh ln⎢ ⎢ 1 + tanh ⎣

−k m(x + L0) ⎤

v eζ 4kBT

RESULTS AND DISCUSSION AFM Measurements with Bare Iron (CIP and Nanoiron). The forces between iron particles and silica are given in Table 1. The force curves obtained in select water chemistries using CIP are shown in Figure 2a,b, while the other force curves are in the Supporting Information (Figures S1−S8). In the absence of CMC coatings, the mean forces of interaction upon approach between silica and either CIP or nanosized iron particles were attractive in Milli-Q water buffered to pH 4 (acetate solutions, Figure S5, Supporting Information), at pH 5.5 in Milli-Q water and in 100 mM NaCl and CaCl2 solutions (Figures S1, S3, and S4, Supporting Information), and in 100 mM CaCl2 buffered to pH 8 with NaHCO3 (Figure S6, Supporting Information). In 100 mM NaCl buffered to pH 8, the forces upon approach were negligible (Figure S7, Supporting Information), whereas they were repulsive in Milli-Q water buffered to pH 8, with either CIP or nanosized iron particles (Figure 2b and S8, Supporting Information). Repulsion upon approach was also measured between silica and CIP in Milli-Q water amended with humic acid at pH 6.75 (experiments with this chemistry were not conducted with nanosized iron). Adhesive forces were obtained from the retraction curves. The effect of the water chemistry on adhesive forces was smaller than the effect on the forces upon approach. Consistent with mean forces of interaction upon approach, average adhesive forces were weakest in Milli-Q water buffered to pH 8 (NaHCO3 solutions) and in 100 mM NaCl buffered to pH 8 with NaHCO3, with either CIP or nanosized iron. To interpret these results it is necessary to analyze each of the forces involved in the interactions between iron particles and silica. In solutions at pH 4 and 5.5 attraction is ascribed to van der Waals forces as well as to electrostatic attraction between iron particles and silica, which have opposite surface charges. The isolectric point (IEP) and the point of zero charge (PZC) for silica are approximately at pH 2 or below,51,54,58−61

2kBT ve

( )e ( )e v eζ 4kBT

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⎥ ⎥ −k m(x + L0) ⎥ ⎦

(1)

where ψDonnan is the Donnan potential, d is the particle separation distance, L0 is the thickness of the polymer layer, ζ is the zeta potential of the bare core of the composite particle, and −L0 ≤ x ≤ 0 is a local coordinate that spans the thickness of the polymer layer at the hard particle surface. In its original formulation, Ohshima’s model did not account for hydration or steric forces. Therefore, in this study Ohshima’s model was further modified to incorporate hydration (eq S5, Supporting Information) and steric forces. Steric forces between two surfaces coated by a polymer layer of thickness L0 are57 ⎡

Fsteric ≈ kBT Γ

⎛ 2L0 ⎞ 3/2⎢⎜ ⎟ ⎢⎣⎝ d ⎠

9/4

⎛ d ⎞3/4 ⎤ −⎜ ⎟ ⎥ ⎝ 2L0 ⎠ ⎥⎦

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binding at the silica surface.62 This is consistent with our observation of a pH-dependent effect for both NaCl and CaCl2 on the iron−silica interaction forces. Humic acid (HA) was also observed to contribute repulsive forces upon approach between iron particles and silica (0.77 ± 0.14 mN/m for CIP, Table 1 and Figure S2, Supporting Information), whereas it had a minor effect on the adhesive force. The repulsion measured in humic acid solutions is in agreement with previous studies, which showed that natural organic matter, of which humic acid was a major constituent,63 hindered nanoscale zerovalent iron attachment onto silica.7 Humic acids bear negative charges even at low pH values because of the dissociation of their acidic groups21 and therefore do not get adsorbed at the silica surface. It was reported that HA adsorption on silica did not occur in the pH range from 2 to 12.64 Negligible HA sorption onto silica was confirmed in the present study by QCM-D experiments. During these experiments the overtones and the dissipation did not vary when the solution flushed through the cell was changed from Milli-Q water to a 10 mg/L HA solution and back to Milli-Q water, indicating negligible HA sorption onto the silica surface over a time period of approximately 5 min (Supporting Information, Figure S15). It is noted that sorption of macromolecules can display time dependency65−67 and that the time for complete adsorption can be longer than that considered in the QCM-D experiments described above. However, sorption of HA onto silica is expected to be negligible even over long time periods or else it would have been detected during our measurements, albeit to a limited extent. HA does not adsorb onto silica, but it can adsorb onto iron particles. The adsorption of negatively charged humic acid to iron particles may neutralize their surface charge or render it negative and shift the point of zero charge to lower pH.19 Furthermore, other studies showed that HA sorption at the iron surface was able to stabilize iron particle suspensions by contributing electrosteric repulsion between particles.22,59 In the light of the above information, it is believed that HA produced electrosteric repulsion between CIP and the negatively charged silica surface (Figure S2, Supporting Information). AFM Measurements with CMC-Coated CIP. The forces between CMC-coated CIP and silica are given in Table 2, while sample force curves collected in the various water chemistries can be found in the Supporting Information (Figures S9−S14). When CIP was coated with CMC, the forces of interaction upon approach between iron particles and silica were repulsive in all chemistries except 100 mM CaCl2 at pH 5.5, in which the

Figure 2. (a) Forces of interaction between CIP and silica in Milli-Q water (pH 5.5) and (b) forces of interaction between CIP and silica in solutions buffered with NaHCO3 (pH 8).

while the point of zero charge for carbonyl iron particles was found to be approximately at pH 10 in a previous study.34 At pH 8, repulsion upon approach was measured in the absence of NaCl or CaCl2 salts. This result may be explained in terms of weaker electrostatic attraction between silica and iron particles, which should be closer to their PZC at pH 8. Reduced electrostatic attraction would allow hydration forces to dominate, producing overall repulsion upon approach. At pH 8, both 100 mM NaCl and 100 mM CaCl2 damped repulsive hydration forces, but CaCl2 had a more significant effect than NaCl. In particular, NaCl could only damp the overall repulsion upon approach (to 0.01 ± 0.05 mN/m for CIP and to −0.05 ± 0.26 mN/m for nanoparticles, Table 1 and Figure S7, Supporting Information), but CaCl2 reversed the overall force from repulsive (0.35 ± 0.08 mN/m for CIP, 1.25 ± 0.48 mN/m for nanoparticles, Table 1 and Figures 2b, S8, Supporting Information) to attractive (−0.18 ± 0.09 for CIP; −0.59 ± 0.16 for nanoparticles, Table 1 and Figure S6, Supporting Information). The ability of cations to weaken repulsive hydration forces is reported to depend on the cation size.18 Therefore, Ca2+ should lead to a more significant damping effect on the repulsive hydration forces compared to Na+, since the hydration radius of calcium cations is larger than that of sodium cations. In a previous study, it was shown that the ability of Ca2+ ions to damp hydration forces was pH- and concentration-dependent.62 At basic pH, the density of negative surface charges is higher than at low pH, leading to stronger electrostatic attraction for positively charged calcium cations. Therefore, the effect of Ca2+ was more significant at high pH values and high CaCl2 concentration due to greater Ca2+

Table 2. Mean Normalized Forces of Interaction upon Approach between CMC-Coated CIP and Silica with 95% Confidence Intervals solution

pH

force upon approach, x = 0 (nN/μm)

adhesive force (nN/μm)

Milli-Qa acetatea 100 mM NaCl 100 mM CaCl2 Milli-Q + NaHCO3 HAa

5.5 4 5.5 5.5 8

0.41 ± 0.31 0.26 ± 0.19 0.04 ± 0.03 −0.02 ± 0.05 0.24 ± 0.05

−0.12 ± 0.09 −0.08 ± 0.03 −0.08 ± 0.04 −0.14 ± 0.07 negligible

6.75

0.74 ± 0.24

−0.05 ± 0.06

a

Adhesive forces were negligible in some cases: confidence intervals were found on the basis of detected values. 13404



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Figure 3. Forces upon approach between CMC−CIP and silica in 100 mM NaCl at pH 5.5, 23 °C: (a) comparison between modified Ohshima’s and AFM forces, (b) contributions of the modified Ohshima’s model forces, (c) detail of Born and steric forces, and (d) detail of the electrostatic force.

forces of interaction upon approach were negligible. Also, repulsion upon approach was weaker in 100 mM NaCl solutions than in Milli-Q water at all pH values (4, 5.5, and 8) and in Milli-Q water amended with HA (pH 6.75). Water chemistry had a minor impact on adhesive forces with the exception of Milli-Q buffered to pH 8, in which adhesive forces were negligible. When adhesive forces were not negligible, the retraction curve was often characterized by a sawtooth shape, due to the rupturing and detachment of the polymer chains from the silica surface. The fact that CMC coatings hinder attachment of iron particles onto silica compared to bare iron particles can be explained in terms of the electrosteric repulsion between negatively charged silica and negatively charged CMC. The degree of ionization of CMC decreased with decreasing pH.44 Therefore, at high pH CMC bears a higher density of negative surface charges than at low pH values, increasing interchain repulsion between chains and possibly promoting extended versus coiled conformations.68 Such conformations would result in higher osmotic pressures when CMC−CIP is brought close to the silica surface. Also, basic chemistries should produce stronger electrostatic repulsion between CMC and negatively charged silica compared to acidic solutions. When cations are present, they can bind to the CMC carboxyl groups neutralizing their negative charge. Cations may sorb at the silica surface, acting as binding sites for CMC chains. Furthermore, when cations reduce the charge of the CMC chains, interchain repulsion between chains will be reduced, yielding lower osmotic pressures when CMC−CIP is brought close to the silica surface. Therefore, NaCl and CaCl2

may damp steric forces in addition to screening electrostatic repulsion. These considerations may explain why repulsion upon approach was damped in NaCl and CaCl2 solutions at pH 5.5. The effect of ionic strength was not probed at pH 4 and 8. However, we believe that increasing the ionic strength at these pH values would have effects similar to those observed at pH 5.5, damping electrostatic and steric repulsion between CMCcoated particles and silica. Nonetheless, even at the highest ionic strengths attraction between CMC−CIP and silica was weaker than when bare iron particles were used. The effect of CMC coating on nanoiron was not probed, but it is expected to be qualitatively similar to the effect on the forces between CIP and silica, producing steric and electrostatic repulsion between nanosized iron particles and silica. These results obtained suggest that CMC coatings would enhance iron particle transport in sandy aquifers, as they result in repulsive iron/silica interaction energies in most water chemistries, whereas many bare iron particles would be attracted to the silica surface. It is noted that this study is concerned with the forces of interaction only and does not examine the effects of other factors that may affect attachment (e.g. porewater velocity, grain size distribution, pore size distribution, and fluid viscosity). XDLVO and Modified Ohshima’s Models. Modeling was conducted to highlight the most important components of the interactions between silica and iron particles, in the presence and absence of CMC polymer coatings. Using the data collected in 100 mM NaCl solutions at pH 5.5, with bare and CMC-coated CIP, both the XDLVO and modified Ohshima’s models successfully described AFM data at 13405



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ACKNOWLEDGMENTS This work was funded by the Natural Sciences and Research Council of Canada through a Strategic Projects Grant (Development of Nanometals for Source Zone Remediation).

separation distances equal to or greater than 1.2 and 2.5 nm, respectively (Figure 3 and Supporting Information, Figure S16). The optimal parameters are reported in the Supporting Information (Tables S1, S3). The fitted parameters for DLVO forces are ion the same order of magnitude as those reported in the literature, with differences attributed to dissimilarities in the experimental conditions. Parameters for hydration forces between iron and silica are not available in the literature, and they are reported here for the first time. This information is of great value for future studies, in which modeling is used to interpret the results of mesoscale experiments (e.g. sand column tests aimed at understanding iron particle transport). The theoretical analysis of the forces showed that hydration forces contributed considerably to the overall interaction when either bare or CMC-coated particles were used. The magnitude of the van der Waals forces was also important, representing the main attractive component of the overall force. A sensitivity analysis conducted on the parameters showed that the XDLVO and modified Ohshima’s model were most sensitive to perturbations in the Hamaker constant and in the parameters appearing in the empirical expression for hydration forces (Supporting Information, Tables S2, S3). In particular, the exponents appearing in the expressions of the hydration forces have the greatest impact on the magnitude of the XDLVO and Ohshima’s forces, while the multiplying coefficients play a less significant role. This result was expected given the importance of van der Waals and hydration forces. Incorporating steric forces in Ohshima’s model appeared to be less important than including hydration forces, since steric interaction contributed weak repulsive forces solely at the shortest separation distances at which polymer strands were compressed. The theoretical analysis described here has highlighted the role of non-DLVO forces in the interactions between silica and iron. This information is of paramount importance, since neglecting non-DLVO forces can lead to a mismatch between theoretical predictions and experimental observations.69−71 In addition, experiments conducted at the small scale shed new light on previous findings from mesoscale sand column experiments and small scale studies conducted with techniques different from AFM. These findings include the role of humic acids in promoting iron transport,7,72,73 the increased mobility of nZVI in the presence of polymeric coatings,73,74 and the impact of pH75 and ionic strength2,76 on iron transport. Finally, the experimental data and model parameters presented can be used to design nZVI formulations for optimum delivery to contaminant source zones.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

AFM force curves for those chemistries for which plots were not given in the main body of the paper; QCM-D for HA sorption onto silica; XDLVO and modified Ohshima modeling, including optimal parameters, sensitivity analysis, and comparison between AFM and model curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: 416-978-3005; fax: 416-978-6813. Notes

The authors declare no competing financial interest. 13406



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