Formation of Iron(III) (Hydr)oxides on Polyaspartate- and Alginate

Nov 16, 2012 - The arrangement of functional groups at the substrate surface (−OH and −COOH) may also contribute. These results provide new inform...
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Formation of Iron(III) (Hydr)oxides on Polyaspartate- and AlginateCoated Substrates: Effects of Coating Hydrophilicity and Functional Group Jessica R. Ray,† Byeongdu Lee,‡ Jonas Baltrusaitis,§ and Young-Shin Jun*,† †

Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

ABSTRACT: To better understand the transport of contaminants in aqueous environments, we need more accurate information about heterogeneous and homogeneous nucleation of iron(III) hydroxide nanoparticles in the presence of organics. We combined synchrotron-based grazing incidence small-angle X-ray scattering (GISAXS) and SAXS and other nanoparticle and substrate surface characterization techniques to observe iron(III) (hydr)oxide [10−4 M Fe(NO3)3 in 10 mM NaNO3] precipitation on quartz and on polyaspartate- and alginate-coated glass substrates and in solution (pH = 3.7 ± 0.2). Polyaspartate was determined to be the most negatively charged substrate and quartz the least; however, after 2 h, total nanoparticle volume calculationsfrom GISAXSindicate that positively charged precipitation on quartz is twice that of alginate and 10 times higher than on polyaspartate, implying that electrostatics do not govern iron(III) (hydr)oxide nucleation. On the basis of contact angle measurements and surface characterization, we concluded that the degree of hydrophilicity may control heterogeneous nucleation on quartz and organic-coated substrates. The arrangement of functional groups at the substrate surface (−OH and −COOH) may also contribute. These results provide new information for elucidating the effects of polymeric organic substrate coatings on the size, volume, and location of nucleating iron hydroxides, which will help predict nanoparticle interactions in natural and engineered systems.

1. INTRODUCTION Iron(III) (hydr)oxide nucleation is an important process in aqueous systems. If formed on mineral or substrate surfaces, these nanoparticles can act as sinks for more effective contaminant removal; however, if formed in solution, they can act as mobile carriers for such inorganic contaminants as arsenic,1 uranium(VI),2 and chromium.3 Acid mine drainage resulting from coal-cleaning processes can release large amounts of aqueous ferric ions in bodies of water, where contaminants can be carried downstream.4 Furthermore, aqueous ferric ions that are transferred to surface coastal waters may act as nutrients for microorganisms such as marine plankton (e.g., bacteria, algae), thus greatly affecting the cycling of carbon.5 Therefore, it is crucial to identify environmental factors and mechanisms influencing iron (hydr)oxide formation and how these factors affect these important processes. Iron(III) (hydr)oxide nanoparticle nucleation can proceed via a surface-induced pathway (i.e., heterogeneous nucleation), in which substrate surface sites facilitate binding of the solute, or via precipitation in bulk solution (i.e., homogeneous nucleation), where solution conditions allow for nanoparticle nuclei condensation.6 Substrate and solution properties, including solute saturation ratio7 and interfacial energy8 can © 2012 American Chemical Society

control nanoparticle nucleation. On the basis of classical nucleation theory, the free energy of formation describes the barrier that must be overcome if homogeneous or heterogeneous nucleation of spherical particles is to occur9 ⎡ 16πγ 3 ⎤ CL ⎥ ΔG = ⎢ f (θ ) ⎢⎣ 3ΔGv 2 ⎥⎦

(1)

where γCL is the crystal−liquid interfacial energy, ΔGv is the free energy change per unit volume [ΔGv = −kBT ln(S)/Ω, and kB is the Boltzmann constant, T is the temperature, S is the saturation ratio, and Ω is the volume per molecule],10 and f(θ) = 1/4 (2 − 3 cos θ + cos3 θ), θ being the contact angle formed between the nucleating crystal and substrate surface. The bracketed term describes the homogeneous nucleation barrier, and this term together with the f(θ) term describes the heterogeneous nucleation barrier. Electrostatic interactions and hydrophilicity/hydrophobicity are additional factors that can Received: Revised: Accepted: Published: 13167

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affect local saturation ratios11 or the contact angle,12 respectively, thus potentially influencing whether iron(III) (hydr)oxide nanoparticle nucleation is a surface-induced process (heterogeneous) or the result of solution precipitation (homogeneous). Organic species in aqueous environments can also affect iron(III) (hydr)oxide nucleation. Polymeric organic surface coatings are abundant in natural and engineered systems and have been shown to affect nanoparticle formation,13,14 particularly with respect to hydrophobic and electrostatic interactions.15,16 Thus, it is necessary to consider effects of organic coatings during iron (hydr)oxide nucleation to better predict aqueous nanoparticle and contaminant mobilization. Although studies on iron(III) (hydr)oxide nucleation at surfaces17,18 and in solution19,20 have been conducted, limitations in measurement techniques have impeded discovery of accurate quantitative information on the effects of organics. In our previous work, using simultaneous grazing incidence small-angle X-ray scattering (GISAXS) and SAXS, we observed the early stage development of heterogeneous and homogeneous iron(III) (hydr)oxide nucleation in the presence of quartz substrates21 and in the presence of inorganic anions.22 However, so far, little research has been conducted with organic surface coatings using such real-time techniques to monitor simultaneous heterogeneous and homogeneous nanoparticle nucleation in situ. Therefore, this current study uses our previously developed simultaneous GISAXS/SAXS setup to monitor the in situ interactions of nucleating iron(III) (hydr)oxide nanoparticles and organic-coated substrates. Complementary techniques, such as atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and electrophoretic mobility measurements, were also used to describe the physicochemical properties of nanoparticles and substrate surfaces. We sought to elucidate the dominant mechanisms governing iron(III) (hydr)oxide nanoparticle formation, particle size, and volume in the presence of three surfaces found in natural and/or engineered systems: quartz, polyaspartate-coated glass, and alginate-coated glass.

Supporting Information. A 4 wt % sodium polyaspartate (2.5 kg/mol, Baypure DS 100/40%,Lanxess) solution and 2 μm sodium alginate (Spectrum Chemicals, 90.8−106% assay, CA) solution were spin-coated (Laurell, WS-400AZ-6NPP/Lite, PA) on glass substrates at 1500 rpm for 30 s. After coating, the glass slides were spin-coated with DI water to remove excess polymer from the coated surface. Coated glass slides were stored in a sealed container prior to use (within 3 days). This method of spin-coating polymer solutions was used in previous studies to generate thin films that fully cover the substrate.28,29 The film thickness of polyaspartate- and alginatecoated substrates (150 and 75 nm, respectively) was determined by AFM. Each of the three substrates were reacted in a solution of 10−4 M ferric nitrate [Fe(NO3)3, J. T. Baker, ACS grade] in 10 mM sodium nitrate (NaNO3, Mallinckrodt, ACS grade) at pH 3.7 ± 0.2. This Fe(III) concentration is typically found in the environment (e.g., at acid mine drainage sites)21 and is optimal for studying simultaneous heterogeneous and homogeneous nucleation. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also performed to determine how iron(III) (hydr)oxide binds with quartz and polymercoated quartz (Supporting Information, Figure S3). 2.2. In Situ Time-Resolved Simultaneous GISAXS/ SAXS Measurements. The substrates were mounted in a specially designed GISAXS/SAXS cell21 and aligned for GISAXS along the incident angle (αi = 0.13°). The beam height (physical dimensions of synchrotron beam) of 100 μm and the operating energy was 12 keV. All GISAXS/SAXS measurements were conducted at the Advanced Photon Source (APS), Argonne National Laboratory, Sector 12-ID-B. Iron and sodium nitrate solutions were made immediately prior to injection in the sample cell. The iron nitrate solution generation marks the start of the reaction. After the GISAXS measurement at the substrate surface, the cell was shifted downward for the SAXS measurement 250 μm above the substrate surface across the 1 cm sample cell window. The 250 μm height was chosen to determine the relationship of nanoparticle formation at the substrate surface (GISAXS) and close to the substrate surface, and to ensure no X-ray scattering interference from GISAXS measurements. The alternating GISAXS and SAXS measurements, each with a 20 s exposure time, were repeated for 2 h at 1 min intervals between measurements. The scattering intensities from particles on the surface and in solution were extracted from GISAXS and SAXS images, respectively. Each experimental system was tested at least three times at the APS, showing good reproducibility. On the basis of our previous work, the temperature change induced by the X-ray beam is negligible. More details on the GISAXS/SAXS measurements and the sample cell can be found in the Supporting Information and our previous work, respectively.21 2.3. X-ray Scattering Data Analysis. The data fitting and analysis are described in detail in the Supporting Information. Briefly, a background signal, usually the first data for the GISAXS and SAXS measurements, was used for data background subtraction. The subtracted 2D images were reduced to 1D images for further analysis. The GISAXS particle size evolution in each sample was obtained by fitting the 1D images with the polydisperse sphere model and Schultz size distribution function, including a structure factor for particle interactions.30 The SAXS intensities were azimuthally averaged over each 2D image over the scattering vector, q (in Å−1), and fitted with the Guinier approximation. All data

2. EXPERIMENTAL SECTION 2.1. Substrate and Sample Preparation. Quartz and polyaspartate- and alginate-coated substrates were used as environmentally relevant model substrates to determine the effect of organic coatings on iron(III) (hydr)oxide formation. Polyaspartate [(C4H4NO3)n, pKa1 = 2.27, pKa2 = 3.6, pKa3 = 4.09, and pKa4 = 5.17]23 has been investigated as a zerovalent iron nanoparticle stabilizer for groundwater remediation24 and is currently under investigation as an environmentally benign alternative for scale inhibition in water treatment.25 Alginate [(C6H8O6)n, pKa1 = 3.38, pKa2 = 3.65]26 is a natural polysaccharide often used to model extracellular matrices (ECMs) in biofilm, which is ubiquitous in aqueous environments.27 Clean (110) surface quartz wafers (MTI Corp.) were cut into 1 cm × 1 cm coupons. The quartz substrate (1 mm thickness) was synthetically grown and cut along the (110) plane and polished by chemical/mechanical etching to near atomic-scale flatness (typical rms roughness about 6.37 ± 0.28 Å). Organiccoated substrates were prepared by cutting glass micro slides (VWR International, Inc.) to 1 cm × 1 cm coupons with a diamond knife, followed by rinsing with acetone, ethanol, and 2-propanol. The glass chemical composition is available in the 13168

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Figure 1. 1D representation of raw GISAXS data for nucleating iron(III) hydroxide nanoparticles in the presence of (A) quartz, (B) polyaspartatecoated glass, and (C) alginate-coated glass.

Contact angle measurements were taken using a Motic Digital Microscope (DM-143). Approximately 20 μL of DI water was transferred to each substrate, and the image was captured using the Motic Image Plus 2.0 software. 2.5. XPS Analysis and Carboxyl Group Surface Density Determination. To determine the relative extent of iron from nanoparticles on the reacted substrates, a custom-designed Kratos Axis Ultra XPS system was used.34 The surface analysis chamber was equipped with an aluminum Kα source using a 500 mm Rowland circle silicon single crystal monochromator giving monochromatic radiation at 1486.6 eV. The X-ray gun was operated at a 15 mA emission current, an accelerating voltage of 15 kV, and an incidence angle of 45°. While the penetration depth of X-rays can be as deep as micrometers, the analytical depth, determined by the inelastic mean free path of the excited photoelectrons, is a few nanometers. Additional XPS measurement information is given in the Supporting Information. To determine the density of −COOH groups on the polymer-coated substrate surface, we used the toluidine blue O (TBO) colorimetric method.35 Replicate polyaspartate- and alginate-coated substrates were submerged in a solution of 0.5 mM TBO in NaOH (EMD Chemicals, ACS grade) at pH 10 for 24 h at 30 °C to promote TBO ionic complexation to surface carboxyl groups. After adsorption, the substrates were rinsed with NaOH (pH 10) to remove excess TBO. The remaining adsorbed TBO on each substrate was desorbed in 5 mL of 50% acetic acid for 10 min under gentle agitation. The TBO concentration was determined by measuring TBO absorbance at 633 nm by a UV−vis spectrophotometer (Agilent Cary 50 UV−vis, CO) against a calibration curve of TBO/50% acetic acid standards. To calculate the carboxyl group density, it was assumed that 1 mol of TBO had complexed with 1 mol of carboxyl groups.36

analysis was performed with the Igor Pro program (V. 6.22A, WaveMetrics, Inc.). 2.4. Electrophoretic Mobility, AFM, and Contact Angle Measurements. Electrophoretic mobility measurements of quartz and polyaspartate- and alginate-coated quartz powders were taken using a Zetasizer instrument (Malvern 1011155 Zetasizer Nanoseries, MA). To prepare polymercoated quartz powder, approximately 2 g of quartz powder (Sigma Aldrich, purum p.a. grade, 40−150 mesh) was added to 4 mL of either 4 wt % polyaspartate or 2 μM alginate solutions, respectively. These solutions were rotated for 24 h, filtered, rinsed with DI water to remove excess polymer, which was not bound to quartz particles, and air-dried prior to measurements. High-resolution transmission electron microscope images are given in the Supporting Information (Figure S4) and provide evidence of polymer coating on quartz particles. The pH of a 10 mM NaNO3 stock solution was adjusted to 3.7, simulating GISAXS/SAXS measurement system conditions. Approximately 2 mL of the stock solution was added to 0.5 g of quartz (or polymer-coated quartz) powder without agitation. After equilibration for 1 h, the electrophoretic mobility of the supernatant containing suspended powder was measured. Electrophoretic mobility measurements of nucleating iron (hydr)oxide nanoparticles (10−4 M Fe(NO3)3 in 10 mM NaNO3) were recorded continuously for 1 h to determine the ζ-potential of nucleating nanoparticles in the reaction systems. Triplicate samples were measured. Substrate surface morphologies were observed using AFM (Nanoscope V Multimode SPS, Veeco). All height, amplitude, and phase contrast images of each sample were collected simultaneously under ambient laboratory conditions in tapping mode. The Nanoscope 7.20 software was used to analyze topographic features and substrate surface roughness and to confirm that the sample coating morphology remained constant during our experimental period. Before reaction, the average root-mean-square values (30 scan locations, 5 μm × 5 μm cross sections of 20 replicate samples) of the surface roughness for the polyaspartate and alginate substrates were 8.37 ± 0.51 and 4.72 ± 0.21 nm, respectively, which should not interfere with the GISAXS measurement quality.31−33 Furthermore, considering similar electron densities between water and polymer coatings compared to nanoparticles, the surface roughness of the polymer-coated substrate should not mask scattering intensities from nanoparticle formation on both polymercoated substrates during GISAXS analysis (detailed description in the Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Quantification of Heterogeneous and Homogeneous Iron(III) Hydroxide Nanoparticle Formation. Simultaneous GISAXS/SAXS measurements allow us to monitor the evolution of iron (hydr)oxide nanoparticle nucleation and size as a function of time and distance above the substrate surface. The GISAXS scattering intensities, I(q), of the nucleating particles are plotted in Figure 1 against the wave vector q (Å−1), which is reciprocally related to the particle size. On the basis of GISAXS scattering intensity data after 2 h, preferential iron(III) (hydr)oxide nucleation occurs on the quartz > alginate-coated > polyaspartate-coated substrate. 13169

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Figure 2. Invariant calculations for (A) GISAXS and (B) SAXS measurements and radius of gyration fitting for (C) GISAXS and (D) SAXS data. Particle size fitting could not be performed on the polyaspartate and alginate substrates prior to 60 min for GISAXS measurements, nor prior to 30 min for the SAXS measurement for all three substrate systems. This is due to low X-ray scattering, which is insufficient for accurate fitting. The grey bars in parts C and D represent the error range for the particle size fittings.

Figure 3. AFM images of unreacted (row 1) and reacted (row 2) GISAXS samples. Columns A, B, and C represent the surface topology of quartz and polyaspartate- and alginate-coated substrates, respectively. The inset in the upper right labeled “glass” is an image of clean glass before spincoating with polymer solutions. Height cross sections taken across the white dotted line in each figure are given below its corresponding image.

To quantitatively compare the total particle volumes, invariant (Q) values were calculated. The invariant, Q (given 37 2 by integrating Q = ∫ ∞ 0 I(q)q dq), calculations for the GISAXS and SAXS measurements are given in Figure 2, parts A and B, respectively. After 2 h, the total particle volume on the quartz substrate was more than twice that on the alginate substrate and around 10 times that on the polyaspartate-coated substrate. Interestingly, the invariant values 250 μm above the substrate surface were largest in the presence of the polyaspartate-coated substrate, followed by the alginate-coated and quartz substrates. The opposite trends in the heterogeneous and homogeneous invariant values suggest that inhibited nucleation on the

polyaspartate surface may promote more particle formation in solution, or fast nucleation on quartz may consume ferric ions close to the surface. Parts C and D for Figure 2 show the particle size (or radius of gyration, Rg) evolution as a function of time for the GISAXS and SAXS measurements, respectively. On the quartz substrate, the particle size increased linearly with time from 1.98 to 6.06 ± 0.07 nm, while on the polyaspartate and alginate substrate, the particle size grew to only 3.30 ± 0.13 and 3.29 ± 0.13 nm. The homogeneous particle size growth was similar within the error range for each system, growing linearly over the 2 h reaction period from 5.61± 0.25 to 15.50 ± 0.19, 5.24 ± 0.35 to 11.51 ± 13170

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0.13, and 6.09 ± 0.06 to 13.66 ± 0.04 nm in the quartz, polyaspartate, and alginate systems, respectively. The AFM images shown in Figure 3 further confirm the GISAXS data results. Rows 1 and 2 represent the unreacted and reacted samples, respectively. The spin-coating method used in this study can be used to generate thin, continuous films.28,29 Over 50 scan spots of replicate samples were measured with AFM to confirm full coverage of polymer coating on the substrates. The layer of polymer coating in images B1 and C1, with patches of polymer on top of the thin layer, was observed in 20 replicate samples. This heterogeneous surface morphology and orientation of coatings have been observed in literature.38 The directionality of polymer patches is a result of spin-coating and not AFM tip artifacts, as confirmed by physically rotating the samples by 90°. Note that these features are subtracted from each GISAXS image; therefore, the intensity and trends shown in Figure 1 come only from particle scattering and not substrate effects. The AFM image particle height distributions are taken at the dashed lines in each figure. The patches of polyaspartate in the unreacted (B1) and reacted (B2) images are similar in morphology. However, the reacted alginate substrate appears to have precipitation (C2) that is not present in the unreacted alginate substrate (C1). This finding suggests that more precipitation occurs on the alginate-coated substrate than on the polyaspartate-coated substrate. Furthermore, the rootmean-square value of the surface roughness remained fairly unchanged after reaction on the polyaspartate substrate (from 8.37 ± 0.51 to 8.45 ± 0.21 nm) but increased from 4.72 ± 0.21 to 7.83 ± 0.15 nm on the alginate substrate based on 50 scan spots from over 20 replicate samples. While it is difficult to clearly discern individual nanoparticles on the polymer-coated AFM samples, the AFM analysis combined with the aforementioned data support the conclusions that precipitation is inhibited on the organic polymer-coated substrates. XPS analysis was also performed to confirm the relative extent of iron oxide nanoparticle formation on different substrate surfaces. The Fe 2p XPS spectra are plotted in Figure 4. The two peaks at 711.4 and 722.8 eV in the Fe 2p spectra correspond to the Fe 2p3/2 and Fe 2p1/2 of the Fe(III) oxidation state, respectively.39 The larger incidence angle for XPS analysis (45°) compared to GISAXS analysis (0.13°) results in a larger X-ray penetration depth,40 and the XPS measurement is less sensitive to nanoparticles on surfaces than GISAXS measure-

ments. However, the Fe XPS spectra clearly reveal that the largest amount of iron exists on the reacted quartz substrate and the least on the polyaspartate substrate. Previously conducted grazing incidence wide-angle X-ray scattering analysis on the freshly precipitated iron(III) (hydr)oxide nanoparticles indicated that no discernible diffraction peaks were observed. This signifies that the nanoparticles are an amorphous iron (hydr)oxide phase. Direct experimental observations of the molecular structures of the newly formed iron hydroxide nanoparticles at the organic-coated surfaces can be an important future research direction but are beyond the scope of this paper. 3.2. Roles of Substrate Surface Hydrophilicity/Hydrophobicity and Functional Groups on Heterogeneous Iron(III) (Hydr)oxide Nucleation. Using additional complementary surface sensitive techniques, we further identified nanoparticle and substrate surface properties. On the basis of the description in section 3.1, the substrate surface properties affect nanoparticle volumes and sizes differently. We hypothesized that three interactions could be responsible for these disparities. First, particle−substrate electrostatic interactions can affect nanoparticle formation. Strong electrostatic interactions between the ferric ions and substrate surface could result in higher saturation ratios close to the surface, favoring heterogeneous nucleation (see eq 1). The electrophoretic mobility (i.e., ζ-potential) results are outlined in Table 1. Table 1. Summary of ζ-Potential Measurements for Iron(III) Hydroxide Nanoparticles, Quartz, and Coated Quartz Suspensions sample 10−4 M FeNO3 + 10 mM NaNO3 10 mM NaNO3 + 10−4 M FeNO3 + suspended quartz powder 10 mM NaNO3 + suspended quartz powder 10 mM NaNO3 + suspended polyaspartate−quartz powder 10 mM NaNO3 + suspended alginate−quartz powder a

ζ-potential (mV) 31.24 ± 1.49 60.6 ± 5.70a −14.30 ± 2.60 −42.92 ± 5.18 −35.27 ± 4.60

Value taken from Hu et al. investigating a similar reaction system.22

Because the iron(III) (hydr)oxide surface charge formed in solution is positive at pH 3.7 (31.24 ± 1.49 mV), it is expected that heterogeneous nucleation is favored on the polyaspartate substrate due to its large negative charge (−42.18 ± 5.18 mV) and is least on the quartz substrate (charge of −14.30 ± 2.60 mV). In fact, the opposite GISAXS heterogeneous nucleation trend was observed. Furthermore, the surface charge of iron(III) (hydr)oxide nanoparticles formed on the quartz surface is even more positively charged (60.6 ± 5.7 mV)22 than above iron (hydr)oxide in solutions, providing further evidence that electrostatics are not the dominant mechanism controlling heterogeneous nucleation. Second, the functional group type and density at substrate surfaces could direct nucleation. The quartz surface hydroxyl groups can form Fe−O−Si complexes and promote iron(III) (hydr)oxide formation.22 Similarly, deprotonated carboxyl groups can covalently bond iron.41,42 The DRIFTS spectra of polymer-coated iron(III) hydroxide powder also revealed carboxyl group peak shifts to lower frequency when mixed with the iron(III) powder (Figure S3, Supporting Information), indicating iron−carboxyl bond formation. To our knowledge, no studies have reported heterogeneous nucleation inhibition

Figure 4. XPS spectra of Fe 2p for clean quartz and reacted quartz and polyaspartate- and alginate-coated substrates. 13171

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close to zero (as in the quartz system), the surface is completely wetted and heterogeneous nucleation of hydrophilic nanoparticles will spontaneously occur.53 While this study’s contact angle measurements are not in high resolution, they can serve as relative values for comparison. Young’s equation can be used to relate contact angle to the system interfacial energies:

on alginate; however, polyaspartate has been shown to inhibit nucleation, despite the presence of carboxyl groups for binding. For example, polyaspartate with molecular weight between 2 and 4 kg/mol (a 2.5 kg/mol solution was used in this study) has been reported to inhibit calcium carbonate, calcium sulfate, and barium sulfate precipitation43 by coating surfaces and blocking nucleation sites.44 The reported silanol surface density for quartz is 5.9−18.8/ nm2 (or 0.98−3.12 nmol/cm2).45 The amphoteric surface hydroxyl groups are protonated (−OH2+) at the system pH of 3.7 ± 0.2, yet Fe3+−surface complexes can still form.46 The total carboxyl group densities at the polyaspartate and alginate substrate surfaces were determined to be 13.48 ± 3.30 and 11.35 ± 2.75 nmol/cm2, respectively. A study by Sreeram et al. investigating iron(III) complexation with alginates demonstrated that iron(III) can also bind to protonated carboxyl groups; however, the binding is weaker and slower compared to deprotonated carboxyl groups.47 Hence, only deprotonated carboxyl groups will be considered in this study. At the system pH, the alginate carboxyl groups at the surface are completely deprotonated (pH is above pKa values of 3.38 and 3.65); therefore, the calculated functional density is approximately equal to the carboxyl group density that can form bonds. At pH 3.7, only half of the carboxyl groups are deprotonated on the polyaspartate surface (between pKa2 = 3.6 and pKa3 = 4.09); therefore, the deprotonated carboxyl group density is only 6.74 ± 1.65 nmol/cm2. In conclusion, alginate has more surface carboxyl groups available for complexing, which could contribute to the increased precipitation compared to polyaspartate. There is a caveat in estimating reactive carboxyl group surface density. Because of possible deviations in reported polymer pKa values [pKa2(polyaspartate) = 3.60, pKa2(alginate) = 3.65], which are similar to the reaction system pH (pH = 3.7 ± 0.2), the actual concentration of deprotonated carboxyl groups may vary. Thus, while this result could contribute to the larger extent of heterogeneous nucleation on alginate than polyaspartate, a definitive connection could not be concluded. Nevertheless, the quartz silanol group density is significantly lower than the polymer carboxyl group density; thus, the densities cannot be responsible for the observed heterogeneous nucleation trend. Finally, the degree of substrate hydrophilicity could control the iron(III) (hydr)oxide formation on the quartz and organiccoated substrates. Quartz is a highly hydrophilic material due to the surface silanol groups.48 Because the iron(III) (hydr)oxide particles are positively charged, protonated, and thus hydrophilic,49 they should preferentially nucleate on the quartz surface, as we observe. Due to its acidic monomers and its reported use as hydrophilic bases and stabilizers for drugs,50 alginate is assumed to be a hydrophilic polymer. Polyaspartate and derivatives have been used for their hydrophobic properties in drug release as components of block copolymers.51 However, polyaspartate has also been reported to be a hydrophilic polymer.52 To determine the hydrophilicity of the substrates in this study, contact angle measurements were performed (Figure S1, Supporting Information). The contact angles of polyaspartate and alginate substrates are roughly 67° ± 1° and 55° ± 1°, respectively. The contact angle of quartz could not be accurately determined due to the strong hydrophilic nature of the surface. These values suggest that quartz is the most hydrophilic surface, followed by alginate- and polyaspartatecoated substrate surfaces. As seen in eq 1, if the contact angle is

γWS − γNS = γNW cos θ

(2)

where γ is the interfacial energy (mJ/m ), θ is the measured contact angle, and subscripts WS, NS, and NW correspond to the water−substrate, nanoparticle−substrate, and nanoparticle−water interfaces, respectively.9 Heterogeneous nucleation will be more spontaneous in systems that minimize the sum of the interfacial energies (γNW + γWS + γNS).6 The nanoparticle− water (γNW) interfacial energy is the same (constant) for all three systems if nanoparticle identities are the same. The water−substrate interfacial energies (γWS) are roughly equivalent to the initial experimental solution−substrate interfacial energies in the reaction system (excluding the effect of NaNO3), and the water−substrate interfacial energy is higher for hydrophobic surfaces.54 Therefore, the sum of water− substrate and nanoparticle−substrate (γWS + γNS) interfacial energies should be lowest for the quartz reaction system (most hydrophilic substrate) and highest in the polyaspartate reaction system (least hydrophilic substrate), which explains the observed heterogeneous nucleation trends. Substrate surface hydrophilicity can also be related to surface roughness55 and functional group availability.56 For contact angle measurements less than 90° (i.e., hydrophilic), Wenzel’s equation suggests that the liquid will spread more due to surface roughness, resulting in a smaller apparent contact angle.57 The contact angle for the polyaspartate substrate (67°) is larger than the alginate substrate contact angle (55°); however, the surface roughness for polyaspartate is larger (8.37 ± 0.51 nm) than for alginate (4.72 ± 0.21 nm). Therefore, surface roughness effects cannot explain the observed trends. In sum, we concluded that interfacial energy, and functional group type and arrangement are the major contributing components of the observed contact angles. Interfacial energy and functional group density calculations have been previously discussed. While the silanol surface density (max 3.12 nmol/cm2) is less than the carboxyl group density (11.35 nmol/cm2) in the coatingsespecially if weak protonated carboxyl group complexing is countedthe physical arrangement of functional groups on the surface could affect heterogeneous nucleation. For example, the carboxyl groups could be branched and more disordered compared to the silanol groups on the (110) surface of quartz; therefore, steric hindrance or bulkiness in the coatings could limit the ease at which iron can complex on the substrate surface.58 Finally, lattice mismatch could also affect heterogeneous nucleation;59 however, this effect was not considered due to the severe mismatch caused by the polymer coating. In conclusion, hydrophilic iron(III) (hydr)oxide nanoparticles prefer forming on quartz and form least on polyaspartate substrates due to the thermodynamic drive to minimize total surface free energy and the potential difference in functional group arrangement or sterics on the substrate surface. 3.3. Environmental Implications. This study provides unique information relating the effects of substrate hydrophilicity to iron(III) (hydr)oxide nanoparticle nucleation, size, and total particle volume evolution. GISAXS, SAXS, AFM, and 2

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UV−vis spectrophotometer. We would also like to thank Ms. Yandi Hu for ζ-potential measurements of quartz powder and iron hydroxide-coated quartz powder, imaging clean quartz with AFM, and valuably discussion; Dr. Soenke Seifert for experimental help during GISAXS beamtime; and Dr. Xingyi Deng for valuable discussion about XPS data analysis. Use of the Advanced Photon Source (Sector 11-BM and 12ID-B) at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

XPS analyses suggest that the heterogeneous nucleation was greatest on the quartz substrate and greatly inhibited in the polyaspartate-coated substrate, while homogeneous nucleation was greatest in the presence of the polyaspartate substrate and least in the presence of the quartz. The final iron(III) (hydr)oxide particle size, both at the substrate surface and in solution, were found to be larger in the quartz reaction system than in the polyaspartate and alginate reaction systems. The degree of substrate hydrophilicity was found to be a more dominant factor in controlling heterogeneous nucleation than electrostatic interactions. A summary of research findings is included in Table S1 of the Supporting Information. Because high concentrations of ferric ions can be found in wastewater,60 the formation and affinity of iron oxide nanoparticles toward ECMs in biofilm may affect biofilm reactor microbial activity in wastewater treatment.61 For example, iron(III) (hydr)oxide nanoparticle formation on biofilm was found to catalyze oxidization of organic pollutants in drinking water.62 Hwang et al. revealed that solutionstabilized surfactant-coated iron oxide nanoparticles significantly reduced simulated wastewater effluent chemical oxygen demand, turbidity, and color.63 Alternatively, the biofilm or extracellular material can affect iron (hydr)oxide nanoparticle formation. Iron(III) (hydr)oxide nanoparticle formation and interactions in the presence of ECMs or polysaccharides have been demonstrated to possess different geochemical reactivity than pure iron(III) (hydr)oxide nanoparticles.64,65 Similarly, near-surface promoted homogeneous ferric ion nucleation may affect other aqueous processes downstream if polyaspartate is used as a dispersing or scaling agent. In addition, our results regarding the importance of hydrophobic interactions over electrostatic interactions in nanoparticle nucleation can be extended to work done with pre-existing nanoparticles. The findings in this study provide unique information about factors affecting iron oxide nanoparticle−contaminant fate and transport, as well as nanoparticle behavior in natural and engineered aquatic systems where organic species and coatings exist.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental description and data summary table, contact angle images (Figure S1), SAXS raw scattering and Lorentz corrected curves (Figure S2), DRIFTS spectra (Figure S3), and HR-TEM micrographs showing polymer coating (Figure S4). 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: (314) 935-4539. Fax: (314) 935-7211. Web site:http://encl.engineering.wustl.edu/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by a Washington University Faculty Start-up Grant and the National Science Foundation’s Environmental Chemical Sciences Program (CHE-1214090). We would like to thank Dr. Srinkath Singamaneni of the Mechanical Engineering & Materials Science Department of Washington University for use of the spin coater, and the Washington University Nano Research Facility for use of the 13173

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