Article pubs.acs.org/est
Fe(III) Hydroxide Nucleation and Growth on Quartz in the Presence of Cu(II), Pb(II), and Cr(III): Metal Hydrolysis and Adsorption Chong Dai† and Yandi Hu*,† †
Department of Civil & Environmental Engineering, University of Houston, Houston, Texas 77004, United States S Supporting Information *
ABSTRACT: Fe(III) hydroxide nanoparticles are an essential carrier for aqueous heavy metals. Particularly, iron hydroxide precipitation on mineral surfaces can immobilize aqueous heavy metals. Here, we used grazing-incidence small-angle Xray scattering (GISAXS) to quantify nucleation and growth of iron hydroxide on quartz in 0.1 mM Fe(NO3)3 solution in the presence of Na+, Cu2+, Pb2+, or Cr3+ at pH = 3.7 ± 0.1. In 30 min, the average radii of gyration (Rg) of particles on quartz grew from around 2 to 6 nm in the presence of Na+ and Cu2+. Interestingly, the particle sizes remained 3.3 ± 0.3 nm in the presence of Pb2+, and few particles formed in the presence of Cr3+. Quartz crystal microbalance dissipation (QCM-D) measurements showed that only Cr3+ adsorbed onto quartz, while Cu2+ and Pb2+ did not. Cr3+ adsorption changed the surface charge of quartz from negative to positive, thus inhibiting the precipitation of positively charged iron hydroxide on quartz. Masses and compositions of the precipitates were also quantified. This study provided new insights on interactions among quartz, iron hydroxide, and metal ions. Such information is helpful not only for environmental remediation but also for the doping design of iron oxide catalysts.
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INTRODUCTION Iron hydroxide nanoparticles formed by precipitation from solution are ubiquitous in many natural and engineered aqueous environments. As iron hydroxide can sequester heavy metals, they play essential roles for the fate of aqueous heavy metals.1,2 For example, iron hydroxide is the main carrier of hazardous heavy metals (e.g., Cu, Pb, Zn, Cd, Ni, Mn, Co) in acid mine drainage (AMD).3−6 Also, during industrial wastewater treatment, iron hydroxide nanoparticles have been widely used to remove heavy metals (e.g., Pb, Zn, Cu, Cr, Cd, Ni) that are discharged from metal plating facilities, mining operations, fertilizer industries, refinery industries, and paper industries.7−9 In these natural and engineered aqueous environments, iron hydroxide nanoparticles can form either in solution or on substrate surfaces as coatings.10−13 The small iron hydroxide nanoparticles suspended in solution will carry the heavy metals and move together freely with the flow; while for iron hydroxide coated on mineral surfaces, retention of the heavy metals on substrate surfaces can occur. To better understand the fate of aqueous heavy metals, it is important to study iron hydroxide precipitation both in solution and on mineral surfaces in the presence of heavy metals. However, because of the technical difficulty to characterize the interfacial processes during Fe(III) hydroxide precipitation on mineral surfaces, previous studies on heavy metals precipitation with Fe(III) hydroxide were conducted only in bulk solutions in the absence of mineral surfaces. These studies have found that the presence of heavy metals during Fe(III) © 2014 American Chemical Society
(hydr)oxide precipitation can significantly affect many properties of the precipitates, such as their solubility, stability, composition, size, shape, crystal structures, and their later phase transformations.7,14−19 In our previous study, aqueous Al(III) can adsorb onto quartz surfaces and form Al−O−Si bond.11 As a result, it significantly affected Fe(III) hydroxide precipitation (e.g., nucleation, growth, and Ostwald ripening) on quartz.11 During Fe(III) hydroxide precipitation on mineral surfaces, in the presence of heavy metals, there can also be interactions between quartz surface and the aqueous metal ions, which may affect the nucleation and growth of Fe(III) hydroxide on mineral surfaces. However, such interactions were not studied yet. The objectives of this study were 2-fold: (1) to measure the nucleation and growth kinetics of Fe(III) hydroxide on quartz in the presence of heavy metals (Cu2+, Pb2+, and Cr3+), and (2) to explore the interfacial interactions among quartz, metal cations, and Fe(III) hydroxide, for understanding the controlling mechanisms of the Fe(III) hydroxide nanoparticle nucleation and growth. To achieve the objectives, in situ GISAXS and quartz crystal microbalance dissipation (QCM-D) experiments were conducted to quantify the size and mass of the precipitates on quartz. Furthermore, other techniques, such Received: Revised: Accepted: Published: 292
August 22, 2014 November 26, 2014 November 28, 2014 November 28, 2014 dx.doi.org/10.1021/es504140k | Environ. Sci. Technol. 2015, 49, 292−300
Environmental Science & Technology
Article
Table 1. Initial Solution Conditions for Fe(III) Hydroxide Precipitation Experimentsa sample name
Fe3+ (mM)
Cu2+ (mM)
Pb2+ (mM)
Cr3+/Al3+ (mM)
Na+ (mM)
NO3− (mM)
IS (mM)b
pHc
SId
FeNa FeCu FePb FeCr FeAle
0.1 0.1 0.1 0.1 0.1
0 1 0 0 0
0 0 1 0 0
0 0 0 0.3 0.5
3 0 0 10 10
3.3 2.3 2.3 11.2 11.8
3.3 3.3 3.3 12 13
3.8 3.8 3.8 3.6 3.7
0.33 0.32 0.33 0.03 0.25
Thermo.tdat was used to perform all GWB calculations. bIonic strength. cpH was calculated by GWB. The measured pH values were 3.81 ± 0.08 for FeNa, FeCu, and FePb solutions, and it was 3.61 ± 0.05 for FeCr solution, which matched well with GWB calculations. dSaturation indices with respect to Fe(OH)3, which is the simplicity of ferrihydrite. SI = Log (Q/Ksp), where Q is the actual dissolved composition, Ksp is the corresponding equilibrium compositions. Ksp(Fe(OH)3) = 10−37.36 at 20 °C was used for SI calculations based on GWB database. The solutions were undersaturated with respect to Cu(OH)2, Pb(OH)2, Cr(OH)3, and Al(OH)3. As the Ksp value used will determine the calculated pH of the solution, the consistence in pH measurements with calculations indicated that the database used here was accurate. More details can be found in the Supporting Information. eData collected from our previous work.11 a
Figure 1. GISAXS scattering intensities generated by particles formed on quartz surfaces from 10−4 M Fe3+ solutions at pH = 3.7 ± 0.1 in the presence of (A) 3 mM Na+, (B) 1 mM Cu2+, (C) 1 mM Pb2+, and (D) 0.3 mM Cr3+. Red arrows show the shifting of the peak positions to a lower q range, indicating particle growth. The black lines are the fitted curves.
Cu2+, Pb2+, and Cr3+ were selected because they are listed as EPA priority pollutants,22 also, they are ubiquitous in industrial wastewater and at acid mine drainage.3,6,23−27 Solutions were prepared with reagent grade Fe(NO3)3·9H2O, Pb(NO3)2, Cu(NO3)2·3H2O, Cr(NO3)3·9H2O, NaNO3, and ultrapure water (resistivity >18.2 MΩ·cm). Solution compositions are shown in Table 1. As a control solution without heavy metals, 3 mM NaNO3 was added and was labeled as “FeNa”. For the solutions containing heavy metals, 1 mM Cu(NO3)2, 1 mM Pb(NO3)2, or 0.3 mM Cr(NO3)3 was added, labeled as “FeCu”, “FePb”, and “FeCr”, respectively. All solutions contained 0.1 mM FeNO3 with pH = 3.7 ± 0.1. Around neutral pH conditions, aggregation of Fe(III) hydroxide can occur, and metals adsorbed on the surfaces of particles can be trapped inside the aggregates; also the sedimentation of the aggregates may limit the colloidal transport of metals.28−31 In this study, the pH values (pH = 3.7 ± 0.1) are typical at acid mine
as QCM-D, dynamic light scattering (DLS), atomic absorption spectrometer (AAS), and X-ray diffraction (XRD), were employed to probe the interfacial processes.
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EXPERIMENTAL SECTION
Substrate and Solution Preparation. Quartz was chosen here as the substrate for Fe(III) hydroxide precipitation experiments in the presence of heavy metals, because quartz is the second most abundant mineral in the Earth’s crust, after feldspar. Also, iron oxide coated quartz (IOCQ) was widely used for heavy metal removal during wastewater treatment.19−21 Quartz single crystal samples (MTI Corporation, geometry: 10 × 10 × 1 mm) cut along (10−10) plane with atomic-level flatness were purchased. The quartz samples were cleaned, and the detailed cleaning procedure can be found in the Supporting Information. 293
dx.doi.org/10.1021/es504140k | Environ. Sci. Technol. 2015, 49, 292−300
Environmental Science & Technology
Article
The IRENA package was used to perform fitting of the experimental data, and the fitted curves (solid lines in Figure 1) match well with the measurements (dots in Figure 1).36 Based on fitting, the time-evolved average radii of particles formed on quartz are shown in Figure 2.
drainage (AMD), and are far from pH(pzc) of Fe(III) hydroxide; therefore, aggregation can be prevented, and the nucleation and growth processes can be better measured. Geochemist’s Workbench (GWB, Release 9.0, Aqueous Solutions LLC) was utilized to calculate the solutions’ pH, ionic strength (IS), and saturation indices (SI) with respect to Fe(OH)3 (Table 1). Thermo.tdat database was utilized in GWB calculations. The calculated pH values (pH = 3.7 ± 0.1) were consistent with measured pH values, indicating that the thermo.tdat database used for GWB calculation was valid for our system. Based on calculations, all solutions were undersaturated with respect to the hydroxides of heavy metals (i.e., Cu(OH)2, Pb(OH)2, or Cr(OH)3). Detailed solution preparation and GWB calculations can be found in the Supporting Information. In Situ GISAXS Measurements. For each experiment (Table 1), a cleaned quartz sample was placed into a homedesigned GISAXS cell, modified from our previously used cell.10,11,13,32 Before measurement, the center of the incident Xray beam was aligned toward the quartz surface. The energy of the incident X-ray was 14 keV. At this energy level, an incident angle of 0.10° was chosen in order to have >99% reflectivity of the incident X-rays, which allowed the incident X-rays to probe only nanoparticles at the quartz surface.33 Then, 0.7 mL of freshly mixed solution (Table 1) was injected into the GISAXS cell. Timing started right after the Fe(III) solution was prepared, and the scattering measurement started about 1 min later. Each experiment was run for ∼30 min. During the measurement, GISAXS data was collected every 2 min. Experiments were conducted at beamline 12 ID-B at the Advanced Photon Source (APS), Argonne National Laboratory (ANL), Lemont, IL. X-ray Scattering Data Analysis. For each run, the first GISAXS image was used as the background scattering to be subtracted by later images. Then, line-cuts were made along the Yoneda wing of the two-dimensional (2D) GISAXS images, where the X-ray scattering signal is the strongest due to the Vineyard effect.34,35 By doing line-cuts, we converted the 2D GISAXS images to one-dimensional (1D) scattering curves of intensity I vs q (scattering factor, reciprocal to particle size with the unit of Å−1). For FeNa, FeCu, FePb, and FeCr, the scattering intensity curves after reactions for different time intervals are shown in Figure 1. All data reductions were performed by GISAXS-SHOP macro, Igor Pro 6.34 (WaveMetrics, Inc., Oregon). To obtain particle size evolutions, we fitted the 1D GISAXS scattering intensity curves (I vs q, Figure 1) with log-normal polydisperse model of noninteracting sphere particles using eq 1: I(q) = I(0) × (Δρ)2
∫
Ex Situ Atomic Force Microscopy (AFM) Measurements. The particles precipitated on quartz were also measured by ex situ AFM measurements (Figure S1, Supporting Information), to complement in situ GISAXS measurements. AFM tapping mode (AFM; Veeco, Inc.) was used at a scan rate of 1.0 Hz. The height, amplitude, and phase images were collected simultaneously. Probes used were 125 μm long with antimony (n) doped silicon tips (TESPA, Brukerprobes). The drive frequency was 320 kHz, and the typical spring constant was 42 N/m. NanoScope Analysis 1.5 was used to analyze the collected images and get topographic information on the samples. Zeta Potential (ζ) Measurements. Two sets of zeta potential measurements were conducted. The first set aimed to study the effects of heavy metals on the surface charges of the precipitates. Each solution in Table 1 was prepared, and zeta potentials (ζ) of iron hydroxide particles formed in solution in the presence of Na+, Cu2+, Pb2+, and Cr3+ were measured. The second set aimed to study the effects of heavy metals on the surface charges of quartz. Because it is difficult to directly measure the zeta potentials (ζ) of single crystal surfaces, the surface charges of quartz powders were measured instead. Solutions containing 3 mM Na+, 1 mM Cu2+, 1 mM Pb2+, or 0.3 mM Cr3+ and suspended quartz powders were prepared without adding Fe3+, and the pH of each solution was adjusted to 3.7 ± 0.1 using HNO3. Then, zeta potentials (ζ) of quartz surfaces in the presence of Na+, Cu2+, Pb2+, or Cr3+ were measured. There is a caveat that the surface charge of quartz powder may be different from quartz single crystals. More detailed information on the solution preparation can be found in the Supporting Information. For each measurement, the freshly prepared solution was injected into a zeta cell (disposable cuvettes, Fisher Scientific), and was placed in a DLS instrument (Nicomp 380 DLS/ZLS, Particle Sizing Systems). All the zeta potential measurements were made every 2 min for 30 min at 20 °C, and the average values and standard deviations of the zeta potentials (ζ) are
n(R , σd)V (R )2
9(sin(qR ) − qR cos(qR ))2 (qR )6
Figure 2. Evolutions of the average radii (Å) of particles precipitated on quartz from 10−4 M Fe3+ solutions in the presence of 3 mM Na+, 1 mM Cu2+, and 1 mM Pb2+. Triplicate size fitting were conducted with different initial values and ranges of the fitting parameters. The average size is shown in Figure 2, and the error bars represent the standard deviations.
dR (1)
where I(0) is related to the number density of particles; Δρ is the electronic density difference (or contrast) between the particle and the surrounding solution; n(R,σd) is log-normal distribution used to represent the size polydispersity of the particles precipitated on quartz, with R and σd being the average radius of the particles and their standard deviation; and V is one particle volume. 294
dx.doi.org/10.1021/es504140k | Environ. Sci. Technol. 2015, 49, 292−300
Environmental Science & Technology
Article
Table 2. Fe(III) Hydroxide Precipitation and Metal Ion Sorption on Quartz Fe(III) hydroxide precipitationa name FeNa FeCu FePb FeCr FeAlg
ζ (mV)
mass (ng/m )
± ± ± ± ±
207.5 ± 4.7 347.1 ± 4.0 140.5 ± 4.5 N/Ac
34.7 39.0 23.9 27.5 40.5
2.8 7.3 6.9 1.2 5.7
2
metal ion sorption onto quartzb
RM/Fe, totald
e
RM/Fe, surface
0.04 ± 0.01 0.29 ± 0.05 N/Ac
