Article pubs.acs.org/Langmuir
Experimental Study of Strontium Adsorption on Anatase Nanoparticles as a Function of Size with a Density Functional Theory and CD Model Interpretation Moira K. Ridley,*,† Michael L. Machesky,‡ and James D. Kubicki§ †
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, United States Illinois State Water Survey, University of Illinois, Champaign, Illinois 61820, United States § Department of Geosciences and The Earth and Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡
S Supporting Information *
ABSTRACT: The effect of particle size on the adsorption of Sr2+ onto monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been evaluated quantitatively with macroscopic experimental studies. The adsorption of Sr2+ onto the anatase particles was evaluated by potentiometric titrations in NaCl media, at two ionic strengths (0.03 and 0.3 m), and over a wide range of pH (3−11) and surface loadings, at a temperature of 25 °C. Adsorption of Sr2+ to the surface of the 20 and 40 nm diameter samples was similar, whereas the Sr2+ adsorption titration curves were shallower for the 4 nm diameter samples. At high pH, the smallest particles adsorbed slightly less Sr2+ than was adsorbed by the larger particles. At the molecular scale, density functional theory (DFT) calculations were used to evaluate the most stable Sr2+ surface species on the (101) anatase surface (the predominant crystal face). An innersphere Sr-tridentate surface species was found to be the most stable. The experimental data were described with a charge distribution (CD) and multisite complexation (MUSIC) model, with a Basic Stern layer description of the electric double layer. The resulting surface complexation model explicitly incorporated the molecular-scale information from the DFT simulation results. For 20 and 40 nm diameter anatase, the CD value for the Sr-tridentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The CD value for the 4 nm sample was smaller than that for the 20 and 40 nm samples, reflecting the shallower Sr2+ adsorption titration curves. The adsorption differences between the smallest and larger anatase particles can be rationalized by water being more highly structured near the 4 nm anatase sample and/or the Sr-tridentate surface species may require more well-developed surface terraces than are present on the 4 nm particles.
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INTRODUCTION Nanoparticles are widespread in the environment and are used widely in technical applications. Interest in and use of nanometer particles arise from the physical and chemical properties of these materials that are highly particle-size dependent in the nanoscale range. For example, engineered metal-oxide nanoparticles are used broadly in fuel cells, in biomedical, electronic, and optical applications, and as catalysts.1−4 Environmental applications of nanoparticles include the use of these materials for water purification systems. Furthermore, nanometer particles are abundant in natural environments.5−7 In natural environments, nanoparticles may play an important role in the fate and transport of radionuclides and metals. Equally, nanomaterials may pose toxicological and environmental risks. The distinctive properties of nanosized particles likely result from their large surface area. Specifically, as nanoparticle size decreases, the ratio of atoms present at the surface increases. On metal-oxide surfaces, undercoordinated metal and oxygen © XXXX American Chemical Society
atoms are highly reactive toward water. The reactivity toward water results in hydroxylated surface functional groups, which impart a pH-dependent surface charge. Surface charge has a direct impact on the binding of ionic species. Recent studies have investigated the surface reactivity of nanosized metal-oxide particles (e.g., anatase, hematite, magnetite, and silica) as a function of particle size.7−17 The results of these studies are ambiguous; nevertheless, they show size-dependent differences in the surface charging behavior, surface charge density, and zeta potential values of the various metal oxides studied. Such differences in surface reactivity will impact directly the ion adsorption behavior of nanometer sized metal-oxide particles. However, while most nanomaterial applications employ a discrete particle diameter, relatively few studies have purposely investigated specific ion adsorption as a function of particle size. Received: October 3, 2014 Revised: December 15, 2014
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DOI: 10.1021/la503932e Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
the macroscopic TiO2 polymorph, strongly adsorbs Sr2+ as an inner-sphere complex.25,35−37 The experimental results of this study are used to quantitatively evaluate the effect of particle size on the adsorption behavior of the nanosized anatase particles. We also present the results of density functional theory (DFT) calculations, performed to identify possible adsorption geometries of Sr2+ on the (101) anatase surface. All experimental data were interpreted using a CD model that was built on the CD-MUSIC model determined previously for the primary charging behavior of the nanoanatase samples in NaCl media.14 The CD model was used to relate the macroscopic Sr2+ adsorption data to the microscopic adsorption geometry indicated by the DFT simulations. Moreover, the DFT simulation results were used explicitly to constrain the CD value for the inner-sphere Sr surface species. The combination of experimental and DFT simulation results within a MUSIC and CD modeling framework allows us to quantitatively evaluate and better elucidate the effects of particle size on the surface adsorption behavior of nanometer-sized metal-oxide particles.
The extensive use of TiO2 nanoparticles, specifically anatase, in photocatalysis processes, for dye-sensitized solar cells and for dental and biomedical implants, has resulted in a number of theoretical and experimental studies examining the adsorption of anions and organic compounds to the surface of anatase nanoparticle catalysts.1−4 Similarly, the ion-binding behavior of iron-oxyhydroxide phases that play an important role in natural environments has been studied extensively. 18,19 In an experimental study by Madden et al.,15 the adsorption of Cu2+ to hematite was higher on 7 nm diameter particles than on 25 nm diameter particles. The increase in adsorption onto the smaller particles was attributed to particle morphology, specifically, edge effects and topographic features. On the other hand, in an experimental study examining the adsorption of Cd2+ to the surface of nanosized anatase particles, Gao et al.1 found that the smallest nanoparticles of 8 nm diameter had the lowest adsorption capacity, whereas nanoparticles ≥20 nm all had similar adsorption capacities. In this case, Gao et al.1 suggest that the differences in adsorption behavior are the result of intraparticle electrostatic effects between the 8 nm diameter particles, thereby reducing the effective reactive surface area. Changes in the ion adsorption behavior of nanoparticles are clearly discernible from macroscopic experimental studies.1,13−15,20 Moreover, the effects of solution chemistry on the ion adsorption behavior of nanoparticles can be evaluated from macroscopic experiments. Specifically, surface complexes may protonate or hydrolyze, or the binding coordination geometry may vary with changes in solution pH, ionic strength, and surface loading. The coordination geometry of surface complexes is best identified at the microscopic scale, either from X-ray techniques or by theoretical study. To relate microscopic and macroscopic observations, electrostatic contributions must be accounted for. Typically, surface complexation models (SCMs) are developed to rationalize adsorption at equilibrium conditions. The refined multisite complexation (MUSIC) model most successfully integrates macroscopic experimental data with molecular-scale surface structural information.21,22 Similarly, the charge distribution (CD) model is based on a structural approach and can account for the ionic charge of inner-sphere complexes. Consequently, a combination of the MUSIC and CD models provides a powerful tool to link microscopic and macroscopic descriptions. To best apply the CD-MUSIC model, paired macroscopic experimental and molecular-scale studies are needed. This approach has been used successfully for several ion−mineral systems, for macroscopic samples,23−29 and, more recently, for nanosized particles.14,18 This article presents the results of a comprehensive suite of experiments examining the adsorption of Sr2+ on the surface of a set of monodisperse anatase (TiO2) particles that are 4, 20, and 40 nm in diameter. Adsorption of Sr2+ by the anatase nanoparticles is evaluated over a broad range of solution conditions (pH, ionic strength, Sr2+ surface loading) in NaCl media, at 25 °C. Strontium adsorption is of great interest because of the presence of the 90Sr radionuclide in radioactive wastewaters. Numerous studies have documented that solution−mineral adsorption processes may be a practical approach for the removal of 90Sr from radioactive wastewaters.30−34 Predominantly, the studies have been batch adsorption experiments and have shown that Sr2+ adsorption depends on solution (i.e., pH and ionic strength) and solidphase composition. Additionally, it has been shown that rutile,
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MATERIALS AND METHODS
Anatase Samples. The three monodisperse anatase samples used in this study have been described previously.13,14 Two samples were obtained from Ishihara Techno Corporation (Osaka, Japan). The samples were designated ST-01 and ST-21 and were approximately 4 and 20 nm diameter powders, respectively. The third anatase sample had a diameter of approximately 40 nm and came from Altair Nanomaterials, Inc. (Reno, NV, USA). All anatase samples were subjected to washing with distilled−deionized water to ensure that the particle surfaces were free of residue from the synthesis procedures and free of other potential water-soluble impurities. The anatase powders were characterized extensively using X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET) N2-adsorption surface area measurements, thermogravimeteric analysis (TGA), high-resolution transmission and scanning electron microscopy (HR-TEM and SEM), and aberration corrected electron microscopy (ACEM). Electron microscopy and XRD characterization verified that the samples were pure, crystalline, monodisperse anatase powders. Imaging results from the ACEM showed visible facets and lattice patterns. The predominant crystal face of all samples, apparent from the lattice spacing, was the (101) plane.14 The (100) plane was also identified, and the prevalent edge facets included the (010) and (001) planes. The N2-BET surface areas of the three samples after washing are given in Table 1.
Table 1. Source and Physical Characteristics of the Three Anatase Powders supplier Ishihara Techno Corporation ST-01 ST-21 Altair Nanomaterials Inc.
particle size (nm)
BET surface area (m2/g)
approximate mass of anatase per titration (g)
4 20 40
300.3 ± 0.41 66.43 ± 0.34 43.98 ± 0.26
0.04 0.22 0.32
Experimental Section. The titration procedure followed throughout the present study was developed by Ridley et al.13,14 specifically for nanoparticle titrations. An essential component of the procedure is the initial dispersion of the particles, achieved using high-intensity (750 W) ultrasonication with an immersible titanium horn. This first step is critical to ensure that the surface titration results reflect the reactivity of the entire surface area of the primary crystallites and not just the surface of close-packed aggregates. The potentiometric titrations were B
DOI: 10.1021/la503932e Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir performed at 25 °C (± 0.1 °C) utilizing a Ross Semimicro combination glass-electrode and Mettler DL70 autotitrator. All experimental solutions were prepared from reagent-grade chemicals and deionized water. All solutions were prepared in NaCl media, with sufficient electrolyte to give ionic strengths of either 0.03 or 0.3 m. For all experiments, the electrode was calibrated using an initial basic solution comprising 5 × 10−4 m OH−, to which an acid calibration solution was added. The acid calibration solution contained sufficient HCl to attain a calibration point close to 0.002 m H+. The titrant solutions comprised NaOH and NaCl. As in previous studies, the ionic strength of the titrant was slightly higher than the corresponding calibration and dispersion solutions,13 making it possible to largely compensate for changes in ionic strength during a titration. The adsorption of Sr2+ onto the anatase surface was determined with test solutions comprising 0.001 m Sr2+, 0.002 m H+, and sufficient NaCl to produce ionic strengths of 0.03 and 0.3 m. The standard titration procedure was to disperse a sufficient mass of anatase powder to provide at least 14 m2 of surface area (Table 1) in 16−20 g of dispersion solution. The dispersion solutions were at the ionic strength of interest and comprised 0.002 m H+. The dispersed sample was transferred to the titration cell, and an appropriate solution comprising Sr2+ was added to total 40 g of the desired test solution. The titration cell was sealed and immersed in a water bath, the headspace was purged with purified argon to prevent CO 2 contamination, and the solutions were stirred mechanically at all times. During each titration, 20 to 30 aliquots of base titrant were added over a pHm range of approximately 8 units (where pHm is the molal H+ concentration). Throughout the titrations, equilibration of the anatase test solution was assumed when the potential drift was
