Yingge Wang

Adsorption of organic matter at mineral/water interfaces: 7. ATR-FTIR and quantum chemical study of lactate interactions with hematite nanoparticles.

The interaction of the l-lactate ion ( l-CH3CH(OH)COO(-), lact(-1)) with hematite (alpha-Fe2O3) nanoparticles (average diameter 11 nm) in the presence of bulk water at pH 5 and 25 degrees C was examined using a combination of (1) macroscopic uptake measurements, (2) in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and (3) density functional theory modeling at the B3LYP/6-31+G* level. Uptake measurements indicate that increasing [ lact(-1)]aq results in an increase in lact(-1) uptake and a concomitant increase in Fe(III) release as a result of the dissolution of the hematite nanoparticles. The ATR-FTIR spectra of aqueous lact(-1) and lact(-1) adsorbed onto hematite nanoparticles at coverages ranging from 0.52 to 5.21 micromol/m2 showed significant differences in peak positions and shapes of carboxyl group stretches. On the basis of Gaussian fits of the spectra, we conclude that lact(-1) is present as both outer-sphere and inner-sphere complexes on the hematite nanoparticles. No significant dependence of the extent of lact(-1) adsorption on background electrolyte concentration was found, suggesting that the dominant adsorption mode for lact(-1) is inner sphere under these conditions. On the basis of quantum chemical modeling, we suggest that inner-sphere complexes of lact(-1) adsorbed on hematite nanoparticles occur dominantly as monodentate, mononuclear complexes with the hydroxyl functional group pointing away from the Fe(III) center.

Probing Ag nanoparticle surface oxidation in contact with (in)organics: an X-ray scattering and fluorescence yield approach.

Characterizing interfacial reactions is a crucial part of understanding the behavior of nanoparticles in nature and for unlocking their functional potential. Here, an advanced nanostructure characterization approach to study the corrosion processes of silver nanoparticles (Ag-Nps), currently the most highly produced nanoparticle for nanotechnology, is presented. Corrosion of Ag-Nps under aqueous conditions, in particular in the presence of organic matter and halide species common to many natural environments, is of particular importance because the release of toxic Ag(+) from oxidation/dissolution of Ag-Nps may strongly impact ecosystems. In this context, Ag-Nps capped with polyvinolpyrrolidone (PVP) in contact with a simple proxy of organic matter in natural waters [polyacrylic acid (PAA) and Cl(-) in solution] has been investigated. A combination of synchrotron-based X-ray standing-wave fluorescence yield- and X-ray diffraction-based experiments on a sample consisting of an approximately single-particle layer of Ag-Nps deposited on a silicon substrate and coated by a thin film of PAA containing Cl revealed the formation of a stable AgCl corrosion product despite the presence of potential surface stabilizers (PVP and PAA). Diffusion and precipitation processes at the Ag-Nps-PAA interface were characterized with a high spatial resolution using this new approach.

Competitive Sorption of Pb(II) and Zn(II) on Polyacrylic Acid-Coated Hydrated Aluminum-Oxide Surfaces

Natural organic matter (NOM) often forms coatings on minerals. Such coatings are expected to affect metal-ion sorption due to abundant sorption sites in NOM and potential modifications to mineral surfaces, but such effects are poorly understood in complex multicomponent systems. Using poly(acrylic acid) (PAA), a simplified analog of NOM containing only carboxylic groups, Pb(II) and Zn(II) partitioning between PAA coatings and α-Al2O3 (1-102) and (0001) surfaces was investigated using long-period X-ray standing wave-florescence yield spectroscopy. In the single-metal-ion systems, PAA was the dominant sink for Pb(II) and Zn(II) for α-Al2O3(1-102) (63% and 69%, respectively, at 0.5 μM metal ions and pH 6.0). In equi-molar mixed-Pb(II)-Zn(II) systems, partitioning of both ions onto α-Al2O3(1-102) decreased compared with the single-metal-ion systems; however, Zn(II) decreased Pb(II) sorption to a greater extent than vice versa, suggesting that Zn(II) outcompeted Pb(II) for α-Al2O3(1-102) sorption sites. In contrast, >99% of both metal ions sorbed to PAA when equi-molar Pb(II) and Zn(II) were added simultaneously to PAA/α-Al2O3(0001). PAA outcompeted both α-Al2O3 surfaces for metal sorption but did not alter their intrinsic order of reactivity. This study suggests that single-metal-ion sorption results cannot be used to predict multimetal-ion sorption at NOM/metal-oxide interfaces when NOM is dominated by carboxylic groups.

Comparison of isoelectric points of single-crystal and polycrystalline α-Al2O3 and α-Fe2O3 surfaces

Abstract The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al2O3 (0001) and (1102) and α-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al2O3 (0001) and (11¯02)

Effect of biofilm coatings at metal-oxide/water interfaces I: Pb(II) and Zn(II) partitioning and speciation at Shewanella oneidensis/metal-oxide/water interfaces

(1 \overline1 02)

Pb, Cu, and Zn distributions at humic acid-coated metal-oxide surfaces

and α-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al2O3 are in good agreement with literature values, whereas the new IEP value for α-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8–8.5) for single-crystal α-Fe2O3 (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Lützenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lützenkirchen et al. (2015) on a fresh α-Fe2O3 (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline α-Al2O3 and α-Fe2O3, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated Ka values of α-Fe2O3 and α-Al2O3 surfaces in contact with water as a function of defect type and concentration suggest that highly reactive surface defect sites (primarily singly coordinated aquo groups) on the Fe- and Al-oxide powders are possibly a major source of the surface charge differences between polycrystalline samples and their oriented single-crystal counterparts studied here. The results of this study provide a better understanding of the surface charging behavior of Fe and Al-oxides, which is essential for predicting complex processes such as metal-ion sorption occurring at mineral/water interfaces.