The colloidal behavior of aluminum oxide nanoparticles (NPs) was investigated as a function of pH and in the presence of two structurally different humic acids (HAs), Aldrich HA (AHA) and the seventh HA fraction extracted from Amherst peat soil (HA7). Dynamic light scattering (DLS) and atomic force microscopy (AFM) were employed to determine the colloidal behavior of the NPs. Influence of pH and HAs on the surface charges of the NPs was determined. zeta-Potential data clearly showed that the surface charge of the NPs decreased with increasing pH and reached the point of zero charge (ZPC) at pH 7.9. Surface charge of the NPs also decreased with the addition of HAs. The NPs tend to aggregate as the pH of the suspension approaches ZPC, where van der Waals attraction forces dominate over electrostatic repulsion. However, the NP colloidal suspension was stable in the pHs far from ZPC. Colloidal stability was strongly enhanced in the presence of HAs at the pH of ZPC or above it, but in acidic conditions NPs showed strong aggregation in the presence of HAs. AFM imaging revealed the presence of long-chain fractions in HA7, which entangled with the NPs to form large aggregates. The association of HA with the NP surface can be assumed to follow a two-step process, possibly the polar fractions of the HA7 sorbed on the NP surface followed by entanglement with the long-chain fractions. Thus, our study demonstrated that the hydrophobic nature of the HA molecules strongly influenced the aggregation of colloidal NPs, possibly through their conformational behavior in a particular solution condition. Therefore, various organic matter samples will result in different colloidal behavior of NPs, subsequently their environmental fate and transport.

Interactions between hydrophobic organic chemicals (HOCs) and dissolved organic matter (DOM) are of environmental significance due to their influence on mobility and bioavailability of HOCs. The linear dissolution concept has been widely used to describe the interactions between HOCs and DOM, but it may not be correct. To date there is no systematic evaluation of nonideal interactions between HOCs and DOM. Therefore, this study employed a dialysis method to investigate sorption, desorption, and competition of two polyaromatic hydrocarbons (PAHs), phenanthrene (PHE) and pyrene (PYR), by two DOMs at pHs of 4, 7, and 11. Nonlinear interactions between PAHs and DOM and desorption hysteresis were consistently observed. The isotherm nonlinearity factor, nvalue, increased significantly after the addition of cosolutes, indicating the occupation of specific binding sites by the cosolute molecules. Significant influence of pH on PAHs-DOM interaction was also observed (higher binding coefficients, stronger desorption hysteresis, and increased nonlinearity at lower pH). This study for the first time systematically showed the nonideal binding behavior of PAHs by DOM. A more complete model rather than linear distribution is required to describe the interactions between HOCs and DOM. Conformation changes of DOM molecules were proposed to explain the interactions between HOCs and DOM.

The colloidal stability of three structurally different humic acid (HA)-coated Al(2)O(3) nanoparticles (HAs-Al(2)O(3) NPs) was studied in the presence of Ca(2+). HAs were obtained after sequential extractions of Amherst Peat Soil. Highly polar HA1-coated Al(2)O(3) NPs exhibited strong aggregation in the presence of Ca(2+). HA3 and HA7-coated NPs showed weaker aggregation due to their increased aliphaticity and low polarity. HA7-Al(2)O(3) NPs displayed the weakest aggregation behavior even at relatively high Ca(2+) concentration. The inverse stability ratio (alpha = 1/W) was the lowest for HA7-Al(2)O(3) NPs, reflecting that strong steric stabilization enhanced colloidal stability. Atomic force microscopy (AFM) of pure Al(2)O(3) NPs on Ca(2+)-saturated mica clearly demonstrated significant aggregation following classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model for hard spheres. On the contrary, weakly polar HA fraction produced approximately 10 nm thick corona of adsorbed layer around each Al(2)O(3) NP, thus stabilizing coated NP suspension through steric effect. Under alkaline conditions and at low ionic strength, adsorbed HA chains swelled, increasing their osmotic potential, which in turn resulted in stabilization of the colloids. Inherent structural variations of natural organic matter (NOM) played a significant part in colloidal stability of the coated NPs. Thus, development of sterically stabilized NPs may have potential application for water remediation in marine and high salinity conditions.

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Ultrasmall monodisperse 2.3 nm CeO2 nanoparticles have been synthesized by simple ammonia precipitation of cerium nitrate in a mixed glycol–water solvent and phase transferred into apolar solvents. Cerium oxide crystal surfaces were passivated with oleic acid (OA) by reflux under ambient pressure conditions. OA molecules were chemisorbed on the ceria nanoparticle surfaces. Preferential growth of the {100} planes has been observed, ascribed to restricted growth of {111} faces due to adsorbed OA. The surfacted CeO2 nanoparticles were dispersed into a stable, colloidal suspension of fine particles in hydrocarbon solvents. The CeO2 nanocrystals were characterized by X-ray diffraction, Fourier transform infrared spectra, and Raman spectral methods. Transmission electron microscopic images and photon scattering studies proved that the colloidal particles in hydrocarbon solvent were indeed monocrystalline with one crystallite per particle, monodispersed with a narrow size distribution.

The catalytic performance of a range of nanocrystalline CeO2 samples, prepared to have different morphologies, was measured using two accepted indicators; oxygen storage and diesel soot combustion. The same powders were characterized in detail by HR-TEM, XRD, XPS, and Raman methods. The study demonstrates that activity is determined by the relative fraction of the active crystallographic planes, not by the specific surface area of the powders. The physical study is a step toward quantitative evaluation of the relative contribution to activity of the different facets. The synthetic protocol permits fabrication of CeO2 nanostructures with preferentially grown active planes, and therefore has potential in developing catalytic applications and in nanocompositing.

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