Vicki H. Grassian

Aggregation and Dissolution of 4 nm ZnO Nanoparticles in Aqueous Environments: Influence of pH, Ionic Strength, Size, and Adsorption of Humic Acid

Metal oxide nanoparticles are used in a wide range of commercial products, leading to an increased interest in the behavior of these materials in the aquatic environment. The current study focuses on the stability of some of the smallest ZnO nanomaterials, 4 ± 1 nm in diameter nanoparticles, in aqueous solutions as a function of pH and ionic strength as well as upon the adsorption of humic acid. Measurements of nanoparticle aggregation due to attractive particle-particle interactions show that ionic strength, pH, and adsorption of humic acid affect the aggregation of ZnO nanoparticles in aqueous solutions, which are consistent with the trends expected from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Measurements of nanoparticle dissolution at both low and high pH show that zinc ions can be released into the aqueous phase and that humic acid under certain, but not all, conditions can increase Zn(2+)(aq) concentrations. Comparison of the dissolution of ZnO nanoparticles of different nanoparticle diameters, including those near 15 and 240 nm, shows that the smallest nanoparticles dissolve more readily. Although qualitatively this enhancement in dissolution can be predicted by classical thermodynamics, quantitatively it does not describe the dissolution behavior very well.

Inhalation Exposure Study of Titanium Dioxide Nanoparticles with a Primary Particle Size of 2 to 5 NM

Background Nanotechnology offers great promise in many industrial applications. However, little is known about the health effects of manufactured nanoparticles, the building blocks of nanomaterials. Objectives Titanium dioxide (TiO2) nanoparticles with a primary size of 2–5 nm have not been studied previously in inhalation exposure models and represent some of the smallest manufactured nanoparticles. The purpose of this study was to assess the toxicity of these nanoparticles using a murine model of lung inflammation and injury. Materials and Methods The properties of TiO2 nanoparticles as well as the characteristics of aerosols of these particles were evaluated. Mice were exposed to TiO2 nanoparticles in a whole-body exposure chamber acutely (4 hr) or subacutely (4 hr/day for 10 days). Toxicity in exposed mice was assessed by enumeration of total and differential cells, determination of total protein, lactate dehydrogenase (LDH) activity and inflammatory cytokines in bronchoalveolar lavage (BAL) fluid. Lungs were also evaluated for histopathologic changes Results Mice exposed acutely to 0.77 or 7.22 mg/m3 nanoparticles demonstrated minimal lung toxicity or inflammation. Mice exposed subacutely (8.88 mg/m3) and necropsied immediately and at week 1 or 2 postexposure had higher counts of total cells and alveolar macrophages in the BAL fluid compared with sentinels. However, mice recovered by week 3 postexposure. Other indicators were negative. Conclusions Mice subacutely exposed to 2–5 nm TiO2 nanoparticles showed a significant but moderate inflammatory response among animals at week 0, 1, or 2 after exposure that resolved by week 3 postexposure.

Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60–180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Adsorption of Organic Acids on TiO2 Nanoparticles: Effects of pH, Nanoparticle Size, and Nanoparticle Aggregation

In this study, the adsorption of two organic acids, oxalic acid and adipic acid, on TiO2 nanoparticles was investigated at room temperature, 298 K. Solution-phase measurements were used to quantify the extent and reversibility of oxalic acid and adipic acid adsorption on anatase nanoparticles with primary particle sizes of 5 and 32 nm. At all pH values considered, there were minimal differences in measured Langmuir adsorption constants, K ads, or surface-area-normalized maximum adsorbate-surface coverages, Gamma max, between 5 and 32 nm particles. Although macroscopic differences in the reactivity of these organic acids as a function of nanoparticle size were not observed, ATR-FTIR spectroscopy showed some distinct differences in the absorption bands present for oxalic acid adsorbed on 5 nm particles compared to 32 nm particles, suggesting different adsorption sites or a different distribution of adsorption sites for oxalic acid on the 5 nm particles. These results illustrate that molecular-level differences in nanoparticle reactivity can still exist even when macroscopic differences are not observed from solution phase measurements. Our results also allowed the impact of nanoparticle aggregation on acid uptake to be assessed. It is clear that particle aggregation occurs at all pH values and that organic acids can destabilize nanoparticle suspensions. Furthermore, 5 nm particles can form larger aggregates compared to 32 nm particles under the same conditions of pH and solid concentrations. The relative reactivity of 5 and 32 nm particles as determined from Langmuir adsorption parameters did not appear to vary greatly despite differences that occur in nanoparticle aggregation for these two different size nanoparticles. Although this potentially suggests that aggregation does not impact organic acid uptake on anatase particles, these data clearly show that challenges remain in assessing the available surface area for adsorption in nanoparticle aqueous suspensions because of aggregation.

Dissolution of ZnO nanoparticles at circumneutral pH: a study of size effects in the presence and absence of citric acid.

Understanding size-dependent processes, including dissolution, of engineered nanoparticles is essential in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size-dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured at circumneutral pH (pH 7.5) and compared. Dissolution was found to be enhanced with smaller ZnO nanoparticles compared to larger-sized particles, even though the nanoparticles were present in solution as aggregates with hydrodynamic diameters on the order of 1-3 μm in size. The presence of citric acid significantly enhanced the extent of ZnO dissolution for all sizes, and the greatest enhancement was observed for the 4 nm particles. Although these results are found to be in qualitative agreement with theoretical predictions, a linearized form of the Kelvin equation to calculate a surface free energy yielded quantities inconsistent with expected values from the literature. Reasons for this inconsistency are discussed and include potential deviations of solubility behavior from classical thermodynamics as a result of a lack of detailed knowledge of surface structure and surface properties, including the presence of different surface crystal facets, and the aggregation state.

Citric Acid Adsorption on TiO2 Nanoparticles in Aqueous Suspensions at Acidic and Circumneutral pH: Surface Coverage, Surface Speciation, and Its Impact on Nanoparticle−Nanoparticle Interactions

Citric acid plays an important role as a stabilizer in several nanomaterial syntheses and is a common organic acid found in nature. Here, the adsorption of citric acid onto TiO(2) anatase nanoparticles with a particle diameter of ca. 4 nm is investigated at circumneutral and acidic pHs. This study focuses on both the details of the surface chemistry of citric acid on TiO(2), including measurements of surface coverage and speciation, and its impact on nanoparticle behavior. Using macroscopic and molecular-based probes, citric acid adsorption and nanoparticle interactions are measured with quantitative solution phase adsorption measurements, attenuated total reflection-FTIR spectroscopy, dynamic light scattering techniques, and zeta-potential measurements as a function of solution pH. The results show that surface coverage is a function of pH and decreases with increasing pH. Surface speciation differs from the bulk solution and is time dependent. After equilibration, the fully deprotonated citrate ion is present on the surface regardless of the highly acidic solution pH indicating pK(a) values of surface adsorbed species are lower than those in solution. Nanoparticle interactions are also probed through measurements of aggregation and the data show that these interactions are complex and depend on the detailed interplay between bulk solution pH and surface chemistry.

Heterogeneous reactions of NO2 and HNO3 on oxides and mineral dust: A combined laboratory and modeling study

This study combines laboratory measurements and modeling analysis to quantify the role of heterogeneous reactions of gaseous nitrogen dioxide and nitric acid on mineral oxide and mineral dust particles in tropospheric ozone formation. At least two types of heterogeneous reactions occur on the surface of these particles. Upon initial exposure of the oxide to NO2 there is a loss of NO2 from the gas phase by adsorption on the particle surface, i.e., NO2(g) → NO2(a). As the reaction proceeds, a reduction of gaseous NO2 to NO, NO2 (g) → NO (g) is found to occur. Initial uptake coefficients γ0 for NO2 on the surface of these particles have been measured at 298 K using a Knudsen cell reactor coupled to a mass spectrometer. For the oxides studied, α,γ-Al2O3, α,γ-Fe2O3, TiO2, SiO2, CaO, and MgO, γ0 ranges from <4×10−10 for SiO2 to 2×10−5 for CaO with most values in the 10−6 range. For authentic samples of China loess and Saharan sand, similar reactivity to the oxides is observed with γ0 values of 2×10−6 and 1×10−6, respectively. For HNO3 the reactivity is 1–2 orders of magnitude higher. Using these laboratory measurements, the impact of heterogeneous reactions of NO2 and HNO3 on mineral dust in tropospheric ozone formation and on O3-precursor relationships is assessed using a time-dependent, multiphase chemistry box model. Simulations with and without heterogeneous reactions were conducted to evaluate the possible influence of these heterogeneous reactions on ambient levels. Results show that values of the initial uptake for NO2 and HNO3, adjusted for roughness effects, must be greater than 10−4 to have an appreciable impact on NOx, HNO3, and O3 concentrations for the conditions modeled here. Thus the measured uptake coefficients for NO2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO3 is sufficiently large to have an effect. Under conditions of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO2 and HNO3) is expected to increase.

Chemistry and Photochemistry of Mineral Dust Aerosol

It has become increasingly clear that heterogeneous and multiphase chemistry of tropospheric aerosols can change the chemical balance of the atmosphere. In this review, we focus on recent laboratory studies of the heterogeneous and multiphase chemistry and photochemistry of mineral dust aerosol, a large mass fraction of the tropospheric aerosol. Mineral dust aerosol contains a mixture of oxides, clays, and carbonates. Molecular-based studies of reactions of these dust components provide insights into the chemistry of Earths atmosphere. We discuss several different types of heterogeneous and multiphase reactions, including (a) ozone decomposition, (b) nitrogen dioxide and nitrate photochemistry, and (c) the dissolution and redox chemistry of Fe-containing dust. We also review some of the important chemical concepts that have recently emerged.

NANOSILVER INDUCES MINIMAL LUNG TOXICITY OR INFLAMMATION IN A SUBACUTE MURINE INHALATION MODEL

BackgroundThere is increasing interest in the environmental and health consequences of silver nanoparticles as the use of this material becomes widespread. Although human exposure to nanosilver is increasing, only a few studies address possible toxic effect of inhaled nanosilver. The objective of this study was to determine whether very small commercially available nanosilver induces pulmonary toxicity in mice following inhalation exposure.ResultsIn this study, mice were exposed sub-acutely by inhalation to well-characterized nanosilver (3.3 mg/m3, 4 hours/day, 10 days, 5 ± 2 nm primary size). Toxicity was assessed by enumeration of total and differential cells, determination of total protein, lactate dehydrogenase activity and inflammatory cytokines in bronchoalveolar lavage fluid. Lungs were evaluated for histopathologic changes and the presence of silver. In contrast to published in vitro studies, minimal inflammatory response or toxicity was found following exposure to nanosilver in our in vivo study. The median retained dose of nanosilver in the lungs measured by inductively coupled plasma - optical emission spectroscopy (ICP-OES) was 31 μg/g lung (dry weight) immediately after the final exposure, 10 μg/g following exposure and a 3-wk rest period and zero in sham-exposed controls. Dissolution studies showed that nanosilver did not dissolve in solutions mimicking the intracellular or extracellular milieu.ConclusionsMice exposed to nanosilver showed minimal pulmonary inflammation or cytotoxicity following sub-acute exposures. However, longer term exposures with higher lung burdens of nanosilver are needed to ensure that there are no chronic effects and to evaluate possible translocation to other organs.

Water, sulfur dioxide and nitric acid adsorption on calcium carbonate: A transmission and ATR-FTIR study

Calcium carbonate (CaCO3) is a reactive component of mineral dust aerosol as well as buildings, statues and monuments. In this study, attenuated total reflection (ATR) and transmission Fourier transform infrared spectroscopy (FTIR) have been used to study the uptake of water, sulfur dioxide and nitric acid on CaCO3 particles at 296 K. Under atmospheric conditions, CaCO3 particles are terminated by a Ca(OH)(CO3H) surface layer. In the presence of water vapor between 5 and 95% relative humidity (RH), water molecularly adsorbs on the Ca(OH)(CO3H) surface resulting in the formation of an adsorbed thin water film. The adsorbed water film assists in the enhanced uptake of sulfur dioxide and nitric acid on CaCO3 in several ways. Under dry conditions (near 0% RH), sulfur dioxide and nitric acid react with the Ca(OH)(CO3H) surface to form adsorbed carbonic acid (H2CO3) along with sulfite and nitrate, respectively. Adsorbed carbonic acid is stable on the surface under vacuum conditions. Once the surface saturates with a carbonic acid capping layer, there is no additional uptake of gas-phase sulfur dioxide and nitric acid. However, upon adsorption of water, carbonic acid dissociates to form gaseous carbon dioxide and there is further uptake of sulfur dioxide and nitric acid. In addition, adsorbed water increases the mobility of the ions at the surface and enhances uptake of SO2 and HNO3. In the presence of adsorbed water, CaSO3 forms islands of a crystalline hydrate whereas Ca(NO3)2 forms a deliquescent layer or micropuddles. Thus adsorbed water plays an important and multi-faceted role in the uptake of pollutant gases on CaCO3.