Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions

Heterogeneous Formation of Organonitrates (ON) and Nitroxy-Organosulfates (NOS) from Adsorbed α‑Pinene-Derived Organosulfates (OS) on Mineral Surfaces

Organonitrates (ON) and nitroxy-organosulfates (NOS) are important components of secondary organic aerosols (SOAs). Gas-phase reactions of α-pinene (C10H16), a primary precursor for several ON compounds, are fairly well understood although formation pathways for NOS largely remain unknown. NOS formation may occur via reactions of ON and organic peroxides with sulfates as well as through radical-initiated photochemical processes. Despite the fact that organosulfates (OS) represent a significant portion of the organic aerosol mass, ON and NOS formation from OS is less understood, especially through nighttime heterogeneous and multiphase chemistry pathways. In the current study, surface reactions of adsorbed α-pinene-derived OS with nitrogen oxides on hematite and kaolinite surfaces, common components of mineral dust, have been investigated. α-Pinene reacts with sulfated mineral surfaces, forming a range of OS compounds on the surface. These OS compounds when adsorbed on mineral surfaces can further react with HNO3 and NO2, producing several ON and NOS compounds as well as several oxidation products. Overall, this study reveals the complexity of reactions of prevalent organic compounds leading to the formation of OS, ON, and NOS via heterogeneous and multiphase reaction pathways on mineral surfaces. It is also shown that this chemistry is mineralogy-specific

Atomic Force Microscopy and X‑ray Photoelectron Spectroscopy Study of NO₂ Reactions on CaCO₃ (101̅4) Surfaces in Humid Environments

In this study, alternating current (AC) mode atomic force microscopy (AFM) combined with phase imaging and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of nitrogen dioxide (NO₂) adsorption on calcium carbonate (CaCO₃) (101̅4) surfaces at 296 K in the presence of relative humidity (RH). At 70% RH, CaCO₃ (101̅4) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a substrate for recrystallization of a nonhydrated calcite phase, presumably vaterite. The adsorption of nitrogen dioxide changes the surface properties of CaCO₃ (101̅4) and the mechanism for formation of new phases. In particular, the first calcite nucleation layer serves as a source of material for further island growth; when it is depleted, there is no change in total volume of nitrocalcite, Ca­(NO₃)₂, particles formed whereas the total number of particles decreases. This indicates that these particles are mobile and coalesce. Phase imaging combined with force curve measurements reveals areas of inhomogeneous energy dissipation during the process of water adsorption in relative humidity experiments, as well as during nitrocalcite particle formation. Potential origins of the different energy dissipation modes within the sample are discussed. Finally, XPS analysis confirms that NO₂ adsorbs on CaCO₃ (101̅4) in the form of nitrate (NO₃^–) regardless of environmental conditions or the pretreatment of the calcite surface at different relative humidity

Histidine Adsorption on TiO₂ Nanoparticles: An Integrated Spectroscopic, Thermodynamic, and Molecular-Based Approach toward Understanding Nano–Bio Interactions

Nanoparticles in biological media form dynamic entities as a result of competitive adsorption of proteins on nanoparticle surfaces called protein coronas. The protein affinity toward nanoparticle surfaces potentially depends on the constituent amino acid side chains which are on the protein exterior and thus exposed to the solution and available for interaction. Therefore, studying the adsorption of individual amino acids on nanoparticle surfaces can provide valuable insights into the overall evolution of nanoparticles in solution and the protein corona that forms. In the current study, the surface adsorption of l-histidine on TiO₂ nanoparticles with a diameter of 5 nm at pH 7.4 (physiological pH) is studied from both macroscopic and molecular perspectives. Quantitative adsorption measurements of l-histidine on 5 nm TiO₂ particles yield maximum adsorption coverage of 6.2 ± 0.3 × 10¹³ molecules cm^–2 at 293 K and pH 7.4. These quantitative adsorption measurements also yield values for the equilibrium constant and free energy of adsorption of <i>K</i> = 4.3 ± 0.5 × 10² L mol^–1 and Δ<i>G</i> = −14.8 ± 0.3 kJ mol^–1, respectively. Detailed analysis of the adsorption between histidine and 5 nm TiO₂ nanoparticle surfaces with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy indicates both the imidazole side chain and the amine group interacting with the nanoparticle surface and the adsorption to be reversible. The adsorption results in no change in surface charge and therefore does not change nanoparticle–nanoparticle interactions and thus aggregation behavior of these 5 nm TiO₂ nanoparticles in aqueous solution

Nitrate Photochemistry on Laboratory Proxies of Mineral Dust Aerosol: Wavelength Dependence and Action Spectra

Nitrate ion adsorbed on the surface of mineral dust particles from heterogeneous reaction of nitric acid, nitrogen pentoxide, and nitrogen dioxide is thought to be a sink for nitrogen oxides. However, it has the potential to release gas-phase nitrogen oxides back into the atmosphere when irradiated with UV light. In this study, the wavelength dependence of nitrate ion photochemistry when adsorbed onto model laboratory proxies of mineral dust aerosol including Al₂O₃, TiO₂, and NaY zeolite was investigated using FTIR spectroscopy. These proxies represent non-photoactive oxides, photoactive semiconductor oxides, and porous aluminosilicate materials, respectively, present in mineral dust aerosol. Nitrate photochemistry on mineral dust particles is governed by the wavelength of light, physicochemical properties of the dust particles, and the adsorption mode of the nitrate ion. Most interestingly, in some cases, nitrate ion adsorbed on oxide particles can undergo photochemistry over a broader wavelength region of the solar spectrum compared to nitrate ion in solution. As shown here, gas-phase NO₂ is the major photolysis product formed from nitrate adsorbed on the surface of oxide particles under dry conditions. The NO₂ yield and the initial rate of production is highest on TiO₂, indicating that nitrate photochemistry is more efficient on photoactive oxides present in mineral dust. Nitrite ion complexed to Na⁺ sites in aluminosilicate zeolite pores is the major photolysis product found for zeolites. Mechanisms for the formation of gas-phase and surface-adsorbed products and a discussion of the wavelength dependence of nitrate ion photochemistry are presented, as is a discussion of the atmospheric implications

Role of Atmospheric CO₂ and H₂O Adsorption on ZnO and CuO Nanoparticle Aging: Formation of New Surface Phases and the Impact on Nanoparticle Dissolution

Heterogeneous reactions of atmospheric gases with metal oxide nanoparticle surfaces have the potential to cause changes in their physicochemical properties including their dissolution in aqueous media. In this study, gas-phase CO₂ adsorption on ZnO and CuO nanoparticle surfaces was studied as a function of relative humidity to better understand the role of CO₂ and H₂O on nanoparticle aging and the influence of this aging process on metal ion dissolution from nanoparticles. Upon nanoparticle exposure to atmospherically relevant pressures of CO₂ under different relative humidity (RH) conditions, temporal variations of surface-adsorbed species were monitored using Fourier transmission infrared spectroscopy (FTIR). Under dry conditions, gas-phase CO₂ readily reacts with surface hydroxyl groups present on the ZnO and CuO nanoparticle surface to form adsorbed bicarbonate, whereas the interaction of CO₂ with surface defect sites and lattice oxygen gives rise to surface-adsorbed monodentate and bidentate carbonate species as well as adsorbed carboxylate. With increasing relative humidity from 0 to 70%, surface speciation gradually changes to that of water-solvated adsorbed carbonate, which was the only detectable surface species at the highest relative humidity investigated (70% RH). High-resolution TEM analysis of reacted ZnO and CuO nanoparticles revealed considerable surface restructuring consistent with the precipitation of crystalline carbonate phases in the presence of adsorbed water. Furthermore, the restructuring of ZnO and CuO nanoparticles during CO₂ exposure is limited to the near surface region. Importantly, the reacted ZnO nanoparticles also show an increase in the extent of their dissolution when placed in aqueous media. Thus, this work provides valuable insights into reactions of atmospheric gases, CO₂ and H₂O, on ZnO and CuO nanoparticle surfaces and the irreversible changes such reactions can induce on nanoparticle surface chemistry and behavior in aqueous media

Surface Adsorption and Photochemistry of Gas-Phase Formic Acid on TiO₂ Nanoparticles: The Role of Adsorbed Water in Surface Coordination, Adsorption Kinetics, and Rate of Photoproduct Formation

Formic acid adsorption and photooxidation on TiO₂ nanoparticle surfaces at 296 K have been investigated using transmission FTIR spectroscopy. In particular, the role of adsorbed water in surface coordination, adsorption kinetics, and photoproduct formation is examined. Gas-phase formic acid adsorbs on the surface at low exposures to yield adsorbed bridged bidentate formate and, at higher exposures, molecularly adsorbed formic acid as well. Upon exposure to water vapor, adsorbed formate becomes solvated by coadsorbed water molecules, and the coordinatin mode changes as indicated by shifts in the vibrational frequencies. Adsorbed water also impacts the adsorption kinetics for formic acid on TiO₂ and increases the adsorption rate, potentially by providing a medium for facile ionic dissociation. Ultraviolet irradiation of adsorbed formate on TiO₂ in the presence of molecular oxygen results in the formation of gas-phase carbon dioxide, which increases in yield in the presence of adsorbed water on the surface. Additionally, the dispersion of TiO₂ nanoparticles in water suspensions is found to change if first exposed to gas-phase formic acid before dispersion. The environmental implications of these results are discussed

Differential Surface Interactions and Surface Templating of Nucleotides (dGMP, dCMP, dAMP, and dTMP) on Oxide Particle Surfaces

The fate of biomolecules in the environment depends in part on understanding the surface chemistry occurring at the biological–geochemical (bio–geo) interface. Little is known about how environmental DNA (eDNA) or smaller components, like nucleotides and oligonucleotides, persist in aquatic environments and the role of surface interactions. This study aims to probe surface interactions and adsorption behavior of nucleotides on oxide surfaces. We have investigated the interactions of individual nucleotides (dGMP, dCMP, dAMP, and dTMP) on TiO2 particle surfaces as a function of pH and in the presence of complementary and noncomplementary base pairs. Using attenuated total reflectance-Fourier transform infrared spectroscopy, there is an increased number of adsorbed nucleotides at lower pH with a preferential interaction of the phosphate group with the oxide surface. Additionally, differential adsorption behavior is seen where purine nucleotides are preferentially adsorbed, with higher surface saturation coverage, over their pyrimidine derivatives. These differences may be a result of intermolecular interactions between coadsorbed nucleotides. When the TiO2 surface was exposed to two-component solutions of nucleotides, there was preferential adsorption of dGMP compared to dCMP and dTMP, and dAMP compared to dTMP and dCMP. Complementary nucleotide base pairs showed hydrogen-bond interactions between a strongly adsorbed purine nucleotide layer and a weaker interacting hydrogen-bonded pyrimidine second layer. Noncomplementary base pairs did not form a second layer. These results highlight several important findings: (i) there is differential adsorption of nucleotides; (ii) complementary coadsorbed nucleotides show base pairing with a second layer, and the stability depends on the strength of the hydrogen bonding interactions and; (iii) the first layer coverage strongly depends on pH. Overall, the importance of surface interactions in the adsorption of nucleotides and the templating of specific interactions between nucleotides are discussed

Heterogeneous Atmospheric Chemistry of Lead Oxide Particles with Nitrogen Dioxide Increases Lead Solubility: Environmental and Health Implications

Heterogeneous chemistry of nitrogen dioxide with lead-containing particles is investigated to better understand lead metal mobilization in the environment. In particular, PbO particles, a model lead-containing compound due to its widespread presence as a component of lead paint and as naturally occurring minerals, massicot, and litharge, are exposed to nitrogen dioxide at different relative humidity. X-ray photoelectron spectroscopy (XPS) shows that upon exposure to nitrogen dioxide the surface of PbO particles reacts to form adsorbed nitrates and lead nitrate thin films with the extent of nitrate formation relative humidity dependent. NO₂-exposed PbO particles are found to have an increase in the amount of lead that dissolves in aqueous suspensions at circumneutral pH compared to particles not exposed. These results point to the potential importance and impact that heterogeneous chemistry with trace atmospheric gases can have on increasing solubility and therefore the mobilization of heavy metals, such as lead, in the environment. This study also shows that surface intermediates that form, such as adsorbed lead nitrates, can yield higher concentrations of lead in water systems. These water systems can include drinking water, groundwater, estuaries, and lakes

Heterogeneous Uptake and Adsorption of Gas-Phase Formic Acid on Oxide and Clay Particle Surfaces: The Roles of Surface Hydroxyl Groups and Adsorbed Water in Formic Acid Adsorption and the Impact of Formic Acid Adsorption on Water Uptake

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth’s crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface −OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed