Gas-phase and air-solid interface behavior of phthalate plasticizer and ozone: The influence of indoor mineral dust

Indoor environments serve as reservoirs for a variety of emerging pollutants (EPs), such as phthalates (PAE), with intricate interactions occurring between these compounds and indoor oxidants alongside dust particles. However, the precise mechanisms governing these interactions and their resulting environmental implications remain unclear. By theoretical simulations, this work uncovers multi-functional compounds and high oxygen molecules as important products arising from the interaction between DEP/DEHP and O3, which are closely linked to SOA formation. Further analysis reveals a strong affinity of DEP/DEHP for mineral dust surfaces, with an adsorption energy of 22.11/30.91 kcal mol-1, consistent with a higher concentration of DEHP on the dust surface. Importantly, mineral particles are found to inhibit every step of the reaction process, albeit resulting in lower product toxicity compared to the parent compounds. Thus, timely removal of dust in an indoor environment may reduce the accumulation and residue of PAEs indoors, and further reduce the combined exposure risk produced by PAEs-dust. This study aims to enhance our understanding of the interaction between PAEs and SOA formation, and to develop a fundamental reaction model at the air-solid surface, thereby shedding light on the microscopic behaviors and pollution mechanisms of phthalates on indoor dust surfaces.

Mechanistic study of the adsorption and penetration of modified SiO2 nanoparticles on cellular membrane

SiO₂ nanoparticles (NPs) are ubiquitous in the atmosphere and have been proved to be harmful to human by bioaccumulation. The bioaccumulation of SiO₂ NPs begins with their partitioning into membrane phospholipids. However, the uptake of SiO₂ particle onto lipid bilayers has been not completely understood. In this study, the uptake process of different modified SiO₂ particles (with adsorbed trace gas molecules, including formic acid (FA), methyl vinyl ketone (MVK), and methacrolein (MAC)) into DPPC bilayers were investigated by molecular dynamics (MD) simulations. Results suggest that after the SiO₂ particle absorbed on the surface of bilayers, it spun by itself to form the most stable adsorption configurations. MAC and MVK molecule tend to be absorbed into DPPC bilayer while FA molecule was more likely located at the surface region of bilayer from the viewpoint of thermodynamics. Besides, it was also found apart from the commonly accepted "Trojan-horse" effect, SiO₂ NPs may increase biohazard via modulating the concentration of pollutants. This work is beneficial for understanding the toxicity and bioaccumulation of FA, MVK, MAC, and SiO₂ NPs at molecular level.

Degradation evaluation of acrylamide in advanced oxidation processes based on theoretical method: Mechanisms, kinetics, toxicity evaluation and the role of soil particles

Acrylamide (AA) is now recognized as an imminent hazardous chemical in the aqueous environment, causing a potential threat to human health. As a neo-formed contaminant (NFC), the degradation measure of AA is largely lacking. In this work, we used quantum chemistry and experimental methods to identify the main degradation mechanism of AA in the UV/H₂O₂ advanced oxidation process (AOP) for the first time. Radical addition reactions dominate the •OH-initiated AA reaction, resulting in few toxic nitrosamines formation. The interaction between AA and the surface model of soil particles (Si_xO_y(OH)_z) is weak, and AA can rapidly migrate down to groundwater via seepage. However, the total rate constants of AA and COM_ADS2-AA with •OH are 2.75 × 10⁹ and 2.09 × 10⁹ M^-1 s^-1, and the removal of AA from aqueous and heterogeneous systems reaches 62.30% and 62.05% within 2 h. Whether in the aqueous-phase or on the surface of soil particles, •OH initiated AA reaction is an efficient way to remove AA. Furthermore, the toxicity of the main by-products of AA show less harmful to three aquatic organisms and rats than AA. UV/H₂O₂ AOP is evaluated as an efficient method to degrade AA while decreasing harm.

Atomistic insights into heterogeneous reaction of formic acid on mineral oxide particles

The heterogeneous reaction between formic acid and mineral dust play an important role in tropospheric chemistry. However, the molecular mechanism on formic acid uptake on mineral dust is not yet to be fully understood. In our work, a comprehensive and multiscale theoretical study (include density functional theory, DFT calculations and reactive molecular dynamics, RMD simulations) has been provided to investigate this heterogeneous reaction at molecular level. The results of DFT calculations show that the SiO₂ and TiO₂ particles have a strong tendency to adsorb formic acid to its surface, and the attractive part of the binding energy was dominated by electrostatic component. RMD simulations show that the uptake of formic acid and water on TiO₂ particles would modified TiO₂ particles, which formed Ti-OH and Ti-OCHO on particle surface. Besides, the formic acid coordination surface modes were dominated by the monodentate formate mode rather than bidentate formate mode. The molecular level study is helpful for understanding the accumulation of formic acid on mineral dust particles and global balance of atmospheric formic acid.

Assessing the role of mineral particles in the atmospheric photooxidation of typical carbonyl compound

Mineral particles are ubiquitous in the atmosphere and exhibit an important effect on the photooxidation of volatile organic compounds (VOCs). However, the role of mineral particles in the photochemical oxidation mechanism of VOCs remains unclear. Hence, the photooxidation reactions of acrolein (ARL) with OH radical (OH) in the presence and absence of SiO₂ were investigated by theoretical approach. The gas-phase reaction without SiO₂ has two distinct pathways (H-abstraction and OH-addition pathways), and carbonyl-H-abstraction is the dominant pathway. In the presence of SiO₂, the reaction mechanism is changed, i.e., the dominant pathway from carbonyl-H-abstraction to OH-addition to carbonyl C-atom. The energy barrier of OH-addition to carbonyl C-atom deceases 21.33 kcal/mol when SiO₂ is added. Carbonyl H-atom of ARL is occupied by SiO₂ via hydrogen bond, and carbonyl C-atom is activated by SiO₂. Hence, the main product changes from H-abstraction product to OH-adduct in the presence of SiO₂. The OH-adduct exhibits a thermodynamic feasibility to yield HO₂ radical and carboxylic acid via the subsequent reactions with O₂, with implications for O₃ formation and surface acidity of mineral particles.

Heterogeneous reactions of isoprene and ozone on α-Al2O3: The suppression effect of relative humidity

The heterogeneous reactions of α-Al₂O₃ particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO^-) ions on the surfaces of α-Al₂O₃ particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-Al₂O₃ particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.

Oxidative and Toxicological Evolution of Engineered Nanoparticles with Atmospherically Relevant Coatings

The health impacts associated with engineered nanoparticles (ENPs) released into the atmosphere have not been adequately assessed. Such impacts could potentially arise from the toxicity associated with condensable atmospheric secondary organic material (SOM), or changes in the SOM composition induced by ENPs. Here, these possibilities are evaluated by investigating the oxidative and toxicological evolution of TiO₂ and SiO₂ nanoparticles which have been coated with SOM from the O₃ or OH initiated oxidation of α-pinene. It was found that pristine SiO₂ particles were significantly more cytotoxic compared to pristine TiO₂ particles. TiO₂ in the dark or under UV irradiation catalytically reacted with the SOM, increasing its O/C by up to 55% over photochemically inert SiO₂ while having negligible effects on the overall cytotoxicity. Conversely, the cytotoxicity associated with SiO₂ coated with SOM was markedly suppressed (by a factor of 9, at the highest exposure dose) with both increased SOM coating thickness and increased photochemical aging. These suppressing effects (organic coating and photo-oxidation of organics) were attributed to a physical hindrance of SiO₂-cell interactions by the SOM and enhanced SOM viscosity and hydrophilicity with continued photo-oxidation, respectively. These findings highlight the importance of atmospheric processes in altering the cytotoxicity of ENPs.

Heterogeneous Reactions of Limonene on Mineral Dust: Impacts of Adsorbed Water and Nitric Acid

Biogenic volatile organic compounds (BVOCs), including the monoterpene limonene, are a major source of secondary organic aerosol (SOA). While gas-phase oxidation initiates the dominant pathway for BVOC conversion to SOA, recent studies have demonstrated that biogenic hydrocarbons can also directly react with acidic droplets. To investigate whether mineral dust may facilitate similar reactive uptake of biogenic hydrocarbons, we studied the heterogeneous reaction of limonene with mineral substrates using condensed-phase infrared spectroscopy and identified the formation of irreversibly adsorbed organic products. For kaolinite, Arizona Test Dust, and silica at 30% relative humidity, GC-MS identified limonene-1,2-diol as the dominant product with total organic surface concentrations on the order of (3-5) × 10¹⁸ molecules m^-2. Experiments with ¹⁸O-labeled water support a mechanism initiated by oxidation of limonene by surface redox sites forming limonene oxide followed by water addition to the epoxide to form limonenediol. Limonene uptake on α-alumina, γ-alumina, and montmorillonite formed additional products in high yield, including carveol, carvone, limonene oxide, and α-terpineol. To model tropospheric processing of mineral aerosol, we also exposed each mineral substrate to gaseous nitric acid prior to limonene uptake and identified similar surface adsorbed products that were formed at rates 2 to 5 times faster than without nitrate coatings. The initial rate of reaction was linearly dependent on gaseous limonene concentration between 5 × 10¹² and 5 × 10¹⁴ molecules cm^-3 (0.22-20.5 ppm) consistent with an Eley-Rideal-type mechanism in which gaseous limonene reacts directly with reactive surface sites. Increasing relative humidity decreased the amount of surface adsorbed products indicating competitive adsorption of surface adsorbed water. Using a laminar flow tube reactor we measured the uptake coefficient for limonene on kaolinite at 25% RH to range from γ = 5.1 × 10^-6 to 9.7 × 10^-7. After adjusting for reactive surface areas, we estimate uptake coefficients for limonene on HNO₃-processed mineral aerosol on the order of (1-6) × 10^-6. Although this heterogeneous reaction will not impact the atmospheric lifetime of gaseous limonene, it does provide a new pathway for mineral aerosol to acquire secondary organic matter from biogenic hydrocarbons, which in turn will alter the physical properties of mineral dust.

Multiphase Ozonolysis of Aqueous α-Terpineol

Multiphase ozonolysis of aqueous organics presents a potential pathway for the formation of aqueous secondary organic aerosol (aqSOA). We investigated the multiphase ozonolysis of α-terpineol, an oxygenated derivative of limonene, and found that the reaction products and kinetics differ from the gas-phase ozonolysis of α-terpineol. One- and two-dimensional NMR spectroscopies along with GC-MS identified the aqueous ozonolysis reaction products as trans- and cis-lactols [4-(5-hydroxy-2,2-dimethyltetrahydrofuran-3-yl)butan-2-one] and a lactone [4-hydroxy-4-methyl-3-(3-oxobutyl)-valeric acid gamma-lactone], which accounted for 46%, 27%, and 20% of the observed products, respectively. Hydrogen peroxide was also formed in 10% yield consistent with a mechanism involving decomposition of hydroxyl hydroperoxide intermediates followed by hemiacetal ring closure. Multiphase reaction kinetics at gaseous ozone concentrations of 131, 480, and 965 parts-per-billion were analyzed using a resistance model of net ozone uptake and found the second-order rate coefficient for the aqueous reaction of α-terpineol + O₃ to be 9.9(±3.3) × 10⁶ M^-1 s^-1. Multiphase ozonolysis will therefore be competitive with multiphase oxidation by hydroxyl radicals (OH) and ozonolysis of gaseous α-terpineol. We also measured product yields for the heterogeneous ozonolysis of α-terpineol adsorbed on glass, NaCl, and kaolinite, and identified the same three major products but with an increasing lactone yield of 33, 49, and 55% on these substrates, respectively.

Hydrogen peroxide enhances the oxidation of oxygenated volatile organic compounds on mineral dust particles: a case study of methacrolein

Heterogeneous oxidation of oxygenated volatile organic compounds (OVOCs) serves as an important sink of OVOCs as well as a source of secondary organic material. However, the roles of gas phase oxidants in these reactions are poorly understood. In this work, we present the first laboratory study of the heterogeneous reactions of methacrolein (MACR) on various mineral dust particles in the presence of gaseous H2O2. It is found that the presence of gaseous H2O2 significantly promotes both the uptake and oxidation of MACR on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, but not on CaCO3. The oxidation of MACR produces organic acids as its major low-molecular-weight product, whose yields are enhanced by a factor of 2-6 in the presence of H2O2. In addition, organic peroxides such as methyl hydroperoxide, peroxyformic acid, and peroxyacetic acid are only formed in the presence of H2O2, and the formation of methyl hydroperoxide indicates that MACR oxidation on the surface involves reaction with OH radicals. A probe reaction using salicylic acid verifies the production of OH radicals from H2O2 decomposition on kaolinite, α-Al2O3, α-Fe2O3, and TiO2, which rationalizes the enhanced MACR oxidation observed on these particles. The uptake coefficients of MACR on kaolinite, α-Fe2O3, and TiO2 in the presence of H2O2 are on the order of 10(-5)-10(-4). Our results provide new insights into the formation and chemical evolution of organic species in the atmosphere.

Kinetics and mechanisms of heterogeneous reaction of gaseous hydrogen peroxide on mineral oxide particles

Recent studies have shown that heterogeneous reactions of hydrogen peroxide (H(2)O(2)) on aerosol surfaces may play an important role in tropospheric chemistry. The data concerning the kinetics and mechanisms of these reactions, however, are quite scarce so far. Here, we investigated, for the first time, the heterogeneous reactions of gaseous H(2)O(2) on SiO(2) and α-Al(2)O(3) particles, two major components of mineral dust aerosol, using transmission-Fourier Transform Infrared (T-FTIR) spectroscopy, and high-performance liquid chromatography (HPLC). It is found that H(2)O(2) molecularly adsorbs on SiO(2), and a small amount of molecularly adsorbed H(2)O(2) decomposes due to its thermal instability. For α-Al(2)O(3), catalytic decomposition of H(2)O(2) evidently occurs, but there is also a small amount of H(2)O(2) molecularly adsorbed on the particle surface. The BET uptake coefficients of H(2)O(2) on both particles appear to be independent of gaseous H(2)O(2) concentration (1.27-13.8 ppmv) and particle sample mass (2.8-6.5 mg for SiO(2) and 8.6-18.9 mg for α-Al(2)O(3)), but are strongly dependent on relative humidity with the values ranging from (1.55 ± 0.14) × 10(-8) and (1.21 ± 0.04) × 10(-7) at 2% RH to (0.61 ± 0.06) × 10(-8) and (0.76 ± 0.09) × 10(-7) at 76% RH for SiO(2) and α-Al(2)O(3), respectively. On the basis of the experimental results and literature data, the potential mechanisms for heterogeneous decomposition of H(2)O(2) were proposed, and the atmospheric implications of these reactions were discussed. It is found that heterogeneous reaction of H(2)O(2) on both mineral oxides plays a significant role in processing mineral aerosols, although its role as a sink for ambient H(2)O(2) is probably limited.