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1. Carbon–titanium dioxide (C/TiO ₂ ) nanofiber composites for chemical oxidation of emerging organic contaminants in reactive filtration applications

   https://doi.org/10.1039/D0EN00975J  

   Greenstein, Katherine E. ; Nagorzanski, Matthew R. ; Kelsay, Bailey ; Verdugo, Edgard M. ; Myung, Nosang V. ; Parkin, Gene F. ; Cwiertny, David M. ( March 2021 , Environmental Science: Nano)

   The recalcitrance of some emerging organic contaminants through conventional water treatment systems may necessitate advanced technologies that use highly reactive, non-specific hydroxyl radicals. Here, polyacrylonitrile (PAN) nanofibers with embedded titanium dioxide (TiO 2 ) nanoparticles were synthesized via electrospinning and subsequently carbonized to produce mechanically stable carbon/TiO 2 (C/TiO 2 ) nanofiber composite filters. Nanofiber composites were optimized for reactivity in flow through treatment systems by varying their mass loading of TiO 2 , adding phthalic acid (PTA) as a dispersing agent for nanoparticles in electrospinning sol gels, comparing different types of commercially available TiO 2 nanoparticles (Aeroxide® P25 andmore »5 nm anatase nanoparticles) and through functionalization with gold (Au/TiO 2 ) as a co-catalyst. High bulk and surface TiO 2 concentrations correspond with enhanced nanofiber reactivity, while PTA as a dispersant makes it possible to fabricate materials at very high P25 loadings (∼80% wt%). The optimal composite formulation (50 wt% P25 with 2.5 wt% PTA) combining high reactivity and material stability was then tested across a range of variables relevant to filtration applications including filter thickness (300–1800 μm), permeate flux (from 540–2700 L m −2 h), incident light energy (UV-254 and simulated sunlight), flow configuration (dead-end and cross-flow filtration), presence of potentially interfering co-solutes (dissolved organic matter and carbonate alkalinity), and across a suite of eight organic micropollutants (atrazine, benzotriazole, caffeine, carbamazepine, DEET, metoprolol, naproxen, and sulfamethoxazole). During cross-flow recirculation under UV-irradiation, 300 μm thick filters (30 mg total mass) produced micropollutant half-lives ∼45 min, with 40–90% removal (from an initial 0.5 μM concentration) in a single pass through the filter. The initial reaction rate coefficients of micropollutant transformation did not clearly correlate with reported second order rate coefficients for reaction with hydroxyl radical ( k OH ), implying that processes other than reaction with photogenerated hydroxyl radical ( e.g. , surface sorption) may control the overall rate of transformation. The materials developed herein represent a promising next-generation filtration technology that integrates photocatalytic activity in a robust platform for nanomaterial-enabled water treatment.« less
2. Rapid Disinfection by Peracetic Acid Combined with UV Irradiation

   https://doi.org/https://doi.org/10.1021/acs.estlett.8b00249  

   Sun, P. ; Zhang, T. ; Mejia-Tickner, B. ; Zhang, R. ; Cai, M. ; Huang, C.-H. ( January 2018 , Environmental science & technology letters)

   This study proposes a novel disinfection process by sequential application of peracetic acid (PAA) and ultra-violet light (UV), on the basis of elucidation of disinfection mechanisms under UV/PAA. Results show that hydroxyl radicals, generated by UV-activated PAA, contribute to the enhanced inactivation of Escherichia coli under UV/PAA compared to PAA alone or UV alone. Furthermore, the location of hydroxyl radical generation is a critical factor. Unlike UV/H2O2, which generates hydroxyl radicals mainly in the bulk solution, the hydroxyl radicals under UV/PAA are produced close to or inside E. coli cells, due to PAA diffusion. Therefore, hydroxyl radicals exert significantly strongermore »disinfection power under UV/PAA than under UV/H2O2 conditions. Pre-exposing E. coli to PAA in the dark followed by application of UV (i.e., a PAA-UV/PAA process) promotes diffusion of PAA to the cells and achieves excellent disinfection efficiency while saving more than half of the energy cost associated with UV compared to simultaneous application of UV and PAA. The effectiveness of this new disinfection strategy has been demonstrated not only in lab water but also in wastewater matrices.« less
3. Radical chemistry in oxidation flow reactors for atmospheric chemistry research

   https://doi.org/10.1039/C9CS00766K  

   Peng, Zhe ; Jimenez, Jose L. ( May 2020 , Chemical Society Reviews)

   Environmental chambers have been playing a vital role in atmospheric chemistry research for seven decades. In last decade, oxidation flow reactors (OFR) have emerged as a promising alternative to chambers to study complex multigenerational chemistry. OFR can generate higher-than-ambient concentrations of oxidants via H 2 O, O 2 and O 3 photolysis by low-pressure-Hg-lamp emissions and reach hours to days of equivalent photochemical aging in just minutes of real time. The use of OFR for volatile-organic-compound (VOC) oxidation and secondary-organic-aerosol formation has grown very rapidly recently. However, the lack of detailed understanding of OFR photochemistry left room for speculation thatmore »OFR chemistry may be generally irrelevant to the troposphere, since its initial oxidant generation is similar to stratosphere. Recently, a series of studies have been conducted to address important open questions on OFR chemistry and to guide experimental design and interpretation. In this Review, we present a comprehensive picture connecting the chemistries of hydroxyl (OH) and hydroperoxy radicals, oxidized nitrogen species and organic peroxy radicals (RO 2 ) in OFR. Potential lack of tropospheric relevance associated with these chemistries, as well as the physical conditions resulting in it will also be reviewed. When atmospheric oxidation is dominated by OH, OFR conditions can often be similar to ambient conditions, as OH dominates against undesired non-OH effects. One key reason for tropospherically-irrelevant/undesired VOC fate is that under some conditions, OH is drastically reduced while non-tropospheric/undesired VOC reactants are not. The most frequent problems are running experiments with too high precursor concentrations, too high UV and/or too low humidity. On other hand, another cause of deviation from ambient chemistry in OFR is that some tropospherically-relevant non-OH chemistry ( e.g. VOC photolysis in UVA and UVB) is not sufficiently represented under some conditions. In addition, the fate of RO 2 produced from VOC oxidation can be kept relevant to the troposphere. However, in some cases RO 2 lifetime can be too short for atmospherically-relevant RO 2 chemistry, including its isomerization. OFR applications using only photolysis of injected O 3 to generate OH are less preferable than those using both 185 and 254 nm photons (without O 3 injection) for several reasons. When a relatively low equivalent photochemical age (<∼1 d) and high NO are needed, OH and NO generation by organic-nitrite photolysis in the UVA range is preferable. We also discuss how to achieve the atmospheric relevance for different purposes in OFR experimental planning.« less
4. Reaction mechanism, energetics, and kinetics of the water-assisted thioformaldehyde + ˙OH reaction and the fate of its product radical under tropospheric conditions

   https://doi.org/10.1039/D0CP00570C  

   Arathala, Parandaman ; Katz, Mark ; Musah, Rabi A. ( May 2020 , Physical Chemistry Chemical Physics)

   The reactions of thioformaldehyde (H 2 CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H 2 CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be −0.8 kcal mol −1 , relative to the separated reactants, which has a ∼1.0 kcal mol −1 lower barrier than the abstraction channel. The H 2 CS + ˙OH reaction assisted by a single water molecule reducesmore »the barrier heights significantly for both the addition and abstraction channels, to −5.5 and −6.7 kcal mol −1 respectively, compared to the un-catalyzed H 2 CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol −1 for both reaction paths. The rate constants for the H 2 CS⋯H 2 O + ˙OH and OH⋯H 2 O + H 2 CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H 2 CS + ˙OH reaction paths for comparison with the H 2 CS + ˙OH + H 2 O reaction in the same temperature range were also computed. The results suggest that the rate of the H 2 CS + ˙OH + H 2 O reaction is slower than that of the H 2 CS + ˙OH reaction by ∼1–4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H 2 CS + ˙OH + H 2 O and H 2 CS + ˙OH reactions were found to be 2.2 × 10 −8 s −1 and 6.4 × 10 −6 s −1 , respectively, calculated using [OH] = 1.0 × 10 6 molecules cm −3 , and [H 2 O] = 8.2 × 10 17 molecules cm −3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H 2 C(OH)S˙ product in the presence of 3 O 2 were also performed. The results show that H 2 CS is removed from the atmosphere primarily by reacting with ˙OH and O 2 to form thioformic acid, HO 2 , formaldehyde, and SO 2 as the main end products.« less
5. Emerging investigators series: the efficacy of chlorine photolysis as an advanced oxidation process for drinking water treatment

   https://doi.org/10.1039/C6EW00029K  

   Remucal, C. K. ; Manley, D. ( January 2016 , Environmental Science: Water Research & Technology)

   The photolysis of hypochlorous acid (HOCl) and hypochlorite (OCl − ) produces a suite of reactive oxidants, including hydroxyl radical (˙OH), chlorine radical (Cl˙), and ozone (O 3 ). Therefore, the addition of light to chlorine disinfection units could effectively convert existing drinking water treatment systems into advanced oxidation processes. This review critically examines existing studies on chlorine photolysis as a water treatment process. After describing the fundamental chemistry of chlorine photolysis, we evaluate the ability of chlorine photolysis to transform model probe compounds, target organic contaminants, and chlorine-resistant microorganisms. The efficacy of chlorine photolysis to produce reactive oxidants ismore »dependent on solution and irradiation conditions ( e.g. , pH and irradiation wavelengths). For example, lower pH values result in higher steady-state concentrations of ˙OH and Cl˙, resulting in enhanced contaminant removal. We also present the current state of knowledge on the alteration of dissolved organic matter and subsequent formation of disinfection by-products (DBPs) during chlorine photolysis. Although the relative yields of DBPs during chlorine photolysis are also dependent on solution conditions ( e.g. , higher organic DBP yields at low pH values), there is conflicting evidence on whether chlorine photolysis increases or decreases DBP production compared to thermal reactions between chlorine and dissolved organic matter in the dark. We conclude the review by identifying knowledge gaps in the current body of literature.« less