10 research outputs found
Plasma-liquid interactions: a review and roadmap
Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas
Predicting Transformation Products during Aqueous Oxidation Processes: Current State and Outlook
Water quality and its impacts on human and ecosystem
health presents
tremendous global challenges. While oxidative water treatment can
solve many of these problems related to hygiene and micropollutants,
identifying and predicting transformation products from a large variety
of micropollutants induced by dosed chemical oxidants and in situ
formed radicals is still a major challenge. To this end, a better
understanding of the formed transformation products and their potential
toxicity is needed. Currently, no theoretical tools alone can predict
oxidatively induced transformation products in aqueous systems. Coupling
experimental and theoretical studies has advanced the understanding
of reaction kinetics and mechanisms significantly. This perspective
article highlights the key progress made concerning experimental and
computational approaches to predict transformation products. Knowledge
gaps are identified, and the research required to advance the predictive
capability is discussed
Computer-Based First-Principles Kinetic Monte Carlo Simulation of Polyethylene Glycol Degradation in Aqueous Phase UV/H<sub>2</sub>O<sub>2</sub> Advanced Oxidation Process
We
have developed a computer-based first-principles kinetic Monte
Carlo (CF-KMC) model to predict degradation mechanisms and fates of
intermediates and byproducts produced from the degradation of polyethylene
glycol (PEG) in the presence of hydrogen peroxide (UV/H<sub>2</sub>O<sub>2</sub>). The CF-KMC model is composed of a reaction pathway
generator, a reaction rate constant estimator, and a KMC solver. The
KMC solver is able to solve the predicted pathways successfully without
solving ordinary differential equations. The predicted time-dependent
profiles of averaged molecular weight, and polydispersitivity index
(i.e., the ratio of the weight-averaged molecular weight to the number-averaged
molecular weight) for the PEG degradation were validated with experimental
observations. These predictions are consistent with the experimental
data. The model provided detailed and quantitative insights into the
time evolutions of molecular weight distribution and concentration
profiles of low molecular weight products and functional groups. Our
approach may be useful to predict the fates of degradation products
for a wide range of complicated organic contaminants
Mechanistic Insight into the Reactivity of Chlorine-Derived Radicals in the Aqueous-Phase UV–Chlorine Advanced Oxidation Process: Quantum Mechanical Calculations
The
combined ultraviolet (UV) and free chlorine (UV–chlorine)
advanced oxidation process that produces highly reactive hydroxyl
radicals (HO<sup>•</sup>) and chlorine radicals (Cl<sup>•</sup>) is an attractive alternative to UV alone or chlorination for disinfection
because of the destruction of a wide variety of organic compounds.
However, concerns about the potential formation of chlorinated transformation
products require an understanding of the radical-induced elementary
reaction mechanisms and their reaction-rate constants. While many
studies have revealed the reactivity of oxygenated radicals, the reaction
mechanisms of chlorine-derived radicals have not been elucidated due
to the data scarcity and discrepancies among experimental observations.
We found a linear free-energy relationship quantum mechanically calculated
free energies of reaction and the literature-reported experimentally
measured reaction rate constants, <i>k</i><sub>exp</sub>, for 22 chlorine-derived inorganic radical reactions in the UV–chlorine
process. This relationship highlights the discrepancy among literature-reported
rate constants and aids in the determination of the rate constant
using quantum mechanical calculations. We also found linear correlations
between the theoretically calculated free energies of activation and <i>k</i><sub>exp</sub> for 31 reactions of Cl<sup>•</sup> with organic compounds. The correlation suggests that H-abstraction
and Cl-adduct formation are the major reaction mechanisms. This is
the first comprehensive study on chlorine-derived radical reactions,
and it provides mechanistic insight into the reaction mechanisms for
the development of an elementary reaction-based kinetic model
Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone Degradation in Aqueous Phase Advanced Oxidation Processes
Advanced oxidation processes (AOPs)
that produce highly reactive
hydroxyl radicals are promising methods to destroy aqueous organic
contaminants. Hydroxyl radicals react rapidly and nonselectively with
organic contaminants and degrade them into intermediates and transformation
byproducts. Past studies have indicated that peroxyl radical reactions
are responsible for the formation of many intermediate radicals and
transformation byproducts. However, complex peroxyl radical reactions
that produce identical transformation products make it difficult to
experimentally study the elementary reaction pathways and kinetics.
In this study, we used ab initio quantum mechanical calculations to
identify the thermodynamically preferable elementary reaction pathways
of hydroxyl radical-induced acetone degradation by calculating the
free energies of the reaction and predicting the corresponding reaction
rate constants by calculating the free energies of activation. In
addition, we solved the ordinary differential equations for each species
participating in the elementary reactions to predict the concentration
profiles for acetone and its transformation byproducts in an aqueous
phase UV/hydrogen peroxide AOP. Our ab initio quantum mechanical calculations
found an insignificant contribution of Russell reaction mechanisms
of peroxyl radicals, but significant involvement of HO<sub>2</sub><sup>•</sup> in the peroxyl radical reactions. The predicted
concentration profiles were compared with experiments in the literature,
validating our elementary reaction-based kinetic model
Computer-Based First-Principles Kinetic Modeling of Degradation Pathways and Byproduct Fates in Aqueous-Phase Advanced Oxidation Processes
In
this study, a computer-based first-principles kinetic model
is developed to predict the degradation mechanisms and fates of intermediates
and byproducts produced during aqueous-phase advanced oxidation processes
(AOPs) for various organic compounds. The model contains a rule-based
pathway generator to generate the reaction pathways, a reaction rate
constant estimator to estimate the reaction rate constant for each
reaction generated, a mechanistic reduction module to reduce the generated
mechanisms, an ordinary differential equations generator and solver
to solve the generated mechanisms and calculate the concentration
profiles for all species, and a toxicity estimator to estimate the
toxicity of major species and calculate time-dependent profiles of
relative toxicity (i.e., concentration of species divided by toxicity
value). We predict concentration profiles of acetone and trichloroethylene
and their intermediates and byproducts in photolysis with hydrogen
peroxide (i.e., UV/H<sub>2</sub>O<sub>2</sub>) and validate with experimental
observations. The predicted concentration profiles for both parent
compounds are consistent with experimental data. The calculated profiles
of 96-h green algae chronic toxicity show that the overall toxicity
decreases during the degradation process. These generated mechanisms
also provide detailed and quantitative insights into the pathways
for the formation and consumption of important intermediates and byproducts
produced during AOPs. Our approach is sufficiently general to be applied
to a wide range of contaminants
Role of Carbonyl Compounds for <i>N</i>‑Nitrosamine Formation during Nitrosation: Kinetics and Mechanisms
N-Nitrosamines are potential human carcinogens
frequently detected in natural and engineered aquatic systems. This
study sheds light on the role of carbonyl compounds in the formation
of N-nitrosamines by nitrosation of five secondary
amines via different pathways. The results showed that compared to
a control system, the presence of formaldehyde enhances the formation
of N-nitrosamines by a factor of 5–152 at
pH 7, depending on the structure of the secondary amines. Acetaldehyde
showed a slight enhancement effect on N-nitrosamine
formation, while acetone and benzaldehyde did not promote nitrosation
reactions. For neutral and basic conditions, the iminium ion was the
dominant intermediate for N-nitrosamine formation,
while carbinolamine became the major contributor under acidic conditions.
Negative free energy changes (–1) and relatively low activation energies (–1) of the reactions of secondary amines with N2O3, iminium ions with nitrite and carbinolamines with N2O3 from quantum chemical computations further support
the proposed reaction pathways. This highlights the roles of the iminium
ion and carbinolamine in the formation of N-nitrosamines
during nitrosation in the presence of carbonyl compounds, especially
in the context of industrial wastewater
Kinetics and Modeling of Degradation of Ionophore Antibiotics by UV and UV/H<sub>2</sub>O<sub>2</sub>
Ionophore antibiotics
(IPAs), one of the major groups of pharmaceuticals
used in livestock industry, have been found to contaminate agricultural
runoff and surface waters via land application of animal manures as
fertilizers. However, limited research has investigated the means
to remove IPAs from water sources. This study investigates the degradation
of IPAs by using ultraviolet (UV) photolysis and UV combined with
hydrogen peroxide (UV/H<sub>2</sub>O<sub>2</sub>) advanced oxidation
process (AOP) under low-pressure (LP) UV lamps in various water matrices.
Three widely used (monensin, salinomycin, and narasin) and one model
(nigericin) IPAs exhibit low light absorption in the UV range and
degrade slowly at the light intensity of 3.36 × 10<sup>–6</sup> Einstein·L<sup>–1</sup>·s<sup>–1</sup> under
UV photolysis conditions. However, IPAs react with hydroxyl radicals
produced by UV/H<sub>2</sub>O<sub>2</sub> at fast reaction rates,
with second-order reaction rate constants at (3.49–4.00) ×
10<sup>9</sup> M<sup>–1</sup>·s<sup>–1</sup>. Water
matrix constituents enhanced the removal of IPAs by UV photolysis
but inhibited UV/H<sub>2</sub>O<sub>2</sub> process. A steady-state
kinetic model successfully predicts the impact of water constituents
on IPA degradation by UV/H<sub>2</sub>O<sub>2</sub> and determines
the optimal H<sub>2</sub>O<sub>2</sub> dose by considering both energy
consumption and IPA removal. LC/MS analysis of reaction products reveals
the initial transformation pathways of IPAs via hydrogen atom abstraction
and peroxidation during UV/H<sub>2</sub>O<sub>2</sub>. This study
is among the first to provide a comprehensive understanding of the
degradation of IPAs via UV/H<sub>2</sub>O<sub>2</sub> AOP
Development of Linear Free Energy Relationships for Aqueous Phase Radical-Involved Chemical Reactions
Aqueous
phase advanced oxidation processes (AOPs) produce hydroxyl
radicals (HO•) which can completely oxidize electron rich organic
compounds. The proper design and operation of AOPs require that we
predict the formation and fate of the byproducts and their associated
toxicity. Accordingly, there is a need to develop a first-principles
kinetic model that can predict the dominant reaction pathways that
potentially produce toxic byproducts. We have published some of our
efforts on predicting the elementary reaction pathways and the HO•
rate constants. Here we develop linear free energy relationships (LFERs)
that predict the rate constants for aqueous phase radical reactions.
The LFERs relate experimentally obtained kinetic rate constants to
quantum mechanically calculated aqueous phase free energies of activation.
The LFERs have been applied to 101 reactions, including (1) HO•
addition to 15 aromatic compounds; (2) addition of molecular oxygen
to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3)
disproportionation of 10 peroxyl radicals, and (4) unimolecular decay
of nine peroxyl radicals. The LFERs correlations predict the rate
constants within a factor of 2 from the experimental values for HO•
reactions and molecular oxygen addition, and a factor of 5 for peroxyl
radical reactions. The LFERs and the elementary reaction pathways
will enable us to predict the formation and initial fate of the byproducts
in AOPs. Furthermore, our methodology can be applied to other environmental
processes in which aqueous phase radical-involved reactions occur
Acid-Catalyzed Transformation of Ionophore Veterinary Antibiotics: Reaction Mechanism and Product Implications
Ionophore
antibiotics (IPAs) are polyether antimicrobials widely
used in the livestock industry and may enter the environment via land
application of animal waste and agricultural runoff. Information is
scarce regarding potential transformation of IPAs under environmental
conditions. This study is among the first to identify the propensity
of IPAs to undergo acid-catalyzed transformation in mildly acidic
aquatic systems and characterize the reactions in depth. The study
focused on the most widely used monensin (MON) and salinomycin (SAL),
and also included narasin (NAR) in the investigation. All three IPAs
are susceptible to acid-catalyzed transformation. MON reacts much
more slowly than SAL and NAR and exhibits a different kinetic behavior
that is further evaluated by a reversible reaction kinetic model.
Extensive product characterization identifies that the spiro-ketal
group of IPAs is the reactive site for the acid-catalyzed hydrolytic
transformation, yielding predominantly isomeric and other products.
Toxicity evaluation of the transformation products shows that the
products retain some antimicrobial properties. The occurrence of IPAs
and isomeric transformation products is also observed in poultry litter
and agricultural runoff samples. Considering the common presence of
mildly acidic environments (pH 4–7) in soils and waters, the
acid-catalyzed transformation identified in this study likely plays
an important role in the environmental fate of IPAs