Shuang Luo

Quantitative Structure–Activity Relationship (QSAR) for the Oxidation of Trace Organic Contaminants by Sulfate Radical

The sulfate radical anion (SO4•–) based oxidation of trace organic contaminants (TrOCs) has recently received great attention due to its high reactivity and low selectivity. In this study, a meta-analysis was conducted to better understand the role of functional groups on the reactivity between SO4•– and TrOCs. The results indicate that compounds in which electron transfer and addition channels dominate tend to exhibit a faster second-order rate constants (kSO4•–) than that of H–atom abstraction, corroborating the SO4•– reactivity and mechanisms observed in the individual studies. Then, a quantitative structure activity relationship (QSAR) model was developed using a sequential approach with constitutional, geometrical, electrostatic, and quantum chemical descriptors. Two descriptors, ELUMO and EHOMO energy gap (ELUMO–EHOMO) and the ratio of oxygen atoms to carbon atoms (#O:C), were found to mechanistically and statistically affect kSO4•– to a great extent with the standardized QSAR model: ln kSO4•– = 26.8–3.97 × #O:C – 0.746 × (ELUMO–EHOMO). In addition, the correlation analysis indicates that there is no dominant reaction channel for SO4•– reactions with various structurally diverse compounds. Our QSAR model provides a robust predictive tool for estimating emerging micropollutants removal using SO4•– during wastewater treatment processes.

Comparison of the reactivity of ibuprofen with sulfate and hydroxyl radicals: An experimental and theoretical study

Hydroxyl radical (•OH) and sulfate radical anion (SO4•-) based advanced oxidation technologies (AOTs) are effective methods to treat trace organic contaminants (TrOCs) in engineered waters. Although both technologies result in the same overall removal of TrOCs, the mechanistic differences between these two radicals involved in the oxidation of TrOCs remain unclear. In this study, we experimentally examined the degradation kinetics of neutral ibuprofen (IBU), a representative TrOC, by •OH and SO4•- at pH3 in UV/H2O2 and UV/persulfate systems, respectively. The second-order rate constants (k) of IBU with •OH and SO4•- were determined to be 3.43±0.06×109 and 1.66±0.12×109M-1s-1, respectively. We also theoretically calculated the thermodynamic and kinetic behaviors for reactions of IBU with •OH and SO4•- using the density functional theory (DFT) M06-2X method with 6-311++G** basis set. The results revealed that H-atom abstraction is the most favorable pathway for both •OH and SO4•-, but due to the steric hindrance SO4•- exhibits significantly higher energy barriers than •OH. The theoretical calculations corroborate our experimental observation that SO4•- has a smaller k value than •OH in reacting with IBU. These comparative results are of fundamental and practical importance in understanding the electrophilic interactions between radicals and IBU molecules, and to help select preferred radical oxidation processes for optimal TrOCs removal in engineered waters.

UV direct photolysis of sulfamethoxazole and ibuprofen: An experimental and modelling study

Photodegradation characteristics of pharmaceuticals and personal care products (PPCPs) during UV irradiation are of practical and scientific importance in selecting operational parameters during water treatment processes. In this study, the molar extinction coefficient (ε), quantum yield (φ), and degradation kinetics of neutral/anionic forms of sulfamethoxazole (SMX) and ibuprofen (IBU) were compared by varying solution pH. The degradation kinetics of the target compounds were observed to reversely correlate to the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values of the target compounds. Then, a kinetic model for predicting the direct photolytic rates at different solution pH was established based on ε and φ of neutral/anionic species. The root mean squared errors for the modeled values suggest that the model exhibits good predictive power. Finally, in order to evaluate the electrical energy consumption during the UV direct photolysis process, the electrical energy per order (EE/O) was assessed. The experimental and modelling results are important to elucidate the mechanism of degradation of target PPCPs under UV irradiation and allow for the selection of optimal conditions in water treatment processes.

Chemical structure-based predictive model for the oxidation of trace organic contaminants by sulfate radical

Second-order rate constants [Formula: see text] for the reaction of sulfate radical anion (SO4•-) with trace organic contaminants (TrOCs) are of scientific and practical importance for assessing their environmental fate and removal efficiency in water treatment systems. Here, we developed a chemical structure-based model for predicting [Formula: see text] using 32 molecular fragment descriptors, as this type of model provides a quick estimate at low computational cost. The model was constructed using the multiple linear regression (MLR) and artificial neural network (ANN) methods. The MLR method yielded adequate fit for the training set (Rtraining2=0.88,n=75) and reasonable predictability for the validation set (Rvalidation2=0.62,n=38). In contrast, the ANN method produced a more statistical robustness but rather poor predictability (Rtraining2=0.99andRvalidation2=0.42). The reaction mechanisms of SO4•- reactivity with TrOCs were elucidated. Our result shows that the coefficients of functional groups reflect their electron donating/withdrawing characters. For example, electron donating groups typically exhibit positive coefficients, indicating enhanced SO4•- reactivity. Electron withdrawing groups exhibit negative values, indicating reduced reactivity. With its quick and accurate features, we applied this structure-based model to 55 discrete TrOCs culled from the Contaminant Candidate List 4, and quantitatively compared their removal efficiency with SO4•- and OH in the presence of environmental matrices. This high-throughput model helps prioritize TrOCs that are persistent to SO4•- based oxidation technologies at the screening level, and provide diagnostics of SO4•- reaction mechanisms.

A novel model to predict gas–phase hydroxyl radical oxidation kinetics of polychlorinated compounds

In this study, a novel model based on aromatic meta-substituent grouping was presented to predict the second-order rate constants (k) for OH oxidation of PCBs in gas-phase. Since the oxidation kinetics are dependent on the chlorination degree and position, we hypothesized that it may be more accurate for k value prediction if we group PCB congeners based on substitution positions (i.e., ortho (o), meta (m), and para (p)). To test this hypothesis, we examined the correlation of polarizability (α), a quantum chemical based descriptor for k values, with an empirical Hammett constant (σ+) on each substitution position. Our result shows that α is highly linearly correlated to ∑σo,m,p+ based on aromatic meta-substituents leading to the grouping based predictive model. With the new model, the calculated k values exhibited an excellent agreement with experimental measurements, and greater predictive power than the quantum chemical based quantitative structure activity relationship (QSAR) model. Further, the relationship of α and ∑σo,m,p+ for PCDDs congeners, together with highest occupied molecular orbital (HOMO) distribution, were used to validate the aromatic meta-substituent grouping method. This newly developed model features a combination of good predictability of quantum chemical based QSAR model and simplicity of Hammett relationship, showing a great potential for fast and computational tractable prediction of k values for gas-phase OH oxidation of polychlorinated compounds.

Quantitative structure–activity relationships for reactivities of sulfate and hydroxyl radicals with aromatic contaminants through single–electron transfer pathway

Sulfate radical anion (SO4•-) and hydroxyl radical (OH) based advanced oxidation technologies has been extensively used for removal of aromatic contaminants (ACs) in waters. In this study, we investigated the Gibbs free energy (ΔGSET∘) of the single electron transfer (SET) reactions for 76 ACs with SO4•- and OH, respectively. The result reveals that SO4•- possesses greater propensity to react with ACs through the SET channel than OH. We hypothesized that the electron distribution within the molecule plays an essential role in determining the ΔGSET∘ and subsequent SET reactions. To test the hypothesis, a quantitative structure-activity relationship (QSAR) model was developed for predicting ΔGSET∘ using the highest occupied molecular orbital energies (EHOMO), a measure of electron distribution and donating ability. The standardized QSAR models are reported to be ΔG°SET=-0.97×EHOMO - 181 and ΔG°SET=-0.97×EHOMO - 164 for SO4•- and OH, respectively. The models were internally and externally validated to ensure robustness and predictability, and the application domain and limitations were discussed. The single-descriptor based models account for 95% of the variability for SO4•- and OH. These results provide the mechanistic insight into the SET reaction pathway of radical and non-radical bimolecular reactions, and have important applications for radical based oxidation technologies to remove target ACs in different waters.

Kinetic and mechanistic aspects of hydroxyl radical‒mediated degradation of naproxen and reaction intermediates

Hydroxyl radical (•OH) based advanced oxidation technologies (AOTs) are effective for removing non‒steroidal anti-inflammatory drugs (NSAIDs) during water treatment. In this study, we systematically investigated the degradation kinetics of naproxen (NAP), a representative NSAID, with a combination of experimental and theoretical approaches. The second-order rate constant (k) of •OH oxidation of NAP was measured to be (4.32 ± 0.04) × 109 M-1 s-1, which was in a reasonable agreement with transition state theory calculated k value (1.08 × 109 M-1 s-1) at SMD/M05-2X/6-311++G**//M05-2X/6-31+G** level of theory. The calculated result revealed that the dominant reaction intermediate is 2‒(5‒hydroxy‒6‒methoxynaphthalen‒2‒yl)propanoic acid (HMNPA) formed via radical adduct formation pathway, in which •OH addition onto the ortho site of the methoxy-substituted benzene ring is the most favorable pathway for the NAP oxidation. We further investigated the subsequent •OH oxidation of HMNPA via a kinetic modelling technique. The k value of the reaction of HMNPA and •OH was determined to be 2.22 × 109 M-1 s-1, exhibiting a similar reactivity to the parent NAP. This is the first study on the kinetic and mechanistic aspects of NAP and its reaction intermediates. The current results are valuable in future study evaluating and extending the application of •OH based AOTs to degrade NAP and other NSAIDs of concern in water treatment plants.

Characterization and quantification of chromate adsorption by layered porous iron oxyhydroxide: An experimental and theoretical study

The inner structure of iron oxyhydroxide agglomerates (IOAs) prepared from hydrolysis of ferric chloride was characterized and correlated to surface complexation of hexavalent chromium, Cr(VI), in a broad range of pH (3-12) and ionic strengths (0.0-5.0M). Evolution of particle size, morphology, and surface activity, combined with density functional theory (DFT) calculations, support the condensation reaction initiated formation of IOAs in three levels: iron nanoparticles to nanolayers to agglomerates. This agglomeration process led to a layered porous structure for aqueous-phase IOAs resulting in a rapid and high removal of Cr(VI) in batch tests. By integrating adsorption results, thermodynamic modeling, and quantum chemical calculations for the adsorption reactions, a quantitative distribution profile for each surface coordination of Cr(VI) ions (i.e., monodentate, bidentate, and hydrogen-bonding) was established. Results of this study are important to understand the fundamental mechanism of IOAs formation in aqueous phase and the intrinsic nature of surface complexations at the mineral-water interface for optimal Cr(VI) removal in hypersaline waste streams.