Ruiyang Xiao

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.

Bioleaching remediation of heavy metal-contaminated soils using Burkholderia sp. Z-90.

Bioleaching is an environment-friendly and economical technology to remove heavy metals from contaminated soils. In this study, a biosurfactant-producing strain with capacity of alkaline production was isolated from cafeteria sewer sludge and its capability for removing Zn, Pb, Mn, Cd, Cu, and As was investigated. Phylogenetic analysis using 16S rDNA gene sequences confirmed that the strain belonged to Burkholderia sp. and named as Z-90. The biosurfactant was glycolipid confirmed by thin layer chromatography and Fourier-transform infrared spectroscopy. Z-90 broth was then used for bioleaching remediation of heavy metal-contaminated soils. The removal efficiency was 44.0% for Zn, 32.5% for Pb, 52.2% for Mn, 37.7% for Cd, 24.1% for Cu and 31.6% for As, respectively. Mn, Zn and Cd were more easily removed from soil than Cu, Pb and As, which was attributed to the presence of high acid-soluble fraction of Mn, Zn and Cd and high residual fraction of Cu, Pb and As. The heavy metal removal in soils was contributed to the adhesion of heavy metal-contaminated soil minerals with strain Z-90 and the formation of a metal complex with biosurfactant.

Kinetics and Mechanism of Sonochemical Degradation of Pharmaceuticals in Municipal Wastewater

A series of six pharmaceuticals were degraded by continuous wave (CW) and pulsed wave (PW) ultrasound at 205 kHz using deionized water, wastewater effluent, and its isolated organic matter matrices. In deionized water, we observed that hydrophobicity is superior to diffusivity (D(W)) for predicting degradation kinetics. Enhancements in degradation kinetics by the PW mode were greatest for the highest DW (i.e., fluorouracil (5-FU)) and K(OW) (i.e., lovastatin (LOVS)) compounds, indicating that a pharmaceutical with either high diffusivity and low hydrophobicity or low diffusivity and high hydrophobicity benefits from additional time to populate the bubble-water interface during the silent cycle of PW ultrasound. Degradation of 5-FU and LOVS were inhibited by wastewater effluent to a greater extent than the other pharmaceuticals. In addition, a pulse enhancement (PE) for 5-FU and LOVS was not present in wastewater effluent. Irradiating 5-FU and LOVS in hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) fractions of effluent organic matter (EfOM) showed that the TPI fraction reduced the PE the most, followed by the HPI and HPO fractions. The smaller size of the TPI over the HPO fraction and higher hydrophobicity of TPI over HPI implicate both size and hydrophobicity of EfOM in hindering degradation of pharmaceuticals.

Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study

The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (k) for reactions with hydroxyl radical ((•)OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of (•)OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D3), octamethycyclotetrasiloxane (D4), and decamethycyclopentasiloxane (D5). Experimentally, we measured k values for D3, D4, and D5 with (•)OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (-2.92 to 0.79 kcal·mol(-1)), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238-358 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from -2.71 to -1.64 kcal·mol(-1), also exhibiting weak temperature dependence. The measured k values were approximately an order of magnitude higher than the theoretical values but have the same trend with increasing size of the siloxane ring. The calculated energy barriers for H-atom abstraction at different positions were similar, which provides theoretical support for extrapolating k for other cyclic siloxanes from the number of abstractable hydrogens.

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.

Effect of pH on the sonochemical degradation of organic pollutants

An increasing number of organic compounds are manufactured, consumed, and discarded every year. Incomplete destruction of these compounds in wastewater treatment plants leads to pollution of natural waters, posing great health and ecological concerns. Ultrasound, as an emerging advanced oxidation technology, can quickly and effectively degrade organic pollutants in waters. To improve removal efficiency of organic pollutants in an ultrasonic system, operational parameters, especially pH, have been frequently evaluated and optimized. This review show that pH-induced changes in volatility, hydrophobicity and Coulombic force between the target compound and cavitation bubbles leads to higher degradation at acidic pH for most compounds. In addition, pH also changes free radical formation and reactivity in water during sonication, thereby altering degradation kinetics of target compounds. However, the influence of pH is not always consistent for various organic pollutants covering a broad range of physicochemical properties and reactivities. A systematic investigation on the pH effect is necessary to elucidate how pH alters cavitation bubble dynamics and collapse, radical yield and reactivity, distribution of target compounds in the vicinity of cavitation bubbles, water matrices transformation, and ultimately the degradation kinetics of organic pollutants. This first systematic review provides valuable insight into the pH effects on organic pollutant sonolysis, helps to improve our mechanistic understanding of the sonochemical system, and sheds light on future application of ultrasound in water engineering.