Xianliang Qiao

Aquatic Photochemistry of Fluoroquinolone Antibiotics: Kinetics, Pathways, and Multivariate Effects of Main Water Constituents

The ubiquity of fluoroquinolone antibiotics (FQs) in surface waters urges insights into their fate in the aqueous euphotic zone. In this study, eight FQs (ciprofloxacin, danofloxacin, levofloxacin, sarafloxacin, difloxacin, enrofloxacin, gatifloxacin, and balofloxacin) were exposed to simulated sunlight, and their photodegradation was observed to follow apparent first-order kinetics. Based on the determined photolytic quantum yields, solar photodegradation half-lives for the FQs in pure water and at 45 degrees N latitude were calculated to range from 1.25 min for enrofloxacin to 58.0 min for balofloxacin, suggesting that FQs would intrinsically photodegrade fast in sunlit surface waters. However, we found freshwater and seawater constituents inhibited their photodegradation. The inhibition was further explored by a central composite design using sarafloxacin and gatifloxacin as representatives. Humic acids (HA), Fe(III), NO(3)(-), and HA-Cl(-) interaction inhibited the photodegradation, as they mainly acted as radiation filters and/or scavengers for reactive oxygen species. The photodegradation product identification and ROS scavenging experiments indicated that the FQs underwent both direct photolysis and self-sensitized photo-oxidation via *OH and (1)O(2). Piperazinyl N(4)-dealkylation was primary for N(4)-alkylated FQs, whereas decarboxylation and defluorination were comparatively important for the other FQs. These results are of importance toward the goal of assessing the persistence of FQs in surface waters.

Polycyclic aromatic hydrocarbons in Dalian soils: distribution and toxicity assessment

Concentrations of 15 polycyclic aromatic hydrocarbons (PAHs) were measured in surface soils collected from Dalian, China, for examination of distributions and composition profiles and their potential toxicity. The sum of 15 PAHs (SigmaPAHs) ranged from 190 to 8595 ng g(-1) dry weight, and showed an apparent urban-suburban-rural gradient in both SigmaPAHs and composition profiles. Using hierarchical cluster analysis (HCA), the sampling sites were grouped into four clusters corresponding to traffic area, park/residential area, suburban and rural areas. The ratios of naphthalene (Nap) and fluorene (Fl) versus fluoranthene (Flu), pyrene (Pyr) and indeno(1,2,3-cd)pyrene (InP) in the four clusters provided evidence of local distillation. The diagnostic ratios indicated the prevalent PAH sources were petroleum combustion and coal combustion in Dalian, and a cross plot of diagnostic ratios distinguished the urban samples from suburban and rural ones. Toxic potency assessment of soil PAHs presented a good relationship with benzo(a)pyrene (BaP) levels, toxic equivalent concentrations based on BaP (TEQ(BaP)) and dioxin-like toxic equivalent concentrations (TEQ(TCDD)). The study highlights that BaP is a good indicator for assessing the potential toxicity of PAHs, and presents a promising toxicity assessment method for soil PAHs.

Quantum chemical investigation and experimental verification on the aquatic photochemistry of the sunscreen 2-phenylbenzimidazole-5-sulfonic acid.

For ecological risk assessment of the large and ever-increasing number of chemical pollutants, it is of importance to develop computational methods to screen or predict their environmental photodegradation behavior. This study developed a computational method based on the density functional theory (DFT) to predict and evaluate the photodegradation behavior and effects of water constituents, taking a sunscreen and personal care product 2-phenylbenzimidazole-5-sulfonic acid (PBSA) as a model compound. Energy and electron transfer reactions of excited state PBSA (PBSA*) with (3)O(2) and water constituents were evaluated. The computational results indicated that PBSA* could photogenerate (1)O(2) and O(2)(-)·, triplet excited state humic/fulvic acid analogs could not photosensitize the degradation, and the anions (Cl(-), Br(-), and HCO(3)(-)) could not quench PBSA* or its radical cation chemically. Experiments employing simulated sunlight confirmed that PBSA photodegraded via the direct and self-sensitization mechanism involving O(2)(-)·. The photodegradation was pH-dependent. The direct and self-sensitized photodegradation was inhibited by fulvic acid. The main photodegradation products were identified, and the pathways were clarified. These results indicate that the DFT-based computational method can be employed to assess the environmental photochemical fate of organic pollutants.

Distinct photolytic mechanisms and products for different dissociation species of ciprofloxacin.

As many antibiotics are ionizable and may have different dissociation forms in the aquatic environment, we hypothesized that the different dissociation species have disparate photolytic pathways, products, and kinetics, and adopted ciprofloxacin (CIP) as a case to test this hypothesis. Simulated sunlight experiments and matrix calculations were performed to differentiate the photolytic reactivity for each dissociation species (H4CIP(3+), H3CIP(2+), H2CIP(+), HCIP(0), and CIP(-)). The results prove that the five dissociation species do have dissimilar photolytic kinetics and products. H4CIP(3+) mainly undergoes stepwise cleavage of the piperazine ring, while H2CIP(+) mainly undergoes defluorination. For H3CIP(2+), HCIP(0), and CIP(-), the major photolytic pathway is oxidation. By density functional theory calculation, we clarified the defluorination mechanisms for the five dissociation species at the excited triplet states: All the five species can defluorinate by reaction with hydroxide ions (OH(-)) to form hydroxylated products, and H2CIP(+) can also undergo C-F bond cleavage to produce F(-) and a carbon-centered radical. This study is a first attempt to differentiate the photolytic products and mechanisms for different dissociation species of ionizable compounds. The results imply that for accurate ecological risk assessment of ionizable emerging pollutants, it is necessary to investigate the environmental photochemical behavior of all dissociation species.

Polybrominated diphenyl ethers in soils of the modern Yellow River Delta, China: Occurrence, distribution and inventory

The Yellow River is the second largest river in China. In this study, the levels of polybrominated diphenyl ethers (PBDEs) in the modern Yellow River Delta (mYRD) were firstly reported. Twenty PBDE congeners in soil/sediment samples from mYRD were measured. The total PBDE concentrations ranged from non-detectable to 18257ngkg(-1) with a mean value of 836ngkg(-1). BDE-209 was the dominant congener, accounting for ∼86.1-99.5% of the total PBDEs. The congener profiles of PBDEs with higher abundances of BDE-153 and BDE-183 were similar to those in sediment of the Bohai Sea, indicating that they shared similar sources. The concentrations and congener patterns varied among different regions. Higher levels of PBDEs were found in the middle area (MA), and more complicated congener compositions were also observed in the MA, whereas lower levels of PBDEs were found in the modern course (MC) and the old course (OC). Much more PBDEs were detected in the top layer (TL) soil where more congeners were also held compared to lower soil layers, implying that more PBDEs were emitted into this area in recent years/decades. Organic matter controlled the PBDE distribution in the soil. Soil in this area might be a source of BDE209 for the Bohai Sea.

Different photolysis kinetics and photooxidation reactivities of neutral and anionic hydroxylated polybrominated diphenyl ethers.

Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) pose potential ecological risks due to their endocrine disrupting effects and extensive sources. It is of great importance to know their environmental transformation for the purpose of ecological risk assessment. Photodegradation is an important transformation pathway of HO-PBDEs. As HO-PBDEs ionize in natural waters, the photochemical reactivities of both neutral and anionic HO-PBDEs need to be unveiled. In this study, six HO-PBDEs were selected as model compounds. Their direct photolysis rate constants (k(d)), quantum yields (Φ), light absorptions (A), second-order reaction rate constants with (1)O(2) (k(1O2)) and (·)OH (k(OH)) at different pH were determined, as well as these parameters for the neutral and anionic HO-PBDEs. The k(d), Φ, A, k(1O2) and k(OH) for the anions are much higher than those for the neutral molecules and vary with the bromination degree. Molecular parameters computed with the density functional theory (DFT) were employed to construct structure-reactivity equations. The ether bond strength, the frontier molecular orbital energy and the charge distribution were found to be the intrinsic structural characters governing the photochemical reactivities. The half-lives range from 0.7 to 60.1h for the photodegradation including the direct photolysis and the reactions with (1)O(2) and ()OH in surface waters at 45°N latitude under the continuous solar irradiation of sunny noon on 15 July. Direct photolysis is the dominant pathway. The photochemical reactivities of other HO-PBDEs at a given pH can also be estimated based on the structure-reactivity equations, which is important for the ecological risk assessment of HO-PBDEs.

The Fragment Constant Method for Predicting Octanol–Air Partition Coefficients of Persistent Organic Pollutants at Different Temperatures

The octanol–air partition coefficient (KOA) is a key physicochemical parameter for describing the partition of organic pollutants between air and environmental organic phases. Experimental determination of KOA is costly and time consuming, and sometimes restricted by lack of sufficiently pure chemicals. There is a need to develop a simple but accurate method to estimate KOA. In the present study, a fragment constant model based on five fragment constants and one structural correction factor, was developed for predicting logKOA at temperatures ranging from 10 to 40°C. The model was validated as successful by statistical analysis and external experimental logKOA data. Compared to other quantitative structure–property relationship methods, the present model has the advantage that it is much easier to implement. As aromatic compounds that contain C, H, O, Cl, and Br atoms, were included in the training set used to develop the model, the current fragment model applies to a wide range of chlorinated and bromina...

Modeling photoinduced toxicity of PAHs based on DFT-calculated descriptors

Quantitative structure-activity relationships (QSARs) were established for photoinduced toxicity of polycyclic aromatic hydrocarbons (PAHs) to two aquatic species. Partial least squares (PLS) regression and molecular structural parameters calculated by density functional theory (DFT) were employed for model development. Two QSAR models were established and their high R(2) and Q(CUM)(2) values indicated their good goodness-of-fit, robustness and internal predictive power. The descriptors that describe the partition behavior, light absorbance, and generation of reactive free radicals were found to be successful in modeling the photoinduced toxicity. The average molecular polarizability (alpha), energy gap (E(GAP)) between the energy of the lowest unoccupied molecular orbital and the highest occupied molecular orbital, lowest triplet excitation energy (E(T1)) and vertical electron affinity at the lowest excited triplet (VEA(T1)) were the main molecular structural factors. Polarizability which determines the partition of PAHs between water and lipid governs the photoinduced toxicity of selected PAHs. Moreover, the photoinduced toxicity increased with the decreasing of E(GAP) probably due to better spectral overlap. The parameter, VEA(T1) that characterizes the ability of PAH anion radical (PAH(*-)) generation from excited triplet state PAH ((3)PAH(*)), is also related with the photoinduced toxicity. This investigation will make us gain more insight into the photoinduced toxicity mechanism and assess the applicability of various DFT-based descriptors to toxicological QSARs.

Probing the Primary Mechanisms Affecting the Environmental Distribution of Estrogen and Androgen Isomers

Land application of animal manure has been identified as a source of natural and synthetic hormone contaminants that are frequently detected down-gradient of agricultural operations. Much research on the environmental fate of hormones has focused on the structural isomers most biologically active in mammals, e.g., the 17β-isomers of the estrogen estradiol (E2) and the synthetic androgen trenbolone (TB). However, recent work has shown that the α- and β-isomers of E2 and TB can cause comparable effects on certain aquatic species. To improve our understanding and ability to predict isomer-specific interactions with environmental sorbents, we measured the association (K(DOC)) of the α- and β-isomers of E2 and TB as well as their primary metabolites (estrone and trendione) with two commercial dissolved organic carbon (DOC) sources by measuring both free and DOC-bound hormone concentrations. We also measured solvent-water partition coefficients partitioning (K(SW)) for the same hormones using hexane, toluene, and octanol. Log K(DOC)(*), log K(OC) (OC-normalized sorption by soils), and K(OW) values are all greater for the β-isomer except between the E2 isomers. Theoretical descriptors reflecting electronic character and solute-solvent interactions were calculated to elucidate isomer-specific behavior. Trends for log K(OW) and log K(DOC) among hormones as well as between isomers are explained reasonably well by computed electrostatic potential and H-bonding parameters.

Integrated fuzzy concentration addition–independent action (IFCA–IA) model outperforms two-stage prediction (TSP) for predicting mixture toxicity

Mixture toxicities were determined for 12 industrial organic chemicals bearing four different modes of toxic action (MOAs) to Vibrio fischeri, to compare the predictability of the integrated fuzzy concentration addition-independent action (IFCA-IA) model and the two-stage prediction (TSP) model. Three mixtures were designed: The first and second mixtures were based on the ratios of each component at the 1% and 50% effect concentrations (EC(1) and EC(50)), respectively; and the third mixture contained an equimolar ratio of individual components. For the EC(1), EC(50) and equimolar ratio, prediction errors from the IFCA-IA model at the 50% experimental mixture effects were 0.3%, 6% and 0.6%, respectively; while for the TSP model, the corresponding errors were 2.8%, 19% and 24%, respectively. Thus, the IFCA-IA model performed better than the TSP model. The IFCA-IA model calculated two weight coefficients from the molecular structural descriptors, which weigh the relation between concentration addition (CA) and independent action (IA) through the fuzzy membership functions. Thus, MOAs are not pre-requisites for mixture toxicity prediction by the IFCA-IA approach, implying the practicability of this method in toxicity assessment of mixtures.