Lushi Lian

Kinetic Study of Hydroxyl and Sulfate Radical-Mediated Oxidation of Pharmaceuticals in Wastewater Effluents

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•-)-mediated oxidation, are alternatives for the attenuation of pharmaceuticals and personal care products (PPCPs) in wastewater effluents. However, the kinetics of these reactions needs to be investigated. In this study, kinetic models for 15 PPCPs were built to predict the degradation of PPCPs in both HO•- and SO4•--mediated oxidation. In the UV/H2O2 process, a simplified kinetic model involving only steady state concentrations of HO• and its biomolecular reaction rate constants is suitable for predicting the removal of PPCPs, indicating the dominant role of HO• in the removal of PPCPs. In the UV/K2S2O8 process, the calculated steady state concentrations of CO3•- and bromine radicals (Br•, Br2•- and BrCl•-) were 600-fold and 1-2 orders of magnitude higher than the concentrations of SO4•-, respectively. The kinetic model, involving both SO4•- and CO3•- as reactive species, was more accurate for predicting the removal of the 9 PPCPs, except for salbutamol and nitroimidazoles. The steric and ionic effects of organic matter toward SO4•- could lead to overestimations of the removal efficiencies of the SO4•--mediated oxidation of nitroimidazoles in wastewater effluents.

Insights into the photo-induced formation of reactive intermediates from effluent organic matter: The role of chemical constituents

In the present study, the formation of triplet states of organic matters (3OM∗) from effluent organic matter (EfOM) under simulated solar irradiation was investigated. EfOM was separated into hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) components. The quantum yield coefficients (ƒTMP) of 3OM∗ were measured for each component and compared to those of reference natural organic matter (NOM). NaBH4 reduction was performed on the EfOM, and the effect of aromatic ketones moieties on triplet formation was also determined. Furthermore, the apparent quantum yield of 1O2 (Φ1O2) and O2•- (ΦO2•-) was measured. Our results suggested that the HPI fraction acted as a sink for 3OM∗. A linear correlation was observed between ƒTMP and Φ1O2 for NOM/EfOM, except for NaBH4-reduced effluent and HPI components. Both ƒTMP and Φ1O2 were positively correlated with the contribution rates of NaBH4-reducible moieties (aromatic ketones) toward 3OM∗. Aromatic ketones were primarily responsible for the production of 3OM∗ from EfOM, whereas quinone moieties played a key role in the production of 3OM∗ in NOM-enriched solutions. Understanding the role of chemical constituents on the photo activity of EfOM/NOM is essential for providing useful insights on their photochemical effects in aquatic systems.

Development of Fluorescence Surrogates to Predict the Photochemical Transformation of Pharmaceuticals in Wastewater Effluents

The photochemical transformation of pharmaceutical and personal care products (PPCPs) in wastewater effluents is an emerging concern for environmental scientists. In the current study, the photodegradation of 29 PPCPs was examined in effluents under simulated solar irradiation. Direct photodegradation, triplet state effluent organic matter (3EfOM*)-mediated and hydroxyl radical (HO•)-mediated degradation are three major pathways in the removal process. With the photodegradation of trace levels of PPCPs, the excitation-emission matrix (EEM) fluorescence intensities of the effluents were also gradually reduced. Therefore, fluorescence peaks have been identified, for the first time, as appropriate surrogates to assess the photodegradation of PPCPs. The humic-like fluorescence peak is linked to direct photolysis-labile PPCPs, such as naproxen, ronidazole, diclofenac, ornidazole, tinidazole, chloramphenicol, flumequine, ciprofloxacin, methadone, and dimetridazole. The tyrosine-like EEM peak is associated with HO•/CO3•--labile PPCPs, such as trimethoprim, ibuprofen, gemfibrozil, atenolol, carbamazepine, and cephalexin. The tryptophan-like peak is associated with 3EfOM*-labile PPCPs, such as clenbuterol, metoprolol, venlafaxine, bisphenol A, propranolol, ractopamine, salbutamol, roxithromycin, clarithromycin, azithromycin, famotidine, terbutaline, and erythromycin. The reduction in EEM fluorescence correlates well with the removal of PPCPs, allowing a model to be constructed. The solar-driven removal of EEM fluorescence was applied to predict the attenuation of 11 PPCPs in five field samples. A close correlation between the predicted results and the experimental results suggests that fluorescence may be a suitable surrogate for monitoring the solar-driven photodegradation of PPCPs in effluents.

UV/chlorine treatment of carbamazepine: Transformation products and their formation kinetics

Carbamazepine (CBZ) is one of the pharmaceuticals most frequently detected in the aqueous environment. This study investigated the transformation products when CBZ is degraded by chlorine under ultraviolet (UV) irradiation (the UV/chlorine process). Detailed pathways for the degradation of CBZ were elucidated using ultra-high performance liquid chromatography (UHPLC)-quadrupole time-of-flight mass spectrometry (QTOF-MS). CBZ is readily degraded by hydroxyl radicals (HO) and chlorine radicals (Cl) in the UV/chlorine process, and 24 transformation products were identified. The products indicate that the 10,11-double bond and aromatic ring in CBZ are the sites most susceptible to attack by HO and Cl. Subsequent reaction produces hydroxylated and chlorinated aromatic ring products. Four specific products were quantified and their evolution was related with the chlorine dose, pH, and natural organic matter concentration. Their yields showed an increase followed by a decreasing trend with prolonged reaction time. CBZ-10,11-epoxide (I), the main quantified transformation product from HO oxidation, was observed with a peak transformation yield of 3-32% depending on the conditions. The more toxic acridine (IV) was formed involving both HO and Cl with peak transformation yields of 0.4-1%. All four quantified products together amounted to a peak transformation yield of 34.5%. The potential toxicity of the transformation products was assayed by evaluating their inhibition of the bioluminescence of the bacterium Vibrio Fischeri. The inhibition increased at first and the decreased at longer reaction times, which was in parallel with the evolution of transformation products.

Development of Novel Chemical Probes for Examining Triplet Natural Organic Matter under Solar Illumination

Excited triplet states of chromophoric dissolved organic matter (3CDOM*) are critical transient species in environmental photochemistry. In the present study, sorbic amine (2,4-hexadien-1-amine) and sorbic alcohol were employed as new probe molecules for triplet measurements and compared to the results measured from sorbic acid under identical conditions. Unlike sorbic acid, sorbic amine and sorbic alcohol were not directly photolyzed under solar irradiation. Photosensitized isomerization of the probes with the conjugated diene structure could yield four geometrical isomers. The separation and quantitative determination of the geometrical isomers were accomplished using HPLC and high-resolution NMR analyses. When photoirradiated Suwannee River natural organic matter (SRNOM) was employed as a source of 3CDOM*, significantly different photosensitized isomerization rates were observed for the diverse charged probes. The bimolecular reaction rate constants between 3SRNOM* and the probes were calculated as (0.42 ± 0.1) × 109 M-1 s-1 for sorbic acid, (1.1 ± 0.1) × 109 M-1 s-1 for sorbic alcohol, and (5.2 ± 0.4) × 109 M-1 s-1 for sorbic amine, respectively. The average apparent Φtriplet was (0.96 ± 0.03)% based on an irradiation range of 290 to 400 nm. We developed highly selective and efficient probes for triplet determination and elucidated the different reaction behaviors of these conjugated dienes containing different charged substituents within the photochemical energy transfer process.

Mechanistic consideration of the photochemical transformation of domoic acid (algal toxin) in DOM-Rich brackish water

Domoic acid (DA) is a neurotoxin generated by several diatom species in harmful algae blooms (HABs). We report the photo-induced transformation products (TPs) and degradation mechanisms of DA in dissolved organic matter (DOM)-rich freshwater and brackish water. High-resolution quadrupole time-of-flight mass spectrometry (QTOF-MS) and the multivariate statistical strategy orthogonal partial least-squares discriminant analysis (OPLS-DA) identified 36 and 23 potential TPs in DOM-rich freshwater and brackish water, respectively. The main reactive sites of DA are the conjugated double bond and proline ring. Isomerization is the predominant transformation pathway induced by excited-state triplet DOM (3DOM∗). The second-order rate constant of the isomerization reaction was measured as (3.8 ± 0.2) × 108 M-1 s-1. The inverse correlation between the dissolved oxygen (DO) concentration and the rate of photo-induced DA isomerization was revealed. Furthermore, under halide-present conditions, halide radicals are mainly responsible for the differentiation of products by quenching hydroxyl radicals and generating unique organic peroxide products. Our results indicated that halide radicals could be important in the photochemical transformation of organic contaminants in high saline environments.