Mechanism insight of acetaminophen degradation by the UV/chlorine process: kinetics, intermediates, and toxicity assessment

Elucidating the performance of UV-based photochemical processes for the removal of trace organic contaminants: Degradation and toxicity evaluation

In this study, the performance of standalone ultraviolet (UV) photolysis and UV-based advanced oxidation processes (AOPs), namely, UV/hydrogen peroxide, UV/chlorine, UV/persulphate, and UV/permonosulphate, were investigated for the degradation of 31 trace organic contaminants (TrOCs). Under the tested conditions, standalone UV photolysis did not achieve effective removal of TrOCs. To improve the degradation efficiency of UV photolysis, four different oxidants were added individually to the test solution. The effect of these oxidants in the absence of UV irradiation was also explored and only chlorine showed promising degradation of some contaminants. During the chlorination of 31 investigated TrOCs, only six demonstrated greater than 50% degradation. The combined UV-based AOPs demonstrated much improved degradation (ranging from 65 to 100%) depending on TrOC-structure and oxidant concentration. The UV/hydrogen peroxide process showed similar degradation of TrOCs, irrespective of the functional groups (i.e., electron withdrawing groups, EWGs and electron donating groups, EDGs) present in their structures. Conversely, the UV/sulphate and UV/chlorine based processes achieved better degradation of the TrOCs with EDGs in their structures. TrOCs degradation improved up to 40% when oxidants concentrations were increased from 0.1 to 1 mM, and further increasing the concentration to 2 mM did not improve degradation. Toxicity evaluation using bioluminescence test (BLT assay) demonstrated that except for UV/hydrogen peroxide, all UV-based AOPs increased the toxicity of the treated effluent, indicating generation of toxic by-products. This study elucidates the performance of four different UV-based AOPs for the removal of commonly detected diverse TrOCs for the first time.

Insight into micropollutant abatement during ultraviolet light-emitting diode combined electrochemical process: Reaction mechanism, contributions of reactive species and degradation routes

Electrochemical process coupling with ultraviolet light-emitting diode for micropollutant abatement was evaluated in the treatment of wastewater containing Cl^-. Four representative micropollutants, atrazine, primidone, ibuprofen and carbamazepine, were selected as target compounds. The impacts of operating conditions and water matrix on micropollutant degradation were investigated. Fluorescence excitation-emission matrix spectroscopy spectra and high performance size exclusion chromatography were employed to characterize the transformation of effluent organic matter in treatment. The degradation efficiencies of atrazine, primidone, ibuprofen and carbamazepine are 83.6 %, 80.6 %, 68.7 % and 99.8 % after 15 min treatment, respectively. The increment of current, Cl^- concentration and ultraviolet irradiance promote the micropollutant degradation. However, the presence of bicarbonate and humic acid inhibit micropollutant degradation. The mechanism of micropollutant abatement was elaborated based on reactive species contributions, density functional theory calculation and degradation routes. Free radicals (HO^•, Cl^•, ClO^• and Cl₂^•-) could be generated by chlorine photolysis and subsequent propagation reactions. The concentrations of HO^• and Cl^• are 1.14 × 10^-13 M and 2.0 × 10^-14 M in optimal condition, respectively, and the total contributions of HO^• and Cl^• for the degradation of atrazine, primidone, ibuprofen and carbamazepine are 24 %, 48 %, 70 % and 43 %, respectively. The degradation routes of four micropollutants are elucidated based on intermediate identification, Fukui function and frontier orbital theory. Micropollutants can be effectively degraded in actual wastewater effluent, and the small molecule compound proportion increases during effluent organic matter evolution. Compared with photolysis and electrolysis, the coupling of the two processes has potential for energy saving in micropollutant degradation, which shed light on the prospects of ultraviolet light-emitting diode coupling with electrochemical process for effluent treatment.

Acetaminophen degradation in aqueous solution by the UV-LED-EC/Cl2 process

In this study, electrochemically generated free chlorine (EC/Cl2) was activated by UV irradiation with a light emitting diode (LED) lamp at 275 nm to degrade acetaminophen (AAP, 2 μM) in aqueous solution. The potential at a RuO2-IrO2/Ti plate anode was set at 1.5 V vs. the Ag/AgCl electrode. Chlorine was in situ generated in the presence of Cl at the anode and then it was transformed into various active species such as OH and reactive chlorine species (RCS) under UV-LED irradiation. The degradation of AAP was investigated using batch tests, evaluating the influence of different experimental conditions such as NaCl concentration, phosphate buffer saline concentration, irradiation time and solution pH, keeping constant the UV-LED power and temperature. Results show that AAP could be completely degraded by the hybrid process with a high mineralization ratio (73%), and the degradation process followed a pseudo-first-order kinetics. The value of the Electric Energy per Order (EEO) = 1.272 kWh m3 order?, which is lower than the energy consumption of some other UV-based processes for AAP degradation. Adding 1 mM HCO3 ions slightly decreased the rate of AAP degradation. Luminescent bacteria experiment revealed that the acute toxicity of the reacted solution could be greatly reduced and the ecological risk was effectively abated. The scavenging assay shows that RCS plays a key role in the AAP degradation. The intermediate products were identified, and possible degradation routes were proposed. The system can advantageously replace conventional UV mercury lamp based ones in the degradation of microorganic pollutants.

Reactive Blue 19 dye removal by UV-LED/chlorine advanced oxidation process

In recent years, advanced oxidation processes (AOPs) have indicated the greatest potential in the removal of stable organic compounds, including dyes. In this study, the ultraviolet light-emitting diodes (UV-LEDs) combined with chlorine was evaluated to remove Reactive Blue 19 (RB19) dye from aqueous solution. The effect of key experimental parameters including pH, initial chlorine concentration, initial dye concentration, and reaction time on the performance of UV-LED irradiation, UV-LED/chlorine, and the chlorination method for the removal of RB19 was studied in this research. Results showed that, more than 99% of RB19 was removed after 30 min of reaction time under optimized conditions (pH = 5, [chlorine] = 300 μM, and [RB19] = 20 mg L^-1) with apparent kinetic rate constant (k_app) of 17.1 × 10^-2 min^-1 in UV-LED/chlorine process. However, for the chlorination method, removal efficiency was 64.7% (k_app = 3.41 × 10^-2 min^-1) with an apparent kinetic rate constant of 0.0341 min^-1. Results also showed that UV-LED irradiation is not effective at all in removing RB19. The scavenging assay showed that OH^• radicals (67.23%) had the highest contribution in RB19 removal in UV-LED/chlorine process while Cl^• (17.82%) and [Formula: see text] (8.56%) had a minor role in the degradation of the dye. The RB19 degradation kinetics analysis revealed that the processes of UV-LED/chlorine and chlorination degradation followed the pseudo-first-order kinetic model. In this study, the impact of chloride, nitrate, bicarbonate, carbonate, sulfate, and sulfite anions on the performance of the process was investigated. It indicated that sulfite anion has the most negative impact on the RB19 removal process. By evaluating the synergistic effect between UV-LED lamp and chlorine, a synergy index of 5.0 was obtained for the UV-LED/chlorine process. The results presented that the UV-LED/chlorine process has a better performance than each of them alone and has the necessary efficiency for RB19 removal. Measuring COD reported its removal efficiency of 98% during the UV-LED/chlorine process under optimized conditions. Experiments continued with textile factory wastewater and indicated 30.9% of its COD removed after treatment when 1.0 μM chlorine was used.

A review on the degradation of acetaminophen by advanced oxidation process: pathway, by-products, biotoxicity, and density functional theory calculation

Water scarcity and the accumulation of recalcitrance compounds into the environment are the main reasons behind the attraction of researchers to use advanced oxidation processes (AOPs). Many AOP systems have been used to treat acetaminophen (ACT) from an aqueous medium, which leads to generating different kinetics, mechanisms, and by-products. In this work, state-of-the-art studies on ACT by-products and their biotoxicity, as well as proposed degradation pathways, have been collected, organized, and summarized. In addition, the Fukui function was used for predicting the most reactive sites in the ACT molecule. The most frequently detected by-products in this review were hydroquinone, 1,4-benzoquinone, 4-aminophenol, acetamide, oxalic acid, formic acid, acetic acid, 1,2,4-trihydroxy benzene, and maleic acid. Both the experimental and prediction tests revealed that N-(3,4-dihydroxy phenyl) acetamide was mutagenic. Meanwhile, N-(2,4-dihydroxy phenyl) acetamide and malonic acid were only found to be mutagenic in the prediction test. The findings of the LC50 (96 h) test revealed that benzaldehyde is the most toxic ACT by-products and hydroquinone, N-(3,4-dihydroxyphenyl)formamide, 4-methylbenzene-1,2-diol, benzoquinone, 4-aminophenol, benzoic acid, 1,2,4-trihydroxybenzene, 4-nitrophenol, and 4-aminobenzene-1,2-diol considered harmful. The release of them into the environment without treatment may threaten the ecosystem. The degradation pathway based on the computational method was matched with the majority of ACT proposed pathways and with the most frequent ACT by-products. This study may contribute to enhance the degradation of ACT by AOP systems.

Acetaminophen degradation by hydroxyl and organic radicals in the peracetic acid-based advanced oxidation processes: Theoretical calculation and toxicity assessment

The research on the mechanisms and kinetics of radical oxidation in peracetic acid-based advanced oxidation processes was relatively limited. In this work, HO^• and organic radicals mediated reactions of acetaminophen (ACT) were investigated, and the reactivities of important organic radicals (CH₃COO^• and CH₃COOO^•) were calculated. The results showed that initiated reaction rate constants of ACT are in the order: CH₃COO^• (5.44 × 10¹⁰ M^-1 s^-1) > HO^• (7.07 × 10⁹ M^-1 s^-1) > CH₃O^{• (}1.57 × 10⁷ M^-1 s^-1) > CH₃COOO^• (3.65 × 10⁵ M^-1 s^-1) >> ^•CH₃ (5.17 × 10² M^-1 s^-1) > CH₃C^•O (1.17 × 10² M^-1 s^-1) > CH₃OO^• (11.80 M^-1 s^-1). HO^•, CH₃COO^• and CH₃COOO^• play important roles in ACT degradation. CH₃COO^• is another important radical in the hydroxylation of aromatic compounds in addition to HO^•. Reaction rate constants of CH₃COO^• and aromatic compounds are 1.40 × 10⁶ - 6.25 × 10¹⁰ M^-1 s^-1 with addition as the dominant pathway. CH₃COOO^• has high reactivity to phenolate and aniline only among the studied aromatic compounds, and it was more selective than CH₃COO^•. CH₃COO^•-mediated hydroxylation of aromatic compounds could produce their hydroxylated products with higher toxicity.

Bioassay- and QSAR-based screening of toxic transformation products and their formation under chlorination treatment on levofloxacin

Levofloxacin (LEV) is a broad-spectrum quinolone antibiotic and widely used for human and veterinary treatment. Overuse of LEV leads to its frequent occurrence in the water environment. In this study, the transformation characteristics of LEV in water during the simulated chlorination disinfection treatment were explored. Fifteen major transformation products (TPs) of LEV were identified, and their plausible formation pathways were proposed. The reaction pathways were strongly dependent on pH condition, and LEV removal was relevant to free available chlorine (FAC) dose. Antibacterial activity of chlorination system was dramatically declined when FAC was more than 3-equivalent (eq) due to the elimination of antibacterial related functional groups. Genotoxicity of chlorination system increased more than 3 times at 0.5-eq of FAC and then decreased with increasing FAC dose, which were in accordance with the relative concentration of toxic TPs estimated by QSAR model. These results implied that the combination of bioassay, QSAR computation and chemical analysis would be an efficient method to screen toxic TPs under chlorination treatment. It is anticipated that the results of this study can provide reference for optimizing operational parameters for water disinfection treatment, and for scientifically evaluating the potential risk of quinolone antibiotics.

Mechanistic insights into paracetamol transformation in UV/NH2Cl process: Experimental and theoretical study

The UV/monochloramine (NH₂Cl) process is an advanced oxidation process that can effectively remove emerging contaminants (ECs). However, the degradation mechanisms of reactive radicals with ECs are not clear. In this work, we combined theoretical calculations with experimental studies to investigate the kinetics and mechanism of radical-mediated degradation of paracetamol (AAP) in UV/NH₂Cl process. The degradation of AAP in UV/NH₂Cl process accords with the pseudo first-order kinetics. Impact factors including NH₂Cl dose, pH, natural organic matter, HCO₃^-, and NO₃^- were evaluated. The reaction mechanisms of AAP with hydroxyl radical (HO^·), reactive chlorine species (RCS), and reactive nitrogen species (RNS) were discussed in detail. Specifically, HO^· attacked AAP mainly through hydrogen atom transfer (HAT) and radical adduct formation (RAF), while Cl₂^·- play a certain role through single electron transfer (SET). ^·NH₂ and Cl^· destructed AAP mainly through HAT. Based on the mechanism analysis, the second-order rate constants of AAP reacts with HO^·, Cl^·, ^·NH₂, ClO^·, Cl₂^·- and ^·NO₂ were calculated through transition state theory as 2.66×10⁹ M^-1 s^-1, 2.61×10⁹ M^-1 s^-1, 1.02×10⁷ M^-1 s^-1, 7.74×10⁶ M^-1 s^-1, 1.32×10⁶ M^-1 s^-1, 1.48×10³ M^-1 s^-1 respectively. The second-order rate constants were then used to distinguish the contribution of radicals to the degradation of AAP. Thirteen transformation products were identified by high-resolution mass spectrometry. Combined active sites with potential energy surface, the detailed reaction pathways were proposed. Overall, this study provides deep insights into the mechanism of radical-mediated degradation of AAP.

Selective degradation of acetaminophen from hydrolyzed urine by peroxymonosulfate alone: performances and mechanisms

Owing to the high concentration of pharmaceuticals in urine, the degradation of these organic pollutants before their environmental release is highly desired. Peroxymonosulfate (PMS) is a desirable oxidant that can be applied to environmental remediation; however, the performance and mechanism of PMS for the degradation of pharmaceuticals in the urine matrix have not been investigated. Herein, PMS was first discovered to efficiently degrade typical pharmaceuticals in hydrolyzed urine (HU) by selecting acetaminophen (ACE) as a target compound. Quenching experiments revealed that singlet oxygen (¹O₂) and hydroxyl radicals (HO˙) were observed in the HU/PMS system, but the principal reactive species (RS) responsible for ACE removal was ¹O₂. The major constituents of HU, including SO₄²⁻ and organics (creatine, creatinine and hippuric acid), hardly affected the elimination of ACE, whereas Cl⁻, H₂PO₄⁻ and NH₄⁺ would accelerate ACE degradation. Besides, HCO₃⁻ slightly inhibited this process. The ACE degradation efficiency was enhanced using photo-irradiation, including sunlight and visible light, although increasing the reaction temperature could, interestingly, hardly accelerate the degradation rate of ACE. Three-dimensional excitation–emission matrices (3D-EEMs) have indicated that other intermediates that have a higher fluorescence intensity might be generated in the HU/PMS system. Finally, nine intermediate products were determined and the degradation pathways of ACE were proposed. Overall, the results of this study illustrated that PMS is an efficient oxidant for the degradation of ACE in HU.

Owing to the high concentration of pharmaceuticals in urine, the degradation of these organic pollutants before their environmental release is highly desired.

Comparative endocrine disrupting compound removal from real wastewater by UV/Cl and UV/H2O2: Effect of pH, estrogenic activity, transformation products and toxicity

Extensive use of endocrine disruptor compounds (EDCs) and their release through various pathways into the environment are emerging environmental concerns. In this context, H₂O₂ and chlorine UV-based treatments were carried out to evaluate their efficiency in the removal of the bisphenol A (BPA), 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) at 100 μg L^-1 from ultrapure water and from wastewater treatment plants (WWTP). Photolysis was performed under different irradiation sources, i.e. UVC and UVA. The effect of H₂O₂ (3 and 30 mg·L^-1), free chlorine concentrations (1 and 2 mg·L^-1) and pH (5, 7 and 9) were also investigated. Toxicity (Raphidocelis subcapitata) and estrogenic activity (yeast estrogen screen - YES assay) were assessed during the processes. Compound removal at optimal operating parameters reached 100% after 15 and 2 min for UVC/H₂O₂ (pH 9 and 3 mg L^-1 of H₂O₂), and UVC/Cl (pH 9 and 2 mg L^-1 of chlorine), respectively. Total organic carbon (TOC) removal achieved 37% and 45% for the H₂O₂ and Cl-UV based process, respectively. The in vitro YES assay indicated that the formed by-products were non-estrogenic compounds, while the toxicity evaluation revealed high cell growth inhibition due to UVC/Cl byproducts. During the UV-based processes, 30 transformation products (TPs) were identified, in which three new chlorinated TPs from E2 and EE2 may be responsible for toxicity effects. EDC degradation by UV/Cl is faster than by UV/H₂O₂, although chlorinated toxic byproducts were also formed during the UV/Cl process.