Photodegradation of nonylphenol in aqueous solution by simulated solar UV-irradiation: The comprehensive effect of nitrate, ferric ion and bicarbonate

Photochemical behaviour and toxicity evolution of phenylbenzoate liquid crystal monomers in water

Liquid crystal monomers (LCMs) are a group of emerging pollutants that pose potential environmental risks because of their ubiquitous occurrence and toxicity. Understanding their environmental transformation is essential for assessing the ecological risk. In this study, we investigated the photochemical transformation kinetics, mechanism, and photo-induced toxicity of three phenylbenzoate LCMs in water. Their apparent photolytic rate constants were within (0.023 - 0.058) min-1, and the half-lives were < 30.0 min, showing lower persistence in water. Dissolved organic matter significantly inhibited their photolysis because of light-shielding effect and quenching of excited triplet states of LCMs. Their photolysis mainly occurred through excited triplet states, and the reactive oxygen species (i.e., ⋅OH, 1O2 and ⋅O2-) contributed to their degradation. The main photolysis pathways were ester bond cleavage, ⋅OH substitution/addition, and defluorination. Experiments and computational simulation revealed that some ·OH addition/substitution products have similar toxicity with LCMs. Additionally, the ∙OH reaction rate constants (kOH) of LCMs were determined to be > 1 × 109 M-1 s-1, evidence for their high reactivity toward ⋅OH. We have further developed reliable methods to estimate kOH of other phenylbenzoate-like LCMs with quantum chemical calculations. These results are useful for understanding the transformation and fate of LCMs in aquatic environments.

Reduction of the environmental impact of wastewater from the pretreatment of biodiesel production: A hybrid proposal for decontamination via photo-electro-Fenton/Fered/O3

Complex wastewater is generated during biodiesel production. We propose a new solution for the treatment of wastewater from enzymatic pretreatment of biodiesel production (WEPBP) by using a hybrid system based on the photo-Fered-Fenton process with O₃ assistance (PEF-Fered-O₃). We applied response surface methodology (RSM) to determine the suitable conditions for the PEF-Fered-O₃ process: a current intensity of 3 A, an initial solution pH controlled at 6.4, an initial H₂O₂ concentration of 12,000 mg L^-1, and an O₃ concentration of 50 mg L^-1. We performed three new experiments under similar conditions with slight changes to the conditions, namely a longer reaction time (120 min) and single or periodic H₂O₂ addition (i.e., small H₂O₂ additions at different reaction times). Periodic H₂O₂ addition provided the best removal results probably by reducing the occurrence of undesired side reactions that cause hydroxyl radical (^•OH) scavenging. With the application of the hybrid system, the chemical oxygen demand (COD) and total organic carbon (TOC) decreased by 91% and 75%, respectively. We also evaluated the presence of metals such as iron, copper, and calcium; electric conductivity; and voltage at 5, 10, 15, 30, 45, 60, 90, and 120 min. We submitted raw and treated WEPBP sludge samples to X-ray diffraction to study the degree of crystallinity. There was a rearrangement of the compounds present in treated WEPBP, possibly caused by oxidation of a large fraction of organic matter. Finally, we evaluated the genotoxicity and cytotoxicity of WEPBP by using Allium cepa meristematic root cells. Treated WEPBP was less toxic to these cells, denoted by improvements in gene regulation and cell morphology. Given the current scenario for the biodiesel industry, applying the proposed hybrid PEF-Fered-O₃ system at suitable conditions provides an efficient alternative to treat a complex matrix, namely WEPBP, to reduce its potential to cause abnormalities in the cells of living organisms. Thus, the negative impacts of the discharge of WEPBP in the environment might be reduced.

Photodegradation behavior and mechanism of dibutyl phthalate in water under flood discharge atomization

Flood discharge atomization is a prevalent hydraulics phenomenon in reservoir scheduling operations, however, its effect on the migration and transformation behavior of pollutants has not been examined. In this study, the behaviors and mechanisms of the direct photodegradation of dibutyl phthalate (DBP) in atomized water and the indirect photodegradation of DBP in the presence of ferric ions and nitrate were investigated. The results showed that the photodegradation rate of DBP was accelerated under atomization conditions by sunlight irradiation. The photodegradation efficiency of DBP in the presence of ferric ions and nitrate under atomization conditions was increased by 2.20 times and 1.82 times compared with no-atomization conditions, respectively. The quencher experiments indicated that the main active species for DBP photodegradation in the presence of ferric ions were hydroxyl radicals (·OH) and superoxide radicals (·O₂^-) with atomization, while the main active species in the presence of nitrate were ·OH, ·O₂^- and electrons (e^-). In addition, the differences were found in the photodegradation products and pathways of DBP between with and without atomization treatment. In the presence of ferric ions, the benzene ring of DBP was opened to produce fumaric acid, while phthalic acid bis(4-hydroxybutyl) ester was produced in the presence of nitrate under atomization conditions. The results of this study provide a scientific basis for assessing the effect of water conservancy projects on the migration and transformation behaviors of pollutants, which is of great theoretical significance and scientific value.

Efficient elimination of nonylphenol and 4-tert-octylphenol by weak electrical stimulated anaerobic microbial processes

The investigation into the degradation of alkylphenol pollutants (APs) has become a hotspot due to their harmful effects on the environment and human health. In this study, microbial electrolysis cells (MECs) were used to degrade nonylphenol (NP) and 4-tert-octylphenol (4-tert-OP). The study found that the degradation rates of NP and 4-tert-OP for a 6-day period were 83.6% and 96.3%, respectively, which were 30.53% and 26.7% higher than those of the group without applied voltage. The double layer area in the degradation of 4-tert-OP was larger than that of NP, and the resistance exhibited by 4-tert-OP (87.47 Ω) in MEC was lower than that of NP (99.42 Ω). Meanwhile, NP had a greater effect on the bioenzyme activity than 4-tert-OP. GC-MS analysis showed that the degradation pathways of both pollutants mainly included oxidation and hydroxylation reactions. Furthermore, the microbial community analysis indicated that the main functional bacteria in NP degradation were Citrobacter, Desulfovibrio and Advenella, and those in 4-tert-OP degradation were Stenotrophomonas, Chryseobacterium, Dokdonella, and the key microbiomes underlying the cooperative relationship. The biotoxicity test indicated that the toxicity of residual substances was significantly reduced. Therefore, the MEC system is efficient and environmentally friendly and has broad application prospects in phenol refractory organics.

Elevated nitrate promoted photodegradation of PAHs in aqueous phase: Implications for the increased nutrient discharge

Polycyclic aromatic hydrocarbons (PAHs) are frequently released in aqueous phase by oil spill or from other sources, and photochemical oxidation is one of their major weathering processes. In this study, the photochemical behavior of phenanthrene (PHE, as a representative PAH) were studied and the effects of nitrogenous compounds were evaluated. The results showed that nitrate was an effective photosensitizer for improving the photodegradation of PHE, but the promoting effect was less effective in seawater due to the presence of halogen ions; the ammonia played a negligible role on PHE degradation. The photochemical ionization was a key process for PHE degradation, it can be retarded due to the quenching of triplet excited state by dissolved oxygen, and the inhibition was most prominent in fresh water. The presence of nitrate increased the steady state concentration of •OH from 2.08 × 10^-15 M to 1.04 × 10^-14 M in fresh water, and from 1.5 × 10^-16 M to 2.08 × 10^-15 M in seawater. The secondary-order reaction rate constant between PHE and •OH (k_•OH,PHE) was determined as 5.70 × 10⁹ M^-1 s^-1. Similar trend was observed for ¹O₂. The contribution of •OH to PHE removal was more prominent in fresh water than in seawater due to the quenching effects of halogen, and the increasing of nitrate enlarged the contribution of •OH. Two possible PHE degradation pathways were proposed based on GC-MS analysis and DFT calculation. The Quantitative Structure-activity Relationship (QSAR) evaluation showed that some degradation intermediates were more toxic than PHE, but the total environmental risk was still diminished due to the low percentage of toxic intermediates. This study provided theoretical and experimental insights into the influence of nitrogenous compounds on the photodegradation of PHAs in water environment.

Photodegradation of oxytetracycline by UV-assisted persulfate and percarbonate processes: kinetics, influencing factors, anion effect, and radical species

In this study, the performance of ultraviolet (UV)-assisted persulfate (PS) and percarbonate (PC) oxidation processes in oxytetracycline (OTC) removal was investigated. UVC lamps were used for the photolysis process and the effect of operating parameters (initial pH, oxidant dose, initial OTC concentration, UV intensity) on OTC removal efficiency was determined. Control experiments were carried out at pH 5.5 and 32 W UV power for 60 min by adding a 4 mM oxidant with 10 mg/L initial OTC concentration. The OTC removal efficiency obtained as a result of only photolysis was 17.3% and the removal efficiency obtained by PS and PC oxidation alone was 18.3% and 12.7%, respectively. The OTC removal efficiencies increased in the combined processes and reached 58.1% and 69.9% for the UV-PS and UV-PC processes, respectively. The reaction rates of the processes were ranked as UV-PC > UV-PS > PS > UV > PC. In the UV-PS and UV-PC processes, the highest removal efficiencies were achieved at alkaline pH values. The OTC removal efficiency was increased with the increase in oxidant dose; however, the efficiency decreased after a certain dose due to the scavenging effect. The removal efficiency also increased as the initial OTC concentration decreased. The UV intensity had a positive effect on OTC removal efficiency. The effect of the water matrix on OTC removal efficiency was investigated while the dominant radical types were determined in UV-assisted processes. The EE/O values for the UV-PS and UV-PC processes were calculated as 211 kWh/m³ and 153 kWh/m³, respectively for 60 min of reaction time. Although similar removal efficiencies were obtained with both UV-assisted processes, the UV-PC process steps forward in terms of being a novel, environmentally friendly, more economic, and promising technology for OTC removal.

Effects of environmental factors on the fleroxacin photodegradation with the identification of reaction pathways

The abuse of fluoroquinolones (FQs) antibiotics leads to bacterial resistance and environmental pollution, so it is of great significance to verify the decomposition mechanism for eliminating antibiotic efficiently and conveniently. The effects of various environmental factors and the fleroxacin (FLE) photodegradation mechanisms were investigated by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS), UV-Vis absorption spectroscopy, fluorescence spectroscopy and quantum chemical calculation. Six possible photodegradation reaction paths on T₁ (excited triplet state) were proposed and simulated. The departure of the piperazine ring and the substitution of F atom at C-6 position by OH group were determined as the main reactions based on the reaction rates and energy barriers of each path. The multi-pathway reactions resulted in the fastest photodegradation rates of FLE at pH 6-7 than other pH conditions. NaN₃ would promote FLE photodegradation by inhibiting the reverse reaction of the separation process of F atom at C-8 and the generation of biphenyl molecules, which was a novel and distinctive phenomenon in this report. ·OH would rapidly combine with the free radicals generated in photolysis processes and made a great contribution to FLE photodegradation. Ca²⁺, Mg²⁺ and Ba²⁺ could stabilize the carboxyl group to impede the photo-competitive process of the decarboxylation reaction, while NO₃^- could generate reactive oxygen species to promote photodegradation.

Comparing the efficacy of various methods for sulfate radical generation for antibiotics degradation in synthetic wastewater: degradation mechanism, kinetics study, and toxicity assessment

In the present study the aim was to investigate and compare various activation processes for amoxicillin degradation. UV radiation, ultrasound, heat, and hydrogen peroxide were selected as the persulfate activation methods. The effects of various parameters such as pH, persulfate concentration, reaction time, AMX concentration, radical scavengers, and anions were thoroughly investigated. The results showed that AMX degradation was following the pseudo-first order kinetic model. The reaction rate of 0.114 min-1 was calculated for the UV/PS process, which was higher than that of the other investigated processes. The AMX degradation mechanism and pathway investigations revealed that sulfate and hydroxyl radicals were responsible for the degradation of AMX by two degradation pathways of hydroxylation and the opening of the β-lactam ring. Competition kinetic analysis showed that the second-order rate constant of AMX with sulfate radicals was 8.56 × 109 L mol-1 s-1 in the UV/PS process. Cost analysis was conducted for the four investigated processes and it was found that 1.9 $m-3 per order is required in the UV/PS process for the complete destruction of AMX. Finally, cytotoxic assessment of the treated effluent on human embryonic kidney cells showed a considerable reduction in AMX-induced cell cytotoxicity, proving that the investigated process is sufficiently capable of completely destroying AMX molecules to nontoxic compounds. Therefore, it can be concluded that UV radiation is much more effective than other methods for persulfate activation and can be considered as a reliable technique for antibiotic removal.

Phototransformation of tetrabromobisphenol A in saline water under simulated sunlight irradiation

The widespread use of halogenated flame retardants in recent years has led to the accumulation of TBBPA in water, which may cause potential harm to living organisms. The phototransformation of the flame retardant TBBPA in alkaline saline water under simulated sunlight irradiation was investigated. The effects of abiotic factors such as the initial concentration of TBBPA, chloride ion concentration, solution pH, inorganic anions and cations, dissolved organic matter (DOM) were studied. The results showed that the phototransformation rate of TBBPA accelerated with the decrease of the initial concentration of TBBPA, the increase of chloride ion concentration and solution pH. The scavenging experiments showed that •OH, ¹O₂, O₂•^- and ³TBBPA* all participated in the phototransformation of TBBPA. The presence of NO₃^-, CO₃^2-, SO₄^2-, Mg²⁺, Ca²⁺, Fe³⁺ and fulvic acid (FA) all inhibited the phototransformation of TBBPA in the present study. The phototransformation products of TBBPA were detected by liquid chromatography-mass spectrometry (LC-MS), and the phototransformation pathways were proposed. This is the first report on the photo-induced generation of halogen exchange products from TBBPA in saline solutions, which will contribute to a better understanding of the environmental behavior and risks of BFRs in water.

Enhanced photodegradation of applied dithianon fungicides on plant leaves by dissolved substances in atmosphere under simulated sunlight

Photolysis of pesticides has been widely investigated for evaluating their environmental behavior and agricultural effectiveness after crop spraying. However, little information about the effects of the water-soluble substances in atmosphere on photodegradation of pesticides is available. In current study, we found that photolysis of applied dithianon fungicides on real plant leaves was much faster than that in sealed stock aqueous suspensions under simulated sunlight. To simulate the natural conditions, for the first time, photodegradation of dithianon in air-saturated solutions containing typical dissolved atmospheric substances (DAS) including CO₂ (HCO₃^-/CO₃^2-), NO₂ (NO₃^-), Fe³⁺ (Fe³⁺-complexes), and humic-like substances (HULIS) exposed to simulated solar irradiations were carried out in lab-scale. Fulvic acid (FA) was used as a surrogate for atmospheric HULIS in this study. The dithianon photodegradation was significantly enhanced in the presence of DAS and the photo-generated reactive species such as ·OH, ¹O₂, CO₃·- and ³FA∗ play important roles according to the results of reactive species quenching, electron spin resonance spectroscopy, and laser flash photolysis experiments. Moreover, the photodegraded intermediates and final products of dithianon on plant leaves have been identified by HPLC-MS analysis, and its possible photodegradation pathways were proposed. This work indicated that, except for direct photolysis, indirect photosensitive degradation induced by the dissolved photo-active substances in atmosphere should be considered for evaluating the degradation of the applied pesticides on crops.

Photochemical reaction of tricresyl phosphate (TCP) in aqueous solution: Influencing factors and photolysis products

Organophosphate triesters (OPEs) have caused great concern as a class of emerging environmental contaminants due to their widespread use and their toxicity to organisms. However, the phototransformation behavior of OPE is still not fully understood, which is important for understanding their environmental fate. In the present study, the photodegradation of tricresyl phosphate (TCP), one of the most widely detected OPEs in aqueous environments, was investigated including the direct photolysis and in the presence of several natural water factors, NO₂^-, Fe³⁺ and humic acid. The degradation process followed the pseudo-first-order kinetics, with rate constant increasing slightly with increasing initial TCP concentration. The presence of NO₂^- and Fe³⁺ was observed to promote the photochemical loss of TCP, while humic acid played a negative role on TCP transformation. Electron spin resonance (EPR) analysis showed that carbon-centered radical was produced in the photolysis process of TCP, and hydroxyl radical contributed to the promotion of rate constant for Fe³⁺ and NO₂^-. Four photolysis products were tentatively identified by HPLC-LTQ-Orbitrap MS analysis, and the possible degradation pathways of TCP were proposed. These findings provide a meaningful reference for the fate and transformation of OPEs in natural water.

Degradation of 4-nonylphenol in marine sediments by persulfate over magnetically modified biochars

In this study, an environmentally friendly and economically viable bamboo biochar (BB) was modified by Fe₃O₄ and was applied for the treatment of real river sediments containing the endocrine disruptor chemical (EDC) 4-nonylphenol (4-NP). The microporosity of Fe₃O₄-BB was clearly observed from the N₂ adsorption isotherms. The catalytic performance of Fe₃O₄-BB is highly dependent on pH and the catalyst dosage. The degradation efficiency of 4-NP (85%) was achieved at pH 3.0 using an initial dosage of 3.33 g L^-1 Fe₃O₄-BB and 2.3 × 10^-5 M persulfate (PS) in a biochar-sediment system. The kinetic behavior of 4-NP degradation with catalysis can be accounted by using the Langmuir-Hinshelwood type kinetic model. The MTT assay results indicated that Fe₃O₄-BB has a low potent cytotoxic effect and is therefore suitable for application in remediation of contaminated sediment.

Effects of inorganic anions on the photolysis of triclosan under UV irradiation

Triclosan is a widely used antimicrobial agent and may pose health risks to many aquatic organisms. Photodegradation is an important transformation pathway for triclosan, but studies on the effects of inorganic anions on photodegradation of triclosan are limited. In the present study, the single and combined effects of NO₃ ^-, Cl^- and HCO₃ ^- on the photolysis of triclosan in aqueous solutions under UV irradiation was evaluated. The results showed that photodegradation of triclosan was inhibited by NO₃ ^- and promoted by HCO₃ ^-, while no significant effect was observed with Cl^-. When Cl^- was added to NO₃ ^-, no effect was observed, but the addition of Cl^- hindered the promotion effect of HCO₃ ^-. The coexistence of NO₃ ^-, Cl^- and HCO₃ ^- inhibited the photolysis of triclosan. These results showed the complex effects of inorganic anions in the photolysis of triclosan and provide useful information for an accurate ecological risk assessment of triclosan in natural waters. It will also help to develop appropriate treatment ways of triclosan.

Photodegradation of diclofenac in seawater by simulated sunlight irradiation: The comprehensive effect of nitrate, Fe(III) and chloride

Diclofenac is one of the most frequently detected pharmaceuticals in various aquatic environments. The photodegradation of diclofenac in the absence/presence of nitrate, Fe(III) and chloride, especially their interactions, were systematically studied. Under the study conditions, photodegradation rate was decreased with increasing nitrate, Fe(III) and chloride concentrations. Nitrate has an synergistic action for Fe(III). Nitrate and chloride have antagonistic effect. An antagonistic action is present between nitrate, Fe(III) and chloride. Moreover, a simple linear model which very well describes the results is given.