Environmental Implications of Hydroxyl Radicals (•OH)

Aquatic photochemistry of sulfamethazine: multivariate effects of main water constituents and mechanisms

The ubiquity of sulfonamides (SAs) in natural waters requires insight into their environmental fate for ecological risk assessment. Extensive studies focused on the effect of univariate water constituents on the photochemical fate of SAs, yet the multivariate effects of water constituents in environmentally relevant concentrations on SA photodegradation are poorly understood. Here, response surface methodology was employed to explore the integrative effects of main water constituents (dissolved organic matter (DOM), NO₃^-, HCO₃^-, Cu²⁺) on the photodegradation of a representative SA (sulfamethazine). Results showed that besides single factors, interaction of factors also significantly impacted the photodegradation. Radical scavenging experiments indicated that triplet-excited DOM (³DOM*) was responsible for the enhancing effect of DOM on the photodegradation. Additionally, DOM may also quench the ³DOM*-mediated oxidation intermediate of sulfamethazine causing the inhibiting effect of DOM-DOM interaction. We also found that HCO₃^- was oxidized by triplet-excited sulfamethazine producing CO₃˙^-, and the high reactivity of CO₃˙^- with sulfamethazine (second-order rate constant 2.2 × 10⁸ M^-1 s^-1) determined by laser flash photolysis revealed the enhancing photodegradation mechanism of HCO₃^-. This study is among the first attempts to probe the photodegradation of SAs considering the integrative effects of water constituents, which is important in accurate ecological risk assessment of organic pollutants in the aquatic environment.

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization techniques, and environmental applications of magnetite and green rust in the environment. The aim is to present a detailed overview of the key aspects related to these mineral phases which can be used as an important resource for researchers working in a diverse range of fields dealing with mixed-valent iron minerals.

Atmospherically Relevant Radicals Derived from the Oxidation of Dimethyl Sulfide

The large number and amounts of volatile organosulfur compounds emitted to the atmosphere and the enormous variety of their reactions in various oxidation states make experimental measurements of even a small fraction of them a daunting task. Dimethyl sulfide (DMS) is a product of biological processes involving marine phytoplankton, and it is estimated to account for approximately 60% of the total natural sulfur gases released to the atmosphere. Ocean-emitted DMS has been suggested to play a role in atmospheric aerosol formation and thereby cloud formation. The reaction of ·OH with DMS is known to proceed by two independent channels: abstraction and addition. The oxidation of DMS is believed to be initiated by the reaction with ·OH and NO₃· radicals, which eventually leads to the formation of sulfuric acid (H₂SO₄) and methanesulfonic acid (CH₃SO₃H). The reaction of DMS with NO₃· appears to proceed exclusively by hydrogen abstraction. The oxidation of DMS consists of a complex sequence of reactions. Depending on the time of the day or altitude, it may take a variety of pathways. In general, however, the oxidation proceeds via chains of radical reactions. Dimethyl sulfoxide (DMSO) has been reported to be a major product of the addition channel. Dimethyl sulfone (DMSO₂), SO₂, CH₃SO₃H, and methanesulfinic acid (CH₃S(O)OH) have been observed as products of further oxidation of DMSO. Understanding the details of DMS oxidation requires in-depth knowledge of the elementary steps of this seemingly simple transformation, which in turn requires a combination of experimental and theoretical methods. The methylthiyl (CH₃S·), methylsulfinyl (CH₃SO·), methylsulfonyl (CH₃SO₂·), and methylsulfonyloxyl (CH₃SO₃·) radicals have been postulated as intermediates in the oxidation of DMS. Therefore, studying the chemistry of sulfur-containing free radicals in the laboratory also is the basis for understanding the mechanism of DMS oxidation in the atmosphere. The application of matrix-isolation techniques in combination with quantum-mechanical calculations on the generation and structural elucidation of CH₃SO_x (x = 0-3) radicals is reviewed in the present Account. Experimental matrix IR and UV/vis data for all known species of this substance class are summarized together with data obtained using other spectroscopic techniques, including time-resolved spectroscopy, electron paramagnetic resonance spectroscopy, and others. We also discuss the reactivity and experimental characterization of these species to illustrate their practical relevance and highlight spectroscopic techniques available for the elucidation of their geometric and electronic structures. The present Account summarizes recent results regarding the preparation, characterization, and reactivity of various radical species with the formula CH₃SO_x (x = 0-3).

Interface Mechanisms of Catalytic Ozonation with Amorphous Iron Silicate for Removal of 4-Chloronitrobenzene in Aqueous Solution

Iron silicate was synthesized and characterized as an efficient ozonation catalyst. Results indicated that iron silicate is a microporous material with poor crystallinity. Fe-O-Si and Fe-O bonds were observed on its surface. The Fe-O bonds belonged to α-Fe₂O₃. Heterogeneous catalytic ozonation test was performed in batch reaction mode, and 4-chloronitrobenzene was used as model organic compounds. Amorphous iron silicate exhibited high catalytic activity, ozone utilization efficiency, and stability in catalytic ozonation. Hydroxyl radical was the dominant oxide species in this process. The reaction mechanism at the solid-water interface indicates that Fe-Si binary oxides on iron silicate surface inhibited ozone futile decomposition. This behavior resulted in enhanced probability of the reaction between ozone and α-Fe₂O₃ on the iron silicate surface to generate hydroxyl radicals, which promoted 4-chloronitrobenzene removal in aqueous solution.

Electrochemical oxidizing digestion using PbO2electrode for total phosphorus determination in a water sample

Total phosphorus is one of the key water quality parameters in environmental monitoring. To precisely determine the total phosphorus, water samples have to be pretreated to convert the various forms of phosphorus to orthophosphate. Conventionally, pretreatment is accomplished by heating, acidification, and oxidation in a digestion equipment, which is dangerous, time-consuming, and complicated. Herein, we propose a novel high-performance electrochemical oxidation protocol for phosphorus digestion based on a PbO₂ electrode. The electrode, which has a hydrophobic and stable surface, was prepared by electrochemical deposition on a titanium substrate and has high hydroxyl radical utilization when digesting total phosphorus. As a result, 90% of sodium glycerophosphate was digested within 30 minutes, and high digestion ratios of acephate, glyphosate, and inland water samples were obtained as well. In addition, this electrochemical digestion protocol does not required heating and acidification steps, which shortens the digestion time. Therefore, a rapid quantification of total phosphorus in the water sample was achieved.

Using electrochemical oxidation for digestion when testing total phosphorus with a digestion ratio of about 90%.

Photochemical transformation of dimethyl phthalate (DMP) with N(iii)(H2ONO+/HONO/NO2−) in the atmospheric aqueous environment

The primary step of photochemical transformation of dimethyl phthalate (DMP) with N(iii) is ˙OH-addition on the aromatic ring of DMP to form a DMP–OH adduct, followed by several decay channels and corresponding rate constants are determined.

Soybean peroxidase immobilized onto silica-coated superparamagnetic iron oxide nanoparticles: Effect of silica layer on the enzymatic activity

Peroxidase immobilization onto magnetic supports is considered an innovative strategy for the development of technologies that involves enzymes in wastewater treatment. In this work, magnetic biocatalysts were prepared by immobilization of soybean peroxidase (SBP) onto different silica-coated superparamagnetic iron oxide nanoparticles. The obtained magnetic biocatalysts were tested for the degradation of malachite green (MG), a pollutant often found in industrial wastewaters and with significant drawbacks for the human and environmental health. A deep physicochemical characterization of the materials was performed by means of X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), High Resolution-Transmission Electron Microscope (HR-TEM) and magnetization measurements among others techniques. Results showed high immobilization yield of SBP onto nanomaterials with excellent properties for magnetic recoverability. A partial loss of activity with respect to free SBP was observed, compatible with the modification of the conformational structure of the enzyme after immobilization. The structural modification depended on the amount (and thickness) of silica present in the hybrid materials and the activity yield of 43% was obtained for the best biocatalyst. Thermal stability and reusability capacity were also evaluated.

F, Walter Z. T, Xochitl DB, Mika S. Towards reliable quantification of hydroxyl radicals in the Fenton reaction using chemical probes

Quantification of hydroxyl radical concentration using two chemical probes was assessed through the Fenton reaction. The probes were 1,2-benzopyrone (coumarin) for fluorescence and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) for electron spin resonance (ESR). The corresponding hydroxylated species, namely 7-hydroxycoumarin (7HC) and 2-hydroxy-5,5-dimethyl-1-pyrroline-N-oxide (DMPO-OH adduct), were monitored by fluorescence and ESR-spin trapping techniques, respectively. The experiments were designed according to the theoretical conditions determined for stable fluorescence and EPR signals. The results demonstrate that: the optimal [chemical probe] : [H₂O₂] ratio predicted by a simplified quasi-steady-state model was in good agreement with the optimal [chemical probe] : [H₂O₂] ratio observed experimentally for [H₂O₂] : [Fe²⁺] = 10, and the proper adjustment of the [chemical probe] : [H₂O₂] ratio at a given concentration of the Fenton's reagent improves the detected amount of hydroxyl radicals. Finally, using DMPO required a higher concentration compared to coumarin to yield the same amount of ˙OH detected but resulted in a more reliable probe for detecting ˙OH under the consideration of this study.

Quantification of hydroxyl radical concentration using two chemical probes was assessed through the Fenton reaction.

Photochemical degradation of the carbapenem antibiotics imipenem and meropenem in aqueous solutions under solar radiation

This paper deals with the photochemical fate of two representative carbapenem antibiotics, namely imipenem and meropenem, in aqueous solutions under solar radiation. The analytical method employed for the determination of the target compounds in various aqueous matrices, such as ultrapure water, municipal wastewater treatment plant effluents, and river water, at environmentally relevant concentrations, was liquid chromatography coupled with hybrid triple quadrupole-linear ion trap-mass spectrometry. The absorption spectra of both compounds were measured in aqueous solutions at pH values from 6 to 8, and both compounds showed a rather strong absorption band centered at about 300 nm, while their molar absorption coefficient was in the order from 9 × 10³-10⁴ L mol^-1 cm^-1. The kinetics of the photochemical degradation of the target compounds was studied in aqueous solutions under natural solar radiation in a solar reactor with compound parabolic collectors. It was found that the photochemical degradation of both compounds at environmentally relevant concentrations follows first order kinetics and the quantum yield was in the order of 10^-3 mol einsten^-1. Several parameters were studied, such as solution pH, the presence of nitrate ions and humic acids, and the effect of water matrix. In all cases, it was found that the presence of various organic and inorganic constituents in the aqueous matrices do not contribute significantly, either positively or negatively, to the photochemical degradation of both compounds under natural solar radiation. In a final set of photolysis experiments, the effect of the level of irradiance was studied under simulated solar radiation and it was found that the quantum yield for the direct photodegradation of both compounds remained practically constant by changing the incident solar irradiance from 28 to 50 W m^-2.

Can Carbamates Undergo Radical Oxidation in the Soil Environment? A Case Study on Carbaryl and Carbofuran

Radical oxidation of carbamate insecticides, namely carbaryl and carbofuran, was investigated with spectroscopic (electron paramagnetic resonance [EPR] and UV-vis) and theoretical (density functional theory [DFT] and ab initio orbital-optimized spin-component scaled MP2 [OO-SCS-MP2]) methods. The two carbamates were subjected to reaction with ^•OH, persistent DPPH^• and galvinoxyl radical, as well as indigenous radicals of humic acids. The influence of fulvic acids on carbamate oxidation was also tested. The results obtained with EPR and UV-vis spectroscopy indicate that carbamates can undergo direct reactions with various radical species, oxidizing themselves into radicals in the process. Hence, they are prone to participate in the prolongation step of the radical chain reactions occurring in the soil environment. Theoretical calculations revealed that from the thermodynamic point of view hydrogen atom transfer is the preferred mechanism in the reactions of the two carbamates with the radicals. The activity of carbofuran was determined experimentally (using pseudo-first-order kinetics) and theoretically to be noticeably higher in comparison with carbaryl and comparable with gallic acid. The findings of this study suggest that the radicals present in soil can play an important role in natural remediation mechanisms of carbamates.

Priority pesticides abatement by advanced water technologies: The case of acetamiprid removal by ozonation

With the aim of exploring treatment alternatives for priority insecticide acetamiprid (ACMP) abatement, the removal of this compound from water by ozonation was studied for the first time, paying special attention to the kinetic, mechanistic and toxicological aspects of the process. The second order rate constants of reactions between ACMP and both molecular ozone (O₃) and hydroxyl radicals (OH) were determined to be 0.25M^-1s^-1 and 2.1·10⁹M^-1s^-1, respectively. On the basis of kinetic results, the degradation of ACMP during ozonation could be well-explained by the reactivity of this pesticide with OH. HPLC/MS analysis of the ozonated ACMP showed ACMP-N-desmethyl, 6-chloronicotinic acid, N'cyano-N-methyl acetamidine and N'-cyano acetamidine as the major transformation products (TPs), all of them formed through amine α carbon oxidation in combination with hydrolysis. Microtox bioassays revealed an increase in the toxicity of the medium during ACMP ozonation process, followed by a decrease to relatively low values. These changes could be attributed to the synergistic effects between TPs as well as to the presence of toxic intermediate aldehydes. Even though adopting strategies to further promote ozone decomposition to hydroxyl radicals appears to be essential, ozonation can be an effective treatment process for ACMP removal and associated toxicity abatement.

Modeling the pH and temperature dependence of aqueousphase hydroxyl radical reaction rate constants of organic micropollutants using QSPR approach

Designing of advanced oxidation process (AOP) requires knowledge of the aqueous phase hydroxyl radical (^●OH) reactions rate constants (k _OH), which are strictly dependent upon the pH and temperature of the medium. In this study, pH- and temperature-dependent quantitative structure-property relationship (QSPR) models based on the decision tree boost (DTB) approach were developed for the prediction of k _OH of diverse organic contaminants following the OECD guidelines. Experimental datasets (n = 958) pertaining to the k _OH values of aqueous phase reactions at different pH (n = 470; 1.4 × 10⁶ to 3.8 × 10¹⁰ M^-1 s^-1) and temperature (n = 171; 1.0 × 10⁷ to 2.6 × 10¹⁰ M^-1 s^-1) were considered and molecular descriptors of the compounds were derived. The Sanderson scale electronegativity, topological polar surface area, number of double bonds, and halogen atoms in the molecule, in addition to the pH and temperature, were found to be the relevant predictors. The models were validated and their external predictivity was evaluated in terms of most stringent criteria parameters derived on the test data. High values of the coefficient of determination (R ²) and small root mean squared error (RMSE) in respective training (> 0.972, ≤ 0.12) and test (≥ 0.936, ≤ 0.16) sets indicated high generalization and predictivity of the developed QSPR model. Other statistical parameters derived from the training and test data also supported the robustness of the models and their suitability for screening new chemicals within the defined chemical space. The developed QSPR models provide a valuable tool for predicting the ^●OH reaction rate constants of emerging new water contaminants for their susceptibility to AOPs.

Exploring ozonation as treatment alternative for methiocarb and formed transformation products abatement

Despite the high toxicity and resistance to conventional water treatments exhibited by methiocarb (MC), there are no reports regarding the degradation of this priority pesticide by means of alternative purification technologies. In this work, the removal of MC by means of ozonation was studied for the first time, employing a multi-reactor methodology and neutral pH conditions. The second-order rate constants of MC reaction with molecular ozone (O₃) and formed hydroxyl radicals (OH·) were determined to be 1.7·10⁶ and 8.2·10⁹ M^-1 s^-1, respectively. During degradation experiments, direct ozone reaction was observed to effectively remove MC, but not its formed intermediates, whereas OH· could oxidize all species. The major identified TPs were methiocarb sulfoxide (MCX), methiocarb sulfoxide phenol (MCXP) and methiocarb sulfone phenol (MCNP), all of them formed through MC oxidation by O₃ or OH· in combination with hydrolysis. A toxicity assessment evidenced a strong dependence on MCX concentration, even at very low values. Despite the OH· capability to degrade MC and its main metabolites, the relative resistance of TPs towards ozone attack enlarged the oxidant dosage (2.5 mg O₃/mg DOC) necessary to achieve a relatively low toxicity of the medium. Even though ozonation could be a suitable technique for MC removal from water compartments, strategies aimed to further promote the indirect contribution of hydroxyl radicals during this process should be explored.

Phototransformation of Acesulfame K in surface waters: Comparison of two techniques for the measurement of the second-order rate constants of indirect photodegradation, and modelling of photoreaction kinetics

By use of photochemical modelling we show that acesulfame K (ACE) can undergo photodegradation in surface waters, mainly by reaction with OH and possibly ³CDOM* (the triplet states of chromophoric dissolved organic matter). With the possible exception of shallow water bodies containing low dissolved organic carbon, we predict ACE to be a refractory compound in environmental waters which agrees well with many literature reports. We used two methods to measure the photoreactivity parameters of ACE, of which one is based on the monitoring of the time evolution of ACE alone and the other is based on the monitoring of both ACE and a reference compound (hereafter, they are referred to as substrate-only and substrate + reference method, respectively). The substrate + reference method can be time-saving, but it is potentially prone to interferences. In this work, ibuprofen and atrazine were used as reference compounds of known behaviour to study the photoreactivity of ACE by competition kinetics in the substrate + reference method. The two methods gave overall comparable results, partially because two different reference compounds instead of only one were used in the substrate + reference method. By so doing, however, one loses part of the time-saving advantage of the substrate + reference method.

Cosmetic wastewater treatment by the ZVI/H2O2 process

The ZVI/H₂O₂ process was applied for cosmetic wastewater treatment. Two commercial zero-valent iron (ZVI) types with different granulations were chosen: Hepure Ferrox PRB and Hepure Ferrox Target. In addition, the pH and stirring method influence on ZVI/H₂O₂ process efficiency was studied. During the ZVI and ZVI/H₂O₂ processes, linear Fe ions concentration increase was observed. The addition of H₂O₂ significantly accelerated the iron dissolution process. The highest COD removal was obtained using finer ZVI (Hepure Ferrox Target) for doses of reagents ZVI/H₂O₂ 1500/1600 mg/L, in a H₂O₂/COD weight ratio 2:1, at pH 3.0 with stirring on a magnetic stirrer. After 120 min of the process, 84.0% COD removal (from 796 to 127 mg/L) was achieved. It was found that the efficiency of the process depends, as in the case of the Fenton process, on the ratio of the reagents (ZVI/H₂O₂) and their dose in relation to the COD (H₂O₂/COD) but does not depend on the dose of the iron itself. Statistical analysis confirms that COD removal efficiency depends primarily on H₂O₂/COD ratio and ZVI granulation, but ZVI dose influence is not statistically significant. The head space, solid-phase microextraction, gas chromatography, mass spectrometry results confirm high efficiency of the ZVI/H₂O₂ process.

Modeling the aqueous phase reactivity of hydroxyl radical towards diverse organic micropollutants: An aid to water decontamination processes

The rate constants of the hydroxyl radical reactions (k_OH) with organic micropollutants (OMPs) in aqueous medium are important in designing the advanced oxidation processes (AOPs) for their removal. In this study, a quantitative structure-property relationship (QSPR) model for the prediction of k_OH of diverse and emerging OMPs was developed in accordance with the OECD guidelines. A large experimental data set (n = 995) comprised of compounds with k_OH values ranging from 7.9 × 10⁵ to 6.8 × 10¹⁰ M^-1 s^-1 was considered and several molecular descriptors were calculated. As a result, five descriptors were found to be important in predicting the k_OH values which related to the electronegativity, topological polar surface area, double bonds, average molecular weight, and halogen atoms in the molecule. The optimal model was validated internally and externally and several statistical stringent parameters were derived. High values of the coefficient of determination (R²) and small root mean squared error (RMSE) in the training (0.954; 0.17) and validation (0.925; 0.14) sets indicated high generalization and predictivity of the developed model. Other statistical parameters derived from the training and validation data also supported the robustness of the model. The proposed model outperformed the earlier QSARs reported for k_OH prediction. Overall, the developed QSPR model provides a valuable tool for an initial assessment of the susceptibility of organic micropollutants to AOPs.

Enhanced antibacterial activity of silica nanorattles with ZnO combination nanoparticles against methicillin-resistant Staphylococcus aureus

Silica nanorattles (SNs) with zinc oxide (ZnO) combination nanoparticles are reported to inhibit methicillin-resistant Staphylococcus aureus (MRSA) for the first time. SNs loaded with ZnO nanoparticles, which can produce free radicals, can cause severe damage to bacteria. ZnO nanoparticles not only provide free radicals in the combined nanostructures, which can inhibit the growth of bacteria, but also form nanorough surfaces with an irregular distribution of spikes on the SNs, which can enhance their adhesion to bacteria. Nanorough silica shell surfaces maintain the high activity and stability of small-sized ZnO nanoparticles and gather ZnO nanoparticles together to enhance production, which improves the efficiency of free radicals against the cytomembranes of bacterial cells. The enhanced adhesion of ZnO@SN nanoparticles to MRSA cells shortens the effective touching distance between free radicals and MRSA, which also improves antibacterial activity. As we expected, the ZnO@SN nanoparticles exhibit a better antibacterial effect than free ZnO nanoparticles against MRSA in vitro and in vivo. We also demonstrate that SNs loaded with ZnO nanoparticles can accelerate wound healing in MRSA skin inflammation models. This method of multilevel functionalization will be potentially applicable to the antibacterial field.

Can radiation chemistry supply a highly efficient AO(R)P process for organics removal from drinking and waste water? A review

The increasing role of chemistry in industrial production and its direct and indirect impacts in everyday life create the need for continuous search and efficiency improvement of new methods for decomposition/removal of different classes of waterborne anthropogenic pollutants. This review paper addresses a highly promising class of water treatment solutions, aimed at tackling the pressing problem of emerging contaminants in natural and drinking waters and wastewater discharges. Radiation processing, a technology originating from radiation chemistry studies, has shown encouraging results in the treatment of (mainly) organic water pollution. Radiation ("high energy") processing is an additive-free technology using short-lived reactive species formed by the radiolysis of water, both oxidative and reducing, to carry out decomposition of organic pollutants. The paper illustrates the basic principles of radiolytic treatment of organic pollutants in water and wastewaters and specifically of one of its most practical implementations (electron beam processing). Application examples, highlighting the technology's strong points and operational conditions are described, and a discussion on the possible future of this technology follows.

Photochemical Water-Splitting with Organomanganese Complexes

Certain organometallic chromophores with water-derived ligands, such as the known [Mn(CO)₃(μ₃-OH)]₄ (1) tetramer, drew our attention as possible platforms to study water-splitting reactions. Herein, we investigate the UV irradiation of various tricarbonyl organomanganese complexes, including 1, and demonstrate that dihydrogen, CO, and hydrogen peroxide form as products in a photochemical water-splitting decomposition reaction. The organic and manganese-containing side products are also characterized. Labeling studies with ¹⁸O-1 suggest that the source of oxygen atoms in H₂O₂ originates from free water that interacts with 1 after photochemical dissociation of CO (1-CO) constituting the oxidative half-reaction of water splitting mediated by 1. Hydrogen production from 1 is the result of several different processes, one of which involves the protons derived from the hydroxido ligands in 1 constituting the reductive half-reaction of water splitting mediated by 1. Other processes that generate H₂ are also operative and are described. Collectively the results from the photochemical decomposition of 1 provide an opportunity to propose a mechanism, and it is discussed within the context of developing new strategies for water-splitting reactions with organomanganese complexes.

Contaminants of emerging concern: a review of new approach in AOP technologies

The presence of contaminants of emerging concern (CECs) such as pharmaceuticals and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), flame retardants (FRs), pesticides, and artificial sweeteners (ASWs) in the aquatic environments remains a major challenge to the environment and human health. In this review, the classification and occurrence of emerging contaminants in aquatic environments were discussed in detail. It is well documented that CECs are susceptible to poor removal during the conventional wastewater treatment plants, which introduce them back to the environment ranging from nanogram per liter (e.g., carbamazepine) up to milligram per liter (e.g., acesulfame) concentration level. Meanwhile, a deep insight into the application of advanced oxidation processes (AOPs) on mitigation of the CECs from aquatic environment was presented. In this regard, the utilization of various treatment technologies based on AOPs including ozonation, Fenton processes, sonochemical, and TiO₂ heterogeneous photocatalysis was reviewed. Additionally, some innovations (e.g., visible light heterogeneous photocatalysis, electro-Fenton) concerning the AOPs and the combined utilization of AOPs (e.g., sono-Fenton) were documented.

Enhanced Phototherapy by Nanoparticle-Enzyme via Generation and Photolysis of Hydrogen Peroxide

Light has been widely used for cancer therapeutics such as photodynamic therapy (PDT) and photothermal therapy. This paper describes a strategy called enzyme-enhanced phototherapy (EEPT) for cancer treatment. We constructed a nanoparticle platform by covalent conjugation of glucose oxidase (GOx) to small polymer dots, which could be persistently immobilized into a tumor. While the malignant tumors have high glucose uptake, the GOx efficiently catalyzes the glucose oxidation with simultaneous generation of H₂O₂. Under light irradiation, the in situ generated H₂O₂ was photolyzed to produce hydroxyl radical, the most reactive oxygen species, for killing cancer cells. In vitro assays indicated that the cancer cells were destroyed by using a nanoparticle concentration at 0.2 μg/mL and a light dose of ∼120 J/cm², indicating the significantly enhanced efficiency of the EEPT method when compared to typical PDT that requires a photosensitizer of >10 μg/mL for effective cell killing under the same light dose. Furthermore, remarkable inhibition of tumor growth was observed in xenograft-bearing mice, indicating the promise of the EEPT approach for cancer therapeutics.

Photochemical Formation of Nitrite and Nitrous Acid (HONO) upon Irradiation of Nitrophenols in Aqueous Solution and in Viscous Secondary Organic Aerosol Proxy

Irradiated nitrophenols can produce nitrite and nitrous acid (HONO) in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high-solute-strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10^-5-10^-4 in the case of 4-nitrophenol) and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite and HONO. The nitrophenols giving the highest photoproduction rates of nitrite and HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew-water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.

Does a Nitrogen Lone Pair Lead to Two Centered-Three Electron (2c-3e) Interactions in Pyridyl Radical Isomers?

Each of the three isomeric pyridyl radicals (2-, 3-, and 4-dehydropyridines) contains a lone pair and an unpaired electron. As a result, a potential two centered-three electron interaction between the radical electron and the lone pair through-space (TS) and/or through-bond (TB) can exist that may influence the stability of the radicals. Due to the change in geometrical positions relative to each other, the strength of interaction can be varied. In this study, we investigated the structural and stability aspects of pyridyl radical isomers with a major emphasis on the interaction of a nitrogen lone pair with the radical center. In order to obtain evidence for such interactions, protonated and N-oxide analogues of the corresponding isomeric pyridyl radicals have been considered in such a way to understand the consequences due to unavailability of the lone pair. Similarly, electron attachment and detachment energies at the radical center (vertical detachment energy, VDE, of corresponding anions and vertical ionization energy, VIE, of radical isomers) have been calculated to find out the interaction trend upon modification at the radical center. Different levels of theory including (U)B3LYP/cc-pVTZ, (U)M06/cc-pVTZ, CBS-QB3, single-point energy calculations at (U)CCSD(T)/cc-pVTZ, and multireference CASSCF/cc-pVTZ methods have been employed in this regard. A closer inspection of geometries, relative stability order, spin density, electrostatic potential, molecular orbitals, NBO analysis, and vibrational analysis have showed a strong and stabilizing TS interaction between the radical center and the lone pair in the case of the 2-pyridyl radical. On the other hand, the 4-pyridyl radical showed stabilizing interactions only via TB coupling, whereas the TS interaction is nonexistent. Despite the presence of both interactions in the case of the 3-pyridyl radical, their overall influence is less effective toward stability.

Oxygen Vacancy Associated Surface Fenton Chemistry: Surface Structure Dependent Hydroxyl Radicals Generation and Substrate Dependent Reactivity

Understanding the chemistry of hydrogen peroxide (H₂O₂) decomposition and hydroxyl radical (•OH) transformation on the surface molecular level is a great challenge for the application of heterogeneous Fenton system in the fields of chemistry, environmental, and life science. We report in this study a conceptual oxygen vacancy associated surface Fenton system without any metal ions leaching, exhibiting unprecedented surface chemistry based on the oxygen vacancy of electron-donor nature for heterolytic H₂O₂ dissociation. By controlling the delicate surface structure of catalyst, this novel Fenton system allows the facile tuning of •OH existing form for targeted catalytic reactions with controlled reactivity and selectivity. On the model catalyst of BiOCl, the generated •OH tend to diffuse away from the (001) surface for the selective oxidation of dissolved pollutants in solution, but prefer to stay on the (010) surface, reacting with strongly adsorbed pollutants with high priority. These findings will extend the scope of Fenton catalysts via surface engineering and consolidate the fundamental theories of Fenton reactions for wide environmental applications.

Microbial response to environmental stresses: from fundamental mechanisms to practical applications

Environmental stresses are usually active during the process of microbial fermentation and have significant influence on microbial physiology. Microorganisms have developed a series of strategies to resist environmental stresses. For instance, they maintain the integrity and fluidity of cell membranes by modulating their structure and composition, and the permeability and activities of transporters are adjusted to control nutrient transport and ion exchange. Certain transcription factors are activated to enhance gene expression, and specific signal transduction pathways are induced to adapt to environmental changes. Besides, microbial cells also have well-established repair mechanisms that protect their macromolecules against damages inflicted by environmental stresses. Oxidative, hyperosmotic, thermal, acid, and organic solvent stresses are significant in microbial fermentation. In this review, we summarize the modus operandi by which these stresses act on cellular components, as well as the corresponding resistance mechanisms developed by microorganisms. Then, we discuss the applications of these stress resistance mechanisms on the production of industrially important chemicals. Finally, we prospect the application of systems biology and synthetic biology in the identification of resistant mechanisms and improvement of metabolic robustness of microorganisms in environmental stresses.

Degradation of Methyl 2-Aminobenzoate (Methyl Anthranilate) by H₂O₂/UV: Effect of Inorganic Anions and Derived Radicals

This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H₂O₂. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H₂O₂ concentration reaches a plateau because of the combined effects of H₂O₂ absorption saturation and ·OH scavenging by H₂O₂. The addition of chloride ions causes scavenging of ·OH, yielding Cl₂·⁻ as the most likely reactive species, and it increases the MA photodegradation rate at high H₂O₂ concentration values. The reaction between Cl₂·⁻ and MA, which has second-order rate constant kCl2•−+MA = (4.0 ± 0.3) × 10⁸ M⁻¹·s⁻¹ (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H₂O₂, because Cl₂·⁻ undergoes more limited scavenging by H₂O₂ compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO₃·⁻ (kCO3•−+MA = (3.1 ± 0.2) × 10⁸ M⁻¹·s⁻¹), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H₂O₂ to partially occur as HO₂⁻, which reacts very quickly with either ·OH or CO₃·⁻ to produce O₂·⁻. The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO₄·⁻ that reacts effectively with MA (kSO4•−+MA = (5.6 ± 0.4) × 10⁹ M⁻¹·s⁻¹). Irradiated H₂O₂ is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans).

Hydrogen peroxide (H2O2) irreversibly inactivates creatine kinase from Pelodiscus sinensis by targeting the active site cysteine

Creatine kinase (EC 2.7.3.2, CK) plays an important role in cellular energy metabolism and homeostasis by catalysing the transfer of phosphate between ATP and creatine phosphate. In this study, we investigated the effects of H₂O₂ on PSCKM (muscle type creatine kinase from Pelodiscus sinensis) by the integrating method between enzyme kinetics and docking simulations. We found that H₂O₂ strongly inactivated PSCKM (IC₅₀=0.25mM) in a first-order kinetic process, and targeted the active site cysteine directly. A conformational study showed that H₂O₂ did not induce the tertiary structural changes in PSCKM with no extensive exposure of hydrophobic surfaces. Sequential docking simulations between PSCKM and H₂O₂ indicated that H₂O₂ interacts with the ADP binding region of the active site, consistent with experimental results that demonstrated H₂O₂-induced inactivation. Our study demonstrates the effect of H₂O₂ on PSCKM enzymatic function and unfolding, and provides important insight into the changes undergone by this central metabolic enzyme in ectothermic animals in response to the environment.

2,4-D abatement from groundwater samples by photo-Fenton processes at circumneutral pH using naturally iron present. Effect of inorganic ions

This study evaluated, at laboratory scale, if the using iron naturally present (0.3 mg L^-1) and adding 10 mg L^-1 of hydrogen peroxide was effective to remove 24.3 mgL^-1 of 2,4-dichlorophenoxyacetic acid (2,4-D) from groundwater samples by simulated solar irradiation (global intensity = 300 W m^-2). Under these conditions, the degradation of 2,4-D reached 75.2 % and the apparition of its main oxidation byproduct 2,4-dichlorophenol (DCP) was observed. On the other hand, pH exhibited an increasing from 7.0 to 8.3 during the experiment. Experiments using Milli-Q water at pH 7.0, iron, and H₂O₂ concentrations of 0.3 and 10 mg L^-1, respectively, were carried out in order to study the effect of ions such as carbonate species, phosphate, and fluoride in typical concentrations often found in groundwater. Ion concentrations were combined by using a factorial experimental design 2³. Results showed that carbonates and fluoride did not produce a detrimental effect on the 2,4-D degradation, while phosphate inhibited the process. In this case, the pH increased also from 7.0 to 7.95 and 8.99. Effect of parameters such as pH, iron concentration, and hydrogen peroxide concentration on the 2,4-D degradation by the photo-Fenton process in groundwater was evaluated by using a factorial experimental design 2³. Results showed that the pH was the main parameter affecting the process. This study shows for the first time that using the photo-Fenton process at circumneutral pH and iron naturally present seems to be a promising process to remove pesticides from groundwater.

UV-based technologies for marine water disinfection and the application to ballast water: Does salinity interfere with disinfection processes?

Water contained on ships is employed in the majority of activities on a vessel; therefore, it is necessary to correctly manage through marine water treatments. Among the main water streams generated on vessels, ballast water appears to be an emerging global challenge (especially on cargo ships) due to the transport of invasive species and the significant impact that the ballast water discharge could have on ecosystems and human activities. To avoid this problem, ballast water treatment must be implemented prior to water discharge in accordance with the upcoming Ballast Water Management Convention. Different UV-based treatments (photolytic: UV-C and UV/H₂O₂, photocatalytic: UV/TiO₂), have been compared for seawater disinfection. E. faecalis is proposed as a biodosimeter organism for UV-based treatments and demonstrates good properties for being considered as a Standard Test Organism for seawater. Inactivation rates by means of the UV-based treatments were obtained using a flow-through UV-reactor. Based on the two variables responses that were studied (kinetic rate constant and UV-Dose reductions), both advanced oxidation processes (UV/H₂O₂ and photocatalysis) were more effective than UV-C treatment. Evaluation of salinity on the processes suggests different responses according to the treatments: major interference on photocatalysis treatment and minimal impact on UV/H₂O₂.

Modelling the photochemical attenuation pathways of the fibrate drug gemfibrozil in surface waters

Gemfibrozil (GFZ) is a relatively persistent pollutant in surface-water environments and it is rather recalcitrant to biological degradation. The GFZ photochemical lifetimes are relatively short in shallow waters with low levels of dissolved organic carbon (DOC), but they can reach the month-year range in deep and high-DOC waters. The main reason is that GFZ undergoes negligible reaction with singlet oxygen or degradation sensitised by the triplet states of chromophoric dissolved organic matter, which are the usually prevalent photochemical pathways in deep and high-DOC sunlit waters. Nitrate and nitrite scarcely affect the overall GFZ lifetimes, but they can shift photodegradation from direct photolysis to the OH process. These two pathways are the main GFZ phototransformation routes, with the direct photolysis prevailing in shallow environments during summer. Under these conditions the GFZ photochemical lifetimes are also shorter and the environmental significance of photodegradation correspondingly higher. The direct photolysis of GFZ under UVB irradiation yielded several transformation intermediates deriving from oxidation or cleavage of the aliphatic lateral chain. A quinone derivative (2,5-dimethyl-1,4-benzoquinone), a likely oxidation product of the transformation intermediate 2,5-dimethylphenol, is expected to be the most acutely and chronically toxic compound arising from GFZ direct photolysis. Interestingly, literature evidence suggests that the same toxic intermediate would be formed upon OH reaction.

On the summertime air quality and related photochemical processes in the megacity Shanghai, China

Summertime surface ozone (O₃) and related secondary formation of fine particles are the major air quality concerns in the megacity of Shanghai. We performed mobile Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) measurements to investigate the spatial distribution of on-road nitrogen dioxide (NO₂) concentrations along the Inner Ring Elevated Road (IRER) in Shanghai. The observations show a ratio of 4 between CE-DOAS averaged on-road NO₂ and the in-situ ambient measurements, illustrating the strong impact of vehicle emissions over the urban area. The air mass transport analysis suggests that the observed episodic ozone events arise from both the abundance of volatile organic compounds (VOCs) precursors in the sampled plume and the regional transport of ozone-rich air masses. Analysis of the sources of PM_2.5 shows that the secondary heterogeneous gas-to-particle conversion of sulfate and nitrate from sulfur dioxide (SO₂) and nitrogen oxides (NO_x) is the largest source of PM_2.5 contributing 44.8±9.2% of the total PM_2.5. Ozone-related photochemical formation of fine particles is estimated to contribute about 22.5±11.9% of the total PM_2.5, which is strongly facilitated by solar radiation in summer. According to our results, nitrous acid (HONO) is the major precursor of hydroxyl radicals (OH) accounting for 40% to 80% of the total OH production during daytime. A significant correlation is found between the HONO levels in the early morning and the daily O₃ and PM_2.5 levels. The summertime measurements indicate that the photolytic reaction of HONO after sunrise increased the abundance of daytime OH and oxidative capacity, resulting in an enhancement of ground level ozone and secondary organic aerosol formation. This study provides quantitative information to better understand photochemical formation of ozone and fine particles in Shanghai during summertime, which is useful for designing collaborative strategies to mitigate emissions of precursor pollutants.

Photosensitized degradation of acetaminophen in natural organic matter solutions: The role of triplet states and oxygen

The photolysis of acetaminophen, a widely used pharmaceutical, in simulated natural organic matter solutions was investigated. The triplet states of natural organic matter (³NOM*) were found to play the dominant role in its photodegradation, while the contributions from hydroxyl radicals and singlet oxygen were negligible. Dissolved oxygen (DO) plays a dual role. From anaerobic to microaerobic (0.5 mg/L DO) conditions, the degradation rate of acetaminophen increased by 4-fold. That suggests the involvement of DO in reactions with the degradation intermediates. With increasing oxygen levels to saturated conditions (26 mg/L DO), the degradation rate became slower, mainly due to DO's quenching effect on ³NOM*. Superoxide radical (O₂^-) did not react with acetaminophen directly, but possibly quenched the intermediates to reverse the degradation process. The main photochemical pathways were shown to involve phenoxyl radical and N-radical cations, finally yielding hydroxylated derivatives, dimers and nitrosophenol. A reaction mechanism involving ³NOM*, oxygen and O₂^- is proposed.

Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

The dual descriptor (Δf) data of azo form (a, RR2) and hydrazone form (b, HRR2) of RR2 dianion. For Δf> 0 (green), the site is favorable for nucleophilic attack, for Δf< 0 (blue), the site is favorable for electrophilic attack. Key bond lengths in Å.

Remarkable enhancement of Fenton degradation at a wide pH range promoted by thioglycolic acid

Thioglycolic acid efficiently recycles Fe(ii) and significantly enhances the Fenton degradation of organic and microbial pollutants at a broad pH range.

Reaction of SO2with OH in the atmosphere

We theoretically investigate the rate constantk(T,p) of the OH + SO₂reaction with experimental accuracy.