Photocatalytic degradation of aromatic pollutants: a pivotal role of conduction band electron in distribution of hydroxylated intermediates

Characteristics and degradation mechanisms of polychlorinated naphthalenes in surface soil in Yangtze River Delta, China

Both thermal and environmental processes are significant factors influencing the existing characteristics, e.g., congener distributions, and existing levels, of polychlorinated naphthalenes (PCNs) in the environment. Soil plays an important role in the life cycle of PCNs, but degradation of PCNs in soils has never been reported. In this study, we collected surface soil samples from 13 cities in the Yangtze River Delta, which is one of the most crowded areas of China and analyzed the samples for 75 PCNs. The long-range transportation from polluted areas was the major source for PCNs in remote areas, but the PCN profiles in remote areas reported in our previous studies were different from those in human settlement in this study, indicating there is a transformation of PCNs after emissions from anthropogenic activities. Two experiments were then designed to reveal the degradation mechanisms, including influencing factors, products, and pathways, of PCNs in surface soils. Based on the experiments, we found that the major factor driving the losses of PCNs in surface soils was volatilization, followed by photo irradiation and microbial metabolism. Under photo-irradiation, the PCN structures would be destroyed through a process of dechlorination followed by oxidation. In addition, the dechlorination pathways of PCNs have been established and found to be significantly influenced by the structure-related parameters.

Hydroxylated transformation products of pharmaceutical active compounds: Generation from processes used in wastewater treatment plants and its environmental monitoring

Pharmaceutical active compounds (PhACs) are organic pollutants detected in wastewater and aquatic environments worldwide in concentrations ranging from ng L-1 to μg L-1. Wastewater effluents containing PhACs residues is discharged in municipal sewage and, subsequently collected in municipal wastewater treatment plants (WWTPs) where are not entirely removed. Thus, PhACs and its transformation products (TPs) are discharged into water bodies. In the current work, the transformation of PhACs under treatments used in municipal WWTPs such as biological, photolysis, chlorination, and ozonation processes was reviewed. Data set of the major transformation pathways were obtained of studies that performed the PhACs removal and TPs monitoring during batch-scale experiments using gas and liquid chromatography coupled with tandem mass spectrometry (GC/LC-MS/MS). Several transformation pathways as dealkylation, hydroxylation, oxidation, acetylation, aromatic ring opening, chlorination, dehalogenation, photo-substitution, and ozone attack reactions were identified during the transformation of PhACs. Especially, hydroxylation reaction was identified as transformation pathway in all the processes. During the elucidation of hydroxylated TPs several isobaric compounds as monohydroxylated and dihydroxylated were identified. However, hydroxylated TPs monitoring in wastewater and aquatic environments is a topic scarcely studied due to that has no environmental significance, lack of available analytic standars of hydroxylated TPs and lack of analytic methods for their identification. Thus, screening strategy for environmental monitoring of hydroxylated TPs was proposed through target and suspect screening using GC/LC-MS/MS systems. In the next years, more studies on the hydroxylated TPs monitoring are necessary for its detection in WWTPs effluents as well as studies on their environmental effects in aquatic environments.

New insight into enhanced photocatalytic selectivity of g-C3N4 by nitrogen vacancy introduction: Experimental study and theoretical calculation

Constructing photocatalyst with both high efficiency and selectivity is highly desired in water treatment process. However, it is difficult to realize the selectivity of photocatalysis due to the non-selective oxidative species produced in this process. Herein, for the first time, the photocatalytic selectivity was achieved on g-C₃N₄ (CN) through N vacancy introduction for effective removal of organic pollutants, and the mechanism of vacancy induced selectivity enhancement was studied. The nitrogen vacancy modified CN (VCN) showed enhanced photocatalytic activity and unique selectivity towards phenolic compounds with electron-donating group, whose kinetic constant for p-aminophenol (p-NH₂) degradation was 5.95 times higher than that over CN. Moreover, VCN photocatalytic system also displayed similar selectivity in binary pollutant systems. Characteristics and theoretical calculation results confirmed the enhanced photocatalytic performance and selectivity of VCN was mainly attributed to the effect of N vacancy. On one hand, electron-deficient N vacancy enhanced the adsorption of the O₂ and phenolic compounds, which promoted the production of O₂^•- and strengthened the photocatalytic surface reaction. On the other hand, the N vacancy preferred to adsorb the electron-donating groups of phenolic compounds, which resulted in their selective removal.

Atomic-level insight into effect of substrate concentration and relative humidity on photocatalytic degradation mechanism of gaseous styrene

Substrate concentration and relative humidity (RH) impact the photocatalytic efficiency of industrial aromatic hydrocarbons, but how they influence intermediate formation and degradation pathway remains unclear. With the help of oxygen isotope tracing method, the effects of these two environmental parameters on degradation mechanism of styrene were revealed at atomic level. Increasing styrene concentration favored product formation, which was however inhibited by RH elevation. Gaseous products were not directly formed in gaseous phase, but originated from desorption of interfacial intermediates. The volatile aldehydes and furans further exchanged their ¹⁶O with ¹⁸O in H₂¹⁸O. Increase of RH showed higher enhancement on ¹⁸O distribution in all products and pathways than that of substrate concentration. Low RH preferred high generation of ¹⁶O₂^•- and ⁽¹⁶⁾¹O₂, dominating reaction to form 1-phenyl-1,2-ethandiol, 2-hydroxy-1-phenyl-ethanon and phenylglyoxal monohydrate in sequence. Successive production of benzyl alcohol, benzaldehyde and benzoic acid through the reaction of styrene with promoted ^•18OH by increasing RH became predominant. Hydration was firstly observed and confirmed as an important gaseous transformation step of aldehyde and furan products. Our findings provide a deep insight into photocatalytic degradation mechanism of aromatic hydrocarbons regulated by environmental parameters to further improve their industrial purification efficiency, and are helpful predicting environmental geochemistry fate of organics and preventing their negative impact on natural environment.

Carbon doped Fe3O4 peroxidase-like nanozyme for mitigating the membrane fouling by NOM at neutral pH

Oxidation is a widely used method in drinking water treatment to mitigate the membrane fouling caused by the natural organic matters (NOM) from the surface water during ultra-filtration (UF) and nano-filtration (NF) processes, and H₂O₂ is one of the common oxidants for it. However, the oxidation capability of H₂O₂ at neutral pH is lower, compared to the acidic and alkaline conditions. In order to improve the efficiency of NOM oxidation at neutral pH, a carbon-doped Fe₃O₄ peroxidase-like nanozyme (CFPN) was synthesized in this study and used as a high-performance catalyst for H₂O₂ to generate hydroxyl radical. The oxygen-containing groups on the carbon structure of CFPN can form an acidic microenvironment, allowing H₂O₂ to produce hydroxyl radical by catalysis in neutral conditions. The results of hydrophilicity analysis, zeta potential, high-performance liquid size exclusion chromatography (HPSEC), Fourier transform infrared spectrum (FTIR) and flux indicated that the hydroxyl radical can oxidize the hydrophobic matters of humic acid (HA) into hydrophilic matters by Fenton reaction or electrophilic addition reaction, which can mitigate the fouling of NF membranes. The results of the same test for the bovine serum albumin (BSA) indicated that the hydroxyl radical can mitigate the fouling of UF membranes by degrading the tertiary and secondary structures of BSA and partly oxidizing the side chain groups. In addition, two types of surface water samples were used to verify the above mechanism, and the results indicated that the hydroxyl radical treatment at neutral pH is a new viable and effective strategy to significantly mitigate the NOM fouling of UF and NF membranes.

Modifications of heterogeneous photocatalysts for hydrocarbon C–H bond activation and selective conversion

This feature article summarizes the recent progress in the modification of heterogeneous photocatalysts for photocatalytic hydrocarbons’ C–H bond activation.

Cu2O@β-cyclodextrin as a synergistic catalyst for hydroxyl radical generation and molecular recognitive destruction of aromatic pollutants at neutral pH

Wastewater systems contain a large number of compounds, such as anthropogenic aromatic pollutants and natural organic matter (NOM), and usually have pH higher than 4. Fenton-like reaction is the most widespread method for removal of organic pollutants, but their reactivity with H₂O₂ may be inhibited by NOM due to the competition of hydroxyl radicals and chelating agents. In this work, Cu₂O@β-cyclodextrin was developed to achieve the collaboration between molecular recognition and Fenton-like catalysis to destruct aromatic pollutants at neutral pH. In Cu₂O@β-CD, covalent CuOC bond was topotaxially converted from CuCl assisted by β-CD at room temperature. Covalently linked β-CD could keep humic acid from interfering catalytic performance of Cu₂O surfaces and inhibit the leaching of copper. A higher catalytic ability was observed for Cu₂O@β-CD with rate constant 0.0331 min^-1 than Cu₂O (0.0064 min^-1) at neutral pH. A mechanism of synergistic catalysis was proposed on the basis of Cu⁺, β-CD and phenoxo-Cu²⁺ complexes in the Cu₂O@β-CD/BPA/H₂O₂ system. The strategy of coupling molecular recognition into Fenton-like reaction provides an efficient and promising approach to the destruction of aromatic pollutants at neutral pH.

Enhanced Photocatalytic Degradation Performance by Fluid-Induced Piezoelectric Field

The introduction of a piezoelectric field has been proven a promising method to enhance photocatalytic activity by preventing photoelectron-hole recombination. However, the formation of a piezoelectric field requires additional mechanical force or high-frequency ultrasonic baths, which limits its potential application on industrial scale. Therefore, it is of great practical significance to design the catalyst that can harvest the discrete energy such as the fluid mechanical energy to form the electric field. Herein, PZT/TiO₂ catalyst with a core-shell configuration was prepared by a simple coating method. By collecting the mechanical energy of water, an internal piezoelectric field was induced. Under 800 rpm stirring, transient photocurrent measured on PZT/TiO₂ electrode is about 1.7 times higher than that of 400 rpm. Correspondingly, the photocatalytic degradation rate and mineralization efficiency of RhB, BPA, phenol, p-chlorophenol much improved, showing the promoting effect of piezoelectric field generated directly from harvesting the discrete fluid mechanical energy.

TiO2 photocatalysis for C–C bond formation

Synergistic utilization of TiO₂-photo-generated holes and electrons is a potential protocol for catalytic C–C bond formation reactions.

Reactions of hydroxyl radicals with benzoic acid and benzoate

Density functional theory was used to study the mechanism and kinetics of benzoic acid with hydroxyl radicals in both gas and aqueous phases as well as benzoate with hydroxyl radicals in the aqueous phase at the M06-2X/6-311+G(d,p) level of theory.

Emerging trends in photodegradation of petrochemical wastes: a review

Various human activities like mining and extraction of mineral oils have been used for the modernization of society and well-beings. However, the by-products such as petrochemical wastes generated from such industries are carcinogenic and toxic, which had increased environmental pollution and risks to human health several folds. Various methods such as physical, chemical and biological methods have been used to degrade these pollutants from wastewater. Advance oxidation processes (AOPs) are evolving techniques for efficient sequestration of chemically stable and less biodegradable organic pollutants. In the present review, photocatalytic degradation of petrochemical wastes containing monoaromatic and poly-aromatic hydrocarbons has been studied using various heterogeneous photocatalysts (such as TiO₂, ZnO and CdS. The present article seeks to offer a scientific and technical overview of the current trend in the use of the photocatalyst for remediation and degradation of petrochemical waste depending upon the recent advances in photodegradation of petrochemical research using bibliometric analysis. We further outlined the effect of various heterogeneous catalysts and their ecotoxicity, various degradation pathways of petrochemical wastes, the key regulatory parameters and the reactors used. A critical analysis of the available literature revealed that TiO₂ is widely reported in the degradation processes along with other semiconductors/nanomaterials in visible and UV light irradiation. Further, various degradation studies have been carried out at laboratory scale in the presence of UV light. However, further elaborative research is needed for successful application of the laboratory scale techniques to pilot-scale operation and to develop environmental friendly catalysts which support the sustainable treatment technology with the "zero concept" of industrial wastewater. Nevertheless, there is a need to develop more effective methods which consume less energy and are more efficient in pilot scale for the demineralization of pollutant.

H2O-involved two-electron pathway for photooxidation of aldehydes on TiO2: an isotope labeling study

Aldehyde pollution has been receiving increasing environmental concern recently. In this study, the photooxidation of aldehydes to carboxylates on the TiO2 surface was studied by an oxygen-isotope-labeling method. The solvent H2O was found to contribute much to the oxygen source of the formed carboxylate, which cannot be explained if the conventional O2-involved free radical chain reaction is the only mechanism for the photocatalytic oxidation of aldehydes. We also observed that, unlike in the TiO2 photocatalytic system, the aldehyde oxidation in homogeneous solutions initiated by single electron oxidant (•)OH and SO4(•-) radicals inserted a small O atom of H2O into the product acids. The detailed experiments, combined with DFT calculation, revealed the existence of a new pathway for the oxidation of aldehyde on TiO2, in which, analogous to oxidation of aldehyde by dehydrogenase, the aldehyde undergoes a hydration first and subsequently is oxidized through a two-electron transfer process. The present study highlights the multielectron characteristic of TiO2 photocatalytic oxidation and can have implications for the oxidation of aldehyde in the environment.

Unraveling the photocatalytic mechanisms on TiO2 surfaces using the oxygen-18 isotopic label technique

During the last several decades TiO₂ photocatalytic oxidation using the molecular oxygen in air has emerged as a promising method for the degradation of recalcitrant organic pollutants and selective transformations of valuable organic chemicals. Despite extensive studies, the mechanisms of these photocatalytic reactions are still poorly understood due to their complexity. In this review, we will highlight how the oxygen-18 isotope labeling technique can be a powerful tool to elucidate complicated photocatalytic mechanisms taking place on the TiO₂ surface. To this end, the application of the oxygen-18 isotopic-labeling method to three representative photocatalytic reactions is discussed: (1) the photocatalytic hydroxylation of aromatics; (2) oxidative cleavage of aryl rings on the TiO₂ surface; and (3) photocatalytic decarboxylation of saturated carboxylic acids. The results show that the oxygen atoms of molecular oxygen can incorporate into the corresponding products in aqueous solution in all three of these reactions, but the detailed incorporation pathways are completely different in each case. For the hydroxylation process, the O atom in O₂ is shown to be incorporated through activation of O₂ by conduction band electrons. In the cleavage of aryl rings, O atoms are inserted into the aryl ring through the site-dependent coordination of reactants on the TiO₂ surface. A new pathway for the decarboxylation of saturated carboxylic acids with pyruvic acid as an intermediate is identified, and the O₂ is incorporated into the products through the further oxidation of pyruvic acid by active species from the activation of O₂ by conduction band electrons.

Catalytic role of TiO(2) terminal oxygen atoms in liquid-phase photocatalytic reactions: oxidation of aromatic compounds in anhydrous acetonitrile

On the basis of experiments carried out with controlled amounts of residual oxygen and water, or by using oxygen-isotope-labeled Ti(18) O2 as the photocatalyst, we demonstrate that (18) Os atoms behave as real catalytic species in the photo-oxidation of acetonitrile-dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO2 . The experimental evidence allows a terminal-oxygen indirect electron-transfer (TOIET) mechanism to be proposed, which is a new pathway that involves the trapping of free photogenerated valence-band holes at Os species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO2 surface and their subsequent healing with oxygen atoms from either O2 or H2 O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO2 surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO2 photocatalytic phenomena.

Fabrication of bidirectionally doped β-Bi2O3/TiO2-NTs with enhanced photocatalysis under visible light irradiation

Stable β-Bi2O3/TiO2-NTs photocatalyst with excellent visible-light-activity is successfully prepared by bidirectional doping. Stake structure of the TiO2-NTs provides a larger specific surface area and makes the contact area between the TiO2-NTs and β-Bi2O3 much larger; The stake structure of TiO2-NTs not only leads to a firmer combination of TiO2-NTs and β-Bi2O3, but also makes them dope one another deeply. The modification of Bi species into TiO2-NTs can form Bi-O-Ti chemical absorption bonds, then a localized impurity level is generated within the band gap. Electrons can be excited and transferred from the Bi(3+) impurity level to the conduction band (CB) of TiO2, similar to narrowing the band-gap of TiO2-NTs, resulting in a red shift of the absorption edge and an enhancement in visible-light activity. During annealing, Bi atoms are partially replaced by Ti atoms. The lattice of β-Bi2O3 is compressed around the Ti impurity, making the lattice dislocate and distort. This dislocation and distortion leads to an increase in the β-Bi2O3 valance band (VB), from 2.02 to 2.28 eV. Accordingly, the weak oxidability of β-Bi2O3 is improved, and its photocatalytic ability is further enhanced. Moreover, this lattice dislocation and distortion changes the Bi-O distances, thus remarkably improving the stability of the β-Bi2O3/TiO2-NTs.